A compact thulium-doped fiber chirped pulse amplifier (FCPA) is presented and used to pump a ZrF4-BaF2-LaF3-AlF3- NaF (ZBLAN) fiber for mid-infrared (MIR) supercontinuum generation. The FCPA is seeded by Raman soliton, which is generated by femtosecond erbium doped fiber laser and a piece of highly nonlinear fiber (HNLF) via soliton self frequency shift (SSFS) process. Additional with fiber stretcher and three stages thulium-doped fiber amplifier, the laser source delivers up to 10 W average power at repetition rate of 10 MHz with pulse width of 252 fs after grating compressor. Supercontinuum generation is realized by coupling the pulses into 10 m ZBLAN fiber with an aspheric lens. Since the laser source operates at anomalous dispersion regime of ZBLAN fiber, the spectral broadening is mainly dominanted by SSFS and stimulated Raman scattering (SRS) in this dispersion regime. Maximum output power of 800 mW corresponding to pulse energy of 80 nJ has been obtained with 5 dB spectrum ranging from 2 μm to 3.5 μm, which is limited by the low pump coupling efficiency of ~10% at this time. This fiber based supercontinuum source may have a wide variety of applications such as medical imaging, micro-spectroscopy and gas detection.
We demonstrate an all fiber mode-locked ultrafast erbium laser oscillator by means of nonlinear polarization evolution (NPE) in polarization maintaining (PM) fibers. Here, the artificial saturable absorber (SA) is formed by a piece of PM active fiber and a Faraday mirror (FM). High repetition rate could be obtained thanks to the introduction of the PM active fiber. The transmission characteristics of the SA are studied through theoretical analyses as well as experimental demonstration. Channeled spectra caused by soliton-soliton interaction and birefringence-induced filter are also studied and discussed. A good agreement is achieved between the theoretical analyses and experimental results.
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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