Gain-Switched Tm-doped Fiber Laser and Amplifier with Simultaneous Mode-Locked-Like Operation
Jacek Swiderski, Maria Michalska, Lukasz Galecki, Wieslaw Pichola, Jacek Kwiatkowski, and Marcin Mamajek
Institute of Optoelectronics, Military University of Technology2 Kaliskiego Street, 00-908 Warsaw, Poland
Abstract—We report on an all-fiber, fast gain-switched, Tm- doped fiber laser and amplifier system operating at 1994.4 nm.
For 26 kHz repetition rate, stable 25-ns pulses with an energy of 0.28 mJ corresponding to a peak-power of 10.5 kW were achieved. Gain-switched pulses with simultaneous mode-locked- like operation were also demonstrated. The peak-power of the highest subpulses from the train was over 30 kW.
Keywords—fiber lasers, gain-switching, pulse amplification
I.INTRODUCTION
Pulsed sources, both bulk solid state [1] or fiber [2] lasers, operating at ~2 µm wavelength range, have recently attracted a great attention due to their wide applications in medicine, remote sensing, spectroscopy, and nonlinear frequency conversion. Some of these applications require pulses of short, usually on the nanosecond scale, duration with accordingly kW peak power and generated at high (>10 kHz) repetition rate. One of the technique allowing achieving pulses of such parameters is fast gain-switching combining with resonant pumping of Tm-doped fiber lasers. Adding an amplification stage it is possible to develop a high-peak power all-fiber laser source meeting the energetic requirements of the above- mentioned applications.
II.RESULTS AND DISCUSSION
The experimental setup consisted of a 1.55-µm, pulsed fiber laser system, a gain-switched thulium-doped fiber laser (TDFL) and a thulium-doped fiber amplifier (TDFA). The system operated at 26 kHz repetition rate and was capable of delivering over 7.5 W of average power at the wavelength of 1994.4 nm. For applied configuration, it provided 2-µm gain- switched pulses with the shortest width of 25 ns. For 28 W of pump power absorbed by the TDFA, the output power of 7.28 W with the slope efficiency of 31.1% was measured (Fig. 1).
The corresponding pulse energy and peak power in this case was 0.28 mJ and 10.53 kW, respectively. The beam quality factor M2 was <1.2 and the output power instability (measured over 10 min runtime) was estimated to be <1%.
During the experiment we noticed that suitable selection of pump energy and pump pulse width leads to a self-started mode-locked-like operation with a full modulation depth, which is shown in Fig. 2. The envelope of the amplified gain- switched pulse contained a train of 20-25 sub-pulses, exactly
spaced at the laser cavity round-trip time (frequency, 190 MHz). The duration of the fully modulated, most intense mode-locked-resembling pulses varied from ~500 to 900 ps and their origin can be identified as the beating of laser longitudinal modes. The peak power of the three highest sub- pulses, estimated assuming a Gaussian shape of each sub- pulse, was determined to be 30.5 kW, 19.5 kW and 18 kW (inset in Fig. 2).
Fig. 1. Average output power and pulse energy vs. absorbed pump power of the TDFA. Top and bottom insets present the shortest recorded 2-µm pulse and far-field beam profile, respectively.
Fig. 2. Output pulse train recorded in the gain-switched pulse envelope for 4.8 W of output power. The inset shows three highest sub-pulses from the train.
REFERENCES
[1] L. Gorajek et. al, “High repetition rate, tunable, Q-switched diode pumped Tm:YLF laser,” Opto-Electron. Rev., vol. 17, pp. 309-317, December 2009.
[2] J. Swiderski, M. Maciejewska, J. Kwiatkowski, and M. Mamajek, “An all-fiber, resonantly pumped, gain-switched, 2 μm Tm-doped silica fiber laser,” Laser Phys. Lett., vol. 10, pp. 015107, December 2012.
This research was partially supported by the Polish National Science Centre.