Development of All-Fiber Format Laser Sources for Nonlinear Microscopy and Spectroscopy

Abstract

Nonlinear microscopy and spectroscopy have become increasingly important techniques that provide improved capabilities for biomedical imaging and probing chemical information. Many types of lasers have been demonstrated for their application in multiphoton microscopy and coherent Raman spectroscopy/microscopy. Traditionally, solid-state lasers are the dominant light sources for such experiments. Compared to the solid-state lasers, fiber lasers have become more attractive because of their robustness, compactness, and low-cost. Nevertheless, due to the limited selection on rare earth metals doped gain fibers, many useful wavelengths are not accessible with current commercially available gain fibers. To solve the issue, optical fiber-based wavelength conversion techniques have been used for tuning the wavelength to the interested spectral region. These fiber-based approaches often utilize free-space coupling tools to achieve the tunability and high output power, which make the fiber laser system become more complex, rather than compact and robust. In this dissertation, four all-fiber format laser sources are presented for the nonlinear microscopy or spectroscopy. The laser sources are developed with commercially available technologies, such as erbium-doped fibers, ytterbium- doped fibers, dispersion-shifted fibers from telecom applications, and spliceable highly nonlinear fibers. The laser sources are designed and constructed without compromising the all-fiber format, which makes them very compact, robust and practical for implementation in clinical settings. For instance, the multiphoton microscope and endoscope using a fiber laser source can be useful tools to obtain high resolution images for in-vivo applications. In Chapter 4 of this dissertation, two all-fiber optical parametric chirped-pulse amplifiers are developed, which can deliver femtosecond pulses near 1300 nm or 1700 nm. These wavelengths are important for deep tissue imaging due to their relatively low scattering loss, and low water absorption. Coherent Raman scattering is known as a nonlinear optical effect that can be utilized for probing the vibrational modes of molecules. This technique is usually used for chemically selective imaging in a variety of applications, such as Raman histology, studying cell metabolism, and pharmaceutical research. In Chapter 5, an all-fiber dual-comb laser source for coherent Raman spectroscopy based on a bidirectional fiber laser is presented. The laser can be used for coherent anti-Stokes Raman spectroscopy in the C-H stretching window. The Raman spectra of various samples have been captured with this all-fiber dual-comb system, which shows a great potential for developing low-cost Raman spectrometer for biomedical researches. In addition to the Raman spectroscopy, in Chapter 6, an all-fiber laser source with two synchronized pulse trains is presented for coherent Raman microscopy. The laser emits two chirped pulse trains around ~1050 nm and ~1550 nm respectively, which covers the important wavenumber band for imaging biological tissues and cells. Coherent Raman imaging of several samples has been demonstrated using this fiber laser. With the demonstrated the imaging capability, the laser presents a promising potential for developing compact, reliable light source for Raman imaging applications.