New Nonlinear Crystal for Infrared OPOs

Breck Hitz

A collaboration of scientists in France, Russia and Germany has demonstrated an optical parametric oscillator (OPO) based on a new nonlinear crystal, lithium selenoindate (LiInSe₂, abbreviated LISe). The investigators believe that their results are a significant step in the development of tunable optical sources in the important 2- to 12-µm spectral range, because LISe can be pumped by readily available 1-µm lasers without two-photon absorption effects, and it exhibits roughly five times larger thermal conductivity than AgGaS₂. Such infrared sources are of interest to spectroscopists because the signatures of most molecules lie in this spectral range, and to the military because heat-seeking missiles and other “smart” weapons rely on signals in this range.

The scientists, associated with the Institut National de Métrologie in Paris, the Institute of Mineralogy and Petrography in Novosibirsk, Russia, and Max Born Institut für Nichtlineare Optik und Kurzzeitspektroskopie in Berlin, evaluated the new crystal in three OPO configurations. First they operated the OPO so it was doubly resonant (i.e., both signal and idler were resonant), then in what they called an idler-resonant configuration in which the signal, while still resonant, was coupled out of the oscillator with ~14 percent efficiency. Finally, they operated the OPO in a true singly resonant configuration in which only the signal was resonant.

Figure 1. In the doubly resonant configuration, CaF₂ plane mirrors M1 and M2 reflected both signal and idler, and a gold mirror reflected the pump for a second pass through the OPO (inset, a). The total output at both wavelengths reached 10 µJ with 6 mJ of pump energy (a). At constant pump energy, the total output energy fell linearly for both single-pass pumping (SPP) and double-pass pumping (DPP) as the repetition rate increased to 40 Hz, and more rapidly after that (b). Images ©OSA.

In each configuration, the investigators pumped the OPO with a Minilite Q-switched Nd:YAG laser from Continuum Inc. of Santa Clara, Calif., whose 10-ns pulses could be varied from a 10- to a 100-Hz repetition rate. Although the laser could produce higher energy, the scientists limited the energy incident on the LISe crystal to 8 mJ to avoid damage to its antireflection coatings.

In the doubly resonant configuration, the CaF₂ plane mirrors reflected more than 99.5 percent of both the signal and idler wavelengths, and a gold mirror and dichroic beamsplitter reflected the pump wavelength back for a second pass though the OPO (Figure 1a, inset). The total output displayed in Figure 1a is the sum of the signal energy at 1537.1 nm and the idler energy at 3456.9 nm. The scientists believe that the rapid decrease in total energy as the repetition rate increased past 40 Hz (Figure 1b) was caused by thermal lensing in the nonlinear crystal.

Figure 2. In the idler-resonant configuration, an intracavity plate coupled out about 14 percent of the signal wavelength (inset). The oscillator produced up to 170 µJ of output at both the signal and idler wavelengths when pumped with 8 mJ.

In the idler-resonant configuration, an intracavity CaF₂ plate, oriented at 45° to the oscillator axis, coupled about 7 percent of the idler going in each direction out of the resonator, so the total output coupling was 14 percent. A silver mirror combined the two outputs into a single beam, which also included a small amount (~5 percent) of energy at the idler wavelength (Figure 2, inset). The total energy at both wavelengths was as great as 170 µJ at 8 mJ of pump energy (Figure 2), but an extrapolation of the data to higher pump energies indicates that significantly higher output energies are possible.

Figure 3. In a singly resonant configuration, only the signal wavelength was reflected from mirror M1 (inset). Energy generated at the ~3.5-µm idler wavelength increased linearly with pump energy, indicating that with properly designed damage-resistant coatings, further output scaling could be expected at higher pump energies.

To maximize the ~3.5-µm idler output, the scientists configured the OPO as a true singly resonant oscillator, with a silver mirror at one end of the resonator and a dielectric mirror that reflected only the signal wavelength at the other (Figure 3, inset). In this configuration, they obtained as much as 92 µJ at the idler wavelength (Figure 3).