Modulators electro-optic intensity modulator

Figure 9.11 An electro-optic intensity modulator (a) schematic of experimental arrangement (b) output transmittance as a function of // / ...

Figure 9.12 An electro-optic intensity modulator using a pair of oriented LiNbOj crystals...

Parker, I.D., et al. 1993. Fabrication of a novel electro-optical intensity modulator from the conjugated polymer, poly(2,5-dimethoxy-p-phenylene vinylene). Appl Phys Lett 62 1519. 

Farwell, M.L., Lin, Z.Q., Wooten, E., and Chang, W.S.C. 1991. An electro-optic intensity modulator with improved linearity. IEEE Phot Tech. Lett. 3 79S2-795. 

In Section 6.2.4, we discuss a polarizers+electro-optics ciystal setup (Fig. 6.3) commonly used for electro-optical intensity modulation purposes. 

The light source can be a xenon lamp associated with a monochromator. The optical configuration should be carefully optimized because the electro-optic modulator (usually a Pockel s cell) must work with a parallel light beam. The advantages are the low cost of the system and the wide availability of excitation wavelengths. In terms of light intensity and modulation, it is preferable to use a cw laser, which costs less than mode-locked pulsed lasers. 

Ashkin and Dziedzic (1977) used the radiation pressure force of a laser beam to levitate microdroplets with the apparatus presented in Fig. 15. A polarized and electro-optically modulated laser beam illuminated the particle from below. The vertical position of the particle was detected using the lens and split photodiode system shown. When the particle moved up or down a difference signal was generated then a voltage proportional to the difference and its derivative were added, and the summed signal used to control an electro-optic modulator to alter the laser beam intensity. Derivative control serves to damp particle oscillations, while the proportional control maintains the particle at the null point. 

Figure 4.12 Longitudinal Pockels cell. The kdp electro-optic crystal between two crossed polarizers makes an intensity modulator.

Mode-locked laser A laser in which many resonant modes are coupled in phase, to yield a train of very short pulses (e.g. ps pulses). The coupling of the modes is obtained by modulation of the gain in the resonator, and can be active (electro-optic modulation of the losses or of the pump intensity), or passive (with a saturable absorber). 

Figure 8. The Pockels effect leads to a change in the refractive index of an electro-optic material due to the application of a static electric field. This can be used to build a Mach-Zehnder intensity modulator, for example, which will have an optical transmission dependent on the electric field applied to the Pockels material.

In order to produce surface-relief electro-optic gratings, Munakata et compared two fabrication methods of SRG inscription. In the first, the SRG was produced with an interference pattern of cw laser, with relatively modest intensities. The gratings so recorded were photo- and thermally erasable, and efficient writing was polarization dependent. In the second method, a phase mask was employed to provide the periodic intensity modulation of a pulsed laser, the 3rd-order harmonic (at 355 nm) of a Nd YAG laser. The SRG was produced with a single laser pulse, allowing a very short fabrication time (less than Is). The direshold for ablation was 500 mj/(em pulse), and the amplitude of the SRG increased with pulse energy. A depth of up to 300 rim could be achieved, leading to a smooth but not sinusoidal surface modulation. 

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... 

---