Precision phase control of femtosecond laser-based
optical frequency combs has produced remarkable and unexpected
progresses in the areas of precision spectroscopy, optical frequency
metrology, and ultrafast optics. A phase stabilized frequency
comb spanning an entire optical octave (> 300 THz) has been
established, leading to a single step, phase coherent connection
between the optical and radio-frequency spectral domains. Optical
frequencies can thus be measured in a straightforward manner by
referencing to millions of marks on a frequency ruler that are
stable at the Hz level while covering most part of the visible
spectrum. The precision comb can also serve as an accurate gear-box
to transfer the oscillatory information of a laser stabilized
by a high quality optical transition down to the microwave/rf
domain, thereby establishing a simple optical atomic clock. We
will present one of the systems based on an optical transition
of iodine molecules, providing an rf clock signal with a frequency
stability comparable to that of an optical standard, and that
is superior to almost all conventional rf sources. To realize
a high-power CW optical frequency synthesizer, a separate widely
tunable single-frequency cw laser has been employed to randomly
access the stabilized optical comb and lock to any desired comb
component. Carrier-envelope phase stabilization of few cycle optical
pulses has recently been realized. This advance in femtosecond
technology is important for both extreme nonlinear optics and
optical frequency metrology. We have now demonstrated stabilization
of the pulse-to-pulse and the absolute carrier-envelope phase
to a level of milli-radians and tens of milli-radians, respectively.
This level of phase coherence is maintained over an experimental
period exceeding a few minutes, paving the groundwork for synthesizing
electric fields with known amplitude and phase at optical frequencies.
Working with two independent femtosecond lasers operating at different
wavelength regions, we can now synchronize the relative timing
between the two pulse trains at the femtosecond level, and also
phase lock the two carrier frequencies, thus establishing phase
coherence between the two lasers. By coherently stitching optical
bandwidth together, a synthesized pulse has been generated. The
simultaneous control of timing jitter (repetition rate) and carrier-envelope
phase can be used to phase coherently superpose a collection of
successive pulses from a mode-locked laser. For example, by stabilizing
the two degrees of freedom of a pulse train to an optical cavity
acting as an coherent delay, constructive interference of sequential
pulses will be built up until a cavity dump is enabled to switch
out the amplified pulse. Furthermore, the synchronization techniques
we developed for pulse synthesis have also made a strong impact
to the field of nonlinear-optics based spectroscopy and imaging,
showing significant improvements in experimental sensitivity and
spatial resolutions. In short, we now appear to have all the experimental
tools required for complete control over coherent light, including
the ability to generate pulses with arbitrary shape, and precisely
controlled frequency and phase, and to synthesize coherent light
from multiple sources.
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©Copyright 1999 KSU Department of Physics |