1 Department of Photonics Engineering, Technical University of Denmark2 Diode Lasers and LED Systems, Department of Photonics Engineering, Technical University of Denmark3 Optical Sensor Technology, Department of Photonics Engineering, Technical University of Denmark4 Risø National Laboratory for Sustainable Energy, Technical University of Denmark5 Copenhagen Center for Health Technology, Center, Technical University of Denmark
Optical coherence tomography (OCT) is a non-invasive imaging technique for visualizing the internal structure of scattering materials, such as biological tissues. It generates two- or three-dimensional images of the sample with cellular (micrometer) resolution. OCT has become an important instrument in the biomedical eld, especially in ophthalmology, where it is used for diagnosing retinal diseases. Using light at 1060nm permits deep penetration into the retina and into the layers beneath, the choroid and the sclera. This wavelength range is also benecial for imaging in eyes affected by cataract. For the 1060nm band, rapidly tunable lasers|so-called swept sources|are available which enable ultra-high speed acquisition of large three-dimensional datasets. However, these light sources require further improvements: higher output power for sufficient signal quality and wider tuning bandwidth for better depth resolution in combination with high tuning speed. We investigate the performance of novel semiconductor laser gain media in fiber-based high-speed swept source prototypes. We demonstrate high output power using a tapered amplifier, and we achieve improved depth resolution with a broadband amplifier and optimization of the light source spectrum. Both technologies are feasible for developing novel high performance swept sources, as we demonstrate by applying the prototypes in OCT imaging. Furthermore, we study numerically how absorption by water in the human eye impairs the performance of retinal imaging. Our simulation reveals a general relationship between the light source bandwidth and the optimal center wavelength, which is supported by our experimental results. This relationship constitutes an important design criterion for future development of high-speed broadband swept sources.