Electrooptical phase modulators

Rickard, M. Nakata, M. Takezoe, FI. Watanabe, J. Clark, N. A. Electronic electrooptic phase modulation using bent-core Uquid crystals. Appl. Phys. Lett. 2005, 87, 261115. 

The second approach is based on FM sideband spectroscopy, a different modulation method, which is well known in microwave spectroscopy, but whose advantages in the optical region have only recently been demonstrated by J. L. Hall et al. (19), and, independently, by G. C. Bjorklund and collaborators (20,21). In this method, the probe beam is sent through an acoustooptic or electrooptic phase modulator which produces two (or more) FM sidebands of such amplitudes and phases that any constructive or destructive interference effects cancel completely. The intensity of the probe beam, before entering the sample, remains therefore exactly constant. If the sample now changes the amplitude or the phase of any of the sidebands or the phase of the carrier, this delicate balance is perturbed, and the light acquires an amplitude modulation, which can be readily observed with a fast photodiode, followed by rf heterodyne detection. 

Modulators are needed to convert electrical signals into optical amplitudes or phase modulations. Polymeric waveguides may be more compatible with integrated optics owing to processing considerations, and will be much cheaper to manufacture. Routing switches, electrooptic shutters, and amplitude modulators are imponant components for optical communications networks. Directional couplers, mode sorters, and Mach-Zender interferometers (Figure 2) are representative of the specific device mechanisms. These devices can op-... 

Lipscomb, G. F., Garito, A. F., and Narang, R. S., An exceptionally large linear electro-optic effect in the organic solid MNA, J. Chem. Phys., 75, 1509 (1981). Sigelle, M, and Hierle, R., Determination of the electrooptic coefficients of 3-methyl 4-nitropyridine 1-oxide by an interferometric phase-modulation technique, J. Appl. Phys., 52, 4199 (1981). 

Other devices that can be fabricated with NLO polymers are for example sensors for electric field. A number of sensors fabricated with EO polymers to detect electric field signals and map the electric field distribution have been implemented in the last years based on Mach-Zehnder intensity and polarization modulators. Other sensors are based on asymmetric Fabry-Perot microcavities to convert phase modulation into amplitude modulation and enhance the samphng signals. Very recently Sun et al. [172] presented a novel broadband electrooptic electric field sensor fabricated with an EO polymer microring resonator coupled to the core of a side-polished optical fiber. A sensitivity of 100 mV/m has been achieved at frequencies up to 550 MHz. 

Materials for Electrooptic Modulation. The fundamental phenomenon of Pockel s effect is a phase change, A( ), of a light beam in response to a low frequency electric field of voltage, V. Relevant relationships for coUinear electrical and optical field propagation are as foUows (1 6) ... 

In an electrooptic material the phase retardation angle is controlled by altering birefringence, which is in turn controlled by the potential of an apphed electric field. An electrooptic device thus acts as a variable phase optical retardation plate, and can be used to modulate the wavelength or intensity of an incident beam. 

Figure 3.38. Principle of the photorefractive effect By photoexcitation, charges are generated that have different mobilities, (a) The holographic irradiation intensity proHle. Due to the different diffusion and migration velocity of negative and positive charge carriers, a space-charge modulation is formed, (b) The charge density proHle. The space-charge modulation creates an electric Held that is phase shifted by 7t/2. (c) The electric field profile. The refractive index modulation follows the electric field by electrooptic response, (d) The refractive index profile.

Figure 17. (a) Transverse electrooptic modulator that rotates polarization of an incoming light beam as a function of applied electric field and (b) a travelling Mach-Zehnder interferometer that introduces a phase shift to the light beam in one arm as a function of applied field. 

Figure 4.14 Three different electrooptic waveguide devices [1] (a) Mach-Zehnder modulator with push-pull arrangement of the electrodes, (b) electrically controlled directional coupler, (c) coupler with phase-reversal electrodes.

Thus, the applied field, E2, changes the effective linear susceptibility (i.e. the dependence of the polarization on the light field, Eft. Since the linear susceptibility is related to the refractive index, the refractive index of the material is also changed by the applied field. This is known as the linear electrooptic (EO) or Pockels effect and can be used to modulate the polarization or phase of light by changing the applied voltage. 

For second harmonic generation (SHG), the tensor is y(2)(—2co co, co) (useful for frequency doubling and parametric down-conversion) while for the linear electrooptic or Pockels71 effect the tensor is y(2)(— co co, 0) (useful for Q-switching of lasers, for phase or amplitude modulators, and for beam deflectors) for optical rectification the tensor is y 2>(0 00, —co) for frequency mixing the tensor is y(2)(— co3 oolr co2) (useful for frequency up-converters, optical parametric oscillators, and spectroscopy). 

X (-0) (I),0) Electrooptic (Pockels) effect Modulators, variable-phase retarders... 

In contrast, the nonlinearities in bulk materials are due to the response of electrons not associated with individual sites, as it occurs in metals or semiconductors. In these materials, the nonlinear response is caused by effects of band structure or other mechanisms that are determined by the electronic response of the bulk medium. The first nonlinear materials that were applied successfully in the fabrication of passive and active photonic devices were in fact ferroelectric inorganic crystals, such as the potassium dihydrogen phosphate (KDP) crystal or the lithium niobate (LiNbO,) [20-22]. In the present, potassium dihydrogen phosphate crystal is broadly used as a laser frequency doubler, while the lithium niobate is the main material for optical electrooptic modulators that operate in the near-infrared spectral range. Another ferroelectric inorganic crystal, barium titanate (BaTiOj), is currently used in phase-conjugation applications [23]. 

Nematic materials are only one member of a large family of a variety of structurally different compounds forming liquid crystalline mesophases. Although only nematics have yet found really widespread use, mostly for display applications, some structurally highly diverse smectic phases also have unique electrooptical characteristics, for example ferroelectricity or antiferroelectricity, which can be modulated by selective fluorination [5, 51]. For 20 years intensive effort has been devoted to making practical use of these phenomena. 

---