Stretched-Pulse Mode-Locked Laser Using Post-Cavity Stretching for Ultrahigh-Speed OCT.

We demonstrate a chirped fiber Bragg grating (CFBG) based stretched-pulse mode-locked (SPML) wavelength-swept laser that uses both intra-cavity and post-cavity stretching. The SPML laser with complementary post-cavity stretching offers versatility in OCT imaging speed and eliminates the need for optical buffering. Using an SPML laser with a 5-m intra-cavity CFBG and a 5-m post-cavity CFBG, providing a wavelength sweep rate of 1075 nm/s, we achieved OCT imaging with a sensitivity of 97.4 dB and a single-sided 6-dB coherence length of 5.6 mm. This demonstrated more than two-fold increase in coherence length with a slight sensitivity drop of 0.6 dB compared to an SPML laser with the same wavelength sweep rate using intra-cavity stretching only. The CFBG-SPML laser with post-cavity stretching offers a compelling option for OCT systems, particularly when long imaging range is required.