NIR and SWIR source generation on fiber optical parametric oscillators for biomedical applications

Abstract

Optical sources in near-infrared (NIR) and short-wave infrared (SWIR) wavelength regions have a wide variety of applications, such as chemical sensing, biomedical imaging and so on. However, due to the lack of effective gain medium, it is difficult to acquire efficient sources covering all those wavelengths based on the emission spectra of rare earth ions. Meanwhile, there are also restrictions for laser construction with filters employed inside the intra-cavity since it always limits the tuning range of output wavelengths while the tuning speed is also governed by the mechanical swept rate. Fiber optical parametric amplification, originating from the third-order susceptibility, offers a wide gain bandwidth with net energy and momentum conserved among the interaction between pump with Stokes / anti-Stokes waves. It has many advantages such as prompt response time, wide gain bandwidth, large gain, high pump efficiency, also sharing features of most fiber laser sources in terms of low cost, alignment-free and invariance to environment disturbance. Furthermore, fiber parametric gain spectrum can be customized by dispersion engineering as well as pump wavelength shifting, and this flexibility makes it a preferable choice for wavelength conversion. Conventional optical parametric amplifiers or oscillators are mostly operated in anomalous dispersion region where a flat and continuous gain spectrum is provided. Thanks to the invention of highly-nonlinear fiber (HNLF) with superior performance in dispersion stabilization, it is possible to construct oscillators in normal dispersion region to generate largely tunable narrowband source in distant wavelength regions when phase-match condition is satisfied by higher-order dispersion. Moreover, the narrow gain spectra work as an inherent intra-cavity filter to limit spectrum linewidth within each roundtrip, facilitating the fiber optical parametric oscillator (FOPO) a filter-free cavity. Based on above discussions, several kinds of fiber laser sources based on FOPO were reported in order to serve different applications in this thesis. Firstly, a widely tunable FOPO was constructed with over 480-nm tuning range in NIR and SWIR, with pump wavelength shifting span of 25-nm in conventional communication range. Secondly, combining the wavelength conversion of parametric process and power amplification of thulium-doped fiber (TDF), high-power widely-tunable sources in SWIR region with many potential applications in bio-chemistry field were generated in cooperation with another researcher for the experiment part. Furthermore, aiming at specific bio-imaging requirement - photoacoustic (PA) imaging for lipid detection, the first versatile fiber laser source in 1.7-μm serving for PA microscopy (PAM) - to the best of our knowledge, has been demonstrated. It has flexible parameters such as adjustable repetition frequency, pulse width and output power, as well as continuous wavelength tuning capability and fast wavelength switching speed, which show high potential of superior function in terms of much higher speed and better flexibility over other existing sources in spectroscopic PAM. And it is also promising for dynamic lipid study in future. All works here are aimed at bridging the wavelength gap for source generation in infrared region and targeting the applications in biomedical treatment.