Electro-acoustic modulator

In Fig. 5 a typical setup for resonant photoacoustic spectroscopy as used in the authors laboratory is shown. The radiation of a cw laser is intensity modulated by a mechanical chopper of high precision. An electro-optic modulation device may also be employed or the laser beam is modulated directly by modulation of its power supply. As already discussed, vibrational excitation with an IR laser, for example, causes a modulated pressure change in the resonator via fast vibrational relaxation. This acoustic signal is detected with a microphone, because these devices provide the highest sensitivity. Detectors employed in calorimetry to measure the heating of a mpte such as thermistors or thermophi are less sensitive and p( sess a slower rise... 

Photoacoustic spectroscopy (PAS) is revolutionary in the way that it is one of the few spectroscopic techniques that is not based on the direct or indirect measurement of electro-magnetic radiation. It is grounded on the ancient observation of Graham Bell, that the exposure of different solid and liquid substances to a rapidly interrupted beam of light results in the emission of acoustic energy at the same frequency as that at which the incident radiation was modulated. 

For a low-frequency, Mach-Zehnder modulator, the impedance is dominated by the electrode capacitance and resistance as shown in Fig. 9.57. A Smith chart plot for a MZM is shown in Fig. 9.58. Unlike the laser diode, a modulator on lithium niobate has no junctions consequently the impedance is independent of modulator bias. As was the case with the diode laser, it is often important to match the modulator impedance to the system impedance, typically 50 or 75 real. However, unlike the diode laser, modulators fabricated in electro-optic materials that are also piezoelectric can have perturbations in their impedance due to coupling of some of the modulation signal into compressional waves. The coupling can be significant at frequencies where the modulator crystal is resonant. The dashed curve in Fig. 9.58 shows an acoustic resonance at 150 MHz. A number of techniques (Betts, Ray and Johnson, 1990) have been developed to suppress these acoustic modes to insignificant levels, as is evidenced by the solid curve in Fig. 9.56. 

The LOAS instrument consists of the following main units A tunable laser, a sample cell with an acoustic transducer (sometimes called a spec-trophone or SP), an amplifier, and a recording system. The acoustic response of the medium can be stimulated by amplitude or frequency modulation of the laser light or by Stark or Zeeman modulation of the absoiption line of the analyte. Both pulsed and cw lasers with mechanical or electro-optical amplitude modulation are often employed in LOAS analytical applications. 

The high polarizabilities of the TiOg-octahedra results in anomalously high values of the dielectric permittivities (see Figure 8.2), as well as the electro-optical, nonlinear optical, piezoelectric and electromechanical coupling coefficients (Cross, 1993). This is at the very heart of the technical application of ferroelectric materials as ultrasonic oscillators, acoustic and optical frequency multipliers, dielectric amplifiers, acoustic and optical frequency modulators, switches, sensors, actuators, and many more. For detailed accounts on these applications, see Uchino (1994, 1996), Cross (1993), and Heywang et al. (2009). 

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