Broadband Sources

The alternative to tunable sources is the absorption spectrometer, composed schematically from a broadband incoherent source, a monochromator and a detector. Additional equipment is also needed, as additional sources for band gap excitation. Each part of the equipment is specific to the spectral range 

---

The transmission coefficient Cl (Qj,t), considering transient (broadband) sources, is time-dependent and therefore accounts for the possible pulse deformation in the refraction process. It also takes account of the quantity actually computed in the solid (displacement, velocity potential,...) and the possible mode-conversion into shear waves and is given by... 

When a broadband source of IR energy irradiates a sample, the absorption of IR energy by the sample results from transitions between molecular vibrational and rotational energy levels. A vibrational transition may be approximated by treating two atoms bonded together within a molecule as a harmonic oscillator. 

Both the modes of operation described in Sect. 5.2 may be used for the detection of chemicals in liquid solution. Because the analyte s absorption line width is very broad, overlapping several (or many) WGMs, no tuning of the microresonator, or locking of a WGM to the scanning laser is necessary. In fact, a broadband source such as a light-emitting diode (LED) may be used. 

In conclusion, when a WGM is excited in a dielectric microresonator, its evanescent component provides a convenient probe of the microresonator s surroundings. Various ways to implement evanescent-wave sensing have been devised, but the emphasis of this chapter has been on microcavity-enhanced absorption spectroscopy. The techniques described here have broad applicability, can even be used with broadband sources, and lend themselves well to further enhancement methods. We are looking forward to continuing our development of these sensors. 

Light from the broadband source is filtered by a wavelength selector and then passed into the sample container. If fluorescence occurs,... 

In an AOTF analyzer the input optical beam to the crystal is polychromatic from a broadband source. For a given acoustic frequency and angle of incidence, only a very narrow band pass of wavelengths from the... 

Assuming that the rate of change of the optical retardation introduced by the interferometer is the same for all the input radiation frequencies (which is normally the case), each individual value of v in the broadband source output contributes a different value of /m in the a.c. component of the detector output (see Figure 3.20). These contributions are, of course, summed by the detector. The combined detector output (see Figure 3.21) arising from the simultaneous measurement of all modulated input signals, is a sum of cosine functions. Such a sum is a Fourier series, and the amplitudes of the individual... 

Finally, it should be noted that dephasing dynamics may also be investigated with temporally incoherent pulses [30-32]. To the extent that the two- and three-pulse Tj measurements described in the preceding depend upon the electric field correlation and not upon the intensity autocorrelations, femtosecond temporal resolution may be obtained with any broadband source. Spectral cross-relaxation and a variety of effects, however, can reduce the effectiveness of this technique. 

Figure 8.33. Temperature dependence of quantum efficiency for front-illuminated silicon CCD. Spectra of a broadband source with the CCD at the indicated temperatures were divided by a spectrum of the same source taken at 25°C. The observed Q at 750 nm was nearly independent of temperature. (Adapted from Andor Technologies product literature.)...

Dispersive Infrared Spectroscopy The dispersive IR spectrometer generally incorporates an IR broadband source, sample cell, a diffraction grating and one or more IR detectors. Dispersive IR instruments may provide simultaneous or sequential measurements. Respectively, the instrument may have a fixed grating and many detectors, or a movable grating and a single detector. In some cases, the grating may be replaced by one or more optical filters to resolve the desired wavelengths. A reference cell and associated optics to perform simultaneous differential analysis are also incorporated to improve sensitivity or reliability of measurement. 

Using results established in previous sections., it can however be shown that this assertion represents only an approximation to the truth. Absorption from a white light or other broadband source in fact allows photon pairs of differing frequencies to be concertedly absorbed by molecules in close proximity (Andrews 1988). In this section, it is shown that this may result in a change to the appearance of absorption spectra, and yield an absorption law that departs from normal Beer-Lambert behavior. One particular case in which such effects may be expected to arise is where supercontinuum laser radiation is employed for spectroscopic purposes. 

In classical optical absorption measurements, the absorption of a sample under different conditions as a function of the energy of the incident electromagnetic radiation is studied. This can be achieved in two ways one can either take a broadband source and use a spectrometer to disperse the electromagnetic spectrum, or use a monochromatic tunable source. There is also a... 

A commercial high-resolution FTS is depicted in Fig. 4.2. The output of the broadband source is focused on a circular aperture (entrance iris). As in the dispersive set-up, the optical beam is made parallel by a collimating mirror, and it intercepts a beam splitter at a non-normal incidence (usually 45 or 60°). One part of the beam is transmitted towards a fixed plane mirror while the other part towards a plane mirror, which can be translated continuously or in steps at a given distance (scan mirror). The beams reflected back by... 

---