Nonlinear optical microscopy based on mode-locked fiber lasers

Abstract

Nonlinear optical microscopy (NLOM) is a set of microscopy techniques that utilize nonlinear interactions between field and matter, including two-photon excited fluorescence (TPEF), second harmonic generation (SHG), coherent Raman scattering (CRS), to name a few examples. Comparing with its linear counterparts such as one-photon fluorescence microscopy, NLOM utilized near infrared (NIR) laser as the excitation light source that experiences less light scattering, which otherwise leads to a poor focus into the sample. Comparing with confocal microscopy, NLOM features the 3D sectioning capability without utilizing pinholes, while the localized excitation suffers from less photo bleaching and enables long-term observation of the dynamics in cells or tissues. In addition, some nonlinear interactions in biological tissues are chemical specific, which allows the chemical-sensitive imaging without exogenous staining. Well known examples include SHG and CRS microscopy.

To maximize the excitation efficiency, NLOM is typically performed using pulsed lasers with picosecond or femtosecond pulse width, which is traditionally generated by solid-state lasers such as mode-locked Ti:sapphire lasers and solid-state optical parametric oscillators. Solid-state laser sources can provide excellent intensity stability, high pulse energy and wide wavelength tuning range. However, the large footprint and environmental sensitive operation hinders the wide applications in clinics and laboratories. As an alternative to the solid-state laser, turn-key mode-locked fiber laser that is compact and alignment-free has been established as an attractive light source for a range of NLOM.

The aim of this thesis is to develop multi-functional mode-locked fiber lasers for NLOM applications and improve the imaging speed with time-stretch techniques. My contributions are divided into three parts:

1) TPEF and SHG microscopy requires femtosecond excitation laser source for optimal signal intensity. Here we develop two femtosecond Yb-doped fiber lasers operating at 1010 nm and 1060 nm, respectively, and we also construct a laser scanning microscope. On this platform we perform TPEF and SHG microscopy on various biological samples.

2) The laser mentioned above generates only a single-color output, which cannot be applied to CRS microscopy. To overcome this limitation, we propose a high-power self-synchronized two-color picosecond fiber laser. Different from dual-color fiber lasers demonstrated previously which exhibit high noise level, our design provides excellent intensity stability, low timing jitter, high modulation depth and low pulse width variation over an extended wavenumber tuning range (2700 – 3550 cm^{-1}). We demonstrate high-contrast and fast CRS imaging on both live cells and biological tissues without utilizing complicated balanced detection schemes for reduction.

3) The majority of nonlinear microscopy is performed based on raster scanning which suffers from limited scanning rate. Here we introduce two ultrafast time-stretch imaging schemes that enable an imaging speed of gigapixels per second. The first one is a green light time-stretch microscope requiring merely 1 GHz sampling rate, which is attributed to the dispersive fiber with very high GVD value. The second work further expands the operating wavelength range of time-stretch imaging systems: it exploits a 10’s-MHz spatiotemporal sweeping fiber array for the ultra-fast optical diagnosis over a multioctave span, ranging from ~400 to ~2000 nm.