LD blowing up fun

[stanwax]

To rule out spikes try running on batteries. Feed the battery power into your driver board and see if this improves the kill rate. Traditonally LDs were anode to the case so if you did ground it the worst that would happen would be that the PSU would be shorted out - low risk to the diode. The trend with DVDRW diodes at least is to make the case the cathode. This means you must be more careful as most drivers (all that ive seen) still control the cathode end of the diode. So if you do earth the case you will kill the diode in a blink. I think this also makes it a little more likely to take a beating from static during operation.

good luck and keep us posted

Rob

[The_Doctor]

My problem had nothing to do with static, or spikes. I solved those problems. I just posted about a very different vulnerability, to retroreflection even under remote circumstances. The vulnerability is extreme, so much so that I'd have thought that a DVD surface might reflect enough light to cause failure, let alone using diodes in unintended ways.

[yaddatrance]

Actually you hit the nail on the head, what you need is just a quarter-wave plate combined with a polarized splitter...

If you just want to play, here's a link to how to (literally) "roll" your own plate... (It's an excerpt from a December '77 Scientific American)

http://www.sas.org/E-Bulletin/2003-0...esAS/body.html

[added] I'm feeling insomniac so I drew a diagram...

[The_Doctor]

Very cool.
Would this protect a bulk output if the beam was mixed polarization after beam-splitter recombination? I'd hate to think what it would cost if I had to do this to protect each diode independantly...

If the source beam was based on recombination with a splitter, it seems the light could get from one diode, to the facet of the other, then return via second mirror bounce to destroy the source cavity. A long shot (literally) but judging by what I've seen, it could happen.

Another thought: Might the returning light be unable to destroy the cavity even without the polarised beam splitter? As it's returning in the other plane, I'm wondering if it might prevent it from forming an extended cavity at all, which I think was the reason the facet was destroyed (the energy density rising even across the metres long space, enough to destroy the OC facet of the diode).

[yaddatrance]

As far as I can tell, the only lasers diodes I've ever seen that needed protection were the reds->IR... bloth blue and green are pumped so they're just peachy... I suppose the nichias might need it, but I haven't really gotten the time to play with one... So practically speaking, you only need one... If your laser is low power enough, you might be able to get away with an absorbing polarizing lens which are much cheaper than a PBS.

This will not work for non-polarized beams... They DO make wonderful premade prisms which do the above and more, but they cost arm+leg...

In terms of damage, it's because the driver is current controlled and has no optical feedback. The problem is the density of light in the cavity. The return beam doesn't have to reinforce to cause damage because it's already powerful.

[The_Doctor]

Actually it's not the PBS cost that bothers me now, so much as the HWP.
(We seriously need a devil smiley imho..)

I looked up a half wave plate in Edmund Optics, and the cost is something no-one should view unless comfortably seated, and planning to spend someone else's money.

Most times in the past when I asked this question, the advice was always 'avoid back reflection'. Seems about right. My plan is to spend time and effort making sure that fast and easy diode replacement is possible, and to lay in enough stock for that.

[jeejeedr]

Maybe a very stupid idea, but something that may improve the effect without the dramatic cost: either a beam splitter cube with only the faces anti reflection coated where the beams enter (so where it should leave in normal operation ) or a glass plate where the diode faced side is anti reflection coated and the other side is highly reflective.

This would reflect partly the reflected beams that would otherwise enter the cavity.

[The_Doctor]

That's actually a good idea, but because light will usually travel back on the path it came on, if relected back, you can't exploit that idea without rotating the polar plane, and that's the difficult and expensive bit. A polarising beam splitter will do the bit you're thinking of, if that rotation is done first, by 45°. (The return trip turns it a further 45°, totalling 90, so the splitter deflects it instead of passing it). It's also sensitive to wavelength, and as diode wavelengths vary with heat, and between diodes, the task might be impossible, or at least inefficient.

One thing I've wondered about is whether reflection itself at certain angles can rotate a polar plane, but I don't think it can. I don't know enough about optics to be sure what effect reflection has on polarised light, other than varying degrees of reflection if the light is strongly polarised already.