The SPADnet FP7 European project is aimed at a new generation of fully digital, scalable and networked photonic components to enable large area image sensors, with primary target gamma-ray and coincidence detection in (Time-of- Flight) Positron Emission Tomography (PET). SPADnet relies on standard CMOS technology, therefore allowing for MRI compatibility. SPADnet innovates in several areas of PET systems, from optical coupling to single-photon sensor architectures, from intelligent ring networks to reconstruction algorithms. It is built around a natively digital, intelligent SPAD (Single-Photon Avalanche Diode)-based sensor device which comprises an array of 8×16 pixels, each composed of 4 mini-SiPMs with in situ time-to-digital conversion, a multi-ring network to filter, carry, and process data produced by the sensors at 2Gbps, and a 130nm CMOS process enabling mass-production of photonic modules that are optically interfaced to scintillator crystals. A few tens of sensor devices are tightly abutted on a single PCB to form a so-called sensor tile, thanks to TSV (Through Silicon Via) connections to their backside (replacing conventional wire bonding). The sensor tile is in turn interfaced to an FPGA-based PCB on its back. The resulting photonic module acts as an autonomous sensing and computing unit, individually detecting gamma photons as well as thermal and Compton events. It determines in real time basic information for each scintillation event, such as exact time of arrival, position and energy, and communicates it to its peers in the field of view. Coincidence detection does therefore occur directly in the ring itself, in a differed and distributed manner to ensure scalability. The selected true coincidence events are then collected by a snooper module, from which they are transferred to an external reconstruction computer using Gigabit Ethernet.
In the field of biomedical imaging there is a strong interest in combining modalities of positron emission tomography
(PET) and magnetic resonance imaging (MRI). An MRI-compatible PET detector module has to be insensitive to the
magnetic field that is why it needs to incorporate avalanche photodiodes (APD) or silicon photomultipliers (SiPM). We
propose a new purely optical characterization method for these devices where no nuclear source is needed. In our method
we use LED sources for both the direct illumination of silicon sensors and fluorescent excitation of the scintillator
material. With this method we can measure the response characteristic and uniformity of pixels in sensor arrays as well
as the optical cross-talk between neighboring pixels. In the same experimental setup we can also emulate the pulse
response of the detector module (i.e. light-spread over the sensor array from a point source in the scintillator material).
We present the detailed construction of the experimental setup and analyze the benefits and drawbacks of this method
compared to the nuclear measurements. The viability of the idea is proven through the characterization of a SiPM array
and a block detector module based on it.
The application of phase-only input data pages has several advantages with respect to conventional amplitude
modulated holographic storage: It avoids the saturation of the storage material by providing a smooth Fourier
plane, improves the response in associative read-out, increases the light efficiency of the recording object wave
and provides the opportunity of data encryption. However, if the information is carried by the phase of object
wave front recovery of the data from the reconstructed beam is problematic with simple intensity sensitive
devices as a CCD camera. To solve this problem we propose a compact phase to amplitude data page conversion
method and apply it to the output of a Fourier holographic data storage system. The phase to amplitude
conversion uses a birefringent crystal to generate two equal intensity copies of the reconstructed data page that
are geometrically shifted by an integer number of pixels with respect to each other each other. The interference
of these two phase modulated images is projected on the detector field of the camera. The interference pattern
contains low and high intensity pixels if the phases of the interfering pixels are opposite and identical
respectively. Using proper data coding, the original data matrix recovered from the intensity pattern of the CCD.
Fourier plane homogeneity, bit error rate and positioning tolerances of the proposed holographic storage method
are investigated by computer modeling and a comparison is provided with amplitude modulated data pages.
We propose a simple and robust method for the recovery of phase data pages. We provide experimental proof of the
concept and investigate its applicability to optical encryption and encrypted holographic storage. Finally we discuss a
possible compact optical implementation of the method.
We present non-volatile readout of thin film polarization Fourier holograms using different wavelengths. We demonstrate application of imaged reference phase coding in portable holographic demonstrator. Experimental results approve applicability of preliminary computer simulations.
High density polarization holographic demonstrator system has been developed using ~2 µm thick azobenzene polyesters on reflective card form media. One possible development of the system is the introduction of phase encoding into the reference arm that provides enhanced security applications. Simulations were carried out with a custom computer program based on mathematical model of the system to generate code sets optimal in terms of code number and security level. The model is suitable also for the prediction of expected tolerances necessary before the definition of a working system. Performed experiments proved applicability of the model for possible system considerations. We also present our concept of extending thin-film holographic principle to multilayered holographic storage of increased capacity.
Tolerancing aspheres and preparing the corresponding drawing indications significantly differ from techniques used at spherical lenses due mainly to surface waviness, an error caused by most asphere fabrication technologies. Standard (ISO) regulations proved to be adequate for several kinds of aspheric lenses (e.g. laser focusing/collimation) made by the traditional diamond turning method, but sometimes are not general enough for recent fabrication techniques (such as CNC polishing of glass aspheres), and today’s more demanding lenses (eyepieces, Fourier objectives, relays etc.). A new, generalized tolerancing technique has been developed to accurately constrain surface waviness, quite independently of fabrication technology, and to provide easy verification of the results. Operation of the method is demonstrated on a Fourier-type objective comprising a glass aspheric lens, by computer simulation and testing of the fabricated prototypes.
We present our results on polarization holographic data storage in thin azobenzene side chain polymers. Two different systems have been demonstrated: a read only system with red diode laser and a read&write system with green frequency doubled solid state lasers. Error free operation have been proved at 2.77 bit/µm2 data density. We have also demonstrated enhanced security holographic storage by applying phase coded reference waves imaged onto the hololographic storage material. We also present the concept of extending the principle to multilayer holographic storage.
The goal of our project is to develop a rewritable holographic memory card system based on thin film polymer media. The data is stored in form of polarization Fourier holograms that present high efficiency even for thin material. The present communication deals with the modeling and the optimization of the system operation. We have developed a computer model based on fast Fourier transformations, taking the parameters of the optical processes into account. The model was used to assess the effect of various parameter sets, optical arrangements and elements on the performance of the system. The results are tested experimentally. Here we present the system model, the main optimization possibilities and other opportunities to utilize the model.
Phase-mostly modulation mode of a Sony LCX016AL-6 liquid crystal display has been found for appropriate elliptical polarization states of the input and output light with calculation using Jones matrices. Phase delay measurement with an accuracy of 2° were performed using a simple diffractive technique at different gray levels. Our measured results agree well with the theoretical calculation with Jones matrices. Both gave about 10% intensity variation and more than 200° phase modulation in the whole range.
Polarization holographic read/write and read only demonstrator systems have been developed using ~2 µm thick azobenzene polyester on a card form media. The thin-film holographic system has practical advantages, e.g. high diffraction efficiency, no cross talk between the holograms, reading in reflection mode, no hardware servo, different wavelengths for writing and reading (non-volatile storage), data encryption possibility, no problem with material shrinkage, etc. The candidate azobenzene polyester has good thermal, room temperature and ambient light stability and good optical properties for the purpose of thin film application. Using thin-film holography the possibilities of multiplexing are limited, however, raw data density as high as 2.77 bit/µm2 has been achieved in an optimized Fourier holographic system using high numerical aperture (NA³ 0.74) objective in a 8f arrangement with sparse code modulation and Fourier-filtering at 532 nm. High density polarization holographic demonstrator systems have been developed using ~2μm thick azobenzene polyesters on reflective card form media. FFT computer simulation of the system including saturation model of the material allows optimization of system components including data density and capacity. A raw density as high as 2.77 bit/μm2 has been achieved without multiplexing in a compact, portable read/write sytem at 532 nm allowing more than 1000 readout without data loss. A separate read only system working at 635 nm realizes non-volatile readout and allows card exchange at a data density of 1.3 bit/μm2. Security level of the presents holographic optical card systems can be further increased by using phase encoded reference beam. Advantageous applications of the proposed encrypted holographic card system are also outlined.
We present the improved demonstrator of our rewritable holographic memory card system. High density optical storage is realized in a non-commercial optical set-up. Fourier transformed recording is used in a polarization holographic arrangement realizing reading and writing from the same side of the data carrier which is a modified plastic card. Holograms containing binary information of 300 x 220 bits are as small as 0.0484 square mm. The storage layer is amorphous polyester providing repeated writing and erasure cycles and thousandfold readouts without loss of information. Alternate read only system providing non-volatile storage can be realized using 635 nm laser diode.
Optical storage properties of thin azobenzene side-chain polyester films were examined by polarization holographic measurements. The new amorphous polyester film is the candidate material for the purpose of rewritable holographic memory system. Temporal formation of anisotropic and topographic gratings was studied in case of films with and without a hard protective layer. We showed that the dominant contribution to the diffraction efficiency comes from the anisotropy in case of expositions below 1 sec even for high incident intensity. The usage of the same wavelength for writing, reading and erasing was tested. The ability of azobenzene polyester for rewriting was found satisfactory after many writing-erasing cycles.
Our goal is to develop a re-writable holographic memory card system based on thin film polymer media on credit card size plastic carriers. Data is stored in our system in form of polarization holograms that present high efficiency and excellent suppression of higher orders even for thin material. Data is written on the card in a parallel way using spatial light modulators to encode the object beam that is Fourier transformed by a custom objective lens and interferes with the reference beam (of orthogonal polarization) on the card. We use reflective carrier in order to read out the data from the same side of the card. This allows us to have a compact system and standard ID 1 type carrier card. The optical system and the data organization are optimized to have a data density higher than 1bit/micrometers 2. We expect to pass the limit of 10 bit/micrometers 2 with the introduction of phase coded multiplexing that would provide more than 2Gbyte capacity if using half the card area as active surface.
We are developing a holographic memory card drive using thin polymer storage layer on credit card sized plastic carriers. The main features of the card are high storage density, re- writability and resistance to harsh environment (e.g. electromagnetic noise, mechanical intolerance, high temperature and humidity). The optical system of this device uses Fourier holograms to record information. The present communication deals with the optimization of the storage density by computer modeling of the system. The model is based on fast Fourier transformations, taking the parameters of the optical processes into account. The model was used to assess the performance of various parameter sets. The results are tested experimentally. The work suggests that storage density higher than 1 bit/micrometers 2 may be achieved in the thin holograms of this memory card at raw bit error rate values below 103.
We introduce a new physical model for description of thin polarization holograms recorded and reconstructed with complex reference beams. Next we apply this model for two fundamentally different arrangements supposed to be used in a data storage system based on phase coded thin polarization holograms. On the basis of the model, we construct some computer programs for simulation of the storage system and optimization of the phase codes. We investigate the feasibility of multiplexed and security encrypted data storage in the form of thin polarization holograms based on the phase encoding method.
We developed a standard credit card-shaped general-purpose data carrier, a reflective Holographic Memory Card (HMC), and the appropriate equipment for its handling. Data recording and retrieval are accomplished by polarisation Fourier holography using a thin layer of photo-anisotropic polymer as the storage material. The data density is about 1 bit/micrometers 2, the maximum storage capacity of the card is around 10 Mbytes assuming a 10 x 10 mm storage area. Data is stored in the form of microholograms, from which 40x40 pieces are recorded on the HMC. The optical system involved performs data writing/reading/erasing and also locates the position of the microholograms. Main components of the optical system are an SLM and CCD for opto-electronic conversion, a frequency-doubled solid-state laser source, a beam shaping system that provides homogeneous illumination of the SLM, an interferometer for hologram construction, special Fourier transforming objectives and a random-phase mask for optimised hologram recording. Our results include conceptual planning, design, fabrication and assembling of the optical system. In the present paper we describe principle of operation including layout of the elements, and explain the operation of the equipment in detail.
We present a novel solution for high-density optical storage of data in thin media. The holographic memory card of Optilink provides sixty-fold data density enhancement compared to present commercial LaserCard devices. The 1 - 2 micrometers thin amorphous polyester storage film is capable of rewritable storage using a single laser source for writing and erasing. The polarization holographic principle used in reflection mode requires demanding optical solutions. Successful data evaluations prove applicability of the new system. Density enhancement up to 16 bit/micrometers 2 with the use of 20 - 30 micrometers thick layer is also outlined.
In most optical storage methods data bits are stored in the form of microscopic pixels on the surface of an appropriate storage material. Some currently developed techniques apply parallel data processing by multiple data bit access simultaneously. Such methods require special imaging systems for data recording and retrieval. In our laboratory a page- organized optical memory card reading/writing equipment is under development. According to the basic principle 256 by 256 data bits are processed at the same time, the corresponding pixels are arranged in a 2D array format. The same objective is used to image the selected data page both at writing in and reading out. This objective performs diffraction limited imaging in an extended field, it has low distortion, and it images each pixel of the same value with the same intensity. To achieve all these specifications a telecentric/inverse telecentric imaging system (a special type of afocal systems) offered a suitable solution. This paper describes the advantages of telecentric/inverse telecentric systems in optical imaging by detailed presentation of our objective. The discussion includes specification and design process of the objective together with our test results performed on the fabricated prototype.
A simple, single-element, afocal, refractive optical device with two aspheric surfaces has been designed and fabricated for transformation the Gaussian intensity profile of a He-Ne laser into a collimated beam of uniform profile. The working principle, the method of design, the method of fabrication are presented. Optical and geometrical properties of the fabricated sample have been tested. Device parameters and simulated behavior are compared with test results in detail.
Simple reflection techniques for optical parameter determination of isotropic or anisotropic multilayers on a lower refractive index substrate are presented here. In case of weak absorption in at least one of the layers characteristic minima appear in the angular dependence of the reflectivity by computer fit of which the layer refractive indices and thicknesses can be determined. In case of a monolayer the achieved accuracy is better than the third and fourth decimal in the real and imaginary part of the refraction index, respectively. The fit of bi-layers has lower accuracy, but has the advantage of refractive index determination of non-absorbing layer(s) in the presence of a weakly absorbing auxiliary layer. Comparison between measured refractive indices by reflectometric and optical waveguiding methods shows agreement within 0.001.
A novel reflection technique for refractive index determination of an optical layer (or multilayer) on a lower refraction index substrate is presented here. In the presence of absorption in at least one of the layers the observed characteristic minima enable high precision determination of the layer indices. The method is demonstrated on different polymer multi-layers. Accuracy limiting factors, such as surface roughness and thickness variation are also discussed.