This self
referenced optical frequency comb allows to synthesize almost any
optical frequency. Hence we call such a device an optical
synthesizer. It is based on a Ti:Sapphire mode locked laser that emits
in
the infrared.
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The Ti:Sapphire
ring laser of the left picuture in close up. The green pump laser was
switched off for this picture. The ring design makes this laser almost
immune for optical feedback that could be some trouble in linear laser
cavities used for the same purpose.
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The nonlinear
interferometer used to self reference the optical synthesizer. It is
used to measure the comb offset frequency by frequency doubling the red
wing of the comb and merge it with the blue wing. A photodetector
placed in the merged beam will record the comb offset frequency wich is
then stabilized to an atomic clock along with the repetition frequency.
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This cesium
atomic clock build by the group of Andre Clairon at BNM-SYRTE/Paris has
been brought to out lab to perform the most precise measurement of a
hydrogen transition frequency reported yet. It provided the
reference for locking the comb offset frequency and the repetiton rate
of our fs laser comb.
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The hydrogen spectrometer operated by Nikolai Kolachevsky and Mark
Fischer. The blue laser light is cretaed by frequency doubling a highly
stable dye laser and is subsequently used to drive the 1s-2s transition
in a 6K hydrogen beam insiide the vacuum vessel visible on the left
side.
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An optical
synthesizer based on a mode locked fiber laser. The advantage here is
there are no part that missallign during operation because of slow
temperature variations. Thefore this synthesizer can operate
continuously for several days.
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Electron
microscopic picture of a photonic crystal fiber made by Philip Russells
group at Bath/England. The air channels around the core permit strong
nonlinear interaction. We have used this property to broaden or
frequency combs such until they cover a complete octave. In this case
the simplest form of self referencing described above (frequency
doubling of the red wing) becomes possible.
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A very broad
spectrum covering the whole visible and beyond is produced by self
phase modulation. The resulting laser beam apears white. The same
prcoess takes place i the photonic crystal fiber.
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