Femtosecond Laser Frequency Combs

As recognized by the 2005 Nobel Prize in Physics, frequency combs based on femtosecond lasers are immensely powerful tools for a wide range of applications. The precise spectral and temporal qualities of stabilized frequency combs have applications in optical clocks based on atomic references, femtosecond stability timing distribution systems, tests of quantum mechanics and fundamental physical constants astrophysical spectrograph calibration, high harmonic generation or soft x-ray generation, and atomic and molecular spectroscopy to name only a few.

Frequency combs are fundamentally based on mode locked laser technology, which has been available since the 1960’s. The fundamental shift was the development first of microstructure fibers that allowed tremendous coherent broadening of the output spectrum from mode locked lasers. However the broader range of applications has come with the maturing of mode locked Ti:Sapphire lasers and chirped mirror technology for dispersion control, as well as impressive advances in Er:fiber laser technology. Currently, frequency combs directly obtained from femtosecond lasers cover a wavelength range from 0.6 µm to 2 µm. Other wavelength ranges may be achieved via frequency conversion of an available comb, a nonlinear optical process that guarantees the transfer of accuracy and stability of the initial comb.

Controlling the absolute frequency of each component of the output spectrum from the mode locked laser is the defining aspect of a frequency comb. The output spectrum from the comb is described by the spacing between the individual frequency components, , as well as the offset of those components from zero frequency. Knowing and controlling these two frequencies, allows all other frequencies generated from the laser to be simply defined as, . This control can be achieved in various ways, and should be tailored to the expected manner in which the comb will be used for best results.

Transforming a Ti:Sapphire femtosecond laser into a frequency comb. In this particular type of construction, the top half of the diagram corresponds to the stabilization scheme necessary for controlling the pulse repetition rate. The lower right corner contains the optics necessary for control of the offset frequency.

One of the more recently proposed applications for frequency comb technology is the calibration of astronomical spectrographs. Highly precise and highly accurate calibration of astronomical spectrographs is necessary to enable astronomers to use the radial velocity method to search for planets outside our solar system and to unravel other cosmological mysteries. In this method, astronomers monitor the light emitted from stars to observe a slight periodic shift in the emitted spectrum caused by the motion of the star induced by an orbiting planet. In this application, both the accuracy and precision of the frequency comb will be utilized to enable searches for earth like planets and solar systems.