Generalized central slice theorem perspective on Fourier-transform spectral imaging at a sub-Nyquist sampling rate.

Fourier-transform spectral imaging captures frequency-resolved images with high spectral resolution, broad spectral range, high photon flux, and low stray light. In this technique, spectral information is resolved by taking Fourier transformation of the interference signals of two copies of the incident light at different time delays. The time delay should be scanned at a high sampling rate beyond the Nyquist limit to avoid aliasing, at the price of low measurement efficiency and stringent requirements on motion control for time delay scan. Here we propose, what we believe to be, a new perspective on Fourier-transform spectral imaging based on a generalized central slice theorem analogous to computerized tomography, using an angularly dispersive optics decouples measurements of the spectral envelope and the central frequency. Thus, as the central frequency is directly determined by the angular dispersion, the smooth spectral-spatial intensity envelope is reconstructed from interferograms measured at a sub-Nyquist time delay sampling rate. This perspective enables high-efficiency hyperspectral imaging and even spatiotemporal optical field characterization of femtosecond laser pulses without a loss of spectral and spatial resolutions.

Towards precise diagnosis time profile of ultrafast electron bunch trains using orthogonal terahertz streak camera.

Ultrafast electron microbunch trains have broad applications in which the individual bunch length and the bunch-to-bunch interval are critical parameters that need to be precisely diagnosed. However, directly measuring these parameters remains challenging. This paper presents an all-optical method that simultaneously measures the individual bunch length and the bunch-to-bunch spacing through an orthogonal THz-driven streak camera. For a 3 MeV electron bunch train, the simulation indicates that the temporal resolution of individual bunch length and the bunch-to-bunch spacing is 2.5 fs and 1 fs, respectively. Through this method, we expect to open a new chapter in the temporal diagnostic of electron bunch trains.

Two-stage reflective self-seeding scheme for high-repetition-rate X-ray free-electron lasers.

X-ray free-electron lasers (XFELs) open a new era of X-ray based research by generating extremely intense X-ray flashes. To further improve the spectrum brightness, a self-seeding FEL scheme has been developed and demonstrated experimentally. As the next step, new-generation FELs with high repetition rates are being designed, built and commissioned around the world. A high repetition rate would significantly speed up the scientific research; however, alongside this improvement comes new challenges surrounding thermal management of the self-seeding monochromator. In this paper, a new configuration for self-seeding FELs is proposed, operated under a high repetition rate which can strongly suppress the thermal effects on the monochromator and provides a narrow-bandwidth FEL pulse. Three-dimension time-dependent simulations have been performed to demonstrate this idea. With this proposed configuration, high-repetition-rate XFEL facilities are able to generate narrow-bandwidth X-ray pulses without obvious thermal concern on the monochromators.

Coherence time characterization method for hard X-ray free-electron lasers.

Coherence time is one of the fundamental characteristics of light sources. Methods based on autocorrelation have been widely applied from optical domain to soft X-rays to characterize the radiation coherence time. However, for the hard X-ray regime, due to the lack of proper mirrors, it is extremely difficult to implement such autocorrelation scheme. In this paper, a novel approach for characterizing the coherence time of a hard X-ray free-electron laser (FEL) is proposed and validated numerically. A phase shifter is adopted to control the correlation between X-ray and microbunched electrons. The coherence time of the FEL pulse can be extracted from the cross-correlation. Semi-analytical analysis and three-dimensional time-dependent numerical simulations are presented to elaborate the details. A coherence time of 218.2 attoseconds for 6.92 keV X-ray FEL pulses is obtained in our simulation based on the configuration of Linac Coherent Light Source. This approach provides critical temporal coherence diagnostics for X-ray FELs, and is decoupled from machine parameters, applicable for any photon energy, radiation brightness, repetition rate and FEL pulse duration.

Attosecond Coherence Time Characterization in Hard X-Ray Free-Electron Laser.

One of the key challenges in scientific researches based on free-electron lasers (FELs) is the characterization of the coherence time of the ultra-fast hard x-ray pulse, which fundamentally influences the interaction process between x-rays and materials. Conventional optical methods, based on autocorrelation, are very difficult to realize due to the lack of mirrors. Here, we experimentally demonstrate a novel method which yields a coherence time of 174.7 attoseconds for the 6.92 keV FEL pulses at the Linac Coherent Light Source. In our experiment, a phase shifter is adopted to control the cross-correlation between x-ray and microbunched electrons. This approach provides critical diagnostics for the temporal coherence of x-ray FELs and is universal for general machine parameters; applicable for wide range of photon energy, radiation brightness, repetition rate and FEL pulse duration.

The detuning effect of crystal monochromator in self-seeding and oscillator free electron laser.

Self-amplified spontaneous emission (SASE) free electron laser (FEL) is capable of generating ultra-short, high power and high brightness X-ray pulses, but its temporal coherence is poor. Self-seeding scheme is an approach to improve the temporal coherence by employing a crystal monochromator. The crystal detuning effect is the phenomenon that the Bragg angle deviates from the middle of the reflection domain due to the refraction effect, and can affect the seed power of hard X-ray self-seeding (HXRSS) FEL. In this paper, we introduce a novel idea to maximize the seed power by tuning the incident angle off the Bragg condition where the Bragg photon energy is corresponding to the central photon energy of the input X-ray pulse. We present the numerical analysis of the detuning effect in different reflecting atomic planes and different asymmetry angles of diamond crystal. Moreover, we analyze how the detuning affects the seed efficiency of HXRSS FEL, and discuss the application to X-ray FEL oscillator (XFELO). We find when the detuning is much smaller than the bandwidth of input X-ray pulse, we can neglect the detuning effect. However, if the detuning is much larger than or comparable with the bandwidth of input X-ray pulse, the detuning effect can not be ignored. This work can give a guidance to HXRSS FEL and XFELO commissioning for high efficiency FEL output.

Angular dispersion enhanced prebunch for seeding ultrashort and coherent EUV and soft X-ray free-electron laser in storage rings.

Prebunching is an effective technique to reduce the radiation saturation length and to improve the longitudinal coherence and output stability in storage-ring-based free-electron lasers (FELs). A novel technique is proposed which uses angular dispersion to enhance the high-harmonic bunching with very small laser-induced energy spread. This technique can effectively reduce the radiation saturation length without significantly reducing the peak power of the FEL. Numerical simulations demonstrate that this technique can be used for the generation of 100 MW scale level, fully temporal coherent femtosecond extreme-ultraviolet and soft X-ray radiation pulses through a 10 m-long undulator based on a diffraction-limited storage ring.