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).
The safety of the thermoelectric device also needs to be considered. The maximum amount of current and voltage for each thermoelectric cooler must be limited by the temperature controller. Otherwise, the TEC can be damaged, resulting in the damage of the laser diode and the entire experimental setup of the instrument.
Finally, it would be ideal if the TEC controller could communicate with the laser diode driver such that the driver can shut itself off whenever it detects a thermal runaway situation; for example, when the temperature of the mount is out of safe operating range. By combining the laser diode driver and the temperature controller in one instrument, the safety of the laser diode can be further improved.
Laser Diode Mount
As it is clear that a proper system design calls for a laser diode driver and a temperature controller, a laser diode mount specifically designed for various laser diode packages has merit. It provides an easy mounting solution with proper thermal contacts and mass for efficient heat dissipation. It ensures proper shielding of electrical connection end-to-end between the laser diode and the controller instruments. Due to the high electrical sensitivity of the laser diode, the cables also require proper shielding to minimize the noise pickup from the environment (Figure 3).
To ensure long-term temperature-stable operation of a laser diode, the laser diode mount must be designed to dissipate the heat generated from the laser diode and the TEC. Commercially available laser diode mounts are equipped with a plate or a mount suitable for the most popular types of laser diode packages.
Optical Measurement and Characterization
One of the most common tasks performed with the laser diode instruments is the L-I-V (light-current-voltage) characterization (Figure 4). This is a combination of the basic I-V measurement, characteristic to a diode, and the light output measurement, characteristic to a laser. Because the laser emits light, measuring optical power is the ultimate measure of how stable the laser diode operates. Often, the laser diode sources have a large divergence angle, making the optical measurements difficult. In tegrating spheres are typically used as they allow for the laser light to be easily captured. It is very common that the researcher or the engineer who works with a laser diode wants to know accurately how much power in watts the laser is outputting or the power measured at a certain point of the beam path. In that case, the detector must be traceable to National Institute of Standards and Technology (NIST) or other similar national standards institutes.
The laser diode can emit light either in continuous wave (CW), in pulses, or in modulation. The CW measurement is straightforward, but pulse or modulation signal measurement can be challenging. Choosing a good detector and selecting a suitable optical power meter with proper specifications will ensure accurate measurements.
A series of quick optical power measurements needs to be made while varying the laser diode current, especially with the L-I-V characterization. Due to the slow response of a thermopile sensor, integrating spheres with a photodiode detector are typically used as they provide faster measurement speed. The challenge is to properly calibrate the detector for high-power lasers. High optical power entered into a sphere eventually turns into heat. The photodiode response varies with temperature; therefore, thermal management of the sphere is another area where care must be given.
A laser diode is a highly sensitive device, even though its use has widely expanded into our everyday lives. As noted, often the laser diode can be instantly killed by a careless touch with a high static discharge, or by current/ voltage transients, which can sometimes be generated. Even if the laser diode emits light, its life can be significantly compromised. Selection of equipment should include a laser diode driver with multiple levels of laser diode protection, a stable temperature controller, and laser diode mount that provides easy mounting while assisting in eliminating potential areas for noise. Ideally, for laser diode characterization, a NISTtraceable optical power meter with integrating sphere should be used to allow for fast and accurate measurements.