Yu. V. Shabalin

Proposal of a compact repetitive dichromatic x-ray generator with millisecond duty cycle for medical applications

Many practical applications of x-rays lie in the important for the society fields of medical imaging, custom, transport inspection and security. Scientific applications besides of fundamental research include material sciences, biomicroscopy, and protein crystallography. Two types of x-ray sources dominate now: conventional tubes and electron accelerators equipped with insertion devices. The first are relatively cheap, robust, and compact but have low brightness and poorly controlled photon spectrum. The second generate low divergent beams with orders of magnitude higher brightness and well-controlled and tunable spectrum, but are very expensive and large in scale. So accelerator based x-ray sources are mainly still used for scientific applications and x-ray tubes - in commercial equipment. The latter motivated by the importance for the society made an impressive progress during last decades mostly due to the fast developments of radiation detectors, computers and software used for image acquisition and processing. At the same time many important problems cannot be solved without radical improvement of the parameters of the x-ray beam that in commercial devices is still provided by conventional x-ray tubes. Therefore there is a quest now for a compact and relatively cheap source to generate x-ray beam with parameters and controllability approaching synchrotron radiation. Rapid developments of lasers and particle accelerators resulted in implementation of laser plasma x-ray sources and free electron lasers for various experiments requiring high intensity, shrt duration and monochromatic x-ray radiation. Further progress towards practical application is expected from the combination of laser and particle accelerator in a single unit for efficient x-ray generation.

Submicrosecond regular and chaotic nonlinear dynamics in a pulsed picosecond Nd:YAG laser with millisecond pumping

We propose and study both numerically and experimentally a feedback-controlled laser system capable of generating regular bursts with a submicrosecond period. Bursting is obtained in a laser that is controlled by a combination of feedbacks in which the negative feedback loop action is delayed by one cavity round trip with respect to the positive one, and the period is adjusted by relative feedback sensitivity. The proper combination of feedbacks is realized in a Nd:YAG laser with millisecond pumping by means of a single optoelectronic negative feedback unit that utilizes the signal reflected from an intracavity Pockels cell polarizer. Regular bursting (microgroups of picosecond pulses) with controlled periods from 25 to 75 cavity round trips is obtained experimentally. The development of chaotic dynamics displayed by the system at a higher pumping level differs from the Feigenbaum scenario.

Laser Physics Research Relevant to Laser-Electron X-Ray Generator

A prototype of laser unit for Laser Electron X-Ray Generator is constructed on the basis of the optoelectronic control. The laser system in which an optoelectronic negative feedback is realized by means of a signal reflected from an intracavity Pockels cell polarizer is proposed and tested. The design provides flexible control over pulse train time structure.

Laser-Electron X-Ray Generator

The possibility of developing a laser-electron x-ray generator based on the Thomson scattering of laser radiation by relativistic electrons and prospects for its application are considered. In its specifications (brightness, average intensity, and dimensions), as well as its construction and operation cost, such a generator is intermediate between x-ray tubes and synchrotron sources. The configuration of channels and experimental stations intended for applications of an x-ray laser-electron generator in studies of the elemental composition and structure of materials is discussed.

Dual Feedback Control in Solid State Lasers: Discrete Maps and Experiments

Combinations of negative and positive feedback loops were investigated on the basis of discrete map analysis, numerical simulation and experimentally with lamp and diode pumped Nd:YAG lasers. Cases with relative feedback delay in two loops equal to one laser cavity round trip time were considered. Dual feedback control allows to achieve: 1) stable operation of both continuous and pulsed lasers in a wide range of active media gain, 2) laser pulse shortening down to several tens of picoseconds and 3) regular burst mode with periods up to hundred laser cavity round trip times.

Generation of a regular sequence of short-pulse microtrains with a discretely varied repetition period

In the Nd:YAG lamp-pumped laser controlled by optoelectronic negative feedback based on an electro-optic DKDP crystal, the mode of generation of a millisecond sequence of short-pulse microtrains with a discretely varied repetition period was implemented. The microtrain repetition periods are fixed and controlled by resonances of shear acoustic oscillations of the crystal. The cavity design allowed selective excitation of the first ten acoustic modes in the period range of 8–0.5 µs.

Harmonic mode-locking and sub-round-trip time nonlinear dynamics of electro-optically controlled solid state laser

Harmonic mode-locking in a solid state laser due to optoelectronic control is studied numerically on the basis of two methods. The first one is detailed numeric simulation taking into account laser radiation fine time structure. It is shown that optimally chosen feedback delay leads to self-started mode-locking with generation of desired number of pulses in the laser cavity. The second method is based on discrete maps for short laser pulse energy. Both methods show that the application of combination of positive and negative feedback loops allows to reduce the period of regular nonlinear dynamics down to a fraction of a laser cavity round trip time.

Application of optoelectronic negative feedback to ordering of the temporal structure of the diode-pumped Nd:YLF laser radiation

The conditions of the generation of radiation with ordered temporal structure are determined for a solid-state laser with external negative feedback. Experiments are performed based on the Nd:YLF laser longitudinally pumped by a high-power diode array. Lasing was controlled using an intracavity electro-optic modulator based on a lithium tantalate crystal. Intensity smoothing and self-mode-locking regimes with generation of several short pulses in the axial interval are implemented by varying the delay in the feedback circuit. Under conditions of amplification above the stability boundary, the secondary electro-optic effect manifestation was detected in the lithium tantalate crystal.

Discrete Maps and the Problem of Round Trip Time Scale Nonlinear Dynamics in Solid-State Lasers

We show numerically and analytically that the control based on the combination of optoelectronic negative and positive feedback loops allows one to obtain new time scale nonlinear dynamics regimes in solid-state lasers. The combination of feedbacks enables the realization of nonlinear dynamics of the logistic map. In lasers with external harmonic modulation of losses the combination of positive and negative feedback loops makes it possible to obtain period doubling bifurcation at the time scale in the range of few up to hundreds laser-cavity round trip times.

Laser cavity round trip time scale regular and chaotic nonlinear dynamics in a picosecond laser controlled with a combination of positive and negative optoelectronic feedbacks

We proposed a simple laser system controlled by optimally adjusted combination of inertial negative and positive feedbacks capable to demonstrate the nonlinear regular and chaotic dynamics on the time scale of the laser cavity round trip time. The numeric simulation proved that such a system can be realized on the basis of the optoelectronically controlled solid state laser.