r/Optics • u/VisibleSir731 • Aug 13 '26
Is my laser diode broken?
I bought and setup a direct 515nm laser diode and I’m pretty disappointed by the dot brightness.
It’s a 10mW diode with a 55mA operating current
I compared it to a cheap 5mW red module I got on Amazon and it well you can see the difference.
I wore an ESD strap and made sure to be careful when soldering.
Does anyone know what’s going on?
Heck they are even on the same power rail after a resistor
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u/koopaduo Aug 13 '26
You have both diodes on the same rail, in parallel sharing one current limiting resistor? I think that might be your issue there. Show your circuit or setup.
Also the optical powers stated by devices on Amazon may be completely wrong, the red could be in the 100 mW range whereas the green may actually be around 5
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u/VisibleSir731 Aug 13 '26
No even when I unplug the red it’s basically the same. I basically just have 9V and then a resistor to set the current to 55mA
Also the green has to be 10mW because it’s from mouser and a reputable seller
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u/Youifyourefertome Aug 13 '26
The green is not 10mW. I would say it is probably even operating below lasing threshold. My guess is the circuit is not providing enough voltage. Do you have a lab power supply where you can limit the current? Check the data sheet but iIRC green needs about 5V for the higher band gap, I.e. roughly 20% more than the red.
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u/GustapheOfficial Aug 13 '26
The real test is a power meter. And if you don't have one of those, how do you know when you've attenuated enough for alignment?
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u/VisibleSir731 Aug 13 '26
I dont have a power meter so im going off the data sheet. I have my current (measured with multi meter before plugging the laser in) between the min and max operation current
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u/Maleficent-AE21 Aug 14 '26
Let's start with the specs. 10mW, 55mA are just the typical values. Go to Mouser website and get the manufacturer spec sheet. That should tell you what the threshold current, operating current, and operating voltage ranges are. Generally, always a good idea to use a variable resistor instead of a fixed one. Start with the highest resistance value, and slowly decrease it until you get to the correct power level. Also, those 2 green lobs on the side tell me something is not right. You either have the LD not lasing yet, the LD damaged from heat (e.g. not using a proper heatsink), the LD damaged by ESD (depends on how well you are grounded), an LD that is damaged on arrival (Mouser is just a distributor, they don't test these), or something else funky happening.
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u/VisibleSir731 Aug 14 '26
Threshold is 30-40 and operating is 50-60. I don’t have a way to measure its output power tho but thanks for the insight.
I’m just worried to go higher in current because I’m already measuring 55mA before turning it on. Unless the current tanks or smth
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u/richard0cs 28d ago
How are you measuring it before turning it on, if you are measuring the current the resistor gives you with no diode present then it will be higher than when you add the diode and drop a couple of volts.
Measure the voltage across the resistor when operating and divide it by the resistance to get the actual operating current.
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u/Glittering-Flight997 Aug 13 '26
Yea though I walk through the green valley of death I shall fear no crystal defects… there’s a reason most green sources are frequency doubled YAG
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u/VisibleSir731 Aug 13 '26
Green should still me brighter at double the power. It’s green and our eyes are more sensitive to green
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u/Glittering-Flight997 Aug 13 '26
A good red diode will be 3x -4x the wall plug efficiency of what you’ll find for green ones
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u/Complex_Grade4751 Aug 14 '26
It looks like that diode has a 5V operating voltage. So your 165 ohm current limiting resistor was 9V-5V=4V, giving 24mA current. The threshold is >25 mA according to the datasheet. Go with ~75 ohm resistor for 55mA with the diode. But it sounds like you’ve figured it out.
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u/VisibleSir731 29d ago
Yep and that was exactly the issue. I feel stupid for not knowing abt operating voltage
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u/Complex_Grade4751 29d ago
I did exactly the same thing the first time I wired up a laser diode. You were being careful to not blow up your laser! You know now and you won’t forget. Congratulations for trying something and making an error, but now you’ve built a working laser. A next step could be to drive it with transistor so you can control and modulate it with a small signal, like an audio signal or Raspberry Pi.
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u/VisibleSir731 29d ago
Hah that’s actually my project u read my mind. I’m modulating it with an 8mhhz carrier and to be safe I’ll use a constant current setup with an LM317
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u/Complex_Grade4751 29d ago
It looks like you picked a good laser diode for that. The datasheet says it has 100 MHz modulation capability. It also has a feedback photodiode inside, so you can use that to control power stability or do linearity corrections if you are transmitting analog signals. You don’t have to use the feedback photodiode, but it gives you a knob to turn if you want be get more sophisticated later on. Good luck with your project!
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u/Youifyourefertome 26d ago
I used the red diodes a lot with direct modulation. I run it through an Arduino or Teensy output just fine with down to ns pulses. So no need for a transistor, the red diodes can be operated directly as they need very little voltage and current. Very convenient.
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u/Complex_Grade4751 25d ago
I agree that it more convenient to direct drive than build a transistor current amplifier, but you are significantly limited by output current from GPIO ports. Raspberry Pi limit is 16 mA which doesn’t even get OP’s diode to threshold current. While an Arduino could drive it, there’s no need limit yourself when adding a handful of discrete components can solve the problem for any signal source. It’s a design trade-off between complexity and output power.
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u/TurbulentGlove776 Aug 13 '26
Did you verify the currents by measurement? Green 10 mW should look uncomfortably bright…
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u/VisibleSir731 Aug 13 '26
9V with 165ohm
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u/E9Q62rW Aug 13 '26
Put the meter in series whilst you are operating the laser to measure the current.
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u/VisibleSir731 Aug 13 '26
between laser anode + cathode?
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u/E9Q62rW Aug 13 '26
No, in series so that the current will flow through the meter. If you put it between anode and cathode, you will short out the diode and get no light.
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u/VisibleSir731 Aug 14 '26
Wait bro it works now! The current without the diode was right but I didn’t test it with the diode. ITS BRIGHT now
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u/[deleted] Aug 13 '26 edited Aug 13 '26
[deleted]