Integration of multiwavelength lasers with fast electro-optical modulators

J.H. Besten, den

Research output: ThesisPhd Thesis 1 (Research TU/e / Graduation TU/e)

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Abstract

Photonic Integrated Circuits (PICs) are of key importance in Wavelength-Division Multiplexing (WDM) networks because of their reduced volume and packaging costs compared to discrete components. The research described in this thesis was focussed on the integration of WDM-lasers and Radio-Frequency (RF) modulators. The WDM-lasers are based on an array of Semiconductor Optical Amplifiers (SOAs) and an Arrayed-Waveguide Grating (AWG). These lasers can be operated as multiwavelength lasers for simultaneous generation of multiple wavelengths, or as tunable laser in which case their digital control is an advantage over precise analog control needed in for instance sampled-grating distributed Bragg reflector lasers. The RF modulators are based on a traveling-wave Mach-Zehnder structure. The integration of both components on one single chip involves engineering trade-offs relating to the optimization of microwave, electrical, optical and fabrication characteristics. An important aspect of the fabrication of PICs is the selection of a suitable integration technology to realize different waveguide types for active (e.g. amplifier) and passive (e.g. AWG) elements. InP epitaxial wafers containing high-quality integrated active and passive regions were developed by JDS Uniphase and the COBRA Research Institute using a three-step metal-organic vapor-phase epitaxy re-growth process. Our PICs use a ridge waveguide design, for which a reactive ion etch process was developed the COBRA cleanroom. As a first step towards the integration of lasers and modulators, we focused on the fabrication of stand-alone devices in compatible structures. The Mach-Zehnder modulator structure was realized in two versions. The first version employed 4-µm-wide phase shifters. This waveguide width enabled a tolerant fabrication process but severely limited the modulator bandwidth due to a high microwave attenuation and a velocity mismatch. A velocity match is important to have an efficient interaction between the modulating microwaves and the optical carrier. Also the modulator impedance of ?? 21W was not matched well to a 50W driver. An additional impedance mismatch caused a standing wave pattern limiting the modulator 3dBe bandwidth to 5GHz. In a next design, we addressed an increase of the modulator impedance and a reduction of both the microwave attenuation and index to achieve a velocity match. All three issues could be accomplished simultaneously, mainly by tuning one design parameter: the waveguide width. The optimum width of 1µm forced us to develop new fabrication steps in order to realize 2- µm-wide metal lines on top of such narrow phase shifters. The result was a traveling-wave Mach-Zehnder modulator that was both velocity- and impedance-matched. The switching voltage was measured to be lower than 5V and the static extinction ratio better than 20dB at 1550nm. The simulated 3-dBe relative optical response was over 50GHz for a device with 2-mm-long phase shifters. The bandwidth deduced from electrical measurements was reduced by a poor quality plated gold to 34GHz. The optical bandwidth measured with a photodiode was reduced to 9GHz by a high contact resistance. These two problems were solved in a second fabrication run. There, a small velocity mismatch limited the modulator bandwidth extracted from S-parameter measurement to 34GHz, enough for 40Gb/s operation. A number of multiwavelength lasers was developed separately in various configurations of three main components: passive waveguides, semiconductor amplifiers (SOA)s and one or more AWGs. We realized a multiwavelength 4-l laser using the basic configuration of an SOAarray and a single AWG in a linear Fabry-Pérot cavity. If no simultaneous operation is required, AWG-based lasers can also be applied as discretely tunable lasers. Then, the properties of the AWG can be exploited to increase the number of generated wavelengths over the number of integrated amplifiers. Using two AWGs and eight SOAs, we realized two linear 16-l digitally tunable lasers with a channel spacing of 100GHz. One of these lasers was measured to have an side-mode suppression ratio of over 40dB and an output power of ?? 1mW at 100mA bias current. This power level was substantially higher than that of earlier published AWG-based digitally tunable lasers with an increased number of wavelengths. This was accomplished by coupling two cavities with a multimode interference coupler into one output waveguide. As an alternative for AWG-based multiwavelength lasers in a linear cavity, we realized several WDM ring lasers, which were the first of their kind. Two 4-l AWG-based ring lasers, a 7-l and a 9-l ring laser were all fabricated in the same technology. One of these ring lasers hold the smallest device size of an AWG-based laser to date (1×1.8mm2). As integrated ring lasers had not been used earlier for multiwavelength operation, we made an extensive study of the stability and the mode-competition mechanism in such lasers. From our investigations it can been concluded that the stability properties of these lasers can be good, but that it is difficult to tap sufficient power out of the ring. AWG-based multiwavelength lasers can be applied as an integrated continuous-wave source for a wavelength converter or a modulator. Integrated modulators are a relevant option since direct laser modulation is limited due to the long laser cavity. We designed and fabricated a 4-l multiwavelength laser integrated with a high-speed Mach-Zehnder modulator. For a device consisting of a 4-l multiwavelength laser with four integrated modulators, we applied a novel concept using a single AWG (de)multiplexer. The realization of these integrated components has been accomplished and we came close to operating devices.
Original languageEnglish
QualificationDoctor of Philosophy
Awarding Institution
  • Electrical Engineering
Supervisors/Advisors
  • Smit, Meint, Promotor
  • Wolter, Joachim, Promotor
  • Leijtens, Xaveer J.M., Copromotor
Award date18 Nov 2004
Place of PublicationEindhoven
Publisher
Print ISBNs90-386-1643-0
DOIs
Publication statusPublished - 2004

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