Standards infrared laser frequencies

The lack of accurate and stable frequency standards in the near-infrared spectral range, and in particular at 1083 nm, is a serious inconvenient to improve the present frequency stability of the He-locked master laser. On the other hand, hyperfine transitions of the iodine molecule has been defined as secondary frequency standard at different visible wavelengths, and in particular at 532 nm, the doubled frequency of the 1064 nm Nd YAG laser. Likewise, our idea has been to lock the master laser frequency to I2 hyperfine transitions at its doubled frequency, 541 nm. 

This frequency mixing in suitable nonlinear mixing elements is the basis for building up a frequency chain from the Cs atomic beam frequency standard to the optical frequency of visible lasers. The optimum choice for the mixer depends on the spectral range covered by the mixed frequencies. When the output beams of two infrared lasers with known frequencies v and V2 are focused together with... 

In this section we discuss some methods of wavelength stabilization with their advantages and drawbacks. Since the laser frequency v = c/A is directly related to the wavelength, one often speaks about frequency stabilization, although for most methods in the visible spectral region, it is not the frequency but the wavelength which is directly measured and compared with a reference standard. There are, however, some stabilization methods in the infrared which do rely directly on absolute-frequency measurements (Sect. 14.7). 

The first frequency measurement of the 15 — 25 resonance made use of a transportable ClU-stabilized HeNe infrared frequency standard at 88 THz [24], built at the Institute of Laser Physics in Novosibirsk/Russia. For calibration it was transported repeatedly to the Physikalisch-Technische Bundesanstalt (PTB) in Braunschweig/Germany where it could be compared with a Cs atomic clock using the PTB frequency chain [25]. 

Presently, twelve reference frequencies covering the visible and infrared regions of the electromagnetic spectrum are recommended by the Comite International des Poids et Mesures (CIPM) for the realization of the metre [1]. Up to now, practical length metrology is performed mainly using the red line of the iodine stabilized He-Ne laser at A = 633 nm with a relative standard uncertainty of 2.5 x Hr11 [2],... 

One interesting reference standard may be the methane stabilized helium neon laser at 3.39 pm. Its infrared frequency can be compared directly with the microwave cesium frequency standard with the help of a relatively short frequency chain [32], An accuracy of 1 part in 1012 or better appears feasible for a transportable secondary standard. 

Primary length measurements are these days based on optical frequency standards. If one needs a unit of length, for example, a wave-length for interferometric measurement, then one divides the optical frequency by the value of the speed of light (299 792 458 m.s ) as defined in the SI metre. The mise en pratique of the metre [44] lists a number of frequency-stabilised lasers at various wavelengths in the visible and near infrared spectral regions. 

C. Freed, J. W. Biellnskl, and W. Lo, "Programmable, Secondary Frequency Standard Based Infrared Synthesizer using Tunable Lead-Salt Diode Lasers," Proceedings of Tunable Diode Laser Development and Spectroscopy Applications," SPIE 438, (1983). 

In addition, other frequency chains have been developed that start from stabilized CO2 lasers locked to the cesium frequency standard in a similar way, but then use infrared color-center lasers to bridge the gap to the l2-stabilized HeNe laser [14.155a]. 

Spectroscopy utilizing tunable laser and microwave sources has been applied widely in exploring atoms, molecules, and condensed matter. Besides the classical areas of optical double resonance and optical pumping the extension of these or related methods to difference frequency measurements in the optical range seems to be of increasing importance. This includes heterodyne techniques. Laser microwave schemes can also play an essential role for the generation of modem frequen( standards. Last but not least, there will be many technical applications like infrared detectors, wavemeters, magnetometers, etc. 

Methods of measuring the phase and frequency of coherent infrared signals have become important in order to extract the maximum information from laser signals. The efficient nonlinear mixing of coherent infrared radiation in standard detector materials has achieved sensitivities approaching the theoretical limit (a few photons per measurement interval) and has stimulated... 

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