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Cambridge University Press eBooks

Coherence theory

Christophér C. Davis

Published Mar 20, 2014• 0 citations
Publisher

Abstract

Introduction In this chapter we will put the concept of coherence on a more mathematical basis. This will involve the formal definition of a number of functions that are used to describe the coherence properties of optical fields. These include the analytic signal, various correlation functions, and the degree of coherence for describing both temporal and spatial coherence. We shall see that the degree of temporal coherence is quantitatively related to the lineshape function and that the degree of spatial coherence between two points is determined by the size, intensity distribution, and location of an illuminating source. We will use the wave equation, and special solutions to the wave equation called Green's functions, to show how spatial coherence varies from point to point. The chapter will conclude with a discussion of how the intensity fluctuations of a source depend on its coherence properties, and we will examine a specific scheme, namely the Hanbury Brown–Twiss experiment, by means of which this relationship is studied. This discussion will involve a discussion of “photon statistics,” namely the time variation of the “detection” of photons from a source. In a quantum-mechanical context, square-law detectors respond to these quantized excitations of the optical field, which we call photons. In classical coherence theory it is advantageous to represent the real electromagnetic field by a complex quantity, both for its mathematical simplicity and because it serves to emphasize that coherence theory deals with phenomena that are sensitive to the “envelope” or average intensity of the field.

DOI
10.1017/cbo9781139016629.023

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Introduction In this chapter we will put the concept of coherence on a more mathematical basis. This will involve the formal definition of a number of functions that are used to describe the coherence properties of optical fields. These include the analytic signal, various correlation functions, and the degree of coherence for describing both temporal and spatial coherence. We shall see that the degree of temporal coherence is quantitatively related to the lineshape function and that the degree of spatial coherence between two points is determined by the size, intensity distribution, and location of an illuminating source. We will use the wave equation, and special solutions to the wave equation called Green's functions, to show how spatial coherence varies from point to point. The chapter will conclude with a discussion of how the intensity fluctuations of a source depend on its coherence properties, and we will examine a specific scheme, namely the Hanbury Brown–Twiss experiment, by means of which this relationship is studied. This discussion will involve a discussion of “photon statistics,” namely the time variation of the “detection” of photons from a source. In a quantum-mechanical context, square-law detectors respond to these quantized excitations of the optical field, which we call photons. In classical coherence theory it is advantageous to represent the real electromagnetic field by a complex quantity, both for its mathematical simplicity and because it serves to emphasize that coherence theory deals with phenomena that are sensitive to the “envelope” or average intensity of the field.
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This article presents a review of coherence properties of electromagnetic fields and their measurements, with special emphasis on the optical region of the spectrum. Analyses based on both the classical and quantum theories are described. After a brief historical introduction, the elementary concepts which are frequently employed in the discussion of interference phenomena are summarized. The measure of second-order coherence is then introduced in connection with the analysis of a simple interference experiment and some of the more important second-order coherence effects are studied. Their uses in stellar interferometry and interference spectroscopy are described. Analysis of partial polarization from the standpoint of correlation theory is also outlined. The general statistical description of the field is discussed in some detail. The recently discovered universal "diagonal" representation of the density operator for free fields is also considered and it is shown how, with the help of the associated generalized phase-space distribution function, the quantum-mechanical correlation functions may be expressed in the same form as the classical ones. The sections which follow deal with the statistical properties of thermal and nonthermal light, and with the temporal and spatial coherence of blackbody radiation. Later sections, dealing with fourth- and higher-order coherence effects include a discussion of the photoelectric detection process. Among the fourth-order effects described in detail are bunching phenomena, the Hanbury Brown-Twiss effect and its application to astronomy. The article concludes with a discussion of various transient superposition effects, such as light beats and interference fringes produced by independent light beams.
Introduction The degree of first-order temporal coherence, a function denoted by g (1) ( τ ), provides information about the coherence length and the power spectral density of a light source. However, without additional information, g (1) ( τ ) has no bearing on intensity fluctuations and higher-order statistics of the emitted light. A quasi-monochromatic laser beam and the beam of light from an incandescent light bulb, provided that the latter is properly filtered to match the spectral line-shape of the former, will have identical degrees of first-order coherence. Any interferometric experiment involving the splitting and superposition of amplitudes would yield identical results for the laser beam and the (properly filtered) thermal light. Therefore, on the basis of such experiments alone, there is no way to distinguish the two light sources. It turns out, however, that the intensity fluctuations of laser light are fundamentally different from those of thermal light. The two sources can, therefore, be distinguished based on their second-order coherence properties. An ideal photodetector produces an electrical signal proportional to the “cycle-averaged intensity” of the E -field (or B -field) of the light beam at the location of the detector. Assuming that the electrical bandwidth of the detector (including all associated circuitry) is greater than the bandwidth of the incident light wave by at least a factor of 2, the output of the detector should accurately represent the intensity fluctuations of the light beam as a function of time.
Introduction In the preceding chapters we have been largely concerned with the simplest coherence effects of optical fields, namely those which depend on the correlation of the field variable at two space-time points ( r 1 , t 1 ) and ( r 2 , t 2 ). As we have seen, these effects include the most elementary coherence phenomena involving interference, diffraction and radiation from fluctuating sources. In this chapter we present an extension of the theory to cover more complicated situations, which have to be described by correlations of higher order, i.e. by correlations of the field variables at more than two space-time points or the expectation values involving various powers and products of the field variables. Situations of this kind have become of considerable importance since the development of the laser and of nonlinear optics. The basic difference between the statistical properties of thermal light and laser light can, in fact, only be understood by going beyond the elementary second-order correlation theory. Many of the higher-order coherence phenomena are most clearly manifest in the photoelectric detection process, which can only be adequately described by the quantum theory of detection or by taking into account the quantum features of the field, both of which will be studied in the succeeding chapters. However, because the field is still described classically in the semi-classical theory of light detection, and also because the classical field description provides a natural stepping stone to the quantum description of field correlations, we will now discuss the general description of field correlations of all orders on the basis of the classical theory of the fluctuating wavefield.
Electromagnetic Wave Theory and Measurement of Light Electromagnetic Wave Theory of Light Classical Superposition Measurement of Light Intensity of Light: Expectation and Fluctuation Measurement of Intensity: Ensemble Average and Time Average Coherence Property of Light-The State of the Radiation Coherence Property of Light Temporal Coherence Spatial Coherence Diffraction and Propagation Diffraction Field Propagation Optical Imaging A Classic Imaging System Fourier Transform via a Lens First-Order Coherence of Light First-Order Temporal Coherence First-Order Spatial Coherence Second-Order Coherence of Light Second-Order Coherence of Coherent Light Second-Order Correlation of Chaotic-Thermal Radiation and the HBT Interferometer The Physical Cause of the HBT Phenomenon Near-Field Second-Order Spatial Coherence of Thermal Light Nth-Order Coherence of Light Nth-Order Near-Field Spatial Coherence of Thermal Light Homodyne Detection and Heterodyne Detection of Light Optical Homodyne and Heterodyne Detection Balanced Homodyne and Heterodyne Detection Balanced Homodyne Detection of Independent and Coupled Thermal Fields Quantum Theory of Light: Field Quantization and Measurement The Experimental Foundation-Part I: Blackbody Radiation The Experimental Foundation-Part II: Photoelectric Effect The Light Quantum and the Field Quantization Photon Number State of Radiation Field Coherent State of Radiation Field Density Operator and Density Matrix Composite System and Two-Photon State of Radiation Field A Simple Model of Incoherent and Coherent Radiation Source Pure State and Mixed State Product State, Entangled State, and Mixed State of Photon Pairs Time-Dependent Perturbation Theory Measurement of Light: Photon Counting Measurement of Light: Joint Detection of Photons Field Propagation in Space-Time Quantum Theory of Optical Coherence Quantum Degree of First-Order Coherence Photon and Effective Wavefunction Measurement of the First-Order Coherence or Correlation Quantum Degree of Second-Order Coherence Two-Photon Interference vs. Statistical Correlation of Intensity Fluctuations Second-Order Spatial Correlation of Thermal Light Photon Counting and Measurement of G(2) Quantum Entanglement EPR Experiment and EPR State Product State, Entangled State, and Classically Correlated State Entangled States in Spin Variables Entangled Biphoton State EPR Correlation of Entangled Biphoton System Subsystem in an Entangled Two-Photon State Biphoton in Dispersive Media Quantum Imaging Biphoton Imaging Ghost Imaging Ghost Imaging and Uncertainty Relation Thermal Light Ghost Imaging Classical Simulation of Ghost Imaging Turbulence-Free Ghost Imaging Two-Photon Interferometry I: Biphoton Interference Is Two-Photon Interference the Interference of Two Photons? Two-Photon Interference with Orthogonal Polarization Franson Interferometer Two-Photon Ghost Interference Delayed Choice Quantum Eraser Two-Photon Interferometry II: Quantum Interference of Chaotic Light Two-Photon Young's Interference Two-Photon Anticorrelation with Incoherent Chaotic Light Two-Photon Interference with Incoherent Orthogonal Polarized Chaotic Light Bell's Theorem and Bell's Inequality Measurement Hidden Variable Theory and Quantum Calculation for the Measurement of Spin 1/2 Bohm State Bell's Theorem and Bell's Inequality Bell States Bell State Preparation
Optical interferometry was at the heart of the revolution which ushered in the new era of twentieth century physics. For example, the Michelson interferometer was used to show that there is no detectable motion relative to the ‘ether’; a key experiment in support of special relativity. It is a wonderful tribute to Michelson that the same interferometer concept is central to the gravity-wave detectors which promise to provide new insights into general relativity and astrophysics in the twenty-first century. Similar tales can be told about the Sagnac and Mach–Zehnder interferometers as discussed in this chapter. We further note that the intensity correlation stellar interferometer of Hanbury- Brown and Twiss was a driving force in ushering in the modern era of quantum optics. We are thus motivated to develop the theory of field (amplitude) and photon (intensity) correlation interferometry. In doing so we will find that the subject provides us with an exquisite probe of the micro and macrocosmos, i.e., quantum mechanics and general relativity. With these thoughts in mind we here develop a framework to study the quantum statistical correlations of light. We will motivate the quantum correlation functions of the field operators from the standpoint of photodetection theory. Many experimentally observed quantities, such as photoelectron statistics and the spectral distribution of the field, can be related to the appropriate field correlation functions. These correlation functions are essential in the description of Young's double-slit experiment and the notion of the power spectrum of light.