Demtroder Laser Spectroscopy Pdf

[Custodio Groves]

Keepingabreast of the latest techniques and applications, this new edition of the standard reference and graduate text on laser spectroscopy has been completely revised and expanded. While the general concept is unchanged, the new edition features a broad array of new material, e.g., ultrafast lasers (atto- and femto-second lasers), coherent matter waves, Doppler-free Fourier spectroscopy, interference spectroscopy, quantum optics and gravitational waves and still more applications in chemical analysis, medical diagnostics, and engineering.

"A detailed survey of the essential ideas and facts, which, because of its clarity and utility, is already a classic... It would be hard to imagine a better book at this level addressed to a wide audience." Applied Optics

"This book is primarily intended as a textbook for a course in modern spectroscopy based on the laser... for the advanced student or for a course designed for physics graduate students working directly in almost any form of spectroscopy, this volume will serve as an excellent text and/or source book. Indeed, this book will also be an excellent source book for senior scientists ... this book has been beautifully produced with clear type, excellent line-drawings, and nicely set equations." (Gary J. Long, Physicalia, Vol. 25 (4), 2003)

"Good news for all students and physicist working in the field. Since long time this book is a standard textbook belonging to highest class of educational books... this third edition of the standard reference and graduate text on laser spectroscopy has been completely revised and expanded... This already classic text can be recommended to students in physics and chemistry, professionals... as well as to engineers and other interested people working in science and engineering." (T. Tschudi, Optik, Vol. 114 (7), 2003)

Spectroscopy denotes methods where the interaction of light with matter is utilized.In many cases, lasers are used as light sources for spectroscopy, which is then called laser spectroscopy (or sometimes laser spectrometry).

In a wider sense, laser spectroscopy can be understood to use laser-like sources, including not only real lasers, but also optical parametric oscillators (OPOs) or the outputs of other nonlinear frequency conversion devices such as frequency doublers or difference frequency mixers.

The spatial coherence of laser light is usually very high, and this leads to further improvements. For example, one can use multipass gas cells, where a large number of round trips can be realized only with a collimated beam having a low beam divergence.

In the following, we consider a range of different techniques of laser spectroscopy, which utilize different properties of laser light. Note, however, that laser spectroscopy is a huge field of research and applications, making it difficult to provide a complete overview.

A frequently used method is laser absorption spectroscopy, where a tunable narrow-linewidth laser (frequently a single-frequency laser) is tuned through some wavelength range, and the light absorption in some sample (i.e., a reduction of optical power of the probe beam) is measured as a function of that wavelength.

Obviously, the spectral resolution is limited by the laser linewidth, which is therefore often minimized with suitable laser designs. Extremely high precision is required and achieved in the area of optical frequency metrology, e.g. for realizing extremely precise optical clocks.

Absorption features are not always directly investigated by measuring wavelength-dependent absorption. Instead, one may exploit more subtle effects on modulated laser beams (frequency modulation spectroscopy) [6] or detect sound waves induced by laser pulses (photoacoustic spectroscopy, see below).

In Raman spectroscopy, one exploits the phenomenon that a medium irradiated with narrow-band continuous-wave laser light does not only scatter light at the same optical frequency (Rayleigh scattering), but partly with slightly reduced frequencies (Raman scattering). The corresponding loss of photon energy stays in the medium; it leads to the excitation of vibrational or rotational modes. By analyzing the optical spectrum of the weak Raman-shifted light (after suppressing Rayleigh-scattered light with a notch filter), one can retrieve information on the vibrational or rotational modes of the medium, also on its temperature.

Raman spectroscopy has a wide range of applications for example in biology and medicine, for distributed temperature sensing in optical fibers, the analysis of artwork and the detection of explosives.

The wavelength-dependent absorption of light in a sample is usually measured through the reduction of optical power of a light beam going through the sample. Photoacoustic spectroscopy, however, is based on a different method: one detects sound waves which are excited by the absorption of intense laser pulses. Such pulses can heat a sample gas or solid material, for example, causing a weak pressure wave which can be detected with a microphone. Processing the microphone signal with a lock-in amplifier, one can achieve a high sensitivity.

Although probably most techniques of laser spectroscopy are based on continuous-wave lasers, there are also various methods where mode-locked lasers are used, which produce trains of ultrashort pulses. Some examples:

Intense laser pulses can vaporize and ionize materials, involving the effect of laser-induced breakdown. The light flash emerging from the vaporized material can be analyzed with a spectrograph, and spectroscopic fingerprints can then be used to identify certain atoms.

For this kind of spectroscopic method, the wavelength of the laser pulses is not particularly important, as the laser light is used only for concentrated delivery of energy. Wavelength resolution is obtained only on the side of photodetection.

A low level of laser noise is often important for precision spectroscopy. Various types of noise, such as intensity noise, phase noise (related to a finite optical bandwidth) or timing jitter, may be relevant. More or less sophisticated schemes for low-noise operation and for the stabilization of lasers are therefore often employed. These themselves often involve techniques of spectroscopy, as far as frequency stabilization is concerned. In extreme cases, a linewidth below 1 Hz is achieved.

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Feature papers represent the most advanced research with significant potential for high impact in the field. A Feature Paper should be a substantial original Article that involves several techniques or approaches, provides an outlook for future research directions and describes possible research applications.

Abstract: Lasers and laser spectroscopic techniques have been extensively used in several applications since their advent, and the subject has been reviewed extensively in the last several decades. This review is focused on three areas of laser spectroscopic applications in atmospheric and environmental sensing; namely laser-induced fluorescence (LIF), cavity ring-down spectroscopy (CRDS), and photoluminescence (PL) techniques used in the detection of solids, liquids, aerosols, trace gases, and volatile organic compounds (VOCs). Keywords: LIF; LEAFS; LIBS; CRDS; photoluminescence; laser; environment; VOCs

Tunable diode lasers are ideal for optical spectroscopy because of their narrow linewidths, large tuning ranges and stable outputs. Because they are more compact and rugged than traditional spectroscopic optical sources, like Ti:Sapphire lasers, dye lasers, color-center lasers, hollow-cathode lamps, and nonlinear systems (e.g. optical parametric oscillators), they have enabled spectroscopic methods to be used not only in laboratory environments but also in the real world. Applications of diode-laser spectroscopy include remote sensing, LIDAR, laser cooling and trapping of atoms,1 frequency standards,2 length standards,3 trace gas detection,4 and process monitoring.5 They can be used to monitor environmentally important species, such as methane, carbon dioxide, and water. In semiconductor manufacturing, they can be used for closed-loop control of deposition processes, including electron-beam, sputtering, molecular-beam epitaxy (MBE), and thermal evaporation, resulting in a significant increase in the yield of existing devices and making possible new and improved ones. Indeed the field has grown so large that a review of the research journals reveals that it has developed its own acronym, TDLAS, which stands for Tunable Diode Laser Absorption Spectroscopy.

There is a wide variety of laser-spectroscopic techniques available to researchers.6 In this application note, we will focus on one particular technique, frequency-modulation spectroscopy, or FMS. FMS is a powerful technique that can achieve a high signal-to-noise ratio with a relatively simple experimental setup. In a typical FMS experiment, the wavelength of a continuous-wave laser is modulated at a particular frequency. As the center wavelength is scanned across the atomic transition, the wavelength modulation is converted into amplitude modulation, giving rise to a modulation in the optical absorption of a sample at the same frequency. (See Figure 1 for a typical absorption line.)

Narrow-band demodulation techniques, such as phase-sensitive detection using a lock-in amplifier, then allow the absorption information to be realized at DC. Because the signal has been moved to a high frequency via modulation, FMS avoids the typical limitations of absorption measurements such as laser-intensity fluctuations, which peak at DC and fall off roughly as 1/f, hence the name 1/f noise. Using this technique, absorption sensitivities can reach the part per million (ppm) level. For example, H2S has been detected at the ppm level in air,7 absorption of yttrium has been measured at the ppm level,5 and methane has been detected with a precision of 1ppb.8 As we will discuss later in this application note, further enhancements of the signal can be achieved by taking advantage of experimental geometries that cancel particular noise sources or undesirable aspects of the signal. Finally, if a reasonable level of attention is given to the optics and electronics, systematic errors can be suppressed and high accuracy can be achieved with FMS.9 This is important, for instance, when trying to determine the exact center of an absorption line. For many applications the center must be found to better than 0.1% of the linewidth.

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