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

Area of Science:

• Photonics and Optical Engineering
• Biomedical Imaging Technology
• Laser Physics

Background:

• Wavelength-swept lasers are crucial for optical coherence tomography (OCT) imaging.
• Stretched-pulse mode-locking (SPML) enhances laser performance but can be limited by intra-cavity components.
• Achieving long coherence lengths in OCT is essential for deep tissue imaging.

Purpose of the Study:

• To demonstrate a novel chirped fiber Bragg grating (CFBG) based SPML laser incorporating post-cavity stretching.
• To evaluate the impact of complementary post-cavity stretching on OCT imaging performance, specifically coherence length and sensitivity.
• To offer a versatile laser source for OCT systems requiring extended imaging ranges.

Main Methods:

• A stretched-pulse mode-locked (SPML) laser cavity was designed using a 5-m intra-cavity chirped fiber Bragg grating (CFBG).
• Complementary post-cavity stretching was implemented using an additional 5-m CFBG.
• The laser's wavelength sweep rate was measured at 1075 nm/μs.
• Optical coherence tomography (OCT) imaging was performed, measuring sensitivity and coherence length.

Main Results:

• The CFBG-SPML laser with post-cavity stretching achieved a wavelength sweep rate of 1075 nm/μs.
• OCT imaging demonstrated a sensitivity of 97.4 dB and a 6-dB coherence length of 5.6 mm.
• A more than two-fold increase in coherence length was observed compared to intra-cavity stretching alone, with a minimal sensitivity drop of 0.6 dB.

Conclusions:

• Complementary post-cavity stretching in CFBG-SPML lasers significantly enhances coherence length for OCT imaging.
• This approach offers versatility in OCT imaging speed and eliminates the need for optical buffering.
• The developed laser is a compelling option for OCT applications demanding long imaging ranges.