Contents I. Introduction
Balanced distributed traveling-wave photodetectors (TWPDs) are interesting devices because of their capability of high speed, high efficiency, and high saturation power handling as well as noise reduction [1]. The light is absorbed in a distributed way along the transmission lines in distributed or traveling-wave photodetectors. The most distinct difference of TWPD with surface illuminated photodetectors is the fact that the bandwidth limitation caused by bandwidth-efficiency product can be avoided in TWPDs, since the light absorption occurs in a distributed fashion. The absorption layer thickness of each TWPD can be kept thin to increase bandwidth, while still achieving high efficiency by adding up photo-generated current in each detector. Therefore, high-speed and high efficiency can be pursued simultaneously in TWPDs by optimizing separately the detector design and optical waveguide design. Balanced photodetectors can reduce relative intensity noise (RIN) and amplified spontaneous emission noise (ASE) from erbium-doped fiber amplifiers, and thus achieve shot-noise limited system performance when used in an optical link. Since the noise figure can be improved by increasing power of the optical carrier, balanced photodectors with high speed and high saturation photocurrents are especially important in applications for analog fiber optic links. TWPDs in GaAs [2] and InP-based material systems [3] have been reported, and there were several reports in monolithic balanced photoreceivers [4]–[5], and discrete balanced photoreceiver [6]. Integration in semiconductor material systems, however, requires complex growth procedures for passive optical waveguide layers. Instead of growing epitxial layers as a passive optical waveguide, the use of a polymer material for the waveguide can have the advantage of cost effectiveness and ease in device fabrication. Integration of multimode polymer waveguides has been demonstrated in polyimide in GaAs [7].
