Known fiber optic communications systems include a laser diode, a modulator and a photodetector diode. Modulators are either direct, modulating the optical wave as it is generated at the source, or external, modulating the optical wave after it has been generated. A problem with fiber optic communications systems is that the dynamic range of the system is largely determined by distortion from the modulator. External modulation of lightwave signals is controlled by adjusting a modulation chirp parameter to a substantially fixed value in a predetermined, controllable manner. This minimizes the transmission power penalty caused by chromatic dispersion in an optical fiber communication system.
External modulation is accomplished, for example, in a dual waveguide device wherein substantially identical input optical beams are supplied to the waveguides and wherein each waveguide is subject to its own individual, mutually exclusive control. Modulation signals are applied to each waveguide via the separate control. Moreover, control signals are applied to each waveguide for adjusting the modulation chirp parameter to a desired non-zero substantially fixed value.
An electro-optical modulator modulates the optical signal with an electromagnetic signal, preferably an RF signal. The RF signal interacts with the optical signal over a predetermined distance. The construction of optical modulators slows the RF signal relative to the optical signal so that it takes the RF signal a longer period of time to travel the interaction distance. Therefore, the RF signal electric field, which modulates the optical signal, varies along the interaction distance. Since the RF signal does not act on the same portion of the optical signal throughout the interaction distance, the optical signal is distorted. The longer the interaction distance, the greater the distortion.
Typical high-speed electro-optical external modulators use a travelingwave electrode structure. Such modulators have a microwave transmission line in the vicinity of the optical waveguide. A microwave signal and an optical signal copropagate for a prescribed distance, thereby acquiring the required optical modulation. To prevent velocity mismatch between the microwave signal and the optical signal in a traveling wave modulator, a thick buffer layer is provided on a wafer to speed up the propagation of the microwave signal. Previously, a silicon dioxide (SiO.sub.2) buffer layer was created through known techniques such as E-beam, sputtering, or chemical vapor deposition (CVD). The buffer layer may be planarized throughout the wafer or may be patterned with electrode structures.
Producing a silicon dioxide buffer layer requires expensive capital equipment and very precise control of production parameters. For example, devices such as evaporators, sputtering machines, gas supply machines or CVD machines cost tens or hundreds of thousands of dollars. Furthermore, most of the time, the SiO.sub.2 material has less oxygen than necessary and requires annealing to gain proper dielectric properties. During annealing, thermal expansion creates stress between the silicon dioxide layer and the optical waveguides. The waveguides can become nonuniformly distributed throughout the wafer or even disappear under this stress. Silicon dioxide is furthermore fairly porous, and absorbs a few percent moisture after a 24-hour boil. It would be advantageous to provide a method of manufacturing optical devices which was less expensive, less complex and yielded higher quality optical devices.
Benzocyclobutene (BCB) is a new class of organic dielectric materials commonly used in multichip module (MCM) technology. As a result of its common use in MCM applications, BCB is a well-known and well-understood material. BCB exhibits several advantages over materials, such as SiO.sub.2, which are conventionally used in integrated optical devices. BCB has lower dielectric loss and a lower dielectric constant, is subject to lower mechanical stress and is much easier to process during production of integrated optical modulators. The ease with which BCB buffer layers are constructed provides a significant advantage over conventional buffer materials. A liquid BCB solution is applied to a wafer cured in a nitrogen atmosphere and patterned with a photoresist or metal mask. No expensive machines, such as CVD machines, are required.
Unfortunately, interface adhesion forces between BCB and thin metal film is poor, resulting in a weak bond between a BCB layer and a metal film layer in an optical device. Furthermore, a velocity matched modulator requires an extremely thin layer of BCB, substantially less than 8000 angstroms, the minimum thickness possible in the BCB 3022 product line. It would be advantageous to provide a method of manufacturing optical devices which use BCB as a buffer layer.