Output Beam Profile Research Articles

Mid-infrared all-fiber superfluorescent source in Er3+-doped fluoride fiber

We report, to the best of our knowledge, the first mid-infrared (MIR) all-fiber superfluorescent source based on a superfluorescent seed and a single-stage amplifier. A 7.8-m-long section of highly-Er3+-doped fluoride fiber is used in the superfluorescent seed and a 4.3-m-long section of lightly-Er3+-doped fluoride fiber acts as the amplifier. Two home-made MIR pump-signal combiners based on side-pumping technology are fabricated directly on two sections of gain fiber. By direct-low-loss fusion splicing of two kinds gain fiber, the power of MIR superfluoresescence main output is amplified to 0.35 W and the power of rear output is amplified to 0.2 W. The output spectrum under maximum power spans from 2680 nm to 2880 nm with 20 dB bandwidth of 60 nm. The measured time domain and output beam profile show that this superfluorescent source keeps relatively stable in microsecond time scale without self-pulsing effect occurrence and has near single-mode gaussian characteristic. This works provides a new way for MIR superfluorescent source, which is of practical interest in a range of applications.

Beam quality prediction for Tm-doped fiber system based on finite-difference beam propagation method

A numerical model is established to predict the beam quality factor M2 of fiber laser for the first time. The finite-difference beam propagation method (FD-BPM) is introduced to simulate the beam propagation of thulium-doped fiber lasers (TDFLs) with given design parameters. The output beam profile can be calculated for an active fiber with complex refractive index profiles and various bending radii. The beam quality factor M2 can be obtained by the calculated beam profile. The numerical simulation results agree with the reported experiment data with a deviation of less than 10%, which validates the accuracy of the model. This model provides a convenient and efficient approach for designing the high-beam-quality TDFLs with reduced experiment efforts.

Comprehensive characterisation of the IBA myQA SRS for SRS and SBRT patient specific quality assurance.

The myQA SRS (IBA) is a new to market 2D complementary metal oxide semiconductor detector array with an active area 140 × 120mm2 and 0.4mm resolution, making it a potential real-time dosimetry alternative to radiochromic film for stereotactic plan verification. Characterisation of the device was completed to assess performance. The dosimetric properties of the device were assessed for 6FF and 6FFF beams from a Varian TrueBeam STx with high definition multileaf collimator. Clinical suitability of the device for Patient Specific Quality Assurance was verified using ten SRS/SBRT plans, compared against other detectors, as well as multi leaf collimator (MLC) tests including picket fence and chair. Gamma analysis was performed using myQA software with criteria of 4%/1mm. The device demonstrated compliance with recommended specifications for basic tests. After the required warm-up period, the maximum deviation in detector signal from initial readings was 0.2%. Short-term and long-term reproducibility was 0.1% (6FF) and 1.0% (6FFF), respectively. Dose linearity was within 0.3% (6FF) and 0.7% (6FFF) and dose-rate dependence within 1.7% (6FF) and 2.9% (6FFF) and were verified with a Farmer type ionization chamber (PTW 30013). Angular dependence was quantified for coplanar and non-coplanar situations. Output factors and beam profiles measured on the device showed agreement within 1% of baseline RAZOR diode (IBA) and CC04 ionisation chamber (IBA) measurements for field sizes 1 × 1 to 10 × 10cm2. The minimum gamma (4%/1mm) pass rates for MLC-pattern tests were 96.5% and 98.1% for the myQA SRS and film, respectively. The average gamma (4%/1mm) pass rates for SBRT and SRS plans were 98.8% and 99.8% respectively. This work represents one of the first studies performed on the commissioning and performance characterisation of this novel device, demonstrating its accuracy and reliability, making it highly useful as a film alternative in stereotactic treatment plan verification.

Comparison of quartz and sapphire optical chambers for infrared laser sealing of vascular tissues using a reciprocating, side-firing optical fiber: Simulations and experiments.

Infrared (IR) lasers are being tested as an alternative to radiofrequency (RF) and ultrasonic (US) surgical devices for hemostatic sealing of vascular tissues. In previous studies, a side-firing optical fiber with elliptical IR beam output was reciprocated, producing a linear IR laser beam pattern for uniform sealing of blood vessels. Technical challenges include limited field-of-view of vessel position within the metallic device jaws, and matching fiber scan length to variable vessel sizes. A transparent jaw may improve visibility and enable custom treatment. Quartz and sapphire square optical chambers (2.7 × 2.7 × 25 [mm3 ] outer dimensions) were tested, capable of fitting into a 5-mm-OD laparoscopic device. A 1470 nm laser was used for optical transmission studies. Razor blade scans and an IR beam profiler acquired fiber (550-µm-core/0.22NA) output beam profiles. Thermocouples recorded peak temperatures and cooling times on internal and external chamber surfaces. Optical fibers with angle polished distal tips delivered 94% of light at a 90° angle. Porcine renal arteries with diameters of 3.4 ± 0.7 mm (n = 13) for quartz and 3.2 ± 0.7 mm (n = 14) for sapphire chambers (p > 0.05), were sealed using 30 W for 5 s. Reflection losses at material/air interfaces were 3.3% and 7.4% for quartz and sapphire. Peak temperatures on the external chamber surface averaged 74 ± 8°C and 73 ± 10°C (p > 0.05). Times to cool down to 37°C measured 13 ± 4 s and 27 ± 7 s (p < 0.05). Vessel burst pressures (BP) averaged 883 ± 393 mmHg and 412 ± 330 mmHg (p < 0.05). For quartz, 13/13 (100%) vessels were sealed (BP > 360 mmHg), versus 9/14 (64%) for sapphire. Computer simulations for the quartz chamber yielded peak temperatures (78°C) and cooling times (16 s) similar to experiments. Quartz is an inexpensive material for use in a laparoscopic device jaw, providing more consistent vessel seals and faster cooling times than sapphire and current RF and US devices.

Mitigating stimulated Brillouin scattering in multimode fibers with focused output via wavefront shaping.

The key challenge for high-power delivery through optical fibers is overcoming nonlinear optical effects. To keep a smooth output beam, most techniques for mitigating optical nonlinearities are restricted to single-mode fibers. Moving out of the single-mode paradigm, we show experimentally that wavefront-shaping of coherent input light to a highly multimode fiber can increase the power threshold for stimulated Brillouin scattering (SBS) by an order of magnitude, whilst simultaneously controlling the output beam profile. The SBS suppression results from an effective broadening of the Brillouin spectrum under multimode excitation, without broadening of transmitted light. Strongest suppression is achieved with selective mode excitation that gives the broadest Brillouin spectrum. Our method is efficient, robust, and applicable to continuous waves and pulses. This work points toward a promising route for mitigating detrimental nonlinear effects in optical fibers, enabling further power scaling of high-power fiber systems for applications to directed energy, remote sensing, and gravitational-wave detection.

High-power and high-quality Gaussian beam in VCSEL via anisotropic modes control

Gaussian beam in the vertical cavity surface emitting laser (VCSEL) hasimportantapplicationin 3D sensing and optical communications, due to its concentrate energy at center and long propagation distance. However, its beam quality under high power tends to be serious degradation and become annular hollow distribution, severely limiting the use of VCSEL. Here, the anisotropic elliptical oxide aperture VCSEL (AEOA-VCSEL) was designed to achieve high-power Gaussian beam with high-quality via anisotropic modes control. The saturation output power reached up to 8.2 mW under low injection current, to our best knowledge, which was the highest power generated in single VCSEL device. Moreover, the AEOA-VCSEL can control the output beam profile only by adjusting injection current, and could also homogenize the mode spacing among adjacent high-order modes. The facile preparation process and low cost makes the device present great potential in augmented and virtual reality (AR/VR), 3D sensing and optical communications.

Clinical Experience with Commissioning a GRID Collimator for Spatially Fractionated Radiation Therapy

In this study, we report our physics commissioning experience for clinically implementing a GRID collimator based Spatially Fractionated Radiation Therapy (SFRT). For SFRT commissioning, we utilized a commercially available brass GRID collimator (DotDecimal, Sanford, FL) that is mounted on a blocking tray for positioning into the accessory mount of the Linac. The brass GRID is 7.62 cm thick and weighs 15.8 kg. The GRID has a total of 149 divergent holes arranged in a hexagonal pattern (1.43 cm in diameter and hole-centers spaced at 2.11 cm when projected at the isocenter distance). The GRID encompasses a maximum field size of 25 × 25 cm at the isocenter. A commercial 48 × 48 × 48 cm water phantom scanning system was used to collect GRID output factors (GRID field to open field ratio), depth dose and beam profile data. Output measurements were performed using a 0.13 cm3 active volume ion-chamber and beam scans were obtained with a diode detector. Data was collected for both flattening and flattening-free beams of nominal energies 6 MV and 10 MV photons. The measurement depths were at dmax (1.5 cm for 6 MV and 2.5 cm for 10 MV), 5-cm and 10-cm respectively. For each energy and depth of measurement, collimator settings were varied from 5 × 5 cm to 28 × 28 cm. From scan profiles at different depths, the valley (lowest) to peak (highest) dose ratios (VPDR) were calculated. A commercial treatment planning system (TPS) was used to test the accuracy of dose calculations with GRID. This was accomplished by importing vendor generated DICOM RT file into the TPS. A block transmission factor of 7% for 6 MV and 10.2% for 10 MV energy beams were applied. All measured data were compared with corresponding TPS calculated data. Test patient treatment plans with GRID were created in TPS and planned distributions were verified using a commercial detector array with 2.5 mm detector spacing. The VPDR, expressed as %, are presented in Table 1. Measured and TPS calculated output factors agreed within 2% for 6 MV and within 3% for 10 MV photon beams. Percent depth doses were lower in magnitude for GRID field compared to open field for all energies studied (for e.g., 6 MV depth dose at 10 cm depth for 10 cm x10cm field 62% for GRID field vs. 66% for open field). Measured and calculated GRID beam profiles agreed within 5% dose difference and 1 mm distance-to agreement. For all test cases, the planned vs. measured dose distributions passed at an average gamma passing rate of 96.5% using a 3% dose difference/ 3 mm distance to agreement and 10% threshold criteria. The Dot Decimal GRID collimator provides a simple way of achieving SFRT in the clinic, albeit heavy to use and has an irradiation field size limitation of 25 cm × 25 cm.

Investigation of High‐Power Spatiotemporal Mode‐Locking with High Beam Quality

AbstractThe developed spatiotemporal mode‐locked (STML) laser has emerged as an effective platform for investigating high‐dimensional nonlinear spatiotemporal dynamics. Additionally, it offers a novel avenue for the design of fiber oscillators capable of operating at high power levels. At present, the focus of STML laser research primarily revolves around this aspect. However, considering practical applications, there is a strong desire to conceive a simple all‐fiber STML oscillator with both high beam quality and high power. In this study, an all‐fiber, high‐power STML oscillator based on multimode fibers is constructed and investigated. By manipulating the pump power and the polarization state, the laser could operate in a dissipative soliton or noise‐like pulse regime, both with average output powers of two operation states reaching the Watt level. Simultaneously, high‐quality output beam profiles are observed in both operation states. To the best of knowledge, this is the first demonstration of high‐power spatiotemporal mode‐locking with high beam quality. The study holds great benefits for advancing the investigations of compact all‐fiber STML lasers which deliver high‐power outputs with superior beam quality and ultimately propels the application of STML lasers. Furthermore, this study contributes to the understanding of the behavior of STML lasers with high power.

Output characteristics’ static fluctuations versus the pump power in 1018 nm fiber oscillators

This paper investigates the static fluctuating behavior of output parameters in 1018 nm fiber lasers using 20/400 µm and 25/400 µm ytterbium-doped fibers (YDFs). It is seen that by increasing the pump power, some static fluctuations is induced in the output characteristics of the lasers, such as output power, back-reflected power, and beam quality factor (M2). The growth of these parameters fluctuates versus the pump power, without any modulation frequency in the temporal behavior of the output beam profile. This effect, which to the best of our knowledge is reported for the first time, occurs at powers much lower than the threshold for dynamic transverse mode instability (TMI). It was found that the static mode-coupling occurs between two lowest-order modes and causes these fluctuations in the lasers’ output parameters. Conducting the experiment for 1080 nm fiber lasers with different lengths of YDF, in addition to confirm the descriptions about how the static fluctuations occurs, shows that this effect occurs in other wavelengths as well.

High power Ho:Y2O3 ceramic laser with controllable output intensity profile at 2.1 µm.

We report on a high-power Ho:Y2O3 ceramic laser at 2.1 µm with controllable output beam profile ranging from LG01 donut, flat-top to TEM00 mode using a simple two-mirror resonator. In-band pumped at 1943nm using a Tm fiber laser beam shaped via a coupling optics comprising a capillary fiber and lens-combination to achieve distributed pump absorption in Ho:Y2O3 and hence selective excitation of the target mode, the laser yields 29.7 W of LG01 donut, 28.0 W of crater-like, 27.7 W of flat-top and 33.5 W of TEM00 mode output for absorbed pump power of 53.5 W, 56.2 W, 57.3 W and 58.2 W, respectively, corresponding to a slope efficiency of 58.5%, 54.3%, 53.8% and 61.2%. This is, to the best of our knowledge, the first demonstration of laser generation with continuously tunable output intensity profile at ∼2 µm wavelength region.

More Than 20 W, High Signal-to-Noise Ratio Single-Frequency All-Polarization-Maintaining Hybrid Brillouin/Ytterbium Fiber Laser

A high-power all-polarization-maintaining (PM) hybrid Brillouin/ytterbium fiber laser (BYFL) at 1064 nm is demonstrated by adopting large-mode-area fiber with core/cladding diameter of 20/400 µm as the hybrid gain medium. Single-frequency laser output with a record output power of 23 W and an excellent optical signal-to-noise ratio of 72 dB is achieved. The BYFL also presents linear-polarized operation with a polarization extinction ratio of over 16.5 dB, excellent output beam profile and long-term stability. Meanwhile, the relative intensity noise of the Brillouin pump laser is significantly reduced with a maximum factor of 36 dB by the BYFL at the high frequency range, while the corresponding frequency noise is reduced by 15–30 dB across the whole examined frequency range. In addition, the estimated laser linewidth was compressed by 7.8 times from 260 kHz to 33 kHz. To the best of our knowledge, this is the highest output power that emitted from any kind of single-frequency fiber lasers and Brillouin lasers. It is believed that the combination of the hybrid gains and larger mode area fiber provides a promising scheme for realizing further power scaling of a single-frequency laser with all-fiber configuration.

Evaluation of Perkin Elmer Amorphous Silicon Electronic Portal Imaging Device for Small Photon Field Dosimetry.

Electronic portal imaging devices (EPIDs) are applied to measure the dose and verify patients' position. The present study aims to evaluate the performance of EPID for measuring dosimetric parameters in small photon fields. In this experimental study, the output factors and beam profiles were obtained using the amorphous silicon (a-Si) EPID for square field sizes ranging from 1×1 to 10×10 cm2 at energies 6 and 18 mega-voltage (MV). For comparison, the dosimetric parameters were measured with the pinpoint, diode, and Semiflex dosimeters. Additionally, the Monaco treatment planning system was selected to calculate the output factors and beam profiles. There was a significant difference between the output factors measured using the EPID and that measured with the other dosimeters for field sizes lower than 8×8 cm2. In the energy of 6 MV, the gamma passing rates (3%/3 mm) between EPID and diode profile were 98%, 98%, 95%, 94%, 93%, and 94% for 1×1, 2×2, 3×3, 4×4, 5×5, and 10×10 cm2, respectively. The measured penumbra width with EPID was higher compared to that measured by the diode dosimeter for both energies. The EPID can measure the dosimetric parameters in small photon fields, especially for beam profiles and penumbra measurements.

Machine Learning for automatic pointing alignment and spatial beam filtering

Constraint Bayesian optimization approach is used to optimize the beam pointing and spatial filtering of a laser beam using the capillary transmission and the output beam profile, as the optimization criteria. We have demonstrated that the developed method was able to robustly find the optimal laser parameters and it will be implemented in the SwissFEL UV photocathode laser in the future.

Pulsed inductive CO2 laser with radio-frequency excitation and influence of the H2 content on the efficiency and lasing temporal characteristics

In 2021, data on the effective pulsed gas discharge inductive CO2 laser with radio-frequency (RF) excitation were published with a pulse output energy of E∼ 1 J (the efficiency η∼ 14.5%) on the gas mixture He:N2:CO2 = 8:2:1. The efficiency of the developed CO2 laser had exceeded the value η∼ 21% at E∼ 350 mJ. At the beginning of 2022, it was shown that xenon addition (Xe = 4%) to the gas mixture made it possible to achieve an efficiency of η∼ 27% at an output energy of E∼ 600 mJ. For the first time, the effect of hydrogen additives in the active medium (He:N2:CO2:H2 and N2:CO2:H2 gas mixtures) was investigated for a pulsed inductive CO2 laser with RF excitation depending on the RF-pumping pulse duration value (τ), which allows the energy and temporal radiation characteristics of the laser to be controlled over a wider range. In addition to those already published, new experimental data have been obtained, namely the output beam profile of the inductive CO2 laser based on He:N2:CO2 = 8:2:1 gas mixture depending on the τ value. The new data will improve our understanding of inductive CO2 laser physics and of the plasma–chemical processes occurring in its active medium. RF current pulses propagated along inductor wires and, thus, an inductive discharge was formed to create a population inversion by IR transitions of CO2 * molecules.

Mathematical Modeling and Optimization of Low Energy Beam Line in 2MV Pelletron Accelerator

The 2 MV 6SDH-2 tandem pelletron accelerator installed at CASP, GC University Lahore has evolved over the past years from acceptance tests to an extremely reliable and convenient research tool in the field of particle accelerator physics, hav-ing capability of providing variety of ion beams from few keV to several MeV in energy. Many electrostatic and magnetic devices are employed to steer an ion beam in an accelerator system all the way from ion source to experimental setup. Mathematical modeling and optimization of low energy beamline has been done for this accelerator. Several ion optics and steering devices are provided in the pre-acceleration beamline for ion beam manipulation and in order to optimize the overall beam transmission. As the quality of the beam from the accelerator depends upon the design of these electrostatic and magnetic devices, optimum settings are derived by manually adjusting voltages and currents provided to low energy beam-line components. From these calibrations and optimization measurements, calculations has been done for mass analysis magnets, steering and focusing components and the results has been represented. From the results, it has been found that there exists a complex relationship between the effects of different voltages and currents in order to tune the desired output current and beam profile at the end-stations. Details of experiments and data analysis are presented.

Investigation of radiation induced static mode degradation in Yb-Ce co-doped pulsed fiber amplifiers

We report on an experimental investigation of radiation induced static mode degradation in Yb-Ce co-doped pulsed fiber amplifiers. For investigating the impact of irradiation on beam quality, radiation-darkened fibers were irradiated by gamma rays. The laser properties of three types of fibers were investigated. With the pump power increased, static mode degradation appeared: M2 factor raised significantly and the output beam profile transformed from LP01 into LP11 shape without high-frequency component. The threshold of static mode degradation decreased with irradiation dose, and related to Yb-Ce co-doped fiber compositions. The doping of Ce benefits to the improvement of the static mode degradation threshold: in the case of the Ce concentration increasing by 35.5%, SMD threshold rose by 16% at most.

Two octave supercontinuum generation in a non-silica graded-index multimode fiber

The generation of a two-octave supercontinuum from the visible to mid-infrared (700–2800 nm) in a non-silica graded-index multimode fiber is reported. The fiber design is based on a nanostructured core comprised of two types of drawn lead-bismuth-gallate glass rods with different refractive indices. This yields an effective parabolic index profile and ten times increased nonlinearity when compared to silica fibers. Using femtosecond pulse pumping at wavelengths in both normal and anomalous dispersion regimes, a detailed study is carried out into the supercontinuum generating mechanisms and instabilities seeded by periodic self-imaging. Significantly, suitable injection conditions in the high power regime are found to result in the output beam profile showing clear signatures of beam self-cleaning from nonlinear mode mixing. Experimental observations are interpreted using spatio-temporal 3+1D numerical simulations of the generalized nonlinear Schrödinger equation, and simulated spectra are in excellent agreement with experiment over the full two-octave spectral bandwidth. Experimental comparison with the generation of supercontinuum in a silica graded-index multimode fiber shows that the enhanced nonlinear refractive index of the lead-bismuth-gallate fiber yields a spectrum with a significantly larger bandwidth. These results demonstrate a new pathway towards the generation of bright, ultrabroadband light sources in the mid-infrared.

Recent Progress in Nonlinear Frequency Conversion of Optical Vortex Lasers

Optical vortices are optical fields that possess a helical phase and orbital angular momentum, which have found the application in micromanipulation, optical communication, orbital angular momentum entanglement, super-resolution imaging, metrology, etc. The urgent need for the wide spreading applications of vortex lasers is to increase the wavelength versatility. In this study, the nonlinear frequency conversion of vortex lasers with a focus on sum frequency generation stimulated Raman scattering, and optical parametric oscillators were meticulously reviewed. The characteristics of the topological charge transfer and output beam profiles of different frequency conversion were discussed. As the precise tuning of optical fields in both temporal and spatial domains shall be the trend of future studies, it is our hope that this review shall serve as a reference for future research. Combining these techniques with the streaming methods to produce optical vortices, i.e., annular pump, off-axis pump, reflection mirror with defect spots, spherical aberration, and birefringence, it is advisable to expand the wavelength and fill the wavelength gap in the ultraviolet, visible, and infrared bands.

Low Dark Current and High Speed InGaAs Photodiode on CMOS-Compatible Silicon by Heteroepitaxy

Top-illuminated proof-of-concept indium gallium arsenide (InGaAs) photodiode (PD) array and high speed InGaAs PDs were realized on (001) silicon (Si) substrate by direct heteroepitaxy using metal&#x2013;organic chemical vapor deposition. The PDs containing InGaAs active layer lattice-matched to InP were grown on Si substrates employing InP-on-Si template with growth technique including GaAs on V-grooved Si, thermal cycle annealing and strained layer superlattice defect filters. Dry etched mesa structure with polyimide passivation was implemented. 8 &#x00D7; 8 InGaAs PD arrays with mesa diameters of 20, 30 and 40 &#x00B5;m were realized on (001) Si, and their performances were benchmarked with identical devices on a native InP substrate. A low dark current density of 0.45 mA&#x002F;cm² and a responsivity of 0.72 A&#x002F;W at 1550 nm were measured for 40-&#x00B5;m-diameter device on Si. Directly modulation has also been performed showing a 3-dB bandwidth up to 11.2 GHz and open eye diagram up to 25 Gbps. The imaging performance of InGaAs PD arrays on Si were also demonstrated for the first time by capturing the output beam profile of a single mode fiber at 1550 nm.

Spatially, Spectrally Single-Mode and Mechanically Flexible 3D-Printed Terahertz Transmission Waveguides

Emerged terahertz transmission waveguides or fibers will enable novel terahertz systems and applications. High-quality output beam profiles, mechanical flexibility and reliability are among the most crucial and challenging characteristics of terahertz transmission waveguides. Here, we design and fabricate the flexible and stretchable transmission waveguides by 3D printing to guide radiation from terahertz (THz) quantum cascade lasers (QCLs) lasing at the frequency of 2.58 THz. Composite silver nanoparticles and polydimethylsiloxane are coated on the inner surface of the 3D-printed polycarbonate/rubber substrate tube. Output beam profiles from the transmission waveguides, which are captured by a room-temperature terahertz camera, demonstrate single-mode spatial intensity distribution. Transmission spectra are measured out from the waveguides and single-mode characteristics of THz QCLs are preserved from threshold to peak bias. More than 300 times of bending and force-strain curves are tested for the 3D-printed flexible terahertz transmission waveguides, the propagation losses exhibit no obvious change, demonstrating a superior mechanical endurance.