Circularly polarized light photoelastic modulator

Experimentally, light from the laser was polarized vertically and passed 45° to the stress axes of the photoelastic modulators producing alternately left and right circularly polarized light at 220 Hz. Three types of measurements were made 1) PAS magnitude spectra at the modulation frequency were recorded by inserting the mechanical chopper and using... 

Figure 12.2. Photoelastic modulator in which an isotropic ZnSe plate is alternately stretched and compressed with Si02 piezoelectric transducers. In general, vertically polarized light (VPL) is converted to elliptically polarized light (EPL). At certain amplitudes, linearly or circularly polarized light may be formed. (Reproduced from [6], by permission of Academic Press copyright 1982.)...

Figure 1 Optical components of a typical MCD/CD Instrument. The modulator, now most commonly a piezoelectrically driven photoelastic device, converts linearly polarized light to a.c. modulated circularly polarized light.

The heart of the polarization-modulated nephelometer is a photoelastic modulator, developed by Kemp (1969) and by Jasperson and Schnatterly (1969). The latter used their instrument for ellipsometry of light reflected by solid surfaces (the application described here could be considered as ellipsometry of scattered light). Kemp first used the modulation technique in laboratory studies but soon found a fertile field of application in astrophysics the modulator, coupled with a telescope, allowed circular polarization from astronomical objects to be detected at much lower levels than previously possible. 

For the measurement of differential absorption spectra in the visible and ultraviolet spectral region, a light modulation technique was introduced over two decades ago [7]. The principle of this measurement also governs the measurement of infrared VCD. This technique involves the high frequency modulation of the exciting beam between left and right circular polarization states via a photoelastic modulator, and measuring the differential absorption with a lock-in amplifier tuned to the modulation frequency. 

Circular dichroism employs standard dispersive or interferometric instrumentation, but uses a thermal source that is rapidly modulated between circular polarization states using a photoelastic or electro-optic modulator. Using phase-sensitive detection, a difference signal proportional to the absorption difference between left- and right-polarized light, AA = — Aj, is recorded as a function of wavenumber. Relative differential absorptions... 

Figure 2 Diagram illustrating the basic optical layout and electronic pathways for the measurement of VCD. The diagram is applicable to both dispersive VCD spectrometers and FT-IR spectrometers that use a photoelastic modulator (PEM) as the source of the polarization modulation of the light beam between left (L) and right (R) circular states.

---