An optical modulator is a device which is used to convert electric signals to optical signals. By applying an electric signal to cause a change of the refractive index of materials, it is possible to control the phase of light and put an electric signal with information on the optical signal. Typical optical modulators are LN and EO polymer modulator.
Organic electro-optic polymer
An organic electro-optic (EO) polymer is a material which dye molecules having an electro-optic effect are bonded and dispersed. An EO polymer has excellent optical properties such as a significantly high EO coefficient and a high-frequency response compared to LN, which is a typical EO material for inorganic dielectrics. In addition, the EO polymer is easy to fabricate waveguide structure by microfabrication process. In recent years, modulator that hybridizes EO polymer and silicon is a promising candidate for high-speed optical communication.
An electro-optic effect is a change of refractive index of materials by applying electric field, and the electro-optic coefficient is a material specific coefficient that represents the magnitude of the change of the refractive index. A material having a large electro-optic coefficient has a large change of the refractive index when an electric field is applied.
A Mach-Zehnder interferometer is a typical interferometer structure which divides the incident light into two paths and combines them again. By controlling the phase of the light after splitting the propagating light, it is possible to constructive and destructive interference when the propagation light is combined. By using the Mach-Zehnder interferometer, the phase modulation of light can be regarded as the intensity modulation. Therefore, the Mach-Zehnder interferometer is generally used as a typical optical modulator for converting an electric signal into an optical signal.
Ridge type waveguide
Ridge type waveguide is one of the types of waveguide structure. The waveguide structure should be designed in a single mode waveguide. Compared to the rectangular waveguide, the ridge type waveguide can be a single-mode waveguide even if the waveguide width is wide.
Figure of merit
A half-wave voltage-length product VπL (unit: Vcm) is generically used for the figure of merit of the modulator. The voltage at which the phase of the propagating light shifts by π is called Vπ. The Vπ is given by
where λ is the wavelength, n is the refractive index, r is electro-optic coefficient, d is the electrode distance, and L is the length of phase shifter. The parameters of Vπ and L in Mach-Zehnder interferometer exhibit a tradeoff. Therefore, the smallest VπL is preferred.
Figure 4 Schematic diagram of an optical phased array
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Optical phased array
An optical phased array is a beam steering device by using light interference (see Figure 4). The optical phased array is consisting of optical modulator array. Using the optical phased array, light beam can be driven at a high speed than a beam steering device which consisting of a mirror or a lens.