Highly coherent broadband mode-locked laser source and its applications

Abstract

The highly coherent laser source encompasses a wide range of applications, including optical metrology and biomedicine. A mode-locked fiber laser-based highly coherent broadband source presents a promising avenue for generating high-coherence broadband pulses with a repetition rate of up to MHz. Regarding swept speed, this solution outperforms broadband vertical-cavity surface-emitting lasers (VCSELs) and Fourier domain mode-locked (FDML) lasers. And it also has a potentially broader spectrum range. However, only a few reported mode-locked fiber lasers could reach a 3-dB bandwidth of one hundred nm; they feature space-free optical components inserted in the cavity or in the laser system, which lowers the compactness of the cavity configuration and makes the construction of the cavity more complicated. Furthermore, there are rare reports about several MHz to tens of MHz highly coherent broadband laser sources with a 3-dB bandwidth of more than 100 nm owing to the pulse spitting caused by the nonlinearity over accumulated. In this context, the objective of this thesis centers on the investigation of highly coherent mode-locked laser sources that possess a 3-dB bandwidth exceeding 100 nm while operating within a repetition rate range spanning from several MHz to hundreds of MHz. Through fine intracavity dispersion management, we successfully showcased the generation of an all-fiber 183.6 MHz mode-locked pulse featuring a 3-dB bandwidth of 114 nm. To further enhance the spectrum bandwidth and flatness, we propose an extra-cavity spectral shaping method which is based on a booster semiconductor optical amplifier (BOA) with broadband gain and polarization sensitivity properties. This technique enhances external spectrum expansion to ~140 nm from 107.2 nm for a 3-dB bandwidth while the 1-dB bandwidth is improved to ~ 100 nm from 8.1 nm. A broadband fiber laser with a repetition rate at tens of MHz can be a balance between the measurement speed and detection range for specific applications such as optical coherence tomography (OCT). Thus, we developed a 71.7 MHz broadband fiber laser realized by the extracavity Raman-scattering utilization. The ultra-broadband laser source has a 10-dB bandwidth of 179 nm. Additionally, the ultra-broadband pulse spectral can be further shaped into one with a 3-­dB bandwidth of 129 nm by extracavity spectral shaping based on the BOA. A several MHz broadband laser source is preferred to obtain a larger detection range while keeping a high resolution. It can also reduce the bandwidth demand of the photodetector and oscilloscope. We demonstrated a high-coherence, broadband, and low-repetition-rate laser source. The low repetition rate is determined by a 6.86 MHz Er-doped fiber laser inserted single-mode-operation, large-mode-area fiber (LMAF). The spectrum width and flatness are realized by combining a high-coherence SC and a BOA spectral for spectral shaping. With the assistance of spectral shaping by the BOA, the SC located at the broadband gain spectrum of the BOA can be shaped to be a 3-dB bandwidth of > 100 nm and a 10-dB bandwidth of >148 nm.