This PDF file contains the editorial “Editorial: Editorials--A Communication Tool” for OE Vol. 33 Issue 12

This PDF file contains the editorial “Guest Editorial: Optics in Ireland” for OE Vol. 33 Issue 12

It is suggested that the measurement of the angular distribution of light transmitted through a thick slab of diffusively scattering material may provide information about the nature of the local scattering events. Monte Carlo simulations indicate that the angular distribution depends on ℓ/ℓ* where ℓ and ℓ* are the true and transport mean free paths, respectively, and this dependence is confirmed analytically using a modified diftusion model. Preliminary experimental results for liquid and solid foams are explicable in terms of strong forward scattering in the individual scattering events.

Ground-based measurements of upper atmospheric parameters, such as temperature and wind velocity, can be made by observing airglow emissions that have a well-defined altitude profile and that are known to be representative of the emitting region. We describe the optical observatory at Maynooth (53.23° N, 6.4° W) at which two instruments, a Fabry-Pérot interferometer and a Fourier transform spectrometer, are used to record atmospheric airglow emissions in Ireland at visible and near-infrared wavelengths, respectively. Descriptions of the instruments, data acquisition, and analysis procedures are provided, together with some sample results.

Two evanescent wave fiber optic sensors for oxygen are reported: one intensity based and the other based on phase fluorimetry. Both sensors employ the quenching by oxygen of the fluorescence from a ruthenium complex trapped in the cagelike structure of a sol-gel-derived porous film on a declad section of multimode optical fiber. The sensors exhibit excellent performance using excitation from new high brightness blue LEDs and establish the viability of low-cost portable sensor devices based on the 501-gel process.

Spectral measurements of the v₁ + v₂ + v₃ combination absorption band of H₂S were made over a 10-nm wavelength range around 1.57 m with two tunable distributed feedback (DFB) laser diodes. Targeting the strongest observed H₂S absorption feature, a detection limit at atmospheric pressure of 50 ppm/m is indicated from the measured detectivity at higher concentration. We also probed the 2v₁ + 2v₂ + v₃ overtone and combination absorption band of CO₂ and investigated the dependence of absorption linewidth on pressure and assessed its implications for expected low-detection limits. These results demonstrate the utility of DFB laser diodes for industrial applications in the area of toxic and trace gas monitoring.

A preliminary investigation into the use of multiwavelength fiber drop analyzer (FDA) for the measurement of viscosity, spectral absorbance, and refractive index is made with a view to obtaining conservative estimates of the instrumental capability of the FDA for these measurands. Some important new insights into drop vibrations are made from studies on the fiber drop traces (FDTs) of mechanically excited damped vibrations in drops with a set volume. A brief description of the feasibility measurements on the first application of the FDA in the diagnosis of disease in synovial fluid is given. Strong experimental evidence is reported for the existence of the surface-guided wave (SGW) peak of the fiber drop trace and some new insights into the nature of the FDT are suggested based on a comparative study of the FDTs from a multiple-wavelength and a single-wavelength FDA. The earlier reported drop period dependence on applied electric field is critically reexamined, a new interpretation of this effect is suggested, and an experimental study of clarification is given. Finally, a brief review of the projected capabilities of the FDA based on the work reported here is provided.

The dependence of absorbance on sample concentration and sample length in evanescent wave spectroscopy, carried out by means of weakly guiding step index optical fiber sensors, is investigated theoretically. The absorbance is found to depend on the product of the sample bulk attenuation coefficient (α), the length of the fiber in contact with the sample (L), and the inverse of the normalized frequency (V) of the sensing section of the waveguide. When the mode volume of the fiber sensor is filled, the absorbance is predicted to vary nonlinearly with concentration (or sample length) at low values of αL/V and to increase as the cube root of αL/V at high values. However, if the mode volume of the sensing section of the fiber is only partially filled, a linear dependence of absorbance on αL/V is indicated. The results of the model are compared with experimental data already published by various groups.

The nonlinear optical response of semiconductor interfaces using three-wave mixing experiments in general, and second harmonic generation (SHG) in particular, has been widely studied, but it is only recently, with the advent of commercial tunable pulsed laser sources, that the spectroscopic aspect of SHG can be more readily exploited. Results from porous Si and Si(100)-Sb are reported that illustrate the potential of spectroscopic SHG as a probe of semiconductor interfaces.

The effect of short- and long-term frequency instability in self- pulsation on all-optical synchronization using a twin-section laser diode is experimentally investigated. Short-term frequency instability broadens the unlocked full width at half maximum (FWHM) of the fundamental of the ii spectrum of the self-pulsating laser diode. We show experimentally that the value of the unlocked FWHM, and thus the level of short-term instability, has a direct effect on the optical power required to maintain synchronization. A novel means of reducing the FWHM is presented, based on a reflective transmission line stub connected to the absorber of the twin-section self-pulsating laser diode in use. A reduction of up to 5 dB in the average optical power required for effective synchronization is observed. Long-term frequency instability can prevent synchronization from taking place because of the limited lock-in frequency range of a self-pulsating laser diode. It is shown that for the devices used here, the dominant cause of long-term instability is temperature. A new method of sensing the temperature in a twin-section laser, called absorber current temperature sensing, which reduces the measured unsycnchronized frequency drift by more than 5:1, is demonstrated. The results have important implications for the design of new all-optical synchronization subsystems, based on self-pulsating laser diodes.

A two-section traveling-wave semiconductor laser amplifier can be utilized as a time-division demultiplexer or channel selector in all-optical networks. Light injected into the amplifier causes changes in the carrier density, which manifests itself as a change in the amplifier electrode voltages that can be processed to obtain the original data signal. Numerical analysis shows that by suitable choice of the electrode bias currents responsitivity and speed are optimized while system transparency is maintained. With optimal device design, demultiplexing speeds of the order of several gigabits per second are feasible.

By suitably modulating a single multielectrode semiconductor laser and processing the optical output, a wideband frequency-swept signal from near-dc to millimeter frequencies can be generated. Initial results reveal that a usable sweep range from 60 MHz to 22 GHz (receiver-bandwidth-limited) can be achieved with low-residual amplitude modulation, with bandwidths of greater than 125 GHz being possible. Generated frequency resolution is limited in laser frequency stability and linewidth.

We have observed time-resolved spectra and spontaneous near-field profiles of vertical cavity surface-emitting lasers and have considered possible reasons for the origin of multiple transverse modes. The results show that, even under low-duty pulsed operation, the evolution of temperature profiles plays a dominant role in determining the transverse mode behavior.

The excitonic transition in II-VI quantum well materials has recently been used as the basis for optical modulators and also as the lasing transition at low temperatures. The central aspect in the use of the exciton resonance in optical devices is the understanding of the exciton linewidth. We present a detailed study of the interactions that affect the linewidth in II-VI semiconductor quantum well materials. The broadening of the resonance with increasing temperature can be controlled by altering the material parameters of the II-VI structures. In so doing the exciton binding energy can exceed the LO-phonon energy and thereby reduce the homogeneous contribution to the measured linewidth. Efforts to reduce well-width fluctuations in the growth of the II-VI quantum well structures, which are responsible for the inhomogeneous linewidth, must also be made to a limit where the room temperature Iinewidth is narrow and homogeneously broadened.

An extended effective mass model is developed to account for the possibility of mixing of the electron wave function from the Γ conduction-band state to the X conduction-band state. The strength of the mixing is determined by a mixing parameter Δ in association with the assumption of a finite interface thickness. For a simple quantum well, the model Δ compares favorably with more complicated empirical pseudopotential models, featuring complete descriptions of the band structure. A multiquantum barrier (MQB) has been designed. Using the single-band effective mass approximation model, for use in visible laser diode structures for enhanced electron confinement, it is clear that within the limitations of the model when mixing effects are accounted for, the degradation in performance is negligible. The MOB design compared with a single barrier of equivalent thickness shows a doubling of the electron reflection coefficient.

A Fabry-Pérot interferometer-based time-resolved spectral measurement system capable of transform-limited performance is described. The measurement system results from a model, developed for the Fabry-Pérot interferometer, from which the mirror reflectivity emerges as the critical parameter in determining both the temporal and spectral response. By employing deconvolution, the spectral response of the system is capable of transform-limited performance. The system is demonstrated by carrying out time-resolved spectral measurements on a multielectrode distributed feedback semiconductor laser under a number of different modulation formats.

Recent work on an acrylamide-based photopolymer holographic recording medium is presented. The lifetime of recorded gratings is improved by the addition of cross-linking monomers. Shelf life is also improved. The effects of various constituents of the photosensitive material are studied to determine an optimum composition, and five xanthene dyes are compared as possible sensitizers for the system. With the most sensitive dye and optimum concentrations of the other constituents an improved formulation is presented with high sensitivity and very high diffraction efficiency. The new formulation also works well in reflection mode.

Directionally sensitive in-plane displacement speckle interferometers in which the modulation of the intensity of a single speckle can be used to monitor the motion of a target surface were reported by Reynolds and Toal in 1992. Difficulties arise due to the phenomenon of speckle motion, which causes a randomly varying intensity at the detector, upon which the interferometric signal is superimposed. This, in turn, makes reliable digital counting systems difficult to implement. Here we report on two methods of overcoming these problems. One employs a simple digital electronic algorithm and the other uses an optical phase-shifting technique. Results obtained for all systems confirm the validity of the principles underlying their design and indicate possible avenues for further improvement.

The small focused spot size and localized heating possible with laser soldering makes it an attractive alternative technique for bonding surface mount devices with small lead pitch onto a printed wiring board. Using a Nd:YAG laser, we have applied this technique to reflow soldering of a test device-a 224-pin quad ceramic chip (lead spacing 25 mil)-onto pretinned substrates. The soldering step was incorporated into a larger workcell, in which an Adept robot was used to place the component in position and also to scan the laser beam, delivered via a fiber, over the leads to be soldered. The mobile laser head was modified to accept a miniature CCD camera coupled to a Cognex vision system, which allowed coincident viewing of the soldering process and postsoldering inspection of the joints.

In situ optical reflectometry at 670 nm is used to monitor the pulsed laser deposition of thin films and multilayers of copper and cobalt. By comparing measured reflectivity with a Fresnel calculation, which includes a Bruggemann description of the dielectric function for partially dense material, the nature of the early stages of the deposition of copper on cobalt can be discerned. It seems that the deposition begins with the nucleation of a partially dense layer, and the formation of fully dense material commences after a deposition corresponding to about 0.3 nm of Cu. Thereafter the thickness of the fully dense layer increases while a partially dense surface layer is maintained. The experiment demonstrates that optical reflectometry can be an effective thickness monitor for the deposition of ultrathin metal films and multilayers and also yields some new information on the nature of the growth.

A facility for producing photoluminescence (PL) images of semiconductor wafers at cryogenic temperatures is described. A custom-designed liquid helium cryostat capable of accepting 50-mm-diam wafers is used together with a scanning optical table for PL excitation and collection. Spectral analysis is achieved by the use of a CCD detector mounted on a standard grating spectrometer. The configuration used enables a full spectrum to be recorded for each point on a wafer with a spatial resolution of approximately 100 μm. The spectral information may be used to produce images of the total PL intensity, the peak PL intensity, and the wavelength at which the peak intensity occurs. Typical PL maps are presented to illustrate the quality of the information.

EUV emission from laser plasmas produced on low Z target material such as plastics and araldites that contain 1 to 10% concentrations of the elements 50 ≤ Z ≤ 70 emit narrow regions of intense quasicontinua in the 6 to 13-nm wavelength range. The typical bandwidth of the emission is 0.5 to 1.5 nm and the features are brighter than any individual lines emitted from the host substrate materials. Similar spectra can be obtained from hydrated salts of these elements, so suitable targets can be prepared as compressed pellets of crystalline or hydrated salts or alternatively from oxides mixed into an araldite matrix. Spectra of tin salts prepared in this way provide an excellent narrowband source for 13.5-nm radiation for use with Mo/Si multilayers. Since the peak shifts toward shorter wavelengths with increasing Z, plasmas of these elements provide "tunable" narrowband EUV radiation sources.

A new multilaser multichannel spectrometer system has been developed that allows a wide range of investigations of the interaction of extreme-ultraviolet (EUV) photons with matter in the form of free atoms or ions, e.g., in gases or plasmas or bound as in solids. The EUV photons are generated by a laser-produced plasma. Applications to the study of photoabsorption by thin foils, gases, and ground- and excited-state atoms and ions are described. The design and performance of a collimated, quasi-monochromatic, intense source of EUV radiation based on the combination of a laser-produced plasma with a EUV multilayer mirror is also reported.

The application of extreme ultraviolet (XUV) radiation to studies of core excited atoms, ions, molecules, and solids has led to the demand for suitable XUV continuum light sources. An excellent, compact, reproducible source is the laser produced plasma, which for a suitable choice of target material can be used to generate a line-free continuum throughout the vacuum ultraviolet (VUV) and XUV from 4 to 200 nm. The characteristics and underlying physics of such sources are discussed and their application to inner shell photoexcitation studies of atoms and ions are briefly described.

A design procedure for triple-clad dispersion-flattened single-mode fibers is developed. Following this procedure, fibers are identified that have lower dopant concentrations in the core than those of previous dispersion-modified fiber designs, a second-mode cutoff wavelength close to the lower operating wavelength of 1300 nm, zeros of dispersion at 1300 and 1550 nm, a small bending loss at 1550nm (for a bend radius of 5.0 cm), and spot sizes that are large enough not to compromise the splicing loss.

The loss and spectral characteristics of fused tapered coupiers depend, respectively, on the taper transition rate and on the size, shape, and length of the coupler waist region. We show how the choice of the fusing and tapering burner is critical in optimizing the behavior of three different coupler types: the single-wavelength splitter, the wavelength division multiplexer, and the wavelength-independent coupler.

A global self-referencing system allows local adaptive optics to be used in a pure optical system for wavefront reconstruction. No conventional, discrete optical components are required.

In an effort to increase achievable postdetection signal-to-noise ratios (SNRs) of continuous-wave, 1-μm all-solid-state ladar systems, a prototype rare-earth-doped optical-fiber amplifier has been included in the optical return signal path of both a heterodyne and a direct-detection ladar system. We provide numerical predictions for SNR increases according to our previously developed theory. We also detail our experimental efforts and provide the results of SNR measurements for four distinct cases: direct ladar detection with and without a fiber amplifier, and heterodyne ladar detection with and without a fiber amplifier. Experimentally measured increases in SNRs for ladar systems incorporating an optical-fiber amplifier are then compared with our earlier predictions. Specifically, we have found that for direct detection with a fiber amplifier in place, the predicted SNR increase is 42.0 dB, and we have measured an increase of 36.5 dB. Similarly, for heterodyne ladar detection with a fiber amplifier, the predicted SNR increase is 3.8 dB, and we have measured an increase of 8.0 dB.

Making use of the beam fanning effect in a BaTiO₃ crystal, we propose two simple setups to perform the function of an optical image-combiner/OR-gate and an optical AND gate, respectively. The optical image combiner combines two patterns, mutually coherent or mutually incoherent, from two input channels into a single coherent one. Experimental results have shown that the response time of these devices ranges from 0.01 to 2.5 s. In the image-combiner/OR-gate device, three optical waves are involved. The first two carry the input signals into a 45-deg-cut BaTiO₃ crystal; the third one brings the output out of the BaTiO₃ crystal to the output plane. The AND-gate device, however, requires only two input beams.

A method is proposed for designing a distortion-invariant correlation phase-only filter by iteratively optimizing the Homer efficiency. Comparisons with conventional filters show some attractive properties of the proposed filters, such as a high correlation peak and good discrimnation ability. An effective improvement for reducing the peak deviation of the phase-only filter is also proposed. The correlation performance of the constructed filters is confirmed by computer simulation.

We present an integral descriptor whose value is invariant with respect to translation, rotation, and scale changes of an object. The descriptors of objects in the input scene can be presented as correlation peaks by means of optical correlators. A triple-invariant pattern recognition method is proposed based on the descriptor. To improve the discrimination, the use of a descriptor vector is suggested.

A total of 300 templates were stored as angularly multiplexed transmission holograms at multiple spots on a photopolymer disk. A novel correlator architecture is described and demonstrated that correlates all the templates against an input image within one disk rotation (≈ 20 ms). The projected performance of the system is 150,000 templates per disk and a processing rate of approximately 6000 correlations per second.

We propose a new way for computing the solid angle subtended by a planar figure. Our method uses a line-integral representation of the solid angle. The path of integration is the boundary of the planar figure. We use this formula to develop an exact expression for the solid angle subtended by a polygon. This expression is simple and easy to compute. For any other shape, we first convert it to a polygonal one and then use the formula for a polygon. We validate our approach by computing the solid angle subtended by circles and ellipses.

The Levenberg-Marquardt version of least squares, namely the damped least-squares method, is widely used in lens design optimization. Several modifications of the approach have been proposed to accelerate convergence of the optimization procedure. Recent developments in nonlinear optimization theory indicate that the basic Gauss-Newton method of least squares can play a useful role for this purpose in many practical applications. Giving a brief outline of the pertinent developments, the paper reports on the feasibility of using the basic Gauss-Newton method of least squares in practical lens design optimization when, at each iteration, a line search procedure follows the least-squares solution to determine the optimum change vector for that particular iteration stage. It is observed that incorporation of the line search procedure provides good convergence even without any damping of the least-squares procedure. Some illustrative numerical results are presented.

The manufacture of segmented active primary mirrors poses difficult fabrication problems for the component off-axis aspheric mirrors. An effort was conducted to develop and demonstrate the necessary fabrication technology to rapidly manufacture the segments of 10-m-class active mirrors. On this project, a 4-m, pie-shaped, 1.7-cm-thick, off-axis parabolic facesheet was fabricated. The key steps of this operation were shaping the segment (prior to all surfacing steps), generating the back and front surfaces using numerically controlled machining, and computer-controlled optical surfacing (CCOS) for vacuum grinding and polishing the front surface. The CCOS polishing rapidly brought the facesheet to within 0.035 μm rms of the proper parabolic surface and produced a superior surface finish with only 8-Å roughness and no visible scratches.

Simulation results are presented for the accumulated phase change in cascaded Kerr phase modulators. The results indicate higher phase modulation for multistage cascaded structures. A scaling scheme for applied optical power that leads to optimum phase modulation is also given for these structures. Cascaded configuration of Kerr modulators can be used for implementing low-power optical switching and amplification.

A new approach for estimating the depth of both diffuse and specular surlaces is presented. The method uses the 1-D local spatial phase of a 2-D image of the object acquired using structured-light illumination. The induced phase modulation of the structured-light pattern is used as a depth cue. For diffuse surfaces, the local phase is used directly to estimate the depth. For specular surfaces, fiduciary level crossings of the phase are used. The use of level crossings has been found to enhance the performance for specular targets. The spatial phase is estimated using both an analytic signal representation and a linear prediction model. The analytic signal representation requires less computation and is shown to give performance comparable to the linear-prediction method. The new approach provides high-resolution depth maps and avoids the difficulty of image registration.

A method for determining the transfer function of a continuous system from sampled responses to single- and multiple-pulse excitation is presented. The method is an extension of the sampled-edge-response method pioneered by the theoretical work of A. Papoulis in 1962 and the application of the theory to optical systems by B. Tatian in 1965. Occasions arise when pulse rather than step-function stimuli are available for system excitations. In such cases, the method presented is useful for determining the system transfer function. The use of antialiasing filters and estimation of non-bandwidth-limited transfer functions are discussed. Practical application of the method to characterizing the system transfer function of a commercial 8- to 12-μm infrared imager is also presented.

This PDF file contains the editorial “Book Rvw: Noise in Digital Optical Transmission Systems," by Gunnar Jacobsen for OE Vol. 33 Issue 12