The Generation Of High Peak-power Tunable Mid-Infrared Laser

The mid-infrared (mid-IR) spectral region, spanning from3μm to5μm, is important for a variety of scientific and technological applications. The mid-IR regime contains the characteristic vibrational transitions of many important molecules. The two atmospheric transmission windows are3μm-5μm and8μm-13μm, in which the Earth’s atmosphere is relatively transparent. High power mid-IR laser are vital for applications in spectroscopy, materials processing, chemical and biomolecular sensing, strong field physics, remote sensing, communication and national defence.Ultrafast intense light sources operating at visible and near-infrared wavelengths have developed steadily into commercially available instruments, such as Ti:Sapphire laser oscillator-regenerative amplifier. However, developments in mid-IR high peak-power laser technology have faced considerable challenges, such as the lack of suitable laser materials used in the chirped pulse amplifier (CPA). Optical parametric chirped-pulse amplifier (OPCPA), proposed by Dubietis et al., support high-power ultrashort pulse generation at near-infrared (near-IR) wavelengths similar to the previous CPA systems. OPCPA offers many advantages over CPA, such as high gain, broad gain-bandwidth and high signal-noise ratio. Moreover, the signal wavelength of the OPCPA can extend to the mid-IR spectral region. Extension of OPCPA technology to mid-IR regime, however, has faced considerable difficulties such as the low diffraction efficiency of mid-IR grating, the lack of large-aperture nonlinear crystals, the generation of tunable mid-IR seed pulse, etc.In order to develop high peak-power mid-IR laser, we propose a promising schame which consists of mid-IR femtosecond OPA and mid-IR noncollinear OPCPA. We present and demonstrate an OPCPA system that generates mid-IR femtosecond pulses with the highest peak power of120GW, allowing wavelength tuning from3.3to3.95μm. To the best of our knowledge, this is the highest peak power ever reported for3-5μm tunable mid-infrared lasers. The major research work can be listed as follows: 1. We propose an efficient frequency conversion scheme based on narrowband and chirped broadband pulses, which can be used to generate broadband mid-IR seed and broadband short-wavelength (blue and ultraviolet) laser. The scheme based on mixing between narrowband and chirped broadband pulses combines the advantages of wider spectral acceptance bandwidth and of alleviating the temporal walk-off, which are both beneficial to higher conversion efficiency. We experimentally demonstrate an efficient sum-frequency mixing scheme based on narrowband and chirped broadband pulses, which offers wider phase matching bandwidth than the broadband second harmonic generation (SHG).2. In order to generate high peak-power (>100GW) mid-IR tunable laser, we propose a schame which consists of mid-IR femtosecond OPA and mid-IR noncollinear OPCPA. The femtosecond OPA acts as the front end of the OPCPA system to generate tunable mid-IR seed. The LiNbO3-based OPCPA stage employs noncollinear geometry to achieve broad phase-matching bandwidth and wavelength tunability. Both stretcher and compressor are designed for the mid-IR pulses, which is crucial for mid-IR dispersion compensation. We design every components of the OPCPA system, including the femtosecond OPA, the stretcher, the OPCPA stage and the compressor. We analyze and compare several different crystals which can be used in the mid-IR regime such as LiNbO3, KTA, LiIO3, KNbO3and PPLN. Particularly, LiNbO3offers many advantages such as large effective nonlinear coefficient, broad phase-matching bandwidth and large aperture, thus LiNbO3has the potential to be used in the mid-IR tunable OPCPA system with TW peak-power level output.3. We experimentally demonstrate a widely tunable OPCPA system that generates mid-IR femtosecond pulses with the highest peak power of120GW. Different from previously reported works, the mid-IR OPCPA scheme adopts seeding from a tunable mid-IR femtosecond OPA and noncollinear phase matching (PM), allowing wavelength tuning from3.3to3.95μm. By employing a high-gain LiNbO3-based parametric amplifier, we demonstrate a large saturated gain over4000in a single-stage amplifier that directly boosts the signal to a chirped-pulse energy of29.5-mJ. After compression, the mid-IR laser has a pulse energy of13.3mJ and pulse duration of111fs, with a peak power of120GW.