Beam quality degradation during the transition from a laser wakefield accelerator to the vacuum is one of the reasons that cause the beam transport distortion, which hinders the way to compact free-electron-lasers. Here, we performed transition simulation to initialize the beam parameters for beam optics transport. This initialization was crucial in matching the experimental results and the designed evolution of the beamline. We experimentally characterized properties of high-quality laser-wakefield-accelerated electron beams, such as transverse beam profile, divergence, and directionality after long-distance transport. By installing magnetic quadrupole lenses with tailored strength gradients, we successfully collimated the electron beams with tunable energies from 200 to 600 MeV.

Using the classical-trajectory Monte Carlo model, we have theoretically studied the angular momentum distribution of frustrated tunneling ionization (FTI) of atoms in strong laser fields. Our results show that the angular momentum distribution of the FTI events exhibits a double-hump structure. With this classical model, we back traced the tunneling coordinates, i.e., the tunneling time and initial transverse momentum at tunneling ionization. It is shown that for the events tunneling ionized at the rising edge of the electric field, the final angular momentum exhibits a strong dependence on the initial transverse momentum at tunneling. While for the events ionized at the falling edge, there is a relatively harder recollision between the returning electron and the parent ion, leading to the angular momentum losing the correlation with the initial transverse momentum. Our study suggests that the angular momentum of the FTI events could be manipulated by controlling the initial coordinates of the tunneling ionization.

We investigate the nonadiabatic spectral redshift of high-order harmonic of He driven by two time-delayed orthogonally polarized laser fields. It is found that the nonadiabatic spectral redshift can be observed by properly adjusting the time delay of the two laser fields when the controlling pulse is added in the raising part of the driving pulse in the vertical direction. That is because the controlling pulse in the vertical direction prevents the ionized electrons from returning to the vicinity of parent ions and then reduces the recombination probability. This leads to the high-order harmonic generated mainly in the falling part of the driving pulse. Meanwhile, we also find that the quantity of redshift can be effectively controlled through accommodating the positive time delays. In addition, this scheme can also be used to produce nonadiabatic spectral blueshift.

The resonator integrated optic gyros (RIOGs) based on the Sagnac effect have gained extensive attention in navigation and guidance systems due to their predominant advantages: high theoretical accuracy and simple integration. However, the problems of losing lock and low lock-in accuracy are the bottlenecks, which restrict the development of digital RIOGs. Therefore, a multilevel laser frequency lock-in technique has been proposed in this Letter to address these problems. The experimental results show that lock-in accuracy can be improved one order higher and without losing lock in a variable temperature environment. Then, a digital miniaturized RIOG prototype (18 cm × 18 cm × 20 cm) has been produced, and long-term (1 h) bias stability of 26.6 deg/h is successfully demonstrated.

In this Letter, an alternative solution is proposed and demonstrated for simultaneous measurement of axial strain and temperature. This sensor consists of two twisted points on a commercial single mode fiber introduced by flame-heated and rotation treatment. The fabrication process modifies the geometrical configuration and refractive index of the fiber. Different cladding modes are excited at the first twisted point, and part of them are coupled back to the fiber core at the second twisted point. Experimental results show distinct sensitivities of 34.9 pm/με with 49.23 pm/°C and 36.19 pm/με with 62.99 pm/°C for the two selected destructive interference wavelengths.

We propose and experimentally validate an optical true time delay beamforming scheme with straightforward integration into hybrid optical/millimeter (mm)-wave access networks. In the proposed approach, the most complex functions, including the beamforming network, are implemented in a central office, reducing the complexity and cost of remote antenna units. Different cores in a multi-core fiber are used to distribute the modulated signals to high-speed photodetectors acting as heterodyne mixers. The mm-wave carrier frequency is fixed to 50 GHz (V-Band), thereby imposing a progressive delay between antenna elements of a few picoseconds. That true time delay is achieved with an accuracy lower than 1 ps and low phase noise.

In this Letter, we have proposed a generalized Gaussian probability density function (GGPDF)-based method to estimate the symbol error ratio (SER) for pulse amplitude modulation (PAM-4) in an intensity modulation/direct detection (IM/DD) system. Furthermore, a closed form expression of SERGGD for PAM-4 has been derived. The performance of the proposed method is evaluated through simulation as well as experimental work. The fitting of probability density functions of the received signal is applied via GGPDF and shape parameters P1 and P2 associated with different PAM-4 levels are determined. The optimum single value of shape parameter P is then calculated to estimate the SER. The mathematical relationship of P with different received optical powers and receiver bandwidths has been determined and verified. The proposed method is a fast and accurate method to estimate SER of a PAM-4 system, which is more reliable and in agreement with the error counting method.

An all-optical intensity modulator based on an optical microfiber coupler (OMC) is presented. The modulator works at 1550 nm wavelength and is modulated directly by heating the coupling region with 980 nm pump light injected through the coupling port of the OMC. The OMC is controlled to have at least a 30 mm long coupling region with diameter smaller than 8 μm, and the uniform waist region diameter is about 3 μm. This is helpful to ensure the optical modulation function based on the light induced thermal effect in the coupling region, while pump light is injected. The modulation response is measured to show good linearity when the 980 nm pump light has a lower intensity (with power below 2.5 mW), which proves that the OMC acts as an all-optical modulator. The bandwidth of the modulator can be at 0.2–50 kHz with the average power of the intensity-modulated pump light about 2 mW, which can be further improved by optimizing the design of the coupler. The demonstrated modulator may have potential value for the application in an all-optical integration system.

Thermally regenerated low-reflectivity fiber Bragg gratings (RFBGs), as one mirror of a resonant cavity, have been introduced as linear-cavity fiber lasers combining with fiber saturable absorbers. The output of lasing presents an optical signal-to-noise ratio of 50 dB and temperature sensitivity coefficient of 15.36 pm/°C for the heating process and 15.46 pm/°C for the cooling process. The lasing wavelength variation and power fluctuation at 700°C are less than 0.02 nm and 0.21 dB, respectively. The RFBG-based fiber laser sensing has displayed good linearity for both the temperature rising and cooling processes, and favorable stability at high temperatures.

We experimentally designed dispersion-managed repeaterless transmission systems with a pre-compensation and post-compensation technique using multi-channel-chirped fiber Bragg gratings. The repeaterless transmission link supports a single channel (1548.51 nm) with a 10 Gbps repeaterless transmission system over 300 km standard single-mode fiber (SSMF). In the system design, two distributed Raman amplifiers (DRAs) were used to improve the signal level propagated along the 300 km SSMF. The co-propagating DRA provided 15 dB on–off gain and the counter-propagating produced 32 dB on–off gain at the signal wavelength. The experiment results show that the post-compensation configuration achieves an optimal performance with a bit error rate at 1×10 9.

Coherent pulse stacking (CPS) is a new time-domain coherent addition technique that stacks several optical pulses into a single output pulse, enabling high pulse energy and high average power. A Z-domain model targeting the pulsed laser is assembled to describe the optical interference process. An algorithm, extracting the cavity phase and pulse phases from limited data, where only the pulse intensity is available, is developed to diagnose optical cavity resonators. We also implement the algorithm on the cascaded system of multiple optical cavities, achieving phase errors less than 1.0° (root mean square), which could ensure the stability of CPS.

A distortion correction method for the elemental images of integral imaging (II) by utilizing the directional diffuser is demonstrated. In the traditional II, the distortion originating from lens aberration wraps elemental images and degrades the image quality severely. According to the theoretical analysis and experiments, it can be proved that the farther the three-dimensional image is displayed from the lens array, the more serious the distortion is. To analyze the process of eliminating lens distortion, one lens and its corresponding elemental image are separated from the traditional II. By introducing the directional diffuser, the aperture stop of the separated optical system changes from the eye’s pupil to the lens. In terms of contrast experiments, the distortion of the improved display system is corrected effectively. In the experiment, when the distance between the reconstructed image and lens array is equal to 120 mm, the largest lens distortion is decreased from 46.6% to 3.3%.

The work proposes a three-laser-beam streak tube imaging lidar system. Besides the main measuring laser beam, the second beam is used to decrease the error of time synchronization. The third beam has n+0.5 pixels’ difference compared to the main measuring beam on a CCD, and it is used to correct the error caused by CCD discrete sampling. A three-dimensional (3D) imaging experiment using this scheme is carried out with time bin size of 0.066 ns (i.e., corresponding to a distance of 9.9 mm). An image of a 3D model is obtained with the depth resolution of <2 mm, which corresponds to ～0.2 pixel.

Traditional one-way imaging methods become invalid when a target object is completely hidden behind scattering media. In this case, it has been much more challenging, since the light wave is distorted twice. To solve this problem, we propose an imaging method, so-called round-trip imaging, based on the optical transmission matrix of the scattering medium. We show that the object can be recovered directly from the distorted output wave, where no scanning is required during the imaging process. We predict that this method might improve the imaging speed and have potential application for real-time imaging.

To reveal the physical mechanism of laser ablation and establish the prediction model for figuring the surface of fused silica, a multi-physical transient numerical model coupled with heat transfer and fluid flow was developed under pulsed CO2 laser irradiation. The model employed various heat transfer and hydrodynamic boundary and thermomechanical properties for assisting the understanding of the contributions of Marangoni convention, gravitational force, vaporization recoil pressure, and capillary force in the process of laser ablation and better prediction of laser processing. Simulation results indicated that the vaporization recoil pressure dominated the formation of the final ablation profile. The ablation depth increased exponentially with pulse duration and linearly with laser energy after homogenous evaporation. The model was validated by experimental data of pulse CO2 laser ablation of fused silica. To further investigate laser beam figuring, local ablation by varying the overlap rate and laser energy was conducted, achieving down to 4 nm homogenous ablation depth.

We systematically study the optimization of highly efficient terahertz (THz) generation in lithium niobate (LN) crystal pumped by 800 nm laser pulses with 30 fs pulse duration. At room temperature, we obtain a record optical-to-THz energy conversion efficiency of 0.43% by chirping the pump laser pulses. Our method provides a new technique for producing millijoule THz radiation in LN via optical rectification driven by joule-level Ti:sapphire laser systems, which deliver sub-50-fs pulse durations.

The confocal microscopy technique was applied for nonlinear optical characterization of single β-barium-borate (β-BBO) nanocrystals. The experimental setup allows measurements of the laser polarization-selective second-harmonic (SH) generation, and the results can be used to determine the nanocrystals’ c-axis orientation, as well as to obtain information about their second-order susceptibility χ(2). The dependence of the SH signal on the laser polarization allowed the discrimination of individual particles from aggregates. The data were fitted using a model that takes into account the BBO properties and the experimental setup characteristics considering (i) the electrostatic approximation, (ii) the effects of the microscope objective used to focus the light on the sample in an epi-geometry configuration, and (iii) the symmetry of χ(2) for the β-BBO nanocrystals. A signal at the third-harmonic frequency was also detected, but it was too weak to be studied in detail.

We experimentally demonstrate a heralded single photon source at 1290 nm by exploiting the spontaneous four wave mixing in a taper-drawn micro/nano-fiber (MNF). Because the frequency detuning between the pump and heralded single photons is ～58 THz, the contamination by Raman scattering is significantly reduced at room temperature. Since the MNF is naturally connected to standard single mode fibers via fiber tapers, the source would be compatible with the existing fiber networks. When the emission rate of heralded signal photons is about 4.6 kHz, the measured second-order intensity correlation function g(2)(0) is 0.017±0.002, which is suppressed by a factor of more than 55, relative to the classical limit.

New techniques for controlling the amplitudes of two orthogonal linearly polarized light are presented. One is based on adjusting the DC voltage into a Mach–Zehnder modulator (MZM) to alter the amplitude of the light traveling on the slow axis of a fiber into the modulator with little changes in the fast-axis light amplitude. Another is based on adjusting the input DC voltages of a dual-polarization MZM operating in the reverse direction, which enables independent control of the two input orthogonal linearly polarized light amplitudes. Experimental results demonstrate that more than 30 dB difference in slow- and fast-axis light power can be obtained by controlling an MZM input DC voltage, and over 24 dB independent power adjustment for light traveling on the slow and fast axes into a dual-polarization MZM.

Supercontinuum generation (SC) of more than one octave spectrum spanning covering from 400 nm to 820 nm was achieved by pumping a piece of aluminum nitride (AIN) single crystal using a nanosecond 355 nm ultraviolet laser. The AlN with a thickness of ～0.8 mm was grown by an optimized physical vapor transport technique and polished with solidification technology. Compared to previously reported ones, the achieved visible SC exhibited the broadest spectrum spanning from bulk materials pumped by a nanosecond pulse laser. The visible supercontinuum in AlN presents new opportunities for bulk material-based white light SC and may find more potential applications beyond typical applications in integrated semiconductive photoelectronic devices.