Abstract: A 10 GHz polarization-maintaining actively mode-locked fiber ring laser was demonstrated. A Fabry Perot laser diode was incorporated into the ring cavity as an all-optical polarizer

C H Kwok

Lixin Xu

P K A Wai

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Novel polarization maintaining actively mode locked fiber 
ring laser 
 
C. H. Kwok, Lixin Xu, and P. K. A. Wai 
 
Photonics Research Centre and Department of Electronic and Information Engineering,  
The Hong Kong Polytechnic University, Hong Kong, China 
 Tel.: +852 2766 6231; Fax: +852 2362 6412; Email: enwai@polyu.edu.hk 
 
Abstract: A 10 GHz polarization-maintaining actively mode-locked fiber ring laser was 
demonstrated.   A Fabry Perot laser diode was incorporated into the ring cavity as an all-optical 
polarizer.  
2003 Optical Society of America 
OCIS codes:  (140.4050) Mode-locked lasers; (140.3520) Lasers, Injection -locked 
 
1. Introduction 
Fiber lasers usually suffer from instability in the output state-of-polarization (SOP) if non-polarization maintaining 
fiber is used. Recently, it is demonstrated that an injection locked Fabry Perot laser diode (FP-LD) can function as 
an all-optical polarizer, where an injected signals with varying SOP can be polarization-stabilized using injection 
locking [1]. In this report we present a novel actively mode-locked fiber ring laser using a FP-LD in the ring to 
stabilize the SOP of the laser output. 
2. Operating principle  
Fabry-Perot laser diodes typically emit in TE polarization because the lasing threshold of TE mode is smaller than 
that of the TM mode [2]. An external beam with varying SOP will injection lock a FP-LD whenever the TE 
component of the signal is above the injection locking threshold.  Once injection locked, the FP-LD output will be 
TE polarized and clamped at the same intensity.  Thus if we incorporate a FP-LD in the cavity of a mode-locked 
fiber ring laser, the FP-LD will realign the randomly varying SOP of the light circulating in the fiber cavity at every 
round trip through injection locking.   The polarization state of the mode-locked laser output will therefore be 
maintained.  
3. Experimental results 
Figure 1 shows the experimental setup for the all-optical polarization maintaining actively mode-locked fiber ring 
laser.  The laser cavity consists of a 12-meter erbium-doped single mode fiber.  The 980 nm pump was coupled into 
the active fiber through a 980/1550 nm wavelength division multiplexer.  The isolators rejected any residual pump 
and guaranteed unidirectional operation of the ring. A polarization sensitive Mach-Zehnder modulator was used as a 
mode-locker offering periodic intracavity loss modulation.  The modulator was dc-biased and driven by a sinusoidal 
signal from a 10 GHz synthesizer through an electrical amplifier.  The signals circulating in the fiber cavity were 
injected into a FP-LD through a polarization independent circulator. The erbium doped fiber amplifiers provided 
extra gain to the cavity and extended the injection locking range.  Figures 2a and 2b show the mode-locked laser 
output without and with the FP-LD.  The amplitude jitters of the output due to the variation of the SOP (Fig. 2a) 
were removed by the use of FP-LD (Fig. 2b).  The 3 dB spectral width was 0.18 nm (Fig. 3) while the FWHM 
pulsewidth was 20 ps.  The time-bandwidth product is 0.45 implying a nearly Gaussian transform limited pulse. 
The output pulse trains were then externally modulated using a 10 GHz polarization dependent modulator. 
Figures 4a and 4c show the eye-diagrams of the modulated output without and with the FP-LD in the laser cavity 
respectively.  The injection locked FP-LD improved the eye diagram and reduced timing jitters by stabilizing the 
polarization in the cavity.  Figures 4b and 4d give the RF spectra at 10 GHz without and with the FP-LD 
respectively.  The additional frequency components at ±50 kHz from the carrier frequency shown in Fig. 4b were 
due to the beating of the two polarization modes in the cavity, thus confirming multi-polarization operation without 
the FP-LD.  
4. Conclusions 
CWA82
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We successfully generated a stable and polarized 10 GHz transform limited pulse train from an actively mode-
locked fiber ring laser using injection locking in a FP-LD to maintain polarization. 
5. References 
[1] W. H. Chung, et al., IEEE Photon. Technol. Lett. , 14, 920 (2002). 
[2] B. M. Yu and J. M. Liu, J. Appl. Phys. , 69, 7444 (1991) 
 
 
 
 
 
 
 
 
EDF 980/1550 WDM 
Pump  
980nm 
Polarization 
stabilized pulses 
Circulator 
1 3 
2 
FP-LD 
MOD CLK 
BPF 
BPF 
EDFA EDFA 
All Optical Polarization Compensator  
(a) (b) 
Fig. 2.  Output temporal pulse shape (a) without (b) with the FP-LD. 
Timebase: 20 ps/div. 
 
10mv/div 
 
10mv/div 
Fig. 1.  Experiment Setup. WDM: Wavelength-division multiplexer. EDF: Erbium-doped fiber. EDFA: Erbium-doped 
fiber Amplifier. BPF: Bandpass filter. MOD: Intensity modulator. FP-LD: Fabry-Perot Laser Diode. 
  
(a) (b) 
 
  
(c)   (d) 
Fig. 4.  Eye diagrams and RF spectra of the mode locked fiber ring 
laser, (a, b) without and (c, d) with, the FP-LD. Timebase: 20 ps/div. 
 
Fig. 3.  Wavelength spectrum of the mode 
locked pulse train. 
0.18 nm 
CWA82


Novel polarization maintaining actively mode locked fiber ring laser

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Novel polarization maintaining actively mode locked fiber ring laser

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