A laser diode or injection laser diode is the most common type of laser producing device. These diodes have a wide range of applications ranging from fiber optic communications, barcode readers to laser scanning and laser printing.
Laser diode driver is an electronic device which is used to supply a constant current to one or more laser diodes. This results in a continuous wave operation of the laser with a constant power output. Since the voltage-current characteristic of a laser diode is highly non-linear, which also depends upon the junction temperature, there is a need to supply constant current apart from constant voltage.
As the temperature of the diode increases, the value of the current also increases. If the voltage is kept constant, this increase in current will further increase the temperature which will add up more current to the already existing high levels of current. This could destroy the diode and stop the whole system.
It is noteworthy that the drive current is responsible for the supply of electrical carriers into the junction. It can be therefore concluded that drive current and not drive voltage is directly linked to the output power. And hence, focusing on keeping a steady supply to the diode is more fruitful.
Handling and Safety
Laser diodes are sensitive devices and even small amount of spike in current can destroy it. If the driver is not made specifically for the laser diode, one should provide a separate controller which can adjust the drive current and set a maximum current limit. The controller should be designed such that it either keeps the current at the maximum allowable value or stops the supply completely. In this way, the risk of damage to the system can be minimized.
Sometimes, diodes are intentionally overdriven i.e. operated at drive current value that is above the recommended levels. In this way, one will get an increased power output. But this practice should be avoided since this will not only risk damage to the device, but also will reduce the life of the diode.
It goes without saying that the laser diode driver itself should be protected against voltage spikes on electrical grid or current transients resulting from improper electrical contacts.
A single diode driver can control a number of laser diodes which is not only an easy but is economical too. In such settings, diodes are always connected in series since this will ensure the same current is supplied to all the diodes. Whereas, in parallel connection, if one diode gets hot, it will lead to more consumption of current and thus it would get even hotter.
Different Power Ranges
They are available in wide range of drive currents ranging from as low as 20mA to as high as 40A. For applications that need low power output, the heat sink is not required, but for high-power diodes, the heat sink is recommended.
High-power diode drivers need to have all its components working at good power efficiency. This is not just to save energy, but because the amount of waste heat generated can be very high and need to be dissipated with the help of forced convection or water cooling system.
Laser diodes should never by ‘hot-plugged’, which means they should never be connected or disconnected directly while the driver is switched off.