KEYWORDS: Crystals, Sensors, Optical engineering, Single mode fibers, Optical filters, Photon polarization, Mirrors, Laser crystals, Transmittance, Signal to noise ratio
A small, portable, high-flux correlated photon-pair source has been designed and constructed from simple opto-mechanical parts. Unique to this device is its straightforward alignment process, together with the direct coupling of signal and idler photons into polarization-maintaining single-mode optical fibers. Spontaneous parametric down-conversion is used to produce photon pairs in β-barium borate (BBO) at a center wavelength of 810 nm. Owing to the applied type-I phase-matching, coincident photons have identical polarization. The estimated fiber-coupling efficiency is 51%, the measured photon and coincidence flux are 636 and 130 kHz/mW, respectively, normalized to pump power (44 mW). The source has an extremely wide wavelength spectrum of 202-nm FWHM, measured at the fiber output, which limits the actual heralding ratio to 20%.
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.
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