Blue frenzy hits the holographer too

wler · Post by **wler** » Thu Jun 17, 2010 10:39 am

Hi Ahmet,

thanks for your message. Yes I will be testing more diodes the moment when they arrive, and also check the spectra with an SFPI when reconfigured to 445nm; fortunately I found mirrrors that seem to fit.

As for ECDL, well there are many factors going in and this was the just first attempt, and I will try other gratings and collimators. One thing that bothers me is that one should have a as-uniform-as-possible wavefront hitting the grating, which is very difficult to obtain. So far I collimated the output so that is was collimated along the fast axis, which is orthogonal to the grooves and thus seems to me more important than the slow axis. The slow axis is then not collimated well and almost certainly the back-coupling efficiency goes down very much. Perhaps some extra beam shaping within the resonator (eg by a cylinder lens) to compensate for the astigmatism can help. Remains to be seen!

All the best,
Wolfgang

Tom B. · Post by **Tom B.** » Sat Jun 19, 2010 7:48 am

For what it's worth, the very minimal official Nichia laser diode data is here:

http://www.nichia.co.jp/en/product/laser_main.html

Their spectrum (when given) looks pretty crappy at full power but as wler and others have seen with red diodes, it can be much cleaner at low power. I wonder why that is? Is it some thermal noise thing? Why do they go chaotic at higher power?

wler · Post by **wler** » Sat Jun 19, 2010 8:06 am

Tom B. wrote:...
Their spectrum (when given) looks pretty crappy at full power but as wler and others have seen with red diodes, it can be much cleaner at low power. I wonder why that is? Is it some thermal noise thing? Why do they go chaotic at higher power?

I guess this is a very complicated nonlinear system and it just becomes unstable and chaotic if the gain becomes larger.

The predominant pattern for most higher power diodes I ever have checked, is that they run predominantly SLM up to like 30mW, in isolated regions up to like twice of that, singular examples can even go higher. For ECDLs it is similar, problems start around 60mW where things become less and less stable and more and more multimode (weak extra modes start to appear), SLM regions becoming very sparse beyond 100mW or so. The practically useful domain is typically 60-80mW for both types of lasers; probably implying that ECDL is not worth the effort.

The surprising observation is that this holds for the blue diodes as well, despite they seem to be multi-emitter diodes and despite their power rating is like 1W (well, one diode tested so far, I got more now and will test those as well).

Tom B. · Post by **Tom B.** » Sun Jun 20, 2010 8:15 am

I'm unclear on what "multi-emitter" means. Is it just a bunch of parallel diodes along the the same die edge? Could you check one out microscopically (maybe with very low forward bias) to see what the emission site spacing is? I guess they could end up being coherent by injection locking - It's hard to see how they could be prevented from interacting in such a setup. I wonder with multi-emiitters if one diode is selected by design to be the master (how could this be done?) or if they constantly compete for the honor which could be a messy process.

wler · Post by **wler** » Sun Jun 20, 2010 8:45 am

Tom B. wrote:I'm unclear on what "multi-emitter" means. Is it just a bunch of parallel diodes along the the same die edge? Could you check one out microscopically (maybe with very low forward bias) to see what the emission site spacing is? I guess they could end up being coherent by injection locking - It's hard to see how they could be prevented from interacting in such a setup.

Me neither - and it is even not 100% sure what kind of the diode actually is. With the second diode I noticed a sequence of 5 or so dots to appear at threshold, when focused in like 5m distance. There are two gaps in between which could mean dead emitters. At more power all smears out and one cannot see the dots any more, but the gaps make dark bands. These don't go away when sending the beam through a spatial filter. As such this diode won't be useful for holography, it appears.

The first diode has no such gaps and works better in all respects (more stable and power). So perhaps the second diode is damaged. But at any rate, the beam quality for these diodes is bad, beam shaping just changes the overall profile but does not improve on the transverse mode structure. I still need to see how to optimally shape the beam and send it through a spatial filter.

One obvious point of concern was that the various emitters may run at different wavelengths. So I was carefully scanning the various lobes across the spectrometer slit. Once there is what I view single mode operation, then the wavelength of the peak does not move across the beam. However when at higher powers several peaks appear, then their mutual balance changes across the beam profile. So it seems that then the various emitters lase "unlocked". That happens above 60mW or so, which is substantially less than the full power the diode can achieve. In a ECDL setup the situation is not essentially different in this respect.

A remaining point of concern is that the emitters may lase at wavelengths that are too close by for me to resolve with the CCD spectrometer. Its resulution is a few pm and this translates to a few Ghz, so the coherence length will be at least a few cm. From my experience with other diodes (which may not be applicable here), it almost never happens that there would be extra modes with such a small spacing. But to settle this definitely one would need to check the lines with a scanning interferometer, but it turned out that the mirrors for mine don't work at 445nm after all, despite initial expectations. So this remains open for now.

I really like to find some literature explaining this kind of diodes in more detail.

All the best,
W

a_k · Post by **a_k** » Sun Jun 20, 2010 10:56 pm

Could it be that those two emitters just have a higher threshold current than the others and only start to lase later? But considering the dark gaps observed even at higher powers they might be dead emitters. Did you compare the total output at the same (high) current for both LDs? If the second one is considerably weaker, this might confirm the assumption of dead emitters to some degree.

I was wondering to what degree a "perfect" equalisation of the divergence in both directions would improve the beam quality. I think the spacing between the emitters could still be disturbing, even without dead emitters. Wouldn't the individual beams need to be combined somehow? Maybe like so: http://lib.semi.ac.cn:8080/tsh/dzzy/wsq ... 70-131.pdf

I don't understand why the emitters are synchronised at all at low powers. What is the mechanism of the synchronisation and why is it power dependant?

There is a website with some information about this LDs: http://www.krazerlasers.com/lasers/445nm/

Ahmet

Colin Kaminski · Post by **Colin Kaminski** » Mon Jun 21, 2010 5:37 pm

It is common to use a seed laser of high spectral purity to trigger another, broader, laser to resonate at a specific frequency. It could be that the emiters are seeding and locking to the same frequency. I am just guessing.

wler · Post by **wler** » Tue Jun 22, 2010 8:20 am

a_k wrote:I don't understand why the emitters are synchronised at all at low powers. What is the mechanism of the synchronisation and why is it power dependant?

I believe this is understood now! Very fascinating. See the thread (RedlumX is me):
http://www.photonlexicon.com/forums/sho ... post152034

-W

Joe Farina · Post by **Joe Farina** » Tue Jun 22, 2010 10:50 am

So it seems to be a single emitter? I wonder if that makes it more attractive for ECDL.

wler · Post by **wler** » Tue Jun 22, 2010 11:30 am

Yes, apparently single emitter. But running a quite non-uniform mode pattern, which may be bad if one wants a homogeneous illumination. The pattern seems to differ between diodes, one diode I tested is sort of OK but the other much worse. I need to see how a useful setup can be made. I was trying to clean the beam by focusing only part of it through a spatial filter, but this looses a lot of power. Perhaps splitting up the elongated beam profile into several, ie ref and object, beams may be a way to mitigate the highly asymmetric pattern.

As for ECDL, this didnt work well as of yet; while I got SLM powers close to 200mW at some point, this was very unstable. But I know from ordinary diodes that a very good collimation is necessary for good backcoupling efficiency, and for the blue diodes such a collimation is difficult because of the large astigmatism. That is, given an ordinary lens, one can collimate well only one axis but then the other is not well collimated. This can be remedied by an extra cylindrical lens. Thus perhaps adding such a lens into the resonator helps to get a flatter wavefront hitting the grating and to improve stability. Many things to try!

-W