Photodiode ThorLabs DET08CL not outputing what's expected

Thread Starter


Joined Oct 18, 2023
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 - 1Hz 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. 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.
  • What I thought might be affecting the results:
    • Optical feedback resulted from the metal finishes on the input side of the photodiode.
    • I used high load impedance at the input to the scope. Maybe I should use 50 Ohms instead.
    • The photodiode is saturated.

Thank you in advance!


Joined Jan 19, 2021
I don't know all that much about a Laser diode but could it be that once it starts it looks like a short to the supply and shuts the supply down? Again, just shooting from the hip here, but can you put the laser input on the scope with the output of the detector just to see if the on time is the same, etc. May not be what you are looking for but maybe more to verify what you're doing? Again, I may be way off the mark here, I don't know much about laser diodes so take it with a grain of salt or maybe a couple of them? ;-)


Joined Jul 5, 2018
If you just leave the diode on (enable on) do you get a continuous output from the laser diode?
If the laser diode has automatic power control, then it could be shutting down from reflection off the photodetector. But I suspect the apc would take way longer than 2 ns. One way to check this would be to separate the laser diode from the photodetector by some distance. Then angle the photodetector slightly so the reflected beam misses the laser diode. The laser diode should then function as expected (assuming there aren't any other issues).
Doubt high impedance versus 50 ohms would make much difference.
I'm also not a laser expert...
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Be sure to check the obvious: the voltage on the 12V bias battery and make sure it is installed in the right direction. The data sheet for the sensor says that for fastest response load it with 50 Ohms which you can do by using the 50 Ohm input on your scope. It will still work with 1Meg input on your scope, just slower. The DET08CL data sheet shows 0.8 to 1.7um response. You could try a simple IR LED (855 or 940nm) in place of your laser diode to get your setup operational. You could drive it from a signal generator or simple 555 timer maybe with a 1 kHz square wave. Once that works, you will have confidence to test with your laser diode.

Art Mezins

Joined May 26, 2019
First, you should always include at least a rough schematic of what you're doing so "we" can see what might be going on. The 50 ohm load is typical of high-speed anything, but optical sensors tend to saturate like a capacitor when hit with a lot of power. Since you can't see the laser's output yourself, make sure it's getting power, so scope the laser diode lead and see what that does. The scope pics are too blurry for me to read and I'm unfamiliar with that specific scope, but it seems that the first (left) pic is on 1ms/div and the second is 200ns/div. If you're not actively discharing the laser, it too has a charge that must deplete which could add to the slump in the off direction. First, see if you can set up the scope to look at the kinds of pulses you expect from the scope using a test setup that is just a fast signal generator. Use that as your bench mark, then try it with you laser test setup and see how close they are. There should be lots of information available online about high speed testing (someone just won a Nobel for attosec pulses!) so check those out. Also, scope manuafacturers AFE are a good source for help, even if you didn't buy the scope from them (they like bragging rights). Just don't expect the answer to jump out at you, you need to search for it! And never take anyone's word for it -- do your own verification and then verify it AGAIN.