High-speed wavelength-swept source at 2.0 μm and its application in imaging through a scattering medium

Fourier Domain Mode Locked Laser and Its Applications

The sweep rate of conventional short-cavity lasers with an intracavity-swept filter is limited by the buildup time of laser signals from spontaneous emissions. The Fourier domain mode-locked (FDML) laser was proposed to overcome the limitations of buildup time by inserting a long fiber delay in the cavity to store the whole swept signal and has attracted much interest in both theoretical and experimental studies. In this review, the theoretical models to understand the dynamics of the FDML laser and the experimental techniques to realize high speed, wide sweep range, long coherence length, high output power and highly stable swept signals in FDML lasers will be discussed. We will then discuss the applications of FDML lasers in optical coherence tomography (OCT), fiber sensing, precision measurement, microwave generation and nonlinear microscopy.

>10 GHz femtosecond fiber laser system at 2.0 μm

We demonstrate a high-power 2.0-μm fiber laser system delivering femtosecond pulses with a fundamental repetition rate of >10 GHz, the highest value so far, to the best of our knowledge. The seed is a self-started fundamentally mode-locked Tm-doped fiber laser that has excellent power and spectral stabilities. The laser system can provide an average power of >600 mW, and the use of soliton-effect-based pulse compression allows the achievement of a pulse duration of 163 fs, leading to a compression factor of ∼ 13. It is anticipated that this new high-power femtosecond fiber laser with a 10-GHz-level fundamental repetition rate can serve as a promising light source for various applications, including laser surgery, micromachining, frequency comb spectroscopy, and nonlinear frequency conversion.

Mo:BiVO4 Nanoparticles-Based Optical Modulator and Its Application in a 2-μm Pulsed Laser

Mo:BiVO4 nanoparticles were employed as an optical modulator in a Q-switched all-solid-state Tm:YAP laser for the first time. The nonlinear optical parameters of Mo:BiVO4 nanoparticles in the 2-μm region were characterized by measuring nonlinear transmission. Saturation intensity was 718 MW/cm2, and the modulation depth was 12.3%. A stable pulse sequence was acquired with a 70.08 kHz maximum repetition rate and an 821 ns pulse width. The maximum output average power was 153 mW, corresponding to 2.18 μJ single pulse energy and 2.67 W peak power. Although the response wavelength of Mo:BiVO4 is in visible light region, our experimental results demonstrates that a saturable absorption effect for wavelengths much longer than visible light (2 μm wavelength) is still possible due to sub-bandgap absorption. Therefore, we experimentally proved that Mo:BiVO4 nanoparticles are a great candidate for use as an optical modulator of a 2-μm pulsed laser.

1.1-µm Band Extended Wide-Bandwidth Wavelength-Swept Laser Based on Polygonal Scanning Wavelength Filter

We demonstrated a 1.1-µm band extended wideband wavelength-swept laser (WSL) that combined two semiconductor optical amplifiers (SOAs) based on a polygonal scanning wavelength filter. The center wavelengths of the two SOAs were 1020 nm and 1140 nm, respectively. Two SOAs were connected in parallel in the form of a Mach-Zehnder interferometer. At a scanning speed of 1.8 kHz, the 10-dB bandwidth of the spectral output and the average power were approximately 228 nm and 16.88 mW, respectively. Owing to the nonlinear effect of the SOA, a decrease was observed in the bandwidth according to the scanning speed. Moreover, the intensity of the WSL decreased because the oscillation time was smaller than the buildup time. In addition, a cholesteric liquid crystal (CLC) cell was fabricated as an application of WSL, and the dynamic change of the first-order reflection of the CLC cell in the 1-µm band was observed using the WSL. The pitch jumps of the reflection band occurred according to the electric field applied to the CLC cell, and instantaneous changes were observed.

Photonic Time-Stretch Technology with Prismatic Pulse Dispersion towards Fast Real-Time Measurements

Photonic time-stretch (PTS) technology enables revolutionary technical breakthroughs in ultrafast electronic and optical systems. By means of employing large chromatic dispersion to map the spectrum of an ultrashort optical pulse into a stretched time-domain waveform (namely, using the dispersive Fourier transformation), PTS overcomes the fundamental speed limitations of conventional techniques. The chromatic dispersion utilized in PTS can be implemented using multiple optical prism arrays, which have the particular advantages of low loss in the extended spectrum outside of the specific telecommunication band, flexibility, and cost-effectiveness. In this article, we propose and demonstrate the PTS technology established for a pair of prisms, which works as a data acquisition approach in ultrafast digitizing, imaging, and measurement regimes.

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 optical imaging at 2.0 μm through second-harmonic-generation-based time-stretch at 1.0 μm

The performance of ultrafast time-stretch imaging at long wavelengths (beyond 1.5 μm) has suffered from low detection sensitivity due to the increasing loss of optical dispersive fibers. Here, we report an ultrafast optical imaging system with a line scan rate of ∼19  MHz at the 2.0-μm wavelength window by combining second-harmonic generation (SHG) with the highly sensitive time-stretch detection at 1.0 μm. In this imaging system, the sample is illuminated by the pulsed laser source at 2.0 μm in the spectrally encoding manner. After SHG, the encoded spectral signal at 2.0 μm is converted to 1.0 μm and then mapped to the time domain through a highly dispersive fiber at 1.0 μm, which provides a superior dispersion-to-loss ratio of ∼53  ps/nm/dB, ∼50  times larger than that of the standard fibers at 2.0 μm (typically ∼1.1  ps/nm/dB). These efforts make it possible for time-stretch technology not only being translated to longer wavelengths, where unique optical absorption contrast exists, but also benefitting from the high detection sensitivity at shorter wavelengths.

Ultra-broadband spatiotemporal sweeping device for high-speed optical imaging

In this work, we propose and demonstrate a spatiotemporal sweeping fiber bundle for ultra-fast optical diagnoses over a multioctave wavelength span, ranging from ∼400  nm to ∼2000  nm. This all-optical spatiotemporal sweeping is realized by precisely controlling the length increment between individual fibers in the fiber bundle. Here, a 200-ps pixel delay increment specifically enables a pixel readout rate of up to 5 GHz. Depending on different configurations of the fiber bundle, either 1D or 2D spatiotemporal sweeping can be realized. Moreover, the high peak power of the short pulse in each pixel can facilitate the highly sensitive optical detection. To showcase its ultra-broadband operation capability, we here perform ultra-fast optical microscopy at three distinctive wavelengths, which are 710 nm, 1030 nm, and 1600 nm, and achieve tens of MHz line-scan rate and few-micrometers resolution for all three experiments. It is anticipated that this inertia-free spatiotemporal sweeping device with ultra-broad bandwidth, GHz pixel readout rate, and high detection sensitivity is promising for ultra-fast optical diagnosis, particularly when hyperspectral characteristics are desired.

Improved Resolution Optical Time Stretch Imaging Based on High Efficiency In-Fiber Diffraction

Most overlooked challenges in ultrafast optical time stretch imaging (OTSI) are sacrificed spatial resolution and higher optical loss. These challenges are originated from optical diffraction devices used in OTSI, which encode image into spectra of ultrashort optical pulses. Conventional free-space diffraction gratings, as widely used in existing OTSI systems, suffer from several inherent drawbacks: limited diffraction efficiency in a non-Littrow configuration due to inherent zeroth-order reflection, high coupling loss between free-space gratings and optical fibers, bulky footprint, and more importantly, sacrificed imaging resolution due to non-full-aperture illumination for individual wavelengths. Here we report resolution-improved and diffraction-efficient OTSI using in-fiber diffraction for the first time to our knowledge. The key to overcome the existing challenges is a 45° tilted fiber grating (TFG), which serves as a compact in-fiber diffraction device offering improved diffraction efficiency (up to 97%), inherent compatibility with optical fibers, and improved imaging resolution owning to almost full-aperture illumination for all illumination wavelengths. 50 million frames per second imaging of fast moving object at 46 m/s with improved imaging resolution has been demonstrated. This conceptually new in-fiber diffraction design opens the way towards cost-effective, compact and high-resolution OTSI systems for image-based high-throughput detection and measurement.

Flexible pulse-stretching for a swept source at 2.0 μm using free-space angular-chirp-enhanced delay

Dispersive pulse-stretching at 2.0 μm has long been hindered by the high intrinsic optical loss from conventional dispersive media. Here a flexible pulse-stretching technique at 2.0 μm is demonstrated over a broad bandwidth with large-scale dispersion and low intrinsic optical loss. The technique employs the newly proposed pulse-stretching scheme, namely, free-space angular-chirp-enhanced delay. Both normal and anomalous temporal dispersion (up to ±500  ps/nm) with low intrinsic loss (<6  dB) over a spectral bandwidth of ∼84  nm at 2.0 μm is obtained with low nonlinear effects. Based on this method, an optical wavelength-swept source at 2.0 μm is realized and applied to spectrally encoded imaging at a line scan rate of ∼19  MHz, proving the potential of this pulse-stretching technique for continuous single-shot measurements at the 2.0 μm wavelength regime, particularly for optical microscopy and spectroscopy.