In this paper, a low F-number freeform off-axis three-mirror system with rectangular field-of-view (FOV) is presented, and the system has an integrated configuration that can be aligned with ordinary methods. The initial structure is solved analytically, and the optimization process is described in detail. This F/1.38 freeform system with a 4° × 5° FOV provides good imaging performance, especially in middle-wave and long-wave infrared bands. The primary mirror and the tertiary mirror are approximately tangent, so these two freeform mirrors can be fabricated on a single substrate. This configuration provides a solution to the practical application of freeform off-axis three-mirror systems. Additionally, a prototype of this system is demonstrated, and the imaging performance of the prototype is tested experimentally. The actual performance of the prototype is discussed and analyzed.

https://iopscience.iop.org/article/10.1088/2040-8978/17/1/015605/pdf

Design of a low F-number freeform off-axis three-mirror system with rectangular field-of-view

The propagation of tightly focused femtosecond laser pulse with numerical aperture of 012 in air is investigated experimentally The formation and evolution of the filament bunch are recorded by time-resolved shadowgraph with laser energy from 24 mJ to 47 mJ The distribution of electron density in breakdown area is retrieved using Nomarski interferometer It is found that intensity clamping during filamentation effect still play a role even under strong external focusing The electron density in some interaction zones is higher than 3×1019 cm-3, which indicates that each air molecule there is ionized

Tightly focused femtosecond laser pulse in air: from filamentation to breakdown.

We report a plasma-based strong THz source generated by using intense femtosecond laser pulses to irradiate solid targets at relativistic intensity >10^18W/cm2. Energies up to 50 microJ/sr per THz pulse is observed in the specular direction when the laser pulses are incident onto a copper foil at 67.5 degree. The source appears to be linearly polarized. The temporal, spectral properties of the THz are measured by a single shot, electro-optic sampling method with a chirped laser pulse. The THz radiation is attributed to the self-organized transient fast electron currents formed along the target surface. Such a strong THz source allows potential applications in THz nonlinear physics.

Strong terahertz radiation from relativistic laser interaction with solid density plasmas

We have demonstrated single-shot measurement of electron diffraction patterns for a single-crystal gold foil using 340 keV electron pulses accelerated by intense femtosecond laser pulses with an intensity of 2×1018 W/cm2. The measured electron beam profile is faithfully reproduced by the numerical simulation of the electron trajectory, providing evidence that the electron pulse spontaneously expands in time owing to the velocity spread produced in the acceleration process, but is not distorted in an irreversible nonlinear manner. This study shows that the laser acceleration is promising for the development of pulse compression methods for single-shot femtosecond electron diffraction.

http://repository.kulib.kyoto-u.ac.jp/dspace/bitstream/2433/86205/1/1.3226674.pdf

Single-shot ultrafast electron diffraction with a laser-accelerated sub-MeV electron pulse

Terahertz (THz) radiation has attracted much attention due to its wide potential applications. Though radiation can be generated with various ways, it is still a big challenge to obtain strong tabletop sources. Plasma, with the advantage of no damage limit, is a promising medium to generate strong THz radiation. This review reports recent advances on strong THz radiation generation from low-density gases and high-density solid targets at different laser intensities.

https://iopscience.iop.org/article/10.1088/1674-1056/21/9/095203/pdf

High power terahertz pulses generated in intense laser—plasma interactions

The dependence of angular distributions of fast electrons generated in the interaction of p-polarized femtosecond laser pulses with foil targets on laser intensities is investigated. A novel fast electron beam along the front target surface is observed for high laser intensity. It is found that the electron acceleration along the target surface is more efficient than those in other directions.

Effective fast electron acceleration along the target surface

In order to solve the problem of uneven spatial frequency sampling and lack of zero frequency sampling in the segmented planar imaging detector for electro-optical reconnaissance (SPIDER), we propose a "checkerboard" imaging system design using a square grid aperture arrangement, show its aperture matching method, and demonstrate that it can achieve the uniform sampling of all integer multiples of fundamental frequency in the highest frequency range with zero frequency in the direction of two orthogonal arrays. Through computer simulation, it is verified that the imaging effect of the "checkerboard" imaging system is superior to that of the SPIDER imaging system and the traditional single aperture imaging system at the same system diameter.

System design for a "checkerboard" imager

Detecting and tracking multiple targets simultaneously for space-based surveillance requires multiple cameras, which leads to a large system volume and weight. To address this problem, we propose a wide-field detection and tracking system using the segmented planar imaging detector for electro-optical reconnaissance. This study realizes two operating modes by changing the working paired lenslets and corresponding waveguide arrays: a detection mode and a tracking mode. A model system was simulated and evaluated using the peak signal-to-noise ratio method. The simulation results indicate that the detection and tracking system can realize wide-field detection and narrow-field, multi-target, high-resolution tracking without moving parts.

/pdf/design-of-a-wide-field-target-detection-and-tracking-system-4sk0hab0fm.pdf

Design of a wide-field target detection and tracking system using the segmented planar imaging detector for electro-optical reconnaissance

TMA (Three Mirrors Anastigmatic) can simplify the configuration of telescope and lighten the weight, so it is very
popular to the design of space-based optical system. According to this concept, an off-axis unobstructed TMA is
calculated. Based on initial configuration parameters, automatic optimization is executed in ZEMAX program, and a new
telescope is obtained with parameters as follow: aperture is 2000mm, f/number is 2.25, FOV is 5°, angular resolution is
better than 7 μrad and image quality meets limited performances.

Optical design of an off-axis telescope: large aperture, small f/number, wide FOV

The propagation of a tightly focused femtosecond laser pulse in air has been investigated. Unlike long-distance self-guided propagation of short laser pulses, a novel oval-like hollow distribution of the laser intensity is observed in the experiments and reproduced by the numerical simulations. The formation of the hollow structures can be explained by the interplay between ionization-induced refraction and Kerr self-focusing.

Qinghua Yu

Papers

Effective fast electron acceleration along the target surface

System design for a "checkerboard" imager

Design of a wide-field target detection and tracking system using the segmented planar imaging detector for electro-optical reconnaissance

Optical design of an off-axis telescope: large aperture, small f/number, wide FOV

Oval-like hollow intensity distribution of tightly focused femtosecond laser pulses in air