IR laser detection

Question

I'm currently trying to characterise a NIR 940nm laser device that I have recently made. The main thing I want to achieve is to see if the laser is truly doing what the enable signal told it to.

I began with a rather slow square wave - 10Hz at 50% duty cycle. I used a ThorLabs DET08CL photodetector that is pointed directly at the laser, with its output connected to an oscilloscope.

I was expecting some square waves on the scope, instead I got this:

The rising edge seemed all good, yet it doesn't stay high for half the period. Instead it begins to fall, where the falling doesn't look like a normal "1ns fall time" per datasheet. My guess was that the optical feedback coming from the laser, reflected by the photodiode may cause it, but I was unsure.

Please let me know if you think this is a good way of characterising the laser waveform, or if you have better ideas.

• Additional info: I convinced myself that the laser is functional, through a less reliable method: I replaced the photodetector with an IR visualizer card, and further dropped the square wave frequency to 1Hz at 50% duty cycle. I could tell that the visualizer flashes, with each blink approxiamtely 0.5 second, with bare eyes. If what's from the oscilloscope was true, I believe that the blinks on the visualizer would be too fast to be captured by eyes. Hence seeking improvements on the photodiode setup, or alternatives.

Answer 1

The Thorlabs documentation for that part has a very nice tutorial explaining how to use photodiodes:

https://www.thorlabs.com/newgrouppage9.cfm?objectgroup_id=1295

You should read the whole thing, but I'll quote the relevant bits here.

If you use the 50 ohm termination they spec, then your bandwidth is 1/(2*pi*50*0.3E-12), or about 10 GHz, and your fall time is 0.35/10GHz = 33 ps. they spec twice that, probably because your wiring is going to add capacitance.

In your case, you are measuring about a million times slower, so probably you have a large load resistance and have added a lot of capacitance, probably from cabling. Try dropping the load resistance to 50 ohms and see if you get the result you expect.