Search

The thin flyer is a small-scale flying object, which is well known as the core functional element of the initiator. Understanding how flyers perform has been a long-standing issue in detonator science. However, it remains a significant challenge to explore how the flyer is formed and functions in the barrel of the initiator via tabletop devices. In this study, we present dynamic and unprecedented images of flyer in barrel via high intensity short-pulse laser. Advanced radiography, coupled with a high-intensity picosecond laser X-ray source, has enabled the provision of state-of-the-art radiographs in a single-shot experiment for observing micron-scale flyer formation in a hollow cylinder in nanoseconds. The flyer was clearly visible in the barrel and was accelerated and restricted differently from that without the barrel. This first implementation of a tabletop X-ray source provided a new approach for capturing dynamic photographs of small-scale flying objects, which were previously reported to be accessible only via an X-ray phase-contrast imaging system at the advanced photon source. These efforts have led to a significant improvement of radiographic capability and a greater understanding of the mechanisms of "burst" of exploding foil initiators for this application.

A polynomial nonlinear filter (PNLF)-based electrical dispersion pre-compensation (pre-EDC) scheme assisted with Gerchberg-Saxton (GS) algorithm is proposed to compensate the chromatic dispersion (CD) for intensity-modulation and direct-detection (IM/DD) optical transmission systems, where PNLF is utilized to fit the nonlinear transfer function of the iterative GS algorithm-based pre-EDC scheme to realize a low-complexity non-iterative CD pre-compensation. The capability of PNLF to fit the nonlinear iterative process enables the PNLF-based pre-EDC scheme to compensate for CD-induced linear distortions and address CD-induced nonlinear distortions, which are typically captured through iterative approaches. Additionally, to further reduce the computational complexity, we also introduce the k-means clustering algorithm to eliminate the weight redundancy and propose a lower-complexity clustered PNLF-based pre-EDC scheme. Simulation results show that PNLF-based and clustered PNLF-based pre-EDC schemes save 76.0% and 97.5% complexity with only 0.3 dB receiver sensitivity penalty at 20% forward error correction (FEC) threshold, compared with GS-based pre-EDC scheme in C-band 56 GBaud 80-km on-off keying (OOK) system. Furthermore, the effectiveness of PNLF-based and clustered PNLF-based pre-EDC schemes is also evaluated through the experimental demonstration. Experimental results show that under C-band 32 GBaud 80-km OOK system, bit error ratio (BER) satisfying 20% FEC threshold is achieved by applying PNLF-based and clustered PNLF-based pre-EDC schemes, which save 78.3% and 94.2% complexity with only 0.4 dB receiver sensitivity penalty compared with GS-based pre-EDC scheme, respectively. The research results indicate that the (clustered) PNLF-based pre-EDC scheme has the great application potential for CD compensation in high-performance and low-cost IM/DD optical transmission systems.

In this paper, a low-complexity optimized detection scheme consisting of a post filter with weight sharing (PF-WS) and cluster-assisted log-maximum a posteriori estimation (CA-Log-MAP) is proposed. Besides, a modified equal-width discrete (MEWD) clustering algorithm is proposed to eliminate the training process during clustering. After channel equalization, optimized detection schemes improve performance by suppressing the in-band noise raised by the equalizers. The proposed optimized detection scheme was experimentally performed in a C-band 64-Gb/s on-off keying (OOK) transmission system over 100-km standard single-mode fiber (SSMF) transmission. Compared with the optimized detection scheme with the lowest complexity, the proposed method saves 69.23% required number of real-valued multiplications per symbol (RNRM) at 7% hard-decision forward error correction (HD-FEC). In addition, when the detection performance reaches saturation, the proposed CA-Log-MAP with MEWD saves 82.93% RNRM. Compared with the classic k-means clustering algorithm, the proposed MEWD has the same performance without a training process. To the best of our knowledge, this is the first time clustering algorithms have been applied to optimize decision schemes.

We report on a high-power Ho:Y 2 O 3 ceramic laser at 2.1 µm with controllable output beam profile ranging from LG 01 donut, flat-top to TEM 00 mode using a simple two-mirror resonator. In-band pumped at 1943nm using a Tm fiber laser beam shaped via a coupling optics comprising a capillary fiber and lens-combination to achieve distributed pump absorption in Ho:Y 2 O 3 and hence selective excitation of the target mode, the laser yields 29.7 W of LG 01 donut, 28.0 W of crater-like, 27.7 W of flat-top and 33.5 W of TEM 00 mode output for absorbed pump power of 53.5 W, 56.2 W, 57.3 W and 58.2 W, respectively, corresponding to a slope efficiency of 58.5%, 54.3%, 53.8% and 61.2%. This is, to the best of our knowledge, the first demonstration of laser generation with continuously tunable output intensity profile at ∼2 µm wavelength region.

We propose a polarized, angle-resolved spectral (PARS) reflectometry for simultaneous thickness and refractive-index measurement of ultra-thin films in real time. This technology acquires a two-dimensional, angle-resolved spectrum through a dual-angle analyzer in a single shot by radially filtering the back-focal-plane image of a high-NA objective for dispersion analysis. Thus, film parameters, including thickness and refractive indices, are precisely fitted from the hyper-spectrum in angular and wavelength domains. Through a high-accuracy spectral calibration, a primary PARS system was built. Its accuracy was carefully verified by testing a set of SiO 2 thin films of thicknesses within two µm grown on monocrystalline-Si substrates against a commercial spectroscopic ellipsometer. Results show that the single-shot PARS reflectometry results in a root-mean-square absolute accuracy error of ∼1 nm in film thickness measurement without knowing its refractive indices.

Stratospheric aerosols play an important role in the atmospheric chemical and radiative balance. To detect the stratospheric aerosol layer, a 1064 nm lidar with high resolution and large dynamic range is developed using a superconducting nanowire single-photon detector (SNSPD). Measurements are typically performed at 1064 nm for its sensitivity to aerosol, whereas detectors are limited by low efficiency and high dark count rate (DCR). SNSPDs are characterized by high efficiency in the infrared wavelength domain, as well as low noise and dead time, which can significantly enhance the signal quality. However, it is still challenging to build an SNSPD with both large active area and high count rate. To improve the maximal count rate (MCR) so as to avoid saturation in the near range, a 16-pixel interleaved SNSPD array and a multichannel data acquisition system are developed. As a reference, a synchronous system working at 532 nm is applied. In a continuous comparison experiment, backscatter ratio profiles are retrieved with resolutions of 90 m/3 min, and the 1064 nm system shows better performance, which is sensitive to aerosols and immune to the contamination of the ozone absorption and density of molecule change in the lower stratosphere.

Fringe projector profilometry (FPP) is an important three-dimensional (3D) measurement technique, especially when high precision and speed are required. Thus, theoretical interrogation is critical to provide deep understanding and possible improvement of FPP. By dividing an FPP measurement process into four steps (system calibration, phase measurement, pixel correspondence, and 3D reconstruction), we give theoretical analysis on the entire process except for the extensively studied calibration step. Our study indeed reveals a series of important system properties, to the best of our knowledge, for the first time: (i) in phase measurement, the optimal and worst fringe angles are proven perpendicular and parallel to epipolar line, respectively, and can be considered as system parameters and can be directly made available during traditional calibration, highlighting the significance of the epipolar line; (ii) in correspondence, when two sets of fringes with different fringe orientations are projected, the highest correspondence precision can be achieved with arbitrary orientations as long as these two orientations are perpendicular to each other; (iii) in reconstruction, a higher reconstruction precision is given by the 4-equation methods, while we notice that the 3-equation methods are almost dominatingly used in literature. Based on these theoretical results, we propose a novel FPP measurement method which (i) only projects one set of fringes with optimal fringe angle to explicitly work together with the epipolar line for precise pixel correspondence; (ii) for the first time, the optimal fringe angle is determined directly from the calibration parameters, instead of being measured; (iii) uses 4 equations for precise 3D reconstruction but we can remove one equation which is equivalent to an epipolar line, making it the first algorithm that can use 3-equation solution to achieve 4-equation precision. Our method is efficient (only one set of fringe patterns is required in projection and the speed is doubled in reconstruction), precise (in both pixel correspondence and 3D reconstruction), and convenient (the computable optimal fringe angle and a closed-form 3-equation solution). We also believe that our work is insightful in revealing fundamental FPP properties, provides a more reasonable measurement for practice, and thus is beneficial to further FPP studies.

We report on power scaling and efficient operation of a Ho:Y 2 O 3 ceramic laser at 2.1 µm in-band pumped with an incoherently beam combined high power and narrow-linewidth Tm fiber source at 1931.2 nm. The 0.5 at.% Ho 3+ doped Ho:Y 2 O 3 ceramic is fabricated in-house with scattering loss of < 0.25% cm -1 . Up to 210.5 W of continuous-wave output power has been generated at 2117 nm for 366 W absorbed pump power shaped with a one-dimensional top-hat profile, corresponding to a slope efficiency of 60.0% with respect to the absorbed pump power. A slope efficiency of 67.5% has been demonstrated with 160 W of output power using a circular beam pump configuration. Results presented in this work verify the superior power scaling capability of a Ho:Y 2 O 3 ceramic laser with high efficiency at ∼2.1 µm.

We demonstrate a widely tunable and passively mode-locked Tm:Y 2 O 3 ceramic laser in-band pumped by a 1627-nm Raman fiber laser. A tuning range of 318 nm, from 1833 to 2151 nm, is obtained in the continuous-wave regime. The SESAM mode-locked laser produces Fourier-transform-limited pulses as short as 75 fs at ∼ 2.06 µm with an average output power of 0.26 W at 86.3 MHz. For longer pulse durations of 178 fs, an average power of 0.59 W is achieved with a laser efficiency of 29%. This is, to the best of our knowledge, the first mode-locked Tm:Y 2 O 3 laser in the femtosecond regime. The spectroscopic properties and laser performance confirm that Tm:Y 2 O 3 transparent ceramics are a promising gain material for ultrafast lasers at 2 µm.

Film is widely used in optoelectronic and semiconductor industries. The accurate measurement of the film thickness and refractive index, as well as the surface topography of the top and bottom surfaces are necessary to ensure its processing quality. Multiple measurement methods were developed; however, they are limited by the requirements of a known dispersion model and initial values of thickness and refractive index. Further, their systems are rarely compatible with surface topography measurement methods. We propose a constrained nonlinear fitting method to simultaneously measure the thickness and refractive index of film in a simple white-light spectral interferometer. The nonlinear phase extracted by the spectral phase-shifting is fitted with the theoretical nonlinear phase obtained by multiple reflection model. The constraints of nonlinear fitting are obtained by the interferometric signal of vertical scanning, reconstructed by the integration of the white-light spectral signal to avoid local minima. The proposed method does not require a priori knowledge of the dispersion model and initial values of thickness and refractive index, and its system is compatible with the vertical scanning interferometry (VSI) method to reconstruct the surface topography of the top and bottom surfaces of film. Three SiO 2 films with different thicknesses are measured, and the results show that the measured refractive index is within the theoretical value range of wavelength bandwidth and the measured thicknesses are closely aligned with the values provided by the commercial instrument. The measurement repeatability of refractive index reaches 10 -3 . Measurements on a polymer film demonstrate that this method is feasible for measuring the film without a priori information.

Optical vortices, beams with spiral wavefronts and screw phase dislocations have been explored in applications in optical manipulation, quantum optics, and the next generation of optical communications. In traditional methods, optical vortices are generated using space light modulators or spiral phase plates, which would sharply decrease the integration of optical systems. Different from previous transverse mode conversion outside the cavity, here we experimentally demonstrate a direct generation of ultrafast vortex beam from a Tm:CaYAlO 4 oscillator by pattern matching of a six-mirror-folded-cavity resonator. By accurately adjusted the angle of the end mirror and the distance L between the M4 and the SESAMs to control the beam diameter of laser incidence on the gain medium in the sagittal and tangential planes, a stable 2 µm ultrafast vortex laser emission of annular Laguerre-Gaussian (LG) mode was obtained with a maximum output power of 327 mW and pulse duration of 2.1 ps. A simple YAG crystal plate was used as handedness selector and a homemade Mach-Zehnder (MZ) interferometer has verified the vortical property of the LG 01 mode. By furtherly controlling the cavity mode pattern matching, other stable transverse-mode operations for TEM 00 , high-order Hermite-Gaussian (HG) transverse mode and doughnut-shaped beams were successfully realized. This work provides a flexible and reliable way to generate mid-infrared ultrafast vortex beams and is of special significance for applications in the areas of molecular spectroscopy and organic material processing amongst others.

A novel joint intra and inter-channel nonlinearity compensation method is proposed, which is based on interpretable neural network (NN). For the first time, conventional cascaded digital back-propagation (DBP) and nonlinear polarization crosstalk canceller (NPCC) are deep unfolded into an NN architecture together based on their physical meanings. Verified by extensive simulations of 7-channel 20-GBaud DP-16QAM 3200-km coherent optical transmission, deep-unfolded DBP-NPCC (DU-DBP-NPCC) achieves 1 dB and 0.36 dB Q factor improvement at the launch power of -1 dBm/channel compared with chromatic dispersion compensation (CDC) and cascaded DBP-NPCC, respectively. Under the bit error rate threshold of 2 × 10 -2 , DU-DBP-NPCC extends the maximum transmission reach by 28% (700 km) compared with CDC. Besides, 3 different training schemes of DU-DBP-NPCC are investigated, implying the effective signal-to-noise ratio is not the proper evaluation metric of nonlinearity compensation performance for DU-DBP-NPCC. Moreover, DU-DBP-NPCC costs 26% lower computational complexity compared with DBP-NPCC, providing a better choice for joint intra and inter-channel nonlinearity compensation in long-haul coherent systems.

We report an all-fiber high pulse energy ultrafast laser and amplifier operating at the short wavelength side of the thulium (Tm) emission band. An in-house W-type normal dispersion Tm-doped fiber (NDTDF) exhibits a bending-induced distributed short-pass filtering effect that efficiently suppresses the otherwise dominant long wavelength emission. By changing the bending diameter of the fiber, we demonstrated a tunable mode-locked Tm-doped fiber laser with a very wide tunable range of 152 nm spanning from 1740 nm to 1892 nm. Pulses at a central wavelength of 1755 nm were able to be amplified in an all-fiber configuration using the W-type NDTDF, without the use of any artificial short-pass filter or pulse stretcher. The all-fiber amplifier delivers 2.76 ps pulses with an energy of ∼32.7 nJ without pulse break-up, due to the normal dispersion nature of the gain fiber, which marks so far, the highest energy amongst fiber lasers in the 1700 nm-1800 nm region.

Benefiting from the excellent capabilities of arbitrarily controlling the phase, amplitude and polarization of the electromagnetic wave, metasurfaces have attracted much attention and brought forward the revolution of fields ranging from device fabrications to optical applications. Cascaded metasurfaces have been demonstrated to correct the monochromatic aberration and enable a near-diffraction-limited focusing spot over a wide incident angle. However, they can only work under the design wavelength and suffer from the axial chromatic aberration at another wavelength. Here, an achromatic metasurface doublet is proposed to eliminate the axial achromatic aberration and enable high-quality focusing with a wide incident angle at distinct wavelengths. It consists of square nanopillar arrays with spatially varying width to simultaneously realize wavelength-dependent phase controls. The constructed metasurface doublet is further verified numerically and near-diffraction-limited foci are exactly formed at the same plane with an incident angle up to 20° for design wavelengths. We expect that our proposed approach can find optical applications in the fields of holograms, photograms, microscopy and machine vision.

Due to the weak birefringence of single mode fibers, solitons generated in fiber lasers are indeed vector pulses and exhibit periodic parameter change including polarization evolution even when there is a polarizer inside the cavity. Period doubling eigenstates of solitons generated in a fiber laser mode-locked by the nonlinear polarization rotation, i.e., period doubling of polarization components of the soliton, are numerically explored in detail. We found that, apart from the synchronous evolution between the two polarization components, there exists asynchronous development depending on the detailed operation conditions. In addition, period doubling of one polarization component together with period-one of another polarization component can be achieved. When the period tripling window is obtained, much complexed dynamics on the two polarization components could be observed.

Several optical instruments have been presented over recent years that use a tilted diffraction grating in the reference arm, describing its effect as introducing a continuous optical delay in the direction of the grating dispersion. Having examined these types of system carefully, we believe this description does not fully capture the manner by which the signal is formed in these instruments, and present here a simple model that clearly demonstrates the essential elements behind the way such systems operate. This analysis not only illustrates the reason for signals of the form seen being generated, but also shows how great care needs to be taken with systems such as these in the interpretation of the results.

We propose a hybrid structure where graphene is inserted to the interface of two one-dimensional photonic crystals (1D PCs). The two PCs are designed to have opposite topological properties, and at the interface, topological edge modes exist. The edge modes exist at both the fundamental frequency (FF) and the third harmonic (TH) frequency. This double resonant structure will enhance the nonlinear responses of graphene greatly, including Kerr nonlinearity and TH generation. We discuss these two kinds of nonlinearities both at terahertz (THz) and near-infrared (NIR) frequencies. The influence of Kerr nonlinearity on the resonant frequencies is considered, when we calculate the TH generation. At THz frequency, low-threshold bistability (about 8MW/cm 2 ) is obtained and the TH generation efficiency of 2.5% is achieved with incident intensity of 10MW/cm 2 . At NIR frequency, the nonlinear conductivities of graphene are about 7 orders lower. Bistability is unlikely to happen with incident intensity below 1GW/cm 2 . The TH generation efficiency is only about 5×10 -6 with incident intensity of 25MW/cm 2 . The proposed structure is more suitable to work as a low-threshold saturable absorber at NIR frequency. These results may be helpful both for a better understanding of graphene's nonlinear responses in a double resonant structure and for potential applications in THz nonlinear devices and NIR nanophotonics.

We report on experimental generation and evolution of circumstance-susceptible, narrow-bandwidth, h-shaped pulse in a thulium-doped fiber (TDF) laser. With typical mode-locking technique based on nonlinear amplifying loop mirror, a type of h-shaped pulse is generated in a net normal dispersion regime for the first time to our best knowledge. Different from pulses with similar profiles achieved in typical anomalous dispersion regime, the h-shaped pulse here exhibits extremely narrow spectral bandwidth and meanwhile becomes highly circumstance-susceptible. Not alike the well-preserved h-shaped profile with anomalous dispersion, here the h-shaped pulse can easily evolve into various other pulse patterns with circumstance variations, including peak-depressed profiles, burst-like emission, multiple h-shaped pulses, and even some highly complex temporal cases. Despite that, the h-shaped pulse broadens as the pump power increasing, being a typical pump-related characteristic dominated by the peak-power-clamping effect. Moreover, it is observed that the h-shaped pulse profile can be re-shaped by incorporating a piece of unpumped TDF into the cavity, i.e., introducing some reabsorption. Our results substantiate the experimental revelation of such a type of particular-profile pulse in the normal dispersion regime, demonstrating some new evolution features facilitated by the dispersion-relevant circumstance-susceptibility.

Raman-scattering-assisted noise-like pulse (NLP) generation was achieved by using an appropriate segment of high nonlinearity fiber in an erbium-doped fiber laser. Broadband spectrum with 203 nm 3-dB bandwidth was obtained, which, to the best of our knowledge, is the broadest bandwidth achieved for NLPs. The broadband operation is the result of tailored cavity design, which optimizes various effects including the Raman scattering effect to maximize the bandwidth of NLPs. Further broadening the NLP spectrum up to 294 nm was achieved by using spectral filtering outside the cavity with a polarization beam splitter.

Conventional optical lenses enable precise foci but suffer from the diffraction limit due to the cutoff of spatial frequencies. Development of a super-oscillatory phenomenon offers an alternative approach to realize far-field sub-diffraction focusing. However, most super-oscillatory lenses exhibit a strong dependence on incident wavelengths, resulting in a narrow-band working frequency due to a fragile super-oscillatory field. Here, for the first time, achromatic super-oscillatory metasurfaces (ASOMs) are proposed to simultaneously steer optical fields at visible wavelengths of 473 nm, 532 nm and 632.8 nm and to achieve focusing at the same axial position with a resolution beyond the diffraction limit. These metasurface-based devices provide dispersionless phase profiles so that the material dispersion can be neglected in the optimization process. In addition, the design strategy can effectively circumvent the axial chromatic aberration observed in previously demonstrated metasurfaces. Constructed ASOMs are further verified numerically and simulated results for one ASOM with spot sizes of 0.706, 0.722 and 0.750 times the diffraction limit at the preset plane are consistent with the designs. Furthermore, benefiting from flexible and arbitrary phase modulations of the metasurface, the proposed method gives more freedom for a design of a super-oscillatory field and enables a lightweight, low-cost and compact optical element to replace the bulky doublet/triplet lens in a conventional optical system.

Wide-bandgap inorganic semiconductors based ultraviolet lasers bring versatile applications with significant advantages including low-power consumption, high-power output, robustness and long-term operation stability. However, flexible membrane lasers remain challenging predominantly due to the need for a lattice matched supporting substrate. Here, we develop a simple laser liftoff process to make freestanding single crystalline ZnO membranes that demonstrate low-threshold ultraviolet stimulated emissions together with large sized dimension (> 2 mm), ultralow-weight (m/A<15 g/m 2 ) and excellent flexibility. The 2.6 μm-thick crack-free ZnO membrane exhibits well-retained single crystallinity and enhanced excitonic emissions while the defect-related emissions are completely suppressed. The inelastic exciton-exciton scattering stimulated emissions with increased spontaneous emission rate is obtained with a reduced threshold of 0.35 MW/cm 2 in the ZnO membrane transferred onto a flexible polyethylene naphthalate substrate. Theoretical simulations reveal that it is a synergetic effect of the increased quantum efficiency via Purcell effect and the improved optical gain due to vertical directional waveguiding of the membrane, which functions as a Fabry-Perot photonic resonator due to the refractive index contrast at ZnO-air boundaries. With simple architecture, efficient exciton recombination and easy fusion with waveguide system, the ZnO membranes provide an alternative platform to develop compact low-threshold ultraviolet excitonic lasers.

Frequency-domain optical parametric amplification (FOPA) is a new scheme that enables extremely broadband amplification of ultraintense pulses. The spatiotemporal coupling property of signal pulses can make the coherent noise of FOPA sharply different from that of conventional OPCPA. This paper presents a first theoretical study on the coherent noise produced in a FOPA system. We reveal that the coherent noise acquires the spatiotemporal coupling, and thus distinguishes the compressed signal pulse not only in time but also in space, which allows the suppression of coherent noise via optical manipulations in the spatial domain. The quantitative impacts of spatiotemporal coherent noise originated from the imperfections in either pump laser or crystal surfaces, are numerically studied. The result provides a new perspective on improving the coherent contrast of ultraintense lasers.

The pulse contrast of state-of-the-art petawatt lasers is limited by coherent noise. This paper reports on a new family of noise, termed nonlinear beat noise, which is generated by the nonlinear mixing of two kinds of coherent noise in optical parametric chirped-pulse amplification (OPCPA). We theoretically study the various nonlinear beat noises and reveal their intensity evolutions in an OPCPA amplifier. The results suggest that nonlinear beat noise will be destructive to the future hundred-petawatt lasers.

A flexible chalcogenide fiber bundle (FB) with a resolution as high as ~31 lp/mm has been fabricated for delivering thermal images of objects at room temperature. The FB is composed of ~200,000 single fibers with a Ge-As-Te-Se glass core 15 μm in diameter and a polyetherimide (PEI) cladding 16.8 μm in diameter. These Ge-As-Te-Se/PEI fibers show good transparency in the 3-12 μm spectral region. The fabricated FB presents a filling factor of ~72% and a crosstalk of ~1%. High-quality thermal images of a human hand were obtained through the FB, demonstrating good potential of the FB for longwave infrared imaging in the areas such as medicine, industry and defense.

We experimentally demonstrated a diode-pumped sub-50 fs Yb,Na:CNGG disordered crystal laser. Pumped by a 980 nm distributed Bragg-reflector tapered diode laser and passively mode-locked with a semiconductor saturable absorber mirror (SESAM), soliton pulses as short as 62 fs and 45 fs were obtained without and with external compression, respectively. The ultrashort pulses had a repetition rate of ~104 MHz at the central wavelength of 1061 nm. To the best of our knowledge, this is the first demonstration of sub-50 fs pulses from the Yb 3+ -doped CNGG type disordered crystal lasers.