SOA 锁模6

18-nm, 10-GHz Continuously Wavelength-Tunable Pulse Generation by Compensated Dispersion Tuning in aMode-Locked SOA Ring LaserGordon K. P. Lei, Mable P. Fok#, and Chester ShuDepartment of Electronic Engineering and Center for Advanced Research in PhotonicsThe Chinese University of Hong Kong, Shatin, N.T., Hong Kong.Email: kplei@.hk# Present address: Department of Electrical Engineering, Princeton University, Princeton, NJ 08544, USA Abstract : We generate a 10-GHz electrically tunable optical pulsed source using compensateddispersion tuning in a mode-locked SOA ring laser. A continuous tuning range of 18 nm isobtained.©2008 Optical Society of AmericaOCIS Code: (140.3600) Lasers, tunable; (140.4045) Mode-locked lasers; (999.9999) Compensated dispersion tuning; IntroductionWavelength tunable pulsed source has found important applications in optical communications and optical sensing [1]. Different approaches have been demonstrated to generate wavelength tunable pulses. Examples include frequency tuning with the use of a dispersion compensating fiber in a semiconductor optical amplifier (SOA) ring laser [2], wavelength tuning in an erbium-doped fiber laser containing a tunable chirped fiber Bragg grating (CFBG) [3], and self-compensated dispersion tuning in a 1 GHz SOA ring laser [4]. In this paper, we experimentally demonstrate a compact and widely tunable 10 GHz pulsed source from a mode-locked SOA ring laser using compensated dispersion tuning. The dispersion is provided by an identical pair of CFBGs. The continuous tuning is achieved by controlling the electrical delay between two RF modulating signals while the mode-locking frequency is kept constant throughout the tuning range. A tuning range of 18 nm is achieved without any polarization readjustment.Principle and Experimental SetupThe experimental setup is shown in Fig. 1. Two SOAs are used as the active media to provide optical gain in the cavity. The two CFBGs offer equal and opposite dispersions for the optical pulses. In principle, a single CFBG can be used for self-compensated dispersion tuning [4]. However, the use of two CFBGs can completely eliminate lasing in the sub-ring cavities. An optical isolator is added after SOA1 to ensure a unidirectional propagation. Light from SOA1 is fed to an electro-optic modulator driven by a 10 GHz RF signal. The modulated light is directed to the first CFBG through an optical circulator. The dispersion of the CFBG is +20 ps·nm-1 and the reflectivity is 97%.Fig. 1: Experimental setup. BPF: Bandpass filter; CFBG1, CFBG2: chirped fiber Bragg grating; EOM1, EOM2: electro-optic modulator; OC1, OC2: optical circulator; PC1, PC2: polarization controller; SOA1, SOA2: semiconductor optical amplifier.With a 30-nm reflection bandwidth from 1535 nm to 1565 nm, the CFBG can be used to broaden input pulses over a wide wavelength range. The broadened pulse is directed to another modulator driven by a delayed RF signal. As different wavelength components are reflected at different positions in the CFBG, there is a time-to-wavelength mapping in the broadened pulse. By controlling the electrical delay between the two RF modulating signals, we can select the wavelength of the output pulse. The gated component in the transmission window is directed to another CFBG through a circulator. The dispersion of the second CFBG is -20 ps·nm-1, thus compensating the dispersion introduced by the first CFBG. A wavelength tunable 10 GHz pulsed source is achieved and 10% power is coupled as the output. The driving frequency and the polarization settings are kept unchanged throughout the tuning range.Results and DiscussionThe relative delay between the RF signals is plotted against the output wavelength in Fig. 2. The tuning slope matches well with the dispersion of 20 ps·nm-1 in the CFBG. The pulse widths at different wavelengths are also measured. The pulse width varies from 22 ps to 29 ps as shown in the same figure. A sample of the pulse train is shown in Fig. 3. The tuning range is 18 nm, spanning from 1536 nm to 1554 nm. No lasing can be observed in the reflection band of 1554 nm – 1565nm. The lasing bandwidth is believed to be limited by the finite optical gains and gain profiles of the SOAs in our setup.Fig. 2: Plot of electrical delay between the 10-GHz RF modulating signals against the output wavelength. The measured pulse widths across the range are also plotted.Fig. 3: 10 GHz pulses generated at 1546 nm.Fig. 4: Illustration of pulse overlapping after dispersion by the CFBG in a 10-GHz pulsed source. The boxes show the transmission windows of EOM2.For a 10 GHz operation, a tunable bandpass filter is required to shift the peak gain of the cavity to the desired wavelength range. The situation is illustrated in Fig. 4. Pulses reflected by the CFBG are broadened to an extent according to the spectral bandwidth of the pulses. However, the pulses are only 100 ps apart. As shown in figure 4, a transmission window selecting λ11 of the pulse P1 will simultaneously select λ6 of the pulse P2 and λ1 of the pulse P3. Thus, a suppression of optical gain in the range away from λ11 is needed to avoid multiwavelength oscillation.Without using the filter, the tuning range is limited to 5 nm for a 10-GHz operation. The range is governed by the non-overlapping region between two broadened pulses. The relationship between the maximum tuning range and the group velocity dispersion is given by:DT =λΔ (1) where Δλ is the maximum tuning range, T is the period of the pulse train and D is the group velocity dispersion provided by the CFBG. The smaller the dispersion, the larger is the tuning range.To demonstrate the tuning capability without using a bandpass filter, we perform additional experiments with a 5 GHz modulating signal derived from a RF pulse generator. As shown in Fig. 5, a tuning range of ~9 nm is achieved. A linear tuning slope is also obtained. The pulse width varies from 22ps to 28ps throughout the tuning range. A sample of the output pulse train is shown in Fig. 6.Fig. 5: Plot of the electrical delay between the 5-GHz RF modulating signals against the output wavelength. The measured pulse widths across the range are also plotted.Fig. 6: 5 GHz pulses generated at 1539 nm.ConclusionWe demonstrate the generation of wavelength tunable pulses from a mode-locked SOA ring laser. The electrical tuning is continuous and is achieved by controlling the relative delay between two RF modulating signals. The tuning offers a potential for high-speed operation. Compensated dispersion is provided by a pair of CFBGs that introduce equal and opposite dispersions to the optical pulses. The tuning range for 5 GHz and 10 GHz pulses are 9 nm and 5 nm, respectively. With the use of a tunable bandpass filter in the setup, the tuning range for 10 GHz pulses has been extended to 18 nm.References[1] B. Dong, S. He, S. Hu, D. Tian, J. Lv, and Q. Zhao, “Time-Division Multiplexing Fiber Grating Sensor With a Tunable Pulsed Laser,” IEEE Photon. Technol. Lett., vol. 18, pp. 2620 – 2622, December 2006.[2] S. Yamashita and M. Asano, “Wide and Fast Wavelength-Tunable Mode-Locked Fiber Laser Based on Dispersion Tuning,” Opt. Express, vol. 14, pp. 9299 – 9306, October 2006.[3] J.H. Lee, Y.M. Chang, Y.G. Han, S.H. Kim, H. Chung and S.B. Lee, “Wavelength and Repetition Rate Tunable Optical Pulse Source Using a Chirped Fiber Bragg Grating and a Nonlinear Optical Loop Mirror,” IEEE Photon. Technol. Lett.,. vol. 17, pp. 34 – 36, January 2005.[4] K. L. Lee, K. Chan, and C. Shu, “Self-Compensated Dispersion Tuning of a Mode-Locked Fiber Laser Using a Linearly Chirped Fiber Grating”, IEEE Photon. Technol. Lett. , vol. 13, pp. 106- 108, February 2001.。