Abstract
While swept source optical coherence tomography (OCT) in the 1050 nm range is promising for retinal imaging, there are certain challenges. Conventional semiconductor gain media have limited output power, and the performance of high-speed Fourier domain mode-locked (FDML) lasers suffers from chromatic dispersion in standard optical fiber. We developed a novel light source with a tapered amplifier as gain medium, and investigated the FDML performance comparing two fiber delay lines with different dispersion properties. We introduced an additional gain element into the resonator, and thereby achieved stable FDML operation, exploiting the full bandwidth of the tapered amplifier despite high dispersion. The light source operates at a repetition rate of 116 kHz with an effective average output power in excess of 30 mW. With a total sweep range of 70 nm, we achieved an axial resolution of 15 microm in air (approximately 11 microm in tissue) in OCT measurements. As our work shows, tapered amplifiers are suitable gain media for swept sources at 1050 nm with increased output power, while high gain counteracts dispersion effects in an FDML laser.
Highlights
Optical coherence tomography (OCT) has become an important technique for biomedical imaging [1]
With a total sweep range of 70 nm, we achieved an axial resolution of 15 μm in air (~11 μm in tissue) in optical coherence tomography (OCT) measurements
With the HI-1060 delay line, the light source allows for Fourier domain mode-locked (FDML) operation with maximal tuning bandwidth for the sweep frequency fsw ranging from 116.78 to 116.86 kHz
Summary
Optical coherence tomography (OCT) has become an important technique for biomedical imaging [1]. In an FDML swept source, dispersion in the delay line limits the achievable tuning bandwidth, because frequency synchronization for all wavelengths simultaneously is impossible [19]. Under these conditions, extended photon cavity lifetime is only possible in a narrow wavelength band, while other spectral components exhibit reduced transmission through the tunable filter [20]. The conventional SOA provides a large small-signal gain, while the tapered amplifier features high saturation power This combination allows for stable FDML operation with high output power and broad bandwidth at the same time. The latter section includes a discussion on the relation of dispersion and gain in FDML lasers
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