Nonlinearly generated broadband ultrafast laser have been increasingly utilized in many applications. However, traditional techniques of characterizing these sources lack the ability to observe the instantaneous features and transitory behaviours of both amplitude and phase. With the advent of the optical time stretch techniques, the instantaneous shotto- shot spectral intensity can be directly measured continuously at an unprecedentedly high speed. Meanwhile, the information of the real-time phase variation, which is carried by the frequency-time mapped spectral signal has yet been fully explored. We present a technique of experimentally measuring the spectral coherence dynamics of broadband pulsed sources. Our method relies on a delayed Young’s type interferometer combined with optical time-stretch. We perform the proof-of-principle demonstrations of spectral coherence dynamics measurement on two sources: a supercontinuum source and a fiber ring buffered cavity source, both with a repetition rate of MHz. By employing the optical time stretch with a dispersive fiber, we directly map the spectral interference fringes of the delayed neighbouring pulses and obtain a sufficiently large ensemble of spectral interferograms with a real-time oscilloscope (80Gb/s sampling rate). This enables us to directly quantify the spectral coherence dynamics of the ultrafast sources with a temporal resolution down to microseconds. Having the ensemble of single-shot interferograms, we also further calculate the cross spectral coherence correlation matrices of these ultrafast sources. We anticipate that our technique provides a general approach for experimentally evaluating the spectral coherence dynamics of ultrafast laser generated by the nonlinear processes e.g. modulation instability, supercontinuum generation, and Kerr resonator.
Optical coherence tomography (OCT) has been utilized for various functional imaging applications. One of its
highlights comes from spectroscopic imaging, which can simultaneously obtain both morphologic and spectroscopic
information. Assisting diagnosis and therapeutic intervention of coronary artery disease is one of the major
directions in spectroscopic OCT applications. Previously Tanaka et al. have developed a spectral domain OCT (SDOCT)
to image lipid distribution within blood vessel . In the meantime, Fleming et al. have demonstrated optical
frequency domain imaging (OFDI) by a 1.3-μm swept source and quadratic discriminant analysis model .
However, these systems suffered from burdensome computation as the optical properties’ variation was calculated
from a single-band illumination that provided limited contrast. On the other hand, multi-band OCT facilitates
contrast enhancement with separated wavelength bands, which further offers an easier way to distinguish different
materials. Federici and Dubois  and Tsai and Chan  have demonstrated tri-band OCT systems to further
enhance the image contrast. However, these previous work provided under-explored functional properties.
Our group has reported a dual-band OCT system based on parametrically amplified Fourier domain mode-locked
(FDML) laser with time multiplexing scheme  and a dual-band FDML laser OCT system with wavelength-division
multiplexing . Fiber optical parametric amplifier (OPA) can be ideally incorporated in multi-band
spectroscopic OCT system as it has a broad amplification window and offers an additional output range at idler
band, which is phase matched with the signal band. The sweeping ranges can thus overcome traditional wavelength
bands that are limited by intra-cavity amplifiers in FDML lasers. Here, we combines the dual-band FDML laser
together with fiber OPA, which consequently renders a simultaneous tri-band output at 1.3, 1.5, and 1.6 μm, for
intravascular applications. Lipid and blood vessel distribution can be subsequently visualized with the tri-band OCT
system by ex vivo experiments using porcine artery model with artificial lipid plaques.
A tri-band spectroscopic optical coherence tomography (SOCT) system has been implemented for visualization of lipid and blood vessel distribution. The tri-band swept source, which covers output spectrum in 1.3, 1.5, and 1.6 μm wavelength windows, is based on a dual-band Fourier domain mode-locked laser and a fiber optical parametric amplifier. This tri-band SOCT can further differentiate materials, e.g., lipid and artery, qualitatively by contrasting attenuation coefficients difference within any two of these bands. Furthermore, ex vivo imaging of both porcine artery with artificial lipid plaque phantom and mice with coronary artery disease were demonstrated to showcase the capability of our SOCT.
Dual-band optical coherence tomography (OCT) can greatly enhance the imaging contrast with potential applications in functional (spectroscopic) analysis. A new simultaneous dual-band Fourier domain mode-locked swept laser configuration for dual-band OCT is reported. It was based on a custom-designed dual-channel driver to synchronize two different wavelength bands at 1310 and 1550 nm, respectively. Two lasing wavelengths were swept simultaneously from 1260 to 1364.8 nm for the 1310-nm band and from 1500 to 1604 nm for the 1550-nm band at an A-scan rate of 45 kHz. Broadband wavelength-division multiplexing was utilized to couple two wavelength bands into a common catheter for circumferential scanning to form dual-band OCT. The proposed dual-band OCT scheme was applied to endoscopic OCT imaging of mouse esophageal wall ex vivo and human fingertip in vivo to justify the feasibility of the proposed imaging technique. The proposed dual-band OCT system is fast and easy to be implemented, which allows for in vivo high-speed biomedical imaging with potential applications in spectroscopic investigations for endoscopic imaging.
Quantitative phase imaging (QPI) has been proven to be a powerful tool for label-free characterization of biological specimens. However, the imaging speed, largely limited by the image sensor technology, impedes its utility in applications where high-throughput screening and efficient big-data analysis are mandated. We here demonstrate interferometric time-stretch (iTS) microscopy for delivering ultrafast quantitative phase cellular and tissue imaging at an imaging line-scan rate >20 MHz—orders-of-magnitude faster than conventional QPI. Enabling an efficient time-stretch operation in the 1-μm wavelength window, we present an iTS microscope system for practical ultrafast QPI of fixed cells and tissue sections, as well as ultrafast flowing cells (at a flow speed of up to 8 m/s). To the best of our knowledge, this is the first time that time-stretch imaging could reveal quantitative morphological information of cells and tissues with nanometer precision. As many parameters can be further extracted from the phase and can serve as the intrinsic biomarkers for disease diagnosis, iTS microscopy could find its niche in high-throughput and high-content cellular assays (e.g., imaging flow cytometry) as well as tissue refractometric imaging (e.g., whole-slide imaging for digital pathology).