Optomechanical Systems Engineering

By Keith J. Kasunic

Covers the fundamental principles behind optomechanical design

This book emphasizes a practical, systems-level overview of optomechanical engineering, showing throughout how the requirements on the optical system flow down to those on the optomechanical design. The author begins with an overview of optical engineering, including optical fundamentals as well as the fabrication and alignment of optical components such as lenses and mirrors. The concepts of optomechanical engineering are then applied to the design of optical systems, including the structural design of mechanical and optical components, structural dynamics, thermal design, and kinematic design.

 Optomechanical Systems Engineering:

• Reviews the fundamental concepts of optical engineering as they apply to optomechanical design
• Illustrates the fabrication and alignment requirements typically found in an optical system
• Examines the elements of structural design from a mechanical, optical, and vibrational viewpoint
• Develops the thermal management principles of temperature and distortion control
• Describes the optomechanical requirements for kinematic and semi-kinematic mounts
• Uses examples and case studies to illustrate the concepts and equations presented in the book
• Provides supplemental materials on a companion website

Focusing on fundamental concepts and first-order estimates of optomechanical system performance, Optomechanical Systems Engineering is accessible to engineers, scientists, and managers who want to quickly master the principles of optomechanical engineering.

• Language: English
• Publisher: Wiley
• Release date: Apr 27, 2015
• ISBN: 9781118919613

Book preview

Preface

A great mechanical engineer’s experience producing camshafts rarely translates into excellence in opto-mechanical engineering.

John Lester Miller

Principles of Infrared Technology

Most of the literature on optomechanical design takes an in-depth look at the many important details needed to develop optical hardware that works—details such as the specifics of alignment mechanisms, screw threads, epoxies, and the like. While useful, this approach also makes it difficult for readers to understand the basic principles behind the designs. Why, for example, is it more difficult to maintain alignment with a fast, off-axis mirror? Or when does the heat from a high-power laser need to be removed with liquid coolant rather than natural air convection? Or is it necessary that a 3-point kinematic mount be used in all situations, or are there designs where a semi-kinematic mount is perfectly acceptable?

This book takes a step back from the detail-level considerations and instead covers the fundamental principles on which optomechanical design rests. It is unique in that the approach taken in the current books—for example, encyclopedic reviews of the many optical instruments that have been built over the years or collections of equations which only those with sufficient background in mechanical engineering can understand—is not used. Instead, it first reviews the basic concepts of optical engineering, showing how the requirements on the optical system flow down to those on the optomechanical design. The concepts and equations needed to design the optomechanical system then follow naturally. While it does utilize case studies as a pedagogical tool, the cases illustrate fundamental principles, not encyclopedic reviews. This, then, is a book on optomechanical systems engineering—a prerequisite for the many details of optomechanical design found in other books.

While this book may seem to be intended only for mechanical engineers, it is also written for the broader audience of engineers, scientists, managers, and technicians with little or no background in mechanical engineering. This includes anyone designing optical, electro-optical, infrared, or laser hardware. In addition to mechanical engineers, a typical reader might be an optical engineer who wants to learn the fundamental principles of optomechanical engineering, as well as electrical, systems, and project engineers who need to do the same but do not have the academic background consisting of courses in mechanics, strength of materials, structural dynamics, thermal management, heat transfer, and kinematics. Even mechanical engineers with training and experience in these disciplines will find this book useful, as Mr. Miller’s comment at the beginning of this preface indicates.

The book begins with an overview of optical engineering, illustrating how the requirements on the optical system determine what the optomechanical system must be able to do. The overview includes optical fundamentals (Chapter 2) as well as the fabrication (Chapter 3) and alignment (Chapter 4) of optical components such as lenses and mirrors. From there, the concepts of optomechanical engineering are applied to the design of optical systems, including the structural design of mechanical and optical components (Chapters 5 and 6), structural dynamics (Chapter 7), thermal design (Chapter 8), and kinematic design (Chapter 9). Finally, the book closes with a summary chapter tying everything together from a systems engineer’s perspective (Chapter 10).

Tucson, ArizonaKeith J. Kasunic

1

Introduction

Despite our best efforts and intentions, Mr. Murphy—a rather jovial fellow who happens to be a bit sensitive to these things, but who also has a fondness for Schadenfreude—will remind anyone developing optical hardware of his inescapable Law, whether they like it or not. For those who are not prepared, the reminder will be unexpected, and schedules, budgets, and careers will eventually be broken; for those who are prepared, his reminder will be much less painful—and soon forgotten, as the customer’s happiness at receiving working hardware reminds us why we are engineers in the first place.

Fortunately, Pasteur’s antidote to Murphy’s near-deadly snakebites—that chance favors the prepared mind—is our opportunity to remove most of the fatalism from engineering. The type of preparation that this book provides goes under the name of optomechanical engineering—an area of optical systems engineering where the rubber meets the road, and thus has the highest visibility to managers and customers alike [1].

It is sometimes said that optomechanical design is a relatively new field, but the truth is a bit more complicated. Not surprisingly, it is as old as optical engineering itself, a field that dates back to at least the early 1600s when the Dutch were assembling telescopes.¹ Notable contributions by people who are otherwise known as great scientists—but should also be recognized as optomechanical engineers—include:

Galileo—While he did not invent the telescope, a practical telescope architecture using refractive (lens) components is named after him.

Isaac Newton—To develop his theories on the nature of light, he invented the first practical telescope using reflective (mirror) components.

James Clerk Maxwell—In addition to his brilliant discovery of the electromagnetic nature of light, he developed a structural theory of trusses and experimented with photoelasticity and kinematic mounts.

Joseph Fourier—He made many contributions in the areas of heat transfer and thermal design, including the discovery of Fourier’s law of conduction and the Fourier transform for analyzing vibrations, heat-transfer problems, and more recently, electrical circuits.

William Thomson (aka Lord Kelvin)—He is best known for his work in thermodynamics, developing the Kelvin temperature scale. In addition, he deserves to be recognized for his work in kinematics, inventing the Kelvin kinematic mount.

In short, these were people who were trying to solve difficult physics problems but were unable to do so until they first solved the instrumentation problems of how to make an apparatus stiff, stable, repeatable, and so on, that is, solve the state-of-the-art optomechanical problems.

More recently, we are still trying to solve difficult problems including the following applications and even quantum optomechanics [2]:

Aerospace: infrared cameras, spectrometers, high-power laser systems, etc.

Biomedical: fluorescence microscopy, flow cytometry, DNA sequencing, etc.

Manufacturing: machine vision, laser cutting and drilling, etc.

In the following sections of this chapter, we first take a look at what a typical optomechanical system might consist of (Section 1.1), the skills needed to engineer such a system (Section 1.2), and the mindset needed to do this efficiently (Section 1.3).

1.1 Optomechanical Systems

If we buy an optomechanical system today, what would we expect to get for our money? Figure 1.1 illustrates a complex biomedical product known as a swept-field confocal microscope—a microscope with some unique features that allow it to image over a wide field-of-view with high resolution [3, 4].

c1-fig-0001

Figure 1.1 A complex optical system such as a swept-field confocal microscope requires a large number of optomechanical components packaged into a small volume.

Credit: LOCI and Laser Focus World, Vol. 46, No. 3 (Mar. 2010) [3].

Given its complexity, the designers of this microscope had to struggle with many issues that are not obvious to the eye, including:

Assembly and alignment—Can the optical components all be assembled in a small package and maintain critical alignments such as the Focus to CCD distance (for which an adjustment is provided)?

Structural design—Are the overall structure and the optical submounts stiff enough to keep things in alignment due to self-weight or shock loading?

Vibration design—Have scan mirror vibrations been isolated from the optics and prevented from causing the optics to move ever so slightly (but more than is acceptable)?

Thermal design—With components such as the piezos and galvanometers dissipating heat in such a small volume, is there even enough surface area to transfer this heat without the external box temperature getting excessively hot?

Kinematic design—If the microscope needs to be repaired, is there a way to remove critical optics that allows them to be replaced in the field, without a major realignment at the factory?

System design—Have all the interactions between the elements been considered, for example, the effects of heat on the optics?

Before getting to these topics, it is important to first understand that common to all optomechanical systems is the use of electromagnetic waves known as light (Fig. 1.2). This refers to the wavelength of these waves—on the order of 1 µm, but extending down to 0.1 µm or so and up to ~30 µm—and distinct from radio waves, with much longer wavelengths. Controlling the curvature and direction of optical wavefronts with lenses and mirrors is what allows us to create optical images, or determine the wavelengths present, or measure the power transported by a wavefront. Keeping mechanical parts aligned and stable to <1 µm—an extremely small dimension equal to ~40 micro-inches (or 0.04 milli-inches, often pronounced in abbreviated form as mils)—is one of the many challenges of optomechanical engineering.

c1-fig-0002

Figure 1.2 Optical electromagnetic wavelengths (light) can be divided into infrared, visible, and ultraviolet bands.

Credit: NASA (www.nasa.gov).

1.2 Optomechanical