POWER  SUPPLIES (CO₂ LASER)

      Most CO₂ lasers (the exceptions being the gas dynamic, which does not need electrical excitation, and the TEA, which is pulsed) are driven from either a DC or an RF pumping source which generates a plasma. A DC excited laser requires only a simple high voltage supply providing enough voltage to initiate and sustain the discharge at moderate currents. Typical figures might be 10 to 30 kV at about 15 to 100 mA for a unit operating with a power output below 100 W. Most lasers can use low-frequency AC as well as DC.

     Like the HeNe laser, CO₂ lasers exhibit maximum output at a finite discharge current, and any increase of current beyond that point results in a decrease in output power. Although output naturally depends on pumping rate, too much pumping results in an increase in heat in the tube, thermal population of the lower lasing levels, and a decrease in output for the laser. As tube length increases, so does the voltage required to excite the discharge. Whereas a 1.5-m tube requires about 30 kV to operate, a longer tube requires even higher voltages. A 3-m tube would probably require 60 kV! Power supplies operating at this voltage become quite costly (more than double that of a 30-kV supply). Voltages this high are also difficult to manage, due to the requirement for better insulation and problems such as corona and arcing. By shortening the active discharge length, voltage is reduced and power supply design is greatly simplified, which is the electrical reason for preferring shorter tubes.

          Another approach to lower discharge length is to install multiple electrodes in a tube: for example, a single anode in the center of the tube and two cathodes. By splitting the effective discharge length in half, voltages across these portions of the tube are reduced to a manageable level (this approach is also used in some long HeNe lasers as well). The complexity and cost of the power supplies are much lower since it is easier and cheaper to obtain electronic components that operate at higher currents and lower voltages than the other way around.

      Regardless of discharge length, the CO₂ laser plasma, like other gas discharges, has a significant negative resistance, which may be overcome using either a ballast resistance (as in an HeNe) or some form of current regulation in the power supply. For a laser operating at a current of 60 mA, the ballast resistance would be required to dissipate an enormous amount of power, making the supply inefficient at best (i.e., a large space heater!). For this reason, most power supplies use some method of active current regulation, but regulating current at the high voltages required by this laser is not an easy task.

         The simplest approach is to use a saturable core reactor to limit current through the tube. These are incorporated into most neon-sign transformers (a popular power source for small lasers) for precisely this reason, since neon signs exhibit similar discharge characteristics and require current limiting. Unfortunately, saturable reactors (called magnetics by some in the industry) work only with AC currents, and the resulting laser would be pulsed, with the discharge extinguishing 120 times a second (for 60-Hz power) at the zero-cross point of the AC line voltage. Because of this, most industrial lasers use continuous DC discharges. Many older industrial lasers (and quite a few of these are still in use today) use vacuum tubes as current regulators since these can handle the high voltages involved. These regulators are often installed between cathodes (often, multiple cathodes, each with a separate regulator, with a single anode in the tube) and the negative output of the power supply.

        The problem of high voltages, and specifically, increasing voltage with tube length, is solved in many small lasers (the majority under 100 W) by using RF excitation of the plasma. These laser tubes often feature long, transverse electrodes driven with RF power; the configuration resembles that of a nitrogen or excimer laser but the cavity optics are quite different. RF excitation solves some problems of DC excitation, such as high voltages (for longer tubes) and sputtering of electrodes, but the power supply electronics are more complex and expensive. For a small, sealed laser, RF is preferred (since sputtering consumes gas, which is a big concern in a sealed system).

     TEA lasers require a unique discharge circuit to generate fast pulses. The circuitry is identical for the excimer laser, in which a capacitor is charged to a high voltage and suddenly discharged through the laser channel using a thyratron switch. Preionization electrodes ensure that gases in the laser channel are ionized to form a pathway for current from the main capacitor.