no 60 - kritzinger

7/29/2019 No 60 - Kritzinger

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Optical add/drop multiplexer using short- and

chirped long-period fibre gratings for dense

wavelength-division multiplexing

Ronnie Kritzinger, Pieter L. SwartDepartment of Electrical and Electronic Engineering

Rand Afrikaans University

PO Box 524, Auckland Park, 2006, South Africa

Tel: +27 11 489 2351 Fax: +27 11 489 2344 E-mail: [email protected]

Abstract We propose and demonstrate the use of a novelwavelength-tuneable optical add/drop multiplexer (OADM)device based on fibre Bragg gratings (FBGs) and chirpedlong-period gratings (CLPGs) having a broad transmission

spectrum. Numerical simulations predict bandwidths in theorder of 40 nm and higher. Fabrication of the symmetricCLPGs, and the method of strain used for tuning of the fibreBragg grating are also discussed.

Index Terms Wavelength-tuneable, Add/drop multiplexer, Fi-bre Bragg gratings, Chirped long-period gratings.

I. INTRODUCTION

DENSE wavelength-division multiplexed (DWDM) net-works require low insertion loss (< 0.5 dB), highchannel isolation (> 30 dB), low back-reflection, wide-range wavelength selectivity, low cross-talk (< 39 dB),and economically viable optical add/drop multiplexers. This

paper proposes the design of a novel broadband add/dropmultiplexing device, that consists of short- and chirped long-

period fibre gratings.

Due to low back-reflection, low insertion loss, ease of

fabrication, and high channel isolation of LPGs [13], trans-

mission grating technology finds application in dispersion

compensation, and in band rejection filters [19][21]. These

types of fibre gratings also find applications in sensing [1],

[11], add/drop multiplexing applications [9], [24], and gain

equalization of erbium-doped fibre amplifiers (EDFAs) [18],

[22].

Long-period gratings are periodic perturbations in an optical

fibre that enable the coupling of power between two co-propagating modes. Fibre Bragg gratings involve the coupling

of a forward propagating guided mode to an identical counter-

propagating mode. Recently, there has been an interest in

transmission gratings that produce high isolation broadband

transmission spectra (bandwidths > 50 nm). It has beenshown that bandwidths in the range of 60 nm 100 nmcan be acheieved [12], [15]. This transmission grating property

will increase the flexibility of a DWDM network, by coupling

of a wide range of wavelengths at once.

In this paper, we propose a means for creating a broadband

add/drop multiplexing device that drops several International

Telecommunication Union (ITU) DWDM channel signals in

a selected wavelength range with a suitable channel spacing,

from one single-mode optical fibre to the next using identical

chirped transmission gratings. A short-period grating, which

is tuned in a third optical fibre, and an optical circulator are

then used to drop a specific wavelength channel. This dropped

channel can later be added to the remaining DWDM channels.

The remainder of the paper is organized as follows. In Sec-

tion II fibre grating theory is briefly discussed, and Section III

gives the proposed design of the OADM. Concluding remarks

are given in Section IV.

II. BACKGROUND

The refractive-index modulation of a fibre grating along the

length of a fibre can be written as [2]:

n(z) = ncoreeff + neff

cos

2

z + (z)

(1)

where ncoreeff is the effective refractive-index of the fundamentalLP01 core mode, is the fringe visibility, is the gratingperiod, neff is the induced index change spatially averagedover the fibre grating period, and (z) denotes the gratingchirp.

A. Fibre Bragg gratings

Since the dominant interaction in a bragg grating is the

reflection of a particular mode of amplitude A(z) into a similarcounter-propagating mode of amplitude B(z), the simplified

coupled-mode equations for FBGs then look as follows [7]:

dA

dz= iA(z) + iB(z)

dB

dz= iB(z) iA(z)

(2)

where A(z) is the forward mode, B(z) is the reverse mode, is defined as the AC coupling coefficient, and is the gen-eral DC self-coupling coefficient. The two modes, A(z) andB(z), represent slowly varying mode envelope functions [8].The spectral dependence and diffraction efficiency of fibre


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gratings can be obtained by using the coupled-mode theory,

since it accurately models the optical properties of most fibre

gratings.

B. Long-period gratings

The coupled-mode equations describing the mode coupling

in transmission gratings are defined as follows [8]:

dA

dz= iA(z) + iB(z)

dB

dz= iB(z) + iA(z)

(3)

The phase-matching condition for LPGs is defined as [8]:

1

2

core nclad

(4)

where is the detuning parameter. core and clad are

the propagation constants for the LP01 core mode and nthcladding mode respectively. r (ncore n

clad) is the

resonant wavelength for coupling to the th (LPl) claddingmode, where the azimuthal order of the mode is given by l.

Chirping the LPG period along the fibre axis causes the

detuning parameter and coupling coefficient to vary fordifferent distances along the fibre axis. In our analysis of

LPGs, coupling is done to a single cladding mode.

In the case of linearly chirped transmssion gratings the

grating period (z) can be expressed as [4]:

(z) = 0

z Lg

2

(5)

where Lg is the grating length, 0 is the grating period atz = Lg/2, and defines the grating chirp. Increasing thegrating period along the fibre length indicates negative chirp,

whereas decreasing the grating period indicates positive chirp.

III. PROPOSED DESIGN AND COMPUTER SIMULATIONS

Fig. 1 illustrates the proposed design of a novel long-period

grating add/drop multiplexer. Two chirped LPGs is placed in

parallel in close contact (< 5 m), to form a cladding-modecoupler without the need of fusion [24]. The chirped LPGs

are fabricated such that the LP01 core mode couples to thefifth cladding mode with a desired chirp. The cladding-mode

propagates in the cladding and a coupling region between the

two optical fibres, exciting a similar cladding mode in the

parallel-placed fibre with an identically inscribed chirped LPG.

The chirp should be chosen such that the desired bandwidth

is obtained, allowing operation as a broadband device.

The tuneable Bragg grating and circulators perform the mul-

tiplexing/demultiplexing operation. The output of the circula-

tor situated on the lefthand side of Fig. 1, can be multiplexed

with the data dropped from the original fibre using appropriate

methods.

CLPG 1

CLPG 2

Coupling region

Non-resonant

light

Circulator

Input

2 1

3

OutputFiber Stretcher

Tunable Fiber

Bragg Grating

Multiplexer

Circulator

2 1

3

Main

Output

Resonant light

Fig. 1. DWDM experimental setup for the add/drop multiplexer configura-tion.

A. Fabrication method for chirped long-period gratings

The exposure of germania-doped silica fibres to a KrF laser

through an amplitude mask, producing a UV-induced index

modulation in the fibre core, is the most common techniqueused to fabricate long-period fibre gratings [13]. This method

does not produce azimuthally symmetric LPGs, and results in

a high polarization-dependent loss of the particular grating.

CO2 laser exposure of a fibre at 10.6 m has beendemonstrated to produce LPGs with polarization insensitivity,

and with high temperature stability [5]. When the fibre was

annealed at 1200C, it was also shown that the spectralproperties of the LPG remained unchanged [6].

In this paper, we will consider fabrication of chirped LPGs

in germania-doped silica fibre, using a pulse-width modulated

25 W CO2 laser to produce azimuthally symmetrically writtenfibre gratings, based on the point-by-point method [14].

Fig. 2. Fabrication setup for azimuthally symmetric chirped LPGs [14].

Fig. 2 illustrates the LPG fabrication process, where a

pair of convex lenses are used to expand the CO2 laserbeam to have a diameter of approximately 37.8 mm. Theexpanded carbon-dioxide beam is then converted to a ring-

shaped beam, by passing the original beam through an annular

spatial filter, decreasing the beam diameter to approximate

25 mm and increasing the f-number to help alleviate the


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alignment difficulty, and increase the focal spot size. The

annular CO2 laser beam is then deflected from a 45 tilted flatmirror parallel to the optical fibre. The deflected laser beam is

focussed on the fibre core by a concave spherical mirror with

diameter 37.8 mm and focal length of approximately 38 mm,for the symmetrical exposure of the fibre.

The flat and concave mirrors have been modified such

that the fibre passes through it horizontally, and the fibre issupported by clamps. When the chirped LPG is fabricated

using the point-by-point method, the translation stage moves in

the axial direction to predetermined positions. The movement

is in synchronization with the shutter in the carbon-dioxide

laser beam.

B. Theoretical Results

The theoretical relationship between the long-period grating

period , and the coupled wavelengths is used to obtain thespecified LPG resonant wavelength desired. Fig. 3 indicates

the variation of resonant wavelength with grating period for

the coupling to different cladding modes. The parameters used

to simulate Fig. 3 are similar to those of Corning single-mode(SMF-28e) fibre, where = 0.005, core radius a1 = 4.1 m,and cladding radius a2 = 62.5 m.

200 250 300 350 400 450 500 550

1400

1450

1500

1550

1600

1650

1700

1750

1800

Grating Period (m)

ResonantWavelength(nm)

LP11LP12LP13LP14LP15

LP16LP17LP18

Fig. 3. Theoretical relationship between resonant wavelengthsR and gratingperiod in the case of core-to-cladding mode coupling.

To obtain the characteristics of LPGs, it is required to plot

these curves for a specific optical fibre, since the properties of

the core/cladding refractive indexes and core/cladding sizes,

differ from fibre to fibre. The slope of all the characteristic

curves are positive as indicated in Fig. 3, but it can be predicted

that for higher order cladding modes LPl ( > 9), the slopesof the curves approach infinity for wavelengths higher than1600 nm.

By chirping the LPG, the transmission spectrum is broad-

ened and will become wider for higher values of grating

chirp . The spectral broadening will not only depend onthe different resonant wavelengths which experience coupling

across the length of the fibre grating, but also on the order of

the cladding mode into which coupling occurs [15], [19].

Fig. 4 illustrates the theoretically obtained transmission

spectra of an LPG of 40 mm, induced index change neff =0.83 103 for different values of chirp dR/dz simulated

for coupling to the fifth cladding mode. The fibre parameters

are the same as those used in Fig. 3.

1500 1510 1520 1530 1540 1550 1560 1570 1580 159010

9

8

7

6

5

4

3

2

1

Wavelength (nm)

Transmission(dB

)

Fig. 4. Transmission spectra of a chirped LPG for different values of gratingchirp. () dR/dz = 0, (- -) dR/dz = 0.24 nm/cm, and ( ) dR/dz =0.24 nm/cm with a tanh-apodization profile.

From Fig. 4 it can be seen that chirping of the gratingperiod enables coupling over a larger range of wavelength.

The transmission spectrum broadens as chirp in increased, and

results in a lower value of transmission loss. If coupling was

done to lower order cladding modes, the resulting transmission

loss would be even lower. This could be very useful in the

add/drop multiplexing device, or even in flattening the gain

spectra of EDFAs. Therefore, by adjusting the induced index

change, length, and chirp, virtually any desired broadband

spectrum can be obtained.

By tuning the fibre Bragg grating used in the proposed

design illustrated in Fig. 1, a specific wavelength channel can

be obtained. A low-cost piezoelectric ceramic fibre stretcher

(PZT) is used for the tuning process by increasing the lengthof the FBG induced with an external voltage, and results in

an increase of overall grating period along the fibre axis.

The fibre Bragg grating is typically fabricated by using a

germania-doped silica fibre exposed to a KrF excimer laser

( = 248 nm) through a phase mask [3]. The change in theresonant wavelength r due to a change in strain is givenas [23]:

strain = r(1 e) (6)

where e is the effective strain-optic constant given by [10]:

e =n2e2

[12 (11 + 12)] (7)

11 and 12 are the components of the strain-optic tensor, is Poissons ratio, and ne is the effective index of thecore. Typical values for a common silica single-mode optical

fibre are: 11 = 0.153, 12 = 0.273, and = 0.17, wherethe wavelength-strain sensitivity at 1550 nm is approximately1.2 pm/ [1], [16].

The simulation results obtained for a 20 mm apodized FBGunder varying external applied strain is illustrated in Fig. 5.

The strain applied ranges from 1600 1600 micro strain


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(), where the resonant wavelength r is 1550 nm with zeroapplied strain.

1548 1548.5 1549 1549.5 1550 1550.5 1551 1551.5 15520

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

Wavelength (nm)

Reflectivity[%]

1600e 800e 800e 1600e

Fig. 5. Resonant wavelength shift for a Kaiser-profile apodized FBG undervarying external strain.

By applying positive strain (increasing voltage on the

piezoelectric ceramic fibre stretcher), the grating period increases, and the central wavelength is shifted to higher

values. This is vice-versa when negative strain is applied.

The relationship between the strain applied and the varying

resonant wavelength r is constant. The wavelength stability isaffected significantly, since Bragg gratings are quite sensitive

to thermal fluctuations [17]. It has been shown that the

wavelength and applied strain sensitivity to the voltage applied

is approximately 1 nm/V, and 85.4 /V respectively, whencoupling is done to the LP08 cladding mode (l = 0) [24].

IV. CONCLUSION

A wavelength-tuneable ADM device for DWDM has beenproposed, where the effect of chirp on the LPG transmission

properties has been studied and illustrated with the aid of

computer simulations. The chirped LPG produces spectra with

wide bandwidth, and could be adjusted by varying the grating

parameters and length. The FBG bonded on a fibre stretcher is

tuned to obtain a desired wavelength channel by adjusting the

applied voltage, and the remaining wavelength channels not

selected could be added to the rest of the spectrum by using

optical circulators.

The proposed OADM device is expected to have low

insertion loss, low cross-talk, and high isolation, making it

suitable for use in optical communication links. This ADM

aparatus is the first known design of its kind, and promises to

make a great impact in DWDM networks.

ACKNOWLEDGMENT

The authors thank Telkom SA Ltd., ATC (Pty) Ltd., Marconi

Communications SA Ltd., THRIP, NLC, NRF and RAU for

financial support.

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Ronnie Kritzinger (M1998) received the B.Ing degree in electrical engi-neering and B.Sc degree in information technology from the Rand AfrikaansUniversity, Johannesburg, in 2002. He is currently working toward the M.Ingdegree, and his primary research interest is optical communications forimplementation in the telecommunications industry.

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