General Technical Discussions

The original post was about the closure of the holography forum, which has now been replaced by the holoforum created by Ahmet.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

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31 Responses to “General Technical Discussions”

  1. Phil Bergeron Says:

    Hi W,
    I too am shocked that the Holoforum.org site closed. I don’t really care what anyone says about me online and would never take action against a website such as the forum because someone slandered me; that is ridiculous. πŸ™‚

    Have you used the new Opnext 642 nm diode that they say on the dats sheet is SLM? I had Mike Lawson build one up and it seems to be MLM. It is STM (uniphase) but sam Goldwasser says there are many modes about 2.7 GHz apart. No garting just the diode and collimating optics. maybe the optics are feeding back into the diode? These are like $150 each-ouch. Any idea if they are SLM out of the can? Phil, 142laser

  2. rxlaser Says:

    Hi Phil,

    which one do you mean? The HL6385DG works relatively well SLM, sometimes free running up to like 60-80mW or so. I found that when they write SLM in the data sheet, it means that typically there is a sweet spot somewhere to operate it, where it runs SLM; it obviously does not mean that the diode runs predominantly SLM. See my web pages for a detailed analysis. I have sold a bunch of such lasers to happy customers.

    In ECDLs they work well too, more stable that is, but not to powers higher than 90-100mW, say.

    If you mean the newer broad ridge high power diodes, I didn’t try since they apparently run multi transverse mode of worse beam quality than the blue Nichias, and even with the 1W Nichias one is not getting much beyond 100mW, so I believe it is not worthwhile.

  3. Kris Meerlo Says:

    Hi Wler,

    Yes that is to bad that the forum is closed, after 2 years off no making
    holograms I want to start again.
    I buy the blue laserdiode but first i want to start with the reddiodes again.
    Wler can you send me the connection from the circuitboard, how to connect the laserdiode and the TEC and powersupply, I buy from you? Because i can not find it back on your site ( the link doesn’t work ).
    Regards,

    Kris.

  4. Phil Bergeron Says:

    The Opnext company has admitted that the SLM claim is a mistake! The diodes are NOT SLM at all and if you find a sweet spot that is SLM you are lucky. They meant single transverse mode. You cannot buy SLM red diodes for $150. Dam! πŸ™‚ Phil

  5. rxlaser Says:

    Well, again, I have built various SLM lasers with these diodes at around 60-90mW. Most of these diodes have a sweet spot somewhere and the tricky thing is to find it.

  6. Kris Meerlo Says:

    Hi Wolfgang,

    I tried this week your your laserdiode controller and it works great.
    I shot 2 holograms without banding the temp is very stable, the type off diode i use, i don’t know, I think it was a 50mw.
    The setup was very simple i use a cardboard shutter and Ultimate plates.
    To check the interference stability I use a collimator tester, it is a very easy method to check te stability off the diode you only need the collimator tester ( Ebay ) and a white card for projection.

    • rxlaser Says:

      Hi Kris,

      great to hear! Yes a simple glass plate like a microscope slide and a lens is sufficient to verify single mode operation for diodes. It seems that many DVD diodes generically run SLM up to about 30-50mW, with some effort in finding sweet spots even higher. My overall recommendation is the Mitsubishi ML101J27.

      Have fun!
      -Wolfgang

  7. Quan Says:

    Thank you so much for the wonderful testing of all these laser diodes. I made holograms 4 years ago using 5mW 650nm diode laser (wonderful result) and tried to use high power diode laser (150-200mW, 660nm, 532nm, 405nm) to make holograms this month. The problems I have are the “sliced bread” in transmission holograms and “dim, sliced bread, wrong color, out of focus” in Denisyuk holograms. I realize that this is caused by the short coherence length of the laser but wonder why 5mW diode laser is much better than 200mW diode laser ( I tried 2 different 200mW 660nm diode laser but all showed “sliced bread”, and the “space” between “sliced bread” is different for two diode lasers). Now I realized why the single longitude mode laser is so expensive and not very powerful. Before come here to read all of your testing graphs, I googled the wikipedia and find the relationship of wavelength difference of different modes is c/(2nL),
    http://en.wikipedia.org/wiki/Longitudinal_mode
    while c is speed of light and L is the length of resonator (mirrors), since the L of diode is 0.02-0.3 mm and the delta wavelength (changes) between different modes are much higher than those of gas lasers (L~20cm, or meters). My calculation of the delta wavelength between muti-modes of 600nm diode for L=0.05mm is about 3.6nm. (delta lamda=((lamde)^2)/(2*L) and that gives about the correct coherence length as the size of “sliced bread”. My result of 532nm is much better and I think that is due to the longer L of the YVO4 crystal for 1064nm laser. I failed to make Denisyuk hologram using 200mW 405nm diode laser for the length of 2 cm of object. It is easy to understand that higher current will lead to higher photon energy of the muti-modes. But it seems that the distribution of different modes should follow the Max Planck’s law. I was happy to see the growing market of high power diode laser but how can we keep the power and still get a single longitudinal mode? Do you think lowering the temperature of the diode will do the job? I mean using liquid nitrogen or dry ice to keep the diode in very cold temperature. Glad to see your web site and blog! Thanks a lot.

    • rxlaser Says:

      Hi Quan, yes I agree with your statements and computations. Many low power diodes can run single mode, and most high power diodes too but not up to high powers. I would say that typical red DVD diodes can run single mode up to approx 30mW, sometimes higher. Some are completely unsuitable, though, see my tests. Blu ray diodes 405nm seem pretty bad in general, but the 445nm diodes much better again.

      Thanks for your interest!

  8. Quan Says:

    Hello, I am very interested in the mode-profile of 150-200 mW diode laser at low temperature (5 degree Celcuis or even below zero). From the manufacture’s data, it is apparent that lower the temperature will increase the efficiency. According to the Boltzmann distribution, d1/d2=exp(-E1/(kt))/exp(-E2/(kt))=exp(delta E/(kt)), So if we lower the temperature even a little bit, the ratio of distribution of different modes will significantly increase. So at lower temperature, the other modes that we don’t want should be lowered. I don’t have the instruments like yours but will try to make holograph using 200mW diode laser at low temperature. If I see the increased depth of “sliced bread”, that means good. I think keeping diode in low temperature is pretty easy and feasible than buying expensive high power single longitudinal mode gas laser.

    • rxlaser Says:

      Hi again, I don’t have experience with low temperatures, because sooner or later there is moisture condensation and ice appears on the diode and optics, so one needs to hermetically seal the whole thing and desiccate, professionals use nitrogen as ambient atmosphere. I have no idea how well diodes behave then, all what I can say that lowering the temp from like 25C to 15C, does already have a positive effect in most cases.

      Quite generally, there seems a magical barrier at approx 100mW for almost all diodes, includung ECDL setups, above which multimode operation appears. I wonder whether this can be improved at low temperatures.

  9. Quan Says:

    Hi, glad to see your quick reply. Yes, the moisture in the air (room temperature) will condense on the surface of cold objects but we still feel very dry in cold winter. The ambient relative humidity is about 30-50% but the pressure of water vapor in the air varies a lot from summer to winter. The reason of condensation is that the pressure of water vapor in ambient temperature is above the saturated water vapor at lower temperature. It’s already cold in north hemisphere at night now and I wonder if you can test the diodes at night in a room without heater or move your instruments out of the room. (Sorry for bad suggestions) Another way to get rid of ambient humidity in a closed small room is put a lot of ice (3-5 kg, -5 degree to -10 degree Celcuis, the colder the better) in the room until the humidity is condensed on the surface of ice and then you can test the profile of diodes in a little bit higher (5 degree C) temperature than the ice. I will try to make hologram with my 200mW 660nm diode laser in really cold winter night in a room without heater.

  10. Quan Says:

    BTW, I forget to mention that don’t use liquid nitrogen or dry ice in a closed room that will cause suffocation and can kill people. But ordinary ice from super cold freezer (-80 C) is best for decrease the humidity of the air in closed room. In another word, you just put big container with higher surface area full of cold ice for condensation to compete the unwanted condensation on diode.

  11. Some thoughts for diode laser for Holography | δ»ŽεˆηΊ§η²Ύζ―η»†θƒžε…‹ιš†ε’Œθ½¬δΈ–η­‰ζ•ˆ Says:

    […] the diode in very cold temperature. Glad to see your web site and blog! Thanks a lot. rxlaser Says: November 13, 2010 at 9:55 am […]

  12. qyuan0366 Says:

    Hi, I have something to tell you about my thoughts and what I did. Since I don’t have the instruments to measure the spectrum of multi modes, I use a simple way to see the mode hop. Put a clean thin glass (2mm) in front of the laser and see the interference pattern of the reflected light about 2 meters away or further. The rings of the interference pattern on the wall is slowly changing at the beginning when diode is turned on (100mW 660nm). But the interference pattern is sharp and clear. The slow changing of the rings is due to the temperature change of the diode. (wavelength change caused by temperature change). After a while, about 2 minutes, the rings are constant and don’t move, suggesting the thermo-equilibrium is reached at the current power. If increasing the power, the rings begin changing again. And will take a while to reach the heat equilibrium within diode. When the power is high, a sudden change of all interference pattern can be seen and the change might last for several seconds to minutes. During this change, the previous sharp and clear interference pattern become “evenly bright” or “less contrast” and some new patterns occur. I think that is due to mode hop. This is common for 660nm 200mW diode laser and also for 200mW 532nm DPSS laser. I carefully watched the interference pattern of 5mW diode for several minutes and no pattern change was observed. So here is the question, is the multi-mode at higher power caused by just high current or due to the high heat? Even if we soak the diode in liquid nitrogen, as long as the diode is on, there is a temperature gradient across the center of diode and the surface of the diode. I wonder the multi-mode of the diode at higher power is due to high heat generation inside of the diode and the actual temperature in the diode center is much higher than the ambient temperature even if the thermo-equilibrium is reached. For 5mW diode, the heat generated in diode is so low and can quickly dissipate. If the temperatures in the center of the diodes are same, there is no reason that 200mW diode runs multi-mode but 5mW diode runs single mode. I calculated the ratio of different modes using Boltzmann distribution. To my surprise, the ratio is not highly dependent on temperature. Here is my calculation:
    For 660nm diode laser, assuming L(length of resonator)=0.1mm, so the wavelength difference between different modes is 2.18nm, the energy difference between two modes next to each other is hc/(2nL)=6.63E(-34)*3E8/(2*1*0.1E(-3))=9.945E-22(J), the ratio of different mode at 20C: n1/n2=exp(delta E/(kt))=exp((9.945E-22)/((1.38E-23)*298))=exp(0.2417)=1.27
    the ratio at 273k is 1.30 and 2.55 at 77k (liquid nitrogen). So the reason that 5mW diode runs in single mode is completely due to the fast heat dissipation rather than the ratio of different mode at low power consumption.
    When you measure the properties of the diodes at 15C at high power, the actual temperature of diode might be as high as 50 or 60C even if the surface temperature of diode is 15C. So when power is above 100mW, it’s so hard to get rid of the heat fast enough and multi-mode occurs. I am pretty sure that diode can get single mode even at 200mW or higher as long as the heat is quickly dissipated. “Quickly” here I mean the temperature in the center of diode must be less than 20C or 25C.

  13. rxlaser Says:

    Hi Qyuan,

    thanks for your comments! I believe these are difficult questions and I am not really quaified to give a meaningful answer, as to what all the causes for multimode behavior at higher currents are and whether this can be helped by low temperatures. Obviously a higher gain will always give a tendency for more modes to lase, but why there seems to be a quite univeral border at around 100mW for practically all diodes incl ECDL setups escapes me. Probably thermal gradients and induced optical inhomogeneities, lensing effects in the material, etc, play a role and I just dont know what effect a very low temperature would have. This would be interesting to investigate.

    But I guess for practically making holograms, dealing with ultracold substances would make things unnecessarily complicated, it is difficult enough to reach thermal stability and avoid drafts and thermal gradients over the holographic setup. SInce one can reach in the order of 100mW with ECDL, which is quite a lot, there is no strong reason to handle liquid nitrogen etc to reach a bit more power!

  14. Quan Says:

    Hi, rxlaser, thanks. I agree with you that “there is no strong reason to handle liquid nitrogen etc to reach a bit more power”. I was thinking that if we can dissipate the high heat from the 1W 445nm diode using liquid nitrogen, we might get over 1W in single mode and it probably can be used for portrait or “non-still” objects without using the vibration-free platform. I am a biochemist and can get liquid nitrogen or dry ice easily in our lab. Also, there is a pulsed Nd:YAG laser with a pulse duration of 4ns and energy of 200mJ/pulse (532nm after SHG) in our lab for laser photolysis study and I am using it for research. I wish I can use it for portrait of people but obviously NOT because of the burning of the eyes. So I am interested in CW semiconductor laser. Let’s do a little bit calculation again here: For slavich emulsion (VRP-M, PFG-03C or PFG-M) the sensitivity at 445nm is about the same as logS=-3.4, which is about 2500uJ/cm^2 for the energy need for exposure. For a 1W laser, the time need for 2500uJ at 100cm^2 is only 0.25 sec. If the 660nm laser can run single mode at 250mW, the time need for 30uJ/cm^2 (sensitivity of red 660nm emulsion from Tianjin, China) at 100cm^2 is 0.012 sec. Such short time will allow the speed of movement of 0.01mm/sec. (for 1/4 wavelength tolerance). I know it might not as good as the prediction here but I just do some calculations for fun before doing the real work.

    I observed the interference pattern again last night and want to tell you more. For 5mW diode, there is also mode hop (interference pattern change) but the rings are still sharp and clear. For my 200mW 532nm DPSS, if it is powered by 2.4V (before the circuit), the interference pattern is pretty stable for quite a long time. But when powered with 3.7 V (before the circuit, the normal working voltage), I see the interference changes often in about 5 minutes. For 200mW 660nm diode, I just saw very bad interference pattern. There was no high-contrast rings like those by 5mW 650nm diode or 532nm DPSS.

  15. Quan Says:

    Hi, rxlaser, how are you recently?
    Find a good post about single and multi mode for laser diodes:
    http://laserpointerforums.com/f54/single-mode-multi-mode-laser-diodes-53695.html

    Also, I just bought a 445nm 1 W Nicha laser diode from ebay. What I found is another problem:
    To make sure I can get single mode at low power, I monitored the current very carefully and it won’t excess 300mA. (Operating current is 1400mA). I use a standard 5.6mm diode housing
    http://cgi.ebay.com/5-6mm-Laser-Diode-House-With-Lens-Laser-Industrial-Lab-/260542376333?pt=LH_DefaultDomain_0&hash=item3ca989298d#ht_2788wt_724
    for heat sink.
    when the current was set as 200mA(monitored with digital multi-meters for voltage and current), 445nm laser is coming out. But the current is slowly going up to 300mA (keeping the voltage constant) in about 3 minutes and 445nm laser was replaced by blue light like LED. If I turn it off and wait for just 3 seconds, the laser is back again but will slowly change to ordinary blue light after a while. I don’t have the problem for my 405nm diode/660nm diode. Does that mean I was running just above the threshold limit or because of above the thermal limit?
    I find the laser modular is warm (40-50C degree) after 3 minutes even if powered by 300mA? Is this normal for 1W 445nm laser diode? It seems that 1W 445nm laser diode has a lower electric-optical conversion efficiency than 405nm/660nm laser diodes.

  16. rxlaser Says:

    Hi Quan,

    you should always run a diode with constant current and not constant voltage (because the forward voltage is strongly temperature dependent). I guess the diode got very hot and then below the lasing threshold. Certainly 50C can be easily reached by 300mA. Better cool it properly, this is good for its life time and also for holography application better have a very stable temperature control.

  17. qyuan0366 Says:

    Hi, rxlaser, thanks for advices.
    I removed the diode from the modular and made an big aluminium heat sink which fully contacts with diode case as much as possible. Yes, as long as the temperature is constant, I didn’t see the increase of the current. (Stays there at 210mA for about 8 minutes). I began using constant voltage rather than constant current when I found the fluctuation of the optical output of 660nm 200mW diode. For some reasons, (660nm 200mW diode, low heat generation than 445nm 1W diode) even if the current is constant, the photometer for optical output measurement is changing up and down after 10 minute stabilization. No optical power fluctuation was found in 5mW 650nm diode. So I tried to risk those in constant voltage.

  18. Kris Meerlo Says:

    Hi Wolfgang,

    I am interested in one off your ECDL 655 or 640nm laserheads.
    Can you tell me the prices?
    Thanks,

    Kris.

    • rxlaser Says:

      Hi Kris,

      I didn’t yet compute as I wanted to have them completely finished; I still need to find a shop which can paint them with a good quality. I will tell!

      Thanks for your interest!
      -Wolfgang

  19. qyuan0366 Says:

    Hi, rxlaser. Sorry for bothering you again for another question. A laser diode behaviors like LED below thresh hold limit. Can we increase the current of a ordinary LED or high power LED (keeping the temperature low with dry ice or liquid nitrogen for fast heat dissipation) to get laser? I don’t know the structural difference between LED and laser diode and wonder how the population inversion can be obtained in laser diode while no population inversion in LED.

    • rxlaser Says:

      Hi Qyuan,

      no a laser diode needs a decent resonator to start with ;-( While a LD can work as expensive LED, sometimes unvoluntarily, it doesn’t work the other way around,

  20. qyuan0366 Says:

    Hi, Wolfgang. More questions for holography. I didn’t use the special polarized laser for holography just the 5mM diode laser. What’s the advantage of using the polarized laser? (except to remove the unwanted reflection between surfaces of the plate) If I split the random polarized laser to two beams of laser with perpendicular polarization planes as object beam and reference beam, using metal object or anything without Brewster’s effect, will I get the interference pattern/hologram? Or for Young’s double slits experiment, if I put two polarize filters on each slit (polarization planes are perpendicular to each other ), can I see the interference pattern on the screen?

  21. rxlaser Says:

    Hi Qyuan,

    laser diodes are always pretty well polarized and this is preferable, as you said, for avoiding reflections within the plate. A non-polarized laser like some HeNe’s don’t have a well defined polarization plane and so it may jump from one day to the next; so if you did some beam splitting or something else that is polarization dependent, you might get different results from one day to the next. So to have reproducible conditions, this is better to be avoided.

    In general reflections at the object depend on the polarization and they can also change it, so that’s why you’d get interference patterns even for orthogonally polarized ref and object beams. In fact it is a useful technique to make the polarization of the object beam tunable by a waveplate, because this allows to fine-tune contrast and suppress strong specular reflections of the object. In order to see the effect beforehand, one could view the scene through a polarizer at the place of the holographic plate.

  22. manfredmueller Says:

    Hi i wonder if using oil instead of air for the optical path from laserdiode to grating in ECDLs would allow for more single line power, since it should have reduced reflectivity at the nitrid/oil surface compared to the nitrid/air surface… already tried that? (could be called Immersion ECDL :D)

    regards

  23. rxlaser Says:

    Hi Manfred,

    no that’s new to me, is this a crazy idea or do some people do this actually? I’d think one would need a very clean fluid and I also wonder whether one could avoid inhomogeneities in the fluid due to heat gradients etc. Interesting idea to ponder about!

    Best,
    W.

  24. manfredmueller Says:

    I think atm its just a crazy idea, but maybe its worth a try? (but make sure you know how to clean the stuff from the oil before beginning πŸ˜€ )
    regarding the oil: a waterclear synthetic oil should do it, since you have dust in the air too, i dont think it needs to be super clean…
    the temperature gradients could be a problem… but i think for short lengths it should be manageable if even necessary…(forced gradient, laminar flow)
    how long would your optical path be?

    regards

  25. rxlaser Says:

    The optical path should be as short as possible, typically 2-3cm; another problem would be the collimator which is between diode and grating; it is usually coated for glass-air transition, so one needs to take the glass-oil transition into account as well.

    I wonder what the reflectivity of the diode facet actually is (for the blue Nichia); since already an AR coated plate (with 1-2% reflection) can reduce the threshold current significantly (like from 190mA to 150mA), I would think that the reflectivity can’t be very high to start with!

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