Jerry R. Hobbs, Associate Editor, Technology
Power supplies are responsible for both the regular operation and the longevity of lasers. Electrical current and the proper amount of voltage are transformed by power-supply components and circuits into a reliable source of energy to operate a laser system. Specific laser designs require tightly controlled and consistent performance from their power supplies.
This month’s Product Focus examines performance specifications of power supplies for diode, solid-state, and gas lasers and offers guidelines for matching power supplies to specific lasers and applications. While many laser manufacturers produce their own power supplies, others recognize that some users, especially OEMs, want to buy nonproprietary power supplies to customize the laser performance for specific applications such as telecommunications, medicine, and industry.* Laser engineers often work with merchant power-supply manufacturers to develop new devices for the research market.
Matching device to laser type
Power-supply designs call for varying combinations of circuitry and components to meet the specifications of different laser applications. No single power-supply design can be optimized for use with all types of lasers. For example, certain diode lasers can be temperature tuned by a power supply to emit exclusively at selected wavelengths. Similarly, a single diode laser can be controlled by one kind of power supply for CW operation and by another kind for pulsed operation. These examples do not hold true, however, for most lamp-pumped solid-state lasers and for gas lasers, which require more complex and device-specific power-supply designs.
Power supplies for diode lasers are often called drivers. Narrow-linewidth diode lasers need low-current-noise drivers. High-power diode arrays draw the highest current and voltage levels. Continuous-wave diode lasers need continuous current, while pulsed or modulated diode lasers require drivers that precisely time delivery of electricity so the optical pulse is consistent.
Lamp-pumped solid-state lasers use power supplies to drive flashlamps in ways that can be compared more closely with gas lasers. Switching power-supply designs, such as capacitor-charging supplies used for lamp-pumped solid-state and excimer lasers, have become more flexible with remote controls via computers, circuit modularity, higher switching rates, and greater processing capabilities.
Gas lasers, such as carbon dioxide (CO2), may use switching power-supply designs common to solid-state lasers but with much higher voltages and longer electrical-pulse durations. Some excimer lasers use more conventional direct current (DC) designs that can be simplified down to a high-voltage transformer, rectifier, and line filter. Waveguide CO2 lasers may use radio-frequency (RF) oscillated DC power supplies.
Innovative electronic devices such as insulated-gate bipolar transistors and switched-resistor regulators and the clever use of application-specific integrated circuits, serve to increase power-supply flexibility for diode, solid-state, and gas lasers. Overall, power-supply designs continue to improve as individual components get better.
Assessing power needs
Most users simply want to know what electrical inputs are required to get the optical outputs desired. And no matter how concrete the definitions of ampere, joule, volt, and watt may be, users should always verify the output and stability of each power supply by testing the devices themselves prior to acceptance. Above all else, power supplies should support the specific laser needs without damage to the device or operator.
Most electrical-supply and temperature-control issues for diode lasers are universal. Drive currents and junction temperatures must be controlled for optimized laser performance. Electronic circuits, called feedback loops, are included to monitor performance and to adjust power-supply components to match a particular diode-laser output and temperature specifications.
Diode-laser devices such as vertical-cavity surface-emitting lasers (VCSELs), master-oscillator/power amplifiers (MOPAs), and frequency-doubled diode-laser systems have special power-supply needs. VCSELs need lower drive currents, greater accuracy, and finer set resolution. MOPAs require independent current drive and temperature controls for each laser section. Modular power supplies are suitable for control of the laser and to temperature-tune the nonlinear crystal in frequency-doubled diode-laser applications. Manufacturers recommend independently isolated outputs to prevent signal crosstalk and noise coupling.
Solid-state and gas-laser power supplies have complex specifications. Output-performance specifications on a capacitor-charging power supply can be difficult to understand; this holds true even when users know how to calculate the charge-discharge cycles needed for a laser application.
The required output power supplied for lamp-pumped solid-state or high-powered pulsed-excimer lasers is usually given in terms of joules per second, which is a function of charge time, repetition rate, output voltage, and component characteristics. During a charge-discharge cycle, the rate of change in voltage is not constant. Manufacturers specify power-supply performance in terms of peak output current, peak charging rate, peak output power, and so on (see figure).
Peak output power, however, should not be calculated as the product of peak output current and maximum output voltage. A more reliable measure of a capacitor-charging power supply is the product of average output current and half the output voltage capability, known as the average output power.
High-voltage-regulated DC power supplies must meet maximum voltage and current output needs for a given laser. On the input side, the voltage and frequency requirements should be broad enough to handle commercial electricity variations without disrupting output. On the output side, these supplies should provide ripple, static-line, and static-load regulation. Circuits are needed to supply current to the plasma-discharge tube, to ionize the plasma, to power cooling fans, and to supply other voltages as needed; for example, the magnets that prevent bore erosion in argon-ion laser tubes need power to create control fields.
Cost is the only power-supply specification that can be generalized. Diode lasers for telecommunications, medical, and industrial use can be connected to robust, application-specific power supplies with built-in circuit redundancies for maximum lifetime with the highest reliability. On the other hand, researchers who only sporadically use diode lasers may be satisfied with a less-expensive general-purpose power supply.
Safety issues
Both optical and electrical hazards attend the operation of any laser. An electrical current of only 4-5 J conducted through the body is enough to kill. And while there are government regulations to protect against optical radiation, electrical requirements are not as strict. For example, a laser must conform to safety standards with interlocks and shutters, but its power supply may not meet electrical safety standards even when functioning properly.
Electro-optic equipment must also conform to industrial fire-prevention codes in many cities and towns before it can be installed. Once you select a particular power supply, be sure to confirm with the manufacturer that the device meets all relevant electrical-safety standards.
ACKNOWLEDGMENT
Thanks to Ken Kaiser, Kaiser Systems Inc. (Beverly, MA), for helpful discussions on specifications for capacitor-charging power supplies.