More Mirrors (and A Little Audio) Mean More Laser Power

Lasers are pretty much magic — it’s all done with mirrors. Not every laser, of course, but in the 1980s, the most common lasers in commercial applications were probably the helium-neon laser, which used a couple of mirrors on the end of a chamber filled with gas and a high-voltage discharge to produce a wonderful red-orange beam.

The trouble is, most of the optical power gets left in the tube, with only about 1% breaking free. Luckily, there are ways around this, as [Les Wright] demonstrates with this external passive cavity laser. The guts of the demo below come from [Les]’ earlier teardown of an 80s-era laser particle counter, a well-made instrument powered by a He-Ne laser that was still in fine fettle if a bit anemic in terms of optical power.

[Les] dives into the physics of the problem as well as the original patents from the particle counter manufacturer, which describe a “stabilized external passive cavity.” That’s a pretty fancy name for something remarkably simple: a third mirror mounted to a loudspeaker and placed in the output path of the He-Ne laser. When the speaker is driven by an audio frequency signal, the mirror moves in and out along the axis of the beam, creating a Doppler shift in the beam reflected back into the He-Ne laser and preventing it from interfering with the lasing in the active cavity. This forms a passive cavity that greatly increases the energy density of the beam compared to the bare He-Ne’s output.

The effect of the passive cavity is plain to see in the video. With the oscillator on, the beam in the passive cavity visibly brightens, and can be easily undone with just the slightest change to the optical path. We’d never have guessed something so simple could make such a difference, but there it is.

10 thoughts on “More Mirrors (and A Little Audio) Mean More Laser Power

  1. I don’t know nuttin’ about nuttin’ so forgive my ignorance. But in theory, could this be done multiple times with the same laser source across great distances – i.e. a “chain” of oscillators that keep bumping the laser power? I wonder if we could have “oscillator stations” positioned every so often across the solar system to push a light sail through space?

    1. You don’t need a “chain of oscillators” to do this: Your light sail just has to be a good enough mirror to return essentially 100% of the light it receives back to its source, and vice-versa. In practice, anything beyond a few tens of kilometres will require impractical size and finesse and mass of mirrors though.

  2. Oh, wow, does that ever bring back memories. In 1986 I built an ultra-precise (for the time) laser, locking a dye laser to an external Invar cavity (“etalon”, actually). It was stable to within a few kHz (i.e. 1 part in 10^11), limited by the thermal Doppler motion of the liquid dye molecules.

    It’s true the circulating power in the cavity is many, many times the output beam power, but it’s not *usable* power: the instant you interrupt it to use it, the stored energy in the circulating beam gets dumped, and the lasing action stops. In my laser the circulating power was on the order of a kilowatt, but the output beam was less than a watt. You need *very* good mirrors in the cavity so they don’t absorb that circulating power.

    Itty bits of dust in the beam (usually) don’t absorb enough to kill the cavity Q, so they can appear to be sitting in a very bright beam, as Les demonstrates.

    The exact frequency the laser resonates at is dependent on how many integral wavelengths fit inside the cavity. The lasing medium must have gain greater than unity at a wavelength supported by the cavity — this sets a minimum length for HeNe lasers, for example, because its gain curve is so narrow — only a few hundred megahertz, or 1 part per million.

    The arrangement that Les shows uses a third (exterior) mirror that bounces the output laser light back into the actual resonating cavity, recirculating the photons. This generally causes a resonance at a slightly different wavelength from the main laser cavity, and multiple simultaneous modes can exist (as Les shows). With a phase sensor and an actuator on the external mirror (and some additional filters), you can lock the two cavities to a single frequency, and end up with a single-frequency, single-mode laser, which is what I did in 1986.

    1. Sweet. I have been thinking bout Dye Laser a lot again recently, as I acquired an interesting Laser to pump my home-made effort, it will need modification though.

      I recently acquired a real Brewster windowed tube, and a set of mirrors to play about with, I have already had first light and it is beautiful! As you say the moment the intracavity beam is interrupted, Lasing stops.

      I think there is quite a bit more investigation to be done on the external cavity design, there are a few question marks hanging over my head, for example does it broaden the line width in a detectable fashion, and how can we measure the actual circulating power in the decoupled cavity.

      1. It’s as simple as you imagine: You estimate the circulating power by measuring the transmission of the coupling (exit) mirror (at the wavelength of interest), then just measuring the exiting power.

        I can’t think of a mechanism where an external cavity can broaden a laser line, unless it is being modulated somehow (piezo, mechanical vibration), or the medium in it is modulating the optical path (non-linear effects, thermal convection). I’d expect the line width of a high-Q laser (like a HeNe) to be dominated by the Doppler broadening from the gas molecules’ thermal motion.

      2. As soon as we apply a field, we couple to a state that is radiatively coupled to the ground state. I figure we can extract at least ten to the twenty-first photons per cubic centimeter which will give one kilojoule per cubic centimeter at 600 nanometers, or, one megajoule per liter!

        1. A bit optimistic, even for a solid state laser. Most of the atoms don’t actually participate. Small commercial Nd:YAG rods, for example, come in around 1 J / cc. The massive National Ignition Facility laser, with its almost 400 cubic *meters* of glass laser amplifiers only gets 4 MJ of final beam energy = 10 *milli*joules per cubic centimeter (albeit after a whole bunch of lossy beam shaping and frequency doubling).

          Maybe a disk laser can do better though.

  3. So, if photons move in waves would this technique create harmonics?

    Would a beam splitter tuned to a particular frequency half way down the length of the beam path be able to make use of the harmonic pulses?

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