Heterodyne frequency measurements and frequency calibration standards for tunable diode lasers

New frequency calibration tables a re r equired to keep abreast of the resolution a ttainable by currently available tunable lasers. One key to the generation of tables with requisite accuracy involves accurate heterodyne f requency measurements; another key consists of reliable fitting and analysis. Coordinated activity in NBS involves selection of suitable molecular calibration candidates, their frequency measure- merit and analysis, arid dissemination of the results in the form of frequency calibration tables. C urrent status of these efforts is described. In t roduc t i on A representative block diagram for tunable diode laser (TDL) - CO, heterodyne measurements is shown in Fig. 1. Either a liquid helium Dewar or a closed cycle cooler was used to refrigerate the TDL. The radia- tion from the TDL was collimat.ed by a 5 cm F/1 ZnS polycrystalline lens. Off-axis parabolic mirrors were used to focus the TDL radiation on the entrance slit (S,) of the monochromator and recollimate t he beam after it emerged from the exit slit (S,). For g eneral use, the entrance slit was replaced with an aperture (which was blackened and shaped to reduce fringes due to slit reflections feeding back to the TDL), and another slit, typically 500 pm or greater, was used on the output. The ZnSe beam splitter was used to divide the p ost monochromator collimated r adiation. One beam was directed through a 50 cm long cell containing DCS and focused onto a PbSnTe photovoltaic detector. The other beam was sent through a 12.5 cm F/5 Ge focusing l ens and mixed with the reference CO, laser beam in a HgCdTe detector/mixer having a 300 MHz 3 dB bandwidth. The detector was followed by a broadband IF ampli- fier (0.1-1200 MHz) with 25 dB gain. The output of the amplifier was fed to a spectrum analyzer, and the frequency of the beat signal was measured with a marker oscillator and frequency counter.