Cihang Kong

Ultrafast time-stretch imaging at 932 nm through a new highly-dispersive fiber

Optical glass fiber has played a key role in the development of modern optical communication and attracted the biotechnology researchers great attention because of its properties, such as the wide bandwidth, low attenuation and superior flexibility. For ultrafast optical imaging, particularly, it has been utilized to perform MHz time-stretch imaging with diffraction-limited resolutions, which is also known as serial time-encoded amplified microscopy (STEAM). Unfortunately, time-stretch imaging with dispersive fibers has so far mostly been demonstrated at the optical communication window of 1.5 μm due to lack of efficient dispersive optical fibers operating at the shorter wavelengths, particularly at the bio-favorable window, i.e., <1.0 μm. Through fiber-optic engineering, here we demonstrate a 7.6-MHz dual-color time-stretch optical imaging at bio-favorable wavelengths of 932 nm and 466 nm. The sensitivity at such a high speed is experimentally identified in a slow data-streaming manner. To the best of our knowledge, this is the first time that all-optical time-stretch imaging at ultrahigh speed, high sensitivity and high chirping rate (>1 ns/nm) has been demonstrated at a bio-favorable wavelength window through fiber-optic engineering.

An Ultrafast Wideband Discretely Swept Fiber Laser

The wavelength sweeping technology has gained its popularity in various research areas for the high resolution and high throughput capabilities. Illuminating with continuously wavelength-swept spectra, traditional spectrally encoded optical systems show low detection sensitivities in either time or spectral domains, due to the optical power divergence. In addition, they can also deliver a nontrivial sampling rate when fast line scan is performed, easily go beyond 50 GS/s, which overwhelms the conventional data processing system. In this paper, we demonstrate a 15-MHz discretely swept source at a bandwidth of ∼70 nm particularly for high-speed spectrally encoded applications. The wideband discretely swept laser exhibits higher peak power, which enhances the detection sensitivity of optical system by more than 3 dB. The discretely sweeping characterization of the proposed laser is also proved to have the potential of reducing the data stream for fast processing without compromising the line-scan rate. It is believed that the efforts made in this paper provide a promising resolution for in situ ultrafast optical diagnosis at a higher sensitivity.

Self-healing highly-chirped fiber laser at 1.0 μm

We demonstrate a MHz wavelength-swept fiber laser with diffraction-free and self-healing properties at the bio-favorable wavelength window of 1.0 μm. This ultrafast wavelength sweeping at a high chirp rate is all-optically realized through a newly-designed dispersive fiber that can provide a dispersion amount up to -1.7 ns/nm. It is 8 times larger than the standard single-mode fiber at this window and by adopting a double-pass configuration, the dispersion amount can be further increased to about -3.5 ns/nm, which is 23 times larger than what has previously been demonstrated. Its beam profile, a 2D Airy function, shows no obvious diffraction within a propagation distance of 2 meters and furthermore, the self-healing property is also verified by blocking the main lobe of the laser beam. This is the first wavelength-swept fiber laser equipped with diffraction-free and self-healing properties at the bio-favorable window. We believe that such effort can enable real-time data processing and a deeper penetration for the high-speed spectroscopic applications in the turbid environment.

Compact fs ytterbium fiber laser at 1010 nm for biomedical applications

Ytterbium-doped fiber lasers (YDFLs) working in the near-infrared (NIR) spectral window and capable of high-power operation are popular in recent years. They have been broadly used in a variety of scientific and industrial research areas, including light bullet generation, optical frequency comb formation, materials fabrication, free-space laser communication, and biomedical diagnostics as well. The growing interest in YDFLs has also been cultivated for the generation of high-power femtosecond (fs) pulses. Unfortunately, the operating wavelengths of fs YDFLs have mostly been confined to two spectral bands, i.e., 970-980 nm through the three-level energy transition and 1030-1100 nm through the quasi three-level energy transition, leading to a spectral gap (990-1020 nm) in between, which is attributed to an intrinsically weak gain in this wavelength range. Here we demonstrate a high-power mode-locked fs YDFL operating at 1010 nm, which is accomplished in a compact and cost-effective package. It exhibits superior performance in terms of both short-term and long-term stability, i.e., <0.3% (peak intensity over 2.4 μs) and <4.0% (average power over 24 hours), respectively. To illustrate the practical applications, it is subsequently employed as a versatile fs laser for high-quality nonlinear imaging of biological samples, including two-photon excited fluorescence microscopy of mouse kidney and brain sections, as well as polarization-sensitive second-harmonic generation microscopy of potato starch granules and mouse tail muscle. It is anticipated that these efforts will largely extend the capability of fs YDFLs which is continuously tunable over 970-1100 nm wavelength range for wideband hyperspectral operations, serving as a promising complement to the gold-standard Ti:sapphire fs lasers.

Fiber chirped pulse amplification of a short wavelength mode-locked thulium-doped fiber laser

Exploiting the promising third near-infrared optical window (1600–1870 nm) for deep bioimaging is largely underdeveloped, mostly because of the lack of stable femtosecond laser sources in leveraging the less scattering loss and locally reduced water absorption. In this letter, we demonstrate the fiber chirped pulse amplification of a short wavelength mode-locked thulium-doped fiber laser (TDFL) at 1785 nm. The mode-locked TDFL (via nonlinear polarization rotation) operates stably at the soliton pulsing regime with a fundamental repetition rate of 46.375 MHz. Utilizing a two-stage fiber amplifier incorporated along the pulse chirping fiber, the power of the laser pulse is boosted up to 690 mW. After dechirping with a diffraction grating pair, laser pulse with a duration of 445 fs, pulse energy of 5.7 nJ, and peak power of 12 kW is achieved. Higher power can be achieved by exploiting low-loss high power fiber components at this special wavelength range.

Synchronized dual-repetition-rate two-color fiber lasers for coherent Raman imaging

We demonstrate a passively synchronized two-color pulsed fiber laser with dual repetition rates, 20 MHz for 1.0 μ and 80 MHz for 782 nm. The wavelength tunability of both synchronized pulse sources is also investigated.