This PDF file contains the editorial “The Color of Knowledge” for OE Vol. 42 Issue 10

A novel and improved waveband-switchable fiber laser based on a semiconductor optical amplifier and sampled fiber Bragg gratings is proposed using high polarization selectivity of a polarization beam splitter. A multiwavelength-switching operation of five laser lines with 0.8-nm wavelength spacing, whose switching displacement was ~7 nm, has been successfully demonstrated. The switching displacement between wavebands, wavelength channels, and their spacings can be flexibly determined by the selected comb filters.

We propose a volume-holographic optical element to perform 3-D object filtering. Theoretical analysis as well as the corresponding experimental measurement of the shifting tolerance of the volume-holographic filter are demonstrated.

An efficient analysis method for ytterbium-sensitized erbium-doped optical waveguide amplifiers (EDWA) is presented. By solving rate equations, the propagation in EDWA is stimulated using the finite difference beam propagation method (FD-BPM). The dependence of gain of EDWA is investigated regarding the waveguide length, erbium concentration, and pump efficiency, etc.

A simple but effective filtering scheme in phase shifting interferometry is proposed. Conventionally, either the phase-shifted fringe patterns, or the resulting wrapped phase field, is chosen for filtering to remove noise. In this letter, we propose filtering the complex field resulting from the combination of the phase-shifted fringe patterns, as a third choice. The filtering can be realized by Fourier transform or windowed Fourier transform. Processing results of speckle fringes are given to show its effectiveness.

An improved approach to locating an object that has undergone an affine transformation is presented. This approach is based on the so-called "smallest square box distance transform" introduced in this paper. Experimental results show that the proposed approach distinctly outperforms Rucklidge's fast methods.

Based on clean techniques and data reconstruction, a novel method to detect invisible moving targets in a foliage penetration ultra-wide-band synthetic aperture radar (FOPEN UWB SAR) image is proposed. The clean techniques are used to suppress clutter inside the region of interest (ROI) and the data reconstruction is used to reject clutter outside the ROI. Radar experimental results shows that a 10-dB gain of signal-to-clutter ratio is obtained in the range compression data domain and the invisible moving targets are detected only using a single-channel FOPEN UWB SAR.

This PDF file contains the editorial “Advances in Optical Waveguide Engineering” for OE Vol. 42 Issue 10

We present the realization and characterization of a distributed feedback (DFB) laser matrix realized on an Er/Yb codoped glass substrate. Output power characteristics such as power efficiency and relative intensity noise (RIN) are presented. Wavelength characteristics including linewidth, wavelength stability versus temperature, and emission wavelength calibration are also investigated. Concerning output power, a 4.5-mW fiber-coupled emission is achieved and a RIN falling below –150 dB/Hz after 4 MHz is measured. The matrix contains 15 lasers emitting at 100 or 25-GHz spaced frequencies with a very narrow linewidth measured to be 3 kHz. Finally, a 1.3-GHz/°C temperature dependency of the comb of wavelength is determined.

Among the various materials currently being investigated for further development of Er³⁺-doped waveguide amplifiers (EDWAs), tellurite glasses are gaining attention because of their broad emission band around 1.5 μm. We report our results on the fabrication of optical waveguides in two kinds of these glasses. Tungsten-tellurite and zinc-tellurite glasses containing sodium and doped with various Er₂O₃ percentages have been prepared and characterized. The Judd-Ofelt analysis was carried out on some of these glasses. Experimentally, broad emission and absorption bands corresponding to transition between ⁴I_13/2 ↔ ⁴I_15/2 were observed as expected. Lifetimes of the ⁴I_13/2 level were also measured. Ion-exchanged planar waveguides were successfully obtained in both types of glasses and characterized by the prism coupling technique; the diffusion process was investigated for the different erbium ion concentrations. All the measured characteristics indicate that both these tellurite glasses are promising for the development of broadband integrated optical amplifiers.

Accurate knowledge of the diffusion coefficients of an ion-exchange glass is crucial for the design, simulation, and optimization of optical waveguide structures created in the glass. We present the measurement of the Stewart coefficient, the maximum refractive index, and the diffusion coefficients for Ag⁺ and Na⁺ ions in IOG-10 glass. With knowledge of these glass properties, we can in turn control the optical and physical properties of the waveguides. Temperature dependence of the optical properties is also presented.

The performance of compact multimode pumped erbium-doped phosphate fiber amplifiers is presented. A fiber amplifier with a small signal net gain of 41 dB at 1535 nm and 21 dB over the full C-band is demonstrated using a newly developed 8-cm-long erbium-doped phosphate fiber excited with a 1-W, 975-nm multimode laser diode. A theoretical model is developed for the multimode pumped amplifier based on modified rate equations and an effective beam propagation method. Close agreement between experimental and modeling results is observed.

Recent advances within the realization of silica-based planar waveguide circuitry are presented. This ranges from the production methods for planar waveguides, including a novel method based on the utilization of focused UV-laser beams for direct waveguide imprinting, to the functionalities that are embedded into the glass materials and waveguide circuitry. Planar waveguide amplifiers, lasers, and the pursuit to obtain highly nonlinear materials to realize purely glass-based switches, modulators, and wavelength converters are also presented. Furthermore, microring resonators are discussed, and finally the latest results within 2-D photonic bandgap structures are reviewed.

A novel rib waveguide-integrated switch is proposed and theoretically discussed. The device is based on the total internal reflection phenomenon (TIR) and the thermo-optic effect (TOE) in hydrogenated amorphous silicon (a-Si:H) and crystalline silicon (c-Si). It takes advantage of a bandgap-engineered a-Si:H layer to explore the properties of an optical interface between materials showing similar refractive indexes but different thermo-optic coefficients. In particular, thanks to modern plasma-enhanced chemical vapor deposition techniques, the refractive index of the amorphous film can be properly tailored to match that of c-Si at a given temperature. TIR may be achieved, however, at the interface by acting on the temperature, because the two materials have different thermo-optic coefficients. The switch is integrated in a 4-μm-wide and 3-μm-thick single-mode rib waveguide. The substrate is a silicon-on-insulator (SOI) wafer with an oxide thickness of 500 nm. The active middle region optimal length is 282 μm. The device performance is analyzed at a wavelength of 1.55 μm. It is shown that the output crosstalk and insertion loss are less than –26 and 3.5 dB, respectively.

We design and fabricate digital thermo-optic switches using low-loss photocuring fluoroacrylate polymers. We use both modeling and experimental design-rule studies to elucidate the contributions to loss and cross talk of the various important building blocks that comprise a Y-branch digital optical switch (YDOS). We present the results of these studies as well as the achieved performance for 1×2, 2×2, and 4×4 switches fabricated with these designs. Average fiber-to-fiber chip- level insertion loss (IL) values for the three designs are 1.1, 1.5, and 2.8 dB, respectively, for 1550-nm illumination. Switching times in every case are below 3 ms. Polarization-dependent loss is less than 0.1 dB at 1550 nm. Fully packaged permanently pigtailed versions of the 2×2 and 4×4 switches are also constructed. For these packaged devices, average insertion losses of 2.0 and 3.3 dB are achieved, and cross talk is maintained at a value less than –45 dB. Recent advances in materials performance have now allowed the insertion loss performance to be further improved, with chip-level IL being reduced to 1.0 and 2.2 dB for 2×2 and 4×4 switches, respectively.

In the last few years, the growth of data traffic has increased the interest in broadband integrated optical devices. Their applications include, for example, a possibility of bidirectional fiber communications in a single fiber, which is achievable by adding the transmission capacity of two optical telecommunication windows for local area networks (LAN) and wide area networks (WAN) or by combining pump and signal wavelengths in rare earth-doped integrated optical amplifiers. A promising technology to realize those devices is ion exchange in glass. This allows the integration of different functions on a glass substrate with efficient results and a better compatibility in fiber systems at a low cost. We propose an original broadband duplexer based on a leaky structure, which involves a nonresonant coupling and ensures a broadband spectral behavior. The broadband duplexer is first presented, and the focus is specially made on the improvement of the output cross talk through the suppression of parasitical back reflections. Theoretical optimization and validation by simulations follow. The spectral response of the optimized structure displays two duplexing bands of at least 200 nm in width with a total output channels' isolation within 0.6- to 0.8-μm range, and a better than 15-dB isolation level within 1.2- to 1.4-μm range, with a maximum of 24-dB isolation at 1.31 μm.

A composite fiber Bragg grating (FBG) structure with several apodized sections is utilized for designing dense wavelength division multiplexing (DWDM) multiband transmission filters. A learning genetic algorithm (LGA) is also developed to determine the optimum design parameters of these filters. By taking advantage of a knowledge base (KB) that stores the FBG parameter sets and the corresponding transmission profile feature sets, our LGA can generate a suitable initial population and perform evolutionary optimization starting from it. This has made the LGA evolve more quickly to more accurate results than the methods without using the KB. The LGA can also store new results into the KB according to its decision procedure and improve its precision of initial prediction as it works through more and more examples.

A novel method has been developed for measuring the rotational angle of a fiber's or a waveguide's polarization axis with respect to a reference angle. The reference angle is the polarization axis of the measuring device. The method also gives the true polarization extinction ratio of the measured fiber or waveguide. The method is suitable for the characterization and rotational alignment of polarization-maintaining waveguides and fibers. In particular, the method can be used to rotationally align the fiber-waveguide interconnections during waveguide characterization. The measuring device is either a linear polarizer or a polarization splitter that is accurately rotated with respect to the device under test. According to the experiments with a polarization-maintaining fiber, the method is very easy and inexpensive to implement, and the angular accuracy can be better than 0.2 deg.

Compared to conventional lenses, gradient index lenses are lighter in weight, smaller in volume, and easy to couple. To facilitate the measurement of a small irregular surface in a limited space, a small optical inspection system is presented, which combines the high axial resolution and absolute position tracking capability of differential confocal microscopy and the compact design of a gradient index lens, and uses the gradient index lens to obtain a small detector 1 mm in diam. The operating principle and construction of the measurement system are detailed, and the design of gradient index lenses is discussed. This differential confocal measurement system features high lateral resolution and depth-discriminated sections. The differential design of the two detectors eliminates both the fluctuation of light source and the electronic noise from probes, and increases the axial resolution as well. With a 1-mm-diam probe an axial resolution of 5 nm is obtained for gauge blocks less than 20 deg. In addition to its application for measurement of a small irregular surface in limited space, the sensor system proposed can be fitted onto a coordinate measuring machine for noncontact measurement of dimensions and surface roughness, and incorporated into a CNC machining center for on-line quality monitoring.

Based on a type of nested cw Raman fiber laser, the loss characteristics of the Raman fiber, fiber Bragg gratings, and splicing in the laser cavity are described. Their impacts on laser performance are analyzed through numerical modeling. The optimal cavity parameters are obtained and agree well with the predicted laser output in the experimental result. Meanwhile, the spectral broadening of the laser output and its influence on the simulation accuracy are discussed for the first time in nested Raman fiber lasers. With the improved modeling, more accurate simulations are realized.

A fiber laser is constructed with an erbium-doped fiber, which has an elliptical core and is single-moded along the minor axis and multimoded along the major axis. The laser can emit simultaneously several stable lines at room temperatures. The maximum number of lasing lines obtained depends on the number of modes supported by the erbium-doped fiber in the cavity. The separation between the lasing lines depends on the fiber birefringence. The stability of the lasing lines in the presence of anisotropic gain effects in a polarization-maintaining fiber is explored.

A public-key image hiding scheme is proposed to offer safe exchange of a stego image on the Internet. In the proposed scheme, the secret image is first compressed by quadtree partition vector quantization (QPVQ) coding into the indices bit stream, which is then embedded into the discrete cosine transform (DCT) of the cover image. In the embedding process, adaptive quantization and appropriate DCT coefficients are used to enhance the imperceptibility of the stago image. On the other hand, the Rivest, Shamir, and Adleman (RSA) algorithm is hired to provide the mechanism of a public-key system for secure communication on the Internet. Our proposed method has two advantages compared with other data hiding techniques. One is the high safety and convenience offered by the RSA system to exchange data on the Internet. The other is that the stego image can pass through high-quality Joint Photograph Experts Group (JPEG) compression without noticeable degradation.

A new method for robust image recovery from noisy transmission based on soft bits and Markov random field modeling is proposed. The method aims to exploit the residual redundancy present in the symbols produced by a source encoder together with knowledge about the statistical properties of natural images. The soft-bit information required for reconstruction can be extracted either directly at the output of a noisy communication channel or at the output of a soft channel decoder (if channel codes are used). In the latter case, the proposed algorithm can also be used in conjunction with iterative channel decoding, resulting in joint source-channel turbo decoding. Examples are presented for the case where no channel codes are employed. The results obtained indicate that the proposed method yields very good performance even under extremely noisy conditions.

This study aims to reduce blocking artifacts in compressed images coded by the block discrete cosine transform (BDCT). The theory of projection onto convex sets (POCS) is applied. Two adaptive constraints are used. The first constraint corresponds to a model that simulates an image before compression using a Delaunay triangular mesh. The mesh is constructed by dividing each block into a set of triangles. The number of triangles in a block is determined by the local block activity. The second constraint is a narrow quantization constraint, where the bound of a BDCT coefficient is also determined by the local block activity. The block activity is measured by the ac components of the BDCT coefficients by incorporating the human visual system model. Our technique produces significantly improved results after a single iteration, and compares favorably with existing methods.

We develop a method for the fabrication of optical structures in GaAs substrates using UV holographic lithography in SU-8 resist, processed to fabricate a mask, followed by chemically assisted ion-beam etching (CAIBE). The technique is based on simple processing steps without procedures of mask transfer, enabling easy fabrication of optical structures. A predevelopment relief behavior is investigated to optimize the processing parameter to form an etching mask in SU-8. By adjusting both exposure dose and time in the postexposure bake (PEB), an SU-8 mask with a flexible duty cycle and high profile quality can be easily produced. Furthermore, an optical structure with a rectangular shaped profile and a 1-μm period in a GaAs substrate is produced by optimizing the processing parameters during the CAIBE process.

Reactively ion etched (RIE) n-GaN epitaxial films with different degrees of surface damage are investigated. To realize prominent effects resulting from RIE damage on electrical and optical properties, the samples are processed at various rf power levels and particularly under relatively high dc self-bias. We find that the restoration of crystal damage in optical properties evaluated by photoluminescence (PL) peak enhancements for samples thermally annealed at 750°C for 3 min could achieve up to 60 to 65% of those before RIE processing. Under high dc self-bias, it is evident that higher rf power produces rougher etched surfaces with more severe damage and higher surface resistance, so the damaged crystal is more difficult to recover. A transmission-line model is used to estimate the variation of ohmic contact resistance. Note that KOH surface treatment prior to contact deposition is more effective in lowering contact resistance than that of a damaged surface restored only by rapid thermal annealing (RTA) processing.

We describe a laser-based profilometry system that demonstrates high-resolution, dynamic surface height measurements in hostile conditions. The robust system measures transient distortions of continuous surfaces under the conditions of high stray light and mechanical disturbances that are typically found in laser drilling and welding. A coherent fringe pattern is projected onto the object surface using a binary phase grating interferometer. A charge-coupled device (CCD) camera fitted with a narrow-bandpass filter, centered on the optical wavelength of the projected fringes, records images of the illuminated workpiece while rejecting unwanted light from the high-power processing laser. The fringe pattern in each image is analyzed by the Fourier transform method, enabling dynamic shape measurement at the camera frame rate. The fringe order is identified uniquely by the dynamic projection of a laser spot aligned with a single fringe. An in-situ calibration produces height maps that are corrected for perspective and lens distortion errors. The unique measurement capability of the system has enabled the first dynamic shape measurements to be made during laser forming. Results are presented for the laser forming of 80×80 mm titanium alloy coupons, over which area a height accuracy of ∓12 μm was achieved.

Three core nonlinear directional couplers (TCNLDCs) with variable coupling coefficients (VCCs) are proposed. Their linear operation is examined analytically. Three types of VCC TCNLDCs are investigated numerically. Their transmission features are compared with those of their two-core counterparts. It is shown that the switching curves of VCC TCNLDCs are sharper than those of their two-core counterparts. Also the features of different types of VCC TCNLDCs are compared to each other and to the constant coupling coefficient (CCC) TCNLDC. It is found that the sidelobes in the postswitching region can be suppressed with VCC TCNLDCs and the peak transmission of VCC TCNLDCs is higher.

We describe the nonlinear optical limiting behaviors of the azo dye methyl-red-doped nematic liquid crystalline (NLC) films and report the phenomena of the formation of an interesting dark spot and a dynamic self-action diffraction ring pattern in the far field beyond the sample cell. We present the method of measuring the optical limiting characteristics of the methyl-red-dye-doped NLC films with typical test results obtained in the experiments. We theoretically analyze the optical limiting mechanisms of the methyl-red-dye-doped NLC films and discuss the advantages and restrictions of the methyl-red dye doped liquid crystal limiters, and propose several possible solutions to overcome these restrictions.

Optimal design is discussed for bidirectionally pumped Raman transmission systems. An optimal design algorithm is proposed for bidirectionally pumped fiber Raman amplifiers (BDP-FRAs), optimal pumping and gain schemes are analyzed for BDP-Raman systems, and the possibility of long-haul transmission is numerically verified for 7200-km 16-×10.66-Gbits/s and 2400-km 16-×42.7-Gbits/s BDP-Raman systems based on a common single-mode fiber (SMF) link.

The laser triangulation technique has been widely used to obtain three-dimensional (3-D) information because of its accuracy. It is a fast, noncontact method for 3-D measurement. However, 3-D data obtained from triangulation are not dense and usually not complete for surface reconstruction, especially for objects with irregular shapes. As the result of fitting surfaces with these sparse 3-D data, inaccuracy in measuring object surface area and volume is inevitable. Accurate surface reconstruction from incomplete 3-D data points becomes an important step toward accurate noncontact surface area and volume measurements of objects moving at high speed. A novel computer vision technique combining laser triangulation and a distance transform is developed to improve the 3-D measurement accuracy for objects with irregular shapes. The 2-D object image boundary points combined with the 3-D data obtained from laser triangulation are used to generate a 3-D wire frame. The distances from each pixel within the object boundary to its nearest boundary point are then used as the constraints for surface approximation. With this additional information from the distance transform, more accurate surface approximation can be achieved. This novel surface approximation technique is implemented and the measurement accuracy is compared with the accuracy using other surface interpolation techniques for the volume measurement of moving objects.

The problem associated with calibrating a structured-light-stripe vision sensor is that the known world points on a calibration target do not normally fall onto the light-stripe plane from the light projector of the vision sensor. Furthermore, usually, only a small number of the world points on the light-stripe plane can be obtained. We propose a novel approach that employs the invariance of double cross-ratio to estimate as many world points on the light-stripe plane as needed to address this problem. Based on the proposed approach, detailed theoretical analyses of the variances of the estimated world points are given as well as the corresponding simulation studies. A calibration target is made according to the proposed approach to provide accurate calibration points. The experiments conducted on a real sensor that consists of a CCD camera and a single-light-stripe-plane laser projector reveal that the proposed calibration approach is of high accuracy and is applicable to many structured-light-based 3-D visual inspection tasks concerning large work pieces.

Three-dimensional computer vision techniques have been actively studied for the purpose of visual traffic surveillance. To determine the 3-D environment, camera calibration is a crucial step to resolve the relationship between the 3-D world coordinates and their corresponding image coordinates. A novel camera calibration using the geometry properties of road lane markings is proposed. A set of equations that computes the camera parameters from the image coordinates of the road lane markings and lane width is derived. The camera parameters include pan angle, tilt angle, swing angle, focal length, and camera distance. Our results show that the proposed method outperforms the others in terms of accuracy and noise sensitivity. The proposed method accurately determines camera parameters using the appropriate camera model and it is insensitive to perturbation of noise on the calibration pattern.

Surface plasmon resonance spectroscopy is an optical technique that is capable of monitoring chemical and physical processes. It is sensitive to small changes of dielectric properties near a metal surface, and has been used to characterize a number of different types of films. This work analyzes the possibility of using the surface plasmon resonance phenomena for gas detection. We present examinations of thin films of phthalocyanines in plasmon systems from the point of view of their application in NO₂ sensors.

A multiplexing method for a fiber optic acoustic sensor is proposed by arranging the working point of Michelson interferometer to improve the temporal resolution of reading the distributed sensor. The slope difference of two series sensors is extracted electrically and is employed to bias the working point. This characteristic is used to separate each sensor from one single data stream by the time difference it takes the acoustic signal to arrive at two sensors. Location application with the verification of the commercial piezoelectric transducer (PZT) proves its efficiency. The stress location is resolved to 5.6 cm at the 54-cm region in one channel.

An improved matched filtering method for pattern recognition is proposed. In this method, functions modulated by the matched filter are quantized to the complexes of a quadrupole in the frequency domain. It is shown that shaper autocorrelation and much more suppressed cross-correlation signals can be obtained over those achieved by ordinary matched filters. It is also demonstrated that video-rate correlation processing with an 256×256 input image size can be performed on a regular personal computer using the proposed method even when the reference pattern is switched for every input image.

We present a profilometry technique for smooth surfaces based on the reflection law. In general, it is difficult to use point detectors for this task due to the spreading of the beam after reflection. Our point detector technique consists of controlling the spreading of the laser beam by analyzing the diffraction of the light through the system. Our analysis enables us to control the spreading of the beam even for a large local curvature of the object under test. The curvature and the distance of the object from the focusing lens are the main parameters to control the resulting spot size of the beam at the detection plane. Once point-like projections are obtained at the detection plane, a novel algorithm is used to calculate the surface profile via computing the local surface slope. The proposed technique is intended for smooth surfaces, such as the testing of convex plates and mirrors or the detection of irregularities in machined parts. It does not require any especial preparation of the surface or extensive alignment of the system. We illustrate our technique by showing experimental measurements of a smooth convex profile. For this, we use a ball bearing.

Equipment with a bifocus objective lens to detect the profile of a supersmooth surface in three dimensions is presented. The basic optical system is an interference system with a fully common optical path. Such an optical structure, which includes the bifocus lens and interference system, can result in the optical common ejection effect. Furthermore, by the electronic common ejection, the instrument can detect the profile within 0.1 nm vertically and 1 μm laterally. The principle of the optical common ejection and the structure of this equipment is described in detail.

The hyperbolic kernel and hyperbolic wavelet were shown to be useful and effective in time-frequency signal processing (Le et al., 2001 and 2003). We introduce the time-frequency wavelet power spectrum as a useful technique to study signal characteristics in which the hyperbolic and sym3 wavelets are employed. The hyperbolic wavelet power spectrum technique is employed for signals including electrocardiogram (ECG), sinusoidal, transient exponential, music, and speech. Comparisons between the Fourier power spectrum technique and hyperbolic wavelet power spectrum technique are also made.

The scheme proposed combines an existing image interpolation algorithm with an artificial neural network (ANN) used to model the residual errors between the interpolated image and the respective original image. Mathematical analysis shows that the performance of the proposed method is superior to that of known single-frame interpolation algorithms. The image restoration results using the presented scheme indicate that the restored images are very similar to the real high-resolution images. We also illustrate that the performance of any single-frame interpolation algorithm can be enhanced by combining the interpolation algorithm into our scheme. Experimental results show the proposed method on generalization and computation complexity is superior to other neural network schemes.

We describe a comparison of target identification performance between targets in the IR and visible bands. Increasing levels of Gaussian blur are applied to 12 varying viewpoint aspects of 12 targets in both the IR and visible spectra. Results are analyzed as a function of blur level and target size to give identification probability as a function of resolvable cycles on target. Probability curves for the IR and visible results are compared and the 50% probabilities of identification (N₅₀) are estimated for both cases. The results show that there is a significant difference in the target acquisition characteristics in the visible and IR wavebands.

An optical technique for evaluating Young's modulus of a vibrating cantilever beam is presented. The technique utilizes two charge-coupled device (CCD) cameras to record and process the images of a vibrating beam. Equations relating the natural frequency with the imaging time are obtained, and Young's modulus as a function of the natural frequency is subsequently determined. The Young's modulus values are compared with those obtained using the conventional strain gauge method. The results show good agreement. The proposed technique is relatively simple and could be used to determine the Young's modulus of a wide variety of beam materials.