Broadband Fourier domain mode-locked laser for optical coherence tomography at 1060 nm

Optical coherence tomography (OCT) in the 1060nm range is interesting for in vivo imaging of the human posterior eye segment (retina, choroid, sclera) due to low absorption in water and deep penetration into the tissue. Rapidly tunable light sources, such as Fourier domain mode-locked (FDML) lasers, enable acquisition of densely sampled three-dimensional datasets covering a wide field of view.

However, semiconductor optical amplifiers (SOAs)-the typical laser gain media for swept sources-for the 1060nm band could until recently only provide relatively low output power and bandwidth. We have implemented an FDML laser using a new SOA featuring broad gain bandwidth and high output power. The output spectrum coincides with the wavelength range of minimal water absorption, making the light source ideal for OCT imaging of the posterior eye segment.

With a moderate SOA current (270 mA) we achieve up to 100nm total sweep range and 12 μm depth resolution in air. By modulating the current, we can optimize the output spectrum and thereby improve the resolution to 9 μm in air (~6.5 μm in tissue). The average output power is higher than 20mW. Both sweep directions show similar performance; hence, both can be used for OCT imaging.

This enables an A-scan rate of 350 kHz without buffering the light source output.