POWER  SUPPLIES (Ruby Lasers​)

       Ruby lasers are almost always pulsed, with only a few research lasers operated in CW mode (and these require extreme cooling); flash lamp pumping is hence the rule for ruby lasers. Being a three-level system, pumping thresholds are quite high, with pump energies of 1000 J or more common in ruby lasers. Often, the flash lamps used with ruby lasers are helical in shape, with the ruby rod at the center of the helix. Helical flash lamps have a comparatively larger volume than linear flash lamps, so can handle the higher energies required for this laser (as opposed to the YAG, which has much lower thresholds and hence usually uses smaller lamps).

          A flash lamp is designed to produce an intense pulse of light, usually in a short time frame ranging from microseconds to 1 ms. The lamp itself consists of a glass tube filled with low-pressure (about 450 torr) xenon gas. Electrodes at either end of the glass tube deliver current to the lamp, and triggering is accomplished either by applying an external high-voltage pulse to the surface of the glass tube or superimposing a high-voltage pulse across the main terminals in the same manner that HeNe tubes are ignited. When the lamp fires, it exhibits a very low resistance consuming all energy from the storage capacitor, producing an intense pulse of light in the process, the spectra of the light emitted being characteristic of the gas used. In the case of ruby, xenon is used since the output is rich in blue light, which is readily absorbed by ruby. Figure 1.1 shows the circuit for a typical flash lamp discharge circuit; all flash lamps, including photographic types, are quite similar in design. Capacitor C1 charges with energy from the power supply until reaching the terminal voltage, usually between 500 and 1000 V. This voltage is present across the flash lamp, but the lamp does not ignite, since the voltage is not sufficient to cause the gas inside to ionize and conduct current. In this particular circuit a trigger pulse of between 4 and 10 kV is applied externally to the glass envelope (via a wire wrapped around the lamp) to ionize gas in the tube and initiate the discharge. To generate the high voltage trigger pulse capacitor C2, a relatively small capacitance, charges from the main power supply through R2 and through the primary of the trigger transformer T1 itself. When the pushbutton is pressed, the left side of C2 is grounded and current flows through the primary of T1. Being a step-up transformer, a high potential

Figure 1.1. Flash lamp circuit.

appears across the secondary of T1 sufficient to ionize gas in the lamp. Once the lamp is ionized, current flows from the main storage capacitor, C1, through inductor L1, which helps form the pulse into one suitable for the lamp in what is called a critically damped LC circuit, and through the lamp, producing an intense pulse of pump light. When the capacitor has dumped all energy into the lamp, the voltage across the lamp falls to a level insufficient to sustain discharge and the discharge current through the lamp ceases. The capacitor then recharges again for the next pulse.

       The external triggering method shown is used with most air-cooled flash lamps, including photographic types as well as lasers, with a very limited firing rate (e.g., one pulse each 10 s). Larger ruby lasers often use water-cooled flash lamps, in which the lamp and rod are both bathed in deionized water for cooling, with deionized water used since it is an insulator and the lamp electrodes are often immersed in cooling water. This somewhat precludes the use of external triggering, so series triggering in which the secondary winding of a trigger transformer inserted into the anode lead is used for water-cooled lamps.