Control of Coherent Light: Time Meets Frequency

Jun Ye

University of Colorado at Boulder
Boulder, CO 80309-0440

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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