Preface

In modern laser world, the word "Pulse" typically covers pulse durations from microseconds to femtoseconds. In principle, it is possible to generate attosecond pulses (10-18 s) by using non-linear processes. Recently, new time ranges were discussed in publications: zeptosecond (10-21 s) and yoctoseconds (10-24 s). To generate pulses from milliseconds to femtoseconds, hundreds of different laser systems have been developed. They can typically generate pulses of specific durations, which are due to laser principles of operation, specific construction, parameters of gain medium, type of modulator, and so on. Smooth tuning of laser pulse duration continues to be a big problem. The shorter the achieved pulse durations are, the more difficult the problem is to determine how to measure such pulses. The solution is inside the laser pulses. Light itself contains information about Time. Generating "light in time", the pulsed lasers combine these two categories as light is a periodical, cyclic process and can be a measure of time, of length, and of frequency.

During the 50 years of their history, the pulsed lasers passed from seconds to zeptoseconds or about 20 orders of magnitude into the short duration's side. This road was difficult. Every time, starting from huge, complex, ineffective, and very expensive machines, the lasers became elegant and economical; they are being transformed to powerful and smart instruments in science, technology, medicine, and everyday life.

Very popular picosecond and femtosecond pulses typically are achieved by modelocking technology. The theoretical concept of this is based on the light waveapproximation. However, in many cases the description of the pulse formation and its evolution is better understandable and easily describable in terms of corpuscular approximation. In introductive chapter "Time and Light", the theory of optical noise is applied for explanation of ultrashort pulses phenomena. This approximation can estimate statistical parameters of generation: probability of single-pulse formation (on the round-trip cavity period), depending on active media gain spectrum width, rate of

This book is devoted to some aspects of ultrashort laser pulses generation, characterization, and applications. As of today, hundreds of books discussing these subjects have been published. More and more techniques go to practical use every year. Shorter and shorter pulses are routinely achievable. New spectral ranges, like Xrays, deep UV, middle and far IR, including TeraHertz bands, became a reality.

#### XII Preface

gain increase, cavity length, output mirror reflectance, and other laser practical parameters.

In total, the book consists of five sections housing seventeen chapters. In such complicated and multidisciplinary area as laser pulse generation and optical pulsematter interaction it is sometimes difficult to specify, which domain of research the chapter belongs to. Conditionally, the chapters have been separated into five interconnected sections:

Introduction: Time and Light – Historical/philosophical/technical overview, optical noise theory of mode-locked lasing, and exotic self-mode-locking technologies;


The scientific editor of this book does not always agree with some concepts, models, and explanations demonstrated in this book. However, the principle of open access publishing is to give possibility for each author to freely demonstrate his/her understanding of phenomena. We know a lot of examples where future Nobel Prize Winners were rejected by solid journals because of negative and non-discussable opinion of the reviewers – the best specialists in specific area. The judge cannot be judged and it is only history that can judge and corrects everyone and everything.

In any case, this book is not a milestone in physics and/or technologies; rather it is like a running train with coaches that will be changed at each new station. The book does not want to go to specific destinations but allows you to sit and think of where your destination could take you. We hope this book will be useful for a wide spectrum of specialists, for professors and students, and for those who are interested in history and in future of the laser technologies.

> **Igor Peshko** Department of Physics and Computer Science, Wilfrid Laurier University, Waterloo, Canada

**Section 1** 
