High-fidelity all-fiber amplification of a gain-switched laser diode

Transcription

1 High-fidelity all-fiber amplification of a gain-switched laser diode Laura Abrardi, Marek A. Gusowski, and Thomas Feurer* Institute of Applied Physics, University of Bern, Sidlerstrasse. 5, CH-3012 Bern, Switzerland *Corresponding author: thomas.feurer@iap.unibe.ch Received 18 April 2014; revised 4 June 2014; accepted 4 June 2014; posted 6 June 2014 (Doc. ID ); published 10 July 2014 We demonstrate a multistage erbium fiber amplifier seeded by a gain-switched laser diode operating at a wavelength of 1550 nm and a repetition rate of 1 MHz. The pulse energy is 0.5 μj, and the pulse duration is 40 ps, resulting in a peak power of 11.4 kw. The three-stage all-fiber amplifier system is designed to avoid any spectral distortions induced by gain saturation or nonlinear effects and high levels of amplified spontaneous emission. The output pulses are close to transform limited with a Gaussian pulse envelope Optical Society of America OCIS codes: ( ) Laser amplifiers; ( ) Lasers, erbium; ( ) Lasers, fiber; ( ) Picosecond phenomena; ( ) Ultrafast lasers Introduction Gain-switched semiconductor laser diodes (SLDs) are very attractive sources of picosecond pulses [1 4]. They are robust, compact, stable, and provide tunable repetition rates from pulse-on-demand up to hundreds of megahertz. At repetition rates of less than 10 MHz, they present themselves as an alternative to picosecond mode-locked lasers, which are more expensive, less robust, and require complex longcavity designs or pulse pickers. The gain-switched laser diodes, however, have several drawbacks that limit their use mostly to telecommunication applications. Their pulse energy is typically only a few tens of picojoules and, consequently, the pulses need to be amplified by several orders of magnitude to reach energy levels available nowadays from mode-locked lasers. When the SLDs are operated at higher power levels, the pulses typically become longer [5,6], and relaxation oscillations can manifest themselves in long pedestals following the main pulse [7,8]. Usually, the output pulses are chirped due to the transient gain dynamics [9 12], and the chirp is generally X/14/ $15.00/ Optical Society of America nonlinear. Moreover, the timing jitter between pulses is larger than, e.g., for mode-locked lasers [13,14]. Despite all these drawbacks, there has been a marked increase in research activities aiming at better stabilization and control of the gain dynamics [15 20], mostly to reduce the timing jitter [14,15] and to improve the pulse quality [21 23]. Many applications, however, require higher pulse energies than provided by such diodes, and amplification becomes essential. Very little literature is found on the amplification of picosecond pulses of SLDs at telecommunication wavelengths around 1550 nm at high repetition rates (tens of MHz) [24], as well as at lower repetition rates [25]. This is largely due to the fact that the low pulse energy combined with the low repetition rate results in an average power so low (around a few μw) that any conventional erbium-doped fiber amplifier operates well below saturation. This is even more of an issue when the SLDs are operated under conditions that lead to clean, almost Gaussian-shaped output pulses, in which case the pulse energy is often less than 1 pj. Moreover, if spectral filtering is used to remove unwanted spectral components and to allow for nearly transform-limited pulse durations, the pulse energy drops even further. In the nonsaturated 10 July 2014 / Vol. 53, No. 20 / APPLIED OPTICS 4611

2 regime, the amplification is always accompanied by a substantial level of amplified spontaneous emission (ASE), often prohibitively high for a laser to be of any practical use. Therefore, the design especially of the first amplification stage is of eminent importance. Once the power level is increased by several orders of magnitude, a standard amplification technology can be used, as demonstrated for amplified mode-locked lasers at 1550 nm in the picosecond regime [26]. Here, we report on the performance of an erbiumdoped all-fiber amplifier that is seeded by a gainswitched distributed feedback SLD (DFB-SLD) at 1 MHz repetition rate. Its multistage configuration boosts the pulse energy to 0.5 μj while preserving a Gaussian-shaped spectrum and a nearly transform-limited pulse duration of approximately 40 ps. First, we describe the multistage design and then continue with a detailed description of the SLD performance and the performance of each of the amplification stages. 2. Amplifier Design Figure 1 shows the scheme of the three-stage amplification in suitably doped optical fibers. It consists of two core-pumped Er-doped fiber amplifiers (EDFA) followed by a commercial ErYb-doped double-clad soft-glass fiber amplifier module (EYDFA). Each Er-doped fiber amplifier stage is pumped by a continuous wave semiconductor laser diode at 976 nm. Between the stages, spectral filtering is applied to remove ASE. All stages use polarization-maintaining (PM) components. The seed laser is a gain-switched DFB-SLD from Advanced Laser Diode Systems [27]. Its output is coupled to a PM fiber and subsequently filtered with a 2 nm wide spectral bandpass. The first amplifier uses a PM Er-doped fiber with a mode-field diameter of 4.8 μm and is followed by a 3 nm wide spectral bandpass filter. The second stage amplifier is an Er-doped fiber with a mode-field diameter of 7 μm, also followed by a 3 nm wide spectral bandpass. The third amplification stage is a commercial ErYbdoped double-clad soft-glass large-mode-area fiber amplifier module Blizzard-1.5 μm from Polar Laser Laboratories [28]. Typically, the pulses out of a low-biased, gainswitched semiconductor laser exhibit a spectral down-chirp [17,18,21], with the detailed features determined by the modulation current and the operation temperature. The temperature not only influences the output wavelength but also the temporal evolution of the output spectrum because it shifts the position of the Bragg wavelength relative to the gain maximum [17]. Through a judicious choice of the parameters, i.e., the current amplitude and the temperature, it is possible to achieve Gaussian-shaped pulses with a duration of approximately 40 ps at full width at half-maximum (FWHM) with negligible pedestals and a pulse energy of 3.5 pj. The spectrum and the temporal intensity profile are shown in Figs. 2 and 3. In order to remove the spectral wing toward shorter wavelengths that is typical for the down-chirped pulses emitted from such laser diodes and to decrease the time-bandwidth product, the seed pulses were filtered with a 2 nm wide spectral bandpass filter. The spectrum after filtering is shown in Fig. 2. The diode has a maximum emission at nm and a low intensity wing toward shorter wavelengths. The center frequency and the bandwidth of the filter have been selected so that they do not affect the temporal profile of the pulse. The Fig. 2. DFB-SLD spectrum after the spectral bandpass. 3. System Performance All spectra presented were recorded with the Optical Spectrum Analyzer AQ6370D Yokogawa. The temporal intensity profiles were recorded with a fast photodiode (30 GHz bandwidth) and an oscilloscope (40 GHz bandwidth) and corrected with the experimentally determined response function. Fig. 1. Schematic of the multistage fiber amplifier seeded by a gain-switched laser diode. Fig. 3. diode. Temporal intensity profile of the gain-switched laser 4612 APPLIED OPTICS / Vol. 53, No. 20 / 10 July 2014

3 Fig. 4. Output spectrum of the first amplifier stage before and after the spectral bandpass. Fig. 6. Output spectrum of the second amplifier stage before and after the spectral bandpass. pulse energy and the average power, however, are reduced to 2.1 pj and 2.1 μw, respectively. The spectrally filtered pulses are subsequently coupled into the first amplifier stage whose performance is summarized in Figs. 4 and 5. Although the maximum gain of this amplifier stage is slightly more than 40 db, we limit the amplification to 16.5 db in order to avoid gain saturation and nonlinear effects that would result in spectral and temporal distortions of the signal. Directly after the fiber amplifier, the signal has a spectral FWHM of 0.12 nm and the signal-to-ase ratio is 0.03 db, i.e., the ASE contributes approximately 50% to the total average power. The average power of the signal and the ASE were estimated assuming that the narrowband signal spectrum resides on top of the broadband ASE spectrum. The spectrum was calibrated with the separately measured average output power. The output of the first amplification stage is subsequently filtered with a 3 nm wide spectral bandpass, and Fig. 4 compares the spectra before and after filtering. After filtering, the signal resides on an approximately 3.5 nm wide ASE background, and the signal-to- ASE ratio has consequently increased to 16 db. The pulse energy and the average signal power after the first stage of amplification and the bandpass filter are 95 pj and 95 μw, respectively. The temporal intensity is unaffected by the amplification and the spectral filtering, as seen in the Fig. 5. After the first amplifier stage, the power level is high enough to permit the second amplification stage to operate in the saturated regime. At its exit, the output pulse energy and average power are 14 pj and 14 mw, respectively, which corresponds to a gain of 21.7 db. As before, we limit the gain level to avoid spectral and temporal pulse distortions due to nonlinear effects. A second bandpass filter with a 3 nm bandwidth further suppresses the ASE, and after the bandpass, the signal-to-ase ratio reaches 26 db, as seen in Fig. 6. Figure 7 presents the temporal intensity of the pulses and demonstrates that the intensity is affected neither by the amplification nor the spectral bandpass. The third amplification stage is a commercial ErYb-doped double-clad soft-glass large-modearea fiber amplifier module (Blizzard-1.5 μm; Polar Laser Laboratories [28]). Seeded with the 14 mw of the previous stage, the system delivers a stable train of pulses with an average power of 0.5 W, which corresponds to pulse energy of 0.5 μj and a peak power of 11.4 kw. Also, this amplification stage can in principle operate at higher gain levels, but here we observe Q switching to occur that can lead to damage in the amplifier. Figure 8 shows the output spectrum of the third amplification stage. The Fig. 5. Temporal intensity profile of the first amplifier. Fig. 7. Temporal intensity profile of the second amplifier. 10 July 2014 / Vol. 53, No. 20 / APPLIED OPTICS 4613

4 Fig. 8. Output spectrum of the third amplifier stage. a signal-to-ase ratio of 14 db was achieved. This high signal-to-ase ratio, the narrow spectral width, and the high degree of polarization make the system an interesting picosecond laser source for a number of applications, especially for nonlinear frequency mixing. It represents a good alternative to modelocked ps laser sources because of its robustness, simplicity, and the option to work at lower repetition rates while providing comparable pulse quality. The high-fidelity amplification and pulse quality of this laser system can be easily power scaled by using further large-mode Er-doped fibers. We would like to thank Dr. Boris Khoury from Advanced Laser Diode Systems for providing us with the DFB-SLD and for several useful discussions. Financial support by the Swiss National Science Foundation through the project NCCR-QP Hybrid Light is greatly acknowledged. Fig. 9. Temporal intensity profile of the third amplifier stage. signal has a FWHM of 0.1 nm and is clearly distinguishable from the ASE contribution originating from the last amplification stage. The signal-to-ase ratio at the output of the boost amplifier is 14 db, i.e., the signal power corresponds to 96% of the total output power. Such signal-to-ase ratio for an amplified gain-switched laser diode at 1 MHz has not been, to the best of our knowledge, reported yet and is comparable only to the most recent results obtained in Yb-doped fiber amplifiers [2]. Figure 9 shows the temporal intensity after the last amplifier stage and demonstrates that no significant temporal distortions are observable. The time bandwidth product is 0.52, approaching the theoretical transform-limited value of 0.44 for a Gaussian pulse. 4. Conclusion We have demonstrated high-fidelity all-fiber amplification of a gain-switched picosecond laser diode at 1550 nm and a 1 MHz repetition rate. Starting with a pulse energy of 2.1 pj and an average power of 2.1 μw from the seed laser, it was possible to optimize a three-stage amplification to produce nearly transform-limited pulses of approximately 40 ps duration with a pulse energy of 0.5 μj. By combining a stepwise increase in fiber core diameter and active dopant concentration with careful spectral filtering, References 1. H. J. Liu and X. F. Li, High power tunable picosecond green laser pulse generation by frequency doubling of an Yb-doped fiber power amplifier seeded by a gain switch laser diode, Laser Phys. 21, (2011). 2. S. Kanzelmeyer, H. Sayinc, T. Theeg, M. Frede, J. Neumann, and D. Kracht, All-fiber based amplification of 40 ps pulses from a gain-switched laser diode, Opt. Express 19, (2011). 3. H. Liu, C. Gao, J. Tao, W. Zhao, and Y. Wang, Compact tunable high power picosecond source based on Yb-doped fiber amplification of gain switch laser diode, Opt. Express 16, (2008). 4. S. Chen, A. Sato, T. Ito, M. Yoshita, H. Akiyama, and H. Yokoyama, Sub-5-ps optical pulse generation from a 1.55 μm distributed-feedback laser diode with nanosecond electric pulse excitation and spectral filtering, Opt. Express 20, (2012). 5. D. Taverner, D. Richardson, L. Dong, J. Caplen, K. Williams, and R. Penty, 158 μj pulses from a single-transverse-mode, large-mode-area erbium-doped fiber amplifier, Opt. Lett. 22, (1997). 6. H.-F. Liu, M. Tohyama, T. Kamiya, and M. Kawahara, Pulse broadening in picosecond amplification by a 1.3 μm InGaAsP traveling-wave amplifier, Appl. Phys. Lett. 63, (1993). 7. S. N. Vainshtein, G. S. Simin, and J. T. Kostamovaara, Deriving of single intensive picosecond optical pulses from a high-power gain-switched laser diode by spectral filtering, J. Appl. Phys. 84, (1998). 8. S. Chen, M. Yoshita, A. Sato, T. Ito, H. Akiyama, and H. Yokoyama, Dynamics of short-pulse generation via spectral filtering from intensely excited gain-switched 1.55 μm distributed-feedback laser diodes, Opt. Express 21, (2013). 9. A. Consoli, J. M. G. Tijero, and I. Esquivias, Time resolved chirp measurements of gain switched semiconductor laser using a polarization based optical differentiator, Opt. Express 19, (2011). 10. J. M. Wiesenfeld, R. S. Tucker, and P. M. Downey, Picosecond measurements of chirp in gain-switched, single-mode injection lasers, Appl. Phys. Lett. 51, (1987). 11. T. L. Koch and R. A. Linke, Effect of nonlinear gain reduction on semiconductor laser wavelength chirping, Appl. Phys. Lett. 48, (1986). 12. K. T. Vu, A. Malinowski, M. A. F. Roelens, M. Ibsen, P. Petropoulos, and D. J. Richardson, Full characterization of low-power picosecond pulses from a gain-switched diode laser using electrooptic modulation-based linear FROG, IEEE Photon. Technol. Lett. 20, (2008) APPLIED OPTICS / Vol. 53, No. 20 / 10 July 2014

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