Volume 6,Issue 6,2010 Table of Contents
2010, 6(6):401-405.
Abstract:
The propagation of femtosecond laser pulses with wavelengths of 1550 nm, 1064 nm, 800 nm and 700 nm, respectively, which are in the normal dispersion region of the nano-structured photonic crystal fiber (N-PCF) with interesting broadband normal dispersion and highly nonlinear properties, is studied. For the effect of chirp variation mainly induced by group velocity dispersion (GVD) and self-phase modulation (SPM), after propagation over a short length, the wave breaking occurs. Namely, oscillatory structures are presented near pulse edges and sidelobes appear in the pulse spectrum. In the case of 800 nm, after the propagation of 20 mm, a super flat spectrum is obtained. The bandwidth of the super flat spectrum is associated with the dispersion length and the nonlinear length. By choosing N-PCF and laser pulse with appropriate parameters, a broadband super flat spectrum in a short length can be achieved.
The propagation of femtosecond laser pulses with wavelengths of 1550 nm, 1064 nm, 800 nm and 700 nm, respectively, which are in the normal dispersion region of the nano-structured photonic crystal fiber (N-PCF) with interesting broadband normal dispersion and highly nonlinear properties, is studied. For the effect of chirp variation mainly induced by group velocity dispersion (GVD) and self-phase modulation (SPM), after propagation over a short length, the wave breaking occurs. Namely, oscillatory structures are presented near pulse edges and sidelobes appear in the pulse spectrum. In the case of 800 nm, after the propagation of 20 mm, a super flat spectrum is obtained. The bandwidth of the super flat spectrum is associated with the dispersion length and the nonlinear length. By choosing N-PCF and laser pulse with appropriate parameters, a broadband super flat spectrum in a short length can be achieved.
2010, 6(6):406-411.
Abstract:
The reflectivity of one-dimensional chalcogenide photonic crystal (CGPC) structure with the first order reflection band in near infrared (NIR) region is theoretically studied. Sb-Se and Ge-S chalcogenide glasses are used as high and low refractive index layers respectively, because these materials have zero absorption in NIR region. The transfer matrix method (TMM) is employed to calculate the reflective spectra of the proposed structure. The theoretical results of reflective spectra of bulk chalcogenide materials with the composition of Sb40Se60 and Ge30S70 for 4, 8, 12 and 15 layers and thicknesses of 117 nm and 183 nm respectively, at normal incidence, are close agreement with the experimental results. Furthermore, by increasing the number of layers of Sb40Se60 and Ge30S70, the reflection bands can be enhanced in the wider range of the NIR region for the polarization at different angles and thus the broadband omnidirectional reflector can be designed.
The reflectivity of one-dimensional chalcogenide photonic crystal (CGPC) structure with the first order reflection band in near infrared (NIR) region is theoretically studied. Sb-Se and Ge-S chalcogenide glasses are used as high and low refractive index layers respectively, because these materials have zero absorption in NIR region. The transfer matrix method (TMM) is employed to calculate the reflective spectra of the proposed structure. The theoretical results of reflective spectra of bulk chalcogenide materials with the composition of Sb40Se60 and Ge30S70 for 4, 8, 12 and 15 layers and thicknesses of 117 nm and 183 nm respectively, at normal incidence, are close agreement with the experimental results. Furthermore, by increasing the number of layers of Sb40Se60 and Ge30S70, the reflection bands can be enhanced in the wider range of the NIR region for the polarization at different angles and thus the broadband omnidirectional reflector can be designed.
2010, 6(6):412-416.
Abstract:
A low-threshold efficient high-repetition-rate eye-safe optical parametric oscillator (OPO) is presented. The OPO is based on an x-cut non-critically phase-matched (NCPM) KTA intra-cavity pumped by an acousto-optically (A-O) Q-switched Nd:YVO4 laser. At 10 kHz, the lowest threshold of 0.75 W and the signal power of 0.6 W are got, corresponding to the single pulse energy of 60 μJ and the peak power of 20 kW. Tuning the frequency, the maximum output power at 1536 nm is 1.03 W at 30 kHz with an optical-to-optical conversion efficiency of 12.26%. The fluctuation of the output power is 2.1% during 2 h operation.
A low-threshold efficient high-repetition-rate eye-safe optical parametric oscillator (OPO) is presented. The OPO is based on an x-cut non-critically phase-matched (NCPM) KTA intra-cavity pumped by an acousto-optically (A-O) Q-switched Nd:YVO4 laser. At 10 kHz, the lowest threshold of 0.75 W and the signal power of 0.6 W are got, corresponding to the single pulse energy of 60 μJ and the peak power of 20 kW. Tuning the frequency, the maximum output power at 1536 nm is 1.03 W at 30 kHz with an optical-to-optical conversion efficiency of 12.26%. The fluctuation of the output power is 2.1% during 2 h operation.
2010, 6(6):417-420.
Abstract:
The design of a novel photonic crystal waveguide power splitter is presented. The proposed power splitter has three output ports, the coupling among three parallel photonic crystal waveguides can be considered as a multimode interference (MMI), and the positions of output waveguides are determined by those of twofold images which are formed by the self-imaging effect of multimode interference. The transmission characteristics of the splitter are investigated by using the finite-difference time domain (FDTD) and the plane wave expansion method. The output optical power in each port can be controlled by adjusting the radius of the dielectric rods in the coupling region, and the coupling effect among output ports is decreased by using the T-shaped output port. The results indicate that 1× 1, 1×2 and 1×3 type power splitters can be realized when the normalized radius of dielectric rods in the coupling region is 0.130, 0.180, 0.152 or 0.221, respectively.
The design of a novel photonic crystal waveguide power splitter is presented. The proposed power splitter has three output ports, the coupling among three parallel photonic crystal waveguides can be considered as a multimode interference (MMI), and the positions of output waveguides are determined by those of twofold images which are formed by the self-imaging effect of multimode interference. The transmission characteristics of the splitter are investigated by using the finite-difference time domain (FDTD) and the plane wave expansion method. The output optical power in each port can be controlled by adjusting the radius of the dielectric rods in the coupling region, and the coupling effect among output ports is decreased by using the T-shaped output port. The results indicate that 1× 1, 1×2 and 1×3 type power splitters can be realized when the normalized radius of dielectric rods in the coupling region is 0.130, 0.180, 0.152 or 0.221, respectively.
5 Design of integrated microlens for collimation of the vertical-cavity surface emitting laser array
2010, 6(6):421-425.
Abstract:
A method based on the theory of transfer matrix to design the integrated microlens for the collimation of vertical-cavity surface emitting laser (VCSEL) array is presented. The integrated microlenses fabricated on the substrate directly and on a certain polymer material which is on the substrate are considered. The relationships between the radius of curvature, beam waist and the divergence angle after collimation are obtained with the help of ZEMAX. The results show that the devices with the divergence angle of 15° (1/e 2) and beam waist of 2 μm can be improved to those with the divergence angle lower than 1°, and the devices with beam waist of 10 μm can be improved to those with the divergence angle lower than 3°, which is a good reference for manufacturing high-power devices with small divergence angle. The conclusions including increasing the thicknesses of both the substrate and polymer material and reducing the diameter of oxidized layer are drawn, which will be an important guidance for experiment research.
A method based on the theory of transfer matrix to design the integrated microlens for the collimation of vertical-cavity surface emitting laser (VCSEL) array is presented. The integrated microlenses fabricated on the substrate directly and on a certain polymer material which is on the substrate are considered. The relationships between the radius of curvature, beam waist and the divergence angle after collimation are obtained with the help of ZEMAX. The results show that the devices with the divergence angle of 15° (1/e 2) and beam waist of 2 μm can be improved to those with the divergence angle lower than 1°, and the devices with beam waist of 10 μm can be improved to those with the divergence angle lower than 3°, which is a good reference for manufacturing high-power devices with small divergence angle. The conclusions including increasing the thicknesses of both the substrate and polymer material and reducing the diameter of oxidized layer are drawn, which will be an important guidance for experiment research.
2010, 6(6):426-428.
Abstract:
The adaptive optics (AO) technology without a wave-front sensor is simple and has more application fields than conventional AO technologies. The theory basis of the AO system based on stochastic parallel gradient descent (SPGD) from the convergence, the stability, and the averaged convergence rate is analyzed, where SPGD is used to control the AO system. The results show that the AO system based on SPGD can obtain good convergence and stability, and the fast convergence rate, which is in inverse proportion to $ \sqrt n $ \sqrt n (n is the number of control parameters).
The adaptive optics (AO) technology without a wave-front sensor is simple and has more application fields than conventional AO technologies. The theory basis of the AO system based on stochastic parallel gradient descent (SPGD) from the convergence, the stability, and the averaged convergence rate is analyzed, where SPGD is used to control the AO system. The results show that the AO system based on SPGD can obtain good convergence and stability, and the fast convergence rate, which is in inverse proportion to $ \sqrt n $ \sqrt n (n is the number of control parameters).
2010, 6(6):429-431.
Abstract:
A laser simulator is proposed which is based on a hardware-in-loop (HWIL) simulation test system. As an important device in the simulation test system, it is used to simulate the laser energy and the divergence angle. The laser simulator simulates the attenuation process of laser energy by the continuous rotation of Glan prisms and the changing of laser spot size through the continuous motion of the laser beam expanding telescope on the straight platform. The dynamic attenuation velocity of the laser simulator is up to 4 dB/s, and the divergence angle adjustment velocity is up to 3 mrad/s. The method is feasible for the simulation test.
A laser simulator is proposed which is based on a hardware-in-loop (HWIL) simulation test system. As an important device in the simulation test system, it is used to simulate the laser energy and the divergence angle. The laser simulator simulates the attenuation process of laser energy by the continuous rotation of Glan prisms and the changing of laser spot size through the continuous motion of the laser beam expanding telescope on the straight platform. The dynamic attenuation velocity of the laser simulator is up to 4 dB/s, and the divergence angle adjustment velocity is up to 3 mrad/s. The method is feasible for the simulation test.
2010, 6(6):432-434.
Abstract:
A tunable photonic crystal (PC) filter is proposed with the twisted nematic (TN) liquid crystal (LC) as a defect layer. The polarization sensitive performance and the eigenmodes are analyzed. The results show that there are two kinds of transmission modes, namely o and e, when the voltage is low. The different modes can be selected by rotating the filter axially. The e mode shifts toward the shorter wavelength side as the applied voltage increases. Then the peaks of two modes merge into each other and the polarization sensitive performance disappears. The photonic band gap is insensitive to the voltage. The tunable range of the filter is 53 nm.
A tunable photonic crystal (PC) filter is proposed with the twisted nematic (TN) liquid crystal (LC) as a defect layer. The polarization sensitive performance and the eigenmodes are analyzed. The results show that there are two kinds of transmission modes, namely o and e, when the voltage is low. The different modes can be selected by rotating the filter axially. The e mode shifts toward the shorter wavelength side as the applied voltage increases. Then the peaks of two modes merge into each other and the polarization sensitive performance disappears. The photonic band gap is insensitive to the voltage. The tunable range of the filter is 53 nm.
2010, 6(6):435-438.
Abstract:
A new collimation system, the heteroaxial double half-elliptical cylinder lens (HDHECL), is proposed based on the raytracing method, and the collimation performance of the HDHECL is discussed and compared with conventional lenses. The simulation results suggest that the refractive index of the HDHECL can be larger than 2, the collimation effect is improved with the increase of the refractive index in a certain range, and the divergence angle in the fast-axis direction can be compressed to less than 0.01 mrad theoretically. These results are of significance in theory and practice.
A new collimation system, the heteroaxial double half-elliptical cylinder lens (HDHECL), is proposed based on the raytracing method, and the collimation performance of the HDHECL is discussed and compared with conventional lenses. The simulation results suggest that the refractive index of the HDHECL can be larger than 2, the collimation effect is improved with the increase of the refractive index in a certain range, and the divergence angle in the fast-axis direction can be compressed to less than 0.01 mrad theoretically. These results are of significance in theory and practice.
2010, 6(6):439-442.
Abstract:
A thermal model of crystal is established. The temperature field differential equation of the diode-end-pumped laser crystal with circular cross-section and new boundary conditions, in which the convection heat transfer is supposed to exist between laser crystal ends and air, is established. The analytical expressions of temperature field, thermal distortion and additional optical path difference (OPD) of crystal are obtained. By numerical calculation, the influence of heat transfer on the thermal effects of laser crystal Nd:YAG is studied. Results show that crystal’s thermal effects, including temperature field, thermal distortion, OPD and thermal focal length, are all weakened as the heat transfer through ends of crystal is strengthened. This conclusion could be used to control thermal effects of laser crystal and improve laser working stability.
A thermal model of crystal is established. The temperature field differential equation of the diode-end-pumped laser crystal with circular cross-section and new boundary conditions, in which the convection heat transfer is supposed to exist between laser crystal ends and air, is established. The analytical expressions of temperature field, thermal distortion and additional optical path difference (OPD) of crystal are obtained. By numerical calculation, the influence of heat transfer on the thermal effects of laser crystal Nd:YAG is studied. Results show that crystal’s thermal effects, including temperature field, thermal distortion, OPD and thermal focal length, are all weakened as the heat transfer through ends of crystal is strengthened. This conclusion could be used to control thermal effects of laser crystal and improve laser working stability.
2010, 6(6):443-445.
Abstract:
A novel approach to generate 60 GHz optical carrier suppression (OCS) millimeter-wave (mm-wave) signal based on the saturated optical parametric amplification (OPA) effect in high non-linear fiber (HNLF) is investigated. In the proposed system, the OPA effect occurs when the signal and pump with 30 GHz frequency interval are set into the high non-linear fiber. By controlling the length of HNLF, OPA effect saturates, and the pump power is delivered in a large extent to the signal and idler light, so a 60 GHz OCS mm-wave is generated. The system does not need high-speed external modulator, high-frequency vibration source or narrow-band filter, which greatly reduces the cost and improves the stability of the radio over fiber (ROF) system. Results show that the 10 Gb/s downstream signal can be transmitted with negligible power penalty.
A novel approach to generate 60 GHz optical carrier suppression (OCS) millimeter-wave (mm-wave) signal based on the saturated optical parametric amplification (OPA) effect in high non-linear fiber (HNLF) is investigated. In the proposed system, the OPA effect occurs when the signal and pump with 30 GHz frequency interval are set into the high non-linear fiber. By controlling the length of HNLF, OPA effect saturates, and the pump power is delivered in a large extent to the signal and idler light, so a 60 GHz OCS mm-wave is generated. The system does not need high-speed external modulator, high-frequency vibration source or narrow-band filter, which greatly reduces the cost and improves the stability of the radio over fiber (ROF) system. Results show that the 10 Gb/s downstream signal can be transmitted with negligible power penalty.
2010, 6(6):446-448.
Abstract:
In this paper, a new model based on an improved Brent Kung (BK) parallel prefix network (PPN) algorithm is proposed and realized in the field programmable gate array (FPGA). This model is employed in the implementation of 20 Gb/s differential quadrature phase-shift keying (DQPSK) precoder in 40 Gb/s polarization division multiplex (PolDM) DQPSK system. In the computation process, the computation complexity (area) optimization with fan-out limited is achieved. In the implementation, 770 FPGA slice registers are utilized, which save about 60% logic resources compared with the previous Kogge Stone (KS) algorithm.
In this paper, a new model based on an improved Brent Kung (BK) parallel prefix network (PPN) algorithm is proposed and realized in the field programmable gate array (FPGA). This model is employed in the implementation of 20 Gb/s differential quadrature phase-shift keying (DQPSK) precoder in 40 Gb/s polarization division multiplex (PolDM) DQPSK system. In the computation process, the computation complexity (area) optimization with fan-out limited is achieved. In the implementation, 770 FPGA slice registers are utilized, which save about 60% logic resources compared with the previous Kogge Stone (KS) algorithm.
2010, 6(6):449-453.
Abstract:
Tianjin University of Technology and Springer-Verlag Berlin Heidelberg 2010 In this paper the solar-blind ultraviolet (UV) light is used as communication medium for wireless sensor networks (WSNs). Three types of single scattering UV communication models are introduced and a directional flooding model is proposed, which are based on the directionality of the UV communication and the traditional flooding routing model. The delay, delay jitter, throughput and energy consumption of the three types of communications of the new model are simulated and compared. The results indicate that the proposed directional flooding model can effectively avoid the unidirectionality and the blindness of the traditional flooding broadcast messages. The energy consumption of nodes in the network is reduced and the life cycle of the network is extended.
Tianjin University of Technology and Springer-Verlag Berlin Heidelberg 2010 In this paper the solar-blind ultraviolet (UV) light is used as communication medium for wireless sensor networks (WSNs). Three types of single scattering UV communication models are introduced and a directional flooding model is proposed, which are based on the directionality of the UV communication and the traditional flooding routing model. The delay, delay jitter, throughput and energy consumption of the three types of communications of the new model are simulated and compared. The results indicate that the proposed directional flooding model can effectively avoid the unidirectionality and the blindness of the traditional flooding broadcast messages. The energy consumption of nodes in the network is reduced and the life cycle of the network is extended.
2010, 6(6):454-457.
Abstract:
The confinement loss and bend loss properties of all-solid photonic band gap fibers with an array of rings doped with high-index material are investigated. The calculated results show that for a specific structure, the confinement loss and the critical bend radius are reduced simultaneously in some band gaps by increasing the inner diameter of ring, which provides a useful guide and a theoretical basis for designing large mode area fibers with low loss.
The confinement loss and bend loss properties of all-solid photonic band gap fibers with an array of rings doped with high-index material are investigated. The calculated results show that for a specific structure, the confinement loss and the critical bend radius are reduced simultaneously in some band gaps by increasing the inner diameter of ring, which provides a useful guide and a theoretical basis for designing large mode area fibers with low loss.
2010, 6(6):458-461.
Abstract:
The study on the acquisition and tracking simulation system in satellite-ground optical communications is presented here. By applying global positioning system (GPS) coordinate calculation, the time needed in initial acquisition of light beacon can be reduced largely. Smith predictor is applied to compensate the mechanical hysteresis of tracking system and to improve the dynamic performance of the system. Tracking experiments over a 16 km distance on the ground are conducted to verify the tracking of light beacon in satellite-ground optical communications. The standard deviation of horizontal coordinates is 35.3568 μrad and the maximum offset is 209.3675 μrad in stable tracking.
The study on the acquisition and tracking simulation system in satellite-ground optical communications is presented here. By applying global positioning system (GPS) coordinate calculation, the time needed in initial acquisition of light beacon can be reduced largely. Smith predictor is applied to compensate the mechanical hysteresis of tracking system and to improve the dynamic performance of the system. Tracking experiments over a 16 km distance on the ground are conducted to verify the tracking of light beacon in satellite-ground optical communications. The standard deviation of horizontal coordinates is 35.3568 μrad and the maximum offset is 209.3675 μrad in stable tracking.
2010, 6(6):462-465.
Abstract:
A fundamental issue in optical burst switching (OBS) networks is to solve the burst contention for the core node. In this paper, a novel priority-based contention solution strategy for OBS networks is proposed. When the contention occurs, the burst priority is considered firstly, and then the burst segmentation method is used for the low priority bursts in this strategy. Ensuring the integrity of high priority bursts, part of the segmented bursts can be transmitted to the destination node via combining wavelength conversion and optical buffer method. Simulation results show that the proposed scheme not only ensures the integrity of high priority bursts, but also reduces the packet loss rate of the low priority bursts maximally, so that it can support good quality of service (QoS) for the network.
A fundamental issue in optical burst switching (OBS) networks is to solve the burst contention for the core node. In this paper, a novel priority-based contention solution strategy for OBS networks is proposed. When the contention occurs, the burst priority is considered firstly, and then the burst segmentation method is used for the low priority bursts in this strategy. Ensuring the integrity of high priority bursts, part of the segmented bursts can be transmitted to the destination node via combining wavelength conversion and optical buffer method. Simulation results show that the proposed scheme not only ensures the integrity of high priority bursts, but also reduces the packet loss rate of the low priority bursts maximally, so that it can support good quality of service (QoS) for the network.
2010, 6(6):466-468.
Abstract:
The propagation of two femtosecond pulses in the region with minimum group-velocity dispersion, influenced by the cross-phase modulation (XPM) in photonic crystal fiber (PCF), is investigated in this paper. The adaptive split-step Fourier method is used to simulate the generalized nonlinear Schr?dinger equations (GNSE). The numerical results show that the combination of the Raman scattering and XPM can make the spectrum and the pulse shape changed, inducing the pulse compression and supercontinuum generation. The Raman effect from molecular vibrations, which can make the pulse spectrum in anomalous-dispersion region to be more symmetrical, is also discussed.
The propagation of two femtosecond pulses in the region with minimum group-velocity dispersion, influenced by the cross-phase modulation (XPM) in photonic crystal fiber (PCF), is investigated in this paper. The adaptive split-step Fourier method is used to simulate the generalized nonlinear Schr?dinger equations (GNSE). The numerical results show that the combination of the Raman scattering and XPM can make the spectrum and the pulse shape changed, inducing the pulse compression and supercontinuum generation. The Raman effect from molecular vibrations, which can make the pulse spectrum in anomalous-dispersion region to be more symmetrical, is also discussed.
18 A high-performance measurement method for artery intimamedia thickness in medical ultrasound images
2010, 6(6):469-472.
Abstract:
A high-performance measurement method for the artery intima-media thickness (IMT) in medical ultrasound images is proposed. The scheme mainly consists of two parts. The first part is the detection process, including adaptive gray stretching for image enhancing, gradient transforming to restrain the over-segmentation, and the watershed algorithm for edge extraction. The intima can be detected automatically and exactly. The second part, combined with regional positioning and boundary fitting stages, is used to measure the IMT. Experimental results show that the proposed scheme can yield a good measurement result, and the calculation time can satisfy the practical requirement.
A high-performance measurement method for the artery intima-media thickness (IMT) in medical ultrasound images is proposed. The scheme mainly consists of two parts. The first part is the detection process, including adaptive gray stretching for image enhancing, gradient transforming to restrain the over-segmentation, and the watershed algorithm for edge extraction. The intima can be detected automatically and exactly. The second part, combined with regional positioning and boundary fitting stages, is used to measure the IMT. Experimental results show that the proposed scheme can yield a good measurement result, and the calculation time can satisfy the practical requirement.
19 Polarizability and abnormal infrared spectra of molecules adsorbed on metal nanostructured surfaces
2010, 6(6):473-476.
Abstract:
Based on the experimental results of abnormal infrared effects (AIREs) for molecules adsorbed on transition-metal nanostructured surfaces, the interactions and the polarizability of the adsorbed molecules are analyzed theoretically. By numerical simulations, the normal absorption spectrum, the Fano-like bipolar spectrum and the anti-absorption spectrum of the adsorbed CO molecules are obtained respectively. The results show that the adsorbed molecules on a metal nanostructured surface can accumulate together dues to the effect of local external electric field, which leads to a change in imaginary part of the molecular polarizability within a certain range of frequency, and even a negative value may appear.
Based on the experimental results of abnormal infrared effects (AIREs) for molecules adsorbed on transition-metal nanostructured surfaces, the interactions and the polarizability of the adsorbed molecules are analyzed theoretically. By numerical simulations, the normal absorption spectrum, the Fano-like bipolar spectrum and the anti-absorption spectrum of the adsorbed CO molecules are obtained respectively. The results show that the adsorbed molecules on a metal nanostructured surface can accumulate together dues to the effect of local external electric field, which leads to a change in imaginary part of the molecular polarizability within a certain range of frequency, and even a negative value may appear.
2010, 6(6):477-480.
Abstract:
The hand vein recognition system based on digital signal processing (DSP) and complex programmable logic device (CPLD) is designed according to the requirements for equipment volume, accuracy and reaction speed. The overall structure and detailed implementation of the system hardware architecture are discussed in this paper. Moreover, the design philosophy and specific realization of system software as well as core algorithms are explored. The recognition system owns many good characteristics, such as high-degree integration, simple structure, flexible programming, convenient application and so on, which make it suitable for circumstances with high requirements for personal identification.
The hand vein recognition system based on digital signal processing (DSP) and complex programmable logic device (CPLD) is designed according to the requirements for equipment volume, accuracy and reaction speed. The overall structure and detailed implementation of the system hardware architecture are discussed in this paper. Moreover, the design philosophy and specific realization of system software as well as core algorithms are explored. The recognition system owns many good characteristics, such as high-degree integration, simple structure, flexible programming, convenient application and so on, which make it suitable for circumstances with high requirements for personal identification.
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