Chapter 5 deals with the theory of passive optical resonators, i.e., where no active medium is present within the cavity. The most widely used laser resonators have either plane or spherical mirrors of rectangular (or more often circular) shape, separated by some distance L. Typically L ranges from a few centimeters to a few tens of centimeters, while mirror dimensions range from a fraction of a centimeter to a few centimeters. Laser resonators thus differ from those used in the microwave field (see, e.g., Sect. 2.2.1.) in two main respects: Resonator dimensions are much greater than the laser wavelength and resonators are usually open, i.e., no lateral surfaces are used. Resonator length is usually much greater than the laser wavelength because this wavelength usually ranges from a fraction of a micrometer to a few tens of micrometers. A laser cavity with length comparable to the wavelength would then generally have too low a gain to allow laser oscillation. Laser resonators are usually open to reduce drastically the number of modes that can oscillate with low loss. In fact in Example 5.1 we see that even a narrow linewidth laser, such as a He-Ne laser, has a large number of modes (≈109) if the resonator is closed. In contrast, on removing the lateral surfaces, the number of low-loss modes is reduced to just a few (≈6 in Example 5.1). In these open resonators, in fact, only the few modes corresponding to a superposition of waves traveling nearly parallel to the resonator axis have low enough losses to allow laser oscillation.
KeywordsSpot Size Spherical Wave Open Resonator Diffraction Loss Lower Order Mode
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