Frequency combs  are very precise tools which are used for applications that require highest accuracy and resolution when measuring  the frequency of light.  A frequency comb has an optical spectrum that consists of equidistant lines in the frequency domain. These lines serve as reference to measure unknown frequencies or to stabilize a laser to a certain frequency. The discrete spectrum in the frequency domain corresponds to a regular train of laser pulses in the time domain. Such output characteristics can be generated from a standard mode-locked laser oscillator. The discrete lines of the spectrum are separated by the repetition rate f_rep of the laser pulses.

In addition, the so called carrier-envelope offset frequency f_CEO is given by the offset of the periodic spectrum from zero. A mode-locked laser is usually called a frequency comb when both degrees of freedom, f_CEO and f_rep, are stabilized to a frequency reference. The discrete spectrum adopts the absolute stability of the reference and can therefore be used as an “optical ruler”. The performance of a frequency comb is thus given by the choice of reference and the ability to stabilize its phase and frequency fluctuations to this reference.

A non-zero carrier-envelope offset frequency f_CEO is caused by a phase slip between the envelope and carrier of subsequent pulses, the carrier envelope phase (CEP) Δ φCE. The typical method to stabilize f_CEO in a standard comb relies on complex self-referencing methods and an active feedback loop to compensate for phase and frequency fluctuations. Their performance is limited by the finite bandwidth of the electronic feedback circuit. As a result, high frequency phase noise cannot be corrected. Another disadvantage of this method is that usually the actuators used for stabilizing f_CEO and f_rep are not fully decoupled, i.e. they have an effect on both parameters. In practice, this means that if one parameter is tightly locked to a reference, the other suffers from additional unwanted broadening.

In contrast, TOPTICA uses proprietary, completely passive CERO-technology to provide unique, low-noise frequency combs. Since the CERO-principle is based on 'difference frequency generation' (DFG), TOPTICA's frequency comb product line is named 'DFC' - Difference Frequency Combs.