Femtosecond optical frequency combs can be used to measure the frequency of any stable laser source, by directly converting the optical frequency to low frequency signals that can be electronically counted.

However, often a significant challenge to measuring the frequency of a laser is that light must be brought from the laser to the measuring equipment. A transportable frequency comb is essential in those cases where a laboratory does not have its own comb system and where it would not be practical for the laser under test to be moved.

Transportable fibre-based
femtosecond comb

At NPL we have developed a transportable frequency comb based around a MenloSystems FC1500 Optical Frequency Synthesizer. This an erbium-doped fibre laser device and produces a frequency comb spanning the range 500 nm to 2.1 µm. Any stable laser with a wavelength within this range can therefore be calibrated using this system.

The frequency comb is stabilised using a high-stability quartz oscillator which is locked to a GPS-disciplined Rapco 2804AR rubidium oscillator. This link to the GPS network provides traceability to the SI second at any location.

All components of this system are mounted on a mobile aluminium frame, measuring 1.21 m (height), 0.95 m (width) and 1.72 m (length).

The performance of this system has been fully evaluated and has been found to give a stability (Allan deviation) of 1 part in 10¹² at 10 s, with an accuracy of approximately 3 parts in 10¹² for averaging times of a few hours (limited by the GPS reference).