Novel optical imaging systems empowered by fiber nonlinearity

Abstract

Fiber nonlinearity is a double-edge sword. It can be deleterious in certain applications such as high-speed optical communications. On the contrary, it can also play an essential role in optical imaging systems by empowering the performance in various scenarios. In optical frequency comb generation, spectrum broadening and pulse reshaping via fiber nonlinearity is demonstrated and further contribute to the imaging performance of two comb-based imaging systems. On the other hand, a phase imaging system based on the phase sensitive parametric process is successfully demonstrated that can readily meet the requirement of multiple applications in biomedical engineering. In optical imaging applications, fast imaging speed is highly desired to enable the real-time observation of dynamics of interest, which benefits the cutting-edge biomedical applications. Driven by such strong motivation, two ultrafast imaging systems based on dual-comb interferometry have been proposed and successfully demonstrated. To enable these two imaging modalities, optical frequency comb has been accordingly developed. Based on electro-optic modulation and nonlinear broadening, a frequency comb centred at 1554 nm with 10-dB bandwidth of ~70 nm and repetition rate of ~8.1 GHz is generated. Specifically, a continuous-wave laser serves as the light seed and is subsequently modulated by amplitude and phase modulators. Afterwards, the light undergoes a nonlinear pulse reshaping and spectrum broadening provided by the nonlinear optical/amplifying loop mirror. By combining two branches of frequency combs with a slightly repetition rate difference, a dual-comb interferometry is hence obtained and in readiness for imaging system application. Firstly, a video-rate dual-comb optical coherence tomography has been demonstrated, which provides the real-time imaging capability and a large imaging depth up to centimeter level. Secondly, we have demonstrated a dual-comb spectrally-encoded confocal microscopy system that features fast and tunable imaging frame rate and low-power imaging capability, while significantly releases the requirements of detection bandwidth and digitization sampling rate. Notably, these two imaging systems manifest the advantages of dual comb interferometry as a light source to probe the spatial structure of sample-under-test. Such dual-comb project brings together the advancement of electro-optic technology and ultrafast optical signal processing to a largely unexplored regime. On the other hand, retrieving phase information is also of great significance in biomedical applications. Different from conventional phase imaging techniques, we propose a quantitative phase imaging based on the phase sensitive parametric process. The system is developed from a spectrally encoded confocal microscopy where the illumination light source is derived from one of the sidebands of a parametric amplifier. The back-reflected spectrally-encoded signal will be launched into a phase sensitive amplifier. As the parametric gain is sensitive to the overall phase between the pump and two sidebands, we can inversely determine the initial phase of the signal by measuring the spectral intensity variation of the sidebands. Moreover, the imaging performance is verified by performing phase imaging on a cubic phase mask and standard microspheres. The detailed study and the proof-of-concept implementation of proposed system will pave the way towards capturing ultrafast phase images and resolving dynamics of biomedical samples in the future work.