Extremely sensitive readout of a stable "etalon of length" is achieved with laser frequency-locking techniques. Rotation of the entire electro-optical system maps any cosmic directional anisotropy of space into a corresponding frequency variation. We found a fractional length change $\frac{\ensuremath{\Delta}l}{l}=(1.5\ifmmode\pm\else\textpm\fi{}2.5)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}15}$, with the expected ${P}_{2}(cos\ensuremath{\theta})$ signature. This null result represents a 4000-fold improvement on the best previous measurement of Jaseja et al.

Improved laser test of the isotropy of space

The problem of obtaining single-frequency output from a long laser is considered, and two methods are investigated experimentally. The first method consists of using an external filter to select one of a number of oscillating modes. The second method consists of suppressing internally the unwanted resonances so that the laser oscillation can only take place at a single frequency. It is shown that with the second method one can in many cases obtain greater power, and experiments are reported in which single-frequency output power of 15 mW was obtained from a 6328 A He-Ne laser, A simple feedback circuit is described for stabilizing the frequency of the laser oscillation.

Stabilized, single-frequency output from a long laser cavity

The merging of continuous wave laser-based precision optical-frequency metrology with mode-locked ultrafast lasers has led to precision control of the visible and near-infrared frequency spectrum produced by mode-locked lasers. Such a phase-controlled mode-locked laser forms the foundation of a "femtosecond optical-frequency comb generator" with a regular comb of sharp lines with well-defined frequencies. For a comb with sufficiently broad bandwidth, it is now straightforward to determine the absolute frequencies of all of the comb lines. This ability has revolutionized optical-frequency metrology, synthesis, and optical atomic clocks. Precision femtosecond optical-frequency combs also have a major impact on time-domain applications, including carrier-envelope phase stabilization, synthesis of a single pulse from two independent lasers, nonlinear spectroscopy, and passive amplifiers based on empty external optical cavities. The authors first review the frequency-domain description of a mode-locked laser and the connection between the carrier-envelope phase and the frequency spectrum to provide a basis for understanding how the absolute frequencies can be determined and controlled. Using this understanding, applications in optical-frequency metrology and synthesis and optical atomic clocks are discussed. This is followed by discussions of time-domain experiments.

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Optical frequency combs: from frequency metrology to optical phase control

Improvements in the frequency stability of the visible helium-neon laser are possible using saturated absorption in iodine vapour. Recent improvements in the design of these lasers are discussed, and the importance of third derivative locking methods in realizing the possible high reproducibility of the iodine reference frequencies is emphasized. Allan Variance measurements of the beat frequency between two stabilized lasers indicate a long term stability of better than 1 part in 1010.

https://iopscience.iop.org/article/10.1088/0022-3735/5/9/025/pdf

Frequency stabilization of the helium-neon laser by saturated absorption in iodine vapour

The band centers, rotational constants, absolute frequencies, and vacuum wavenumbers for12C16O 2 ,13C16O 2 ,12C18O 2 ,13C18O 2 ,12C16O18O,14C16O 2 , and14C18O 2 have been simultaneously computed from 590 beat frequency measurements between pairs of adjacent 00\deg1-[10\deg0,02\deg0] I and II band CO 2 laser transitions. The input data included the 56 beat frequencies measured between adjacent12C16O 2 rotational lines by Petersen et al. The absolute frequencies are directly related to the cesium frequency standard via the 29 442 483.315 (0.025) MHz 10.18 μm I-R (30) and the 32 134 266.891 (0.024) MHz 9.33 μm II-R(10) transitions in12C16O 2 which were measured by Evenson et al. The results described in this paper will provide the means for setting up easily reproducible secondary frequency standards in the 8.9-12.3 \mu m wavelength band.

Absolute frequencies of lasing transitions in seven CO 2 isotopic species

Wave-Length Stabilization of an Optical Maser

WE report here the completion of a determination of the speed of light at the National Physical Laboratory1. The value was obtained from the product of the measured frequency and the wavelength, determined through up-conversion, of the radiation from a CO2 laser stabilised to the R(12) transition of CO2 at 9.3 µm.

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Measurement of the speed of light

A servocontrolled 1-m plane-parallel Fabry-Perot interferometer has been developed at NPL for the precise intercomparison of laser wavelengths. This instrument has been used to measure the wavelength ratio of a 679-nm radiation and that from a 633-nm iodine-stabilized He–Ne laser, achieving an accuracy of 2.9 parts in 1011. The 679-nm light was derived from a stabilized CO2 laser radiation by upconversion, and the wavelength of this 9.3-μm laser radiation can be calculated from the visible wavelength result. Frequency measurements on the same CO2 laser radiation have already been made in this laboratory, so that the experiment reported here leads to a precise value for the speed of light in vacuum and to the value of 473, 612, 380.5 ± 0.3 MHz for the absolute frequency of the visible radiation from a He–Ne laser stabilized to component d of127I2.

Frequency determination of visible laser light by interferometric comparison with upconverted CO(2) laser radiation.

This, and part II following, describe a determination of the speed of light made by measuring the frequency and wavelength of radiation from a CO$\_{2}$ laser. This laser was operated on the 9.3 $\mu $m R(12) transition, and stabilized by reference to fluorescence in an external CO$\_{2}$ absorption cell. The laser frequency was measured via a sequence of harmonic-mixing stages involving the HCN and H$\_{2}$O lasers as transfer oscillators, and was found to be 32 176 079 482 $\pm $ 14 kHz ($\pm $ 4.2 parts in 10$^{10}$). A correction to this value of -10 kHz is necessary to obtain the centre frequency of the CO$\_{2}$ reference transition. The wavelength measurements and value of $c$ are described in part II.

Measurement of the speed of light I. Introduction and frequency measurement of a carbon dioxide laser

WAVELENGTH measurements, which are part of an accurate determination of the speed of light1, have been made on the radiation from a carbon dioxide laser, stabilised to the R(12) transition at 9.3 µm by saturated fluorescence in an external CO2 cell, which has a measured frequency2 reproducible to better than one part in 109. The problems of a direct intercomparison of infrared and visible wavelengths were avoided by mixing the CO2 radiation with light from a 10-mW He–Ne laser at 0.63 µm to give a difference frequency sideband at a wavelength of 0.68 µm. This mixing process, known as up-conversion, was performed in a cooled, single crystal of proustite3,4. The wavelength of the CO2 radiation was then determined using the relationship λ9.3 = λ0.63/(1—R), in which it is assumed that c is the same for the two visible radiations. The wavelength measurements were thus of the ratio R = λ0.63/λ0.68 and the value of λ0.63 with respect to the primary length standard.

W. R. C. Rowley

Papers

Wave-Length Stabilization of an Optical Maser

Measurement of the speed of light

Frequency determination of visible laser light by interferometric comparison with upconverted CO(2) laser radiation.

Measurement of the speed of light I. Introduction and frequency measurement of a carbon dioxide laser

Accurate wavelength measurement on up-converted CO 2 laser radiation