Transmitting Volume Bragg Gratings Research Articles

Crystallization heat treatments for the fabrication of volume Bragg gratings based on photo-thermo-refractive glass

Photo-thermo-refractive (PTR) glass is a functional optical material with broad applications in holographic optics. In this work, we examine the effect of crystallization heat treatment on the growth and distribution of sodium fluoride (NaF) nano-crystals in PTR glass using DSC, XRD, SEM, and TEM. The optical properties of the crystallized PTR glass are analyzed using a spectrophotometer. The results indicate that the NaF nano-crystals precipitated in PTR glass matrix are in the form of polycrystalline, and the growth of which is highly dependent on temperature. The growth of NaF nano-crystals saturates during the crystalline phase transition, resulting in decreased distribution uniformity. Higher crystallization temperature and longer crystallization time lead to an increase in absorption and scattering losses in the visible band. However, the crystallized PTR glass exhibits favorable absorption and scattering loss characteristics in the near-infrared band. By adjusting the crystallization treatments within the temperature range of 530 °C–570 °C for 10–120 min, refractive index modulation (RIM) values of the transmitting volume Bragg gratings (TVBG) ranging from 50 to 350 ppm can be achieved, which allows for maximum relative diffraction efficiencies exceeding 90% at 532 nm and 1053 nm using different crystallization processes. This research enhances the understanding of NaF nano-crystal growth mechanisms and provides insights into the application of holographic optical elements based on PTR glass in both the near-infrared and visible bands.

High-efficiency, low-crosstalk multiplexed volume Bragg grating for non-mechanical angular magnification

The demand for compact transmitting volume Bragg gratings (TVBGs) for non-mechanical angular magnification is growing quickly in the industrial and defense areas. However, channel crosstalk limits the angular width and diffraction efficiency of multiplexed TVBGs. In this work, a crosstalk criterion is first proposed to evaluate the channel crosstalk quantitatively. Based on this criterion, for ±45° non-mechanical angular magnification, 12-channel high-efficiency low-crosstalk TVBGs are achieved by reducing the half-width at first zero (HWFZ) of the angular selectivity curve. Compared to the unoptimized scheme, the measured average relative diffraction efficiency of all optimized TVBGs reach 92 %, and the maximum HWFZ is reduced from 12.3496 to 1.7628 mrad. Our approach paves the way to develop high-performance multiplexed devices.

Versatile approach to laser beam shaping and analyzing by holographic phase masks

A new technology that includes a flexible holographic recording system using photo-thermo-refractive (PTR) glass is demonstrated for creating so-called ‘holographic phase masks’ or HPMs. These diffractive optical elements are permanently recorded in the PTR glass, can be multiplexed as with normal holograms, do not require electric power to operate, and can perform near arbitrary beam phase transformations. The holographic setup includes a spatial light modulator that enables recording transmitting volume Bragg gratings (VBGs) with arbitrary phase patterns encoded into them. As a result, the desirable phase pattern is introduced in a beam that is diffracted by the VBG. These HPMs are tunable within the visible and near IR spectral regions and can be made achromatic, with spectral widths of up to 50 nm. Furthermore, they tolerate laser power densities on the order of several kW cm−2. Applications of HPMs include high-power, broadband laser beam shaping and modal analysis of complex beams profiles, both of which are demonstrated here.

Transmitting volume Bragg grating with longitudinal moiré apodization

The apodized transmitting volume Bragg grating (TVBG) with the amplitude of the refractive index modulation (RIM) dropped to zero at both the surfaces is discussed. The proposed method for apodization is based on the recording of two identical uniform TVBGs with slightly different directions of their Bragg wave vectors in a single wafer. As a result, slow sinusoidal moire modulation occurs in the direction perpendicular to the averaged direction of the two wave vectors of Bragg modulation. The cutting of a specimen at the planes of the two closest zeros of the moire envelope produces the apodized TVBG. The longitudinal sinusoidal semi-period profile of RIM requires its specific maximum value to achieve 100% diffraction efficiency at the exact Bragg condition. Such an apodized TVBG demonstrates a significant suppression of the sidelobes in the diffraction efficiency depending on the angular or the wavelength detuning, in comparison to the diffraction operation of a uniform TVBG.

Wavefront shaping optical elements recorded in photo-thermo-refractive glass.

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.

2.5 kW average power, two-channel spectral-beam-combined output based on transmitting volume Bragg grating

Spectral beam combination based on volume Bragg gratings is an effective approach to obtaining high power laser output. In spectral beam combining system, spectral channel spacing will affect the number of non-combined sub-beams and the overall combined output power due to the finite available gain bandwidth. Based on coupled wave theory, a two-channel high power spectral beam combining model is proposed. By appropriately relaxing the requirements for the spectral channel spacing and line-width of sub-beams, the higher combined output power can be obtained but the spectral density does not significantly decrease. In this work, a 2-channel spectral beam combining system is demonstrated to present a 2.5 kW combined power with combining efficiency 85% by employing a transmitting volume Bragg grating. The combining system has a high spectral density of 0.51 kW/nm with 5 nm spectral spacing between channels. The output can keep a good beam quality when the combined power is less than 1 kW, while the significant degradation of combined beam quality occurs when output power is 1.5 kW and is restricted mainly by the dispersion properties and thermal effects of volume Bragg gratings. During this 2-channel beam combining process, no special active cooling measure is used. Interactions between laser radiation and the grating are verified. Thermal absorption of high power laser radiation in the grating will cause the temperature to remarkably increase, resulting in the thermal expansion of the grating period, which leads to the degradations of diffraction efficiency and the spectral selectivity. Research is also focused on the surface distortion, and the results indicate that the thermal-induced wave-front aberrations of the non-combined sub-beams lead to the deterioration of beam quality. Transmitted and diffracted beams experience wave-front aberrations to different degrees, leading to distinct beam deterioration.

Wavefront distortion optimized with volume Bragg gratings in photothermorefractive glass.

The wavefront characteristics in 2D angular filtering on the basis of two orthogonal transmitting volume Bragg gratings (VBGs) is presented. The experimental results show that middle-high frequency wavefront distortions are efficiently suppressed with VBGs. The peak-valley value of the beam at a wavelength of 1053 nm reduces from 2.075λ to 0.209λ, and the root mean square value reduces from 0.207λ to 0.041λ. The wavefront power spectrum density shows that the wavefront distribution of the beam in medium and high frequencies is corrected by the VBGs. Additionally, the far-field distribution and focusing properties of the beam are improved. The beam Strehl ratio increases from 0.43 to 0.96, and the encircled energy improves from 95% energy at 4.01 mrad to 95% energy at 1.26 mrad.

Diffraction of volume Bragg gratings under high flux laser irradiation.

Diffraction property of transmitting volume Bragg gratings (VBGs) recorded in photo-thermo-refractive glass (PTR) is studied under the irradiation of a continuous-wave fiber laser with flux of 1274 W/cm2. Dependence of temperature characteristics of VBGs prepared by different crystallization temperatures is presented. When temperature of VBGs rises up to 33°C, there are a 2.7% reduction and 1.59% ripple of diffraction efficiency for VBGs. The period variation caused by the thermal expansion of VBGs is used to explain the reduction of diffraction efficiency, and experimental results are in agreement with theoretical analysis.

Wavelength combining with volume Bragg gratings in photo-thermo-refractive glasses

Wavelength combining with a transmitting Volume Bragg Grating recorded in photo-thermo-refractive glasses is present. The combining condition is determined with theoretical simulation. The combining efficiency of 81.4% is obtained in the experiment, and it is influenced by the beam quality of the incident beam. The wavefront characteristics of combining beam are improved.

透射型体布拉格光栅对畸变激光束的空间滤波研究

Using transmitting volume Bragg gratings (TVBG) as a basis, an experiment on one-dimensional spatial filtering of a deformed laser beam is designed. The deformed laser beam results from a He-Ne laser beam modulated by an amplitude modulation plate with a spatial frequency of 7.2 mm ?1. Results show that when the central wave vector of the deformed beam satisfies the Bragg law of TVBG, the spatial profile of the -1st forward-diffracted order is similar to that of the undeformed He-Ne laser beam due to the TVBG with a spatial frequency selective bandwidth of less than 5.0 mm?1. The higher frequency components of the deformed beam are filtered out in the optical near field. Thus, the TVBG cleanup of the spatiallydeformed laser beam is realized experimentally.

大模面积双包层光纤激光频谱组束的实验效果

Based on the design scheme of spectral beam combining using transmitting volume Bragg grating, the experimental results for spectral beam combining of two fiber lasers are reported. The gain mediums used in this experiment were two large mode area erbium-ytterbium co-doped double-cladding fibers, pumped by semiconductor lasers. The output powers of the two fiber lasers were 0.39 W and 0.53 W respectively. With grating diffraction efficiency lower than 60%, the maximal output power of 0.64 W with absolute combining efficiency of about 69.6% was achieved. The influencing factors of the combining efficiency in the experiment are analyzed.

Ultrashort laser pulse diffraction by transmitting volume Bragg gratings in photo-thermo-refractive glass

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.

Spectral beam combining of fiber lasers based on a transmitting volume Bragg grating

Based on the principle of spectral beam combining using a transmitting volume Bragg grating, a simple experimental setup was designed, in which an approach to control the incidence angles of beams using a focusing lens is presented. The experimental results for the spectral beam combining of two fiber lasers are reported. The gain media used in this experiment were large-mode area Er 3+/Yb 3+ co-doped double-clad fibers, pumped by semiconductor lasers. The output powers of the two fiber lasers were 0.39 and 0.53 W, respectively. With a grating diffraction efficiency of about 60%, a maximum combined power of 0.64 W with an absolute combining efficiency of about 69.6% was achieved.