Emmy Noether Research Group: Fourier Domain Mode Locking (FDML)

The mechanism of Fourier Domain Mode Locking (FDML) was proposed, developed and implemented during my postdoctoral visit at M.I.T.. It represents a completely new operation regime of lasers. Not the amplitude or phase, but the spectrum is modulated, complementary to standard mode locked lasers. A sequence of clean optical frequency sweeps is generated, which have the potential to be compressed to a train of ultrashort laser pulses. An entire sweep is optically stored within the cavity. FDML has the potential to revolutionize biomedical imaging in optical coherence tomography (OCT), overcome limitations of pulsed lasers and offer new possibilities in spectroscopy. In spite of the fact that a fully operational prototype does already exist, the mechanism of FDML itself is hardly understood yet. Goal of the proposed project is a complete theoretical understanding of FDML to investigate experimental limitations and problems and to do research on possible applications of FDML in biomedical imaging and spectroscopy. Research on the mechanism of Fourier Domain Mode Locking could have a lasting impact on the future of laser technology.

The objective of the Emmy Noether Project was to investigate Fourier Domain Mode Locking (FDML) in lasers with a view to gaining a physical understanding and developing new applications in biomedical imaging and sensing. Fourier Domain Mode Locking (FDML) is a novel operating mode in lasers in which a sequence of very pure optical frequency sweeps is generated. The light is "buffered" in a kilometer-long optical fiber. The resulting light waveforms can be used, among other applications, to "scan" biological tissue with very high spatial resolution using the principle of optical coherence tomography (OCT). FDML lasers allow for significantly higher imaging speeds.
During the course of the project, significant progress was made both in understanding FDML operation and in its application to OCT. Using FDML lasers, we were able to build OCT systems with depth scanning rates of up to 20 million lines per second. This is approximately 50 times faster than current research systems from other groups and 200 times faster than commercial systems with comparable image quality. Using a light wavelength of 1050 nm for use in ophthalmology, these fast OCT systems enabled, for the first time, high-resolution in vivo OCT imaging of the human retina over an angle of 70°. This technological advancement enables entirely new examination protocols, diagnostic options, and follow-up studies. Using a light wavelength of 1300 nm for use in cardiology, FDML-based OCT was tested for imaging coronary arteries. In vitro measurements were performed using a completely new catheter developed by a cooperation partner.
With 3 million depth scans per second and a spiral movement of the catheter optics at 192,000 revolutions per minute, a coronary artery can be imaged along its entire length in 0.8 seconds. Since this acquisition time is less than the duration of a heartbeat, it is expected that most image distortions and artifacts of current intravascular OCT images will no longer occur in the future. In addition to the application of FDML lasers for OCT, significant progress has also been made in the field of FDML laser physics. In collaboration with a research group at the Technical University of Munich, the first theoretical description of FDML operation was developed. Furthermore, a short-pulse laser was constructed that, for the first time, enables high pulse energy directly from a semiconductor. This takes advantage of the fact that the kilometers-long fiber optic cable in the FDML laser temporarily stores optical energy as a light field. Such lasers could represent robust and efficient short-pulse light sources in the future. The experiments also discovered a special operating mode of the FDML laser in which intensity and phase noise of the emitted light field virtually disappear. This so-called "sweet spot" and the physical effects that lead to it are not yet understood.