Output Beam Research Articles

Design of a compact off-axis two-mirror freeform optical antenna for shaping and transmitting an elliptical beam emitted by laser diode.

Elliptical Gaussian beams generated by laser diodes (LDs) often exhibit asymmetrical divergence angle distribution, which limits their practical applications. In this study, we propose what we believe is a novel approach to shape and collimate the elliptical output beam from a LD. The design process involves the construction of two freeform reflective surfaces on a reference circle using a three-dimensional point-by-point iterative method, based on the law of conservation of energy, the vector reflection theory, and Fermat's principle. The output beam's maximum divergence angle is effectively compressed to 3.1579mrad. The design is compact with a folded optical path and antenna size of 368.8c m 3. This paper presents a comprehensive design and optimization process, along with an in-depth analysis of the system's performance, thereby offering novel insights for emerging optical design practitioners.

Realization of collimated specific profiles in rotation-symmetrical beam shaping system with divergent light source

A simple numerical method is proposed for the design of two aspherical surfaces, each comprising multiple segmented refractive planes, for generating a collimated beam with a specific irradiance profile in a beam shaping system with a divergent light source. However, in real-world manufacturing, this performance improvement is obtained at the expense of a greater cost and complexity. Accordingly, a second algorithm is proposed which maximizes the number of rays passing through the central regions of the refractive planes in the second aspherical surface and hence minimizes the total number of segments required to achieve the same beam shaping performance. The feasibility of the proposed method is demonstrated through the design of two aspherical lenses for generating collimated output beams with ring- and triangle-like irradiance profiles, respectively. The experimental results show that the beam profiles are in close agreement with the desired irradiance distributions. In general, the results indicate that the proposed method provides a versatile and efficient approach for achieving the desired collimated profile in beam forming systems with a divergent light source.

Picosecond Pulse Tapered Fiber Amplifier Operated near 1030 nm with Peak Power up to 1 MW

We demonstrated an optimization of a picosecond fiber amplifier based on Yb-doped tapered fiber in a spectral range of 1030 nm. Nonlinear effects limiting peak power scaling (stimulated Raman scattering and four-wave mixing) were studied and factors affecting their threshold were established, such as gain, diameter profile along the length of taper, output mode field diameter, and numerical aperture of a pump. By determining the optimal amplification regime and manufacturing advanced tapered fibers, we amplified 13 ps pulses to a record-high peak power of 1 MW at a wavelength of 1029 nm directly at the output of the fiber at an average power of 13.8 W. Four-wave mixing was the limiting factor, and the total fraction of deleterious components in the output spectrum was ~2%. The quality of the output beam was close to being diffraction limited (M2 < 1.2).

4.6 kW linearly polarized and narrow-linewidth monolithic fiber amplifier based on a fiber oscillator laser seed

In this work, a record output power of 4.6 kW linearly polarized and narrow-linewidth fiber amplifier based on an optimized fiber oscillator laser (FOL) seed was realized by employing a homemade polarization-maintaining Yb-doped fiber (PMYDF), corresponding to a slope efficiency of 79.5% and a 3 dB linewidth of 0.3452 nm. Through an effective strategy relying on decreasing the transmission fiber length from 200 m to 120 m and adding a chirped and tilted fiber Bragg grating (CTFBG), the stimulated Raman scattering (SRS) effects were well-suppressed. By applying the forward combiner with the interconnection between the pump arms into the MOPA system, the MI threshold is increased by more than 560 W and the slope efficiency of the upgraded MOPA system is boosted by 5%. During the experimental process of power amplification, the polarization extinction ratio (PER) remains higher than 15 dB, and a near-diffraction-limited output beam at the laser power of 2980 W was measured with the M2x = 1.314 and M2y = 1.311.

Advanced LD pumped 3.3 J/1 Hz nanosecond Nd:glass preamplifier for SG-II upgrade laser facility.

We demonstrate a laser-diode-pumped multipass Nd:glass laser amplifier with a range of advanced characteristics. The amplifier exhibits high extraction efficiency, enables arbitrary shaping of spatial beam intensity, and effectively suppresses frequency modulation to amplitude modulation conversion. Our approach achieves excellent beam quality via thermal lensing and thermal depolarization compensation. When a 1.82 mJ/5 ns laser pulse was injected into the amplifier, the output energy reached up to 3.3 J with a repetition rate of 1 Hz at a central wavelength of 1053.3 nm. The near-field modulation of the amplified output beam was below 1.2, and the far-field focusing ability of the beam was 90% at 2.9 times the diffraction limit. This laser amplifier system holds potential for integration as a preamplifier within the SG-II upgrade high power laser facility.

Performance comparison of conventional and striation-based beamformers for underwater bearing detection of pulse sources.

The multi-path and dispersion properties of shallow water waveguides make conventional beamforming (CBF) face issues such as beam shift, broadening, splitting, output distortion, and array gain reduction. In this paper, the striation-based beamforming (SBF) is investigated to address these issues. SBF differs from CBF by utilizing frequency-shift processing along interference striations. The performance difference between CBF and SBF is compared. It demonstrates that under ideal waveguide modeling with pulse sources, SBF can achieve a beam output response that is close to the plane wave condition. The speed term of SBF's response is approximately independent of modal indexes, which equips SBF to form a unique beam output and guarantee the beam resolution. The processing of consistent signals along the striation maintains the optimal signal correlation, which makes SBF ensure the output fidelity and array gain. To shift the mainlobe of SBF to the source azimuth, the time delay related to the waveform truncation point can be introduced to pre-compensate the array signals. There exist two theoretical accuracy limits to using the truncation. First, truncation time corresponds to the waveform point at r0/c (r0 is the source range), and the mainlobe of SBF is directed to the source azimuth. Second, truncation corresponds to the pulse peak point, and the azimuth estimation accuracy of SBF gets close to CBF. Simulations and experimental results are given as illustrations.

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.

Modeling of hydrocarbon-free potassium flowing-gas diode-pumped amplifier

А 3D CFD model coupled with wave optics model for laser beam propagation is used to calculate the performance of a hydrocarbon-free K end pumped flowing-gas diode pumped alkali amplifier (DPAA). The K DPAA efficiency and output beam quality were calculated as a function of various parameters and the results were compared with those calculated for end pumped Rb DPAA with hydrocarbon-free buffer gas. For moderate pump intensity, both the efficiency and beam quality deteriorate at low gas velocity. The efficiency strongly depends on the pressure of the buffer helium, reaching a maximum at pressure close to atmospheric and falling with a further increase in pressure. The maximum efficiency of the K DPAA (∼0.45) is approximately 4 times higher than that of the Rb one, achieved at the same optimal pump power of 5 kW. Addition of CH4 to the He buffer in the K DPAA decreases the amplifier efficiency and reduces the beam quality.

Binary coding metasurface for broadband and flexible generation of acoustic vortex beams

Considerable efforts have recently focused on sound vortices imprinted with orbital angular momentum (OAM) yet whose generation generally relies on sophisticated phase modulation, whether through traditional phased arrays or emerging metamaterial methods. Here, we propose and numerically demonstrate a mechanism for broadband generation of acoustic vortices in a simple, flexible, and high-efficiency way through binary-phase-based chirality modulation enabled by building a binary coding metasurface. The metasurface with a theoretically derived phase profile that is implemented with two types of meta-structures is capable of twisting the incident plane wave into a vortex beam with a desired order in a broad band and at the same time enables steering the vortices' propagation direction freewheelingly. The effectiveness of our proposed mechanism is verified by numerically demonstrating the broadband generation of vortex beams carrying different OAMs through a monolayered binary coding metasurface. We further demonstrate the generality and flexibility of our mechanism for generating the multiplexed vortex beams as well as modulating the propagation direction of the output beam by judiciously designing the 1-bit coding sequences of the metasurface. We anticipate our design with capability and simplicity to have far-reaching implications in OAM-enabled applications ranging from high-capacity acoustic communication to contactless particle manipulation.

Generation of cylindrical vector beam pulses by using multi-wavelength EDFL with two-mode fiber filter

A multi-wavelength erbium-doped fiber laser (MW-EDFL) operating in the L-band with cylindrical vector beam outputs is proposed and experimentally investigated. The multi-wavelength lasing is realized by exploiting a two-mode fiber filter (TMFF) as the comb filter and a single-mode fiber spool with 2-km in length to suppress the mode competition. The multi-wavelength combs with channel spacing of 0.36 nm, 0.58 nm, and 1.12 nm are achieved by regulating the interference length of the TMFF, and up to 18 lasing channels are obtained with a wavelength spacing of 0.36 nm. Good long-term stability is manifested within a 100-minute duration, as the maximum peak power fluctuation of 1.96 dB and the maximum wavelength shift of 0.03 nm are measured. By adjusting the polarization controller associated with the two-mode long-period grating as a high-efficiency mode converter, radially and azimuthally polarized cylindrical vector beams of high purity are thus generated.

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.

Quality Control Analysis of Linear Accelerator Radiotherapy Device in the Department of Radiotherapy at Pasar Minggu District Hospital

Radiation accidents pose a significant risk, often stemming from inadequate calibration of radiation output, procedural lapses, and faults in radiation monitoring systems. Addressing these risks is pivotal, particularly in radiotherapy, where the precise functioning of linear accelerator (Linac) devices is crucial. This study delves into the quality control procedures applied to the Linac radiotherapy device within the Department of Radiotherapy at Pasar Minggu District Hospital. Drawing upon the renowned reference standards set by the AAPM Task Group 142 and BAPETEN Regulation No. K2N.2/MT-08, this research, conducted in November 2022, rigorously evaluated the mechanical, dosimetry, and safety aspects of the Linac device. The examination encompassed critical elements, including gantry angles, optical distance indicators, laser precision, collimator angles, light field size, and photon and electron beam outputs. Safety features such as door interlocks, audiovisual indicators, and radiation alert systems were scrutinized. The analysis reveals a reassuring finding: the Linac device at Pasar Minggu District Hospital adheres commendably to tolerance limits specified by reference standards across all measured parameters, indicating robust performance in mechanics, dosimetry, and safety. This meticulous quality control regimen has proven highly effective in ensuring the device's operational integrity and safety, affirming its reliability for precise radiotherapy treatments.

Prediction on X-ray output of free electron laser based on artificial neural networks

Knowledge of x-ray free electron lasers’ (XFELs) pulse characteristics delivered to a sample is crucial for ensuring high-quality x-rays for scientific experiments. XFELs’ self-amplified spontaneous emission process causes spatial and spectral variations in x-ray pulses entering a sample, which leads to measurement uncertainties for experiments relying on multiple XFEL pulses. Accurate in-situ measurements of x-ray wavefront and energy spectrum incident upon a sample poses challenges. Here we address this by developing a virtual diagnostics framework using an artificial neural network (ANN) to predict x-ray photon beam properties from electron beam properties. We recorded XFEL electron parameters while adjusting the accelerator’s configurations and measured the resulting x-ray wavefront and energy spectrum shot-to-shot. Training the ANN with this data enables effective prediction of single-shot or average x-ray beam output based on XFEL undulator and electron parameters. This demonstrates the potential of utilizing ANNs for virtual diagnostics linking XFEL electron and photon beam properties.

Quartz-Enhanced Photoacoustic Sensor Based on a Multi-Laser Source for In-Sequence Detection of NO2, SO2, and NH3.

In this work, we report on the implementation of a multi-quantum cascade laser (QCL) module as an innovative light source for quartz-enhanced photoacoustic spectroscopy (QEPAS) sensing. The source is composed of three different QCLs coupled with a dichroitic beam combiner module that provides an overlapping collimated beam output for all three QCLs. The 3λ-QCL QEPAS sensor was tested for detection of NO2, SO2, and NH3 in sequence in a laboratory environment. Sensitivities of 19.99 mV/ppm, 19.39 mV/ppm, and 73.99 mV/ppm were reached for NO2, SO2, and NH3 gas detection, respectively, with ultimate detection limits of 9 ppb, 9.3 ppb, and 2.4 ppb for these three gases, respectively, at an integration time of 100 ms. The detection limits were well below the values of typical natural abundance of NO2, SO2, and NH3 in air.

Development of the heavy ion RFQ for CAFE2

A radio frequency quadrupole (RFQ) accelerator which accelerates particles with a mass-to-charge ratio up to 3 to over 1.33 MeV/u was developed. This RFQ was designed based on the existing 162.5 MHz solid-state power sources of the Chinese Accelerator Driven System (C-ADS) injector II. The frequency is relatively high to a heavy ion RFQ which means a weak transverse focusing strength and brings challenges to the beam loss and quality control. The average radius r0 of the RFQ was appropriately chosen to make a compromise between the transverse focusing strength and the maximum surface field. The beam dynamics parameters were carefully optimized to improve the quality of the RFQ output beam so that the beam could be well transported in the following superconducting structure. Beam dynamics simulations and an error study have been performed to evaluate the RFQ and a method which is more suitable for the analyses of small acceptance RFQs has been adopted. And the results of the beam commissioning have been proved to agree well with the simulations.

Non-imaging metasurface design for collimated beam shaping.

Non-imaging optical lenses can shape the light intensity from incoherent sources to a desired target intensity profile, which is important for applications in lighting, solar light concentration, and optical beam shaping. Their surface curvatures are designed to ensure optimal transfer of energy from the light source to the target. The performance of such lenses is directly linked to their asymmetric freeform surface curvature, which is challenging to manufacture. Metasurfaces can mimic any surface curvature without additional fabrication difficulty by imparting a spatially-dependent phase delay using optical antennas. As a result, metasurfaces are uniquely suited to realize non-imaging optics, but non-imaging design principles have not yet been established for metasurfaces. Here, we take an important step in connecting non-imaging optics and metasurface optics, by presenting a phase-design method for beam shaping based on the concept of optimal transport. We establish a theoretical framework that enables a collimated beam to be redistributed by a metasurface to a desired output intensity profile. The optimal transport formulation leads to metasurface phase profiles that transmit all energy from the incident beam to the output beam, resulting in an efficient beam shaping process. Through a variety of examples, we show that our approach accommodates a diverse range of different input and output intensity profiles. Last but not least, a full field simulation of a metasurface has been done to verify our phase-design framework.

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.

Ultra‐Low Threshold Single‐Mode Upconversion Lasing in a Strong Coupled Microcavity via Four‐Photon Absorption

AbstractWith the development of ultra‐fast optical technology, the nonlinear interaction between light and matter has attracted great attention. Upconversion laser generation through multiphoton absorption (MPA) enables numerous critical applications such as quantum signal processing, biophotonics, and 3D microfabrication. Herein, an ultra‐low threshold single‐mode four‐photon absorption (4PA) upconversion lasing at room‐temperature (RT) by constructing a strong‐coupling microcavity is demonstrated. The planar microcavity significantly enhances the 4PA nonlinear interaction, which facilitates the obtainment of low‐threshold upconversion lasing with an excellent output divergence angle and beam quality. In addition, the temporal dynamic mechanism and polarization characteristics of single‐mode upconversion lasing are also comprehensively discussed. The work provides a feasible approach to fabricate a low‐threshold single‐mode upconversion laser through high‐order nonlinear MPA processes.

Three-dimensional printing of a beam expander to enable the combination of hundred-micron optical elements and a single-mode fiber.

We use a flexible two-photon photopolymerization direct laser writing to fabricate an integrated diffractive lens system on a fiber tip to expand the output beam of the fiber. The results show that the micro-integrated beam expander based on double lenses (axial size of about 100 μm) has a magnification of 5.9 and a loss of 0.062 dB. Subsequently, we demonstrate the fabrication of a spiral phase plate (diffractive optical elements) and micro-lens arrays (refractive optical elements) on an integrated beam expander, and their optical properties are measured and analyzed, respectively. This Letter is an exploration of the future integrated micro-optical systems on an optical fiber tip.

Using harmonic beam combining to generate pulse-burst in nonlinear optical laser

The ultrashort lasers working in pulse-burst mode reveal great machining performance in recent years. The number of pulses in bursts effects greatly on the removal rate and roughness. To generate a more equal amplitude of pulses in burst with linear polarization output and time gap adjustable, we propose a new method by the harmonic beam combining (HBC). The beam combining is commonly used in adding pulses into the output beam while maintaining the pulse waveform and beam quality. In the HBC, dichroic mirrors are used to combine laser pulses of fundamental wave (FW) into harmonic wave (HW), and nonlinear crystals are used to convert the FW into HW. Therefore, HBC can add arbitrarily more HW pulses to generate pulse-burst in linear polarization with simple structure. The amplitude of each pulse in bursts can be adjusted the same to increase the stability of the burst, the time gap of each pulse can be adjusted precisely by proper time delay. Because HBC adds pulses sequentially, the peak power density of the burst is the same as each pulse, pulses can be combined without concern of back-conversion which often occurs in high peak power density. In the demonstration, the extendibility of HBC was verified by combining two beams with a third beam. The combined efficiency rates were larger than 99%, and the beam quality of each beam was maintained at M 2 ≈ 1.4.