Near-field Beam Research Articles

Diode-pumped medium-aperture-size square Nd,Y:CaF2 rod amplifier for Inertial Confinement Fusion laser drivers

We demonstrate, for the first time, an 12 mm × 12 mm 0.5%Nd,5% Y:CaF2 crystal rod having a uniformly-distributed fluorescence spectrum and capable of operating as an amplifying medium at high repetition frequencies. A small gain of 2.7 is experimentally achieved at repetition frequency of 10 Hz for a pump center wavelength of 802 nm, power and absorption efficiency, 61.2 kW and 63.7%, respectively. Spatial-uniformity degradation of the output near-field beam distribution is observed, which should be attributed to the inhomogeneity of Nd,Y:CaF2 crystal. For a pump power of 61.2 kW, the stored energy of Nd,Y:CaF2 amplifier is 3.73 J. When the input energy is 50 mJ, the output laser energy is 1.4 J of extraction efficiency up to 37.53% after four-pass amplification.

Measurement of the Phase and Intensity Profile of Surface Plasmon Laser Emission

We study the near- and far-field radiation patterns of surface plasmon (SP) lasers in metal hole arrays and observe radially polarized vortex-vector laser beams in both near and far field. Besides the intensity profile, also the complementary phase profile is obtained with a beam block experiment, where we block part of the beam in the near field, measure the resulting changes in the far field, and retrieve the phase using an iterative algorithm. This phase profile provides valuable information on the feedback mechanisms and coherence of the laser and shows that our SP laser operates in a phase-slip mode instead of a pure dark mode. To explain our observations, we extend the standard model for distributed feedback lasers by introducing a position dependence in the optical gain and refractive index.

Reconfigurable Near-Field Beam Pattern Measurement System From 0.03 to 1.6 THz

With the discovery and utilization of great observation sites, on-ground radio astronomical observations have been extended from the millimeter-wave (mm-wave) range to around 1.6 THz. The radiation collected by the telescope reflector antenna is coupled to the receiver by means of the receiver optics. These optics must be designed to match the in-coming fields from the telescope and properly characterized by amplitude and phase measurements. This paper presents a reconfigurable near-field beam pattern measurement system which can characterize magnitude and phase patterns of antennas and optics from the mm-wave to the THz region. Measurements at 900 GHz and 1.37 THz are presented for two different configurations, using different phase-lock components, and measured under cryogenic and room-temperature conditions, respectively.

Spatial beam shaping for high-power frequency tripling lasers based on a liquid crystal spatial light modulator

We propose and demonstrate a spatial beam shaping method to achieve high-quality near-field for a high-power frequency tripling laser system by using a liquid crystal spatial light modulator (SLM). Considering the nonlinear relationship between the output 3ω intensity and the input 1ω intensity of the frequency conversion system and the transmittance nonuniformity of the whole laser system, we introduce an efficient spatial beam shaping method that improves the output near-field beam quality of frequency tripling laser dramatically. Results show that the near-field peak-to-mean value of the frequency tripling laser improves from 1.83:1 to 1.42:1 after spatial beam shaping within four shots. This method provides effective guidance for spatial beam shaping of high-power frequency tripling laser systems.

Shannon entropy method of small-scale self-focusing assessment in high-power laser systems.

Through analysis of near-field beam profiles, we propose a method using Shannon entropy to assess the development of small-scale self-focusing during laser propagation and amplification in high-power laser systems. In this method, the entropy curve that corresponds to increasing B integral displays an evident turning point at which small-scale self-focusing starts to rapidly develop. In contrast to classical methods using contrast, modulation, or power spectral density, the proposed method provides the B integral criterion more clearly and objectively. This approach is an optimization method that can be utilized in the design and operation of high-power laser systems.

高功率激光系统中缺陷引起的近场调制分析

In high power laser device, near-field beam quality is evaluated through the intensity distribution. However, the measured distribution is a steady state, which can not reflect the evolution features of intensity modulation caused by noise disturbances. Some area encountering serious intensity modulation may be ignored in transmission process. Near-field transmission characteristics of the beam disturbed by noise disturbance are studied. Meanwhile, deeper understanding of the limitations of the measured near-field intensity distribution is obtained. To simplify the analysis of the near-field intensity distribution at different distance under the influence of single local defect, the characteristics of near-field intensity modulation is represented by equivalent Fresnel number. The results show that once a weak modulated information appears in the distribution of measured nearfield intensity, there may be a more serious modulation area before the measurement position. A highest modulation can be up to nine times for a phase type defects with pi delay corresponding to incident intensity.

Near-Field Directive Beams From Passive and Active Asymmetric Optical Nanoantennas

Core-shell, plasmonic-based nanoantennas have been shown to be resonant, resulting in extremely large enhancements of their scattered power. Nevertheless, they primarily radiate as electric dipoles due to their subwavelength sizes. We report how, through the introduction of asymmetry into these nanoantennas, it is possible to achieve highly directive optical beams in their near fields. In particular, we perforate the outer metallic coating of a core-shell nanoparticle and add gain material into its core region. It is demonstrated that this active holey-coated nanoparticle configuration effectively captures the incident field, amplifies it, and then channels this power into an intense near-field beam. The performance characteristics of these nanobeamers are compared to the nanojet behaviors experimentally realized with multimoded, electrically large dielectric particles. The intensities of the near-field power flow generated by these highly subwavelength nanobeamers are shown to be significantly larger than those produced by their multimode, micron-sized nanojet counterparts. Because of these enhanced field localization properties, a sensor array example is considered to demonstrate the efficacy of augmenting its detector elements with these nanobeamers.

A sidelobe suppressing near-field beamforming approach for ultrasound array imaging.

A method is proposed to suppress sidelobe level for near-field beamforming in ultrasound array imaging. An optimization problem is established, and the second-order cone algorithm is used to solve the problem to obtain the weight vector based on the near-field response vector of a transducer array. The weight vector calculation results show that the proposed method can be used to suppress the sidelobe level of the near-field beam pattern of a transducer array. Ultrasound images following the application of weight vector to the array of a wire phantom are obtained by simulation with the Field II program, and the images of a wire phantom and anechoic sphere phantom are obtained experimentally with a 64-element 26 MHz linear phased array. The experimental and simulation results agree well and show that the proposed method can achieve a much lower sidelobe level than the conventional delay and sum beamforming method. The wire phantom image is demonstrated to focus much better and the contrast of the anechoic sphere phantom image improved by applying the proposed beamforming method.

Improvement of beam quality in unstable resonators by a conical output coupler

A novel conical output coupler for positive-branch unstable resonators has been proposed to improve the output beam quality. With the application of an inner cone annular scraper and an outer cone reflector, a realization of the output beam from unstable resonator with solid core is introduced. The feasibility of this conical output coupler with solid beam is examined numerically and experimentally with the help of a commercial fast transverse flow (FTF) CO2 laser. The characteristics of the output coupling beam in near field and far field are discussed. Under the definition of 86.5 % power content, the M pc 2 factors of original annular beam and improved solid beam are 5.22 and 1.16 in simulation. In experiment, the M pc 2 factor of the original annular beam from the commercial FTF CO2 laser is more than 10, while the M pc 2 factor of the improved solid beam is 3.82. Both simulation and experimental results show that the beam quality can be improved a lot by introducing this output coupling configuration.

High-quality near-field beam achieved in a high-power laser based on SLM adaptive beam-shaping system.

We demonstrate a high-power laser system with a high-quality near-field beam by using a liquid-crystal spatial light modulator (SLM). An efficient spatial beam shaping algorithm is discussed which can improve the output nearfield beam quality effectively. Both small-signal and large-signal amplification situation of the laser are considered in the beam shaping algorithm. The experimental results show that the near field fluence modulation of output is improved from 1.99:1 to 1.26:1 by using the liquid-crystal SLM. Obvious uniform spatial fluence distribution and near-field beam quality improvement are observed.

Near-Field Beamforming Source Localization for Gear Transmission in a Semi-Anechoic Room

In this study, we discussed the sound sources localization and characterization for a spur gear transmission, which deals with a vibro-acoustic surveillance for predictive maintenance. The technique uses is based on the near-field Beam forming using Bartlett and Capon estimators. The measurements done on the gear transmission with a rectangular phased array of microphones were performed in acoustics and vibration laboratory (LVA) of the INSA of Lyon, in a semi-anechoic room. The gear transmission was considered sometimes open and sometimes closed. The results are presented as an image showing the acoustic field distribution radiated by the gear transmission at the most energetic frequencies. Both estimators have satisfactory results, but even better for the method with high resolution capon.

Thermal lens evolution and compensation in a high power KGW Raman laser

The transient thermal lens in a high-average power double metal tungstate Raman laser has been investigated. An external cavity potassium gadolinium tungstate (KGW) laser designed for second-Stokes output was burst-pumped with up to 46 W of average power at a pulse repetition rate of 38 kHz. At low duty-cycle, the laser generated up to 18 W of on-time average Raman power with a conversion efficiency of 40%. At high duty cycle, efficiency is reduced and the near-field beam profile expands in the X1' crystal direction over a period of tens of milliseconds. The evolution of the spatial beam properties occurs in response to the development of a highly astigmatic thermal lens with fast-axis susceptibility of approximately -1.7 m-1 per watt of Raman output power. We show that the likely cause for astigmatism is primarily photo-elastic in origin. Beam circularization was achieved by incorporating an intracavity convex cylindrical lens.

Fabrication and characterization of disordered polymer optical fibers for transverse Anderson localization of light.

We develop and characterize a disordered polymer optical fiber that uses transverse Anderson localization as a novel waveguiding mechanism. The developed polymer optical fiber is composed of 80,000 strands of poly (methyl methacrylate) (PMMA) and polystyrene (PS) that are randomly mixed and drawn into a square cross section optical fiber with a side width of 250 μm. Initially, each strand is 200 μm in diameter and 8-inches long. During the mixing process of the original fiber strands, the fibers cross over each other; however, a large draw ratio guarantees that the refractive index profile is invariant along the length of the fiber for several tens of centimeters. The large refractive index difference of 0.1 between the disordered sites results in a small localized beam radius that is comparable to the beam radius of conventional optical fibers. The input light is launched from a standard single mode optical fiber using the butt-coupling method and the near-field output beam from the disordered fiber is imaged using a 40X objective and a CCD camera. The output beam diameter agrees well with the expected results from the numerical simulations. The disordered optical fiber presented in this work is the first device-level implementation of 2D Anderson localization, and can potentially be used for image transport and short-haul optical communication systems.

Near-field beam focusing by a single bare subwavelength metal slit with the high-index transmission space

We theoretically demonstrate that a single bare subwavelength metal slit without any surrounding corrugations can have a capability to steer the incident light into focusing patterns by introducing a high index in the transmission half-space. The focusing properties are identified to depend on both the slit width and the output permittivity. The underlying physics lies in the interference of quasi-cylindrical waves scattered from the slit, and our proposed model agrees well with the simulation results. This finding is believed to inspire some novel ideas for the nano-optics design.

Analysis of the chromaticity of near-field binary beam shapers

The chromatic properties of binary beam shapers inducing a spatially dependent transmission on the near field of an optical source after far-field filtering and imaging are derived. Beam shapers using pixels with two transmission states are highly versatile and have been experimentally realized with distributions of amplitude pixels, phase pixels, or polarization pixels that can be designed using a half-toning algorithm. The amplitude of the shaped beam in an image plane has a precisely controlled continuous profile given by the local density of the two pixel types in the object plane. The wavelength dependence of the field transmission corresponding to the full range of design transmission between 0 and 1 is studied analytically to assess the general performance of the corresponding beam shapers for broadband sources, with an example of a specific 2D transmission profile relevant to laser engineering. Amplitude beam shapers have no significant chromaticity, but have low damage threshold. The high-damage-threshold twisted nematic liquid crystal and phase shapers induce a wavelength-dependent transmission and phase on the shaped beam.

Near-field shaping with two binary diffractive optical elements in tandem

Diffractive optical elements (DOEs) are commonly used for beam shaping in the far field. In the present work, we develop an algorithm for near-field beam shaping using a double DOE system with binary modulation, so that it can be easily implemented on a glass substrate. In a first stage, the first amplitude DOE is equal to the target intensity distribution and the second phase DOE is obtained using a simple and fast iterative technique. In order to improve the optical efficiency of the system, we also analyze the behavior of the algorithm when the first DOE is not equal to the target image, but a widened version of it. Several numerical simulations are obtained, and an analysis of tolerances is performed.

Microscopic optoelectronic defectoscopy of solar cells

Scanning probe microscopes are powerful tool for micro- or nanoscale diagnostics of defects in crystalline silicon solar cells. Solar cell is a large p-n junction semiconductor device. Its quality is strongly damaged by the presence of defects. If the cell works under low reverse-biased voltage, defects emit a light in visible range. The suggested method combines three different measurements: electric noise measurement, local topography and near-field optical beam induced current and thus provides more complex information. To prove its feasibility, we have selected one defect (truncated pyramid) in the sample, which emitted light under low reverse-biased voltage.

Dynamics and pulse-package oscillations in broad-area semiconductor lasers with short optical feedback

Pulse-package oscillations in broad-area semiconductor lasers subjected to short optical feedback are experimentally observed. The pulse-package oscillation consists of a frequency component that corresponds to an external optical feedback loop with an envelop of periodic low-frequency fluctuations. However, the periodicity induced by optical feedback does not always improve the time-averaged beam profiles of the laser oscillations. We also investigated time-averaged near-field beam profiles in relation with the optical feedback dynamics.

Assessment of far and near-field beam patterns

Assessment of far and near-field beam patterns

Improvement of near-field beam quality for high-power laser pulses by a two-photon-absorption organic compound

A compound was prepared by introducing a dicyanomethylene group in a benzylidene cyclopentanone dye. The two-photon absorption properties and optical-limiting behavior of the compound in N,N-dimethylformamide (DMF) were investigated. An ultrashort laser source with 130-fs pulse width and 1064-nm wavelength was used. The compound exhibits obvious optical-limiting behavior and two-photon absorption is the predominant mechanism causing the observed behavior. The compound in DMF was applied to improve the near-field beam quality for the pump laser system of an ultra-intense and ultrashort laser facility. The results show that the near field of the pump laser beam becomes more homogeneous after passing through the compound solution and high beam quality is achieved.