LASER  STRUCTURE (Ruby Laser​)

        The structure of a ruby laser can vary from a simple design with integral mirrors deposited directly onto the faces of the rod, to a complex design featuring a plethora of optical elements, such as Q-switches, mode-limiting apertures, and single frequency selectors within the cavity. The optical train of a relatively complex ruby laser, a double-pulse ruby laser used for holography, is depicted in Figure 1.1. The cavity resonator itself consists of a dielectric high reflector and an etalon for an output coupler, the etalon being used to ensure single-frequency operation of the oscillator. Whereas the etalons as transmission filters with transmission peaks corresponding to the FSR of the etalon, this particular etalon is a reflector, reflecting wavelengths separated by the FSR. In that respect, it simply replaces the broadband OC normally used in a laser. The single-frequency operation of this laser increases the coherence length to about 10 m, desirable when using this laser as a source for holography. Aside from frequency stability, spatial quality is also ensured through the use of a variable aperture in the optical train, which serves to limit the transverse mode of the laser to TEM₀₀ mode.

       The laser is Q-switched and uses a Pockels cell EO modulator to generate fast pulses. For this laser, two pulses are produced in rapid succession by opening the Q-switch twice. An EO switch is used since it is faster than an AO modulator and allows true modulation: It can be opened partially, allowing the first pulse to be produced without draining the entire energy of the rod. Energy left in the rod is then used to generate the second pulse. Pulses in a laser of this type must usually be balanced, so that they have the same energy. This balancing procedure also illustrates the utilization of energy stored in the rod. The procedure begins by setting the EO modulator to dump all energy from the rod during the second pulse (i.e., the switch is opened fully during the second pulse and is closed completely during the first pulse). At this stage only the second pulse appears in the output. The switch is opened gradually for the first pulse, and the laser is test fired, with the energy of each pulse monitored. The process is repeated with the switch opened slightly more for the first pulse, until eventually the first pulse extracts one-half of the energy stored in the rod and the resulting two pulses are balanced.

       Although an EO Q-switch usually employs two polarizers, this particular laser uses only one polarizer, consisting of a stack of quartz plates at

Figure 1.1. Optical train of a ruby laser.

Brewster’s angle to enhance the degree of polarization. The second polarizer is the ruby rod itself, in which the gain is highly dependent on the orientation of light passing through it, so gain is highest when amplifying light of only one polarization. By careful alignment of the rod (which has a definite crystal axis easily determined by rotating it while viewing through a polarizer), so that the optical axis is rotated 90 degrees from that of the polarizing stack and placing the Pockels cell between these polarizing elements, the cell can be used to switch intracavity light. This particular laser also incorporates a separate amplifier rod, which boosts the output power by a factor 5- to 10-fold. The amplifier rod is twice the length of the oscillator rod and is pumped with correspondingly higher energy than the oscillator.