6.5 W of average power have been generated by a mid-infrared ZnGeP2 (ZGP) optical parametric oscillator (OPO)
pumped directly by a Q-switched Tm3+-doped single-oscillator fiber laser. The Tm3+-fiber pump laser based on a silica
polarization-maintaining (PM) double-clad fiber provided average powers of up to 23 W at pulse widths of 65 ns at
40 kHz repetition rate. The ZnGeP2 OPO produces 45 ns mid-IR pulses. The OPO slope efficiency was 40% and the
optical-to-optical conversion efficiency 32%.
The generation of mid-infrared (mid-IR) supercontinuum (SC) radiation, ranging from 2 - 5 μm, is subject of intense research due to its wide range of applications. A very popular host media for mid-IR SC generation are soft glass fibers owing to their low-loss transmission in the mid-IR wavelength regime, particularly fluoride fibers are very attractive for high-power operation. In this research study, a diode-pumped Q-switched mode-locked (QML) thulium (Tm3+)-doped double-clad silica fiber laser is used to pump a ZrF4-BaF2-LaF3-AlF3-NaF (ZBLAN) fiber for mid-IR SC generation. The QML regime of the fiber laser is actively generated by two acousto-optic modulators. The Tm3+-fiber laser provided up to 23.5 W (26 W) of average output power in QML (continuous wave) regime with a slope efficiency of 36 % (32 %). The measured beam quality has been close to the diffraction-limit in QML regime. The system delivered mode-locked pulses with a duration of 7.5 ps, measured with a commercial autocorrelator system, at a repetition rate of 46 MHz. The Q-switched envelopes had a width between 50 and 150 ns depending on the output power level and the adjustable repetition rate. Mid-IR SC with an average output power in all spectral bands of 4.5 W have been achieved with more than 3 W/ 1.7 W/ 1 W/ 0.36 W after a long-wave-pass filter with a 3 dB-edge at 2.15 μm/ 2.65 μm/ 3.1 μm/ 3.5 μm.
This paper describes new laser sources and non linear conversion setups for 2 μm and mid-IR generation based
on fiber technologies developed at ISL. Especially for jamming heat-seeking missiles, these novel designs allow
to propose future compact, efficient and integrable laser systems. The specialty of the ISL technology lies in the
use of single 2 μm fiber laser oscillators, which deliver the full output peak power to pump optical parametric
oscillators or nonlinear fibers. No multi-stage amplifiers at 2 μm or 1.55 μm are necessary to efficiently pump
non linear converters to obtained useful energies in the mid-infrared spectral range. This technology leads to
efficient, simple and promising setups to be implemented in flying platforms. The best results achieved in continuous-wave (CW), Q-switched (QS) and mode-locked (ML) regimes with fiber lasers based on Tm3+-doped and Tm3+,Ho3+-codoped fibers are presented. Up to 70 W of average power was achieved around 2 μm with a Tm3+-doped fiber in CW regime. In ML regimes, at a repetition rate of 66 MHz, 50 W of average power was reached. In QS regime, up to 32 W of average power was generated around 2 μm with a polarization maintaining Tm3+-doped fiber at a repetition rate of 40 kHz. With a Tm3+,Ho3+-codoped fiber, up to 25 W of average power was obtained around 2070 nm in Q-switched regime. For example at 50 kHz, the pulse duration was around 50 ns at the maximum output power. The M2 was estimated to be less than 1.2.
The emission from QS fiber lasers was used to directly pump OP-GaAs and ZGP OPOs. For example, in band II,
up to 6.5 W of averaged power was recently obtained from a ZGP OPO pumped by a Tm3+-doped fiber laser. At
40 kHz repetition rate, the pulse duration was around 65 ns at the maximum output power. For 3 W of averaged
output power, the M2 of the signal beam was estimated to be less than 2.1 and less than 2.4 for the idler beam.
Using a mode-locked Tm3+-doped fiber laser to pump a ZBLAN fiber at an injection efficiency of ~60%, an
overall supercontinuum power of up to 2.2 W from a pump power of 5.4 W was achieved. The power above 2650 nm was 0.7 W.