Maybe that was the problem, as my thermistor/external TE controller had no shielding at all.a_k wrote:Joe, did u use an NTC as temperature sensor? If so, noise certainly becomes a problem if it is connected to an external TE controller without shielding because the signals are so week.
405nm DCG
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Joe Farina
405nm DCG
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Colin Kaminski
405nm DCG
What I do is wrap a strand of copper wire onto my solderng iron and leave a pig tale sticking out. I then tin the whole thing and only use the tip of the wire to solder. Then I allign the wire wrap wire with the pin and just use a little solder. I have not overheated a diode or broken a diode with the wire since I started working this way.a_k wrote:Joe, did u use an NTC as temperature sensor? If so, noise certainly becomes a problem if it is connected to an external TE controller without shielding because the signals are so week. I don't know how much it matters if you connect the LD to a distant constant current driver with unshielded cable. I have not noticed anything negative so far.
Wires surely act as antennas and with all the RF emitting equipment that's present almost everywhere it could become problematic, especially if you were using an LD's built in photo diode as feedback element, because then weak signals would have to travel without shielding between LD and the controller.
Colin, wire wrap has the additional advantage that the LD does not get heated up while soldering, althought that's not a big problem if done fast.
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a_k
405nm DCG
In the meantime i received two more violet LDs. They produced far stronger fringes as the first one and the fringes remained equally strong between 0 and 90% of the power range. So it looked like one very broad continuous stable region instead of the series of more or less broad good regions i've seen with other LDs.
A series of DCG Denisyuks were made and unfortunately they all show the "sliced bread" phenomenon. Various current settings were tried but the result couldn't be improved. It looks like the LD tested is not running (almost) single mode. I hope the second LD will do better.
I have ordered 2 + 1 more LDs from two different sources for testing, hoping that they will be as suitable for holography as the first one had been.
What i don't understand is why the first LD with weaker fringes did produce such good results and the second one whith such strong fringes only has a coherence length of a few mm.
Both LDs might be from the same producer, as they do have similar digital stamps:
A series of DCG Denisyuks were made and unfortunately they all show the "sliced bread" phenomenon. Various current settings were tried but the result couldn't be improved. It looks like the LD tested is not running (almost) single mode. I hope the second LD will do better.
I have ordered 2 + 1 more LDs from two different sources for testing, hoping that they will be as suitable for holography as the first one had been.
What i don't understand is why the first LD with weaker fringes did produce such good results and the second one whith such strong fringes only has a coherence length of a few mm.
Both LDs might be from the same producer, as they do have similar digital stamps:
- 19-P0012.jpg (19.98 KiB) Viewed 2833 times
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Sergio
405nm DCG
a_k, If there is no sign of mode hop the laser is multimode, since I imagine a short cavity with a high crystal refraction index probably the coherence is milimetric, with such large gain and Fabry-Pérot large wavelength (= Short FP cavity value) separation would be easy to discriminate axial modes with retroactive feedback as an ECDL, it may be the case of using a non-optimized microlens onto first laser so the self aligned lens formed plane convex FP cavity easily, did you use such lens now?
Anyway for a MM laser would be easy form a ECDL single mode cavity with a inexpensive single grating.
Anyway for a MM laser would be easy form a ECDL single mode cavity with a inexpensive single grating.
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Joe Farina
405nm DCG
Thanks for posting your results, Ahmet.
So it looks like you've tested two diodes (which appear to be the same kind of diode, right?), and the first one had a useable coherence length, while the second didn't. Unfortunately this is not uncommon. I read somewhere that even diodes which are from the same batch have different characteristics, not to mention variations between batches. Also, as we know, even single diodes change as they age.
But I'm still very encouraged that you so quickly got those superb results with the first diode. Actually, rather than try to reason it out, I would just approach the problem in this way: just keep buying these cheap diodes as money allows, and try to make holograms with them. Then, keep the good ones and sell the rest to the pointer people. The fact that the first one you tried worked implies that it's not a very isolated occurence, at least I hope so.
The problem of fringe appearance versus coherence has been discussed on this forum previously. My own feeling (after considerable experimentation and frustration) is that it's pointless to rely on fringe appearance, and the only thing to do is make a hologram and see if it works.
One thing I would be concerned about is the heat problem. I'm not sure what happened to your first diode, but heat dissipation can be a pretty tricky thing. I would suggest being absolutely certain that heat is being drawn away from the diode. At the very least, the base of the diode must have a layer of good-quality heat sink grease, and this layer must be firmly sandwiched between the diode and a very substantial heatsink. In no way, shape, or form would I rely on a standard cylindrical diode module to provide heat-sinking. The true master of laser diode holography, Tom B., uses a container of water to provide a huge amount of thermal mass to draw heat away from the diode. It would be very worthwhile to review his setup (though I'm not sure where to look, perhaps his website or old posts on the forum).
So it looks like you've tested two diodes (which appear to be the same kind of diode, right?), and the first one had a useable coherence length, while the second didn't. Unfortunately this is not uncommon. I read somewhere that even diodes which are from the same batch have different characteristics, not to mention variations between batches. Also, as we know, even single diodes change as they age.
But I'm still very encouraged that you so quickly got those superb results with the first diode. Actually, rather than try to reason it out, I would just approach the problem in this way: just keep buying these cheap diodes as money allows, and try to make holograms with them. Then, keep the good ones and sell the rest to the pointer people. The fact that the first one you tried worked implies that it's not a very isolated occurence, at least I hope so.
The problem of fringe appearance versus coherence has been discussed on this forum previously. My own feeling (after considerable experimentation and frustration) is that it's pointless to rely on fringe appearance, and the only thing to do is make a hologram and see if it works.
One thing I would be concerned about is the heat problem. I'm not sure what happened to your first diode, but heat dissipation can be a pretty tricky thing. I would suggest being absolutely certain that heat is being drawn away from the diode. At the very least, the base of the diode must have a layer of good-quality heat sink grease, and this layer must be firmly sandwiched between the diode and a very substantial heatsink. In no way, shape, or form would I rely on a standard cylindrical diode module to provide heat-sinking. The true master of laser diode holography, Tom B., uses a container of water to provide a huge amount of thermal mass to draw heat away from the diode. It would be very worthwhile to review his setup (though I'm not sure where to look, perhaps his website or old posts on the forum).
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a_k
405nm DCG
Sergio, i did observe one of the LDs for an extended period of time and could not see a single mode hop. All other (100mW) LDs i have used before did have several more or less broad stable operating regions and the closer the upper limit was, the fewer and narrower the good regions got.
With this LD the fringes are very strong without the slightest visible change up to high power levels. Because i could not see any difference in fringe contrast, i made several denisyuks scanning the whole power spectrum. Unfortunately all holograms were of the sliced bread variety. I do strongly suspect that the LD is running multimode.
I did take a couple DCGs without collimation optics, but the "sliced bread" thing was still present.
Could you please elaborate a bit about ECDL single mode cavity? Would it be possible to discriminate the modes outside of the LDs resonator, because this is not an external cavity laser? As you can imagine i'm very interested in a not too complicated or expensive way of separating a single mode. Do you know where to get such a grating?
Loss of power is not important because with 100mW of 405nm we can waste a lot and still have a decent light source. This 10x7cm plate i made today took an exposure time of 2 secs.
Joe, this LD was extracted from the same type of new PHR803-T as the first one. And it has the same label at the same position of the part, so chances are high that they are different batches of the same product or even different members of the same batch. I'm trying to find out the producer and the type of these LD's hoping to get some specific data.
It's my actual strategy to buy more LDs from different suppliers, keep the ones that are ok, and try to sell the rest to the laserpointer ppl. They are still good enough for them for popping balloons
Until now i have been using the fringes to find the usable operating regions. The broader they were, the less hassle was involved. And the stronger the fringes were, the better the holograms got. This is what i experienced so far. But this last LD does not fit the picture at all.
The LD produces roughly 500mW of heat at the highest current setting. The aixiz module is certainly not sufficient to operate this LD, it gets warm to the touch very quickly. The first LD died a fraction of a second after it had been turned on. It did not have time to heat up, so whatever the cause of its death was, it's safe to assume that it was not releated to unsufficient cooling.
I completely agree with you that its very important to keep the LD cool. Usually i put the aixiz module into an aluminium cube, which sits on top of a large aluminium block. Fro all interfaces thermal paste is used. Even with the violet LD, the temperature of the block never reaches more than 0.5° - 1° C above room temp. The large aluminium block also means that it takes about 10 minutes until everything has the same temperature.
It's a clever idea of TomB to use a water container, because it will reduce the slow thermal shift of an LDs emitted wavelength and so lead to a better coherence length. This consideration gets more important the longer the exposure times are. But with exposure times of only a few seconds it will get a bit less important, because slow events need time and if you are fast enough, there is no time for them to happen
Let's hope that the next LDs will be as good as the first was.
With this LD the fringes are very strong without the slightest visible change up to high power levels. Because i could not see any difference in fringe contrast, i made several denisyuks scanning the whole power spectrum. Unfortunately all holograms were of the sliced bread variety. I do strongly suspect that the LD is running multimode.
I did take a couple DCGs without collimation optics, but the "sliced bread" thing was still present.
Could you please elaborate a bit about ECDL single mode cavity? Would it be possible to discriminate the modes outside of the LDs resonator, because this is not an external cavity laser? As you can imagine i'm very interested in a not too complicated or expensive way of separating a single mode. Do you know where to get such a grating?
Loss of power is not important because with 100mW of 405nm we can waste a lot and still have a decent light source. This 10x7cm plate i made today took an exposure time of 2 secs.
- dscn2427.jpg (60.61 KiB) Viewed 2771 times
It's my actual strategy to buy more LDs from different suppliers, keep the ones that are ok, and try to sell the rest to the laserpointer ppl. They are still good enough for them for popping balloons
Until now i have been using the fringes to find the usable operating regions. The broader they were, the less hassle was involved. And the stronger the fringes were, the better the holograms got. This is what i experienced so far. But this last LD does not fit the picture at all.
The LD produces roughly 500mW of heat at the highest current setting. The aixiz module is certainly not sufficient to operate this LD, it gets warm to the touch very quickly. The first LD died a fraction of a second after it had been turned on. It did not have time to heat up, so whatever the cause of its death was, it's safe to assume that it was not releated to unsufficient cooling.
I completely agree with you that its very important to keep the LD cool. Usually i put the aixiz module into an aluminium cube, which sits on top of a large aluminium block. Fro all interfaces thermal paste is used. Even with the violet LD, the temperature of the block never reaches more than 0.5° - 1° C above room temp. The large aluminium block also means that it takes about 10 minutes until everything has the same temperature.
It's a clever idea of TomB to use a water container, because it will reduce the slow thermal shift of an LDs emitted wavelength and so lead to a better coherence length. This consideration gets more important the longer the exposure times are. But with exposure times of only a few seconds it will get a bit less important, because slow events need time and if you are fast enough, there is no time for them to happen
Let's hope that the next LDs will be as good as the first was.
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Sergio
405nm DCG
Ahmed the coherence length will depend on spectral laser gain bandwidth , if you could measure it you can confirm the hypothesis of mm laser, in this case the modes will not hop but "ooze".a_k wrote:Sergio, i did observe one of the LDs for an extended period of time and could not see a single mode hop. All other (100mW) LDs i have used before did have several more or less broad stable operating regions and the closer the upper limit was, the fewer and narrower the good regions got.
Right, with the Zebra depth you can compute the coherence lenght if i remember.With this LD the fringes are very strong without the slightest visible change up to high power levels. Because i could not see any difference in fringe contrast, i made several denisyuks scanning the whole power spectrum. Unfortunately all holograms were of the sliced bread variety. I do strongly suspect that the LD is running multimode.
ECDL is an extended cavity diode laser, by using a Littrow grating configuration you can obtain a forced feedback lasing that will filtrate only one axial mode and depending on current stability and temperature control a very stable WL.Could you please elaborate a bit about ECDL single mode cavity? Would it be possible to discriminate the modes outside of the LDs resonator, because this is not an external cavity laser? As you can imagine i'm very interested in a not too complicated or expensive way of separating a single mode. Do you know where to get such a grating?
I think you are interested in a student simple design that can be followed on these links:
http://optics.ph.unimelb.edu.au/atomopt/diodes.html
http://pagesperso-orange.fr/redlum.xohp/laser/ECDL.html
http://www.its.caltech.edu/~ph76a/laserprimer.pdf
The grating can be obtained at Edmund Scientific or similar shop and costs around $58, also available are cheap mirror mounts that you would need to attach the grating.
So the extracted power at single mode would be very interesting due efficient energy extracting, note that for a single mode diode laser there is no sense in attach a grating, only with a real MM laser seems feasible, I guess.Loss of power is not important because with 100mW of 405nm we can waste a lot and still have a decent light source. This 10x7cm plate i made today took an exposure time of 2 secs.
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a_k
405nm DCG
Sergio, thanks for the explanations and the link. I knew about wler's experiments with ECDL before and also about the difficulties he ran into, because a very accurate temperature stabilization is needed for an extended cavity to function properly.
The geometry needed for using a grating to form an ECDL might be simple, and the grating readily available, but the stability requirements are not so easy to fulfill.
Besides that, the LD at hand is an ordinary one, not an external cavity type. External cavity LDs have one end face AR-coated, ordinary ones do not have this AR coating. Do you think it would be still possible to use a grating to separate modes? You might have noticed that wler did use a Rohm RLD65PZB5 *with* AR coating. I have no means for replacing or removing an LDs facet coating.
About the zebra strips, there is no need to measure the coherence length, because if the strips are millimeters apart, the coherence length will be of the same order. The hologram that i attached before shows two things to me: There are at least two concurring modes and they get in phase and out of phase every 3mm or so. This would mean that they are roughly 0.05nm apart (incidence angle of the beam was 30deg).
The geometry needed for using a grating to form an ECDL might be simple, and the grating readily available, but the stability requirements are not so easy to fulfill.
Besides that, the LD at hand is an ordinary one, not an external cavity type. External cavity LDs have one end face AR-coated, ordinary ones do not have this AR coating. Do you think it would be still possible to use a grating to separate modes? You might have noticed that wler did use a Rohm RLD65PZB5 *with* AR coating. I have no means for replacing or removing an LDs facet coating.
About the zebra strips, there is no need to measure the coherence length, because if the strips are millimeters apart, the coherence length will be of the same order. The hologram that i attached before shows two things to me: There are at least two concurring modes and they get in phase and out of phase every 3mm or so. This would mean that they are roughly 0.05nm apart (incidence angle of the beam was 30deg).
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Sergio
405nm DCG
Ahmet You are right, but I remember a paper related to those Australian work that even without AR coating is possible obtain a mode control, remember also that the grating is used at a somewhat low first order reflectanceBesides that, the LD at hand is an ordinary one, not an external cavity type. External cavity LDs have one end face AR-coated, ordinary ones do not have this AR coating. Do you think it would be still possible to use a grating to separate modes? You might have noticed that wler did use a Rohm RLD65PZB5 *with* AR coating. I have no means for replacing or removing an LDs facet coating.
Forced oscillation may surpass those loss with non AR surface.
Sergio.