Optical Fiber Technology 5, 209᎐214 Ž1999. Article ID ofte.1998.0294, available online at http:rrwww.idealibrary.com on
Apodized Distributed-Feedback Fiber Laser D. Yu. Stepanov, J. Canning, and L. Poladian Australian Photonics Cooperati¨ e Research Centre, Optical Fibre Technology Centre, The Uni¨ ersity of Sydney, 101 National Inno¨ ation Centre, Australian Technology Park, E¨ eleigh, New South Wales 1430, Australia E-mail:
[email protected]
and R. Wyatt, G. Maxwell, R. Smith, and R. Kashyap B55 125, BT Labs., Martlesham Heath, Ipswich IP5 7RE, United Kingdom Received August 14, 1998; revised October 29, 1998
Enhanced side-mode suppression in a distributed-feedback ŽDFB. structure with gain which is optically pumped to produce lasing action can be achieved when the structure is apodized. Side-mode suppression exceeding 50 dB in a phase-shifted apodized DFB fiber laser is experimentally ob served. 䊚 1999 Academic Press
1. INTRODUCTION
Wavelength-division multiplexing ŽWDM. systems are being introduced to increase the information capacity of existing fiber-optic links. They require laser transmitters with accurate wavelength selection and high wavelength stability. A laser source in a WDM network must also have a high fiber-coupled output power. The widely used distributed-feedback ŽDFB. and distributed Bragg reflector ŽDBR. semiconductor diode lasers ŽSDL. both have difficulties of wavelength stability and selectivity and require active temperature stabilization. The advances in manufacturing DFB fiber lasers now offer an alternative to semiconductor sources for telecom applications. Fiber lasers in general are fully fiber-compatible, allowing for very low coupling losses. The DFB fiber lasers, particularly, have a number of additional advantages over their semiconductor counterparts. The potential of DFB fiber lasers as low-noise, narrow-linewidth sources for WDM systems has been demonstrated recently in digital transmission 209 1068-5200r99 $30.00 Copyright 䊚 1999 by Academic Press All rights of reproduction in any form reserved.
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tests w1x. Further, with a passive temperature-compensated package w2x the wavelength stability of DFB fiber lasers could be set better than 1 GHz within the temperature range from y20 to q80⬚C as was demonstrated for fiber Bragg gratings w3x, while stabilization of semiconductor diode lasers even within 10 GHz requires a significant effort w4x. This important technological advantage of DFB fiber lasers could be used to reduce costs per wavelength channel. Another broad application area for fiber lasers in general, and DFB fiber lasers in particular, is optical sensing. Narrow linewidths of the order of kHz, high powers in a number of wavelength ranges including the eye-safe 1.5 m region, low weight, and environmental stability in a simple package make them attractive for a number of remote sensor applications including Doppler-shift velocimetry, range finding, and gas detection. Complex amplitude and frequency profiles are readily achievable in DFB fiber lasers due to the flexibility of the fabrication process, opening ways for engineering optimization of the fiber devices and obtaining new properties. For example, apodization of the DFB structure would significantly improve side-mode suppression, important to improve the device signal-to-noise ratio. In this paper, we report an Er-doped apodized DFB fiber laser with more than 50 dB side-mode suppression, resolution limited by the noise floor. Apodization is shown numerically to provide a significant increase in the lasing threshold for side resonances compared to that for an unapodized -shifted DFB structure.
2. DFB FIBER LASER FABRICATION AND CHARACTERIZATION
Pieces of Er-doped germanosilicate fiber 10 cm long were spliced to standard Corning SMF28 single-mode optical fiber, which is not single-mode at 980 nm. This resulted in problems as higher order spatial modes at the pump wavelength were excited and the pumping efficiency of the doped fiber suffered from poorer overlap of these higher order modes with the doped fiber core. The doped fiber had a cutoff wavelength of 1050 nm and a pump absorption of ; 175 dBrm. The spliced samples were placed in a hydrogen atmosphere at 200 atm pressure and 80⬚C temperature for 24 h. After hydrogenation the samples were kept in a refrigerator at y40⬚C. Grating structures were fabricated using a computer-controlled interferometric setup which had a high degree of flexibility in setting the Bragg wavelength and a wide choice of standard and user-defined profiles of the local grating strength. We applied super-Gaussian profiles G Ž x . s exp yln 2 Ž 2 xrL .
2n
of order n, with the full width at half maximum ŽFWHM. L and x defined relative to the center of the grating. These profiles were found to apodize the grating spectral response. Although a lower order super-Gaussian profile would suppress
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grating sidelobes further, an order of 3 was used to better utilize the doped fiber gain. With 65 mm of the FWHM the apodization profile dropped down to zero at the ends of the doped fiber. A 244-nm light beam from a frequency-doubled Ar-ion laser was scanned along a phase mask at a controllable constant speed. Introducing a phaseshift in the middle of the structure to achieve single-longitudinal mode operation of the DFB laser was realized using a modification of the post-processing technique w5x. Whilst phaseshifts could be introduced by applying appropriate profiles on the local grating strength and phase, the post-processing technique allowed tuning the position of the transmission notch appearing in the bandgap of the grating in situ. The resulting transmission spectra of the grating structure before and after post-processing are shown in Fig. 1. The fabricated grating structure was packaged and subsequently characterized. The optical spectrum of the laser, when pumped with 36 mW of launched 980-nm pump power, is shown in Fig. 2. The optical spectrum was measured using an optical spectrum analyzer ŽOSA. with 0.07 nm resolution. The central wavelength was measured to be 1537.129 nm Ž; 195 THz. and the side-mode suppression was better than 50 dB. The lasing threshold was measured to be about 25 mW. Approximately 20% of pump power was absorbed in the Er-doped fiber, resulting in up to 0.4 mW output with slope efficiency of about 10%. If there were several laser modes the OSA would not resolve them because the separation between the modes is below the OSA resolution due to the long grating length. However, better than 50 dB side-mode suppression ratio was confirmed by looking at the radiofrequency spectrum of the optical beat signal in self-heterodyne measurements. This measurement revealed the presence of two orthogonal polarization states which produced an optical beat at about 1.41 GHz corresponding to the fiber birefringence of about 7 = 10y6 . No other beats have been observed. The two polarization modes can be seen in the scanning Fabry᎐Perot spectrum of the laser output shown in Fig. 3.
FIG. 1. Transmission spectra of the grating written in the Er-doped fiber before Ždashed line. and after Žsolid line. post-processing.
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FIG. 2. Optical spectrum of the DFB fiber laser output.
FIG. 3. Scanning Fabry᎐Perot spectrum of the DFB fiber laser output.
3. CALCULATION OF LASER MODE THRESHOLDS
The calculated lasing thresholds for -shifted apodized and unapodized DFB structures, and the results of the simulations are shown in Fig. 4. The apodized grating described in Section 2 was simulated using the spectral data of Fig. 1 and the lasting thresholds were found by calculating losses in the simulated structure resonances. In the simulation of the unapodized grating the length of the structure equal to the FWHM of the apodized one Ži.e., 65 mm. was used. This allowed the
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FIG. 4. Calculated lasing thresholds for -shifted apodized ŽB. and unapodized Ž⽧. grating structures.
resonances to appear at approximately the same frequencies. It is evident from Fig. 4 that the thresholds for an apodized structure are strongly suppressed compared to those for the unapodized case. It may be argued that the thresholds for the side resonances are still much larger than that for the central one and, therefore, lasing action will not commence at those side modes as the gain will be clamped to the minimum threshold. However, three-level solid-state lasers such as Er-doped lasers often exhibit relaxation oscillations at start-up and due to environmental perturbations, with the instantaneous gain exceeding the threshold gain at the beginning of the spike w6x. This may cause the threshold condition for a side mode to be satisfied resulting in active gain competition. Lasing of the structure will eventually become very sensitive to environmental perturbations. An improvement in the laser performance should be expected if the side-mode thresholds are increased.
4. CONCLUSION
In conclusion, we demonstrated an apodized distributed-feedback fiber laser with side-mode suppression in excess of 50 dB. This was achieved by apodization of the grating strength profile to reduce the finesse of side-mode resonances. This is consistent with numerical calculations showing that apodization increases the lasing threshold for the side modes.
ACKNOWLEDGMENTS The authors acknowledge S. Law and D. Thorncraft for packaging the fiber laser and B. Smith is for assistance in grating fabrication.
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