Frequency scanning

Fig. 5.3 Experimental setup. Light from a frequency scanned cw diode laser is launched into a bitapered fiber to excite WGMs of the microresonator. The microresonator is held in a PZT fixture for tuning and locking purposes inset). Reprinted from Ref. 4 with permission. 2008 Optical Society of America...

Fig. 5.5 Experimental setup. The diode laser is frequency scanned by one waveform generator, while the other controls the modulation. The light couples from a tapered fiber into and back out of microsphere WGMs, and the throughput is detected. A polarizing beamsplitter (PBS) separates throughput of the two polarizations. A diode pumped solid state laser can be used as an external heat source for the microsphere, and the vacuum chamber allows control over the ambient pressure. Reprinted from Ref. 5 with permission. 2008 International Society for Optical Engineering...

The sensing methods summarized thus far are intended for absorption detection of molecules in the ambient, but molecules (or indeed thin films) on the microresonator surface can also be detected. In particular, if the surface is covered to such an extent that the optical energy absorbed heats the microresonator, the resulting thermal bistability in the frequency-scan response can be used to determine the absorption and/or thickness of the thin-film coating. This and surface characterization by measurement of the thermal accommodation coefficient were described in Sect. 5.5. These methods offer quite precise measurement, provided that certain reasonable and easily implemented assumptions are satisfied. 

Development of laser sources was followed by the use of special monochromators that can resolve the more-intense, elastically scattered light (Rayleigh line) from the weak, inelastically scattered, Raman signal. The requirement of frequency matching in the double or triple monochromators presents a challenging, coupling problem for frequency-scanning systems. 

Photons from a tunable dye laser are absorbed by a metastable state of the ion. Spontaneous emission at several wavelengths follows. The ionizing electron energy is varied over the region of interest. At each setting the laser is frequency scanned across the Doppler profile of the io.i-beam... 

Fig. 2.3 Typical isothermal frequency scans for PE-PEE diblocks with indicated compositions in different ordered phases (Zhao ei at. 1996). Qualitative differences between the low frequency rheological response for distinct ordered structures similar to these are observed for other diblocks. S = BCC spheres, C = hex cylinders, G = Ia3d gyroid, HPL = hexagonal perforated layer, L = lamellae. (A) G (x) G . Structural assignments of the ordered phases were made using TEM and SAXS.

The data recorded as the laser frequency is scanned consists of the fluorscence signal from the PMT, a Doppler-free I2 spectrum and frequency markers from the etalon. The etalon provides a calibration of the frequency scan. The Doppler-free I2 spectra provides an absolute frequency reference used to correct for small laser frequency drifts, separator voltage drifts and to determine the absolute acceleration voltage of the separator for the Doppler shift corrections. We are thus able to record data over long periods of time, e.g. 3 hours, and maintain a reasonable resolution of 100 MHz. Some of the first online data recorded with this system is shown in Figure 2. The overall detection efficiency has been measured to be 1/1000, i.e. one detected photon per 1000 atoms, for the largest transition in the nuclear spin 1/2 isotopes. 

When the oscillator frequency v, is varied (frequency scan ), the frequency of the rotating magnetic field will come into resonance with the precessing Larmor frequencies vL of the protons, induce phase coherence, and tip the net magnetiza-... 

The t scale disappeared with the development of frequency-scan instruments and of the pulsed FT mode, which is essentially an instantaneous frequency scan. The terms upfield and downfield are now obsolete and have been replaced, respectively, by shielded (lower S, or to the right) and deshielded (higher S, or to the left). 

At York University, Toronto, microwave measurements have been revived by Storry and Hessels [9,10], that could benefit of lasers to excite the 23P level, instead of the lamps used by Hughes and coworkers. Also, the detection of laser-induced fluorescence from 23P levels makes another basic difference with respect to Hughes experiments, which, together with a microwave frequency scan, eliminated the lineshape asymmetries. In this experiment, a moderate magnetic field is also used to select the transitions between the desidered Mj sublevels. The... 

The Fig. 2 experimental points result from the averaging of 20 pulses recorded for each frequency. The frequency scan Is performed randomly to avoid systematic drift effects. The width at half maximum of the resonance 120 kHz, corresponds to the theoretical limit due to the finite transit time across the waist of the microwave Gaussian beam. The observed Zeeman components have not the same weight. This is just related to the resonant 447 GHz microwave source whose polarization is partially elliptical. 

Figure 9a (inset) shows the zero-field ODMR spectrum for Pd(2-thpy)2 detected at the electronic origin of the emission spectrum at 18,418 cm Due to the frequency scan of the microwave radiation, one observes a signal at 2886 MHz (0.0962 cm ) with a line width (fwhm) of 20 MHz. No other zero-field ODMR signal could be observed in the frequency range from 300 MHz up to 8000 MHz. However, for a system of three triplet substates, one would expect to... 

The time required for each frequency scan can be seen in Figure 21.6(a). The average time required for each scan was 1194 s (0.33 h). The time required for a measurement at each individual frequency is illustrated in Figure 21.6(b). At low frequencies, the time required generally corresponds to three or four cycles, but at high frequencies, a much larger number of cycles is needed to account for the smaller signal-to-noise level. 

Fig. 7 An example of a frequency scan showing the change in a materials behavior as frequency varies. Low frequencies allow the material time to relax and respond, hence, flow dominates. High frequencies do not and elastic behavior dominates. (From Ref.. ) (View this art in color at www.dekker.com.)...

As the frequency is increased in a frequency scan, the Newtonian region is exceeded and a new relationship develops between the rate of strain, or the frequency, and the viscosity of the material. This region is often called the power law region and can be modeled by ... 

The frequency scan is used for several purposes that is discussed here. One very important use, that is very straightforward, is to survey the material s response over various shear rates. Different properties are required at these regimes, and to optimize one property may require chemical changes that harm the other. Similarly, changes in polymer structure can show these kinds of differences in the frequency scan. Branching affects the response to different frequencies. ... 

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