Integrated Optics Interferometric Modulators

INTEGRATED OPTICS INTERFEROMETR1C MODULATORS Douglas R. ThayerLos Alamos National Laboratory Post Office Box 1663, P-14, MS D410, Los Alamos New Mexico 87545INTRODUCTIONMany of the applications of photonics to broadband diagnostics experiments require modulating light in response to an electrical input signal. The two general methods which are employed for this purpose are direct modulation of a source and modulation of a source by an external device. Direct modulation of a source (light emitting diode, laser diode, etc.) involves the relatively straightforward application of well known electrical tech niques and, depending on the required bandwidth, linearity, and dynamic range, may be imple­ mented with a few components at low cost. These techniques are treated in detail in other chapters.External modulation of a continuous-wave (cw) source can provide wider bandwidth and enhanced performance relative to direct modulation in some applications, and has received considerable attention in recent years. The external modulators described in this chapter are the directional coupler and Y-branch interferometer, both of which use the electrooptic properties of certain materials to alter the transmission of light in optical wave guides. These modulators have been made in a variety of materials (e.g. LiNb03, LiTa03, GaAs, and InP) and in several configurations. In addition to their realization as research devices, several types have recently become available as commercial products. Other modulator types have also been implemented (total internal reflection, waveguide cutoff, polarization shift­ ing)1 , but will not be discussed here. Due to the wide interest and rapid progress in this field, the reader interested in fabrication details and design trade-offs should refer to the literature and recent review articles such as the excellent one provided by Alferness.1WIDE BANDWIDTH APPLICATIONS OF INTEFEROMETR1C MODULATORSMost of the commercial applications being considered for external modulators are related to the telecommunications industry, where the emphasis is on digital transmission and switching techniques. The obvious characteristics desired are low switching voltage and power, high speed operation, and large extinction ratios or isolation ratios between optical waveguides. Directional couplers and 3 dB coupler Mach-Zehnder interferometers in the materials cited above have demonstrated the feasibility of these applications. Since most of the wide bandwidth photonic systems described in this volume are directed toward transmitting and recording analog signals from transient phenomena, the following discussion will describe the application of the Y branch Mach-Zehnder interferometer to these systems. A general schematic of this application is shown in Fig. 1. The basic ele­ ments are a narrow optical line-width cw source, the electrooptic modulator, the fiber optic transmission medium, and the receiver/recorder. System and component parameters that will be discussed are: modulator characteristics; optical source characteristics; optical power and polarization effects; modulator and fiber optic light losses; and system con­ siderations.