The efficiency of the injection and recombination processes in InGaN/GaN LEDs is governed by the properties of the active region of the devices, which strongly depend on the conditions used for the growth of the epitaxial material. To improve device quality, it is very important to understand how the high temperatures used during the growth process can modify the quality of the epitaxial material. With this paper we present a study of the modifications in the properties of InGaN/GaN LED structures induced by high temperature annealing: thermal stress tests were carried out at 900 °C, in nitrogen atmosphere, on selected samples. The efficiency and the recombination dynamics were evaluated by photoluminescence measurements (both integrated and time-resolved), while the properties of the epitaxial material were studied by Secondary Ion Mass Spectroscopy (SIMS) and Rutherford Backscattering (RBS) channeling measurements. Results indicate that exposure to high temperatures may lead to: (i) a significant increase in the photoluminescence efficiency of the devices; (ii) a decrease in the parasitic emission bands located between 380 nm and 400 nm; (iii) an increase in carrier lifetime, as detected by time-resolved photoluminescence measurements. The increase in device efficiency is tentatively ascribed to an improvement in the crystallographic quality of the samples.
The issues and challenges of growing GaN-based structures on large area Si substrates have been studied. These include
Si slip resulting from large temperature non-uniformities and cracking due to differential thermal expansion. Using an
AlN nucleation layer in conjunction with an AlGaN buffer layer for stress management, and together with the interactive
use of real time in-situ optical monitoring it was possible to realise flat, crack-free and uniform GaN and LED structures
on 6-inch Si (111) substrates. The EL performance of processed LED devices was also studied on-wafer, giving good EL
characteristics including a forward bias voltage of ~3.5 V at 20 mA from a 500 μm x 500 μm device.
The performance of a series of near-UV (~385 nm) emitting LEDs, consisting of high efficiency InGaN/AlInGaN QWs in the active region, was investigated. Significantly reduced roll-over of efficiency at high current density was found compared to InGaN/GaN LEDs emitting at a similar wavelength. The importance of optical cavity effects in flip-chip geometry devices has also been investigated. The light output was enhanced by more than a factor of 2 when the light-emitting region was located at an anti-node position with respect to a high reflectivity current injection mirror. A power of 0.49 mW into a numerical aperture of 0.5 was obtained for a junction area of 50 micrometers in diameter and a current of 30 mA, corresponding to a radiance of 30 W/cm2/str.
Simultaneous strain and temperature measurement system with fiber Bragg grating was presented in this paper. The light
from broadband source (BBS) was coupled into sensing probe through 3dB coupler1. Reflective light of two FBGs was
split through coupler2 and went into chirped gratings with different pass-band. Demodulation method adopted chirped
grating and long period grating edge linear filtering technology. It can send each reflected spectrum to different edge
filter. It makes every FBG's reflective spectrum was demodulate. The central reflected wavelength of two FBGs was
1546.15nm and 1554.17nm respectively. Through simulation experiment, we can get that (formula available in manuscript). Strain measurement ranged from 0 to 2000 με. Temperature measurement ranged from 0 to 200°C.
Sampled fiber Bragg grating(SFBG) has caused extensive research interest due to its special filtering characteristic, strict
wavelength interval, compact structure, easy integration and low cost etc. Based on coupling-mode theory, the reflective
spectrum of sampled fiber Bragg grating was analyzed using transmission matrix method. Strain and temperature sensing
characteristics of sampled fiber Bragg grating were discussed in the paper. Wavelength shift and transmission intensity
vary linearly with strain and temperature. The change of strain and temperature can be determined simultaneously by a
single sampled fiber grating. In the experiment, the strain and temperature of sampled fiber Bragg grating change
simultaneously, the photoelastic coefficients of sampled fiber Bragg grating were P11=0.121, P12=0.27. The Poisson ratio
of optical fiber core stuff was 0.17, the thermal expansion coefficient was 5.5×10-7/°C, and the calorescence coefficient
of optical fiber was 8.3×10-6/°C. We can get that A, B, C and D were separately -0.00055nm/ε, 0.013nm/°C, -
0.00033/ε, -0.00011/°C by calculating. Strain measurement ranged from 0 με to 2500με. Temperature measurement ranged from 0°C to 300°C.
Transmission spectrum of phase-shift long period gratings is analyzed by transfer matrix method. Through simulation
experiments, the effects of grating parameters (period and length, average index modulation, the phase-shift position) on
the phase-shift long period Gratings are discussed. The rule of each parameter effect the transmission spectrum is
discussed. We draw a conclusion that if we select suitable grating parameters, we can get the special transmission
spectrum for the application.
The distributed surface acoustic wave (SAW) strain sensing system based on FPGA was presented in this paper. Equal
strength beam SAW strain sensor was adopted. Through special device sealing, it not only protected the surface
electrode but also did not influence the sensitivity of the sensor. Equal precision frequency meter based on FPGA
realized the frequency measurement of measuring signal. Equal precision frequency meter had high measuring precision.
It can keep constant measuring precision during the whole frequency domain. Measuring frequency ranged from 0.1 to
100MHz. Relative error was 1 ppm.
Fiber grating works as a novel kind of smart sensing element. It adopts wavelength encoding. It is prone to multi-points multiplexing and it is compatible with normal fiber system completely. It has been got extensive and deeply research in fiber sensing field because such advantages. In order to realize simultaneous strain and temperature measurement, solving temperature and strain's cross sensitivity is the key problem in fiber grating sensors researching. Simultaneous strain and temperature measurement using fiber Bragg grating(FBG) is presented in this paper. Different cladding diameter fiber solve temperature and strain's cross sensitivity problem. Demodulation system adopts arrayed-waveguide grating (AWG). It has merits of high wavelength resolution, integrated, small series-mode interference and low insertion loss. Light source adopted clock pulse broadband source. The system integrates time division multiplexing and wavelength division multiplexing to solve the multiplexing addressing problem of FBG sensing network. It proved feasibly according theoretical analyzing. Measuring strain range from 0 to 2500 με. Measuring temperature range from 25 to 120°. Error of strain is ± 17 με and error of temperature is ±1°C.
Sampled fiber Bragg grating (SFBG) has caused extensive research interest due to its special filtering characteristic, strict wavelength interval, compact structure, easy integration, and low cost etc. Based on coupling-mode theory, the reflective spectrum of sampled fiber Bragg grating is analyzed using transmission matrix method. Strain and temperature sensing characteristics of sampled fiber Bragg grating are discussed in the paper. Wavelength shift and transmission intensity vary linearly with strain and temperature. Strain and temperature changes can be determined simultaneously by a single sampled fiber grating. In the experiment, the strain and temperature of sampled fiber Bragg grating change simultaneously, the photoelastic coefficients of sampled fiber Bragg grating are P11=0.121, P12=0.27. The Poisson ratio of optical fiber core stuff is 0.17, the thermal expansion coefficient is 5.5×10-7/°C,and the calorescence coefficient of optical fiber is 8.3×10-6/°C. By calculation, we have got that A, B, C and D separately -0.00276, 0.045, 0.72, 11.3, remain in the strain measuring range from 0 to 1200με, and the temperature measuring range from 20 to 110°C.
In this paper, acoustic emission wave vibration acceleration measurement system is presented. In the system the distributed Fiber Bragg grating(FBG) measuring probe is used. In order to fulfill wavefront filtering, space and frequency domain filering, frequency modulation continuous wave (FMCW), wave division multiply (WDM) and time division multiply (TDM) techniques are adopted, and optical wavelet filter is designed. Thus influences of transverse sensitivity, temperature noise and intrinsic noise of fiber can be eliminated automatically, and disturbance of imbalance and nonlinear optical signal can also be restricted efficiently. The system realize distributed measurement of the acoustic emission wave vibration acceleration. The range of measured acceleration is 4.3m/s2~340m/s2, the range of frequency response is quasi-static~1000, the resolution is 7.5x10-7nm/√Hz. The presented measured system is virtually significant theoretically and practically in some fields such as micro-vibration measurement and fault diagnosis of marine platform, large-scale building, concrete dam and satellite launch pad, etc.