A traveling-wave electroabsorption modulator with a large optical cavity and intrastep quantum wells

This paper reports a novel traveling-wave electroabsorption modulator (TWEAM) with a large optical cavity waveguide and an intrastep quantum well structure designed to achieve a high bandwidth, high saturation power and better fiber-matched optical profile, which is good for high coupling efficiency. The optical mode characteristic shows a great improvement in matching the circular mode of the fiber and the saturation power of 21 dBm, and a 3 dB bandwidth of 23 GHz was achieved for the fabricated TWEAM.

Crosstalk analysis of a fiber laser sensor array system based on digital phase-generated carrier scheme

A detailed study on analyzing the crosstalk in a wavelength division multiplexed fiber laser sensor array system based on a digital phase generated carrier interferometric interrogation scheme is reported. The crosstalk effects induced by the limited optical channel isolation of a dense wavelength division demultiplexer (DWDM) are presented, and the necessary channel isolation to keep the crosstalk negligible to the output signal was calculated via Bessel function expansion and demonstrated by a two serial fiber laser sensors system. Finally, a three-element fiber laser sensor array system with a 50-dB channel-isolation DWDM was built up. Experimental results demonstrated that there was no measurable crosstalk between the output channels.

Fiber laser hydrophone based on double diaphragms: Theory and experiment

A fiber laser hydrophone with enhanced sensitivity is demonstrated. Two diaphragms with a hard core fixed at each center are used as the sensing element. Theoretical analysis shows that the Young's modulus of the diaphragm and the radius of the hard core have significant effect on the acoustic sensitivity. Experiments are carried out to test this effect and the performance of the hydrophone. The experimental result agrees well with the theoretical result, and a sensitivity of 7 nm/MPa has been achieved.

Mode coupling and vertical radiation loss for whispering-gallery modes in 3-D microcavities

The characteristics of whispering-gallery modes (WGMs) in 3-D cylindrical, square, and triangular microcavities with vertical optical confinement of semiconductors are numerically investigated by the finite-difference time-domain (FDTD) technique. For a microcylinder with a vertical refractive index 3.17/3.4/3.17 and a center layer thickness 0.2 mu m, Q-factors of transverse electric (TE) WGMs around wavelength 1550 nm are smaller than 10(3), as the radius R < 4 mu m and reach the orders of 10(4) and 10(6) as R = 5 and 6 mu m, respectively. However, the Q-factor of transverse magnetic (TM) WGMs at wavelength 1.659 mu m reaches 7.5 x 10(5) as R = 1 mu m. The mode coupling between the WGMs and vertical radiation modes in the cladding layer results in vertical radiation loss for the WGMs. In the microcylinder, the mode wavelength of TM WGM is larger than the cutoff wavelength of the vertical radiation mode with the same mode numbers, so TM WGMs cannot couple with the vertical radiation mode and have high Q-factor. In contrast, TE WGMs can couple with the corresponding vertical radiation mode in the 3-D microcylinder as R < 5 mu m. However, the mode wavelength of the TE WGM approaches (is larger than) the cutoff wavelength of the corresponding radiation modes at R = 5 mu m (6 mu m), so TE WGMs have high Q-factors in such microcylinders too. The results show that a critical lateral size is required for obtaining high, Q-factor TE WGMs in the 3-D microcylinder. For 3-D square and triangular microcavities, we also find that the Q-factor of TM WGM is larger than that of TE WGM.

Effects of coupling lens on optical refrigeration of semiconductors

Optical refrigeration of semiconductors is encountering efficiency difficulties caused by nonradiative recombination and luminescence trapping. A commonly used approach for enhancing luminescence efficiency of a semiconductor device is coupling a lens with the device. We quantitatively study the effects of a coupling lens on optical refrigeration based on rate equations and photon recycling, and calculated cooling efficiencies of different coupling mechanisms and of different lens materials. A GaAs/GaInP heterostructure coupled with a homo-epitaxial GaInP hemispherical lens is recommended.

Magnetic coupling properties of mn-doped ZnO nanowires: First-principles calculations

Based on the density functional theory, we study the magnetic coupling properties of Mn-doped ZnO nanowires. For the nanowires with passivated surfaces, the antiferromagnetic state is found and the Mn atoms have a clustering tendency. When the distance between two Mn atoms is large, the system energetically favors the paramagnetic or spin-glass state. For the nanowires with unpassivated surfaces, the ferromagnetic (FM) coupling states appear between the two nearest Mn atoms, and the zinc vacancies can further stabilize the FM states between them. The electrons with enough concentration possibly mediate the FM coupling due to the negative exchange splitting of conduction band minimum induced by the s-d coupling, which could be useful in nanomaterial design for spintronics. (C) 2008 American Institute of Physics.

Fiber Bragg grating pressure sensor with enhanced sensitivity

A novel fiber Bragg grating (FBG) pressure sensor with the enhanced sensitivity has been demonstrated. A piston-like diaphragm with a hard core in the center is used to enhance the sensitivity. Both the theoretical analysis and the experimental result show that the radius of the hard core has significant effect on the pressure sensitivity. When the radius of the hard core is 1.5 mm, a pressure sensitivity of 7.23 nm/MPa has been achieved.

Design and optimization of photonic crystal coupling layer for bi-color quantum well infrared photodetectors

Finite difference time domain (FDTD) method is used for the simulation and analysis of electromagnetic field in the top coupling layer of GaAs/AlGaAs quantum well infrared photodetector (QWIP). Simulation results demonstrated the coupling efficiencies and distributions of electromagnetic (EM) field in a variety of 2D photonic crystal coupling layer structures. A photonic crystal structure for bi-color-QWIP is demonstrated with high coupling efficiency for two wavelengths.

Excitonic bright-to-dark transition induced by spin-orbit coupling

The center-of-mass motion of quasi-two-dimensional excitons with spin-orbit coupling is calculated within the framework of effective mass theory. The results indicate that the spin-orbit coupling will induce a controllable bright-to-dark transition in a quasi-two-dimensional exciton system. This procedure can work as a way to increase the lifetime of excitons. (c) 2008 American Institute of Physics.

A new phototransistor with uni-travelling-carrier and optically gradual coupling properties

Based on appropriate combination of different band-gap InGaAsP, a new edge-coupled two-terminal double heterojunction phototransistor (ECTT-DHPT) was designed and fabricated, which is double heterojunction, free-aluminium, and works under uni-travelling-carrier mode and optically gradual coupling mode. This device is fully compatible with monolithic micro-wave integrated circuits (MMIC) and heterojunction bipolar transistor (HBT) in material and process. The DC characteristics reveal that the new ECTT-DHPT can perform good optoelectronic mix operation and linear amplification operation by optically biased at two appropriate value respectively. Responsivity of more than 52 A/W and dark current of 70 nA (when V-EC = 1 V) were obtained.

DISCRETELY TUNABLE FIBER RING LASER USING FBG TUNABLE FILTER AND MACH-ZEHNDER INTERFEROMETER

A discretely tunable Er-doped fiber-ring laser using a fiber Mach-Zehnder interferometer (MZI) and a tunable fiber Bragg grating (FBG) is proposed. In this scheme, the combination of MZI and FBG acts as a discrete wavelength selector. Analysis of its transmission function shows that discrete wavelength tuning can be realized, and experiments demonstrate 64 single-mode outputs with a mode spacing of 181.7 pm, and the output power is quite stable in the whole tuning range. (C) 2009 Wiley Periodicals, Inc. Microwave Opt Technol Lett 51 2595-2598, 2009; Published online in Wiley InterScience (www. interscience.wiley.com). DOI 10.1002/mop.24690

FREQUENCY RESPONSE OF FIBER RING RESONATOR AND ITS APPLICATION IN DFB LASER WAVELENGTH VARIATION MEASUREMENT

Frequency response of a fiber ring resonator (FRR) composed of an ordinary optical coupler and a segment of optical fiber is theoretically and experimentally investigated. The frequency response equation based oil small-signal modulation is derived and studied in detail. It is shown that the shape of the frequency response curve is very sensitive to the wavelength; as a result, the FRR can be applied to measure the wavelength of a lightwave source with high resolution. With this method, we demonstrate the measurement of tiny changes of wavelength of a DFB laser. (C) 2009 Wiley Periodicals. Inc. Microwave Opt Technol Lett 51 2444-2448, 2009 Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.24608

Fabrication and optical optimization of spot-size converters with strong cladding layers

A silicon-on-insulator (SOI) optical fiber-to-waveguide spot-size converter (SSC) overlaid with specially treated silica is investigated for integrated optical circuits. Unlike the conventional process of simply depositing the hot silica on silicon waveguides, two successive layers of silicon dioxide were grown on etched SSC structures by PECVD (plasma-enhanced chemical vapor deposition). The two layers have 0.8% index contrast and supply stronger cladding for an incident light beam. Additionally, this process is able to reduce the effective refractive index of the input mode to less than 1.47 (extremely close to that of the fiber), substantially weakening the unwanted back reflection. Exploiting this technology, it was demonstrated that the SSC showed a theoretical low mode mismatch loss of 1.23 dB for a TE-like mode and has an experimental coupling efficiency of 66%.

Magnetic coupling properties of rare-earth metals (Gd, Nd) doped ZnO: First-principles calculations

The electronic structure and magnetic coupling properties of rare-earth metals (Gd, Nd) doped ZnO have been investigated using first-principles methods. We show that the magnetic coupling between Gd or Nd ions in the nearest neighbor sites is ferromagnetic. The stability of the ferromagnetic coupling between Gd ions can be enhanced by appropriate electron doping into ZnO Gd system and the room-temperature ferromagnetism can be achieved. However, for ZnO Nd system, the ferromagnetism between Nd ions can be enhanced by appropriate holes doping into the sample. The room-temperature ferromagnetism can also be achieved in the n-conducting ZnO Nd sample. Our calculated results are in good agreement with the conclusions of the recent experiments. The effect of native defects (V-Zn, V-O) on the ferromagnetism is also discussed. (C) 2009 American Institute of Physics. [DOI 10.1063/1.3176490]

Elimination of environmental noise in interferometric wavelength shift demodulation for dynamic fiber Bragg grating sensor array

We present a novel reference compensation method for eliminating environmental noise in interferometric wavelength shift demodulation for dynamic fiber Bragg grating (FBG) sensors. By employing a shielded wavelength-division-multiplexed reference FBG in the system the environmental noise is mea, sured from the reference channel, and then subtracted from the demodulation result of each sensor channel. An approximate 40 dB reduction of the environmental noise has been experimentally achieved over a frequency range from 20 Hz to 2 kHz. This method is also suitable for the elimination of broadband environmental noise. The corresponding FBG sensor array system proposed in this paper has shown a wave-length resolution of 7 x 10(-4) pm/root Hz. (c) 2009 Elsevier B.V. All rights reserved.