Page 1 of 12 1234511 ... LastLast
Results 1 to 10 of 118

Thread: Modulation efficiency

  1. #1
    Join Date
    Jan 2008
    Location
    Stockholm, Sweden
    Posts
    799

    Default Modulation efficiency

    The classical specifications of lasers gives us useful data like CW power and beam characteristics. Most of this data is collected under static conditions, i.e. operation at full power after warm-up. For most of us, the lasers in our projectors are typically not operated under static conditions - they are modulated. This modulation can significantly change the characteristics of the system, especially when DPSS lasers are involved.

    Now that I finally have a nice power meter at work I'm able to do some measurements on my lasers. Here I present some preliminary results that I hope we can discuss.

    First I will show what a typical result using the method described later looks like. Because the lasers used are of different different powers I'm showing the relative power instead, which is simply the momentary power divided by the full power of the laser.

    Not surprisingly the diode laser handles the modulation best. The two DPSS lasers are similar, but both have rather low power compared to the diode laser.

    To better be able to compare the results you can look at the area under the graph, which is simply the emitted energy (as energy = power*time). It is easily obtained by integration.

    140 mW red diode: 22.1 J
    430 mW green laserwave: 40.3 J
    130 mW blue viasho: 9.16 J

    We see that for example the blue viasho, which at full power reaches nearly the same power as the diode laser manages to squeeze out less than half of the energy of the diode. If respect is taken to the the lasers' respective full power output and assume that the diode is ideal (100% efficient) it is possible to calculate a "modulation efficiency". For the green laser that efficiency is 59% and the blue is merely 45% efficient. Interesting!

    What practical implications does it have though? Well, I don't want to jump to conclusions, because there are some more measurements I'd like to do. However, two immediate thoughts that occur to me are:

    • The ~20% loss in PCAOMs might not be so bad after all
    • An explanation to why people get away with using less red (and 405 nm) power from diode lasers than predicted by color models



    Looking forward to some comments now...


    Equipment used:
    Ophir Nova II power meter with a 3A-P-FS thermopile head
    Pangolin Flashback 3 SE
    Laserwave 532nm 400mW DPSS laser
    Viasho 473nm 100mW DPSS laser
    Opnext HL6385DG 642nm direct injection diode with FlexMod2 driver
    DZ's color correction board

    Method:
    Rising and falling edge delay were set to 0 and gain to maximum on the color correction board. The offset was then adjusted to just below the lasing threshold. Each laser was allowed to warm up at full power and the output power recorded. After the warm-up was completed the LAStudio show "Show must go on" was played by the FB3 at default settings (single color) through DZ's color correction board and the power logged.

    The (radiant) energy Q is given by integrating the radiant flux Φ over time. The modulation efficiency is calculated like:
    Q*Φ[ref,CW]/(Q[ref]*Φ[CW])
    Where Φ[ref,CW] is the full power radiant flux under and Q[ref] the radiant energy of the reference diode and Φ[CW] the full power radiant flux of the tested laser.

  2. #2
    Join Date
    May 2006
    Location
    Native Floridian
    Posts
    3,127

    Default

    The ~20% loss in PCAOMs might not be so bad after all
    I completely agree, I wonder though, would having the solid state lasers on full 100% of the time severely reduce the life of the laser.

  3. #3
    Join Date
    Feb 2008
    Location
    East Sussex, England
    Posts
    5,248

    Default

    Pangolin Flashback 3 SE
    Do you perhaps also need to factor in the issue of the FB3 having 'excessive' something (almost Bill's own words ) going on thats causing lower power than you might see with a QM or other DAC?

  4. #4
    Join Date
    Jan 2009
    Posts
    1,622

    Default

    I was just discussing this very same thing with Buffo in another thread recently. Everyone knows that a good mix of RGB with the proper depth can provide up to 16.7 million color hues. To my knowledge the only way to actually realize anywhere near the full color spread possible is with a PCAOM.. The lousy response of DPSS and many diode lasers to direct modulation makes it impossible to get that kind of color spread in the real world. Truthfully, anything other than a purely linear and predictable laser response to direct modulation would ruin any hopes of millions of colors. Thousands of colors might be possible with very fine modulation response and good color correction, but really I cant recall seeing more than perhaps a few hundred hues from even the best solid state projectors I've ever seen, both in videos and in real life.

    @DZ: to the best of my knowledge, the lifetime rating on any laser is based on 100% duty cycle unless some other duty cycle is directly specified. For almost all show lasers, that means CW at full power.

  5. #5
    Join Date
    Jul 2005
    Location
    Flying over a town near you
    Posts
    1,404

    Default

    Quote Originally Posted by DZ View Post
    I completely agree, I wonder though, would having the solid state lasers on full 100% of the time severely reduce the life of the laser.
    Actually this would be ideal living for a DPSS unit. Diode structures have improved and the rated MTTF is just that...operation at desired power...below upper limits. The stress of power input flexing the thermal limits of sub-par crystal cuts and diode modulation problems would almost be unheard of. After the laser comes to operating normal temps....it is very stable at that point with regards to the point of the design in the laser. I think a valid test is in order.
    You are the only one that can make your dreams come true....and the only one that can stop them...A.M. Dietrich

  6. #6
    Join Date
    May 2006
    Location
    Native Floridian
    Posts
    3,127

    Default

    Hmm, I also think a test is in order. Any manufacturer out there willing to put up a pair of DPSS's for this experiment? 100% duty cycle on one and 50% duty cycle on the other. My money's on the 50% duty cycle laser lasting the longest.

  7. #7
    Join Date
    Jan 2009
    Posts
    1,622

    Default

    ^No doubt. In the past there's been concern with facet damage from temperature changes and rapidly changing photon flux caused by direct modulation. I don't believe it's an issue, personally, especially with today's diode designs, but a head to head test could be interesting. There is always a good amount of potential for premature death, though, and this possibility might complicate results. Since laser manufacturers typically rate their system's MTTF as CW operating time, as long as the systems exceed 10,000 hours CW they meet spec.

  8. #8
    Join Date
    Jan 2008
    Location
    Stockholm, Sweden
    Posts
    799

    Default

    Something which is apparently relevant to this test is how the laser was operating before the test. I chose to bring the lasers from full power to playing the show in the previous test, but what happens if your show calls for a short break from the lasers? Here is the blue viasho that has been idling at the lasing threshold for 2 minutes after playing the last show compared to taking it straight from full power.



    We can see some small differences at the beginning, but after that they are a very close match. Running the numbers shows that the efficiency has dropped from 45% to 43%, not enough to cause a significant shift in color.

    Doing the same for the laserwave green definitely gives some interesting results. In this test the pause was only 1 minute, but that was enough to give a very significant change.



    I think that this kind of behavior is problematic. How could I ever calibrate the colors for a system containing this laser, when I'll get different results depending on what the projector was displaying 5 minutes ago? Such a large change in output power will easily shift a perfect yellow to an orange color.

    Looking at the numbers, the modulation efficiency drops from 59% to 40%. The effect is even more pronounced at the beginning of the show, where the numbers are 68% and 33% respectively.

    I'm interested in hearing fixes to this problem I have, but perhaps that's the subject of another thread.

    Quote Originally Posted by norty303 View Post
    Do you perhaps also need to factor in the issue of the FB3 having 'excessive' something (almost Bill's own words ) going on thats causing lower power than you might see with a QM or other DAC?
    I don't know, should I? These lasers are receiving the same signal and I'm comparing the DPSS lasers to the diode laser. Even if the FB3 does something to lower the power, that effect should be the same for all lasers unless there's a problem with them, which is kind of what I'm looking for.

  9. #9
    Join Date
    Feb 2008
    Location
    East Sussex, England
    Posts
    5,248

    Default

    Originally Posted by norty303 Do you perhaps also need to factor in the issue of the FB3 having 'excessive' something (almost Bill's own words ) going on thats causing lower power than you might see with a QM or other DAC?
    I don't know, should I? These lasers are receiving the same signal and I'm comparing the DPSS lasers to the diode laser. Even if the FB3 does something to lower the power, that effect should be the same for all lasers unless there's a problem with them, which is kind of what I'm looking for
    I can only say I don't know because the 'something' is currently an unknown. However, if that 'something' is excessive or increased modulation, then the FB3 test is going to show the problem up to be bigger than it neccessarily is. E.g. the relative power will be lower overall for those lasers that modulate badly (or at least have slow rise times)

  10. #10
    Join Date
    Dec 2006
    Location
    Netherlands
    Posts
    983

    Default

    Tocket, very interesting! could you also show the modulation signal in the diagrams.
    The warm-up issue can also play part at a very small time scale, i.e., the length of a frame. It is the same problem but I previously characterized that as a duty-cycle dependency problem.
    As you point out the main issue is color: with a low green duty cycle* in a frame my yellow would indeed come out orange.
    I didn;t and don't have access to such a nice meter so I used the eye's sensitivity to color changes to eyeball the quality of modulation.
    (the eye is much more sensitive to color changes that brightness changes)

    *the green laser would be operating colder, reducing the optical output for the same modulation input.

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •