A compact narrowband tunable mid-IR CW source with >30 mW for spectroscopy between 3.1 and 3.5 μm

We demonstrate spectroscopy on methane using a commercially available CW optical parametric oscillator pumped by a 780 nm tapered amplifier. This source produces >30 mW output from 3.1 to 3.5 μm with a line width <2 MHz. The device has no moving parts other than a mechanical shutter on the output during normal operation. Mode-hop free tuning is accomplished by fast temperature control of the seed DFB diode laser. Coarse tuning is accomplished by crystal temperature tuning of a periodically poled MgO doped lithium niobate crystal. Cavity length is fixed by temperature stabilization. The wavelength of the signal and pump were measured with a silicon based wavemeter and the calculated idler wavelength compared to results from direct wavelength measurement with a HgCdTe based wavemeter. The results were found to be within the error of the HgCdTe based wavemeter. Next, the output was propagated through a 20 cm gas cell with sapphire windows pressurized at 10 Torr partial pressure for both methane and acetylene. Power was reduced using a calcium fluoride wedged window. The reduced reflected power from the window was measured using a HgCdTe photodetector. Our results show good agreement with HITRAN.