This PDF file contains the editorial “Designing Optical Systems, Part I: Analysis” for OE Vol. 43 Issue 07

The Fourier transform method for demodulation of carrier fringes has been extensively developed and widely used in optical metrology. However, the Fourier transform being a global operation, it has poor ability to localize the signal properties and hence the result of FTM is not ideal. A windowed Fourier transform method is thus proposed, with advantages of signal localization and noise filtering. An example demonstrates the improved result compared to the traditional Fourier transform.

We report dispersions resulting from a slot device (SD) in silica-based arrayed-waveguide grating (AWG). A SD is used to produce flattened passbands and we show the dependence on the bandwidth, the crosstalk, the ripple, and the chromatic dispersion (CD) of the passband in the presence of such a device. A comparison with other known techniques is also given. The device has been developed on a high index silica-based PLC platform but can be implemented on higher index contrast platforms.

Optical label switching based on combined differential phase shift keying (DPSK) modulation and subcarrier multiplexing is experimentally demonstrated at 10 Gbit/s DPSK encoded payload and 100 Mbit/s amplitude modulated subcarrier-multiplexed labeling. This scheme is spectral efficient and robust to fiber dispersion.

This PDF file contains the editorial “Illumination Engineering” for OE Vol. 43 Issue 07

Elliptical reflectors are widely used in illumination engineering as a means to concentrate the light of a source in the secondary focal plane. To collect as much light from the source as possible, the reflector must cover a wide angular range of the emitted radiation, hence forward and backward reflections on the reflector must be discussed and conditions for paraxial optics do not apply. We develop a framework of relations that describe the image of an extended source in the secondary focal plane of the ellipsoid. After deriving basic relations between object and image spaces, the propagation of rays depending of their origin and on their direction is investigated. As a result, the function and the aberrations of the reflector can be expressed in terms of an angular dependent magnification for longitudinal and lateral coordinates. With that, one cannot not only calculate luminance and illuminance distributions in the secondary focal plane in a straightforward manner, but can easily understand their properties and their dependence on ellipse parameters. On the basis of the developed relations, implications for the design of "elliptical" short arc high-intensity-discharge (HID) lamps for video projection are discussed.

The simultaneous multiple surface (SMS) method in 3-D geometry is presented. Given two orthotomic input ray bundles and another two orthotomic output ray bundles, the method provides an optical system with two free-form surfaces that deflects the rays of the input bundles into the rays of the corresponding output bundles and vice versa. In nonimaging applications, the method enables controlling the light emitted by an extended light source much better than single free-form-surface designs, and also enables the optics contour to be shaped without efficiency losses. The method is also expected to find applications in imaging optics.

An analytical model of light propagation in rectangular light pipes, termed the flux confinement diagram (FCD), is developed. Based on the edge ray concept of nonimaging optics, the FCD is a construction that describes the angular distribution of flux propagating in a light pipe depending on a light pipe's index of refraction and its geometry. With the FCD model, the angular "mode" of a ray can be defined at any plane in the system. The FCD model is developed here and used to describe flux input coupling, transport, and output coupling in light-pipe illumination systems. The practical example of a flux loss prediction due to a geometrical change (a bend) is examined using the FCD and compared to a ray-tracing analysis. We discuss how this model of flux propagation is a useful tool to aid in "first-order" design and layout of light-pipe illumination systems.

We propose a novel design method for reflective nonimaging concentrators that is based on maximizing system performance beyond standard nonimaging metrics of transfer efficiency and proscribed illumination distribution. This new technique enables system operation characteristics to influence the algorithm, including nongeometrical parameters. This algorithm is termed non-edge-ray design (NERD) because we found that edge rays do not always give optimal illumination design. The case of a diode-pumped, Nd:YAG solid state laser is provided as a confirmation of the design utility. The pump-to-mode configuration, which includes a nonimaging pump cavity reflector, is the focus of this investigation. The merit function of the design process includes not only the transfer efficiency from an extended, 2-D laser diode array, but also the mode coupling of the absorption distribution within a laser rod to its desired output mode. Standard edge-ray design is shown to limit performance, with both numerical and experimental results. Two alternative pump cavity reflectors are developed and the improvements in the outputs from the lasers in TEM₀₀ modes are presented. A periodic cavity sees over 8% improvement in optical efficiency at an output power of 10 W, while an averaging cavity provides nearly 5% improvement.

The simultaneous multiple surface (SMS) method is used to design air-gap RXI-type lenses, which efficiently produce a very narrow beam from high-powered LEDs. These designs are compact, with both front surface and reflecting back surface calculated simultaneously from periphery inward, as profiles of circular symmetry, via applying the edge-ray principle to the chip geometry. The light source is a Lambertian-glowing cube 1.2 mm square and 0.15 mm high, as viewed through its clear (n = 1.54) package dome, with emission down to 95 deg from the symmetry axis. A given exit-aperture diameter defines a minimum, étendue-limited collimation angle, α = arc sin (chip-width/diameter). At the center of the back surface there is a cavity surrounding the source. The front surface reflects that source flux to the back surface, which reflects it back forward again, accomplishing the optical folding thereby. The back surface is shaped so that the light it reflects forward will be refracted out the front surface to become the collimated output beam.

Given a set of available LEDs or other light sources with known specifications including spectrum, total luminous flux (in lumens), and efficacy (in lumens per watts), we show how to select that combination which yields light of the desired (photometric) color and, in addition, maximizes various objectives such as efficacy, luminous flux, and color rendering index.

Lightpipes are essential to automotive interior illumination systems. These light distribution systems are constrained by geometrical complexity, which makes both analysis and fabrication difficult. Their development depends on empirical procedures that are often inefficient and time-consuming. We review the successful modeling procedure of one complete automotive illumination light-pipe system, which includes a light source and a light pipe. The success of modeling is verified by comparing results to experimental measurement. A simple method of light-pipe evaluation based on nonsequential ray trace optical modeling for the automotive illumination industry is demonstrated. Extensive modeling of the light sources for these systems is also included.

The Wavien's dual paraboloid reflector system improves the output brightness of an illumination system by preserving the brightness of the arc. We present steps for optimizing a projection engine illumination system using this reflector system and an output tapered light pipe. The output from the light pipe is then fed into a supercube polarization recovery system for its freedom from problematic wave plates. Ray-tracing software is used to optimize the components, including the dual paraboloid reflector parameters and the supercube polarization recovery system. The model shows agreements with the experimental results and is suitable for designing and predicting future system performance.

We discuss the impact of spatial coherence on the defocusing properties of a retinal projection display. The paper demonstrates that there is an optimum degree of spatial coherence that significantly increases the depth of focus as compared with a fully incoherent or a fully coherent illumination. To this end, a theoretical analysis of the effect of light coherence on image contrast is presented and the influence of coherence effects on image quality is discussed. Measurements of both the contrast function (CF) and the text readability when defocusing are presented using a bench model of the display system. The text readability is evaluated using a combination of objective image quality criteria and human experiments. The results reveal that for reading text targets the best depth of focus can be achieved not by fully coherent illumination, but by partially coherent light of a spatial coherence level σ between about 0.35 and 0.5. Further, it is shown that the optimum σ level depends slightly on the targeted text size.

A generalized nonparaxial theoretical framework based on the scalar diffraction theory is developed to describe the propagation of an optical field through a linear optical system with quasi-monochromatic spatially incoherent illumination. Software implementation of this theoretical framework on single and multiple processor platforms was developed and simulated results of the imaging process through optical aberration-corrected optics are presented for both in-focus and out-of-focus imaging, validating the first-order nonparaxial model.

We present the design, performance, and tests of a new generation of cooled large-area (5 cm²) optical composite bolometers with a pure germanium absorber, permitting a good efficiency from near-IR to x-rays. With a sensitivity often better than photomultipliers or semiconductor diodes, they allow fluorescence measurements of cold targets with no window, no infrared background, good optical couplings, and a flat response on a large spectral band. Performance obtained at 25 mK is very promising: noise equivalent power as low as 4×10^–17 W/Hz^½ in the continuous mode, energy threshold about 50 eV in the pulse detection mode, and time constant τ~3 ms. These detectors of low mass (0.25 g) have been successfully used for detecting the fluorescence emitted by much more massive bolometers, having, for example, a BGO (92 g) or a CaWO₄ (54 g) target. The simultaneous detection of heat and light in these double bolometers permits the identification of each event in the massive target (α decay, γ, or cosmic ray interaction, neutron recoil...). Thanks to the consecutive excellent subtraction of the radioactive and cosmic ray background, it is a powerful tool developed by several groups for fundamental research: study of very rare decays of atoms, measurement of internal very low radioactivity content in single crystals, direct detection of dark matter recoils in massive fluorescence targets, and detection of solar neutrino fluorescence events in liquid ⁴He. Recently obtained results are reviewed: the first detection of the rare alpha decay of ²⁰⁹Bi, and new scintillation data on Al₂O₃ (sapphire), LiF, or TeO₂ at 20 mK. By cooling at 10 mK, sensitivity can yet be increased by more than one order of magnitude.

We present a multiscale image approach for contrast enhancement based on a composition of contrast operators. Contrast operators are built by means of the opening and closing by reconstruction. The operator that works on bright regions uses the opening and the identity as primitives, while the one working on the dark zones uses the closing and the identity as primitives. To select the primitives, a contrast criterion given by the top-hat transformation is proposed. Thus, the regions verifying the criterion remain intact as in the original image, whereas the other regions are attenuated by the opening or closing. This choice enables us to introduce a well-defined contrast in the output image. Moreover, by applying these operators by composition according to the scale (size) parameter, the output image not only preserves a well-defined contrast at each scale, but also increases the contrast at finer scales. Because of the use of connected transformations to build these operators, the principal edges of the input image are preserved and enhanced in the output image. Finally, since the contrast operators do not increase the gray level of the image, these operators are improved by applying an anamorphosis to the regions verifying the criterion.

We derive a modified version of cubic convolution interpolation for the enlargement or reduction of digital images by arbitrary scaling factors. The proposed scaling scheme is used to resize various format pictures in the transcoding system, which transforms the bitstream compressed at a bit rate, such as the HD bitstream, into another bit rate stream, for example, the SD bitstream. The transcoding is performed in spatial domain. In many applications such as the transcoder, the resolution conversion is very important for changing the image size while the scaled image maintains high quality. The scaling process consists of two steps: fitting the original data with a continuous function, and resampling the function on a new sampling grid. We focus on the modification of the scaler kernel according to the relation between formats of the original and the resized image. In the modification, various formats defined in MPEG standards are considered. We show experimental results that demonstrate the effectiveness of the proposed interpolation method. The algorithm exhibits significant improvement in the minimization of information loss when compared with the conventional interpolation algorithms.

Small target detection and tracking are important for laser radars in many applications such as laser weapons, directed infrared countermeasures (DIRCM), fire control, target recognition, and free-space laser communications. The detection and tracking performance depends on the mode of detection, SNR, target signal statistics, beam jitter, and turbulence induced irradiance variations. We show results of the root mean square (rms) tracking error versus SNR primarily for direct detection systems using a strong glint return. For the general case of a certain signal and noise probability density functions (pdf) it is hard to obtain analytical solutions for the mean and variances of the estimates for the rms tracking error. We have therefore used numerical simulations to illustrate how the pdf and SNR will affect the tracking accuracy. Gamma functions and other pdf's can be used to characterize the signal distributions to get a first hand on tracking performance. The results are presented as tracking errors versus the angular spot size of the laser beam in the tracking detector plane. We also investigate the beam optimization problem for target detection and "power in bucket," which is maximizing the laser energy at the target. We find that there are optimum beam sizes (w) versus the rms jitter (σ) and that optimum w/σ (minimizing the false alarm rate for a given detection probability P_d) typically fall in regions 1 to 3 depending on probability of detection and the representative pdf for the application in mind.

The switching characteristics of a semiconductor optical amplifier (SOA)-assisted Sagnac gate are analyzed in terms of their critical performance parameters for full duty cycle operation from 10 to 40 GHz. Within this frame, the influence of the control pulse width, as well as of the SOA gain recovery time on the switching energy and the contrast ratio, is examined through numerical simulation. The obtained results show that full switching operation at 40 GHz or higher is feasible either by deploying gain recovery reduction techniques in bulk SOAs, or other alternative technologically advanced optical devices, such as quantum-dot SOAs.

We set up a compact high-resolution linear charge-coupled-device (CCD)-based spectrometer for the characterization of optoelectronic materials and devices. The spectrometer is controlled by a personal computer. Detailed design diagrams of the mechanical structure and electronic hardware are described. Spectral resolution of the CCD measurement system of ~0.7 Å/pixel is determined from spectral calibration using two laser sources. Electroluminescent measurements of several selective optoelectronic components, as well as the transmission measurements of GaSe and GaSe_0.9S_0.1 layered semiconductors, are performed utilizing the new spectrometer. The experimental results are analyzed and discussed.

A compact large Gaussian mode field diameter single-mode fiber is developed. This expanded single-mode fiber consists of splicing fibers with various lengths and characteristics. The principle of this optical arrangement is to excite the first mode of propagation of the multimode graded index to preserve a Gaussian field. The coupling properties are numerically analyzed, considering the effects of gap, radial offset, and tilt. The fiber connections loss between the same kinds of expanded single-mode fibers with a Gaussian mode field diameter of 18 μm is measured with an average of 0.55 dB. This large mode field diameter is preserved on a long length of fiber >20 cm.

Genetic algorithms are applied to the design optimization of a lightguide. Physically impossible offspring generated by the genetic algorithm evolution are removed by a simple logical test before further iterations. The performance of the lightguide and the evolution of the design are measured by a single criterion combining both uniformity of illumination and brightness. The process produces a nonintuitive solution, and experimental results match the theoretical predictions and show an improvement in performance of the final design.

Two-micron detectors are critical for atmospheric CO2 profiling using the lidar technique. InGaAs and HgCdTe detectors are commercially available for this wavelength but they lack sufficient gain, which limits their detectivity. The characterization results of a novel AlGaAsSb/InGaAsSb phototransistor for 2-μm application are reported. The device was developed by AstroPower, Inc. for NASA Langley Research Center. Spectral response measurements showed the highest responsivity in a 1.9- to 2.1-μm region with a maximum value of 2650 A/W at 2 μm. A 2-μm detectivity of 3.9×10¹¹ cm Hz^1/2/W was obtained, which corresponds to noise equivalent power of 4.6×10^–14 W/Hz^1/2.

A desktop vision system is presented for complete 3-D model reconstruction. It is fast (3-D reconstruction in under 20 min), low cost (uses a commercially available digital camera and a rotation stage), and accurate (about 1 part in 500 in the working range). Partial 3-D shapes and texture information are acquired from multiple viewing directions using rotational stereo and shape-from-focus (SFF). The resulting range images are registered to a common coordinate system, and a surface representation is created for each range image. The resulting surfaces are integrated using an algorithm named region of construction. Unlike previous approaches, the region of construction algorithm directly exploits the structure of the raw range images. The algorithm determines regions in the range images corresponding to nonredundant surfaces that can be stitched along the boundaries to construct a complete 3-D surface model. The algorithm is computationally efficient and less sensitive to registration error. It also has the ability to construct complete 3-D models of complex objects with holes. A textured 3-D model is obtained by mapping texture information onto the complete surface model representing the 3-D shape. Experimental results for several real objects are presented.

A transmission-line model of a two-stage all-fiber soliton-effect femtosecond-pulse compressor is designed using comprehensive simulation tools. The compressor is predicted to have compression factors as high as 24, with low peak power (<0.5 nJ pulse energy at a 180-fs pulse width) and high repetition rate (~40 GHz) without many of the disadvantages of heretofore-designed compressors. Prior optical pulse compressors are primarily based on solid state lasers or fiber Bragg gratings. Solid state lasers are relatively large, have high pulse energies (~10 nJ), and lack the precision needed for many applications, such as in medical diagnosis and treatment. Fiber Bragg grating compressors are limited in attainable compression factors and intensity levels and have restricted use with other fiber Bragg gratings. Other fiber compressors face third-order dispersion (TOD), self-steepening (SS), intrapulse stimulated Raman scattering (ISRS), and higher order dispersion (HOD), which limits their use. The suggested design addresses a requirement for the modeling of an all-fiber optical pulse compressor that answers the preceding objections. The designed compressor, has no third-order or higher order dispersion, and is not affected by ISRS and SS effects, which can otherwise reduce the quality of the compressed pulses.

A new wavefront sensor data processing algorithm is described and analyzed. The wavefront sensing concept is similar to a Shack-Hartmann type wavefront sensor, but uses an image projection correlation algorithm as opposed to a centroiding approach to estimate optical tilt. This allows the wavefront sensor to estimate tilt parameters while guiding off of point sources and extended objects such as the surface granulation of the sun. The projection-based cross-correlating scheme differs from a 2-D correlation-based tilt estimation approach in that the images are vectorized on the focal plane array itself prior to readout. This on-chip preprocessing approach allows the wavefront sensor data to be compressed, which results in a large reduction in the amount of data read out of the focal plane array while maintaining the desired bandwidth of the adaptive optical system. An implementation of the projection-based wavefront sensor algorithm is described in detail, showing important signal processing steps on and off of the focal plane array of the sensor. The algorithm design is tested in simulation for speed and accuracy by processing simulated solar and astronomical datasets. Timing analysis is presented, which shows how the collection and processing of image projections is computationally efficient and lends itself to a wavefront sensor design that can produce both competitive speed and tilt estimation accuracy.

We describe a novel electron-impact x-ray source based on a high-speed liquid-metal-jet anode. Thermal power load calculations indicate that this new anode concept potentially could increase the achievable brightness in compact electron-impact x-ray sources by more than a factor 100 compared to current state-of-the-art rotating-anode or microfocus sources. A first, successful, low-power proof-of-principle experiment is described and the feasibility of scaling to high-brightness and high-power operation is discussed. Some possible applications that would benefit from such an increase in brightness are also briefly described.