A tunable cavity-locked diode laser source for terahertz photomixing

An all solid-state approach to the precise frequency synthesis and control of widely tunable terahertz radiation bg differencing continuous-ware diode lasers at 850 nm is reported in this paper. The difference frequency is sy nthesized by three fiber-coupled external-cavity laser diodes. Two of the l asers are Pound-Drever-Hall locked to different orders of a Fabry-Perot (FP ) cavity. and the third is offset-frequency locked to the second of the cav ity-locked lasers using a tunable microwave oscillator. The first cavity-lo cked laser and the offset-locked laser produce the difference frequency who se value is accurately determined by the sum of an integer multiple of the free spectral range of the FP cavity and the offset frequency, The dual-fre quency 850-nm output of the three laser system is amplified to 500 mW throu gh two-frequency injection seeding of a single semiconductor tapered optica l amplifier As proof of precision frequency synthesis and control of tunabi lity the difference frequency is converted into a terahertz Hare bq optical -heterodyne photomixing in low-temperature-grown GaAs and used for the spec troscopy of simple molecules, The 3-dB spectral power bandwidth of the tera hertz radiation is routinely observed to be less than or similar to 1 MHz. A simple, but highly accurate, method of obtaining an absolute frequency ca libration is proposed and an absolute calibration of 10(-7) demonstrated us ing the known frequencies of carbon monoxide lines between 0.23-1.27 THz.