Ensuring Safe Operation and Accurate Characterization of Laser Diodes
- Sunday, 01 July 2012
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.