Nonlinear Optical Technology E-Book

Elsa M. Garmire

• Herausgeber: John Wiley & Sons
• Kategorie: Fachliteratur
• Sprache: Englisch

Veröffentlichungsjahr: 2024

Beschreibung

NONLINEAR OPTICAL TECHNOLOGY Comprehensive resources describing today's Nonlinear Optics (NLO) technology, its applications, and concepts behind the technology Taking shape at the unique interdisciplinary engineering school at Dartmouth College, Nonlinear Optical Technology explores the importance of NLO in terms of how it permeates a vast number of applications such as fiber optics, biomedicine, sensors (especially Internet of Things), microscopy, spectroscopy, and machining, under the assumption engineers of all stripes may end up working in technical areas impacted by Nonlinear Optics (NLO) and would benefit from learning about the field. Each section follows a set format, beginning by describing some exciting new technology made possible by NLO. This part is followed by a description of the background information necessary for students to understand the basic NLO concepts for that application. The author occasionally includes personal experiences as a pioneer in this field where it provides additional understanding and motivation. Each section ends with a description of other developments in technology that use the same NLO concept. Bringing together disparate topics in NLO under a straight-forward rubric based on applications, Nonlinear Optical Technology includes information on: * Extending lasers (with NLO technology), covering new colors (harmonic generation, stimulated raman, and stimulated brillouin) and pulsed lasers (saturable absorption and ultra-high harmonic generation) * Information technology, covering telecommunications (fiber optics NLO and photonic NLO) and data storage (NLO in nanostructures and photonic crystals) * Sensors, covering distributed sensing (brillouin scattering in fibers) and localized sensors (NLO in photonics) * Materials interaction, covering machining (nonlinear absorption), spectroscopy (four-wave mixing), and microscopy (two-photon absorption) Serving as a comprehensive standalone resource on the subject for engineers and students without requiring pre-knowledge of advanced concepts, Nonlinear Optical Technology is an essential resource for those in fields that intersect with NLO applications and integration, as well as anyone who wishes to self-teach NLO concepts in general.

Leseprobe

Nonlinear Optical Technology

From The Beginning

Elsa M. Garmire

Thayer School of Engineering,Dartmouth CollegeHanover, NH, USA

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Published by John Wiley & Sons, Inc., Hoboken, New Jersey.Published simultaneously in Canada.

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Library of Congress Cataloging-in-Publication DataNames: Garmire, Elsa M., 1939– author. | John Wiley & Sons, publisher.Title: Nonlinear optical technology : from the beginning / Elsa M. Garmire.Description: Hoboken, New Jersey : Wiley, [2024] | Includes index.Identifiers: LCCN 2024005319 (print) | LCCN 2024005320 (ebook) | ISBN 9781119508359 (cloth) | ISBN 9781119508373 (adobe pdf) | ISBN 9781119508342 (epub)Subjects: LCSH: Nonlinear optics.Classification: LCC QC446.2 .G377 2024 (print) | LCC QC446.2 (ebook) | DDC 621.36/94–dc23/eng/20240310LC record available at https://lccn.loc.gov/2024005319LC ebook record available at https://lccn.loc.gov/2024005320

Cover Design: WileyCover Image: © MirageC/Getty Images

To Jarus Quinn of the Optical Society of AmericaFor inspiring me to leadershipAndTo Robert Russell, my spouseFor inspiring me to courage

AndTo disadvantaged individuals worldwide who dream of advanced education. May they overcome adversity and contribute to global harmony.

Raman Laser Image, photographed in color (1964) Elsa M. Garmire, PhD.

Preface

Nonlinear optics (NLO) began in 1962 as a revolutionary new technology arising from the newly developed ruby laser. The ruby laser emitted a new kind of coherent light never seen before; previous lamps emitted incoherent light. Ordinary light is characterized by the energy and momentum of its photons. Coherent light consists of electromagnetic waves, characterized by the amplitude and phase of their electric fields.

Nonlinear optics occurs when intense, coherent light waves interact in a nonlinear transparent material. NLO was first demonstrated only a year after the first laser by focusing the ruby laser's high‐power coherent light into crystal quartz.

This author was fortunate to enter graduate school at MIT in 1961 as the first student of Prof. Charles Townes (Nobel Prize, 1964), who was new to MIT. My first year was spent in “beta‐testing” the second commercially sold ruby laser and choosing NLO as a topic for study. This amazing opportunity allowed me to contribute, along with other colleagues, to the beginnings of NLO. Over the next six years, our NLO research group contributed to new facets of nonlinear optics: nonlinear refractive index, stimulated Raman Scattering, and stimulated Brillouin scattering.

After MIT, optical nonlinearities remained a major focus of my career. Arriving at Caltech as a research scientist (no women faculty or students allowed), I collaborated with Prof. Amnon Yariv on NLO for mode‐locking lasers. By 1970, we pivoted as pioneers into the new field of integrated optics (now called photonics), envisioning the integration of miniature semiconductor lasers, modulators, and detectors all onto a single substrate. My contribution at Caltech was to design and build the first university‐based equipment for GaAs/AlGaAs epitaxy and device fabrication for integrated optics.

Transitioning to USC in 1975, I led a research group studying nonlinear optics in semiconductors over the next 20 years. My 30 PhD students and assorted others contributed to understanding, fabricating, design, and study of early nonlinear photonic technology. Optical bistability studies led to enhanced nonlinearities at the band edge in semiconductors, quantum wells, and multiple n‐i‐p‐i structures.

In 1995, Dartmouth College tapped me as dean of Thayer School of Engineering, where I became entranced by their teaching approach: engineering‐across‐the‐curriculum. With no departments, faculty explained their subjects to students from any technical background. This requires a deep understanding of the breadth of a field as well as a focus on the key concepts, a new skill for me to learn.

By 2000, a second generation of NLO technology applications opened up because of improvements in lasers, optical fibers, and optical materials and fabrication. The need for understanding NLO in twenty‐first century applications became apparent across multiple technologies. My Dartmouth graduate students contributed to some of these.

In retirement, I have collected multiple facets of NLO technology into a single textbook that should be understandable for engineers and scientists of many backgrounds. Hence, this book introduces each facet of NLO from its historical beginning, with the most basic explanations. Each chapter then connects this fundamental understanding to NLO technology as used in today's applications.

Each technical chapter introduces a different aspect of an optical nonlinearity: second harmonic generation, optical parametric amplification and oscillation, higher‐order harmonics, nonlinear refractive index, self‐phase modulation, spatial solitons, four‐wave mixing, stimulated Raman and Brillouin scattering, and nonlinear absorption.

For students new to NLO, each technical chapter is preceded by a summary section that introduces the concepts, explains why this facet of nonlinear optics is important, and describes a few applications. For students without a background in optics, four appendices are available: linear optics, materials and linear optics, optical resonators, and waveguides.

Acronyms

ADP

ammonium dihydrogen phosphide

A‐S

anti‐Stokes

BOTDA

Brillouin optical time delay analyzer

CARS

coherent anti‐Stokes Raman Scattering

CCD

charge coupled device

CLSM

confocal laser scanning microscope

CPA

chirped pulse amplifier

CSP

cadmium silicon phosphide (CdSiP2)

CW

continuous wave

dB

deciBell

DBR

distributed Bragg reflector

DC

direct current

DFG

difference frequency generation

DPSS

diode pumped solid state (lasers)

DWDM

dense wavelength division multiplexing

EFISH

electric field‐induced SHG

EM

electromagnetic

EUV

extreme ultraviolet

FBG

fiber Bragg gratings

FPI

Fabry–Perot interferometer

FROG

frequency‐resolved optical gate

FWHM

full width half maximum

FWM

four‐wave mixing

GRIN

graded index

HHG

high harmonic generation

IR

infrared

KDP

potassium dihydrogen phosphide

KLM

Kerr lens mode‐locking

KTP

potassium titanyl phosphate

LIGO

light interferometer gravitational observatory

LN

lithium niobate

MOPA

master oscillator parametric amplifier

Nd

neodymium

NIR

near infrared

NLO

nonlinear optics

NEW

nonlinear wave equation

OFC

optical frequency cascade

OKE

optical Kerr effect

OPA

optical parametric amplifier

OPCPA

optical parametric chirped pulse amplifier

OPL

optical path length

OPO

optical parametric oscillator

OTDR

optical time domain reflectometry

PCF

photonic crystal fibers

PCM

phase conjugate mirror

PD

(dielectric) polarization density

PM

phase match

PMT

photomultiplier tube

PPLN

periodically poled lithium niobate

PW

(dielectric) polarization wave

QM

quantum mechanics

QND

quantum non‐demolition

QPM

quasi‐phase‐matching

RHS

right‐hand side

RT

room temperature

SA

saturable absorption

SBS

stimulated Brillouin scattering

SESAM

semiconductor saturable absorber mirror

SFG

sum frequency generation

SH

second harmonic

SHG

second harmonic generation, second‐order nonlinear coefficient 7

SI

standards internationale

SLM

spatial light modulator

SPM

self‐phase modulation

SRS

stimulated Raman scattering

SXR

soft x‐ray

TDM

time division multiplexing

THG

third harmonic generation

TPA

two‐photon absorption

TWM

two‐wave mixing

USP

ultrashort pulses

UV

ultraviolet

VUV

very ultraviolet

WDM

wavelength division multiplexing

XPM

cross‐phase modulation

YAG

yttrium aluminum garnet

YALO

yttrium aluminum lanthanum oxide

YLF

yttrium lanthanum fluoride

Introduction: Why Nonlinear Optics?

Nonlinear Optics Is the Child of Lasers: Lasers were born in May, 1960, as flashes of red light that dazzled observers and awed the inventors! Light unlike any created before! So powerful that it drilled holes in razor blades and ionized the air! The first time since the creation of earth had such dazzling light been seen!

Laser light’s dazzle comes from the fact it emits coherent light. How can we imagine coherent light? Why does coherent light dazzle? Coherence means that all parts of a light beam have a phase relation with every other part of the beam that is fixed for all time. The fixed phase controls exactly what is happening to the beam. It can now be focused to an extraordinarily small point – bright enough that laser light alone can break down air, creating a startling (and dangerous) spark!

We explain the difference between ordinary

incoherent light and

coherent light this way: Think of ordinary

incoherent light

as a cloud of photons, each traveling in a different direction; like a crowd of individual people on New Year’s Eve, milling around in Times Square. These “photons” move randomly in many directions, individually or as small groups. Such crowds may be uncomfortable; motion is blocked by innumerable collisions and it is difficult to get out of the high density.

Coherent light

is comparable to a parade of well‐trained soldiers, marching in rows with synchronous steps and swinging arms. Their periodic motion mimics photons in a coherent wave. Each has the same periodic arm‐swing (frequency) and phase‐fronts (defining lines of identical physical motion). All individuals, identically marching can be impressively powerful. No wonder militaristic countries advertise their strength with vast fields of marching soldiers. Want to be powerful? Be coherent, not incoherent. What hopes for the future reside in coherent waves!

In 1961 lasers were the only source of powerful coherent optical waves. The laser revolution opened up opportunities for nonlinear optics (NLO), which requires powerful, coherent light beams. This book describes how this new field developed, from its original concepts through its developing technology to its wide application today. We learn how powerful coherent light interacts nonlinearly with transparent materials. We also present some of the fascinating science and engineering that resulted from applying the tools of nonlinear optics.

Why Is Nonlinear Optics Important? Lasers were the first technology that provided the coherent optical waves that enable NLO. From some primitive early experiments until today, laser technology has expanded into vastly new realms never before reached by human technology. New realms in time‐, frequency‐, and intensity‐scales that cannot be reached by any other means. Advanced NLO technologies have replaced traditional optical systems and offered numerous opportunities for technology development. Through NLO, coherent laser light has been transformed to new wavelengths (throughout the visible and extending to extreme UV, to IR, and further to terahertz radiation); light pulses have been shortened to femtoseconds (10−15 s) and NLO provides the only way to measure such short pulses. Nonlinear optics is behind several Nobel prizes: observing gravity waves for the first time required NLO to “squeeze” noise out of laser light to well below quantum predictions.

Important commercial applications such as optical fiber telecommunication systems were originally limited by optical nonlinearities in fibers, but NLO technology overcame all those impediments. Now, after 50 years of research, state‐of‐the‐art NLO devices and systems form the basis of ultrahigh‐speed data switching. World‐record laser intensities (W/cm2) are generated by NLO amplifiers in ultrahigh power laser facilities around the world. Practically anywhere lasers are used, NLO can make them better: physics, chemistry, biology, medicine; with commercial, industrial, and basic scientific applications.

Author’s Intent: Nonlinear optics requires coherent light. From a classical point of view, coherence means the light from lasers is truly “wavelike.” Even today, lasers remain the only direct sources of coherent light. Fortunately, nonlinear optics provides a lossless means to transform one form of coherent light into another while conserving energy and momentum. Read how this book extends traditional “linear optics” (low‐power, weak light) into the realm of “nonlinear optics” (high‐power, coherent) in dielectric nonlinear materials. Our approach includes both science and engineering viewpoints.

Readers will learn about the “magic” of NLO when powerful, coherent light travels through certain nonlinear transparent materials that transform coherent light in ways that appear magic! Each of the major NLO technologies is explained here, from beginning concept through analysis to final demonstration.

The book title says “from the beginning,” a phase that has several meanings; one means beginning from 1962. The second means beginning with the physics of light interacting with atoms and molecules in materials, describing technical challenges that were overcome, and ending with examples of today’s technology.

Yet a third meaning for “from the beginning” describes the author’s personal involvement in the exciting state of nonlinear optics from its beginning. I was fortunate enough to be a part of the first five years of nonlinear optics, as an MIT graduate student and post‐doc from 1961 to 1966. My PhD research pioneered stimulated Raman and Brillion scattering, and my post‐doc investigated nonlinear refractive index that introduced spatial solitons and four‐photon‐mixing.

This book is designed to be understandable by all interested users, no matter their technical field. No complex variables are introduced so the mathematics is straightforward. All concepts needed to understand nonlinear optical interactions are explained using classical concepts. Since NLO technology is rapidly evolving into new technical areas, I foresee numerous scientists and engineers having questions about NLO. This book is for those who may be curious about how NLO technology has changed the world.

The technical level is assumed to be science/engineering seniors or first‐year graduate students. Design issues are emphasized, particularly in problem assignments, offering possibilities of credit for engineering design.

Finally, it is planned that readers can teach themselves the material (along with access to the internet). Most topics have considerable supplemental information in Wikipedia, and other similar resources. I envisage some readers to have moved into this field later in life and are looking for a source “from the beginning.” Alternatively, this book offers key words that can lead readers to additional open sources on the internet to find much greater detail. Wikipedia is often a good source.

Textbook Contents: The purpose of studying NLO is to investigate how coherent light can interact with nonlinear dielectric materials and their technological implications. The background of the field includes understanding: (i) properties of coherent light; (ii) properties of dielectric materials; (iii) characteristics of the light–material interaction; (iv) practical applications.

Typical NLO textbooks assume the reader already understands these basic concepts in the linear regime and then emphasize the nonlinear interactions. As this book is designed for beginners whose background might be in fields broader than physics or engineering (chemistry and biology), we provide self‐contained appendices introducing necessary basic concepts.

My approach is to simplify concepts down to their basics, most of which were discovered in the 1960s. Students will come to understand how the earliest experiments opened up this entirely new field. This will be the basis for understanding how advancements in laser technology have led to revolutions in NLO. Femtosecond pulses have enabled reaching instantaneous nonlinearities at modest power levels. Lasers that include nonlinear optical fibers have offered exciting new commercial applications. Nonlinear optics in photonic crystals, in quantum, and in 2D materials is becoming practical, and nonlinearities are even finding opportunities in nanomaterials. This book provides the basics, but details are beyond this course level.

Three Branches of Nonlinear Optics: Nonlinear optics technology, as a whole, neatly divides into three branches, each of which is treated separately. While the underlying principles are the same for all three, their differences diverge immediately into three separate branches. The technical chapters are separated by branch, each containing three to four chapters.

Branch #1 is second‐order nonlinearities that can change the frequency of monochromatic coherent light using specific nonlinear crystals;

Branch #2 is third‐order nonlinearities that occur in all media, creating an intensity‐dependent refractive index (as well as third harmonic);

Branch #3 is optical‐loss nonlinearities. These are: inelastic scattering of coherent light from molecular vibrations (stimulated Raman scattering); similar scattering from acoustic waves (stimulated Brillouin scattering); nonlinear loss (two‐photon and saturable absorption).

Nonlinear Optics Summary: This textbook provides a summary of the nonlinear optics first, before providing technical chapters that delve into the details of each approach to nonlinear optics. This summary is intended as an overall high‐level introduction to these NLO technologies and motivations for their importance. It is expected that students will read the relevant summary before studying the technical chapter. After understanding the technical chapter, reading its summary again should cement necessary understanding.

Technical Chapters: The technical chapters describe each technology completely, beginning with explaining the way that high‐intensity light creates nonlinear coherent motion within molecules. The molecular motion creates an oscillating polarization density that radiates new frequencies, and/or mixes wave directions. If these new frequencies match waves available in ambient light waves, there will be gain and the new waves will grow. We numerically calculate expected performance for each of these nonlinearities.

Each chapter focuses on a specific nonlinear optical process, including detailed explanations of the nonlinear processes and mathematical proofs relevant to those explanations. Depending on the level of the student and the course, details of the proofs can be ignored. However, they are there to fulfill anyone’s curiosity about what is really happening. Each chapter describes technology that uses that specific process.

Appendices for “Just‐in‐Time Learning”: Each new concept will be introduced only when it is needed, along with the background required to understand the new idea. This book is organized with background information provided in appendices. The basic information on Linear Optics, needed to understand Nonlinear Optics, is presented in Appendices A and B, which describe how light and transparent dielectric materials interact with light in the linear world. Somewhat more specialized are C, describing resonant optical cavities and D, describing waveguides. Hopefully these appendices, provided at the end, will provide useful background material when needed.

Why Did NLO Development Wait So Long? The first operating laser was reported in 1960 and the first NLO experimental results were just a year later. Most of the basic NLO concepts were understood within the first seven years. However, it took a long time to “tame” laser technology enough to make NLO “practical.”

The first commercial use of nonlinear optics was probably the first “green laser pointer.” These quotation marks are because these are not green lasers. In fact, they are infrared lasers that excite the second order optical nonlinearity to convert invisible infrared light into green light. This second‐harmonic process is extraordinarily efficient, which is why the “green laser pointer” uses extraordinarily small batteries.

Originally, NLO technology had very limited options, was difficult to control, and was studied in a limited number of laboratories. Over the years, literally every aspect of the technology needed major improvement.

Important steps included: (i) more practical lasers; (ii) shorter laser pulses; (iii) better NLO materials; (iv) reproducibility by computer control; (v) avoiding laser damage; (vi) mode‐locking for ultra‐short pulses (USP); (vii) measurement technology for USP; (viii) theoretical support for quantum theory; (ix) engineering design for optical system technology; (x) improved optical fibers.

This list is not complete, nor up‐to‐date. By 2000, it’s fair to say that NLO was a technology with some well‐defined applications, particularly with high‐intensity lasers. However, since then a great deal of progress has been made in the fabrication of artificial materials, designed for NLO applications. All these developments, and opening of more commercial applications, have broadened the desirability of including optical nonlinearities in optical systems design.

Nonlinear Optics Contributes to Photonics: Technology is at a cusp in the field called “photonics,” defined here as the application of light generation, detection, and manipulation to improve optical systems. Those who have learned the concepts in this textbook will be prepared to contribute to any of these categories.

Light generation with NLO begins with second harmonic oscillators, optical parametric oscillators, stimulated Raman scattering, stimulated Brillouin scattering. Pulse generation is controlled by Q‐switching and mode‐locking.

Light detection of ultra‐short‐pulses requires NLO, whose processes are orders of magnitude faster than electronics. Noise floors can be driven well below the quantum limit by NLO, which are processes that introduce no system loss.

Light manipulation occurs from a nonlinear refractive index; nonlinear wave‐mixing among two or more waves creates changes in both time and space, which have impact on frequencies and directions of propagation. These applications are limitless.

Nonlinear optics is developing all‐optical techniques to contribute to photonics: faster, smaller, and more compact and robust, with more functionality. I hope you will find NLO as exciting as I have. I look forward to seeing what gets built next!