Frequency Combs

An Optical Frequency Comb (OFC) constructed from a stabilized mode-locked laser produces a spectrum of equally spaced optical frequencies.  OFCs are used for high-accuracy optical applications where precise & accurate control over/or knowledge of the wavelengths of a laser is required. The repetition rate of the laser pulses equals the spacing of the frequency comb lines.

Optical Frequency Combs are often generated using a mode-locked Erbium-doped fiber laser. When the OFC is properly stabilized against a well-defined frequency reference, the frequency comb optical spectrum consists of lines separated exactly equally in the frequency domain. This spectrum of “comb teeth” can be used to reference wavelengths or stabilize secondary lasers far from the original laser wavelength.  Frequency combs enable the most precise optical clocks thus far invented.  They are also the engine for Dual Comb Spectroscopy, which utilizes two OFCs with nearly identically spaced comb teeth to record high-resolution spectra over a broad swath of wavelengths rapidly – rivaling Fourier transform infrared spectroscopy (FTIR) in sensitivity and resolution.  The stability of the reference laser can be transferred from the SWIR to the NIR, visible, and even the microwave spectrum making the OFC an extremely useful tool at many wavelengths.

The repetition rate of the mode-locked laser pulses can alternatively be stabilized with respect to a GPS-disciplined radio frequency (RF) reference.  [See Application Note here.]  This is done by comparing the repetition rate of the laser pulses to the RF reference and then using a feedback loop to lock the comb repetition rate, typically by adjusting the cavity length.

Once the repetition rate of the mode-locked laser pulses is stabilized, the OFC can be used to measure the frequency of other optical sources. This is done by beating a “tooth” of the frequency comb with the unknown optical source and measuring the beat frequency. The beat frequency is equal to the difference in frequency between the precisely characterized comb tooth and the laser under investigation.

Other applications include the generation of low-phase noise microwaves and facilitating quantum sensing and computing.