National Physical Laboratory

Femtosecond Optical Frequency Combs Femtosecond Optical Frequency Combs

NPL's femtosecond combs are used to measure the frequencies of optical standards relative to our caesium fountain primary frequency standard and to compare optical standards operating at different frequencies.

A femtosecond comb is based on a mode-locked laser that emits a periodic train of ultrashort pulses. In the frequency domain, its output spectrum corresponds to a comb of millions of equally spaced modes with frequencies:

fn = n frep + f0

where frep is the pulse repetition rate, f0 is the carrier-envelope offset frequency and n is the mode number.

By locking the frequencies frep and f0 to a stable frequency reference, a precise frequency 'ruler' is created in which all comb modes are phase coherent with the frequency reference. The frequency of any stable laser within the bandwidth of the comb can then be measured by observing a beat with the nearest comb mode.

Femtosecond graph

NPL's femtosecond combs

NPL has three femtosecond combs, which play a key role in our optical atomic clock development programme.

Two are home-built systems based on commercial mode-locked Ti:sapphire lasers operating at repetition rates of about 90 MHz and 800 MHz. These systems incorporate a novel type of self-referencing scheme based on Wollaston prisms, which exhibits significantly lower noise than conventional arrangements.

The third comb is a commercial system based on a mode-locked erbium-doped fibre laser. This produces comb spectra spanning the entire spectral region from 500 – 2100 nm. It is designed to be transportable, enabling optical frequency measurements to be carried out away from NPL using a GPS-disciplined oscillator as a frequency reference.


The NPL frequency combs are used to measure the frequencies of our optical clocks relative to our caesium fountain primary frequency standard and to compare optical clocks operating at different frequencies.

Frequency combs can be used to transfer the stability of an optical atomic clock to a 1.5 µm transfer laser. This enables the frequencies of remotely located optical clocks to be compared via optical fibre networks. We have also shown that frequency combs can be transmitted over fibre networks with high fidelity, allowing optical and microwave frequencies to be disseminated simultaneously.

A femtosecond comb referenced to an ultrastable laser can provide a microwave output signal with fractional frequency stability superior to that of the best commercially available microwave oscillators. Within a European collaboration, NPL is investigating the potential of this technology for development of a robust, compact and ultrastable microwave source.

The combination of femtosecond combs and accurate atomic frequency references also makes possible increasingly sensitive tests of fundamental physics and measurements of fundamental constants. The goal of one experiment underway at NPL is to determine the Rydberg constant with improved accuracy by spectroscopy of atomic hydrogen. In another we aim to set stringent limits on any possible present-day variation of the fine structure constant by using the combs to measure the frequency ratio between two optical clock transitions in a single 171Yb+ ion.

In collaboration with the University of Oxford, we are also exploring the potential of direct frequency comb spectroscopy as a method for trace gas detection.

Project team

  • Helen Margolis
  • Giuseppe Marra
  • Luke Johnson
  • Maurice Lessing
  • Patrick Gill

Selected publications

  • Absolute frequency measurement of the 2S1/2 - 2F7/2 electric octupole transition in a single ion of 171Yb+ with 10-15 fractional uncertainty
    S.A. King, R.M. Godun, S.A. Webster, H.S. Margolis, L.A.M. Johnson, K. Szymaniec, P.E.G. Baird and P. Gill
    New Journal of Physics 14, 013045 (2012)
  • Dissemination of an optical frequency comb over fiber with 3x10-18 fractional accuracy
    G. Marra, H.S. Margolis and D.J. Richardson
    Optics Express 20, 1775-1782 (2012)
  • Common-path self-referencing interferometer for carrier-envelope offset frequency stabilization with enhanced noise immunity
    V. Tsatourian, H.S. Margolis, G. Marra, D.T. Reid and P. Gill
    Optics Letters 35, 1209-1211 (2010)
  • Frequency measurement of the 2S1/2 - 2D3/2 electric quadrupole transition in a single 171Yb+ ion
    S. Webster, R. Godun, S. King, G. Huang, B. Walton, V. Tsatourian, H. Margolis, S. Leas and P. Gill
    IEEE Transactions on Ultrasonics, Ferroelectrics and Frequency Control 57, 592-599 (2010)
  • Hertz-level measurement of the optical clock frequency in a single 88Sr+ ion
    H.S Margolis, G.P. Barwood, G. Huang, H.A. Klein, S.N. Lea, K. Szymaniec and P. Gill
    Science 306, 1355-1358 (2004)


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