Welcome

This is a test to see whether it works… everybody has a blog now and why not me. I get lots of technical questions by email, and it might be easier to have discussions in a blog rather then by mail. So please feel free to drop by, english and german spoken here.

So what happened here during the last two years? Almost nothing, I didn’t have much time for my hobby but plan to be more active again. There is a major incomplete project, namely of stabilized 650nm diode laser modules suitable for holography. I had made a prototype which works well (stable single longitudinal mode at approx 50mW), and next I would build some more of them; but this requires some PCB and mechanical work for which I don’t have time right now.

Moreover I’d be eager to do some holography again, but this requires setting up my optical table which is completely blocked right now by the spectrum measurement apparatus I need for the diode lasers… to bad that I don’t have space for more than one concurrent project.

On the long run, I will try a single mode high power DPSS laser; I have already collected lots of parts like Yag rods and disks, KTP’s, diodes from 2 to 60W, 100A power supplies. All sorts of cooling, temperature stabilization, pump optics design, thermal lensing, green noise, etc, issues will come up and this will be a huge effort as far as I can see. But as a fellow holographer has said…. adversary builds character 😉

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11 Responses to “Welcome”

  1. javier Calderon Says:

    It seems really interesting what you do.

    I live in London and create Art employing different electronic media. So it would be great to have you around here to create a piece for this city.

    please contact me if you would like to come London to collaborate on a work of art.

  2. rxlaser Says:

    Hi Javier,

    well I am not doing holograms anytime soon as I am into building lasers for the time being! But there are plenty of fine holography artists in UK, I recommend a posting at http://holographyforum.org/phpBB2/index.php for finding them.

    Thanks for your interest!
    All the best,
    W.

  3. Phil Bergeron Says:

    Gi,
    Thanks for all the great information. Can you recomment a blue diode that is very easy to get low power (around 15 mW) SLM from with no optival feenbacl; just temperature and current? Thanks, Phil, 142laser, laser nerd 🙂

  4. rxlaser Says:

    Hi Phil,

    unfortunately not, I never touched any other blue diode expect for the Casio/Nichia(?) ones. Very likely the Osram ones would do but I really don’t know!

    Best,
    W

  5. 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 sit and blog! Thanks a lot.

  6. Mark Z Says:

    Nice work…..wanted to find out what it might take to set up your light meter circuit (or something similar) for a device I am building which would be used to characterize narrow band low and high power LEDs. Both the fully featured version with OPT202W and negative voltage generator.or the minimal version with OPT101P might work for this application. Or perhaps some other photodiode… Ideally this device would scan through the spectra to locate peak output for the various emitters to be tested. Its range would be from 400 to 700+ nm. I was thinking that the setup might be most efficient and accurate in the photovoltaic mode. Also contemplating a Hamamatsu PD with a range of 380-100 nm.. Any thoughts? Or suggestions Best M

  7. rxlaser Says:

    Hi Mark,

    if you need to scan the wavelength and find peaks, etc, then you need a spectrum analyzer, a single photodiode wouldnt help. Anyway photodiodes don’t do well for broad sources like LEDs (since their sensitivity depends on the wavelength).

    It just happens that this weekend I set up a spectrometer and I think this is precisely what you would need for finding peak wavelengths of LED’s etc. These spectrometers can be obtained here: http://www.science-surplus.com/products/spectrometers

    See also here for my take on these:
    http://redlum.xohp.pagesperso-orange.fr/electronics/spectrometer.html

    Probably you’s need to get one with a 900l/mm grating in order to get the spectral range you are aiming for.

    All the best,
    W.

  8. Mark Z Says:

    W, thanks for the info on the spectrometer which is definitely a great piece of equipment to have.

    I plan on using a grating chamber. The light entering the chamber would be reflected then refracted off a grating after which it would exit the chamber. The LEDs to be measured would range from 20mA to 400mA.

    Here is where the idea for the photo detector comes in… The grating angle is adjustable and there is a calibrated dial which displays the wavelength at any angle in nm. When calibrated the accuracy on the dial is around 2 tenths of a nm. While photo-diodes are more responsive to the red end of the spectrum, the OPT-101 or OPT-202 type you designed the board for do appear to be responsive in the shorter wavelengths.

    Not sure if your design could be adapted to have more than one setting to also test shorter wavelength LEDs and lasers in the 400nm range?

    There are most likely a few additional perimeters which would need to be specified to make this work properly.

    So fay my understanding is that a device using the photovoltaic mode of operation will be more precise and generate less noise.

    And so this would work if the amplifier can pick up the signal on the lower end.

    There are also larger area photo diodes which are relatively linear and responsive from 320-1100 nm, however, this means a completely different amplifier design. I am not sure this is the best direction given the OPT parts all have integrated amplifiers.

    Thanks for your reply

    Best Mark

  9. rxlaser Says:

    Hi Mark,

    are you using an already existing mechanical setup? To get such a high accuracy is very non-trivial. I guess you use a commercial spectrometer with mechanical grating rotation?

    I am not sure what you precisely mean by “Not sure if your design could be adapted to have more than one setting to also test shorter wavelength LEDs and lasers in the 400nm range?”

    The wavelength range for the OPT sensors is fixed, so I dont know what kind of setting you mean?

    Anyways, I’d think that those OPT sensors are fine for the purpose of light metering as I described, but most likely you’d get better performance with a specifically designed circuit (noise and speed), where the size is not important. In particular there are UV-enhanced Photodiodes which would work below 400nm (eg from Hamamatsu I think). Such circuits are not much more complicated as the one with the OPT sensors, see eg. http://redlum.xohp.pagesperso-orange.fr/electronics/transimp.html
    This one should do fine!

    Best,
    W.

    • Mark Z Says:

      Dr. W, lase

      I am working with what appears to be an old scanning spectrometer that was built in the 60s. There are no electronics inside anymore. when i scan red narrow band light sources with the device using a generic photo-detector setup I borrowed, the device exhibits a peak granularity of about 2 tenths of a nm.

      The metal contraption has an adjustable slit which can be set to look at much higher power devices like your lasers for instance, although it currently is not able to measure the lower power blue emitters, It can get a reading on higher power cyan emitters.

      I started looking at some of the Hamamatsu devices that you mentioned. There is another part number S1133-01…. It has a spectral response of 320-1100 nm. with a photo sensitivity that varies from 0.58 A/W at 960 nm to 0.19 A/W at 400 nm. It has a higher than average terminal cap. of 700pF. The output looks to be fairly linear across the visible range

      Wondering if this can be setup with an unbiased photovoltaic mode of operation….. the photo diode which would sacrifice speed for precision. But I am not as expert in the electronics as you are (although I struggle through and somehow find ways of making things work)

      Adapting a photo-diode like this one for a full vis light scanning application might include a second setting to boost the sensitivity and extend the dynamic range of the photo-diode

      The idea here would be to enable better characterization of lower power illuminants and emitters which are in the blue and close to the uv range. So perhaps there is a way to set the detector to a higher sensitivity when measuring blue and uv devices.

      Hope this helps to answer some of your questions….

      Best Mark

      • rxlaser Says:

        Hi Mark,

        I see! Well there is no problem to have the gain of the amplifier switcheable so that it has more sensitivity in the blue region. If you are not aiming for speed (eg for fast scanning), then this can be done by a very simple circuit, eg the one I mentioned in the previous post. It also uses a rotary switch for modifying the gain in steps of 10. I guess the only crucial thing is to obtain a photodiode what has enough sensitivity in the UV.

        All the best,
        W.

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