Fourier domain mode locking

Basic Setup of an FDML Laser

Fourier domain mode locking (FDML) is a laser modelocking technique that creates trains of frequency swept laser pulses. ^[1]

A Fourier domain mode locked laser consists of a ring cavity of length $L$ with tunable optical bandpass filter and gain element. For FDML operation, the filter tuning frequency $f$ needs to match the inverse cavity roundtrip time $1/\tau = c/L$ or a i-th harmonic thereof,

$f_i=i\frac{c}{L}$,

with the group speed of light $c$ in the cavity.

A basic FDML laser configuration - as shown in the figure - consists of a:

• semiconductor optical amplifier (SOA) as gain medium
• fiber Fabry Perot tunable filter (FFP-TF) as bandpass filter
• polarization controller (PC) to optimize the polarization for the SOA
• delay fiber (DELAY) to meet the requirement that the roundtrip time is a multiple of the inverse filter tuning frequency
• fiber coupler (FC) to couple out a fraction of the light power
• isolator (ISO)

A main application for FDML lasers is optical coherence tomography.

New work suggests that FDML lasers are suitable for the generation of short pulses in the range of 60 picoseconds.^[2] Furthermore, the work suggests that FDML lasers could overcome the limitations of conventional pulsed laser operation. Here, the energy of the light field is optically stored in the delay fiber, enabling high energy pulses to be produced directly with a low-power semiconductor laser. Shorter pulses in the femtosecond regime might be possible in the future.

References

1. R. Huber, M. Wojtkowski, and J. G. Fujimoto, "Fourier Domain Mode Locking (FDML): A new laser operating regime and applications for optical coherence tomography," Opt. Express 14, 3225-3237 (2006)
2. Christoph M. Eigenwillig, Wolfgang Wieser, Sebastian Todor, Benjamin R. Biedermann, Thomas Klein, Christian Jirauschek and Robert Huber: Picosecond pulses from wavelength-swept continuous-wave Fourier domain mode-locked lasers In: Nature Communications 4, 1848 (2013).