Eleven-Wavelength Switchable Fiber Ring Laser with a Dispersion

Optics Communications 281 (2008) 5842–5845

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Optics Communications

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Eleven-wavelength switchable ﬁber ring laser with a dispersion compensation ﬁber and a delayed interferometer

Fei Wang a,b, Xin-Liang Zhang a,*, Jian-Ji Dong a,YuYua, Zheng Zhang a a Wuhan National Laboratory for Optoelectronics and School of Optoelectronic Science and Engineering, Huazhong University of Science and Technology, Wuhan 430074, China b School of Mathematics and Physics, Chongqing Institute of Technology, Chongqing 400050, China article info abstract

Article history: We demonstrate a fiber ring laser with a dispersion compensation fiber (DCF) and a delayed interferom- Received 14 May 2008 eter (DI) with temperature control, which is able to switch eleven wavelengths one by one. In ring cavity, Received in revised form 15 July 2008 DCF supplies different effective cavity lengths for different wavelengths, DI generates a wavelength comb Accepted 24 July 2008 corresponding to the ITU grid, a flat-gain erbium-doped fiber amplifier (EDFA) provides uniform gain for each lasting wavelength, and a semiconductor optical amplifier (SOA) not only acts as active modulator, but also alleviates homogeneous broadening effect of EDFA. Stable pulse trains with a pulsewidth about Keywords: 40 ps at 10 GHz have been obtained by injecting external optical control signals into the laser. Wave- Optical communications length switching process among eleven wavelengths is achieved by merely tuning an intracavity optical Wavelength switchable lasers Mode-locked lasers delay line. Semiconductor optical amplifier Ó 2008 Elsevier B.V. All rights reserved.

1. Introduction temperature control, which is able to switch eleven wavelengths one by one. In ring cavity, DCF supplies different effective cavity Wavelength switchable laser sources can be used in wavelengths for different wavelengths, DI generates a wavelength comb length-division-multiplexed (WDM) transmission systems, opti- corresponding to the ITU grid, a flat-gain EDFA provides gain for cally controlled phase array radars and fiber sensors [1].To each channel, a band pass filter (BPF) with a relative broad band- realize wavelength switching of fiber laser, various techniques width determines the operation wavelength range and a semicon- have been reported, such as the cavity with cascaded fiber Bragg ductor optical amplifier (SOA) modulated by external optical grating (FBG) [1–4], the cavity with a multimode FBG (MM-FBG) injection signal not only acts as active modulator, but also allevi- [5–7], the cavity with a Fabre-Pérot laser diode (FP-LD) [8], the cav- ates homogeneous broadening effect of EDFA due to inhomoge- ity with a FP etalon filter [9], the cavity with a flat-top fiber comb neous gain character of SOA. Since this laser is mode-locked by filter [10], the cavity with a spectral polarization-dependent loss an external optical signal, potentially ultrashort optical pulses at element [11], the cavity with a multisection high-birefringence high repetition rates can be readily obtained, and the mode-locked (HiBi) fiber loop mirror [12], the cavity with dispersion-manage- lasing wavelength can be conveniently switched among any of the ment [13] and so on. FBG is frequently utilized to acquiring wave- transmitted wavelengths via the DI by merely adjusting the delay length switching, which is an ideal device to select lasing time of the intracavity tunable optical delay line (TODL). Unlike wavelengths. However, the number of switchable wavelength using cascaded FBG in switching wavelength laser, the number of was limited by the number of cascaded FBG. switchable wavelength is limited by the number of cascaded Actively mode-locked fiber lasers have been developed for FBG. To increase the number of switchable wavelength, more wavelength switching, because various wavelength selection de- FBG must be cascaded. But, device loss rapidly increases with cas- vices can be used in ring cavity, and its output wavelength can caded FBG increasing. Theoretically speaking, using DI instead of be tuned easily. In addition, the erbium-doped fiber amplifier cascaded FBG, the number of switchable wavelength is not limited (EDFA) may be designed to achieve flat gain over 40 nm wave- by DI itself. Of course, the corresponding measures should be em- length range, and the equal amplitude at the different wavelength ployed to suppress overmuch wavelength oscillation in switching can be achieved. But, special measures need be adopted to sup- wavelength laser. press wavelength competing caused by its homogeneous gain. In this paper, we present a fiber ring laser with a dispersion compensation fiber (DCF) and a delayed interferometer (DI) with 2. Experimental setup

* Corresponding author. Tel.: +86 27 87792242x802; fax: +86 27 87792367. Fig. 1 depicts the experimental setup of the wavelength switch- E-mail address: [email protected] (X.-L. Zhang). able mode-locking ﬁber ring laser. The laser cavity is formed by a

Fig. 1. Experimental setup of the wavelength switchable mode-locking ﬁber ring laser.

DCF, a fiber DI, a broad bandwidth BPF, a SOA, a flat-gain EDFA, a TODL, a circulator, an isolator and a 90:10 optical coupler. The external optical signal at 1563.65 nm is generated by external modulating a distributed-feedback laser diode (DFB-LD) using two LiNbO3 Mach–Zehnder modulators. Before injecting into ring cavity, the signal power can be controlled by a followed EDFA and an attenuator (ATT). An optical field generated from spontane- ous emission are modulated by the external optical signal using the cross gain modulation (XGM) effect in SOA [14,15]. The SOA is a strained multiple quantum-well device with an active region of 450 lm and its small signal gain is about 13 dB at 100 mA biased current. The optical field intensity is amplified via the EDFA with the small signal gain of 25 dB to overcome cavity loss. The DCF supplies different effective cavity lengths for different wavelengths. The total dispersion of the DCF is about 84 ps/nm, and it serves as a comb filter. The transmission characteristics of 25 ps fiber DI Fig. 3. Mode-locked output pulses at a repetition rate of 10 GHz. (a) Pulse at 22 are shown in Fig. 2. The spacing between adjacent peaks is waveform. (b) Spectra of the pulses. 0.32 nm, and the extinction ratio (ER) is 12 dB. The BPF determines the region of lasing wavelength and suppress overmuch wavelength oscillation. Its central wavelength is 1559 nm and is with 3. Results and discussion a 3-dB bandwidth of 3.74 nm. TODL is utilized to adjust the cavity length for all possible oscillation wavelength, and its maximum de- In our experiment setup, the SOA has a MQW active region, its lay time is 300 ps. The isolator allows the light to propagate unidi- gain is polarization-independent and its bias current is 100 mA. rectionally, and a majority of the light is extracted out by using a When the external signal that power is 5 dBm and pulse width is 90:10 output coupler. The optical waves are monitored by an optical spectrum analyzer (OSA, Anritsu MS9710C), and a communications signal analyzer (CSA, Tektronix 8000B).

Fig. 2. Measured transmission characteristics of 25-ps DI. The spacing between adjacent peaks is 0.32 nm. Fig. 4. Wavelength switching characteristics by tuning the optical delay line. 5844 F. Wang et al. / Optics Communications 281 (2008) 5842–5845

Fig. 5. Waveforms of the mode-locked pulses on the different wavelength.

55 ps is injected into ring cavity, steady harmonic mode-locking is acquiring more wavelengths switching, DCF with a bigger disper- acquired by adjusting TODL. If injecting power of external signal is sion should be employed in cavity. The range of switched wave- too low, optical field in cavity can’t be modulated effectively. lengths of this laser can be extended to cover the whole gain Therefore, a continuous wave (CW) is exported from the cavity. region of the EDFA (40 nm) by replacing BPF with the different cen- When injecting power is higher than 5 dBm, gain of the SOA is rap- tral wavelength and bandwidth. The channel spacing also allows to idly saturated, and mode-locked pulses no more happen to change be changed by replacing the DI with the different delay time. The obviously. In addition, a broader injecting pulses results in nar- wavelength switching speed of the laser is dominated by the rower mode-locked pulses. Fig. 3a shows the pulse waveform of mechanical motion of ODL. Therefore, if an electrically controlled mode-locked output pulses at a repetition rate of 10 GHz. The fun- ODL is used, switching time can be shortened. Waveforms of the damental frequency of the fiber ring laser is about 0.275 MHz at mode-locked pulses on the different wavelength corresponding the wavelength 1559.88 nm, and the corresponding optical length to Fig. 4 are shown in Fig. 5. The clear pulse profiles indicate that of the ring cavity is calculated to be 1.091 km. Injection pulse fre- obtained mode-locked pulses are steady. The peak power and quency is 10.0001 GHz, thus, the laser operated at around the FWHM of mode-locked pulses versus wavelengths are shown in 36,364th harmonic. The pulse width is about 40 ps. Fig. 3b shows Fig. 6. The pulse widths of the output trains obtained are within the corresponding optical spectrum of the pulses. The lasing wavelength is 1559.88 nm and 3-dB spectral width is 0.17 nm. There- fore, the bandwidth–pulsewidth product is 0.83, indicating that the pulses are chirped. The chirp of the pulses may arise from the time variation of the charge density in the SOA. The side- mode-suppression ratio (SMSR) is better than 25 dB. To acquire switching of the mode-locked lasing wavelength among any of the transmitted wavelengths via the DI, the delay time of the intracavity ODL need to be adjusted. The wavelength switching characteristics are shown in Fig. 4. It shows clearly that eleven output wavelengths can be switched one by one, which is k1 = 1557.57 nm, k2 = 1557.90 nm, k3 = 1558.22 nm, k4 = 1558.56 nm, k5 = 1558.88 nm, k6 = 1559.22 nm, k7 = 1559.55 nm, k8 = 1559.88 nm, k9 = 1560.22 nm, k10 = 1560.56 nm, and k11 = 1560.93 nm, respectively. Its average spacing is 0.32 nm, which is decided by the spacing between adjacent peaks of DI comb spectra. Spectral widths of the output trains at the different wavelength are same (3 dB bandwidth is 0.17 nm), which can not be acquired via other filters, such as FBG. Each wavelength can steadily last, which owe to DI being with temperature control. Only several wavelengths can be switched without DCF in ring cavity, whereas, for Fig. 6. The peak power and FWHM of mode-locked pulses versus wavelengths. F. Wang et al. / Optics Communications 281 (2008) 5842–5845 5845

38.5–47.5 ps, and shorter pulses can be obtained, if the length of 2006ABB017) and the Program for New Century Excellent Talents DCF is optimized. The peak powers of the output trains obtained in Ministry of Education of China (Grant No. NCET-04-0715). The are within 1.0–1.6 mW. authors also gratefully acknowledge the support from the Commis- sion of Education of Chongqing City of PR China (KJ080607). 4. Conclusion References We have proposed and successfully demonstrated a ﬁber ring [1] J. He, K.T. Chan, IEEE Photon. Technol. Lett. 15 (2003) 798. laser, which is able to be conveniently switched among any of [2] N.J.C. Libatique, R.K. Jain, IEEE Photon. Technol. Lett. 11 (1999) 1584. the transmitted wavelengths via the DI by merely adjusting ODL. [3] X.M. Liu, X.Q. Zhou, X.F. Tang, J. Ng, J.Z. Hao, T.Y. Chai, E. Leong, C. Lu, IEEE Active mode locking of the lasing light wave is obtained in a Photon. Technol. Lett. 17 (2005) 1626. [4] Q. Mao, J.W.Y. Lit, IEEE Photon. Technol. Lett. 14 (2002) 612. MQW–SOA by gain modulation caused by external injecting optical [5] S.G. Fu, L.B. Si, Z.C. Guo, S.Z. Yuan, Y. Zhao, X.Y. Dong, Appl. Opt. 46 (2007) 3579. control signals. Synchronously, homogeneous broadening effect of [6] X.H. Feng, H.Y. Tam, P.K.A. Wai, IEEE Photon. Technol. Lett. 18 (2006) 1088. EDFA was alleviated by SOA with inhomogeneous gain. The pro- [7] L. Su, C. Lu, J.Z. Hao, Z.H. Li, Y.X. Wang, IEEE Photon. Technol. Lett. 17 (2005) 315. posed scheme has some advantages such as potential high repeti- [8] P.C. Peng, J.H. Lin, S. Chi, IEEE Photon. Technol. Lett. 16 (2004) 1023. tion rate, good stability, ease to be switched. [9] H.L. Liu, H.Y. Tam, W.H. Chung, P.K.A. Wai, N. Sugimoto, IEEE Photon. Technol. Lett. 17 (2005) 986. [10] Y.W. Lee, H.T. Kim, J. Jung, B. Lee, Opt. Express 13 (2005) 1039. Acknowledgements [11] Y.W. Lee, B. Lee, IEEE Photon. Technol. Lett. 15 (2003) 795. [12] S. Hu, L. Zhan, Y.J. Song, W. Li, S.Y. Luo, Y.X. Xia, IEEE Photon. Technol. Lett. 17 This work is supported by National High Technology Develop- (2005) 1387. [13] K.L. Lee, C. Shu, IEEE Photon. Technol. Lett. 15 (2003) 513. ing Program of China (Grant No. 2006AA03Z414), the Science Fund [14] F. Wang, G.Q. Xia, Z.M. Wu, Opt. Commun. 257 (2006) 334. for Distinguished Young Scholars of Hubei Province (Grant No. [15] F. Wang, X.L. Zhang, J.J. Dong, Opt. Commun. 281 (2008) 2868.