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Blaaberg S.,Norsk Elektro Optikk | Mork J.,Technical University of Denmark
IEEE Journal of Quantum Electronics | Year: 2014

A detailed analysis of the noise spectrum of a semiconductor optical amplifier excited by an amplitude-modulated input signal is presented. This extends the well-established theory for the case of continuous-wave input signals and is relevant for various applications within optical signal processing. One important example is the analysis of noise in microwave photonic elements based on slow-light propagation in semiconductor optical amplifiers. Expressions for the noise spectra are derived and the dependence on important operation parameters, such as input power, modulation depth, and modulation frequency, is investigated and explained. We find several interesting modifications to the spectra compared with the continuous-wave case. In particular, the side-bands present in the input-signal lead via four-wave mixing effects to additional structure in the spectra as well as additional noise components. © 2014 IEEE. Source

Hoye G.,Norwegian Defence Research Establishment FFI | Fridman A.,Norsk Elektro Optikk
Optics Express | Year: 2013

Current high-resolution push-broom hyperspectral cameras introduce keystone errors to the captured data. Efforts to correct these errors in hardware severely limit the optical design, in particular with respect to light throughput and spatial resolution, while at the same time the residual keystone often remains large. The mixel camera solves this problem by combining a hardware component - an array of light mixing chambers - with a mathematical method that restores the hyperspectral data to its keystone-free form, based on the data that was recorded onto the sensor with large keystone. A Virtual Camera software, that was developed specifically for this purpose, was used to compare the performance of the mixel camera to traditional cameras that correct keystone in hardware. The mixel camera can collect at least four times more light than most current high-resolution hyperspectral cameras, and simulations have shown that the mixel camera will be photon-noise limited - even in bright light - with a significantly improved signal-to-noise ratio compared to traditional cameras. A prototype has been built and is being tested. © 2013 Optical Society of America. Source

Fridman A.,Norsk Elektro Optikk | Hoye G.,Norsk Elektro Optikk | Loke T.,Norsk Elektro Optikk
Proceedings of SPIE - The International Society for Optical Engineering | Year: 2013

Current high-resolution hyperspectral cameras attempt to correct misregistration errors in hardware. Usually, it is required that aberrations in the optical system must be controlled with precision 0.1 pixel or smaller. This severely limits other specifications of the hyperspectral camera, such as spatial resolution and light gathering capacity, and often requires very tight tolerances. If resampling is used to correct keystone in software instead of in hardware, then these stringent requirements could be lifted. Preliminary designs show that a resampling camera should be able to resolve at least 3000-5000 pixels, while at the same time collecting up to four times more light than the majority of current high spatial resolution cameras that correct keystone in hardware (HW corrected cameras). A Virtual Camera software, specifically developed for this purpose, was used to compare the performance of resampling cameras and HW corrected cameras. For the cameras where a large keystone is corrected by resampling, different resampling methods are investigated. Different criteria are suggested for quantifying performance, and the tested cameras are compared according to these criteria. The simulations showed that the performance of a resampling camera is comparable to that of a HW corrected camera with 0.1 pixel residual keystone, and that the use of a more advanced resampling method than the commonly used linear interpolation - such as for instance high-resolution cubic splines - is highly beneficial for the data quality of the resampled image. Our findings suggest that if high-resolution sensors are available, it would be better to use resampling instead of trying to correct keystone in hardware. © 2013 SPIE. Source

Fridman A.,Norsk Elektro Optikk | Hoye G.,Norsk Elektro Optikk
Optics Express | Year: 2015

Spectral data acquired with traditional push-broom hyperspectral cameras may be significantly distorted due to spatial misregistration such as keystone. The mixel camera is a new type of push-broom hyperspectral camera, where an image recorded with arbitrary (even large) keystone is reconstructed to a nearly keystone-free image. The key component of the mixel camera is an array of light mixing chambers in the slit plane, and the precision of the image reconstruction depends on the light mixing properties of these chambers. In this work we describe how these properties were measured in a mixel camera prototype. We also investigate the potential performance of the mixel camera in terms of spatial co-registration, based on the measured response of the mixing chambers to a point source. The results suggest that, with the current chambers, a perfectly characterized mixel camera should have residual spatial misregistration that is equivalent to 0.02-0.03 pixels keystone. This compares favorably to high resolution instruments where keystone is corrected in hardware or by resampling. © 2015 Optical Society of America. Source

Hoye G.,Norsk Elektro Optikk | Loke T.,Norsk Elektro Optikk | Fridman A.,Norsk Elektro Optikk
Optical Engineering | Year: 2015

We propose a method for measuring and quantifying image quality in push-broom hyperspectral cameras in terms of spatial misregistration caused by keystone and variations in the point spread function (PSF) across spectral channels, and image sharpness. The method is suitable for both traditional push-broom hyperspectral cameras where keystone is corrected in hardware and cameras where keystone is corrected in postprocessing, such as resampling and mixel cameras. We show how the measured camera performance can be presented graphically in an intuitive and easy to understand way, comprising both image sharpness and spatial misregistration in the same figure. For the misregistration, we suggest that both the mean standard deviation and the maximum value for each pixel are shown. We also suggest how the method could be expanded to quantify spectral misregistration caused by the smile effect and corresponding PSF variations. Finally, we have measured the performance of two HySpex SWIR 384 cameras using the suggested method. The method appears well suited for assessing camera quality and for comparing the performance of different hyperspectral imagers and could become the future standard for how to measure and quantify the image quality of push-broom hyperspectral cameras. © 2014 The Authors. Source

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