Developing optical parametric oscillators (OPOs) that produce pulses at wavelengths beyond 5 mm has been difficult due to high idler absorption losses in the midinfrared. Although substantially boosting the power of the pump laser is one solution, this alternative rules out the use of small, convenient pulsed or continuous-wave lasers. Scientists at the University of St. Andrews (Fife, Scotland) have developed a low-power alternative.
Based on periodically poled lithium niobate synchronously pumped by a modelocked Ti:sapphire laser, their device uses a semimonolithic cavity design and hemispherical focusing to achieve operation at pump power thresholds of 17 mW and midinfrared idler powers of up to 64 mW in the 3.9- to 6-mm spectral range. Pulse-repetition rates reach to 322 MHz. The OPO produces a 280-mW signal output at 35% extraction efficiency, emitting transform-limited signal pulses of 0.4-ps duration and 1- to 1.14-mm wavelength. Minimizing the number of cavity components and coated surfaces--for example, coating the output mirror directly onto the nonlinear crystal--led to a low threshold and high stability. The maximum idler output was 64 mW at 4.3 mm, with output decreasing to 10 mW at 5.5 mm and 0.5 mW at 5.9 mm. Contact Majid Ebrahimzadeh at [email protected].