In solar applications, traditional crystalline silicon photovoltaic (PV) cells are the most commonly used technology to harvest solar energy. The efficiency of Si PV is fundamentally limited to around 33% and in practice, these cells have an outdoor efficiency of less than 22%. Concentrated PV technology uses multi-junction PV cells that collect a broader spectrum of the sun with high efficiency (>40% has been reported). However, due to the different semiconductors used, multi-junction cell costs are higher than traditional PV cells. Increasing the solar concentration not only reduces the cost of electricity produced by multi-junction cells, by reducing the required area, but can also maximize the IV efficiency of the cells. There exist different methods to concentrate solar energy such as large parabolic mirrors, which have tracking challenges due their size and weight; or spherical lens arrays, which have limited optical geometrical concentration ratios. In this respect, freeform optical devices can be used to enhance the optical throughput for multi-junction cells and reduce the space required to achieve large concentration ratios. In this work, we discuss a novel optical design combining aspherical lens arrays and arrays of optical waveguides, which constitute broadband, freeform non-imaging optical devices. We compare different waveguide designs which have been optimized using non-sequential ray tracing software. The relationship between the optical surface quality and the optical efficiency is also investigated. Finally, we present the results of the experimental characterization of these waveguides under laboratory conditions using different techniques to measure optical throughput and stray light losses.
Holography can offer unique solutions to the specific problems faced by automotive optical systems. Frequently, when possibilities have been exhausted using refractive and refractive designs, diffraction can come to the rescue by opening a new dimension to explore. Holographic optical elements (HOEs), for example, are thin film optics that can advantageously replace lenses, prisms, or mirrors. Head up display (HUD) and LIDAR for autonomous vehicles are two of the systems where our group have used HOEs to provide original answers to the limitations of classical optic. With HUD, HOEs address the problems of the limited field of view, and small eye box usually found in projection systems. Our approach is to recycle the light multiple times inside a waveguide so the combiner can be as large as the entire windshield. In this system, a hologram is used to inject a small image at one end of a waveguide, and another hologram is used to extract the image several times, providing an expanded eye box. In the case of LIDAR systems, non-mechanical beam scanning based on diffractive spatial light modulator (SLM), are only able to achieve an angular range of few degrees. We used multiplexed volume holograms (VH) to amplify the initial diffraction angle from the SLM to achieve up to 4π steradian coverage in a compact form factor.
We present a technique to record refreshable holographic stereograms continuously. We eliminated the translation stage that shifts the recording beams back and forth and replaced it with an uninterrupted transparent belt holding holographic lenses. The belt is driven along a perimeter, shifting the lens laterally in front of a photorefractive screen without reversing direction. The holographic lenses focus the object beam onto holographic pixels and are permanently recorded in a thin photopolymer. The photopolymer material is flexible enough for the lenses to follow the curvature of the belt when it goes around the tensioning rollers. The hogel data are uploaded sequentially onto a spatial light modulator to form the object beam. The rotation of the belt in one single direction allows for a continuous operation and a much faster recording speed than with a translation stage that needs to reverse direction at the end of its travel span.
We measured the diffraction efficiency response of two photorefractive polymer devices according to the duration of the single laser pulse used to record the hologram. The pulse duration was varied from 6 nanoseconds to 1 second, while the pulse energy density was maintained constant at 30 mJ/cm<sup>2</sup>. This changed the peak power from 5 ×10<sup>9</sup> mW to 30 mW. We observed a strong reciprocity failure of the efficiency according to the pulse duration, with a reduction as large as a factor 35 between 1 second and 30 <i>μs</i> pulse duration. At even lower pulse duration (< 30 <i>μs</i>), the efficiency leveled out and remained constant down to the nanosecond exposure time. The same behavior was observed for samples composed of the same material but with and without buffer layers deposited on the electrodes, and different voltages applied during the holographic recording. We explained these experimental results based on the charge transport mechanism involved in the photorefractive process. The plateau is attributed to the single excitation of the charge carriers by short pulses (<i>τp</i> < 30 <i>μs</i>). The increase of efficiency for longer pulse duration (<i>τp</i> > 30 <i>μs</i>) is explained by multiple excitations of the charge carriers that allows longer distance to be traveled from the excitation sites. This longer separation distance between the carriers increases the amplitude of the space-charge field, and improves the index modulation. The understanding of the response of the diffraction efficiency according to the pulse duration is particularly important for the optimization of photorefractive materials to be used at high refresh rate such as in videorate 3D display.
Photorefractive (PR) polymers change their index of refraction upon illumination through a series of electronic phenomena that makes these materials one of the most complex organic systems known. The refractive index change is dynamic and fully reversible, making PR materials very interesting for a large variety of applications such as holography and 3D display. In order to improve the recording speed and achieve videorate for our stereographic display application, we have introduced a new type of electrode geometry where the anode and cathode are in the same plane and are shaped as interpenetrating combs. This type of electrode geometry does not require the sample to be tilted with respect to the writing beams to record the hologram, which is a significant advantage. To monitor the highly non-homogeneous field resulting from this configuration, we used a multiphoton microscope to directly observe the chromophore orientation in situ upon the application of an electric field. Most recently, we developed a fast repetition rate laser (10kHz) where the pulse width can be adjusted from microseconds to milliseconds so that, in conjunction with a ns Q-switched Nd:YAG laser and an externally chopped CW laser, the diffraction efficiency of the material could be measured over 9 orders of magnitude. This measurement helps us better understand the mechanism of grating buildup inside photorefractive polymers.
Presented here is a 32 × 32 optical switch for telecommunications applications capable of reconfiguring at speeds of up to 12 microseconds. The free space switching mechanism in this interconnect is a digital micromirror device (DMD) consisting of a 2D array of 10.8μm mirrors optimized for implementation at 1.55μm. Hinged along one axis, each micromirror is capable of accessing one of two positions in binary fashion. In general reflection based applications this corresponds to the ability to manifest only two display states with each mirror, but by employing this binary state system to display a set of binary amplitude holograms, we are able to access hundreds of distinct locations in space. We previously demonstrated a 7 × 7 switch employing this technology, providing a proof of concept device validating our initial design principles but exhibiting high insertion and wavelength dependent losses. The current system employs 1920 × 1080 DMD, allowing us to increase the number of accessible ports to 32 × 32. Adjustments in imaging, coupling component design and wavelength control were also made in order to improve the overall loss of the switch. This optical switch performs in a bit-rate and protocol independent manner, enabling its use across various network fabrics and data rates. Additionally, by employing a diffractive switching mechanism, we are able to implement a variety of ancillary features such as dynamic beam pick-off for monitoring purposes, beam division for multicasting applications and in situ attenuation control.
We present here the use the DMD as a diffraction-based optical switch, where Fourier diffraction patterns are used to steer the incoming beams to any output configuration. We have implemented a single-mode fiber coupled N X N switch and demonstrated its ability to operate over the entire telecommunication C-band centered at
1550 nm. The all-optical switch was built primarily with off-the-shelf components and a Texas Instruments
DLP7000™with an array of 1024 X 768 micromirrors. This DMD is capable of switching 100 times faster than currently available technology (3D MOEMS). The switch is robust to typical failure modes, protocol and bit-rate agnostic, and permits full reconfigurable optical add drop multiplexing (ROADM).
The switch demonstrator was inserted into a networking testbed for the majority of the measurements. The testbed assembled under the Center for Integrated Access Networks (ClAN), a National Science Foundation (NSF) Engineering Research Center (ERC), provided an environment in which to simulate and test the data routing functionality of the switch. A Fujitsu Flashwave 9500 PS was used to provide the data signal, which was sent through the switch and received by a second Flashwave node. We successfully transmitted an HD video stream through a switched channel without any measurable data loss.
Digital micromirror devices (DMDs) by their high-switching speed, stability, and repeatability are promising devices for fast, reconfigurable telecommunication switches. However, their binary mirror orientation is an issue for conventional redirection of a large number of incoming ports to a similarly large number of output fibers, like with analog micro-opto electro-mechanical systems. We are presenting here the use of the DMD as a diffraction-based optical switch, where Fourier diffraction patterns are used to steer the incoming beams to any output configuration. Fourier diffraction patterns are computer-generated holograms that structure the incoming light into any shape in the output plane. This way, the light from any fiber can be redirected to any position in the output plane. The incoming light can also be split to any positions in the output plane. This technique has the potential to make an “any-to-any,” true nonblocking, optical switch with high-port count, solving some of the problems of the present technology.
The complexity of photorefractive polymers arises from multiple contributions to the photo-induced index grating. Analysis of the time dynamics of the two-beam coupling signal is used to extract information about the charge species responsible for the grating formation. It has been shown in a commonly used photorefractive polymer at moderate applied electric fields, the primary charge carriers (holes) establish an initial grating which, however, are followed by a subsequent competing grating (electrons) that decreases the two-beam coupling efficiency. We show by upon using higher applied bias fields, gain enhancement can be achieved by eliminating the electron grating contribution and returning to hole gratings only.
Digital micro-mirror devices (DMD) by their high switching speed, stability, and repeatability are a promising devices for fast, reconfigurable telecommunication switches. However, their binary mirror orientation is an issue for conventional redirection of a large number of incoming ports to a similarly large number of output fibers like with analog MEMS.
We are presenting here the use the DMD as a diffraction based optical switch, where Fourier diffraction patterns are used to steer the incoming beams to any output configuration. Fourier diffraction patterns are computer generated holograms that structures the incoming light into any shape in the output plane. This way, the light from any fiber can be redirected to any position in the output plane. The incoming light can also be split to any positions in the output plane. This technique has the potential to make an "any to any", true non-blocking, optical switch with high port count, solving some the problems of the present technology.
Photorefractive composites derived from conducting polymers offer the advantage of dynamically recording holograms
without the need for processing of any kind. Thus, they are the material of choice for many cutting edge applications,
such as updatable three-dimensional (3D) displays and 3D telepresence. Using photorefractive polymers, 3D images or
holograms can be seen with the unassisted eye and are very similar to how humans see the actual environment
surrounding them. Absence of a large-area and dynamically updatable holographic recording medium has prevented
realization of the concept. The development of a novel nonlinear optical chromophore doped photoconductive polymer
composite as the recording medium for a refreshable holographic display is discussed. Further improvements in the
polymer composites could bring applications in telemedicine, advertising, updatable 3D maps and entertainment.
The electron transporting molecule tris(8-hydroxyquinoline) aluminum (Alq<sub>3</sub>) was introduced into a photorefractive
composite in a low density to study the effects of electron traps on the performance. Compared to a control sample, Alq<sub>3</sub>
samples exhibited higher dielectric strength, over-modulation at reduced voltage, and increased writing speed. Transient
measurements indicated grating revelation via decay of a competing grating. The dynamics are consistent with a bipolar
charge transport model. Overall, Alq<sub>3</sub> improves the sensitivity, trapping, and breakdown voltage without significant
losses in absorption or phase stability.
The very first demonstration of our refreshable holographic display based on photorefractive polymer was published in
Nature early 2008<sup>1</sup>. Based on the unique properties of a new organic photorefractive material and the holographic
stereography technique, this display addressed a gap between large static holograms printed in permanent media
(photopolymers) and small real time holographic systems like the MIT holovideo. Applications range from medical
imaging to refreshable maps and advertisement. Here we are presenting several technical solutions for improving the
performance parameters of the initial display from an optical point of view. Full color holograms can be generated
thanks to angular multiplexing, the recording time can be reduced from minutes to seconds with a pulsed laser, and full
parallax hologram can be recorded in a reasonable time thanks to parallel writing. We also discuss the future of such a
display and the possibility of video rate.
Two-beam coupling (TBC) in a photorefractive polymer using transmission and reflection geometries is
investigated. With drift (due to an applied electric field) and diffusion, a linearized analysis suggests a phase shift
between the intensity grating and the induced refractive index grating different from the ideal value of 90 degrees,
which is supported by experimental results using a transmission grating geometry. In a self-pumped reflection
grating geometry, which is also experimentally studied, the phase shift can be closer to 90 degrees due to a shorter
grating period. Absorption and absorption gratings during TBC is also experimentally investigated.
We summarize the performances measured at room temperature and in cryogenic conditions of a set of NIR Volume
Phase Holographic Gratings (VPHGs) which can then be used in astronomical instrumentations. VPHGs are novel
optical components which can replace standard transmission gratings. Instead of a surface modulation a diffraction index
modulation printed in a volume of material generates the diffraction according to the required specifications. Results on
transmission and wavefront deformation are presented and compared in the two temperature regimes. These results were
achieved along the run of the Joint Research Action 6 of OPTICON FP6 programme whose participating institutions are
Osservatorio Astronomico di Brera (INAF), Instituto de Astrofísica de Canarias, Centre Spatial de Liege, Politecnico di
Milano and European Southern Observatory.
We describe the redesign and upgrade of the versatile fiber-fed Bench Spectrograph on the WIYN 3.5m telescope. The
spectrograph is fed by either the Hydra multi-object positioner or integral-field units (IFUs) at two other ports, and can
be configured with an adjustable camera-collimator angle to use low-order and echelle gratings. The upgrade, including
a new collimator, charge-coupled device (CCD) and modern controller, and volume-phase holographic gratings
(VPHG), has high performance-to-cost ratio by combining new technology with a system reconfiguration that optimizes
throughput while utilizing as much of the existing instrument as possible. A faster, all-refractive collimator enhances
throughput by 60%, nearly eliminates the slit-function due to vignetting, and improves image quality to maintain
instrumental resolution. Two VPH gratings deliver twice the diffraction efficiency of existing surface-relief gratings: A
740 l/mm grating (float-glass and post-polished) used in 1st and 2nd-order, and a large 3300 l/mm grating (spectral
resolution comparable to the R2 echelle). The combination of collimator, high-quantum efficiency (QE) CCD, and VPH
gratings yields throughput gain-factors of up to 3.5.
Volume phase holographic gratings (VPHGs) possess unique properties that make them attractive for numerous applications. After reviewing major VPHG characteristics through theory, we discuss some aspects of the dichromated gelatin recording material and the holographic recording process. The large-scale VPHG research facility set up at the Center Spatial de Liège enables production of VPHGs up to 380 mm in diameter, with fringe frequencies from 315 to 3300 lp/mm. We describe the work that has been undertaken in our laboratory to remove the last limitations inherent in VPHGs.
To increase the size of the volume phase holographic gratings the Centre Spatial de Liege can produce, mosaic technic has been tested and characterized. This method consists of assembling VPH gratings recorded and processed independently into one larger grating. By this way, the final grating size becomes virtually unlimited and dispersive elements can accommodate the largest telescope beams.
The second research line about VPH gratings was the high line frequency domain: ν > 3000 lp/mm. Actually, for these frequencies, diffraction according to TE and TM modes is maximum for different wavelengths. However, it is possible to tune the index modulation to three times what is usually required to use the first diffraction TE peak. In this case, the second TE maximum matches the
first TM maximum and unpolarized light is so entirely diffracted. This article also summarizes our prospects in the field of very high index modulation gratings where Δ<i>n</i> as high as 0.14 has been reached; cryogenic temperature operation for which we have demonstrated our VPH gratings stand -180°C without any Blaze modification; and wavefront correction by post-polishing to minimize diffracted beam aberrations. With this latter technique, λ/6 wavefront over 10 cm diameter has been obtained in the first trial.
The recent interest of the astronomer community for volume phase holographic gratings is directly related to the enhancement of spectrograph throughput since this kind of grating can rise higher diffraction efficiency. Indeed, dichromated gelatine technology has demonstrated capability for 70-90% efficiency. From the heritage of several diffractive and holographic projects and applications, the Centre Spatial de Liege has recently decided to invest in the large-scale DCG grating technology. This paper will present the new facility which is now fully operational, its capability and first results obtained.
The recent interest of the astronomer community for volume phase holographic gratings is directly related to the enhancement of spectrograph throughput since the grating can rise higher diffraction efficiency. Indeed, dichromated gelatin technology has demonstrated capability for 70-90% efficiency. From the heritage of several diffractive and holographic projects and applications, the Centre Spatial de Liege has recently decided to invest in the large-scale DCG grating technology. This paper will present the new facility presently under construction. The goal is to be ready to respond to the market demand in 2002 with a capacity for producing 30 cm dia. holographic gratings. The challenge is not the size itself but the quality control in each process step. Thanks to the heritage of space instrumentation, CSL is trained to fulfill requirements on product and quality control. Large clean rooms are equipped with DCG coating machine, optical bench, development lab, and conditioning processes. The grating period may range from 325 to 3000 lp/mm. Low frequencies are especially hard to holographically record because it induces a cumbersome set-up. The working wavelength of DCG gratings is limited by the gelatin transmissivity (from 350 nm to 2 micrometers ). But the actual limitation factor in the IR is the refractive index modulation, equivalent to etching depth on ruled gratings: working wavelength of 1.5 micrometers means a need for 3 times the modulation of a visible grating. Large efforts are needed to insure that IR volume-phase gratings can reach efficiency higher than alternative grating technologies. In that field, this paper presents experimental results on small grating samples. A realistic performance goal is discussed to advise the astronomer community of our near-future products.
We have used theoretical models to give an account of the photoinduced reorientation of the azo-dye molecules in polymers and the related macroscopic effects that are diffraction efficiency and photoinduced birefringence. Measurements have been carried out for three doped polymers presenting different behaviors according to the writing intensity and the sample temperature. Interpolation of the experimental data reveals that the limiting factor for the amplitude of the diffraction efficiency is principally the temperature of the samples, whereas then the sensitivity of the compounds seems to be only driven by the nature of the dye.