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Laser Linewidth

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(joined February 2015)
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Hello, in a number of the laser and optical transmitter components the line width is a required variable. How is it defined? Is it FWHM or width 20dB down?

Thank you.

Responses (7):

    • #17386
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      Thomas Freir
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      Hello, in a number of the laser and optical transmitter components the line width is a required variable. How is it defined? Is it FWHM or width 20dB down?

      Thank you.

    • #17397
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      Damian Marek
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      Hi Thomas,

      Actually the linewidth is defined from the expected phase noise of the laser. Attached is a screenshot from our documentation on the CW Laser linewidth.

      Regards

      Attachments:
    • #17436
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      Thomas Freir
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      Thank you for the reply Damian,
      I am trying to model a CWDM system using typical values for transmitters/lasers from manufactures data sheet. Linewidth is typically specified in nanometers usually 20dB from the peak. I have yet to identify a data sheet for CWDM lasers that specifies the phase noise. How would you suggest I proceed?

      • #17488
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        Damian Marek
        Participant

        Hi Thomas,

        I had to run this by the OptiSystem team because I was not completely sure! The answer is that the phase is modeled, as I said, as a phase noise which also means the instantaneous frequency is modeled by a probability distribution. This translates to the linewidth corresponds “roughly” to a FWHM of the spectrum. You can verify this with the OSA component in OptiSystem and the CW Laser source. Un-check the Resolution Bandwidth setting in the visualizer other wise you will get varying answers from the averaging.

        To learn more about how we model the linewidth you can check out this commonly cited paper:

        [1] C. Henry, “Theory of the linewidth of semiconductor lasers,” IEEE J. Quantum Electron., vol. 18, no. 2, pp. 259–264, Feb. 1982.

    • #17451
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      Ravil
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      Hi Thomas, the spectral linewidth at FWHM refers to the width of the signal at the half power from its peak level. The spectral linewidth at 20dB refers to the linewwidth at “the bottom” of the signal (1% of peak power) which is usually considered as a total linewidth of the signal and used for CWDM spectral grid calculations.

    • #17452
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      Ravil
      Participant

      Damian, do you happen to know which delta_f is used in your attached document? the one at 20dB or FWHM?

    • #17453
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      Ravil
      Participant

      Thomas, a good source to undestand the concept is this book:
      G. Agrawal “Fiber-Optic Communication Systems”

      +2
    • #43933
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      muhammad jawad
      Participant

      the emission range of wavelengths of a light source is called its linewidth

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