Feature Of The Week 12/6/09: Cascaded Wavelength Swept Amplified Spontaneous Emission Source for Optical Coherence Tomography
Feature Of The Week 12/6/09: Due to increasing interest in Optical Coherence Tomography (OCT) using wavelength swept lasers (SS-OCT), huge efforts have been made by both research and commercial organizations around the world to push the performance of such sources and with it OCT system capabilities. This includes sweep repetition rate, power, new wavelengths, coherence length, and more. One of the drawbacks of standard wavelength-swept lasers is the fact that lasing has to build up from amplified spontaneous emission repetitively within several roundtrips of the light, limiting the maximum achievable sweep speed.
Dr. Robert Huber and other researchers from the Ludwig-Maximilians-Universität in Munich have been doing leading work in area of optical coherence tomography (OCT) in general and in particular the area of high performance wavelength swept lasers for OCT systems. This includes the invention of the Fourier Domain Mode Lock Laser (FDML) which set several benchmarks for SS-OCT.
The FDML laser concept circumvented traditional sweep rate limitations and achieved very high imaging speeds. In FDML systems, several kilometers of optical fiber are inserted into the resonator, enabling the synchronization of the wavelength filter resonance to the roundtrip time of the light in the laser cavity. This enabled the entire chirped profile to be synchronously circulating within the laser cavity. However, in the wavelength range of 800nm and 1060nm, relevant for retinal OCT-imaging, higher dispersion and increased loss in the long fiber can complicate FDML operation.
Christoph Eigenwillig, Benjamin Biedermann, Wolfgang Wieser and Robert Huber recently present an alternative approach to realize high speed swept source OCT: a wavelength swept amplified spontaneous emission (ASE) source. In order to achieve sufficient output power, ASE light alternately passes a linear cascade of several different gain elements and different filter elements. Multiple filtering is necessary to prevent excessive amplification of unfiltered ASE background. In order to ensure optimum operation of the source and enable high imaging speed, an accurate, phase-shifted drive-signal for the different wavelength filters is required to account for the propagation time of light between the filters. A wavelength swept ASE source at 1300nm is demonstrated providing a high average output power of ~50mW for sweep rates up to 340 kHz at 100nm full sweep range. Unlike FDML operation, no long delay fiber is required. Therefore, the wavelength swept ASE source could be a promising approach for high speed ophthalmic SS-OCT at 800 nm or 1060nm.