This PDF file contains the editorial “Teaching Physics” for OE Vol. 43 Issue 04

A thermo-optic Mach-Zehnder (MZ) variable optical attenuator based on silicon waveguides with a large cross section was designed and fabricated on silicon-on-insulator (SOI) wafer. Multimode interferometers were used as power splitters and combiners in the MZ structure. In order to achieve a smooth interface, anisotropic chemical etching of silicon was used to fabricate the waveguides. Isolating grooves were introduced to reduce power consumption and device length. The device has a low power consumption of 210 mW and a response time of 50 µs.

We propose and demonstrate an L-band two-wavelength wavelength-division-multiplexing (WDM) transport system employing chirped fiber gratings (CFGs) to reduce the cross-phase modulation-induced crosstalk, and using dispersion compensation fiber (DCF) to reduce the fiber dispersion. Good performances of composite second order and composite triple beat were obtained accompanied by acceptable carrier-to-noise ratio in our proposed systems.

To the best of our knowledge, proposed and demonstrated for the first time is an optical power independent optical beam profiler instrument. The instrument uses a pair of symmetrically located power detection photodetectors along with an electronically programmed two-dimensional digital micromirror device to implement spatial beam profiling. The instrument also features high-repeatability all-digital controls and fast millisecond reconfigurations. The instrument can have a critical impact in test, measurement, and monitoring systems for various optical beams, including high-power laser beams.

In studying hydrodynamic instabilities between two miscible fluid mixtures, one often faces the problem of assigning a reliable value to very small diffusion coefficients. It is also difficult that for diffusion between two binary mixtures, such as water and glycerin, water and salt, etc., no complete data are available in the literature. To measure the isothermal diffusivity of liquid binary mixtures, we propose an improved version of digital projection moiré. The system uses a simple and flexible fringe generator realized with a diffractive optical element (DOE). The fringe patterns are projected onto the bottom of a ground glass plate. The phase object (diffusion cell) is placed in front of the ground glass (in other words, in front of the fringe pattern), which is imaged by a digital video camera. Grating patterns, during the evolution of diffusion phenomena, are captured by a charge-coupled device (CCD) camera and stored in a computer at different times. With the aid of fast Fourier transform (FFT) and signal demodulating techniques, the images are processed to obtain the diffusion coefficients. The theoretical basis of the technique is presented. We also report experiments conducted for validating the system.

To find the optimal driving voltage and bandwidth for electro-optic polymer-waveguide-based directional couplers, four electrode structures are investigated. They are the microstrip line (MSL), the asymmetric coplanar waveguide with ground (ACWG), the coplanar waveguide with ground (CPWG), and the edge-coupled microstrip line (CMSL). UV15:Polymethyl methacrylate/dispersed-red 1:UV11-3-based directional couplers are evaluated. The CPWG and the ACWG have almost the same driving voltages, which are about twice the driving voltage of the CMSL. The MSL has the largest driving voltage, which is 20 to 25% higher than the diving voltages of the CPWG and ACWG. Simulation results further conclude that the MSL has the largest bandwidth, and its bandwidth is about 1.6 times that of the bandwidth of the CMSL, which has the narrowest bandwidth. The bandwidths of the ACWG and CPWG are 1.4 and 1.2 times that of the CMSL, respectively. The driving voltages for the four different devices are experimentally confirmed.

We have proposed and experimentally demonstrated a new tunable laser structure, which is based on Fabry-Perot (FP) lasers with external lightwave injection. The wavelength tuning can be obtained by adjusting the bias currents of FP lasers. The wavelength tuning time of <2 ns, 3.3-nm tuning range, and the side-mode suppression ratio (SMSR) of >19 dB have been achieved experimentally. In addition, the SMSR performance has also been investigated. This tunable laser has the advantage of simple architecture, potentially low cost, data direct modulation, and fast wavelength tuning, and is expected to benefit the applications of fast wavelength tuning.

The angle-polished method for side-pumping technology that is useful in high-power fiber lasers and fiber amplifiers is investigated. By geometric optics, the simple model for calculation of the coupling efficiency is derived. Numerical simulations show that both the polished angle of multimode fiber and the refractive index of index-matching material are pivotal to the side-coupling efficiency. By experiments, coupling efficiency of 80.2% and maximum coupling power of 1.12 W are measured for a 200-μm multimode fiber and a double-clad fiber with a 400/340-μm D-shaped inner cladding. Slope efficiency of 56% and output power of 282 mW are demonstrated in a D-shaped Yb-doped double-clad fiber laser by the side-pumping system with the angle-polished method.

The minimum spanning tree (MST) matching algorithm has been used for searching the partial matching points extracted from fingerprint images. The method, however, has some limitations. To obtain the relationship between the enrolled fingerprints and the input fingerprints the MST is used to generate the tree graph that represents the unique graph for the given minutiae. From the graph, the matching points are estimated. However, the shape of the graph highly depends on the positions of the minutiae. If there are some pseudo minutiae caused by noise, the shape of the graph will be totally different. To overcome the limitations of the MST, we propose the center of rotation method (CRM) that finds true partial matching points. The proposed method is based on the assumption that the input fingerprint minutiae are rigid body. In addition, we employ the polygon wrapping the minutiae for fast matching speed and reliable matching results. In conclusion we have been able to confirm fast performance and high identification ratio by using CRM.

The selection of initial blocks is one of the important issues in finite-state vector quantizers (FSVQs) for images. Conventional FSVQs use blocks located at fixed positions as initial blocks, which then are coded by the codewords in the super codebook. We propose an adaptive-initial-block (AIB) scheme to determine the initial blocks for gradient-match (GM) and side-match (SM) vector quantizers, which are two significant classes of FSVQs. According to the next-state functions used in GMVQs and SMVQs, the blocks with large boundary matching errors in original VQs are selected as the initial blocks. The error propagation effects can be reduced, and the coding performance is significantly improved. The simulation results show that the peak signal-to-noise ratios of coded images can be 2 dB higher than that of conventional GMVQs and SMVQs when the state codebook sizes are much smaller than that of the super codebook.

We present a new approach for compressing interference multispectral remote sensing images. A new rectangle region-of-interest (ROI) coding algorithm is integrated with an embedded-block-coding optimized truncation (EBCOT) method to make a distortion tracking of the spectral region. Accordingly, a rate-distortion optimizer is designed to give an adaptive lifting of the rate-distortion slopes. This allows the peak signal-to-noise ratio (PSNR) of the reconstructed multispectral region to be guaranteed higher than 38 dB for different image conditions. In this coding scheme, the lifting procedure is carried out in a code stream organization stage instead of after the DWT, as suggested by JPEG2000. As a result, the coarse lifting step situation due to the nature of bitplane shift methods can be improved, which enables flexible adjustment of compression fidelity in the ROI and background (BG), and improves the quality of the BG and the whole image. The Experimental results are encouraging.

An antialiasing image subsampling method using Fourier-based windowing is presented. To reduce resolution, directly subsampling an image results in aliased data. To avoid aliasing, the original image has to be low-pass filtered within the bandwidth of the subsampled image prior to subsampling. However, applying a simple rectangular filter causes ringing effect (ripples), which is also called the Gibbs phenomenon. Conventional finite impulse response (FIR) or infinite impulse response (IIR) filters use spectral windows (e.g., Gaussian, Hamming, Hanning, or Butterworth windows, etc.) to yield a smooth transition band at the cutoff frequency. Yet this approach attenuates the higher frequency contents of the subsampled image and results in the widening of the transition band of the impulse response. An alias-free image subsampling method is developed using a power window to reduce the spatial domain ringing effect and to preserve most of the spatial frequency contents of the subsampled image.

We describe a novel interpolation algorithm to find the optimal image intensity function generating an optimal gray-level estimation of interpolated pixels of digital images. The new approach is based on the proposed image block mapping method and least-square support vector machines (LSSVM) with Gaussian radial basis function (RBF) kernels. With the mapping technique, the interpolation procedure of the LSSVM is actually accomplished in the same input vector space. A number of different scale interpolation experiments are carried out. The experimental results demonstrate that the performance of the proposed algorithm is competitive with many other existing methods, such as cubic, spline, and linear methods. The peak signal-to-noise ratio of the image reconstructed by the proposed algorithm is higher than those obtained by the spline. And the estimated accuracy of the proposed algorithm is similar to that of the cubic algorithm, while the computational requirement is lower than the latter.

We use an extension of normalized convolution to smooth height maps from interferometry using confidence values. The latter are often used for dichotomous good/bad decisions only, with all bad data being discarded. To minimize loss of information, we weight each pixel individually by the inverse of its expected variance. The relation between supplied confidence values and empirical variances is found by regression. The width of the smoothing kernel—as small as possible to prevent loss of spatial resolution, as large as necessary to average out noise—is adjusted locally so as to yield a smoothed image with a prespecified uncertainty that is homogeneous throughout. In our experimental investigations using metrological data from a white light interferometric sensor, the variable-width mask leads to images with somewhat lower absolute deviation from an average image than the fixed-width masks we use for comparison.

A Jamin interferometer-based arrangement for measurement of single-pass wavefront distortion, introduced by optical components such as laser rods, parallel plates, optical windows, etc., is discussed. Phase shifting interferometry is applied using a wedge-type phase shifter. Fabrication techniques for the Jamin plates and wedge phase shifter are described. Results are shown for a laser rod. Advantages and disadvantages of the configuration are discussed.

Electronic speckle shearing pattern interferometry (ESSPI) is a technique that has been used for surface strain and vibration analysis, assuming that simple derivatives are obtained by this technique. However, large shears are usually introduced in the interferometer to increase its sensitivity and to analyze small amplitudes of vibration, limiting this technique to the calculation of approximated derivatives. As strain is related to the exact derivative of the in-plane and out-of-plane deformations, the association with the approximated derivatives can result in reduced accuracy in the quantification of strain. We propose the use of a recently developed phase-recovering technique in stroboscopic ESSPI for improving the overall accuracy of the estimated amplitude of vibration. A successful mode comparison is revealed between a controlled ESSPI experiment with a large shear and finite element (FE) modeling.

An IR diffraction interferometer is proposed for coplanarity measurement of high-density solder bump patterns. The method utilizes long-wavelength (λ= 10.6 μm) coherent infrared laser light, which serves to reduce the apparent roughness of test objects and enables the regularly spaced solder bump arrays to produce well-defined diffracted wavefronts. A single diffracted wavefront is isolated by an optical system and directed to interfere with a reference wavefront to produce a whole-field map of bump topography. An optical configuration similar to classical Fizeau interferometry is implemented to prove the concept.

In order to overcome the limitations of conventional Fourier transform and Gabor transform analyzing nonstationary signals, dilating Gabor transform is applied to analyze the optical fringes of 3-D shape measurement. The dilating Gabor transformation is introduced by using a changeable window of Gaussian function in a conventional Gabor transform. This phase analysis method provides more accurate results than Fourier transform and Gabor transform. Simulation and experimental results are presented that demonstrate the validity of the principle.

An Rb-stabilized diode laser has been developed for use in a high-precision interferometer. The light source is a commercially available external-cavity tunable diode laser. The laser frequency is stabilized to a Doppler-free absorption line of Rb by the third-harmonic technique. The laser emits an output beam with a high power (more than 7 mW) and fast frequency modulation (10 kHz). The relative optical frequency uncertainty of 4.3×10^–10 is achieved for a 0.01-s averaging time.

Vibration-induced jitter degrades the pointing and imaging performance of precision optical systems. Practical active jitter reduction is achieved by maintaining beam alignment with mirror-positioning control systems. In the presence of time-varying or uncertain disturbances, jitter control systems using fixed-gain feedback control loops cannot operate without significant limitations on their performance. A feedback control technique called Q-parameterization can adapt to time-varying disturbances by adjusting its parameters in real time to maintain optimal performance. Adaptive feedback jitter control using Q-parameterization is experimentally verified on an optical testbed, increasing jitter reduction compared to an H₂-optimal fixed-gain controller.

New classes of codes, namely extended modified prime sequence codes (EMPSC) and difference codes, both having an ideal in-phase cross-correlation (IPCC) value of one, are suggested for spectral-amplitude-coding optical code-division multiple-access communication systems. The general structure of the system is discussed with simpler encoder and decoder architectures based on fiber Bragg gratings. Since the influence of multiuser interference can be completely removed due to ideal IPCC, the performance analyses of the proposed codes have been carried out by incorporating the effects of phase-induced intensity noise (PIIN), shot noise, and thermal noise at the receiver, employing Gaussian approximation. Assuming typical system parameters, evaluation results show that with an increase in code weight, the maximum number of accommodative users in the system can be increased as well as significant improvements in bit error probability for a given number of active users. For the same code weight, the difference codes can support a greater number of simultaneous users than the other. It is found that the effects of PIIN dominate compared to shot noise and thermal noise at large powers, whereas the effects of PIIN and shot noise are highly reduced at low powers, so that thermal noise becomes a more influential limitation factor. In comparison with earlier codes, the error performance of EMPSC is similar to that of modified quadratic congruence codes, while difference codes resemble those codes developed based on Steiner systems.

A portable transceiver for indoor wireless link that employs a transmitter of eye-safe infrared light-emitting diodes and a receiver of photodiode arrays with multichannel transimpedance-summer architecture is presented. The transmitter can attain a wide field of view (up to 55 deg half-angle) and high speed (up to 35 MHz) to support different intensity modulation schemes. The receiver can receive signals with a bit error rate (BER) of 10^–4 at a plane of 2 m away from the transmitter, even at a point ±50 deg off the transmitter's vertical axis. The bit rate of the transceiver can achieve up to 40 Mbit/s in an indoor nondirected infrared wireless link, and can be extended to 100 Mbit/s when light-emitting diodes (LEDs) with higher cut frequency are used. The system is able to transmit real-time uncompressed video 320×240 in frame size.

Optical phased arrays using birefringent liquid crystals can efficiently steer a laser communication beam to a small angle. The algorithm presented derives a thickness for the liquid crystal cell and computes a sequence of voltages for the device electrodes that best deflects the beam.

Two-dimensional holographic frequency-division demultiplexing using an acousto-optic modulator is proposed. In the proposed scheme, the image multiplexing side and the demultiplexed side are connected by one optical transmission facility. By using a parallel-aligned nematic liquid-crystal spatial light modulator (PAL-SLM) as a thin hologram, we successfully demonstrated image demultiplexing with carrier frequencies of approximately 80 MHz.

The nonlinear interactions between two ultrashort pulses with temporal widths of less than 100 fsec are numerically studied in all-optical phase-shift switches. Specifically, the influence of self- and cross-Raman scattering on the switching performance is investigated. The energy exchange resulting from cross-Raman scattering during a collision is found to affect the phase-shift resulting from cross-phase modulation. It is also shown that a fiber with a low birefringence can be used in a switching scheme based on a higher-order soliton pulse due to the large self-Raman delay difference between the fundamental soliton pulse and the higher-order soliton pulse.

According to the spectral response formula of multialkali photocathode, a simulation of spectral response of super S₂₅ and new S₂₅ photocathodes is carried out. Some characteristic parameters are obtained. The internal mechanism of good properities and some technical methods, which could probably be used in production, are discussed through analyzing the results of the simulation, which is referential to the research in the theory and technological process.

A high-accuracy absolute measurement system of the refractive index of air is proposed. The measurement of the instantaneous index of refraction is achieved by comparing two laser optical paths with a differential laser interferometer. The vacuum optical path length is varied via a motor-driven trapezoidal vacuum cavity moving through a known distance. Compared to other measurement methods, the sealed vacuum cavity requires no vacuum pumping between measurements, and allows this method to function well in industrial applications. The measurement accuracy is better than 5×10^–8.

We compare images from the Enhanced Thematic Mapper Plus (ETM+) sensor on Landsat-7 and the Advanced Land Imager (ALI) instrument on Earth Observing One (EO-1) over a test site in Rochester, New York. The site contains a variety of features, ranging from water of varying depths, deciduous/coniferous forest, grass fields, to urban areas. Nearly coincident cloud-free images were collected one minute apart on 25 August 2001. We also compare images of a forest site near Howland, Maine, that were collected on 7 September 2001. We atmospherically corrected each pair of images with the Second Simulation of the Satellite Signal in the Solar Spectrum (6S) atmosphere model, using aerosol optical thickness and water vapor column density measured by in situ Cimel sun photometers within the Aerosol Robotic Network (AERONET), along with ozone density derived from the Total Ozone Mapping Spectrometer (TOMS) on the Earth Probe satellite. We present true-color composites from each instrument that show excellent qualitative agreement between the multispectral sensors, along with gray-scale images that demonstrate a significantly improved ALI panchromatic band. We quantitatively compare ALI and ETM+ reflectance spectra of a grassy field in Rochester and find ≤6% differences in the visible/near infrared and ~2% differences in the short-wave infrared. Spectral comparisons of forest sites in Rochester and Howland yield similar percentage agreement except for band 1, which has very low reflectance. Principal component analyses and comparison of normalized difference vegetation index histograms for each sensor indicate that the ALI is able to reproduce the information content in the ETM+ but with superior signal-to-noise performance due to its increased 12-bit quantization.

We describe a simple way to effectively measure the height of a micro-solderball of a micro-ball grid array (micro-BGA) on a bumped wafer. The proposed method derived using a white-light illumination source can perform a speedy whole field inspection of the micro-solderballs. The optical system consists of a white-light telecentric configuration and a long-working-distance microscope with a telecentric objective. The optical axes of the illuminating light and microscope are symmetrically arranged with respect to the normal of the test surface and hence the technique is applicable to optical measurement on a mirror-like test surface, such as a silicon wafer. The height of a solderball is computed from a simple formula based on the geometry of the specimen and the system configuration. Experimental results demonstrate that the height of a micro-solderball can be readily measured with accuracy in the order of micrometers.

Coatings with layer structures of Ag/glass, Ag/Cr/glass, and Ag/Cr-Ag/Cr/glass deposited with magnetron sputtering are investigated. The results indicate that the performance for reflectance, hardness, adherence, and humidity durability of the silver coatings with Ag/Cr/glass and Ag/Cr-Ag/Cr/glass structures are better than pure silver film. In addition, the silver coatings with an Ag/Cr-Ag/glass structure present more advantages than that of the Ag/Cr/glass. Reasons are analyzed accordingly.

We propose a spectral matching imager using light-emitting diodes (LEDs) of narrowband spectral power distributions. Each spectral channel of the LED illumination undergoes amplitude modulation (AM) with one of an orthonormal set of carrier signals. This imager, which employs the correlation image sensor (CIS) as its imaging device, performs spectral correlation pixelwise between a reference spectrum and that of the object in every frame by demodulating the orthonormal AM carriers via temporal correlation. This sensing principle enables a higher efficiency in the use of illumination power than the spectral matching imager we originally proposed. A twelve-channel AM-coded multispectral light source is developed by assembling commercial LEDs. Under this LED light source, experiments were carried out on pairs of glass pieces of a similar color, but with different spectral transmittance functions. The results confirmed that the proposed imager successfully performed spectral matching on each pair of glass pieces in real time.

A quasi-continuous-wave diode array side-pumped Q-switched Yb:YAG slab laser has been demonstrated. A KD*P crystal is used as an electro-optic Q-switch and 3.07-mJ output energy with a pulse width as short as 56.27 ns (FWHM) is obtained. The Yb:YAG crystal doped density is 10 at.% with the dimension 8×3×1 mm. The pulse width and energy fluctuations are 3.44% and 3.36% respectively. The relative Q-switched theory is analyzed and rate equations are calculated. The influence of Q-switch delay time on pulse performance is also studied. The final experimental results coincide well with the theoretical results.