Radio over free space optical link using a directly modulated two-electrode high power tapered laser

The analog modulation performance of a high-power two-electrode tapered laser is investigated. A 25dB dynamic range for 2.4GHz 802.11g signals is achieved with a 26dB loss budget, showing a >1km free space range is possible. © 2010 Optical Society of America.

Free-space locomotion with thread formation

The paper presents a new concept of locomotion for wheeled or legged robots through an object-free space. The concept is inspired by the behaviour of spiders forming silk threads to move in 3D space. The approach provides the possibility of variation in thread diameter by deforming source material, therefore it is useful for a wider coverage of payload by mobile robots. As a case study, we propose a technology for descending locomotion through a free space with inverted formation of threads in variable diameters. Inverted thread formation is enabled with source material thermoplastic adhesive (TPA) through thermally-induced phase transition. To demonstrate the feasibility of the technology, we have designed and prototyped a 300-gram wheeled robot that can supply and deform TPA into a thread and descend with the thread from an existing hanging structure. Experiment results suggest repeatable inverted thread formation with a diameter range of 1.1-4.5 mm, and a locomotion speed of 0.73 cm per minute with a power consumption of 2.5 W. © 2013 IEEE.

Echolocation signals of the free-ranging Yangtze finless porpoise (Neophocaena phocaenoides asiaeorientialis)

This paper describes the high-frequency echolocation signals from free-ranging Yangtze finless porpoise in the Tian-e-zhou Baiji National Natural Reserve in Hubei Province, China. Signal analysis showed that the Yangtze finless porpoise clicks are typical high-frequency narrow-band (relative width of the frequency spectrum Q=6.6 &PLUSMN; 1.56, N=548) ultrasonic pulses. The peak frequencies of the typical clicks range from 87 to 145 kHz with an average of 125 &PLUSMN; 6.92 kHz. The durations range from 30 to 122 μ s with an average of 68 &PLUSMN; 14.12 μ s. The characteristics of the signals are similar to those of other members of the Phocoenidae as well as the distantly related delphinids, Cephalorhynchus spp. Comparison of these signals to those of the baiji (Lipotes vexillifer), who occupies habitat similar to that of the Yangtze finless porpoise, showed that the peak frequencies of clicks produced by the Yangtze finless porpoise are remarkably higher than those produced by the baiji. Difference in peak frequency between the two species is probably linked to the different size of prefer-red prey fish. Clear double-pulse and multi-pulse reverberation structures of clicks are noticed, and there is no indication of any low-frequency (< 70 kHz) components during the recording period. © 2005 Acoustical Society of America.

High-temperature AlN interlayer for crack-free AlGaN growth on GaN

This paper presents a study of the transformation of high-temperature AlN (HT-AlN) interlayer (IL) and its effect on the strain relaxation of Al0.25Ga0.75N/HT-AlN/GaN. The HT-AlN IL capped with Al0.25Ga0.75N transforms into AlGaN IL in which the Al composition increases with the HT-AlN IL thickness while the total Ga content keeps nearly constant. During the HT-AlN IL growth on GaN, the tensile stress is relieved through the formation of V trenches. The filling up of the V trenches by the subsequent Al0.25Ga0.75N growth is identified as the Ga source for the IL transformation, whose effect is very different from a direct growth of HT-AlGaN IL. The a-type dislocations generated during the advancement of V trenches and their filling up propagate into the Al0.25Ga0.75N overlayer. The a-type dislocation density increases dramatically with the IL thickness, which greatly enhances the strain relaxation of Al0.25Ga0.75N. (c) 2008 American Institute of Physics.

InAs/GaSb superlattices for photodetection in short wavelength infrared range

The first report of a short wavelength infrared detector based on type II InAs/GaSb superlattices is presented. Very short period superlattices containing InAs (2ML)/GaSb (8ML) superlattices (SLs) were grown by molecular-beam epitaxy on GaSb substrates. The photoluminescence showed a cut-off wavelength at 2.1 mu m at 10 K and 2.6 mu m at 300 K. Room-temperature optical transmittance spectra shows obvious absorption in InAs (2ML)/GaSb (8ML) SL in the range of 450-680 meV, i.e. 1.8-2.7 mu m. The cut-off wavelength moved from 2.3 mu m to 2.6 mu m with temperature rising from 77 K to 300 K in photoresponse spectra. The blackbody response R-v exponentially decreased as a function of 1/T in two temperature sections (130-200 K and 230-300 K). The blackbody detectivity D-bb(center dot) was beyond 1 x 10(8) cmHz(1/2)/W at room temperature. (C) 2009 Elsevier B.V. All rights reserved.

Chirp-free optical modulation using a silicon push-pull coupling microring

We propose a silicon ring-based optical modulation method to perform chirp-free optical modulations. In this scheme, we locate the light to be modulated at the resonance of the ring and tune the coupling coefficient between the ring and the straight waveguide by using a push-pull coupling structure. The chirp-free phase modulation can be achieved by varying the coupling coefficient in a large range, which can modify the coupling condition of the ring such that the input light experiences an abrupt phase shift of pi at the output. If the coupling coefficient is adjusted in a small range such that the coupling condition of the ring is kept unchanged, only the intensity of the light will be modulated. This leads to chirp-free intensity modulation. Our simulations performed at 10 Gbits/s confirm the feasibility of the proposal. (C) 2009 Optical Society of America

New type of multimode-interference-type thermo-optic variable optical attenuator

A thermo-optic variable optical attenuator (VOA) based on a Mach-Zehnder interferometer and multimode-interference coupler is fabricated. Not a single-mode but a multimode waveguide is used as the input and output structures of the optical field, which greatly reduces the coupling loss of the VOA with a normal single-mode fiber. The insertion loss of the fabricated VOA is 2.52 to 2.82 dB at the wavelength of 1520 to 1570 nm. The polarization dependent loss is 0.28 to 0.45 dB at the same wavelength range. Its maximum attenuation range is up to 26.3 dB when its power consumption is 369 mW. The response frequency of the fabricated VOA is about 10 kHz. (C) 2004 Society of Photo-Optical Instrumentation Engineers.

Photoluminescence characterization of nanocrystalline ZnO array

High-density and uniform well-aligned ZnO sub-micron rods are synthesized on the silicon substrate over a large area. The morphology, and structure of the ZnO sub-micron rods are investigated by x-ray diffraction, transmission electron microscopy and Raman spectra. It is found that the ZnO sub-micron rods are of high crystal quality with the diameter in the range of 400-600 nm and the length of several micrometres long. The optical properties were studied bill photoluminescence spectra. The results show that the intensity of the ultraviolet emission at 3.3 eV is rather high, meanwhile the deep level transition centred at about 2.38 eV is weak. The free exciton emission could also be observed at low, temperature, which implies the high optical quality of the ZnO sub-micron rods. This growth technique provides one effective way to fabricate the high crystal quality ZnO nanowires array, which is very important for potential applications in the new-type optoelectronic nanodevices.

Structure and magnetic properties of ScFe6Ga6-type RCo5Ga7 (R = Y, Tb, Dy, Ho and Er)

The structure and magnetic properties of the RCo5Ga7 (R = Y, Tb, Dy, Ho and Er) compounds with the ScFe6Ga6-type structure have been studied. The stability of RCo5Ga7 is closely related with the ratio of the metal radii R-RE/R-(Co,R-Ga). With R-RE/R-(Co,R-Ga) less than or equal to 1.36, the compounds can be stabilized in the ScFe6Ga6-type structure. The lattice of RCo5Ga7 shrinks as the atomic order of R increases, and it is consistent with the lanthanide contraction. The structure analysis based on X-ray diffraction patterns reveals that in the orthorhombic RCo5Ga7 (Immm), R occupies the 2a site, and Co enters into the 8k and the 4h sites, and Ga is at the 4e, 4f, 4g, 4h and 8k sites. The interatomic distances and the coordination numbers of RCo5Ga7 are provided from the refinement results. The short interatomic distance (less than 2.480 Angstrom) between the Co ions results in the negative magnetic interaction, which does not favor ferromagnetic ordering. The magnetic moment of YCo5Ga7 is absent, and RCo5Ga7 (R = Tb, Dy, Ho and Er) may have long-range magnetic ordering with the paramagnetic Curie temperature lower than 5 K. (C) 2004 Elsevier Inc. All rights reserved.

Synthesis of high quality n-type CdS nanobelts and their applications in nanodevices

High quality n-type CdS nanobelts (NBs) were synthesized via an in situ indium doping chemical vapor deposition method and fabricated into field effect transistors (FETs). The electron concentrations and mobilities of these CdS NBs are around (1.0x10(16)-3.0x10(17))/cm(3) and 100-350 cm(2)/V s, respectively. An on-off ratio greater than 10(8) and a subthreshold swing as small as 65 mV/decade are obtained at room temperature, which give the best performance of CdS nanowire/nanobelt FETs reported so far. n-type CdS NB/p(+)-Si heterojunction light emitting diodes were fabricated. Their electroluminescence spectra are dominated by an intense sharp band-edge emission and free from deep-level defect emissions. (c) 2006 American Institute of Physics.

Multimode-interference-type thermo-optic variable optical attenuator with a response frequency of 10 kHz

A thermo-optic variable optical attenuator based on a multimode interference coupler principle is fabricated. The propagation loss of the fabricated device is 1.6 to 3.8 dB at the wavelength range 1510 to 1610 nm, which is very near the calculated value (1.2 dB) by the finite difference beam propagation method. The maximum power consumption is 363 mW and the dynamic attenuation range is 0 to 26 dB. The response frequency of the fabricated attenuator is about 10 kHz. (C) 2003 Society of Photo-Optical Instrumentation Engineers.

Type-II SiGe/Si MQWS (multi-quantum wells) and self-organized Ge/Si islands grown by UHV/CVD system

Type-II SiGe/Si MQWs (Multi-Quantum Wells) and Self-Organized Ge/Si Islands were successfully grown by a homemade ultra-high vacuum/chemical vapor deposition (UHV/CVD) system. Growth characteristics and PL (photoluminescence) spectra at different temperature were measured. It demonstrated that some accumulation of carriers in the islands results in the increase of the integrated PL intensity of island-related at a certain temperature range.

Metal-free growth of Si/SiO2 nanowires by annealing SiOx (x < 2) films deposited by PECVD

A new metal catalysis-free method of fabricating Si or SiO2 nanowires (NWs) compatible with Si CMOS technology was proposed by annealing SiOx (x < 2) films deposited by plasma -enhanced chemical vapor deposition (PECVD). The effects of the Si content (x value) and thickness of SiOx films, the annealing process and flowing gas ambient on the NW growth were studied in detail. The results indicated that the SiOx film of a thickness below 300 rim with x value close to 1 was most favorable for NW growth upon annealing at 1000-1150 degrees C in the flowing gas mixture of N-2 and H-2. NWs of 50-100nm in diameter and tens of micrometers in length were synthesized by this method. The formation mechanism was likely to be related to a new type of oxide assisted growth (OAG) mechanism, with Si nanoclusters in SiOx films after phase separation serving as the nuclei for the growth of NWs in SiOx films > 200nm, and SiO molecules from thin SiO, film decomposition inducing the NW growth in films < 100nm. An effective preliminary method to control NW growth direction was also demonstrated by etching trenches in SiOx films followed by annealing.

Study on biomass circulation and gasification performance in a clapboard-type internal circulating fluidized bed gasifier

We investigated the solid particle flow characteristics and biomass gasification in a clapboard-type internal circulating fluidized bed reactor. The effect of fluidization velocity on particle circulation rate and pressure distribution in the bed showed that fluidization velocities in the high and low velocity zones were the main operational parameters controlling particle circulation. The maximum internal circulation rates in the low velocity zone came almost within the range of velocities in the high velocity zone, when uH/umf = 2.2-2.4 for rice husk and uH/umf = 3.5-4.5 for quartz sand. In the gasification experiment, the air equvalence ratio (ER) was the main controlling parameter. Rice husk gasification gas had a maximum heating value of around 5000 kJ/m3 when ER = 0.22-0.26, and sawdust gasification gas reached around 6000-6500 kJ/m3 when ER = 0.175-0.24. The gasification efficiency of rice husk reached a maximum of 77% at ER = 0.28, while the gasification efficiency of sawdust reached a maximum of 81% at ER = 0.25.

The design and operation of a new clapboard-type internal circulating fluidized-bed gasifier

The design and operation of a new clapboard-type internal circulating fluidized-bed gasifier is proposed in this article. By arranging the clapboard in the bed, the gasifier is thus divided into two regions, which are characterized by different fluidization velocities. The bed structure is designed so that it can guide the circulating flow passing through the two regions, and therefore the feedstock particles entrained in the flow experience longer residence time. The experimental results based on the present new design, operating in the temperature range of 790 degrees C-850 degrees C, indicate that the gas yield is from 1.6-1.9 Nm(3)/kg feedstock, the gas enthalpies are 5,345 kJ/Nm(3) for wood chip and 4,875 kJ/m(3) for rice husk, and a gasification efficiency up to 75% can be obtained.