Archive for June, 2010

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.