Vol 38, No 4 (2017)

We consider the evolution of qubit states for the Demkov problem in the presence of dephasing processes in the spin tomographic-probability representation. We present an explicit solution of the spin tomogram in terms of the ₁F₂ hypergeometric function. We calculate the tomographic Shannon and q entropies through the solution of the master equation in the form of tomographic-probability distribution of the qubit states obtained.

We develop the wave-optics approach for calculating the diffraction distribution of gradient refractiveindex lenses and observing the diffraction pattern of gradient refractive-index lenses in the experiments. The results of our calculation are in good agreement with the experimental results obtained. We show that the diffraction can be regarded as a method to check the quality of the refractive-index distributions of gradient refractive-index lenses.

In this work, we develop a high-power optical-fiber transmitter for lidar applications. The device can be utilized for measurements of the methane concentration and determination of the parameters of methane absorption line at wavelengths near 1,650 nm. The transmitter is based on a fiber Raman amplifier with an active fiber length converting the pumping radiation with a wavelength of 1,540 nm to the desired wavelength region of about 1,650 nm. We conduct the theoretical modeling of the Raman amplifier operation and demonstrate the advantage of the two-stage amplifier design over the single-stage design. In the modeling, the total power of a single Raman amplifier in the twostage design is 4.5 W with a pumping power of 5 W and an active fiber length of 1,000 m. Then we experimentally investigate the developed single Raman amplifier and demonstrate a satisfactory agreement between its parameters and the results of the theoretical modeling. The achieved average power of the transmitter at the output stage is over 7 W with two single Raman amplifiers joined in parallel. The distributed-feedback (DFB) master laser utilized in the transmitter is modulated by a linear-frequency signal with simultaneous application of a nonsynchronous high-frequency signal to eliminate the noise due to stimulated Brillouin scattering.

To study the characteristics of a high-energy proton beam produced by the interaction between the ultra-intense pulse laser and a composite target, we set up an experiment using the \SILEX-I" laser device. Using the CR39-type nuclear track detector (NTD), HD810-type radiochromic films (RCF), and Thomson ion spectrometer, we measure the density, space distribution, yield, and energy spectrum of the proton beam, which are mainly produced in the normal direction of the target back. The results show that the proton-beam launch direction has nothing to do with the ultra-intense laser pulse incident direction; the proton-beam space distribution is within the disc, silk or ring structure, the launch field angle of the proton beam is small, and the high-energy proton beam has a cut-off energy, which is related to the thickness of the target. These experimental results can provide important parameters and reference for the development of the laser proton imaging and treatment device.

We report the effect of preliminary γ-irradiation of polyethylene (PE) and ethylene-propylene copolymer (CEP) on the kinetics of polymer ablation under CO₂ laser irradiation. The rate of PE ablation exceeds the rate of CEP ablation at all doses of γ-irradiation. The ablation rate of the polymers can be approximated by a linear function in the initial stage, and the rate reaches a constant value in the second stage of ablation. The rate of laser ablation increases linearly with the dose of preliminary γ-irradiation in both stages of the kinetics. For PE ablation, the duration of the linear increase in rate decreases with increase in preliminary radiolysis dose. The morphology of the crater surface formed during the laser ablation of γ-irradiated polymers is characterized by a more diverse structure.

Terahertz wave radiation has important applications in medical instrumentation, optical communication, and so on. In this paper, we demonstrate a dual-wavelength (659.6 nm/669.4 nm) light output of a compact intracavity frequency-doubling Nd:YAG laser. We achieve a maximum output power of 1.3 W at central wavelengths of 659.6 and 669.4 nm with linewidths of 57 and 74 pm, respectively. The power instability at the maximum output power is less than 1%, and the beam quality is 1.5. This dual-wavelength laser provides a potential source for generating a coherent terahertz wave radiation of 6.46 THz by the nonlinear optical difference frequency method.

We report on the mid-infrared (MIR) and terahertz (THz) wave spectra of D₂O gas pumped using a fundamental transverse mode transversely excited atmospheric CO₂ laser with an emission wavelength of 9.26 μm. We obtain MIR emission lines at center wavelengths of 9.262, 9.491, and 9.752 μm, which correspond to the vibrational-energy-level-transition lines of D₂O. We observe an intense THz stimulated Raman emission line of 385 μm and a weak cascade-transition line of 359 μm for transitions from rotational levels 4₂₂ to 4₁₃ and 4₁₃ to 4₀₄ in the first-excited vibration state of the D₂O molecule gas. We establish a four-energy-level system for modeling the laser kinetics of the dual-wavelength (385 and 359 μm) superradiation THz laser. For the optically pumped D₂O gas 385 μm THz laser, in considering the cavity effect and insertion loss of a THz cavity oscillator, an approximation treatment of the THz laser kinetics can be made based on a three-energy-level system.