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Extreme Fiber Coupled Laser Diode Power Loss

  1. Nov 19, 2016 #1
    What could cause a laser source with fiber coupled laser diodes to have extreme loss of power? The laser diodes have a wavelength of 658nm and have an output rated at 60mW but it seems as though the optical channels actually output ranges between 58 to 70mW.

    During some troubleshooting I noticed that one of my optical channels had a loss of -13dB (95%) so I cleaned the fibers to make sure there was no blockage but when I remeasured I was only given a small gain of maybe +30dB (0.1%). I decided to spot check other channels and noticed that they also had significantly dropped in power.

    I thought that perhaps the sources had been on for too long and that perhaps the diodes where being affected by heat, I shut everything down and proceeded to check the systems the next day after they had cooled but unfortunately the output was still extremely low.
  2. jcsd
  3. Nov 19, 2016 #2


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    They appear to be close to the specified output. What output do you expect from a 60mW diode?

    I do not understand your dB to % conversions. What do the % represent?
    How can +30dB be a small gain when it represents 1000 times as much power?
  4. Nov 19, 2016 #3


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    I'm going to agree with Baluncore

    your misunderstanding / misinterpretation of the figures is leading you to believe there is a problem when one doesn't exist

  5. Nov 21, 2016 #4
    I was using a guide that was helping me convert dB to percentage, I must of made a mistake.

    Either way I still don't understand why my source channels are outputting less then 10mW when after considering loss they should be outputting around 50mW and are rated at 60mW.

    For example when I first measured one channels output power I was given 54mW yet when I recently did a spot check on the same channel I measured 4mW.

    Would a source that has been powered on for a day cause the diode to lose power?

    Could noise cause this problem?
  6. Nov 21, 2016 #5


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    Powering off for a time should not make a difference. If it does then that is an important clue. Monitor the temperature of the module.

    It might be matching of the fibre to your power meter that is a problem. How is the connection made ?

    Some laser diodes have an optical power monitor built onto the back of the diode. Are you able to use that sensor output to independently monitor the laser diode output before the fibre ?

    I believe we can identify the problem if we are given a little more information. It would help if we knew the make and model of the fibre coupled laser diode module, and the make and model of the power meter you are using.
  7. Nov 21, 2016 #6
    This is the link to the laser diode spec sheet

    The fiber coupled laser diode uses a FC connector
    The fiber jumper used to connect to the power meter is ST to FC

    The source that we are using is a recent custom build and the sensor is supposed to be part of the next build

    I am using a Thorlab PM200 power meter
    [/PLAIN] [Broken]
    as well as a Thorlab Photodiode Amplifier
    Last edited by a moderator: May 8, 2017
  8. Dec 5, 2016 #7
    It seems that the issue was due to the bulkhead connector.
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