A study of the active thermal control for the high energy detector on the HXMT

A thermal control system (TCS) based on the resistance heating method is designed for the High Energy Detector (HED) on the Hard X-ray Modulation Telescope (HXMT). The ground-based experiments of the active thermal control for the HED with the TCS are performed in the ambient temperature range from -15 to 20 ℃ by utilizing the pulse width to monitor the interior temperature of a NaI(Tl) crystal. Experimental results show that the NaI(Tl) crystal's interior temperature is from 17.4 to 21.7 ℃ when the temperature of the PMT shell is controlled within (20±3) ℃ with the TCS in the interesting temperature range, and the energy resolution of the HED is maintained at 16.2% @ 122 keV, only a little worse than that of 16.0% obtained at 20 ℃. The average power consumption of the TCS for the HED with a low-emissivity shell is about 4.3 W, which is consistent with the simulation.     

Length Dependence of Thermal Conductivity of Single-Walled Carbon Nanotubes

Dependence of the thermal conductivity on the length of two armchair single-walled carbon nanotubes （SWNTs） is studied by the nonequilibrium molecular dynamics （MD） method with Brenner Ⅱ potential. The thermal conductivities are calculated for （5, 5） and （7, 7） SWNTs with lengths ranging from 22 to 155nm. The results show that the thermal conductivity of SWNTs is sensitive to the length and it does not converge to a finite value when the tube length increases up to 155nm, however it obeys a power law relation.     

Thermal resistance matrix representation of thermal effects and thermal design of microwave power HBTs with two-dimensional array layout

Based on the thermal network of the two-dimensional heterojunction bipolar transistors(HBTs) array, the thermal resistance matrix is presented, including the self-heating thermal resistance and thermal coupling resistance to describe the self-heating and thermal coupling effects, respectively.For HBT cells along the emitter length direction, the thermal coupling resistance is far smaller than the self-heating thermal resistance, and the peak junction temperature is mainly determined by the self-heating thermal resistance.However, the thermal coupling resistance is in the same order with the self-heating thermal resistance for HBT cells along the emitter width direction.Furthermore, the dependence of the thermal resistance matrix on cell spacing along the emitter length direction and cell spacing along the emitter width direction is also investigated, respectively.It is shown that the moderate increase of cell spacings along the emitter length direction and the emitter width direction could effectively lower the self-heating thermal resistance and thermal coupling resistance,and hence the peak junction temperature is decreased, which sheds light on adopting a two-dimensional non-uniform cell spacing layout to improve the uneven temperature distribution.By taking a 2 × 6 HBTs array for example, a twodimensional non-uniform cell spacing layout is designed, which can effectively lower the peak junction temperature and reduce the non-uniformity of the dissipated power.For the HBTs array with optimized layout, the high power-handling capability and thermal dissipation capability are kept when the bias voltage increases.     

Thermal resistance matrix representation of thermal effects and thermal design in multi-finger power heterojunction bipolar transistors

The thermal resistance matrix including self-heating thermal resistance and thermal coupling resistance is presented to describe the thermal effects of multi-finger power heterojunction bipolar transistors. The dependence of thermal resistance matrix on finger spacing is also investigated. It is shown that both self-heating thermal resistance and thermal coupling resistance are lowered by increasing the finger spacing, in which the downward dissipated heat path is widened and the heat flow from adjacent fingers is effectively suppressed. The decrease of self-heating thermal resistance and thermal coupling resistance is helpful for improving the thermal stability of power devices. Furthermore, with the aid of the thermal resistance matrix, a 10-finger power heterojunction bipolar transistor (HBT) with non-uniform finger spacing is designed for high thermal stability. The optimized structure can effectively lower the peak temperature while maintaining a uniformity of the temperature profile at various biases and thus the device effectively may operate at a higher power level.     

Power dissipation characteristics of great power and super high speed semiconductor switch

The power dissipation characteristics of pulsed power switch reversely switched dynistors （RSDs） are investigated in this paper. According to the expressions of voltage on RSD, derived from the plasma bipolar drift model and the RLC circuit equations of RSD main loop, the simulation waveforms of current and voltage on RSD are acquired through iterative calculation by using the fourth order Runge-Kutta method, then the curve of transient power on RSD versus time is obtained. The result shows that the total dissipation on RSD is trivial compared with the pulse discharge energy and the commutation dissipation can be nearly ignored compared with the quasi-static dissipation. These characteristics can make the repetitive frequency of RSD increase largely. The experimental results prove the validity of simulation calculations. The influence factors on power dissipation are discussed. The power dissipation increases with the increase of the peak current and the n-base width and with the decrease of n-base doping concentration. In order to keep a low power dissipation, it is suggested that the n-base width should be smaller than 320μm when doping concentration is 1.0×10^14cm^-3 while the doping concentration should be higher than 5.8×10^13cm^-3 when n-base width is 270μm.     

Designing power heterojunction bipolar transistors with non-uniform emitter finger lengths to achieve high thermal stability

With the aid of a thermal-electrical model,a practical method for designing multi-finger power heterojunction bipolar transistors with finger lengths divided in groups is proposed.The method can effectively enhance the thermal stability of the devices without sacrificing the design time.Taking a 40-finger heterojunction bipolar transistor for example,the device with non-uniform emitter finger lengths is optimized and fabricated.Both the theoretical and the experimental results show that,for the optimum device,the peak temperature is lowered by 26.19 K and the maximum temperature difference is reduced by 56.67% when compared with the conventional heterojunction bipolar transistor with uniform emitter finger length.Furthermore,the ability to improve the uniformity of the temperature profile and to expand the thermal stable operation range is strengthened as the power level increases,which is ascribed to the improvement of the thermal resistance in the optimum device.A detailed design procedure is also summarized to provide a general guide for designing power heterojunction bipolar transistors with non-uniform finger lengths.     

Thermal stress reduction of GaAs epitaxial growth on V-groove patterned Si substrates

We investigate the thermal stresses for GaAs layers grown on V-groove patterned Si substrates by the finite-element method. The results show that the thermal stress distribution near the interface in a patterned substrate is nonuniform,which is far different from that in a planar substrate. Comparing with the planar substrate, the thermal stress is significantly reduced for the Ga As layer on the patterned substrate. The effects of the width of the V-groove, the thickness, and the width of the SiO₂ mask on the thermal stress are studied. It is found that the SiO₂ mask and V-groove play a crucial role in the stress of the Ga As layer on Si substrate. The results indicate that when the width of V-groove is 50 nm, the width and the thickness of the SiO₂ mask are both 100 nm, the Ga As layer is subjected to the minimum stress. Furthermore,Comparing with the planar substrate, the average stress of the Ga As epitaxial layer in the growth window region of the patterned substrate is reduced by 90%. These findings are useful in the optimal designing of growing high-quality Ga As films on patterned Si substrates.     

2-μm mode-locked nanosecond fiber laser based on MoS_2 saturable absorber

We demonstrated a 2-μm passively mode-locked nanosecond fiber laser based on a MoS_2 saturable absorber(SA).Owing to the effect of nonlinear absorption in the MoS_2 SA, the pulse width decreased from 64.7 to 13.8 ns with increasing pump power from 1.10 to 1.45 W. The use of a narrow-bandwidth fiber Bragg grating resulted in a central wavelength and 3-dB spectral bandwidth of 2010.16 and 0.15 nm, respectively. Experimental results show that MoS_2 is a promising material for a 2-μm mode-locked fiber laser.     

Power of all-fiber amplifier increasing from 1030 W to 2280 W through suppressing mode instability by increasing the seed power

In this work, we investigate suppressing mode instability in detail by varying the seed power in a large mode area all-fiber amplifier with a fiber core diameter of 25 μm. The transverse mode instability(TMI) thresholds are systematically measured for different seed power. Our experimental results reveal that increasing the seed power has a positive influence on enhancing the output power before the TMI effect appears, and finally the TMI threshold is approximately doubled from1030 W to 2280 W when the seed power is increased from 27 W to 875 W. Almost 84.7% slope efficiency is reached with different seed power before the TMI threshold power. During our operation, we also find that in this type of LMA fiber the beam quality of the amplifier is degraded gradually instead of a sudden change as the pump power increases.     

Evaluation of thermal resistance constitution for packaged AlGaN/GaN high electron mobility transistors by structure function method

The evaluation of thermal resistance constitution for packaged AlGaN/GaN high electron mobility transistor (HEMT) by structure function method is proposed in this paper.The evaluation is based on the transient heating measurement of the AlGaN/GaN HEMT by pulsed electrical temperature sensitive parameter method.The extracted chip-level and package-level thermal resistances of the packaged multi-finger AlGaN/GaN HEMT with 400-μm SiC substrate are 22.5 K/W and 7.2 K/W respectively,which provides a non-invasive method to evaluate the chip-level thermal resistance of packaged AlGaN/GaN HEMTs.It is also experimentally proved that the extraction of the chiplevel thermal resistance by this proposed method is not influenced by package form of the tested device and temperature boundary condition of measurement stage.     

Dispersion flattened photonic crystal fiber with high nonlinearity for supercontinuum generation at 1.55 μm

A robust design for a photonic crystal fiber (PCF) based on pure silica with small normal dispersion and high nonlinear coefficient for its dual concentric core structure is presented.This design is suitable for flat broadband supercontinuum (SC) generation in the 1.55-μm region.The numerical results show that the nonlinear coefficient of the proposed eight-ring PCF is 33.8 W -1 ·km -1 at 1550 nm.Ultraflat dispersion with a value between -1.65 and -0.335 ps/(nm·km) is obtained ranging from 1375 to 1625 nm.The 3-dB bandwidth of the SC is 125 nm (1496–1621 nm),with a fiber length of 80 m and a corresponding input peak power of 43.8 W.The amplitude noise is considered to be related to SC generation.For practical fabrication,the influence of the random imperfections of airhole diameters on dispersion and nonlinearity is discussed to verify the robustness of our design.     

Noise in a coupling electromagnetic detecting system for high frequency gravitational waves

This paper discusses the basic categories of noise in detecting high frequency gravitational waves in the microwave band (～0.1--10 GHz), which contain shot noise from the laser and the thermal radiation photons, thermal noise from statistical fluctuation of the thermal photons and fluctuation of the temperature, radiation press noise on the fractal membrane, the noise caused by the scattering of the Gaussian Beam (GB) in the detecting tube and noise in the microwave radiometers. The analysis shows that a reasonable signal-to-noise ratio may be achieved for a detecting device with the fixed power of GB (10⁵ W), only when the temperature of the environment is no more than T=1 K, and the optimal length of the microwave radiometers is about 0.3 m.     

Evaluation of thermal resistance constitution for packaged A1GaN/GaN high electron mobility transistors by structure function method＊

The evaluation of thermal resistance constitution for packaged A1GaN/GaN high electron mobility transistor （HEMT） by structure function method is proposed in this paper. The evaluation is based on the transient heating measurement of the A1GaN/GaN HEMT by pulsed electrical temperature sensitive parameter method. The extracted chip-level and package-level thermal resistances of the packaged multi-finger A1GaN/GaN HEMT with 400μm SiC substrate are 22.5 K/W and 7.2 K/W respectively, which provides a non-invasive method to evaluate the chip-level thermal resistance of packaged A1GaN/GaN HEMTs. It is also experimentally proved that the extraction of the chip- level thermal resistance by this proposed method is not influenced by package form of the tested device and temperature boundary condition of measurement stage.     

Performance of gain-switched all-solid-state quasi-continuous-wave tunable Ti:sapphire laser system

We have made a gain-switched all-solid-state quasi-continuous-wave （QCW） tunable Ti：sapphire laser system, which is pumped by a 532 nm intracavity frequency-doubled Nd：YAG laser. Based on the theory of gain-switching and the study on the influencing factors of the output pulse width, an effective method for obtaining high power and narrow pulse width output is proposed. Through deliberately designing the pump source and the resonator of the Ti：sapphire laser, when the repetition rate is 6 kHz and the length of the cavity is 220 mm, at an incident pump power of 22 W, the tunable Ti：sapphire laser from 700 to 950nm can be achieved. It has a maximum average output power of 5.6W at 800nm and the pulse width of 13.2 ns, giving an optical conversion efficiency of 25.5% from the 532 mn pump laser to the Ti：sapphire laser.     

Thermal stability test and analysis of a 20-actuator bimorph deformable mirror

One of the important characteristic of adaptive mirrors is the thermal stability of surface flatness. In this paper, the thermal stability from 13℃ to 25℃ of a 20-actuator bimorph deformable mirror is tested by a Shack--Hartmann wavefront sensor. Experimental results show that, the surface P--V of bimorph increases nearly linearly with ambient temperature. The ratio is 0.11μm/℃ and the major component of surface displacement is defocused, compared with which, astigmatism, coma and spherical aberration contribute very small. Besides, a finite element model is built up to analyse the influence of thickness, thermal expansion coefficient and Young's modulus of materials on thermal stability. Calculated results show that bimorph has the best thermal stability when the materials have the same thermal expansion coefficient. And when the thickness ratio of glass to PZT is 3 and Young's modulus ratio is approximately 0.4, the surface instability behaviour of the bimorph manifests itself most severely.     

Oxide-aperture-dependent output characteristics of circularly symmetric VCSEL structure

The influence ot oxidation aperture on the output characteristics ot the circularly symmetric vertical-cavity-surtaceemitting laser(VCSEL) structure is investigated.To do so,VCSELs with different oxide aperture sizes are simulated by the finite-difference time-domain(FDTD) method.The relationships among the field distribution of mode superposition,mode wavelength,output spectra,and far-field divergence with different oxide apertures are obtained.Further,VCSELs respectively with oxide aperture sizes of 2.7 μm,4.4 μm,5.9 μm,7 μm,8 μm,9 μm,and 18.7 μm are fabricated and characterized.The maximum output power increases from 2.4 mW to 5.7 mW with oxide aperture increasing from 5.9 μm to 9 μm.Meanwhile,the wavelength tuning rate decreases from 0.93 nm/mA to 0.375 nm/mA when the oxide aperture increases from 2.7 μm to 9 μm.The thermal resistance decreases from 2.815℃/mW to 1.015℃/mW when the oxide aperture increases from 4.4 μm to 18.7μm.It is demonstrated theoretically and experimentally that the wavelength spacing between adjacent modes increases with the augment of the injection current and the spacing becomes smaller with the oxide aperture increasing.Thus it can be reported that the aperture size can effectively reduce the mode overlaying but at the cost of the power decreasing and the wavelength tuning rate and thermal resistance increasing.     

Light intensification effect of trailing indent crack in fused silica subsurface

A finite-difference time-domain algorithm was applied to solve Maxwell’s equations to obtain the redistribution of an electromagnetic plane wave in the vicinity of a trailing indent crack(TIC).The roles of five geometrical parameters playing in light intensification were calculated numerically under the irradiation of a 355-nm normal incidence laser.The results show that the light intensity enhancements between the nearest neighbor pits were remarkable,which may lead to damage.The calculated results reveal that the light intensity enhancement factor(LIEF)can be up to 11.2 when TIC is on the rear-surface.With the increase of the length as well as the depth of pits,LIEF increased.Conversely,with the increase of the axis of pits,LIEF gradually declined to a stable status.It was observed that there exists an optima width or gap,which enables LIEF to be increased dramatically and then decreased gently.By comparison,results suggest that the worst cases occur when the depth and the length are both very large,especially if the width equals to 2?and the gap equals the width.This work provides a recommended theoretical criterion for defect inspection and classification.     

Optimization of a solar-blind and middle infrared two-colour photodetector using GaN-based bulk material and quantum wells

This paper calculates the wavelengths of the interband transitions as a function of the Al mole fraction of Al_xGa_1-xN bulk material. It is finds that when the Al mole fraction is between 0.456 and 0.639, the wavelengths correspond to the solar-blind (250~nm to 280~nm). The influence of the structure parameters of Al_yGa_1-yN/GaN quantum wells on the wavelength and absorption coefficient of intersubband transitions has been investigated by solving the Schr?dinger and Poisson equations self-consistently. The Al mole fraction of the Al_yGa_1-yN barrier changes from 0.30 to 0.46, meanwhile the width of the well changes from 2.9~nm to 2.2~nm, for maximal intersubband absorption in the window of the air (3~μm <λ <5~μm). The absorption coefficient of the intersubband transition between the ground state and the first excited state decreases with the increase of the wavelength. The results are finally used to discuss the prospects of GaN-based bulk material and quantum wells for a solar-blind and middle infrared two-colour photodetector.     

Beam experiments with a non-intercepting beam induced f luorescence profile monitor for the ADS LINAC

An accelerator-driven subcritical system(ADS) project was launched in China in 2011, aiming to design and build an ADS demonstration facility with the capability of more than 1000 MW thermal power. The driver linac is defined to be a 10 m A current of high energy protons at 1.5 Ge V in continuous wave operation mode. To meet the extremely high power and intense beam accelerator requirements, non-interceptive monitors for the beam transverse profile are required for this proton linac. Taking advantage of the residual gas as active material, the Beam Induced Fluorescence(BIF) monitor exploits gas-excited fluorescence in the visible spectrum region for transverse profile measurements. The advantages of this non-intercepting method are that nothing is installed in the vacuum pipe,component design is compact and there is no need for expensive signal processing electronics. Beam experiments have been performed under constant beam conditions. The helium spectrum has been verified with different optical filters, showing that a proper optical band-pass filter covering 400–500 nm is necessary for fluorescence experiments with helium. By changing gas pressure, it is shown that gas pressure is proportional to the signal amplitude but has no influence on detected profile width. Finally, a comparison experiment between the BIF monitor and a wire scanner shows that the detected profile width results of both methods agree well.     

Effect of lateral structure parameters of SiGe HBTs on synthesized active inductors

The effect of lateral structure parameters of transistors including emitter width, emitter length, and emitter stripe number on the performance parameters of the active inductor(AI), such as the effective inductance Ls, quality factor Q,and self-resonant frequency ω_0 is analyzed based on 0.35-μm Si Ge Bi CMOS process. The simulation results show that for AI operated under fixed current density JC, the HBT lateral structure parameters have significant effect on Ls but little influence on Q and ω_0, and the larger Ls can be realized by the narrow, short emitter stripe and few emitter stripes of Si Ge HBTs. On the other hand, for AI with fixed HBT size, smaller JCis beneficial for AI to obtain larger Ls, but with a cost of smaller Q and ω_0. In addition, under the fixed collector current IC, the larger the size of HBT is, the larger Ls becomes, but the smaller Q and ω_0 become. The obtained results provide a reference for selecting geometry of transistors and operational condition in the design of active inductors.