OPTICS  AND  CAVITIES (Ruby Lasers)

      Some compact ruby lasers use integral mirrors fabricated directly onto the ends of the rod. This is often a coating of silver (as used in the original laser developed by Maiman) or, more recently, a dielectric coating. The rear of the rod is coated to reflect 100% (or as close as possible), and the front of the rod is coated as an output coupler with a partial transmission, the optimal transmission being a function of the length of the rod and the pumping rate. Larger ruby lasers as well as those for special-purpose applications (the type of ruby laser primarily found nowadays) use external optics.

        All solid-state laser crystals exhibit, to a varying degree, an effect called thermal lensing. Imagining a ruby laser pumped with an intense light and cooled with flowing water, a thermal gradient will develop since the outside of the rod will be cool and the inside of the rod, relatively warm. Changes in rod temperature result in minor variances in the indexes of refraction of the material. These changes result in a spherical lensing effect, which affects the intracavity beam exiting the rod. Localized heating may also cause thermal lensing effects in a square crystal, such as those used for many diode-pumped solid-state lasers (discussed later). As such, a laser rod exhibits a focal length that can be measured. Cavity reflectors used in a ruby laser are often slightly concave to compensate for this effect.

       Most ruby lasers manufactured at present are purpose-built, so many have special optics designed for a specific purpose. Double-pulse lasers used for holography, for example, often take extreme measures to ensure beam quality. Such lasers may feature a standard HR with an etalon for an OC, which reflects only certain wavelengths, those wavelengths separated by the FSR of the etalon. This scheme allows single-frequency operation of the otherwise spectrally wide ruby output and hence a longer coherence length required for holography.

      Often, a MOPA configuration is used with two optically pumped rods in which one rod is used as an oscillator and a second as an amplifier. The oscillator is a complete laser with HR, OC, Q-switch, and intracavity optics as required. The oscillator usually produces as clean a beam as possible, which then passes through an amplifier to increase the output power by up to 10 times that of the oscillator output. Often, the amplifier has a longer rod and more pump power than the oscillator. Having a long upper lasing level lifetime, Q-switching is easily done with a ruby laser. AO or EO Q-switches may be used with a ruby laser, with faster (and more controllable) EO modulators a popular choice for this laser.