We report on free-space photonic switches that are based on switched volume holographic diffraction grating for RF and telecommunications applications. For example, a cascade of n independently controlled electro-optic (EO) gratings can be configured in a binary tree structure to route a free-space optical carrier to on of 2n possible output channels. These gratings are comprised of liquid crystal composite materials, which exhibit low loss, high speed, and high switching contrast. Thus, this switching technology enables the construction of compact photonic switching systems with the potential for low insertion loss and low crosstalk. In this paper we describe grating based systems that are fanout capable, due to the inherent analog nature of the switched grating materials. Finally, we present the performance characteristics of the monolithic photonic switching systems constructed from arrays of switched gratings.
A cascade of n independently controlled gratings can be used to route an optical carrier through one of 2 inch evenly spaced time delay paths. The resulting optical systems include digital time shifters for phased arrays with the potential for improving attainable performance in terms of insertion loss, crosstalk, and compactness. We describe results from an effort in which these characteristics of free-space optical time delay system based on switched- volume-diffraction gratings were modeled and investigated experimentally. In one experiment, a 1 by 4 router, which constitutes the front end of a 2-bit photonic time delay circuit, was used to validate the low insertion loss and miniaturization capabilities of this technology. We fabricated electrically switched gratings which demonstrated 20 dB contrast and a response time of 15 microseconds. Realistic loss and crosstalk parameters were used in detailed systems modeling to show that practical system can be built using this technology with very low insertion loss and crosstalk. Various configurations are described, including a multi-pass device that may replace many single channel time shifters with a single optical system.
Switched-volume-diffraction gratings are used to form free- space optical time delay systems with digitally selectable delays. In these systems, a cascade of n independently controlled gratings provides 2' evenly spaced time delay paths. An important feature of the approach described here is that the technology allows the use of compact micro-optic packaging, which in turn allows independent time delay channels to be tightly stacked. In one such scenario, the optical system for 75 5-bit optical time delay modules, each with a maximum selectable time delay of 1 ns, can be packed in a 3-inch cube. Recent results of theoretical modeling and experiments are presented which show that these systems have potential for excellent channel isolation, crosstalk suppression, and low insertion loss. Optical systems are discussed in the context of phased array applications: various configurations are described, including a multi-pass entire array driver that may replace many single channel time shifters with a single optical system.
Free space optical systems are described that use switchable grating technology and nonlinear materials to form digital time shifters. A cascade of n independently controlled gratings provides 2n evenly spaced time delay paths. Featured characteristics including potential for excellent switch isolation, spurious beam and crosstalk suppression, reduction in complexity, and low insertion loss are discussed in the context of phased array applications. Varied configurations are discussed including hybrid free- space/guided wave configurations for long time delays; transmission and double-pass digital optical time shifters; and entire array drivers that replace many single-channel time shifters with a single configuration. Using free space micro-optics, many independent time shifter configurations can be compactly stacked. A novel noise suppression device is discussed that enhances the channel isolation and signal purity of the systems.
A cascaded optical logic system is reported in which an array of vertical-cavity surface-emitting lasers (VCSELs) selectively controls an array of symmetric self-electro- optic effect devices (S-SEEDs). This configuration allows interconnect masks to be reconfigured dynamically by selectively disabling microlasers through a host controller. A single-array system in which an optical input is sequenced through successive rows of an S-SEED array is demonstrated. The system is operated by setting the initial states of the S-SEEDs using the VCSELs and then optically scrolling the data from row to row.
Telescopes, microscopes, and similar compound systems often require achromats for objectives, since the longitudinal chromatic aberration from a singlet objective is so large that eyepieces cannot easily correct it. Such achromats can limit large-aperture systems because the curvatures required of their components are much stronger than the curvatures of singlets with the same net optical power.
The hybrid diffractive-refractive telescope formed by combining a diffractive eyepiece with an unachromatized refractive objective is shown to eliminate the need for such bulky objectives, since a
diffractive eyepiece is capable of correcting the large longitudinal chromatic aberrations of singlet refractive objectives. By splitting the holographic eyepiece into two elements, paraxial lateral color may
also be corrected. First-order design considerations in these hybrid telescopes are presented, and a practical hybrid telescope layout is developed in which 1) primary chromatic aberration is eliminated, 2)
paraxial lateral color is corrected, and 3) a useful eye relief is obtained.