<title>Two-frequency MOPA diode laser system for difference-frequency generation of coherent THz waves</title>

Abstract

We developed a tunable, cavity-locked diode laser source at 850 nm for difference-frequency generation of coherent THz- waves. The difference frequency is synthesized by three fiber-coupled external-cavity diode lasers, where tow of the lasers are locked to adjacent modes of an ultra-stable Fabry-Perot cavity and the third laser is offset-phase- locked to the second cavity-locked laser using a tunable microwave oscillator. The first cavity-locked laser and the offset-locked laser produces the difference frequency, whose value is precisely determined by sum of integer multiple of free spectral range of the Fabry-Perot cavity and the offset frequency. The difference-frequency signal is amplified to 500 mW by the master oscillator power amplifier technique, simultaneous two-frequency injection-seeding with a single semiconductor optical amplifier. Here we demonstrate the difference-frequency generation of THz waves with the low- temperature-grown GaAs photomixers and its application to high-resolution spectroscopy of simple molecules. An absolute frequency calibration was carried out with an accuracy of approximately 10<SUP>-7</SUP> using CO lines in the THz region.