Thermally-induced Birefringence Research Articles

Thermal Birefringence-Based beam shaping in high energy lasers

Repetition-rate high-energy laser systems with high beam quality have been attracting growing attentions in various applications. However, the degradation of beam quality caused by thermal effects is nearly unavoidable in high-energy laser amplifiers operating at high repetition rates. One primary thermal effect is thermal-induced birefringence, which typically induces polarization changes at various spatial positions within the beam. This study explores the potential of leveraging the thermal-induced birefringence effect to manipulate the polarization distribution of the beam and, consequently, control the spatial intensity distribution of the beam by using an additional polarizer. This method demonstrates promise for beam pre-compensation, mitigating thermal-induced depolarization, and other laser applications in high-energy lasers with high repetition rates.

Influence of acousto-optic frequency shifter's thermal-induced birefringence on laser frequency-shifted feedback system

The acousto-optic frequency shifter (AOFS) is the key component of the laser frequency-shifted feedback (LFSF) system, and its thermal-induced birefringence (TIB) seriously affects the stability of the system. In this paper, the influence of the AOFS's TIB on LFSF system is analyzed, and the method of suppressing this influence is proposed. The theoretical model of LFSF considering the AOFS's TIB is established, and the influence on both amplitude and phase of frequency-shifted signal is analyzed. When the AOFS starts working, its TIB effect is obvious due to the accumulation of working heat, which makes the polarization state of its diffraction light change dynamically in most cases. The laser feedback imaging system and laser feedback displacement measurement system are built separately to evaluate the amplitude and phase stability. By principal axis alignment or quasi-common-path compensation, the amplitude variation, phase fluctuation, phase jump and phase temperature drift caused by the AOFS's TIB can be eliminated or suppressed. This study improves the startup stability and anti-ambient temperature interference ability of the LFSF system, which also has guiding significance to other heterodyne interference systems.

Orthogonally polarized radiation from an electro-optically Q-switched laser diode side-pumped Er:YSGG laser with dual cavities

A laser diode-side-pumped electro-optically Q-switched Er:YSGG laser that emits orthogonally polarized laser radiation at 2.79 µm is demonstrated. This laser has a dual cavity structure to compensate for the thermal-induced birefringence effect; additionally, negative curvatures were ground at the crystal ends of Er:YSGG to compensate for the thermal lensing effect. The laser achieved a maximum repetition frequency of 20 Hz, with a pulse energy and pulse width of 71 mJ and 58 ns, respectively. The maximum pulse energy was 81 mJ, and the pulse width was 46 ns at 17 Hz, these conditions yielded a peak power of 1.76 MW, and an optical-to-optical efficiency of 1.27%.

Fabrication, microstructure and laser performance of Nd3+-doped Lu3Al5O12 transparent ceramics

Nd:LuAG transparent ceramics were fabricated by a solid-state reaction method using commercial high-purity oxide powders as raw materials. The microstructure, optical transmittance, fluorescence spectrum and laser performance of 0.8 at.% Nd:LuAG transparent ceramics were investigated. It is found that the relative density and grain size for Nd:LuAG ceramics increase with the increase of sintering temperature. The specimen sintered at 1830°C for 30h is dense and homogeneous in structure with an average grain size of about 39.6μm, whose in-line transmittances are 82.8% at 1064nm and 81.8% at 400nm. The main fluorescence emission peak is centered at about 1063.5nm and the fluorescence lifetime is 240μs. The optimum laser properties are obtained with the 300μm pumping beam and 25% output coupler, corresponding to a slope efficiency of 43.4% and an optical–optical efficiency of 39.3%. In addition, the depolarization caused by the thermally-induced birefringence was also investigated.

Thermally-induced birefringence in solid-core photonic crystal fiber lasers

The authors report photo-thermal effects in solid-core photonic crystal fiber lasers (PCFLs) using a finite element method. Thermal stresses and birefringence due to self-heating of PCFLs in the core and the cladding regions were simulated as a function of induced heat per meter. Simulations were devoted to three kinds of PCFLs with various d/Λ ratios (d is the hole diameter and Λ is the hole-to-hole distance) where large-mode-area solid-core PCFLs with air-holes triangular lattice as cladding and polymer coating were considered. For thermal modeling, all cooling mechanisms such as convection and radiation have been taken into account. We showed that thermally-induced birefringence has rather strong impact on the phase difference between perpendicular polarization components and so on depolarization.

Birefringence in injection‐compression molding of amorphous polymers: Simulation and experiment

AbstractThe influence of the processing variables on the residual birefringence was analyzed for polystyrene and polycarbonate disks obtained by injection‐compression molding under various processing conditions. The processing variables studied were melt and mold temperatures, compression stroke, and switchover time. The modeling of flow‐induced residual stresses and birefringence of amorphous polymers in injection‐compression molded center‐gated disks was carried out using a numerical scheme based on a hybrid finite element/finite difference/control volume method. A nonlinear viscoelastic constitutive equation and stress‐optical rule were used to model frozen‐in flow stresses in moldings. The filling, compression, packing, and cooling stages were considered. Thermally‐induced residual birefringence was calculated using the linear viscoelastic and photoviscoelastic constitutive equations combined with the first‐order rate equation for volume relaxation and the master curves for the Young's relaxation modulus and strain‐optical coefficient functions. The residual birefringence in injection‐compression moldings was measured. The effects of various processing conditions on the measured and simulated birefringence distribution Δn and average transverse birefringence <nrr−nθθ> were elucidated. Comparison of the birefringence in disks manufactured by the injection molding and injection‐compression molding was made. The predicted and measured birefringence is found to be in fair agreement. POLYM. ENG. SCI., 2013. © 2013 Society of Plastics Engineers

Frozen-in birefringence and anisotropic shrinkage in optical moldings: II. Comparison of simulations with experiments on light-guide plates

The thermally-, flow-induced and total birefringence components and anisotropic shrinkages in LGP moldings were simulated by using a combination of a CV/FEM/FDM technique nonlinear viscoelastic and photoviscoelastic constitutive equations and orientation functions, as described in Part I of this study. The simulated results were compared with measurements on LGP moldings of a polystyrene (PS) and two optical grade polycarbonates (PCs) OQ1030 and OQ3820 having low and high molecular weights. The thermally-induced birefringence was simulated by a combination of constrained and free cooling during molding. In LGP moldings of PS, the simulated thermally-induced birefringence indicated a minor variation with location in the mold plane, a parabolic shape in the core region and an increase towards the wall. Compared to the flow-induced birefringence, the thermal birefringence provided a minor contribution to the total transverse birefringence Δn12. In LGP moldings of PCs, the simulated thermally-induced birefringence showed a significant variation with location in the mold plane, nearly constant value in the core region and high value in the wall region. In LGP moldings of both PCs, the contributions of the thermally- and flow-induced birefringence to the total transverse birefringence Δn12 were significant. The effect of processing conditions on the development of the normal birefringence in LGP moldings of PCs was ranked from most to least: the packing pressure, mold temperature, melt temperature, injection speed and packing time. However, in LGP moldings of PS the packing time effect was significant due to a longer gate freezing time. Simulated and measured normal birefringence along the flow direction was in fair agreement, but simulations were unable to describe the observed birefringence maximum arising near the gate. The averaged luminance of LGP moldings exhibited some correlation with the averaged normal birefringence. LGP moldings of PC OQ1030 indicated a pronounced maximum in the simulated transverse flow birefringence in the core but a low value near the wall. In contrast, the LGP molding of PC OQ3820 showed a high simulated birefringence near the wall and a low value of maximum in the core. The simulated and measured total transverse birefringence in LGP moldings was in fair agreement. LGP molding of both PCs showed similar tendency in shrinkage variation with processing conditions. However, the thickness shrinkage was higher in LGP moldings of PC OQ3820. The effect of processing conditions on the development of shrinkage in LGP moldings of both PCs was ranked from most to least: the packing pressure, melt temperature, mold temperature, injection speed and packing time. In LGP moldings of PS, the thickness shrinkage slightly increased with increasing melt temperature and significantly increased with reducing packing time. A good agreement between the simulated and measured anisotropic shrinkages in LGP moldings at various processing conditions was observed.

Aperture obstruction influences birefringence evaluation of a Nd : YAG laser system with tandem-set rods

In this study, we derive a theoretical and experimental expression for evaluating the thermally induced birefringence (TIB) in a Nd : YAG laser system with double rods in tandem. The analytical solution determines the optimum oscillator geometry corresponding to the minimum depolarization loss while the so-called aperture obstruction is considered in the calculation model. It can be seen that the experimental results closely follow the theoretical evaluation for both the depolarized degree and the birefringent distribution of a probe beam. We also find that the traditional evaluation method, which neglects the thermally induced lens effect and aperture truncation during the optical design, leads to an obvious deviation in determining the optimum arrangement of a laser system for compensation of birefringence. The method presented in this paper is thought to be useful in analysing any high-powered oscillators or amplifiers with high pulse energy.

Thermal stress induced birefringence in double cladding fiber with non-circular inner cladding

Thermal stress induced birefringence due to the thermoelastic difference between the outer cladding and the non-circular inner cladding of a double cladding fiber (DCF) is theoretically investigated. A modified slab model is proposed to estimate the influence of the outer cladding on the birefringence property of the core. Numerical simulation is carried out to investigate the thermoelastic and geometric contributions of the cladding to thermal induced birefringence in rectangular and D-shaped ytterbium-doped DCFs. The thermoelastic relations deduced in the modified slab model are also qualitatively verified. Research shows that the influence of the outer cladding on the thermal-induced birefringence in DCF varies greatly depending on the choice of outer cladding materials, inner cladding shapes and temperature variation.

A crystalline-orientation self-selected linearly polarized Yb:Y3Al5O12 microchip laser

Crystalline-orientation and pump dependent polarization states of Yb:Y3Al5O12 microchip lasers were observed experimentally. Linear polarization was observed at six crystalline orientations in the (111) plane relative to the beam propagation direction along the [111] crystalline axis of Yb:Y3Al5O12 crystal. The extinction ratio of the linear polarization decreases and the laser tends to oscillate at random polarization state at high pump power levels. Linear polarization states of Yb:Y3Al5O12 microchip laser was selected by the anisotropic spectroscopic properties of the Yb3+-ion in cubic Y3Al5O12 crystal. The random polarization oscillation at high pump power was caused by the strong thermal-induced birefringence and depolarization.

Thermally-induced-birefringence effects of highly Nd 3+-doped Y 3Al 5O 12 ceramic lasers

Thermally-induced-birefringence effects of Nd:YAG ceramic lasers are accurately measured. Depolarization for the ceramics is found to be nearly the same with that for (1 1 1)-cut single crystals if Nd 3+ concentration is the same. Under non-lasing condition, the thermally-induced birefringence becomes larger as Nd 3+ concentration increases due to severer quenching. Under lasing, however, depolarization is measured to be one-third of that under non-lasing because the stimulated emission reduces the nonradiative process, which decreases the thermal load. Moreover, direct pumping into the emitting level by use of a 885 nm pump source intrinsically reduces the heat generation compared with the conventional 808 nm pumping; we have experimentally verified that the depolarization and the thermal load are reduced by 30% by the direct pumping.

A Flash-Lamp-Pumped Nd:YAG Laser with Dual-Telescopic Optics Configuration

A flash-lamp-pumped Nd:YAG laser that contains two laser rods and dual-telescopic elements is investigated theoretically and experimentally. The two intracavity telescopes of the resonator are appropriately chosen and adjusted to achieve a large-volume TEM00 mode inside the laser rods, and to minimize the variational effects of the thermal lens in the rods. The drift of the optimal operating point of the resonator caused by variation in the pump energy is compensated by adjusting defocusing of the two telescopes over a broad pump energy range. It is experimentally shown that our resonator geometry with appropriate parameters obtained the output energy of 560 mJ with 100 #x03BC;s duration and slope efficiency of 3.5#x0025;, at a pump energy of 16J, even though the misalignment tilting angle of the end mirror reached #x00B1;5mrad. The bi-focusing effect caused by the thermally-induced stress birefringence was compensated by a 90#x00B0; quartz rotator. Output energy was increased by 18#x0025; at the pump energy of 16J. The misalignment tilting angle of the end mirror with intracavity telescope was 4 times larger than that without intracavity telescope.