Optical spatial modulation （OSM） is a multiple-transmitter technique that can provide higher data rates with low system complexity as compared with single-input single-output systems. Orthogonal frequency division m...Optical spatial modulation （OSM） is a multiple-transmitter technique that can provide higher data rates with low system complexity as compared with single-input single-output systems. Orthogonal frequency division multiplexing （OFDM） is widely implemented to achieve better spectral efficiency in wireless channels. Asymmetrically clipped optical OFDM （O-OFDM） and DC-biased O-OFDM are two well-known O-OFDM techniques suitable for intensity-modulation direct-detection optical systems. In this work, sample indexed spatial OFDM （SIS-OFDM） is proposed to combine OSM and O-OFDM in a novel way and achieve significant per- formance gain. By assigning time-domain samples of the O-OFDM transmit symbol to different transmitters, SIS-OFDM achieves much better spectral efficiency and reduces computational complexity at the transmitter as compared with previous work that combines OSM with O-OFDM in the frequency domain. We also consider the impact of optical source biasing on overall performance, and the relative performance of imaging receiver （ImR） versus non-imaging receiver （NImR） design for our proposed SIS-OFDM technique. Results indicate that for an Ntx x Nrx multiple-input multiple-output configuration where Nix = N = 4, SIS-OFDM using ImR can achieve up to 135 dB of signal-to-noise ratio gain over comparable system using a NImR. Also, using Nc number of O-OFDM subcarriers provides up to Nsc × log2（Ntx） additional bits per symbol of spectral efficiency over technioues that combine OSM and O-OFDM in the freollencv domain.展开更多
As an additive manufacturing technique,the laser-aided direct energy deposition(L-DED)method has been widely used for component repair(also called laser repair).With significant differences from traditional manufactur...As an additive manufacturing technique,the laser-aided direct energy deposition(L-DED)method has been widely used for component repair(also called laser repair).With significant differences from traditional manufacturing techniques,the laser repair process has the characteristics of point-by-point deposition and has a high-temperature gradient in the repair area,resulting in the formation of heterogeneous thermal deformation and residual stress after cooling.High stress may lead to the appearance of cracks in the repair area and may seriously influence the bearing capacity of the repaired parts.Therefore,the characterization of the temperature and deformation fields of the components during laser repair is important for the analysis of the mechanism of damage evolution in the repair area,optimization of the process parameters,and improvement of the mechanical properties of the repaired components.Because of the demand for the simultaneous measurement of the temperature and deformation fields,using a multispectral camera and a self-designed three-peak filter,a temperature-deformation field measurement technique was developed and applied to in situ monitoring during the L-DED manufacturing process.In the actual measurement process,the synchronous measurement of the temperature field of the laser molten pool and the deformation field of the side surface of the repaired component were realized using the images of multiple channels in the multispectral camera.The experimental results verify that a three-peak filter can effectively eliminate the influences of glare and overexposure on the recorded multispectral images.Moreover,the amplitude of the displacement field and the temperature gradient of the repaired component will increase with the increase in laser power,which may affect the final molding of the repaired component.This work extends the function of the multispectral camera to measure the temperature and deformation fields and provides a new measurement method for further optimizing the process parameters of laser repair.展开更多
Recent noteworthy developments in the field of two-dimensional（2D） correlation spectroscopy are reviewed.2D correlation spectroscopy has become a very popular tool due to its versatility and relative ease of use.The...Recent noteworthy developments in the field of two-dimensional（2D） correlation spectroscopy are reviewed.2D correlation spectroscopy has become a very popular tool due to its versatility and relative ease of use.The technique utilizes a spectroscopic or other analytical probe from a number of selections for a broad range of sample systems by employing different types of external perturbations to induce systematic variations in intensities of spectra.Such spectral intensity variations are then converted into2 D spectra by a form of correlation analysis for subsequent interpretation.Many different types of 2D correlation approaches have been proposed.In particular,2D hetero-correlation and multiple perturbation correlation analyses,including orthogonal sample design scheme,are discussed in this review.Additional references to other important developments in the field of 2D correlation spectroscopy,such as projection correlation and codistribution analysis,were also provided.展开更多
基金supported by the Engineering Research Centers Program of the National Science Foundation under Grant No.EEC-0812056
文摘Optical spatial modulation （OSM） is a multiple-transmitter technique that can provide higher data rates with low system complexity as compared with single-input single-output systems. Orthogonal frequency division multiplexing （OFDM） is widely implemented to achieve better spectral efficiency in wireless channels. Asymmetrically clipped optical OFDM （O-OFDM） and DC-biased O-OFDM are two well-known O-OFDM techniques suitable for intensity-modulation direct-detection optical systems. In this work, sample indexed spatial OFDM （SIS-OFDM） is proposed to combine OSM and O-OFDM in a novel way and achieve significant per- formance gain. By assigning time-domain samples of the O-OFDM transmit symbol to different transmitters, SIS-OFDM achieves much better spectral efficiency and reduces computational complexity at the transmitter as compared with previous work that combines OSM with O-OFDM in the frequency domain. We also consider the impact of optical source biasing on overall performance, and the relative performance of imaging receiver （ImR） versus non-imaging receiver （NImR） design for our proposed SIS-OFDM technique. Results indicate that for an Ntx x Nrx multiple-input multiple-output configuration where Nix = N = 4, SIS-OFDM using ImR can achieve up to 135 dB of signal-to-noise ratio gain over comparable system using a NImR. Also, using Nc number of O-OFDM subcarriers provides up to Nsc × log2（Ntx） additional bits per symbol of spectral efficiency over technioues that combine OSM and O-OFDM in the freollencv domain.
基金the National Natural Science Foundation of China(Grant Nos.12032013 and 11972209)the National Key Research and Development Program of China(Grant No.2017YFB1103900)。
文摘As an additive manufacturing technique,the laser-aided direct energy deposition(L-DED)method has been widely used for component repair(also called laser repair).With significant differences from traditional manufacturing techniques,the laser repair process has the characteristics of point-by-point deposition and has a high-temperature gradient in the repair area,resulting in the formation of heterogeneous thermal deformation and residual stress after cooling.High stress may lead to the appearance of cracks in the repair area and may seriously influence the bearing capacity of the repaired parts.Therefore,the characterization of the temperature and deformation fields of the components during laser repair is important for the analysis of the mechanism of damage evolution in the repair area,optimization of the process parameters,and improvement of the mechanical properties of the repaired components.Because of the demand for the simultaneous measurement of the temperature and deformation fields,using a multispectral camera and a self-designed three-peak filter,a temperature-deformation field measurement technique was developed and applied to in situ monitoring during the L-DED manufacturing process.In the actual measurement process,the synchronous measurement of the temperature field of the laser molten pool and the deformation field of the side surface of the repaired component were realized using the images of multiple channels in the multispectral camera.The experimental results verify that a three-peak filter can effectively eliminate the influences of glare and overexposure on the recorded multispectral images.Moreover,the amplitude of the displacement field and the temperature gradient of the repaired component will increase with the increase in laser power,which may affect the final molding of the repaired component.This work extends the function of the multispectral camera to measure the temperature and deformation fields and provides a new measurement method for further optimizing the process parameters of laser repair.
文摘Recent noteworthy developments in the field of two-dimensional（2D） correlation spectroscopy are reviewed.2D correlation spectroscopy has become a very popular tool due to its versatility and relative ease of use.The technique utilizes a spectroscopic or other analytical probe from a number of selections for a broad range of sample systems by employing different types of external perturbations to induce systematic variations in intensities of spectra.Such spectral intensity variations are then converted into2 D spectra by a form of correlation analysis for subsequent interpretation.Many different types of 2D correlation approaches have been proposed.In particular,2D hetero-correlation and multiple perturbation correlation analyses,including orthogonal sample design scheme,are discussed in this review.Additional references to other important developments in the field of 2D correlation spectroscopy,such as projection correlation and codistribution analysis,were also provided.
