Fiber Lasers

Fundamentals with MATLAB® Modelling

Johan Meyer, Justice Sompo, Suné von Solms, Johan Meyer, Justice Sompo, Suné von Solms, Justice Mpoyo Sompo, Johan Meyer, Justice Mpoyo Sompo, Suné von Solms

• Publisher: CRC Press
• Year: 2022
• Print ISBN: 9780367543488
• eBook ISBN: 9781000531985
• Edition: 1
• Topic: Technology & Engineering
• Subtopic: Electrical Engineering & Telecommunications
• Index: Technology & Engineering

Over the past two decades, the use of fiber lasers in engineering applications has gradually become established as an engineering discipline on its own. The development of fiber lasers is mainly the result of studies from various domains like photonics, optical sensing, fiber optics, nonlinear optics, and telecommunication. Though many excellent books exist on each of these subjects, and several have been written specifically to address lasers and fiber lasers, it is still difficult to find one book where the diverse core of subjects that are central to the study of fiber laser systems are presented in simple and straight forward way.

Fiber Lasers: Fundamentals with MATLAB Modelling, is an introduction to the fundamentals of fiber lasers. It provides clear explanations of physical concepts supporting the field of fiber lasers. Fiber lasers' characteristics are analyzed theoretically through simulations derived from numerical models. The authors cover fundamental principles involved in the generation of laser light through both continuous-wave (CW) and pulsing. It also covers experimental configuration and characterization for both CW and Q-switching. The authors describe the simulation of fiber laser systems and propose numerical modelling of various fiber laser schemes. MATLAB® modelling and numerical computational methods are used throughout the book to simulate different fiber laser system configurations.

This book will be highly desirable and beneficial for both academics and industry professionals to have ample examples of fiber laser approaches that are well thought out and fully integrated with the subjects covered in the text. This book is written to address these needs.

1 Fundamentals of Fiber Lasers

DOI: 10.1201/9781003256380-1

1.1 Introduction

Fiber lasers were proposed right after the first demonstration of a working laser by Theodore Maiman in the 1960s (Maiman 1960). The possibility of using silica fiber as a gain medium for lasers was introduced by Elias Snitzer et al. (1989). Soon after, they made the first demonstration of a working silica fiber laser (Snitzer and Koester 1963). Since then, fiber lasers have evolved to be one of the most successful technologies in photonics.

Fiber lasers are low cost and relatively easy to produce sources of excellent beam quality light in the infrared, visible, and ultraviolet ranges of the optical spectrum. For this reason, they are desirable for applications in telecommunication, medicine, manufacturing industry, and metrology. Several types of fiber lasers have been developed to meet an always-increasing demand from various industries. These include continuous wave, pulsed, narrow-linewidth, single frequency, tuneable and high-power fiber lasers. The fiber laser basic structure consists of a gain medium made of an optical fiber doped with rare-earth ions and a feedback mechanism brought about by a resonant cavity formed by either physical mirrors or the frequency selective fiber Bragg grating (FBG), printed into the core of the fiber.

In this introductory chapter, we will provide a short overview of the important properties of fiber lasers as well as the milestones in their development. We will highlight why fiber lasers are such a desirable light source. We will briefly discuss the different types of fiber lasers. Finally, we will mention some of the most important applications of fiber lasers.

1.2 Interest of Lasers in Fiber Form

The telecommunications industry became first interested in Erbium-doped fiber because of their excellent amplifying characteristics in the 1550 nm region. With the maturation of fabrication techniques of fiber optics such as Modified Chemical Vapour Deposition (MCVD) (Cognolato 1995) and the availability of powerful and reliable semiconductor lasers diodes emitting in the absorption band of most rare-earth ions, it became possible to manufacture Erbium-Doped Fiber Amplifiers (EDFA) (W Naji et al. 2011), useful for long haul optical communication systems.

Obtaining a laser from these amplifiers only required adding a feedback mechanism to them. This can be achieved using mirrors in a Fabry-Perot configuration. These mirrors can be of various types, the simplest being the coating of the end faces of cleaved optical fiber with reflective material. The reflection at the air-glass interface is enough to trigger oscillation because of the particularly high gain available in the rare-earth-doped fiber gain medium. A more effective way of obtaining feedback in fiber lasers is by using fiber Bragg gratings (Hill and Meltz 1997). Fiber Bragg gratings have the advantage of being frequency-selective allowing the realization of single longitudinal mode fiber lasers (Jauncey et al. 1988).

The core diameter of a standard single-mode fiber can vary from 3 to 10 micron; therefore, significant light intensity can develop with relatively low propagating power. The fiber geometry results in high confinement of the pump and laser fields over long distances thereby providing long interaction lengths between the rare-earth dopant and the pump field. The consequence of the foregoing is the high population inversion which means high optical gains at relatively low pump powers. The high gain also allows the use of high loss elements such as etalons and diffraction gratings without a significant increase of threshold or reduction of output power. Such components are often essential to achieve single longitudinal mode operation of the fiber laser (Huang et al. 2005).

The broadening of the absorption bands of rare-earth ions provides an incomparable opportunity to absorb a large band of optical frequencies. Therefore, fiber lasers possess the ability to convert the low-quality output radiation from low-cost laser diodes, transmitting at several wavelengths, into a high-brightness coherent source, desirable for applications such as remote sensing (Fu et al. 2012) and fiber-based communications systems (Gangwar and Sharma 2012). On the other hand, the emission spectrum is also broad, allowing the design of lasers emitting over several wavelengths as well as tuneable lasers. Tuneable fiber lasers are designed by incorporating a wavelength selective element into the cavity making it possible to achieve tunability over wavelengths of 50 nm or more (Chen et al. 2005). Furthermore, the flexibility of the fiber enables long cavity lengths to be established while taking up a small volume of space. Long cavities result in narrow-linewidth fiber lasers which are preferred in several applications (Yarutkina et al. 2013). The compatibility of the fiber with several optical components such as couplers, isolators, and wavelength division multiplexers (WDMs) makes it possible to have a compact and robust design because the laser can be made in an all fiber configuration without light leaving the fiber, avoiding tedious alignment of bulk optics. Fiber lasers also offer several design possibilities that allow the control of optical properties such as dispersion and polarization, resulting in a large performance improvement possibility.

The small surface to volume ratio of optical fibers, which is 10 to 50 times larger than that of other types of solid-state lasers, is at the origin of good heat dissipation making fiber lasers excellent candidates for high power lasers. This surface to volume ratio prevents detrimental phenomena resulting from heat build-up such as thermal aberrations (Paun et al. 2009).

The doping of the glass matrices with various rare-earth ions opens the possibility for multiple operation wavelengths in the near and far-infrared spectrum. The most used dopants are Neodymium and Ytterbium (1 µm) (Zervas 2014), Erbium, Erbium, and Ytterbium (1.5 µm) (Song et al. 2009), Thulium and Holmium (around 2 µm) (Hanna et al. 1988). Fiber lasers built using these dopants offer a variety of output wavelengths, some of which are of great interest in telecommunications and other applications. For example, the output in the 1.3 to 1.5-micron spectral region which corresponds to low-loss transmission windows for silica glass has been extremely useful in the past decade (Agrawal 2012). Nowadays, the 2 to 3-micron spectral region is subject to intense research because lasers at these wavelengths have potential application as light sources for future generations of low-loss mid-infrared telecommunications fiber systems (Pollnan and Jackson 2001). Another benefit of fiber lasers results from their inherent wave guiding property which allows easy coupling of fibers and various optical components.

1.3 Chronological Review of Fiber Lasers

The origin of fiber lasers can be traced back to early years of laser technology when Snitzer and Koester published results for a multi-component fiber laser in 1964 (Snitzer and Koester 1963). Soon after, intensive research was conducted to investigate the possibility of using fiber lasers in optical information processing (Luo et al. 2017) as well as optical amplification (Giles and Desurvire 1991). It became obvious that fiber lasers have huge potential after Kao and Sham speculated on the possibility of using fiber optics in telecommunication. However, due to the weak output power of earlier devices, the concept did not find practical application and was regarded as a mere laboratory interest. As a result, from 1975 to 1985 little research was published. The regain of interest in fiber lasers was stimulated by a conjunction of factors, amongst them the improvement in the fiber manufacturing techniques, availability of high-quality laser diodes emitting at a wavelength corresponding to the absorption wavelength of rare-earth ions, new host materials and the improvement in passive optical components including isolators, couplers, and fiber Bragg gratings. The research group at Southampton University contributed significantly to the field during that period. Using an extension of MCVD technologies, they were able to manufacture doped fibers allowing the demonstrating of Q-switching, mode-locking, and single longitudinal mode operation (Nagel et al. 1982). Soon afterwards, the British Telecom Research Laboratory made significant advances in understanding the core concepts of fiber laser technology such as gain, excited-state absorption, and the relation between host material and the range of lasing wavelength. Specifically, they pioneered the use of Fluorozirconate (Kaczmarek and Karolczak 2007) glass as host to increase the range of emission wavelength.

1.3.1 Erbium and the Ytterbium Co-doping

Later, Erbium-doped fiber lasers were reported. Erbium has an important property which is lasing in the 1530 to approximately 1580 nm region corresponding to the second telecommunication window. However, because of luminosity quenching resulting from Erbium ion clustering (Auzel and Goldner 2001), the output power of Erbium-doped fiber lasers was limited to a few milliwatts. In fact, the Erbium ions in glass result in detrimental effects at high concentrations. At doping concentrations around 24 × 10²⁵ ions/m³, because of the low solubility of the ions in the amorphous glass, Erbium ions tend to form clusters and exchange energy among themselves. This form of energy exchange tends to reduce the conversion efficiency of the laser. The energy efficiency reduction is commonly known in technical terms as luminosity quenching. Such luminosity quenching phenomena include mainly cooperative up-conversion (CUC) (Hwang et al. 2000) and pair induced quenching (PIQ) (Federighi and Di Pasquale 1995). These two phenomena along with excited-state absorption (ESA) (Barmenkov et al. 2009) contribute significantly to the degradation of the overall efficiency of the fiber laser because the energy supposed to contribute to stimulated emission and participate in lasing action is now spent in energy exchange phenomena.

To prevent these up-conversion phenomena from taking place, several techniques have been used when designing the glass of the fiber. Such techniques involve using phosphate or aluminosilicate glass with higher solubility for Erbium ions rather than silica glass (Taccheo et al. 1999). An experiment conducted and reported in a 2001 paper confirmed efficiency of up to 20% when using phosphate glass compared to the case of silica glass (Moghaddam et al. 2011). In addition to using phosphate glasses, co-doping Erbium with Ytterbium (Federighi and Di Pasquale 1995) ions proved to be very beneficial in t...

Table of contents

• Cover
• Half Title
• Title Page
• Copyright Page
• Dedication
• Contents
• Preface
• Chapter 1: Fundamentals of Fiber Lasers
• Chapter 2: Optical Fibers
• Chapter 3: Rare-Earth Ions and Fiber Laser Fundamentals
• Chapter 4: Mathematical Methods for Fiber Lasers
• Chapter 5: Continuous-Wave Silica Fiber Lasers
• Chapter 6: Q-switched Fiber Laser
• Chapter 7: Narrow Linewidth Fiber Lasers
• Chapter 8: High-Power Fiber Lasers
• Index