KEYWORDS: Computer programming, Receivers, Silicon, Multiplexers, Waveguides, Simulation of CCA and DLA aggregates, Photonic integrated circuits, Data communications, Analog electronics, Signal detection
The time slots of M-ary pulse-position-modulation (PPM) can be replaced by M optical CDMA codes: Code/Pulse-Position-Swapping (C/PPS) or Code-Shift-Keying (CSK). Features of C/PPS are that it: (1) retains the intensity-modulation/
direct-detection (IM/DD) and multiple-bits-per-symbol features of PPM; (2) supports multiple access
communications; (3) supports bandwidth-on-demand; (4) supports tailoring quality of service; (5) translates the received
symbol to its corresponding bit sequence without using look-up-tables (LUTs)); (6) and has a common electronic bandwidth
for all data rates. In this paper we describe the design of C/PPS encoders and decoders based on the specific
example of 4x4 fast frequency hopping (FFH) sonar codes.
Code/pulse-position-swapping (CPPS) is a communications scheme that substitutes pulse-position-modulation (PPM)
symbols with optical-code-division-access (O-CDMA) codes. CPPS retains the multiple bits per symbol communication
of M-ary PPM and the asynchronous multiple access of O-CDMA. Additionally, CPPS has the advantages of granular
communications, common electrical bandwidth for all users independent of data rates, compatibility with free-space or
guided (fiber and waveguide) communication links, and compatibility with intensity modulation/direct detection. The
transmitted symbols (codes) of CPPS are translated from a deserialized bit stream that has been divided into words of
length log2M. Thus the receivers associate the detected symbol with the original bit sequence by means of an
electronically implemented look-up-table (LUT). This paper describes the architecture and design of a direct translating
receiver based on map-coding, which uses optical processing to output the transmitted bit sequence without the need for
a LUT. Analyses and computations characterize the receiver concept in terms of bit errors (mistranslations).
This paper explores the architecture and design of an optically-implemented 64-ary PPM transmitter
and direct-translating receiver that effectively translates incoming electrically-generated bit streams
into optical PPM symbols (and vice-versa) at > 1 Gb/s data rates. The PPM transmitter is a cascade
of optical switches operating at the frame rate. A corresponding receiver design is more difficult to
architect and implement, since increasing data rates lead to correspondingly shorter decision times
(slot times and frame times). We describe a solution in the form of a time-to-space mapping
arrayed receiver that performs a translating algorithm represented as a code map. The technique for
generating the code map is described, and the implementation of the receiver as a planar lightwave
circuit is given. The techniques for implementing the transmitter and receiver can be generalized for
any case of M-ary PPM.
NASA is developing technology for 64-ary PPM using relatively large PPM time slots (10 ns) and relatively simple
electronic-based receiver logic. In this paper we describe photonics-based receiver options for the case of much higher
data rates and inherently shorter decision times. The receivers take the form of virtual (array or quadrant) arrays with
associated comparison tests. Previously we explored this concept for 4-ary and 16-ary PPM at data rates of up to 10
Gb/s. The lessons learned are applied to the case of 64-ary PPM at 1.25 Gb/s. Various receiver designs are compared,
and the optimum design, based on virtual arrays, is evaluated using numerical simulations.
M-ary pulse position modulation (M-ary PPM) is an alternative to on-off-keying (OOK) that transmits multiple bits as a
single symbol occupying a frame of M slots. PPM does not require thresholding as in OOK signaling, instead
performing a comparison test among all slots in a frame to make the slot decision. Combining PPM with optical code
division multiple access (PPM/O-CDMA) adds the benefit of supporting multiple concurrent, asynchronous bursty PPM
users. While the advantages of PPM/O-CDMA are well known, implementing a receiver that performs the comparison
test can be difficult. This paper describes the design of a novel array receiver for M-ary PPM/O-CDMA (M = 4) where
the received signal is mapped onto an xy-plane whose quadrants define the PPM slot decision by means of an associated
control law. The receiver does not require buffering or nonlinear operations. In this paper we describe a planar
lightwave circuit (PLCs) implementation of the receiver. We give detailed numerical simulations that test the concept
and investigate the effects of multi-access interference (MAI) and optical beat interference (OBI) on the slot decisions.
These simulations provide guidelines for subsequent experimental measurements that will be described.
Optical CDMA (O-CDMA) based on matrix codes (e.g., Wavelength/Time, or W/T, matrices) can have a higher capacity and spectral efficiency (bit/s/Hz) than WDM. We have ben exploring the implementation of W/T matrices with WDM components. The wavelength part of the W/T matrices is generated by a WDM laser array consisting of multiple independent laser cavities (each at a distinct ITU frequency) coupled through a semiconductor optical amplifier (SOA). These integrated laser arrays are known as MFLs. The time-like part of the W/T matrix codes is produced by Bragg grating arrays. The W/T matrix implementation requires that the wavelengths be synchronously on-off-keyed (OOF) with a return-to-zero (RZ) modulation of less than 25% duty cycle, and that the synchronicity be retained at the point of correlation (the receiver end). In this paper we explore on- off-keying the MFL SOA in order to achieve a synchronous RZ pulse-train which can then be externally data-modulated. Particular emphasis was placed on determining the smallest achievable pulse-widths and highest pulse repetition rates for the MFLs, since the long MFL cavity lengths may preclude fast modulation formats.
Optical code division multiple access (O-CDMA) uses very narrow transmission pulses and is thus susceptible to fiber optic link impairments. When the O-CDMA is implemented as wavelength/time (W/T) matrices which use wavelength division multiplexing (WDM) sources such as multi-frequency laser transmitters, the susceptibility may be higher due to: (a) the large bandwidth utilized and (b) the requirement that the various wavelength components of the codes be synchronized at the point of the data modulation and encoding as well as after (optical) correlation. A computer simulation based on the nonlinear Schroedinger equation (developed to study optical networking on the National Transparent Optical Network (NTON)) was modified to characterize the impairments on the propagation and decoding of W/T matrix codes over a link of the NTON. Three critical impairments were identified by the simulation: group velocity dispersion (GVD); the flatness of the optical amplifier gain; and the slope of the GVD. Subsequently, experiments were carried out on the NTON link to verify and refine the simulations as well as to suggest improvements in the W/T matrix signal processing design. The NTON link measurements quantified the O-CDMA dispersion compensation requirements. Dispersion compensation management is essential to assure the performance of W/T matrix codes.
KEYWORDS: Satellites, Ranging, Signal to noise ratio, Data communications, Single mode fibers, Satellite communications, Receivers, Data processing, Radar signal processing, Radar
Clusters of small satellites which orbit in tight constellations and cooperatively perform radar, surveillance, data collection, and bent pipe communications, are being explored for potentially replacing the use of a single, larger, more complex satellite. Two basic requirements of these clusters are: communications must be available between any satellite pair, and range and position between any satellite pair must be continually monitored for computing the antenna configuration. While up/down links will most likely be RF, compact, lightweight, low power, lasercom direct detection optical packages can perform the crosslinking and position location operations. In this paper, a lasercom architecture is presented that utilizes pairs of relatively wide beam divergence optical beams to minimize pointing and tracking requirements and a point-to- point network topology. Data transfer is accomplished by the use of pulsed optical Code-Division-Multiplexing signaling formats, with address codes assigned to each satellite. A given satellite can establish a link with any other satellite by encoding data onto the proper address code. Some basic link budgets will be presented to scope the optical design in terms of power, data rate, and constellation size. Range and position can be obtained from the same lasercom architecture, using the address codes as ranging markers.
Although much of today's space lasercom development is concentrated in digital communications, there still remain applications where the analog lasercom link is preferred. An analog lasercom link transmits RF or IF carriers instead of decoded bits. The most important application is in commercial bent pipe satellite constellations, where modulated carriers are to be relayed between satellites, avoiding the use of costly and risky onboard demod - remod digital processors. In this paper some basic tradeoffs are reported which aid in scoping the performance, limitations, and shortfalls of an analog crosslink. The analysis assumes fiber based technology at either 1.55 or 1.3 microns for the crosslink hardware, using optical intensity modulation (IM) and direct photodetection. When multiple carriers are used to IM the laser, the inherent nonlinearity of the modulation causes power suppression that limits the available bandwidth (number of carriers) that can be relayed using as single optical wavelength. With single wavelength bandwidth limited, the crosslink capacity can only be improved by the use of multiple wavelengths. This tradeoff of suppression and wavelengths is considered here.
A novel method of interconnecting high-performance computers, predicated on the bit-parallel transmission of data, using monolithic, mode-locked WDM array sources, ten wavelengths wide, operating at greater than 20 GHz, is being developed. The byte-wide data is coupled to a single-mode fiber of appropriate dispersion characteristics, and the byte synchronism is maintained by means of a 'shepherd soliton'. The shepherding condition requires that the photonic components between the WDM array and the single- mode fiber, not introduce any skew which cannot be corrected by the shepherd soliton. This requires that the passive component skew be within the capture range of the shepherd pulse. In this paper we describe the impact of the shepherding condition on the design of the passive WDM components, and we describe a suitable coupler design, based on planar lightwave circuits and the corresponding planar lightwave circuit design rules.
New coding and modulation techniques formulated specifically for employment with wavelength division multiplexing (WDM) components and systems to significantly increase the number of virtual channels are described and analyzed. It is generally accepted that technical and economic considerations limit a conventional WDM system to less than 32 wavelength channels and, in the near term, most likely to 4 or 8. We assume a 4-wavelength WDM system and impose WDM coding or modulation in which the WDM wavelengths are interpreted as unit vectors in a quasi-orthogonal space. We describe how various combinations of these unit vectors define signatures of the wavelength-space type (WDM-vectors), the wavelength-space-time type (WDM-matrices), or the hybridized WDM-optical CDMA type, each potentially capable of yielding a rich address space, even when the number of WDM wavelengths is small. The topologies, implementation, and performance of these codes is discussed for network and interconnection systems.
It is well known that either Wavelength Division Multiplexing (WDM) or pulsed Code Division Multiple Accessing (CDMA) can be used as a basic accessing format in structuring an optical communication network or high speed parallel data processor. Each such system has its own inherent advantages and disadvantages when used as the principle format. When the two concepts are combined into a single hybrid WDM/CDMA format, some of the disadvantages of each format can be offset by the presence of the other, producing a more robust networking system with improved communication performance. In this paper these hybrid concepts are presented, and some of their hardware and architecture impacts are developed.
This paper describes a new interconnect and local area network transmission concept for computer communications based on spectrally encoding one or more computer words into a wavelength datagram. At physical and data link level, this system resembles an optical ribbon cable, except that all the bits pass on one fiber optic waveguide. At the network level, such fiber optic link segments can be interconnected all-optically using 2x2 optical switches into ShuffleNet or other architectures that permit a photonic packet to pass from source to destination without being incumbered with the extra delay and bandlimiting associated with electronic switching and regeneration. Unique properties of such a system include low latency (<10ns), very high bandwidth (<100Gbit/s per port), precise time alignment (<10ps) of the individual word bits over km distances, and dynamic scalability to support cluster computing and distributed supercomputing. Novel system elements disclosed in this paper include: (J) a bit parallel wavelength (BPW) fiber optic link that uniquely maintains wavelength channel time alignment, (2) an innovative parallel stepped wavelength optical transmitter that time synchronizes each laser diode element at its optical output, (3) a spectral encoder/decoder that adds fault tolerance and optical message addressing capability, and (4) a technique for transmitting and maintaining time aligned multi-X solitons as parallel bits through fiber media. Applications to teraflop high performance parallel computing and DoD input/output (I/O) bound applications are described.
The theoretical foundations for code division multiple access (CDMA) are similar for microwave and optical implementations, and the advantages of this form of multiple access communications are similar, also. The advantages include the ability to support bursty, asynchronous, concurrent communications and to tolerate multipath interference. Optical implementations of CDMA are different in that they tend to require guided lightwaves (e.g., fiber cable), the coding is imposed on the intensity rather that on the amplitude of the lightwave, the various codes are not strictly orthogonal, and the receiver uses direct detection. This gives rise to the term 'optical CDMA'. These pseudo- orthogonal CDMA codes usually require wide bandwidths to represent the encoded data, so that optical CDMA is sometimes considered bandwidth inefficient. Two classes of optical CDMA codes which ar sufficiently bandwidth efficient for wireless communications applications are (1) incoherent spectral CDMA and (2) optical matrix CDMA. In addition, the wireless communications applications may need to support various data rates (for the various services), and these two classes of optical CDMA are both very tolerant of data rate variations. An appropriate topology for applications such as a local area network (LAN) is a star and, for multimedia data distribution, a tree.
A 5V/5mW power by light (PbL) power supply has been designed and developed. The design is based on a 50mw 800nm laser diode a segmented GaAs photocell and a DC/DC converter which acts as a voltage regulator. This paper discusses the system concept and elements as well as the measured efficiencies of the components and the system configuration. The paper also discusses the scalability of the design concep
There are government and industry trends towards avionics modularity and integrated avionics. Key requirements implicit in these trends are suitable data communication concepts compatible with the integration concept. In this paper we explore the use ofCode Division Multiple Access (CDMA) techniques as an alternative to collision detection and collision avoidance multiple access techniques.
Systematic relationships among codes code weights and code length have been developed for pseudo orthogonal codes. Special emphasis has been placed on identifying minimum code lengths applicable to code division multiple access systems for given codes and weights. These derived relationships are applied to temporal spatial and spectral CDMA systems to estimate their link budgets complexity and performance in various communication applications. The construction and benefits of a new class of hybrid pseudo orthogonal codes is discussed.
KEYWORDS: Pulsed laser operation, Time multiplexed optical shutter, Magnetism, Polarization, Physics, Signal detection, Optical storage, Data storage, Magnetic sensors, Sensors
We have measured the polarization signal from erasure through laser induced magnetization reversal (under the influence of a bias magnetic field) by using the incident laser pulse to read while writing. The measurements depict the physics of TMO during laser induced heating. The data indicate that the writing mechanism may not be symmetric with bias field.
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