Narrow-bandwidth, picosecond, 1064-nm pumped optical parametric generator for the mid-IR based on HgGa2S4

Abstract

We report on optical parametric generation in HgGa 2 S 4 pumped by 16-ps, 1064-nm pulses at 250 kHz with idler tuning from 3.85 to 10 µm. Seeding at the signal wave ensures narrow-bandwidth, quasi-Fourier-limited, high beam-quality operation.OCIS codes: (190.4970) Parametric oscillators and amplifiers; (160.4330) Nonlinear optical materials. IntroductionOptical parametric generators (OPGs) and amplifiers (OPAs) are widely used for frequency down-conversion of powerful (amplified) ultrafast laser sources operating in the near-IR part of the spectrum into the mid-IR. However, the application of nonoxide nonlinear optical crystals to generate idler wavelengths beyond 4-5 µm, the upper transmission limit of oxide materials, is problematic in this regime for few reasons [1]. One of the major restrictions comes from two-photon absorption (TPA) at the pump wavelength because non-oxide crystals show smaller band-gap energy E g . It is related to the availability of amplified ultrafast laser sources: The most mature and powerful picosecond (ps) laser sources based on the Nd 3+ -ion emit near 1 µm. This represents the shortest pump wavelength used so far in such OPG/OPAs [1]. Most of the demonstrated mid-IR OPGs were pumped, however, at longer wavelengths, primarily near 2.9 µm, by less mature and efficient lamp-pumped Er-lasers/amplifiers operating at very low (few Hz) repetition rates [1].In earlier work on 1-µm pumped ps OPG/OPA using AgGaS 2 (AGS) pumped by ~20 ps Nd:YAG laser pulses, idler wavelengths as long as 10 µm were reached, at pulse duration ~8 ps and typical bandwidths of 10 cm -1 , which corresponds roughly to 5 times the Fourier limit [2]. Away from degeneracy, however, the conversion efficiency dropped getting few times lower compared to difference-frequency generation (DFG) [3]. Higher total conversion efficiency and an order of magnitude increase in the idler pulse energy (e.g., 130 µJ at 5.3 µm) were reported in [4]. These OPG/OPA experiments were performed with single pulses (repetition rate ~1 Hz) from lamp-pumped laser systems [2,4]. The interest in ultrafast OPG/OPAs pumped near 1 µm was recently revived after the discovery of the CdSiP 2 (CSP) crystal possessing extremely high nonlinearity which offers non-critical phase-matching (PM) [1]. In [5] a single pass CSP OPG was pumped by 8.7 ps Nd:YVO 4 laser pulses at 100 kHz. Pump to idler conversion efficiency ~2.5% was achieved for an idler wavelength of 6.204 µm resulting in an average idler power of 154 mW, however, thermal lensing effects occurred beyond this pump level caused by the still relatively high residual absorption of CSP at 1.064 µm. With time-bandwidth product TBP~9.3 measured for the signal, the pulses were also far from Fourier-limited. In another experiment, a non-critically phase-matched CSP OPG was temperature tuned from 6.153 to 6.731 µm delivering maximum idler energy of 33 µJ at 6.234 µm (or ~1.6% conversion efficiency to the idler) with 20 ps, 5 Hz pump pulses from a Nd:YAG laser system but the TBP reported was roughly 100 tim...

High-energy, near- and mid-IR picosecond pulses generated by a fiber-MOPA-pumped optical parametric generator and amplifier

“…3(a), the signal linewidth was reduced to 0.046nm leading to a ~100 times increase in spectral density. Furthermore, the corresponding pulse duration increased to 150 ps, as a consequence of the saturated gain in the OPA operation regime [8], as seen in Fig. 3(b).…”

“…Typically, this has been achieved using solid-state near-IR pump lasers based on gain media such as Ti:sapphire or Yb-doped tungstates [3][4][5]. Additionally, in order to reach the μJ-level mid-IR pulse energies that are interesting for organic materials processing techniques such as resonant infrared pulsed laser deposition (RIR-PLD) [6,7], systems are normally operated at Hz to kHz repetition rates [8][9][10].…”