Sabino Piazzolla

First-harmonic diffusion model for holographic grating formation in photopolymers

We present a model with which to describe and predict the formation of gratings during exposure in holographic photopolymers. This model combines the action of photopolymerization and of free-monomer diffusion during holographic exposures. We consider the free-monomer density to be spatially varying, during exposure, with a single first-harmonic term out of phase with respect to the intensity interference pattern. Examples of behavior predicted by the model include the variation of the saturation diffraction efficiency with recording exposure intensity and with beam intensity modulation, as well as the variation of recorded grating modulation during dark diffusion transient. The model is supported by experiments carried out by exposure of DuPont HRF-150-38 holographic photopolymers.

Holographic grating formation in photopolymers.

We introduce a model describing real-time grating formation in holographic photopolymers, under the assumption that the diffusion of free monomers is much faster than the grating formation. This model, which combines polymerization kinetics with results from coupled-wave theory, indicates that the grating formation time depends sublinearly on the average holographic recording intensity, and the beam intensity ratio controls the grating index modulation at saturation. We validate the model by comparing its predictions with the results of experiments in which DuPont HRF-150X001 photopolymer was used.

Dynamics during holographic exposure in photopolymers for single and multiplexed gratings

Under the assumption that at low recording intensity the grating formation process is much slower than the free monomer diffusion, we present a model that describes the dynamics of the holographic grating formation in photopolymers. The model indicates how the beam intensity modulation influences the saturation value of the grating modulation, while the average recording intensity is related in a nonlinear fashion to the process time constant. An extension of this model describes and predicts the recording of a single grating in the presence of an additional mutually incoherent light beam, and the simultaneous recording of angularly mutliplexed gratings. The model is validated by comparing with experimental results obtained using DuPont HRF-150-38 photopolymer.

Single-step copying process for multiplexed volume holograms.

A technique for copying the index or absorption modulation of a multiplexed volume holographic element into a second volume holographic medium is presented. The technique utilizes a set of coherent but mutually incoherent optical sources and can perform the copy process in a single exposure step. The concept is demonstrated experimentally for the case of three angularly multiplexed holographic gratings in dichromated gelatin.

Integrated network architecture for sustained human and robotic exploration

The National Aeronautics and Space Administration (NASA) Exploration Systems Mission Directorate is planning a series of human and robotic missions to the Earths Moon and to Mars. These missions will require telecommunication and navigation services. This paper sets forth presumed requirements for such services and presents strawman lunar and Mars telecommunications network architectures to satisfy the presumed requirements. The paper suggests that a modest ground network would suffice for missions to the near-side of the Moon. A constellation of three Lunar Telecommunications Orbiters connected to a modest ground network could provide continuous redundant links to a polar lunar base and its vicinity. For human and robotic missions to Mars, a pair of areostationary satellites could provide continuous redundant links between a mid-latitude Mars base and Deep Space Network antennas augmented by large arrays of 12-m antennas

Statistics of link blockage due to cloud cover for free-space optical communications using NCDC surface weather observation data

Cloud opacity is one of the main atmospheric physical phenomena that can jeopardize the successful completion of an optical link between a spacecraft and a ground station. Hence, the site location chosen for a telescope used for optical communications must rely on knowledge of weather and cloud cover statistics for the geographical area where the telescope itself is located. In this work, the effects of cloud cover on an optical link are statistically described, considering ten observation sites at locations in the southwestern United States, From California to Texas. The data used for the preparation of this work are surface observation data provided by the National Climatic Data Center (NCDC). NCDC provides hourly information on the cloud coverage of an observation site. Using proper algorithms, these data give a statistical description of link blockage over the ten selected observations sites. Statistics averaged over a number of years for each observation site are presented. Cloud coverage statistics for two and three site diversity are also given for a ground network of optical telescopes. Finally, it is shown quantitatively how the use of two or three telescopes can improve the probability of completion of an optical link and how to select the right locations for a ground network of telescopes in the southwestern United States.

Analysis of telescope site selection for optical deep space network

The successful design of an optical deep space network (ODSN) greatly depends on the selection of optimal telescope sites. At the highest system level, there are two main factors to consider in the design of a global optical communications network for deep space applications: telescope size (i.e., aperture size) and the distance between stations. The size of the individual telescope aperture needs to be selected based on mission needs (e.g., maximization of received photons per bit). At the same time, because of weather effects and Earth rotation, a number of telescopes have to be placed within certain distances around the Earth in order to achieve global coverage. The distance between the adjacent telescopes is driven by other secondary factors, which are basically derived requirements from: 1) outage tolerance; 2) continuity in data stream; 3) operational cost; and 4) minimal requirements on the spacecraft payload design. To perform properly, ground stations must be placed on high-altitude peaks (for better visibility and high atmospheric transmission) around the Earth. However, the scarcity of peaks, along with geopolitical issues, may cause difficulties in the selection of the telescope sites in a global network. In an optical deep space link, the characterization of the atmospheric channel requires great attention. In fact, cloud opacity is the first evident impairment to the successful closure of a space-to-ground (and vice versa) optical link. Likewise, aerosol distribution in the atmosphere can significantly increase the optical thickness of the atmosphere with a detrimental attenuation of the laser signal. Moreover, an optical communication/tracking network must operate during daytime, and in this case, an increase of background sky radiance can dramatically affect the receiver performance by increasing system noise. Therefore, we present an analysis of site selection for an optical deep space network as performed by the ODSN study team at JPL. Given a set of mission requirements, we illustrate how the high-level requirements, along with the properties of the atmospheric channel, can be used to determine the site selection and the architecture of an ODSN. Moreover, we characterize candidate sites for a global optical network and their possible suitability for global architectures such as the linear dispersed optical subnet (LDOS) and cluster optical subnet network (COS).

Optical communications from planetary distances

Future planetary campaigns, including human missions, will require data rates difficult to realize by microwave links. Optical channels not only provide an abundance of bandwidth, they also allow for significant size, weight, and power reduction. Moreover, optical-based tracking may enhance spacecraft navigation with respect to microwave-based tracking. With all its advantages, optical communications from deep space is not without its challenges. Due to the extreme distance between the two ends of the link, specialized technologies are needed to enable communications in the deep space environment. Although some of the relevant technologies have been developed in the last decade, they remain to be validated in an appropriate domain. The required assets include efficient pulsed laser sources, modulators, transmitters, receivers, detectors, channel encoders, precise beam pointing technologies for the flight transceiver and large apertures for the ground receiver. Clearly, space qualification is required for the systems that are installed on a deep space probe. Another challenge is atmospheric effects on the optical beam. Typical candidate locations on the ground have a cloud-free line of sight only on the order of 60-70% of the time. Furthermore, atmospheric losses and background light can be problematic even during cloud-free periods. Lastly, operational methodologies are needed for efficient and cost effective management of optical links. For more than a decade, the National Aeronautics and Space Administration (NASA) has invested in relevant technologies and procedures to enable deep space optical communications capable of providing robust links with rates in the order of 1 Gb/s from Mars distance. A recent publication indicates that potential exists for 30-dB improvement in performance through technology development with respect to the state-of-the-art in the early years of this decade. The goal is to fulfill the deep space community needs from about 2020 to the foreseeable future. It is envisioned that, at least initially, optical links will be complemented by microwave assets for added robustness, especially for human missions. However, it is expected that as optical techniques mature, laser communications may be operated without conventional radio frequency links. The purpose of this paper is to briefly review the state-of-the-art in deep space laser communications and its challenges and discuss NASA-supported technology development efforts and plans for deep space optical communications at JPL.