Boon Yi Soon

One-dimensional photonic crystal optical limiter

We explore a new passive optical limiter design using transverse modulation instability in the one-dimensional photonic crystal (PC) using x(3) materials. The performance of PC optical limiters strongly depends on the choice of the materials and the geometry and it improves as the duration of the incident pulse is extended. PC optical limiter performance is compared with that of a device made from homogeneous material. We identify three criteria for benchmarking the PC optical limiter. We also include a discussion of the advantages and disadvantages of PC optical limiters for real world applications.

Spatio-temporal instabilities for counter-propagating waves in periodic media

Nonlinear evolution of coupled forward and backward fields in a multi-layered film is numerically investigated. We examine the role of longitudinal and transverse modulation instabilities in media of finite length with a homogeneous nonlinear susceptibility c((3)). The numerical solution of the nonlinear equations by a beam-propagation method that handles backward waves is described.

Using the joint transform correlator as the feature extractor for the nearest neighbor classifier

Financial transactions using credit cards have gained popularity but the growing number of counterfeits and frauds may defeat the purpose of the cards. The search for a superior method to curb the criminal acts has become urgent especially in the brilliant information age. Currently, neural-network-based pattern recognition techniques are employed for security verification. However, it has been a time consuming experience, as some techniques require a long period of training time. Here, a faster and more efficient method is proposed to perform security verification that verifies the fingerprint images using the joint transform correlator as a feature extractor for nearest neighbor classifier. The uniqueness comparison scheme is proposed to improve the accuracy of the system verification. The performance of the system under noise corruption, variable contrast, and rotation of the input image is verified with a computer simulation.

Joint wavelet transform correlation with separated target and reference planes.

In recent years, we realize the usefulness of feature extraction for optical correlator and hereby, we investigate the capability of Laplace operator in feature extraction of multiple targets. The first-order terms and the false alarm terms in the correlation output would be removed using electronic power spectrum subtraction technique. Most importantly, the entire magneto-optic SLM is completely utilized for displaying only targets in the input scene. A new cost efficient hardware implementation is proposed and aforementioned result of the proposed system is evaluated through computer simulation.

The fast Fourier transform method and ill-conditioned matrices

We study the problem of finding numerical solutions of the linear algebraic equation, a*x=b, where a denotes an NxN ill-conditioned coefficient matrix. It is well-known that Gaussian elimination methods coupled with pivoting strategies are ineffective in this setting due to round-off error. We propose a new and simple application of the fast Fourier transform (FFT) method. Other viable methods, such as the QR method (QRM) or the singular value decomposition method (SVDM), have been proposed in the literature. The goal of this paper is to investigate the performance of the proposed method and compare it to other popular methods. The comparison is illustrated by computer simulation results using MATLAB.

Efficient edge enhancement using real time optical correlation

In recent years, feature extraction using an optical correlator shows promising edge enhancement potential. Hereby, we investigate the performance of the Laplace and Sobel operators in feature extraction for multiple targets. The undesired terms such as the zero-order and false alarm terms are removed by the power spectrum subtraction technique. Also, the separated target and reference planes method is incorporated into the joint wavelet transform correlator to maximize the space-bandwidth product of a magneto-optic spatial light modulator. A cost effective and efficient optoelectronic hardware is proposed for realtime optical correlation. The aforementioned result is verified by computer simulation for both cases; in noise free and in overlapping additive noise corruption situations using both the Laplace and Sobel operators. The Laplace operator could be implemented with lesser steps and is most appropriate for real-time correlation. The Sobel operator yields better edge information and has higher tolerance to noise corruption but required additional correlation steps at the expense of more computation time.

One-dimensional photonic bandgap optical limiter design

We propose a compact, passive optical limiter incorporating nonlinear materials in a multi-layer photonic bandgap (PBG) structure. A coupled-mode theory that includes interaction between counter-propagating optical pulses and diffraction describes the complex transmission and reflection coefficients. Two materials with different nonlinear refraction and absorption coefficients make up the individual layers. Transverse modulational instabilities appear as the intensity increases; this effect is exploited to improve the optical limiter performance by inserting an aperture in the system. Investigating the role that material combinations play in determining the transmission characteristics optimizes the device design. Our results indicate that PBG devices can improve the dynamic range by more than a factor of two over homogeneous material designs.

Optical security verification for blurred fingerprints

Optical fingerprint security verification is gaining popularity, as it has the potential to perform correlation at the speed of light. With advancement in optical security verification techniques, authentication process can be almost foolproof and reliable for financial transaction, banking, etc. In law enforcement, when a fingerprint is obtained from a crime scene, it may be blurred and can be an unhealthy candidate for correlation purposes. Therefore, the blurred fingerprint needs to be clarified before it is used for the correlation process. There are a several different types of blur, such as linear motion blur and defocus blur, induced by aberration of imaging system. In addition, we may or may not know the blur function. In this paper, we propose the non-singularity inverse filtering in frequency/power domain for deblurring known motion-induced blur in fingerprints. This filtering process will be incorporated with the pow spectrum subtraction technique, uniqueness comparison scheme, and the separated target and references planes method in the joint transform correlator. The proposed hardware implementation is a hybrid electronic-optical correlator system. The performance of the proposed system would be verified with computer simulation for both cases: with and without additive random noise corruption.

Edge detection using joint transform correlation

A new joint wavelet transform correlation based technique is proposed for feature extraction such as detection of edges in an unknown input scene. We exploited a modified version of the Roberts and Sobel wavelet filters as the reference images for extracting the edges of an unknown input scene. The performance of the proposed technique using the aforementioned wavelet filters are evaluated and compared using numerical simulations.