I was looking for laser diodes to buy and I noticed many of them have a "monitor diode" included. What is the purpose of a monitor diode? Would not the current (of the laser diode) be a better way to monitor it?
Would not the current (of the laser diode) be a better way to monitor it?
No, the monitoring diode is an accurate and fundamental method of ensuring the calibration of many laser diodes. For instance, if you buy a laser diode from a reputable supplier, you can pay a little more and get a precise figure for the light power output based on the photodiode output current. The supplier may typically produce information like: -
output power = 4.70 mW at a PD current of 250 uA when reverse biased at -5 volts
If you buy and use a lot of laser diodes, this information is crucial else, you need to setup your own test rig and calibrate it yourself. Many folk don't want to do that.
Then, the photodiode can be used in a simple closed loop system to ensure that in critical applications, the laser diode light output remains constant across a wide range of operational conditions.
Operating current versus temperature versus light output is too complex and unreliable for accurate control of the laser diode light output. In other words, the photodiode output current is a much more reliable method of determining laser diode output power than any other method.
It it normally used as feedback to measure if you are losing light over time due to laser degradation and this in turn can be used as AGC to maintain constant light output.
Just measuring the current or even both Vf and current is not enough due to said degradation. Also, lasers have a knee below which the operate as an LED more or less and above it they start to lase. You won’t capture that without the monitor diode being present.
To some degree you can measure coupling/fiber insertion.
Can also be used to create a poor man’s laser microphone. YouTube link to Applied Science: youtube.com/watch?v=1MrudVza6mo
Just to make it explicit, their wording is a tiny bit lazy. Monitor photodiode would be clearer.
How it's integrated depends on the laser package (even big benchtop lasers may have monitor diodes, but they're discrete components). Crucially unlike measuring the input to the laser diode (as you would measuring its current), it measures the output light.