6. Interference & Diffraction
6. Interference & Diffraction
In many spectroscopic measurements, the incident, transmitted or emitted radiation beams are dispersed by frequency (or wavelength) to increase the selectivity and/or information content of the measurement. We’ve already seen why broadband light can be separated into its constituent frequencies using a prism, i.e., the dispersion (frequency dependence) of the refractive index. However, the use of diffraction or interference phenomena provides much better wavelength selection because these exploit the wave nature of light. Diffraction and interference are closely related phenomena; in fact, diffraction can be considered interference with scattering. So to understand diffraction, it is best to begin discussing interference.
Interference occurs when two beams are superimposed. The superposition principle states that constituent electric fields are additive:
Fig. 6A shows how two beams “interfere” with one another for several values of kΔz. For kΔz=0, the figure on the top left, the total electric field is twice that of the individual electric fields. This is constructive interference. As the phase shift increases to π/2 (90o), the figure on the top right, the composite electric field is less than twice the constituent fields and is phase-shifted from both of them. When kΔz= 2π/3 (120o), lower left figure, the composite electric field is only as large as the constituent fields and its phase is between the two. Finally, as the phase shift reaches one-half the wavelength, kΔz= π (180o) lower right figure, the amplitude of the composite electric field is zero. This last case is called destructive interference. A key point to understand is that two waves must have a well-defined phase relationship to give rise to interference effects. Waves with fixed phases are said to be “coherent”. The most common way of achieving coherence is to split a beam of incoherent light. This is counterintuitive, but is based on the idea that components of the wave will constructively interfere with each other when a beam is split and recombined. A second point to remember is that while electric fields are additive, irradiances are not. Consequently, the irradiance of a beam produced by destructive interference is small rather than precisely zero.
Interference filters are convenient when there is only one wavelength of interest. A common application of interference filters is in inexpensive optical instruments where the various emission lines of a mercury lamp are selected for absorbance or fluorescence measurements. Interference filters typically will select a wavelength within 1 to 10 nm, depending on the construction of the filter. The more selective filters have stronger reflections on either side of the cavity to enhance the interference by making the incoming and reflected electric fields closer to one another in amplitude. Holographic interference filters are devices that have many cavities evenly spaced at the λ/2 intervals. Holographic printing, which is produced by crossed laser beam etching, produces nearly perfect cavity spacing.
Fabry-Perot interferometers are devices that have adjustable cavity spacings so that the wavelength transmitted by the cavity may be scanned. (While there is some confusion around the nomenclature, Fabry-Perot etalons are interferometers in which cavity spacings are fixed. Their performance is similar that of interference filters, though the construction is different; for example, the dielectric is usually air.) An entire spectrum can be observed by varying (scanning) the cavity spacing of the interferometer. Unlike the interference filter, the cavity spacing of the Fabry-Perot interferometer is much larger than the wavelength of light. The optical wavelengths transmitted are thus very high order modes of the interferometer. The finesse, F, and coefficient of finesse, CF, which measure how selective an etalon or filter is for a particular wavelength, depend on the reflectivity of the cavity walls.
The bandwidth of the transmitted bands and resolving power of the cavity are related to F and CF. The full-width at half maximum (intensity) for transmitted bands and resolving power (ratio of the mean of two spectral bands that are separated by the device divided by the difference in their wavelengths) of an etalon (or interferometer position) are
These expressions imply that the bandwidth of transmitted bands decreases and separation between adjacent bands increases as the reflectivity of the cavity walls approaches unity. The amount of light transmitted by an interferometer is
where Φm is the maximum power in an individual fringe, k is the wavevector (see Section 1) and Δz is the difference in the optical pathlengths of adjacent beams leaving the device. In the interferometer (or etalon) Δz=2dcos(θ). (See text by Fig. 6B for definitions.)
In the Michelson interferometer light is split at a 50% beamsplitter then recombined to produce interference. In the case of a monochromatic source, whenever the difference in the path lengths in the two arms is a multiple of the wavelength produced by the source, constructive interference occurs at the detector producing a large signal. When the path length is an odd multiple of one-half the wavelength, destructive interference occurs and the detector signal is small. Pulling the moving mirror at constant speed produces an oscillating signal (called an interferogram) whose frequency is depends on the frequency of the source radiation. In fact, the frequency of the interferometer signal is proportional to the speed of the moving mirror and inversely proportional to the wavelength of the radiation. Scanning (pulling/pushing) the movable mirror thus allows straightforward determination of the frequency of the source radiation. In a later section we will see that the spectrum of a polychromatic source can be obtained by Fourier transformation of the output of the Michelson interferometer.
Diffraction
A number of monochromator and spectrograph (exit aperture rather than exit slit) designs are used to incorporate diffraction gratings into the optical train of spectrometers. The Czernzy (pronounced Churny) -Turner is very commonly used design for reflective gratings, but the compactness of the Littrow design is convenient for many applications. The resolution of these devices depends on the slits and curved mirrors used to direct the light to and from the grating as well as the spacing of grooves on the grating, as the following equation shows
where Wslit is the slit width, f is the focal length of the curved mirror, d is the groove spacing and β is the angle at which the diffracted ray leaves the grating. The monochromator throughput,Υ(λ), also depends on these (and related) factors.
The etched surface diffraction grating and the holographic volume grating are permanent gratings. Useful transient gratings also can be written into materials. For example acousto-optic gratings may be written in quartz. Quartz is a piezo-electric material whose density, and thus refractive index, changes with applied voltage. A high frequency, i.e., RF, oscillating voltage induces an oscillating density gradient in the quartz. This grating will diffract radiation at the refractive index boundaries. The acousto-optic grating is transient because it disappears when the RF power is off. Ingle & Crouch discuss modulators, which modify beam power using this technology. A more recent development is their use as tunable Bragg diffraction gratings. The entire visible spectrum can be scanned in fractions of seconds by varying the RF frequency.
Interference
(6.1)
(6.2)
(6.3)
(6.4)
(6.5)
Last Updated: 3/17/10