GaAs-based components for photonic integrated circuits

Semiconductor lasers based on GaAs/AlGaAs double heterostructures have recently received increasing interest for atomic spectroscopy applications. For example GaAs/AlGaAs double heterostructure laser radiation at the wavelengths of 780 nm and 1064 nm is used for rubidium spectroscopy, and for molecular iodine spectroscopy (at 532 nm using frequency-doubling techniques), respectively. Optical systems used for these applications should feature compact, mechanically stable, and highly efficient components in order to be suitable for operation in harsh environments (e.g. quantum optic precision experiments in drop towers or in space). State-of-the-art optical systems use micro-integrated laser modules with a small footprint. However, passive components that are necessary for the manipulation of the laser light such as phase and amplitude modulators, fiber couplers, and beam splitters are only commercially available on macro-scales. These components are then integrated into the laser systems platforms where additional opto-mechanical components are indispensable. Because of their excessive request for space, commercially available laser systems and electro-optic setups are not suitable for deployment in the field or in space. The miniaturization of passive components for reduction of mass and form factor of electro-optical systems as well as for improving the overall robustness and reliability of the latter is a prerequisite for fieldand space deployment of quantum sensors. This work presents GaAs/AlGaAs double heterostructure electro-optic phase modulators for the first time at the wavelength of 780 nm and at the wavelength of 1064 nm. The specifications of the phase modulators meet the micro-integration requirements into hybrid laser modules and spectroscopy modules with respect to optical aperture and electrical interface. Details related to design and fabrication of the phase modulators are included. The characterization of the electric and electro-optic properties of the phase modulators is then presented. The GaAs/AlGaAs double heterostructure of the phase modulators is further used to develop, design, and realize waveguide couplers. Multi-mode interference (MMI) couplers for applications at the wavelength of 780 nm are presented for the first time. MMI couplers can replace fiber-based couplers in complex optical systems and are core building blocks for photonic integrated circuits (PICs). The compatibility of the double heterostructure phase modulators and MMI couplers for implementation of PICs is then demonstrated through a monolithicallyintegrated Mach-Zehnder-intensity modulator at 780 nm for the first time. Further, a method based on heterodyne interferometry is developed for the indepth investigation of phase modulators including phase and amplitude modulation response, including modulation efficiency, residual amplitude modulation, and signal distortion in GaAs-based phase modulators. During hybrid integration of a phase modulator chip into an electro-optical hybrid system, this method can be applied to optimize the coupling efficiency in real time while at the same time reducing the residual amplitude modulation. The method further provides the means to separate linear and the non-linear effects in the phase modulation signal. It is shown how this provides the means to determine separately linear and quadratic electro-optics in GaAs/AlGaAs double heterostructures. For the first time, the quadratic electro-optic coefficient is determined without having to calculate the contribution of free carriers effects to phase modulation. This thesis work is organized as follows: First, the fundamentals of guided wave optics are presented (chapter 2). Electromagnetic wave propagation in planar waveguides is described, conditions for guided optical modes are provided, and concepts for waveguide couplers are discussed. Then the electro-optic properties of GaAs are presented and phase modulation in GaAs/AlGaAs double heterostructures due to electro-optic effects is introduced (chapter 3). Next, the design of GaAs/AlGaAs double heterostructure phase modulators (chapter 4) and waveguide couplers (chapter 5) is presented. For the design of the phase modulators we study the state-of-the-art GaAs-based electro-optic phase modulators at the wavelength of 1.31 nm and transfer these concepts to the wavelengths of 780 nm and 1064 nm. Efficient GaAs/AlGaAs double heterostructures with phase modulation efficiencies larger than 15 deg /(V ·mm) are designed. The design of low loss and polarization maintaining waveguide couplers and phase modulators then allows for the development of a Mach-Zehnder intensity modulator (chapter 6) at the wavelengths of 780 nm. In the experimental part of this work, after fabrication (chapter 7), the electrooptic performance of these devices is characterized experimentally. The performance of phase modulators, waveguide couplers, and MZI modulators is determined (chapter 8). The modulation efficiency of 16 deg /(V ·mm) is demonstrated for phase modulators at 780 nm and at 1064 nm. The propagation losses amount to 1.2 dB/cm at 780 nm and to 4.3 dB/cm at 1064 nm (which corresponds to an improvement beyond state-of-theart for GaAs-based phase modulators at 1064 nm). For the Mach-Zehnder-intensity modulator, the extinction ratio of more than 10 dB and excess loss of less than 3 dB are demonstrated. Further, the novel heterodyne analysis method for in-depth characterization of the electro-optic performance of phase modulators is developed and implemented (chapter 9). As an application of the heterodyne analysis method, the linear and the quadratic electro-optic coefficients of GaAs/AlGaAs double heterostructures are determined. For the linear electro-optic coefficient we find results that are in agreement with literature. However, the results for the quadratic electro-optic coefficients differ from values given in the literature. The discrepancy is discussed and suggestions to solve it are provided.