Ultrafast optical switches are one of the crucial components in next generation all-optical communication networks. One
promising candidate is the optical switch based on saturable intersubband (ISB) absorption in doped
InGaAs/AlAs/AlAsSb coupled double quantum wells (CDQW). We employ the density matrix theory and modified
optical Bloch equations with the relaxation-time approximation to simulate the time evolution of optical nonlinearities in
quantum well (QW) structures. The absorption saturation characteristics are derived afterwards. The theoretical estimates
are used to interpret the corresponding experimental results. Furthermore, several impact factors related to dynamic and
saturation characteristics of the materials are discussed theoretically. The studies provide useful clues to optimize the
QW material and device design.
In view of an electrically pumped photonic crystal-based semiconductor optical amplifier (SOA), we investigate optical mode propagation in 2D PhC waveguides in the presence of metal contacts for carrier injection. Our photonic crystal (PhC) devices are manufactured in the InP/InGaAsP material system. For the loss measurements, we have fabricated contact strips as narrow as 300nm with a sub-50nm placing accuracy on top of W3 waveguides. We study the influence of their position and width on optical power transmission through passive waveguides with respect to viability for future active devices. Our experimental results are complemented by numerical studies (FDTD, plane-wave expansion method).
Progress in photonics by monolithic integration for higher functional density, performance and reduced cost faces challenging hurdles due to technological and functional heterogeneities. Advanced local material growth techniques are enabling concepts towards high-density photonic integration, unprecedented performance and multi-functionality and ultimately optical systems-on-a-chip.
For example mode-locked laser diodes (MLLDs) are key devices for ultra-short pulse generation for all-optical Tbit/s communication networks. MLLDs suffer from material compromises and will benefit from the possibility to design the gain, absorber and passive-waveguiding sections independently. We have proposed and demonstrated the integration of a saturable absorber with a fast absorption recovery time based on an InP/InGaAsP uni-traveling-carrier structure (UTC) to achieve pulses below 1 ps with repetition rates up to 40 GHz. The use of the UTC absorber instead of the commonly employed reverse-biased gain material requires however the heterogeneous growth of multiple layer stacks on the same chip with the butt-coupled regrowth technique.Critical for the MLLD performance are the reflections and the optical coupling between the different monolithic integrated layer structures of passive, absorbing and amplifying sections. 2D FDTD simulations of the optical waveguides demonstrate that to minimize reflections an angled interface between the different structures is preferable and can lead to reflection coefficients as low as 10^-6. To obtain an angled interface we used a wet chemical etching process sequence of selective and non-selective etchants, which is sensitive to crystal orientation and yields a 55° tilted interface. In addition we can conclude from our simulations that in order to minimize both, insertion loss and reflections, a bending of the light guiding layers has to be prevented. Bendings can lead to measured losses of 5-7 dB per interface whereas correctly aligned light guiding layers results in losses of 1.5 dB and intensity reflections below 10^-5 per interface. The bendings originate from different growth rates near and far away from masked areas during regrowth due to reactants diffusion on the SiO2 mask. The bending can be minimized by optimizing the mask under etch of the SiO2 mask and low pressure MOVPE growth. We demonstrate operation of mode-locked laser diodes with an integrated UTC absorber and pulse durations below 1 ps.
The monolithic integration of photonic circuits will open new perspectives for optical communication networks. It will enable higher transmission rates, new functionalities, higher functional densities, leading to all-optical networks and reduced cost for telecommunication. Mode-locked laser diodes (MLLDs) will play an important role for short pulse generation in Tb/s networks for the transmitters, as well as for clock recovery for the receivers and optical regenerators. To overcome the limitations of conventional monolithically integrated MLLDs, where the pulse width is limited by the relatively slow absorption recovery, we demonstrate an ultrafast semiconductor saturable absorber based on the uni-traveling-carrier (UTC) concept. The UTC absorber is designed to be monolithically integrated in InGaAsP/InP mode-locked laser diodes grown by MOVPE. The absorber shows a saturation energy of Esat,abs of 1pJ at 1.55μm and a voltage-dependent recovery time of 2ps at 2V reverse bias. The importance of an optimum absorption-bandgap to absorber-length ratio is demonstrated to keep the saturation energy low. The voltage-dependent absorption and absorption recovery time make this absorber ideal for hybrid mode-locking and synchronization to an external RF-source.
Monolithic photonic integration offers unsurpassed perspectives for higher functional density, new functions, high per-formance, and reduced cost for the telecommunication. Advanced local material growth techniques and the emerging photonic crystal (PhC) technology are enabling concepts towards high-density photonic integration, unprecedented per-formance, multi-functionality, and ultimately optical systems-on-a-chip. In this paper, we present our achievements in photonic integration applied to the fabrication of InP-based mode-locked laser diodes capable of generating optical pulses with sub-ps duration using the heterogeneous growth of a new uni-traveling carrier ultrafast absorber. The results are compared to simulations performed using a distributed model including intra-cavity reflections at the sections inter-faces and hybrid mode-locking. We also discuss our work on InP-based photonic crystals (PhCs) for dense photonic integration. A combination of two-dimensional modeling for functional optimization and three-dimensional simulation for real-world verification is used. The fabricated structures feature more than 3.5μm deep holes as well as excellent pattern-transfer accuracy using electron-beam lithography and advanced proximity-effects correction. Passive devices such as waveguides, 60° bends and power splitters are characterized by means of the end-fire technique. The devices are also investigated using scanning-near field optical microscopy. The PhC activity is extended to the investigation of TM bandgaps for all-optical switches relying on intersubband transitions at 1.55μm in AlAsSb/InGaAs quantum wells.
In short-distance optical links, the development of driving circuits for vertical-cavity surface-emitting lasers (VCSELs) requires precise and computationally efficient VCSEL models. A small-signal model of a VCSEL is computationally efficient and simple to implement; however, it does not take into account the nonlinear output behavior of the VCSEL. In contrast, VCSEL models that are highly based on first principles cannot be implemented in standard circuit device simulators, because the simulation of eye diagrams becomes too time consuming. We present another approach using VCSEL models, which are based on the 1-D rate equations. Our analysis shows that they combine efficient extraction and short simulation time with an accurate calculation of eye diagrams over a wide range of ambient temperatures. As different implementations of the rate equations exist, tradeoffs between three different versions are presented and compared with measured GaAs oxide-confined VCSELs. The first model has a linear and the second a logarithmic function of the gain versus the carrier density. The third model considers the additional transport time for carriers to reach the active region with quantum wells. For parameter extraction, a minimum set of parameters is identified, which can be determined from fundamental measurements.
We report on the investigation of planar photonic crystal waveguide
transitions with a scanning near-field optical microscope (SNOM) in
collection mode. An abrupt and a gradual taper design intended to
couple light from a W3 (three missing rows of holes) to a W1 waveguide
were fabricated in a InGaAsP slab waveguide. SNOM measurements reveal
that a taper design can efficiently funnel light into the W1
waveguide. For both designs a suppressed coupling of light into the W1
waveguide is measured for a frequency which corresponds to a mode
crossing which we determined by 3D plane wave simulations.
We investigate the patterning accuracy limits of electron-beam lithography with different proximity-effect correction (PEC) methods applied to the fabrication of planar photonic crystal structures (PPCS). Energy-intensity distribution simulations reveal that conventional energy-equalization PEC techniques present a lower limit of the best attainable hole-radius variation of 1% for a generic PPCS, while a method proposed by Watson (midpoint-equalization PEC) should inherently account for beam broadening and theoretically can reach perfect accuracy. Simulation results are verified experimentally. Additionally, we introduce a new method to determine the beam-broadening parameter . We compare energy-equalization PEC and midpoint-equalization PEC regarding the impact of geometrical key parameters of PPCS on achievable patterning accuracy, and show that proximity effects impose severe limitations on the patterning of structures with large fill ratios and/or small lattice constants. Furthermore, we perform a sensitivity analysis of both PEC methods on the proximity parameters and show that overestimation of the backscatter efficiency can actually improve the lithographic accuracy of the energy-equalization method and mimic the midpoint-equalization PEC method to a certain degree.
We investigate the patterning-accuracy limits of proximity-effect
corrected (PEC) electron-beam lithography applied to the fabrication
of photonic crystals (PhC's). Energy-intensity distribution
simulations reveal that conventional dose-modulation PEC techniques
present a lower limit of the best attainable hole-radius variation of
approximately 1% for a generic PhC structure, while a PEC method proposed by Watson theoretically should yield perfect correction. Simulation results were verified experimentally and additionally we introduce a new method to determine the beam-broadening parameter α. We analyzed the impact of geometrical key parameters of PhC's on achievable patterning accuracy and showed that proximity effects impose severe limitations on the patterning of structures with large filling factors and/or small lattice constants. Furthermore, we performed a sensitivity analysis on the proximity parameters and showed that overestimation of the backscatter efficiency can actually improve the lithographic accuracy and
mimic the Watson-PEC method to a certain degree.
A high-temperature, high sensitive borehole seismometer with fibre optic interferometric displacement sensors has been constructed and practically tested Its parameters are suited for monitoring of seismic vibrations at working temperatures from 20°C to 300°C. Long term stability of performance characteristics has been confirmed for the seismic receiver and the fibre optic components at elevated temperatures and for several km of fibre optic signal transmission lines. Seismic vibrations could be monitored by coupling the seismic mass via spherical reflectors to a single-mode fibre optic Michelson interferometer. Optical phase processing based on wavelength modulation of tuneable DFB laser source yields <0. 1nm sensitivity of seismic vibrations within 0.1. .30Hz information bandwidth. Laboratory test measurements at elevated temperatures agree well with the results of an electrodynamic seismometer.
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