Volume Bragg Gratings: Fundamentals and Applications in Laser Beam Combining and Beam Phase Transformations

Two major volume Bragg grating (VBG) applications will be presented and in particular laser beam combining and holographically encoded phase masks. Laser beam combining is an approach where multiple lasers are combined to produce more power. Spectral beam combining is a technique in which different wavelengths are superimposed spatially (combined) using a dispersive element such as a volume Bragg grating. To reduce the complexity of such combining system instead of multiple individual VBGs, it will be demonstrated that a single holographic element with multiple VBGs recorded inside could be used for the same purpose. Similar multiplex volume holographic elements could be used for coherent beam combining. In this case, the gratings operate at the same wavelength and have degenerate output. Such coherent combining using gratings written in photothermo-refractive (PTR) glass will be discussed. The chapter also demonstrates that binary phase profiles may be encoded into volume Bragg gratings, and that for any probe beam capable of satisfying the Bragg condition of the hologram, this phase profile will be present in the diffracted beam. A multiplexed set of these holographic phase masks (HPMs) can simultaneously combine beams while also performing mode conversion. An approach for making HPMs fully achromatic by combining them with a pair of surface gratings will be outlined

Scaling the spectral beam combining channels in a multiplexed volume Bragg grating

In order to generate high power laser radiation it is often necessary to combine multiple lasers into a single beam. The recent advances in high power spectral beam combining using multiplexed volume Bragg gratings recorded in photo-thermo-refractive glass are presented. The focus is on using multiple gratings recorded within the same volume to lower the complexity of the combining system. Combining of 420 W with 96% efficiency using a monolithic, multiplexed double grating recorded in PTR glass is demonstrated. A multiplexed quadruple grating that maintains high efficiency and good beam quality is demonstrated to pave a way for further scaling of combining channels

Moire volume Bragg grating filter with tunable bandwidth

We propose a monolithic large-aperture narrowband optical filter based on a moire volume Bragg grating formed by two sequentially recorded gratings with slightly different resonant wavelengths. Such recording creates a spatial modulation of refractive index with a slowly varying sinusoidal envelope. By cutting a specimen at a small angle, to a thickness of one-period of this envelope, the longitudinal envelope profile will shift from a sine profile to a cosine profile across the face of the device. The transmission peak of the filter has a tunable bandwidth while remaining at a fixed resonant wavelength by a transversal shift of incidence position. Analytical expressions for the tunable bandwidth of such a filter are calculated and experimental data from a filter operating at 1064 nm with bandwidth range 30-90 pm is demonstrated

Three-Dimensional Low-Index-Contrast Photonic Crystals Fabricated Using A Tunable Beam Splitter

This paper describes the design and implementation of a tunable beam splitter for fabricating three-dimensional low-index-contrast photonic crystals using four-beam interference. Here, a central console is used to split a single laser beam into four beams that are focused onto the sample in an umbrella-like configuration using three adjustable mirrors. The design facilitates simple and precise adjustments of the beam angles and polarization, which can be used to readily optimize fabrication conditions for different photosensitive materials and lattice structures. Structures fabricated using this tunable beam splitter at two different incident angles of 18.5° and 27° resulted in lattices with hexagonal symmetries having well-defined nanometre-scale features that are in close agreement with the feature sizes predicted theoretically. © 2006 IOP Publishing Ltd

Simultaneous Laser Mode Conversion And Beam Combining Using Multiplexed Volume Phase Elements

An approach is presented where a single volume phase element is used to accomplish spectral beam combining of multiple beams while simultaneously converting higher order modes to the fundamental mode. © OSA 2013

Femtosecond Laser Transverse Mode Conversion By An Achromatic Volume Phase Mask

Ultrafast femtosecond laser systems have enabled many breakthroughs in the fields of science and technology. However due to the large spectral bandwidth necessary for creating short pulses, it is quite difficult to manipulate their transverse mode structure. Here we present successful femtosecond transverse mode conversion from the fundamental mode TEM00 to modes TEM01, TEM10, and TEM11 with the use of an achromatic phase mask based on diffractive optics. The femtosecond source had ∼12.8 nm of bandwidth and pulse duration of \u3c100 fs

Three-Dimensional Diffractive Micro- And Nano-Optical Elements Fabricated By Electron-Beam Lithography

The broad development of the micro- and nano-technologies in the past few years increased the need of techniques capable of fabricating sub-micron structures with arbitrary surface profiles. Out of the several fabrication approaches (HEBS lithography, laser writing, etc.) the electron beam writing stands out as the one capable of the highest resolution, superior alignment accuracy and very small surface roughness. These characteristics make the technique greatly applicable in the fields of photonics and micro-opto-electro- mechanical-systems (MOEMS). Here we describe the specificity of fabricating 3D diffractive micro- and nano-optical elements using Leica EBPG 5000+ electron beam system. Parameters like speed of writing, dose accumulation, pattern writing specifics, etc. affect greatly the electron-beam resist properties and the desired 3D profile. We present data that can be used to better understand the different dependencies and therefore achieve better profile and surface roughness management. The results can be useful in future developments in the areas of integrated photonic circuits and MOEMS

High-Energy Wavelength Tunable Dual-Channel Tm:Ylf Laser

A dual channel high energy solid state laser using intracavity volume Bragg gratings is presented. CW operation showed a maximum output power of 5.7 W with a slope efficiency of 48%. Implementation of a Cr:ZnSe saturable absorber with 80% transmission achieved q-switching regime with 160 ns long pulses and combined energy of 4 mJ. Each channel had less than 1 nm spectral width in the range between 1880 and 1908 nm. The spectral difference between the two channels was tuned from 5 to 20 nm, corresponding to 0.4 to 1.7 THz, providing an alternative method for generating narrowband terahertz radiation

Effect Of Aberrations In A Holographic System On Reflecting Volume Bragg Gratings

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. © 2013 Optical Society of America

High Brightness, Sub-Nanosecond, And Compact Passively Q-Switched Laser With Intracavity Volume Bragg Gratings

A high brightness, high energy passively Q-switched (PQS) Nd:YAG laser is presented using volume transmitting Bragg gratings (TBGs) as angular filters. A planar cavity with a length of 1 cm, an 800 μm diameter pump beam, a 20% transmission Cr:YAG saturable absorber, and a 40% output coupling resulted in output pulse energies greater than 1 mJ and durations near 600 ps. In order to increase brightness without altering cavity length, a TBG with narrow angular selectivity placed in the cavity to suppress the higher order transverse modes allowing for near diffraction limited output