Xiao Yuan

Near field angular filtering with volume Bragg gratings in photothermorefractive glass

Angular filtering with volume Bragg gratings in photothermorefractive glass was described. The coupled-wave theory was used to discuss the angular filtering based on Transmission Volume Bragg Gratings (TBGs). The results showed that the cutoff frequency was improved, the refractive index modulation had a greater impact on the output beam and the deviation from Bragg condition for incident beam should be less than 0.1mrad. The influence on output beam quality with TBGs parameters was analyzed. Experimental results showed that the near-field distribution of output beam through the angular filtering is better than that of the incident laser beam, and the low-frequency loss of near-field angular filtering is less than 1.8%.

Design of wideband single-polarization single-mode photonic crystal fiber

Abstract. A novel structure of single-polarization single-mode photonic crystal fiber (SPSM-PCF) is proposed by introducing two large semicircular holes on both sides of the core region. The characteristics of SPSM fibers are numerically analyzed using the finite-element method. The SPSM operating range can be adjusted with the parameters of the elliptical ratio γ and the spacing Λ1 between the semicircular hole and the adjacent circular hole. The SPSM bandwidth of about 0.7 μm and operating range from 1.0 to 1.7 μm for the PCF have been obtained with the silica–air holes structure. An all-solid PCF structure is proposed, and the SPSM bandwidth of about 0.30 μm is predicted. With the increase in refractive index difference between the substrate and filling materials the SPSM operating range is blue-shifted, and with the decrease in the refractive index difference the bandwidth of the SPSM is reduced.

Effect of polarizer parameters on measuring Verdet constant of magneto-optical glass

The traditional measurement for Verdet constant of magneto-optic glass is to measure the Faraday rotation angle indirectly by using optical power meter, which is based on the Faraday rotation effect. There is a relatively large measurement error, because the method neglects the influence of the extinction ratio of polarizer and the axis angle. In our research, a method based on Jones matrix and simulation is studied and used to improve the measurement accuracy. The extinction ratio of polarizer and the axis angle are discussed during the measurement. The experiment system is built for measuring the Verdet constants of ZF7 glass and Ce3+/Tb3+ co-doped glass, by using the He-Ne laser light source, and different polarizers. The results show that the changes of the polarizer’s extinction ratio mainly affected the Verdet constant measurement. The lower extinction ratio, the larger measurement error. The extinction ratio more than 1:1000 for the polarizer is necessary for an accurate measurement, which has important value in terms of reference and guidance for the accurate measurement of Verdet constant of magneto optic glass.

Dispersion characteristics of Tellurite glass photonic crystal fiber

In this paper, we have fabricated the tellurite glass (70TeO2-20ZnO-5Al2O3-5La2O3, mol%, TZAL) by using high temperature melting method. Considering the material dispersion, the dispersion properties of TZAL glass photonic crystal fiber (PCFs) for various structures are analyzed and precisely described based on the vector finite element method (FEM). A novel structure with three-ring TZAL Glass PCF is proposed by introducing large elliptical holes in the inner ring. The simulation results show that ZDW decreases from 1.586μm to1.485μm when the numbers of elliptical holes increases from two to six. ZDW of horizontally disposed ellipses of PCF is a litter lower than that of vertically positioned ellipses. Furthermore, with optimizing parameters of pitch period and diameter of air hole, ZDW is reduced to 1.396μm. The dispersion characteristics can be flexibly designed and adjusted. The approach and results can be guidance for design, manufacture the photonic crystal fibers.

Band-stop angular filtering based on volume Bragg gratings. (Conference Presentation)

The uniform near-field distribution and focusing characteristics of laser beam, which is related with the spatial frequencies in laser beams, are very important for high power laser applications, such as laser processing and laser fusion. The traditional pinhole filter can be used to improve the near-field uniformity, but may lead to the pinhole-closure and back-reflection. The angular filter based on transmitting volume Bragg gratings (TBGs) recorded in the photo-thermorefractive (PTR) glass could be used to improve the near-field beam quality. However, the incident beam must satisfy the Bragg condition and the optical axis of filtered beam is deflected, which makes the laser system very difficult to align. The band-stop angular filter with two TBGs may be a good method to solve the above problems. In this paper, the band-stop angular filtering is performed and characterized. The band-stop angular filtering is demonstrated with a YLF laser with the wavelength of 1053 nm. The TBGs used in the experiment has the angular selectivity of 1.35mrad, the period of 1.97μm and the diffraction efficiencies of about 92%. Since part of the characteristic spatial frequencies was cleared out with the band-stop angular filter, there was an intensity drop on the edge of the filtered beam. The optical axis for the incident and output beams keeps basically coaxial after filtering, which can be used as a plug-and-play device in the high-power laser systems. The characteristic spatial frequency of 1.98mm-1 corresponds to the TBG deviation angle of 1.35mrad, and the spatial frequencies around the characteristic frequency of 1.98mm-1 were reduced to 20% compared to that of the original beam. The desired bands in the laser beam can be filtered with different TBGs, which has potential applications in high power lasers.

Frequency-to-amplitude modulation in angular filtering based on volume Bragg gratings (Conference Presentation)

Issue of frequency-to-amplitude modulation (FM-to-AM) conversion is one of the key scientific problems in the development of high power lasers, especially in fusion laser drivers. In large aperture and high power lasers, the sinusoidal phase modulated pulses are used to avoid stimulated Brillouin scattering and to obtain spatially-averaged focal spot on the target. Propagation through optical components in the laser chains is not optimal and slightly filters the parts of the optical spectrum. Therefore, the frequency modulation (FM) is partly converted into the amplitude modulation (AM), and this AM will lead to higher-order nonlinear effects or even cause damages to optical elements due to instantaneous ultrahigh intensity in laser propagation process. Volume Bragg gratings (VBGs) with programmable angular selectivity from 0.1mrad to 10mrad and high efficiency of 99 % is an effective filtering element to clean up the spatial modulations in laser beams. The FM-to-AM conversion is studied with a sinusoidal phase modulation laser pulse with the bandwidth of 0.30nm and 0.15nm, and is demonstrated with an YLF laser with the wavelength of 1053 nm and pulse width of 3ns. The experimental results show that FM to AM conversion level is increasing with the decrease of the spectrum selectivity bandwidth of the VBGs. At the VBG spectrum selectivity bandwidth of 7.9 nm, the FM to AM conversion level is reduced to about 6% for 0.3 nm and 3.5% for 0.15nm, which can be used in high power laser for controlling the beam spot on the target.

Analysis of wavefront distortion for laser beams corrected with volume Bragg gratings in photothermorefractive glass

The wavefront characteristics in the angular filtering based on the transmitting volume Bragg gratings (VBGs) recorded in the photothermorefractive glass are discussed. The wavefront distortions were introduced in the super-Gaussian beam and the wavefront distribution of the diffracted beam was analyzed. Stimulation results show that the low frequency wavefront distribution with the period larger than 40mm is difficult to be corrected while the high frequency wavefront with period smaller than 0.1mm is easy to be corrected with VBGs. The middle-high frequency wavefront distortion with period between 5mm and 0.25mm can be effectively corrected with VBGs with the appropriate angular selectivity, and the PV values can be reduced 167 times and 1000 times than that of modulated beam, respectively.

Theoretical study of transverse mode selection in laser resonator with volume Bragg gratings

Volume Bragg grating (VBG) can be used in laser resonator to control the transverse distribution due to its excellent Bragg selectivity. The coupled-wave theory is used to analyze the angular selectivity of VBG, and the output modes of the volume Bragg resonator are simulated with the fast Fourier transform (FFT) method and the coupled-wave theory. In this paper, the volume Bragg grating is inserted into a plane-parallel resonator, the intensity distribution and diffraction losses for the mode of TEM00, TEM10 and TEM20 are simulated, and the loss difference for different modes at different angular selectivity of VBGs are discussed. At the VBG angular selectivity of 3 mrad, the diffraction loss for fundamental mode is of 6.3%, while the diffraction loss for TEM10 and TEM20 mode are 19.8% and 32.7%, respectively. Therefore, TEM10 and TEM20 can be easily suppressed if the gain is between 6.3% and 19.8%, and a fundamental transverse mode can be obtained. Besides, the simulation results show that the intensity distribution profile of the transverse modes become smooth with the insertion of VBG, but the diffraction losses of transverse modes are increasing, and the diffraction loss increases with the order number of transverse modes increasing. Moreover, the loss difference between modes is getting large under the effect of VBG. The high loss difference between different modes is good for transverse mode selection, and VBG with reasonable angular selectivity in laser resonator will force the multi-mode to operate in a single transverse mode, which may has potential applications in lasers.

Laser-induced damage on fused silica with photo-acoustic method

The surface damage processes of fused silica are studied by a new photo-acoustic probe with Anti-Emi (Electron-Magnetic Interference), easy-adjusted and non-damage for the samples, and the damage thresholds is detected according to the rapid increase of the acoustic signals. Experimental results show that the damage threshold of fused silica samples is 13.86 J/cm2 at the wavelength of 1064 nm and the pulse width of 10 ns. This work may provide an effective technical support for the laser-induced damage detection.

Simulation of wavefront optimization with volume Bragg grating in photothermorefractive glass

Optimization for the wavefront distortion based on volume Bragg gratings (VBGs) recorded in photothermorefractive glasses (PTR) is simulated. A beam with different wavefront distortions is the incident on VBGs at the Bragg angle. The wavefront distribution of diffracted beam through two orthogonal VBGs is simulated based on the angular spectrum theory and the coupled wave theory. Four VBGs with different angular selectivity are used to simulate and optimize the wavefront distribution. The simulation results show that the wavefront distribution can be optimized with the VBGs of suitable angular selectivity. The PV value and the RMS value can be improved. The far field quality of laser beams is optimized.