Optical frequency comb
Optical frequency comb
With this tool we have a laser light source that produces a spectrum of equally spaced sharp modes with known frequencies. These modes will be employed in a locking scheme to stabilize all laser frequencies used in the group's experiments.MotivationThe field of metrology has experienced huge progress through the use of optical frequency combs as ultrastable frequency references. In 2005 the Nobel Prize in physics was awarded for the development of the comb. Relative frequency stabilities in the optical domain up to 10-16 have been reached . We aim at transferring the stability of the comb to our lasers and attain their phase-locked operation.
We use a femtosecond fiber laser system (Menlo Systems) to generate laser pulses with less than 100 fs duration, at a central wavelength of 1550 nm. These pulses are separated by their 4 ns round trip time in the laser cavity. In the frequency domain this corresponds to a spacing of 250 MHz between adjacent modes. The pulses are first frequency-doubled to around 780 nm and then launched into a photonic crystal fiber. The main nonlinear process inside this fiber is the self-phase modulation of the pulse that causes a broadening from ~550 nm to ~900 nm.
The frequency n of a single mode in the spectrum is given by a multiple of the repetition rate (m·nrep) plus the carrier-envelope offset frequency (nCEO). By stabilizing both of these to an RF reference such as an atomic clock, one transfers the long-term stability of that reference to the optical region, such that all comb modes are stable in phase with respect to each other. As all laser sources are stabilized to the comb, this phase coherence is transferred to the laser frequencies. This allows us to excite certain transitions in the ion where phase coherence between different light fields plays a critical role.
We stabilized one of our laser systems to the frequency comb by a phase locked loop. The current task is to stabilize the two characteristic frequencies of the comb, nrep and nCEO, via a phase lock to our atomic clock.
The long-term objective is to stabilize all the lasers which excite the various transitions in the ion to the comb spectrum. On the other hand we are setting up a 729 nm laser with a very narrow linewidth, which will help reducing the phase noise in the comb lock scheme. Another application will be to excite the Ca ion directly with the pulse trains coming from the comb.
 R. Holzwarth et al., Phys. Rev. Lett., 2000, 85, 11