Vladimir E. Sherstobitov

Efficient and compact short pulse MOPA system for laser-produced-plasma extreme-UV sources employing RF-discharge slab-waveguide CO2 amplifiers

Recent studies of fundamental issues of target material format and laser radiation parameters have revealed the attractiveness of LPP EUV source technology based on Sn target and multi-kW CO2 laser driver. In recent work we have reported 8kW of average power at 100kHz repetition frequency and 20ns pulse duration produced by our MOPA CO2 laser driver built on a chain of Fast Axial Flow (FAF) amplifiers. However, the oscillator power is insufficient to saturate the input stages and significant amount of available laser energy (>80%) is untapped. In this paper we report a step towards an improvement of laser driver power and efficiency. For the first time, to our knowledge, the performance of a novel multi-pass pre-amplifier based on RF-excited slab waveguide CO2 laser technology has been numerically modeled. The calculations show the feasibility of this approach. We carried out amplification experiments to validate the numerical model. In our experiments we have obtained power gain of 10 at 13-pass configuration from a slab of 60x600mm2 geometry at 20ns pulse length and 100kHz repetition frequency at diffraction-limited output and no self-oscillation. The experiment has validated the numerical model, which will be used at this stage to design and optimize a pre-amplifier for our current FAF amplifier chain. Furthermore, these results enable us to design and optimize next generation of LPP laser driver based entirely on compact slab-waveguide amplifiers.

Correction of segmented mirror aberrations by phase conjugation and dynamic holography

A novel approach is proposed for using the phase conjugation phenomenon for dynamic correction of distortions in beam forming laser systems. The method is based on the well-known application of a special diffraction structure on the surface of corrected optical elements ensuring nonreciprocal beam propagation in a system including a phase conjugate mirror. The main feature of the proposed approach is that it uses the diffraction structure which is not static (as usually) but is written dynamically by a small auxiliary laser source irradiating the corrected element surface simultaneously with the working beam. This makes it possible to extend the capabilities of the known method with respect to scaling-up the size of compensated mirrors and allows replacement of a large solid mirror by a segmented one. The novel method of correction has been tested experimentally by the use of 1.06 μm radiation for recording the dynamic hologram and a 10.6 μm laser beam as the working beam, distorted by a two-segment mirror with a dynamic hologram on its surface and corrected by a phase conjugate mirror. The means of dynamic hologram recording in different spectral ranges are discussed.

Phase conjugation in a high-power E-beam-sustained CO2 laser

The experimental demonstration of phase conjugation technique as applied to aberration correction in a high-power gas-flow CO2 laser is reported. A four-wave mixing in 34SF6 was used to obtain high efficient phase conjugation of repetition rate electron- beam-sustained CO2 laser radiation. For the first time, the nearly diffraction-limited performance of the double-pass CO2 amplifier with the phase-conjugate mirror for the output energy of about 1.2 KJ and average power of 40 KW is demonstrated.

Feasible configurations of a repetitively pulsed TEA CO2 laser with small angular divergence and 1- kJ pulse energy

The results are presented of expert evaluations aimed at developing the optimal concept of creation of a compact TEA CO2 laser with a self-sustained discharge capable of generating the repetitive bursts of ten radiation pulses with 1 kJ-energy in each and the pulse repetition rate of 30 Hz. Particular emphasis is placed on the problems of achieving as small angular divergence and beam wandering as possible.

Compensation for distortions of a telescope's primary mirror by means of a volume dynamic hologram

We describe a number of experiments devoted to compensation for aberrations in the primary mirror of a telescope using a volume dynamic holography technique, where the writing and reading of the hologram are carried out independently. Three sets of experiments demonstrating holographic aberration correction have been performed. In all cases transmitting holograms were formed in photorefractive crystals such as BSO and SBN. In experiments with a small diameter primary mirror the image is formed by a multicolor point-like object. The system includes a unit which compensates for the hologram dispersion. The aberration correction of a six- segmented primary mirror of 150 mm in diameter is demonstrated in conditions where there is a pronounced difference in the wavelengths of the recording beams and that of the source observed (Δλ=0.12 μm). The experiments on observation an extended object of continuous spectrum (Δλ=0.35 μm) with a single primary mirror of 150 diameter have been also made. All these experiments indicate an efficient compensation of distortions of the primary mirror by the hologram.

Self Q-switching and mode locking in broad area lasers subject to transverse mode selective feedback

Numerical and experimental results of output dynamics investigations of AR-coated broad area lasers (BALs) above laser threshold are presented. The BALs are subject to feedback from a free-space external Fourier-optical 4f-setup with a spatial reflective filter in the Fourier-plane for transverse mode selection. It is shown theoretically and experimentally that under certain pump current conditions the BALs are operating in a repetitive self-pulsation mode. Pulse duration is approx. 1 ns at repetition rates of 200 to 500 MHz. Using the same setup active mode-locking of a BAL is achieved experimentally. Pulse durations of 103 ps are obtained. The Gaussian-like fundamental and higher order transverse modes up to mode No. 4 can be adjusted while the laser is operating in a mode-locked state. Experimentally, the simultaneous combination of mode-locking, transverse mode selection, and pulse shaping of a BAL in a modified 4f-setup implementing a spectral filter is investigated. Employing an optimized spectral sinc-like function as amplitude and phase filter the mode-locked BAL emits nearly square-shaped pulses with a pulse duration of 705 ps, while running close to the Gaussian-like transverse mode.

Optical Problems of Laser Radiation Transport in the LOTV Concept

In the scope of the LOTV concept optical problems have been considered of a laser power transport from an airborne laser, flying at a 10‐kilometer altitude, to a space “tugboat” with a laser propulsion engine (LPE), intended for payload orbital transfer from LEO to GEO. For a transmitter telescope size of 1–1.5 m the range of optimal sizes of a receiver collector as well as the range of optimal specific impulses of LPE have been determined for the radiation wavelength of 0.5 μ and 1 μ, and payloads of 3–5 t. The optimization was carried out by searching a trade‐off between propellant mass saving and the orbit transfer time reduction.

On a Possibility of Laser Beam Control in LOTV Mission by Means of Nonlinear and Coherent Optics Techniques

In the context of LOTV Mission (Laser Orbital Transfer Vehicle) the analysis of laser beam delivery efficiency has been carried out for power beaming with a 200‐kW‐solid‐state laser from an aircraft to a space “tugboat” having a beam receiving collector of 10‐meter‐size. Application of nonlinear‐optics and adaptive optics techniques of laser beam control for correction of optical distortions in the propagation path caused by vibration and thermal deformation of optical elements as well as by atmospheric turbulence is discussed. Possible architecture of laser beam transport systems including a laser beacon on the receiver, a repetitively pulsed solid‐state laser in MOPA‐PCM configuration and a beam director telescope of 1‐meter size is considered. The results of computer simulation of a MOPA‐PCM system as well as the system as a whole are presented. The requirements to optical elements of the system are discussed.

Fourier optical transverse mode selection in broad-area lasers: experiment and simulation

Broad area lasers (BALs) with external Fourier-optical cavities with spatial filter for transverse mode selection are investigated experimentally and numerically. Nearfield and farfield distributions are calculated and compared to experimental results. Two different BALs, one with a high (10%) and the other with a very low residual reflectivity of the output facet (facing the external resonator) are operated in external Fourier-optical 4f setups. For the BAL mentioned first, transverse mode selection is obtained at low pump currents; the emission of the fundamental or a specific higher order transverse mode can be stabilized. In the latter BAL transverse modes can be selectively excited up to pump currents more than 200% above laser threshold. Numerical calculations reveal, that if this BAL within an external 4f-cavity is pumped even higher above laser threshold, it starts to operate in a self-Q-switched mode. Pulse duration and repetition rates are in the range of 2-4ns and 100-200MHz, respectively. Also, the concept and technology for fabrication of compact, miniaturized hybrid integrated Fourier-optical external cavities based on polymer waveguide mirrors is described. Experimental results of transverse mode selection in BALs with hybrid integrated-optical cavities are presented.

Laser resonators with helical optical elements

The features of fundamental modes in large-aperture unstable resonators with helical mirrors are for the first time to our knowledge investigated via computer simulation. It is shown that a large-aperture laser can oscillate at a single transverse mode with axially symmetric intensity and nonzero topological charge determined by the charge of the helical coupling mirror. Energy efficiency of such a laser does not differ from that for conventional unstable cavity.