|
Bent waveguides are a very important structure in integrated optics [1]. They are used to change the propagation of the light guided in optoelectronic devices. The inherent limitation of bent waveguide structures is that its associated radiation loss increases with branching angle [2]. To reduce the radiation loss, a bent waveguide with small branching angle must be reluctantly constructed due to the sacrifice of dimensional compactness. To achieve compact integrated optical devices and modules with low loss, a variety of low-loss and wide-angle bent waveguide structures have been reported [3]-[8]. Among of them, the microprism-type bent structure is a promising one for practical application [4]-[8J. In a conventional bent waveguide, phase compensation rule was used to design microprism-type structures. To tilt the entire phase front of the guided eigen mode wave, a microprism was designed to compensate for the phase difference between phase fronts in front of and behind the bent region. However, for the conventional design, only the innermost and outmost optical paths of the microprism were considered for phase compensation. Since the other optical paths are ignored, the designed microprism was trangular. To avoid the compensation error, the whole optical paths propagating through microprism must be considered. Since the optical path changes its direction gradually and continuously within bent or tapered region [9], instead of abrupt change as assumption of conventional analysis, a curved path is adopted to simulate more actual situation. The systematic design and analysis of the so-called full phase compensation microprism-type bent waveguides are investigated. It is very difficult to construct the ideal microprism configuration in practical fabrication process due to inevitable fabrication tolerances. For example, the vertices of the designed microprism shape are distorted and flattened, referred as "flattened-vertex" tolerance, during practical photolithography processes. Besides, the accurate refractive indices of the designed structure are unattainable in practical deposition, referred as "Variant-index" tolerance. For the pattern alignment during the photolithography process, the microprism pattern may be tilted from exact position, referred as "pattern-tilt" tolerance. All of the dependences of the transmission efficiency on these tolerances in fabrication processes are studied.
|
