We report a mid-infrared frequency comb generator, which produces up to 3 W of output power. The comb mode spacing is 208 MHz, the spectral bandwidth is ~300 GHz, and the center wavelength is tunable between 3 and 3.4 μm. The comb generation is based on intracavity difference frequency mixing between near-infrared pump and signal beams of a continuous-wave-pumped optical parametric oscillator. The signal beam, which is resonant in the cavity, acquires a comb structure through cascading quadratic nonlinearities in a periodically poled lithium niobate crystal. This comb structure is transferred to the spectrum of the mid-infrared idler beam via the difference frequency mixing process.
We have developed and characterized a new iodine spectrometer for absolute frequency measurements of I<sub>2</sub> transitions at the 633-nm region. This region is important especially in length metrology. To obtain good frequency accuracy, the various sources of frequency shifts in the used saturation spectroscopy configuration have been minimized by optimizing the laser beams for spatial and spectral quality. This is done by injection locking a microlens-coupled diode laser with nearly Gaussian output beam to a stable external-cavity diode laser. The use of the injection-locking scheme also reduces the detrimental effects of optical feedback, as well as residual amplitude modulation related to laser frequency modulation via injection current.