The design and the realization of a cost effective reflective polarimetric fiber optic thermometer are discussed for several
applications. The temperature dependent birefringence of a polarization maintaining fiber is used to deduce the sensor
head temperature from measured polarization intensities. Measurements from a fabricated and packaged prototype show
that the sensor features a non-ambiguous temperature range of >160°C and an accuracy of ±2°C.
A highly accurate reflective interferometric fiber-optic current sensor for alternating and direct currents up to 500 kA is
investigated. The magnetic field of the current introduces a differential phase shift between right and left essentially
circularly polarized light waves in a fiber coil wound around the conductor. Technology adopted from fiber gyroscopes
is used to measure the current-induced phase shift. The sensor achieves accuracy to within ±0.1% over at least two
orders of magnitude of current and for temperatures from -40 to 80°C with inherent temperature compensation by means
of a non-90°-retarder. The paper analyzes the influence of key parameters on the sensor accuracy as well as linearity as a
function of magneto-optic phase shift. Particularly, we consider residual birefringence in the sensing fiber and its effect
on the high-current performance of the sensor as well as optimum parameters for the temperature compensation scheme.
Applications of the sensor are in high-voltage substations and in the electrolytic production of metals such as aluminum.
Patterning of deeply subwavelength artificial nanomaterials (photonic crystals, plasmonic metamaterials) for the
visible or near-infrared optical spectrum is a challenging task. Electron-beam lithography is often the method
of choice thanks to its combination of flexibility, accuracy and availability in many research laboratories. We
present an analytical model for large and dense arrays of photonic nanostructures which allows to predict the
maximum fill ratio (radius divided by nearest neighbor distance) before the onset of resist shrinkage between
the individual elements. The model includes geometrical parameters of the design (lattice constant, lattice
symmetry), resist properties (resist contrast) and proximity parameters (beam broadening, backscatter range,
backscatter efficiency). It is shown that the resist contrast has a significant impact on the achievable maximum
fill ratio even for large nearest neighbor distances and that the beam broadening, i.e. the quality of the EBL
equipment, is of paramount importance. The background energy level which is determined by the backscatter
efficiency and the lattice symmetry is shown to have a weaker influence on the maximum fill ratio. The derived
model can be used as a guideline in the project planning stage to predict achievable fill ratios at a planned lattice
constant and consequently an assessment whether a desired functionality at a certain wavelength is possible.
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.
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 have studied tapers that couple light from a conventional ridge
waveguide into a planar photonic crystal (PhC) waveguide. Tapering
is achieved by changing the PhC waveguide width either in steps or
gradually. Lag effects in fabrication provide an additional
tapering due to the fact that the hole depths scale with the
corresponding hole diameter. Our analysis deals with the
out-of-plane loss that arises within such taper sections. The PhC
consists of a triangular lattice of air holes introduced into an
InGaAsP/InP slab structure. For conceptual studies we use the 2D
multiple multipole method (MMP) in conjunction with an extended
phenomenological model. This model covers the out-of-plane
scattering providing a loss parameter and an effective index
correction for the holes under consideration. This realistic 2D
model is retrieved from full-wave 3D FDTD simulations and
measurements.
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.
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