||An optical cavity
or optical resonator
is an device that forms a standing wave (in case of a linear
arrangement) or a traveling wave (in case of ring-like arrangement) for
light waves. Optical cavities are a major component of lasers,
surrounding the gain medium and thus providing feedback of the laser
light. They are also used in optical parametric oscillators, in some
interferometers or in ultra-sensitive detection systems. Optical
resonators are typically formed by a set of mirrors arranged in a way
to reflect light onto itself, i.e. light of a specific frequency will
be trapped inside the resonator. Depending on the
the mirrors, Q-factors of the order
of ten to one hundred billions can be achieved.
major drawback of optical resonators formed by mirrors is their
bulkiness and sensitivity to environmental influences. One has to go
great lengths to sufficiently isolate an optical resonator from
environmental influences to be able to either reach the quantum or
thermal noise limit.
Entirely without mirrors, optical cavities can also be formed by what
is known as total internal reflection.
In this case, light is trapped inside a transparent medium with a
refractive index that is higher than the refractive index of the
environment. In the simplest case, such resonators can be produced by
melting the tip of an optical
which results in a tiny glass sphere measuring only a few 100 microns
in diameter. The high purity of the fiber material allows optical
Q-factors in excess of a billion. Therefore, practically by melting a
piece of glass, optical resonators with Q-factors in excess of ten
billions can be obtained. This is thousands of times better
the world's best quartz crystals. A more elaborate way of producing
toroidal structures and many interesting applications can be found here.
Such cavities are interesting candidates for a large variety of
applications, such as cavity quantum electro dynamics,
sensing of single atoms or molecules or for a variety of cavity optomechanics
experiments. If one could fully exploit the high Q-factors of such
structures, tiny ultrastable optical oscillators might become feasible.
vision of a spherical microresonator obtained from melting the end of
an optical fiber. The diameter of the sphere is approximately 0.3mm.
The red line on the right is an optical fiber that carries the light to
and from the optical resonator.