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Microchip lasers are very compact and efficient coherent light sources; however, their applications are restricted due to poor beam quality and limited brightness, especially in high-emission power regimes. We used a thin-film-based angular filtering mirror to substitute one of the planar resonator mirrors to solve this limitation. The angle-selective reflection suppresses the high-order transverse modes and enables lasing on the lowest-order ones, thus improving the spatial quality of the emitted beam. Our experimental results, supported by numerical simulations, demonstrate decrease of beam quality parameter M 2 and a twofold increase in the brightness of emitted radiation. • Microchip laser applications are restricted due to poor beam quality and limited brightness. • Thin-film-based angular filtering mirror can substitute one of the planar microchip resonator mirrors and solve the brightness limitation in microchip laser. • We demonstrate twofold increase in the brightness of emitted radiation by experiment and confirm by numerical simulation. [ABSTRACT FROM AUTHOR]

Analytical model of Cs diode-pumped alkali laser with unstable confocal resonator is reported. Two of the parameters characterizing the quality of the output beam were studied: the axial luminous intensity in the far field and the power in the bucket (PIB). The far-field axial light intensity and PIB were compared with their values for a truncated Gaussian reference beam whose aperture was limited by the aperture of the large resonator mirror and 99% of the Gaussian beam energy passed through the aperture of this mirror. For large values of the unstable resonator magnification factor m > 5, the far-field axial light intensity is approximately 40% larger than that of the Gaussian reference beam. Analysis of the PIB curves showed that for large m > 5, the values of the integrated fractional encircled power within a sufficiently large diffraction angles for the unstable resonator exceed its values for the reference Gaussian beam. [ABSTRACT FROM AUTHOR]

• A three-field transient numerical model including fluid, light, and solid is developed. • The field synergy principle is utilized to interpret the variation trends of beam quality. • The mechanism of thermal blooming effect in basic structural units of high-power laser emission system is analyzed. A three-field transient numerical model, incorporating fluid dynamics, light propagation, and solid mechanics, is developed to investigate the impact of convective heat transfer on the thermal blooming effect in high-power laser systems. The numerical model is capable of capturing the interactions between any two fields and is validated through comparison with experimental data. The strength of convective heat transfer is quantitatively evaluated with the help of Field Synergy Principle. Based on this, the interplay between convective heat transfer and the laser is thoroughly examined. The results reveal that, due to variations in heat transfer performance, the entire laser transmission duration can be divided into three periods in the horizontal unit and two periods in the l -type unit. Moreover, a higher strength of convective heat transfer is observed in the horizontal unit compared to the l -type unit. Additionally, a lateral movement of the thermal plume within the l -type unit is found to cause lateral stretching of the spot in the far field. In accordance with the specific laser system, the influences of two mirror materials, namely sapphire and fused silica, are also considered. These findings have practical implications for optimizing high-power laser systems, particularly in terms of controlling thermal distortion. [ABSTRACT FROM AUTHOR]

A spectral beam combining (SBC) system by using a concave blazed grating with a Rowland-circle mount with the radius of 0.2 m was firstly built by us. Theoretically, on the basis of 5-element fiber lasers, the combining result for a plane and concave blazed grating optimized by using the recursive method were compared. The result shows that the energy density (ED) for the concave grating at the focus point is far higher than that for the plane grating, the beam quality for the concave grating is slightly inferior than that for the plane grating. Experimentally, two lasers with the wavelengths of 1544.6 nm and 1561.2 nm were combined. For the plane grating, at a point 153 mm form the grating, the measured ED was 2.07 mW/mm2 with the beam quality factor(M 2) of 1.28. While for the concave grating, the measured ED at the same position was 691.2 mW/mm2 with M 2 of 1.92. The simulation and experiment results indicate that SBC using a concave grating due to its higher ED has more advantages in directed-energy applications such as defense and manufacturing. [ABSTRACT FROM AUTHOR]

• For the first time, an all-fiber (6 + 3) structure was proposed for the fabrication of a ring spot signal combiner, and samples were successfully fabricated. • The samples produced showed a significant improvement in transmission efficiency compared to the cascaded 3 × 1 − (6 + 1) × 1 structure. • For the first time in the test method proposed return light test, and the sample test successfully passed. Aiming at the problem that the ring beam mode adjustable signal combiner with a single center port structure limits the output power of the laser beam in the inner ring of the laser. A new all-fiber structure (6 + 3) × 1 signal combiner is designed, which can independently carry more than 3 kW of power at each input port, and the output fiber is a ring-shaped dual-core fiber. The tapered fiber bundle adopts the (6 + 3) × 1 structure, where the three input ports in the center form a tapered fiber bundle to transmit the laser beam to the inner core of the output fiber, and the remaining six ports in the outer ring transmit the beam to the outer core of the output fiber, and the beams of the inner and outer cores can be transmitted independently, which effectively increases the output power of the center port. Combiner using (6 + 3) × 1 structure of the fiber transmission efficiency of more than 98 %, the center of the M2 is 5.32, can withstand the role of the return light. The structure of the output beam energy is more uniform, can effectively prevent sputtering, so that it can be applied to ultra-precision welding and other fields. At the same time, the structure also provides a feasible new solution to increase the center M2 of the beam splitter. [ABSTRACT FROM AUTHOR]

The mid-IR Fe:ZnSe laser has gained widespread attention due to its enormous potential with high-energy output. However, applications of Fe:ZnSe laser are limited by the beam quality. To optimize the beam quality of high-energy pulse Fe:ZnSe laser, this study presents a method of using spatial filtering in the resonator. Based on the theory of the ABCD transfer matrix and simulation of resonator stability and beam radius properties, the structure of the resonator was designed. Additionally, we study the output properties of Fe:ZnSe laser with different aperture diameters. We demonstrate when the aperture diameter D = 0.5 mm, the Fe:ZnSe laser output energy is 45.7 mJ and the beam quality is M x 2 = 2.11 , M y 2 = 2.29 , which is the best beam quality of using a pulse-pumped source to our knowledge. This method will guide for optimizing the beam quality of high-energy pulse mid-IR laser. [ABSTRACT FROM AUTHOR]

А 3D CFD model coupled with wave optics model for laser beam propagation is used to calculate the performance of a hydrocarbon-free K end pumped flowing-gas diode pumped alkali amplifier (DPAA). The K DPAA efficiency and output beam quality were calculated as a function of various parameters and the results were compared with those calculated for end pumped Rb DPAA with hydrocarbon-free buffer gas. For moderate pump intensity, both the efficiency and beam quality deteriorate at low gas velocity. The efficiency strongly depends on the pressure of the buffer helium, reaching a maximum at pressure close to atmospheric and falling with a further increase in pressure. The maximum efficiency of the K DPAA (∼0.45) is approximately 4 times higher than that of the Rb one, achieved at the same optimal pump power of 5 kW. Addition of CH 4 to the He buffer in the K DPAA decreases the amplifier efficiency and reduces the beam quality. [ABSTRACT FROM AUTHOR]

High power semiconductor disk lasers (SDLs) with near-diffraction limited beam quality were demonstrated. The influence of the incident angle, operation temperature, cavity length and curvature radius of the output coupler mirror on the beam quality were investigated. The condition for near-diffraction limited beam quality was analyzed. Choosing the pump spot size 10%–20% smaller than the fundamental mode size is the roughly optimized condition for achieving near-diffraction limited beam quality in SDLs. Finally, the SDL with an output power of 12.9 W and M2<1.2 was realized. [ABSTRACT FROM AUTHOR]

Filter-based spectral beam combining provides a promising approach for power scaling and brightness enhancement, especially for broad-linewidth fiber lasers. In this paper, we have demonstrated the beam combining experiments of two high power broad-linewidth fiber lasers using an edge filter as the combining element and achieved 10.25 kW combined power with an efficiency of 97%. Due to the thermal lens effect, the combined beam quality shows obvious signs of deterioration, which occurs in all cases of high power spectral beam combination system, including grating-based and prism-based schemes. The surface deformation and the thermal behavior are monitored in real-time so as to research the mechanism of the combined beam quality deterioration during the combining process. The primary reason for the decline of beam quality could be attributed to the thermal induced change in the refractive index, which not only results in an inhomogeneous wave-front aberration but causes the different focal waists of the incident channels. To counterbalance these aberrations, an air-cooling device and a passively thermally compensated optical system based on CaF 2 windows are integrated into the system. In this case, the thermal lensing effect is minimized and the beam quality deterioration factor (BQD) of the combined output beam has been improved 52% in horizontal direction and 101% in vertical direction, showing the great potential of the filter-based approach to achieve a combined beam with high power and excellent beam quality. • New 10kW class spectral beam combining scheme based on edge filter is proposed. • Deterioration mechanism of beam quality in FSBC system is firstly analyzed. • The effect of various optimization schemes is investigated experimentally. • The FSBC firstly achieves 10 kW level output power with a beam quality below 10. [ABSTRACT FROM AUTHOR]

Abstract The proposed Compact Intense Fast NEutron Facility (CIFNEF) will have a wide range of applications, including the investigation of the exotic structure of neutron-rich nuclei, data for the nuclear fission, and the simulation of the neutron field in a star. To satisfy aims of producing high-intensity fast neutrons and forward neutrons with very low environmental background, the CIFNEF linac should have the capacity of accelerating continuous wave (CW) deuteron (D +), hydrogen (H 2 +) and lithium (7Li 3 +) beams to 2.5 MeV/u with maximum beam currents of 10 mA, 5 mA and 10 uA, respectively. Based on the above requirements, we proposed a novel compact linac using a combination of RFQ and DTL structures. The dynamics of RFQ and DTL are completed to meet all requirements and start-to-end simulation results show that the three ion species can be accelerated to the final energy with transmission efficiency above 99% as well as good beam quality with lower emittance growth. In addition, we performed error sensitivity analysis and combined error study to evaluate the error tolerance limits of the obtained design. [ABSTRACT FROM AUTHOR]

We study the effect of lateral error on the coupling efficiency and beam quality of a Gaussian beam coupled into a large-core multimode fiber. The formula for the effect of lateral error on coupling efficiency is derived in detail and verified with both simulations and experiments. The effect of lateral error on beam quality is also analyzed with the use of ZEMAX and verified with our experimental setup. The results show that lateral error can change the path of the light coupled into the fiber to such a large extent that it influences the power distribution of the Gaussian beam. [ABSTRACT FROM AUTHOR]

The inhomogeneity of the refractive index distribution in optics material degrades the output laser nearfield beam quality. The modulation effect of the nearfield influenced by the refractive index distribution in the crystal material is studied theoretically in this work. The internal refractive index distribution of the transmissive optics is analyzed by measuring the transmitted wavefront and the output laser nearfield after passing through the defective crystal is measured in a high power laser system. The experimental results show that due to the diffraction effect during laser transmission, the internal refractive index defect of the optics material causes the intensity modulation in the nearfield of the output laser. [ABSTRACT FROM AUTHOR]

Abstract The effect of alignment error on the coupling efficiency and beam quality of a Gaussian beam coupled into a large-core multimode fiber is studied in this paper. The equations for evaluating the effect of alignment error on the coupling efficiency are derived separately, and verified with the method of simulation. The calculation and simulation results obtained are highly consistent. In the same way, the effects of alignment error on the beam power distribution are discussed. The results show that the lateral error will change the path of partial light to a large extent, and have a greater influence on the power distribution of the Gaussian beam than the longitudinal error and angular error do. [ABSTRACT FROM AUTHOR]

In this article, a formulation was developed for designing a stable disk laser resonator and achieving a high power and a beam quality close to the fundamental mode with a short effective length. In this regard, by taking a matrix approach and using Gaussian beam propagation based on the ABCD law for Gaussian beams, the fundamental-mode spot size inside a resonator and its sensitivity to misalignment, as well as dynamic stability in multiple folded resonators, were analyzed in order to achieve a short effective length. In the presented method, the optimal geometrical parameters of the resonator were obtained by applying the length limits, permissible level of sensitivity to misalignment, and range of dynamic stability with the help of MATLAB software coding. Using this design algorithm, a Z-shaped resonator with the arm length of 50 cm and fundamental-mode spot size of 2.4 mm on the active media was designed, while the resonator had a good dynamic stability and low sensitivity to misalignment. Considering these specifications, it was possible to design a compact disk laser with a power of 1 kW, which had a beam quality close to the fundamental mode. To the best of our knowledge, the laser disk resonator described in this paper, which is resistant to misalignment and has a short effective length, has not been previously documented in the literature. The model developed in this research is a general model that can be used in much more complex structures with any numbers of optical elements in other lasers. • Misalignment sensitivity model is developed for multi-element stable resonators. • Different I-, V-, Z- and W- shaped stable resonators are investigated. • Misalignment tolerable, compact disk laser resonator is designed. [ABSTRACT FROM AUTHOR]

Although lasers generated by stimulated Raman scattering (SRS) were reported to have better beam quality in comparison with pump laser; once pump laser intensity exceeded the threshold of laser-induced-breakdown (LIB), Raman laser beam quality was deteriorated dramatically. In this study, the backward SRS (BSRS) process of methane pumped by 532 nm laser was applied to produce high beam quality seed Raman laser beam, and stimulated Raman amplification with configuration of collimated seed laser and pump laser counter-propagation (BSRA) was used to increase the pulse energy of Raman laser. With a seed Raman laser of 51.4 mJ, ∼127.2 mJ Raman laser could be produced by BSRA process. Raman laser beam quality achieved M x 2 = 2.52 and M y 2 = 2.40, which were better than pump laser. The technique could be applied repeatedly; pulse energy and peak power of Raman laser could be further improved. This work provides of possible method to achieve high peak power (large pulse energy) Raman lasers with excellent beam quality. [ABSTRACT FROM AUTHOR]

• Ring-assisted fiber (RAF) is a promising candidate for fiber communication and high-power application due to its novel structure. • In this manuscript, the beam quality factor (BQF) of RAF is investigated for the first time. • We systematically studied the effects of ring parameters and wavelengths on the BQF of RAF. • The results will provide a comprehensive understanding on the beam quality of RAFs, and thus offer a guidance to the design of this novel fiber for better application. Ring-assisted fiber (RAF) introduces a high refractive index ring into the cladding of the fiber, which brings the fiber more flexibility for communication and high-power application. In this paper, for the first time, we investigate how the additional ring affects the beam quality factor. The theoretical optical field model of RAF is established and verified at the first step. Then both the M2 factor and the β factor under different cases are numerically calculated, based on their general definition and the theoretical filed model. The results show that a larger thickness of the ring will greatly deteriorate the beam quality, but the distance between the ring and the core of RAF brings little influence on the beam quality. Besides the ring parameters, the beam quality factors of the pure and mixed modes under different wavelengths are also investigated systematically. Our results will not only provide a comprehensive understanding on the beam quality of RAFs, but also offer a guide to the design of this novel fiber for further applications. [ABSTRACT FROM AUTHOR]

Abstract In order to obtain a 1053 nm output laser with a high beam quality, the mode distribution of the YLF solid laser with a plane-plane resonator is studied. According to the Collins theory, the influences of the position and size parameters of a Gaussian aperture on the mode of the output beam is simulated and analyzed. Compared with the simulation results without a Gaussian aperture, the Gaussian aperture in the cavity can significantly improve the quality of the output laser. Based on the theoretical simulations, a laser with optimal structural parameters is built. The profiles and mode distributions of the laser output beams with a Gaussian aperture and without a Gaussian aperture in the cavity are measured, respectively. When the radius of the light spot on the film of the Gaussian aperture is 4 mm and the distance between the Gaussian aperture and total-reflection mirror is 30 mm, a 1053 nm laser output with the values of M2 in the x and y directions of 1.9 and 1.8, and maximum output power of 9.7 W is obtained. The experimental results demonstrate that the Gaussian aperture in the cavity can improve the mode distribution of the cavity and obtain the laser output with a higher beam quality, which is basically consistent with the results of theoretical simulations. [ABSTRACT FROM AUTHOR]

Based on the generalized Huygens Fresnel principle, the propagation equation of fundamental Gaussian vortex beam with phase distortion is derived. To quantitatively analyze the influence of random phase distortion on the quality of vortex beam, the variance of light intensity relative deviation is proposed as the evaluation parameter. The influences of some key factors on the quality of a vortex beam are emphatically discussed. The results show that with the increasing of distortion strength the beam quality becomes worse. On the contrary, with the increasing of the topological charge, the beam quality improves gradually. [ABSTRACT FROM AUTHOR]

The nearfield beam quality of the high power laser system is of great importance in order to achieve a very good effect in the specific position. We review various aspects of evaluating this nearfield beam quality, present results obtained in a high power laser system, and discuss the valuable reference of beam quality improvement in the spatial shaping system for high power lasers. [ABSTRACT FROM AUTHOR]

A novel spectral beam combining (SBC) system based on a concave grating with a Rowland-circle mount was designed and built. Theoretically, the aberrations, as well as the normalized intensity distribution of a diffracted beam with the wavelength of 1550 nm were calculated, and the grating mount was optimized by using the recursion approach. The results show that the optimized mount is similar to paraboloid shape, under the optimized mount, the aberration is significantly reduced, and the intensity distribution of the diffraction beam is almost consistent with the intensity distribution of the output of a fiber. Further, the combining result of 5-element fiber lasers was simulated, which indicates that the beams of the fiber lasers can be made to spatially overlap. Experimentally, for the output beams of two fiber lasers with the wavelengths of 1536.4 nm and 1543.6 nm, the measured diffracted powers are 0.336 W and 0.368 W with the beam quality factors (M x 2) of 1.78 and 1.82. Further, the combined power and M x 2 were measured as well, which are 0.659 W and 2.52 with the combining efficiency of about 71%. • In this paper, we designed and built a novel spectral beam combining (SBC) system based on a concave grating with a Rowland-circle mount. • Theoretically, the diffraction and the combining output results were analyzed. • Experimentally, we built an SBC experimental platform and conducted the SBC experiments of two laser beams. • The results demonstrate that the SBC of the two beams can be well accomplished. [ABSTRACT FROM AUTHOR]

Advanced acceleration technologies are receiving considerable interest in order to miniaturize future particle accelerators. One such technology is the dual-grating dielectric structures, which can support accelerating fields one to two orders of magnitude higher than the metal RF cavities in conventional accelerators. This opens up the possibility of enabling high accelerating gradients of up to several GV/m. This paper investigates numerically a quartz dual-grating structure which is driven by THz pulses to accelerate electrons. Geometry optimizations are carried out to achieve the trade-offs between accelerating gradient and vacuum channel gap. A realistic electron bunch available from the future Compact Linear Accelerator for Research and Applications (CLARA) is loaded into an optimized 100-period dual-grating structure for a detailed wakefield study. A THz pulse is then employed to interact with this CLARA bunch in the optimized structure. The computed beam quality is analyzed in terms of emittance, energy spread and loaded accelerating gradient. The simulations show that an accelerating gradient of 348 ± 12 MV/m with an emittance growth of 3.0% can be obtained. [ABSTRACT FROM AUTHOR]

Purpose To investigate the feasibility of using the ratio of dose-area product at 20 cm and 10 cm water depths ( DAPR 20,10 ) as a beam quality specifier for radiotherapy photon beams with field diameter below 2 cm. Methods Dose-area product was determined as the integral of absorbed dose to water ( D w ) over a surface larger than the beam size. 6 MV and 10 MV photon beams with field diameters from 0.75 cm to 2 cm were considered. Monte Carlo (MC) simulations were performed to calculate energy-dependent dosimetric parameters and to study the DAPR 20,10 properties. Aspects relevant to DAPR 20,10 measurement were explored using large-area plane-parallel ionization chambers with different diameters. Results DAPR 20,10 was nearly independent of field size in line with the small differences among the corresponding mean beam energies. Both MC and experimental results showed a dependence of DAPR 20,10 on the measurement setup and the surface over which D w is integrated. For a given setup, DAPR 20,10 values obtained using ionization chambers with different air-cavity diameters agreed with one another within 0.4%, after the application of MC correction factors accounting for effects due to the chamber size. DAPR 20,10 differences among the small field sizes were within 1% and sensitivity to the beam energy resulted similar to that of established beam quality specifiers based on the point measurement of D w . Conclusions For a specific measurement setup and integration area, DAPR 20,10 proved suitable to specify the beam quality of small photon beams for the selection of energy-dependent dosimetric parameters. [ABSTRACT FROM AUTHOR]

The profile of transmitting beams in inter-satellite optical links significantly impacts communication performance, shaping the general Gaussian transmitting beam into a far field top-hat profile may improve the performance of the inter-satellite laser communication system. The calculations show that this scheme would at least reduce beaconless acquisition time by nearly 63%, and may reduce the outage probability of communication link several orders of magnitude. In addition, the tracking error allowance of communication terminals would at least increase to 1.65 times while keep communication link stable. Use of top-hat beam is quite helpful to build and operate robust inter-satellite optical communication terminals at lower technical risks and cost. [ABSTRACT FROM AUTHOR]

The aim of this study was to quantitatively characterize a micro focus x-ray tube that can operate in both continuous and pulsed emission modes. The micro focus x-ray source (Model L9181-06, Hamamatsu Photonics, Japan) has a varying focal spot size ranging from 16 µm to 50 µm as the source output power changes from 10 to 39 W. We measured the source output, beam quality, focal spot sizes, kV accuracy, spectra shapes and spatial resolution. Source output was measured using an ionization chamber for various tube voltages (kVs) with varying current (µA) and distances. The beam quality was measured in terms of half value layer (HVL), kV accuracy was measured with a non-invasive kV meter, and the spectra was measured using a compact integrated spectrometer system. The focal spot sizes were measured using a slit method with a CCD detector with a pixel pitch of 22 µm. The spatial resolution was quantitatively measured using the slit method with a CMOS flat panel detector with a 50 µm pixel pitch, and compared to the qualitative results obtained by imaging a contrast bar pattern. The focal spot sizes in the vertical direction were smaller than that of the horizontal direction, the impact of which was visible when comparing the spatial resolution values. Our analyses revealed that both emission modes yield comparable imaging performances in terms of beam quality, spectra shape and spatial resolution effects. There were no significantly large differences, thus providing the motivation for future studies to design and develop stable and robust cone beam imaging systems for various diagnostic applications. [ABSTRACT FROM AUTHOR]

High brightness, high power tapered diode-laser regularly consists of an index-guided ridge-waveguide section and a gain-guide tapered section. The gain match between the ridge and tapered sections were studied in this paper. The gain peak wavelength of the ridge section is expected to be larger than that in the tapered section because of the higher optical power density by numerical simulation. It is found that the gain peak mismatch between the ridge and tapered sections results in broadened optical spectra and low output powers according to the experimental results. The mismatch is suppressed by lowering the junction temperature of the ridge section which shifts the gain peak of the ridge section to a shorter wavelength. Alternatively, the shift of the gain spectra was also realized by increasing the transparency carrier density with an additional optical loss in the ridge waveguide introduced by He + ion implantation. Tapered diode lasers with such designs have a beam quality factor M 2 (1/e2) = 1.1 with an output power of 8 W and a narrow spectra width of 2 nm. • The spectral gain match in tapered diode laser was demonstrated numerically and experimentally for the first time. • High electro-optical efficiency of 56.6% was obtained at 8 W with an inject current of 10 A. • High beam quality of M 1 / e 2 2 = 1.1 was obtained at output power of 8 W. [ABSTRACT FROM AUTHOR]

When the high-power lasers propagate in seawater, the thermal blooming (TB) effect can cause wavefront distortion, which severely affects the beam quality. Here we proposed a potential approach to reduce the thermally-induced wavefront distortion by using the self-rotating beam in seawater. This approach using the self-rotating beam can directly rotate the intensity distribution of the laser beam and thus break the accumulation path of thermally-induced wavefront distortion, contributing to the improvement of beam quality. Results show that, the self-rotating beam with an appropriate rotation frequency can well mitigate the thermal blooming effect in seawater, exhibiting excellent performance in reducing beam expansion, beam centroid drift and beam quality degradation. • An approach of using self-rotating beam to suppress thermal blooming effect of seawater is proposed. • Physical mechanism why self-rotating beam can suppress thermal blooming is explained. • Influence of rotation frequency, power density, beam width and helical charge on beam quality is obtained. [ABSTRACT FROM AUTHOR]

This paper proposes a novel segmented cladding fiber structure for large mode area properties. In this structure a thin ring is placed between the high index core and nonuniform cladding. It is called Single-Ring Segmented-Cladding Fiber (SR-SCF). The novel fiber offers the possibility of single-mode(SM) operation from 1 µm to 1.7 µm with a large core diameter. With illustrations, the fiber has a better SM operation than segmented-cladding fiber (SCF) is demonstrated. A large effective area of 1000 µm 2 is achieved. The SM operation with very high suppression of the higher order modes can arise by 76%. Moreover, mode spacing between the adjacent modes (LP 01 and LP 11 ) is also improved significantly. Besides, the bending property is analyzed. It is found that the fiber is insensitive to bending angle ranging from −180° to 180° at bending radius of 30 cm. The proposed fiber will play an important role in developing high power fiber laser, fiber amplifier and high power delivery application. [ABSTRACT FROM AUTHOR]

In this paper thermal effects of the disk active medium and associated effects on the beam quality of laser are investigated. Using Collins integral and iterative method, transverse mode of an unstable resonator including a Yb:YAG active medium in disk geometry is calculated. After that the beam quality of the laser is calculated based on the generalized beam characterization method. Thermal lensing of the disk is calculated based on the OPD (Optical Path Difference) concept. Five factors influencing the OPD including temperature gradient, disk thermal expansion, photo-elastic effect, electronic lens and disk deformation are considered in our calculations. The calculations show that the effect of disk deformation factor on the quality of laser beam in the resonator is strong. However the total effect of all the thermal factors on the internal beam quality is fewer. Also it is shown that thermal effects degrade the output power, beam profile and beam quality of the output laser beam severely. As well the magnitude of each of affecting factors is evaluated distinctly. [ABSTRACT FROM AUTHOR]

The M 2 factor and other parameters often used in evaluating the beam quality of lasers are not sufficient. Based on the beam characteristics of semiconductor laser, the paper presents a new expression for the beam quality on semiconductor laser. The new beam quality factor can not only evaluate the beam quality of the semiconductor laser but also evaluate the beam quality of the semiconductor laser chips via collimating and focusing lenses. The results give us grounds to make the following conclusions: the new beam parameter product succeeds in its universality and adaptability in collimating and focusing lenses of a multi-emitter semiconductor laser made by stacking several signal emitter chips. [ABSTRACT FROM AUTHOR]

In this paper, it is found that the behavior of the effect of thermal blooming on the beam quality of truncated laser beams is quite different from that of untruncated ones. The beam spreading of truncated laser beams is less affected by thermal blooming than that of untruncated ones. There exists the oscillatory behavior of beam width versus the truncated parameter due to thermal blooming. When there exists the cross wind, under steady-state thermal blooming, the astigmatism of truncated laser beams is better than that of untruncated ones, and the centriod position is furthest away from the propagation axis for a certain value of the truncated parameter. [ABSTRACT FROM AUTHOR]

The thermal lens effect of 2.1 μm Cr, Tm, Ho: YAG (CTH:YAG) solid-state laser under high pumping power condition is analyzed, and a symmetric spherical resonator which is insensitive to thermal focal length change is proposed to improve the beam quality of Fabry–Perot (F–P) resonator. Then the gradient-reflectivity mirror is introduced as output mirror to optimize the resonator mode and beam quality. Based on the scalar diffraction theory, the Fox-Li numerical iteration method and fast Fourier transform (FFT) algorithm are used to calculate the resonator mode and output power distribution of resonators with Gaussian, super-Gaussian and parabolic gradient mirror, respectively. By comparing the cavity loss and beam quality, one can find that the symmetric spherical resonator with a super-Gaussian mirror can provide the best output beam quality, it has the minimum cavity loss of 0.1907, the minimum far-field divergence angle of 1 mrad and the maximum power in the bucket (PIB) of 89.42%. [ABSTRACT FROM AUTHOR]

We have demonstrated efficient generation of high beam quality, high-energy and broadband tunable femtosecond mid-infrared pulses using a three-stage collinear optical parametric amplifier (OPA). The white-light continuum (WLC) seeded OPA setup, based on KTA crystal in three stages and pumped by a femtosecond laser pulse at 800 nm, is capable of producing idler wavelength ranging from 2.4 μm to 4.0 μm with energy up to 82 μJ at 3.27 μm, which corresponds to signal energy of 350 μJ at 1060 nm. The output pulse has excellent intensity distribution with measured beam quality factor M 2 ~1.1 for signal and M 2 ~1.7 for idler. To our knowledge, this is the best beam quality reported in 3–5 μm femtosecond OPA until now. The achieved mid-infrared pulse also has a good energy stability with a fluctuation of 1.01% rms over half an hour. [ABSTRACT FROM AUTHOR]

The properties of standard Laguerre–Gaussian beam (sLGB) and elegant Laguerre–Gaussian beam (eLGB) through the misaligned complex hard edge apertures (MCHEAs) are investigated. Analytical formulae for the far field distributions of sLGBs and eLGBs through the different hard edge apertures are derived based on the generalized Huygens–Fresnel diffraction integral equation. The propagation properties and beam qualities of sLGBs and eLGBs are studied comparatively. It is shown that the divergence angle of sLGBs with the low order modes is less than that of eLGBs. However, the energy distributions of eLGBs with the high order modes after through the multiple hard edged apertures are more convergent in comparison with sLGBs, and eLGBs have the better beam quality than sLGBs. The study results are useful for the propagations and applications of sLGBs and eLGBs through the complex optical systems. [ABSTRACT FROM AUTHOR]

• Laser performance of LD side-pumped Er:LuYSGG bonding crystal was demonstrated. • Maximum output power of 20.76 W is achieved and improved effectively by bonding. • M 2 factors of 6.80/6.74 of the bonding crystal are better than that of un-bonding one. • Er:LuYSGG bonding crystal is a promising 2.8 μm gain element operated at high power. We demonstrate the laser performance of 970 nm LD side-pumped LuYSGG/Er:LuYSGG/LuYSGG bonding crystal in a frequency range of 50–800 Hz. The thermal distribution simulation implies that the bonded Er:LuYSGG crystal possesses a higher cooling efficiency compared with the un-bonded one. The maximum average power of 20.76 W is realized at a frequency of 150 Hz and a pulse width of 200 μs, corresponding to a slope efficiency of 23.94 % and optical-to-optical conversion efficiency of 19.22 %. A single laser wavelength of 2797.5 nm is observed. The M 2 factors of the bonding rod are 6.80/6.74, indicating a relatively high beam quality. Therefore, the LuYSGG/Er:LuYSGG/LuYSGG bonding crystal exhibits an improvement of laser performance operated at 2.8 μm due to excellent heat dissipation ability, which is a promising gain element and can be applied to the harsh radiation environment. [ABSTRACT FROM AUTHOR]

In the course of performance acceptance testing, benchmarking or quality control of X-ray imaging systems, it is sometimes necessary to harden the X-ray beam spectrum. IEC 61267 specifies materials and methods to accomplish beam hardening and, unfortunately, requires the use of 99.9% pure aluminium (Alloy 1190) for the RQA beam quality, which is expensive and difficult to obtain. Less expensive and more readily available filters, such as Alloy 1100 (99.0% pure) aluminium and copper/aluminium combinations, have been used clinically to produce RQA series without rigorous scientific investigation to support their use. In this paper, simulation and experimental methods are developed to determine the differences in beam quality using Alloy 1190 and Alloy 1100. Additional simulation investigated copper/aluminium combinations to produce RQA5 and outputs from this simulation are verified with laboratory tests using different filter samples. The results of the study demonstrate that although Alloy 1100 produces a harder beam spectrum compared to Alloy 1190, it is a reasonable substitute. A combination filter of 0.5 mm copper and 2 mm aluminium produced a spectrum closer to that of Alloy 1190 than Alloy 1100 with the added benefits of lower exposures and lower batch variability. [ABSTRACT FROM AUTHOR]

We present the generation of high-quality and beam parameter controllable hollow beams with a liquid-core optical fiber (LCOF) filled with CS 2 by adjusting off-axis orientation and LCOF structure. The dependences of the dark spot size (DSS), ring beam width (RBW) and transverse field intensity distribution of the hollow beam on the incidence angle of laser beam, the length and core diameter of the LCOF are investigated experimentally, and analyzed theoretically by using Zemax simulation. The results show that the quality of the hollow beam is related to the times of total internal reflection of skew rays in the fiber core; the DSS and RBW are closely controlled by the incidence angle and core diameter. High-quality thin-ring hollow beam is achieved experimentally by the LCOF of the core diameter of 100 μm and the length of 1 m with the incidence angle of less than 10°. The hollow beam generated has high stability, and is weakly dependent on the bending of the LCOF. [ABSTRACT FROM AUTHOR]

We analyze the off-axial aberrations of a slightly inclined Gaussian beam in a thin radial graded-index (GRIN) lens. We assume that the endfaces of the lens are flat and the aperture stop is generally not in contact with the lens surface. First we formulate the diffraction of a complex-source-point spherical wave through a thin GRIN lens, while the terms of up to fourth order in aperture variables are taken into account. Then we find a ray-optical solution to the diffracted beam which is represented in terms of aberration coefficients of the Seidel type. Finally we evaluate the effect of off-axial aberrations on the quality of the diffracted Gaussian beam. The quality of an oblique Gaussian beam is much degraded by increasing the angle of inclination (or the spot size) of the beam and taking the aperture stop out of the lens surface. [ABSTRACT FROM AUTHOR]

The detector array parameters will greatly affect the measurement accuracy of beam quality at target. In this study, the relations of the parameters with encircled power ratio and the beam radius at target were further analyzed. Since the beam at target is a Gauss-like beam, the effects of detector array's spatial resolution, effective testing area, systematic dynamic range, and surface response homogeneity on measurement accuracy of beam quality were analyzed quantitatively with numerical simulation. The results show that among all influence factors, the measurement accuracy of beam radius was greatly affected by the detector's normalized size and dynamic range, but slightly by normalized resolution and surface response homogeneity. These results were successfully applied in design of detector array and achieved good effect. The experimental data and theory are well consistent. [ABSTRACT FROM AUTHOR]

Neutron radiography is one of the main applications of research reactors. It is a powerful tool to conduct nondestructive testing of materials. The parameters that affect the quality of a radiographic image must be considered during the design of a neutron radiography system. Hence, this study aims to investigate the effect of geometric factors on the quality of the neutron radiography system. The results show that the performance of the mentioned system can be increased by regulating the geometric factors. [ABSTRACT FROM AUTHOR]

In this study, the efficiency and beam quality deteriorations in double-cladding fiber (DCF) amplifiers induced by core misalignment of series fusion splices are investigated. The losses of guide modes and power conversions between fundamental mode and high order modes (HOM) are well described according to mode coupling theory. Furthermore, a typical fiber amplifier with three fusion splices and bending mode-selection is established to discuss the mechanism of deteriorations in total systems. Mode competition theory is also introduced to describe different modes powers and beam quality changes with fiber length. Based on these calculations, the accumulations of deterioration caused by series fusion splices are intuitively illustrated. These simulations of laser performance show good accordance with the experimental results. An effective method to significantly enhance the performance of the fiber amplifier is also shown and discussed. [ABSTRACT FROM AUTHOR]

Abstract: The stability of the higher output power with high-quality beams is very important parameter in material processing. This paper presents using the S parameter and Gauss beam parameters matching to design efficient resonant cavity of two-rod Nd:YAG. The fundamental mode beam radius in rod, fundamental mode beam radius on the mirror, beam quality factor, and the value of the degree of the symmetry of the TEM00 mode volume as a function of the between neighbor rods distance, laser rod end face to mirror distance, and input power are simulated. The design method of resonant cavity is illustrated and compared with previously published experimental measurements for different pump power and the length of the cavity parameters and a good agreement has been found. [Copyright &y& Elsevier]

There are two types of X-ray devices: transmission-type and reflection-type. These two types rely on different techniques to generate X-rays. This study aims to explore physical characteristics of transmission-type anode X-ray device. The newly developed transmission-type X-ray device (NanoRay Biotech) was assessed, and the beam quality of the device was determined by measuring the half-value layer (HVL) and energy spectrum. Furthermore, this study measured the percentage depth dose (PDD) and flatness and symmetry using ionization chambers in the PTW MP3-s water tank, and all of the measurements of the field size on the entrance surface of the water phantom was set 10 by 10 cm2. The study also discusses the stability and linearity of the tube voltage (kVp) and the tube current (mAs) of the X-ray tube, and investigates the effect of aluminum (Al) filter on beam quality. The results show that at the same depth, when the source-to-surface distance was reduced, the PDD declined, while the symmetry of the lateral dose profile was within 3%. In addition, the dose output of the X-ray device increased with an increase in kVp or mAs. The HVL was 1.55 and 0.41 mm with and without the Al filter, respectively. The introduction of the filter helped reduce low-energy photons by 23.4% and increased the ratio of the characteristic radiation by 2.22% in the energy spectrum. With the filter, the device can effectively reduce low-energy photons, thereby decreasing the surface dose and increasing the ratio of characteristic radiation. The rapid decline in PDD made the device suitable for the clinical treatment of shallow tumors and the intraoperative radiation therapy for tumor beds. • The transmission-target X-ray tube does not exhibit a heel effect. • Transmission X-ray tubes are suitable for the clinical treatment of shallow tumors. • The Al filter helped the transmission X-ray tube reduce low-energy photons by 23.4%. [ABSTRACT FROM AUTHOR]

For the nonlinear beam propagation case, the influence of coma aberration on the beam quality is studied when a laser beam propagates upwards through Earth's inhomogeneous atmosphere. The coma aberration results in a degradation of the beam quality. However, the influence of coma aberration on the beam quality becomes weak because of the nonlinear self-focusing effect in the atmosphere. In particular, a laser beam with coma aberration propagates along a curve path, which may be useful to avoid the obstacles in the path. Furthermore, the expression of the maximum intensity position at the space-debris target is obtained. On the other hand, for the laser beam with coma aberration, the optimal intensity at the space-debris target is also investigated, and the expression of the optimal initial beam power is also obtained. As the coma aberration becomes worse, the optimal intensity decreases but its optimal initial beam power increases. [ABSTRACT FROM AUTHOR]

Abstract: Aim: This study aimed at calibrating a new set of GR-200A thermoluminescent dosimeters (TLDs) in low and medium orthovoltage energy photon therapy beams and in a diagnostic beam of known beam quality, in order to determine their response and to establish if the same set of TLDs could be used across both environments for in-vivo dosimetry purposes. Methods and materials: A set of 20 TLDs was used for this study. An Oven type PCL3 was used to anneal the TLDs. The response of the TLDs was read using the Reader type LTM manufactured by Harshaw Bicron, United State of America. Vacuum tweezers were used to transfer the TLDs at the time of measurements and calibration. TLDs were kept in a subdued ultra-violet environment between the annealing and irradiation process. TLDs were placed on a 30 × 30 × 17.6 cm3 Polymethylmethacrylate (PMMA) phantom during irradiation. A calibrated Orthovoltage machine was used to deliver a known absorbed dose to the TLDs using 95 kVp (3.00 mm Al HVL), 180 kVp (1.00 mm Cu HVL), and 300 kVp (3.00 mm Cu HVL) therapy beams. A cylindrical ionization chamber (PTW 30001) and an electrometer (PTW 10008) were used to confirm the absorbed dose delivery of the orthovoltage machine at the time of measurement. Likewise, a calibrated LX40 radiotherapy Simulator was used to deliver a known diagnostic absorbed dose to the TLDs using an 80 kVp beam of 2.97 mm Al HVL. A TM77334 ionization chamber was used similarly to confirm the absorbed dose. The TLDs were also irradiated on the PMMA phantom to generate element correction coefficient and readers calibration factor necessary for calibration of the reader before actual calibration of the TLDs. The accepted variation in raw response of the individual TLDs from the average of the batch was compared and a deviation of less than ±20% was considered within tolerance. A 10% tolerance was subsequently considered suitable for the measurement of absorbed dose. Results: Of the 20 TLDs calibrated in the 95 kVp therapy beam (3 mm Al half-value layer), 17 were within the accepted response level (i.e. ±20% deviation), 17 in the 180 kVp therapy beam (1 mm Cu half-value layer), 16 in the 300 kVp therapy beam (3 mm Cu half-value layer) and 15 in the diagnostic beam of 80 kVp (2.97 mm Al half-value layer). 16 of the 17 TLDs were within ±10% dose tolerance at 95 kVp whereas all the TLDs that were within the accepted response level at the 180 kVp and 300 kVp, were within the ±10% dose tolerance. 12 of the 15 TLDs at the diagnostic beam energy were within the ±10% dose tolerance. Three of the TLDs were therefore rejected at all energies. Conclusion: The study concludes that the same set of GR-200A TLDs could be used across both kilovoltage therapy and diagnostic fluoroscopy environments for in-vivo dosimetry purposes. [Copyright &y& Elsevier]

Abstract: It is necessary for high power ultraviolet lasers to propagate through a long air path during experiments of inertial confinement fusion (ICF). When the product of gain efficiency and propagation distance is beyond the stimulated rotational Raman scattering (SRRS) threshold, the high power ultraviolet laser beam is subject to loss of energy and a decrease in beam quality, further resulting in the reduction of conversion efficiency of triple-harmonic, and even causing the optical components to be destroyed. This paper studies the four-dimension numerical modeling of SRRS effects, with emphasis on the variation of beam aperture and corresponding beam distributions in near field. The research showed that the variation of the pump beam aperture made little effect on SRRS transform efficiency ratio with the same incident energy and the same propagation distance, the transverse intensity distribution of beams became more even with an increase in the beam aperture. We have made the deduction that high power laser's SRRS effects through a long air path could be depressed by increasing the beam aperture on the condition of equal incident energy and propagation distance. [Copyright &y& Elsevier]

Highlights: [•] A metallized polymer film was tested on a heliostat at Sandia. [•] Reflectivity tests and beam quality tests were performed. [•] Soiling rate and impact of specularity at long focal distance was evaluated. [•] Specular reflectivity of SMF1100 was 2–4% lower than that of silvered glass. [•] Soiling rates were similar, but power from SMF1100 was less. [Copyright &y& Elsevier]

Abstract: Periodic checks of megavoltage electron beam quality are a fundamental requirement in ensuring accurate radiotherapy treatment delivery. In the present work, thermoluminescent dosimeters (TLDs) positioned on either side of a lead sheet at the surface of a water equivalent phantom were used to monitor electron beam quality using the electron backscattering method. TLD100 and TLD100H were evaluated as upstream detectors and TLD200, TLD400 and TLD500 were evaluated as downstream detectors. The evaluation assessed the test sensitivity and correlation, long and short term reproducibility, dose dependence and glow curve features. A prototype of an in-air jig suitable for use in postal TLD dose audits was also developed and an initial evaluation performed. The results indicate that the TLD100-TLD200 combination provides a sensitive and reproducible method to monitor electron beam quality. The light weight and easily fabricated in-air jig was found to produce acceptable results and has the potential to be used by radiation monitoring agencies to carry out TLD postal quality assurance audits, similar to audits presently being conducted for photon beams. [Copyright &y& Elsevier]

Abstract: A novel multicore ytterbium doped fiber laser is designed, with the target of maximizing both the effective mode area and the beam quality, by means of a complete home-made computer code. It can be employed to construct high power and Quasi-Gaussian beam lasers. The novel laser configuration exploits a single mode multicore fiber and does not need Talbot cavity or other in-phase mode selection mechanisms. This is an innovative solution, because to the best of our knowledge, for the first time, we have designed a truly single-mode multicore fiber laser. For comparison we have optimized two other laser configurations which are well known in literature, both employing a multimode multicore fiber and a Talbot cavity as a feedback for the in-phase supermode selection. All three multicore fibers, constituted by the same glass, are doped with the same ytterbium ion concentration and pumped with the same input power. Multimodal fiber lasers exhibit lower beam quality, i.e. a higher beam quality factor M 2, with respect to the single mode one, even if suitable Talbot cavities are designed, but they are very competitive when a more compact laser cavity is required for the same output power. The novel single mode nineteen core laser exhibits a simulated effective mode area A eff =703μm2 and a beam quality factor M 2 =1.05, showing better characteristics than the other two lasers. [Copyright &y& Elsevier]

Abstract: Thermal effects and beam wavefront properties of a diode-pumped Q-switch Nd:YAG laser have been investigated. The influence of thermal lens effect on laser output mode and beam quality is discussed by means of transmission matrix optics. The functional relations between the thermal lens focal length and the beam spot size for a plano-concave stable resonator have been calculated numerically. Further experiment on the output beam quality of a laser diode pumped acousto-optic Q-switched laser has been also performed by using a Hartmann–Shack (H–S) wavefront sensor and Zernike mode reconstruction theory. In this way, the PV and RMS values of the wavefront aberration, the first 35 orders Zernike aberrations, can be acquired, and point spread function (PSF) of the beam and the distribution of circle energy can also be obtained by calculation, so the output beam quality can be known fully. The experimental results show that the wavefront aberration of laser is mainly includes the defocus Z 3, the low-order astigmatism Z 4 and Z 5, the spherical aberration Z 10, etc. because of the thermal effect of gain medium, which will lay an experimental foundation for solid-state laser optimum design. [Copyright &y& Elsevier]

Abstract: The phase aberration caused by thermal deformations in a high power inner optical system is simulated and the wavefront pre-compensation for correcting the aberration is studied experimentally. Choosing Strehl ratio (SR) and the root-mean-square (RMS) of distorted phase as the characteristic parameters, the beam quality of outgoing laser is calculated when the wavefront of incident laser is compensated. Based on the computed results, the laboratory experiment is designed to model the thermal deformations aberration and the correction process by using two Liquid Crystal on Silicon Spatial Light Modulators (LCoS-SLM). The negative influence of astigmatism and defocusing on laser beam quality in far field is obvious and the astigmatism induces the non-axisymmetrical divergence of intensity distribution. After wavefront pre-compensation the power in the bucket (PIB) value of outgoing laser beam is close to the flat wavefront laser source. Both simulated and experimental results indicate that the method of wavefront pre-compensation can greatly correct the aberrations caused from thermal deformation, and the laboratory experiment is a feasible method to modeling the inner optical system with phase aberration. [Copyright &y& Elsevier]