Dissertation Defense: Yukun Qin "Development of all-fiber format laser sources for nonlinear microscopy and spectroscopy"

    Monday, March 1, 2021 - 1:00pm



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    Nonlinear microscopy and spectroscopy have become increasingly essential techniques for biomedical imaging and probing chemical information. Many lasers have been demonstrated for their capability in multiphoton microscopy and coherent Raman spectroscopy/microscopy. Traditionally, solid-state lasers are the dominant light sources for such experiments. Compared to solid-state lasers, fiber lasers have become more attractive because of their robustness, compactness, and low-cost. Nevertheless, due to the limited selection of rare earth metals doped gain fibers, many useful wavelengths are not accessible with current commercially available gain fibers. Optical fiber-based wavelength conversion techniques have been used for tuning the wavelength to the desirable spectral region to solve the issue.

    In this dissertation, four all-fiber laser sources are presented for 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. These laser sources are designed and constructed without compromising the all-fiber format, making them very compact, robust, and practical for implementation in various environmental settings.

    The first two all-fiber laser sources are based on optical parametric chirped-pulse amplifiers. The lasers were developed by utilizing degenerate four-wave-mixing (FWM) process in optical fibers. We used ~1550 nm pump and two different dispersion-shifted fibers to provide parametric gain near 1300 nm or 1700 nm, respectively.  These laser sources address key needs for multiphoton microscopy because 1300 nm and 1700 nm have low scattering loss and low water absorption which are important features for deep tissue imaging. The amplifiers provided ~40 dB gain around the phase-matched spectral window, which can amplify a small signal (~1 mW) to watt level average power. With the all-fiber optical parametric chirped-pulse amplifier, we demonstrated watt-level output average power; moreover, by using a conventional grating pair compressor, the signal pulses can be compressed down to ~ 350 fs. Multiphoton imaging of several samples is demonstrated with this laser source.

    The third laser source is an all-fiber dual-comb laser source based on a bidirectional fiber laser. The laser has two asynchronized outputs; one pulse train was centered near ~1550 nm (Stokes), and another was centered ~1060 nm (Pump). The Stokes and Pump have an identical chirp-rate. Thus, a narrowband difference frequency excitation can be created when the two pulses overlap spatially and temporally. Since the Stokes and Pump have different repetition rates, self-sampling can be realized at the repetition rate difference. The laser can be used for coherent anti-Stokes Raman spectroscopy in the C-H stretching window. Raman spectra of various samples have been captured with this all-fiber dual-comb system, which shows great potential for developing a low-cost Raman spectrometer for biomedical research.

    The fourth laser source is an all-fiber laser source with two synchronized output pulse trains at ~1550 nm and ~1060 nm, respectively. The laser is similar to the third laser source but with the identical repetition rate for both Stokes and Pump. Compared to the previous dual-comb source, this laser can excite the Raman resonances at the laser’s repetition rate (~100 MHz), which is a good candidate for microscopic application. Coherent Raman imaging for several samples has been demonstrated using this fiber laser.