Optical wavelength-swept sources around 2.0-µm wavelength window for ultrafast imaging applications

Abstract

Inspiring developments in optical imaging over the last few decades have created new opportunities in scientific, industrial, and medical fields. In terms of the imaging speed or temporal resolution, time-stretch imaging based on broadband wavelength-swept sources with spectrally encoded imaging system has been proposed for ultrafast optical imaging capable of observing transient dynamics with high throughput. However, the operating wavelength windows of time-stretch imaging are limited to those with available low-loss dispersive fibers for wavelength-sweeping, i.e., the telecommunication window at ~ 1550 nm, near-infrared regimes at 1000 nm and 800 nm. Further translating time-stretch imaging to longer short-wavelength infrared (SWIR) window at 2.0 µm has thus been unfeasible, primarily owing to the exceedingly large transmission loss in conventional optical silica fibers. Nevertheless, the “retina-safe” wavelength window at 2.0 µm, which is close to the far end of the transparent window of silica fibers, has attracted extensive research interests in recent years because of its intrinsic property of high maximum optical power permissible to human eyes and the broad gain bandwidth offered by the developed thulium-doped silica fiber at this wavelength regime. Thus, it has been proved valuable for a wealth of applications in metrology, spectroscopy, sensing and imaging. In this regard, this thesis aims to extend the application area of this wavelength regime at 2.0 µm to ultrafast measurement and mainly offers solutions for feasible high-speed wavelength-swept sources at 2.0 µm based on different dispersive methods for ultrafast time-stretch imaging. In general, the wavelength-swept sources at 2.0 µm are demonstrated in two main directions here: 1) the development of the original fiber laser source; 2) different dispersive methods for wavelength sweeping at 2.0 µm. 1) A supercontinuum source is first employed to conduct the spectrally encoded confocal microscopy (SECM) system. The limiting factors imposed on the imaging resolution are analyzed. Experimentally obtained microscopic images are presented to show the advantages of illumination at this long SWIR wavelength in terms of imaging penetration and depth of field. However, the noise-initiated supercontinuum source is not suitable for wavelength sweeping in time-stretch imaging. Therefore, a mode-locked thulium-doped fiber laser (TDFL) at 2.0 µm based on additive-pulse mode locking (APM) is then conducted. The superior spectral and temporal stability, together with the broad bandwidth of this TDFL indicate its potential for ultrafast time-stretch imaging. 2) Three dispersive methods are demonstrated in this thesis for wavelength sweeping at 2.0 µm. First, a chirped fiber Bragg grating (CFBG) is utilized to introduce chromatic dispersion at 2.0 µm. Second, a flexible technique based on the free-space angular-chirp-enhanced delay (FACED) device is proposed. The working principle of FACED is introduced in terms of its capability for dispersion tuning and achieving large dispersion-to-loss ratio (DLR). Finally, an alternative method by combining second harmonic generation (SHG) with the advanced low-loss, highly dispersive fiber at 1.0 µm is demonstrated. It first converts the spectrally encoded signal at 2.0 µm to 1.0 µm wavelength regime, then takes full advantages of the low-loss dispersive fiber and sensitive off-the-shelf single-pixel photodiodes at 1.0 µm.