Polarization handling in photonic integrated circuits

Abstract

Photonic Integrated Circuits (PICs) are usually polarization dependent. A changing polarization of the light coupled into these circuits can severely degrade their performance. On-chip manipulation of the polarization can help to improve this and to add extra functionality based on polarization. The aim of this thesis is to develop a generic integration technology with polarization handling capability. The main effort of the work focusses on extending the standard technology for PICs with a new type of polarization converter. Furthermore a novel type of polarization splitter has been developed that consists of a passive Mach Zehnder Interferometer and polarization converters. Thus by only adding a polarization converter, the generic platform with polarization handling, including a polarization splitter is obtained. Moreover by the addition of a spot size converter packaging of the PICs becomes feasible. The polarization can be applied to add functionality. For example the performance of a wavelength converter can be optimized using the polarization. Wavelength converters are key components in optical telecommunications networks, but the available devices have several problems. Firstly they need expensive tunable wavelength filters at the output, and secondly they are highly polarization dependent. The application of polarization handling is demonstrated by a new type of integrated wavelength converter: POLARIS (POlarization LAbelling for Rejection and Isolation of Signals). This wavelength converter uses the polarization of the light to label the original and the converted signals. By using a polarization splitter, the two signals can be separated and filtered. This approach can also be used in all-optical switches. In this way tunable filters and polarization dependence are avoided. On-chip polarization manipulation can be used in a number of other circuits as well to enable a broad variety of functions and improvements (for example: polarization independent optical amplifiers, on-chip polarization controller, a laser with a switchable output polarization). To demonstrate the generic integration platform, the development and realization of polarization converters and polarization splitters, together with standard passive (waveguides, couplers) and active (semiconductor optical amplifiers) components is needed. The standard components are designed and a standard fabrication process is developed in which all these components can be integrated. Two generations of polarization converters are realized. The first device has an efficient and short design, but it proved to be difficult to integrate it with active components. A second generation converter is designed, fabricated and characterized. This device is well suited for integration and has a high conversion. Furthermore, two types of polarization splitters are demonstrated. Also these devices need to fit in the standard fabrication. One design is a relatively long device, tolerant to fabrication variations, but leading to complications with integration. A second design is shorter and consists only of a passive Mach Zehnder interferometer with polarization converters in the arms. This splitter fits exactly in the integration scheme, so this is the device of choice for the generic integration technology. Moreover an array of Mach Zehnder Interferometers with SOAs in the arms is designed and fabricated. This circuit can be used in wavelength converters and all-optical switches. The device is integrated with spotsize converters to enable packaging. With the packaged device wavelength conversion up to 40 Gb/s is demonstrated. The POLARIS concept is demonstrated by simulations and experimentally verified. An integrated version of POLARIS is designed. The generic integrated polarization handling technology is demonstrated by realizing this circuit. The realization clearly showed that the integration scheme is useable, because working examples of all relevant components were present on the chip. Unfortunately due to time constraints not all processing steps were sufficiently optimized, leading to a too low yield of working components; therefore no POLARIS operation could be shown with the integrated device. This thesis describes the theory, design, fabrication and characterization of polarization handling components, as well as passive and active components, integrated in InP/InGaAsP. A generic integration technology for Photonic Integrated Circuits is developed. Circuits constructed with the components of this technology can be made polarization insensitive and can have additional functionality based on polarization.