• Research within SCOOP
• Modeling
• Fabrication and Processing
• Basic Characterization
• Systems Characterization

Functionalities that are currently pursued include:

• Optical add/drop multiplexing,
• Optical signal generation, including short-pulse generation,
• Optical regeneration,
• Clock-recovery, and
• Wavelength conversion.

Interferometric structures of the Michelson and Mach-Zehnder type as well as modulators of the electro-absorption type will be explored for the achievement of these functionalities. Also the mode-locked laser is a very promising component for performing various functions in all-optical networks, i.e. clock-recovery and demultiplexing.

The potential of low-dimensional structures (quantum dots and quantum wires) for improving the device performance will be assessed. The fabricated devices will be assessed in systems test beds and to the extent possible they will go into systems demonstrators.

Parallel to the experimental efforts on fabricated devices, modeling is undertaken for device optimisation and for a more complete understanding of the behaviour of devices in systems.

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Modeling
We model and analyze semiconductor devices for high-capacity optical communication systems and networks, with a particular emphasis on devices for optical signal processing. Our efforts comprise projects spanning from basic materials physics, including novel nanostructures, over active waveguides, to photonic integrated devices and subsystems.

As the bitrate and the level of device integration increases, further demands are placed on the correct physical description of the devices. In order to assess the device characteristics that are critical in system and network applications, and in order to compare directly with the outcome of systems experiments, it is important also to develop simplified models and efficient numerical codes, which allow simulation of real (sub)systems including, e.g., fiber transmission.

At this point we have made extensive modeling of Mach-Zehnder interferometers with respect to short switching windows that can be used for demultiplexing of ultra-high bit-rate OTDM signals.

We also have a working model that enables us to model and design quantum well electroabsorption modulators to give the desired properties such as high speed and high ssaturation power combined with a small drive voltage. It is an extensive model that takes the quantum mechanical effects of the quantum wells into account (bound carriers, modified bandstructure etc.).

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Fabrication and Processing
We have fabricated the first generation of devices at the process facilities at GiGA. The semiconductor material for these is all InP-based and the wafers have been grown using the MOVPE facilities also at GiGA.

The devices are ridge waveguide devices and include lasers and amplifiers with linear, non-linear and angled waveguides as well as Michelson and Mach-Zender type interferometers, and electroabsporption modulators.

The process include standard optical lithography to define the ridges, the electrical isolation between contact pads, and the pads themselves. Dry etch (RIE) followed by wet etch is used to define the mesa for the ridges. The electrical contact pads are realised through e-beam evaporation of various metals onto the the samples. The wafer is then thinned to 100 µm and metalised on the substrate side. After cleaving and AR coating, the devices are separated and mounted using sofisticated methods onto copper heatsinks for thermal control. Tapered fibres or high NA lenses are then used to couple light in and out of these components.

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Basic Characterization
The group is internationally renowned within fundamental research on optics and ultrafast dynamics of semiconductor nanostructures.

We use ultra-fast spectrally and temporally resolved spectroscopy to determine basic properties of the devices. Using pump-and-probe measurements, we are able to extract important time constants, and to determine the overall quality of our devices.

The time constants is important input to the modeling group, which can then use this information to gain more insight into the important processes taking place in the devices.

The component activity is to a high extent systems driven and performed together with the Systems characterization part of SCOOP.

The research also concerns MBE-growth of high-quality heterostructures forming low-dimensional structures like quantum wells and quantum wires, and again we perform advanced characterization of these structures with methods of high spectral, temporal and spatial resolution. Our samples are grown at the III-V Nanolab at the Niels Bohr Institute at the University of Copenhagen.

Apart from the SCOOP devices, we currently focus on mainly two areas:

• Growth and characterization of zero-dimensional quantum dot systems
• Fabrication and characterization of all-optical or optoelectronic components based on III-V semiconductors

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Systems Characterization
Very extensive research has been performed within the field of Optical Time Division Multiplexing (OTDM) for generating ultra-high bit-rate signal at and in excess of 40 Gbit/s within the Systems Competence Area. To generate OTDM signals, very high quality short pulse sources are required, as well as high precision multiplexers. Typically, to generate a 40 Gbit/s pulse train the maximum pulse width is ~10 ps and the ratio between the center of the pulse and the pulse tail power in the center of the adjacent pulse should be well in excess of 20 dB to avoid a large impact from interferometric cross-talk. Furthermore, as the channel separation is performed in the time domain and not the wavelength domain, active signal processing is a necessity to perform, e.g., demultiplexing.

The aim of SCOOP is to design and develop components for ultra high bit-rate signal processing. The manufactured components are aimed at the following applications:

• Semiconductor Optical Amplifiers (SOAs)
  • wavelength conversion using cross-gain modulation
  • demultiplexing using four wave mixing
  • transmission impairment equalization by means of optical phase conjugation
• Electroabsorption modulators
  • external modulation of pulse-sources
  • demultiplexing
  • wavelength conversion using cross-absorption modulation
  • clock-recovery schemes
• Interferometric Michelson and Mach-Zehnder structures
  • demultiplexing
  • wavelength conversion using cross-phase modulation

The Systems Competence Area performs basic static system related characterization of the manufactured devices such as, e.g., gain versus input power (SOAs) and absorption versus reverse bias (EAs). From these investigations device behavior in larger system content is predicted and further device optimizations are performed. After optimization and further testing the components are implemented in the above described larger system test bed for thorough assessment of their dynamic performance. The device under test can be inserted anywhere or replace any components in the test bed giving a very large flexibility during the testing phase.

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