Source frequency modulation

Frequency-Modulation Spectroscopy. Frequency-modulation spectroscopy (tins) is a high sensitivity null-background infrared technique for measuring absorbances down to 10 with fast acquisition speeds. Fms involves frequency-modulating a laser source at COq to produce a carrier frequency having sidebands at cJq where is an integral multiple of the modulation frequency. Dye lasers and many other single-line sources can... 

The light striking the detector contains two signals the dc (or average value) is given by 7o(A.) (A,), where. / (A,) is the dc reflectance of the material, while the modulated value (at frequency 2 ) is lQ k)AR k), where AR k) is the change in reflectance produced by the modulation source. The ac signal from the detector, which is... 

The smaller the duty cycle the larger the number of harmonics and the less power available per modulation frequency of the excitation. 30 In this case care must be taken to not place to much power in the unused high-frequency region by decreasing the duty cycle of the excitation. A good rule of thumb for externally modulated sources is the following ... 

In phase-modulation fluorometry, the pulsed light source typical of time-domain measurements is replaced with an intensity-modulated source (Figure 10.5). Because of the time lag between absorption and emission, the emission is delayed in time relative to the modulated excitation. At each modulation frequency (to = 2nf) this delay is described as the phase shift (0, ), which increases from 0 to 90° with increasing modulation frequency. The finite time response of the sample also results in demodulation to the emission by a factor m which decreases from 1.0 to 0.0 with increasing modulation frequency. The phase angle (Ow) and the modulation (m, ) are separate... 

Fig. 10.19a) and the appearance of breaks shown in Fig. 10.19b. This is an important result because in systems, living or engineered, it may be the only means of the declaration of success, that is, the actual location of the target (modulation source). It also shows that the range is longer for low frequencies (Fig. 10.19b, lines a and c). 

Fig. 10.19 (a) Experimental dependence of coherence spectra on distance (10-120 cm) between the modulation source and the sensing array, (b) Ratios of intensities ao/ai at fundamental frequency and at its first harmonic, plotted as the function of distance. The breaks indicate disappearance of the higher harmonics with distance. They indicate softening of plume edges with distance due to diffusion... 

In a theoretical study of the advantages to be gained from nondispersive systems, Chester and Winefordner (28) have studied frequency-modulated sources in nondispersive atomic fluorescence, and have demonstrated a multiplex disadvantage for nondispersive atomic fluorescence systems. These studies have indicated that the limiting noise is flame background noise. 

Precise frequencies relative to a frequency standard (at frequency oiq) can be provided by comb generation. If we amplitude or frequency modulate (at frequency 2) a source at the standard frequency, then spectral components at frequencies ioq n 2 (n an integer) are generated. If n and 2 are chosen so that ioq + n 2 a w(unknown) then w(unknown) can be precisely determined by heterodyne methods. The challenge is to make (n 2) very large. 

One of the most convenient methods for determining lifetime, as well as one of the best suited to low-cost applications, involves using frequency-modulated excitation (49). Upon excitation by a frequency modulated source, the finite lifetime of the emitter causes a phase-shift and demodulation of the emission relative to the excitation waveform as shown schematically in Fig. 6. 

A laser system that delivers pulses in the picosecond range with a repetition rate of a few MHz can be considered as an intrinsically modulated source. The harmonic content of the pulse train - which depends on the width of the pulses - extends to several gigahertz. The limitation is due to the detector. For high frequency measurements, it is absolutely necessary to use microchannel plate photomultipliers (that have a much faster response than usual photomultipliers). The highest available frequencies are then about 2 GHz. As for pulse fluorometry, Ti sapphire lasers are most suitable for phase fluorometry, and decay times as short as 10-20 ps can be measured. 

Tvvo vidcl used approaches are used for lifetime measurcnienis. ilie lime-domain approach and the frt i/iu niy-domain approach. In tinte-domain measurements. a pulsed source is employed and the time-depcndcnr decay of fluorescence is measured. In the frequency-domain method, a sinusoidallv modulated source is used to excite the sample. The phase shift and demodulation of the fluorescence emission relative lo the excitation waveform provide the lifetime information. ( onimercial instrumentation is available to implement both techniques. ... 

In this technique, the ENDOR response is observed as changes in the EPR absorption-mode (in-phase) signal as an rf source is swept. Commonly the EPR signal is detected without field modulation, but the rf frequency is modulated at frequencies around 10 kHz. The signal is decoded at this modulation frequency by a phase-sensitive detector. As a consequence of this scheme, the ENDOR signal appears as the derivative display. The standard commercially available spectrometer is set up in this fashion. In this type of ENDOR experiment, the degree of desaturation of the EPR signal depends on the complicated interplay of all relaxation... 

Figure 2.11 Schematic response to source frequency sweeping of a transmission cavity containing an absorbing gas. a represents the cavity with the gas absorption profile superimposed at the cavity resonance, b shows the absorption detected at the fundamental frequency and c that at the second harmonic of the modulation frequency. The mirror spacing in b and c would be adjusted to keep the cavity resonance synchronous with the source frequency. The ordinate scale has been offset and exaggerated for clarity, but the abscissa scale is the same for all three traces...

There are alternative means of source frequency stabilisation " one is to couple an oscillator to a resonant cavity machined from a temperature insensitive alloy as part of a spectrometer, e.g. Zhu et alP Oscillator modules are available commercially, e.g. Farran," Elva-l, " that have low phase noise and can be employed as local oscillators in narrow band spectrometer designs. Thirup et aV... 

When the signal is frequency modulated (Figure 3.11a) modulation sidebands still appear, but their phase is such that no signal results on demodulation by heterodyne detection. Only if the cavity or sample itself shows absorption dependent on the source frequency is some of the applied FM converted to amplitude modulation (Figure 3.11b) and a signal detected. 

Figure 3.11b Comparison of the spectral linewidth and maximum deviation of the FM source frequency. The frequency deviation at the MMW frequency would be typically 250 kHz and linewidth HWHM > 1-2 MHz. The carrier sweeps between the deviation limits at the source modulation rate, 1 kHz typically...

In an FM MMW spectrometer the spectral source frequency is modulated at a certain rate /, typically 1 kHz. This gives rise to sidebands of the spectral source frequency above and below the carrier frequency. The frequency modulated MMW carrier has in its modulation envelope phase and amplitude relationships to the carrier. Mixing in the non-linear junction of the detector yields the modulation signals altered by their interaction with the cavity and gas inside it, with their preserved amplitude and phase relationship to the original modulation signals. Those properties are measured by passing the heterodyne mixer output and the thermal noise contribution from the mixer, to a filtered phase-sensitive detection system, with the original modulation as reference. 

Of the many possible forms of signal modulation, that of source FM has the advantage of being simple to apply and requiring no additional MMW components or structural modifications to the absorption cavity. The penalty is the considerable attention that must be paid to the interaction between source, cavity and sample to obtain reproducible results that are capable of analysis. For a start, the use of a cavity presupposes that the source frequency be kept in synchronism with the cavity resonance whenever data are taken, and in respect of achieving this source FM possesses a distinct advantage over other modulation methods. 

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