Two-Window Heterodyne Methods to Characterize Light Fields by Frank Reil Department of Physics Duke University
Total Page:16
File Type:pdf, Size:1020Kb
Copyright °c 2003 by Frank Reil All rights reserved Two-Window Heterodyne Methods to Characterize Light Fields by Frank Reil Department of Physics Duke University Date: Approved: Dr. John E. Thomas, Supervisor Dr. Daniel J. Gauthier Dr. Robert Behringer Dr. Moo-Young Han Dr. Bob Guenther Dissertation submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy in the Department of Physics in the Graduate School of Duke University 2003 abstract (Physics) Two-Window Heterodyne Methods to Characterize Light Fields by Frank Reil Department of Physics Duke University Date: Approved: Dr. John E. Thomas, Supervisor Dr. Daniel J. Gauthier Dr. Robert Behringer Dr. Moo-Young Han Dr. Bob Guenther An abstract of a dissertation submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy in the Department of Physics in the Graduate School of Duke University 2003 Abstract In this dissertation, I develop a novel Two-Window heterodyne technique for mea- suring the time-resolved Wigner function of light fields, which allows their complete characterization. A Wigner function is a quasi-probability density that describes the transverse position and transverse momentum of a light field and is Fourier- transform related to its mutual coherence function. It obeys rigorous transport equations and therefore provides an ideal way to characterize a light field and its propagation through various media. I first present the experimental setup of our Two-Window technique, which is based on a heterodyne scheme involving two phase-coupled Local Oscillator beams we call the Dual-LO. The Dual-LO consists of a focused beam (’SLO’) which sets the spatial resolution, and a collimated beam (’BLO’) which sets the momental resolution. The resolution in transverse posi- tion and transverse momentum can be adjusted individually by the size of the SLO and BLO, which enables a measurement resolution surpassing the uncertainty principle associated with Fourier-transform pairs which limits the resolution when just a single LO is used. We first use our technique to determine the beam size, transverse coherence length and radius of curvature of a Gaussian-Schell beam, as well as its longitudinal characteristics, which are related to its optical spectrum. We then examine Enhanced Backscattering at various path-lengths in the turbid medium. For the first time ever, we demonstrate the phase-conjugating properties of a turbid medium by observing the change in sign of the radius of curvature for a iv non-collimated field incident on the medium. We also perform time-resolved mea- surements in the transmission regime. In tenuous media we observe two peaks in phase-space confined by a hyperbola which are due to low-order scattering. Their distance depends on the chosen path-delay. Some coherence and even spatial prop- erties of the incident field are preserved in those peaks as measurements with our Two-Window technique show. Various other applications are presented in less de- tail, such as the Wigner function of the field inside a speckle produced by a piece of glass containing air bubbles. v Acknowledgments First, I would like to thank John Thomas for being a great advisor. His love of physics has been contagious and his insight inspirational; this helped me change my mind when I had thought about switching to economics. From a human side, John has always been a very decent person who was fun to be around. Thank you also to all my friends I made at Duke. Among my fellow graduate students I would first like to mention Jon Blakely, with whom I had many political discussions. He’s a perfect example that you can disagree with someone on a great many political topics and be good friends with the same time. The same is true for Rob Macri, who I’ve been with to China Inn numerous times over the past years. I would also like to mention Steve-O Nelson, Shigeyuki Tajima, Konstantin Sabourov and Bob Hartley, with whom I hung out on many occasions, when the busy schedule in graduate school permitted, as well as Meenakshi Dutt, Marina Brozovic and Michael Stenner, which I liked to socialize with in the department. I’m also thankful to all my fellow co-workers in our group who provided a very pleasant environment to work in: My predecessor Adam and post-doc Samir, who left in 1999 and 2000; then Ken, Mike, Stephen and Lee, and also the students who joined the group after me, Staci and Joe. I would also like to thank my friends at home who I kept in contact with over the years, most of all my good buddy of 17 years, Martin Nolte. He is one of the funniest and most decent people I know, and manages to organize circle of friends vi reunion-parties each time I visit home. Most of all I’m thankful to my parents back in Germany who have instilled my early interest in science by giving me vividly illustrated books of astronomy and paleontology when I was about 5, while never pushing me to do or read stuff I didn’t like to. Oftentimes my father would sat down with me to translate some of those huge numbers appearing next to the pictures of Jupiter, Saturn and the Andromeda galaxy into some hand-drawn figure which was more accessible to my mind at that stage - and still is :-). Next came the electronics kit I got for chrismas when I was 8. I wasn’t allowed to use the soldering iron by myself since my father was afraid I could hurt my fingers which would have interfered with me playing piano. One night, when everyone was asleep, I went down to the basement anyway to finish that multivibrator - needless to say, I ended up with a couple of blisters on my fingers. Despite that first, rather unpleasant experience with experimental physics, a decade of distorted radio reception for my poor mother during her favorite radio shows ensued, which she bravely put up with in the name of science - thank you, Mama! Also thanks to my sister Margarete who has been a great sibling and friend over the years we grew up together. vii For my beloved parents and sister viii Contents Abstract iv Acknowledgments vi List of Figures xvii 1 Introduction 1 1.1 Wigner functions and their measurement . 3 1.2 Thesis organization . 10 2 Optical Tomography Methods 12 2.1 Introduction . 12 2.2 Time domain methods . 13 2.3 Frequency domain methods . 15 2.4 Optical techniques using fluorescence . 16 2.4.1 Multiphoton Fluorescence Microscopy . 16 2.4.2 Fluorescence Lifetime Imaging (FLIM) . 17 2.5 Resolution-enhancing techniques . 17 2.5.1 Near-field Scanning Optical Microscopy (NSOM) . 17 2.5.2 Deconvolution Microscopy . 18 2.6 Optical spectroscopy for chemical analysis . 20 2.6.1 Infrared Spectroscopy . 20 ix 2.6.2 Raman spectroscopy . 20 2.7 Confocal Microscopy . 21 2.8 Optical Coherence Tomography (OCT) . 23 2.8.1 Optical Coherence Microscopy (OCM) . 24 2.8.2 Color Doppler Optical Coherence Tomography (CDOCT) . 25 3 Wigner Functions 27 3.1 Introduction . 27 3.2 Basic properties of Wigner functions . 29 3.3 Wigner functions of spatially incoherent light . 29 3.4 Wigner functions of a (partially) coherent beam................................... 30 3.5 Integrals of Wigner functions . 32 3.6 Propagation through linear optical systems . 33 3.6.1 General Luneburg’s first order systems . 35 4 The Single-Window technique 39 4.1 Introduction . 39 4.2 Experimental setup with laser light source . 40 4.3 Balanced detection scheme . 44 4.4 Real time noise suppression . 45 4.5 Automated data acquisition . 46 4.6 Experimental setup with SLD light source . 47 4.7 Measurement of smoothed Wigner functions . 49 4.7.1 Mean square beat signal for transversely coherent light . 54 x 4.7.2 Averaging over fields . 56 4.7.3 Resolution of the Single-Window technique . 57 5 The Two-Window technique 58 5.1 Introduction . 58 5.2 Experimental Setup . 59 5.3 Measurement of true Wigner functions . 61 5.3.1 Extraction of the true Wigner function from SB(x; p) . 65 5.3.2 Relation between SB(x; p) and the Signal field . 67 5.4 Interpretation of the measured beat signals . 69 5.4.1 Ideal Dual-LO . 71 5.4.2 The quadrature signals . 73 5.4.3 Non-ideal Dual-LO . 75 5.4.4 Measurement of Gaussian beams for ideal and real Dual-LO 78 5.5 Acousto-optic modulators (AO) . 79 5.6 Phase-locked loop . 80 5.7 4f-system . 82 5.8 Overview of complete system . 84 6 The Superluminescent diode 87 6.1 Introduction . 87 6.2 General parameters of the SLD . 88 6.2.1 Determination of the SLD’s longitudinal coherence length . 89 6.3 Collimation of SLD light . 91 6.4 Drift . 93 xi 6.5 Temperature stabilization . 94 6.6 SLD power supply . 95 7 Characterization of a transverse beam profile - Theory 97 7.1 Introduction . 99 7.1.1 Single-Window method . 99 7.1.2 Two-Window method . 102 7.2 R=1, σs and σg unknown . 103 7.2.1 General beat voltage . 104 7.2.2 Beat voltage for a close-to perfect SLO (A ¿ B) . 107 7.2.3 Beat voltage for our special BLO (A = B) . 108 7.2.4 Complex beat signal SB(dx; px) . 109 7.2.5 Extraction of σs and σg from the quadrature signals . 110 7.3 σg=1, σs and R unknown . 116 7.3.1 General beat voltage . 116 7.3.2 Beat voltage for a perfect SLO (A ¿ B) .