Our low repetition rate Ti:sapphire-based femtosecond optical frequency comb is generated by a Kerr-lens mode-locked Ti:sapphire laser. In the frequency domain, the output of this laser is a comb of modes, spaced by the repetition rate of the laser. This comb is centred at around 810 nm, and covers a spectral range of around 30 nm to 35 nm FWHM. To broaden the frequency comb to span a full optical octave, about 30 mW of the laser output is coupled into a short length of microstructure fibre.

The frequency of the mth mode of the comb is given by f_m = m f_rep + f₀, where f_rep is the repetition rate of the laser and f₀ is the frequency offset of the whole comb with respect to the frequency origin.

The frequency f_rep is determined by splitting off a small portion of the laser output and detecting the intermode beat spectrum using an avalanche photodiode. The intermode beat signal at the 9th harmonic of#160;f_rep is used to stablise and count the repetition rate.

The offset frequency f₀ is determined using the self-referencing technique. Comb modes in a bandwidth of a few nm around 1060 nm are frequency doubled in a single pass through a KTP crystal, and recombined with modes in an equivalent bandwidth around 530 nm. The resultant beat frequency f₀ is detected using an avalanche photodiode after spectral filtering by a diffraction grating, and is filtered and amplified using an analogue tracking oscillator to obtain a countable signal. The offset frequency is stabilised by adjusting the tilt of the end mirror in the cavity using a piezo-electric stack.

Self-referencing and beats arrangement used on the low repetition rate Ti:sapphire-based comb

The beat frequency f beat between an optical standard and the comb mode nearest in frequency is detected by an avalanche photodiode after spectral filtering using a grating in a similar manner to the offset frequency f₀. Again a tracking oscillator is locked to the beat to obtain a countable signal.

The rf synthesizers and counters used on the frequency comb are all referenced to the 10 MHz output of a hydrogen maser which forms part of the clock ensemble used to generate the timescale UTC(NPL). For measurements requiring the highest level of accuracy, the offset of the hydrogen maser output from exactly 10 MHz is determined by comparison with the NPL caesium fountain.

We have used this low-repetition rate femtosecond comb to measure the frequencies of a number of optical frequency standards. These include the strontium ion standard at 674 nm, the ytterbium ion standard at 467 nm, iodine-stabilised HeNe lasers at 633 nm and an acetylene-stabilised laser at 1.5 μm.