1 764 IEEE JOURNAL OF SELECTED TOPICS IN QUANTUM ELECTRONICS, VOL. 13, NO. 3, MAY/JUNE 2007 Continuous-Wave Operation of Semiconductor Optical Amplifier-Based Multiwavelength Tunable Fiber Lasers With 25-GHz Spacing Varghese Baby, Lawrence R. Chen, Senior Member, IEEE, Serge Doucet, and Sophie LaRochelle, Member, IEEE Abstract We demonstrate wide bandwidth operation in a semiconductor optical amplifier (SOA)-based fiber laser with a wavelength spacing of 25 GHz. A broadband multiwavelength filter based on superimposed and chirped fiber Bragg gratings is used for wavelength selection. Three different cavity configurations are presented and compared: 1) a single-soa standing-wave cavity; 2) a single-soa ring cavity; and 3) a double-soa ring cavity. Up to 41 wavelengths are obtained with a signal-to-noise ratio of 35 db and a wavelength tunability of 10 nm. Index Terms Fiber lasers, multiwavelength lasers, optical gratings, semiconductor optical amplifiers (SOAs). I. INTRODUCTION THE ADVENT OF wavelength division multiplexing (WDM) networks has made the need for multiwavelength sources very compelling. Distributed feedback (DFB) lasers have been extensively used as sources for single-channel communications, and therefore, as a natural extension, WDM sources were built using an array of DFB lasers . However, as channel spacing in the WDM systems continue to decrease for accommodating the bandwidth demand, the constraints on wavelength drifts of the laser sources become tighter. It is generally believed that a single multiwavelength source will be more robust to wavelength drifts than an array of single-wavelength sources . In addition to telecom applications , multiwavelength lasers have also been used in other applications such as time-resolved spectroscopy, fiber optic sensing, optical instrumentation, microwave photonics, characterization of photonic components such as amplifiers and filters, and light detection and ranging (LIDAR) . The use of fiber-based solutions avoids the higher coupling loss associated with other solutions. The key attributes required for multiwavelength sources include large number of channels, high-output power, stable operation in power and wavelength, and tunable operation over a wide wavelength range with application-specific wavelength spacing . Multiwavelength lasers operating in the 1500-nm region have been developed using different gain mechanisms and media such Manuscript received November 11, 2006; revised March 15, This work was supported in part by the Canadian Institute for Photonics Innovations. V. Baby is with Marusyk, Miller and Swain LLP, Ottawa ON KIP 5PQ, Canada ( L. R. Chen is with the Photonics Systems Group, Department of Electrical and Computer Engineering, McGill University, Montreal QC H3A 2A7, Canada ( S. Doucet and S. LaRochelle are with the Department of Electrical and Computer Engineering, Universite Laval, Quebec QC G1K 7P4, Canada ( Digital Object Identifier /JSTQE as erbium-doped fiber amplifiers (EDFAs) , semiconductor optical amplifiers (SOAs) , and schemes based on stimulated Raman scattering (SRS) gain  and stimulated Brillouin scattering (SBS) gain . In addition, hybrid gain mechanisms using a combination of the above mechanisms have also been used . The homogeneous linewidth broadening of the EDF medium limits the narrowest spacing of multiwavelength lasing to a few nanometers . While various schemes such as cryogenic cooling, frequency shifting, careful gain equalization, spatial-spectral multiplexing, polarization-hole burning, intracavity four-wave mixing in nonlinear fibers, etc.,   have been used to overcome this limit, they add more complexity (and therefore, cost) to these lasers. In contrast, SOAs have inhomogeneous linewidth broadening which allows narrower spacing for multiwavelength lasing operation . Previous experiments have demonstrated semiconductor fiber lasers with different numbers of simultaneous wavelengths and wavelength spacing. Various wavelength selection schemes have been used including delay interferometers, arrayed waveguide gratings, serial and parallel arrays of fiber Bragg gratings (FBG), Fabry Pérot filters, and high birefringence (HiBi) loop filters  . Tunability has been obtained using control-of-phase modulators in HiBi loop filters , polarization control in Lyot filters and birefringent sampled gratings , and the control of digital micromirrors in grating-based cavities . Simultaneous lasing of 75 wavelengths with a spacing of 40 GHz was demonstrated using the SOA-based fiber lasers . Recently, we reported preliminary results on continuouswave (CW) wavelength-tunable lasing operation of 35 wavelengths with 25-GHz (200 pm) spacing in a standing-wave configuration, using a 25-GHz-spaced comb filter based on a transmission grating filter (TGF) as the wavelength selection mechanism . While more recent work has shown narrower spacing , the operation was limited to simultaneous lasing of only three fixed wavelengths. In this paper, we present a detailed study of three different lasing cavities using the 25-GHz-TGF: a standing-wave cavity and two ring cavities. The results show lasing operation with high number of wavelengths (up to 41), large signal-to-noise ratios (SNRs) ( 35 db), and wavelength tunability (10 nm). II. EXPERIMENTAL SETUP The key elements common to all our lasing cavities were the use of TGF for 25-GHz-spacing wavelength selection and the use of the SOAs as gain media, which allowed such narrow spacing lasing. The TGF was fabricated by superposing two linearly X/$ IEEE
2 BABY et al.: CONTINUOUS-WAVE OPERATION OF SOA-BASED LASERS WITH 25-GH SPACING 765 Fig. 1. Spectral response. (a) TGF comb filter. (b) TGF comb filter zoomed in. (c) HiBi loop filter with 8-nm 3-dB pass band. (d) C/L coupler. chirped FBGs to form a distributed Fabry Pérot resonator , . The chirped gratings were written in hydrogen-loaded photosensitive fiber using the phase mask scanning method  and by displacing the phase mask by the required amount in between successive exposures. In our case, for a free spectral range (FSR) of 25 GHz (0.2 nm), the displacement was 4.06 mm. UV exposure was made using a CW 244-nm laser with an average power of 50 mw. For our writing setup, the phase mask was 12.5 cm long with a central period of nm and a chirp of 2.5 nm/cm. While the FSR was designed to be 25 GHz, the average index nonuniformities along the grating affect the effective index of the distributed cavities with the result that the FSR varies between 24 and 26 GHz over the entire TGF spectrum band. The extinction amplitude of the distributed Fabry Pérot resonator is 24 db with finesse between 20 and 24. These characteristics are equivalent to that of a Fabry Pérot resonator formed by using two mirrors with reflectivities between 85% and 88%. For the phase mask chirp used, these reflectivities correspond to the superimposed gratings with peak index modulations of 5.5 l0 4 for each grating. Fig. 1(a) and (b) shows the spectral response of the TGF, which has a 200-pm-spaced comb structure across the entire C-band. The insertion loss of the device was about 8 db with a loss variation of 2 db between resonance peaks. In addition to the TGF, a HiBi fiber-based loop filter  was also used within the cavity to tune the band of lasing operation. The sinusoidal spectral transfer function of the HiBi loop filter also enhances the SNR of the lasing wavelengths by restricting the lasing to a small band. Polarization controllers (PCs) within the HiBi loop filter can be adjusted to tune the spectral transfer function while the bandwidth of the loop filter is determined by the length of the HiBi fiber inside the loop . The HiBi fiber used for all our experiments was a PANDA fiber from Fujikura (SM-13P) with a measured birefringence n at 1550 nm and a maximum specified loss of 1 db/km. Two HiBi fiber lengths of approximately 0.8 and 0.45 m corresponding to 8- and 15-nm pass bands, respectively, were used in our setup. Fig. 1(c) shows the spectral transfer function of an 8-nm HiBi Fig. 2. Schematic of the different lasing cavities. (a) Standing wave cavity. (b) Single-SOA ring cavity. (c) Double-SOA ring cavity. C/L: C-L band coupler. TABLE I PARAMETERS OF THE USED SEMICONDUCTOR OPTICAL AMPLIFIERS loop filter. The insertion loss was 3 db. The noise present in the longer wavelength region was due to the lower EDFA power in that region used as source for the measurements. Fig. 1(d) shows the response of the C-L band coupler that was used to avoid spurious lasing in the L-band (the TGF operated only over the C-band). The three lasing cavities demonstrated are shown in Fig. 2. Different SOAs were tried in the cavities partly for performance comparison and partly due to availability constraints. Table I shows the relevant SOA parameters. A. Standing-Wave Cavity The SOA used was a Covega BOA 1004 and incorporated a highly efficient InP/InGaAsP quantum well-layered structure with a ridge waveguide design. The cavity loss was 14 db. As with the other lasing configurations, the cavity PCs were used to optimize the lasing operation. B. Single-SOA Ring Cavity Standing-wave cavities suffer from spatial hole burning effects resulting in gain saturation at lower intracavity average powers. To overcome this effect, the ring cavities were tried. The SOA used was a Covega SOA 1117 that had a low-polarizationdependant gain of 1dB.
3 766 IEEE JOURNAL OF SELECTED TOPICS IN QUANTUM ELECTRONICS, VOL. 13, NO. 3, MAY/JUNE 2007 Fig. 3. Multiwavelength output from the standing-wave cavity using the 8-nm HiBi loop filter. (a) Different settings of the PC in the HiBi loop filter. (b) Closer look at the lasing output for a specific setting. C. Double-SOA Ring Cavity In addition to the Covega SOA 1117, a second SOA, CIP NL- OEC 1550, was used to further improve the gain, and to facilitate schemes for power equalization, which is explained later. The latter was a high-confinement factor device with an InP-buried heterostructure design. Its small-signal gain was 30 db and the gain peak was around 1560 nm. For both the ring cavities, the cavity loss was measured to be 16.2 db, and was independent of the HiBi fiber lengths. The independence in the loss of the HiBi loop filter with HiBi fiber length is consistent with the specified loss of 1 db/km and the small lengths of HiBi fiber used in these filters. III. EXPERIMENTAL RESULTS A. Standing-Wave Cavity Fig. 3(a) shows the tunable multiwavelength lasing obtained using the standing-wave configuration with the 8-nm HiBi loop filter. The spectral measurements were taken using an optical spectrum analyzer (OSA; Agilent 83447) with 0.06-nm resolution and a sensitivity of 58 dbm. The overall tuning range of l0 nm was obtained by adjusting the PC within the HiBi loop filter. Fig. 3(b) gives a closer look for one scenario: 25 wavelengths are obtained with a minimum SNR of 35 db and a power uniformity of 7 db. Fig. 4. Multiwavelength output from the standing-wave cavity using the 15-nm HiBi loop filter. (a) Different settings of the PC in the HiBi loop filter. (b) Closer look at the lasing output for a specific setting. Fig. 4 shows the results obtained by the use of the 15-nm HiBi loop filter. Once again, 10 nm tuning range was obtained. As shown in Fig. 4(b), 35 wavelengths were obtained with a minimum SNR of 35 db and a power uniformity of 9 db. The output power stability of the lasing lines for the case of the 8-nm HiBi loop filter was 2.5 db over 1 h, as shown in Fig. 5(a). These measurements were made by subtracting the spectral powers at various time intervals from those at a reference time point. More accurate power stability measurements are expected to be made from tracking the powers of the individual peaks. The polarization states of the individual wavelengths of the comb were analyzed using an external 0.2-nm pass band fiber Bragg grating for the isolation of individual wavelengths. A typical result for three of the wavelengths, for the case of the 8-nm HiBi loop filter is shown in Fig. 5(b). This shows elliptical polarization with different azimuthal angles and different degrees of ellipticity that are stable over time. B. Single-SOA Ring Cavity Figs. 6 and 7 show the lasing output of the single-soa ring cavity for two different settings of the PC in the 8-nm and
4 BABY et al.: CONTINUOUS-WAVE OPERATION OF SOA-BASED LASERS WITH 25-GH SPACING 767 Fig. 5. (a) Spectral power fluctuation after 1 h. (b) Azimuthal angle of three wavelengths isolated using an external FBG, with time (measured with a polarimeter Profile PAT 9000B). Fig. 8. Lasing output of the double-soa ring cavity for two different settings of the polarization controller of the 8-nm HiBi filter. Fig. 6. Lasing output of the single-soa ring cavity for two different settings of the polarization controller of the 8-nm HiBi filter. Fig. 9. Lasing output of the double-soa ring cavity for two different settings of the polarization controller of the 15-nm HiBi filter. TABLE II PERFORMANCE COMPARISON OF DIFFERENT LASING CONFIGURATIONS Fig. 7. Lasing output of the single-soa ring cavity for two different settings of the polarization controller of the 15-nm HiBi filter. 15-nm HiBi loop filters, respectively. The measurements were made with a resolution bandwidth of 0.06 nm and a sensitivity of 60 dbm. We obtained a maximum of 22 and 39 simultaneous lasing wavelengths within a power deviation of 10 db for the 8-nm and 15-nm HiBi loop filters, respectively. The power stability while using the 8-nm HiBi loop filter was measured to be less than ±1 db over a period of 1 h. The reduced output power in each individual lasing wavelength for the 15-nm HiBi loop filter was expected due to the spreading of the total SOA gain across the larger loop filter bandwidth. C. Double-SOA Ring Cavity Figs. 8 and 9 show the results for the double-soa ring cavity for two different settings of the PC in the 8-nm and 15-nm HiBi loop filters, respectively. We obtained a maximum of 30 and 41 simultaneous lasing wavelengths within a power deviation of l0 db for the 8-nm and 15-nm HiBi loop filters, respectively. As in the case of the single-soa cavity, a reduction in the power of the lasing wavelengths is observed with the use of the 15-nm HiBi loop filter. The power fluctuations for the laser were within ±2 db over 1 h for both the 8-nm and 15-nm HiBi loop filters. IV. DISCUSSION Table II summarizes the performance of the three lasing configurations that we investigated. All three cavities show potential for broad-bandwidth lasing with good stability. Up to 10 nm wavelength tunability was achieved in all three cases using the HiBi loop filters as the tunable-wavelength-selection component in the lasing cavity. The tuning range was limited by the HiBi loop filter, as the control of PC within the HiBi loop leads to both a tuning in the center wavelength of the filter and in a variation of the extinction
5 768 IEEE JOURNAL OF SELECTED TOPICS IN QUANTUM ELECTRONICS, VOL. 13, NO. 3, MAY/JUNE 2007 Fig. 10. Tunable multiwavelength lasing using a thin-film-filter-based Fabry Perot filter with 5-nm 3-dB bandwidth. ratio of the filter . A different experiment was, therefore, done using a thin-film-based filter with a 3-dB bandwidth of 5 nm and tunability across the entire C-band. Fig. 10 shows the tuning of the laser from 1525 to 1560 nm using this filter, and confirms that the tuning range of 10 nm was only limited by the tuning capabilities of the HiBi filter. However, due to the narrower bandwidth of the filter, the lasing spectrum shows greater nonuniformity. Observations of the lasing output for all three configurations over a period of an hour did not show any wavelength drifts though the measurements are limited by the low resolution of the OSA. Linewidth measurements are also limited by the OSA resolution. However, the linewidth of each of the comb wavelengths is expected to be approximately 0.1 nm, which indicates that the output is not single mode since mode spacing in all three cavities is expected to be 10 MHz. The multimode nature of the individual wavelengths of the comb output for all three lasing configurations was confirmed on an oscilloscope by the presence of beating noise between the different modes. One of the requirements of the multiwavelength source is the power uniformity between the different output wavelengths. In other fiber laser demonstrations using SOAs as the gain media, power nonuniformity in multiwavelength lasing output has been reduced by various gain-clamping techniques for SOAs such as the use of an amplifier assist ring for gain-clamped operation , a feedback mechanism , a linear optical amplifier obtained by gain-clamping of an SOA using a vertical-cavity surface-emitting laser , and four-wave mixing in dispersionshifted fiber  or photonic crystal fiber   as a secondary process for generating more wavelengths. In our experiments, attempts to reduce the power nonuniformity using the amplifier-assist-ring configurations  and feedback mechanisms  did not yield any improvement. We believe that this is because the primary cause for the power nonuniformity in our experiments is the variation in insertion loss of the TGF with wavelength ( 2 db) and not the gain characteristics of the SOAs itself. Future work will concentrate on improvement of power levels, power uniformity, and number of wavelengths using TGFs with lower loss and greater uniformity. V. CONCLUSION In conclusion, we have demonstrated multiwavelength (considering only experiments with more than three wavelengths) lasing with the narrowest spacing to date with fiber lasers based on SOAs. The wavelength selection mechanism was based on the use of a superimposed and chirped grating-based transmission grating filter for comb selection, and the use of HiBi loop filters for wavelength band selection. Up to 41 wavelengths are obtained with a minimum SNR of 35 db and a tuning range of 10 nm. Tunability across the entire C-band, albeit with lower number of wavelengths and greater power nonuniformity was achieved using a thin-film-based filter. All results show good power stability over time, with less than ±2 db variation over 1 h. 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Ham, Stable and broad bandwidth multiwavelength fiber ring laser incorporating a highly nonlinear photonic crystal fiber, IEEE Photon. Technol. Lett., vol. 17, no. 12, pp , Dec Varghese Baby was born in Trivandrum, Kerala, India, in He received the B.Tech. degree in electrical engineering from the Indian Institute of Technology, Madras, in 2000, and the M.S. and Ph.D. degrees in electrical engineering from Princeton University, Princeton, NJ, in 2002 and 2006, respectively. He has been a Postdoctoral Research Fellow with the Photonics Systems Group, Department of Electrical and Computer Engineering, McGill University, Montreal, QC, Canada, since January Currently, he is working with Marusyk, Miller and Swain LLP, Ottawa, ON, Canada. His current research interests include fiber lasers, fiber optic sensors, and all-optical networking. Lawrence R. Chen (S 93 M 00 SM 06) was born on February 17, 1973, in Red Deer, AB, Canada. He received the B.Eng. degree in electrical engineering and mathematics from McGill University, Montreal, QC, Canada, in 1995, and the M.A.Sc. and Ph.D. degrees in electrical and computer engineering from the University of Toronto, Toronto, ON, Canada, in 1997 and 2000, respectively. Since 2000, he has been with the Photonics Systems Group, Department of Electrical and Computer Engineering, McGill University, where he is currently an Associate Professor. His current research interests include ultrafast photonics and fiber optics, optical pulse shaping and signal processing, fiber lasers and amplifiers, optical code division multiple access, and fiber gratings. Serge Doucet received the Bachelor s and M.Sc. degrees in electrical engineering from Universite Laval, Quebec, Canada, in 2003 and 2006, respectively. His current research interests include the design and development of fiber Bragg gratings mostly affected by dispersion compensation and the development of distributed lattice-coupled cavities fiber Bragg grating to make tunable multichannel chromatic dispersion compensator for high-bit-rate optical systems. Sophie LaRochelle (M 00) received the Bachelor s degree in engineering physics from Universite Laval, Quebec, Canada, and the Ph.D. degree in optics from the University of Arizona, Tucson, in 1987 and 1992, respectively. From 1992 to 1996, she was a Research Scientist with the Defence Research and Development, Valcartier, Canada, where she worked on electrooptical systems. Currently, she is a Professor with the Department of Electrical and Computer Engineering, Universite Laval, where she holds a Canada Research Chair in optical fiber communications and components. Her current research interests include active and passive fiber optic components for optical communication systems including fiber Bragg gratings, optical amplifiers, multiwavelength, and pulsed fiber lasers. Dr. LaRochelle is a member of the Optical Society of America.