1) Alleged new, cheap 635/638nm diodes on ebay. These were actually 658nm diodes and also not new. I returned them and so far am still waiting for a refund…..

2) Mitsubishi ML520G71 300mW/635nm laser diode. Unsuitable both in free-running and ECDL mode.

3) HL63133DG 170mW/638nm laser diode. Excellent in ECDL mode, so far the best diode! Clean stable output beyond 100mW can be achieved. See the pic at 635nm and around 100mW:

4) Update: The diode currently sold as Opnext HL45023TG on ebay turned out to be the PL450 from Osram. It has quite similar data, so most wouldn’t complain, but for purposes of single longitudinal mode operation there may be a major difference, potentially. Let’s hope for it, because the diode seems pretty useless for holography. I found it completely unsuitable in free running mode, and (after a lot of tinkering) quite OK in ECDL mode. Initial attempts produced less than than 20mW in single mode operation, but with some optimizations more than 40mW was achieved. And the beam quality is good, especially compared to the common 445nm/1W diodes which do not provide much more power in stable single mode operation.

5) Update2 (Nov 6):
Here a brief movie (2MB) showing single mode operation from the CCD spectrum analyzer at around 40mW (popping sounds indicate mode jumps):

There was a moderately good ebay deal to be had and so I decided to bring a Coherent Sapphire 488-20 laser head plus driver board back from my trip to the US – along with 20kg of other laser goodies . This is an interesting, though not too powerful optically pumped semiconductor laser (OPSL). The board looked new but the head was used and sold as “probably not up to specs”. Indeed so, I measured 9.6mW instead of 20mW, but I figured out, by some reverse engineering, how to bring it up back to specs, ie, 20mW with clean stable single longitudinal-mode operation. Adding a housing for the driver board and a few analog and digital controls, et voila… system ready for holography! For details see here.

As soon as I have found a good way to have the die cast aluminum cases painted, I will sell off a few lasers, to make room for new projects. Here some eye candy:

PS: Drivers are sold put, most lasers too. I am not sure
whether I will build more, as the effort is barely worthwhile ;-( Just in case, keep an eye on my blog!

I changed the title and topic of the thread, as various general and interesting comments were made that had nothing to do with the closure of the forum. I think in this way the information will be easier accessible.

From top to bottom the current was 221mA, 225mA, 300mA, and 400mA. I didn’t turn the diode up to more than 500mA where it was yielding 430mW (with non-optimal Lens-27), because single mode cannot be expected at high powers anyway. Note the beautiful single mode at 221mA!

PS1: I now was completing the first systematic landscape scan and the result is shown here. A sweet spot at 233mA and 14.9C yielded 64mW! I tested also an ECDL setup but so far the power wasn’t significantly higher, but this can probably be improved as well.

PS2: A first first test of an ECDL didn’t pan out quite well, there are many parameters into this game indeed and perhaps things can be improved. Indeed a test with better collimator, different grating and careful adjustment, more than 160200mW single longitudinal mode was achieved, but this was quite unstable. Stay tuned for more results.

In fact beam quality appears the main downside of these diodes and may be the deciding factor of their suitability for holography. Firsts tests of shooting through a spatial filter were the opposite of exciting…. probably, for a holography application, best would be to knife-edge split up the beam and use the pieces as object and reference beams.

PS3: Right now I was spending time in designing a higher power version of my super stable driver boards, which would be optimized for ECDLs using the 445nm diodes. Since I was running out of the HY5600 TEC controllers I was forced to cook up a completely new circuit, this time however PWM based and with readily available parts. It runs at least as well as the old circuit.

PS4: I now verified reports here and here that even a simple glass plate can give sufficient feedback in order to provoke stable single mode operation; indeed the results look promising, see here for details.

Below a pic of a test setup with glass plate feedback:

PS5: After a couple of weeks where I have spend quite some effort to try various different configurations for glass plate feedback, the following scheme seems to emerge. Within limits the amount of feedback does not matter much, what changes is how much the threshold is current is lowered. But with a reduced threshold also the maximal current for stable or single mode operation is reduced. It turns out that no matter what I tried, the maximal power for stable single mode operation stays always at about 60-80mW. This is not much different from the free running diode (except that the proportion of single mode zone modes is higher and the general stability is much better).

Also with grating feedback the generic situation is not much different, but there are lucky expections. But even there higher power operation is quite a gamble.

So here a brief summary: single mode operation with the blue Nichia diode is generically possible up to 60-80mW, quite independently of the setup. With sacrifices on stability and mode purity, more than 200mW are possible, also relatively independent of the feedback. However, there is then a tendency for several supermodes to lase simultaneusly. For an extended summary see here.

Both lasers work reasonably well, but the main problem ist that at higher powers, the zones of single mode operation of typical DVD diodes becomes very small and thus the lasers can drift out over time. To combat/control this problem, I had developed a noise detector which works very well for free-running, non-ECDL lasers. As for ECDL lasers, I found that they sometimes run multi-mode without the presence of extra noise in the light output. This would be of course an important problem for day-to-day use, and I am spending most of the time with trying to get arund it. It becomes worse with increasing power, and problematic beyond 70-80mW for common diodes.

It seems that one needs to very carefully select a diode, adjust the operation point and the optical properties of the resonator. Particularly well behaved are the diodes that also work well free running, like the ML101J27 and the HL6385DG. It seems that one can typically achieve up to approx 80 and 100mW, resp., in relatively stable single mode operation. But I need to spend more time investigating; there is a lot of variation between individual diodes and the number of parameters in the game is so large that I don’t yet have a clear picture. Moreover I am running out of the Opnext HL6385DG diodes I got moderately priced via a group buy.

Other things I was busy with were finding better collimators; so far I mostly used the familiar Lens-27 from Roithner, which has an extremely high efficiency but a relatively poor beam quality; some holographers had complained about this. I found that the G-650-1 (obtained from here) has a much better profile while having only slightly less power. Some of my findings are here.

The boards are 79€ each plus shipping, with suitable Peltier element and thermistor 4€ more. The 640nm lasers go for 275€ and the 658nm between 180€ and 200€ depending on the power.

Note added (Jan 10):
I got some more lasers, the new ones have a noise detection feature built in, which is supposed to help to detect instabilities due to backreflections etc. More details are here. The extra for each laser is 20€. I sell separately the ready-to-use boards for 20€ as well, you can use them with your own lasers (a glass plate beam splitter is needed in addition).

Note added (Feb 15):
All lasers are sold out, I am not sure whether I will build more of this kind anytime soon. Right now I am looking into ECDLs to see whether they have a substantial advantage over bare diodes such as to justify the higher costs. So far it seems that the achievable stable power is not much higher as compared to bare single mode diode lasers, and the main difference, a very small line width, is not relevant for ordinary holography purposes. Stay tuned.

I immediately ran tests on one of them, with disappointing results at first, but subsequent more careful tests revealed single mode regions at high power and low temperatures, see here. This now seems very promising and I will test more of those diodes, also in ECDL setups – stay tuned.

PS 09.09.09: I found, not unexpectedly, that the HL6385 does fine in an ECDL setup, for details see here.

I also used another sample of those diodes in an commercial ECDL laser that was originally designed for 780nm. And voila, it worked again very well. Some info on this (quite simple) conversion of this laser is here.

Finally, I wanted to see how a “blu-ray” 406nm diode works in an ECDL setup; before I had found that the bare diode by itself is totally unsuitable. I found that in my ECDL configuration the diode can run single longitudinal mode but only to powers of approx 15mW. See for details here.

PS. Aug 28: I was playing with a Mitsubishi ML101J27 in an ECDL and it worked fine .. until I changed polarization and the stronger feedback killed the diode despite I thought was careful. Good that I learned this lesson with that diode and not with an Opnext HL6285… one lesson is to determine the diffraction efficiency of the grating, for both polarizations, and then estimate the maximal drive beforehand. But there is no really safe way to find out what the maximal power of an ECDL is. At any rate, I plan to try an Opnext soon.

I now have received the PCBs for the drivers, and in an evenings’ work eagerly completed the first prototype:

Next thing is to check that all still works properly and conduct extensive measurements. If all goes well, I would be able to produce 4-5 complete units plus a few drivers separately, and put them up for sale. Please contact me if you are interested.