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LOA based multiwavelength Laser Source K. K. Qureshi, W. H. Chung and H.Y. Tam Photonics Research Center, Department of Electrical Engineering The Hong Kong Polytechnic University, Hong Kong, China Tel: +852-2766-6190, Fax: +852-2330-1544, Email:
[email protected]
P. K. A. Wai Photonics Research Center, Department of Electronic and Information Engineering The Hong Kong Polytechnic University, Hong Kong, China Tel: +852-2766-6231, Fax: +852-2362-8439, Email:
[email protected]
Abstract: We propose a multiwavelength laser source based on a Linear Optical Amplifier (LOA). Thirty eight wavelengths covering C+L-band with 0.8 nm channel spacing are generated at room temperature. Simultaneous tuning of multiwavelengths is also demonstrated. 2005 Optical Society of America
OCIS codes: (250.5980) Semiconductor optical amplifiers; (140.3510) Lasers, fiber
1. Introduction Multiwavelength laser sources have potential applications in instrument testing, sensing and wavelength division multiplexing (WDM) systems. This type of source is desired as it is an efficient and economical way to increase the transmission capability of the WDM systems. There are different methods to get simultaneous multiwavelength outputs such as multiwavelength Raman lasers [1,2], multiwavelength generation using Semiconductor Optical Amplifiers (SOA) [3] and multiwavelength Erbium doped fiber lasers [4,5]. The mutiwavelength Er-doped fiber lasers are useful but the outputs are not stable at room temperature due to homogeneous broadening of lasing modes. In this paper, we propose and demonstrate a simple configuration of multiwavelength laser source in the C+L-band, constructed with a Linear Optical Amplifier (LOA) acting as an inhomogeneous gain medium in the cavity. The perpendicular laser field in the LOA helps to clamp the mode competition more effectively than the parallel laser field in the case of SOA [6]. Comparison of the transient response between an LOA and an SOA verified that SOA based multiwavelength laser source is more prone to spectral fluctuations compared to LOA based source. We also performed a power stability test on one of the filtered wavelength of this laser source. 2. Gain Trasient of LOA In order to demonstrate that LOA based laser source has negligible mode competition as compared to the SOA based laser source we measured the switching transient response of the LOA and compared it with that of an SOA. For this measurement we used a pump and a probe technique as reported in [7]. The comparison between these two gain mediums was made by operating them at the same value of gain. The SOA was operated 2 dB below its saturation output power at a biasing current of 102 mA whereas the LOA was operated at 4 dB below its saturation output power at a biasing current of 130 mA. In this pump-probe method the pump beam at a wavelength of 1545.2 nm was modulated at low frequency of 20.9 KHz to simulate adding and dropping of wavelengths in a WDM system. The probe signal at a wavelength of 1548.0 nm was monitored at the falling edge of the pump beam. At the output of the LOA or the SOA the surviving channel i.e the probe beam was measured after a narrow-band tunable filter with a fast photodetector. In the case of SOA at the falling edge of the pump signal the probe beam jumps instantly by more than 1.4 dB whereas in the case of LOA the probe beam changes by less than 0.15 dB as shown in the Figure 1. This clearly explains that the effect of adding or dropping wavelength on the surviving wavelengths is negligible in the case of LOA when compared to an SOA. It also revealed that the LOA is not susceptible to instantaneous input power changes, which makes it a superior choice over SOA which experiences gain compression that leads to distortion and crosstalk in dynamic WDM systems [8]. LOA is not only useful for dynamic WDM systems but can also be used as a gain medium in a laser source configuration where different wavelengths should be amplified by the cavity gain media and the gain or loss of one wavelength should not affect the other wavelengths. In the next section we explain the use of LOA as a gain medium for stable multiwavelength generation in the C+L-band.
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3. Experimental Results Figure 2 shows a schematic diagram of the proposed configuration of the laser. It consists of an 80:20 coupler, a polarization controller (PC), a polarization independent isolator, a thin film etalon filter and a C-band Linear Optical Amplifier (LOA). The thin film etalon filter has a FSR of 100 GHz and exhibits absolute wavelength accuracy of +/1.25 GHz over operation temperature and C+L-band wavelength range. The operation temperature for this filter is from 0 to 70 °C and operation wavelength is from 1525 nm to 1620 nm. The LOA has a typical bandwidth gain flatness of 1.4 dB. The optical isolator in the cavity ensures unidirectional operation of the ring cavity and avoids unwanted reflections and the polarization controller to achieve high signal to noise ratio. The laser output is taken from an 80:20 fiber coupler placed in the ring, which provides 20% for the output and 80% for the feedback function. The total length of the cavity was approximately 3 meters and the round trip loss of the cavity was estimated to be less than 3.2 dB. The output spectrum of the multiwavelength laser is shown in Figure 3(a), which indicates a total of more than 38 wavelengths within C and L-band (with at least 20 dB SNR) with a channel spacing of 0.8 nm. The laser wavelength spacing was controlled by the cavity comb filter (thin film etalon filter). About 20 lasing wavelengths in the L-band exhibit optical signal to noise ratio of greater than 40 dB. Although the LOA used in the cavity is meant for C-band, but since the carrier density of the LOA decreases due to strong optical feedback (80%) which shifts the gain profile towards the L-band of the spectrum. Therefore, the lasing spectrum can be tuned by varying the loss in the laser cavity. A variable optical attenuator was inserted inside the multiwavelength laser cavity. The attenuation is adjustable, using an electrical voltage with a dynamic range of 40 dB in the 1550 nm region. The multiple lasing wavelengths were tuned simultaneously as the attenuation level was adjusted. As the attenuation level was increased the multiwavelength comb blue shifted (shorter wavelength region). Figure 3(b-d), shows the output spectra of the multiwavelength laser when the attenuation levels are 3 dB, 5 dB and 7 dB, respectively. Other parameters such as the biasing current applied to the LOA, and the output coupling ratio were kept constant during the tuning process. The experimental results indicates that the maximum tuning range is about 22 nm (1548 ~1570 nm), which corresponds to the shift of the longest lasing wavelength as the attenuation level is increased from 0 to 7 dB. This is a very effective method for wavelength tuning. In order to measure the stability of the laser source we filtered out one of the laser channel and observed its output power over 3 hours using an optical power meter. Inset of Figure 4 shows the spectrum of the filtered wavelength. The optical SNR of the lasing output is very large (> 50 dB) which is higher than previously reported multiwavelength sources. The power fluctuation observed by power meter was within 0.5 dB during the test as shown in Figure 4, which confirmed that this laser source is very stable at room temperature and may become useful for DWDM applications and component characterization. 4. Summary In summary, we have demonstrated a stable multiwavelength operation of an LOA based laser working at room temperature. A total of more than 38 lasing wavelengths were obtained with the channel spacing of 0.8 nm. By using an optical variable attenuator in the cavity, simultaneous tuning of multiwavelengths was achieved with a tuning range of around 22 nm. The laser exhibits high optical SNR of greater than 40 dB. We also observed that the power fluctuation of a single wavelength channel was less than 0.5 dB. 5. References [1] [2] [3] [4] [5] [6] [7] [8]
E. Yamada, H. Takara, T. Ohara, K. Sato, T. Morioka, K. Jinguji, M. Itoh and M. Ishii, “ A high SNR, 150 ch supercontinum CW optical source with precise 25 GHz spacing for 10 Gbit/s DWDM systems” Opt. Fiber Commun. Conf. 2001, Anaheim, CA, 1, ME2-1 – ME2-3. F. Koch, P. C. Reeves-Hall, S. V. Chernikov and J. R. Taylor, “ CW, multiple wavelength, room temperature, Raman fiber ring laser with external 19 channel, 10 GHz pulse generation in a single electro-absorption modulator”, Opt. Fiber Commun. Conf. 2001, Anaheim, CA, 3, WDD7-1 – WDD7-3. N. Pleros, C. Bintjas, M. Kalyvas, G. Theophilopoulos, K. Yiannopoulos, S. Sygletos, and H. Avamopoulos, “Multiwavelength and power equalized SOA laser sources,” IEEE Photon. Technol. Lett., 14, 693-695, (2002). N. Park, and P.F. Wysocki, “24-line multiwavelength operation of erbium doped fiber ring laser,” IEEE Photon. Technol. Lett., 8, 14591561, (1996). S. Yamashita and T. Baba, “Multiwavelength fiber lasers with tunable wavelength spacing” Opt. Fiber Commun. Conf. 2001, Anaheim, CA, 3, WA8-1 - WA8-3. F.W. Tong, W. Jin, D.N. Wang, and P.K.A. Wai, “Multiwavelength fibre laser with wavelength selectable from 1590 to 1645 nm”, Electron Lett, 10, 594-595, (2004). D.A. Francis, S.P. Dijaili, J.D. Walker, “A single chip linear optical amplifier,” Opt. Fiber Commun. Conf. 2001, Anaheim, CA, Postdeadline Paper PD13-1. L. H. Spiekman, G. N. van den Hoven, T. van Dongen, M. J. H. Sander- Jochem, J. J. M. Binsma, J. M. Wiesenfeld, A. H. Gnauck, and L. D. Garret, “Recent advances in WDM applications of semiconductor optical amplifiers,” in Proc. 26th Eur. Conf. on Opt. Commun. 2000, 1, invited paper, Munich, Germany, 35–38.
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Fig.2. Experimental setup for multiwavelength laser. LOA: Linear Optical Amplifier; PC: polarization controller.
Fig.1. Gain transients for SOA and LOA when the pump signal is turned OFF.
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Fig.3. Multiwavelength tuning with variable optical attenuator. The attenuation levels are (a) 0 dB (b) 3 dB (c) 5 dB (d) 7 dB -17
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Fig.4. Stability test of a single wavelength channel. Inset shows the spectrum of the channel.
