Continuous-wave excitation response

Resonance spectroscopy is a low power, CW application used for quality inspection. It is used to test the quality of mass-produced components because it is fast and the cost per test is low. A test sample is injected with CW ultrasound or sound at one transducer and the response at a second transducer is measured. A spectmm is built-up by stepping over many frequencies. Continuous wave excitation fills the sample under test with ultrasoimd, allowing waves from relatively distant parts of the sample to reach the receiver and contribute to the spectmm. A whole sample canbe tested without any scanning and interpretation is done automatically by computer, typically using a trained artificial neural network, resulting in short testing times. The time to complete a full test of, 100 test frequencies on an automotive component could be as short as 1 sec so that more than 1000 parts per hour can be tested. Resonance spectroscopy is suitable for 100% quality assurance of mass-produced parts. 

In the typical setup, excitation light is provided by a pulsed (e.g., nanosecond) laser (emitting in the visible range, e.g., at 532 nm, if Mb is investigated), while the probe is delivered by a continuous-wave (cw) laser. The two beams are spatially overlapped in the sample, and the temporal changes in the optical properties (such as optical absorption or frequency shift) that follow the passage of the pump pulse are registered by a detector with short response time (relative to time scale of the processes monitored), such as a fast photodiode. 

Continuous wave instruments involve a considerable waste of time. A solution to the inefficiency of single - frequency observation is to excite all of the nuclei in a sample simultaneously and to observe the total response of the sample. This is done by periodical, intense, short RF pulses. A RF pulse excites a finite band width of frequencies. The detector observes a pattern called a free induction decay (FID). An example is presented in figure C.2. Fourier Transforming this FID, yields the classical NMR spectrum. 

Impulse-response and transfer functions can be measured not only by pulse excitation, but also by excitation with monochromatic, continuous waves (CW), and with continuous noise or stochastic excitation. In general, the transformation executed by the system can be described by an expansion of the acquiired response signal in a series of convolutions of the impulse-response functions with different powers of the excitation [Marl, Schl]. Given the excitation and response functions, the impulse-response functions can be retrieved by deconvolution of the signals. For white noise excitation, deconvolution is equivalent to cross-correlation [Leel]. 

Fig. 4.1.1 Interrelationship between excitation (left) and response (right) in spectroscopy (a) Excitation with continuous waves (CW excitation) directly produces the spectrum, (b) For pulsed excitation, the spectrum is obtained by Fourier transformation of the impulse response, (c) For stochastic excitation, the impulse response is derived by cross-correlation of excitation and response signals.

For this reason ki(t) is also called the impulse-response function. For excitation of the linear response in NMR, that is, for excitation with small flip-angle pulses, k t) is identical to the FID (Fig. 4.1.1(b)). If the input is a weak continuous wave with adjustable frequency co, then x(t) = exp in>r, and the response is given by the input wave attenuated by the spectrum K (to) of the impulse-response function ki (r),... 

IR measurements on doped Si and shown that the subsurface mobile carriers can be probed by their response to an IR near-field with a spatial resolution of 30nm [48]. The group of Havenith presented a scanning near-field infrared microscopy (SNIM) system this is an IR s-SNOM set-up based on a continuous-wave optical parametric oscillator (OPO) as an excitation source with a much wider tunability compared to the usually applied CO2 lasers [49]. With this set-up, a subsurface pattern of implanted gallium ions in a topographically fiat silicon wafer was imaged with a lateral resolution of <30 nm. 

The technique can be carried out using either a continuous wave (CW) or a pulsed spectrometer. The RE energy is used to excite the nuclear magnetization. The measurement is the response of the spin system to this excitation. In CW the nuclear magnetization is irradiated at a... 

With frequency domain FLIM the light source is a continuous wave laser as opposed to a pulsed laser. The continuous wave laser is modulated via an acousto-optical modulator and the sample is excited by a sinusoidally modulated light. The fluorescence response is also sinusoidally modulated at the same frequency but it is delayed in phase and is partially demodulated. For a single exponential decay the lifetime of the donor chromophore can be quickly calculated by either the phase shift (j) (rp) or the modulation ratio M (r ,) using the following equations ... 

In the previous section, we established a correspondence between the transient time-domain response (exponentially damped cosine wave) to a sudden "impulse" excitation and the steady-state frequency-domain response (Lorentzian absorption and dispersion spectra) to a continuous excitation. The Fourier transform may be thought of as the mathematical recipe for going from the time-domain to the frequency-domain. In this section, we shall introduce the mathematical forms of the transforms, along with pictorial examples of several of the most important signal shapes. 

The resonance spectrum, obtained by continuously exciting the object over a wide frequency range (sine-wave fi quency sweep) and measuring its response, provides an acoustic signature of the object. The measurements can be made with direct contact transducers, such as piezoelectric crystals, or with a non-contact setup using a speaker for excitation and a laser vibrometer for response measurement. Typicalfy, the frequency sweep range used for chemical munitions lies between 1 KHz and 30 KHz and the entire frequency sweep can be carried out in less than 60 seconds. 

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