What are Laser Diode Drivers?

A Laser diode driver (LDD) is a driving circuit/device used to provide suitable constant current input for laser diodes & to control it. Ideally, the current input should be linear, noiseless, and accurate. This device is designed to deliver the exact amount of current needed for the laser such that its output is ideal for the intended application.

Laser diodes have a plethora of applications including spectroscopy, remote sensing, medical diagnostics, particle sizing & counting, materials processing, etc. These applications require laser diodes to meet certain threshold performance characteristics. Proper design of the system including the laser diode can ensure that they meet these thresholds. The quality of the current input into the diode is one of the factors that ensure a better performance profile for the laser diode. Laser diode drivers can ensure that the current input is stable. Such a system will feature low noise, high wavelength & power stability, laser diode safety, and safer user operation.

LDDs can operate in either a constant current or a constant power mode. In constant current mode, a user can set an operating current value for the LDD. Feedback is taken from the output current of the driver and compared with the set operating current value to find the error in it. After which the LDD corrects the output current to minimize this error and hence ensures a constant current operation. Similarly, in the constant power mode, the LDD maintains a constant optical power for the laser diode.

Laser Diode Safety

Ensuring laser diode safety is a very important function of LDD. Some of these safety features are maximum current limit, time delay, interlocking, slow start, and brownout protection.

Allowing too much current to pass through a laser diode will damage it. To overcome this, users can set a maximum operating current limit. Generally, LDDs will choose to either cap the current at the maximum level or to turn it off upon reaching this limit based on the design. 

User safety is also equally important while working with the laser. For this purpose, LDDs offer a time delay between the application of electric power to the system and the beginning of the laser operation. Also, protective interlocking packaging is used, upon removing which the power supply to the laser diode is cut off.

LDDs have a slow start to ensure that the diode is not damaged due to thermal shock arising from the sudden start of the system. Also, a decrease in the input current in the case of a DC supply can affect the performance of the driver. For this reason, LDDs have brownout protection which will turn off the output.  

Temperature control

The change in device temperature will cause an increase in injection current in the laser diode and LDD like in any other semiconductor devices. This would cause instabilities or shifts in the output wavelength and power due to factors like temperature-dependent shifts in bandgap energy. This shift may not be large but can affect the results obtained in very sensitive applications. So, proper control of operating temperature is crucial for laser diodes and can be done using LDDs

Laser diode modulation

Apart from an arbitrary constant current output, LDDs can also provide modulated current outputs and hence a modulated pulsed laser output. This is attained by turning on and off the constant current output of an LDD. 

The modulated pulsed operation of an LDD is different from the normal pulsed operation of lasers. The peak power in a pulsed laser can reach higher values than a modulated optical output based on LDDs. This is because the optical power for a time period is incorporated into the laser pulse of a pulsed laser. While for a modulated pulse, the optical power output is just the power generated during the time when the laser is on with its instantaneous power not exceeding the maximum power limit. Also, the modulated pulse can only reach the microsecond regime while a pulsed laser can have a femtosecond regime operation.

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