Serial time-encoded amplified microscopy (STEAM) based on a stabilized picosecond supercontinuum source

Ultrafast discrete swept source based on dual chirped combs for microscopic imaging

An inertial-free, ultrafast frequency comb source based on two chirped optical frequency combs (OFCs) is proposed and experimentally demonstrated. The high linearity frequency sweeping is realized by the Vernier effect between the two OFCs rather than any mechanical motion component, so that good stability and reliability are ensured and no recalibration or resampling process is required. Swept rate up to 1 MHz is realized while keeping a narrow instantaneous linewidth of 0.03 nm, thanks to the extra-cavity frequency sweeping method. The wavelength step is proportional to the swept rate (3.8 pm at 10 kHz), and can be tuned by changing the repetition rate difference between the two OFCs. This swept source is applied for high-speed wavelength encoded imaging and achieves 4.4-μm spatial resolution at a 329-kHz frame rate. Compared with the traditional time-stretch microscopy, the signal acquisition bandwidth decreased from 3.8 GHz to below 90 MHz to achieve the same spatial resolution. Furthermore, the exposure time for a specific wavelength is much longer due to the discrete sweeping feature, which is a benefit for higher sensitivity. This discrete swept source provided a promising low-cost option for high-speed biomedical imaging systems and high-accuracy spectroscopy.

Ultrafast time-stretch microscopy based on dual-comb asynchronous optical sampling

The ultrafast time-stretch microscopy based on a single-pixel detector has become a hotspot of the research, owing to its high sensitivity compared to those pixel sensors. However, gigahertz or tens of gigahertz acquisition bandwidth is required for this scheme, resulting in great expense for the whole imaging system and hindering its wide applications. In this Letter, a dual-comb asynchronous optical sampling is applied for the conventional time-stretch microscopy, whose ultrafast temporal axis is magnified by 9200 times. The acquisition bandwidth requirement is thus greatly relaxed, and 320 kHz bandwidth successfully resolves 2.3 μm spatial resolution with tens of kilohertz frame rate.

Time-bandwidth compression of microwave signals

We report and demonstrate a reconfigurable photonic anamorphic stretch transform to realize time-bandwidth product (TBP) compression for microwave signals. A time-spectrum convolution system is employed to provide an ultra-high nonlinear dispersion up to several nanoseconds per gigahertz, which is required for processing nanosecond-long microwave signals. The group delay of the system can be engineered easily by programming a WaveShaper. Based on the proposed scheme, the TBP of a double pulse microwave signal is compressed by 1.9 times. Our proposal can provide a more efficient way to sample, digitize and store high-speed microwave signals, opening up entirely new perspectives for generation of many critical microwave signal processing modules.

Ultrafast measurements of optical spectral coherence by single-shot time-stretch interferometry

The palette of laser technology has significantly been enriched by the innovations in ultrafast optical pulse generation. Our knowledge of the complex pulse dynamics, which is often highly nonlinear and stochastic in nature, is however limited by the scarcity of technologies that can measure fast variation/fluctuation of the spectral phase (or coherence) and amplitude in real-time, continuously. To achieve this goal, we demonstrate ultrafast interferometry enabled by optical time-stretch for real- time spectral coherence characterization with microsecond-resolution. Accessing the single-shot interferograms continuously, it further reveals the degree of second-order coherence, defined by the cross-spectral density function, at high speed-a capability absent in any existing spectroscopic measurement tools. As the technique can simultaneously measure both the high-speed variations of spectrally resolved coherence and intensity, time-stretch interferometry could create a new arena for ultrafast pulse characterization, especially favorable for probing and understanding the non-repetitive or stochastic dynamics in real-time.

Gain-guided soliton fiber laser with high-quality rectangle spectrum for ultrafast time-stretch microscopy

The ultrafast time-stretch microscopy has been proposed to enhance the temporal resolution of a microscopy system. The optical source is a key component for ultrafast time-stretch microscopy system. Herein, we reported on the gain-guided soliton fiber laser with high-quality rectangle spectrum for ultrafast time-stretch microscopy. By virtue of the excellent characteristics of the gain-guided soliton, the output power and the 3-dB bandwidth of the stable mode-locked soliton could be up to 3 mW and 33.7 nm with a high-quality rectangle shape, respectively. With the proposed robust optical source, the ultrafast time-stretch microscopy with the 49.6 μm resolution and a scan rate of 11 MHz was achieved without the external optical amplification. The obtained results demonstrated that the gain-guided soliton fiber laser could be used as an alternative high-quality optical source for ultrafast time-stretch microscopy and will introduce some applications in fields such as biology, chemical, and optical sensing.

High-performance multi-megahertz optical coherence tomography based on amplified optical time-stretch

As the key prerequisite of high-speed volumetric structural and functional tissue imaging in real-time, scaling the A-scan rate beyond MHz has been one of the major pursuits in the development of optical coherence tomography (OCT). Along with a handful of techniques enabling multi-MHz, amplified optical time-stretch OCT (AOT-OCT) has recently been demonstrated as a viable alternative for ultrafast swept-source OCT well above MHz without the need for the mechanical wavelength-tuning mechanism. In this paper, we report a new generation of AOT-OCT demonstrating superior performance to its older generation and all other time-stretch-based OCT modalities in terms of shot-to-shot stability, sensitivity (~90dB), roll-off performance (>4 mm/dB) and A-scan rate (11.5 MHz). Such performance is mainly attributed to the combined contribution from the stable operation of the broadband and compact mode-locked fiber laser as well as the optical amplification in-line with the time-stretch process. The system allows us, for the first time, to deliver volumetric time-stretch-based OCT of biological tissues with the single-shot A-scan rate beyond 10 MHz. Comparing with the existing high-speed OCT systems, the inertia-free AOT-OCT shows promises to realize high-performance 3D OCT imaging at video rate.

Spectrally-resolved statistical characterization of seeded supercontinuum suppression using optical time-stretch

Real-time experimental measurements of the spectrally-resolved noise properties of supercontinuum (SC) have been challenging because of the lack of ultrafast optical spectrometer technologies. Understanding the SC noise is increasingly important because it not only can gain new insight of the complex spectral dynamics of SC generation, but also provides clues to search for stable SC source. Driven by the intense interest in the active seeding mechanism for SC generation, we experimentally demonstrate real-time spectrally-resolved, broadband, statistical characterization of minute continuous-wave (CW) seeded SC, enabled by an ultrahigh-speed spectral acquisition technique called optical time-stretch (OTS). The shot-to-shot statistical analysis shows that the seeded SC exhibits a general compromise between SC bandwidth and spectral stability. OTS also allows us to experimentally identify the seeding condition for SC suppression, in which the spectral broadening is mainly contributed by the cascaded parametric process that delays Akhmediev Breather breakup process and subsequent soliton self-frequency shift. Additionally, the characteristic spectral signature of the Raman solitons, which are becalmed by the minute CW seed, can be clearly captured in real-time by OTS operated at a spectral acquisition rate as high as 20 MHz. We anticipate the OTS technique could provide further new insights for understanding more complex mechanisms of seeded-SC generation which can be examined experimentally.

Multiwavelength time-stretch imaging system

A high-speed microscopic imaging system based on a multiwavelength source and time-stretch technique is proposed and demonstrated. We realize an imaging system at 1D scan rate of 80 MHz with 20 resolvable points. This scheme breaks the bottleneck of large bandwidth and high repetition rate in mode-lock lasers and has great potential for imaging system integration.

Asymmetric-detection time-stretch optical microscopy (ATOM) for ultrafast high-contrast cellular imaging in flow

Accelerating imaging speed in optical microscopy is often realized at the expense of image contrast, image resolution, and detection sensitivity--a common predicament for advancing high-speed and high-throughput cellular imaging. We here demonstrate a new imaging approach, called asymmetric-detection time-stretch optical microscopy (ATOM), which can deliver ultrafast label-free high-contrast flow imaging with well delineated cellular morphological resolution and in-line optical image amplification to overcome the compromised imaging sensitivity at high speed. We show that ATOM can separately reveal the enhanced phase-gradient and absorption contrast in microfluidic live-cell imaging at a flow speed as high as ~10 m/s, corresponding to an imaging throughput of ~100,000 cells/sec. ATOM could thus be the enabling platform to meet the pressing need for intercalating optical microscopy in cellular assay, e.g. imaging flow cytometry--permitting high-throughput access to the morphological information of the individual cells simultaneously with a multitude of parameters obtained in the standard assay.

Simple approach for fast real-time line scan microscopic imaging

A simple fast line scan microscopic imaging approach based on a wavelength-space-time mapping technique has been proposed. With a lab-made subpicosecond pulse laser with 10 dB bandwidth of 12 nm, we experimentally demonstrate a free-space optical apparatus designed for fast line scan imaging of microscopic objects. This system has a spatial resolution of 22 μm, field-of-view of 2.5 mm, and line scan rate of 20.9 MHz. By imaging a modified unitraveling carrier photodetector, we demonstrate the application of semiconductor device inspection for speeding up quality control.

Anamorphic transformation and its application to time-bandwidth compression

A general method for compressing the modulation time-bandwidth product of analog signals is introduced. As one of its applications, this physics-based signal grooming, performed in the analog domain, allows a conventional digitizer to sample and digitize the analog signal with variable resolution. The net result is that frequency components that were beyond the digitizer bandwidth can now be captured and, at the same time, the total digital data size is reduced. This compression is lossless and is achieved through a feature selective reshaping of the signal's complex field, performed in the analog domain prior to sampling. Our method is inspired by operation of Fovea centralis in the human eye and by anamorphic transformation in visual arts. The proposed transform can also be performed in the digital domain as a data compression algorithm to alleviate the storage and transmission bottlenecks associated with "big data."

Optical time-stretch confocal microscopy at 1 μm

We demonstrate optical time-stretch confocal microscopy in the 1 μm spectral window for high-speed and high-resolution cellular imaging. In contrast to the prior demonstrations of time-stretch imaging, which all operated in the telecommunication band, the present work extends the utility of this imaging modality to a wavelength regime (~1 μm), which is well known to be the optimal diagnostic window in biophotonics. This imaging technique enables us to image the nasopharyngeal epithelial cells with cellular resolution (<2 μm), at a line scan rate of 10 MHz, and with a field of view as wide as ~0.44 mm × 0.1 mm. We also theoretically and experimentally characterized the system performance. As the low-loss dispersive fibers for the time-stretch process as well as other essential optical components for enhancing the imaging sensitivity are commonly available at 1 μm, time-stretch confocal microscopy in this wavelength range could usher in realizing high-speed cell imaging with an unprecedented throughput.