Fourier Domain Mode Locking

The English-language term Fourier domain mode locking ( English , FDML) describes a laser mode coupling technique with which chirps are generated, i.e. wave trains with continuously varying wavelengths .

The technique is used in particular in optical coherence tomography .

idea

In the simplest case, lasers with tunable wavelengths consist of a broadband laser-active medium (gain element), an optical bandpass filter with a variable transmission frequency for the selection of the wavelength and a resonator which usually contains the first two elements mentioned. The tuning speed in a conventional resonator is limited by the time that is required to build up lasing from the ASE subsurface ( amplified spontaneous emission ) at the respective wavelength . As a rule, the light to be amplified even has to pass through the gain medium several times until saturation or at least a sufficiently high intensity is built up. The FDML concept was developed to circumvent this limitation. The tuning frequency is increased by lengthening the ring resonator by a few kilometers in order to synchronize the circular frequency of the light with the tuning frequency of the filter. Light with a certain wavelength that leaves the filter needs exactly the time to circulate in the cavity that the filter may need to go through the entire tuning range once. Thus, many light pulses with different wavelengths can pass through the cavity several times and are always amplified, since the incoming pulse serves as a seed for stimulated emission in the gain medium.

Basic structure of an FDML laser

Layout and function

A FDML laser essentially consists of three components: an optical storage ring as a resonator of length , a tunable optical bandpass filter and a gain element . For FDML operation it is necessary to set the filter tuning frequency to the inverse cycle time of light in the optical storage ring , ${\displaystyle L}$ ${\displaystyle f}$ ${\displaystyle 1/\tau =c/L}$

${\displaystyle f_{i}=i{\frac {c}{L}}}$,

where is the group velocity of light in the storage ring and represents the -th harmonic . ${\displaystyle c}$ ${\displaystyle i}$ ${\displaystyle i}$

A simple FDML laser setup, as shown in the graphic, consists of:

• a semiconductor optical amplifier (SOA) as a light source
• a fiber Fabry-Perot filter (FFP-TF) as an optical band pass filter
• Pole paddles (PC) to optimize the polarization for the SOA
• an optical fiber (DELAY), the length of which must be adjusted so that the light cycle time corresponds to a multiple of the inverse filter driver frequency
• a fiber coupler (FC) to couple out part of the circulating laser light
• optical isolators (ISO)

Pulse compression

Recent studies have demonstrated that FDML lasers are also suitable for generating short laser pulses with a duration of 60 to 70 picoseconds . The advantage over conventional pulse compression is that the energy is stored optically in the fiber and not in the optical amplifier medium. This means that laser pulses can now be generated directly with a low-power semiconductor laser. This technology can potentially be further refined so that pulse durations in the femtosecond range could be implemented in the future.

Individual evidence

1. ^ R. Huber, M. Wojtkowski, JG Fujimoto: Fourier Domain Mode Locking (FDML): A new laser operating regime and applications for optical coherence tomography . In: Optics Express . tape 14 , no. 8 , 2006, p. 3225-3237 , doi : 10.1364 / OE.14.003225 .
2. ^ Robert Huber: Fourier domain mode locking: new lasers for optical coherence tomography . In: SPIE Newsroom . 2009, doi : 10.1117 / 2.1200901.1440 .
3. ↑ 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 . 2013, doi : 10.1038 / ncomms2870 .

Web links

• Fourier domain mode locking: new lasers for optical coherence tomography (engl.)