A 2 ?m, gain-switched Tm-doped fiber laser and amplifier system with output average power of 9 W at 50 kHz

Abstract

Abstract-We demonstrate a gain-switched thulium-doped fiber laser and amplifier system (TDFL&A) operating at a wavelength of 1994.7nm. A pulsed fiber Master Oscillator Power Amplifier (MOPA) at 1.55μm was used to gain-switch the Tm fiber laser via in-band pumping. In the next step, the gain-switched pulses were amplified in a cladding-pumped Tm-doped fiber amplifier pumped by up to 30 W at 793nm. For the pulse repetition frequency (PRF) of 100kHz, the laser system delivered the maximum average output power as high as 9.03W with a slope efficiency of 36.4%. For the PRF of 50 kHz, stable 25-ns pulses with energy of 0.18mJ corresponding to a peak-power of 6.8kW were achieved. The performance of the laser system is described.Lasers operating at ~2-µm have already proved to be good sources of coherent radiation used in medicine, spectroscopy, gas sensing, direct energy systems or nonlinear frequency conversion systems [e.g., 1-7]. Particularly, Tm 3+ -doped fiber lasers (TDFLs) have attracted great attention of many research groups recently. They seem to be especially suitable for producing 2-µm laser radiation due to: (1) strong absorption spectrum that has good overlap with the emission spectrum of commercially available 793-nm laser diodes, (2) the energy-level structure of Tm 3+ ions providing the quantum efficiency of a laser close to 200% through the cross relaxation process and (3) wide tunability over the emission band spanning from ~1.7 to 2.1µm. TDFLs can effectively generate continuous wave (CW) output power (exceeding even 1 kW level [8]) as well as pulses of a nanosecond [9][10] or picosecond [11][12] width.Many applications require 2-µm pulses of short, usually on the nanosecond scale, duration with appropriate kW peak power and generated at high (>10 kHz) PRF. Such values can be directly achieved by simple Q-switching of laser losses [e.g., 13-15]. However, in the case of all Qswitched lasers the pulse width is dependent on repetition rate, which may be a problem for some applications, especially for those where a high degree of control on pulse parameters is needed. Therefore, an alternative method of pulse operation seems to be gain-switching where pulse operation is realized by laser gain on/off switching via pump power modulation [12,16]. In particular, fast gain-switching, together with in-band * E-mail: jswiderski@wat.edu.pl pumping, can provide stable 2-µm pulses of short duration at PRF of a few tens of kHz [17]. Furthermore, a gain-switching technique offers simple laser geometry (there is no need to use additional in-cavity components, which is convenient for ensuring an all-fiber configuration) and a narrow bandwidth of laser output (by using fiber Bragg gratings).To realize fast gain switching of TDFLs effectively, they can be pumped by short (<200ns) pulses at a wavelength of ~1.5µm [17][18] or ~1.9µm [19]. This allowed obtaining 2-µm pulses as short as 10ns [17] and pulses with energy of up to 1.3mJ [16]. This paper presents results on a 2-µm, fast gain switched, narrow-band Tm 3+ -doped ...