Generation of octave-spanning mid-infrared using a BGSe nonlinear crystal

Dr.JINWEI ZHANG and his team using a Cr:ZnS laser system delivering 28-fs pulses at a central wavelength of 2.4 µm is used as the pump source, which drives the intra-pulse difference frequency generation inside the BGSe crystal. As a result, a coherent broadband mid-infrared continuum spanning from 6 to 18 µm has been obtained. It shows that the BGSe crystal is a promising material for broadband, few-cycle mid-infrared generation via frequency down conversion with femtosecond pump sources.

Introduction

Mid-infrared (MIR) light in the range of 2-20 µm is useful for chemical and biological identification due to the presence of many molecular characteristic absorption lines in this spectral region . A coherent, few-cycle source with a simultaneous coverage of the broad MIR range can further enable new applications such as mirco-spectroscopy , femtosecond pump-probe spectroscopy , and high-dynamic-range sensitive measurements Until now numerous schemes have

been developed to generate coherent MIR radiation, such as synchrotron beam lines, quantum cascade lasers, supercontinuum sources , optical parametric oscillators (OPO) and optical parametric amplifiers (OPA). These schemes all have their own strengths and weaknesses in terms of complexity, bandwidth, power, efficiency, and pulse durations. Among them, intra-pulse difference frequency generation (IDFG) is attracting growing attention thanks to the development of high-power femtosecond 2 µm lasers that can effectively pump small-bandgap non-oxide nonlinear crystals to generate high-power broadband coherent MIR light. Compared to the normally used OPOs and OPAs, IDFG allows a reduction in system complexity and enhancement of reliability, as the need to align two separate beams or cavities at high precision is removed. Besides, the MIR output is intrinsically carrier-envelope-phase (CEP) stable with IDFG .

Fig 1

Transmission spectrum of the 1-mm-thick uncoated BGSe crystal provided by DIEN TECH. The inset shows the actual crystal used in this experiment.

Fig 2

Experimental setup of the MIR generation with a BGSe crystal. OAP, off-axis parabolic mirror with an effective focus length of 20 mm; HWP, half-wave plate; TFP, thin-film polarizer; LPF, long-pass filter.

In 2010, a new biaxial chalcogenide nonlinear crystal, BaGa4Se7 (BGSe), has been fabricated using the Bridgman-Stockbarger method . It has a wide transparency range from 0.47 to 18 µm (as shown in Fig. 1) with nonlinear coefficients of d11 = 24.3 pm/V and d13 = 20.4 pm/V. The transparency window of BGSe is significantly broader than ZGP and LGS although its nonlinearity is lower than ZGP (75 ± 8 pm/V). In contrast to GaSe, BGSe can also be cut at the desired phase-matching angle and can be anti-reflection coated.

The experimental setup is illustrated in Fig. 2(a). The driving pulses are initially generated from a home-built Kerr-lens mode-locked Cr:ZnS oscillator with a polycrystalline Cr:ZnS crystal (5 × 2 × 9 mm3 , transmission=15% at 1908nm) as the gain medium pumped by a Tm-doped fiber laser at 1908nm. The oscillation in a standing-wave cavity delivers 45-fs pulses operating at a repetition rate of 69 MHz with an average power of 1 W at a carrier wavelength of 2.4 µm. The power is amplified to 3.3 W in a home-built two-stage single-pass polycrystalline Cr:ZnS amplifier (5 × 2 × 6 mm3 , transmission=20% at 1908nm and 5 × 2 × 9 mm3 , transmission=15% at 1908nm), and the output pulse duration is measured with a home-built second-harmonic-generation frequency-resolved optical grating (SHG-FROG) apparatus.

Conclusion

They demonstrated a MIR source with the BGSe crystal based on the IDFG method. A femtosecond Cr:ZnS laser system at the wavelength of 2.4 µm was used as the driving source, enabling a simultaneous spectral coverage from 6 to 18 µm. To the best of our knowledge, this is the first time broadband MIR generation has been realized in a BGSe crystal. The output is expected to have few-cycle pulse durations and also to be stable in its carrier-envelope phase. Compared to other crystals, the preliminary result with BGSe shows a MIR generation with comparable broad bandwidth (wider than ZGP and LGS) although with a lower average power and conversion efficiency. Higher average power could be expected with further optimization of the focus spot size and crystal thickness. A better crystal quality with higher damage threshold would also be beneficial for increasing the MIR average power and conversion efficiency. This work shows that BGSe crystal is a promising material for the broadband, coherent MIR generation.

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Post time: Dec-07-2020