Polarization multiplexed photonic integrated circuits for 100 Gbit/s and beyond

In this thesis, a fully integrated dual-polarization (DP) optical transmitter is proposed, developed and demonstrated. The transmitter proposed in this work consists of two key building blocks: a polarization rotator (PR) and two electro-absorption modulators (EAM), laid out in a serial configuration. Furthermore, a laser source is monolithically integrated. All devices are realized in the semiconducting material system of indium phosphide (InP) and its related quaternary alloys (InGaAsP). The technology used for fabrication is referred to as generic, meaning that it is not intended for fabrication of a specific device but rather to enable as many different applications as possible, much like modern silicon processes in electronics. The thesis begins with the theoretical implications of optical polarization in integrated waveguides. Known principles on how to make integrated polarization rotating devices are discussed and it is shown that they prove to be too sensitive on typical fabrication tolerances. It is shown that the Jones formalism, originally intended for free space optics, can be used to describe integrated waveguides on the micro-scale. Based on this formalism, an optimization technique is derived that lowers the sensitivity to fabrication errors by 25% and brings down theoretical optical losses from 0.8 dB to 0.1 dB. The same approach is used to design devices that rotate polarization not by 90°, but by 45°. Next, new device structures are proposed for polarization resolved reception and transmission of optical signals. The iSTOMP (integrated STOkes MaPper) uses an interferometric structure loaded with several PRs and allows the complete characterization of the polarization ellipse of incoming signals. The mathematical groundwork is given as well as a geometric way of understanding the device. For data transmission, a serial DP EAM design is proposed. It consists of two EAMs interconnected by a single 90° PR. The DP EAM makes use of the fact that EAMs in InP typically only modulate one polarization, so that the cascade of devices enables DP modulation. The EAMs make use of multi-quantum well (MQW) layers, thin sheets of semiconductor that break the symmetry of the InP crystal. The electronic and optoelectronic effects in MQW-based EAMs are studied to verify that the proposed DP EAM can indeed be demonstrated with good performance. In particular, a polarization resolved model of the dominating light-matter interaction, the quantum-confined Stark effect (QCSE), is given. Finally, a design of an EAM is derived. System-level simulations of the DP EAM are carried out to conclude the theoretical part of the thesis. It is shown that for an implementation penalty below 1 dB, the PR has to have an extinction ratio above 16 dB. To carry out detailed analysis of the fabricated devices, a new approach for polarization resolved measurements is given. It makes use of the Müller/Stokes formalism and is implemented in an experimental setup. This setup uses only optical fibers and no free space optics, thus it is suitable for automated measurements of many devices. The polarization in this setup is shown to be accurate within 2° across the entire Poincaré sphere and C-band. This setup allows a fast and stable characterization of the fabricated PRs, EAMs and the DP EAM. It is shown how the polarimetric measurement equation can be solved easily in the fiber-based setup and how decomposition of Müller matrices can give insights into the various devices. It is shown further that for devices with strong polarization dependence, phase effects like the chirp parameter can be deduced from Müller measurements. Fabricated PRs with polarization extinction ratios of up two 25 dB (rotation within ±4° around 90°) and losses below 1 dB are achieved. The fabricated EAMs show a very strong polarization dependence of over 20 dB, just like theory suggests. In the first fabricated generation, electro-optic bandwidths of up to 17 GHz are measured. These devices are capable of transmitting 39 Gbit/s. The new DP EAM is characterized and it is shown that it can indeed modulate two distinct states of polarization. In a system experiment using 28 GBaud PAM-4 signaling, error-free transmission of 100 Gbit/s is shown over 80 km of fiber. Finally, a fully integrated transmitter is demonstrated, comprising a distributed feedback (DFB) laser, a 45° PR and the aforementioned DP EAM. This is the first demonstration of a monolithically integrated transmitter PIC capable of polarization multiplexing in InP.

Author dc:creator

• Baier, Moritz Friedrich

Advisor dc:contributor.advisor

• Schell, Martin

Licence dc:rights.uri

• https://creativecommons.org/licenses/by/4.0/

Language dc:language.iso

en

Identifier URI

http://dx.doi.org/10.14279/depositonce-7286

OAI identifier oai:identifier

oai:depositonce.tu-berlin.de:11303/8125