Design and integration of MZI modulators and AWG-based multiwavelength lasers in InP

The first Mach-Zehnder interferometer (MZI modulator integrated with an arrayed-waveguide grating (AWG) based laser shall be demonstrated. A novel 4-channel device as small as 4x8 mm2 is realized, employing only one AWG instead of three needed conventionally. Integration of two required epitaxial layerstacks was realized by growing one waveguide layer, locally removing it and regrowing a second layer in the etched region. The modulator design is optimized using rigorous field simulation of the complex semiconductor layer stack. Almost perfect velocity match and low microwave loss are obtained by 1 μm-narrow waveguides with a 2 μm signal electrode on top. INTRODUCTION Dense integration of lasers with other components is a hot research topic since the laser output serves as an optical carrier for on-chip processes such as light conversion or modulation. Examples of advanced photonic integrated circuits (PICs) are a distributed feed-back laser (DFB) with an electro-absorption modulator (EAM) [1], a distributed Bragg reflector (DBR) laser with a Mach-Zehnder interferometer (MZI) modulator [2], and an arrayed-waveguide grating (AWG) based laser (AWGL) with a wavelength converter [3]. Our goal has been to demonstrate the first MZI modulator integrated with an AWGL (total device size: 4×8 mm2). An AWGL has the advantage over a DBR or DFB laser that it can not only be utilized as a widely-tunable laser, but also as a multiwavelength source that generates multiple wavelengths simultaneously. This enables the integration of multiple modulators with only one AWGL to create an emitter of several wavelengths that can be modulated independently. — For integrating of both an active (λg = 1550 nm) and passive (λg = 1250 nm) film layer in the AWG-based Fabry-Perot laser, we applied a three-step MOVPE butt-joint coupling technique. This also enables a doping profile in the passive sections that is optimal for the electro-optical phase-shifting Mach-Zehnder arms. Furthermore, since the Mach-Zehnder modulators require a layerstack with a semi-insulating substrate, the semiconductor optical amplifiers (SOAs) in the laser are equipped with a lateral n-contact instead of a conventional bottom contact. The design of an MZI modulator is governed by two key properties: good velocity match between optical wave and microwave as well as low microwave loss. Hence a highly accurate model for microwave and optical propagation in an electro-optic modulator is needed that includes the loss in the modulating electrodes and in the inhomogeneous semiconductor layer stack. For this purpose the method of lines (MoL) is employed, a full wave simulation tool especially suited for complicated layer stacks [4]. In order to have a high modulation efficiency as well as a low microwave attenuation, the p-doped waveguide cladding should be as narrow and highly doped as possible. For this reason, the modulators are realized as 1 μm-narrow deeply etched waveguides with a 2 μm signal electrode on top. This reduces the microwave index and we obtain an almost perfect velocity match with the optical wave. Moreover, the impedance of the modulator increases and excellently matches with the driver. This paper describes the development and realization of a novel 4-channel AWGL with four MZI modulators, four SOAs (one for each wavelength) and only one AWG. A conventional approach would need three AWGs: one in the multiwavelength laser and two at both ends of the modulators, for demultiplexing and multiplexing the wavelengths before and after modulation, respectively. This introduces complications: a large chip space is needed and all (de)multiplexers should be accurately tuned to each other. Our new approach performs all required multiplexing and demultiplexing operations with a single AWG. We hope to present first measurement results at the conference. 2This work was carried out under the framework of the European OBANET IST-2000-25390 project and the Dutch NRC-Photonics programme.