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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.

All-solid-state tube lasers (ASSTLs) with zigzag beam path combine the attractive properties of rod, slab, and disk lasers. However, the analysis on the beam quality of tube lasers is rarely reported. In order to explore how and how seriously the static errors including fabricating and alignment errors degrade the beam quality of a tube laser, a theoretical model of a nonideal ASSTLs constructed in master oscillator power amplifier (MOPA) configuration has been built up by taking the static errors into consideration. On the basis, the influence of the fabricating and alignment errors on the static beam quality of the ASSTLs was analyzed in detail, and the annular aberration of the output beam was discussed as well. Simulation results indicate that the static beam quality of tube lasers strongly depends on the fabricating and alignment errors. These static errors would severely degrade the output beam quality of the ASSTLs and even result in coma and defocus aberrations. This work may provide valuable references for the design, processing, and applications of ASSTLs.

With rising current, the lateral far-field angle of high-power broad-area lasers widens (far-field blooming), which can be partly attributed to non-thermal effects due to carrier-induced refractive index and gain changes that become the dominant mechanism under pulsed operation. To analyze the non-thermal contribution to far-field blooming, we use a traveling wave-based model that properly describes the injection of the current into and the diffusion of the carriers within the active region. Although no pre-assumptions regarding the modal composition of the field is made and filamentation is automatically accounted for, the highly dynamic time-dependent optical field distribution can be very well represented by only a few modes of the corresponding stationary waveguide equation obtained by a temporal average of the carrier density and field intensity. The reduction of current spreading and spatial holeburning by selecting proper design parameters can substantially improve the beam quality of the laser.

The propagation characteristics of a Hermite-Gaussian correlated Schell-model (HGCSM) beam in the turbulence of biological tissue are analyzed. The average intensity, spectral degree of coherence, and the dependence of the propagation factors on the beam orders, transverse coherence width, fractal dimension, characteristic length of heterogeneity, and small length-scale factor are numerically investigated. It is shown that the HGCSM beam does not exhibit self-splitting properties on propagation in tissues due to the strong turbulence in the refractive index of biological tissue. The larger the beam orders, the fractal dimension, and the small length-scale factor are, or the smaller the transverse coherence width and the characteristic length of heterogeneity are, the smaller the normalized propagation factor is, and the better the beam quality of HGCSM beams in turbulence of biological tissue is. Moreover, under the same condition, the HGCSM beam is less affected by turbulence than of Gaussian Schell-model (GSM) beam. It is expected that the results obtained in this paper may be useful for the application of partially coherent beams in tissue imaging and biomedical diagnosis.

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.

We investigate the frequency-resolved intensity noise spectrum of an Yb-doped fiber amplifier down to the fundamental limit of quantum noise. We focus on the kHz and low MHz frequency regime with special interest in the region between 1 and 10 kHz. Intensity noise levels up to $$\ge$$ ≥ 60 dB above the shot noise limit are found, revealing great optimization potential. Additionally, two seed lasers with different noise characteristics were amplified, showing that the seed source has a significant impact and should be considered in the design of high power fiber amplifiers.

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.

An efficient MgO: PPLN singly resonant optical parametric oscillator (SROPO) pumped by a Nd: YVO4/YVO4 2nd-Stokes Raman laser at 1313 nm was demonstrated. The maximum conversion efficiency of 54.8% was obtained at the pump power of 1.16 W, which was higher than that of the OPO pumped by 1st-Stokes Raman laser at 1176 nm, due to the better beam quality, narrower line width and pulse width of the 2nd-Stokes Raman laser. The maximum output power of 1.57 W at 1855.5 nm with the pulse repetition frequency (PRF) of 40 kHz was achieved under the pump power of 4 W. The corresponding conversion efficiency was 39.7%, with the pulse width of 2.53 ns, linewidth of 0.45 nm, and M2 of 1.87. Meanwhile, with the crystal period ranging from 27.91 to 29.98 μm, the tuning range of signal wave was 1778.6–1861.8 nm.

Broad-area diode lasers with increased brightness and efficiency are presented, which are fabricated using an enhanced self-aligned lateral structure by means of a two-step epitaxial growth process with an intermediate etching step. In this structure, current-blocking layers in the device edges ensure current confinement under the central stripe, which can limit the detrimental effects of current spreading and lateral carrier accumulation on beam quality. It also minimizes losses at stripe edges, thus lowering the lasing threshold and increasing conversion efficiency, while maintaining high polarization purity. In the first realization of this structure, the current block is integrated within an extreme-triple-asymmetric epitaxial design with a thin p-doped side, meaning that the distance between the current block and the active zone can be minimized without added process complexity. Using this configuration, enhanced self-aligned structure devices with 90 µm stripe width and 4 mm resonator length show up to 20% lower threshold current, 21% narrower beam waist, and slightly higher (1.03×) peak efficiency in comparison to reference devices with the same dimensions, while slope, divergence angle and polarization purity remain almost unchanged. These results correspond to an increase in brightness by up to 25%, and measurement results of devices with varying stripe widths follow the same trend.

A sealed miniaturized Tissue Equivalent Proportional Counter (mini-TEPC) able to work in gas-steady modality was developed at the Legnaro National Laboratories of the Italian National Institute of Nuclear Physics (LNL – INFN, Legnaro, Italy). The aim of the present work is to compare the response of this mini-TEPC with that of a silicon microdosimeter based on a monolithic telescope. Pairwise measurements were performed at the 62 MeV proton clinical Spread Out Bragg Peak (SOBP) of CATANA at the Southern National Laboratories of INFN (LNS – INFN, Catania, Italy). The dose mean lineal energy values were derived from the spectra measured with the two detectors and compared with the total dose-averaged LET calculated by means of Geant4 Monte Carlo simulations. Finally, the possibility to apply the Microdosimetric Kinetic Model (MKM) to reproduce RBE variations with depth along the Spread Out Bragg Peak was investigated.

This work presents a review on the effect of transverse mode instability in high-power fiber laser systems and the corresponding investigations led worldwide over the last decade. This manuscript includes the description of the experimental observations, the physical origin of this effect, as well as some of the proposed mitigation strategies., Comment: (c) 2020 Optical Society of America. One print or electronic copy may be made for personal use only. Systematic reproduction and distribution, duplication of any material in this paper for a fee or for commercial purposes, or modifications of the content of this paper are prohibited

On-line image guidance using magnetic resonance (MR) imaging is expected to improve the targeting accuracy of proton therapy. However, to date no combined system exists. In this study, for the first time a low-field open MR scanner was integrated with a static proton research beam line to test the feasibility of simultaneous irradiation and imaging. The field-of-view of the MR scanner was aligned with the beam by taking into account the Lorentz force induced beam deflection. Various imaging sequences for extremities were performed on a healthy volunteer and on a patient with a soft-tissue sarcoma of the upper arm, both with the proton beam line switched off. T 1-weighted spin echo images of a tissue-mimicking phantom were acquired without beam, with energised beam line magnets and during proton irradiation. Beam profiles were acquired for the MR scanner's static magnetic field alone and in combination with the dynamic gradient fields during the acquisition of different imaging sequences. It was shown that MR imaging is feasible in the electromagnetically contaminated environment of a proton therapy facility. The observed quality of the anatomical MR images was rated to be sufficient for target volume definition and positioning. The tissue-mimicking phantom showed no visible beam-induced image degradation. The beam profiles depicted no influence due to the dynamic gradient fields of the imaging sequences. This study proves that simultaneous irradiation and in-beam MR imaging is technically feasible with a low-field MR scanner integrated with a static proton research beam line.

We report on kW-class dense wavelength beam combining of a laser diode module consisting of ten broad-area laser diode bars by using a novel multi-laser cavity approach based on a thin-film filter (TFF) as a dispersive optical element. The wavelength-stabilized output of the TFF cavity is beam combined upon a −1st order transmission grating. Hereby, a cylindrical telescope is used for linear dispersion-matching between the TFF and the combiner grating. On the basis of simulations of the resulting beam quality deterioration, we are able to optimize the cavity and the combiner setup for optimal beam quality preservation. We demonstrate a highly efficient direct diode laser with 1.1-kW output power and a symmetrical beam parameter product of about 6mm × mrad (95 % power content) in both beam axis.

Frequency doubling of an infrared laser radiation in non-linear optical crystals is a widely used technique to obtain light in the visible range. The second harmonic generation process is influenced by several well-known parameters. In this article we study the effect of group delay dispersion on the second harmonic generation process for femtosecond pulses. We show, both through simulation and experiments, that for certain parameters even a small amount of chirp can have a detrimental effect on the conversion efficiency as well as the second harmonic beam quality. We also check the effect of higher order dispersion. By properly accounting for those effects the crystal length and focusing conditions can be optimized to reach high conversion efficiency, while maintaining low sensitivity to chirp variations and good beam quality.

Experimental results of the implementation of our concept of neutral gain modules (NGM) in high-power thin-disk lasers are presented. To demonstrate power scalability and high brightness, series of NGMs comprising up to four Yb:YAG thin-disk laser modules have been integrated in various ways into stable and unstable resonators. Furthermore, to compensate for the parabolic component of the remaining thermal lens effects in the high-power regime, an intracavity deformable mirror was embedded. On the basis of the underlying concept, in our experiments an output power of nearly 3 kW with a beam propagation factor $$M^2 = 5$$ as well as a brightness up to $$B = 21\,{\rm GW}/{\rm cm ^2} \, {\rm sr }$$ could be demonstrated. Compensating systematically for the nonparabolic components of the phase aberrations, the concept also enables a further enhancement of the beam quality. The principle is transferable to other laser configurations, e.g., master oscillator power amplifier systems.

A monolithic silicon device consisting of a matrix of micrometric cylindrical diodes (about 2 μm in thickness and 9 μm in diameter) coupled to a residual energy measurement stage E (about 500 μm in thickness) was proposed and studied for assessing the quality of a therapeutic proton beam. The device was placed at different depths inside a polymethyl-methacrylate phantom and irradiated with a modulated 62 MeV proton beam at the Centro di AdroTerapia e Applicazioni Nucleari Avanzate (CATANA) of the Laboratori Nazionali del Sud (LNS, Catania, Italy) of the Istituto Nazionale di Fisica Nucleare (INFN). At each phantom depth, the energy imparted in the two detector stages was measured event-by-event in coincidence mode. The distributions of the energy imparted to the cylindrical diodes were corrected for tissue-equivalence by applying an optimized procedure. In order to perform a comparison with literature data measured with a cylindrical TEPC, the distributions derived with the silicon detector were corrected for shape-equivalence. The agreement with the microdosimetric spectra measured with the TEPC was satisfactory above the detection limit imposed by the electronic noise of the silicon-based system.

The application of standard unstable resonators does not allow for an independent adjustment of the resonator magnification and the output coupling. Either you get a high magnification together with a high output coupling, or vice versa. Certain laser types, like e.g. thin-disc lasers or chemical oxygen iodine lasers, permit only quite low optimum output couplings. The corresponding low resonator magnification is equal to a poor beam quality. In order to apply unstable resonators with a high magnification also to low gain media an additional mirror surface retroreflects a part of the out coupled radiation back into the cavity. The output coupling is reduced efficiently, whereas the resonator magnification stays high. Accordingly low gain media can be operated with high power extraction in combination with a good beam quality. Numerical and experimental investigations are shown. The experiments are performed with a chemical oxygen iodine laser operating at a wavelength of 1.315 μm and demonstrate the feasibility of this resonator design.

The BQ‐CHECK phantom (PTW Freiburg, Germany) has been designed to be used with a 2D ion chamber array to facilitate the quality assurance (QA) of electron and photon beam qualities (BQ). The BQ‐CHECK phantom has three wedges covering the diagonal axes of the beam: two opposed aluminum wedges used to measure electron energy and a single copper wedge used to measure photon energy. The purpose of this work was to assess the suitability of the BQ‐CHECK phantom for use in a routine QA program. A range of percentage depth dose (PDD) curves for two photon beams and four electron beams were measured using a MP3 plotting tank (PTW Freiburg). These beams were used to irradiate a STARCHECK array (PTW Freiburg) with and without the BQ‐CHECK phantom on top of the array. For photons, the ratio of the signals from two chambers underneath the copper wedge was used as an effective TPR measurement (TPReff) and, for electrons, the full width at half maximum of the profile (EFWHM) underneath the aluminum wedges was used as an electron energy constancy measurement. PDD measurements were compared with TPReff and EFWHM to assess the sensitivity of the BQ‐CHECK phantom. The clinical tolerances of TPReff were determined for 6 MV (0.634–0.649), and 10MV (0.683–0.692). For electrons, the clinical tolerances of EFWHM were determined for 6 MeV (94.8–103.4 mm), 8 MeV (105.5–114.0 mm), 10 MeV (125.4–133.9 mm) and 12 MeV (138.8–147.3 mm). Electron and photon energy metrics are presented which demonstrate that the BQ‐CHECK phantom could be used to form part of an efficient routine monthly QA program. Acceptable beam quality limits for various nominal beam energies were established and at these limits, modified profiles were acquired using the STARCHECK array. From the modified profiles, EFWHM and TPReff were determined for the electron and photon beams, respectively. It was demonstrated that both EFWHM and the TPReff have a linear relationship with conventional beam quality metrics. PACS numbers: 87.56.bd, 87.56.‐v

High-power high-brightness laser diode arrays comprising 25 tapered laser oscillators have been fabricated. The devices, based on recently developed low-modal gain epitaxial layer-structures, deliver a maximum output power of more than 25-W continuous-wave. A high beam quality uniformity is achieved with an average beam quality factor of M/sup 2/=2.6 for each individual emitter. Compared to conventional broad-area laser diode arrays the brightness of each emitter is improved by more than an order of magnitude in the slow-axis direction. These arrays have the potential to produce optical power densities as high as 1 MW/cm/sup 2/.

A numerical analysis of laser resonators with aberrations is presented. The analysis shows that aberrations lead to large diffraction losses of laser resonators which are laid out to produce diffraction-limited beam quality. Static or dynamic compensation of the aberrations is possible and would yield much higher output power.

A rectangular negative branch off-axis hybrid resonator was coupled to a 10-kW class Chemical Oxygen-Iodine Laser (COIL). Various schemes of geometrical coupling between resonator and gain medium were investigated. The extracted power was 6.6 kW and reached about 70% of the output power for the COIL device in an optimized conventional stable resonator configuration. Experimentally measured margins for the sensitivity of resonator setup and alignment were found in close agreement with numerical calculations. Power density distributions were measured in the near field and in the far field. The divergence of the emitted laser beam in the unstable direction was lower than 2 times diffraction limited.

Eva Rodriguez-VidalIban QuintanaJon EtxarriUrko AzkorbebeitiaDeitze OtaduyIK4-TEKNIKERPolo Tecnologico de EibarCalle Inaki Goenaga, 520600 Eibar, Gipuzkoa, SpainE-mail: erodriguez@tekniker.esFrancisco GonzalezFernando MorenoUniversity of CantabriaGroup of OpticsApplied Physics DepartmentAvenida de los Castros, s/n39005, Santander, SpainAbstract. Laser transmission welding (LTW) of thermoplastics is a directbonding technique already used in different industrial applications sectorssuch as automobiles, microfluidics, electronics, and biomedicine. LTWevolves localized heating at the interface of two pieces of plastic to bejoined. One of the plastic pieces needs to be optically transparent tothe laser radiation whereas the other part has to be absorbent, beingthat the radiation produced by high power diode lasers is a good alterna-tive for this process. As consequence, a tailored laser system has beendesigned and developed to obtain high quality weld seams with weldwidths between 0.7 and 1.4 mm. The developed laser system consistsof two diode laser bars (50 W per bar) coupled into an optical fiberusing a nonimaging solution: equalization of the beam parameter product(BPP) in the slow and fast axes by a pair of step-mirrors. The powerscaling was carried out by means of a multiplexing polarization technique.The analysis of energy balance and beam quality was performed consid-ering ray tracing simulation (ZEMAX®) and experimental validation. Thewelding experiments were conducted on acrylonitrile/butadiene/styrene(ABS), a thermoplastic frequently used in automotive, electronics and air-craft applications, doped with two different concentrations of carbon nano-tubes (0.01% and 0.05% CNTs). Quality of the weld seams on ABS wasanalyzed in terms of the process parameters (welding speed, laser powerand clamping pressure) by visual and optical microscope inspections.Mechanical properties of weld seams were analyzed by mechanicalshear tests. High quality weld seams were produced in ABS, revealingthe potential of the laser developed in this work for a wide range of plasticwelding applications.

The properties of a laser beam coupled out of a standard unstable laser resonator are heavily dependent on the chosen resonator magnification. A higher magnification results in a higher output coupling and a better beam quality. But in some configurations, an unstable resonator with a low output coupling in combination with a good beam quality is desirable. In order to reduce the output coupling for a particular resonator, magnification fractions of the outcoupled radiation are reflected back into the cavity. In the confocal case, the output mirror consists of a spherical inner section with a high reflectivity and a flat outer section with a partial reflectivity coating. With the application of the unstable resonator with reduced output coupling (URROC), magnification and output coupling can be adjusted independently from each other and it is possible to get a good beam quality and a high power extraction for lasers with a large low gain medium. The feasibility of this resonator design is examined numerically and experimentally with the help of a chemical oxygen iodine laser.

The application of high energy lasers for material processing requires a high beam quality. This is e.g. determined by the image quality of the optical elements used for beam guidance and shaping. Until now, in the design of the optical elements several influencing parameters that occur during laser processing have been largely neglected. Reasons for this are the limited knowledge on the operational performance of optical elements for high power lasers (up to 25 kW) and the difficult measurement of the occurring deformations and displacements. Responsible for the deterioration of the desired image properties of the optical elements are, on the one hand, manufacturing inaccuracies, and, on the other hand, statical, dynamical, and thermal forces. Statical forces result from the mounting of the optical components and the applied coolant pressure. Dynamical forces can occur due to vibrations of the mechanical components of the beam guidance and fluctuations of the coolant pressure. The abs orption of a small part of the incident laser radiation leads to a heat-up of the optical element, resulting in a time and space-dependent deformation of the optics. The statical, dynamical, and thermal deformations and displacements occur simultaneously. A systematic analysis of the operational behavior requires a separated consideration of these parameters. Only such a proceeding can lead to a valuation of the influencing parameters. Using holographic interferometry, such a separated consideration of the different influences becomes possible. In comparison to other interferometric methods, the holographic interferonetry also allows not only the investigation of the optical component, e.g. a mirror, but also of the other components of the optical element. The results of this investigation lead to first conclusions about the operational behavior of the considered optical elements.

(AlGaIn)(AsSb) ridge-waveguide tapered diode lasers with separately contacted ridge and tapered sections, emitting at 1.93 µm, have been analyzed in pulsed mode with respect to their high-power capability and wavelength tunability. Operating the ridge section above saturation, a variation of the current through this section resulted in a change in lasing wavelength, while changing the current injected into the tapered section at a constant ridge current allowed to vary the output power at constant lasing wavelength. Furthermore, the optical power required to saturate the tapered amplifier section has been derived from a comparison of the experimental characteristics with beam propagation method calculations.

Various schemes of geometrical coupling between optical resonator and gain medium were investigated for a 10-kW class Chemical Oxygen-Iodine Laser (COIL). Starting from theory, different types of resonator layouts were designed and optimized for COIL with a rectangular gain medium and an output coupling of about 10%. Hybrid resonators match these coupling conditions more easily than concentric unstable resonators. Compared to the negative branch type, the positive branch hybrid resonator shows very high sensitivity to the optical alignment in the unstable direction but avoids a focal line within the resonator. The obtained output power of both hybrid resonators is compared to the output power of the COIL device in a conventional stable resonator configuration. Measured margins for the sensitivity of resonator setup and alignment were found in close agreement with numerical calculations. Power density distributions were measured in the near field and in the far field. The divergence of the emitted laser beam in the unstable direction was nearly diffraction limited.

High brightness performance of chemical oxygen iodine lasers (COIL) requires resonator concepts that enable efficient power extraction from low gain medium while the beam quality is close to the diffraction limit. The resonator shall provide straightforward alignment procedures and stable output performance for efficient laser operation. Such resonator configurations are theoretically designed and implemented in a 10 kW-class COIL. Different resonator concepts are pre-evaluated by numerical methods with regard to their design and alignment sensitivities. Promising candidates are tested for their alignment performance and brightness parameters. Among others, hybrid resonators are found to be of straightforward optical architecture meeting the above challenges. Theoretical predictions and experimental results are in excellent agreement. Therefore, further promising design approaches, like multi-pass hybrid resonators or the modified negative-branch unstable resonator, will be depicted and discussed within this paper.