A MHz−Level Swept Source by Time Stretching the Shot−Noise Limited Supercontinuum Based on a Mode−Locked Laser and the Application in Optical Coherence Tomography

The performance of swept−source optical coherence tomography (SS−OCT) is fundamentally limited by a critical trade‐off between achievable imaging range and imaging speed. Current supercontinuum broadening swept source is usually above tens of MHz, which severely restricts the detection distance and application scope of the SS−OCT system. Here, a MHz−level mode−locked laser supercontinuum−based SS−OCT is first presented, with an axial resolution of 11.4 µm and a long imaging range with a 6 dB sensitivity roll−off length of 119 mm. A low−repetition−rate mode‐locked laser with 5.7 MHz is built as the seed laser based on low‐nonlinear large‐mode‐area fiber (LMAF), which is then used to achieve low‐noise and high‐coherence spectral broadening through all‐normal dispersion (ANDi) fiber. A single−pulse operation with long−term stability and self−starting can be achieved without any active feedback control. Notable, the average cross−correlation coefficient between different scanning cycles is as high as 0.997, and the coefficient of variation (CV) is 0.074%, indicating that the constructed swept source has excellent repeatability and high coherence. This work demonstrates the possibility of generating the low−repetition−rate mode−locked laser and MHz swept laser with high coherence and low noise characteristics, which contributes to the development of ultrafast laser and applications of SS−OCT.