Laser-based Mid-infrared Sources and Applications

Konstantin L. Vodopyanov

• Publisher: Wiley
• Year: 2020
• Print ISBN: 9781118301814
• eBook ISBN: 9781119011989
• Edition: 1
• Topic: Technology & Engineering
• Subtopic: Electrical Engineering & Telecommunications
• Index: Technology & Engineering

An important guide to the major techniques for generating coherent light in the mid-infrared region of the spectrum Laser-based Mid-infrared Sources and Applications gives a comprehensive overview of the existing methods for generating coherent light in the important yet difficult-to-reach mid-infrared region of the spectrum (2–20 ?m) and their applications.

The book describes major approaches for mid-infrared light generation including ion-doped solid-state lasers, fiber lasers, semiconductor lasers, and laser sources based on nonlinear optical frequency conversion, and reviews a range of applications: spectral recognition of molecules and trace gas sensing, biomedical and military applications, high-field physics and attoscience, and others. Every chapter starts with the fundamentals for a given technique that enables self-directed study, while extensive references help conduct deeper research.

Laser-based Mid-infrared Sources and Applications provides up-to-date information on the state-of the art mid-infrared sources, discusses in detail the advancements made over the last two decades such as microresonators and interband cascade lasers, and explores novel approaches that are currently subjects of intense research such as supercontinuum and frequency combs generation. This important book:

• Explains the fundamental principles and major techniques for coherent mid-infrared light generation

• Discusses recent advancements and current cutting-edge research in the field

• Highlights important biomedical, environmental, and military applications

Written for researchers, academics, students, and engineers from different disciplines, the book helps navigate the rapidly expanding field of mid-infrared laser-based technologies.

1
Mid‐IR Spectral Range

1.1 Definition of the Mid‐IR

Infrared radiation was unknown before the year 1800 when Friedrich Wilhelm Herschel – a German‐born musician, who moved to England to work as a music band conductor, but later became obsessed with astronomy and eventually landed the position of the King's Astronomer − discovered infrared radiation. He made this finding while exploring sunlight, dispersed into its colors by a glass prism, with the aid of a liquid thermometer with a blackened bulb to absorb radiation (a prototype of a modern microbolometer). His experimentation led to the conclusion that there must be an invisible form of light beyond the visible spectrum [1].

Further experiments showed that this invisible radiation is electromagnetic radiation with a lower frequency than the red in the visible spectrum. Modern science further divides the infrared spectral region into near‐infrared, mid‐infrared, and far‐infrared.

According to the Encyclopedia Britannica, the “middle infrared” (mid‐infrared or mid‐IR) region of the electromagnetic spectrum covers, in wavelength, the portion between 2.5 and 50 μm (6–120 THz in frequency or 200–4000 cm⁻¹ in wavenumbers).¹ (The wavenumber is the inverse of the vacuum wavelength, λ, expressed in cm⁻¹; it is also equal to the optical frequency divided by the speed of light, ν/c.)

However, the definitions of the “mid‐IR” vary substantially in the technical literature, depending on a field‐specific community. For example, the detector‐based community subdivides the IR into four spectral bands, based on transmission windows of the atmosphere,² as can be seen in Figure 1.1: shortwave infrared (SWIR), 1–3 μm; mid‐wave infrared (MWIR), 3–5 μm; longwave infrared (LWIR), 8–14 μm; and very‐long wave infrared (VLWIR), 14–30 μm.

Figure 1.1 Transmission spectrum of a 100‐m path in the “standard” atmosphere (spectral resolution 4 cm⁻¹). The plot uses data from the HITRAN database (US model, mean latitude, summer, and zero elevation) [2]. The labels indicate the molecules that are responsible for transmission dips in the corresponding spectral regions.

Also, it is not uncommon in the current literature to refer to mid‐IR as “multi‐terahertz range,” especially when the authors generate few‐cycle mid‐IR transients combined with electro‐optic methods of their detection, which is typical for terahertz science.

This book loosely defines the mid‐IR range as 2–20 μm. This definition allows, on the short‐wavelength side, to encompass a few categories of solid‐state and fiber lasers, as well as certain types of microresonator‐, nonlinear fiber‐, and waveguide‐based sources. On the long‐wavelength side, 20 μm is a suitable practical limit set by the atmospheric transparency.

Heat energy is often transferred in the form of infrared radiation, which is given off from an object as a result of atomic and molecular motion. The mid‐IR region overlaps with the spectral range of heat (blackbody) radiation at temperatures close to room temperature. Based on the Planck's law, the peak of the infrared radiation (in terms of power per unit wavelength) emitted by a human body at 310 K is at λ ≈ 9.35 μm. Overall, our body emits 52 mW of mid‐IR radiation per square centimeter; that radiation can be easily detected by a thermal microbolometer‐based camera, as shown in Figure 1.2.

However, this book is about coherent laser sources, and the difference between the diffuse light of a heated body and a monochromatic laser‐like beam is that the latter has a well‐defined frequency and phase.

Figure 1.2 This is what the author's laboratory looks like in the mid‐IR at 8–12 μm.

1.2 The World's Second Laser

Interestingly, the world's second laser – after the Maiman's ruby laser – was a mid‐IR solid‐state laser based on trivalent uranium‐doped calcium fluoride (U³⁺:CaF₂) [3]. It was operating at a wavelength λ = 2.49 μm and was developed by Peter Sorokin and Mirek Stevenson at the IBM research labs, in the same year as the Maiman's ruby laser, 1960.

The laser was pumped by a pulsed flashlamp and was cooled by liquid helium. The energy‐level diagram of U³⁺:CaF₂ is shown in Figure 1.3. Broadband pumping in the visible part of the spectrum causes transitions to excited U³⁺ bands. These pumping transitions are followed by rapid, nonradiative transitions to the two metastable upper laser levels. The thick arrow shows the 2.49‐μm laser transition observed by the authors. The laser oscillation takes place in a transition from a metastable state to a level that is approximately 515 cm⁻¹ above the ground state. At liquid helium temperatures, this state is depopulated by at least a factor of 10¹⁰ relative to the ground state. Hence, this was the first demonstration of a four‐level solid‐state laser.

Figure 1.3 Energy‐level diagram of trivalent uranium in calcium fluoride [3]. Broadband pumping light applied in the blue and green visible spectrum causes transitions to excited bands. These pumping transitions are followed by rapid, nonradiative transitions to the two metastable upper laser levels. The thick arrow shows the 2.49 μm laser transition observed by the authors.

Source: reproduced from figure 1 of [3], with permission of APS.

Furthermore, the 1960 work by Sorokin a...

Table of contents

• Cover
• Table of Contents
• About the Author
• Preface
• 1 Mid‐IR Spectral Range
• 2 Solid‐state Crystalline Mid‐IR Lasers
• 3 Fiber Mid‐IR Lasers
• 4 Semiconductor Lasers
• 5 Mid‐IR by Nonlinear Optical Frequency Conversion
• 6 Supercontinuum and Frequency Comb Sources
• 7 Mid‐IR Applications
• Index
• End User License Agreement