A far infrared (FIR) frequency synthesis technique using saturated-absorption stabilized CO 2 lasers and a point-contact diode has been used to measure frequencies of a number of strong CW H 2 O, D 2 O, and CH 3 OH laser lines. The first frequency measurements of the 79-μm H 2 O, the 73- and 108-μm D 2 O, and 11 CO 2 -pumped CW12CH 2 16OH laser lines are reported. This measurement is the first demonstration of the general usefulness of CO 2 lasers for accurate synthesis of FIR frequencies.

Correction to "Far infrared frequency synthesis with stablilized CO 2 lasers: Accurate measurements of the water vapor and methyl alcohol laser frequencies"

The principles of operation of atomic frequency standards are described. Details are given of their physical construction. Their characteristics as well as their availability, principal uses and impact are specified. The basic concepts involved in atomic frequency standards and the terminology commonly used in this field are summarized. The role of quartz crystal oscillators which are slaved to active and passive atomic frequency standards is emphasized. The traditional atomic frequency standards are reviewed, i.e. the hydrogen maser, the caesium beam tube and the passive rubidium cell frequency standards.

https://iopscience.iop.org/article/10.1088/0022-3735/9/9/001/pdf

Atomic frequency standards and clocks

Reviews nonlinear devices that have been used or are potentially useful for laser frequency measurement in the optical region, extending from the submillimetre to the ultraviolet. The devices are grouped mainly in two classes: point-contact junctions and bulk nonlinear crystals or gases. Their physical principles and limitations are discussed, and attainments when constructed of various materials are listed. Ways of measuring frequencies with them are considered.

http://iopscience.iop.org/article/10.1088/0022-3735/9/11/001/pdf

Application of nonlinear devices to optical frequency measurement

It is verified experimentally that frequency differences between CO2 laser lines and the harmonics of these differences can be used as intermediate frequencies to bridge the gap between microwave frequency standards and infrared frequencies. Criteria for the design of a microwave to 30-THz frequency chain are discussed, and experimental details of the measurement of five CO2 isotope laser frequencies to an accuracy of ±2 parts in 109 are presented. The values obtained for the CO2 isotope transitions are 26334.776614 GHz for P(40) of 13C16O2, 27979.469512 GHz for R(6) of 13C18O2, 28665.704581 GHz for P(6) of 12C16O2, 29442.483293 GHz for R(30) of 12C16O2 and 33328.991337 GHz for R(8) of 12C18O2.

Measurement of the Absolute Frequencies of CO2 Laser Transitions by Multiplication of CO2 Laser Difference Frequencies

Early determinations of the speed of light, based on astronomical observations or terrestrial time-of-flight experiments, were largely superseded in the 1940's by measurements of the frequency and wavelength of microwave radiation. The results were limited by the uncertainty in the wavelength measurements, and it was apparent that greater accuracy could be achieved by using radiation of shorter wavelength. It was, however, not until the development of lasers, and the nonlinear optical techniques made possible by their high output intensities, that frequency measurements could be extended towards the visible region of the spectrum. Stimulated by a demand for a more precise knowledge of the speed of light for application in space research, geodesy, and metrology, a new series of determinations has recently taken place. The measurements were made upon the radiations from a number of different stabilized-laser systems operating in the near infrared and visible spectral regions. Several different interferometric techniques were developed for the wavelength measurements. The results have led to a new recommended value for the speed of light, 299 792 458±1.2 m s−1, and to the possibility of re-defining the unit of length.

Laser wavelength measurements and the speed of light

It is proposed that the frequency differences between CO2 laser ines and the harmonics of these differences might be used as intermediate frequencies to bridge the gap between microwave frequency standards and infrared frequencies to simplify absolute measurements of the latter. Tungsten-nickel point-contact diodes would be used as harmonic generators and mixers. The preliminary results of an experimental feasibility study are presented. A description of a novel two-frequency laser, built for this study, is given. It is shown that beat frequencies between differences up to 715 GHz and microwave frequency harmonics up to the twelfth can be observed; interactions with at least the third harmonic of a CO2 frequency difference are possible, producing signals with an S/N of 51 dB in a 2.2-s time constant receiver; attempts to observe a beat between a CO2 frequency and the seventh harmonic of a difference were not successful.

Hans D. Riccius

Papers

New Frequency Measurements and Techniques in the 30-THz Region