We propose a novel adjustment structure and method for an InP-based polarization beam splitter/combiner by using the characteristic whereby the sign at the front of this term changes depending on the light propagation direction. To confirm the proposed principle of adjustment for our InP-based Mach–Zehnder interferometer polarization beam splitter/combiner, we fabricated a test sample that had an npin high-mesa waveguide structure with InGaAlAs/InAlAs multiple quantum wells. By using the test sample, we demonstrated the easy adjustment of an InP-based polarization beam splitter/combiner with the individual modulation of the TE and TM modes and showed that we can output TE/TM polarization at any port as desired.
InP-based modulators/demodulators have become very attractive recently because of their small chip size, low driving voltage, and potential for monolithic integration with semiconductor active devices, such as laser/photo-diodes.1–10) And polarization-division multiplexing (PDM) is mainly used as a method for doubling the transmission capacity of digital coherent technology.1–10) Although several types of InP-based PDM components including a polarization beam splitter/combiner (PBS/PBC) have been reported,11–22) there have been few reports regarding the monolithic integration of PBS/PBCs on InP-based modulators/demodulators.9,10)This is because the integration of InP-based PBS/PBCs is hindered by their tight fabrication tolerances caused by their higher refractive index. To overcome this, a promising way to realize monolithic integration is to activate phase shifters placed on the InP-based PBS/PBC. However, even this approach presents a difficulty and the adjustment takes a long time. Therefore, there is a real need for an easy adjustment method.
In this paper, we propose a novel structure for an InP-based PBS/PBC using a Mach–Zehnder interferometer (MZI) that allows the easy individual adjustment of the TE mode properties of the PBS/PBC. Our proposed structure and adjustment method provides a high yield PBS for monolithic integration. In Sect. 2, we point out the difficulty of fabricating a PBS/PBC on an InP waveguide. After an explanation of the refractive index changes in an InP waveguide when an electric field is applied, we propose a novel structure and principle for an easily adjustable InP-based PBS/PBC. In Sect. 3, we describe a test sample fabricated to verify our concept. We then confirm that the sign in front of the Pockels effect terms for the refractive index change depends on the light propagation direction by comparing the experimental and fitted data with an estimation of the linear and the quadratic electro-optic coefficients of the waveguides that we used in the test sample. In Sect. 4, we demonstrate the easy adjustment of the PBS/PBC properties by using the proposed structure and method23) and show that we can output TE/TM polarization at any desired port.
- InP; PDM
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