Longitudinal Modes

Discussion in 'Lasershow Designer 2000' started by Dreamlasers_wu, Nov 2, 2005.

  1. Dreamlasers_wu

    Dreamlasers_wu New Member

    The output frequencies of a laser are determined by several factors. First, the gross wavelength is determined by the energy uncertainty (broadening) of the laser transition, which determines the wavelength and overall linewidth. Nonetheless, at any given instant, only a relatively few frequencies within this overall envelope are allowed to oscillate. These "longitudinal modes" result from the boundary conditions that, in a conventional two-mirror lasers, the amplitude of the wave must be zero at the mirror surface (i.e., that the oscillating wave is a standing wave). This means only those laser frequencies that meet the criteria

    n = Nc/2L

    can operate, where c is the speed of light, L is the effective cavity length, and N is an interger. Adjacent modes are typically orthogonally polarized.

    The illustration below shows the lasing envelope of a helium neon laser operating at 632.8nm with a cavity spacing of 23cm. This results in a mode spacing of 640MHz. Since the width of the gain curve (FWHM) is only 1400MHz, only two longitudinal modes can operate at any given time. If the laser were twice as long, four longitudinal modes could operate simultaneously.

    Since the allowable longitudinal modes are a function of cavity length, the frequency will change as the cavity length changes. In lasers where only a few longitudinal modes can operate, these changes will cause outpout power to fluctuate as the modes sweep under the gain curve
    :D :D
  2. Dreamlasers_wu

    Dreamlasers_wu New Member

    Do you know dreamlasers?

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    Shanghai Dream Lasers Technology Co., Ltd. has been the professional manufacturers and original suppliers of DPSS Laser, Precision optics components and Crystals for several years. Our DPSS Lasers are featured with high reliability, high stability, high efficiency, low noise, excellent laser beam quality and low cost. Our lasers are specifically designed for OEM, scientific and industrial use.

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  3. Pangolin

    Pangolin Staff Member

    Re: Do you know dreamlasers?

    Dear Wu,

    Thanks for the explanation. That is very interesting. Often times with DPSS lasers, especially blue lasers, we see the output of these lasers appear very "noisy" insomuch as power jumping from full to pretty low (or zero) at a relatively high frequency (tens of killohertz) and often pretty random. Modulating the power of the laser exacerbates this condition. Is this due to the laser "mode hopping", or is this due to thermal effects inside the laser which change as a result of the modulation? Can you comment about this in general, and whether or not this kind of phenomenon can also be seen with your lasers?

    William Benner
    Pangolin Laser Systems
  4. luminavp

    luminavp Active Member

    What text book did you lift this from ? ;)

  5. Pangolin

    Pangolin Staff Member

    Now Lumin, be nice :)

    Wu sounds like he knows a lot about lasers -- certainly more about the physics than myself! Even if he is quoting from a book, which may or may not be true, he deserves a degree of respect :N

  6. laser_faq

    laser_faq New Member

    Re: Do you know dreamlasers?

    I have a little experiences with noisy of DPSS lasers. The light feedback control and the temperature are key to the noisy. Before making ajustment of the temperature of the KTP and LD, you must gurrantte the light feedback is on the right GAIN and OFFSET. You can see the influence using a scope when you ajust the temperature. Decrease or Increase the temperature of LD or KTP will generate the diffrent waveform on the scope. you can find a best waveform by ajsuting the temp.
  7. Pangolin

    Pangolin Staff Member

    Re: Do you know dreamlasers?

    Thanks for the feedback. Yes, I could see that, especially if the light feedback is obtained through the laser diode itself, and not externally, that this could be a problem. I have also heard that light re-entering the laser diode can cause problems, even if that light is not monitored by the built-in PIN photodiode. I heard that some companies employ an optical isolator (quarter wave plate and polarizing element) to prevent such light from re-entering the laser diode. And lastly, I have recently heard of one company succesfully battling this by employing an etalon within the laser. This seemed particularly necessary in blue laser diodes.