… been a busy fall

December 5, 2010

Well the title says it all, I have spent the last couple of months in developing/building/testing a new series of driver boards, and complete ECDLs. While the boards and red ECDLs run very well, the 445nm ECDLs still make headache for stability reasons; 40-60mW can be done pretty well but everything beyond that is hard to keep stable over extended periods of time.

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!

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General Technical Discussions

October 24, 2010

The original post was about the closure of the holography forum, which has now been replaced by the holoforum created by Ahmet.

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.

Blue frenzy hits the holographer too…

June 12, 2010

There has been for the last 10 days or so an incredible frenzy over the 1W 445nm Nichia diodes that can be harvested from certain projectors; see the turmoil at the various light show and ballon popper forums. Well the holographer, being not particularly interested in using his lasers as lamps in discotheques, or in wielding 1W laser pointers in public, is more interested in the mode stucture of these little beasties. So I had to get a few of those, and today, within the hour of receiving it, I set it up and checked the mode structure with my CCD spectrum analyzer (due to lack of a 1 Amp capable driver I had first to mod my ol’ proven SDL800 such as to allow it to run blue diodes with higher forward voltage). And voila – without any systematic search, I found that the diode runs single longitudinal mode up to 55mW (with Lens-27 at 221mA and 15.4C) ! This is truly exciting, not the least because the diodes are transverse multimode and apparently have many emitters. Here is a quick sample overview:

Nichia-spec

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.

ECDL update, etc.

March 21, 2010

After I was done with my free-running diode lasers, I was focusing on building a robust ECDL as a prototype and reference model. Some preliminary info is here. Concurrently I was playing with the remodelled commercial ECDL described here. Here a pic of the latest prototype:

ECDL proto

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.

Lasers and driver boards for sale

September 4, 2009

Finally, after many weeks of extensive burn-in and long-term mode stability tests, I now have a few lasers ready to go, as well a couple of highly stable diode and TEC driver boards for DIY holography lasers; details are here. Some of the lasers were sent off today for testing in real-life holography. I will report when I know more. If you are interested, contact me.

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.

Opnext HL6385DG….

June 24, 2009

I just got a few of those 150mW/642nm diodes, which are characterized by “single longitudinal mode” in their data sheet. There was a special price for them during this months; and also a group buy at photonexicon.com.
Obviously of potential interest to holographers…the warm color would be well suited for color holography, and it would work well with optics intended for HeNe lasers such as waveplates.

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.

ECDL reloaded

June 3, 2009

My first attempt at an external cavity diode laser (ECDL) didn’t work nicely at all, it didn’t run single longitudinal mode and thus was completely useless for holography purposes. Recently I just put in other types of diodes, notably a high power “open can” DVD burner diode, and immediately things worked out perfectly! See for details here. I wonder about the great difference – perhaps it is just the missing output window of the diode?

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.

Prototyping.. updated

April 24, 2009

Well the last few weeks were quite productive and not only I completed a series of measurements, but I also finalized the development of a stable laser diode driver and TEC controller. All went very smooth without major obstacles, apart from a surprise with trim pot noise.

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.

Open can and blu-ray DVD diodes…

April 4, 2009

I have acquired some more laser diodes for testing their suitability for holography.

The first thing to try out was to measure the mode spectrum of one of those high power “open can” DVD burner diodes, which can do several hundered milliwatts when tortured. Well, the news is –not unexpected– that those diodes are totally unsuitable for holography. see here. Moreover I found the the familiar Sony SLD1236VL and SLD1239JL-54 diodes a little bit better, but not too much.

Summarizing, for all DVD diodes I tested the rule seems to be that they can run single mode at up to 70-90mA, which yields approx 20-30mW. There may be exceptional stable spots higher up, eg. I got approx 50mW with one Rohm diode, for example. The best diode so far was the Mitsubishi ML101J27 which got up to approx 80mW (with some weak other modes; true single mode up to approx 50 mW). Thorlabs sells it as single mode diode, probably for good reason, however there is no mention of this in the data sheet.

There is a number of declared single longitudinal mode diodes, esp from Opnext/Hitachi, some with powers exceeding 100mW. Most likely they would be the way to go, I will try to get a handle on them and do some measurements; unfortunately the more powerful ones are quite pricey. But it seems there is no easy way around that if one aims for more than 30-50mW.

Note added: I now checked a GH04P21A2GE/PHR-803T “blu-ray” diode and the results are here. In a nutshell: totally unsuitable for holopgrapy!

Finally…

February 7, 2009

.. I found time and will to put together laser diode driver, TEC controller, data conditioning circuitry and USB interface in one case. One of the DAC’s had been damaged when cleaning up the mess of circuits and loose wires, and the prospect of replacing that tiny SMD device (with the danger of making the whole PCB unusuable) had reduced my incentive to go on with this project since two years…well there were other reasons too. But it wasn’t that bad after all. So now everything works again and I go on developing a stable single mode red diode laser suitable for holography.

Next thing was to rebuild Colin’s laser head with extra emphasis on thermal stability. Just right now automated scans are performed for determining the best operating point (current and temperature). On the pic above you see the laser head in front of my universal diode controller incl digital measurement machinery.

I now also performed long term stability measurements, and posted the results on my revamped web site. After a few minutes warm-up time, the laser runs stable single mode without jumps for hours; the price is a moderate 32mW output compared to the 100mW+ the Rohm diode can do.