Multiple diode question

[allthatwhichis]

Aren't you going to run into polarity issues with more than 2 beams?

[p1t8ull]

Aren't you going to run into polarity issues with more than 2 beams?

I'm using 3 polarised beam-splitting cubes to combine 2 beams at a time (making 3 beams from 6 diodes) then simple front surface mirrors to run those 3 beams as close as possible to each other, making a slightly triangular spot. I don't see this a problem as most reds end up either square, elliptical or a short line...

After testing the cubes and collimating optics (both of which yield significant losses as they are cheap surplus items) I should get >500mw of 655nm (with TE cooling) in a 3mm beam with 1mrad divergence - which closely matches my green and blue beam profiles.

It's engineering the optically stable beam combiner to hold it all together that's proving to be a brain strain

Shame I couldn't get my little pinkies on some 200mw diodes... one day maybe.

Pit.

[steve-o]

If one diode shorted, there may be a microsecond or 2 before the power supply compensates for the current increase (depending on the PS)-- blowing a few more diodes out!
(just my .02)
Steve

[The_Doctor]

If one diode shorted, there may be a microsecond or 2 before the power supply compensates for the current increase (depending on the PS)-- blowing a few more diodes out!
(just my .02)
Steve

Possible, but I doubt they'll fail that way. It takes a violent overcurrent to short a semiconductor junction. Even if laser diodes are more sensitive that most other types, you won't see that much increase when using a constant current drive. Even a spike won't last long enough to do it. What would happen is the energy density in the cavity would rise, destroying the front facet (OC mirror) in a tiny fraction of a second. (Not sure how fast, but microseconds or less, I think). The lasing would stop, and the extra energy that wasn't getting out as light would go as heat through the mounting. It could be an extra 30% or so, but not the tens or hundreds of times increase needed to melt the semiconductor junction. Also, I doubt it could be enough to cause cavity failure in the others in the chain. Although the output heat rises in the failed diode, it's current won't spike because there won't be enough change in the forward voltage, so there won't be a current spike in the other diodes if it's just a front facet being destroyed. Even if there was a slight rise in momentary current, it would probably be very short, not more than 50% rise at most above safe CW rating, so probably within the pulse rating for DVD writer diodes.

The one way a meltdown could happen before cavity destruction is if heatsinking was really bad. As the metal cases of series chains of diodes have to be electrically isolated from each other, this IS a risk in badly designed mountings, but it's still not the most likely way to fail.


p1t8ull, I like the idea, I'm wanting to try a triangular form too. But I think it would be cheaper and easier to combine the beams into two triangles with a pair of mirrors per group, then combine through a single polariser cube.

[p1t8ull]

I think it would be cheaper and easier to combine the beams into two triangles with a pair of mirrors per group, then combine through a single polariser cube.

I had thought about that way of doing it, and you're right, it would be cheaper and probably easier. If I didn't have the three 5mm PBS then I would've gone that route. Three of those little blighters cost more than I'm prepared to spend on cutting-edge cocking about

As I did have 3 cubes available, I decided to combine first, then merge in an attempt to reduce the distance the beam travels, making the design more compact and therefore (hopefully) more likely to stay in alignment. I have managed to squeeze 2 diodes, the collimating optics, one cube and a mirror within a 24x35mm area. The optics are quite large, so they occupy most of that.

Pit.

[allthatwhichis]

All this and no pictures...

[steve-o]

It takes a violent overcurrent to short a semiconductor junction.

That's a fairly broad statement, Doctor.
Apparently you've never worked with germanium diodes

Steve

[buffo]

Apparently you've never worked with germanium diodes

How many laser diodes use germanium as a substrate?

Adam

[illuzion]

Welp,
I see a few concerns here, my main one is that although it is possible to place diodes in series, and drive them with constant current, you are making one serious assumption, is that all of your Laser diodes have exactly the same lasing current threshold (point at which they will begin lasing), and that point is what sets the amount of current you can go up from there to achieve maximum output. If one diode has a lower threshold (very likely), you will end up with 2 options, #1, turn down the current so that the weakest diode is driving full output, reducing the efficiency (output) of every other diode in the chain since it is not able to put out full power, #2, the current it set at point where most diodes are outputting full power, if one diode's threshold current changes (easy as temperature change in most cases), you would probably burn optical cavity components (mirrors) within the diodes, and make an expensive LED. If you look at datasheets, the spec current vs actual lasing current can vary as much as 50%!!! (mainly on smaller diodes), hey why do you think most of them have a photodiode for optical feedback for as opposed to driving them constant currrent?? Unless you are sampling (optically) the output of each of your diodes and running a closed loop current control based on this, I don't think its a terribly good idea. Other issue is that as you add more diodes in series, you increase the forward voltage drop of the chain, this is a problem for modulation, and now the rise and fall time of the power supply is longer, due to a wider voltage swing, and that will slow down modulation.
Just my .02
illuzion

[steve-o]

Very well put Illusion!