Accurate Absolute Frequency Measurements on Stabilized CO2 and He-Ne Infrared Lasers

In our laboratory, we have measured the frequencies of CO2 and He-Ne lasers near 30 and 88 THz, stabilized, respectively, by saturated fluorescence in CO2 and saturated absorption in CH4. Our measurement system includes a stable free-running optically pumped CH3OH laser at 4.25 THz replacing the noisy H2O laser used as a transfer oscillator in early experiments. As a result of the reduced mixing orders (≤9), beat notes between lasers are now observed with ≈30-dB signal-to-noise (S/N) ratios in a 100-kHz bandwidth. Therefore, beat frequencies can be measured accurately with digital counters and simultaneous counting of the frequencies involved largely eliminates the uncertainties due to transfer oscillators. The measurements are referred to the cesium beam frequency standard. The results are processed by a desktop calculator which also controls the measurement process.