As the demand for multiple radio frequency carrier bands continues to grow in space
communication systems, the design of a cost-effective compact optical transmitter that
is capable of transmitting selective multiple RF bands is of great interest, particularly for
NASA Space Communications Network Programs. This paper presents experimental
results that demonstrate the feasibility of a concept based on an optical wavelength
division multiplexing (WDM) technique that enables multiple microwave bands with
different modulation formats and bandwidths to be combined and transmitted all in one
unit, resulting in many benefits to space communication systems including reduced
size, weight and complexity with corresponding savings in cost. Experimental results will
be presented including the individual received RF signal power spectra for the L, C, X,
Ku, Ka, and Q frequency bands, and measurements of the phase noise associated with
each RF frequency. Also to be presented is a swept RF frequency power spectrum
showing simultaneous multiple RF frequency bands transmission. The RF frequency
bands in this experiment are among those most commonly used in NASA space
The role of the Advanced Air Transportation Technologies program undertaken at the NASA Glenn Research Centers has been focused mainly on the improvement of air transportation safety, with particular emphasis on air transportation communication systems in on-board aircraft. The conventional solutions for digital optical communications systems specifically designed for local/metro area networks are, unfortunately, not capable of transporting the microwave and millimeter RF signals used in avionics systems. Optical networks capable of transporting RF signals are substantially different from the standard digital optical communications systems. The objective of this paper is to identify a number of different communication link architectures for RF/fiber optic transmission using a single backbone fiber for carrying VHF and UHF RF signals in the aircraft.
To support these architectures, two approaches derived from both hybrid RF-optical and all-optical processing methodologies are discussed with single and multiple antennas for explicitly transporting VHF and UHF signals, while the relative merits and demerits of each architecture are also addressed. Furthermore, the experimental results of wavelength division multiplexing (WDM) link architecture from our test-bed platform, configured for aircraft environment to support simultaneous transmission of multiple RF signals over a single optical fiber, exhibit no appreciable signal degradation at wavelengths of both 1330 and 1550 nm, respectively. Our measurements of signal to noise ratio carried out for the transmission of FM and AM analog modulated signals at these wavelengths indicate that WDM is a fiber optic technology which is potentially suitable for avionics applications.
In this paper, we present the findings of a study on the radio frequency (RF) signal switching and distribution techniques in a civil aviation aircraft. Using the Boeing Aircraft 777 as a model, method and mode of RF signal switching and distribution were investigated. The aim is to evaluate system performance and if possible determine methods of improvement. The performance of the system was measured in terms of savings in system parameters such as weight, size and length of the associated components. Instead of using coaxial cables or twisted pair wire for routing the RF signals, optical fibers cables were suggested as a method of improvement. During this study, the difficulty of achieving this objective became obvious due to the complexity of the problem. However, suggestions were made on possible methods of improvements.
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.
We have investigated crosslinkable polyimides for both passive and active electro-optic devices. These fluorinated polyimides are soluble in the imidized form and are both thermally and photo-crosslinkable leading to easy processability into waveguide structures and the possibility of stable electro-optic properties. We have fabricated channel and slab waveguides and investigated the mechanism of optical propagation loss using photothermal deflection spectroscopy and waveguide loss spectroscopy, and found the losses to arise from residual absorption due to the formation of charge transfer states. The absorption is inhibited by fluorination yielding propagation losses as low as 0.4 dB/cm in the near infrared. Channel waveguides formed by a simple wet etch process are observed to have no excess loss over slab structures. We have produced electro-optic polymers by doping with the nonlinear optical chromophores, DCM and DADC; and a process of concurrent poling and thermal crosslinking. Multilayer structures have been investigated and poling fields optimized in the active layer by doping the cladding with an anti-static agent.
We have investigated a promising class of polyimide materials for both passive and active electro-optic devices, namely crosslinkable polyimides. These fluorinated polyimides are soluble in the imidized form and are both thermally and photo-crosslinkable leading to easy processability into waveguide structures and the possibility of stable electro-optic properties. We have fabricated channel and slab waveguides and investigated the mechanism of optical propagation loss using photothermal deflection spectroscopy and waveguide loss spectroscopy, and found the losses to arise from residual absorption due to the formation of charge transfer states. The absorption is inhibited by fluorination leading to propagation losses as low as 0.3 dB/cm in the near infrared. Because of the ability to photocrosslink, channel waveguides are fabricated using a simple wet-etch process. Channel waveguides so formed are observed to have no excess loss over slab structures. Solubility followed by thermal cross-linking allows the formation of multilayer structures. We have produced electro-optic polymers by doping with the nonlinear optical chromophores, DCM and DADC; and a process of concurrent poling and thermal crosslinking. Multilayer structures have been investigated and poling fields optimized in the active layer by doping the cladding with an anti-static agent. The high glass-transition temperature and cross-linking leads to very stable electro-optic properties. We are currently building electro-optic modulators based on these materials. Progress and results in this area also are reported.
Optically Processed Beam Forming Networks (OPBFNs) have been identified as an emerging technology for phased array antenna applications requiring rapidly reconfigurable multiple beams. OPBFNs have the potential for a considerable decrease in weight and volume over typical phased array antenna architectures. The potential compactness and flexibility of OPBFNs has made them an attractive candidate for an antenna role in the Mars Environmental Survey (MESUR) project. The NASA Lewis Research Center (LeRC) and the Jet Propulsion Laboratory (JPL) are jointly investigating the use of OPBFNs in this application. As its part in this joint effort, LeRC is developing an OPBFN testbed to evaluate OPBFN architectures and components, as well as developing system modeling programs to simulate OPBFN performance. JPL's role in the project is to develop a photonic transceiver that feeds the OPBFN. This paper discusses the modeling and development of the OPBFN testbed at LeRC.