ABSTRACT
We demonstrate that 3-mm-thick, periodically poled L i N b O 3 enables energy scaling of a nonresonant optical parametric oscillator operated in the narrowband mode with a volume Bragg grating at the signal wavelength. Utilizing the full available pump power at 1064 nm, we obtained maximum average powers of 2.25 and 2.08 W for the signal (1.922 µm) and idler (2.383 µm) pulses at 10 kHz, at a total conversion efficiency of 32.8%, which represents a fourfold increase in terms of peak powers over our previous work. The signal and idler spectral linewidths were â¼1n m, with pulse lengths of â¼6n s and an idler beam propagation factor of â¼5.
ABSTRACT
We present a tunable (6.62-11.34 µm), singly-resonant, cascade optical parametric oscillator with intracavity pumping of BaGa2GeSe6 in the second stage and spectral narrowing realized by a Volume Bragg Grating acting on the signal wave of the first stage which serves as a pump for the second stage. The maximum energy achieved near 8 µm in the narrowband regime is 1.1 mJ at 100â Hz (spectral width: â¼20â cm-1, pulse duration: â¼7â ns). The overall conversion efficiency from 1 to 8 µm for broadband and narrowband operation is 4.0% and 3.1%, respectively, corresponding to quantum efficiencies of 31% and 23%.
ABSTRACT
The paper presents an overview of the benefits of recording phase masks into the bulk of photo-thermo-refractive glass. We demonstrate that both binary and gray-scale phase masks can be encoded into the medium, and that such masks can be used for mode conversion and beam shaping with near-theoretical efficiency. We further demonstrate that by encoding the phase mask profile into a transmitting volume Bragg grating, it is possible to create tunable and achromatic phase masks without requiring a complex phase pattern.
ABSTRACT
While conventional complex phase masks are chromatic, we present an achromatic holographic phase mask capable of performing optical beam transformations in a spectral range exceeding 1000 nm. The system consists of a holographic phase mask fabricated by encoding the desired phase profiles into volume Bragg gratings, inserted in between two surface gratings. This device automatically adjusts each spectral component diffracted by the surface grating to the Bragg angle of the volume Bragg grating and equalizes phase incursion for all diffracted wavelengths. Transverse mode conversion is demonstrated and compared with theory for multiple narrow line laser sources operating from 488 to 1550 nm and for a broadband femtosecond source.
ABSTRACT
A novel photothermal process to spatially modulate the concentration of sub-wavelength, high-index nanocrystals in a multicomponent Ge-As-Pb-Se chalcogenide glass thin film resulting in an optically functional infrared grating is demonstrated. The process results in the formation of an optical nanocomposite possessing ultralow dispersion over unprecedented bandwidth. The spatially tailored index and dispersion modification enables creation of arbitrary refractive index gradients. Sub-bandgap laser exposure generates a Pb-rich amorphous phase transforming on heat treatment to high-index crystal phases. Spatially varying nanocrystal density is controlled by laser dose and is correlated to index change, yielding local index modification to ≈+0.1 in the mid-infrared.
ABSTRACT
Recording of volume Bragg gratings (VBGs) in photo-thermo-refractive glass is limited to a maximum refractive index change about 0.002. We discuss various saturation curves and their influence on the amplitudes of recorded gratings. Special attention is given to multiplexed VBGs aimed at recording several gratings in the same volume. The best shape of the saturation curve for production of the strongest gratings is the threshold-type curve. Two-photon absorption as a mechanism of recording also allows increasing the strength of multiplexed VBGs.
ABSTRACT
An external volume Bragg grating (VBG) is used for transverse and longitudinal mode stabilization of a broad area diode laser (BAL) with an on-chip transverse Bragg resonance (TBR) grating. The internal TBR grating defines a transverse low-loss mode at a specific propagation angle inside the BAL. Selection of the TBR mode was realized via the angular geometry of an external resonator assembly consisting of the TBR BAL and a feedback element. A feedback mirror provides near diffraction limited and spectral narrow output in the TBR mode albeit requiring an intricate alignment procedure. If feedback is provided via a VBG, adjustment proves to be far less critical and higher output powers were achieved. Moreover, additional modulation in the far field distribution became discernible allowing for a better study of the TBR concept.
ABSTRACT
Photo-thermo-refractive (PTR) glass is a photosensitive multi-component silicate glass that is commercially used for the recording of volume holographic elements and finds many applications in advanced laser systems. Refractive index decrement in this glass is observed after UV exposure followed by thermal development. This procedure also causes the appearance of Ag-containing particles that can then be optically bleached by using the second harmonic of a Nd:YAG laser. Despite the broad usage of this method, its mechanisms are still unclear. In this paper, a systematic study of the short pulse laser-induced destruction of Ag-containing particles' kinetics versus incident energy per pulse and dosage is presented. We show that no bleaching of Ag-containing particles occurs for an energy density in laser pulses below 0.1 J/cm2 while above 1 J/cm2, the efficiency of bleaching saturates. Efficiency of bleaching depends on the type of particles to be bleached (Ag, AgBr ). Using a simple model of short pulse laser interaction with nanoparticles embedded in glass, the temperature of the Ag-containing particles reached during the laser interaction is shown to be large enough to produce complete dissipation of these particles which is expected to be the main mechanism of short pulse laser-induced destruction of Ag-containing particles.
ABSTRACT
Volume Bragg gratings serve an important role in laser development as devices that are able to manipulate both the wavelength and angular spectrum of light. A common method for producing gratings is holographic recording of a two collimated beam interference pattern in a photosensitive material. This process requires stability of the recording system at a level of a fraction of the recording wavelength. A new method for measuring and stabilizing the phase of the recording beams is presented that is extremely flexible and simple to integrate into an existing holographic recording setup and independent of the type of recording media. It is shown that the presented method increases visibility of an interference pattern and for photo-thermo-refractive glass enables enhancement of the spatial refractive index modulation. The use of this technique allows for longer recording times that can lead to the use of expanded recording beams for large aperture gratings.
ABSTRACT
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%.
ABSTRACT
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.
ABSTRACT
The effect of aberrations in the recording beams of a holographic setup is discussed regarding the deterioration of properties of a reflecting volume Bragg grating. Imperfect recording beams result in a spatially varying grating vector, which causes broadening, asymmetry, and washed out side lobes in the reflection spectrum as well as a corresponding reduction in peak diffraction efficiency. These effects are more significant for gratings with narrower spectral widths.
ABSTRACT
Experimental and modeled dependencies of the induced refractive index on dosage of UV exposure in photo-thermo-refractive glass for different thermal treatment regimes are presented. Resulting spatial profiles of refractive index modulation in a reflecting Bragg grating recorded by a holographic technique are computed, and corresponding diffraction efficiencies are modeled. It is shown that nonlinearity of the photosensitivity response is responsible for spatial distortions of a recorded grating that result in a decrease of the diffraction efficiency.
ABSTRACT
Permanent binary phase masks with planar surfaces and high tolerance to laser radiation are recorded in the volume of photo-thermo-refractive glass using the contact copying technique and binary amplitude master masks. Conversion of a Gaussian beam to higher order modes is shown.
ABSTRACT
Volume Bragg gratings (VBGs) in photothermorefractive (PTR) glass are widely used for laser beam control including high-power laser systems. Among them, spectral beam combining based on VBGs is one of the most promising. Achieving 100+ kW of combined laser beams requires the development of PTR glass and VBGs with an extremely low absorption coefficient and therefore methods of its measurement. This paper describes the calorimetric method that was developed for measuring a low absorption coefficient in PTR glass and VBGs. It is based on transmission monitoring of the intrinsic Fabry-Perot interferometer produced by the plane-parallel surfaces of the measured optical elements when heated by high-power laser radiation. An absorption coefficient at 1085 nm as low as 5×10(-5) cm(-1) is demonstrated in pristine PTR glass while an absorption coefficient as low as 1×10(-4) cm(-1) is measured in high-efficiency reflecting Bragg gratings with highest purity. The actual level of absorption in PTR glass allows laser beam control at the 10 kW level, while the 100 kW level would require active cooling and/or decreasing the absorption in PTR Bragg gratings to a value similar to that in virgin PTR glass.
ABSTRACT
A master-oscillator power-amplifier with stimulated Brillouin scattering (SBS) beam cleanup or wavefront reversal typically incorporates a Faraday isolator to outcouple the Stokes light, limiting the power scalability. Volume Bragg gratings (VBGs) have the potential for scaling to higher powers. We report here the results of tests on a VBG designed to resolve wavelengths 0.060 nm apart, corresponding to the 16 GHz frequency shift for SBS backscattering at 1064 nm in fused silica. Such an element may also find use in between stages of fiber amplifiers, for blocking the Stokes wave.
ABSTRACT
A new class of Fabry-Perot filters produced by a multilayer dielectric mirror deposited on top of a reflecting volume Bragg grating is described. The first fabricated prototype for the 852 nm region demonstrates a 30 pm bandwidth, 90+% transmission at resonance, and a good agreement with theoretical simulation.
ABSTRACT
An experimental demonstration of a moiré reflecting Bragg grating in photo-thermo-refractive glass is carried out. This narrowband filter is obtained by the recording of two reflecting Bragg gratings with different periods. Filters with central wavelength at 1550 nm, bandwidth of 50 pm, and transmission higher than 95% are demonstrated. The methods to decrease bandwidth to 1 pm are finally investigated.
ABSTRACT
We report on the characteristics of ultrashort laser pulse diffraction by transmitting volume Bragg gratings recorded in photo-thermo-refractive glass. The angular and spectral selectivity properties of these phase volume gratings are shown to obey the predictions of a modified Kogelnik's coupled wave analysis that accounts for polychromatic beams. Spatio-temporal distortions such as angular dispersion and pulse front tilt that occur after diffraction by a single volume grating are studied and corrected by using a volume grating pair. The angular filtering properties of volume gratings offer potential as spatial filtering elements inside ultrafast laser cavities.
ABSTRACT
Three conditions for non-collinear third harmonic generation by a PTR glass volume Bragg grating are demonstrated using infrared ultrashort pulse illumination. Each condition corresponds to a different angle of grating orientation and a separate generation mechanism. We identify the mechanisms as corresponding to sum-frequency generation, Bragg diffraction of 3 omega, and a non-resonant Bragg condition involving three omega photons interacting with a nonlinear grating vector. Theoretical modeling is performed using wave vector additions and the results are compared to experimental measurements.
