This PDF file contains the editorial “Keeping Current” for OE Vol. 41 Issue 05

We present a 100-x 100-pixel retina that can detect differences between an image under analysis and a reference image. The retina is realized in standard 0.6-?m CMOS technology with three layers of metal from Austria Micro Systems. Its total area is 34 mm2 with a fill factor of about 37%.

A ring resonator with multiple reflections as a tunable filter is proposed and analyzed in this paper. A coupler and two identical Bragg gratings are used to implement the internal reflections so an active filter with a wide tuning range controlled through a coupler coupling constant is derived. Simple equations to design the device are included.

A directly modulated 1.3-?m 77-channel AM-VSB video and 63-channel simulated 256-QAM digital signals lightwave long-haul transmission system using a praseodymium-doped fiber amplifier (PDFA) and split-band and wavelength-division multiplexing (WDM) techniques is demonstrated. A high system performance of carrier-to-noise ratio (CNR) >50 dB, composite second order (CSO) >65.5 dB, and composite triple beat (CTB) >64 dB for AM-VSB is obtained, accompanied with carrier-to-noise and nonlinear distortion ratio (C/(N + NLD))>38 dB for 256-QAM.

This PDF file contains the editorial “Polarization” for OE Vol. 41 Issue 05

A system for generating polarization mode dispersion is developed to reproduce a close approximation to the polarization mode dispersion (pMd) of fiber links in the laboratory. A multistage system of rotating wave plates and combinations of birefringent elements generates a mixture of first-order, second-order, and higher order PMD suitable for testing OC-192 and OC-768 hardware. The PMD emulator can be operated in either static or time-varying modes. In the static mode, first-order PMD is generated with the differential group delay (DGD) continuously adjustable from 0 to 160 ps. In the dynamic mode, time-varying PMD containing first and higher orders is generated with an approximately Maxwellian DGD distribution. The PMD emulator generates PMD variations at adjustable frequencies up to 400 Hz.

The high throughput, low divergence, and quasimonochromatic characteristics of lasers make them desirable as light sources for numerous applications. Because of their extensive use, much work has been done to characterize laser noise. For laser applications in polarim- etry, a majority of this work has focused on laser output intensity fluctuations, which are primarily influenced by shot noise and flicker noise. Methods and algorithms have been developed to optimize signal to noise ratio (SNR) for polarimetric systems within both the shot-noise and flicker-noise limits. However, for systems that are extremely polarization state sensitive, such as those used for polarization imaging, the current optimization techniques still result in laser noise contributions that can contribute 10% or greater error to detected intensities. One potential source of these errors is a "wobble" in the polarization vector centered about the preferred polarization state of the laser output. We present a Mueller matrix theoretical analysis of this wobble phenomena. Our proposed solution for its elimination includes the introduction of a quarter- wave plate in the system. For this solution, we present experimental data that supports its viability.

We present a theoretical and experimental study of radially symmetric aberration caused by the differential transmission and phase shift of p- and s-polarized components of an axial beam passing through spherical lenses and plane parallel plates. We give a general description of the aberrations for an axial beam. The extinction is calculated as a function of the numerical aperture for uncoated lenses and for plane parallel plates. In our theoretical analysis, the polarization of output rays is described as a function of the input ray parameters, the shape factor, and refractive index of the lenses used. For rays that are inclined to the optical axis, optimal lens shape factors that minimize the rays' polarization aberrations are found. Techniques for measurement of radially symmetric birefringence in a lens system are described. Finally, we discuss strategies for polarization rectification and introduce new designs including meniscus rectifiers and a liquid crystal rectifier that can actively compensate a wide variety of polarization aberrations. Good correlations between theory and experimental results for microscope optical systems with coated and uncoated optical elements are found. Our results enable us to suppress depolarization and remove anomalous diffraction in a modern microscope equipped with high-numerical-aperture lenses.

The use of segmented mirrors for the primary elements of large telescope systems is becoming increasingly popular owing to the cost effectiveness of the design. Likewise, the use of nonoptically flat glass in large collectors can further reduce cost. Unfortunately, both nonoptical flatness and segment misalignment result in phase errors. If the segmented mirror is not imaged at the detector plane, errors in the measured polarization state may result. We have developed a computer simulation of an optical system, using angular propagation to propagate a two-component-wave electric field from element to element. We use this simulation to study the effects of mirror surface errors on the polarization state of an optical beam. We report herein on the effects of surface warping and of segment piston and tilt on the Stokes parameters of an optical beam reflected from a segmented mirror. When Stokes parameters are averaged across the detector plane, we find no significant error in these averages due to mirror surface errors. However, Stokes parameters at points in the detector plane can have significant errors if there are significant variations in the Stokes parameters of the incident field and there are significant errors on the mirror surface.

The analysis of polarimeter instrument matrix plays a key role in the problem of polarimeter optimization, which is widely discussed in the literature. The dimensionality and form of the instrument matrix are determined by the construction of the probing channel [polarization state generation (PSG)] and the receiving channel [polarization state analyzer (PSA)] of a polarimeter. We study the effect of structural features of the instrument matrix on error in the Mueller matrix measurement for the type of polarimeter whose PSA is a complete Stokes polarimeter. We show that the structuring of the instrument matrix due to the decreasing of number of input polarizations and optimization of the set of input polarizations result in a considerable decrease in the number of necessary measurements and an increase of the precision of the Mueller matrix element measurement.

The performance limitations of an imperfectly calibrated four- channel polarimeter in the presence of photon-counting detection noise is examined for arbitrary light levels. The theory described here provides a framework for developing polarimeter designs with minimal noise sensitivity and to evaluate the noise performance of existing designs. The polarimeter decomposes the input field into four channels, each of which is detected by the photon-counting sensor. We theoretically describe the propagation of detection noise through the polarimeter calibration matrix. The variances of both the mutual phase delay and the orthogonal intensities are given for photon-counting noise following Poisson statistics, and additive Gaussian detection noise. The variances of these parameters depend on the average number of photons incident on the polarimeter, the detector read noise, and errors in the polarimeter calibration matrix. A particular polarimeter design, whose calibration matrix is known exactly, is examined for moderate, low, and very low light levels. Theoretical performance curves are shown for various sensor parameters and light levels, and are compared to simulated results. Very good agreement between theory and simulation is shown. The simulation validates the use of the Gaussian probability density function for the parallel (inphase) and normal components of the phase fluctuations, and provides an accurate theoretical prediction of phase delay fluctuations for arbitrary light levels. The phase-delay noise cloud is illustrated for several cases.

We describe an instrument for measuring linear retardance in transparent optical components using two photoelastic modulators. The instrument contains a He-Ne laser (632.8 nm), a polarizer, two low- birefringence photoelastic modulators at different frequencies, an analyzer, and a silicon photodiode detector. A sample is placed between the two modulators. The detector signals corresponding to linear retardance in a sample are analyzed using lock-in amplifiers. A computer program calculates and displays both the retardation magnitude and the angle of the fast axis. The reported instrument is essentially a polarimeter specifically designed for measuring low-level linear retardance in high-quality optical components. It provides a retardation sensitivity of better than 0.005 nm (0.003 deg or 5 x 10-5 rad with a He-Ne laser at 632.8 nm).

We investigate the polarimetric scattering properties of highly absorbing and highly reflective rough surfaces in the IR. We obtain Mueller matrix measurements from these surfaces and compare the data to a Fresnel model. From the data, we determine that a rough, highly absorbing surface has a higher degree of polarization compared to a rough, highly reflective surface. In addition, we present a full polarimetric version of the microfacet model and discuss its properties. This closed-form result for the polarimetric bidirectional reflectance distribution function (BRDF) is derived from the microfacet model.

Reflectance R, ellipsometric parameters (?,?), depolarizations D, and cross-polarized depolarization Dv of specular reflection, and scattering by a rough stainless steel surface are measured using a null ellipsometer. The three polarization elements (Px ,Py ,Pz ) of a principal Mueller matrix are obtained from D, ?, and ?. The measured ? and ? of specular reflection for incident angle ?>70 deg are fit to the Fresnel equations to obtain n and k. The measured specular R(? ) is fit to Beckmann’s scattering theory to obtain the root mean square (rms) roughness ?. The fit ? (= 392 nm) is of the same order as the stylus measured ? (= 484 nm). The D of specular reflection is small (< 0.07). Scattering measurements are made with variable sample orientation at a detection direction 40 deg backward from the incident direction. The Dv of scattering is very small (< 0.002). The values of ?, ?, Px , Pz , and Dv of scattering are about constant for an off-specular angle (OSA) within ±60 deg and can be explained by the facet model with single scattering. The measured bidirectional reflectance distribution function (BRDF) in this region can be converted to slope angle distribution with OSA52 slope angle. The values of ?, ?, Px , Pz , and Dv of scattering deviate significantly and symmetrically from a constant level for |OSA|> 60 deg, for which the simple facet model with single scattering does not apply.

The ability to perform polarimetric imaging throughout the visible and infrared (IR) wavebands has improved considerably in the past decade. Systems now exist that enable measurements to be made of all four Stokes parameters arising from each pixel in the image. The question of whether polarimetric imaging offers an advantage over conventional imaging methods for discrimination of plant type in scenes of natural vegetation remains to be answered. Although the size of a leaf may be below the spatial resolution of an imaging system, the polarimetric properties of individual leaves may affect the data observed from a tree or forest canopy. We report the results of measurements of the polarized hemispherical directional reflectance (HDR), which is related to the directional emissivity, and bidirectional reflectance distribution function (BRDF) from two examples of leaves. To completely characterize the polarimetric properties of a leaf, and ultimately a leaf canopy, an extensive measurement of the polarized BRDF and HDR of individual leaves is required. This is necessary because of the large range of possible relative orientations of the illumination, leaf and observer, and the range of polarization states of incident radiation. We report a limited set of laboratory measurements designed to investigate whether any gross po- larimetric difference exist between two dissimilar types of plant leaves in the visible, near IR (NIR), and IR spectral wavebands. Laurel (prunus laurecatious) has a wax surface creating a gloss or glabrous appearance to the leaf. The surface of mullein (verbascum thapsus) is highly pubescent with a dense layer of hair over the adaxial surface, creating a highly diffuse surface reflectance. Significant differences are found between the two species of leaf in the measured polarized directional reflectance and emissivity.

An instrument is presented that combines the functions of a reflectometer, a spectrometer, and a polarimeter. A Fourier transform spectrometer serves as a light source for a dual rotating retarder polarimeter. Operation of the instrument results in measurement of the complete Mueller matrix of a sample in reflection over a wide spectral band. Calibration and error compensation is described. Polarization characteristics of materials in reflection are determined.

Linear polarization of thermal infrared (TIR) radiation occurs when radiation is reflected or emitted from a smooth surface (such as the top of a landmine) and observed from a grazing angle. The background (soil and vegetation) is generally much rougher and therefore shows less pronounced linearly polarized radiation. This difference in polarization is utilized to enhance detection of landmines using TIR cameras. A setup has been constructed for the acquisition of polarized TIR images. This setup contains a polarization filter that rotates synchronously to the frame sync of the camera. Either a long wave infrared (LWIR) or a mid wave infrared (MWIR) camera can be mounted behind the rotating polarization filter. The synchronization allows a sequence of images to be taken with a predefined constant angle of rotation between the images. Using this image sequence, three independent Stokes images are calculated, consisting of the unpolarized radiance, the difference between vertically and horizontally polarized radiances, and the difference between the two diagonally polarized radiances. A model has been developed that describes the polarization due to reflection of and emission from a smooth surface. This model predicts the linear polarization for a landmine "illuminated" by a source that is either hotter or cooler than the surface of the landmine. The measurement setup has been used to validate the model. The measurements agree well with the model predictions and can be used for estimating the real part of the refractive index of the rubber surface of the landmine. Besides the indoor measurement, outdoor measurements have also been performed. The results of these measurements show that under the given conditions the majority of landmines can be observed in the polarized radiance, whereas they are not clearly visible in the normal (unpolarized) radiance or the visual image.

A compact, lightweight, robust, and field-portable spectropolarimetric imager is developed to acquire spectropolarimetric images both in the laboratory and outdoors. This imager uses a tellurium dioxide (TeO2) acousto-optic tunable filter (AOTF) as an agile spectral selection element and a nematic liquid-crystal variable retardation (LCVR) plate as a tunable polarization selection device with an off-the-shelf charge- coupled device (CCD) camera and optics. The spectral range of operation is from 400 to 800 nm with a 10-nm spectral resolution at 600 nm. Each spectral image is acquired with two retardation values corresponding to the horizontal and vertical incident polarizations. The operation of the imager and image acquisition is computer controlled. We describe the instrument and its operation and present results of our measurements.

The integration and calibration of a narrow-band imaging po- larimeter is described. The system is designed to exploit subtle spectral details in visible and near-IR polarimetric images. All of the system components were commercial off the shelf. This device uses a tunable liquid crystal filter and a 16-bit cooled CCD camera. The challenges of calibrating a narrow-band imaging polarimeter are discussed. We describe examples of image data collected in the laboratory, which show that spectral polarimetric data is superior to unpolarized intensity data in facilitating the extraction of detail in shadowed regions below a vegetative canopy. In particular, we introduce a polarization-based normalized difference vegetation index (NDVI) algorithm that demonstrates significant contrast enhancement between a man-made object and a foilage background.

We present and analyze a technique for snapshot imaging spectropolarimetry. The technique involves the combination of channeled spectropolarimetry with computed tomography imaging spectrometry (CTlS). Channeled spectropolarimetry uses sideband modulation to encode the spectral dependence of all four Stokes parameters in a single spectrum. CTIS is a snapshot imaging spectrometry method in which a computer-generated holographic disperser is employed to acquire dispersed images of the target scene, and both spatial and spectral information is reconstructed using the mathematics of computed tomography. The combination of these techniques provides the basis for a snapshot imaging complete Stokes spectropolarimeter that can be implemented with no moving parts. We review design considerations for the spectropolarimeter and present preliminary simulation results.

Spectrally varying long-wave infrared (LWIR) polarization measurements can be used to identify materials and to discriminate samples from a cluttered background. Two LWIR instruments have been built and fielded by the Air Force Research Laboratory: a multispectral LWIR imaging polarimeter (LIP) and a full-Stokes Fourier transform infrared (FTIR) spectral polarimeter (FSP), constructed for higher spectral resolution measurements of materials. These two instruments have been built to gain an understanding of the polarization signatures expected from different types of materials in a controlled laboratory and in varying field environments. We discuss the instruments, calibration methods, general operation, and measurements characterizing the emitted polarization properties of materials as a function of wavelength. The results show that we are able to make polarization measurements with a relative accuracy of 0.5% degree of polarization (DOP) between two different instruments that are calibrated with the same techniques, and that these measurements can improve the understanding of polarization phenomenology.

The adaptive optics for any telescope in the 25- to 100-m class will be complex. It is believed that adaptive optics should, to the maximum extent, be designed as an integrated part of a telescope. The proposed Swedish 50-m Extremely Large Telescope is considered here to illustrate the principle of integrated adaptive optics. Two alternative designs both using the Ritchey-Chretien telescope system and laser guide star (LGS) reference sources are presented. The first design employs trombone optics, which bring the laser guide star images back to the normal Ritchey-Chretien focal surface (referred to as the RC-focus) from the LGS focal surface (referred to as the LRC-focus), and a layer- oriented wavefront sensor system optically performing the averaging ''shift and add" in the final focus. According to this procedure, sensed wavefronts are overlapped with a certain mutual shift and added for estimation of wavefront average slope values, resulting in actuator commands for driving the shape of the deformable mirrors. The second design employs a numerical ''shift and add" procedure and has two wavefront sensors. The first one performs LGS sensing in an intermediate focus (LRC-focus), giving the input data for an analytical algorithm for deriving the mirror deformations to correct for atmospheric turbulence. By using an artificial laser source at the intermediate focus, the shape of the second deformable mirror is controlled by a second wavefront sensor in the final focus. The capability of the analytical algorithm to derive the mirror corrections from the measured wavefronts ensures proper functioning of the adaptive optics system. This system has a simpler optical design compared to the first design.

We propose and demonstrate the use of an adaptive membrane mirror to increase the efficiency of coupling light into optical singlemode fibers. The membrane mirror has an active area of 12 mm in diameter and is electrostatically activated by 37 electrodes. Two optimization algorithms are compared. The first one is a maximization algorithm, while the second one is an evolutionary algorithm. These algorithms are used either to optimize the electrode voltages or to optimize a set of low-order Zernike polynomials describing the membrane deformation.

Based on the stopband width measurement, the coupling coefficient of distributed feedback (DFB) lasers can be estimated with reasonably good accuracy. However, to characterize distributed Bragg reflector (DBR) lasers, many researchers have used the calculated coupling coefficient, since the stopband width from the reflectivity profile of the lasers is more difficult to clarify for this estimation compared to that of DFB lasers. It is especially true when grating losses due to scattering and absorption are large and difficult to measure for the surface grating devices. In this case, estimation of the coupling coefficient becomes more problematic or even erroneous. This work uses a novel method that involves implementing the relationship between the real and effective grating lengths and mode spacing under the subthreshold to determine the coupling coefficient and grating losses of DBR lasers. Through a self-consistent approach, for DBR lasers with a geometry of La =420 ?m and Lg=380 ?m, where La and Lg are the active-section and grating-section lengths, respectively, a coupling coefficient of 45 cm21 and grating losses of 38 cm-1 at the lasing wavelength of 833 nm were estimated with relatively low uncertainty of 10%.

We demonstrate a distributed Bragg reflector (DBR) waveguide laser array by using an Ag film ion-exchange technique. We achieve an output power of 11 mW at 1540 nm for a coupled pump power of 145 mW, with a threshold of 60 mW and slope efficiency of 13%. The thin film ion exchange produces the highest index change possible at the surface, due to the ion exchange technique. Hence a wide array of wavelengths can be implemented in a single chip. We demonstrate a lasing wavelength range of 2.1 nm (1548.6 to 1550.7 nm) in an array with a single grating. Since the index change due to our process is large, we can fine tune the wavelengths of the array to fall on International Telecommunication Union (ITU) grid wavelengths by annealing. We demonstrate fine tuning of the wavelength for a channel from 1540.2 to 1540 nm, ITU specified wavelength, by annealing.

We present a method for measuring the intensity variation regarding the measured physical attributes of intensity-based fiber Bragg grating sensors. These sensors can be configured for multiplexing, thermal insensitivity, and immunity to noise from the light source and/or the lead-in/out fibers caused by the external load. To achieve these goals, the intensity-based optical fiber sensor is implemented between a pair of fiber Bragg gratings. Based on the ratio or dBm difference from the two reflecting intensities of the reference and the sensing fiber Bragg gratings, the appropriate measurable parameters associated with the intensity variations can be detected. Different from transmission type intensity-based sensors, a reflective type based on a pair of fiber Bragg gratings is introduced. To validate the proposed method, a transducer made by using an etched-cladding fiber is deposited onto the metal thin film for sensing metal corrosion. Based on the evanescent field of a fiber and a pair of fiber Bragg gratings, multiple embedded corrosion sensors could be designed for civil structural health monitoring. Also, the proposed method could be applied to a biosensing design on a transducer.

We present a fiber optic relative humidity sensor. The sensor was fabricated by coating a thin polyvinyl alcohol/CoCl2 film on a 200-?m core plastic-cladding fiber; the film was deposited after removing the cladding of the fiber. It was found that bending the fiber leads to drastic changes in the response behavior. The sensor was compared against a commercially available relative humidity sensor and was found to be sensitive to relative humidity ranging from 3 to 90%. The sensor response is usually very fast, but varies at different humidity levels and has a good dynamical range. The sensor response is repeatable and fully reversible, and should be useful for monitoring relative humidity in harsh industrial environments.

It is well known that laser beams spread as they propagate through free space due to natural diffraction, and that there is additional spreading when optical waves propagate through atmospheric turbulence. Previous studies on Gaussian beams have mainly involved the lowest order mode (zero order). The study of higher order mode Gaussian beams has involved Hermite-Gaussian and Laguerre-Gaussian beams for rectangular and cylindrical geometry, respectively. These studies have developed expressions for the field and intensity in free space, in addition to developing new definitions of beam size in the receiver plane for the higher order modes. We calculate the mean intensity of higher order mode Gaussian beams propagating through atmospheric turbulence, and, based on previously developed definitions for beam radius, we calculate the additional beam spreading due to random media. It is shown that higher order mode Gaussian beams experience less percentage of additional broadening due to atmospheric fluctuations than the zero-order mode beams.

We present a fast predictive search algorithm for vectorquan- tization (VQ) based on a wavelet transform and weighted average Kalman filter (WAKF). With the proposed algorithm, the minimum distortion code word can be found by searching only a portion of the wavelet transformed code book. If the minimum distortion code word found falls within a predicted search area obtained by the WAKF algorithm, the relative address that is shorter than the absolute address for a full search range is sent to the decoder. Simulation results indicate that the proposed algorithm achieves a significant reduction in computations and about a 30% bit-rate reduction, as compared to conventional full search VQs. In addition, the reconstructed quality is equivalent to that of the full search algorithm.

A single fringe phase shifting technique is presented for extracting the whole-field displacement distribution from single fringe patterns. By making a small modification in the traditional Fourier transform (FFT) method for fringe processing, the proposed method is capable of maintaining the fringe sign information. An additional approach is introduced that utilizes knowledge of the fringe sign in identifying fringe skeletons with high precision. A combination of these methods of fringe pattern processing was used to examine the whole-field displacement distribution in double cantilever beam (DCB) fracture specimens prepared from bovine maxillary molars. Phase maps and precise fringe skeletons were generated using the proposed techniques from fringe patterns obtained using moire interferometry. Both aspects of the displacement measurements were used to determine the local displacement distribution near the crack tip, and to study the mechanics of fracture.

We describe in detail the design, construction, and characterization of an efficient frequency doubled and stabilized all-solid-state Ti: sapphire laser. The laser frequency has been locked to the resonance of a Fabry-Perot cavity, and doubled in a Brewster-cut potassium niobate crystal placed inside a power enhancement cavity. Up to 200 mW of single frequency blue light with fast frequency instabilities of 40-kHz rms and a drift of 10 MHz/hour has been generated. The spectral distributions of amplitude and frequency noise for the free-running laser have been measured and compared with the case of pumping from an argon ion laser. Our laser is well suited in atomic physics for high resolution spectroscopy and for laser cooling and trapping using transitions in the blue-violet region, as we demonstrate with the calcium resonant transition, at 423 nm.

A new frequency-modulated continuous wave modulation scheme, which gives correct results even when the Doppler shift is larger than the frequency difference associated with the range, is presented and tested with a tunable laser diode and fiber-based system. By inserting a constant frequency region in the modulation scheme, both the magnitude and the sign of all beat frequencies can be determined. When they are known, the correct frequency difference as a result of the range can be calculated. This new scheme gives more freedom when choosing the modulation parameters of the laser, because increasing the modulation frequency, and/or the frequency sweep, to avoid ambiguities resulting from a large Doppler shift no longer becomes necessary. This is especially useful when using a somewhat cheaper laser diode source, since the maximum obtainable modulation frequency and frequency sweep can be somewhat limited. The suggested modulation scheme makes it possible to use some laser diodes in an application they otherwise would not be suited for.

Twisted nematic liquid crystal spatial light modulators have been widely used for such applications as optical information processing and real-time holography. It is important to experimentally determine the physical parameters of the modulators to control their modulation characteristics. The parameters are usually calculated from a set of measured intensity transmittances. We mathematically prove that the calculation always suffers from ambiguities, and there exist two distinct origins. Neglecting absolute phase is one source of ambiguity; the other relates to the quasisinusoidal dependence of the complex amplitude transmittance on the parameters. Jones matrix formalism is used to analyze the ambiguity, and interesting examples are presented.