concentration quenching effect in rare-earth doped glasses · 2017. 7. 21. · iii declaration of...

UNIVERSITY OF ARIZONA

MASTER’S THESIS

Concentration quenching effect in

rare-earth doped glasses

Author:

Chun XIA

Supervisor:

Dr. XiuShan ZHU

A thesis submitted in fulfillment of the requirements

for the degree of MASTER OF SCIENCE

in the

Photonics Sciences and Technology Group

College of Optical Science

May 11, 2017

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iii

Declaration of AuthorshipI, Chun XIA, declare that this thesis titled, “Concentration quenching effect in rare-

earth doped glasses” and the work presented in it are my own. I confirm that:

• This work was done wholly or mainly while in candidature for a research de-

gree at this University.

• Where any part of this thesis has previously been submitted for a degree or

any other qualification at this University or any other institution, this has been

clearly stated.

• Where I have consulted the published work of others, this is always clearly

attributed.

• Where I have quoted from the work of others, the source is always given. With

the exception of such quotations, this thesis is entirely my own work.

• I have acknowledged all main sources of help.

• Where the thesis is based on work done by myself jointly with others, I have

made clear exactly what was done by others and what I have contributed my-

self.

Signed: Chun Xia

Date:05/09/2017

v

“Thanks to my solid academic training, today I can write hundreds of words on virtually any

topic without possessing a shred of information, which is how I got a good job in journalism.”

Dave Barry

vii

University of Arizona

Abstract

Faculty Name

College of Optical Science

MASTER OF SCIENCE

Concentration quenching effect in rare-earth doped glasses

by Chun XIA

Concentration quenching effect in ytterbium (Yb) and neodymium (Nd) doped phos-

phate glasses, thulium (Tm) doped germanate glass, and praseodymium (Pr) doped

tellurite glass were studied. The fluorescence and lifetime of these rare-earth doped

glasses with different concentrations were measured. Ion pair and clustering are

included in the model to explain the quenching effect occurring in highly doped

glasses. This study will help us in designing and fabricating high unit gain optical

fibers.

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ix

Acknowledgements

I would like to take the time to first thank those who have helped me develop and

understand lasers fundamentally, most notably, my advisor Xiushan Zhu. Their abil-

ity to patiently answer my questions was invaluable to the experience I developed

throughout this project.

People to also thank are my colleagues for their wonderful collaboration, this project

would have been impossible without the support of them, especially JingWei Wu,

JunFeng Wang and YunXiu Ma for opinions and ideas to point me in the right di-

rections, Lenoid Kotov and Patrick Keiffer always helping me out when running

out of experimental equipments, and MingHong Tong for discussing the working

principles of multiple lasers and energy levels.

Finally and most importantly, I would like to thank my parents, who I love dearly,

for always being there for me under any experience I was facing in life.

xi

Contents

Declaration of Authorship iii

Abstract vii

Acknowledgements ix

List of Figures xv

List of Tables xix

1 Introduction 1

1.1 History . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

1.2 Areas of Interest . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

2 Spectroscopy of rare earth ions 9

2.1 Energy level broadening mechanisms . . . . . . . . . . . . . . . . . . . 10

2.2 Non-radiative transitions . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

2.3 Ion-ion interactions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

2.3.1 Cross relaxation . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

2.3.2 Up-conversion energy transfer . . . . . . . . . . . . . . . . . . . 14

2.3.3 Fluorescence quenching . . . . . . . . . . . . . . . . . . . . . . . 15

2.4 Mechanisms Affecting the Lifetime . . . . . . . . . . . . . . . . . . . . . 15

2.4.1 Spontaneous Emission . . . . . . . . . . . . . . . . . . . . . . . . 16

2.4.2 Self absorption . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

2.4.3 Phonon Decay . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

2.5 Modeling of clustering effect induced concentration quenching . . . . 17

3 Experimental arrangement and techniques 21

3.1 Absorption cross section of rare-earth doped glasses . . . . . . . . . . . 21

xii

3.2 Emission cross section . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22

3.2.1 Construction of the spectrometry system . . . . . . . . . . . . . 22

3.2.2 Alignment procedure . . . . . . . . . . . . . . . . . . . . . . . . 26

3.3 Fluorescence lifetime measurements . . . . . . . . . . . . . . . . . . . . 27

OPO laser method . . . . . . . . . . . . . . . . . . . . . . . . . . 29

Steady state illumination method . . . . . . . . . . . . . . . . . . 29

3.3.1 The fluorescence decay . . . . . . . . . . . . . . . . . . . . . . . . 31

4 Ytterbium doped phosphate glass 35

4.1 Ytterbium rare-earth ion doped glasses . . . . . . . . . . . . . . . . . . 35

4.1.1 Energy level and absorption spectrum . . . . . . . . . . . . . . . 36

4.1.2 Emission spectrum . . . . . . . . . . . . . . . . . . . . . . . . . . 36

4.1.3 Quenching effect and emission cross section . . . . . . . . . . . 39

4.1.4 Fluorescence lifetime measurements with short pulse method . 40

4.1.5 Lifetime measuremnt with Switched illumination method . . . 43

4.1.6 Summarize . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45

5 Neodymium doped phosphate glass 47

5.1 Energy level and absorption cross section . . . . . . . . . . . . . . . . . 47

5.2 Quenching effect and emission spectrum . . . . . . . . . . . . . . . . . 49

5.3 Concentration quenching effect in Neodymium doped fiber of differ-

ent concentration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51

5.4 Fluorescence lifetime measurements with short pulse method and switched

illumination method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51

6 Thulium doped germanium glass 57

6.1 Energy level and absorption spectrum . . . . . . . . . . . . . . . . . . . 57


6.3 Fluorescence lifetime measurements with short pulse method and switched

illumination method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 59

7 Praseodymium doped tellurite glass 65

7.1 Energy level and absorption spectrum . . . . . . . . . . . . . . . . . . . 65

7.2 Absorption Cross section . . . . . . . . . . . . . . . . . . . . . . . . . . . 66

xiii


7.4 Fluorescence lifetime measurements with short pulse method . . . . . 67

8 Conclusion 69

8.1 Fluorescence lifetime in rare-earth doped materials . . . . . . . . . . . 69

8.1.1 Perspective work . . . . . . . . . . . . . . . . . . . . . . . . . . . 70

Bibliography 71

xv

List of Figures

2.1 Rare-earth ions of focus in this thesis . . . . . . . . . . . . . . . . . . . . 9

2.2 Broadening effect when rare-earth ions are doped into host materials . 10

2.3 Non-radiative decay rate as a function of energy gap for various glass

and host materials. The vertical lines indicate the energy gap between

the lowest Stark level of the ith excited manifold, to the highest Stark

level from the next lowest, jth manifold.[20] . . . . . . . . . . . . . . . . 12

2.4 Cross relxation of Tm3+ between the 3H4 and 3H6 manifold . . . . . . 14

3.1 Absorption and emission processes . . . . . . . . . . . . . . . . . . . . . 22

3.2 Experimental setup for fluorescence spectrum measurement . . . . . . 23

3.3 Schematic diagram for fluorescence spectrum measurement . . . . . . 23

3.4 Relative position of fiber laser and objective lens . . . . . . . . . . . . . 27



3.7 Fluorescence decay acquired by the 1012 TDS oscilloscope . . . . . . . 30

3.8 The power fluctuation of the 808 nm diode laser. . . . . . . . . . . . . . 31

3.9 Fitting procedure for the fluorescence decay waveforms.The red dots

are normalized raw data acquired with oscilloscope and the black line

are the fitted line with the least square method. . . . . . . . . . . . . . . 33

4.1 Splitted energy level of ytterbium . . . . . . . . . . . . . . . . . . . . . . 36

4.2 Absorption spectrum for ytterbium doped rare-earth ion for different

concentrations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37

4.3 Absorption cross section for ytterbium doped rare-earth ion of differ-

ent concentrations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38

4.4 Experimental data plot for absorption spectrum and emission spectrum 39

xvi

4.5 Calculated absorption cross section and emission cross section with

zeroline wavelength at 974.7 nm . . . . . . . . . . . . . . . . . . . . . . 40

4.6 Normalized emission spectrum for ytterbium doped rare-earth ion for

different concentrations . . . . . . . . . . . . . . . . . . . . . . . . . . . 41

4.7 Pulse train behavior of CONTINUUM Surelite OPO laser’s idle signal. 41

4.8 Lifetime for Ytterbium doped glasses of different concentrations pumped

with varing pump power. . . . . . . . . . . . . . . . . . . . . . . . . . . 42

4.9 Lifetime measurement of Ytterbium doped phosphate glass with dif-

ferent concentration using short pulse method and switched illumi-

nation method. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44

4.10 Absorption spectrum for ytterbium doped rare-earth ion for different

concentrations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44

5.1 Neodymium rare-earth ion energy level diagram . . . . . . . . . . . . . 47

5.2 Absorption spectrum for Neodymium doped rare-earth ion of vary-

ing concentrations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 48

5.3 Absorption cross section for Neodymium doped rare-earth ion of vary-

ing concentrations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49

5.4 Absorption and emission spectrum plot for neodymium rare-earth ion 50

5.5 Normalized emission spectrum for Neodymium doped phosphate glass

of different concentration . . . . . . . . . . . . . . . . . . . . . . . . . . 50

5.6 The ASE spectrum for fiber of different lengths pumped with 808 nm

diode laser source. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52

5.7 The ratio of the peaked value with that of the 1 cm long fiber . . . . . . 52

5.8 The measured lifetime with different concentrations pumped with 808

nm short pulse of varying pump power. . . . . . . . . . . . . . . . . . . 53

5.9 The measured lifetime with varying concentration pumped with 875


5.10 A comparison between lifetime measured with short pulse method

and function generator method . . . . . . . . . . . . . . . . . . . . . . . 54

xvii

6.1 Simplified energy level diagram of Thulium ion, the possible emission

from the 3F3 energy level is showed together with the peak central

wavelengths . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 58

6.2 Absorption spectrum for Thulium doped rare-earth ion of varying

concentrations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 59

6.3 Absorption cross section for Thulium doped rare-earth ion of varying

concentrations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 60

6.4 Emission spectrum plot for neodymium rare-earth ion pumped with

793 nm laser diode. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61

6.5 The measured lifetime with varying concentration pumped with 1800


6.6 A comparison between lifetime measured with 808 nm and 1800nm

pump wavelength . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 62

6.7 The data analysis for fluorescence lifetime measurement with func-

tion generator method. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63

6.8 The inverse of the measured lifetimes are plotted with concentration. . 64

7.1 The involved energy level of Ytterbium and Praseodymium for 1.3

µm emission . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 66

7.2 Absorption cross section of Praseodymium trivalent state. . . . . . . . 67

7.3 The inverse of the measured lifetimes are plotted with concentration. . 68

xix

List of Tables

1.1 Transition Wavelength for Rare-earth Ions . . . . . . . . . . . . . . . . . 5

5.1 Neodymium concentration resulting in lifetime reduction for the 1064

nm emission. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49

6.1 Fluorescence lifetime of Thulium manifolds in silica glass, the non-

radiative can then be calculated by the difference between the recip-

rocal of the radiative and observed lifetime. Note: N/A - not reported 57

1

Chapter 1

Introduction

The field of fiber optics is now around over forty-five years old, nonetheless, it still

draws growing and considerable interest and attention. This is largely due to the

accelerated rate of development and the commercialization of optical fiber related

devices and optics-based technology[1] . There are many driving forces behind this

undiminished interest such as the benefits that optical fibers provided in telecom-

munications, increasing need in higher output power and smaller size for integrated

devices[2].

One of the most significantly applications of fiber optics is in transoceanic links,

long haul and local optical communication network. The optical telecommunica-

tions went through a revolution after the erbium-doped optical fiber amplifier was

introduced in 1987. In order to explore the potentials of becoming more compact

and easy to integrate, highly doped erbium doped fibers were pervading to mini-

mize the gain length and dispersion introduced by the amplifying fiber[3]. Strong

interest also arose when researches were trying to fabricate highly doped erbium

doped waveguide, heavily doped erbium doped fiber in mode-locked fiber laser as

well as high unit gain thin disk laser. That resulted in a deeper studies of the rela-

tionship between EDF(Erbium doped fiber) performance and erbium ion concentra-

tion. However, as the concentration goes higher, many negative effects occur such

as higher lasing threshold, non-flat gain curve and nonsaturable absorption which

stop it from having a high quantum efficiency as a result of the pair and clustering

effects[4][5].

2 Chapter 1. Introduction

Four rare-earth ion doped glasses are studied in this thesis, which are Ytterbium(Yb),

Neodymium(Nd), Thulium(Tm) and Praseodymium(Pr). The ytterbium is attrac-

tive for it’s application in all solid-state diode pumped lasers emitting at 1 µm due

to its clean energy level and low thermal loading[6]. Neodymium is one the most

suitable material as to pumping at 1 µm due to its sharp absorption line. In order

to further increase the pump absorption, the use of highly concentrated, shorter

Neodymium doped laser material is a must[7]. Thulium draws people’s interest

for its telecommunication purpose at around 1.8 µm as well as another near infrared

emission around 2.3 µm for gas detection, medicine and remote sensing. In order

to generate laser radiation most efficiently for different base material, a map be-

tween gain for interested wavelength range and concentration is needed for lower

pump power and higher output power[8]. The praseodymium based fiber ampli-

fier(PDFA) has motivated intense interest in 1.3 µm fiber amplifiers. While a strong

quenching effect occurs as dopant level goes higher[9].

1.1 History

Since the first LASER was first demonstrated by Theodore Maiman in 1960 using

Ruby, Izuo Hayashi and Morton B. Panish of Bell Labs design the first semiconduc-

tor laser that operates continuously at room temperature in 1971. Charles H. Henry

invents the quantum well laser[10], which requires much less current to reach lasing

threshold than conventional diode lasers and which is exceedingly more efficient in

1972. Multiple laser designs were carried out to meet the demand for better effi-

ciency, smaller size and higher power[11]. The fiber laser has a history almost as

long as that of the laser itself and quickly draw people’s attention after it’s appear-

ance. Since its invention in 1963 by Elias Snitzer, the fiber laser required almost two

decades of development before the first commercial devices appeared on the market

in the late 1980s.These lasers used single-mode diode pumping, emitted a few tens

of milliwatts, and attracted users because of their large gains and the feasibility of

single-mode continuous-wave (CW) lasing for many transitions of rare-earth ions

1.1. History 3

not achievable in the more-usual crystal-laser version. The most well-known appli-

cation of fiber-laser technology is in 1550-nm erbium-doped fiber amplifiers[12].

Today, optical fiber communication has been established as one of the most promis-

ing technologies for medium and long distance data transmissions.(Ji, 2005). One of

the biggest issue limits the speed of optical communication is BER(Bit Error Rate),

in order to compensate the loss of the wave-guide, an optical amplifier is needed.

Doped fiber amplifier(DFA) is an optical amplifier which uses rare-earth doping

material which are: Erbium, Praseodymium , Europium, Neodymium, Terbium,

Lutetium, Ytterbium, Holmium, Lanthanum and Thulium, inside the fiber. The

first rare earth doped material of Neodymium Nd3+ used in a singe-mode fiber was

demonstrated in 1960 (James, 1991)[13].

The previous studies have usually assumed that fiber parameters are not dependent

on erbium ion concentration in the fiber[14]. Recently, rare earth element doped

fibers are also commonly used in fiber lasers. As they are becoming more compact

and integrated. Strong interest in heavily doped erbium-doped waveguides and in

the applications of heavily doped EDF in mode-locked fiber lasers resulted in sev-

eral recent studies of the dependence of EDF performance on erbium ion concentra-

tion[15]. Now, research efforts has shifted to the optimization of the erbium-doped

fiber design as a function of concentration, while the performance of erbium-doped

fiber amplifier has a strong dependence on the erbium ion concentration, which is

then improved both experimentally and theoretically[16].

This thesis presents the experimental setup for the measurements of desired up-

per energy level lifetimes and fluorescence spectrum. The fluorescence lifetimes of

rare earth ion, Praseodymium, Ytterbium, Thulium and Neodymium, doped galsses

were studied in this thesis to determine whether the fluorescence lifetime could be

a probe to determine quenching effect for different concentrations of rare-earth ions

and an accurate modeling of the pairing effect and clustering effect which confines

the performance of high doping conditions. The lifetime of different base materials

were also studies as a comparison purpose.

Rare earth ions have a long history in optical application, they distinguish them


from other optically active ions for the following perspective: First, the absorption

and emission transitions are relatively insensitive to different host materials. Second,

they have a relatively long lifetimes of metastable states. All these properties results

from the nature of the electron distribution and lead to excellent performance of rare

earth ions in multiple optical applications.

For many laser applications, however, watts of optical power rather than milliwatts

are required, which leads to a rapid penetration of fiber devices into many different

applications formerly dominated by other lasers. Fiber laser has many advantages

that differentiate them in terms of practicality, functionality and performance. All

the features listed below have played an important role as to the establishment of

the commercial interest, driving the relatively rapid development and practical de-

ployment of fiber lasers:

Fully fiberized cavities, which allows robust and compact system designs without

careful alignment of free-space components. Relative broad gain linewidths (up to

20 THz), giving a wide wavelength tunability and ultrashort pulse duration. Robust

transverse mode working at mono-mode fibers. It allows a significant degree of

freedom from the thermally induced mode distortions compared with the bulk solid-

state lasers. Possibility of high gains, offering the option of master oscillator power

amplifier(MOPA) schemes[4].

A fiber laser is a kind of laser in which the active gain medium is an optical fiber

doped with different kinds of rare-earth elements such as erbium, ytterbium, thulium,

holmium and neodymium. The same system is also related to amplifier. The dop-

ing fiber will provide light amplification instead of lasing with the optical resonator

cavity not exist.

1.2 Areas of Interest

Fundamental to these applications are the selection of rare-earth ions, doping base

materials, dopant concentration and dopant distribution within the active region of

the fiber. Former researches has published many papers studying the concentration

1.2. Areas of Interest 5

TABLE 1.1: Transition Wavelength for Rare-earth Ions

Operating range(nm) Dopant ion Transition Type of transition455 Tm3+ 1D2→ 3F4 UC, ST480 Tm3+ 1G4→ 3H6 UC, ST490 Pr3+ 3P0→ 3H4 UC, ST520 Pr3+ 3P1→ 3H5 UC, ST550 Ho3+ 5S2 , 5F4→ 5I8 UC, ST550 Er3+ 4S 3

2→ 4I 3

2UC, ST

601 - 618 Pr3+ 3P0→ 3H6 UC, ST631 - 641 Pr3+ 3P0→ 3F2 UC, ST

651 Sm3+ 4G 52→ 6H9/2 UC, ST

707 - 725 Pr3+ 3P0→ 3F4 UC, ST753 Ho3+ 5S2, 5F4→ 5I7 UC, ST

803 - 825 Tm3+ 3H4→ 3H6 UC, ST850 Tm3+ 4S3/2→ 4I11/3 UC, ST

880 - 886 Pr3+ 3P1→ 1G4 UC, ST902 - 916 Pr3+ 3P1→ 1G4 UC, ST900 - 950 Nd3+ 4F3/2→ 4I9/2 UC, ST970 - 1040 Y b3+ 5F5/2→ 5F7/2 UC, ST980 - 1000 Er3+ 4I11/12→ 4I15/2 UC, ST1000 - 1150 Nd3+ 4I3/2→ 4I11/2 UC, ST1060 - 1100 Pr3+ 1D2→ 3F4 UC, ST1260 - 1350 Pr3+ 1G4→ 3H5 UC, ST1320 - 1400 Nd3+ 4F3/2→ 4I13/2 UC, ST

1380 Ho3+ 5S2, 5F4→ 5I5 UC, ST1460 - 1510 Tm3+ 3H4→ 3F4 UC, ST

1510 Tm3+ 1D2→ 1G4 UC, ST1500 - 1600 Er3+ 4I13/2→ 4I15/2 UC, ST

1660 Er3+ 2H11/2→ 4I9/2 UC, ST1660 Er3+ 2H11/2→ 4I9/2 UC, ST1720 Er3+ 4S 3

2→ 4I 9

2UC, ST

1700 - 2015 Tm3+ 3F4→ 3H6 UC, ST2040 - 2080 Ho3+ 3H4→ 3H5 UC, ST2250 - 2400 Tm3+ 3H4→ 3H5 UC, ST

2700 Er3+ 4I11/2→ 4I13/2 UC, ST2900 Ho3+ 5I6→ 5I7 UC, ST

profile of rare-earth elements across the fiber core and come up with mature fiber

manufacture techniques and processes to realize a delicate control of dopant radial

distribution whether it’s a step profile or some other novel profile designs. It then

enables us to do modeling and characterizing laser and amplifier response without

worries about the concentration distribution having an impact on parameters such

as gain, saturation and efficiency.


The gain response of an erbium-doped fiber could behave significantly different de-

pending on the transverse distribution profiles of the fiber design. Furthermore,

the gain is a function of overlapping factor between the dopant area and the signal

mode-field area. In addition, a non flat gain curve could be possible to obstruct the

realization of a constant gain for the whole input bandwidth.

The lasing threshold for laser, gain and efficiency for laser and amplifier also depend

strongly on the dopant level. Take the EDFA(Erbium doped optical fiber amplifier)

as an example, in order to get satisfying gain for erbium-doped fiber amplifiers op-

erating in the C-band, the erbium concentration of only a few hundred parts per

million(ppm) is needed, while a significantly longer length of fiber is required when

operating in the L-band(1565 - 1610 nm) because of the lower gain in the L-band.

To shrink the size of the amplifier, an easy way is to increase the dopant concentra-

tion level for higher efficiency since more pump is absorbed. Alternatively, shorter

length of fiber and less pump power are required as a result of higher gain per meter

resulting in lower cost of materials. The same is true for lasers and other devices or

applications. However, many negative effects could occur as the dopant concentra-

tion increases, take laser as an example, some paper claim that as the concentration

goes higher, the lasing threshold goes bigger and the lasing efficiency falls off. It’s

because in previous models, doped rare-earth ions are treated as separate individ-

uals, but as the concentration of doped rare-earth ion increases, the decrement of

ion-ion separation directly enables the ion-ion interactions to occur[17].

Quenching effect could results from many different energy transfer channels, some

is concentration dependent such as mltiphonon decay and sidebands but the others

are concentration dependent such as cross-relaxation and cooperative upconversion

process. Most of the impetus given to the understanding of the processes involved in

ion-activated solids for the last sixteen years are due to the development of new flu-

orescent light sources, either coherent or incoherent,and the need to improve them.

Pairing effect was first put forward around 1980s to describe the behavior of en-

ergy transfer inside the pumped fiber and lasing process with rate equation[18].

But that suffocate the case for ultra high dopant condition, researches then put for-

ward clustering effect to further explain the behavior of energy transfer processes

1.2. Areas of Interest 7

as the distance between neighboring ions becomes even smaller which enables en-

ergy transfer between clusters. That provides a decent explanation to explain the

non saturation effect. It is then practical to increase the dopant concentration to

the most efficient level when taking account of the gain and quenching effect into

consideration instead of arbitrarily increasing dopant concentration. Take erbium-

doped pure silica fibers without modifiers(additional components incorporated to

modify the structure of the glass and increase the solubility of rare-earth ions) as an

example, the optimum dopant concentration of erbium should be less than 100 ppm

while great deterioration of the lasing threshold and gain characteristics occurs with

concentrations around and below 1000 ppm[19].

The behavior of the rare-earth ions is not only a function of dopant concentration,

but also greatly influenced by base material such as silica and phosphate. Thus an

universal physical parameter to describe the behavior and determine these parame-

ters of dopants are sought.

9

Chapter 2

Spectroscopy of rare earth ions

Aiming to model and predict the behavior of rare-earth elements doped fiber de-

vices, to reach a good understanding of spectroscopic properties of rare-earth ions

is an necessity. This chapter reviews the physics of fundamental atomic structure of

rare-earth ions and there interaction with doped base materials such as glass.

Rare-earth elements family has a group of 15 elements known as Lanthanides, they

are most stable when in triply ionized form. The rare-earth ions have an electrical

structure identical to xenon plus some number of 4f electrons ranging from 1-14

when they are at the trivalent ionization state, i.e. 1s2, 2s2, 2p6, 3s2, 3p6, 3d10, 4s2,

4p6, 4d10, 4fN , 5s2, 5p6, 6s0, 5d0(6s and 5d electrons are removed for trivalent state).

The remaining 4f shell of electrons have a bigger radius, and therefore has a shield

effect from the outer environment which results in 4f → 4f optical transitions be-

ing relative sharp and insensitive to the change of host materials. The rare earth

elements of particular interests for this thesis are highlighted in figure 2.1.

FIGURE 2.1: Rare-earth ions of focus in this thesis

10 Chapter 2. Spectroscopy of rare earth ions

2.1 Energy level broadening mechanisms

For a single isolated rare earth ion, a very sharp emission indicates that the energy

level is very thin and narrow. But the well defined set of energy levels cannot be

completely described when doped into different host materials. The energy levels

are then broadened due to several energy broadening mechanisms.

The first mechanism is termed homogeneous broadening mechanism which applies

equally to all ions wherever the site is in a system. The most common and simple

form of broadening is lifetime broadening as a result of Heisenberg’s uncertainty

principle. Heisenberg’s principle states that the characteristic lifetime of an given

energy manifold τ and the energy of that particular manifold will have a well known

relationship of ∆Eτ ≥ h̄/2. Since the energy manifolds of rare-earth ions have a

typical lifetime of 10−3 - 10−4s , the amount of broadening will then have a number

ranging from 10−8 - 10−4 cm−1. While a bigger contribution to the homogeneous

broadening mechanism of an energy manifold actually comes from the transitions

between Stark levels within the same energy manifold. The size of the Stark levels

will determine whether the energy transfer between the ions and host materials can

occur. For example, if the energy separation has a amount as small as a few 100

cm−1, the energy can then be transferred to the host material’s vibration mode when

process such as absorption, emission occur.

FIGURE 2.2: Broadening effect when rare-earth ions are doped intohost materials

2.1. Energy level broadening mechanisms 11

Inhomogeneous broadening arises because different rare-earth ions contribute not

identically due to the sites occupied by the rare-earth ions not identical. It could be

easily observed in crystals especially when a large amount of defects or imperfeca-

tions exist in the crystal structure. But the impact is acturally very small compared

to that of glass materials. In glass, inhomogeneous broadenging become the most

dominant broadening mechanism. The disordered distribution nature of glasses

provides all possible environments to the rare-earth ions, each with a characteris-

tic set of field parameters. It could be favored or discarded at the same time. As to

EDFA, a broad gain band is favored while it’s just the opposite as to lasing. Both

the absorption and emission spectra of rare-earth ions measured in glass exhibit a

continuous broadband spectrum where the position of the individual Stark levels

of the manifold are masked by the inhomogeneous linewidth, and this also shows

considerable differences to that of crystals.

The Hamiltonian could be written for an individual ion under the condition of weak

interaction of the 4f electrons from other ions.

H(t) = H0 +HElectron−Electron +HSpin−Orbit +Hmaterialfield

, where H0 is the Hamiltonian of an free ion in complete isolation, HElectron−Electron

is the Hamiltonian which describes the interaction between the 4f electrons with

each other with the form of Coulomb interaction once the centrally symmetric con-

tribution has been removed, HSpin−Orbit describes the interactions between spin and

orbit momentum.

The Hamiltonian can also be written for an individual rare-earth ion

H(t) = H0 + Vbasematerial + VEM (t) + Vion−ion(t)

, whereH0 still describes the Hamiltonian of an ion of complete isolation, Vbasematerial

is the Hamiltonian describing the interaction between base material environment

and a rare-earth ion. VEM (t) describes the corresponding behavior when ion inter-

acts with electromagnetic field, Vion−ion(t) describes the interaction of rare-earth ions

themselves. The first two terms in the equation have a static perturbations which are


time independent while the last two terms are time dependent.

2.2 Non-radiative transitions

As predicted by Judd-Ofelt theory, the radiative lifetimes as well as the transition

strengths are independent of the energy separation between the absorption and

emission states. This contradicts with the experimental results observed through-

out the 1960s that fluorescence of different host materials could only be detected

with energy separations not smaller than 1000 cm−1. This verified the existence of

another possible energy transfer channel termed non-radiative decay. Non-radiative

transitions occur with the help of phonon(quanta of vibrational energy in the host

material) for both absorption and emission processes whereas radiative transitions

involve the emission and absorption of photons. The non-radiative decay could oc-

cur efficiently and rapidly when electronic energy separations are close enough to

be bridged by emission or absorption of phonons.

FIGURE 2.3: Non-radiative decay rate as a function of energy gapfor various glass and host materials. The vertical lines indicate theenergy gap between the lowest Stark level of the ith excited manifold,

to the highest Stark level from the next lowest, jth manifold.[20]

The non-radiative processes could then be favored or suppressed by carefully select-

ing of the host material.

2.3. Ion-ion interactions 13

2.3 Ion-ion interactions

The radiative and non-radiative processes described in the previous section thus far

have only taken the circumstance of a single rare-earth ion into consideration. That

works well for concentration that is significantly low since the ions are distributed

evenly throughout the glass matrix, the relatively large distance prevents the in-

teraction between neighboring ions. However, when the dopant concentration is

increased, pairing effect or clustering effect could occur for ultra high concentration.

The term Vion−ion will then be taken into consideration of the dynamic Schrödinger

equation. Various forms of ion-ion interactions are discussed in the following sec-

tions.

2.3.1 Cross relaxation

The energy exchange can occur in different forms during the interacting process.

For most of the cases, the energy transfer process happens between a pair of ions,

with the ion located at the excited state the excited ion the ’donor’ ion and the one

receiving the photon energy the ’acceptor’ ion. Cross relaxation is a very special case

of ion-ion energy transfer since the energy from and excited donor is transferred to a

neighboring ion, promoting the acceptor ion to a higher energy level with the donor

ion demoted to a lower energy state. It happens with a higher possibility if both the

donor and the acceptor have approximately the same energy gap. A very special

case is that the donor and the acceptor have the same energy gap, it is then termed

as resonant since the energy gaps are matched. The process can also occur with the

help of phonons if an energy mismatch exists between the energy gaps, this type of

energy transfer is then termed as phonon assisted. It’s worth mentioning that the

energy transfer process is faster if the phonons are emitted than absorbed.[47]

Depending on the application, the effects of cross relaxation could be beneficial and

detrimental at the same time. If ions needs to be prompted to higher energy level

with the help of cross relaxation, then the process could be extremely favorable.

However, if one desires population inversion for the lasing circumstance, then the


initially excited donor ions are quenched resulting a higher lasing threshold and a

reduction in the lasing efficiency.

FIGURE 2.4: Cross relxation of Tm3+ between the 3H4 and 3H6 man-ifold

One of the most well-known cross relaxation process is in Tm3+ between the 3H4

and 3H6 manifold. The ion is first excited to the 3H4 upper energy level, it then

interacts with a nearby ion locating at the 3H6 ground state. The energy mismatch

between the transitions of 3H4→ 3F4 and 3H6→ 3F4 in silica is around 600-700 cm−1

and one phonon is required to assist the energy transfer process. The process could

be extremely useful as to increase the quantum efficiency of 2 µm lasing transition

between 3H6→ 3F4.[60]

2.3.2 Up-conversion energy transfer

A special case of cross relaxation is termed as energy transfer up-conversion(ETU).

Initially, both the donor and the acceptor ions are pumped to the excited state, part

of or all of its energy is then transferred to a nearby acceptor ion from the donor ion.,

prompting the acceptor ion to a higher energy level. For example, when the erbium

doped fiber being pumped at 980 nm or Ytterbium doped fiber being pumped at 915

nm, green light could be observed. It’s the fluorescence from the higher energy level

which is realized by up-conversion process. The energy mismatch could also be

2.4. Mechanisms Affecting the Lifetime 15

compensated with phonons, it becomes more obvious as pairing effects dominates

inside the optical fiber.

2.3.3 Fluorescence quenching

Fluorescence quenching is a process that could occur with any of the aforementioned

energy transfer channels. Most of the time it’s considered as an side effect since it

results in reduced excitation. After the energy is transferred from the donor ion to

the acceptor ion, the latter does not fluoresce, the energy could either be used to

fluoresce at a different wavelength or relaxes non-radiatively. The acceptor acts as

an energy sink and is then called the deactivator which results in a deduction of

quantum efficiency and an increment of the lasing threshold.

Quenching effect becomes stronger when the interaction between ions stronger as

the distance between neighboring ions becomes smaller. A strong quenching effect

will occur for high dopant concentration conditions, it is then termed as concentra-

tion quenching.

2.4 Mechanisms Affecting the Lifetime

The measured upper energy level lifetime could be affected by many processes whether

to increase or decrease the lifetime. The measured lifetime is the net result of all these

processes of energy transfer channels after prompted to the excited state. In gen-

eral, the measured lifetime could be calculated from the lifetime decay and could be

calculated by the equation below which consists of the radiative and non-radiative

rates:

τ =1

ARad +ANon−Rad

where τ is the measured fluorescence lifetime decay rate. Typically, the radiative de-

cay rates consists of two parts, the natural spontaneous emission and self-absorption.

The non-radiative decay could happen because of any mechanisms aforementioned


in the previous sections. But resonant energy transfer and cooperative energy trans-

fer could happen to very special circumstances. Whereas the phonon and impurity-

assisted concentration quenching effect is an universal quenching effect happens to

all the rare-earth ions at high dopant concentration. The following sections give a

more detailed description of these mechanisms.

2.4.1 Spontaneous Emission

The spontaneous emission probability ARad is the property of the upper excited en-

ergy level itself, and gives rise to the natural radiative lifetime. In experiment, ARad

could be determined by measuring the fluorescence lifetime as the dopant concen-

tration goes to zero. Alternatively, it can be calculated from the absorption spectrum

and can be given by [102]

ARad =1

τRad=

8πcn2

λ4pN

g

g′

∫σabs(λ)dλ

where c is the speed of light, n is the refractive index at the mean absorption wave-

length, λpis the peak absorption wavelength, N is the dopant concentration, g and

g′ are the degeneracies of the upper and lower energy levels respectively and σabs is

the absorption coefficient.

2.4.2 Self absorption

Self-absorption is also termed as self trapping or radiation-trapping, it’s a process

where an emitted photon through the spontaneous emission process is re-absorbed

by an ion located at the ground state instead of contributing to luminescence light.

The re-population of the excited state cause the effective lifetime to increase. This

process can occur for multiple times before the photon finally escaped the material.

This effect can only happen to resonant energy transfer where there is no energy

gap between them. This potentially needs the absorption ions of the same type as

the emitting ion, hence the term ’self absorption’. The bigger the absorption coeffi-

cient and the broader the σabs is, the stronger the self absorption has an impact on

2.5. Modeling of clustering effect induced concentration quenching 17

measured lifetime. The extend of the overlapping region between absorption and

emission spectrum will give us an idea of the likelihood of self-absorption. Self-

quenching can be described with a simple calculation.

ω =1

τe− 1τ0

where τe is the measured lifetime, τ0 is the intrinsic radiative lifetime and ω is the

additional contribution to the lifetime through the self-absorption energy transfer.ω

can then be used to describe the contribution to the lifetime by self-absorption, and

it has a linear relationship with the concentration.

2.4.3 Phonon Decay

Phonon is the bridge when interaction between dopant and base material would

occur. If the ground state manifolds and excited state manifolds have an energy gap

10200 cm−1, this would require at least 10 phonons to bridge the energy gap. Thus

it’s confident to say that the phonon assisted process is negligible to enable a high

radiative efficiency.

But for 2 µm emission, the phonon energy level of the base material has to be suffi-

ciently low to enable the radiative energy transfer.

2.5 Modeling of clustering effect induced concentration quench-

ing

As early as year 1993, researchers found the nonsaturable absorption effect in heav-

ily erbium-doped fibers which cannot be explained in the previous put forward pair-

ing effect.

The clustering and pairing effect in high concentration doped silica fibers is now

a well-accepted phenomenon, the modeling of clusters and pairs has considered a

fraction of the dopants exists in the form of ion pairs for simplicity purpose. The

main idea behind it is that an immediate energy transfer leads to an instantaneous


non-radiative relaxation of one ion at excited state. But in the case of dopant with

the concentration exceeds 40wt %, in such a material, the interaction between only

two ions cannot be applied to explain the nonsaturable experimental result.

The basic hypothesis requires the instantaneous energy transfer between two rare-

earth ions both excited to the same metastable state. But the probability of a pair with

two excited ions being excited at the same time is very small. Thus, a new hypothesis

needs to be made which is that each ion of a cluster can efficiently transfer its energy

to any of the other ions of the same cluster. Suppose a cluster consists of ions with a

number of n, then a succession of (n-1) fast relaxations leads to a situation that all of

the ions in a cluster but one are deexcited.

In an typical Erbium-doped single-mode fiber, the 980 nm absorption could be satu-

rated with a pump power at the order of some milliwatts. The physical meaning of

the power value corresponds to a saturation pump intensity Isaturation

Isaturation = h ≥ /στ

where≥ is the frequency of the pump power, σ is the emission cross-section between

two energy levels, and τ the metastable energy level lifetime, which promotes half of

the population of the corresponding rare-earth ions to their metastable energy level.

The presence of the clustering effect magnifies the non-radiative decay process, the

cluster ions are maintained in their fundamental state by their fast relaxations. Since

the excited state ions decay nearly simultaneously to the ground state, it’s impossible

to reach s state of saturation with half of the electrons promoted to the higher energy

level since the rapid relaxation constantly maintained one atom of each pair in the

ground state. And the nonsaturable absorption effect grow dramatically with the

rare-earth ion concentration.

where the Wij denotes the absorption rate between the i and j energy level and Wji

denotes the stimulated emission rate, and they have a relationship of,

Wij = Wji =σijIijh 6=ij

,

2.5. Modeling of clustering effect induced concentration quenching 19

Iij represents the intensity of the h 6=ij fluorescence light energy. γij stands for

the spontaneous relaxation rate corresponding to the i and j energy level. The lower

number denotes lower energy level, and higher number denotes higher energy level.

The rate equation for a homongeneously distributed scenario has an form showed

below.∂ah0∂t

= −(W10 +W20)ah0 + (W10 + γ10)ah1 +W20ah2 , (2.1)

∂ah1∂t

= W10ah0 − (W10 + γ10)ah1 + γ21ah2 , (2.2)

∂ah2∂t

= W20ah0 − (W20 + γ21)ah2 . (2.3)

Below is the rate equation for the clustering scenario.

∂ac0∂t

= −(W10 +W20)ac0 + (W10 + γ10 + Γac1)ac1 +W20ac2, (2.4)

∂ac1∂t

= W10ac0 − (W10 + γ10 + 2Γac1)ac1 + γ21ac2, (2.5)

∂ac2∂t

= W20ac0 + Γ(a

c1)

2 − (W20 + γ21)ac2. (2.6)

The clustering effect could play a negative effect in the fiber amplifiers, but it could

also have a positive influence from the effective energy transfer perspective. For

example, a highly Ytterbium doped fiber suffers from concentration quenching, but

it could be favored as to the Ytterbium - Erbium codoped fiber. Since the heavily

doped levels favor cluster formation, then the energy transfer between Ytterbium

and Erbium could be enhanced.

21

Chapter 3

Experimental arrangement and

techniques

In this chapter, a detailed experimental setup description for absorption spectrum,

lifetime measurement, emission spectrum and the theories behind it are explained.

The rare-earth ion lifetime is measured with two different methods, which are OPO

short pulse pump method and the square wave modulated switched illumination

method.

3.1 Absorption cross section of rare-earth doped glasses

To analyze the absorption properties when rare-earth ions are doped into glasses, a

broadband light source is required to cover all the wavelength range of interest, for

example, from 400 - 3000 nm. The light is first split into two beam, with the first as a

reference, and the second shined through samples.

The absorption spectrum is extremely helpful to determine the wavelength selection

as the excitation wavelength for lifetime and emission spectrum measurement by

looking at the corresponding absorption peak. The broadening effect of the absorp-

tion spectrum could also help to determine the quenching effect. The magnitude and

the width of the peak could also give us an idea of the possibility of non-radiative

energy transfer.

22 Chapter 3. Experimental arrangement and techniques

FIGURE 3.1: Absorption and emission processes

Room temperature absorption spectrum were measured with a Cary-5000 scan UV-

Vis-NIR spectrophotometer. The transmission spectrum is then obtained after base-

line correction.

3.2 Emission cross section

Fluorescence spectroscopy is a type of electromagnetic spectroscopy that analyzes

fluorescence from a sample excited by higher energy level. It involves using a beam

of light that excites the electrons to higher energy levels. Then the electrons can drop

down to one of the lower energy levels. So, in a typical fluorescence measurement,

the excitation wavelength is fixed and the detection wavelength varies. An emis-

sion map is measured by recording the emission spectra resulting from a range of

excitation wavelengths and combining them all together.

3.2.1 Construction of the spectrometry system

The experimental setup of the luminescence measurement system is shown in Fig

3.2.

The monochromator MS257 consisting of four 6000 lines/mm gratings that can op-

erate in spectrum region from 400 - 9000 nm. The first grating operates at 900 - 3000

3.2. Emission cross section 23

FIGURE 3.2: Experimental setup for fluorescence spectrum measure-ment

nm region, the second grating operates at 425 - 1600 nm region and the third grating

operates at 2500 - 9000 nm region.

FIGURE 3.3: Schematic diagram for fluorescence spectrum measure-ment

In the common Czerny–Turner design, the broad-band illumination source is aimed

at an entrance slit. The amount of light energy available for use depends on the

intensity of the source in the space defined by the slit and the acceptance angle of

the optical system. The slit is placed at the effective focus of a curved mirror so that

the light from the slit reflected from the mirror is collimated (focused at infinity). The

collimated light is diffracted from the grating and then is collected by another mirror,


which refocuses the light, now dispersed, on the exit slit . In a prism monochromator,

a reflective prism takes the place of the diffraction grating, in which case the light is

refracted by the prism.

At the exit slit, the colors of the light are spread out (in the visible this shows the

colors of the rainbow). Because each color arrives at a separate point in the exit-slit

plane, there are a series of images of the entrance slit focused on the plane. Because

the entrance slit is finite in width, parts of nearby images overlap. The light leaving

the exit slit contains the entire image of the entrance slit of the selected color plus

parts of the entrance slit images of nearby colors. A rotation of the dispersing ele-

ment causes the band of colors to move relative to the exit slit, so that the desired

entrance slit image is centered on the exit slit. The range of colors leaving the exit slit

is a function of the width of the slits. The entrance and exit slit widths are adjusted

together.

Fiber laser is mounted with 740 series(Newport) butterfly mount. The tempera-

ture and current is then controlled by LDC-3900(ILXLight) modular laser-diode con-

troller. The fluorescence light is then gathered by the objective lens with NA of 0.4.

After modulated by MC1000A(Thorlabs) optical chopper, the light is then focused

by another lens with the focal length of 45mm to collect the signal light into the slit.

A list of experimental details are listed below:

(a) The fluorescence is most often measured at a 90◦ angle relative to the excita-

tion light. This geometry is used instead of placing the sensor at the line of the

excitation light at a 180◦ angle in order to avoid interference of the transmit-

ted excitation light. No monochromator is perfect and it will transmit some

stray light, that is, light with other wavelengths than the targeted. An ideal

monochromator would only transmit light in the specified range and have a

high wavelength-independent transmission. When measuring at a 90◦ angle,

only the light scattered by the sample causes stray light. This results in a better

signal-to-noise ratio, and lowers the detection limit by approximately a factor

10000, when compared to the 180◦ geometry. Furthermore, the fluorescence

can also be measured from the front, which is often done for turbid or opaque

3.2. Emission cross section 25

samples.

(b) Correction of all these instrumental factors for getting a ‘standard’ spectrum

is a tedious process, which is only applied in practice when it is strictly neces-

sary. This is the case when measuring the quantum yield or when finding the

wavelength with the highest emission intensity for instance.

(c) As mentioned earlier, distortions arise from the sample as well. Therefore,

some aspects of the sample must be taken into account too. Firstly, photode-

composition may decrease the intensity of fluorescence over time. Scattering

of light must also be taken into account. The most significant types of scat-

tering in this context are Rayleigh and Raman scattering. Light scattered by

Rayleigh scattering has the same wavelength as the incident light, whereas

in Raman scattering the scattered light changes wavelength usually to longer

wavelengths.

(d) Other aspects to consider are the inner filter effects. These include reabsorp-

tion. Reabsorption happens because another ion absorbs the photon emitted

by other ions. Another inner filter effect occurs because of high concentrations.

The inner filter effects change the spectrum and intensity of the emitted light

and they must therefore be considered when analysing the emission spectrum

of fluorescent light.

(e) A lock in amplifier is a type of amplifier that can extract a signal with a known

carrier eave from an extremely noisy environment. Depending on the dynamic

reserve of the instrument, signals up to 1 million times smaller than noise com-

ponents, potentially fairly close by in frequency, can still be reliably detected.

It is essentially a detector followed by low-pass filter that is often adjustable

in cut-off frequency and filter order. Usually sine and cosine demodulation is

performed simultaneously, which is sometimes also referred to as dual-phase

demodulation. This allows the extraction of the in-phase and the quadrature

component that can then be transferred into polar coordinates. The signal can

than be extracted and analyzed.


(f) A monochromator is an optical device that transmits a mechanically selectable

narrow band of wavelengths of light chosen from a wider range of wave-

lengths available at the input. Diffraction grating is used to separate the colors

of light. The light enters through the hypotenuse face and is reflected back

through it, being refracted twice at the same surface. Nowadays, the Czerny-

Tnrner design is commonly implemented in the monochromator design. The

broad-band illumination source (A) is aimed at an entrance slit(B). The amount

of light energy available for use depends on the intensity of the source in the

space defined by the slit and the acceptance angle of the optical system. The

slit is placed at the effective focus of a curved mirror so that the light from the

slit reflected from the mirror is collimated. The collimated light is diffracted

from the grating and then is collected by another mirror, which refocuses the

light, now dispersed, on the exit slit. At the exit slit, the colors of the light are

spread out. Because each color arrives at a separate point in the exit-slit plane,

there are a series of images of the entrance slit focused on the plane. Because

the entrance slit is finite in width, parts of nearby images overlap. The light

leaving the exit slit contains the entire image of the entrance slit of the selected

color plus parts of the entrance slit images of nearby colors. A rotation of the

dispersing element caused the band of colors to move relative to the exit slit,

co that the desired entrance slit image is centered on the exit slit. The range of

colors leaving the exit slit is a function of the width of the slits. The entrance

and exit slit widths are adjusted together.

3.2.2 Alignment procedure

Since the luminescence light itself is very weak, the more light is gathered, the bigger

the SNR is for the spectroscopy measurement. An objective lens of 0.4 NA is selected.

The sample is put in front of the small region of the focal point. The collimated light

is then refocused by another lens with the focal length of 45 mm. If the sample is

moved by 1 um, the light spot at the slit will be moved up or down for more than

100 µm. The laser is then mounted in a three dimensional linear stage. With the

Cartesian coordinate defined in the experimental setup, z movement will have an

3.3. Fluorescence lifetime measurements 27

FIGURE 3.4: Relative position of fiber laser and objective lens

impact on whether the light could be gathered on the detector, and x movement will

have an impact on whether the light could be gathered by the slit. In real experiment,

output is extremely sensitive to z direction.

The procedures are shown below to achieve the best alignment:

(a) Mark the height of the laser on the slit of the monochromator.

(b) The objective lens and the second lens are then put into the system separately

after making sure the gathered light can hit the same spot marked on the slit

(c) The height of the detector is then determined and positioned with a three di-

mensional linear stage after center the wavelength at the peak wavelength of

the laser.

(d) The laser is angle cleaved and then rotated 90◦ to excite the edge of the glass

samples.

3.3 Fluorescence lifetime measurements

To study the fluorescence lifetimes of certain energy manifolds, a tunable laser is fa-

vored to give the freedom of excitation wavelength selection, optical filters are used


to select the wavelength of interest, also, an accurate measurement of a fluorescence

lifetime requires the detector to respond sufficiently fast. A general rule of thumb is

the response time should be less than 1/10 of the lifetime which is to be measured.

When the fluorescence lifetime of a particular manifold is being measured, it is pre-

ferred that only a small amount of ions from the ground state are excited to that

manifold to avoid the undesired competing effects. To avoid this unwanted regime,

a short excitation pulse is used to probe the energy manifolds, which limits the peak

power per pulse and does not allow the manifold to saturate.

While the second technique is the steady-state illumination, the illumination is turned

on long enough until the upper energy level is saturated. This method is less accu-

rate compared to the short pulse method, but it has two advantages. First, it shares

the same experimental set up as the emission spectrum measurement. The lumi-

nescence decay could be easily measured if a detector is put just in front of the slit.

Second, it’s cheap. An OPO laser is very expensive and takes a lot of space, while

the diode laser is just thousands of dollars, but the diode laser has to be one that can

be modulated.

Energy levels can be excited either directly or indirectly. Direct excitation involves

the desired energy level being pumped with a wavelength at or near the peak ab-

sorption for that manifold and monitoring the decay at a similar wavelength. Since

no other energy transfer process is involved, this technique can then provide an ac-

curate amount of fluorescence lifetime.

The indirect pumping, on the other hand, relies on mechanism like cross relaxation

and up-conversion to populate the energy level. The population process can exhibit

characteristics of the higher or lower energy level feeding it. If one of them has an

significantly long lifetime, the measured lifetime is then not accurate.

Two common forms of fluorescence measurements are briefly summarized with

their defining equations, advantages and set up details.



OPO laser method

The short pulse is first tuned to the wavelength corresponding to the to be measured

manifold, after being focused with a short focal length lens, it is focused of the edge

of the glass samples, the tiny light spot should be as close to the edge as possible

while making sure most of the excited light does not shine directly into the objective

lens. A filter is then put in front of the detector. The strongest singal should be

obtained before doing the measurement by adjusting the position of the glass with

the three dimensional linear stage to get the best signal to noise ratio.

Steady state illumination method

This method share the same experimental setup as the emission spectrum measure-

ment set up. First is the frequency selection of the signal square wave. The frequency

has to be at least ten times lower than the inverse of the upper energy level lifetime

in order to get the full fluorescence lifetime decay. When the voltage is at low level,

the power is on until the upper energy level is saturated, the pump laser is sudden-

lly turned off after a falling edge of the square wave is detected, the fluorescence is

then measured. The average function of the oscilloscope could significantly increase

the signal to noise ratio.



FIGURE 3.7: Fluorescence decay acquired by the 1012 TDS oscillo-scope

The fluorescence decay is then shown in Figure 3.7. The black plot indicates the

behavior of the fluorescence light of a period; the blue plot shows the rising slope

and the dropping slope of the square wave from the function generator.


FIGURE 3.8: The power fluctuation of the 808 nm diode laser.

3.3.1 The fluorescence decay

The decay of the excited state to the ground state can be expressed as

I(t) = I0etτ

Where, I0 is the intensity at time zero(upon excitation) and τ is the lifetime. This is

defined as the time for the intensity to drop by 1/e or to around 37%. In terms of

rate constants(kr – radiative rate, knr – non radiative rate) the lifetime can be written

as below, which can be compared to the fluorescence quantum yield(φ)

τ =1

kr + knr, φ =

krkr + knr

The fluorescence (FL) signal is multiparametric and can be considered as follows,

along with the measurements that can elucidate them,

FL = f(I, λexc, λem, P, x, t)


Where; I = intensity - measurement is quantum yield(φ) λexc=excitation wavelength-

measurement of absorption spectrum λememissionwavelength – measurement of fluo-

rescence spectrum, P -pump power, x=position-measurement by fluorescence mi-

croscopy, t=time-measurement of fluorescence lifetime.

Although the decay law is based on first order kinetics, in practice, many fluores-

cence decays are more complex. Often populations of excited states are in an inho-

mogeneous environment and quenching processes and other environmental influ-

ences can lead to multi- or non exponential decay behavior.

The lifetime of the upper energy level is an intrinsic property and, within certain

constraints, independent of concentration. This means that changes in concentra-

tion, whether caused by cross-relaxation or concentrating the sample, would not

affect the lifetime value. While in the real lifetime measurement, where a change in

intensity of the recorded emission would be observed. It’s not an absolute measure-

ment, but a relative one. Fluorescence is an ideal probe, as the fluorescence decay

could be highly influenced by its environment on the presence of other interactions,

which can affect knr. Thus the fluorescence lifetime is useful in elucidating the con-

centration quenching effect.

The raw data is first normalized and then plotted with a logarithmic vertical axis to

verify if only one lifetime exist in the scenario. The data is then fitted with the least

square method.

Lifetime spectroscopy is a fundamental prerequisite for an accurate determination

of lifetime. Two techniques are used in the present work for measuring the carrier

lifetime. The carrier lifetime measurements are based on the dynamics of excited

upper energy level which is generated optically. Concerning the time dependence

of the illumination, two operating regimes can be distinguished.

(a) The first regime involves a sharp pulse of illumination that is rapidly turned

off and a subsequent determination of the luminescence intensity without illu-

mination. This is a traditional but useful transition technique.

(b) The second regime is the steady-state illumination, the illumination is turned


FIGURE 3.9: Fitting procedure for the fluorescence decay wave-forms.The red dots are normalized raw data acquired with oscil-loscope and the black line are the fitted line with the least square

method.

on long enough until the upper energy level is saturated, the lifetime is then

measured from the decay.

35

Chapter 4

Ytterbium doped phosphate glass

Effects such as excited-state absorption, cross-relaxation, upconversion, and concen-

tration quenching are present in rare-earth ion doped materials, and can lead to

reduced laser efficiency because alternative paths exist to deplete the excited-state

population.

The quenching effect for Neodymium, Ytterbium, Thulium rare-earth ion doped

glasses of different concentrations are verified from lifetime perspective and emis-

sion spectrum intensity perspective. Either the reduction of upper energy level life-

time or the reduced normalized lasing intensity could indicate that the quenching

effect exists to the interested glass samples.

4.1 Ytterbium rare-earth ion doped glasses

The advantage of the Y b3+ ion as a lasant ion derive from its simple electric struc-

ture. There are only two energy level manifolds, the ground 2F7/2 state and an ex-

cited 2F5/2 state, which are separated by approximately 10,000 cm−1. Effects such as

excited-state absorption, cross relaxation, upconversion, and concentration quench-

ing have relative low impact at low concentration.

36 Chapter 4. Ytterbium doped phosphate glass

FIGURE 4.1: Splitted energy level of ytterbium

4.1.1 Energy level and absorption spectrum

The splitting effect explains why the absorption spectrum of the upper energy level

has a broad continuous profile instead of single absorption lines. Also, the absorp-

tion intensity on the contrary tells us the absorption cross-section, which physically

means the probability of absorption for various wavelengths.

For the laser we have in lab, both 915 nm and 975 nm could be used as the excitation

light source, both are used for lifetime measurement, but only the latter one is used

for luminescence measurement since 975 nm is also the emission peak for ytterbium

ion.

4.1.2 Emission spectrum

The emission spectrum is the spectrum of electromagnetic radiation emitted due to

an atom making a transition from the higer excited state to a lower energy state. The

collection of different transitions, leading to different radiated wavelengths, make

up an emission spectrum. The intensity of the emission spectrum can thus give us

a basic idea of population distribution when light is pumped to higher energy state.

This can also related to other properties of the object through the Stefan-Boltzmann

law.

4.1. Ytterbium rare-earth ion doped glasses 37

FIGURE 4.2: Absorption spectrum for ytterbium doped rare-earth ionfor different concentrations

From the two absorption peaks of the absorption spectrum, it seems that there are

two major upper energy levels being populated and have the biggest population at

the same time.

Since the energy level is broadened because of rotational and vibrational energy of

the base material, the Boltzmann distribution will have a impact on redistribution of

probability on the upper energy level, which is then described by

NfNi

= exp(− ∆EkBT

)

It describes the relative population of energy states with a continuous function due

to the effect of temperature. At lower temperature, the lower energy states are more

greatly populated. At higher temperature, there are more higher energy states pop-

ulated, but each is populated less.

From Beer-Lambert law:

I = I0exp(−σnL)


FIGURE 4.3: Absorption cross section for ytterbium doped rare-earthion of different concentrations

with σ the absorption cross-section, n the intensity of the rare-earth ions and L the

path length. If L is expressed in cm, the absorption cross-section then has a unit of

cm−2. The absorption cross-section can then solved with the formula:

σabsorption =ln(T )

nL

With T the transmission ratio, the emission cross-section σemission can then be calcu-

lated by:

σemission = σabsorptionexp(E0 − hνkT

)

The E0 has a corresponding wavelength of 974.6 nm wchich is very close to 975 nm

peak given by the measured peak emission wavelength.


FIGURE 4.4: Experimental data plot for absorption spectrum andemission spectrum

4.1.3 Quenching effect and emission cross section

If no quenching effect exists, all ions share the same absorption cross-section σabsorption

and the same emission cross-section σemission. Since the fluorescence light is gath-

ered from the side of the glass samples, the fluorescence light is then proportional

to the number density. Then a linear relationship between the number density of

rare-earth ions and the peak emission intensity. The normalized intensity is then

defined by the ratio between peak emission intensity and number density. When no

quenching effect exists, the normalized intensity should be a constant, while a dip

should be observed with the existance of quenching effect.

As shown in the plot, the normalized intensity stays the same for glass samples with

the concentration of 2%, 4% and 6%. But a dramatic drop down of 40% was observed

with the 13.55% concentration samples.


FIGURE 4.5: Calculated absorption cross section and emission crosssection with zeroline wavelength at 974.7 nm

4.1.4 Fluorescence lifetime measurements with short pulse method

FL = f(I, λexc, λem, P, x, t). The measured lifetime could be a function of the exci-

tation wavelength, emission wavelength, excitation power, position and time. The

controlled valuable method is used to study each situation. The results are thus

shown below:

(a) FL = f(I, λexc, λem, P, x, t)

First, to determine whether the glasses with the concentrations of 2 %, 4 %, 6

%, 13.55 % has a pump power dependence on lifetime measurement, samples

were pumped with 915 nm short pulse from the idle signal of CONTINUUM

Surelite OPO laser. Each pulse width is around 5 ns.

Since the lifetime is potentially a function of pump power, the behavior of

pulse power from the OPO laser has to be characterized first before even started.

The behavior of the pulse train from the OPO laser is shown below.


FIGURE 4.6: Normalized emission spectrum for ytterbium dopedrare-earth ion for different concentrations

FIGURE 4.7: Pulse train behavior of CONTINUUM Surelite OPOlaser’s idle signal.


FIGURE 4.8: Lifetime for Ytterbium doped glasses of different con-centrations pumped with varing pump power.

A big fluctuation effect is observed, it’s caused by the mismatch of the two

laser cavity, but since the peak power is pretty stable, the trigger level of the

oscilloscope can then be chosen carefully in order to select out the pulse with

certain power.

The samples are then pumped with 1 mW, 2 mW, 3 mW separately. As is

shown in the plot below, the lifetime for each concentration has a biggest fluc-

tuation of 10 µs, while the lifetime has a order of ms, the small lifetime dif-

ferences can then be neglected. The lifetime is then not a power of the pump

power for Ytterbium ion doped glass samples.

(b) FL = f(I,λexc, λem, P, x, t)

Since the to be measured lifetime can also be a function of pump wavelength,

the aforementioned samples are then pumped with 915 nm and 975 nm short

pulse for the same pump power. A negligable lifetime difference behavior

is also observed as shown in fig1.4. Then, in this case, the lifetime is not a

function of excitation wavelength.


(c) FL = f(I, λexc,λem, P, x, t)

Two emission peaks located at 975 nm and 1030 nm. To study whether the flu-

orescent light of two wavelengths share the same lifetime, the lifetime of of the

whole emission spectrum is compared with that of 1030 nm, and this is realized

by a 1 µm filter.

As is shown at Fig4.8, the fluctuation of measured lifetime is also negligable.

Then, the measured lifetime is not a function of emission wavelength as long

as all the emitted light share the same upper energy level.

(d) FL = f(I, λexc, λem, P,x, t)

The lifetime measurement is totally repeatable whatever the height of the edge

or when it is tested.

4.1.5 Lifetime measuremnt with Switched illumination method

The experimental setup is exactly the same as the emission spectrum measurement

of the next section except that a detector is positioned in from of the slit of the

monochromator. After a finer adjustment of x and z direction to get the highest

signal, the data is then recorded for lifetime measurement.

The laser is then turned on and off by a function generator, the falling edge of the

laser last for around 50 µs. When the upper energy level is saturated, the laser is

then turned off and the fluorescence light is then measured and analyzed with oscil-

loscope.

It’s worth to mention is that the OPO method is a more accurate way to measure

lifetime, the goal of this section serves as a comparison purpose. The error of the

function generator method comes from two aspects: with the first the turning off

time of the laser and the second that it’s the average lifetime that is measured.

As shown from the plot, a bigger fluctuation happens to the function generator

method, but the two plots are showing similar behavior, it’s then safe to say the

result of the OPO method is trustable.


FIGURE 4.9: Lifetime measurement of Ytterbium doped phosphateglass with different concentration using short pulse method and

switched illumination method.

FIGURE 4.10: Absorption spectrum for ytterbium doped rare-earthion for different concentrations


4.1.6 Summarize

(i) The lifetime of glass samples with different concentrations are measured with

OPO meathod and function generator method separately. The low concen-

tration samples, with the concentration of 2%, 4% and 6%, has a lifetime of

1.32 ms. The sample with the 13.55% concentration has a lifetime of 1.32 998µs.

(ii) The lifetime experiences negligible impact from changing pump wavelength,

pump power, or detection emission wavelength.

(iii) The emission cross-section is calculated from Boltzmann theory and shows a

good accordance with the shape of emission spectrum with the zero line at

974.7 nm.

(iv) Quenching effect is observed only on the 13.55% doped glass sample with the

dropping down on the lifetime and normalized emission intensity.

47

Chapter 5

Neodymium doped phosphate

glass

Neodymium is one the most suitable material as to pumping at 1 µm due to its sharp

absorption line especially for Nd:YAG laser application. In order to further increase

the pump absorption, the use of highly concentrated, shorter Nd:YAG components

is a must.

5.1 Energy level and absorption cross section

FIGURE 5.1: Neodymium rare-earth ion energy level diagram

48 Chapter 5. Neodymium doped phosphate glass

Figure 5.1 illustrated 4F3/2 → 4I11/2 emission for the Nd3+-doped glasses. Because

of its importance in high-power and high-energy laser applications, this is probably

the most thoroughly characterized transition for both glass and crystalline hosts.

This band Provides four-level operation at room temperature. The terminal state

lies roughly 10 kT above the ground state, leading to a thermal population of the

4I11/2 of only 1 ion in 104.

FIGURE 5.2: Absorption spectrum for Neodymium doped rare-earthion of varying concentrations

Because of its importance in high-energy lasers, the 1060 nm transition is used both

for lasing and amplifier. The behavior of the absorption spectrum, emission spec-

trum and lifetime measurements were carried out to study the quenching effect for

Neodymium doped optical glasses.

Since one of the peak absorption locates at 803 nm, a fiber laser with center wave-

length of 808 nm is then used as the excitation source.

5.2. Quenching effect and emission spectrum 49

FIGURE 5.3: Absorption cross section for Neodymium doped rare-earth ion of varying concentrations

5.2 Quenching effect and emission spectrum

As the concentration goes up, the decreasing of lifetime indicates the reduction in the

quantum efficiency. It can occur through any of the aforementioned energy transfer

processes. A delicate concentration should be taken into consideration leveraging

between smaller size and higher quantum efficiency.

The following drawn table illustrates the reduction of lifetime for the 1064 nm emis-

sion with varying concentration:

TABLE 5.1: Neodymium concentration resulting in lifetime reductionfor the 1064 nm emission.

Glass type Quenching Concentration(1020cm−3)

Silicate 3.9 - 6.0Phosphate 3.9 - 8.6

Fluorophosphate 3.0 - 4.0Fluorozirconate 4.2Fluoroberyllate 3.8-5.3


FIGURE 5.4: Absorption and emission spectrum plot for neodymiumrare-earth ion

FIGURE 5.5: Normalized emission spectrum for Neodymium dopedphosphate glass of different concentration

5.3. Concentration quenching effect in Neodymium doped fiber of different

concentration51

When at low concentration, the two-ion cross-relaxation mechanism is responsible

for the quenching effect. This hold the assumption that the doped rare-earth ions

are distributed evenly throughout the glass and do not cluster, this is true for low

concentration and for multicomponent glasses.

But clustering does occur even at low concentration in cilica, for the available data,

phosphates are the most resistant to quenching and the fluuorophosphates are the

least.

Another quenching process involves the energy transfer between rare-earth ions and

OH1 complex, it can trap the emitted photon and thus extremely effective at quench-

ing effect.

5.3 Concentration quenching effect in Neodymium doped fiber

of different concentration

Neodymium doped fiber with different lengths are tested to compare the impact of

concentration on fiber performance. The four fiber have different lengths, which are

1 cm, 2cm, 4cm and 12 cm, with the corresponding concentration 0.25 %, 0.5 % , 1 %,

3 %. But the number of doped neodymium ions is the same. The results are shown

in Fig5.6 and Fig5.7.

The ASE noise level for the peak value is about 3.5 times, 2.5 times, 1.5 times stronger

than that of the 1cm fiber.

5.4 Fluorescence lifetime measurements with short pulse method

and switched illumination method

FL = f(I, λexc, λem, P, x, t). The measured lifetime could be a function of the exci-

tation wavelength, emission wavelength, excitation power, position and time. The

controlled valuable method is used to study each situation. As mentioned before,


FIGURE 5.6: The ASE spectrum for fiber of different lengths pumpedwith 808 nm diode laser source.

FIGURE 5.7: The ratio of the peaked value with that of the 1 cm longfiber

5.4. Fluorescence lifetime measurements with short pulse method and switched

illumination method53

FIGURE 5.8: The measured lifetime with different concentrationspumped with 808 nm short pulse of varying pump power.

the same CONTINUUM Surelite OPO laser is used for lifetime measurement. The

results are thus shown below:

(a) FL = f(I, λexc, λem, P, x, t)

First, to determine whether the glasses with the concentrations of 0.25 %, 0.5

%, 1 %, 3 % has a pump power dependence on lifetime measurement, sam-

ples were pumped with 808 nm and 875 nm short pulse from the idle signal of

CONTINUUM Surelite OPO laser separately. Each pulse width is around 5 ns.

The samples are then pumped with 1 mW, 2 mW, 3 mW at 815 nm and 875 nm

separately. As is shown in the plot, the lifetime for each concentration has

a biggest fluctuation of 12.3 µs, while the lifetime has a order of hundreds of

µs, the small lifetime differences can then be neglected. That’s true for both

excitation wavelength. The lifetime is then not a power of the pump power for

Ytterbium ion doped glass samples.

When the samples were excited by 875 nm pulse with the power at 4 mW,


FIGURE 5.9: The measured lifetime with varying concentrationpumped with 875 nm short pulse of varying pump power.

FIGURE 5.10: A comparison between lifetime measured with shortpulse method and function generator method

5.4. Fluorescence lifetime measurements with short pulse method and switched

illumination method55

multi-lifetime is observed when the long-last tail was shown on a logarithmic

coordinate. This indicates that the measured lifetime is a function of pump

power, it becomes less accurate as the pump power goes up, while when the

concentration is low, it doesn’t vary with the pump power.

This indicates that the when the lifetime becomes a function of power, quench-

ing effect exists. In order to get an accurate measurement, the excitation power

should be as low as possible.

(b) FL = f(I,λexc, λem, P, x, t)

Since the to be measured lifetime can also be a function of pump wavelength,

the aforementioned samples are then pumped with 808 nm and 875 nm short

pulse for the same pump power. A negligable lifetime difference behavior

is also observed as shown in fig1.4. Then, in this case, the lifetime is not a

function of excitation wavelength.

The result is then verified with the function generator method, with the pump

wavelength 808 nm. The results are then be verified to be trustable.

57

Chapter 6

Thulium doped germanium glass

Thulium is the thirteenth lanthanide element with twelve electrons locating at 4f

shell. Thulium doped fiber amplifier is the most promising candidate for the s-band

its is far superior to the other amplifiers with respect to power conversion efficiency.S

band TDFA from 1460-1500 nm which is from lower s band to middle s band wave-

concentration quenching effect in rare-earth doped glasses · 2017. 7. 21. · iii declaration of...

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