TUNABLE LIGHTSOURCE WITH CASCADING PHASE-SHIFTING FILTERING
Tunable lasers or wavelength swept light sources are used to achieve a rapid wavelength tuning. A powerful concept enabling ultra-high tuning rates is e.g. Fourier Domain Mode-Locking (FDML). Based on this principle, fast tunable lasers can be built (FDML-lasers, relying on a resonator optical feedback). A fundamental element for FDML-lasers is the optical fiber, which can be from 100m to km long. Because of their length, these Fibers must show a very little light intensity loss. This requirement prevents the usability of FDML-lasers for shorter wavelengths (for instance in the range of 800 nm). Another limit is the chromatic dispersion of the fiber, which is acceptable for longer wavelength (1050-1550 nm) but becomes higher for shorter ones.
The present invention is based on cascading phase-shifting filtering. According to this concept, the light transmitted from a first optical filter, goes through an optical gain medium and reaches a second optical filter after a certain time. The second filter is driven such that it is in the same state as the first, after the time that the light needs to travel from the first to the second filter. In this way, the same wavelength is transmitted from both filters. The cascading phase-shifting filtering overcomes several restrictions shown by FDML and shows the follwoign advantages:
- It does not use a kilometer long glass fibrer, so dispersion and damping for higher frequencies which are observed in the glass fibres are no issue. For this reason, tunable light sources down to 600 nm can be obtained.
- The special setup of the light source also allows for the use of non-periodic waveforms and decreases the time to tune to a new frequency significantly. Speeds up to 340 000 wavelength sweeps per second over round 100 nm with 0.1 nm linewidth have been reached (see the figure below).
This invention is particularly relevant for Optical Coherent Tomography (OCT), in particular for its application in ophthalmology, where a wavelength-range of 800 nm is needed. Other possible applications are in the profilometry, and in the Bragg-gratings based sensorics.
A prototype has been built and succesfully tested.
 C. M. Eigenwillig, B. R. Biedermann, W. Wieser, and R. Huber, “Wavelength swept amplified spontaneous emission source“, Optics Express 17, 18794-18807 (2009).
 EP 2 364 106 B1, US 8,810,901 B1.