Novel optical sources and signal processing methods for optical imaging modalities

Abstract

Optical coherence tomography (OCT) and optical microscopy (including linear and nonlinear optical microscopy) are two important optical imaging modalities for biomedical imaging applications. An optical imaging system generally consists of an optical source, a light-sample interaction mechanism, and a signal processing unit. In this thesis, a type of novel optical sources and optical signal processing methods are developed to improve system performance of OCT and optical microscopy in terms of imaging speed, imaging contrast, and imaging range. These improvements are achieved through increasing the repetition rate of the optical source, enhancing detection sensitivity, and reducing optical signal bandwidth by leveraging broadband high-energy fiber mode-locked laser, fiber optical parametric amplifier, and dual optical frequency combs. In optical source part, an ultrafast all-fiber inertial-free swept source built upon a broadband fiber mode-locked laser in conjunction with time-stretch technique was demonstrated. A swept source OCT was constructed based on this light source whose repetition rate (44.5 MHz) was one order of magnitude higher than that in conventional OCT systems. Besides, a high-energy (3.9 nJ) fiber mode-locked laser at 1.6-µm window was built for a wavelength-tunable optical source with wavelength tuning range from 1.6 µm to 1.8 µm through soliton self-frequency shift in optical fiber. In addition, a high energy pulse generation scheme for three-photon fluorescence microscope by using this fiber laser and a chirped pulse amplifier was also exhibited. These fiber sources at L-band and longer wavelength can explore a wider scope in deep bio-tissue imaging area for lower water absorption. In optical signal processing part, to enhance detection sensitivity, a scheme by using fiber optical parametric amplifier as the pre-amplifier before the photodetector in optical imaging systems was proposed. The effectiveness by using this sensitivity enhancement scheme was proved in theory based on a semi-classical model, and it was also verified experimentally in a swept-source OCT and time-stretch microscope. The sensitivity enhancement capability with amplified signal band (single band) is determined by the gain of fiber optical parametric amplifier, and 3-dB sensitivity can be further improved by using both amplified signal and phase-conjugated idler together (dual band). This sensitivity enhancement scheme can be potentially applied in the scenarios where ultrafast broadband signal at low-power level is being handled. To reduce optical signal bandwdith at signal processing section, a type of dual optical frequency combs were developed based on electro-optic modulators and nonlinear devices (nonlinear optical loop mirror and a nonlinear amplified loop mirror). A video-rate dual-comb OCT with centimeter imaging range was demonstrated based on this dual-comb source. By down-converting the interference signal from optical domain to radio-frequency domain through dual-comb beating, the down-converted bandwidth of the interference signal in dual-comb OCT was at least two orders of magnitude lower than that in conventional OCT systems. In summary, a high performance optical imaging system normally relies on advanced optical sources and signal processing methods. Fiber mode-locked laser, fiber optical parametric amplifier, and dual optical frequency combs provide powerful and versatile solutions to construct high performance OCT and optical microscopy systems.