KEYWORDS: Video, Video compression, Video processing, Transform theory, Video coding, Standards development, Computer programming, Semantic video, 3D video compression, Image compression
Today's video codecs have evolved primarily to meet the requirements of the motion picture and broadcast industries,
where high‐complexity studio encoding can be utilized to create highly‐compressed master copies that are then broadcast
one‐way for playback using less‐expensive, lower‐complexity consumer devices for decoding and playback. Related
standards activities have largely ignored the computational complexity and bandwidth constraints of wireless or Internet
based real‐time video communications using devices such as cell phones or webcams. Telecommunications industry
efforts to develop and standardize video codecs for applications such as video telephony and video conferencing have
not yielded image size, quality, and frame‐rate performance that match today's consumer expectations and market
requirements for Internet and mobile video services. This paper reviews the constraints and the corresponding video
codec requirements imposed by real-time, 2-way mobile video applications. Several promising elements of a new mobile
video codec architecture are identified, and more comprehensive computational complexity metrics and video quality
metrics are proposed in order to support the design, testing, and standardization of these new mobile video codecs.
In this paper, we discuss the potential for advanced modulation and electronic signal processing techniques to remove key barriers to commercially viable, near-term deployment of multi-Tb/s link capacities utilizing 40 Gb/s optical transport.
We have measured and analyzed the room-temperature capacitance-voltage (C-V) characteristics of In0.35Ga0.65As/GaAs MQW laser structures with different doping levels in the active region. Average doping densities in the well-barrier regions were directly extracted from the as- measured carrier profiles. A model for he C-V measurement, including the self-consistent solution of Poisson and Schroedinger equations, was developed. The carrier profiles obtained from the simulated C-V characteristics do not correspond to the free carrier profiles since the local charge neutrality hypothesis does not hold for QW structures. Thus, the true carrier distribution can only be determined from a full quantum-mechanical simulation of the laser structure. We have determined, form the comparison between experimental and simulated profiles, a conduction band offset (Delta) Ec/(Delta) Eg of 0.81. We have also applied C-V measurements to samples with interdiffused QWs, and obtained the characteristic interdiffusion length.
John Ralston, Eric Larkins, K. Eisele, S. Weisser, Susann Buerkner, A. Schoenfelder, Juergen Daleiden, Konrad Czotscher, Ignacio Esquivias, Joachim Fleissner, R. Sah, Martin Maier, Willy Benz, Josef Rosenzweig
Optimized molecular-beam epitaxial growth of pseudomorphic MQWs, the application of abrupt and spatially-localized carbon doping, and the development of short-cavity coplanar ridge-waveguide structures with high-quality chemically-assisted ion-beam etched facets have been combined to fabricate GaAs-based MQW lasers which have achieved damping-limited direct modulation bandwidths exceeding 40 GHz. More detailed measurements indicate intrinsic modulation bandwidths exceeding 60 GHz for devices with p-doped active regions. The reduced linewidth enhancement factor, (alpha) , observed in these lasers also indicates their suitability for low-chirp high-speed direct modulation. The laser design has been further incorporated into a complete technological process for the monolithic integration of GaAs MQW lasers and HEMT-based laser-driver circuits capable of operation in data rates up to 20 Gb/s. Using the impurity-free interdiffusion process, large shifts in the lasing wavelength have been achieved with no strain relaxation and while maintaining the high-speed modulation properties of the pseudo-morphic InGaAs/GaAs MQW lasers, demonstrating the feasibility of fabricating high-speed multi-wavelength laser arrays.
We describe short-cavity In0.35Ga0.65As/GaAs multiple quantum well (MQW) lasers with undoped and p-doped active regions. The epilayer structure consists of four 5.7 nm QWs separated by 20.1 nm barriers in a GaAs core. The cladding layers consist of Al0.8Ga0.2As. In the case of p-doped devices a 4.5 nm carbon (C)-doped region (2.5 multiplied by 1019 cm-3) was inserted above each QW, separated by a 3.1 nm GaAs spacer, resulting in a modulation-doped core region. Using a CAIBE process, short-cavity ridge-waveguide lasers are fabricated in a triple-mesa geometry suitable for on-wafer probing. The best device (6 multiplied by 130 micrometers squared) with an undoped active region attained a damping-limited direct modulation bandwidth exceeding 40 GHz at a cw bias current of 160 mA. In contrast, the p-doped devices, demonstrating a maximum bandwidth of 37 GHz, are still limited by power dissipation. (alpha) -factors as low as 1.4 and 1.5 for undoped and p-doped devices, respectively, are extracted from measurements of the sub- threshold gain spectra. In addition, we demonstrate eye diagrams at 25 Gbit/s (limited by the pulse pattern generator) for these laser diodes. A complete characterization of dc and rf properties of these lasers is presented.
Ignacio Esquivias, Beatriz Romero Herrero, S. Weisser, Konrad Czotscher, John Ralston, Eric Larkins, Julia Arias, A. Schoenfelder, Michael Mikulla, Joachim Fleissner, Josef Rosenzweig
The transport of carriers along the confinement region, the carrier capture into, and the carrier escape out of the quantum wells (QWs) are limiting processes affecting the high-frequency properties of QW lasers. The influence of these processes on the laser performance depends mainly on the ratio of the effective carrier transport/capture time and the effective escape time. We present experimental results about the escape times for GaAs/AlGaAs and InGaAs/GaAs high-speed QW lasers with varied geometrical dimensions (cavity width and length), number of QWs, In-concentrations, and p-doping levels in the active region, as extracted from electrical impedance measurements in the sub-threshold regime. In addition to the expected increase of the escape time with increasing QW barrier height, we observe an important increase in the escape time for lasers with p-doping. The escape time dependences on the carrier concentration and on the temperature are determined and discussed.
A GaAs/AlxGa1-xAs multiple quantum well laser with a 3 dB electrical modulation bandwidth of 16 GHz has been developed. Optimized design of the waveguide, including implementation of high average Al mole fraction (xeff equals 0.8) GaAs/AlAs binary short-period superlattice cladding layers, together with a coplanar electrode geometry, has resulted in a vertically compact laser structure suitable for integration.
Far infrared photothermal ionization spectroscopy of well-center doped
shallow donors in GaAs/AlCaAs'multiple-quantum-well structures has been carried
out to investigate the electron - optical-phonon interaction. The ls-2p(m=+1)
and ls-3p(m=-i-1). transitions are tuned through the resonant region with the GaAs
optical phonons by magnetic fields up to 23 .5T . Both two -level and three -level
resonance measurements show similar anti-level-crossing behavior with extremely
large interaction gaps. The lower branches are well below (-50 cm ) the
resonant energies involving the bulk zone centr LO phonons. These anomalous
features strongly suggest that electrons in the wells interact with phonon modes
at several different frequencies, or a phonon band, in contrast to the single
frequency LO phonons in the bulk. A partial explanation may be found in terms
of electron - interface-phonon and electron - confined-LO-phonon interactions.
Other possible mechanisms are also discussed.
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