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Signal beam combiners play a pivotal role in enhancing the output power of fiber lasers, which have wide-ranging applications from industrial processing to medical and military uses. This paper explores the influence of fiber twist angle and non-uniform symmetric arrangement on the performance of 19 × 1 fiber signal combiners. A simulation model was developed to analyze the impact of these parameters under adiabatic tapering conditions and the principle of brightness conservation. The model allowed for a systematic investigation of how varying twist angles and non-uniform spacings affect the combiner's performance metrics, such as transmission efficiency and beam quality. The study found that an optimal balance between high transmission efficiency (up to 98.5%) and good beam quality (minimum M2 factor of 1.06) can be achieved when the twist angle is kept below 60° and the non-uniform spacing is maintained within 10–30 μm. These conditions ensure minimal degradation of the beam quality while maximizing the transmission efficiency of the combiner. These findings offer valuable insights into the optimization of signal combiner design, which is critical for advancing high-power fiber laser systems. By carefully controlling the fiber twist angle and non-uniform spacing, designers can achieve superior performance in fiber laser applications, thereby enhancing the overall efficiency and reliability of these systems. This research contributes to the broader field of optical engineering by providing a deeper understanding of the underlying principles that govern the performance of signal combiners. [ABSTRACT FROM AUTHOR]

The nonlinear self-focusing effect on the beam quality of powerful incoherent array beams propagating upwards in the turbulent atmosphere is investigated. A quadratic approximation of the nonlinear phase is proposed, and the analytical propagation formulae of incoherent array beams in the turbulent atmosphere are derived and confirmed. It is shown that the beam quality at the target degrades as the fill factor fc increases due to an increase of self-focusing, but it becomes better as the number N of sub-beams increases because of a decrease of self-focusing. Furthermore, the detrimental self-focusing may be even eliminated when fc is small enough or N is large enough. On the other hand, it is found that in the turbulent atmosphere, an incoherent array beam can achieve a higher threshold critical power, a higher intensity and a higher energy focusability at the target than a single laser beam. [ABSTRACT FROM AUTHOR]

Quantum cascade lasers (QCLs) in the mid-infrared (MIR) hold significant potential for widespread applications in both military and civilian contexts. However, the utility of present single-chip QCLs is hampered by issues such as low output power and subpar beam quality. This study addresses these limitations by employing spectral beam combining (SBC) based on a diffraction grating, with an aim to enhance both power and beam quality of MIR QCLs. Coaxial power synthesis of three single-chip QCLs (around 4.75 μm) is achieved experimentally, importantly, the beam quality did not decrease after combining, essentially maintaining the same quality as before combining. It is proposed that there are four main factors affecting the combining efficiency, namely operating optical power (or driving current) of the chips, individual differences in QCL chips, diffraction grating, and reflectivity of feedback mirror, and their effects on the combining efficiency are discussed separately. This study confirms that SBC is an effective way to obtain high-power and high-beam quality MIR QCL sources, and lays a research foundation for more beam spectral combining. [ABSTRACT FROM AUTHOR]

Small-field dosimetry used in special procedures such as gamma knife, Cyberknife, Tomotherapy, IMRT, and VMAT has been in evolution after several radiation incidences with very significant (70%) errors due to poor understanding of the dosimetry. IAEA-TRS-483 and AAPM-TG-155 have provided comprehensive information on small-fields dosimetry in terms of code of practice and relative dosimetry. Data for various detectors and conditions have been elaborated. It turns out that with a suitable detectors dose measurement accuracy can be reasonably (±3%) achieved for 6 MV beams for fields >1×1 cm2. For grid therapy, even though the treatment is performed with small fields created by either customized blocks, multileaf collimator (MLC), or specialized devices, it is multiple small fields that creates combined treatment. Hence understanding the dosimetry in collection of holes of small field is a separate challenge that needs to be addressed. It is more critical to understand the scattering conditions from multiple holes that form the treatment grid fields. Scattering changes the beam energy (softer) and hence dosimetry protocol needs to be properly examined for having suitable dosimetric parameters. In lieu of beam parameter unavailability in physical grid devices, MLC-based forward and inverse planning is an alternative path for bulky tumours. Selection of detectors in small field measurement is critical and it is more critical in mixed beams created by scattering condition. Ramification of small field concept used in grid therapy along with major consideration of scattering condition is explored. Even though this review article is focussed mainly for dosimetry for low-energy megavoltage photon beam (6 MV) but similar procedures could be adopted for high energy beams. To eliminate small field issues, lattice therapy with the help of MLC is a preferrable choice. [ABSTRACT FROM AUTHOR]

Signal beam combiners play a pivotal role in enhancing the output power of fiber lasers, which have wide-ranging applications from industrial processing to medical and military uses. This paper explores the influence of fiber twist angle and non-uniform symmetric arrangement on the performance of 19 × 1 fiber signal combiners. A simulation model was developed to analyze the impact of these parameters under adiabatic tapering conditions and the principle of brightness conservation. The model allowed for a systematic investigation of how varying twist angles and non-uniform spacings affect the combiner's performance metrics, such as transmission efficiency and beam quality. The study found that an optimal balance between high transmission efficiency (up to 98.5%) and good beam quality (minimum M2 factor of 1.06) can be achieved when the twist angle is kept below 60° and the non-uniform spacing is maintained within 10–30 μm. These conditions ensure minimal degradation of the beam quality while maximizing the transmission efficiency of the combiner. These findings offer valuable insights into the optimization of signal combiner design, which is critical for advancing high-power fiber laser systems. By carefully controlling the fiber twist angle and non-uniform spacing, designers can achieve superior performance in fiber laser applications, thereby enhancing the overall efficiency and reliability of these systems. This research contributes to the broader field of optical engineering by providing a deeper understanding of the underlying principles that govern the performance of signal combiners.

Quantum cascade lasers (QCLs) in the mid-infrared (MIR) hold significant potential for widespread applications in both military and civilian contexts. However, the utility of present single-chip QCLs is hampered by issues such as low output power and subpar beam quality. This study addresses these limitations by employing spectral beam combining (SBC) based on a diffraction grating, with an aim to enhance both power and beam quality of MIR QCLs. Coaxial power synthesis of three single-chip QCLs (around 4.75 μm) is achieved experimentally, importantly, the beam quality did not decrease after combining, essentially maintaining the same quality as before combining. It is proposed that there are four main factors affecting the combining efficiency, namely operating optical power (or driving current) of the chips, individual differences in QCL chips, diffraction grating, and reflectivity of feedback mirror, and their effects on the combining efficiency are discussed separately. This study confirms that SBC is an effective way to obtain high-power and high-beam quality MIR QCL sources, and lays a research foundation for more beam spectral combining.

The development of electron beam applications motivates electron gun research. In addition to the beam energy and current, its quality also plays a significant role in the spread of electron guns. Herein, we present the design of an electron gun for use as an electron source in a linear accelerator with high beam current and quality. In this design, starting with the Pierce geometry, a stay-by-stay optimization based on the physics governed by the beam behavior has been carried out to control the emittance growth and get a beam of outstanding quality. In particular, non-linear transverse force, space charge effect, and spherical aberration are considered. This resulted in a threefold improvement in the beam quality as measured by the beam emittance.

Ionization chambers were calibrated regarding absorbed dose to water in MeV X-ray fields of a medical linear accelerator, with nominal maximum energies of 6 MeV and 15 MeV. Whereas, absorbed doses to water at a depth of 10 cm below the water surface were measured using the same ionization chamber (referred to as IC-1) in two different manners (i.e., applying the old beam quality correction factor kQ0 from the old IAEA/TRS-398 protocol, referred to as D1-old10 and using the new one, kQ from the updated protocol, referred to as D1-new10. The calibration factors of IC-1, ND1,D,w, Q were deduced (for beam qualities Q of MeV X-ray fields with maximum energies of 6 MeV and 15 MeV). The identical procedure was applied to two other ionization chambers (IC-i, i=2,3) to obtain absorbed doses to water at a depth of 10 cm (referred to as Di-new10) using the new beam quality correction factor. A comparison between D1-new10 and Di-new10 was conducted to derive the calibration factors of the IC-i, NiD,w,Q. The combined standard uncertainties of NiD,w,Q (i =1,2,3) were estimated within 1.22% and 2.30%, respectively.

A continuous-wave (CW), high-power, quasi-three-level Nd:YAG laser operating at 946 nm is reported. The beam quality of the laser is greatly improved. The laser consists of a composite Nd:YAG rod end pumped by a fiber-coupled diode laser inside a simple concave-plane cavity. At an incident pump power of 31 W, a maximum CW output of 9.98 W was obtained at 946 nm, with a beam quality factor of M2~5. The corresponding optical-to-optical efficiency was 32.2% with respect to the incident pump power. To the best of our knowledge, this is the highest output power at 946 nm with such a beam quality ever generated by diode-pumped Nd:YAG laser with bulk crystals. [ABSTRACT FROM AUTHOR]

The high-energy/high-intensity laser facility PHELIX of the GSI Helmholtzzentrum für Schwerionenforschung in Darmstadt, Germany, has been in operation since 2008. Here, we review the current system performance, which is the result of continuous development and further improvement. Through its versatile frontend architecture, PHELIX can be operated in both long- and short-pulse modes, corresponding to ns-pulses with up to 1 kJ pulse energy and sub-ps, 200 J pulses, respectively. In the short-pulse mode, the excellent temporal contrast and the control over the wavefront make PHELIX an ideal driver for secondary sources of high-energy ions, neutrons, electrons and X-rays. The long-pulse mode is mainly used for plasma heating, which can then be probed by the heavy-ion beam of the linear accelerator of GSI. In addition, PHELIX can now be used to generate X-rays for studying exotic states of matter created by heavy-ion heating using the ion beam of the heavy-ion synchrotron of GSI.

Suppressing mode degradation is the key issue for high-power laser delivery; however, diagnosing mode degradation in its entirety, ranging from the contents and origins to locations, has always been a major obstacle. Here, a versatile approach for tracing the origins of mode coupling is demonstrated through addressing the differential intermodal dispersions of fiber modes. Full recognition for modal contents and the origins of mode degradation are experimentally completed in a two-mode fiber laser delivery system, which assists a significant improvement of beam quality M 2 from 1.35 to 1.15 at the highest power of over 300 W. This method yields a quantitative characterization for manipulating the individual mode of dual-mode coupling origins or their combinations. This work points toward a promising strategy for the online tracing of mode coupling in cascade fiber links, thus enabling further pursuit of seeking extreme beam quality in high-power fiber laser systems.

Thin-film beam combining technology is an effective approach to improve output power while maintaining beam quality. However, the lack of comprehensive research into the key factors affecting the beam quality in systems makes it challenging to achieve a practical combined beam source with high brightness. This paper clearly established that the temperature rise of dichroic mirrors (DMs) and sub-beam overlapping precision are the main factors affecting the beam quality of the system, with quantified effects. Based on this understanding, a combined light source of four channels of 3 kW fiber lasers was achieved, and an output power of 11.4 kW with a beam quality of M2 x = 1.601 and M2y = 1.558, using three high-steepness low-absorption DMs and the active control technique. To the best of our knowledge, this is the best beam quality for a 10 kW light source. This study offers a solution for practical high-power laser sources in the tens of kilowatts range.

This article introduces a novel counter-directional pump/signal combiner with a built-in mode field adapter. This combiner offers additional functionality to address the problem of mode field mismatch between the gain fiber and delivery fiber, which occurs when using a large-core (50 μm) signal fiber to suppress stimulated Raman scattering. The combiner exhibits negligible degradation in beam quality as a result of implementing mode field matching and employing an effective feedback alignment technique. The beam quality of the mode field adapter integrated in the combiner is improved by 10%, compared to the conventional approach, thanks to the reduction in splicing points and the prevention of fusion loss caused by tapering-induced cladding diameter mismatch. The combiner demonstrates an average pump coupling efficiency of 98.5% and a temperature rise coefficient of less than 10 °C/kW without active cooling. Furthermore, an integrated device based on the combiner is fabricated to shorten the length of delivery fiber, therefore mitigating the effects of stimulated Raman scattering. All those techniques mentioned above are utilized in a narrow linewidth laser system, resulting in an output of approximately 5840 W with a nearly single-mode characteristic. [ABSTRACT FROM AUTHOR]

Copyright of Laser Technology is the property of Gai Kan Bian Wei Hui and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

The Vertical Cavity Surface-Emitting Laser (VCSEL) has led to the rapid development of advanced fields such as communication, optical sensing, smart cars, and more. The accurate testing of VCSEL beam quality is an important prerequisite for its effective application. In this paper, a method for measuring the divergence angle of the VCSEL far field spot based on transmissive Fourier ptychography is proposed. First, a single CCD multi-angle VCSEL far-field spot acquisition system is designed. Second, based on the proposed Fourier ptychographic algorithm with synchronous optimization of embedded optical transfer function, a resolution-enhanced phase image of the spot is reconstructed and the boundary extracted by the Sobel operator of the phase image is defined as the boundary position of the beam waist. In this way, the beam waist radius of the laser beam is calculated. Finally, the divergence angle of the laser beam is measured via the radius of the beam waist. Compared with the traditional Gaussian beam definition method, the method proposed in this paper has higher accuracy in divergence angle measurement. The experimental results show that this method can improve the divergence angle measurement accuracy by up to 9.7%. [ABSTRACT FROM AUTHOR]

This study aimed to determine the optimal radiographic conditions for detecting lesions on digital chest radiographs using an indirect conversion flat-panel detector with a copper (Cu) filter. First, we calculated the effective detective quantum efficiency (DQE) by considering clinical conditions to evaluate the image quality. We then measured the segmentation accuracy using a U-net convolutional network to verify the effectiveness of the Cu filter. We obtained images of simulated lung tumors using 10-mm acrylic spheres positioned at the right lung apex and left middle lung of an adult chest phantom. The Dice coefficient was calculated as the similarity between the output and learning images to evaluate the accuracy of tumor area segmentation using U-net. Our results showed that effective DQE was higher in the following order up to the spatial frequency of 2 cycles/mm: 120 kV + no Cu, 120 kV + Cu 0.1 mm, and 120 kV + Cu 0.2 mm. The segmented region was similar to the true region for mass–area extraction in the left middle lobe. The lesion segmentation in the upper right lobe with 120 kV + no Cu and 120 kV + Cu 0.1 mm was less successful. However, adding a Cu filter yielded reproducible images with high Dice coefficients, regardless of the tumor location. We confirmed that adding a Cu filter decreases the X-ray absorption efficiency while improving the signal-to-noise ratio (SNR). Furthermore, artificial intelligence accurately segments low-contrast lesions. [ABSTRACT FROM AUTHOR]

On the basis of the principle of spectral beam combining (SBC), a simple combining system using a reflecting volume Bragg grating as a combiner was designed in which two focusing lenses were used to control the incident angles of input beams, and two concave lenses were used to solve the problem of beam divergence caused by focusing lenses. The effects of the beam divergence angle (DA) on diffraction efficiency and beam quality were analyzed theoretically. The simulation results showed that a larger DA will reduce diffraction efficiency and lead to deterioration of beam quality. In the SBC experiment, the performances of the output beam with and without concave lenses were compared. The experimental results showed that, when the concave lenses were not inserted, the measured output power was 503 mW with a combining efficiency of 85.3% and the beam quality factor of 1.68. The experiment further showed that the combined power was 566 mW with a combining efficiency of 96% and a beam quality factor of 1.31 when concave lenses were used. [ABSTRACT FROM AUTHOR]

Purpose: To quantify the potential error in outputs for flattening filter free (FFF) beams associated with use of a lead foil in beam quality determination per the addendum protocol for TG‐51, we examined differences in measurements of the beam quality conversion factor kQ when using or not using lead foil. Methods: Two FFF beams, a 6 MV FFF and a 10 MV FFF, were calibrated on eight Varian TrueBeams and two Elekta Versa HD linear accelerators (linacs) according to the TG‐51 addendum protocol by using Farmer ionization chambers [TN 30013 (PTW) and SNC600c (Sun Nuclear)] with traceable absorbed dose‐to‐water calibrations. In determining kQ, the percentage depth‐dose at 10 cm [PDD(10)] was measured with 10×10 cm2 field size at 100 cm source‐to‐surface distance (SSD). PDD(10) values were measured either with a 1 mm lead foil positioned in the path of the beam [%dd(10)Pb] or with omission of a lead foil [%dd(10)]. The %dd(10)x values were then calculated and the kQ factors determined by using the empirical fit equation in the TG‐51 addendum for the PTW 30013 chambers. A similar equation was used to calculate kQ for the SNC600c chamber, with the fitting parameters taken from a very recent Monte Carlo study. The differences in kQ factors were compared for with lead foil vs. without lead foil. Results: Differences in %dd(10)x with lead foil and with omission of lead foil were 0.9 ± 0.2% for the 6 MV FFF beam and 0.6 ± 0.1% for the 10 MV FFF beam. Differences in kQ values with lead foil and with omission of lead foil were −0.1 ± 0.02% for the 6 MV FFF and −0.1 ± 0.01% for the 10 MV FFF beams. Conclusion: With evaluation of the lead foil role in determination of the kQ factor for FFF beams. Our results suggest that the omission of lead foil introduces approximately 0.1% of error for reference dosimetry of FFF beams on both TrueBeam and Versa platforms. [ABSTRACT FROM AUTHOR]

A method of optimizing the stability of the resonant cavity by adding an additional crystal into the laser resonant cavity is studied for the first time. The laser performance can be effectively improved by adding crystals with appropriate refractive index into the resonant cavity. In this study, a Nd3 + :Gd0. 1 Y0. 9 AlO3 (Nd:GYAP) crystal laser device was built, and LBO crystal were added into the resonant cavity. The effects of LBO crystal on the laser properties of b-cut and c-cut crystals were compared respectively. The laser performance with and without LBO crystal was characterized in terms of output power, laser wavelength, beam quality and polarization characteristics. When LBO crystal is inserted into the laser resonant cavity, the slope efficiency of b-cut Nd:GYAP increases from 18. 9% to 24. 3%, and that of c-cut Nd:GYAP increases from 2. 87% to 10. 07% without changing the spectrum and beam quality. The maximum output power of b-cut crystal increases from 0. 931 W to 1. 254 W, and that of c-cut crystal increases from 63 mW to 134 mW. However, the polarization of the output laser deflects to some extent due to the optical rotation effect. This research provides a way to improve the slope efficiency and output power of lasers in the cases of the cavity length must be extended such as tuning and mode-locking operations. [ABSTRACT FROM AUTHOR]

Wavelength selection has a major impact on spectral beam combining (SBC) of laser beams that propagate in free space and in the atmosphere. We derive analytical expressions for the intensity and four characteristic quality parameters of free-space SBC beams. We show that the peak intensity, the mean-square beam width, and the M 2 factor of an SBC beam at the target are all larger than those of one single laser beam of the same power and average wavelength. The peak intensity can be made higher by increasing the wavelength spacing but reducing the mean wavelength. However, when an SBC laser beam propagates in the atmosphere, the beam quality depends on the absorption coefficient of the multi-wavelength field due to thermal blooming. A thermal distortion number is introduced for an SBC beam and its validity is confirmed. We show that the power proportion of a high-absorption element within an SBC beam should be as low as possible to reduce the thermal blooming effect. [ABSTRACT FROM AUTHOR]

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 continuous-wave (CW), high-power, quasi-three-level Nd:YAG laser operating at 946 nm is reported. The beam quality of the laser is greatly improved. The laser consists of a composite Nd:YAG rod end pumped by a fiber-coupled diode laser inside a simple concave-plane cavity. At an incident pump power of 31 W, a maximum CW output of 9.98 W was obtained at 946 nm, with a beam quality factor of M2~5. The corresponding optical-to-optical efficiency was 32.2% with respect to the incident pump power. To the best of our knowledge, this is the highest output power at 946 nm with such a beam quality ever generated by diode-pumped Nd:YAG laser with bulk crystals.

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.

Deformable mirrors (DMs) are widely used in high-power laser systems to improve the output beam quality. However, under high-power laser irradiation, the heat accumulates on the DM mirror surface and results in high-power laser induced distortions (LID), which will degrade the wavefront correction performance of the DM. In order to suppress the negative impact of the LID on the DM’s performance, we introduce the dual magnetic connection DM (DDM), in which the mirror and the base are connected by two-layer magnets. Configuration of the DDM is presented. The simulation is established to analyze the LID characteristics as well as the wavefront correction performance of the DDM. An experiment is established to investigate the LID of the DDM in practical conditions. Simulation and experiment results indicate that under high-power laser irradiation, the DDM could effectively suppress the LID and maintain good wavefront correction capability.

Background: In a recent study, we reported beam quality correction factors, fQ, in carbon ion beams using Monte Carlo (MC) methods for a cylindrical and a parallel-plate ionization chamber (IC). A non-negligible perturbation effect was observed;however, the magnitude of the perturbation correction due to the specific IC subcomponents was not included. Furthermore, the stopping power data presented in the International Commission on Radiation Units and Measurements (ICRU) report 73 were used, whereas the latest stopping power data have been reported in the ICRU report 90. Purpose: The aim of this study was to extend our previous work by computing fQ correction factors using the ICRU 90 stopping power data and by reporting ICspecific perturbation correction factors. Possible energy or linear energy transfer (LET) dependence of the fQ correction factor was investigated by simulating both pristine beams and spread-out Bragg peaks (SOBPs). Methods: The TOol for PArticle Simulation (TOPAS)/GEANT4 MC code was used in this study. A 30 × 30 × 50 cm3 water phantom was simulated with a uniform 10 × 10 cm2 parallel beam incident on the surface. A Farmer-type cylindrical IC (Exradin A12) and two parallel-plate ICs (Exradin P11 and A11) were simulated in TOPAS using the manufacturer-provided geometrical drawings. The fQ correction factor was calculated in pristine carbon ion beams in the 150-450 MeV/u energy range at 2 cm depth and in the middle of the flat region of four SOBPs. The kQ correction factor was calculated by simulating the fQo correction factor in a 60Co beam at 5 cm depth. The perturbation correction factors due to the presence of the individual IC subcomponents, such as the displacement effect in the air cavity, collecting electrode, chamber wall, and chamber stem, were calculated at 2 cm depth for monoenergetic beams only. Additionally, the mean dose-averaged and track-averaged LET was calculated at the depths at which the fQ was calculated. Results: The ICRU 90 fQ correction factors were reported. The pdis correction factor was found to be significant for the cylindrical IC with magnitudes up to 1.70%. The individual perturbation corrections for the parallel-plate ICs were <1.0% except for the A11 pcel correction at the lowest energy. The fQ correction for the P11 IC exhibited an energy dependence of >1.00% and displayed differences up to 0.87% between pristine beams and SOBPs. Conversely, the fQ for A11 and A12 displayed a minimal energy dependence of <0.50%. The energy dependence was found to manifest in the LET dependence for the P11 IC. A statistically significant LET dependence was found only for the P11 IC in pristine beams only with a magnitude of <1.10%. Conclusions: The perturbation and kQ correction factor should be calculated for the specific IC to be used in carbon ion beam reference dosimetry as a function of beam quality. [ABSTRACT FROM AUTHOR]

Nowadays, fiber laser has been widely used in industrial processing, especially in welding, cutting and other fields, and the appearance of adjustable ring-mode fiber laser improves effectively the quality of processing. In this paper, a (6 + 1) × 1 adjustable ring-mode fiber signal combiner is developed based on the technology of fiber cladding corrosion. The test results show that under the same injection condition, the beam quality transmitted through the central port of the combiner is degraded by only 8.3% compared with that transmitted through a single 50/250 μm fiber. It is proven to be feasible to maintain the beam quality of the (6 + 1) × 1 combiner by fiber corrosion technology. In order to improve the power of the central port of the (6 + 1) × 1 adjustable ring-mode combiner, a 3 × 1 fiber signal combiner and the central port of (6 + 1) × 1 combiner are cascaded. The output beam quality is M2 = 4.45 and the overall transmission efficiency is greater than 95%. This combiner can choose the mode of the output beam according to the actual application requirements, so as to achieve a better application effect. [ABSTRACT FROM AUTHOR]

In order to solve the problem of atmospheric influence on the far-field measurement of the quality of a laser beam, we proposed a direct wavefront measurement system based on the Hartmann detection principle, which can measure large apertures and high-power laser beams. The measuring system was composed of a lens array and a detector. The wavefront detection of a large aperture laser beam could be realized by controlling the distance between the lenses and the size of the lens. The influence of different duty cycle factors on the accuracy of the wavefront reconstruction under the same arrangement and different arrangement conditions was simulated and analyzed. The simulation results showed that when the sub-lenses of the system were not in close contact, the reconstruction accuracy of the duty factor of 0.8 was close to that of the case of the duty factor of 1. Within a certain detection range, the hexagonal arrangement of 19 lenses and the arrangement of 8 × 8 lens arrays had a high wavefront restoration accuracy; both were lower than 0.10 λ. The system proposed in this paper was suitable for measuring a large aperture laser beam, providing a new idea for measuring and analyzing the quality of large aperture laser beams. It also has an important significance for improving the measurement accuracy of the beam quality. [ABSTRACT FROM AUTHOR]

The spatial distribution of beams with orbital angular momentum in the far field is known to be extremely sensitive to angular aberrations, such as astigmatism, coma and trefoil. This poses a challenge for conventional beam optimization strategies when a homogeneous ring intensity is required for an application. We developed a novel approach for estimating the Zernike coefficients of low-order angular aberrations in the near field based solely on the analysis of the ring deformations in the far field. A fast, iterative reconstruction of the focal ring recreates the deformations and provides insight into the wavefront deformations in the near field without relying on conventional phase retrieval approaches. The output of our algorithm can be used to optimize the focal ring, as demonstrated experimentally at the 100 TW beamline at the Extreme Light Infrastructure - Nuclear Physics facility. [ABSTRACT FROM AUTHOR]

We measured the divergence angles of supercontinuum fiber sources from various types of silica fibers in the spectral range from 0.5 to 2.3 μ m, covering almost the entire range of supercontinuum that can be generated from silica fibers. The divergence angle of the supercontinuum directly output from the photonic crystal fibers (PCFs) increases monotonously with the increase of the wavelength. The divergence angle of the supercontinuum after transmitting through single-mode fibers, including HI1060, SMF-28 and 10/130 fibers, increases firstly and decreases then with the increase of the wavelength. The turning point is different for the three kinds of fibers. The beam parameter product (BPP) increases monotonously with wavelength for both the PCF and the single-mode fibers. Theoretical calculations are also made for the mentioned four types of fibers. The divergence angle of supercontinuum after transmitting through multimode fibers with 50, 100 and 200 μ m core diameters is also measured, which varies in a certain range without obvious rules depending on the wavelength. [ABSTRACT FROM AUTHOR]

GaAs-based, highly-efficient, kW-class, 1-cm laser bars with high peak power Popt and improved beam quality in quasi-continuous-wave mode are presented. The use of an extreme-triple-asymmetric (ETAS) epitaxial layer structure diminishes power saturation of high-power bars at high driving current. The resulting ETAS bars with 4 mm cavity produce a record 1.9 kW peak power, limited by available current supply, with a maximum power conversion efficiency ηE = 67% at THS = 25 °C heat-sink temperature. Both Popt and ηE have been increased further by operating the bars at THS = −70 °C. Sub-zero operation raises the Popt to 2.3 kW and the maximum ηE to 74%. A second configuration of ETAS bars with optimized lateral layout is further realized to obtain narrow lateral beam divergence θ up to 2 kA driving current, without sacrificing Popt and ηE. A 2–3° lower θ (95% power level) is observed over a wide operating range at room temperature. A high degree of polarization is also maintained across the whole operatingrange.

Progress in epitaxial design is shown to enable increased optical output power Popt and power conversion efficiency ηE and decreased lateral far-field divergence angle in GaAs-based distributed Bragg reflector (DBR) broad-area (BA) diode lasers. We show that the wavelength-locked power can be significantly increased (saturation at high bias current is mitigated) by migrating from an asymmetric large optical cavity (ASLOC) based laser structure to a highly asymmetric (extreme-triple-asymmetric (ETAS)) layer design. For wavelength-stabilization, 7th order, monolithic DBRs are etched on the surface of fully grown epitaxial layer structures. The investigated ETAS reference Fabry-Pérot (FP) BA lasers without DBRs and with 200 µm stripe width and 4 mm cavity length provide Popt = 29 W (still increasing) at 30 A in continuous-wave mode at room temperature, in contrast to the maximum Popt = 24 W (limited by strong power saturation) of baseline ASLOC lasers. The reference ETAS FP lasers also deliver over 10% higher ηE at Popt = 24 W. On the other hand, in comparison to the wavelength-stabilized ASLOC DBR lasers, ETAS DBR lasers show a peak power increment from 14 W to 22 W, and an efficiency increment from 46% to 60% at Popt = 14 W. A narrow spectral width (< 1 nm at 95% power content) is maintained across a very wide operating range. Consistent with earlier studies, a narrower far-field divergence angle and consequently an improved beam-parameter product is also observed, compared to the ASLOC-based lasers.

In this paper, the optical field transmission characteristics of an all-fiber signal combiner with a dumbbell shape and its application in laser beam combining are analyzed theoretically. Based on the three-layer waveguide model, the mode characteristics of a tapered multi-core fiber and a tapered single core fiber are analyzed respectively. On this basis, the working principle of an all-fiber signal combiner with a dumbbell shape is introduced from the perspective of mode evolution. The finite-difference beam propagation method is used to numerically analyze optical field envolution process in the combiner. Influences of the waist diameter, taper length and geometric arrangement are analyzed in detail. The calculation results show that an all-fiber signal combiner with a dumbbell shape can realize high brightness laser beam combining and the beam quality can reach the theoretical limit.

Evaluating the laser quality accurately is one of the most important and fundamental physical issues for laser sources, and the beam quality of lasers from the large mode area few-mode fibers have been haunted by the presence of high order mode for many years. This paper presents a modification to the M2 factor, which can be used to evaluate the mode content of fiber lasers accurately and efficiently, no matter whether the fiber modes are superposited coherently or incoherently. By mathematical derivation, the origin of the influence of relative phase on the M2 factor has been determined mathematically. A modification to the second moment of the beam intensity profile has been proposed, which eliminates the impact of uncontrollable relative phase on the second moment, and subsequently restores the one-to-one mapping between mode content and M2 factor even for coherent superposition cases. Also presented are the results of numerical simulations, which support the validity of the modified M2 factor to evaluate the mode content of the high power fiber lasers. With modified M2 factor being less than 1.1, the power fraction of LP11 mode content is unique and determined to be less than 3%.

This article introduces a novel counter-directional pump/signal combiner with a built-in mode field adapter. This combiner offers additional functionality to address the problem of mode field mismatch between the gain fiber and delivery fiber, which occurs when using a large-core (50 μm) signal fiber to suppress stimulated Raman scattering. The combiner exhibits negligible degradation in beam quality as a result of implementing mode field matching and employing an effective feedback alignment technique. The beam quality of the mode field adapter integrated in the combiner is improved by 10%, compared to the conventional approach, thanks to the reduction in splicing points and the prevention of fusion loss caused by tapering-induced cladding diameter mismatch. The combiner demonstrates an average pump coupling efficiency of 98.5% and a temperature rise coefficient of less than 10 °C/kW without active cooling. Furthermore, an integrated device based on the combiner is fabricated to shorten the length of delivery fiber, therefore mitigating the effects of stimulated Raman scattering. All those techniques mentioned above are utilized in a narrow linewidth laser system, resulting in an output of approximately 5840 W with a nearly single-mode characteristic.

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We establish a model based on the dispersion of the diffractive grating to investigate the influence of the spectral envelope and wavelength stability on the combined beam quality M 2 of the spectral beam combining (SBC) system. We obtain the analytic expressions of combined beam quality corresponding to the second-moment-spectral width of the beamlets, which can be used to forecast the combined beam quality for any integrable spectral envelope shape of beamlets in an SBC system. Additionally, we find that the center wavelength shift of beamlets has a significant impact on the combined beam quality. The second-moment-based width of the center wavelength shift distribution among all beamlets should be less than 0.5 times the spectral width. Here, the spectral width is the weighted root mean square of all beamlets’ spectral width, where the weight is the power of each single channel. [ABSTRACT FROM AUTHOR]

Antimonide semiconductor laser is a new type of laser with unique advantages in the 2 μm band. However, employing FP cavities causes multiple transverse modes to degrade beam quality despite achieving higher power output. In this paper, an antimonide semiconductor laser operating in 2 μm band is realized by utilizing fiber coupling and combining. Fiber combining results in higher output power, while the uniform patterns in both near-field and far-field are obtained, and the beam quality is improved. The experimental results illustrate that the output power reaches 1.2 W after 7-channel beam combination, and the near-field distribution is approximately Gaussian, while the far-field distribution is a flat-top.

The Vertical Cavity Surface-Emitting Laser (VCSEL) has led to the rapid development of advanced fields such as communication, optical sensing, smart cars, and more. The accurate testing of VCSEL beam quality is an important prerequisite for its effective application. In this paper, a method for measuring the divergence angle of the VCSEL far field spot based on transmissive Fourier ptychography is proposed. First, a single CCD multi-angle VCSEL far-field spot acquisition system is designed. Second, based on the proposed Fourier ptychographic algorithm with synchronous optimization of embedded optical transfer function, a resolution-enhanced phase image of the spot is reconstructed and the boundary extracted by the Sobel operator of the phase image is defined as the boundary position of the beam waist. In this way, the beam waist radius of the laser beam is calculated. Finally, the divergence angle of the laser beam is measured via the radius of the beam waist. Compared with the traditional Gaussian beam definition method, the method proposed in this paper has higher accuracy in divergence angle measurement. The experimental results show that this method can improve the divergence angle measurement accuracy by up to 9.7%.

• 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]

A high brightness GaN laser diode array with an output power of 78 W and an average lateral beam-quality factor of 3.5 for 38 emitters has been realized by the new design method of lateral-corrugated waveguides (LCWGs). The LCWG we propose was optimized by calculating both output power and beam-quality so as to maximize lateral brightness. As a result, the maximum output power of 117 W was obtained and a lateral brightness increased 1.6 times compared to the previous LCWG array. Finally, a high brightness GaN laser source was achieved by combining the beams from 38 emitters for GaN laser processing. [ABSTRACT FROM AUTHOR]

We present our recent experimental results on the pulsed regimes of Raman conversion of highly multimode laser diode (LD) pump radiation into the 1st and higher order Stokes radiation in multimode graded-index fibers. Three different linear cavities of Raman fiber laser with the modulation of losses (by acousto-optic modulator, AOM) or gain (by LD current) are explored and compared. An LD with wavelength of 976 nm is used for pumping enabling Raman lasing at wavelength of the 1st (1018 nm) and 2nd (1064 nm) Stokes orders. At ~27.2-kHz repetition rate corresponding to the laser cavity round-trip frequency (i.e., in the mode-locking regime), nanosecond pulses have been observed for both Stokes orders having the highest peak power of ~300 W in the scheme with bulk AOM and the shortest duration of 5–7 ns in the scheme with fiber-pigtailed AOM. At the same time, the beam quality of generated pulses is greatly improved as compared to that for pump diode (M2 > 20) reaching the best value (M2 = 2.05) for the 2nd order Stokes beam in the scheme with the gain modulation and demonstrating also the most stable regime. [ABSTRACT FROM AUTHOR]

The beam quality determines the focusing and transmission characteristics of the laser, which associated strongly with the light field mode in the laser resonant cavity. The study of optical field mode is of great significance to the design of laser resonator, external light path and beam shaping. With the development of laser technology, there are various new complex resonant cavity and optical elements. It is usually difficult to solve the light field mode in the resonant cavity by analytical methods, and the numerical calculation method can more easily solve the light field mode of the complex cavity. Numerical calculation method is an effective way to solve complex cavity light field mode. Among them, Matlab is popular for its complete graphics processing function, the results visualization and powerful calculation function. In this paper, numerical solutions of various resonator, including to parallel plane cavity, square mirror confocal cavity and circular mirror confocal cavity, are systematically simulated based on Matlab analysis software, and the relationships between light field modes and Fresnel number are discussed in different resonators. Then, the characteristics of mode distribution and mode selection of different cavities were obtain, which would provide an important basis for improving the beam quality through mode selection. [ABSTRACT FROM AUTHOR]

High‐order Hermite–Gaussian (HG) lasers oscillating at multiple wavelengths are extensively needed for generating vortex lasers carrying orbital angular momentum (OAM), which have wide applications on optical communication, high capacity storage, and high resolution imaging. Off‐axis pumping or off‐axis resonator has been used to generate high‐order HG0,n lasers, however, low optical efficiency limits practical applications of these lasers. Here, direct generation of multiple wavelength high‐order HG0,n lasers in a rectangular beam pumped Raman microchip laser constructed with Yb3+:Y3Al5O12 (Yb:YAG) and yttrium vanadate (YVO4) crystals is demonstrated. The order n of HG0,n mode lasers can be tuned from 1 to 9. HG0,n (n > 3) mode Raman lasers oscillate at multiple wavelengths and shift to longer wavelength with n. The output power is 137 mW for multiwavelength HG0,9 mode Raman laser when the input pump power is 2.7 W. The optical conversion efficiency is as high as 5.1%. Excellent beam quality with My2 close to theoretical values (My2 = 2n+1) for high‐order HG0,n lasers has been achieved. Topological charge tunable Laguerre–Gaussian (LG) vortex lasers with high beam quality are generated from HG0,n mode lasers with an astigmatic mode converter. This work demonstrates that rectangular beam pumping is an effective method for developing efficient multiwavelength high‐order HG0,n mode Raman lasers. [ABSTRACT FROM AUTHOR]

Progress in epitaxial design is shown to enable increased optical output power Popt and power conversion efficiency ηE and decreased lateral far-field divergence angle in GaAs-based distributed Bragg reflector (DBR) broad-area (BA) diode lasers. We show that the wavelength-locked power can be significantly increased (saturation at high bias current is mitigated) by migrating from an asymmetric large optical cavity (ASLOC) based laser structure to a highly asymmetric (extreme-triple-asymmetric (ETAS)) layer design. For wavelength-stabilization, 7th order, monolithic DBRs are etched on the surface of fully grown epitaxial layer structures. The investigated ETAS reference Fabry-Pérot (FP) BA lasers without DBRs and with 200 µm stripe width and 4 mm cavity length provide Popt = 29 W (still increasing) at 30 A in continuous-wave mode at room temperature, in contrast to the maximum Popt = 24 W (limited by strong power saturation) of baseline ASLOC lasers. The reference ETAS FP lasers also deliver over 10% higher ηE at Popt = 24 W. On the other hand, in comparison to the wavelength-stabilized ASLOC DBR lasers, ETAS DBR lasers show a peak power increment from 14 W to 22 W, and an efficiency increment from 46% to 60% at Popt = 14 W. A narrow spectral width (< 1 nm at 95% power content) is maintained across a very wide operating range. Consistent with earlier studies, a narrower far-field divergence angle and consequently an improved beam-parameter product is also observed, compared to the ASLOC-based lasers. [ABSTRACT FROM AUTHOR]

GaAs-based, highly-efficient, kW-class, 1-cm laser bars with high peak power Popt and improved beam quality in quasi-continuous-wave mode are presented. The use of an extreme-triple-asymmetric (ETAS) epitaxial layer structure diminishes power saturation of high-power bars at high driving current. The resulting ETAS bars with 4 mm cavity produce a record 1.9 kW peak power, limited by available current supply, with a maximum power conversion efficiency ηE = 67% at THS = 25 °C heat-sink temperature. Both Popt and ηE have been increased further by operating the bars at THS = −70 °C. Sub-zero operation raises the Popt to 2.3 kW and the maximum ηE to 74%. A second configuration of ETAS bars with optimized lateral layout is further realized to obtain narrow lateral beam divergence θ up to 2 kA driving current, without sacrificing Popt and ηE. A 2–3° lower θ (95% power level) is observed over a wide operating range at room temperature. A high degree of polarization is also maintained across the whole operatingrange. [ABSTRACT FROM AUTHOR]

The Kerr self-focusing and group-velocity dispersion (GVD) effects play important roles when a high-power pulsed laser beam propagates from the ground through the nonlinear and dispersive atmosphere to space orbits. For an annular pulsed laser beam, the Kerr self-focusing and GVD effects on the beam quality on the target is studied in this paper. Due to self-focusing, the maximum intensity Imax on the target arrives when the beam power P = Popt (Popt is called the optimal beam power), and the intensity on the target will decrease as the beam power P increases further when P > Popt. The Popt of nanosecond and picosecond pulsed beams is lower than that of femtosecond pulsed beams, while the maximum intensity Imax on the target of nanosecond and picosecond pulsed beams is higher than that of femtosecond pulsed beams. The Popt and Imax of annular pulsed beams are higher than those of Gaussian pulsed beams. In addition, the B integral and the Strehl ratio are also adopted to describe the beam quality on the target. On the other hand, because of the interplay of Kerr self-focusing and GVD effects, the temporal pulse splitting and self-healing may take place on propagation, and the temporal pulse splitting may take place more than once. The results obtained in this paper are useful for laser ablation propulsion's applications in space.