Optical foveated imaging using liquid crystal spatial light modulators has received considerable attention in the recent
years as a potential approach to reducing size and complexity in wide-angle lenses for high-resolution foveated imaging.
In this paper we propose a very compact design for an F/2.8 visible monochromatic foveated optical system covering a
total field-of-view of 80 degrees and capable of achieving a resolution in excess of 100 MPixels. The diffraction
efficiency and image quality of the foveated optical system are estimated. The foveated optical system is compared to
equivalent conventional wide-angle lenses in terms of size, complexity and image quality. Fabrication and assembly
tolerances as well as limitations of the current transmissive LC SLM technology are taken into consideration.
A unique technology for the fabrication of high-quality and robust beam delivery optics for fiber lasers is presented.
CO2 lasers are used to reshape the spherical surface of plano-convex fused-silica rod-lenses, and then fuse the optical
fiber directly to these lenses. A specific fiber collimating system is presented and analyzed in terms of aberrations,
insertion loss, M2, and return loss. Test results are compared to the theoretical modeling, demonstrating the accuracy
and repeatability of this technology.
Optical foveated imaging using liquid crystal (LC) spatial light modulators (SLMs) has received considerable attention in recent years as a potential approach to reducing size and complexity in fast wide-angle lenses. We cover a theoretical study quantifying the diffraction efficiency and image quality of foveated optical systems (FOSs) based on transmissive LC SLMs. A practical design example of a fast wide-angle FOS based on the current transmissive LC SLM technology is proposed.
Foveated imaging addresses the need for compact wide-angle imagers capable of high-resolution and compressed data
transmission. The principle behind foveated imaging is to cover a wide field-of-view (FOV) with a relatively simple and
compact low-resolution lens, and use a liquid crystal spatial light modulator (SLM) to correct wavefront aberrations at
any selected field point. The SLM correction provides a
high-resolution fovea that can be actively moved anywhere
within the FOV. While most research has focused so far mainly on SLM performance, the general trend being to
increase SLM resolution and modulation depth, the actual lens design and system optimization aspects were often
neglected. In this paper, we propose a wide-angle lens design intended for foveated imaging applications, and discuss
typical tradeoffs. Taking this design as an example, we present a method to estimate the smallest SLM resolution
required to correct the wavefront error effectively, showing that with the appropriate design, this resolution can be
reduced up to 10 times compared to current designs. Increasing the SLM resolution beyond this point and increasing the
modulation depth above one wavelength is not necessary, and will actually reduce the performance of the imaging
system. We also demonstrate the importance of fabrication tolerances, and we propose a method to calibrate the SLM in
order to cancel out all additional wavefront aberrations introduced by fabrication and assembly errors.
Aspheric and diffractive surfaces in infrared materials are traditionally fabricated by single point diamond turning,
which is a high-cost, low-throughput process, not suitable for low-cost, high-volume applications. Precision molding of
chalcogenide glasses is a novel process we developed to allow the efficient fabrication of quality infrared optics in large
volumes. In this paper we present the advantages and particularities of designing thermal imaging lenses for
high-volume applications using precision molded chalcogenide glasses. As an example, we present a compact 19 mm
F/1.1 infrared lens design for a 320 × 240 uncooled detector array operating from 8 to 14 microns. The excellent image
quality and transmission of tested prototypes prove that precision molding of chalcogenide glasses is an ideal optical
fabrication technology for the high-volume production of infrared optics.
Shack-Hartmann sensors are widely used to measure wavefront aberrations. We present the fundamental and specific engineering steps in the design of Shack-Hartmann wavefront sensors. Typical performance requirements such as sensor dynamic range, sensitivity and accuracy are defined and discussed. We investigate the trade-offs between these performance metrics and the factors affecting the trade-offs. A first order approach for selecting the optimal parameters of the sensor central piece, the lenslet array, is presented. We also propose a quick tolerance analysis method that can predict the wavefront measurement error due to misalignments, using only the ray-tracing software.
This paper presents the design, analysis, and fabrication of a telecentric f/1.3 thermal imaging lens. The 14.8 mm wideangle
lens provides a 62° diagonal field-of-view, and was designed to operate over the 8-14 μm infrared spectral band. Focus can be manually adjusted from 0.5 m to infinity, maintaining constant image quality over the entire range. A compact air-spaced doublet design limits the overall length to 34 mm and the maximum diameter to 28 mm. Lens materials were chosen to minimize chromatic aberrations, reduce cost, and fit within the molded chalcogenide glass
manufacturing capabilities. Combining a molded aspheric chalcogenide lens with a polished spherical Germanium lens eliminated the need for a diffractive surface to correct chromatic aberrations, and reduced the fabrication cost. Vignetting was purposely introduced at the extreme fields to compensate for the effects of aberrations on the relative illumination variation across the field-of-view. Athermalization of the lens was achieved mechanically over the entire operating temperature range (- 40 to + 80°C).
Numerous optical systems, such as telescopes, adaptive optics systems, and aberrometers, are equipped with wavefront sensors, which often use sampling devices measuring the slope of the wavefront at discrete points across the pupil (e.g. Shack-Hartmann sensors). The accuracy of the sampled output signal is always affected by the fabrication and alignment tolerances of the wavefront sensing optical system. Typically, it is a requirement to express the measurement error in terms of input wavefront, so the optical ray intercept error has to be converted into wavefront measurement error. This conversion cannot be obtained directly from a conventional tolerance analysis because of the wavefront breaking by the sampling device. The tolerancing method proposed in this paper solves the problem of converting conventional merit function degradation into input wavefront measurement error. The proposed method consists of two parts. First, a Monte Carlo tolerance analysis based on a specific merit function is performed, and a 90% border system is selected. Then, an optimization is applied to the 90% border system, by varying a "dummy" phase surface introduced at the entrance pupil of the system. A concrete example is presented.
This paper presents the optical design and experimental demonstration of a compact, foveated, wide field-of-view (FOV) imaging system using two lenses and a liquid crystal spatial light modulator (SLM). The FOV of this simple doublet system is dramatically improved by the SLM, which can be programmed to correct all the geometrical aberrations at any particular field angle. The SLM creates a variation in the image quality across the entire FOV, with a diffraction-limited performance at the field angle of interest (similar to the foveated human vision). The region of interest can be changed dynamically, such that any area within the FOV of the system can be highly resolved within milliseconds. The wide FOV, compactness, and absence of moving parts make this system a good candidate for tracking and surveillance applications. We designed an f/7.7 system, with a 60° full FOV, and a 27 mm effective focal length. Only two lenses and a beam splitter cube were used along with a reflective SLM. The theoretical wavefront aberration coefficients were used to program the SLM, which was placed in the pupil plane of the system. A prototype was built and the system was experimentally demonstrated using monochromatic light and a CCD camera.
Pulses of arbitrary temporal shape can be generated by spectrally filtering a short pulse. Frequency selective reflectors, such as fiber Bragg gratings, can be designed to obtain the desired pulse shape. The required distribution of the refractive index modulation, amplitude and phase, can be calculated using inverse scattering techniques. For weak gratings, under the Born approximation, the impulse response of the grating is directly related to the refractive index distribution. The specified refractive index can be photo-written in an optical fiber using standard phase-mask scanning techniques. Two Bragg gratings were specially designed to shape a train of gaussian pulses into antisymmetric Hermite-Gauss pulses. The first grating had a length of 40 mm producing a spectral response over 0.05 nm (peak-to-peak). This grating was interrogated by 100 ps pulses produced by a CW tunable laser with an external modulator. The second grating (L = 10 mm and ?? = 0.18 nm) was interrogated with a mode-locked fiber laser (7 ps). The pulses were characterized in the frequency and time domain. The antisymmetric pulses were propagated in standard fiber to verify the presence of the phase shift between the two lobes. These Hermite-Gauss pulses could be used to study antisymmetric dispersion-managed soliton pulses.
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