an opto-vlsi based tunable fiber ring laser

8/7/2019 An Opto-VLSI Based Tunable Fiber Ring Laser

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An Opto-VLSI Based Tunable Fiber Ring Laser

Rong Zheng, Zhenglin Wang and Kamal Alameh

Centre of Excellence for MicroPhotonics Systems, Electron Science Research Institute

Edith Cowan University, 100 Joondalup Drive, Joondalup, WA 6027, AUSTRALIAEmail: [email protected]

Abstract

We demonstrate a novel Opto-VLSI tunable fiber ring

laser structure in which an Opto-VLSI processor driven

by steering phase holograms dynamically selects the

lasing wavelengths. A proof-of-concept tunable fiber

ring laser which has a wavelength tuning range of more

than 4 nm around 1530 nm and a side-mode suppression

ratio more than 25 dB is demonstrated at room

temperature. The output power of the lasing wavelengths

can be controlled within 0.5 dB uniformity by

reconfiguring the phase hologram of the Opto-VLSI

processor. Dual-wavelength laser tuning is also

experimentally demonstrated.

Index Terms Opto-VLSI processing, Erbium-doped

fiber (EDF), tunable fiber laser.

1 Introduction

Tunable fiber lasers operating in the 1550 nm window

have attracted great research interests recently becauseof their applications in different areas such as optical

communications, gas sensors for pollution monitoring,

high resolution spectroscopy and photonic true-time-

delay (TTD) beamforming systems [1-3]. Various

techniques have been proposed to realize the single-

longitudinal-mode operation in erbium-doped fiber

(EDF) ring lasers. To realize single-frequency operation

and decrease mode hopping in erbium-doped fiber ring

lasers, fiber ring resonators with intra-cavity Fabry-Perot(FFP) filter have been extensively investigated for

wavelength tuning in a fiber ring laser. However, an FFP

alone is insufficient to stabilize both the lasing

wavelength and power of a fiber ring laser. A passive

multiple-ring cavity or a compound ring resonatorcomposed of a dual-coupler fiber ring to guarantee

single-longitudinal-mode laser oscillation has been

proposed to realize single-frequency operation [4-6].

Other methods including integrating two cascaded FFP

filters of wide different free spectral ranges (FSRs) into

the cavity or adding an extra ITU-grid periodic filter in

the optical loop[7-9] have also been demonstrated to

provide full tunability and single-longitudinal-mode

operation.

In this paper, we present a new tunable fiber ring

laser structure that employs an Opto-VLSI processor asthe wavelength-selection element. A proof-of-concept

tunable fiber ring laser is experimentally demonstrated

over a wavelength tuning range from 1528 nm to 1532

nm with a side-mode suppression ratio higher than 25dB and output power uniformity better than 0.5 dB.

Tunable dual-wavelength laser operation is also

experimentally verified with the same configuration at

room temperature.

2 Opto-VLSI Tunable Ring Laser

Structure

The Opto-VLSI-based tunable ring laser structure is

shown in Fig.1. It consists of an Opto-VLSI processor

that realises wavelength division multiplexing (WDM)

equalization through adaptive optical beam steering, ahigh dispersion blazed grating that diffracts the

wavelength components of the light onto the active area

of the Opto-VLSI processor and an EDF ring with

forward pumping.

The reconfigurable Opto-VLSI processor comprises

an array of liquid crystal (LC) cells driven by a Very-Large-Scale-Integrated (VLSI) circuit can generate

digital holographic diffraction gratings that dynamically

steer, multicast, and/or shape optical beams. The use of

an Opto-VLSI processor in conjunction with an EDF

ring laser can achieve dynamic WDM equalization thus

generating multiple wavelengths without moving parts

[10]. In an optically-amplified cavity, this dynamic

WDM equalization feature enables independent control

of the loss at different wavelengths and allows

arbitrarily-selected wavelengths to lase at room

temperature.

Proceedings of the Third IEEE International Workshop on Electronic Design, Test and Applications (DELTA’06)

0-7695-2500-8/05 $20.00 © 2005 IEEE



The optical isolator shown in Figure 1 is placed in

the ring cavity to realize a unidirectional oscillation that

prevents spatial hole burning in the erbium-doped fiberlaser gain medium and also to improve the laser output

power stability. The laser output power is obtained from

the zeroth order diffracted beam from the Opto-VLSI

processor.

Fiber

collimator

Opto-VLSI

processor

Diffraction

Grating

WDM Coupler

Erbium-doped

fiber

Isolator

Circulator

980nm

Pump

OSA

Polarization

Controller

PC

λ1

λN

Figure 1. Tunable Opto-VLSI fiber laser configuration

The broadband amplified spontaneous emission

(ASE) noise from the Er-doped fiber is routed through a

circulator and a polarization controller, and collimated to

about 1mm diameter by a fiber collimator and launched

to the dispersion grating, which diffracts the differentwavelength components of the ASE signal along

different incident angles onto the active area of the

Opto-VLSI processor. By driving the Opto-VLSI

processor with appropriate steering phase holograms,

one particular wavelength or several wavelengthcomponents can be reflected back along their incidence

paths, thus locking the oscillating modes of interest and

preventing the lasing of other modes by keeping their

power levels below the lasing threshold. The wavelength

tuning range is mainly determined by two parameters,

namely, (i) the angular dispersion of the grating, whichdetermines the diffraction angle and the spacing between

the different wavelength components, and (ii) the

maximum steering angle and the working area of theOpto-VLSI processor.

3 Experimental Results

The experimental setup of the tunable Opto-VLSI fiberring laser is shown in Fig.1. A 40-micron pixel size,

128x128-pixel nematic LC Opto-VLSI processor, which

has a maximum steering angle of around ±1.0 degree at

1530 nm, was used in conjunction with a high-dispersiongrating to realize dynamic wavelength selection. The

high dispersion blazed grating (having 1200 lines/mm

and a blazed angle of 70º at 1530 nm) was used to

spread the EDF gain spectrum and diffract a specificwavelength range onto the Opto-VLSI processor. The

laser gain medium was a 14m long EDF of core

diameter 4.9mm, Numerical Aperture (NA) 0.23 and

cutoff wavelength 800 nm. Its peak absorption near 1530

nm is around 5 dB/m. Forward 980nm laser pumping

was employed which provided an EDF small-signal gain

of more than 20 dB over the C-band. The 980nm pump

laser was coupled to the ring laser signal through a

wavelength division multiplexed (WDM) coupler of

insertion loss 0.5 dB at 1530 nm. An isolator was placedin the ring to (i) maintain unidirectional oscillation,

which prevents spatial hole burning in the EDF laser

gain medium, and (ii) improve the laser output power

stability. The measured total fiber-to-fiber coupling loss

was about 2 dB and the insertion loss of Opto-VLSI

processor was around 10 dB, which was mainly due to a

low mirror reflectivity and a low fill factor. The laser

output was obtained from the signal reflected off the

blazed grating and monitored by an optical spectrum

analyzer (OSA). The total length of the fiber ring cavity

was about 20 m.

Fig. 2 demonstrates the tuning capability of the

Opto-VLSI ring laser shown in Figure 1. The output

spectrum is shown for different phase holograms

uploaded onto the Opto-VLSI processor. A 20x40 pixel

block was driven by a phase hologram thatindependently attenuates the wavelength component

falling on it through variable beam steering. By

reflecting the wavelength component back along,

around, or away from its incidence path, a low, medium,

or high loss, respectively, was achieved. By changing

the position as well as the phase hologram of the pixel

block, the lasing wavelength was dynamically tuned. In

all scenarios of Figure 2, the lasing wavelength was

reflected off the Opto-VLSI processor along its incident

path so that the attenuation was kept at a minimum level.

The output optical signal signal-to-noise ratio measured

by an OSA (with a resolution bandwidth of 0.06 nm)

was greater than 25 dB. The polarization controller was

used to optimize the diffraction efficiency of the Opto-VLSI processor and to enforce single-polarization

lasing.

Fig 3 shows the output power of the Opto-VLSI fiber

ring laser at 1529.15 nm versus the pump power. It is

obvious that the ring cavity starts to lase at pump

threshold power of 15 mw.


0-7695-2500-8/05 $20.00 © 2005 IEEE



1525 1526 1527 1528 1529 1530 1531 1532 1533 1534 1535-50

-45

-40

-35

-30

-25

-20

-15

Wavelength(nm)

Output power

(dBm

)

(a)

bFigure 5. (a) Phase hologram and measured spectra of

the Opto-VLSI fiber ring laser when operated in dual-

wavelength mode, (b) measured spectra for different

tuning scenarios.

4 Conclusions

A novel tunable Opto-VLSI fiber ring laser that can

operate at room-temperature has been proposed and

demonstrated in this paper. The tunable fiber ring laser

used a reconfigurable Opto-VLSI processor as a WDM

equalizer to realize wavelength tuning and gain control.

Experimental results have shown that an output laser

signal tunable over a wavelength span of 4 nm with

signal-to-noise ratio better than 25 dB can be achieved.

The measured uniformity of the laser output power wasbetter than 0.5 dB and the stability was excellent at room

temperature. The tunable laser had a wavelength tuning

resolution as small as 0.05 nm and can be step-tuned at

50 GHz ITU grid spacing. A tunable dual-wavelength

laser output has also been demonstrated where two

output wavelengths were simultaneously selected to lase

within the fiber ring cavity.

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an opto-vlsi based tunable fiber ring laser

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