Laser Diode Response Time

Discussion in 'General Electronics Chat' started by bilalbajwa, May 18, 2011.

  1. bilalbajwa

    Thread Starter Member

    Feb 21, 2007
    Guys i am doing a project in which i need to know the response time of the laser diode to the sent pulse. In other words, i am trying to determine how fast a laser diode can emit light of certain intensity to a sent pulse.

    Here is what i have done so far:

    Burst of PULSE Generated Using Waveform Generator:
    Period: 185.1 micro-sec
    Duty Cycle: 80%
    Cycle: 1
    Amplitude: 6V Dc
    Period of Burst: 1 Second

    So every second a pulse of period 185.1 usec 80% duty cycle is sent to the laser diode. Now the weird part is i can see the laser diode blink once a second. This is doubtful because i am sending it such a fast pulse and i shouldn't be able to even see the blink. If i can see it that means its in milliseconds. So my question here is how fast is human eye.

    My whole purpose of the experiment is i want to know whats the fastest pulse i can send to the laser diode and still see the light coming from it at certain intensity.

    In order to perform this experiment, i made a black and totally dark box and installed laser diode on one end and LDR on the opposite end. LDR gave an acceptable response at low frequency however, as the frequency of pulsing increased the LDR became unsuitable.

    I want to now use a photodiode or photo transistor. What's the difference? Can anyone post an example (part number) of each. Moreover, some phototransistor have 3 legs and some has two. Whats going on?

    So let me summarize my experiment.
    I want to know the fastest pulse that i can send to the laser diode and still able to see the light at particular intensity. May be what i am asking is the smallest response time of the laser diode. Can someone suggest what would i be using to note that naked eye, LDR, PhotoDiode or Phototransistor (whats the difference). The laser diode is common <5mW keychain pointer.
  2. beenthere

    Retired Moderator

    Apr 20, 2004
    You might find that investigating persistence of vision will give useful information -
  3. KJ6EAD

    Senior Member

    Apr 30, 2011
    The kind of specifications you're asking about would be in the datasheet for each of the devices you mention. If you know the manufacturer and model number of the device, you can search for the data. If not, you can only determine the rise time, sensitivity, etc. by experimentation, but that brings the limitations of whatever instrumentation you use to make the measurements into the realm of variables as well.

    You need to start with some known quantity, either the emitter, detector or some measurement tool and work from that point forward.

    Hamamatsu Photonics is a well known maker of such devices. Perhaps a browse of their products will give you some ideas: In this example datasheet for photodiodes, you can see spectral sensitivity, rise time, etc. clearly in numerical and graphical form:

    Edmund Optics is a good source for lasers and other optical equipment:
  4. Adjuster

    Well-Known Member

    Dec 26, 2010
    The human eye is really rather slow in its response, but a short burst of light can nevertheless be seen if it is energetic enough. If pulses of equal peak power are made shorter and shorter, a point will be reached where they will appear to get dimmer rather than shorter.

    You have the electrical detectors you listed in ascending order of speed already: LDRs are not especially fast, photo-transistors are generally faster, and potentially photo-diodes may be faster still. Don't expect much speed from a photovoltaic solar panel, but some photo-diodes intended for telecommunications or for optical test instruments can be very fast in the right conditions.

    The photo-diode has some advantages because it can be more easily optimised for its job as a detector, for instance with a P - Intrinsic -N (PIN) construction. Additionally, the signal is obtained directly from the current generated by the optical input, without any of the extra distorting effects which may be present in a photo-transistor. This may make it a better choice for use as a waveform monitor. One major disadvantage of a photo-diode is its relatively low output.
  5. bilalbajwa

    Thread Starter Member

    Feb 21, 2007
    Thanks guys for your reply

    @ Adjuster
    Sir very useful information. The fact you mentioned

    "If pulses of equal peak power are made shorter and shorter, a point will be reached where they will appear to get dimmer rather than shorter. "

    This is exactly what i observed. I had no idea why the light is dimming. Will you please explain why this happens. Does it have something to do with Persistence of vision.

    When i saw this phenomenon happening and following the logic that a pulse sent in micro seconds is too short and i shouldn't be seeing the light from laser diode which appeared to be in duration of milliseconds. I stopped trusting my eyes and this was the reason i wanted to hook up a detector and observe the sent pulse and detector response simultaneously on the oscilloscope. For this reason i need a very fast response detector. When i did some research i found the fact that Photo-diode are very fast compared to photo transistors i dont know why is that the case (pl shed some light.).

    OK, i found these three part numbers economical and that satisfy the requirement. Will you please look at these and suggest a best option.

    One more question: I guess you are familiar with Persistence of Vision clocks. I was wondering how fast the leds are blinking in those type of clocks; moreover, how fast the shaft is rotating. What i am trying to guess here is our eyes potential to see the few microseconds apart blinking leds. Do you think our eyes can see a led(super bright led) blink in microseconds. I performed the experiment using the function generator and sent a pulse of 185usec with 80% dutycycle and i was able to see the pulse but again it appeared to be in milliseconds.

    I highly appreciate your help.
  6. bilalbajwa

    Thread Starter Member

    Feb 21, 2007
    Guys anyone please reply.
  7. ErnieM

    AAC Fanatic!

    Apr 24, 2011
    Of these three the PNZ335 is fastest. RTFM.

    Too bad they list it as as a discontinued type.

    Once we were in the information gathering stage for a potential battery powered product with a LED indicator. We were interested in seeing how short a pulse we could apply and still see a flash on the LED. While I don't remember the number it was way below anything I would expect, perhaps a 5 to 10 mS length from a blue LED could be clearly observed from 40 feet away.

    Think I'm going to repeat the experiment so I have some hard numbers.
  8. bilalbajwa

    Thread Starter Member

    Feb 21, 2007
  9. Adjuster

    Well-Known Member

    Dec 26, 2010
    Even the slower BPW46 photo-diode may well be good enough for your purposes. It is quoted as having 100ns rise and fall times, so could detect pulses down to less than 1μs. Such pulses would be pretty dim, at least if the laser was only working at its normal continuous rated power level.

    When we see a light shining steadily, or a long pulse of light, its brightness is related to its power level. For a short isolated flashes, however, brightness is more related to the amount of energy in each pulse. With a light flashing fast enough to appear steadily illuminated, it is the average power which determines brightness.

    In the case of the persistence of vision clocks (or other similar displays) the eye is not able to separate the rapidly succeeding pulses in a time sense, but the rapid motion of the display LEDs causes light from different pulses to fall on different parts of the light-sensitive interior of the eye. You might be able to estimate the length of the flashes used in such displays by considering the speed of the moving arm carrying the LEDs.

    Photo-transistors are not generally regarded as very fast devices, nor particularly linear. They are used to obtain a relatively big response, by transistor action amplifying photo-current arising in the collector-base junction. Their response time can be quite long because of the same response-speed limitations that apply to any simple transistor amplifier. There may be further limitations relative to a good photo-diode because the photosensitive area may not be so well optimised for fast response.
  10. bilalbajwa

    Thread Starter Member

    Feb 21, 2007
    Thanks everyone and specially Adjuster. Sir just out of curiosity are you in optics field? Would you please recommend a book where i can learn this(optics) stuff!