# What is the purpose of a "monitor diode" inside a laser diode case?

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?

• It's also good for self-mixing intererometry because then you do not needs optics like beam splitters to produce a beam for the target and a beam to mix with, and redirect both to a photodiode for monitoring youtube.com/watch?v=MUdro-6u2Zg Jun 8 at 13:34
• Laser diodes are highly nonlinear devices driven by positive feedback. Nonlinearity or positive feedback individually make predicting output from input challenging, but here you actually have both, so a lot of the time it's hopeless to predict output from just the input current. Jun 8 at 13:51
• @user1850479 You should post that as an answer. I think it's far more direct than the current answers. Jun 8 at 13:53

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.

• You are right, but on the web store, the word is Monitor Diode, so I used the same. I never worked with Laser Diodes before, but I know a little about electronics. Jun 9 at 14:10
• @JulioOtuyama that looks like a typical example of manufacturers using wording that's obvious if you were close enough to the design of the part to know about its internals, perfectly fine for many customers, but overall could be more helpful - a common enough occurrence Jun 9 at 14:39