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Lasing over the EM spectrum

  1. Aug 24, 2004 #1
    I am wondering just which parts of the EM spectrum can we and can we still not lase in. For the purpose of this thread, I consider the visible pretty much covered.

    But what about the IR, is there any thing between 3 and 10 microns?

    What is lasable at longer wavelengths? Are masers available for certain lines only or do they cover a continuum? What about radio? ("raser"?) It that lasable?

    And what about the short wavelengths, where are we at in this area?

    Cheers.
     
  2. jcsd
  3. Aug 24, 2004 #2
    I dont understand what lase means, and cant find a definition on google, please clarify.
     
  4. Aug 25, 2004 #3

    Claude Bile

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    The shortest wavelength that is attainable through lasing that I am aware of is in the UV, at around 190 nm. Past this is impossible for two reasons:

    Solid laser mediums become opaque around 250 nm, only a very few select material are transparent up to 190 nm.

    Gasesous medium begin to ionise well before one gets to 190 nm.

    It is possible to frequency double, or frequency triple a laser to get at higher wavelengths, but to do this, you need some sort of nonlinear crystal anyway, which suffers the same difficulty in that they are opaque at wavelengths shorter than 190 nm.

    In the IR between 3 and 10 microns, it is certainly possible to create a laser that emits these wavelengths, however they are not overly useful, so they are not very prevalent.

    Masers do not cover a continuum, but they do have a large spectral linewidths. Radio waves are not laseable because there are no transitions that can be made to lase via a population inversion.

    Claude.
     
  5. Aug 27, 2004 #4
    Thanks. I know masers are older, but I never bothered to find out how they work, Are they gas transitions? What population inversion is involved?

    I have heard of X-ray lasers (not sure which wavelength specifically), would these be made with doubling or tripling 190 nm?

    ArmoSkater87, strictly speaking, "to lase" is probably slang for producing a laser beam. It involves amplifying light though stimulated atom transitions. Beams produced this way have special properties (esp. narrow linewidth and correlation). With linewidth spreading, it is possible to reach many wavelegths that don't necessarily come from the stimulated transitions.

    I am wondering if some regions of the EM spectrum cannot be covered by this type of beam. Or whether it is actually not possible to produce by any means a coherent laser-beam-like wave for some wavelength.
     
  6. Aug 27, 2004 #5

    Nereid

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    Astronomers has detected lots and lots of natural masers, including water masers. I don't know, off the top of my head, how far into the radio natural masers occur, or have been observed (the site gives a brief idea of the physical conditions, and mechanisms, for natural astrophysical masers).

    Re far UV and X-ray lasing: it's important to distinguish between continuous lasers and 'one-pass' lasers. There were reports (IIRC) that the US had tested nuclear devices which produced intense, one-pass X-ray lasers; I expect the details are military secrets.

    Could appropriate population inversions be created such that lasing could occur in the gamma part of the EM spectrum? Maybe in the collapse of flares on magnetars?
     
  7. Aug 27, 2004 #6

    ZapperZ

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    Actually, if you include free electron lasers in this, there is no limit to the wavelength, in principle. You can literally dial in whatever wavelength you want either by changing the energy of the electrons in the beam, or by adjusting the insertion device (wiggler or undulator) in the beamline.

    The only thing preventing us from having all these lasers is the technical difficulties. I think we have achieved deep UV and IR lasers with FEL. Some of these are what is called SASE FEL (self-amplified spontaneous emission FEL). There is a push right now to get an X-ray FEL using this technique.

    Zz.
     
  8. Aug 27, 2004 #7

    Chronos

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    The US military had a design for orbiting X-ray laser as far back as the 60's, as I recall. They were proposed as an anti-missile defense. It never got off the drawing board because it was a one shot deal [it was powered by a nuclear detonation which was very hard on the equipment], hence very expensive, and targeting the thing was a nightmare [high probability of missing].
     
  9. Aug 29, 2004 #8

    Claude Bile

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    Gonzolo:

    Since X-Rays have wavelengths typically of the order of angstroms, frequency doubling or tripling would not cut it, though it is possible to obtain higher harmonics (51st harmonic generation for example has been acheived, though with low efficiency). The problem using NLO is essentially the transmission of the crystal doing the doubling, no crystal that I am aware of has transmission past 190 nm.

    Masers work the same way as lasers (i.e. via population inversion), however the population inversion is between two vibrational energy levels rather than two electronic energy levels. As such only molecules with a permanent dipole moment, such as water can have purely vibrational spectra and can thus 'mase'.

    Nereid:

    A 'one-pass' laser is not a laser as such, rather it is termed an amplifier. Although the laser acronym implies light amplification, it is technically light oscillation (though the acronym 'loser' dosen't sound so cool).

    The important difference between light amplification and light oscillation is the spectrum. A laser typically has a fine linewidth with a shape determined primarily by the cavity, while light amplification has a spectrum that is determined by the input beam and the gain profile of the medium. Thus so called 'one-pass' X-Ray lasers are not really lasers at all.

    In order to get a laser, the most important requirement is a population inversion between two energy levels. High energy physics isn't my realm of expertise. Perhaps create a population inversion using electron + positron as one state and the annihilated pair (i.e. nothing) as a lower state?

    There are so many possibilities I am not prepared to say that it is not possible. The major problem as I see it is confinement, X-Rays and Gamma rays are highly penetrating and thus very difficult to control, but then again I'm not prepared to say it is impossible.
    --
    Finally, as to free electron lasers, the generally consensus among experts in the field is that it is just an expensive pipe dream. Interesting physics, but not much in the way of practicality. Optical Parametric Oscillators (or OPO's) are a much more viable alternative to obtain a tuneable laser output.

    Claude.
     
  10. Aug 30, 2004 #9

    ZapperZ

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    Strange. The general consensus among "experts" in the field from MY perspective is that it IS going to be extremely valuable, especially for the life sciences and for production of the next generation of high-powered lasers. The LCLS (Linac Coherent Light Source) being built at SLAC right now promises to do just that - provide tunable, intense, and high-brightness X-ray beam for various users in a variety of areas of study.

    This beast is going to be as common as a synchrotron light source.

    Zz.
     
  11. Aug 30, 2004 #10
    Thanks, that answers the main questions I had.

    Now, I have to know more about this LCLS.
     
  12. Aug 30, 2004 #11

    ZapperZ

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