I have a huge task in front of me and I am actually lost. I want to drive this laser diode. I've worked with laser dirvers before and I used LM317 for simple circuits. This time I'd like to safely drive this laser with 5 Mhz modulation up to 150 mA and I also want to control the amout of current I apply with a variable resistor and monitor this current and the current laser uses with an LCD screen. This will be connected to a butterfly laser mount. I'll need a TEC Controller too but I think I can handle that. So basically what I want will look like this and the completed system will look like this. My question is, what is the best configuration for such a laser driver system? I know there are similar questions but I did not understand what to do with those to modify them for my system.
It seems to me that your best option is to drive the laser with a current source and use the in-built monitor photodiode to regulate the light output to the average level you desire by using feedback techniques to modify the current. This is the standard approach because it overcomes such issues as the laser output extinguishing (as it warms) because the threshold lasing current alters with temperature: -
Of course it does require you to have kept a record of what the specific sensitivity of the photodiode was when you bought the device. The data sheet gives very wide limits but a lot of supplier's of higher-end lasers will have calibrated it. If not then you need to get the photodiode calibrated.
Regards the picture, where it says "input pulses" this is where you would feed your modulation - it can be digital in nature or an analogue signal - it all depends on how you would design the pulsing and biasing circuit. I'm not sure what your modulation is so I can't help much more on this except to say that there are two methods that I know of.
- For injection of high speed data you can use an inductor in series with the current feed and inject a small p-p voltage on-top of the forward voltage applied to the laser. Inductors are used so that you are not reliant on the current source's transistor's capacitance slowing the data edges down too fast.
- You can superimpose slow speed data (5 MHz falls into this bracket) directly to the output of the "adder" shown in the picture.
There are many ways to skin a cat.
THIS ANSWER GIVES NO SAFETY RECOMMENDATIONS REGARDING SAFE OUTPUT LEVELS FROM LASER DIODES - SAFETY IS IMPORTANT WITH LASERS AND YOU NEED TO RESEARCH THIS SEPERATELY.
What's the application? Do you need the modulation to be the full 150 mA? Lots of times you can use a bias Tee, input the DC current through one leg and the RF modulation through another. If you look at your laser you can see it's designed to do that. Pin 3 is DC and RF via pin 12.