Small Divergence Angle Research Articles

Investigation on the effect of beam divergence angle upon output waveform based on stimulated Brillouin scattering optical limiting

This paper investigates the effect of beam divergence angle on output waveform based on stimulated Brillouin scattering optical limiting. Output waveforms in the case of different pump divergence angles are numerically simulated, and validated in a Nd:YAG seed-injected laser system. The results indicate that a small pump divergence angle can lead to good interaction between pump and Stokes, and a platform can be easily realized in the transmitted waveform. In contrast, a peak followed by the platform appears when the divergence angle becomes large.

Study of GaN-Based Photonic Crystal Surface-Emitting Lasers (PCSELs) With AlN/GaN Distributed Bragg Reflectors

GaN-based 2-D photonic crystal (PC) surface-emitting lasers (PCSELs) with AlN/GaN distributed Bragg reflectors are fabricated and investigated. A clear threshold characteristic under the optical pumping at room temperature is observed at about 2.7 mJ/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> with PC lattice constant of 234 nm. Above the threshold, only one dominant peak appears at 401.8 nm with a linewidth of 0.16 nm. The laser emission covers whole circular 2-D PC patterns of 50 mu m in diameter with a small divergence angle. The lasing wavelength emitted from 2-D PC lasers with different lattice constants occurs at the calculated band edges, showing different polarization angles due to the light diffracted in specific directions, corresponding exactly to Gamma -, <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">K</i> -, and <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">M</i> -directions in the <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">K</i> space. The PCSEL also shows a spontaneous emission coupling factor beta of 5 times 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-3</sup> and a characteristic temperature of 148 K. Furthermore, the coupled-wave model in 2-D hexagonal lattice is applied to distinguish the discrepancy in threshold power and the corresponding coupling coefficient. The results show that the lasing actions within Gamma, <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">K</i> , and <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">M</i> modes have a substantial relation between the threshold energy density and the coupling coefficient.

Quantum cascade lasers with integrated plasmonic antenna-array collimators

We demonstrated in simulations and experiments that by defining a properly designed two-dimensional metallic aperture-grating structure on the facet of quantum cascade lasers, a small beam divergence angle can be achieved in directions both perpendicular and parallel to the laser waveguide layers (denoted as theta perpendicular and theta parallel, respectively). Beam divergence angles as small as theta perpendicular=2.7 degrees and theta parallel=3.7 degrees have been demonstrated. This is a reduction by a factor of approximately 30 and approximately 10, respectively, compared to those of the original lasers emitting at a wavelength of 8.06 microm. The devices preserve good room temperature performance with output power as high as approximately 55% of that of the original unpatterned lasers. We studied in detail the trade-off between beam divergence and power throughput for the fabricated devices. We demonstrated plasmonic collimation for buried heterostructure lasers and ridge lasers; devices with different waveguide structures but with the same plasmonic collimator design showed similar performance. We also studied a device patterned with a "spider's web" pattern, which gives us insight into the distribution of surface plasmons on the laser facet.

Small divergence edge-emitting semiconductor lasers with two-dimensional plasmonic collimators

Using quantum cascade lasers with a two-dimensional metallic aperture-grating structure defined on the facet the authors demonstrate a collimated laser beam with small divergence angle perpendicular and parallel to the laser waveguide layers (2.7° and 3.7°, respectively). These values represent a reduction by a factor of ∼30 and ∼10, respectively, compared to those of the original 8.06-μm- wavelength laser without plasmonic collimation. The devices preserve good room temperature performance with output power as high as 53% of that of the original unpatterned lasers.

High-power laser diodes emitting light above 1100 nm with a small vertical divergence angle of 13°

Laser diodes with highly strained InGaAs quantum wells, emitting at 1130 nm, embedded in a GaAs waveguide were investigated. This Letter reviews the design of the vertical structure for enclosing high output power in angles smaller than 18 degrees . Example designs were processed to 200 microm stripe-width lasers with an 8-mm-long optical cavity. When these are mounted on C mounts, they give an output power of 38 W under quasi-cw operation from a single emitter.

Numerical study of the Ne-like Cr x-ray laser at 28.6 nm

We investigate the Ne-like Cr x-ray laser at 28.6nm by using a modified 1D lagrangian hydrodynamic code MED103 coupled with an atomic physics data package and a 2D ray tracing code as a post-processor. The laser pumping configuration includes two prepulses and one main pulse. The first prepulse normally irradiates the target, while the second prepulse and the main pulse irradiate the target at grazing-incident angles. We predict that saturation can be achieved for the Ne-like Cr x-ray lasers with a total pumping energy of 125mJ. Good beam qualities with no deflecting angle and a small divergence angle of 5mrad are observed.

Fiber-Optic Vibration Measurement for Low Refractive-Index Media Using Wavefront Splitting Interferometry

A fiber-optic vibration measurement technique based on the wavefront-splitting interferometry is presented in this paper. Due to a small divergence angle of Gaussian laser beam emitting from a single-mode fiber, the sensing beam propagating toward the specimen has an incident angle of nearly 90°, so that the reflection coefficient of specimen surface is relatively high even for low-refractive-index materials. In preliminary tests, overhead-projector transparency was used to demonstrate the vibration measurement with our fiber-optic vibration sensor up to 4 kHz. The Young's modulus of the transparency was also determined by using the vibration method, and the result agreed well with that measured by using a conventional testing machine.

GaN-based two-dimensional surface-emitting photonic crystal lasers with AlN∕GaN distributed Bragg reflector

GaN-based two-dimensional (2D) surface-emitting photonic crystal (PC) lasers with AlN∕GaN distributed Bragg reflectors are fabricated and demonstrated. The lasing threshold energy density is about 3.5mJ∕cm2 per pulse under optical pumping at room temperature. Only one dominant emission wavelength of 424.3nm with a narrow linewidth of 1.1Å above the threshold is observed. The laser emission covers whole circularly 2D PC patterns (50μm in diameter) with a small divergence angle. The lasing wavelength emitted from 2D PC lasers with different lattice constants occurs at the calculated band-edges provided by the PC patterns. The characteristics of large area, small divergence angle, and single mode emission from the GaN-based 2D surface-emitting PC lasers should be promising in high power blue-violet emitter applications.

Radiation pressure acceleration of thin foils with circularly polarized laser pulses

A new regime is described for radiation pressure acceleration of a thin foil by an intense laser beam of above 1020 W cm−2. Highly monoenergetic proton beams extending to giga-electron-volt energies can be produced with very high efficiency using circularly polarized light. The proton beams have a very small divergence angle (<4°). This new method allows the construction of ultra-compact proton and ion accelerators with ultra-short particle bursts.

Effect of Nozzle Angle on Flow Field and Temperature Distribution in a Billet Mould when using Swirl Flow

Recently, interesting effects have been noted in studies of swirl flow, particularly regarding billet moulds when considering a specific divergent angle of the immersion nozzle. Therefore, in the present work a numerical analysis and water model study of the mould region of a continuous casting apparatus are performed with changing the outlet divergent angles of the immersion nozzle using swirling flow in the pouring tube, to control the heat and mass transfer in the continuous casting mould. To make our studies consistent with the previous research, which was done based on a square billet, this time we investigate round billets. The results show that the distance from the meniscus of the centres of both the lower and upper circulation loops decreases systematically with increasing the divergent angle. This, in turn, leads to: (i) a more active heat and mass transport near the meniscus (particularly over 100°); (ii) a gradual change from a concentric circulation to a more clearly logarithmic spiral from the mould wall to the nozzle on the meniscus, which leads to more active heat and mass transfer; (iii) a decreased penetration depth of nozzle outlet flow (even at a comparatively small divergent angle such as 20°) and a superheat dissipation in the melt.

How Far Can Ki‐Energy Reach?—A Hypothetical Mechanism for the Generation and Transmission of Ki‐Energy

‘Ki-energy’, which can be enhanced through the practice of Nishino Breathing Method, was reported to have beneficial health effects. Although Ki-energy can play an important role in complementary and alternative medicine (CAM), as yet it is unknown how Ki-energy is generated, transmitted through air and received by another individual. We previously proposed that Ki-energy may include near-infrared radiation, and that the wavelength was between 800 and 2700 nm. Since Ki-energy is reflected by a mirror, we believe that the ‘Ki-beam’ has a small divergence angle. It can also be guided in a desired direction. The acrylic mirror reflection experiment suggests that the wavelength may be between 800 and 1600 nm. Using a linear variable interference filter, we found that Ki-energy may have a peak around 1000 nm. We have also observed that ‘sensitive’ practitioners responded to Ki sent from a distance of 100 m. All of these results suggest that (i) Ki-energy can be guided as a directional ‘beam’ with a small divergence angle; (ii) the beam can be reflected by a mirror and (iii) Ki-energy may have a specific wavelength. Since these properties are characteristics of the laser radiation, we propose a quantum physics-based mechanism of ‘Light Amplification by the Stimulated Emission of Radiation’ (i.e. LASER) for the generation of Ki-energy. Volunteers responded to Ki even with a blindfold. This suggests that the skin must be detecting Ki-energy. We propose that the detector at the skin level may also have the stimulated emission mechanism, which amplifies the weak incident infrared radiation.

Surface Emitting THz Wave Parametrical Oscillator UsingMgO:LiNbO3

Room-temperature operation terahertz (THz) wave source isdemonstrated using three MgO:LiNbO3 crystals which have anoncollinear arrangement. The experimental results show that the THzwave can be tunable from 0.8 THz to 3.0 THz, and the peak energy outputis 103 pJ/pulse at 1.5 THz. The noncollinear cavity configuration makesthe THz beam have Gaussian-like spatial distribution, small divergenceangle, perpendicularly eradiated from the crystal surface. The beamquality factor M2 is measured to be Mx2 = 1.15,Mz2 = 1.25 for characterizing the THz wave beam. Experiments alsoshow that the THz beam can be focused by using a polyethylene lens, andthe focal spot size is close to the diffraction limit.

OPTICAL PHASE-CONJUGATION PROPERTY OF THREE-PHOTON EXCITED BACKWARD STIMULATED EMISSION

Three-photon pumped cavityless lasing experiments have shown that the forward and backward stimulated emission (cavityless lasing) beams manifest different divergence angles and spatial/temporal fluctuation behaviors. When a dye solution as the gain medium is pumped by a pulsed laser beam with ~ 1500-nm wavelength and ~ 150-fs pulse duration, the backward stimulated emission beam exhibits a smaller divergence angle and much better spatial/temporal stability than the forward stimulated emission beam. These apparently unexpected results can be explained reasonably by assuming that the backward stimulated emission beam exhibits an optical phase-conjugation property, so that influences from a random spatial/temporal change of the refractive-index in the lasing medium can be automatically removed for this backward lasing beam. A physical model of wave-front reconstruction via pump-beam induced holographic grating in an aberrated gain medium is proposed, which is supported by a rigorous theoretical analysis.

Characteristics of compact‐size D lens collimator for micro‐optics devices applications

AbstractA compact‐size D lens collimator was fabricated and characterized. High circular rate 98%, small divergence angle 0.186°, and low return loss 59.8 dB were achieved in experiment to present the excellent performance of D lens collimator. Insertion loss sensitivity due to variation in working distance, operating wavelength, and misalignment tolerance were demonstrated in experiment and verified with theoretical simulation by OSLO. It is worth to note that the compact‐size D lens collimator exhibits low insertion loss, less than 0.3 dB, not only for large working distance range 10–30 mm but also for wide operating wavelength band 1560 ± 50 nm. When compared with gradient index lens collimators, the D lens collimator is potentially suitable for the application in the design of micro‐optics devices. © 2007 Wiley Periodicals, Inc. Microwave Opt Technol Lett 49: 1946–1949, 2007; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.22581

A new positron source with high flux and excellent electron-optical properties

Positron annihilation spectroscopy is a well established research tool to study the surface and bulk electron distributions of solids and liquids. These are extracted from the energy and angular distributions of the two 511keV X-rays, produced during the annihilation of a thermal positron and an electron from the sample. Positron investigations and monitoring, however are currently not used in an industrial environment due to the lack of a sufficiently intense positron sources to record distribution functions with good statistics within minutes. Most positron spectrometers have radioactive sources which produce only modest intensities (106e+/s). An improvement by at least a factor of 100 is needed to become viable for on-line positron metrology. We propose to combine several technologies to generate a positron beam with good electron-optical properties, such as a small divergence angle and small beam diameter and a flux of 108e+/s or more. Positrons from a 10Ci beta source will be moderated with a stack of 12 layers of tungsten meshes. The thermalized positrons will be accelerated into a deflection focusing analyzer (DFA) which focuses the positrons into a small (1–2mm2) area of a second moderator. A rare gas solid will be used to thermalize the positrons once more. The moderating area forms the small emitter source for a accelerating gun that generate a beam of mono-energetic positrons of any desired energy.

Vertical beaming of wavelength-scale photonic crystal resonators

We report that $> 80%$ of the photons generated inside a photonic crystal slab resonator can be funneled within a small divergence angle of $\pm 30^\circ$. The far-field radiation properties of a photonic crystal slab resonant mode are modified by tuning the cavity geometry and by placing a reflector below the cavity. The former method directly shapes the near-field distribution so as to achieve directional and linearly-polarized far-field patterns. The latter modification takes advantage of the interference effect between the original waves and the reflected waves to enhance the energy-directionality. We find that, regardless of the slab thickness, the optimum distance between the slab and the reflector closely equals one wavelength of the resonance under consideration. We have also discussed an efficient far-field simulation algorithm based on the finite-difference time-domain method and the near- to far-field transformation.

Single-mode blue-violet laser diodes with low beam divergence and high COD level

We demonstrate GaN-based high-power single transverse-mode laser diodes (LDs) emitting at 405 nm. LD structures are designed to exhibit a high level of catastrophic optical damage and small beam divergence angle. By the control of refractive index profiles, we achieved a vertical beam divergence angle of as low as 17.5/spl deg/ and maximum output power of as high as 470 mW under continuous-wave operation condition. In addition, nearly fundamental transverse-mode operation is demonstrated up to 500-mW pulsed output power by far-field investigation.

Processing of Diamond for Integrated Optic Devices Using Q-Switched Nd:YAG Laser at Different Wavelengths

In the present investigation, a Q-switched Nd:YAG laser is used to study the various aspects of diamond processing for fabricating integrated optic and UV optoelectronic devices. Diamond is a better choice of substrate compared to silicon and gallium arsenide for the fabrication of waveguides to perform operations such as modulation, switching, multiplexing, and filtering, particularly in the ultraviolet spectrum. The experimental setup of the present investigation consists of two Q-Switched Nd:YAG lasers capable of operating at wavelengths of 1064 nm and 532 nm. The diamond cutting is performed using these two wavelengths by making the “V”-shaped groove with various opening angle. The variation of material loss of diamond during cutting is noted for the two wavelengths. The cut surface morphology and elemental and structural analysis of graphite formed during processing in both cases are compared using scanning electron microscopy (SEM) and laser Raman spectroscopy. Both the Q-Switched Nd:YAG laser systems (at 1064 nm and 532 nm) show very good performance in terms of peak-to-peak output stability, minimal spot diameter, smaller divergence angle, higher peak power in Q-switched mode, and good fundamental TEM 00 mode quality for processing natural diamond stones. Less material loss and minimal micro cracks are achieved with wavelength 532 nm whereas a better diamond cut surface is achieved with processing at 1064 nm with minimum roughness.

Biexciton lasing of submicron-sized ZnO particle in a Fabry-Perot cavity

We have observed the lasing of submicron-sized ZnO particle in a Fabry-Perot cavity at low temperatures. A thin alkali-halide crystal including a small amount of ZnO particles are sandwiched between two quartz plates arranged as a pair of opposite mirrors, that is, a Fabry-Perot cavity. Under low excitation due to a He–Cd laser, emissions of free and bound excitons and their phonon replicas are recognized markedly, while a biexciton emission band appears under high-dense excitation by a pulsed N2 laser. Biexciton lasing is demonstrated as a narrowed emission by focusing the excitation light beam on an appropriate position being a ZnO particle in the cavity. When a cavity length satisfies the resonant condition, the lasing occurs in the spectral region of the biexciton emission band and with the very small divergence angle. The obtained threshold of excitation-power density is about 0.50MW∕cm2. We discuss lasing in terms of stimulated emission caused by population inversion between biexciton and exciton states.

Loss cone boundary measurement using diagnostic neutral beam and neutral particle analyzer in a compact helical system

A horizontally scannable diagnostic neutral beam (DNB) has been installed on the compact helical system (CHS) in order to study the confinement of energetic ions with different pitch angles by varying the injection angle. The DNB has been designed to provide energetic ions as a test particle source with (1) no heating to background plasma and (2) small divergence angle. A charge-exchange neutral particle analyzer (NPA) to measure energetic ions injected by the DNB is also scannable and varies the observation angle on the equatorial plane in CHS. A combination of horizontally scannable DNB and NPA provides information on whether the energetic ions with different pitch angle are confined in the plasma or immediately lost. The experimental results are consistent with the prediction of single particle full orbit calculation in CHS.