The diffraction characteristics of hump volume Bragg grating in photo-thermo-refractive glass are analyzed with coupled-wave theory. Results show that the diffraction efficiency of hump volume Bragg gratings (HVBGs) is increasing with the product of refractive index modulation and grating thickness. Keeping the refractive index modulation and grating thickness, the interval between the hump peaks enlarges with the grating period increasing. Keeping the grating period and the product of refractive index modulation and grating thickness, the interval between the hump peaks enlarges with the grating thickness decreasing. Moreover, the angular selectivity of HVBGs is getting better with the larger interval.
Ray tracing method is used to calculate additional phase and group delay dispersion (GDD) of femtosecond laser diffracted by transmitting blazed gratings. The dependence of GDD with grating parameters is discussed. For the pulse durations of 100fs and 30fs, the pulse broadening and the propagation process through the transmitting blazed grating are analyzed with the Fourier optics. Stimulation results show that the pulse broadening is related to the grating period and the propagation distance diffracted from the transmitting blazed grating.
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.
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.
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.
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.
A type of configuration with four identical chirped volume Bragg gratings (CVBGs) in parallel combination was proposed to improve dispersion, and the maximum group delay of nanoseconds can be reached at an oblique incidence. The diffraction properties of CVBGs at oblique incidence were simulated with the transfer matrix method. The performance of this configuration on dispersion, including group delay dispersion and cubic dispersion, was well studied with detailed numerical simulation. With optimization of the CVBG structural parameters, including the grating thickness, spatial chirp rate, and refractive index modulation, the configuration can be applied in large dispersion applications.
Aiming to the rectangle beam shape in most high power laser, the influences of the several kinds of geometrical aberrations
in slit spatial filtering system have been simulated based on the diffraction theory of aberration. The aberration tolerances
of all kinds of geometrical aberrations in the lenses and incident beam are obtained.
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%.
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 Λ 1 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.