1.7-μm mode-locked fiber laser for nonlinear optical microscopy

Abstract

Optical microscopy is a valuable method for biological science. At first, conventional linear microscopy was only capable of examing thin samples such as individual cells and thin tissue sections. In recent years, nonlinear optical microscopy (NLOM) has opened up new possibilities for imaging thick and highly scattering biological tissues, including live animals. NLOM also provides intrinsic 3D sectioning without the need for pinholes, as only the region in focus with the highest light intensity is effectively excited. Compared to confocal microscopy, NLOM has less photo bleaching, allowing for long-term observations of cellular and tissue dynamics. Unlike linear microscopy, which uses visible light excitation, NLOM utilizes near infrared (NIR) lasers excitation light that experiences less scattering and enables deeper penetration. Additionally, the lower energy of NIR photons minimizes damage to biological tissues. Among NIR waveband, 1.3-μm and 1.7-μm have been proved to be two significant windows for biomedical imaging considering the scattering and water absorption in tissues. Besides, recent developments indicate that ultrafast fiber lasers operating at 1.7-μm wavelength may have significant applications in biomedical imaging and spectroscopic analysis. This is due to the potential for deeper tissue penetration and the presence of abundant molecular absorption in this spectral wave band. Initially, nonlinear frequency conversion, specifically soliton self-frequency shift, was suggested as a method to produce ultrashort pulses at the 1.7-μm wavelength to overcome the absence of an effective gain medium. However, the amount of frequency shift depends on the peak power of the pulses, leading to a contradiction between the output pulse energy and the desired wavelength. Recently, thulium-doped fiber (TDF), commercially available and capable of emitting a broad range of wavelengths from 1600 nm to 2100 nm in the 3F4 − 3H6 transition, has been shown to be an alternative for achieving effective optical gain in the 1.7-μm wavelength range. This thesis aims to enhance the specificity and depth of all-fiber 1.7-μm mode-locked fiber lasers for NLOM applications. My contributions are categorized into three parts: 1) Building up a ps-level two-color self-synchronized fiber laser that operates with sufficient power in cell-silent regime (~2100 cm-1) for coherent Raman scattering (CRS) microscopy. The whole laser system includes a master cavity, a slave cavity, and amplifiers. 2) Generating pump and Stokes beams in free space, which includes the wavelength selection of Stokes beam and frequency doubling to generate pump beam. To verify the CRS microscopy, we get CARS imagings of 1-Hexyne pure sample. The whole system has potential for further CRS imaging of tissues and cells. 3) Building up a compact all-fiber 1.7-µm mode-locked fiber laser for three photon microscopy (3PM). The cavity is based on SESAM, incorporating a nonlinear optical loop mirror (NOLM), and can be self-started. Various factors, including the gain and mode-locking mechanisms are discussed for low-repetition-rate pulses.