Phase-shifting lensless Fourier-transform holography with a Chinese Taiji lens.

Compared to off-axis holography, phase-shifting technology takes full use of the space-bandwidth product of the detector array. Here we introduce the Chinese Taiji into diffractive lenses and generate the phase-shifting focal spots, which not only can be used for a spherical reference wave, but also make it possible to realize the lensless Fourier-transform holography. The experimental results of a 1951 U.S. Air Force resolution test target verify the validity of the above theory in the optical region. Owing to the amplitude-only diffractive lenses, these kinds of lenses will have great potential in x-ray holography or biochemical microscopy for the next generation of synchrotron radiation and free electron lasers in the future.

Design and calibration of hot-electron spectrometer array for angle-resolved measurement.

A hot-electron spectrometer array with two-dimensional distribution has been designed with a wide-angle range and high-energy resolution to measure the spatially resolved electron spectra for high-power-laser plasma interaction experiments. It consisted of 19 identical electron spectrometers set in three directions with an interval of 10°. Each electron spectrometer was designed with a uniform magnetic field to detect electrons in the range from 20 to 500 keV. The spectrometers were calibrated using electrons from an accelerator. In an experiment, the spatially resolved electron energy spectra, which approximately had a Maxwell distribution, were obtained from an aluminum foil target irradiated by a 0.53-µm laser pulse.

Characterization of ultrashort pulses by time-frequency conversion and temporal magnification based on four-wave mixing at 1 µm.

In order to characterize ultrashort pulses in real time at 1 µm wavelength, a temporal imaging structure based on the four-wave mixing effect in highly nonlinear fibers is implemented and analyzed both theoretically and experimentally. It is found that both time-frequency transfer and the temporal magnification process can be realized approximately in one structure. The pulse widths of the signal laser measured by the time-frequency transfer and the temporal magnification process are 3.2 ps and 3.1 ps, respectively, which are nearly the same and are in agreement with the result of the autocorrelator. The temporal magnification factor is 33, and the temporal resolution is 380 fs. The method based on the temporal magnification process is inherently real time and single shot, which makes it suitable for applications in the measurement of high-power ultrashort pulses. The four-wave mixing time lens promises future applications in the characterization of the single-shot high-power short laser.

Tunable compensation of GVD-induced FM-AM conversion in the front end of high-power lasers.

Group velocity dispersion (GVD) is one of the main factors leading to frequency modulation (FM) to amplitude modulation (AM) conversion in the front end of high-power lasers. In order to compensate the FM-AM modulation, the influence of GVD, which is mainly induced by the phase filter effect, is theoretically investigated. Based on the theoretical analysis, a high-precision, high-stability, tunable GVD compensatory using gratings is designed and experimentally demonstrated. The results indicate that the compensator can be implemented in high-power laser facilities to compensate the GVD of fiber with a length between 200-500 m when the bandwidth of a phase-modulated laser is 0.34 nm or 0.58 nm and the central wavelength is in the range of 1052.3217-1053.6008 nm. Due to the linear relationship between the dispersion and the spacing distance of the gratings, the compensator can easily achieve closed-loop feedback controlling. The proposed GVD compensator promises significant applications in large laser facilities, especially in the future polarizing fiber front end of high-power lasers.

Suppression of FM-to-AM modulation by polarizing fiber front end for high-power lasers.

FM-to-AM modulation is an important effect in the front end of high-power lasers that influences the temporal profile. Various methods have been implemented in standard-fiber and polarization-maintaining (PM)-fiber front ends to suppress the FM-to-AM modulation. To analyze the modulation in the front end, a theoretical model is established and detailed simulations carried out that show that the polarizing (PZ) fiber, whose fast axis has a large loss, can successfully suppress the modulation. Moreover, the stability of the FM-to-AM modulation can be improved, which is important for the front end to obtain a stable output. To verify the model, a PZ fiber front end is constructed experimentally. The FM-to-AM modulation, without any compensation, is less than 4%, whereas that of the PM fiber front end with the same structure is nearly 20%. The stability of the FM-to-AM modulation depth is analyzed experimentally and the peak-to-peak and standard deviation (SD) are 2% and 0.38%, respectively, over 3 h. The experimental results agree with the simulation results and both prove that the PZ fiber front end can successfully suppress the FM-to-AM conversion. The PZ fiber front end is a promising alternative for improving the performance of the front end in high-power laser facilities.

Noncritically phase-matched fourth-harmonic generation of Nd:glass lasers and design of final optics assembly.

The noncritically phase-matched (NCPM) fourth-harmonic generation (FHG) with partially deuterated dihydrogen phosphate (KD*P) crystal at an Nd:glass laser radiation wavelength of 1053.1 nm has been confirmed. NCPM FHG has been achieved in 70% and 65% deuterated KD*P crystal at the temperature of 17.7°C and 29.3°C, respectively. The angular acceptance of 70% and 65% deuterated KD*P crystals fixed at their NCPM temperature were measured, which were 53 and 55 mrad, respectively. The application of the NCPM FHG in a high-power laser facility for inertial confinement fusion is also discussed. Based on the theoretical analysis, the NCPM KD*P can be placed after the focus lens; thus, the laser-induced damage of a fused-silica lens at ultraviolet can be avoided.

Studies on design of 351 nm focal plane diagnostic system prototype and focusing characteristic of SGII-upgraded facility at half achievable energy performance.

In order to obtain the intensity distribution of a 351 nm focal spot and smoothing by spectral dispersion (SSD) focal plane profile of a SGII-upgraded facility, a type of off-axis imaging system with three spherical mirrors, suitable for a finite distance source point to be imaged near the diffraction limit has been designed. The quality factor of the image system is 1.6 times of the diffraction limit tested by a 1053 nm point source. Because of the absence of a 351 nm point source, we can use a Collins diffraction imaging integral with respect to λ=351 nm, corresponding to a quality factor that is 3.8 times the diffraction limit at 351 nm. The calibration results show that at least the range of ±10 mrad of view field angle and ±50 mm along the axial direction around the optimum object distance can be satisfied with near diffraction limited image that is consistent with the design value. Using this image system, the No. 2 beam of the SGII-upgraded facility has been tested. The test result of the focal spot of final optics assembly (FOA) at 351 nm indicates that about 80% of energy is encompassed in 14.1 times the diffraction limit, while the output energy of the No. 2 beam is 908 J at 1053 nm. According to convolution theorem, the true value of a 351 nm focal spot of FOA is about 12 times the diffraction limit because of the influence of the quality factor. Further experimental studies indicate that the RMS value along the smoothing direction is less than 15.98% in the SSD spot test experiment. Computer simulations show that the quality factor of the image system used in the experiment has almost no effect on the SSD focal spot test. The image system can remarkably distort the SSD focal spot distribution under the circumstance of the quality factor 15 times worse than the diffraction limit. The distorted image shows a steep slope in the contour of the SSD focal spot along the smoothing direction that otherwise has a relatively flat top region around the focal spot center.

Demonstration of a high-energy, narrow-bandwidth, and temporally shaped fiber regenerative amplifier.

We report a high-energy and high-gain fiber regenerative amplifier for narrow-bandwidth nanosecond laser pulses that uses a Yb-doped photonic crystal fiber. The input pulse energy is 270 pJ for a 3.5 ns laser pulse with 0.3 nm (FWHM) bandwidth. At a pump laser power of 8.6 W at 974 nm, pulse energies up to 746 µJ with 1.2% (rms) energy stability are generated. To the best of our knowledge, this is the highest energy obtained in a fiber-based regenerative amplifier. A high-energy, nearly diffraction-limited, single-mode beam with a high gain of 64 dB shows promise for future application in the front ends of high-power laser facilities.

Performance of target irradiation in a high-power laser with a continuous phase plate and spectral dispersion.

We report on the performance of target irradiation at the SG-II high-power laser facility with a continuous phase plate (CPP) and the technique of smoothing by spectral dispersion (SSD). Simulative and experimental results are presented, where the irradiation uniformity and energy concentration of the target spots are analyzed. The results show that the designed CPP can focus the spot energy into the desired region and shape a profile with steep edge and flat top, but the actual performance of the fabricated CPP needs some improvements. It is also proved that the CPP is insensitive to the long-scale wavefront distortion in the incident beam. The one-dimensional SSD configuration evidently works in smoothing the fine-scale intensity modulation inside the target spot.

Accuracy of single-shot autocorrelation measurements of petawatt laser pulses.

At the Shen Guang II (SGII) Petawatt Laser Facility, measurements of large-energy, single-shot laser pulses sometimes feature asymmetric autocorrelation signals, causing uncertainty in the measurement of compressed pulses. This study presents a method for defining and describing the asymmetry of autocorrelation signals. We discuss two sources of asymmetry: the nonuniform distribution of the near field excited by a beam, and the rotation of autocorrelator arms from the cylinder lens. The pulsewidth of an asymmetric autocorrelation signal is shorter than its real width. After updating the autocorrelator, the single-shot autocorrelator for the SGII petawatt laser exhibits a measurement uncertainty of below 12.3%. Recommendations on reducing asymmetry in large-energy, single-shot autocorrelation are discussed.

Uniform irradiation of adjustable target spots in high-power laser driver.

For smoothing and shaping the on-target laser patterns flexibly in high-power laser drivers, a scheme has been developed that includes a zoom lens array and two-dimensional smoothing by spectral dispersion (SSD). The size of the target pattern can be controlled handily by adjusting the focal length of the zoom lens array, while the profile of the pattern can be shaped by fine tuning the distance between the target and the focal plane of the principal focusing lens. High-frequency stripes inside the pattern caused by beamlet interference are wiped off by spectral dispersion. Detailed simulations indicate that SSD works somewhat differently for spots of different sizes. For small spots, SSD mainly smooths the intensity modulation of low-to-middle spatial frequency, while for large spots, SSD sweeps the fine speckle structure to reduce nonuniformity of middle-to-high frequency. Spatial spectra of the target patterns are given and their uniformity is evaluated.

Design of ultrabroadband internal reflection gratings with high efficiency.

A high-efficiency, ultrabroadband dielectric internal reflection grating with rhombus-shaped grooves is designed by a rigorous coupled-wave analysis, and an effective method for predicting spectral bandwidths of gratings from their efficiency maps is presented. The grating can be fabricated from a single dielectric material, and its reflection diffraction efficiency of the -1st order can reach more than 0.99. More importantly, an ultrabroadband top-hat diffraction spectrum with efficiency exceeding 0.98 over 170 nm wavelength wide is achieved, which makes the gratings suitable for applications associated with broadband illumination, such as ultrashort pulses.

Design of rectangular-groove fused-silica gratings as polarizing beam splitters.

The application of rectangular-groove fused-silica gratings as polarizing beam splitters (PBSs) under Littrow incidence is investigated. Based on the simple modal method, two different cases of PBS gratings are designed. The achieved solutions, which are independent on the incident wavelength, are verified by the rigorous coupled-wave analysis and expressed in several polynomials instead of listing one or two numerical solutions. More importantly, on the basis of the designed PBS gratings, a porous fused silica antireflective film is introduced to improve their performances. Theoretical results indicate that such modified rectangular-groove PBS gratings exhibit higher diffraction efficiencies (over 0.99) and larger spectral bandwidths.

Individual far-field model for photon sieves composed of square pinholes.

A photon sieve can be composed of a large number of square pinholes. By taking the related coordinate transform into account, we present here an individual far-field model for a photon sieve composed of many square pinholes whose edges are symmetrically vertical or parallel to the polar coordinate. In particular, a simple analytical expression for the diffracted far field of an individual square pinhole is given, and the focusing contribution from an individual square pinhole is further discussed. The obtained results can be used for the analysis, design, and simulation of a high numerical aperture photon sieve composed of the above-mentioned square pinholes.

Fractional Fourier transform of flat-topped multi-Gaussian beams.

The fractional Fourier transform (FRFT) of the flat-topped multi-Gaussian beam (FMGB) is investigated based on the three kinds of FRFT optical systems: Lohmann I, Lohmann II, and quadratic graded-index systems. The analytical expressions for the FRFT of the FMGB are derived based on the propagation of the FMGB through the three systems. By introducing a hard-edge aperture function, the analytical expressions for the FRFT of the FMGB carried out by the apertured FRFT optical systems are presented. The FRFT characteristics of the FMGB for the three kinds of FRFT optical systems with and without apertures are discussed in detail. Results show that the three types of FRFT optical systems have the same function when the apertures are ignored but that significantly different characteristics are exhibited when the apertures appear.

Propagation of flat-topped multi-Gaussian beams through an apertured ABCD optical system.

The generalized analytical expression for the propagation of flat-topped multi-Gaussian beams through a misaligned apertured ABCD optical system is derived. Using this analytical expression, the propagation characteristics of flat-topped multi-Gaussian beams through a spatial filter are investigated. The analytical formula of the electric field distribution in the focal plane is also derived for revealing the effects of the misalignment parameters clearly. It is found that different misalignment parameters have different influences on the electric field distributions of the beam focus spot and the output beam characteristics. The intensity distribution of the beam is mainly determined by the misalignment matrix element E, and the phase distribution is affected by the misalignment matrix elements G and E.

Propagation of flat-topped multi-Gaussian beams through a double-lens system with apertures.

A general model for different apertures and flat-topped laser beams based on the multi-Gaussian function is developed. The general analytical expression for the propagation of a flat-topped beam through a general double-lens system with apertures is derived using the above model. Then, the propagation characteristics of the flat-topped beam through a spatial filter are investigated by using a simplified analytical expression. Based on the Fluence beam contrast and the Fill factor, the influences of a pinhole size on the propagation of the flat-topped multi-Gaussian beam (FMGB) through the spatial filter are illustrated. An analytical expression for the propagation of the FMGB through the spatial filter with a misaligned pinhole is presented, and the influences of the pinhole offset are evaluated.

Characteristics of beam alignment in a high power four-pass laser amplifier.

Beam alignment is used to control the pointing, rotation, and position of a beam automatically. We give an overview of the beam alignment system for a four-pass amplifier system. The coupling relationships between dependent quantities and deviations thereof for parameters of key optical elements are analyzed using matrix optics techniques. The rotation characteristics of a near field are discussed. The characteristics of the four-pass adjustment in the cavity spatial filter are shown and the output characteristics are provided. Finally, we develop an adjustment procedure.

Two-dimensional performance of uniform irradiation with the use of an edge-softened lens array and spectral dispersion.

A lens array composed of edge-softened elements is used to improve on-target irradiation uniformity in the Shenguang II Laser Facility, with which a Fresnel pattern of suppressed diffraction peaks is obtained. Additional uniformity can be reached by reducing short-wavelength interference speckles inside the pattern when the technique of smoothing by spectral dispersion is also used. Two-dimensional performance of irradiation is simulated and the results indicate that a pattern of steeper edges and a flat top can be achieved with this joint technique.