Ensuring Safe Operation and Accurate Characterization of Laser Diodes
- Sunday, 01 July 2012
Laser diodes are very sensitive to electrostatic discharge (ESD), current/ voltage transients, and temperature changes, and extra care must be taken to ensure the laser diode is protected during all operating conditions. Unlike general-purpose power supplies or current supplies, laser diode drivers, mounts, and cables greatly improve the protection of laser diodes and allow low noise operation. A temperature controller helps stabilize the temperature of the laser diode packages, which provides for safe operation and wavelength stability of the laser. For fast and reliable characterization, an optical power meter and integrating sphere-based photodiode detectors are often employed where a good understanding of optical measurements is required.
Laser Diode Driver
Engineers or scientists who do not use a laser diode (LD) regularly often operate it by connecting unshielded wires between a general-purpose power supply and a laser diode. A laser diode is extremely sensitive to static discharge and any sudden surge of electricity. Using a general-purpose power supply, neither the voltage supply nor the current supply will properly eliminate the transient electric surges, which could lead to killing the device. A properly designed laser diode driver is equipped with a number of features that ensure the safety of not only the laser diode, but also the operator.
Protection features should include: LD current limit, power line transient protection, monitor photodiode current limit, compliance voltage limit, output power level limit, thermoelectric cooler (TEC) current limit, high and low temperature limits, voltage limits, sensor open, and TEC open detection.
In addition to the traditionally available laser diode protection schemes, many controllers tie the operation of the laser current driver and the temperature controller together (Figure 1). If the linking option is enabled, the laser diode driver will automatically shut off in case the operation of the temperature controller is terminated, thus avoiding overheating of the laser diode. During operation, the front panel controls can be disabled to prevent any changes in the output by accidental maladjustment of the control knob.
After the safety issue is addressed, the next thing to consider is controlling the amount of current noise circulating through the laser diode. Due to the proliferation of technologies employing laser diodes, many new applications require unprecedented levels of low current noise, while demanding higher optical power output. In applications like materials processing and defense, very high optical output power is an essential feature, imposing real safety concerns on the operator. It is very important to eliminate stray laser light, and the laser diode driver should include an interlock feature to disable the output if a user enters the area.
A laser diode generates a large amount of heat density due to its small size and imperfect conversion rate. When the electrical power is converted to generate the laser light, the energy not converted to photons turns to heat, which must be dissipated through the package to avoid permanent thermal damage of the device. Laser diodes are often packaged together with an active cooling element, such as a thermoelectric cooler. Lowpower laser diodes are packaged with high thermal-conducting materials with no active cooling. Even if the laser diode package itself may not have a TEC, the mounting device may still need an active cooling element to properly dissipate the heat away from the laser diode and provide wavelength stability.
A thermoelectric cooler is fundamentally an electrical device and is less sensitive to ESD and current/voltage transients than laser diodes so it can be controlled by a simple power supply. However, to ensure maximum temperature stability, a specially designed temperature controller should be used. Temperature controllers have the ability to use a temperature sensor, usually a thermistor, to provide feedback to the output to adjust the polarity and effort of the output in order to stabilize the temperature. Typically, the feedback loop uses a proportional, integral, and differential (PID) algorithm to stabilize the temperature. The PID settings must be flexible to allow the user to adjust the constants to optimize the package design. Some temperature controllers, such as the ILX Lightwave LDT-5900C series, incorporate an auto-tune function to calculate the PID constants for the package (Figure 2).