Andrey Yu. Rodionov

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.

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.

1 mJ single-rod fiber Er:glass laser for rangefinding

We demonstrate a compact Er:glass single-rod fiber laser for rangefinding with 1 mJ energy Q-switched at 1.54μm. Double-pass pumping with 16 W power and 5 ms pulse duration was used. Active medium was enveloped with diffuse reflector. Thus efficient output power operation achieved. Free-running mode output pulse energy was 12 mJ with a slope efficiency of 16%. Transparent glass-ceramics containing Co2+:MgAl2O4 nanocrystals were selected as the optimal passive gate to ensure Q-switching in a temperature range and transverse mode selection. The Q-switch mode had steady operation at 1 Hz repetition rate with thermal effects playing no visible role.

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.

Technology optimization techniques for multicomponent optical band-pass filter manufacturing

Narrowband optical devices (like IR-sensing devices, celestial navigation systems, solar-blind UV-systems and many others) are one of the most fast-growing areas in optical manufacturing. However, signal strength in this type of applications is quite low and performance of devices depends on attenuation level of wavelengths out of operating range. Modern detectors (photodiodes, matrix detectors, photomultiplier tubes and others) usually do not have required selectivity or have higher sensitivity to background spectrum at worst. Manufacturing of a single component band-pass filter with high attenuation level of wavelength is resource-intensive task. Sometimes its not possible to find solution for this problem using existing technologies. Different types of filters have technology variations of transmittance profile shape due to various production factors. At the same time there are multiple tasks with strict requirements for background spectrum attenuation in narrowband optical devices. For example, in solar-blind UV-system wavelengths above 290-300 nm must be attenuated by 180dB. In this paper techniques of multi-component optical band-pass filters assembly from multiple single elements with technology variations of transmittance profile shape for optimal signal-tonoise ratio (SNR) were proposed. Relationships between signal-to-noise ratio and different characteristics of transmittance profile shape were shown. Obtained practical results were in rather good agreement with our calculations.

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.

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.

Optimal pumping for eye-safe Er:YAG laser

We report on theoretical investigation of quasi-three level Er:YAG laser. We propose a numerical model of the laser design with side pump by 1471 nm laser diodes. The model describes the dynamical propagation of the pump in the cavity and the kinetic parameters of the active medium.

Objective for monitoring the corona discharge

Remote optoelectronic probing is one of the most actual aspects of overhead electric line maintenances. By installing such systems on a helicopter (for example) it becomes possible to monitor overhead transmission line status and to search damaged parts of the lines. Thermal and UV-cameras are used for more effective diagnostic. UV-systems are fitted with filters, that attenuate visible spectrum, which is an undesired type of signal. Also these systems have a wide view angle for better view and proper diagnostics. For even more effectiveness, it is better to use several spectral channels: like UV and IR. Such spectral selection provides good noise reduction. Experimental results of spectral parameters of the wide view angle multispectral objective for such systems are provided in this report. There is also data on point spread function, UV and IR scattering index data and technical requirements for detectors.