[time-nuts] optically excite a quartz crystal?

Attila Kinali attila at kinali.ch
Sun Apr 20 19:50:37 UTC 2014

On Sun, 20 Apr 2014 10:35:08 -0700
<cdelect at juno.com> wrote:

> After reading about how the BVA oscillators avoid the problems of "on
> crystal" electrodes I was wondering if anyone has tried to optically
> excite a quartz crystal in an oscillator?
> (Use a modulated laser to drive the bare crystal, and a photodetector
> setup to detect and provide feedback?) 
> Seems like it might work. Any comments?

I am not really sure about that.

(Disclaimer: my knowledge about solid state physics and piezo-electric
devices is at best rudimentary, so please correct me if i'm wrong)

The oscillations of the quartz crystal are deformations of the
crystal lattice. This deformation is induced by applying an electrical
field and coupled into the lattice over the piezo-electric effect.

The wavelength of the electromagnetic field is usually much much larger
than the dimensions of the crystal involved. As such, the field can be
seen as constant trough the crystal. I.e. the field induced strain on
the lattice is constant trough the whole length.

On the other hand, the wavelength of lasers is in the order of a couple
thousand times the lattice constant (approx 0.5nm). I.e. the field of
a laser within a quartz crystal wouldn't be constant if one would be
to use a crystal in the sweet spot region between 1MHz and 10MHz.
Using two lasers with a ~10MHz frequency difference and using two
photon absorbtion will probably yield to a very small energy coupling
to induce any measurable oscillation, if it is possible at all
(i don't know of any effect that would translate a two photon absorption
into lattice oscillations).

Thus, i don't think it would be possible to induce oscillations in a quartz 
crystal using a laser based system.

On the other hand, there are currently experiments running to use lasers
to generate RF frequency refernces coupled to the interogation of atomic
clocks (see e.g. [1]) and the results are comparable to ultra low noise
crystal oscillators.

				Attila Kinali

[1] State-of-the-Art RF Signal Generation From Optical Frequency Division.
by Hati, Nelson, Barnes, Lirette, et. al., 2013

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