This PDF file contains the editorial “A Prison Cell” for OE Vol. 45 Issue 10

Coherent interferometric absolute distance metrology enables different techniques for length metrology. Measurements are made without ambiguity, by using either one or several synthetic wavelengths resulting from the beating of two or more wavelengths or, in the case of frequency sweeping interferometry (FSI), from a frequency sweep. These techniques require accurate measurements of the frequency variation, a task which is usually performed with a Fabry-Pérot (FP) interferometer. Sensor performances are highly dependent on the FP free spectral range (FSR), which must be accurately known. We describe a method to measure the FSR of an FP using a tunable laser in a Michelson interferometer, using both absolute and relative distance measurements. The optical setup is based on a frequency-sweeping sensor and a controlled translation table. An uncertainty of 45 ppm in the measurement of a 1-GHz FSR is obtained. The method is indicated for the calibration of the FP in the frequency-sweep-range-measuring subsystem of a frequency sweeping sensor absolute distance sensor as it does not require any addition hardware, configuring a self-calibration approach.

Considering the state of the art of wavelength conversion technology, it is likely to dictate a more limited and sparse deployment of wavelength converters in the optical burst switching (OBS) networks. Without wavelength conversion capabilities at optical switches, the start wavelength selection method becomes an important issue to avoid burst contention. We present a new start wavelength selection method, the minimum relative group combined interference level (Min-RGCIL) algorithm, which is a modified version of a proposed method called first-fit-TE (traffic engineering). The performance study indicates that the new method achieves better performance than the original one and is robust under Poisson and self-similar traffic.

A global optimization process is developed for designing an objective lens for multiple-configuration applications. As a demonstration, it is shown that diffraction-limit performance can be achieved for a pickup head used for both compact disks (wavelength 780 nm) and digital versatile disks (wavelength range 635 to 650 nm) with different working distances and focal lengths, at a numerical aperture of 0.6.

A compact mechanical package is developed for a standard microscope that implements a solid immersion lens (SIL) on a retractable bimorph swing arm. With the compact package mounted on an inverted microscope, far-field and near-field images are obtained at the same location by moving the SIL with the swing arm. With white-light incoherent illumination, the resolution of this system for observing digital versatile discs is around 200 nm with an effective numerical aperture of 1.5. Imaging with the SIL is compared with an atomic force microscopy scan.

Camera calibration is essential in order to perform 3-D reconstruction from uncalibrated images. This paper proposes a new technique of camera calibration using a homography among three planar patterns taken from a single image. It is implemented more easily and simply than conventional methods. Experimental results support the hypothesis and show that the performance of the proposed method is better than that of conventional methods. Examples of 3-D reconstruction are demonstrated using the proposed algorithm on sequences of images.

Polymer waveguides are successfully fabricated by a simple method, which is incorporated with a lithography-galvanic (LIGA)-like and micromolding process. The refractive index of the UV polymer used in these experiments is changed by an extremely low electric field. The measured propagation loss is 0.32 dB/cm for 1.3 µm and 0.30 dB/cm for 1.55 µm. This process is easy, simple, and suitable for mass production.

The principle for utilizing a full-external-cavity He-Ne laser as a displacement sensor is presented. Inserting a quartz plate into the cavity, we split one laser frequency into two orthogonally polarized beams (o light and e light). When one cavity mirror is moved along the laser axis, we obtain power-tuning curves for o light and e light, in which equal-intensity points appear periodically, one period corresponding to λ/2 displacement. Moreover, four different polarization states in the laser output appear periodically. Attaching the moving mirror to the measured object, we realize displacement measurement with resolution of λ/2 through counting the number of equal-intensity points. We measure displacements less than λ/2 by means of a PZT mounted to the other cavity mirror. An increasing voltage is applied to the PZT to displace the mirror. Once the two beams reach the neighboring equal-intensity point, we note down the voltage variation of the PZT to get the true displacement. We discriminate displacement directions by sensing the order of appearance of the four polarization states. This system can be expected to afford a measurement range of ≈ 35 mm and a resolution of 10 nm, and has the capability of self-calibration. The potential error factors are also discussed.

Absorbing mirrors are optical thin film systems consisting of an absorbing metal layer held by a transparent interlayer in defined distance to a (metal) mirror layer. In the reflected light, such three-layer systems show vivid color effects due to the absorption in the metal. This absorption is modified and enhanced by interference effects. The interference condition can be tuned by the thickness of the transparent interlayer. We show by experimentation and by stratified medium model calculations the importance of a nanoscale granular structure in the absorbing metal layer for obtaining a wide spectral range of the color effect.

This paper presents the effect of anode number on the operating parameters in a fast-axial-flow cw CO₂ laser. The discharge stability and uniformity, input power, output temperature at tube exit, and output power of the laser have been investigated experimentally for different anode numbers.

The operation of a passively mode-locked erbium-doped fiber ring laser in two-color pulse mode has been experimentally observed. The experiments demonstrate that the wavelengths of two-color pulses can be switched among three wavelength bands. Through carefully controlling the polarization states of the laser cavity, various modulation patterns of the two-color pulses are obtained, which are due to the interaction of two different-wavelength pulses. Single- and three-color pulses are also observed. Multiwavelength oscillation is ascribed to the wavelength-dependent transmission induced by birefringence, and can synchronously mode-lock through nonlinear polarization rotation.

The influence on an outcoupled mode by introducing intracavity tilt perturbation in an unstable resonator is developed. The intra-cavity mode properties and Zernike aberration with the intracavity mirror's misadjustment are calculated theoretically. The experimental results about the relations of intracavity perturbation and the properties of mode aberration are presented by adopting a Shack-Hartmann (H-S) wavefront sensor. The results show that the intracavity perturbation of the concave mirror has a more remarkable effect on outcoupled beam quality than that of the convex mirror. For a large Fresnel-number resonator, the tilt angle of the intracavity mirror has a near-linear relationship with the extracavity Zernike tilt coefficient. The ratio of the tilt aberration coefficient approaches the magnification of the unstable resonator if equivalent perturbation is applied to the concave and convex mirrors, respectively. Furthermore, astigmatism and defocus also increase with tilt aberration of the mode. So an intracavity phase-corrected element used in an unstable resonator will be most effective when placed as near as possible to the concave mirror plane. Based on these results, a control system for correcting intracavity tilt aberrations is established, and the aberration-corrected results are presented.

Fluctuations in output of a pulsed dye laser are inevitable and seriously affect the realization of their full potential as a narrow-bandwidth tunable source. High-repetition-rate dye lasers require the flow of the dye solution to be at sufficiently high speed across the pump region so as to replace the heated medium and move it away from the dye laser axis. This introduces instabilities and fluctuations in the lasing medium, thereby affecting the bandwidth and wavelength of the dye laser. We report the effect of optical inhomogeneity in the gain medium on the bandwidth of a rhodamine 6G dye laser transversely pumped by a copper vapor laser. It is observed that the bandwidth of the dye laser decreases with increasing Reynolds number within the laminar region of flow.

The defects responsible for damage in KDP and DKDP crystals are investigated by analyzing the density of damage sites and their individual morphology under different irradiation wavelengths and fluences. The experimental results indicate the presence of two defect species initiating damage over different wavelength ranges. Disparities in the morphology of damage sites initiated are also observed over each wavelength range.

We present an adaptive control scheme for laser-beam steering by a two-axis microelectromechanical systems (MEMS) fast steering mirror. Disturbances in the laser beam are rejected by a µ-synthesis feedback controller augmented by the adaptive control loop, which determines control gains that are optimal for the current disturbance acting on the laser beam. The variable-order adaptive controller is based on an adaptive lattice filter that implicitly identifies the disturbance statistics from real-time sensor data. Experimental results demonstrate that the adaptive controller significantly extends the disturbance-rejection bandwidth achieved by the feedback controller alone. The experimental results also illustrate the value of the variable-order capability of the adaptive controller.

Absorption coefficients of highly transparent optical materials (glass) have been measured using a z-scan technique. Absorption coefficients as low as 10⁻⁵ cm⁻¹ and lower can be measured by scanning a substrate of a transparent optical material in the focal region of a lens. Glass substrates with relatively high absorption coefficients are used for calibration and for verifying the results of measurements. The sensitivity of this technique proves to be high enough to reveal differences between absorption coefficients of different substrates made of the same material (BK7). We have suggested and used a novel procedure, scanning nonlinear lens profiling, for characterization of homogeneity of glass substrates and other transparent optical materials. Optical nonlinearity of glass substrates allows fast and high-precision measurement of the power and power density of high-power near-IR laser beams without large temperature increases and related problems.

The method of buried landmine detection based on using elastic waves in the ground and a laser Doppler vibrometer (LDV) as a vibration sensor has shown excellent performance in field tests. To increase the speed of measurements, a multi-beam laser Doppler vibrometer (MB-LDV) was developed. The system is based on a heterodyne interferometer and is capable of simultaneously measuring the vibration of the ground at 16 points over a span of 1 m with a velocity resolution of less than 1 µm/s. Both digital in-phase and quadrature (I&Q) and analog phase-locked loop (PLL) demodulation have been used for signal processing. The MB-LDV can create a velocity image of the ground surface either in "stop-and-stare" mode or in a continuously scanning mode. The continuously scanning operation results in an increased velocity noise floor due to speckle noise. The speckle noise floor increases with the increase of the speed of the laser beam and can degrade the velocity image of a mine. To overcome the effects of speckle noise, the excitation source must provide a ground vibration velocity higher than the velocity noise floor of the vibrometer. The MB-LDV has been tested at landmine test lanes and shows the ability to detect buried landmine within a one-square-meter area in a time of less than 20 s.

A fiber-optic biosensor for glucose concentration detection has been designed, based on electrostatic self-assembly. The end face of a multimode fiber is coated with poly(allylamine hydrochloride) in combination with Prussian blue and the enzyme glucose oxidase. The glucose concentration that can be measured ranges between 0.06 and 2 mM. The initial velocity of change of the output biosensor signal has been found to be linear in the glucose concentration. The sensor recovers after immersion in ascorbic acid. The pH values analyzed range between 5 and 7. The sensitivity of the device has been increased by adjusting the number of bilayers and by adding some capping bilayers at the end of the structure.

We systematically compare the transmission performance of three different modulation formats — return-to-zero (RZ), chirped RZ (CRZ), and vestigial sideband RZ — on an economically possible transmission link using standard single-mode fiber (SSMF) and all erbium-doped fiber amplifiers. This link is composed of wavelength-division multiplexed channels at 10 Gbit/s over 24×100-km amplifier spans without using new types of fibers or Raman amplifiers. The generation of each modulation format in the experiments is optimized for more reasonable comparison, and their optical characteristics are investigated by comparison with simulation results. Optimum input powers into SSMF and dispersion compensation fiber for each modulation format signal were explored, and their transmission performance was compared. In our cost-effective transmission link, the CRZ signal showed the lowest input power and best performance among all the formats investigated.

A new multiple-access technology based on optical code-division multiple access (OCDMA) for passive optical networks (PONs) is presented. The design of the optical line terminator and the optical network unit for the OCDMA PON are studied in detail. Its bit-error-rate performance is studied under consideration of multiple-user interference and various noise contributions. Simulation results indicate that the proposed scheme is feasible and that the novel design can improve the performance of optical access networks.

Time-slot interchange is investigated using an optical buffer with a large variable delay based on an active-vertical-coupler crosspoint switch. The delay could range from 1 to 999 time slots in our scheme, and the length of the time slot is correspondingly flexible. The experiment could be performed at a data rate of 10 Gbit/s. The results show the power penalty is about 1.5 and 5.6 dB, respectively, when the packets pass through two buffers and through one buffer nine times.

We demonstrate a capacity upgrade to a 40-Gb/s channel rate with reconfigurable optical add-drop multiplexing (ROADM) operation from a 32×10-Gb/s–based unrepeated WDM link. The 16×40-Gb/s channels were added at the ROADM node without traffic interruption in the expressed channels. The worst channel of the 40-Gb/s rate shows 1.1 dB of Q-value margin to the forward error correction (FEC) limit. The expressed 10-Gb/s channels also satisfy the transmission limit of the bit error ratio (BER) 10^–15 without FEC operation.

An optical dynamic spectral encoding code division multiple access (CDMA) communication system is proposed. In this system, an electrically controlled tunable optical filter (TOF) is used to encode the modulated broadband light source. The code depends on the function set to the controller. Two-dimensional code, named functional code, is also proposed based on a shifted sine function. The function defines the dynamic coding pattern of the central wavelength of the transmitted narrowband optical signal. Thus, the system will allow for an easy reconfiguration of the transmitter without the need for a sophisticated encoder. At the receiver, a synchronized TOF with the same function is used as a decoder. The system is modeled and analyzed taking into account the multiple access interference, phase-induced intensity noise (PIIN), and thermal noise. The performance of this system is shown to be better compared with a fast frequency hopping (FFH) system and a spectral amplitude coding (SAC) system that uses either a Hadamard code, a modified quadratic congruence (MQC) code, or a modified frequency hopping (MFH) code.

The authors analyze electromagnetic wave propagation through optical fibers consisting of a glass core surrounded by a high-refractive-index dielectric thin film, in turn surrounded by a glass cladding. The asymmetry of the structure due to different core and cladding refractive indices results in two types of guided modes: they propagate either in the core and film or only in the film. The dispersion equations are derived, and mode classifications are established for transverse and hybrid modes. The cutoff conditions of transverse modes are given. Numerical simulations are performed to obtain the effects of film index and thickness on the cutoff frequencies and dispersion curves. It is shown that a high-index film causes the modes to become evanescent in the core and guided only in the film. These modes form two types of linearly polarized modes, according as they follow the TE or the TM mode propagation curves of asymmetric slab waveguide. Investigation of power flow along the propagation direction shows that the higher the refractive index of the film, the higher the power density in the film.

A novel method to construct a quality map, called modulation–phase-gradient variance (MPGV), is proposed, based on modulation and the phase gradient. The MPGV map is successfully applied to two phase-unwrapping algorithms—the improved weighted least square and the quality-guided unwrapping algorithm. Both simulated and experimental data testify to the validity of our proposed quality map. Moreover, the unwrapped-phase results show that the new quality map can have higher reliability than the conventional phase-derivative variance quality map in helping to unwrap noisy, low-modulation, and/or discontinuous phase maps.

The article proposes a scheme to break a Catch-22 loop in an optical-figure–wavefront measurement. For instance, to measure the tilt-independent optical figure of a nominal optical flat at cryogenic temperatures, one requires a cryogenic Dewar-window system for a Fizeau interferometer mainframe outside the Dewar to see through. The problem is: how to calibrate the window system in situ using the yet-to-be-calibrated nominal optical flat, and vice versa, in only one cryogenic cooldown? The proposal includes: (a) interferometric phase-map measurements with the test piece slightly offset in different transverse directions, and (b) an unconventional numerical integration scheme, starting with one-dimensional bidirectional integrations, to synthesize the two-dimensional wavefront distortion function. The numerical scheme helps minimize the nonuniformity in noise-power distribution that results from integrating phase-map data, and thus the associated uncorrelated random noise. The numerical integration scheme represents a non-Newtonian concept specifically for noise-carrying experimental data. The algorithm that determines the algorithm for the domain-shape-specific integration scheme is described in detail.

Separation of the surface strain gradient as a function of x and y was achieved using three-dimensional (3-D) digital holographic interferometry. The optical setup is capable of independently determining the x, y, and z displacements of the object surface. The magnitude contributions of the dependent, or interconnected, strain gradients are calculated from the normal strain gradients. In order to simplify the mathematical model, objects with smooth surfaces were considered and also only small static loads were applied to deform them. The 3-D information thus gathered is analyzed in order to obtain the x and y strain gradients. Each gradient is then processed to obtain a pair of mutually dependent gradient maps. These maps are shown for the x and y axes in a 3-D sparse vector map that changes according to the strain present on the object.

A digital speckle shear pattern interferometer using a cyclic-path shear arrangement with a polarization phase-shifting method for the deformation-derivative measurement is presented. The cyclic-path arrangement, being a common path, improves the stability of the system. The proposed arrangement can be used to obtain the lateral as well as the radial slope contours. Experimental results on a centrally loaded diaphragm with its edge rigidly clamped, in lateral and radial speckle shear configurations, are presented.

A method based on a hybrid genetic algorithm is proposed to obtain the wavefront aberrations of a real interferogram. By solving an optimization problem, the proposed method fits a set of Zernike polynomials to the experimental data. The results show that, compared to a conventional genetic algorithm, the hybrid genetic algorithm not only improves the searching ability but also accelerates the convergence. The proposed method is robust to spatial variation of the illumination.

A digital speckle shearing interferometer is developed using a dynamic binary phase grating realized by an electrically addressed, twisted nematic liquid-crystal spatial light modulator as the shearing element. The standard phase-shifting algorithm can be easily achieved by changing the grating pattern displayed on the spatial light modulator. The method has a number of advantages including variable sensitivity, no moving part, and no requirement for calibration of the value of the phase shift. The error due to the unexpected diffraction order is also discussed.

Ultraviolet (UV) observations of the Earth's upper atmosphere are essential for meeting operational requirements for space weather specification and prediction. Such observations provide valuable information about neutral and ion density variations. Current operational sensors measure the limb profiles by mechanically scanning the field of view across the limb. This mechanical scan mechanism requires significant power and can fail, and the high counting rates during observations near the peak of the limb profile require high-speed detectors to accommodate the counting rates when using the high-sensitivity sensors. This paper describes an instrument that can provide limb observations of the UV airglow by aligning the slit perpendicular to the limb. To measure the limb profile without scanning requires a combination of wide field of view and high spatial resolution that previous instruments have been unable to provide. This approach would require significantly less resources (power, weight, etc.) than current sensors, while providing similar performance. A preliminary scattering analysis of the instrument is also included.

We describe the design and characterization of a compact dual-polarization lidar that uses a liquid crystal variable retarder (LCVR) to discriminate between backscattered polarization states on alternate laser pulses (at 30 Hz). Measurements of the polarization discrimination of the system, including the liquid crystal and a Schmidt-Cassegrain receiver telescope, show that depolarization ratios can be determined with an additive error of less than 0.4%. The source is a Nd:YAG laser with a wavelength of 532 nm, pulse energy of 118 mJ, and pulse-repetition frequency of 30 Hz. The normal operating range is 15 km, with a 1.5-m range resolution. The full-angle receiver field of view is variable up to 8.8 mrad. Sample data from atmospheric clouds demonstrate the use of lidar depolarization measurements for distinguishing between ice and liquid water in thin clouds with low multiple scattering (with cloud phase verified using radiosonde profiles of atmospheric temperature and humidity). Also shown is a lidar observation of a depolarizing layer over Bozeman, Montana, identified as subvisual cirrus, aerosols transported from in or near China, or a combination thereof.

A simple method for the determination of the refractive index of liquids is demonstrated by using digital holographic interferometry (DHI). A number of experiments are performed to measure the refractive indices of liquid samples. Experimental results reveal that the deviation in refractive index measured by this method and that measured by Abbe refractometer are within experimental limits.

We design and develop a low-cost pyroelectric detector-based IR motion-tracking system. We study the characteristics of the detector and the Fresnel lenses that are used to modulate the visibility of the detectors. We build sensor clusters in different configurations and demonstrate their use for human motion tracking.

We propose a highly efficient content-lossless compression scheme for Chinese document images. The scheme combines morphologic analysis with pattern matching to cluster patterns. In order to achieve the error maps with minimal error numbers, the morphologic analysis is applied to decomposing and recomposing the Chinese character patterns. In the pattern matching, the criteria are adapted to the characteristics of Chinese characters. Since small-size components sometimes can be inserted into the blank spaces of large-size components, we can achieve small-size pattern library images. Arithmetic coding is applied to the final compression. Our method achieves much better compression performance than most alternative methods, and assures content-lossless reconstruction.

A copyright protection technique without using the original image in the process of copyright verification is demanded in most applications. In addition, robustness, imperceptibility, security, and unambiguity are also essential requirements for a copyright protection technique. In this paper, a robust copyright protection scheme for digital images, based on the low-band characteristic, is proposed to meet these requirements. Experimental results show that the proposed scheme not only can clearly verify the copyright of the digital images, but also is more robust than existing schemes against several attacks such as JPEG lossy compression, noise adding, sharpening, and blurring.

We investigate the implementation and application of the RC5 block cipher algorithm for digital images and provide testing, verification, and encryption efficiency of the RC5 block cipher for digital images. We describe briefly the basic design parameters of the RC5 block cipher and its implementation for digital images. A complete specification for the method of application of the RC5 block cipher to digital images is given. Several test images are used for inspecting the validity of the encryption and decryption algorithms. Also, we provide and introduce a mathematical measure for encryption efficiency, which we will call the encryption quality instead of visual inspection, and apply it to several images. The encryption quality of the RC5 block cipher algorithm is investigated along its several design parameters, such as word size, number of rounds, and secret key length, and the optimal choices for the best values of these design parameters are given.

The APEX method is a noniterative direct blind deconvolution technique that can sharpen certain kinds of high-resolution images in quasi real time. The method is predicated on a restricted class of blurs, in the form of 2-D radially symmetric, bell-shaped, heavy-tailed, probability density functions. Not all images can be usefully enhanced with the APEX method. However, the method is found effective on a broad class of galaxy images, including Hubble space telescope advanced camera for surveys (ACS) color imagery. APEX-detected optical transfer functions that successfully sharpen these images are very far from Gaussian, and of a type seldom found in the imaging literature. Several examples are given where significantly sharper and visually striking reconstructions are obtained, with sharpening confirmed by the tripling or quadrupling of image gradient norms.

We present a method for enhanced 3-D tracking using an interaction matrix. We developed two constraints to avoid the singularities and local minima of the interaction matrix. These constraints were combined with the minimization of tracking errors to identify suitable system configurations for enhanced tracking. Experiments were conducted to validate the method.

Imaging sensors have been deployed for a variety of nonintrusive diagnostics in photogrammetry, videometry, and other pertinent gross-field visualization. Especially, three-dimensional (3-D) remote sensing based on stereoscopic vision has become increasingly important in many research and industrial applications. Typical applications can be particle tracking in flow visualization, motion/deformation detection in dynamics and stress analysis, and robot vision in automation and quality control, to name a few. The use of an appropriate calibration technique for image sensing is thus essential in both laboratory and field applications. To provide a robust and reliable calibration capability for stereoscopic 3-D detection, we develop a hybrid technique that is based on the use of artificial neural networks and a conventional physical-mathematical model. The hybrid technique is advantageous in procedural simplicity; that is, ease in hardware setup and speed in data processing. Our results show that the hybrid approach can improve the accuracy in predicting the object space coordinates by about 30% compared to those based on a purely physical-mathematical model. It appears that the hybrid technique can combine the merits of both physical-mathematical model and artificial neural networks to improve the overall performance.

We present a high-speed 3-D spatiotemporal shape measurement technique by means of structured light. Current methods use a constant number of images that do not take into account the available temporal continuity of the measured object. That is, they focus on acquiring and processing as quickly as possible a fixed number of images to solve for the correspondence problem and later obtain the 3-D shape by triangulation. The number of images used imposes the use of some spatial support. The major contribution of our research is a new spatiotemporal scheme that, depending on the object's movement, adaptively uses the maximum number of projected images consistent with the local temporal continuity, therefore solving the correspondence problem with the minimum possible spatial support for each position. This is achieved by the use of a hybrid color pattern composed of an analog sinusoidal periodic code in the red channel and a digital binary spatial code in the blue channel that is cyclically displaced. No subpixel calculation is used and it is possible to implement error correction strategies that make the method fast and reliable, enabling dynamic online 3-D measurement of objects in movement.

Design of an enhanced surface plasmon polaritons (SPPs)–based nanostructure for the purpose of beam shaping is discussed. An indentation with depth-tuned grooves is presented to realize the beam shaping and extraordinary transmission. The nanostructure is directly fabricated using focused ion beam (FIB) milling on an Ag thin film coated on quartz with a thickness of 200 nm. A large measurement error is found during geometrical characterization of the nanostructures by use of an atomic force microscope (AFM) working in tapping mode. Apex wearing and 34 deg full cone angle of the probe generate the measurement errors during the characterization of nanostructures with a feature size of 200 nm and below. To solve this problem, an FIB trimmed AFM probe is employed in the geometrical characterization. The results show that the error is improved greatly using the trimmed probe. The desired excitation of the SPPs is derived using an optical fiber coupled CCD spectrometer after the modified geometrical characterization. The designed structure can be used as an optical probe for future inspection and detection use.