Electro-optical modulation

Figure Cl.4.9. Usual cooling (carrier) and repumping (sideband) transitions when optically cooling Na atoms. The repumper frequency is nonnally derived from the cooling transition frequency with electro-optic modulation.

Circular dichroism employs standard dispersive or interferometric instmmentation, but uses a thermal source that is rapidly modulated between circular polari2ation states using a photoelastic or electro-optic modulator. Using phase-sensitive detection, a difference signal proportional to the absorption difference between left- and right-polari2ed light, AA is recorded as a function of wavenumber. Relative differential absorptions... 

In video microscopy, for instance, background is normally subtracted using differential interference contrast (DIC) [18]. This technique, which requires a number of manipulations from the user, may now be automated using a new method called polarization-modulated (PMDIC) [19,20], It requires the introduction of a liquid crystal electro-optic modulator and of a software module to handle difference images. PMDIC has been shown to bring improvements in imaging moving cells, which show a low contrast, as well as thick tissue samples. 

Hill, R. A. Knoesen, A. Mortazavi, M. A., Corona poling of nonlinear polymer thin films for electro optic modulators, Appl. Phys. Lett. 1994, 65, 1733 1735... 

Historically, the first instrument for the determination of lifetime was a phase fluorometer (designed by Gaviola in 1926) operating at a single frequency. Progress in instrumentation enabled variable modulation frequency by employing a cw laser (or a lamp) and an electro-optic modulator (0.1-250 MHz), or by using the harmonic content of a pulsed laser source (up to 2 GHz). These two techniques will now be described. 

Phase fluorometers using a continuous light source and an electro-optic modulator... 

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. 

Figure 6.11. Traveling wave single-mode LiNbOj electro-optic modulator.

However, unlike diode lasers, an external acousto-optic or electro-optic modulator is required for phase-modulation lifetime fluorometry to be performed with a HeNe laser. 

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. 

Electro-optical modulators are other examples whose efficiency is enhanced in the presence of ion-radicals. These devices are based on the sandwich-type electrode structures containing organic layers as the electron/hole-injecting layers at the interface between the electrode and the emitter layer. The presence of ion-radicals lowers the barrier height for the electron or hole injection. Anion-radicals (e.g., anion-radicals from 4,7-diphenyl-l,10-phenanthroline—Kido and Matsumoto 1998 from tetra (arylethynyl) cyclooctatetraenes—Lu et al. 2000 from bis (1-octylamino) perylene-3,4 9,10-bis (dicarboximide)s— Ahrens et al. 2006) or cation-radicals (e.g., cation-radicals from a-sexithienyl—Kurata et al. 1998 l,l-diphenyl-2-[phenyl-4-A/,A- /i(4 -methylphenyl)] ethylene— Umeda et al. 1990, 2000), all of them are electron or hole carriers. 

Crystalline lithium niobate was for many years the only material that could be used in devices, an example of its use being in electro-optic modulators for interfacing with fibre optic technologies. Unfortunately, this material is not very satisfactory because it is difficult to grow, and hence expensive, and only shows modest electrooptic properties. 

Electro-optic modulation (Pockels effect) using a poled polymer is shown schematically in Figure 5.33, where an optical signal confined in a wave-guide is modulated by the application of an external voltage, known as a Mach-Zender Interferometer. 

Figure 5.33 Electro-optic modulation in a wave-guide using a poled polymer.

Show the Pockels effect (electro-optic modulation)... 

Fig. 1 Setup for transient holographic grating measurements. The electro-optic modulators (EOMs) are used for 180°-phase shifts of the holographic grating. The piezo mirror serves for phase matching between the diffracted beam and the coherent reference wave generated by the local oscillator. The setup is a modified version of the one described in [87]...

The EO effect is a second-order nonlinear optical (NLO) effect. Only non-centrosymmetrical materials exhibit second-order NLO effects. This non-centrosymmetry is a condition, both at the macroscopic level of the bulk arrangement of the material and at the microscopic level of the individual molecule. All electro-optic modulators that are presently used by telecom operators are ferro-electric inorganic crystals. The optical nonlinearity in these materials is to a large fraction caused by the nuclear displacement in the applied electric field, and to a smaller fraction by the movement of the electrons. This limits the bandwidth of the modulator. The nonlinear response of organic materials is purely electronic and, therefore, inherently faster. 

Figure 4.17 Demonstration of the limited linear part of electro-optical modulators. In digital optocommunication applications, the full modulation depth (on/off) is used...

If electro-optic modulators and directional couplers are to be driven by the electrical outputs from very large-scale integration (VLSI) semiconductor chips, then the required drive or VK voltages of EO devices must be less than 6 V (for... 

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