Objective   Widely tunable, high-energy, stable, compact, high-beam-quality, mid-infrared 3-5 μm light sources based on optical parametric oscillator (OPO) and optical parametric amplifier (OPA) systems, known as the fingerprint region, are of considerable importance in applications including remote sensing, atmospheric monitoring, spectroscopy analysis, and photoelectric detection surveys. In particular, it is desirable to utilize high-energy, mid-infrared light sources with large wavelength tunability for highly sensitive and selective photoacoustic trace-gas sensing, in which most molecules have strong vibrational transitions. At present, the technologies available that can achieve the desired laser output in the widely tunable and highly-energized mid-infrared region of 3-5 μm are primarily quantum and inter band cascade lasers (QCLs) and OPOs. Although OPO technology has been around for a long time, it is still an excellent light source choice for the widely tunable mid-infrared region. It provides selectivity owing to its large wavelength tunability, high energy, increased beam quality, and compact, cost-effective devices for the generation of mid-infrared light in the 2-5 μm spectral range.

  Methods   Experimental setup for the high beam quality, idler-resonant MgO: PPLN-OPO is shown (Fig.1). A solid-state Nd:YAG laser (pulse duration: 25 ns, wavelength: 1.064 μm, PRF: 50 Hz, maximum pulse energy: 21 mJ, spatial form: Gaussian profile) was used as the pump source of the OPO. The pump beam was observed by a conventional CCD camera. The spatial forms of the signal and idler outputs were measured by using a Spiricon pyroelectric camera III (Fig.2). The energy scaling of the compact idler-resonant OPO has been investigated with the increasing pump energy (Fig.3). Spectral properties of a compact idler-resonant OPO have been measured by spectrometer (SpectraPro HRS-500, 300 line/mm) (Fig.4). Idler output energies as a function of the idler wavelength at a pump energy of 21 mJ was shown (Fig.5). To validate the high beam quality idler output, the beam quality factor (M²) of the mid-infrared idler output was measured by means of the knife-edge method (Fig.6).

  Results and Discussions   The maximum signal and idler output energies of 3.2 mJ and 1.12 mJ were obtained at a pump energy of 21 mJ, corresponding to the slope efficiency of 24% and 9%, respectively (Fig.3). The wavelengths of the signal and idler outputs were tuned in the ranges of 1.505-1.566 μm and 3.318-3.628 µm by changing the MgO: PPLN crystal temperature in the range of 25-200 ℃ (Fig.4), and the beam quality factor of the mid-infrared idler output was measured by means of the knife-edge method, resulting beam quality factors were estimated as 1.2 and 1.2 along the horizontal and vertical directions, respectively (Fig.6).

  Conclusions   We have successfully demonstrated the high-beam-quality, idler-resonant tunable optical parametric oscillator based on single-grating MgO: PPLN crystal. A maximum idler output energy of 1.12 mJ and signal output energy of 3.2 mJ was achieved at a pump energy of 21 mJ, and the beam quality factors of 1.2 and 1.2 in the two orthogonal directions, respectively. The wavelengths of the signal beams and idler beams outputs were tuned in the ranges of 1.505-1.566 µm and 3.318-3.628 µm by changing the MgO: PPLN crystal temperature in the range of 25-200 ℃. In the future work, by using a PPLN crystal with multiple gratings, a broader range of wavelength tuning is expected.