Oleksiy Andrusyak

Spectral Combining and Coherent Coupling of Lasers by Volume Bragg Gratings

The use of volume Bragg gratings (VBGs) recorded in photo-thermo-refractive (PTR) glass for laser beam control is described. These new optical elements provide extremely narrow spectral and angular selectivity and have a high level of resistance to high-power pulsed and continuous-wave laser radiation. These features of PTR volume gratings are used for transverse and longitudinal mode selection, passive coherent coupling, and spectral beam combining (SBC) of semiconductor, solid state, and fiber lasers.

Efficient power scaling of laser radiation by spectral beam combining

The possibility of achieving multikilowatt laser radiation by spectrally combining beams using volume Bragg gratings (VBGs) is shown. The VBGs recorded in a photothermorefractive glass exhibit long-term stability of all its parameters in high-power laser beams with power density >1 MW/cm2 in the cw beam of total power on a kilowatt level. We consider an architecture-specific beam-combining scheme and address the cross-talk minimization problem based on optimal channel positioning. Five-channel high efficiency spectral beam combining resulted in a >750 W near-diffraction-limited cw beam has been demonstrated experimentally.

Ultimate efficiency of spectral beam combining by volume Bragg gratings.

Spectral beam combining (SBC) by volume Bragg gratings (VBGs) recorded in photo-thermo-refractive (PTR) glass is a powerful tool for laser applications that require higher radiance than a single laser unit can achieve. The beam-combining factor (BCF) is introduced as a tool to compare various beam-combining methods and experiments. It describes the change of radiance provided by a beam-combining system but is not affected by the initial beam quality of the combined lasers. A method of optimization of VBGs providing the maximum efficiency of SBC has been described for an arbitrary number of beams. An experiment confirming the proposed modeling for a two-beam SBC system by a single VBG has demonstrated a total combined power of 301 W with a channel separation of 0.25 nm, combining efficiency of 97%, close to diffraction limited divergence with M(2)=1.18, BCF of 0.77, and spectral radiance of 770 TW/(sr·m(2)·nm), the highest to date for SBC.

Applications of volume Bragg gratings for spectral control and beam combining of high power fiber lasers

Volume Bragg gratings (VBGs) recorded in photo-thermo-refractive (PTR) glass are used in a wide range of high-power laser applications due to their unique spectral response and excellent optical and thermo-mechanical properties. Experimental results of applications of narrow-band reflecting VBGs to spectral beam combining (SBC) and wavelength control of fiber lasers are presented. Output power of 770 W from a system combining five fiber lasers with 91.7% efficiency is demonstrated with spectral separation between channels of 0.5 nm around 1064 nm and no distortions in diffracted beams. Similar system with 0.25 nm channel separation around 1550 nm is demonstrated with the same efficiency and M2 of the spectrally-combined beam < 1.15. A novel compact monolithic multi-channel beam combiner based on stacked tilted VBGs is suggested. Absolute efficiency exceeding 90% is reported for a four-channel device with 0.7 nm spectral separation of channels. We show that a linear stack of monolithic combining elements enables compact spectrally-combined laser systems with output power of 10-100 kW. A common-cavity approach to multi-channel spectral beam combining of high-power lasers is demonstrated. In this configuration wavelengths of the sources are passively controlled by a combination of a common output coupler and intra-cavity VBGs, which also act as combining elements. Laser wavelengths are automatically selected to match resonant wavelengths of respective gratings and provide maximum combining efficiency. Stable operation of a passively-controlled system combining two amplifiers with 0.4 nm spectral separation is demonstrated. Wavelengths of amplifiers are shown to automatically follow Bragg condition of VBGs during heating of gratings.

Thermal tuning of volume Bragg gratings for spectral beam combining of high-power fiber lasers

High-radiance lasers are desired for many applications in defense and manufacturing. Spectral beam combining (SBC) by volume Bragg gratings (VBGs) is a very promising method for high-radiance lasers that need to achieve 100 kW level power. Laser-induced heating of VBGs under high-power radiation presents a challenge for maintaining Bragg resonance at various power levels without mechanical realignment. A novel thermal tuning technique and apparatus is presented that enables maintaining peak efficiency operation of the SBC system at various power levels without any mechanical adjustment. The method is demonstrated by combining two high-power ytterbium fiber lasers with high efficiency from low power to full combined power of 300 W (1.5 kW effective power), while maintaining peak combining efficiency within 0.5%.

Thermal tuning of volume Bragg gratings for high power spectral beam combining

A tabletop kW-level spectral beam combining (SBC) system using volume Bragg gratings (VBGs) recorded in photothermo- refractive (PTR) glass was presented at the last meeting [1]. Diffraction efficiency of VBGs close to 100% was demonstrated. However, when using VBGs for spectral beam combining, it is important to ensure high diffraction efficiency for the diffracted beam and low diffraction efficiency for the transmitted beams simultaneously. The unique, unmatched properties of VBGs allow spectral beam combining achieving this condition at wavelengths with less than 0.25 nm separation. We present modeling of reflecting VBGs for high power SBC that takes into account laser spectral bandwidth, beam divergence, PTR-glass scattering losses, and grating non-uniformity. A method for optimization of VBG parameters for high-efficiency SBC with an arbitrary number of channels is developed. Another important aspect of spectral beam combiner design is maintaining high diffraction efficiency as the temperature of beam-combining VBGs changes during operation due to absorption of high power radiation. A new technique of thermal tuning of large aperture VBGs, designed to maintain high efficiency of beam combining without mechanical adjustment over a wide range of laser power, is developed. Finally, these tools are used to demonstrate a robust and portable 5-channel SBC system with near diffraction limited spectrally-combined output beam.

Control of filamentation for enhancing remote detection with laser induced breakdown spectroscopy

We report on the use of a novel phase element to control the far-field intensity pattern generated by a high-peak-power, femtosecond laser. The pre-determined intensity pattern results in a well defined location of the filaments formed by the propagation of these beams through the atmosphere. This enhancement of the localization and repeatability of the intensity distribution can be extremely beneficial for laser induced breakdown spectroscopy (LIBS) of remote regions of interest.

Single-pulse and burst-mode ablation of gold films measured by quartz crystal microbalance

Femtosecond ablation has several distinct advantages: the threshold energy fluence for the onset of damage and ablation is orders of magnitude less than for traditional nanosecond laser machining, and by virtue of the rapid material removal of approximately an optical penetration depth per pulse, femtosecond machined cuts can be cleaner and more precise than those made with traditional nanosecond or longer pulse lasers. However, in many materials of interest, especially metals, this limits ablation rates to 10-100 nm/pulse. We present the results of using multiple pulse bursts to significantly increase the per-burst ablation rate compared to a single pulse with the same integrated energy, while keeping the peak intensity of each individual pulse below the air ionization limit. Femtosecond ablation with pulses centered at 800-nm having integrated energy of up to 30 mJ per pulse incident upon thin gold films was measured via resonance frequency shifts in a gold-electrode-coated quartz-crystal oscillator. Measurements were performed using Michelson-interferometer-based burst generators, with up to 2 ns pulse separations, as well as pulse shaping by programmable acousto-optic dispersive filter (Dazzler from FastLite) with up to 2 ps pulse separations.

Coherent and spectral beam combining of fiber lasers using volume Bragg gratings

Five-channel spectral beam combining (SBC) using volume Bragg gratings (VBGs) in photo-thermo-refractive (PTR) glass with 0.5 nm spectral separation between channels and combined power >750 W has been recently reported. We report on improvements in this technique with the use of thermal control of VBGs that allows precise high-power alignment required for dense SBC with 0.25 nm spectral separation of channels. Experimental results of passive coherent beam combining (CBC) of fiber lasers using multiplexed VBGs are presented and analyzed. Methods for achieving 100 kW class systems using novel hybrid architectures that combine both coherent and spectral beam combining are discussed.

Passive coherent locking of fiber lasers using volume Bragg gratings

We introduce a novel technique of coherently locking fiber lasers using volume Bragg gratings (VBGs) recorded in photo-thermo-refractive (PTR) glass as a passive multiplexer between channels. A two-channel coherently-locked Ybdoped fiber laser system with a narrow linewidth (~2.5 pm) and linear polarization (PER >20 dB) is demonstrated at a level of ~ 4 W (limited by pump). Scaling of this technique to coherently lock multiple (>2) fiber laser channels is discussed.