Entangled photon pairs are created by a system consisting of a 1064 nm pump diode laser that is fiber coupled to a high generation rate photon pair source. The source is a dual element periodically poled potassium titanyl phosphate (KTP) waveguide that up-converts 1064 nm photons to single 532 nm photons in the first stage. In the second stage, the green photons are down converted to energy entangled photon pairs at 800 nm and 1600 nm. The output photon pairs are guided by fiber to sorting optics where they are separated and sent into high-efficiency photon detectors. In particular, the 1600 nm photons are detected by a superconducting nanowire with efficiency over 60% and dead time less than 50 ns. Detector output electrical signals are sent to a time tagger with bin resolution as narrow as 25 ps for coincidence counting. The ultimate goal of this setup is to demonstrate a singlesource, high efficiency, high data rate, quantum communication system to enable Earth-space quantum networks. Of particular interest is a source of entangled photons that is amenable to utilization in aircraft and spacecraft under rigorous flight and environmental conditions. Test results that characterize the entangled photon pair creation and detection capabilities of our system will be presented.
Jeffrey Wilson, Dalton Chaffee, Nathaniel Wilson, John Lekki, Roger Tokars, John Pouch, Tony Roberts, Philip Battle, Bertram Floyd, Alexander Lind, John Cavin, Spencer Helmick
A high generation rate photon-pair source using a dual element periodically-poled potassium titanyl phosphate (PP KTP) waveguide is described. The fully integrated photon-pair source consists of a 1064-nm pump diode laser, fiber-coupled to a dual element waveguide within which a pair of 1064-nm photons are up-converted to a single 532-nm photon in the first stage. In the second stage, the 532-nm photon is down-converted to an entangled photon-pair at 800 nm and 1600 nm which are fiber-coupled at the waveguide output. The photon-pair source features a high pair generation rate, a compact power-efficient package, and continuous wave (CW) or pulsed operation. This is a significant step towards the long term goal of developing sources for high-rate Quantum Key Distribution (QKD) to enable Earth-space secure communications. Characterization and test results are presented. Details and preliminary results of a laboratory free space QKD experiment with the B92 protocol are also presented.
The aberration introduced by the primary optical element of a lightweight large aperture telescope can be corrected with a diffractive optical element called the liquid crystal spatial light modulator. Such aberration is usually very large, which makes the design and modeling of such a system difficult. A method to analyze the system is introduced, and the performance limitation of the system is studied through extensive modeling. An experimental system is demonstrated to validate the analysis. The connection between the modeling data and the experimental data is given.
We describe the application of smectic A (SmA) liquid crystals for beam deflection. SmA materials can be used in digital beam deflectors (DBDs) as fillers for passive birefringent prisms. SmA prisms have high birefringence and can be constructed in a variety of shapes, including single prisms and prismatic blazed gratings of different angles and profiles. We address the challenges of uniform alignment of SmA, such as elimination of focal conic domains. Fast rotation of the incident light polarization in DBDs is achieved by an electrically switched 90-deg twisted nematic (TN) cell.
Gruneisen has shown that small, light weight, liquid crystal based devices can correct for the optical distortion caused by an imperfect primary mirror in a telescope and has discussed the efficiency of this correction. In this paper we expand on that work and propose a semi- analytical approach for quantifying the efficiency of a liquid crystal based wavefront corrector for this application.
2-D optical phased array antennas formed by a liquid crystal on silicon (LCOS) spatial light modulator are described for free-space laser communication and high-resolution wavefront control. The device consists of an 2-D array of 1024×768 phase modulator elements, each with controlled voltage, and can induce a phase shift from 0 to 2 for wavelengths up to the near IR. When the device is used as a wavefront corrector, 18.7 waves peak-valley (at 632.8 nm) of aberration in the optical system is corrected to a residual of 1/9 wave peak-valley, or 1/30 wave rms. The Strehl ratio improved from 0.006 to 0.83 after correction. An additional linear phase ramp was added to the correction phase ramp to simultaneously correct and steer the laser beam. Continuous steering over ±4 mrad in the X-Y plane with a steering accuracy higher than 10 µrad has been obtained. The 1-D beam-steering efficiency is 80% at the maximum steering angle of 4 mrad. These results suggest that an LCOS device can be used to achieve very high-resolution wavefront control at very high efficiency.
The effect of fringing electric fields in a liquid crystal (LC) Optical Phased Array (OPA), also referred to as a spatial light modulator (SLM), is a governing factor that determines the diffraction efficiency (DE) of the LC OPA for high resolution spatial phase modulation. In this article, the fringing field effect in a high resolution LC OPA is studied by accurate modeling the DE of the LC blazed gratings by LC director simulation and Finite Difference Time Domain (FDTD) simulation. Influence factors that contribute significantly to the DE are discussed. Such results provide fundamental understanding for high resolution LC devices.
2-D Optical Phased Array (OPA) antenna based on a Liquid Crystal On Silicon (LCoS) device can be considered for use in free space optical communication as an active beam controlling device. Several examples of the functionality of the device include: beam steering in horizontal and elevation direction; high resolution wavefront compensation in large telescope; beam shaping with computer generated kinoform. Various issues related to the diffraction efficiency, steering range, steering accuracy as well as magnitude of wavefront compensation is discussed.
A spatial light modulator, which is capable of high-resolution wavefront compensation and high accuracy beam steering, has been demonstrated using a Liquid Crystal On Silicon (LCOS) microdisplay with 1024×768 XGA resolution. When the device is used as a wavefront corrector, about 18.7 waves (peak-to-valley at 632.8nm) of aberration in the optical system is corrected to a residual of 1/9 wave (peak to valley) or 1/30 wave rms. Measurement of the far field beam profile confirmed the strehl ratio improved from 0.006 with the wavefront correction off, to a strehl ratio of 0.83 after correction. An additional linear phase ramp was added to the correction phase ramp to simultaneously correct and steer the laser beam. We demonstrated we can steer the beam continuously in the range of ±4 mrad in X-Y plane, with a steering accuracy better than 10μrad, or about 1/10 the diffraction limited beam divergence. The quality of the steered beam remains very high during the steering as the ellipticity of beam is smaller than 1±0.04, focused beam waist is 1.3x the diffraction limited beam waist and strehl ratio remains higher than 0.66. The 1-D beam steering efficiency is 80% at the maximum steering angle of 4 mrad, which agrees very well with our Finite Difference Time Domain (FDTD) simulation result of diffraction efficiency 86% at maximum steering angle. These results suggest that an LCOS device can be used to achieve very high-resolution wavefront control at very high efficiency.
Dynamic holography has been demonstrated as a method for correction aberrations in space deployable optics, and can also be used to achieve high-resolution beam steering in the same environment. In this paper we consider some of the factors affecting the efficiency of these devices. Specifically, the effect on the efficiency of a highly collimated beam from the number of discrete phase steps per period on steering resolution is also considered. We also present some results of Finite-Difference Time-Domain (FDTD) calculations of light propagating through liquid crystal blazed gratings. Liquid crystal gratings are shown to spatially modulate both phase and amplitude of propagating light.
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