Continuously distributed phase shifts in chirped DFB lasers usingbent waveguides

The authors have implemented chirped distributed feedback (DFB) grating lasers with phase shifts [λ/8, 2λ/8 and 3λ/8] distributed continuously over various axial distances, using their recently proposed method for producing bent optical waveguides. The singlemode stability of the lasers with distributed phase shifts was found to be considerably higher than for lasers with abrupt phase shifts     

Continuously chirped DFB gratings by specially bent waveguides fortunable lasers

We have implemented and studied a new type of tunable multiple-section semiconductor distributed feedback (DFB) laser using tailored chirped DFB gratings. Arbitrarily and continuously chirped DFB gratings are defined by bent waveguides on homogeneous grating fields with ultrahigh spatial precision. The mathematical bending functions are optimized in this case to provide enlarged wavelength tuning ranges. We present the results of model calculations, the technological device realization and experimental results of the DFB laser characterization e.g. a tuning range of 5.5 nm without wavelength gaps and high side mode suppression ratio     

A tractable large-signal dynamic model-application to stronglycoupled distributed feedback lasers

The modulation characteristics of DFB semiconductor lasers have been studied using a transfer matrix method combined with an appropriate rate equation analysis. The model takes into account longitudinal mode spatial hole burning, as well as the nonuniform current injection resulting from the axially varying Fermi voltage, and can be used for the efficient simulation of static, small-signal, and large-signal dynamic properties. The program is applied to the interpretation of experimental data from a strongly coupled InGaAsP/InP DFB laser. The experimental high-frequency properties of this device are well described by the simulations     

Record tuning range of InP-based multiple air-gap MOEMS filter

Continuously tunable Fabry-Perot filters based on multiple InP/air-gap MOEMS technology are presented. Record wavelength tuning >112 nm with an actuation voltage of only 5 V is demonstrated. The FWHM remains constant over the entire tuning range. The stopband covers both the second and third optical telecommunication window (1250-1800 nm).     

Continuously tunable 1.55-/spl mu/m VCSEL implemented by precisely curved dielectric top DBR involving tailored stress

We present an optically pumped and continuously tunable 1.55-/spl mu/m vertical-cavity surface-emitting laser (VCSEL). The device shows 26-nm spectral tuning range, 400-/spl mu/W maximum output power, and 57-dBm side-mode suppression ratio. The VCSEL is implemented using a two-chip concept. The movable top mirror membrane is precisely designed to obtain a tailored air-gap length (L'=16 /spl mu/m) and a radius of curvature (ROC=4.5mm) in order to efficiently support the fundamental optical mode of the plane-concave resonator. It consists of a distributed Bragg reflector (DBR) with periodic, differently stressed silicon nitride and silicon dioxide multilayers implemented by plasma-enhanced chemical vapor deposition. The lower InP-based part, comprising the InP-InGaAsP bottom DBR and the active region, is grown monolithically using metal-organic vapor phase epitaxy.     

Role of ground reaction torque and other dynamic measures inpostural stability

To compare the sensitivity of different balance parameters of movement of the center of pressure (COP) and ground reaction torque (GRT) measurements, data from balance tests are presented. To examine dynamic parameters and possible quantitative interpretations, data are presented for an identical test condition (feet tandem, eyes closed) for a traumatic brain-injured patient and a norm. It is shown that the plot of the COP position during a balance test, the stabilogram, was a less sensitive measure of balance than dynamic parameters of radial velocity or GRT     

Vortex dynamics in Bi2Sr2CaCu2O8-thin films in the presence of columnar defects

With heavy ion irradiation we create continous amorphous columnar defects in Bi₂Sr₂CaCu₂O₈-thin films. With regard to a reliable comparison of irradiation effects three of four identical striplines on the same samples were exposed to different irradiation procedures. We performed irradiations as well parallel as under different angles with respect to the film -axis. Beside an enlarged normal state resistivity after irradiation the films suffer a T_c-reduction proportional to the volume of the damaged material. The activation energy ascertained from resistive transitions shows best enhancement for magnetic field values close to the matching field. Measurements of the transport critical current density, the activation energy and the irreversibility field in different field orientations show no anisotropic flux pinning due to the induced columnar defects. This can be explained by the two dimensional behaviour of the Bi₂Sr₂CaCu₂O₈-thin films.