Let me try to explain some of the jargon. There are basically two uses
of LiNbO3 in optics: as a modulator or as a nonlinear crystal.
A modulator is something that lets different amounts of light through
itself depending on a signal applied to the modulator - usually an
electrical signal. You can kind of think of it as an electrically
controlled shutter. If you apply a voltage to a LiNbO3 modulator, the
"shutter" is closed and no light can get through. If you apply no
voltage, then the shutter is open and light can get through. One of
the reasons that lithium niobate is useful is that you can apply a
very quickly changing voltage to it and it will turn on and off very
quickly. That's what a "high-speed modulator" is. A LiNbO3 modulator
can typically turn on and off at around 40 GHz - i.e. 40 billion times
a second.
The reason you might want to do this is, say, for sending an optical
signal down an optical fiber - you can shine a laser into a LiNbO3
modulator and turn a fast-changing electrical signal into an optical
one. This is very useful for optical communications.
There are two methods of making modulators out of LiNbO3, using either
electro-optics or acousto-optics. Electro-optics uses the fact that a
LiNbO3 crystal will change the polarisation of light that passes
through it depending on how strong a voltage is being applied to the
crystal. If you also add a polarising filter, this can make a
modulator.
The other method is acousto-optics, which relies on LiNbO3's
piezoelectric properties. Lithium niobate is piezoelectric, which
means it will expand and contract slightly if you apply a high voltage
to it. If you continously apply an alternating electric signal to it,
the expansion and contraction causes a sound wave in the crystal. When
light passes through the crystal, it gets bent or deflected by a
process called Bragg Diffraction due to the sound wave. If you turn on
and off the driving signal, you turn on and off the sound wave, and
therefore can turn on and off the ability of the crystal to deflect
light. This is also a form of modulation.
The second major use of LiNbO3 crystals in nonlinear optics. LiNbO3,
along with a few other rare crystals such as potassium dihydrogen
phosphate (KDP) and beta-barium borate (BBO) has an interesting
property that light passing through it can be changed in wavelength,
or color. There are several different things that can be done, but the
most common is probably "frequency doubling" or "second harmonic
generation" (SHG) - doubling the frequency of light (which is the same
thing as halving the wavelength of light).
For instance, one of the commonest ways to make a high-power green
laser is to take an infra-red laser such as a Nd:YAG (wavelength =
1064 nm) and pass the output through a nonlinear optical crystal such
as one made from LiNbO3. The light is halved in wavelength to produce
green light (wavelength = 532 nm).
By varying the size of the crystal and other things you can also shift
the wavelength of the light to other colors. In general the technique
is called "frequency mixing" or wave-mixing.
Hope this is useful to you. |
