Author name used in this publication: M. S. DemokanAuthor name used in this publication: H. Y. Tam2005-2006 > Academic research: refereed > Publication in refereed journalVersion of RecordPublishe

Demokan, S

Liu, H

Tam, HY

Zhang, A

IEEE Photonics Technology Letters

PolyU Institutional Repository

IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 17, NO. 12, DECEMBER 2005 2535Stable and Broad Bandwidth MultiwavelengthFiber Ring Laser Incorporating a HighlyNonlinear Photonic Crystal FiberAiling Zhang, Heliang Liu, M. S. Demokan, and H. Y. TamAbstract—A stable and broad bandwidth multiwavelength fiberring laser suitable for use in dense wavelength-division-multi-plexing systems is demonstrated. A highly nonlinear photoniccrystal fiber is inserted in the ring cavity to stabilize the power fluc-tuation and broaden the output spectrum. A thin-film Fabry–Pérotetalon filter is used to get stable wavelength lasing. A Sagnac filteris employed to achieve uniform power distribution at each wave-length at the laser output. A laser with a power fluctuation about0.1 dB in each wavelength within 20 min and with wavelength shiftof less than 0.016 nm within 10 min is achieved. A stable 24-wave-length lasing operation with wavelength spacing of 0.8 nm withinthe 3-dB spectral range of 1573–1594 nm and signal-to-noise ratiobetter than 48 dB is observed experimentally.Index Terms—Dense wavelength-division multiplexing(DWDM), erbium-doped fiber, Fabry–Pérot (FP) etalon filter,four-wave mixing (FWM), multiwavelength fiber lasers, nonlinearoptics, photonic crystal fiber (PCF).I. INTRODUCTIONMULTIWAVELENGTH fiber lasers have aroused in-terest because of their potential applications in densewavelength-division-multiplexing (DWDM) systems. Therequirements for DWDM sources include a large number ofchannels over a broad bandwidth, uniformly distributed powerover channels, precise and stable wavelength spacing, andsmall power fluctuation at each channel. These multiwave-length sources can also find applications in fiber-optic sensorsand optical testing and measurements. Both the erbium-dopedfiber amplifier (EDFA) and the semiconductor optical amplifier(SOA) have been used as the gain medium in developing themultiwavelength fiber laser. The recently developed SOA-basedmultiwavelength lasers have a typical bandwidth of 20-30 nmand around 0.6-dB peak power fluctuation [1], [2]. EDFA-basedmultiwavelength lasers often have a smaller bandwidth andworse stability than the SOA-based ones due to their homoge-nous line broadening characteristics at room temperature.Several schemes have been proposed to mitigate the modecompetition existing within the EDFA at room temperature,which include adding a frequency shifter or phase shifter intothe ring cavity [3]–[5] or adding a length of highly nonlinearManuscript received July 20, 2005; revised September 12, 2005. This workwas supported by the Research Committee of The Hong Kong Polytechnic Uni-versity (Project A-PG55).The authors are with the Photonics Research Center and Department of Elec-trical Engineering, The Hong Kong Polytechnic University, Hung Hom, HongKong (e-mail: eealzhan@polyu.edu.hk).Digital Object Identifier 10.1109/LPT.2005.859542Fig. 1. Schematic of the setup for a multiwavelength fiber laser which utilizesa highly nonlinear PCF and an FP etalon serving as a comb-filter.photonic crystal fiber (PCF) into the ring cavity [6]. The emer-gence of PCFs with high nonlinearity and small dispersionslope has enabled the nonlinear effect, such as four-wavemixing (FWM), to happen more easily in a short length ofPCF and with low optical power. A dual wavelength fiberlaser stabilized by the two degenerate FWM in PCF has beendemonstrated [6]. The power fluctuation is greatly reduced byadding a 51-m-long PCF into the ring cavity. However, in [6],only two wavelengths are generated. In order to generate morewavelengths, the two fiber Bragg filters are replaced with athin-film filter.In this letter, a stable and broad bandwidth multiwavelengthlaser source using a highly nonlinear PCF as the nonlinear gainmedium is demonstrated. The wavelength spacing of the lasersource is anchored on the ITU frequency grid for DWDM ap-plication by using a Fabry–Pérot (FP) thin-film filter. The non-linear effect in the PCF will help broaden the multiwavelengthbandwidth and stabilize the power fluctuation at each wave-length. The thin-film filter used has a smaller wavelength drift,which will result in stable lasing wavelength at each channel.The power distribution over each channel can be made to benearly uniform by a filter. As a result, we get a wavelength- andpower-stable, broad bandwidth multiwavelength laser sourcewith wavelength spacing around 0.8 nm and with uniform powerdistribution in each wavelength. A detailed explanation follows.II. EXPERIMENTAL SETUPThe schematic of the experimental setup for the multiwave-length fiber ring laser is shown in Fig. 1, which consists ofa high-power EDFA, a short length of highly nonlinear PCF,a thin-film FP etalon filter, and a polarization controller (PC).1041-1135/$20.00 © 2005 IEEEAuthorized licensed use limited to: IEEE Xplore. Downloaded on October 21, 2008 at 23:11 from IEEE Xplore.  Restrictions apply.2536 IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 17, NO. 12, DECEMBER 2005The saturation power of the EDFA is about 27 dBm, which isused as the gain medium in the ring laser. Isolators are used toprovide unidirectional lasing oscillation, which are included in-side the EDFA. There is no gain flattening component inside theEDFA. The length of the highly nonlinear PCF is about 20 m,which is used to broaden the spectrum and stabilize the powerfluctuation of the ring laser by the FWM effect, as will be ex-plained later. The total insertion loss of the PCF, including thetwo splices to singlemode fibers, is around 1 dB. The nonlinearcoefficient of the PCF is about 30.6 W km , the zero-dis-persion wavelength of the PCF is around 1550 nm, and thedispersion slope near the zero-dispersion wavelength is verysmall [ 0.0004 ps/(nm km)]. The stimulated Brillouin scat-tering threshold is measured to be about 200 mW. The PC is usedto control the states of the polarization in the laser cavity. The FPfilter is used to provide periodic loss in the spectrum domain togenerate multiwavelength lasing. Normally, the maximum han-dling power of commercial pigtailed thin-film filters is around20 dBm. In our experiment, the power in the cavity is largerthan 27 dBm. In order to avoid damaging the thin-film filter, afiber bench from Optics For Research is used to increase thebeam diameter to 0.5 mm, which is more than 2.5 times largerthan the typical beam diameter of commercial pigtailed devices( 0.2 mm). So the intensity gets smaller for the same power.As a result, the maximum power allowed to pass through the FPetalon film is increased by more than six times. The loss spec-trum of the FP filter including the loss introduced by the fiberbench (around 1.15 dB) is observed. The insertion loss at eachwavelength peak is around 1.8 dB. The FP filter has an extinc-tion ratio larger than 12 dB. The wavelength spacing of the FPfilter is around 0.8 nm. The peak wavelengths and wavelengthspacing can be slightly changed by rotating the FP etalon by asmall angle.III. RESULTS AND DISCUSSIONFirst, a 90/10 coupler, where 10% of the optical poweris extracted as output, was used in the laser configuration.The output spectrum of the multiwavelength laser with themaximum biasing current of the pump of the EDFA is shownin Fig. 2(a). The total output power is around 15 dBm. Themaximum extinction ratio of the multiwavelength laser is about55 dB. If we define the number of effectively excited wave-lengths as those whose powers are within 20 dB of the powerof the strongest wavelength, then the spectrum of the excitedwavelengths, from Fig. 2(a), is between 1560 and 1587 nm.The spectrum bandwidth is about 27 nm. The measured 3-dBbandwidth of one channel is about 0.067 nm. The broadenedspectrum is caused by the high nonlinearity in the PCF. Whenseveral wavelengths are excited in the ring laser, those wave-lengths with sufficient power will generate new wavelengthsthrough FWM in the highly nonlinear PCF. The newly gener-ated wavelengths could then generate more wavelengths. Thespectrum will become even broader if we increase the valueof , where is the nonlinear coefficient of the PCF,is the power passing through PCF, and is the effective fiberlength of the PCF. In order to increase the optical power inthe laser cavity, the 90/10 coupler is replaced by a 99/1 couplerFig. 2. Output spectrum of the laser with different couplers.Fig. 3. Sagnac filter spectrum.to couple 99% of the output power back to the laser cavity.The output spectrum of the laser becomes wider, as shownin Fig. 2(b). The total output power is around 5.12 dBm. Themaximum extinction ratio of the multiwavelength laser is stillabout 55 dB. The spectrum of the excited wavelengths withpower drop within 20-dB ranges from 1560 to 1599 nm, as seenin Fig. 2(b). The spectrum range is about 39 nm which is largerthan that obtained by using a 90/10 coupler. The measured3-dB bandwidth of one channel is about 0.063 nm, which is alittle bit smaller than that obtained with a 90/10 coupler. Thewavelength spacing is 0.8 nm for both cases. It is also clearthat the output power distribution over wavelengths becomesslightly more flat when a 99/1 coupler is used. However, thechannel with the most power is at 1571 nm, irrespective of thecoupler used.In addition to wavelength and power stability, uniform powerdistribution over each wavelength is also desirable. In order tomake the power distribution over different channels nearly uni-form, a Sagnac loop filter [7] with a transmission spectrum thatis a mirror image of the laser output spectrum is designed andinserted at the laser output to flatten its spectrum [shown inFig. 2(b)]. The Sagnac loop filter was constructed with a 3-dBcoupler, a polarization-maintaining (PM) fiber, and a PC. TheSagnac filter bandwidth can be adjusted by changing the lengthof the PM fiber. Fig. 3 shows the transmission of the designedAuthorized licensed use limited to: IEEE Xplore. Downloaded on October 21, 2008 at 23:11 from IEEE Xplore.  Restrictions apply.ZHANG et al.: STABLE AND BROAD BANDWIDTH MULTIWAVELENGTH FIBER RING LASER 2537Fig. 4. Output spectrum after filtered by a Sagnac filter.Fig. 5. Power stability of the multiwavelength laser at wavelength 1560.7 nm.Sagnac loop filter by using a 10-cm PM fiber. The loss peaks canbe tuned by changing the polarization state of the PC. In orderto flatten the peak at 1571 nm of the laser output in Fig. 2(b), theloss peak is tuned to 1571 nm by the PC. The filter is connectedat the output of the laser. After the filter the laser spectrum is flat-tened because the loss of the Sagnac filter at 1571 nm is muchhigher than the adjacent wavelengths, whereas the spectrum ofthe laser is most powerful at 1571 nm but less so at other wave-lengths. Therefore, a balance is established. Fig. 4 focuses onthe flat part of the laser spectrum. From Fig. 4, we see that the3-dB bandwidth of the multiwavelength laser is around 19 nmwith 24 DWDM channels. The extinction ratio of the laser isbetter than 48 dB. It can be observed from Fig. 4 that the opticalpower at 1571 nm is much lower than that at 1585 nm. This isbecause the extinction ratio of the Sagnac filter is too large. Thebandwidth of the multiwavelength laser can be even larger if thefilter spectrum is optimized. The uniform power distribution canalso be realized by the insertion of a custom-designed filter di-rectly inside the laser cavity.The output power fluctuations of the laser, using either the90/10 coupler or the 99/1 coupler, are studied by filtering outthe channel at 1560.7 nm with a bandpass filter. The Sagnacfilter is sensitive to the environment which causes its transmis-sion spectrum to drift. In our setup, the Sagnac filter was prop-erly protected to reduce any external disturbances to the filter. Inaddition, due to the short fiber-length in the Sagnac filter and be-cause the filter has a large bandwidth (free-spectral range), driftin the filter transmission spectrum would introduce only a rel-atively small variation in the laser output spectrum. In this partof the experiment, the Sagnac filter is not included in the setup.The power fluctuations measured by a power meter are shown inFig. 5. The power fluctuations for both cases are around 0.1 dBwithin 20 min. The wavelength drift of the output laser is alsostudied by monitoring one wavelength. The wavelength driftsabout 0.016 nm within 10 min, which is much smaller than thechannel spacing.IV. CONCLUSIONA stable and broad bandwidth multiwavelength fiber ringlaser for use in DWDM systems has been demonstrated. Theoutput spectrum is broadened and power fluctuation is stabi-lized by means of the nonlinear effect in a highly nonlinearPCF. Stable wavelengths are achieved by using a thin-filmFP etalon filter. The power distribution over channels is madenearly uniform by using a Sagnac filter at the output of the lasersuch that the 3-dB bandwidth of the laser is around 19 nm with24-wavelengths. The wavelength drift is less than 0.016 nmwithin 10 min and the power fluctuation in one wavelength isabout 0.1 dB within 20 min.ACKNOWLEDGMENTThe authors wish to thank P. K. A. Wai for fruitfuldiscussions.REFERENCES[1] H. Dong, G. Zhu, Q. Wang, H. Sun, N. K. Dutta, J. Jaques, and A. B.Piccirilli, “Multiwavelength fiber ring laser source based on a delayedinterferometer,” IEEE Photon. Technol. Lett., vol. 17, no. 2, pp. 303–305,Feb. 2005.[2] Y.-G. Han, G. Kim, J. H. Lee, S. H. Kim, and S. B. Lee, “Lasing wave-length and spacing switchable multiwavelength fiber laser from 1510 to1620 nm,” IEEE Photon. Technol. Lett., vol. 17, no. 5, pp. 989–991, May2005.[3] F. Ahmed, N. Kishi, and T. Miki, “Multiwavelength Erbium-doped fiberFabry–Pérot laser and its uniform spectral lines power operation,” IEEEPhoton. Technol. Lett., vol. 17, no. 4, pp. 753–755, Apr. 2005.[4] J. Yao, J. Yao, Z. Deng, and J. Liu, “Multiwavelength Erbium-dopedfiber ring laser incorporating and SOA-based phase modulator,” IEEEPhoton. Technol. Lett., vol. 17, no. 4, pp. 756–758, Apr. 2005.[5] A. Bellemare, M. Karasek, M. Rochette, S. LaRochello, and M. Tetu,“Room temperature multifrequency erbium-doped fiber lasers anchoredon the ITU frequency grid,” J. Lightw. Technol., vol. 18, no. 6, pp.825–831, Jun. 2000.[6] X. Liu, X. Yang, F. Lu, J. Ng, X. Zhou, and C. Lu, “Stable and uniformdual-wavelength erbium-doped fiber laser based on fiber Bragg gratingsand photonic crystal fiber,” Opt. Express, vol. 13, no. 1, pp. 142–147,Jan. 2005.[7] X. Fang and R. O. Claus, “Polarization-independent all-fiber wave-length-division multiplexer based on a Sagnac interferometer,” Opt.Lett., vol. 20, no. 20, pp. 2146–2148, Oct. 1995.Authorized licensed use limited to: IEEE Xplore. Downloaded on October 21, 2008 at 23:11 from IEEE Xplore.  Restrictions apply.

Stable and broad bandwidth multiwavelength fiber ring laser incorporating a highly nonlinear photonic crystal fiber

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