Spectrometers and Monochromators

Abstract

The first section of this chapter introduces three main options for compact fiber-optic spectrometers available on the market: the Czerny-Turner, the Wadsworth, and the Rowland schemes. This is followed by detailed derivation of the basic diffraction grating equation in blazed configuration and analysis of this formula and the formulas for angular and linear dispersions. Detailed figures with graphical representations are helpful to understand basic phenomenology and geometry of the spectrometers. Focusing optical systems provide imaging capabilities, however, aberrations are imminent. The most important practical problem that may be encountered is the order-sorting filter. The purpose and design principle of this element is comprehensively explained. The Wadsworth and the Rawland schemes are analyzed in full detail, emphasizing the curvature of the spectrum, which is particularly important in compact spectrometers that use flat linear detector arrays. Newly purchased spectrometer frequently requires calibration, which can be made with the help of an argon-mercury calibration module. Most frequently used argon and mercury calibration lines are summarized and listed in the table for quick reference. But it is not enough: general understanding of how the entire spectrum looks like is needed to correctly identify the lines. For reference purpose, a typical spectrum is graphically presented and calibration procedure is explained. Some particular doublets of the mercury spectrum may serve as a mean for estimating spectral resolution of a particular spectrometer, and the nomogram for that is provided. Some ways of increasing energy efficiency of fiber-optic spectrometers and typical mistakes are explained, like the use of fiber bundles with random organization of fibers. The second section is devoted to imaging spectrometers—devices that not only measure spectrum but also link it to spatial position of the source in one dimension. Interpretation of such a map may be tricky without initial experience, therefore the experimental pictures in two and three dimensions are presented with comprehensive explanation. The option of using imaging spectrometers for spectral interferometry (Chap. 6) is a very efficient way to measure one-dimensional profiles of microscopically patterned surfaces. The optical schemes and impressive examples are presented and explained in detail. The next section explains the concept, design, and typical features of gated intensified spectrometers. Starting from the principle, design, and parameters of microchannel image intensifiers, the reader is guided through mathematics, explaining advantages of optical amplification, to design features of a gated spectrometer that can be built in the laboratory. Experimental results, presented here, show that, with optical amplification around 4000, the intensified spectrometer offers significantly better signal-to-noise ratio than ordinary spectrometers. This section ends with experimental results, showing temporal resolution of spectral measurements equal to about one meter of optical fiber. The fourth section explains principles, features, and practical know-how of Fourier-transform spectrometers in visible domain. Although the idea of the Fourier spectroscopy is widely known, its implementation in visible domain is commonly considered as unreliable. The facts presented in this section oppose this stereotype. Explanation begins with explicit mathematical formulas that directly connect the Fourier transform array to the spectrum. This mathematical part is followed by the example of experimental spectrometer, assembled from standard optical elements available from ordinary vendors. Peculiarity of Fourier spectroscopy is that the resultant spectrum appears self-calibrated if the scanning stroke is known. Compensated and non-compensated beamsplitters, spectral resolution, importance of scanning linearity, phase measurements are explained. Typical experimental modulation curves and restored spectra are presented, including high-frequency spectra with phase-sensitive detection. The next section introduces the Fabry-Perot interferometer—actually a spectrometer with exceptionally high spectral resolution, commonly used to analyze mode structure of lasers. The section begins with the simplified theory of the interferometer with flat mirrors, introducing the Airy formula, followed by explanation of its practical use. Interferometers with flat mirrors are extremely unstable, therefore in practice they were superseded by confocal versions. The nature of unique stability of confocal interferometers is explained in full detail with comprehensive graphical representation. This theoretical part is followed by description of the design of commercially available interferometers, practical recommendations, and experimental oscilloscope traces. The last section of this chapter compares the two most frequently used types of monochromators: the grating monochromators and interference filters. Discussion of monochromators covers the following topics: formula of the monochromatic condition, linearity of tuning, sine-bar mechanism, higher-order ghosts in broad-band spectrum, experimental results. Their rivals—interference filters—are actually interferometers Fabry-Perot but on much smaller scale of separation. As such, they are also governed by the Airy formula, predicting multiple transmission peaks. To suppress them, practical devices are combined with colour glass filters and additional multilayer cavities. Typical experimental transmission curves finalize this chapter.

With all the variety of spectrometers on the market, it is better to know what to request from the vendor in order not to have problems later in the laboratory. And, do not be afraid to make your own device or modify the existing one if necessary.