Wavelength and Frequency Measurements

To obtain laser action it is necessary not only to have an appropriate population inversion but also to ensure that the rate of stimulated emission, which depends on the intensity of the local radiation field, is high compared with loss mechanisms. This condition can often be met in pulsed systems even in the absence of a resonator, in so-called mirrorless lasers. In cw lasers, however, a resonator is invariably necessary to increase the intensity of radiation which is circulating in the laser medium the intensity of the circulating radiation can be calculated by multiplying the output intensity of the laser by the inverse of the transmission coefficient of the output coupling optics. 

The guide wavelength can differ markedly from the free-space wavelength A and so wavelengths measured on this basis must be carefully corrected. This requires an absolute identification of the mode in question, which is not so in the Fabry-Perot resonator case, where modes with fixed values of p and / reappear with the same periodicity (very nearly) as do modes with any other fixed values of p and /. For example in Fig. 3.4, which is a cavity scan made with a circular metallic waveguide laser of 19mm diameter, many different periodicities 

As well as the Fabry-Perot, other types of interferometers have of course been used. Noteworthy was the measurement, as early as 1969, of the wavelength of laser lines to an accuracy of 10 using a long path-length Michelson interferometer [3.13]. Frequencies had already been measured for some of the lines involved so that a calculation of the speed of light was possible (see below). Noteworthy also is the use of the CO2 laser radiation, whose wavelength is accurately 

The property of a Josephson-junction whereby the frequency of the illuminating radiation is converted into a voltage, with a conversion factor based on fundamental constants, means that it too can be used as a spectrum analyzer well suited to (sub)millimetre lasers [3.16]. 

For a number of purposes, the accuracy obtainable by the interferometric measurement of wavelength is not adequate. The most obvious of these purposes is molecular spectroscopy of the lasing molecule itself, which was discussed in Sect. 2. When used as a local oscillator in an astronomical receiver one would also like to know the laser frequency to within a few megahertz so as to know the radial velocity of the observed objects to within a few km/s. In metrology too, where the laser might be used in a chain to link microwave measurements with those made in the optical, high precision is necessary. For such purposes heterodyne measurements, which yield the frequency directly, are to be preferred, and these are now discussed. 

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In the first part of this book, I have attempted to fill a gap in the literature by reviewing the experimental and theoretical techniques used to acquire the fundamental data on millimetre and submillimetre lasers. That is, I have devoted sections to molecular spectroscopy and line identification (Chap. 2), wavelength and frequency measurement (Chap. 3), and power (i.e. intensity) measurement (Chap. 4), including a listing of those lines for which power has indeed been reliably measured. I conclude Part I with some notes on pump lasers (Chap. 5). I hope that even experienced readers will enjoy these sections, but their primary purpose is to provide an easy introduction to the literature for those less familiar with the subject. 

Wavelength and frequency measurements have been exhaustively compiled (in Part II of this book) along with molecule and pump identification. Part I contains a short review of the relevant measurement techniques in each of these areas and, in addition, a review of power measurements. 

When these wavelength and frequency measurements are combined the result for the velocity of light, given in Table 13.3, is about two orders of magnitude more accurate than the previously accepted value. This result has been... 

Figure 12.11 Electromagnetic waves are characterized by a wavelength, a frequency, and an amplitude, (a) Wavelength (A) is the distance between two successive wave maxima. Amplitude is the height of the wave measured from the center. (b)-(c) What we perceive as different kinds of electromagnetic radiation are simply waves with different wavelengths and frequencies.

Figure 4-12 Illustrations of the wavelength and frequency of water waves. The distance between any two identical points, such as crests, Is the wavelength, A. We could measure the frequency, v, of the wave by observing how often the level rises and falls at a fixed point In its path—for instance, at the post—or how often crests hit the post.

The significant feature of wave motion is its repetitive nature, which can be characterized by the measurable properties of wavelength and frequency. Wavelength (X) is the distance between corresponding points on adjacent waves. The unit for wavelength is a distance unit. 

The goal of the basic infrared experiment is to determine changes in the intensity of a beam of infrared radiation as a function of wavelength or frequency (2.5-50 im or 4000—200 cm respectively) after it interacts with the sample. The centerpiece of most equipment configurations is the infrared spectrophotometer. Its function is to disperse the light from a broadband infrared source and to measure its intensity at each frequency. The ratio of the intensity before and after the light interacts with the sample is determined. The plot of this ratio versus frequency is the infrared spectrum. 

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