Microwave photonic filters are photonic subsystems designed with the aim of carrying equivalent tasks to those of an ordinary microwave filter within a radio frequency (RF) system or link, bringing supplementary advantages inherent to photonics such as low loss, high bandwidth, immunity to electromagnetic interference (EMI), tunability, and reconfigurability. There is an increasing interest in this subject since, on one hand, emerging broadband wireless access networks and standards spanning from universal mobile telecommunications system (UMTS) to fixed access picocellular networks and including wireless local area network (WLAN), World Interoperability for Microwave Access, Inc. (WIMAX), local multipoint distribution service (LMDS), etc., require an increase in capacity by reducing the coverage area. An enabling technology to obtain this objective is based on radio-over-fiber (RoF) systems where signal processing is carried at a central office to where signals are carried from inexpensive remote antenna units (RAUs). On the other hand, microwave photonic filters can find applications in specialized fields such as radar and photonic beamsteering of phased-arrayed antennas, where dynamical reconfiguration is an added value. This paper provides a tutorial introduction of this subject to the reader not working directly in the field but interested in getting an overall introduction of the subject and also to the researcher wishing to get a comprehensive background before working on the subject.

A tutorial on microwave photonic filters

This work presents an overview of progress and developments in the field of fiber optic sensor technology, highlighting the major issues underpinning recent research and illustrating a number of important applications and key areas of effective fiber optic sensor development.

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Fiber optic sensor technology: an overview

This paper is concerned with the probability, P[T, r(τ)], that a stationary Gaussian process with mean zero and covariance function r(τ) be nonnegative throughout a given interval of duration T. Several strict upper and lower bounds for P are given, along with some comparison theorems that relate P's for different covariance functions. Similar results are given for F[T, r(τ)], the probability distribution for the interval between two successive zeros of the process.

The one-sided barrier problem for Gaussian noise

Transport equations are derived for particles, momentum, and energy of electrons in a semiconductor with two distinct valleys in the conduction band, such as GaAs. Care is taken to state and discuss the assumptions which are made in the derivation. The collision processes are expressed in terms of relaxation times. The accuracy is improved by considering these to depend on the average kinetic energy rather than the electron temperature. Other transport equations used in the literature are discussed, and shown to be incomplete and inaccurate in many cases. In particular, the usual assumption that the mobility and diffusion constant depend locally on the electric field strength is shown to be incorrect. Rather, these quantities should be taken as functions of the local average velocity of electrons in the lower valley.

Transport equations for electrons in two-valley semiconductors

Photonic signal processing offers the prospect of realizing extremely high multigigahertz sampling frequencies, overcoming inherent electronic limitations. This stems from the intrinsic excellent delay properties of optical delay lines. These processors provide new capabilities for realizing high time-bandwidth operation and high-resolution performance. In-fiber signal processors are inherently compatible with fiber-optic microwave systems and can provide connectivity with built-in signal conditioning. Fundamental principles of photonic signal processing, including sampling, tuning, and noise, are discussed. Structures that can extend the performance of photonic signal processors are presented, including methods for improving the filter shape characteristics of interference mitigation filters, techniques to increase the stopband attenuation of bandpass filters, and methods to achieve large free spectral range. Several photonic signal processors, including high-resolution microwave filters, widely tunable filters, arbitrary waveform generators, and fast signal correlators, are discussed. Techniques to solve the fundamental noise problem in photonic signal processors are described, and coherence-free structures for few-tap notch filters are discussed. Finally, a new concept for realizing multiple-tap coherence-free processor filters, based on a new frequency-shifting technique, is presented. The structure not only eliminates the phase-induced intensity noise limitation, but can also generate a large number of taps to enable the achievement of processors with high performance and high resolution.

Photonic signal processing of microwave signals

A method for analysis of wave propagation on structures consisting of periodic metal strips is presented. The method is an extension of earlier theories based on Legendre polynomial expansions of the electric field. The extension allows analysis of strips on any material or layers of materials, provided the electromagnetic properties can be expressed in terms of a wavenumber-dependent permittivity. Layers of dielectrics, piezoelectrics, semiconductors, and metals are typical of the materials of practical interest in connection with surface wave acoustics. Strip configurations such as interdigital transducers, multistrip directional couplers and reflectors, and strip coupled amplifiers can be analyzed by the present approach. Prescriptions for calculating the amplitudes of the space harmonics of the electric field strength are given. Closed-form expressions for strip current and voltage are developed.

A method for analyzing waves in structures consisting of metal strips on dispersive media

Dispersion relations for acoustic surface waves in piezoelectrics with periodic metal strips applied to the surface are developed. Particular emphasis is given to the modification in effective piezoelectric coupling due to strip geometry, and the details of the dispersion relations around the stop bands. The general results are applied to discuss devices such as multistrip directional couplers and strip-coupled amplifiers.

Acoustic surface waves in piezoelectric materials with periodic metal strips on the surface

The erasure, which occurs during readout of holograms in photochromic or photorefractive media, imposes limitations on the amount of information which can be stored. It is shown that storage of several holograms by varying the angle of incidence of the reference beam is particularly unfavorable. It is much more advantageous to use a large number of bits per page or to store the holograms in separate parts of the sample. If erasure is prevented by some kind of physical or chemical fixing or biasing process, the storage capacity is limited by the bit rate required during readout. This limitation may also be severe, in some cases reducing the storage capacity to several orders of magnitude below the diffraction-limited value.

Limitations on holographic storage capacity of photochromic and photorefractive media

A distributed sensor system including an optical source having a short coherence length for optionally continuously monitoring each sensor in the system. In one preferred embodiment, an array of fiber-optic sensors are organized in a ladder configuration, with the sensors positioned in spaced relation and defining the rungs of the ladder. Light transmitted through the sensors is multiplexed onto a return arm of the ladder, with sensor spacing being such that interference between light from different sensors is prevented. The multiplexed signals are received by an optical fiber receiver which couples the multiplexed light with an interfering optical reference signal to produce a phase difference signal representing conditions influencing selected sensors. Embodiments are disclosed for use of either pulsed or continuous wave light sources. In another preferred embodiment, the sensors comprise a plurality of interferometers connected in series configuration by a common optical fiber, which provides multiplexed output signals from the sensors to another plurality of interferometers comprising receivers. Functional equivalents of the series configuration provide sensors and receivers comprising birefringent or two-mode fiber. The optical path length differences between each pair of sensor arms are selected to prevent interference between the multiplexed sensor output signals from the various sensors. The optical path lengths through the sensors and receivers are structured so that each receiver produces a phase difference signal relating to conditions affecting light transmission through a specific sensor. A phase and amplitude modulation technique is disclosed for providing heterodyned output signals from the distributed sensor system.

Kjell Blotekjaer

Papers

Transport equations for electrons in two-valley semiconductors

A method for analyzing waves in structures consisting of metal strips on dispersive media

Acoustic surface waves in piezoelectric materials with periodic metal strips on the surface

Limitations on holographic storage capacity of photochromic and photorefractive media

Distributed sensor and method using coherence multiplexing of fiber-optic interferometric sensors