KEYWORDS: Fiber lasers, Signal to noise ratio, High power lasers, Optical isolators, Laser applications, Acoustics, High power fiber amplifiers, Optical amplifiers, Laser damage threshold, Polarization
We demonstrate a high-power, high signal-to-noise ratio single-frequency 1 μm Brillouin all-fiber laser with high slope efficiency. The Brillouin laser system consists of a high-power single-frequency fiber laser and a single-pass Brillouin ring cavity. The high-power single-frequency fiber laser is one-stage master-oscillator power amplifier with the maximum output power of 10.33 W, the signal-to-noise ratio of 50 dB and the slope efficiency of 46%. The Brillouin fiber laser is pumped by the amplified laser with a linewidth of 33 kHz and an output power of 2.61 W limited by the damage threshold of the optical isolator. By optimizing the length of the Brillouin ring cavity to 10 m, stable singlefrequency Brillouin fiber laser is obtained with 3 kHz linewidth owing to the linewidth narrowing effect. At the launched pump power of 2.15 W, the Brillouin fiber laser generates maximum output power of 1.4 W with a slope efficiency of 79% and the optical signal-to-noise ratio of 77 dB.
We demonstrate on chirped pulse amplification of a dissipative soliton thulium-doped fiber laser. The system consists of an all-fiber seed laser, a fiber-based stretcher, two-stage fiber amplifier and a free space grating compressor. The oscillator works in the normal dispersion regime and delivers up-chirped pulses with output power of 3 mW at repetition rate of 29.3 MHz. The spectrum of the seed laser is located at 1938 nm with a 10 dB bandwidth of 50 nm. The output pulses are directly stretched in ~50 m normal dispersion fiber to 72 ps pulse duration. In the pre-amplifier and power amplifier, both forward pumping and backward pumping are tested in the experiment. Output power of 7 W has been achieved in the power amplifier with backward pumping corresponding to a pulse energy of 239 nJ, which has an amplification slope efficiency of 37.8%. The PER at the highest average output power was measured to be 19.5 dB. The amplified up-chirped pulses could be dechirped to a duration time of 121 fs with energy of 161 nJ using a pair of fused silica transmission gratings.
The research on high-power ultrafast thulium-doped fiber lasers at 2 μm wavelength and mid-infrared (mid-IR) supercontinuum sources at 2~5 μm wavelength is one of these hot spots because of their potential applications in eye-safe radar, material processing, and electro-optical countermeasure system, which has an irreplaceable role compared to other wavelength fiber lasers. Here, we review the recent work of our group at 2 μm high-power ultrafast thulium-doped fiber lasers and 2~5 μm high-power mid-IR supercontinuum sources. Firstly, we demonstrate a high-power picosecond-pulsed thulium-doped all-fiber master-oscillator power-amplifier (MOPA), which yielded 120 W of average output power at central wavelength of 1963 nm with pulse duration of 16 ps. And then, we demonstrate a 203 W average power all-polarization-maintaining (all-PM) thulium-doped fiber MOPA system delivering picosecond pulses without using conventional chirped pulse amplication (CPA) technique. The pulse duration of 15 ps at 611.5 MHz repetition-rate results in a peak-power of 22 kW in the final PM thulium-doped fiber power amplifier. The polarization extinction ratio (PER) at the highest average output power was measured to be >15 dB, further power scaling is limited by available pump power. Finally, we demonstrate a high-power mid-IR supercontinuum generation with up to 21.8 W average power and spectrum spanning from 1.9 to beyond 3.8 μm in a single-mode ZBLAN fiber pumped by a 2 μm amplified picosecond pulses from a single-mode thulium-doped all-fiber MOPA.
We report high power all fiber mid-infrared (mid-IR) supercontinuum (SC) generation in a single-mode ZBLAN (ZrF4- BaF2-LaF3-AlF3-NaF) fiber with up to 21.8 W average output power from 1.9 to beyond 3.8 μm pumped by amplified picosecond pulses from a master oscillator power amplifier (MOPA) based on small-core single-mode thulium-doped fiber (TDF) with injected seed pulse width of 24 ps and repetition of 93.6 MHz at 1963 nm. The optical-optical conversion efficiency from the 793 nm pump laser of the last stage thulium-doped fiber amplifier (TDFA) to mid-IR SC output is 17%. It is, to the best of our knowledge, the highest average power mid-IR SC generation in a ZBLAN fiber to date. In addition, a noise-like fiber oscillator based on a nonlinear loop mirror (NOLM) with wavepacket width of ~1.4 ns and repetition rate of 3.36 MHz at 1966 nm is also used as a seed of the MOPA for mid-IR SC generation in the ZBLAN fiber. At last, a mid-IR SC from 1.9 to beyond 3.6 μm with average output power of 14.3W, which is limited by injected noise-like pulses power, is generated. The optical-optical conversion efficiency from the 793 nm pump laser of the last stage TDFA to mid-IR SC output is 14.9%. This proves the amplified noise-like pulses are also appropriate for high power mid-IR SC generation in the ZBLAN fiber.
We demonstrate high average power all-fiber single-polarization thulium(Tm)-doped nanosecond laser based on all-fiber master-oscillator power-amplifier (MOPA).The seed source employed is a directly modulated discrete-mode (DM) diode at 2 μm with pulse width of 15.8 ns and tunable repetition rate range of 400 kHz to 4 MHz. The pulse duration reduced to 6 ns as the gain reshaping during amplification. The MOPA yields 160 W maximum average powers at 4 MHz repetition with 6 ns pulse width, with respect to pulse energy of 40 μJ and peak-power of 6.7 kW. The slope efficiency for the last stage of PM LMA Tm-doped fiber amplifier was 48.8% with respect to launched pump power. The polarization extinction ratio (PER) of system measured at maximum output power is beyond 15 dB. The beam quality factor M2 was measured to be 1.5 at average output power of 160 W. To the best of our knowledge, this is the highest average power 2 μm nanosecond pulses laser from laser diode seeded all-fiber MOPA systems to date.
We have demonstrated a high-power all-polarization-maintaining thulium-doped all fiber picosecond pulsed master oscillator power amplifier. The thulium-doped all fiber oscillator was mode locked by a semiconductor saturable absorber mirror to generate average output power of 100 mW at a repetition rate of 611.5 MHz in a short linear cavity. The first PM thulium-doped fiber preamplifier produced 4.5 W average output power for 17 W incident pump power, and the pulse width was measured to be 18 ps. In the second PM thulium-doped fiber preamplifier and the final PM thulium-doped fiber power amplifier, a segment of 4.5 m LMA PM thulium-doped double-clad fiber were used as the gain medium. Both the thulium-doped active fiber has a core diameter of 25 μm, a core NA of 0.09, inner cladding diameter of 400 μm and a NA of 0.46. The second PM fiber preamplifier produced 73 W average output power for 135 W incident pump power. In the final PM fiber power amplifier, the maximum average output power was up to 203 W at the available pump power of 300 W, the slope efficiency for the final PM fiber power amplifier was 50.7%. The PER at the highest average output power was measured to be <15 dB. The pulse width was 15 ps and the central wavelength was 1985 nm, which corresponds to peak power 22 kW. To the best of our knowledge, this is the highest average output power ever reported for an all fiber ultra-short-pulsed laser at 2 μm wavelength region.
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