Advanced Knife Edging

[planters]

I have been wrestling with the knife edging of a few 445nm diodes that pass thru an anamorphic prism pair as the expansion "correction" optics. For convenience and to eliminate an extra reflection in the optical train, I set the pair to deviate the expanded beam about 10 degrees in order to catch the next mirror in the beam path. I noticed the typical lll generated l l l as expected, however the order was mixed, yet not for all the groupings ( I have four groups of three diodes). Furthermore, the total expansion width for each of these groupings was SUBSTANTIALLY different. Some barely looked expanded and others wouldn't fit the scanners. But, the far field beams were identical. Curious, I measured the wavelength of each of the diodes and that explained it. The range of wavelength was large, 10nm and the groupings that showed the enhanced performance had a steady and continuous change in wavelength from left to right while the worst reversed this trend and the mixed were, well mixed.

Adjusting the prism pair for zero deviation and the effect disappeared. So, by repositioning the diodes to take advantage of this wavelength effect I have been able to do what I found to be impractical with the red diodes and that is wavelength multiplex. The reds vary little in wavelength without temperature control. However, with a sufficient number of diodes to select from, ordering them correctly according to their wavelengths and then adjusting the deviation of the prism pairs to produce the necessary paralleling of the beams that are slowly converging onto the prism, I believe it might be possible to combine an almost limitless number of blues onto a fixed scanner size. I need to check the wavelength variation of the M series as this was studied in the A series and I don't know if this amount of variability is present in the more powerful diodes.

This is pretty significant.

[logsquared]

That is an excellent observation. Do you find the wavelength stays roughly the same throughout the current range and temp. variation of the individual diodes?

[polishedball]

I'll work on posting up some pictures but I have (3) P73's knifed and corrected. The proof of concept was just tore down to build it out as a module not just breadboard. I do TEC cool. The combination of 6 diodes can yield 5W and land on oversized mirrors.

Planters at SELEM we discussed some of these ideas including how you stated stacking them instead of laying them next to one another. The stacking was a fail near field for me.

Anyhow not trying to hijack your thread, but I was playing with this as well.

Are you using prisms or mirrors for your knife edge? I was not happy till switching to prisms to get a lot better and tighter edge, eliminating the small line between.

[planters]

Prisms only.

With the diodes thermally grounded to a base plate and not actively cooled, the property remains visually unchanged all the way to full power. This makes sense as the wavelength variation can not be so wild that hot and cool diodes in a projector will shift to the same magnitude as in 10nm or more. In additi9on, the published delta lambda with temperature is 0.02nm/C and I also found this in early experiments with cooling, they just don't shift. The only limit I can see is that eventually the line width of each beam will eventually come to dominate and the deterioration of spot size due to dispersion will equal the benefit due to the increased allowance for near field beam expansion. But, this is not detectable at the three diode level.

[badger1666]

I would be keen to learn how you get 3 p73's together and still keep them on
Oversize mirrors ? Do you mean like big mirror 5mm beam dt 40's

I'll work on posting up some pictures but I have (3) P73's knifed and corrected. The proof of concept was just tore down to build it out as a module not just breadboard. I do TEC cool. The combination of 6 diodes can yield 5W and land on oversized mirrors.

Planters at SELEM we discussed some of these ideas including how you stated stacking them instead of laying them next to one another. The stacking was a fail near field for me.

Anyhow not trying to hijack your thread, but I was playing with this as well.

Are you using prisms or mirrors for your knife edge? I was not happy till switching to prisms to get a lot better and tighter edge, eliminating the small line between.

[krazer]

I have been in contact with a few people using the (pretty large, as you said I have seen ~15nm variation in XJ-a130 diodes) wavelength variations, and having some luck with them, although I haven't seen anyone try using prisms to combine diodes of different wavelengths yet. I would be careful though, because the wavelength definitely does shift with power, I generated the below plot for diodes in the stock heatsink

Which shows a ~4nm shift going from .5a to 1.5a

[planters]

Well it does work as I said at the three diode level. Maybe this is due to the parallel nature of the change as your graph shows, but I am surprised as my measurements when chilling to -30 C showed a barely detectable shift. It may be due to a non-temperature effect as the power varies. I suspect that this is the case. Did you look at line width vs current? I am curious as to the real world line width in any case, but maybe the lambda your seeing if unresolved, is the recrutment of longer wavelength/higher threshold regions within the diode that are also responsible for higher mode operation. What do you think?

[krazer]

I am fairly sure that it is mostly due to the current, which is to be expected for a semiconductor laser due to the gain (and its wavelength dependence) dependence on the carrier concentrations. I have also seen that there is virtually no temperature shift (although power does change a bit), which is surprising to me, although I am not too familiar with the physics in GaN aside from the fact that it is 'different' than GaAs due to the huge bandgap, among other factors. Below is a figure from my undergrad notes for an InGaAs laser showing a considerable wavelength shift (0.93ev to 0.95ev => 1333nm to1305nm) for increased carrier density (this is roughly proportional to the drive current).


The spectromer I was using did not have enough resolution to make out the bandwidth of the diode accurately, although there are mode hops which can put a several nm shift over a small current change (<100ma), but as far as I can tell in general it is pretty small (<2nm). I have one plot I took on the OSA at school at http://www.krazerlasers.com/lasers/445nm/G0002.BMP (it was taken at 300ma, I didn't have any good cooling solution at the time), showing that it is pretty ugly. In general I found that at higher currents (~1amp) it usually condensed down to something less than 2nm wide, although there were definitely diodes which showed 'very' multimode behavior (>5nm) at certain higher currents. I also did all of this testing years ago, so current diodes may be much better in that regards, the power certainly seems more consistent from diode to diode than it did for the first few months.

Do you have access to a spectrometer? I remember there being a pretty clear shift of a 'mostly' gaussian wavelength profile (ie, smaller than my spectrometer can resolve, on the order of a 2nm), which moved toward longer wavelengths with higher current, and occasional regions where the spectrum was wide enough that I could resolve 2 peaks (at least a 4nm separation). The testing script did a gaussian fit to the recorded spectrum, and if the fit was bad (as in the case with 2 peaks) threw away the result.

[planters]

No. I don't have a good, high enough resolution spectrometer. But, I wish I did because I suspect additional bands will appear. The trade off will occur when there is excessive dispersion to allow dense multiplexing. Not only will each spot begin to widen due to the wavelength spread, but with modulation, the group (although it will probably remain tight due to the common trend) will drift away from alignment with the other colors. However, if limited to the three that I have been able to multiplex then at least a three fold increase ( and maybe more) in brightness will be achievable without noticeable misdirection.

[krazer]

That question turned out to be a bit more complicated that I originally expected, see http://www.photonlexicon.com/forums/...ams-Dispersion

Do you know if your prisms are made of BK7? If that is the case I am a little skeptical of how effective the procedure will be, although if they are SF7 or something with a bit more dispersion at 445nm the numbers all seem about right. Can you see the beams shift at all as you ramp the current? My numbers show that there should be about 1/10 to 1/5 the shift as there would be for 2 diodes at the extreme ends of the spectrum (440 vs 450nm) so it will be hard to see, but it would be neat to see it actually there. I would actually be a bit more interested to see if it is possible to use this to combine a few 635nm diodes, or better yet combine 660 and 645 without needing an expensive/lossy dichro.