Lasers Al Fresco: Fun With Open-Cavity Lasers

Helium-neon lasers may be little more than glorified neon signs, but there’s just something about that glowing glass tube that makes the whole process of stimulated emission easier to understand. But to make things even clearer, you might want to take a step inside the laser with something like [Les Wright]’s open-cavity He-Ne laser.

In most gas lasers, the stimulated emission action takes place within a closed optical cavity, typically formed by a glass tube whose ends are sealed with mirrors, one of which is partially silvered. The gas in the tube is stimulated, by an electrical discharge in the case of a helium-neon laser, and the stimulated photons bounce back and forth between the mirrors until some finally blast out through the partial mirror to form a coherent, monochromatic laser beam. By contrast, an open-cavity laser has a gas-discharge tube sealed with the fully silvered mirror on one end and a Brewster window on the other, which is a very flat piece of glass set at a steep angle to the long axis of the tube and transparent to p-polarized light. A second mirror is positioned opposite the Brewster window and aligned to create a resonant optical cavity external to the tube.

To switch mirrors easily, [Les] crafted a rotating turret mount for six different mirrors. The turret fits in a standard optical bench mirror mount, which lets him precisely align the mirror in two dimensions. He also built a quick alignment jig, as well as a safety enclosure to protect the delicate laser tube. The tube is connected to a high-voltage supply and after a little tweaking the open cavity starts to lase. [Les] could extend the cavity to almost half a meter, although even a waft of smoke was enough obstruction to kill the lasing at that length.

If this open-cavity laser arrangement seems familiar, it might be because [Les] previously looked at an old-school particle counter with such a laser at its heart.

11 thoughts on “Lasers Al Fresco: Fun With Open-Cavity Lasers

  1. Outstanding demo from Les. Great fun.

    I don’t know if Les has a high-resolution spectrometer, but it’s very instructive to look at the spectrum coming out of the tube. We think of a laser being monochromatic but that’s not quite true. The laser amplifier (i.e., the gaseous HeNe gain medium) has positive gain over a bandwidth a GHz or so at the centre frequency of 474 THz (632.8 nm), but the exact frequency (wavelength) is set by the resonator cavity: the round trip path in the cavity must be an exact integer number of wavelengths in order for the resonance to occur.

    If the resonating cavity is long enough (more than 20 cm or so), multiple oscillating modes can exist simultaneously, as long as they remain in the positive-gain 1-2 GHz wide region of the spectrum. Each mode separated by a frequency spacing dictated by the cavity length: a longer cavity can support more modes.

    A 50 cm long cavity (1 m round trip) would support modes 300 MHz apart, and on the spectrum analyzer you would see 3-5 simultaneous peaks withing the gain curve.

    What’s really cool is to watch those peaks sweep through the gain curve as the laser cavity expands during warmup. So a given 39-cm cavity might support modes with 1234567, 1234568 and 1234569 wavelengths, and as the cavity warms and expands the 1234567 wavelength mode will wink out at the lower end of the gain curve, and a new one with 1234570 wavelengths will appear at he higher frequency end of the gain curve, 1.15 GHz higher.

    What’s even cooler is you can temperature-stabilize the tube, monitoring two adjacent peaks, and lock the frequency very precisely to a known frequency, within a kHz or so, or few parts per trillion. It makes a fantastic frequency reference.

    A minor detail is that adjacent peaks in the spectrum are orthogonally polarized, so this won’t work with Les’s external-cavity laser as-is, because the second mode won’t exit the tube (indeed, it won’t even have enough gain to be supported, due to loss in the Brewster window). This only works with tubes with internal mirrors. However, it also means you don’t need a fancy spectrum analyzer to make it work: just a polarizing beamsplitter will separate the two adjacent modes, and two photodiodes will give you the signals to go into a PID controller for the heater. You’ll want a fairly short laser (15-20 cm) for this, so only two or three modes are supported, so you don’t have to go through the trouble of disambiguating modes.

    1. Thanks!
      Longitudinal modes is on my todo list.
      I have a few long He-Ne tubes, but they all have an internal brewster plate. There is one that might not though.
      I don’t have a high res spectrometer,or even a decent spectrum analyzer but I’m sure a review of old literature will point to a way round that!

      1. You might be able to find an old acoustic wave (acousto-optical cell) analyzer head really cheap: input is the driving frequency ramp and output goes to a scope in X-Y mode. Hard to use, no built-in display, and you need to calibrate with a know reference for absolute frequency (we used iodine vapour absorption lines), but spectacular resolution: sub-MHz is easy to get. Kids these days want a digital display and number readout, so nobody uses these things any more, except maybe the telecom fibre guys.

  2. The recent article about the “almost fatal book”, i.e. The Scientific American Book of Projects for the Amateur Scientist” on CD may well include the Amateur Scientist articles on building HeNe lasers from scratch. With Brewster windows at both ends and open cavities. Since lasers were very expensive back then, my high school let me build a glass blowing and vacuum systems lab to try to do so. Never got that far but oh what fun!

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