Tunable External Cavity Diode Laser

  1. http://optics.ph.unimelb.edu.au/atomopt/publications/littrow_rsi_vol72_p4477_2001.pdf

    The picture of the ECDL I explain can be found from the above web site.

    The purpose of piezo is to change the cavity length of the diode laser, so that the wavelength can be varied.

    From the picture, there are two PZT. One is PZT stack, and the other one is PZT disk.


    PZT disk: change the cavity length.

    I have one question. I wonder why when I turn on the laser, my photodiode can only detect the signal of the laser only if I turn on the PZT stack?

    Thank you.
     
  2. jcsd
  3. NoTime

    NoTime 1,570
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    Really just a guess, but looking at the arrangement in the pdf, with the stack off the mirror tilts and the beam alignment would be skewed.
    Lasers require the mirrors to be parallel.
     
  4. Redbelly98

    Redbelly98 12,039
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    Are you saying the laser is definitely on and produces a laser beam, but the photodiode gives no signal? And the beam (if there is one) is hitting the photodiode?

    Or do you mean there really is no laser beam, even though the laser is being powered?

    That mirror is not one of the laser cavity mirrors. Note how it is 45 degrees to the beam.

    It's the grating that functions as a cavity mirror here. Some wavelength is reflected back to the diode, and that determines the wavelength of the laser. Changing the grating angle changes what wavelength is reflected back to the diode.

    Regards,

    Mark
     
  5. First, thank you for your response.

    Here is the link that I uploaded to youtube for easy explanation.


    Redbelly98, you're right, the diffraction grating and the mirror are parallel to each oher and lie 45 degrees to the grating mount.

    As you can see from the video, the diode output beam hit the diffraction grating. The zeroth order diffracted beam will be diffracted to the mirror and travel to the vapor glass cell. Whereas, the 1st order beam is reflected back to the diode itself.

    The absorption spectrum of the atoms are shown on the oscilloscope.

    As you can see, the function generator is connected to the PZT stack. The generator gives out a triangle wave of certain voltage to the PZT stack. The PZT stack as mentioned earlier is to tune the wavelength of the laser by rotating the angle of the diffraction grating.

    I couldn't explain why the on and off of the funciton generator make the pattern disappear??

    Thank you.
     
    Last edited by a moderator: Sep 25, 2014
  6. Redbelly98

    Redbelly98 12,039
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    When you don't have a ramp voltage going to the stack, you are no longer scanning the wavelength. So you won't see the atomic spectrum, instead you should see a constant photodiode signal due to the constant wavelength of the laser.

    Can you answer three questions for me (to help me better understand what is happening):

    1. When the PZT stack voltage is off, is the photodiode signal zero or non-zero?
    2. When the PZT stack voltage is off, is there a beam coming out of the diode that you can see (either visibly or with eg. an IR sensor card)?
    3. Was the oscilloscope on AC or DC mode in the video?
     
  7. Thank you, I think I understand what you mean. You mean in a single wavelength, it's hardly any absorption. But by varying the wavelength in a small size. there'll be different absorption regarding how closed the wavelength to the energy level difference. So, you can see a pattern.

    Am I understanding what you said correctly?

    However, to your explanation, I also find it a bit weird. When there's no pattern on the scope(pzt stack off), I still can see a fluorescence line in the vapor cell by using infrared conversion viewer which means absorption occurs as the electrons are jumping down to lower level giving out lights.

    Answers to your questions:
    1. photodiode diode signal is zero(coz I use AC for oscilloscope)
    2. Yes.
    3. AC. (So, when I put in DC mode, I can see a shift of the whole horizontal line vertically when PZT stack is off)


    Thank you.
     
  8. Redbelly98

    Redbelly98 12,039
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    Not quite. I mean the absorption will be a fixed, constant amount.

    Yes. The amount of absorption depends on the wavelength, so:
    Vary the wavelength --> vary the absorption signal
    Constant wavelength --> constant absorption signal

    I think so.

    Okay, that's helpful to know. So the laser is on, and the wavelength is close enough to the absorption peak to produce fluorescence. If you turn the adjustment screw located at the PZT stack (with PZT stack still off), you should see the fluorescence get brighter or dimmer as you turn that screw.

    Replies to your answers:
    1. No, the photodiode signal is unknown when you measure in AC mode. Use either DC mode, or a voltmeter mode, to measure what the signal is.
    2. Good, I had wondered originally if you really meant the laser does not come on, but now I see that it is in fact on.
    3. Okay. That means there is a signal from the photodiode, since you see a nonzero voltage when the scope is in DC mode.

    Everything seems to work just as it should. If anything is still unclear, feel free to ask.
     
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