What is this thing

ive seen these on many lasers. What are they? ive seen IR filters and have added them to lasers. is this a type of IR filter?

another view

It's an IR filter mounted at an angle after the crystals. It is used to reflect some of the light back into the photo diode. The photo diode is the thing that the two wires go to, from the driver (PD+ PD- contacts).

This way, the driver circuit can adjust the current through the pump diode to maintain a constant output. It's an interesting design. Usually, if APC (automatic power control) is used, they just use the photo diode, that is integrated into the IR pump diode. But that just regulates the amount of IR, and the green still changes with the temperature (as the crystals change in efficiency).


This approach could theoretically keep the green output constant (well, the mixture of the green and some IR, if they wanted to regulate the power of the green alone, the PD would have to be after the filter), but only if the laser is low powered and doesn't heat up too much. If the crystals would lose too much efficiency, and the driver would try to compensate with a higher current through the diode, it could actually make things worse or even kill the diode if there are no upper current limits built into the circuit.

If the laser is built from components that are used below their maximum ratings however, this approach can work very good. It's rarely found these days, used to be in some older pointers or so i heard.. Where did you find it?


There are some New Wish new arrivals at KD that would seem to use this same type of regulation. At least it looks like it from the picture, but they usually use one pic for all power lasers of the same type. It's probably only present in the lower power models again. When a laser is operating at the upper most limits, the best thing to do is to maintain a constant current through the diode, and the rest depends on the crystals and the heatsinking.

ohhhh...... I get it... thats smart. i have a few of these... all newwish. i got the pics from DX site, since my camera has gotten really shitty at close photos as these. Thanks for the info!!

I brought this up a few months ago.  Still don't truly know it's benefits of it's existence.  I've removed these from some of my DX 30's and 5mW pens and 5mW modules.  Although it has an apparent IR filter, it still leaks quite a bit.  And once I removed them, it seems as the power increased just a tiny bit, probably from removing the filter and stopping the loss through reflection.

Yeah, an IR filter blocks some green as well.

What i find interesting is, that your lasers still worked after that... Obviously, it is yet another copied design, where the circuit is hacked not to do much but drop a little voltage, and all the components on it are there just to "warm up".

It's funny, that they copied everything. I mean, the IR filter would be enough, no need for it being at an angle or the wires and the PD, if they don't use the photo diode for regulation.


Unless of course there is another reason for the power increase you noticed.. Without a PD, the circuit might try to increase the current (because it's trying to get a feedback from the PD but can't) and the built in upper current limit stops it at the max.

I'm just guessing here, as i haven't tested this myself yet, and i really don't know which possibility is more likely.


Do you still have any of these that you took out?

while your on the topic, if you shine a 532 nm green through a KTP does it turn back into 1064nm or does it just stay the same or somethinf else ??

Nothing happens. It stays the same.

The wavelength range in which these crystals are efficient at conversion is VERY narrow. So narrow in fact, that if the wavelength of the diode changes too much with heat, the power of the green output can drop rapidly.

If these crystals could convert any wavelength, we could have any color lasers at high powers, but the crystals for blue for example are different and have very low efficiency at the moment. The crystals for yellow are even worse. That's why you don't see powerful blue or yellow pointers.


You need a much more powerful pump to achieve the same amount of blue, and even more for yellow and temperature stability is even worse than with green lasers. I don't know the specific numbers tho.

brewster's angle

If one wanted to disable the PD how would you do it safely?
My LG Galileo has the same IR filter & photo diode setup and I wanted to see if disabling it would increase power. Could I just cut the wires connecting the PD to the circuit?

cut either or both to disable APC

RA_pierce said:

If one wanted to disable the PD how would you do it safely?
My LG Galileo has the same IR filter & photo diode setup and I wanted to see if disabling it would increase power. Could I just cut the wires connecting the PD to the circuit?

Click to expand...

You can not know how this will affect the output! Why would you do that? It could even kill your laser! I don't know what kind of laser it is, but i would imagine it is expensive.

What i wrote above were just my guesses about the circuit. If you don't know exactly how it works, you shouldn't cut anything.


If you want more power, the only smart thing to do would be to measure the current it is using now, build your own driver and increase the current a little. Too much, and you can kill the pump diode. Besides, why would you want to remove the IR filter? It doesn't really affect the brightness that much and you probably don't want to shine IR all around.



If the driver has an upper current limit, cutting the PD wires would make the diode run at that maximum current, but to be really sure, it would be better to go with a constant current driver. What power diode does your laser use?

It's a Galileo, so it was expensive ($80). But It was already broken so I just use as a laser to experiment with. It only outputs ~1mW. It was pot modded and output ~50mW but I cooked it in the sun...

Anyway, I cut the connection to the APC and it didn't make any visual difference at all.
But I can't determine whether cutting it really would have made a difference because it was already broken to begin with.

Oh if it's broken, than it really doesn't matter, i guess.

If you modded it, you most likely killed the diode or burnt the crystals. But you can get 50mW green lasers for cheap..

One of my favourite lasers actually looks a little similiar to the Galileo. Must be a cheap copy... I'm talking about the "KD50" (EDWINA) laser. I have three, any my best one is putting out 74mW, another one did 47 and climbed with heat to 60, the third was bad, only 25-35mW. Unfortunatelly i killed the weaker two by carelessness.

But for the price, it's a great laser, even if you have to buy two to get a good one sometimes. Powers under 100mW are easy to get for cheap. Over that, it's usually just more expensive and has more problems, but not really much more powerfull.

The KD50 has some smaller design problems that are easy to fix, but for a better quality build, you could look into the New Wish new arrivals on KD or Rominsen and similiar on DX.


My #1 most favourite laser is of course my CNI 150mW (179mW peak, 169 average, equivalent of Nova X-150).



Anyway, what i wanted to say was: If you want a more powerful laser, buy a more powerful laser.

I don't think the PD regulates any current in the DX models. The power difference is very min. Like when you compare the dot through perfectly clean glass and without. I think it's just the beam being less obstructed and slightly IR filtered. My DX "5", I call it, should have been sold as a 30 or even 50. I haven't yet taken it appart, but when I do, I anticapate I will find the PD in it. I call it 'DX "5" ' because the thing is WAY overpowered to be a 5, burning thin black plastic, poping balloons @ 1' @ 5sec.

The "5"mWs vary in power a lot..

There was a guy complaining on DX, as he wanted to sell 5mW greens, but couldn't rely on them having the correct power.

If the manufacturer really wanted them to be 5mW, they would have to turn the power down. It would also be very stable around 5mW in that case due to less heat. And the people who are obsessed with pot "modding" would finally have a laser to do it with again.