Fourier-domain mode-locking (FDML)

Fourier-domain mode-locking (FDML) is a novel laser operation scheme. In FDML, the cavity round-trip time is synchronized to the wavelength-tunable filter driving signal. In this way, FDML overcomes the typical speed limitations of other wavelength-swept laser concepts.

Standard swept laser

Current standard high-speed, frequency-swept laser sources typically consist of a broadband gain medium with a tunable optical bandpass filter in the cavity. The maximum achievable frequency tuning rate is limited by the characteristic time constant for building up laser activity inside the cavity. This non-stationary operation, corresponding to a temporally varying distribution of energy between the longitudinal modes of the laser cavity, has many drawbacks, including increased amplitude noise, low power, broad instantaneous linewidth, or short instantaneous coherence length.

standard swept laser 512

A standard swept laser configuration with gain element and optical bandpass filter. Only a very small wavelength range is lasing at each time instant.

tuning speed limit 480

Therefore, lasing has to rebuilt for each new wavelength position, ultimately limiting sweep speed.

FDML laser

These problems can be overcome by extending the laser cavity and periodically driving the optical bandpass filter synchronously with the optical round-trip time of the propagating lightwave in the cavity. This produces a quasi-stationary operation where light from one frequency sweep propagates through the cavity and returns to the optical bandpass filter at the exact time when the transmission of the filter is at the same spectral position. In the frequency domain, this produces a fixed phase relation between the longitudinal cavity modes such that the transient electric field at the optical bandpass filter has only frequency components that match the transient filter transmission, while all other frequency components destructively interfere. We refer to this technique as Fourier domain mode locking (FDML).

FDML is complementary to standard mode locking. In FDML, the spectrum, rather than the amplitude of the field, is modulated. A dynamic spectral window function (wavelength window which changes in time), rather than a temporal one (time window with no wavelength dependence), is applied. As a result, the laser generates a sequence of narrowband optical frequency sweeps at the cavity repetition rate or a harmonic thereof. This frequency-swept output can also be thought of as a sequence of highly chirped, long pulses, with a fixed phase relationship between successive frequency sweeps.

In FDML, all wavelengths are stored in a long delay line, eliminating the sweep speed barrier.


Members of Optores have performed pioneering work on FDML and swept lasers, such as:

  1. Swept laser design and the sweep speed limit.
  2. The original paper on FDML.
  3. First description of FDML and OCT at MHz and even multi-MHz speeds.
Also see articles by our customers at the bottom of the FDML laser product page.