Field Beam Quality Research Articles

Impact of the central refractive index dip of fibers on high-power applications

Central refractive index dip is a common phenomenon in the fibers fabricated by the modified chemical vapor deposition (MCVD) technology, which is the main fabrication technique for high-power laser fibers. In this paper, we present a numerical analysis of the dip effect on high-power-related parameters for the first time, to the best of our knowledge. Three aspects including mode field parameter, beam quality, and bending performance are studied under different dip parameters and bending radii. It is found that the dip is possible to increase the effective mode area and the bending loss, which offers a flexible way to suppress the non-linear effects and filter the higher-order modes by optimizing the dip parameters. Besides, different from the mode area and bending loss, beam quality exhibits an interesting trend when the dip radius increases. The results could facilitate a comprehensive understanding of the dip fiber properties, which also offer guidance to evaluate and design the fiber with central refractive index dip for high-power applications.

Off-axis pumped Tm:YLF vortex laser with continuously tunable wavelength

In this paper, HG mode conversion and etalon are used to achieve a 1.9 μm vortex beam output in a Tm:YLF laser while achieving a certain range of wavelength continuous tuning. In theory, the wavelength tuning characteristic and tuning capability of the etalon are analyzed. In the experiment, the HG1,0 mode laser is achieved by off-axis pump, and the optical field distribution and beam quality are relatively consistent with the theoretical state. After the HG1,0 mode laser passes through the π/2 mode converter, the LG1,0 vortex beam output is realized. The optical field distribution and beam quality of the vortex beam are relatively close to the theoretical state. The threshold of the laser at LG1,0 mode is 3.2 W, the slope efficiency is 37.7%, the optical–optical conversion efficiency is 34%, and a laser output of 8.6 W is obtained at the highest pump power of 25.3 W. On this basis, the continuous wavelength tuning of the LG1,0 mode vortex beam from 1937 nm to 1921 nm was achieved by rotating the etalon. Through comparison, it can be found that the optical field distribution and output power of each wavelength are relatively close during the wavelength tuning process. Finally, the topological charge of the vortex beam is measured by the interference of the vortex beam with the spherical wave and the plane wave. This article is of great significance for the further research of multi-dimensional tuning to realize large-capacity optical communication.

Quantifying the ionization quenching effect in organic plastic scintillators used in MV photon dosimetry

Miniature organic plastic scintillators attached to optical fiber cables have found important uses in medical dosimetry, in particular for small-field dosimetry and field output factor measurements in megavoltage linear accelerator beams. Plastic scintillators are well suited for these applications due to their high degree of water equivalence and their small size. Plastic scintillators, however, are known to suffer from ionization quenching, meaning that the light production per absorbed dose decreases if the dose deposition is caused by low energy electrons. As MV beams also lead to dose deposition by low energy electrons, ionization quenching will also affect, for example, field output factor measurements in MV beams if the fraction of dose delivered by low energy electrons changes with field size or beam quality. While the influence of ionization quenching on scintillator dosimetry in beams of heavy charge particles or kV x-ray has been subject to much research, the effect in MV beams is less well studied, and it is often implicitly assumed that there is no effect. The purpose of this work therefore was to quantify the influence of ionization quenching on plastic scintillators, specifically BCF-60, for the two application in MV photon dosimetry: (i) field output factor measurements for field sizes from 0.5×0.5 cm2 to 10×10 cm2 for a 6 MV beam and (ii) ionization chambers beam quality correction factor (kQ-factors) measurements for beams between 4 MV and 15 MV at a fixed 10×10 cm2 field size. The quenching was quantified using Monte Carlo modelling of the MV beams, a variation of Birks formalism that accounted for the detailed dose deposition by secondary electrons, and quenching parameters established previously using experiments in kV x-ray beams. This enabled the computation of the light yield in a “quenching free” scintillator and therefore a computation of a correction factor due to ionization quenching. Ionization quenching was found to have a small but statistically significant influence on both of the studied applications: (0.6±0.2)% for the field output factor measurements between 0.5×0.5 cm2 and 10×10 cm2 and about (2±0.4)% for the kQ-factor application for beams between 4 MV and 15 MV. The modelling results were in agreement with experimental measurements. The results support that the ionization quenching effect has a small effect on field-output factor measurements and it can probably be neglected during clinical measurements. For direct scintillator-based measurements of beam quality correction factors for ionization chambers, ionization quenching is larger and this effect needs to be accounted for if anyone wants to measure or verify ionization chamber kQ-factors using organic plastic scintillators.

The characteristics of small field beam quality and output factor of 6 MV FFF

Flattening filter free linear accelerator (FFF LINAC) has been installed in Indonesia. To ensure the accuracy of FFF irradiation, we evaluate the characteristic of FFF in regular and small fields. We employed the Monte Carlo (MC) simulation of FFF LINAC 6 MV as a standard reference in this study. Then, we compared the calculation of FFF beam at central and lateral axis to the measurement and TPS at 10 × 10, 1 × 1, 2 × 2, 3 × 3, 4 × 4 cm2 field sizes. Output factor (OF) and beam quality such that TPR20,10 and penumbra of FFF LINAC were evaluated. TPR20,10(S) for 10 × 10 cm2 was 0.01 differ between MC and measurement whereas it varied from 0.588 to 0.703 for small fields. Dose difference of lateral axis was agreed within 3% except for the penumbra region. Output factor of small fields measurement indicated that field size of 1 × 1 cm2 had a large discrepancy to MC according to this works. The TPR20,10(10) of MC, TPS, and measurement was not a significant difference, while the OF was tending to a large deviation in 1 × 1 cm2. The results showed that our FFF LINAC could be used for small field until 2 × 2 cm2.

210. GLAaS absolute dose calibration algorithm with an Elekta iViewGT Electronic Portal Imaging Device (EPID)

Purpose Radiotherapy with intensity modulation (IMRT and VMAT) implies the setup of adequate QA programs to guarantee safe treatments. Concerning this issue, amorphous silicon EPID technologies are particularly interesting due to their high spatial resolution, large area, stability, dynamic range, and real-time acquisition capabilities. In this context, the GLAaS [1] algorithm has proved capable to meet a variety of applications with a simple direct calibration process to convert raw measurements into absorbed-dose-to-water; nevertheless, GLAaS was never validate with a different technology apart from Varian-EPIDs. To test its feasibility out of single manufacture environment, GLAaS was applied to Elekta iViewGT images. Methods GLAaS accounts for pixel-by-pixel response changes in time differentiating on field and/or segment sizes and beam quality, i.e. primary or transmitted radiation (e.g. below MLC), basis. Dosimetric data from a 6 MV Elekta-Synergy were collected at dmax and SDD = 160 cm (EPID position), for primary and transmitted radiation to be correlated with iViewGT pixel reading (R). For open and modulated fields, comparison to TPS dose maps (Monaco-XVMonteCarlo) was assessed through profiles and gamma analysis. Results For a given beam, the response of iViewGT was confirmed to be linear (D(Gy) = m ∗ R + q), as already established for Varian EPIDs. Similarly, the variable slope response to primary radiation was modelled according to the following empirical algorithm: m pr ( OF ) = a · OF + b; OF ( EwwF ) = [ c + d · ln ( EwwF ) - 1 where EwwF is the equivalent field size of each segment based on IC data and OF is the EPID reading measured output factor. For transmitted radiation the relation mtr = k · mpr was adopted. Download : Download high-res image (325KB) Download : Download full-size image Conclusions The extension of GLAaS to iViewGT offers the opportunity to easily set-up flexible and reliable verification, also in highly demanding conditions as for SRS and SBRT treatments; the compatibility with different technologies could encourage multi-centers studies concerning treatment delivery and dose calculation problematics.

Influence of phase error of optical elements on optical path design of laser facilities

Optical path design of high power laser facilities should consider several optimization measures such as those that are related to image transmission, ghost avoidance, and stray light management. According to the diffraction optical propagation theory, we study the the influences of wavefront characteristics of large aperture optical components on optimizing the design parameters of optical path in view of increasing the output load. The results show that the arrangement interval of the last stage optical drive can be very useful in improving the output load of the laser facilities if it is controlled to be over 6 m long. In general, a large aperture optical element with a phase error peak value of 0.34 can reduce the near field beam quality of a high-power laser by about 10% and give rise to a maximum decrease of about 21% when the phase error reaches 1.36. Superposition of multiple optical elements with different phase error distribution characteristics can reduce the negative effect of the mid frequency phase error. However, the nonlinear effect of large aperture optical components can aggravate the influence of the intermediate frequency phase error on the damage resistance capacity of the device. Under the premise that the damage threshold of the large caliber optical element is limited to 20 J/cm2, the using of a laser facility with a compact optical path, with an input laser energy density controlled to be below 16.8 J/cm2, will avoid damaging the optical components efficiently. A relatively flexible optical layout can further increase the average energy density of the final output laser and is very beneficial to the enhancing of the output load capacity of the laser facility.

Measured and Monte Carlo simulated surface dose reduction for superficial X-rays incident on tissue with underlying air or bone.

Measurement of surface dose reduction effects for superficial x-rays incident on tissue with underlying air or bone and comparison with Monte Carlo simulations of such effects. Further to investigate the correlation between surface dose reduction and changes in Compton backscatter spectra with tissue-bone separation. An Advanced Markus chamber with entrance window facing downstream on the surface of a solid water phantom was used to investigate changes in surface dose with an underlying air or bone interface located at various depths below the surface. Chamber readings were obtained for interface depths ranging from 1 to 100 mm using the 50 kV, 100 kV and 150 kV beams of an Xstrahl 150 x-ray unit, with field diameters (ϕ) = 2.5 cm and 5 cm. For each beam quality and field size the dose correction factor, DCF(t), namely the ratio of measured dose (t) to dose (t = 100 mm) was determined. Monte Carlo simulations of DCF(t) for air and bone interfaces in tissue are used to validate corresponding measured data. For a given beam and field size, the difference between simulated spectra with an air or bone interface at t = 3 mm was used to determine the Compton backscatter from bone at the surface. For air, DCF(t < 40 mm) is less than unity and for a given t DCF(t) decreases as beam energy increases from 50 kV to 150 kV. Conversely for bone DCF(t < 40 mm) increases for a given t with increasing beam energy. In particular for t = 1 mm, ϕ = 5 cm, DCF for air(bone) are 0.90(0.92) and 0.86(0.96) for 50 kV and 150 kV, respectively. For the same tube potentials corresponding factors, ϕ = 2.5 cm, for air(bone) are 0.94(0.96) and 0.92(0.99). Calculated DCF(t) based on Monte Carlo simulations are consistent with experimental observations to within 2%. Monte Carlo simulations of x-ray spectra demonstrate the presence of Compton backscatter from underlying bone in tissue. With bone at 3 mm depth calculated backscatter spectra at the tissue surface suggest that surface dose is influenced by the proximity of bone and that this effect depends on beam quality. This work demonstrates the feasibility of using an Advanced Markus chamber with entrance window facing downstream to investigate surface dose reduction with underlying air or bone in tissue. As the field size decreases and beam quality increases surface dose with underlying bone tends to full backscatter values even though tissue thicknesses are below those normally associated with full backscatter. Conversely with underlying bone close to the surface dose will increasingly fall below full backscatter values as the beam energy is reduced and field size is increased.

Advances in high power high beam quality diode lasers

Semiconductor laser enjoys its benefits such as small volume, light weight, long operation life, various selectable wavelength and direct current driving, meanwhile suffers from its beam quality which is making it hard for direct applicdations. Researchers around the world realize that it is beam quality has the same importance as power, and acquiring high power and high beam quality is the key issue in semiconductor laser induxtry. The question of how to improve the beam quality of high power semiconductor lasers is attracting increasing attention from researchers home and abroad. Considering the application fields which require high power high beam quality, such as industry processing and national defense, this paper discussed the research progress on both diode laser unit devices and laser beam combining sources. First of all, the relationships between single laser emitter′s structureand themodeof laser units, including lateral mode, transverse mode and longitudinal mode, are analyzed. Lateral mode is the main factor which limits the high beam quality for high power semiconductor lasers. Ridge waveguide is the main method adopted to realize single lateral mode. A method called longitudinal photonic bandgap crystal is introduced to manipulate the transverse mode of a single laser unit, which can acquire large optical near field and high beam quality even in high current input. To control longitude mode, usually to acquire single longitude mode in single semiconductor laser unit, distribute Bragg reflectors and distribute feedback structures using gratings in fabrication is also introduced in this section. And then we summarize the methods and some results of controlling modeusedinternationalresearchers. Further more, we introduced the lasers beam technology, including beam shaping and beam combining, considering all conditions of single emitters beam combining, mini bar beam combining, centimeter bar beam combining, laser stacks beam combining and laser systems beam combining. In beam combining technology, we introducted the method of traditional beam combining, including polarizetion beam combining and wavelength beam combining. Then the method of using dense wavelength division multiplexing with volume Bragg gratings to realize beam combinign is also introduced. Spectrum beam combining method using a Bragg diffractive grating is then introduced, which may realize a high beam quality of a single emitter′s beam spot with a laser bar′s optical power. Furthermore, coherent beam combining considering each laser unit′s wavelength, polarization and phase to realize high power high intensity farfield is also introduce. Then, the lastest international reported of high power, high beam quality diode laser combining sources are introduced, and characteristics of laser beam technology and development trends arediscussed and analyzed. Finally, the developments of high power high beam quality diode lasers are prospected. Right now, our country still have a distance below internal research in high quality semiconductor laser chip and beam combining source. The semiconductor laser chips′ power and beam quality needed to be improved. The technology of 10 kW level beam combining and optical fiber coupling technology is still urged to be acquired. Grating fabrication in optical beam combining technology and chip fabrication are also needed to be conquered.

More than 400 W random fiber laser with excellent beam quality.

We report on the demonstration of a high power, high beam quality random fiber laser. It is a half-open cavity based on 130-m-long large mode area step-index fiber with a core diameter of 20μm and a numerical aperture of 0.08. The random laser cavity, whose central wavelength is 1120nm, is pumped by a 1070nm Yb-doped fiber oscillator. The maximum output power is 418W with respect to the injected pump power of 588W, corresponding to the optical-to-optical efficiency of 71%. The output power, spectral, and temporal characteristics are carefully presented and discussed, showing that such a cavity can export high power random laser stably and effectively. The far field beam quality is also measured, and the profile is nearly a Gaussian shape. This work provides an impactful method to advance the output power of random fiber lasers.

SU-G-BRB-05: Automation of the Photon Dosimetric Quality Assurance Program of a Linear Accelerator

Purpose: To develop an automated method to calculate a linear accelerator (LINAC) photon radiation field size, flatness, symmetry, output and beam quality in a single delivery for flattened (FF) and flattening-filter-free (FFF) beams using an ionization chamber array. Methods: The proposed method consists of three control points that deliver 30×30, 10×10 and 5×5cm2 fields (FF or FFF) in a step-and-shoot sequence where the number of monitor units is weighted for each field size. The IC Profiler (Sun Nuclear Inc.) with 5mm detector spacing was used for this study. The corrected counts (CCs) were calculated and the locations of the maxima and minima values of the first-order gradient determined data of each sub field. Then, all CCs for each field size are summed in order to obtain the final profiles. For each profile, the radiation field size, symmetry, flatness, output factor and beam quality were calculated. For field size calculation, a parameterized gradient method was used. For method validation, profiles were collected in the detector array both, individually and as part of the step-and-shoot plan, with 9.9cm buildup for FF and FFF beams at 90cm source-to-surface distance. The same data were collected with the device (plus buildup) placed on a movable platform to achieve a 1mm resolution. Results: The differences between the dosimetric quantities calculated from both deliveries, individually and step-and-shoot, were within 0.31±0.20% and 0.04±0.02mm. The differences between the calculated field sizes with 5mm and 1mm resolution were ±0.1mm. Conclusion: The proposed single delivery method proved to be simple and efficient in automating the photon dosimetric monthly and annual quality assurance.

光学元件表面缺陷相对位置分布对近场光束质量的影响

光学元件表面缺陷相对位置分布对近场光束质量的影响

终端靶场中相位畸变对近场光束质量的影响

终端靶场中相位畸变对近场光束质量的影响

Numerical investigation of beam quality in unstable optical resonators and the effects of practical imperfections

Field characteristics and beam quality of confocal positive branch unstable resonator under some practical imperfections has been investigated. Based on Fresnel–Kirchhoff integral, the near and far field intensity of the resonator was calculated numerically. Using the second order moment method, the beam quality factor, M2 of the resonator was also calculated. Variety of practical distorting affects, mainly misalignment, off-axis, and thermal deflection of the mirrors, as well as gain non-uniformity were considered in these calculations. The results show that the beam quality factor is more sensitive to the off-axis and misalignment distortions while the non-uniform gain has significant effect only for unusual conditions. The effects of geometrical parameters, i.e. Fresnel number as well as magnification factor on the beam quality were also investigated.

Film dosimetry for fluoroscopic procedures: potential errors

Kodak EDR 2 film was calibrated across the range of beam qualities and exposure rates typically used in our cardiac catheterisation laboratory. Its dose-response curve was successfully modelled up to its saturation point of 1 Gy. The consistency of the film's response with film batch, time between exposure and processing processor and day-to-day variations in performance, was investigated. The effects of field size, exposure rate, beam quality and use of the dynamic wedge filter were quantified. The overall uncertainty in dose was estimated to be between -20% and +40%, at 160 mGy. This uncertainty increases as the film approaches its saturation point. In addition, some unusual artefacts were observed.

PHASE CONJUGATION IN A HIGH POWER REGENERATIVE LASER AMPLIFIER SYSTEM

We discuss the design and performance of a master oscillator/regenerative power amplifier laser system using a stimulated Brillouin scattering phase conjugator to control beam aberrations within the amplifier cavity. The master oscillator is a self-seeded Q- switched Nd:YLF laser which provides up to 40 mJ of single frequency light in a 10 ns to 15 ns pulse for amplification. The power amplifier uses a regenerative ring design with approximately 10 total passes and a phase conjugate mirror after the 5th pass. The conjugator is necessary to help produce a plane wavefront and thus a uniform intensity distribution within the ring, a requirement to avoid damage at the high intensities. The energy at the phase conjugate mirror exceeds 1 J/pulse and thus careful attention is paid to the tradeoff between input energy, conjugator reflectivity and near field spatial fidelity of the return. The laser output is in the range of 20 J/pulse, 2 GW peak with average power of 70 W. Measurements have been made at high power of the near field and far field beam quality.

Transverse pattern modifications in a stable apertured laser resonator

The effects of hard apertures on the transversal mode distribution and the output near and far field distributions are investigated both theoretically and experimentally. Wavefronts propagating in different directions inside a CO(2) laser cavity were measured at the same plane and found to have different transversal amplitude profiles. The influence of intracavity aperture's location and diameter on the near field distribution and far field beam quality were measured and calculated. Quantitative figures of merit are introduced to evaluate the departure of the transversal mode patterns from ideal Gaussians.

The build-up of backscatter in the energy range 1 mm Al to 8 mm Al HVT (radiotherapy beams)

Little information is available on the influence of the thickness of underlying material on the magnitude of the backscatter factor and investigations were therefore undertaken. Radiation beam qualities between 1 mm Al to 8 mm Al half-value thickness (HVT) and source-to-surface distances (SSD) applicable to contemporary superficial radiotherapy were used. Measurements of the backscatter were made in a polystyrene phantom using a purpose-designed parallel-plate ionisation chamber built into the phantom surface. The effect of the thickness of underlying material on the build-up was investigated as a function of field size, beam quality, and SSD. The backscatter build-up curves were found to rise more slowly with increasing field size, increasing HVT and longer SSD; and the experimental data were fitted by an exponential function. A set of equations is proposed correlating the parameters influencing the backscatter behaviour and allowing the fractional backscatter factor to be calculated for a given field size, HVT, SSD and thickness of underlying material.

Recombination in parallel plate free-air ionization chambers.

Consideration is given to the choice of the formula which best describes recombination between parallel plates. This formula is modified so that it may be applied to free-air ionization chambers. The modifications entail determining (i) the effective width of a circular cross-section X-ray beam, (ii) the effect of space charge and (iii) the effect of ion spread and distribution. The modified formula is checked experimentally with a small free-air chamber by varying field strength, dose-rate, beam quality and size and shape of the X-ray beam. Good confirmation is obtained.