Laser-Induced Damage of Optical Materials

Roger M. Wood

• Publisher: CRC Press
• Year: 2003
• ISBN:
  9781000687330
• Edition: 1
• Topic: Physical Sciences
• Subtopic: Optics & Light

The laser power handling capacities of optical systems are determined by the physical properties of their component materials. At low intensity levels these factors are not important, but an understanding of damage mechanisms is fundamental to good design of laser products operating at high power. Laser Induced Damage of Optical Materials presents

Chapter 1

Introduction

When a low-intensity light beam passes through a transparent substrate or falls on a reflecting mirror or on an absorbing medium, little or no effect may be observed. When the intensity of the beam is increased, however, a whole range of reversible interactions may become obvious. These include temperature rise, expansion, distortion, strain, nonlinear transmittance and absorption, electro-optic effects, second-harmonic generation, optical parametric oscillation and self-focusing. When the intensity of the beam is increased further these phenomena give way to non-reversible changes in the material or component. These effects include surface and bulk annealing, surface melting, material softening and bending, cracking, pitting, bulk melting, vaporization and violent shattering. Although these effects will occur whenever a light beam is powerful enough (either in average or peak energy or power density) the advent of the laser has given prominence to them as common occurrences. It will be assumed, for the rest of this book, that the radiation source is a laser. This will make it easier to calculate the magnitude of the effects since laser beams are usually monochromatic and have precise temporal and spatial characteristics.

The main limitation in the design and utilization of lasers (and in particular miniaturization) is the tendency of the high laser flux generated in the laser cavity to change the optical performance characteristics of the internal optical components. The change in the optical performance of individual components, from those measured as free-standing components, may be due to any one of a number of factors. These effects may be either individual or cumulative. They may limit the transmission of the system, degrade the performance or may lead to laser-induced damage.

The change in component performance may be due to:

thermally induced strain and distortion of the optical elements,

a change in the beam divergence of the transmitted beam due to thermal lensing,

melting, in particular of the dielectric coatings,

laser-induced damage of absorbing specks in the optical materials (e.g. Pt speck in laser glass),

self-focusing in the optical elements,

laser-induced damage of the optical element surfaces due to weak coatings, contaminated surfaces and coatings, or digs/scratches on the coatings/surfaces.

These problems have been experienced over the whole range of laser wavelengths and pulse duration. For example they are well known in the following.

Diode lasers—Most diode lasers exhibit a change of divergence with input power. Degradation of the laser output characteristic occurs due to damage to the internal facets because of a drift of internal defects and impurity ions.

Low-power He/Ne lasers—There is a gradual diminution of the power output with time because of a build-up of contamination on the mirrors leading to a change in the window characteristics with time.

Medium/high-power Nd:YAG lasers—Birefringence effects affect the design of both the laser resonator and the system. The life and output of the laser is usually limited by damage to the anti-reflectance and high-reflectance coatings. There is usually a change of output beam divergence with input power and nonlinear input/output characteristics.

Optically complicated SHG and dye laser systems—Reflections and multiple beams affect the design of these systems.

High-power pulsed and continuous wave CO₂ lasers—Melting, contamination/degradation of the mirror surfaces, distortion/birefringence and change of the beam divergence with input power are all affected by the design, construction and specifications.

Ultra-high-power, short-pulse lasers—Multi-photon ionization and self-focusing limit the energy per pulse.

The highest laser powers/energies are found inside the laser oscillators and amplifiers (until the output powers are focused on a workpiece or target). It is therefore clear that knowledge of the laser-induced damage thresholds of the individual laser and optical materials making up the resonator is necessary before a laser system can be contracted to a minimum size. It is also necessary to ensure that these irradiation levels, including those pertaining to air-breakdown, do not occur before the laser beam reaches its target. In order to gain this knowledge it has been necessary to measure, reliably, the component power and energy handling capabilities of the optical materials involved in a laser resonator and subsequent optical system. Only when this was done could the design of damage-resistant laser/optical systems be finalized.

One of the most pertinent aspects of laser system design and deployment is that of the specification of the optical components which make up the optical system. It is hoped that this treatise will make it clear that both the mounting configuration and the overall design of the system affect the specifications of the individual components. It should also be clear that unless the system designers understand the reasons why they put certain restrictions on the specification they are likely to find inconsistency in the overall operation of the laser system. A system may work well as constructed but when, as happens to all high-power systems, laser-induced damage or even simple degradation of the output occurs, the operator may have problems in obtaining components which allow the original specification to be reached. The replacement components may be to the nominal specification put on by the original laser manufacturer but inferior characteristics and even laser-induced damage may occur. This may be due to an incomplete specification or, as has been seen in many cases, may be the result of the component manufacturer complying with the laser manufacturer’s specification to too high a tolerance. One example of this is taken from the supply of focusing lenses for a 10.6 jum, CO₂ continuous wave (cw) laser. The original laser had operated well but with a slowly degraded output for over a year. It was finally decided to refurbish the laser system and when it was stripped down it was obvious that the final focusing lens was damaged and its transmission was degraded. A replacement lens was ordered from a reputable component manufacturer, delivered and fitted into the system. The laser power to the workpiece was momentarily restored to something approaching its original level although there was some doubt as to the power density delivered to the workpiece. After only a short time of operation the lens cracked. After a considerable amount of recrimination and investigation it was found that, if the component supplier finished the lens diameter to the minimum in the specification (as opposed to the naturally inclined maximum), the lens worked perfectly and the full output power and the original focused power density could be obtained. Subsequent investigation indicated that the tightness of the fit and the consequent differential expansions of the lens and the lens holder caused strain and distortion in the lens. This resulted in a degradation of the lens focusing characteristic (larger focused spot size), the necessity for a slightly higher input energy and a resultant strain in the component that led to catastrophic damage.

Thermally induced distortion of the optical components comprising the resonator can degrade the optical performance at powers well below those required to melt or fracture the optical component. This subject is one of the main problems confronting system designers. The problem comprises the twin effects of a change in the optical figure of the window or lens and the introduction of both birefringence and extra beam divergence.

The necessity for measuring and improving the power handling capacity of optical materials in order to construct more compact laser systems, comes:

1.in the military context, where the deployment of equipment depends critically on its ability to fit into a constricted space and on its input power demands,

2.in the industrial and medical contexts where large systems are cumbersome and where downtime is a serious issue, and

3.in the extreme cases, such as laser-induced fusion, where such high powers need to be generated and the system costs are so enormous that a constant search has to be made to identify components that can handle the high peak powers and energies involved.

The output characteristics of a laser system are affected by the design of the complete system. This means that the temporal and spatial output characteristics are a function of not only the basic resonator design but also the temperature of the system and the input power level. This makes it necessary either to always use a laser system at a constant input power level or to measure and adapt the laser output to the application. Any effect that modifies the output of a laser is important in ensuring the optimum use of the laser in its particular application. These effects include absorption, distortion and laser-induced damage. Experience shows that most lasers are subject to at least one of these effects and if they are to be run for an extended period then care must be taken to minimize all three. Non-uniform absorption of the laser beam and consequent nonlinear or differential expansion by a component or component holder leads to wavefront distortion, the production of extraneous modes, changing beam divergence, strain and cracking. Running a laser resonator even at low output levels can lead to contamination of the mirror and windows, alteration of the resonator conditions and either cessation of lasing or catastrophic damage to the optical components. Running a laser at high output levels can lead to immediate single-shot damage and running a laser over an extended period or at a high pulse repetition frequency can lead to cumulative or pulserepetition frequency (prf )-dependent laser-induced damage.

This book has been written in order to help engineers, managers etc. to understand that they cannot expect to generate increasingly high powers and energies from their laser systems without coming to some sort of compromise between these powers/energies and the laser system characteristics. The first laser systems were bulky and yielded what today would be regarded as ridiculously low outputs. The size and inferior output characteristics of these systems inhibited the use of these lasers in many industrial and military applications. The limitations of these lasers pressurized the system manufacturers both to increase efficiency and to reduce system size. The developments over the years have led to an impressive range of laser types, wavelengths, powers and energies and there are now relatively small systems that generate enough power/energy to fulfil a range of applications. These applications include laser-induced fusion, cutting, welding and drilling of engineering materials, medical, communications, and military (see chapter 8 for a summary of these). However, the sheer success of the engineering effort put into laser development has resulted, in many cases, in a series of ‘black boxes’ and it has been noticeable that there is now a new generation of laser engineers ‘who knew not Joseph’ (The Bible, Exodus 1 v 8). The engineers who developed the present laser systems had to overcome or live with the constraints inherent with generating high laser powers and energies. In many cases the present generation have come to accept the engineering benefits without understanding why the detail is so important. There is now growing up two series of laser engineers, those who understand the laser systems and those who use the same systems. This book is designed to be a bridge between these two schools of engineers.

It has been said that the history of a man begins with his father and mother. Similarly the mechanisms and morphology of laser-induced damage in optical materials can be traced back to the electronic and physical parameters of these materials. The book therefore starts by considering the geometrical and physical properties of optical and laser materials. These include the origin of the wavelength dependent absorption as well as the many other sources of light absorption, which lead to a change in the material optical and thermal characteristics.

Chapter 3 includes a discussion of the optical nonlinear mechanisms, which occur at medium power levels below the threshold for laser-induced damage. Nonlinear effects, whilst not necessarily leading directly to laser-induced damage, may however lead to increased absorption and local temperature rise. All materials exhibit some absorption and, if the incident energy/power is high enough, will melt. The threshold at which this will occur is both pulse length and wavelength dependent as well as being a function of the material and laser spot sizes and the ambient temperature. The pertinent material parameters will be the absorption, at the laser wavelength, the specific heat, the conductivity and the melting temperature. If the laser pulse is high powered but of short duration then avalanche ionization and/or multiphoton absorption may take place. These effects, which are only experienced under short-pulse, high-power conditions, may in fact occur at lower irradiation levels than those required for melting. However this is only observed in the case of low-absorption, highly transparent optical materials.

Chapter 4 describes the processes by which laser-induced damage occurs and generates the expressions that can lead to the calculation of the laser-induced damage threshold (LIDT) of a material.

Chapter 5 then goes on to describe the material optical quality parameters which lower this LIDT below the theoretical value. These parameters include bulk, sub-surface and surface quality. This chapter includes the methods and techniques that have been developed to minimize this damage.

Chapter 6 emphasizes the contribution which dielectric coatings have in increasing the transmission of a laser system. This includes comments on the role of dielectric coatings in minimizing laser-induced damage.

Chapter 7 covers a range of important topics, such as the LIDT of gases and liquids, the power handling capacity of fibre-optics, the significance of the units of measurement and scaling laws.

Chapter 8 covers the measurement techniques for measuring the relevant material characteristics and the various ISO standards that have been developed over the past 10 years.

A series of appendices lists just some of the range of LIDTs of optical materials over a range of wavelengths and pulse lengths that have been made over the past 30 years. It is hoped that these will be useful to those who need to know what maximum power/energies their optical materials, components or systems can withstand. They should also indicate that there is a great gulf between the theoretical and measured performance for many materials. It is hoped that by the time the reader gets to look at these appendices that he/she will understand the reasons for the discrepancies.

I commented in the previous version of this book that there is a plethora of research papers and conference reports relating to the subject of laser-induced damage. Much of this, albeit very worthy in its own context, goes unnoticed because of the very magnitude of the data. In many cases the lessons of the past have gone unlearned because there is just so much detail that it takes a post-doctoral dissertation to even summarize it. At the same time the understanding of the subject is still increasing and my earlier book on the subject (Wood 1986) is now slightly out of date and incomplete. Much of the literature on the subject is to be found in the mainstream scientific publications and mention should be especially made of the Proceedings of the Annual Boulder Damage Symposia. The earlier years’ proceedings were published as NBS Special Publications and more lately as SPIE Proceeding reports. It is also possible to approach the subject via the SPIE Milestone Series of selected papers on laser-induced damage in optical materials (Wood 1990). However, although the committed student will find these publications invaluable I have thought it sensible to write this book as a summary of what has been done and to point those interested in going farther back to the key literature without drowning them in the minutiae of the subject.

I unreservedly acknowledge my debt to my earlier colleagues at GEC Hirst Research Centre, Wembley, UK, where we had fruitful collaboration in the subject for over 30 years, to the sponsors of that work from the UK Ministry of Defence, to my international colleagues and in particular the organizers of the Boulder Damage Symposia and more latterly to my colleagues involved under ISO TC 172 in the formulation of international standards for laser and laser-related measurements. In particular I acknowledge th...

Table of contents

• Cover
• Half Title
• Series Page
• Title Page
• Copyright Page
• Table of Contents
• Preface
• 1 Introduction
• 2 Optical effects at low power/energy levels
• 3 Optical effects at medium power/energy levels
• 4 Damage theory
• 5 Surfaces and sub-surfaces
• 6 Coatings
• 7 Special topics
• 8 Measurement techniques
• Appendices: 1 Dielectric breakdown fields
• 2 Summary of literature surveys of typical damage threshold in MW mm-2 under 10 ns Q-switched 1.06 um irradiation
• 3 Damage thresholds measured at the GEC Hirst Research Centre (MW mm-2) 1.064 um irradiation, 10 ns
• 4 (a) Summary of LIDT measurements made at 10.6 um at the GEC Hirst Research Centre on infrared window substrates and coatings (pulsewidth, τ = 100 ns)
• 5 Summary of LIDTs of infrared transmitting materials
• 6 LIDTs of metal mirrors
• 7 LIDTs of coated substrates
• 8 Summary of the LIDTs of ultraviolet transmitting materials
• 9 Summary of damage thresholds under HF/DF laser irradiation
• 10 Continuous wave laser damage, 10.6 μm
• 11 Comparison of the LIDTs of λ/4 single-layer coatings on fused silica
• References
• Index