Output Radiance Research Articles

Reduction of speckle effect using galvanometer scanners in a laser-based uniform source for radiance responsivity measurement

Narrow linewidth lasers play an important role in high-spectral and high-precision radiometric calibration. In this paper, a standard uniform light source for radiometric calibration of imaging detectors and devices is designed and realized based on narrow linewidth laser and intergrating sphere. Because the speckle effect caused by laser has a great influence on the radiance transfer, galvanometer scanners were employed to reduce the speckle effect. CCD method was used to measure the nonuniformity of the middle row/column of the integrating sphere exit, and nonuniformity reduced from 2.6 % to 0.72 % when the galvanometer scanners were turned on. By superimposing 9 different regions of static speckle image, the non-uniformity is greatly reduced, which is consistent with the experimental results. The output radiance stability of the source is measured with an improvement from 8.4 × 10-5 to 2.7 × 10-5. Finally, the nonuniformity of the source is measured to be 0.15 % by a spot luminance meter.

Wavelength beam combining by spectrally selective polarization transformation

Efficient wavelength beam combining is demonstrated using spectrally selective polarization transformation in carefully designed stacks of birefringent plates combined with polarization-selective beam splitters. This beam-combining scheme is shown to enable a high-throughput power and intensity scaling of blue-diode-laser arrays. The wavelengths of individual diode lasers in the laser array are shifted with respect to each other by a few nanometers, by fine temperature adjustment, for the highest overall output radiance, limited by etendue conservation.

Channel density and efficiency optimization of spectral beam combining systems based on volume Bragg gratings in sequential and multiplexed arrangements.

Spectral-beam-combining (SBC) systems utilizing multiple volume Bragg gratings must be carefully analyzed to maximize channel density and efficiency, and thus output radiance. This analysis grows increasingly difficult as the number of channels in the system increases, and heuristic optimization techniques are useful tools for exploring the limits of these systems. We explore three classes of multigrating SBC systems: cascaded, where each grating adds a new channel to the system in sequence; sandwiched, where several individual gratings are placed together and all channels enter the system at the same facet; and multiplexed, where all of the gratings occupy the same holographic optical element (HOE). Loss mechanisms differ among these three basic classes, and our optimization algorithm shows that the highest channel density for a given minimum efficiency and fixed operating bandwidth is achieved for a cascaded grating system. The multiplexed grating system exhibits the lowest channel density under the same constraints but has the distinct advantage of being realized by a single HOE. For a particular application, one must weigh channel density and efficiency versus system complexity when choosing among these basic classes of SBC systems. Additionally, one may need to consider the effects of finite-width input beams. As input beam radius is reduced, angular clipping effects begin to dominate over spectral interference and crosstalk effects, limiting all three classes of SBC systems in a similar manner.

Global sensitivity analysis of the spectral radiance of a soil–vegetation system

We analyzed the sensitivity of Top-Of-Atmosphere (TOA) radiance and surface reflectance of a soil–vegetation system to input biophysical and biochemical parameters using the coupled Soil–Leaf-Canopy radiative transfer model SLC and MODTRAN. We applied variance-based global sensitivity analysis for different atmospheric conditions and observation configurations. Among 23 input parameters, crown coverage, leaf area index, leaf inclination distribution function and soil moisture were found to be the most influential parameters driving the output variance of the radiance between 400 and 2500nm with a few exceptions. Hapke's soil parameters and the canopy layer dissociation factor were recognized to have marginal influence on the output radiance. It is also found that a large portion of uncertainty in the output radiance is driven by the interaction effects among input parameters in the visible (~550nm), whereas the red-near infrared (~670nm), seems to have fewer interaction effects. The effect of solar/view direction is found to be significant on TOA radiance sensitivity to the input parameters. The results also confirmed that the sensitivities of surface reflectance are comparable to the TOA radiance sensitivities when the atmosphere is clear and visibility is high. Since coupled surface-atmosphere RT models can be computationally intensive, this work also introduces an improvement to the design and sampling of screening methods for efficient sensitivity analysis of computationally expensive models. The improvement is based on three elements: a) generating sample points by Sobol's sequence generator; b) variational analysis in the parameter space using the winding stairs method; c) use of mean and variance sensitivity measures. The results with 1200 model runs demonstrated high correlation (92%) with variance-based global sensitivity analysis using 49,152 model runs, in determining the most influential and non-influential parameters.

Planar Waveguide Laser Optimization and Characterization Employing Real-Time Beam Quality Measurement

We present a methodology for optimizing the output radiance and characterizing a planar waveguide laser utilizing a technique offering “real-time” beam quality measurement. This approach is applicable to lasers in general employing the simultaneous measurement and comparison of a re-imaged cavity-mode beam-waist (near-field) and the power distribution in the far-field (specifically in the Fourier plane of a lens). This approach provides the laser's beam parameter product, and hence M2 beam propagation factor. By exploiting this technique we optimized the performance of a quasi-four-level Nd:YAG planar waveguide laser with an extended stable-cavity configuration, whilst characterizing the in-plane thermal lens. This laser produced a record radiance level of >; 9 TWm-2/sr-1 for this 946-nm transition, with an effective thermal lens focal length >; 400 mm at all incident pump powers up to the maximum level of 87 W.

Theoretical modeling of generation of hat-top intensity profile from Gaussian beam by means of a two-stage beam shaper

Both theoretical and experimental results of an optical beam shaping system are investigated in this report. The described system is a two-stage beam shaping device including a fiber-bundle and a prism-duct. A source light is used to illuminate the fiber-bundle and the image of output beam is captured by a CCD camera. The fiber-bundle output beam shape shows a linear arrangement of circular spots lights, which are placed in a rectangular cross section of about 21.37mm×2.44mm. In another study, the photograph picture of the prism output beam is taken by a digital camera. The prism output beam cross section is a square shape with a dimension of about 4×4mm2. According to the experimental results, the prism duct converted a Gaussian beam profile with multiple-peak distribution to a hat-top beam profile with the uniform intensity distribution. For theoretical investigations, using ZEMAX software a simulation is performed to analyze the beam shaping design. By proper modeling the output beam shape and radiance profiles in position space and angle space of the fiber-bundle and the prism duct are investigated. Theoretical radiance profiles are obtained by using simulated images and results are in agreement with the experimental results.

Simulation and Design on a New Uniform Radiation Source for Calibration of Satellite Remote Sensors

High-precision calibration of space sensors is a complex task. For calibration, integrating sphere system is the main equipment to produce large area uniform radiation source. Such integrating spheres are usually very huge with large diameters, generally more than one meter. Furthermore, such a large integrating sphere costs very high. In order to reduce the size of integrating sphere and cost, the necessary pre-simulation is very necessary. In this paper, a new semi-integrating sphere with 1300mm diameter was designed by using of ray-tracing algorithm on different special structures to improve the design efficiency. The results showed that, 1. The output of semi-integrating sphere system is more sensitive to its inner light source location and the aperture. 2. The practical measurement of semi-integrating sphere is approaching to the simulation findings. Although the opening diameter of semi-integrating sphere reaches to 50% of the diameter of semi-integrating sphere, the output radiance uniformity is still larger than 95% and with better performance.

Simulation and Design on a New Uniform Radiation Source for Calibration of Satellite Remote Sensors

High-precision calibration of space sensors is a complex task. For calibration, integrating sphere system is the main equipment to produce large area uniform radiation source. Such integrating spheres are usually very huge with large diameters, generally more than one meter. Furthermore, such a large integrating sphere costs very high. In order to reduce the size of integrating sphere and cost, the necessary pre-simulation is very necessary. In this paper, a new semi-integrating sphere with 1300mm diameter was designed by using of ray-tracing algorithm on different special structures to improve the design efficiency. The results showed that, 1. The output of semi-integrating sphere system is more sensitive to its inner light source location and the aperture. 2. The practical measurement of semi-integrating sphere is approaching to the simulation findings. Although the opening diameter of semi-integrating sphere reaches to 50% of the diameter of semi-integrating sphere, the output radiance uniformity is still larger than 95% and with better performance.

Flat-top beam profile generated using a fiber-bundle prism-coupled beam shaper

In this paper, an optical beam shaping system is theoretically and experimentally investigated. The optical system design software ZEMAX is used to simulate and analyze the reported beam shaping design. By using this software ray tracing diagrams are presented with the aim of studying the direct beam propagation, total reflection rays, and the lost rays. The prism duct output beam shape and radiance profiles in both position space and angle space are also studied. For experimental investigation, a two-stage beam shaping design including a fiber-bundle and a prism duct is used. A source light is used for the fiber-bundle illumination and the photograph image of the output beam is taken by a digital camera. The fiber-bundle output beam cross section is a rectangular shape with a dimension of 25.65 mm width and 2.44 mm height. In another experiment, the prism output beam is captured by a CCD camera. The prism output beam shape depends on the prism exit face, which is a rectangle (4.15 mm × 3.55 mm) for this case. The image date of the prism output beam is converted to a response curve, which is approximately a flat-top profile. The experimental image profiles are compared with the simulated image profiles and there is a good agreement between the observed results.

Lambertian radiance and transmission of an integrating sphere

The importance of Lambertian transmission, rather than total transmission, is argued and expressions for both are given. Exact expressions for output radiance are given in terms of both total sphere area and port area. A formula for choosing sphere size to give the maximum Lambertian transmission is developed. Port fractions in the range of 0.1-0.2 are recommended.

Multimode-diode-pumped gas (alkali-vapor) laser

We report what we believe to be the first demonstration of a multimode-diode-pumped gas laser: Rb vapor operating on the 795 nm D1 resonance transition. Peak output of approximately 1 W was obtained using a volume-Bragg-grating stabilized pump diode array. The laser's output radiance exceeded the pump radiance by a factor greater than 2000. Power scaling (by pumping with larger diode arrays) is therefore possible.

LED-based spectrally tunable source for radiometric, photometric, and colorimetric applications

A spectrally tunable light source using a large number of LEDs and an integrating sphere has been designed and is being constructed at the National Institute of Standards and Technology. The source is designed to have a capability of producing any spectral distribution, mimicking various light sources in the visible region by feedback control of individual LEDs. The output spectral irradiance or radiance of the source will be calibrated by a reference instrument, and the source will be used as a spectroradiometric as well as a photometric and colorimetric standard. A series of simulations have been conducted to predict the performance of the designed tunable source when used for calibration of display colorimeters. The results indicate that the errors can be reduced by an order of magnitude when the tunable source is used to calibrate the colorimeters, compared with measurement errors when the colorimeters are calibrated against Illuminant A. The source can also approximate various CIE daylight illuminants and common lamp spectral distributions for other photometric and colorimetric applications.

Integral design method for nonimaging concentrators

We present an integral design method for maximizing concentration onto a given absorber shape. This technique uses a variable-acceptance edge-ray principle to transform nonuniform input and output radiance distributions. It easily recovers familiar designs that transform uniform radiance distributions. The method is simpler to use and more general than previous nonimaging design techniques, such as string methods and flow-line approaches. We show how this technique is adapted to satisfy diverse boundary conditions, such as satisfying total internal reflection or design within a material of graded index. Presented are an analytic solution to the classic θ1–θ2 concentrator and a novel two-stage, two-dimensional solar collector with a fixed circular primary mirror and nonimaging secondary. This newly developed secondary provides a 25% improvement in concentration over conventional nonimaging secondaries.

Gallium arsenide laser-array-on-silicon package

A monolithic array of AlGaAs lasers has been packaged together with an array of fiber lightguides on a substrate of silicon. The components have been optimized for maximum lightguide output radiance consistent with reliable cw laser operation. Coupling efficiencies up to 80% have been achieved between laser and lightguide. Output powers up to 70 mW cw have been observed from a 50-microm core diameter lightguide of 0.15 numerical aperture. Eight-device array multispot packages have been fabricated with 10 mW/spot, limited by laser quality and thermoelectric cooler capacity. Fabrication tolerances and device electrical and optical crosstalk are discussed.