Stability of the solar constant makes the Sun an attractive on-orbit calibration source for radiometers operating at visible and near JR wavelengths. Direct viewing of the Sun provides a radiance or irradiance that is significantly above the dynamic range of most Earth Observing System (EOS) radiometers, thereby requiring attenuated viewing of the Sun. To provide radiometric repeatability, the attenuator used must be stable over time at all in-band wavelengths, uniformly flood the radiometer aperture and field of view, and be invariant over the range of solar viewing angles. The Earth Radiation Budget Experiment (ERBE) radiometers flown in the mid -1980's carried a mirror attenuator mosaic (MAM) to attenuate the solar energy. This device, consisting of specularly reflective, closely packed concave hemispheres with a black mask covering the area between the spheres was successfully used to calibrate the ERBE shortwave (0.3 to 3.5 rim) and total (0.3 to >50 pm) radiometer channels. For CERES, the calibration accuracy requirements have been tightened (±1% shortwave, total channel, end-of-life, la). While the stability and uniformity demonstrated by the ERBE MAM are sufficient for CERES, the variation with solar incidence angle is not. Improvements to the ERBE design have been made for CERES and sample MAM's have been fabricated and tested. The results of this study as well as the features and details of the MAM design are adse 1. BACKGROUND Accurate radiometric measurements of the Earth's albedo and thennal emission are necessaiy for monitoring and predicting the effects of global warming. In the early '80's, the Earth Radiation Budget Experiment (ERBE) instruments were flown to measure the balance between absorbed solar irradiance, and thermal emission by the Earth to space1.2.34. Absolute radiometric calibration of the ERBE sensors was of critical importance because subtle differences in the reflected solar energy and emitted thermal radiance could have a profound impact on predictions for the Earth's average temperature. An extensive ground calibration was performed on each instrument prior to launch and in-flight calibration sources were designed to fly with each instrument. One such in-flight calibration source was a unique solar diffuser designed to attenuate the incident solar radiation by diffuse reflection to a broadband radiance level of approximately 90 W/m2/sr, a level near the middle of the ERBE instrument dynamic range.5 Since variation of the solar constant over time is small and well characterized, the attenuated reflectance of the Sun pmvided a repeatable source for periodically checking the gain of the ERBE shortwave (0.3 to 3.5 pm) and total (0.3 to >50 pm) channel sensors.The Clouds and the Earth's Radiant Energy System (CERES) instrument currently under development is the next advancement beyond the ERBE instruments, designed to fly on several platforms including Tropical Rain Measurement Mission (TRMM) and Earth Observing System (EOS) AM and PM over the next decade.6 Overall radiometric calibra...
Antiguided arrays have been optimized for single-spatial-mode operation to high output power. Diffraction-limited beam operation to cw power levels of 0.5 W have been reproducibly obtained. The high efficiency operation (wall plug efficiency = 20%) and demonstrated reliability make these practical devices, and useful for many applications requiring a high output power spatially coherent beam. Two-dimensional optical waveguide calculations demonstrate the mechanisms responsible for the high mode selectivity of these structures. It is also made clear why coupled-mode theory failed to predict how to obtain single-lobe phase-locked array operation, and what are the conditions to obtain stable, in-phase mode behavior.
The mode control and mode distribution properties of ring resonators employing self-imaging and a 90 °mode rotation are discussed. Such resonator designs are shown to have a very uniform distribution for the dominant mode and superior mode eigenvalue magnitude separation. These properties are very useful for designing resonators with a high quality factor, i.e., those with low magnifications.
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