IEEE TRANSACTIONS ON INSTRUMENTATION AND MEASUREMENT, VOL. IM-29, NO. 4, DECEMBER 1980 Accurate Absolute Frequency Measurements on Stabilized CO2 and He-Ne Infrared Lasers ANDRE CLAIRON, BRAHIM DAHMANI, AND JACQUES RUTMAN, MEMBER, IEEE Abstract-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 CH30H laser at 4.25 THz replacing the noisy H20 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 301-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 mea- surements are referred to the cesium beam frequency standard. The results are processed by a desktop calculator which also controls the measurement process. INTRODUCTION THE ADVENT OF nonlinear point contact diodes work- ing in the submillimeter and infrared regions has extended absolute frequency measurements by harmonic mixing into these regions of the electromagnetic spectrum where lasers are used as coherent signal sources. The first laser frequency measurement was performed in 1967 on the discharge HCN gas laser near 890 GHz [1]. Successive harmonic mixing ex- periments have lead to successful frequency measurements of the H20, C02 and He-Ne discharge gas lasers, respectively, near 10, 30, and 88 THz [2], [3]. Of special interest for metrology are the frequency measurements performed on CO2 and He-Ne lasers using frequency stabilization by saturated fluorescence or saturated absorption, since these devices can be considered as secondary frequency standards in the infrared. These first measurements used the HCN and H20 discharge gas lasers as intermediate oscillators in the laser chain. Metal-insulator-metal (MIM) diodes were employed with success as nonlinear mixers up to 197 THz [4], [5], but did not extend to the measurement of the He-Ne line at 260 THz (1.15 gm). Nonlinear crystals have therefore been used to reach this frequency by addition of lower laser frequencies and also to reach the visible region by frequency doubling [6]. For frequencies lower than about 3 to 4 THz, other nonlinear de- vices such as point-contact Josephson junction or Schottky diodes can be used [7], [8]. In the meantime, optically pumped submillimeter lasers had been discovered and their interest for frequency metrology pointed out [91, both as a possible future standard (assuming Manuscript received June 28, 1980. This work was supported by Research Contracts from the Bureau National de Metrologie, Paris, France. The authors are with the Laboratoire Primaire du Temps et des Frequences, 61, avenue de l'Observatoire, Paris, France 75014. the development of adequate frequency stabilization tech- niques) or as an intermediate low noise oscillator in a laser frequency measurement chain. Recently, the use of the 4.25-THz CH30H laser, as a transfer oscillator between a 99-GHz klystron and the CO2R(32) laser, with a Josephson junction as harmonic mixer between the klystron and the CH30H laser, has been reported at NPL; this scheme has produced a new and precise measurement of the frequency of the He-Ne(CH4) laser [10]. In our laboratory, we have completed new measurements of the frequency of stabilized CO2 and He-Ne lasers with a scheme wherein the noisy H20 discharge laser used in earlier chains has been replaced by a free-running optically pumped CH30H waveguide laser at 4.25 THz as intermediate oscil- lator between a phaselocked HCN waveguide laser' and the saturated fluorescence stabilized 13CO2P(28) laser developed in our laboratory. A NEW LASER CHAIN INCLUDING AN OPTICALLY PUMPED (FIR) CH30H LASER Time domain frequency stability measurements made in our laboratory on free-running optically pumped CH30H wave- guide lasers working at 4.25 THz (70.5 Am) have clearly demonstrated their high-frequency stability with a two-sample standard-deviation of ay (T = 50 ms) t 2 10-12 in a 5-MHz bandwidth [11]. The frequency of the CH30H line at 70.5 ,um can be used as an intermediate step between the HCN fre- quency and the frequency of some of the CO2 laser lines. Specifically, we have used the following relationships: 5 -HCN -CH30H t 202.129 GHz VC02 -7 VCH30H 8.943 GHz (1) (2) where vi denotes the frequency of the useful laser lines: VHCN 891 GHz, VCH3OH 4.25 THz, and VCO2 t 29.771 THz for the '3CO2P(28) line. The difference frequencies appearing in (1) and (2) can be downconverted to tens of megahertz by heterodyning, re- spectively, with the third harmonic of an 67.4-GHz klystron and with the fundamental of an X-band klystron the signals of which are radiated on to the diodes. Point contact W-Ni diodes are used for both harmonic mixing experiments. The complete klystron and laser chain linking the He- Ne(CH4) optical frequency standard to the cesium beam I Developed by MM. Auvray, Gastaud, Pyee, and Sentz at the Paris VI University (Laboratoire d'Electronique et de R6sonance Magnetique). 0018-9456/80/1200-0268$00.75 © 1980 IEEE 268