Fundamental Performance Limits of Active Polymer Integrated Optics
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First, the n3r33 coefficients are not as large as in other inorganic technologies. Second, singlemode poled polymer waveguides must usually mode-match to optical fiber, leading to a large separation (- 20 .tm) between the electrodes and a correspondingly smaller modulation field for a given drive voltage. This means that it is advantageous to seek devices and applications where the planar feature of the technology, not the EO response, is the main advantage. 65
Mat. Res. Soc. Symp. Proc. Vol. 392 0 1995 Materials Research Society
The length of device required for an EO modulator to fully switch or modulate an optical signal from logic-level electrical signals is of order a few cm in any material system so far demonstrated. The width required is only a few tens of microns. This not only leads to significant space required for an EO transmitter array, but also means the electrodes necessary to drive the modulators must be transmission lines at frequencies around 0.3 GHz or above. There is incentive to decrease the active device length not only to occupy less area, but also to allow the drive electrodes to be designed as simple RF stubs for significantly higher impedance. One approach to this is to increase the EO sensitivity of the materials, and this has long been one of the great promises of polymer EO materials and is being pursued by many groups around the world. Another approach is to make active complementary taps'°. In this approach, a large amount of optical energy is fed into the modulator, and the modulator redirects a small fraction of that energy into one or the other of the two outputs. Since only a small portion of the light is actually being modulated, the active device length can be much shorter, of order 1 mm in current materials, for a given switching voltage. This lets the electrical load of the modulators be modeled as stub elements into the multi-GHz range, rather than requiring transmission lines as in the longer modulators, thereby reducing the power dissipated in both the modulator and the output driving it. The signal level is indicated by which of the two outputs that most of the output signal is directed towards. Since a relatively constant fraction of energy is removed from the supply beam, it can be used as the supply for subsequent complementary modulators with little additional noise. Since the signal is represented by the relative levels in two complementary channels, there can be greater tolerance to the additional noise that is introduced. Eventually, the supply beam will become too noisy and/or too weak for further tapping and must be dumped. This means that such a scheme makes much less efficient use of optical energy than other schemes and is only practical in systems where large amounts of single-mode light can be supplied economically, i.e. where wall-plug power is plentiful and power dissipation is the primary limitation on improved system performance. EO polymer waveguide taps may be cascaded and tiled to provide an active optical tap array. This device permits efficient co
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