Submicrosecond speed optical coherence tomography system design and analysis by use of acousto-optics

A novel high-speed no-moving-parts optical coherence tomography (OCT) system is introduced that acquires sample data at less than a microsecond per data point sampling rate. The basic principle of the proposed OCT system relies on use of an acousto-optic deflector. This OCT system has the attractive features of an acousto-optic scanning heterodyne interferometer coupled with an acousto-optic (AO) variable optical delay line operating in a reflective mode. Fundamentally, OCT systems use a broadband light source for high axial resolution inside the sample or living tissue under examination. Inherently, AO devices are Bragg-mode wavelength-sensitive elements. We identify that two beams generated by a Bragg cell naturally have unbalanced and inverse spectrums with respect to each other. This mismatch in spectrums in turn violates the ideal autocorrelation condition for a high signal-to-noise ratio broadband interferometric sensor such as OCT. We solve this fundamental limitation of Bragg cell use for OCT by deploying a new interferometric architecture where the two interfering beams have the same power spectral profile over the bandwidth of the broadband source. With the proposed AO based system, high (e.g., megahertz) intermediate frequency can be generated for low 1/f noise heterodyne detection. System issues such as resolution, number of axial scans, and delay-path selection time are addressed. Experiments described demonstrate our high-speed acousto-optically tuned OCT system where optical delay lines can be selected at submicrosecond speeds.     

一种新型生物芯片扫描平台的研究

扫描工作台是生物芯片扫描仪中的一个关键组成部分,扫描时间是衡量扫描仪性能的一个重要指标。本文中我们报导一种新型的扫描仪工作平台,采用直线电机作为驱动的工作台,取消了从电动机到工作台间机械传动的环节,提高了系统刚度,降低了振动噪声,缩短了采用光学系统静止机构的扫描仪的扫描时间,为快速高精度激光共聚焦生物芯片扫描仪的研制提出了新的实施方案。     

Liquid helium cooled sample stage for scanning electron microscope

A cryogenic stage is described for use with a Cambridge S4-10 scanning electron microscope. The stage is operated with liquid helium in direct contact with the back of the sample to be irradiated by the electron beam. This bath cryostat principle will work at temperatures between 1.5 K and 4.2 K with good cooling performance. The installation of the stage within the microscope does not require any modification of the microscope chamber or the detector arrangement. A precision adjustment of the sample perpendicular to the electron beam is achieved by micrometric screws.     

Resolution improvement with dispersion manipulation and a retrieval algorithm in optical coherence tomography

We propose and demonstrate what is to our knowledge a novel technique of improving the spatial resolution of an optical coherence tomography (OCT) system given a non-Gaussian light source spectrum. By using dispersive materials in the reference arm of the OCT system, the resultant dispersion variation led to a full-width at half maximum (FWHM) of the interference fringe envelope smaller than the Fourier transform-limited value of a Gaussian spectral shape with the same spectral FWHM, at the expense of significant tails. The effects of the tails, which would blur the OCT images, were tremendously reduced with a retrieval algorithm. Simulation results and processed OCT scanning images have shown the capability of the proposed technique.     

Characterization of a novel bioactive composite using advanced X-ray computed tomography

A novel bioactive, porous silica–calcium phosphate nanocomposite (SCPC) that can be used to treat large bone defects in load-bearing positions has been tested and has shown great potential for applications in tissue engineering. Porosity is essential to the performance of the composite material as a tissue engineering scaffold, as porous scaffolds provide a physical, 3-D template to support new tissue formation. However, porosity characterization using conventional techniques such as porosimetry or scanning electron microscopy requires extensive preparation of samples and may destroy important features during preparation and analysis stage. In this work, the new composite is characterized using an advanced high resolution X-ray computed tomography, which is a non-destructive testing technique that allows construction of the 3-D topology of the microstructure. The results clearly show the effectiveness and versatility of this technique in characterizing the porous architecture of the novel composite biomaterial. The pore distribution, morphology and interconnectivity in the composite scaffolds were found to be ideal for use in tissue engineering applications.     

Investigation of glass–fibre reinforced polymers by polarisation-sensitive, ultra-high resolution optical coherence tomography: Internal structures, defects and stress

In this paper, we report on optical coherence tomography (OCT) for the investigation of glass–fibre reinforced polymer (GFRP) materials. OCT, a contactless and non-destructive imaging technique, provides depth-resolved information on internal structures within semi-transparent materials. In our study, we apply a highly advanced OCT technique: we use a transversal scanning, ultra-high resolution, polarisation-sensitive (UHR-PS)-OCT for the analysis of various GFRP samples. UHR-imaging with resolution in the micron range is shown to provide information on the scale of single fibres. PS-OCT, an extension of classical OCT, is sensitive to birefringence and consequently to anisotropies and stress within a material. Our system is therefore applied to the investigation of defects located within the fibre layers of loaded and damaged GFRP samples. Imaging of matrix fracture, cracks, and internal stress, demonstrates the high potential of UHR-PS-OCT for the investigation of damage in GFRP materials, with special emphasis on detecting the early stages of defect formation.     

PSO Based Multi-Objective Approach for Controlling PID Controller

CSTR (Continuous stirred tank reactor) is employed in process control and chemical industries to improve response characteristics and system efficiency. It has a highly nonlinear characteristic that includes complexities in its control and design. Dynamic performance is compassionate to change in system parameters which need more effort for planning a significant controller for CSTR. The reactor temperature changes in either direction from the defined reference value. It is important to note that the intensity of chemical actions inside the CSTR is dependent on the various levels of temperature, and deviation from reference values may cause degradation of biomass quality. Design and implementation of an appropriate adaptive controller for such a nonlinear system are essential. In this paper, a conventional Proportional Integral Derivative (PID) controller is designed. The conventional techniques to deal with constraints suffer severe limitations like it has fixed controller parameters. Hence, A novel method is applied for computing the PID controller parameters using a swarm algorithm that overcomes the conventional controller's limitation. In the proposed technique, PID parameters are tuned by Particle Swarm Optimization (PSO). It is not easy to choose the suitable objective function to design a PID controller using PSO to get an optimal response. In this article, a multi-objective function is proposed for PSO based controller design of CSTR.     

Vacuum-tight sample transfer stage for a scanning electron microscopic study of stabilized lithium metal particles

A vacuum-tight transfer stage is described, that enables air or moisture sensitive specimens to be safely transferred from a protective environment, like that in a dry box, to a scanning electron microscope without exposing the sample to ambient conditions. The transfer stage concept described here is, in fact, applicable to the protected transfer of hygroscopic or air-sensitive samples to any vacuum-based analysis equipment where the transfer device opens automatically under vacuum to reveal the sample. This device concept is also applicable to the transfer of samples under vacuum from one glove box to another where the transfer chamber can be easily opened manually to reveal the sample. An example of one application of the device is provided by a controlled exposure study of stabilized lithium metal particles (SLMP) at various stages of shelf life. We found that a reaction coating formed on “fresh” SLMP after it was exposed to 100 Pa air for as short as 15 m. Such a reaction to air exposure was not observed on “aged” SLMP with a thickened carbonate surface coating after 4 and 6 months of shelf life. The case study of the surface coating on SLMP clearly demonstrated the excellent vacuum performance of the novel transfer stage.     

Artificial fingerprint recognition by using optical coherence tomography with autocorrelation analysis

Fingerprint recognition is one of the most widely used methods of biometrics. This method relies on the surface topography of a finger and, thus, is potentially vulnerable for spoofing by artificial dummies with embedded fingerprints. In this study, we applied the optical coherence tomography (OCT) technique to distinguish artificial materials commonly used for spoofing fingerprint scanning systems from the real skin. Several artificial fingerprint dummies made from household cement and liquid silicone rubber were prepared and tested using a commercial fingerprint reader and an OCT system. While the artificial fingerprints easily spoofed the commercial fingerprint reader, OCT images revealed the presence of them at all times. We also demonstrated that an autocorrelation analysis of the OCT images could be potentially used in automatic recognition systems.     

Optical coherence tomography of glass reinforced polymer composites

Optical coherence tomography (OCT) is a nondestructive and noncontact technique to image microstructure within scattering media. The application of OCT to highly scattering materials such as polymer composites is especially challenging. In this work, OCT is evaluated as a technique to image fiber tows and voids in two materials: an epoxy E-glass-reinforced composite and a vinyl-ester E-glass-reinforced composite. Features detected using OCT are compared with optical microscopy. Fiber architecture and voids of glass-reinforced polymer composites can be successfully imaged using OCT. The quality of the OCT image is strongly affected by the refractive index mismatch between the fibers and reinforcement. The largest sources of noise in the images arise from fiber lens effects, interference from within the sample, and a very large reflection at the surface.     

Liquid-helium stage in a scanning electron microscope

A liquid-helium stage to be inserted into a scanning electron microscope (SEM) is described. The sample chamber, which is filled with liquid helium, has a volume of about 8 × 7 × 5 cm³. Translation of the cooled sample in x, y, and z directions is possible during electron-optical observation. Since mounting the liquid-helium stage to the SEM can be performed in less than 30 min, switching the SEM from regular operation at room temperature to the low-temperature mode and back is relatively easy. After cooling the system to 4.2 K, a 3 litre helium reservoir allows experimentation near 4.2 K for about 5 h.     

A Sliding Mode Flux-Linkage Controller With Integral Compensation for Switched Reluctance Motor

A novel flux-linkage controller using sliding mode technique with integral compensation (SM-I) is proposed for torque ripple minimization of a switched reluctance motor (SRM). The proposed SM-I controller inherits the advantages of proportion–integration (PI) and conventional SM controller. These make it feasible for the flux-linkage controller to reduce torque ripple by correctly selecting the flux ramps in the limit of available dc-link voltage. Moreover, since the controller is not model-based, it avoids the complexity of mathematical modeling and is easily implemented. Experimental results demonstrate that the proposed controller performs better and can be used as an alternative for nonlinear SRM drive systems.      

A new method to obtain Fourier transform infrared spectra free from water vapor disturbance

Infrared absorption bands due to water vapor in the mid-infrared regions often obscure important spectral features of the sample. Here, we provide a novel method to collect a qualified infrared spectrum without any water vapor interference. The scanning procedure for a single-beam spectrum of the sample is divided into two stages under an atmosphere with fluctuating humidity. In the first stage, the sample spectrum is measured with approximately the same number of scans as the background. If the absorbance of water vapor in the spectrum is positive (or negative) at the end of the first stage, then the relative humidity in the sample compartment of the spectrometer is changed by a dry (or wet) air blow at the start of the second stage while the measurement of the sample spectrum continues. After the relative humidity changes to a lower (or higher) level than that of the previously collected background spectrum, water vapor peaks will become smaller and smaller with the increase in scanning number during the second stage. When the interfering water lines disappear from the spectrum, the acquisition of a sample spectrum is terminated. In this way, water vapor interference can finally be removed completely.     

Fabrication and characterization of three-dimensional InGaAs/GaAs nanosprings

This paper presents the use of a novel fabrication technique to produce three-dimensional (3D) nanostructures. The process is based on conventional microfabrication techniques to create a planar pattern in an InGaAs/GaAs bilayer that self-assembles into 3D structures during a wet etch release. The nanostructures are proposed to function as nanosprings for electromechanical sensors. Nanomanipulation inside a scanning electron microscope (SEM) was conducted to probe the structures for mechanical characterization. The results were validated by simulation.     

Design, Identification, and Control of a Flexure-Based XY Stage for Fast Nanoscale Positioning

The design, identification, and control of a novel, flexure-based, piezoelectric stack-actuated XY nanopositioning stage are presented in this paper. The main goal of the design is to combine the ability to scan over a relatively large range (25times25 mum) with high scanning speed. Consequently, the stage is designed to have its first dominant mode at 2.7 kHz. Cross-coupling between the two axes is kept to -35 dB, low enough to utilize single-input--single-output control strategies for tracking. Finite-element analysis (FEA) is used during the design process to analyze the mechanical resonance frequencies, travel range, and cross-coupling between the X- and Y-axes of the stage. Nonlinearities such as hysteresis are present in such stages. These effects, which exist due to the use of piezoelectric stacks for actuation, are minimized using charge actuation. The integral resonant control method is applied in conjunction with feedforward inversion technique to achieve high-speed and accurate scanning performances, up to 400 Hz.     

利用WDM光纤耦合器的光纤光栅传感解调技术

根据 WDM 光纤耦合器波长解调方案的工作原理、偏振特性以及影响系统波长分辨力的因素,提出一种改进的利用 WDM 光纤耦合器的光纤光栅传感解调技术。该技术在原技术的基础上,采用偏振控制器控制入射光偏振状态,提高了解调的精度和稳定性。对 WDM 光纤耦合器的多次波长扫描结果表明,采用偏振控制器后,其波长误差可减小到 5pm 左右。实验采用 1540/1560nm的 WDM 光纤耦合器对单点光纤光栅应变传感器进行静态解调,结果表明:按此技术开发的解调系统具有 0.01nm 波长分辨力和 10nm 的波长线性解调范围。     

A bend-testing stage for the scanning electron-microscope

The design of a stage to bend materials in a scanning electron-microscope (Stereoscan, Cambridge Scientific Instruments Ltd) is described together with examples of its use in the fracture of composite materials. The bend module, when fitted to a large modular stage, can be used with existing scanning electron microscopes and is capable of bending, under increasing load or cyclic loading conditions, rectangular specimens of a maximum size 44×4×1.5 mm, to a maximum strain of 1.5%. Using the module with a standard display system the non-catastrophic stages of deformation and failure can be followed at higher magnification than has previously been possible. When the module is used in conjunction with a fast scanning and display system, deformation and fracture processes may be recorded at high magnifications while the specimen is being strained.At present on leave of absence at Cavendish Laboratory, Cambridge.Patent applied for.     

Enzyme-assisted nanolithography

We have chemically immobilized alkaline phosphatase molecules onto the apex of a tip of an atomic force microscope. When the substrate BCIP is dephosphorylated by alkaline phosphatase, it will precipitate in the presence of NBT. By bringing the tip in the vicinity of a suitable sample, we could locally deposit this complex on the sample. Thus we combined the activity of an enzyme with the accuracy in positioning a tip in scanning probe microscopy to demonstrate a novel technique referred to as enzyme-assisted nanolithography. By use of other enzymes, this method will open the possibility to chemically modify surfaces on a nanometer scale.     

Dispersion compensation in high-speed optical coherence tomography by acousto-optic modulation

We report studies of the analyses of and compensation for group dispersion to improve the axial resolution of high-speed optical coherence tomography (OCT) by acousto-optic modulation (AOM). Theoretical modeling and experiments reveal that the high-order group dispersion induced by acousto-optic crystals broadens the measured coherence length (Lc) and thus degrades the axial resolution of OCT imaging. Based on our experimental studies, we can compensate for the dispersion to less than 50% broadening of the source Lc by adjusting the grating-lens-based optical delay in the reference arm and can further eliminate it by inserting like acousto-optic crystals in the sample arm of the OCT system. The results demonstrate that this AOM-mediated OCT system permits high-performance OCT imaging at A-scan rates of as much as 4 kHz by use of a resonant scanner. Because of its ultrastable direct frequency modulation, this AOM-mediated OCT system can potentially improve the performance of high-speed Doppler OCT techniques.