This PDF file contains the editorial “Setting Tolerances” for OE Vol. 42 Issue 03

We propose a new algorithm which constitutes an improvement over conventional video shot boundary detecting methods based on frame differences and histogram differences. Instead of using absolute frame differences, block-by-block based relative frame differences are employed. Frame adaptive thresholding values combined with selectively applied histogram differences are also employed for better detection.

A thermo-optic variable optical attenuator based on a multimode interference coupler principle is fabricated. The propagation loss of the fabricated device is 1.6 to 3.8 dB at the wavelength range 1510 to 1610 nm, which is very near the calculated value (1.2 dB) by the finite difference beam propagation method. The maximum power consumption is 363 mW and the dynamic attenuation range is 0 to 26 dB. The response frequency of the fabricated attenuator is about 10 kHz.

A new technique is proposed to test beamsplitters using two-wave mixing in photorefractive crystal. The method uses the diffraction efficiency of two-wave mixing in Bi₁₂TiO₂₀ (bismuth titanium oxide). Detailed qualitative and quantitative analysis is given.

A liquid-crystal spatial light modulator (SLM) can be used to alter the phase of the wavefront to achieve a deflection or a change in the shape of a laser beam. Ideal blazed gratings have perfect diffraction efficiency, and all the light is deflected. Encoded onto a pixelated spatial light modulator, however, the spatial limitations and phase quantization result in a stepped, nonideal blazed grating and decreased diffraction efficiency. The far-field intensity distribution was measured for different deflection angles using 4 and 32 phase levels. The result has been analyzed and used to construct a simple model of the studied SLM. Simulations of the far field based on this model show good agreement with the experimental results.

A new design scheme for a polymeric waveguide thermo-optic beam deflector is presented and experimentally demonstrated. The refractive indices of the waveguide materials can be thermally tuned by applying electric current through a metal thin film electrode deposited and patterned using standard lithography on top of the waveguide. In doing so, the geometry of the electrode determines the performance and reliability of the beam deflector. We form a pair of heating electrodes sitting opposite to each other like interlocked sawteeth that have a folded-thin-strip substructure, and hence demonstrate an advanced beam deflector that has biangular sweeping capability, low power consumption, less wavefront distortion, and high reliability. A full sweep angle of 56.5 mrad (3.24 deg) and 1×8 switching capability at a 1550 nm wavelength are attained with an average power consumption of 247 mW per switching between adjacent resolvable spots. Switching from the zero-bias spot to the first resolvable spot exhibits a response time of 2 ms in both rising and falling.

We introduce the iteratively clustered interpolation algorithm to compute a structured printer inverse look-up table from irregularly sampled experimental color data. The algorithm is based on a gradient optimization method, with initial points generated through an iterative technique. Experimental results with a digital color printer are provided to illustrate the algorithm.

We propose a new approach to object matching and shape reconstruction across two views. The approach relies on the topological relationship among the feature points and a planarity approximation for the object surfaces in carrying the object from the model view to the scene view. The correspondences are then refined by fitting an active contour model to the transferred feature points on the scene view, which automatically assumes the shape of the object on correct matching. For the purpose of 3-D reconstruction, it could be shown that while the establishment of such correct correspondences alone would result in ambiguities under the assumption of weak perspective projection, the adoption of a regularization criterion, in the form of a depth-continuity constraint, and a set of geometric constraints associated with selected subset of surfaces and contours of the object, would overcome this problem and lead to the recovery of the approximate 3-D structure of the object.

Edges in grayscale digital imagery are detected by localizing the zero crossings of filtered data. To achieve this objective, truncated time- or frequency-sampled forms (FSFs) of the Laplacian-of-Gaussian (LOG) filter are employed in the transform domain. Samples of the image are transformed using the discrete symmetric cosine transform prior to adaptive filtering and the isolation of zero crossings. This paper evaluates an adaptive blockwise filtering procedure, based on the FSF of the LOG filter, which diminishes the edge localization error, emphasizes the signal-to-noise ratio around the edge, and extends easily to higher dimensions. Theoretical expressions and applications to digital images are presented.

This paper proposes a new digital method to compensate for the aberration of an electron objective lens in electron holography. In this method, the object wavefront in the exit pupil plane is numerically reconstructed from a digitized electron hologram, and is corrected by multiplying it with the conjugated phase-error function. Then, an aberration-free image can be obtained by calculating the Fresnel integral of this corrected wavefront. In comparison with traditional methods, this method is much more convenient and accurate. Some verifying experiments are also presented in this paper.

A temperature optical fiber sensor based on 2,4,5-triphenylimidazole, also known as Lophine, with thermochromic and hydrophobic properties, is presented. To our knowledge this material has never been used before in this kind of application. The sensor is a low-cost device with a simple fabrication process. Its operation requires inexpensive LED sources and standard multimode fibers. Furthermore, it has been designed for specific immersed applications, e.g., in fish bowls, nurseries, or washing machines. Experimental results in different environmental conditions are presented and discussed. The fabricated sensors show a sensitivity of 0.06 dB/°C with no sign of hysteresis and no cross-sensitivity to humidity. The work is completed with a brief study of the thermochromic material.

A novel demodulation technique for performing dynamic deformation measurements using a path-unbalanced Michelson interferometer is reported. The method is based on the rf amplitude modulation of a low-coherence source, and demodulation is achieved by tracking in the frequency domain the position of the minimum of the detected intensity. This technique is particularly suitable for deformation measurements in civil engineering structures where deformations of the order of few millimeters over the sensor length are expected. The method features a measurement range of at least 10 mm, sensitivity better than 10 μm, and dynamic deformation measurements with a bandwidth up to 100 Hz.

A fiber optic-based off-focus surface topography sensor is proposed based on beam reflection. A specially arranged fiber optic probe with an objective microlens is used for the surface off-focus detection, and a CCD to reduce the inconsistency of the discrete detectors and the difficulties of the multichannel simultaneous signal acquisition. According to the unique signal analysis methods, surface topography as well as distance between the measured surface and sensor probe can be obtained at one measurement setup. Sensor structure and the measurement principle are described, and theoretical analysis and preliminary experimental results indicate the validity and feasibility of the proposed method.

We investigate the characteristics of an asymmetric directional coupler made of side-polished fiber and multimode planar waveguide incorporating a thin metal film. The influence of the metal film thickness and the refractive index of the superstrate on the spectral and polarization properties of the coupler is measured and explained based on the weakly coupled model. It is shown that the coupler behaves like a band-rejection filter or a polarizer, depending on the device parameters. The application possibilities of the sensors to measure the thickness of thin metal film and the refractive index of the liquid are also observed.

Like many other semiconductor devices, the ohm contact technology between HgCdTe and a metal electrode is a key part in infrared focal plane array (IRFPA) technology, on which loophole p-n junctions are detectors. In the study of HgCdTe loophole p-n junction, we found that the larger the p-n junction, the larger the leakage current. Therefore, a new etching electrode processing method was proposed. By this method, the detector and electrode can be formed at the same time, and the technical process can be simplified compared with the chemical corrosion electrode method. But an abnormal character of the detector was found during testing. Thus, the electrode, which is twenty times larger than the detector, was designed, and the technical electrode also takes the etching processing. We found that the abnormal character disappears, and the new method was testified as feasible. The ohm contact between p-HgCdTe and a metal electrode was realized.

The diffraction characteristics of a broadband light source from a circular aperture in the near field are theoretically clarified via a prompt and precise mathematical derivation. We provide a rigorous way of treating in the case of a time-dependent Gaussian-shaped pulsed beam passing through a circular aperture and give a more accurate and concrete criteria for judging the timing for the spectral red and blue shifts and the splitting of light distributions. In addition, the distortion of the space-time diffraction intensity distribution is also presented in detail. Hence, the results offer a clear understanding about the Gaussian beam quality of the output of optical resonator in the near-field diffraction.

In radial gradient index rods (GRIN rods) stress birefringence may appear due to the variation in chemical composition. This results in image degradation in addition to that produced by aberrations. We propose an interferometric method to evaluate the effects of birefringence and the aberrations at the same time. It uses the four-step phase stepping technique twice with two different polarizations of the reference beam. The resulting information consists of two phase maps, one containing information about the aberrations and the other about the birefringence effects given as a relative phase delay between the radial and azimuthal components of the test beam. The main feature is that this relative phase delay is evaluated locally over the aperture at the exit pupil.

Out-of-gamut problems exist in color image processing and color reproduction in different devices. Extensive research has been done on transforming out-of-gamut colors to the inside of the output device's color gamut for color reproduction. We focus on the out-of-gamut problem in color image processing. Luminance, hue, and saturation (LHS) and YIQ color coordinate systems are investigated. Efficient techniques for luminance and saturation processing and gamut clippings are presented. The efficient technique of luminance clipping for LHS is developed based on scaling, and the efficient technique of luminance clipping for YIQ is developed based on shifting. However, the efficient technique of saturation clipping for both LHS and YIQ is a combination of scaling and shifting. Such efficient techniques reduce the complexity of forward and backward transformations between the color representing the color coordinate system (RGB) and the processing color coordinate system (LHS or YIQ) significantly. Illustrations show the result of RGB, luminance, and saturation clippings. Saturation clipping is the best in terms of maintaining the hue and contrast of the original image. Experiments are also conducted to measure the efficiency of the proposed efficient techniques. It shows that the processing time is reduced by 27% comparing with the conventional technique using forward and backward color transformations.

This paper deals with measurement and characterization of IR sensor performance. Alternative methods have been investigated in order to determine the performance more reproducibly and accurately. Furthermore, the new methodologies reduce the subjectivity of the performance parameters such as the standard minimum resolvable temperature difference (MRTD) and tend to reduce the time (and thus cost) of measurement. The three methods described concern the use of (1) and IR zoom collimator, (2) a test device with line targets, and (3) an extended target combined with a TV camera instead of a human observer and a double-line source, resolving the ambiguity beyond the Nyquist frequency in staring focal plane array cameras. In the IR zoom collimator a variety of targets are used in four different orientations; the observer has to report not only the recognition, but also the orientation of the target. Instead of variation of the temperature difference of the target, its range is varied by zooming in and out. In the rapid objective MRTD tester, the performance of the eye is simulated with a simple formula. The complete MRTD is obtained in about two minutes. In the double-slit method the best phase of the detector matrix is taken in comparison with the two line sources; instead of the MRTD curve, a new type of performance curve is measured: the minimum resolvable intensity difference (MRID), from which the range performance is obtained similarly to the classical MRTD method. The paper contains examples of measurements with each of the methods.

We present work on the design and physical performances for a large-aperture multisegment amplifier, which is an engineering prototype amplifier for our technical-integration-line facility. The amplifier consists of 4×2 apertures with each clear aperture about 300×300 mm. Each amplifier module consists of eight laser slabs, arranged four high, two wide, and one long, and twenty high-pulse power flashlamps with an input electric energy of about 23 kJ per lamp. These twenty flashlamps are arranged in a 6-8-6 scheme. The central eight flashlamps will irradiate their energy to the laser slabs in two directions to increase the pumping transfer efficiency. Experimental results indicated that the energy stored in the upper laser level is about 0.24 J/cm³, which corresponds to an energy storage efficiency of about 3.0%. Finally, with an 1.053-μm laser probe we obtained a small-signal gain coefficient of 5.0% cm^–1.

The vibration mode shape measurement technique utilizing a continuous scanning laser Doppler vibrometer (SLDV) is addressed. Continuous scanning capability is added to the conventional discrete laser Doppler vibrometer by reflecting the laser beam on the surface of the object using two oscillating mirrors. Bow scanning resulting from the proposed scanning mechanism is compensated by controlling the two mirrors. If a continuous sinusoidal scanning method is used, it can be shown that the velocity output signal from the SLDV is modulated to give the spatial velocity distribution in terms of coefficients that are obtained from the Fourier transformation of the time-dependent velocity signal. Using the Chebyshev series form, the analysis of vibration mode shape techniques for 2-D scanning is presented and discussed. The performance of the proposed SLDV is presented using the experimental results of the vibration mode shape of a plate.

The U.S. Army and the U.S. Air Force are investigating laser range-gated shortwave IR (LRG-SWIR) imaging systems for use in target identification. When coupled to an electron-bombarded CCD, the imaging system can obtain high-resolution images at long ranges. Speckle, an image artifact inherent in laser-illuminated imaging systems, results from interference patterns caused by the coherent illumination. Laser speckle degrades target identification performance but can be reduced by averaging successive LRG-SWIR images. This research is a first attempt at quantifying target identification performance degradation associated with laser speckle. The research begins with a laboratory experiment to verify a speckle model that includes power spectral density and intensity probability density functions. An LRG-SWIR sensor simulation is developed that includes coherent illumination resulting in speckle target images. A field demonstration is performed to verify the fidelity of the simulation. The simulation is then applied to the U.S. Army Night Vision and Electronic Sensors Directorate (NVESD) target identification set with various levels of image averaging and blur. Observer performance results are analyzed in terms of target identification probability and the effects of various levels of blur and speckle are characterized.

A novel method for implementing header address recognition by using spectral-hologram-based correlator is proposed as a cost-effective way of realizing routing in internet protocol over photonic networks. Due to a difference of spectral response between commercially available holographic media and optical fibers, a second-harmonic optical header-address signal is generated and used both in the synthesis of the spectral hologram and in the recognition of the header addresses. The spectral hologram is employed to store the header addresses. The header-address recognition is performed by taking correlations between an incoming second-harmonic optical address and those stored in the spectral hologram. The system performance is analyzed.

A numerical method that uses the Wigner distribution function to compute the impulse response along the optical axis provided by an aberrated optical system is evaluated. The technique is compared with a classical method used to compute the response integral. Test systems with pupils of nonconventional shape are considered. It is shown that, in general, the same degree of accuracy for the computation of the polychromatic merit functions is achieved faster by use of this novel method.

A calculation method for evaluating UV optical filters, suitable for fluorescence detectors (FD), such as the one from the Auger Project and other similar cosmic ray observatory setups, is presented. We have considered the adopted geometry of a telescope based on a spherical mirror and a camera consisting of a photomultiplier pixel array. In the calculations, an angular weighting factor is introduced, which affects the results of the optical filter performance. Two optical filters, of the interference and absorption type, are evaluated using this method. Experimental measurements of the spectral transmittance of the filters for various angles of incidence are made by a double beam spectrophotometer. To calculate an overall performance, the most dominant spectral lines of the atmospheric fluorescence are used, modified by the atmospheric transmission coefficients. This performance is determined by the detection efficiency of the pixel detector (photomultiplier) with the optical filters used.

A theoretical investigation of the feasibility of planar channel waveguide amplifiers and Fabry-Pérot lasers, made of praseodymium (Pr³⁺)-doped chalcogenide glass, is performed by means of a homemade computer code. The buried channel waveguide amplifier, having both the core and the substrate doped with a Pr³⁺ concentration N = 3000 ppm, shows a gain close to G = 7 dB for an input signal power of P_s = 1 μW at the wavelength λ_s = 1310 nm and a pump power of P_p = 200 mW at the wavelength λ_p = 1020 nm. The Fabry-Pérot laser employing this buried channel waveguide, terminated by an input mirror having reflectivity R₁ = 98% and an output mirror having reflectivity R₂ = 41%, exhibits a threshold pump power P_th = 16 mW and a slope efficiency S = 8%.

Novel monolithic optical receivers in 0.8-μm standard MOS technology with non-return-to-zero data rates in excess of 531 Mbit/s at 850 nm are introduced. At 638 nm the data rates are above 622 Mbit/s. The innovative integrated double and pin photodiodes allowing this high speed without any modification in a CMOS process are described, and results for their quantum efficiency and for their transient response are presented. Furthermore, the improvement of the sensitivity of the photodiodes and of the optoelectronic integrated circuits (OEICs) by an antireflection coating is discussed. The circuit topology of several OEICs is described. A transimpedance input stage with an active MOS feedback resistor is implemented. Analytical calculations for the bandwidth and for the overall transimpedance of several amplifiers as well as measured results are presented. Finally, an OEIC in a 1.0-μm CMOS technology with a polysilicon resistor instead of a MOS feedback resistor with a data rate of 1 Gbit/s for 638 nm is shown. The sensitivity of this OEIC is improved by at least 9 dBm over that of published OEICs.

This paper proposes for the first time an optical touch switch based on total internal reflection (TIR). Key features include ease of implementation, prevention of the light beam from becoming incident directly on the user's eyes, and ability to accept both strong and weak mechanical forces from users. Experimental proof of the concept in the touch-off, release-on mode using a Dove prism shows that a small electrical load (e.g., an LED) in the on state can be turned to the off state by just placing a finger with a measured small force of 0.3 N. The optical loss of 0.4 dB is also measured; it is due to back reflection from the Dove prism. In addition, two methods for removing the finger from the touching surface are studied, indicating that slide-out is suitable in practice for operating the TIR-based optical touch switch.

Theoretical and experimental work to integrate parabolic intracavity lens into a high-power tapered ridge-waveguide semiconductor laser diode is carried out. A finite difference beam propagation model is used to simulate the laser operation with and without the lens. Focused ion-beam etching is used to integrate the lens with the laser cavity. The lens is designed to compensate for the phase curvature of the incident mode on the output facet. This leads to improved linearity in the light-current characteristics of the device, showing an increase of up to 47% in power slope efficiency, 2-μm broadening in near field pattern, and 1-deg narrowing in the far-field pattern.

A novel semiconductor optical amplifier (SOA) optical gate with a graded strained bulk-like active structure is proposed. A fiber-to-fiber gain of 10 dB when the coupling loss reaches 7 dB/factet and a polarization insensitivity of less than 0.9 dB for multiwavelength and different power input signals over the whole operation current are obtained. Moreover, for our SOA optical gate, a no-loss current of 50 to 70 mA and an extinction ratio of more than 50 dB are realized when the injection current is more than no-loss current, and the maximum extinction ratio reaches 71 dB, which is critical for crosstalk suppression.

We discuss the plate-guiding method of growing micrometer-thick single crystal films of electro-optic organic material N-(4-nitrophenyl)-(L)-prolinol. This approach has the advantages of full control over the thickness of the film and selection of a proper seed for initial crystal growth. Characterization of the electro-optic properties of the films is performed using the intensity modulation technique with a low (of the order of 10 V) alternating driving voltage. Both the fundamental and double-frequency responses are used for the characterization. The electro-optic effect is pseudolongitudinal since the configuration of the experiment is that of a longitudinal intensity modulator. The main contribution to the effect originated from the transverse component of the external electric field. By purely electro-optic means we determine the orientation of the dielectric axes and measure the half-wave voltage, the figure of merit, and electro-optic coefficients r₁₂ and r₂₂ to be 3.24 kV and 99.2, 461, and 154 pm/V, respectively. We also find that electro-optic coefficients r₆₁ and r₆₃ are two orders of magnitude less than r₁₂ and r₂₂.

Research on object recognition invariant under out-of-plane rotations has so far yielded limited results. The problem becomes even more complex when in addition scale changes must also be taken into account. We develop a new object recognition method invariant to translations, rotations, changes of pose, and scale. The method is based on angular wedge sampling about the centroid of the object, yielding translation-, rotation-, and scale-invariant features. A modified feature space trajectory classifier is used to obtain out-of-plane rotation invariance. The method is successfully tested on models of military land vehicles and is optically implementable.

Meteorological remote sensing images may contain a huge amount of information, but the matter of extracting and turning them into usable forms is generally quite complex. A wide range of operations is still performed by human experts, as active intelligence and human skills are required. Automatic procedures usually take charge of low- and middle-level tasks, relieving human experts from part of the work. An example of a partly automated task is the tracking of meteorological structures, which is usually performed using techniques based on local correlation between pixel intensity values. We test a technique that is based on the global shape of the observed phenomenon instead. As opposed to traditional techniques based on local correlation, this technique uses the whole shape of the observed object to infer local correspondences. Two modal shape descriptions have been used as the basis for this shape-based technique, and both descriptions are useful for analyzing, classifying, and tracking cloud structures in satellite images. Results using Meteosat and GOES datasets are proposed, discussing the advantages and disadvantages of shape analysis techniques in different meteorological applications. A comparison of interframe matching is provided between the methods discussed here and the traditional block-matching method.

A new method of spatial coherence profilometry is demonstrated. The surface profile is measured by shifting the spatial degree of coherence gradually in its own space of existence, and modulating its phase angle. In each point of the sample we analyze the change of light intensity versus the phase of a Fresnel zone pattern used as the intensity distribution of an incoherent quasimonochromatic source. The tilt of the surface is measured by gradually shifting the Fresnel zone plate on its transverse plane. This shift of the light source rotates the spatial degree of coherence around the coordinate origin until the condition of maximum interference visibility is fulfilled. The method works without any mechanical movement and a quasimonochromatic light illuminates the interferometric system. Experimental demonstration of the new method is presented.

Integrating an epitaxially grown monocrystalline garnet cathode ray tube's (CRT's) high resolution and a liquid crystal light valve's (LCLV's) large screen and high brightness, we develop a CRT optically addressed LCLV projection display system. The CRT's phosphor screen is green chrome yttrium aluminum garnet (Cr:YAG) fabricated by liquid phase epitaxy. The LCLV's fabrication and the optical system's design are given. The projection display system shows good performances.

This paper discusses contrast degradation in projection displays that use the Digital Micromirror DeviceTM (DMDTM) of Texas Instruments. Various possible mechanisms for the degradation of the contrast ratio are investigated by using optical ray-trace simulations and performing some experiments. It is found that the scattering of light at the DMD is the principal source of contrast degradation. This scattering is implemented in our simulations of the DMD projector in the optical ray-trace program ASAP. This enables us to simulate the contrast ratio in a realistic way. Two methods to improve the contrast ratio are proposed. Both methods, however, reduce the total light transmission of the projector. These methods are illustrated with our simulation model, and quantitative results are given.

Multiple series of data were acquired specifically to relate the number of resolvable cycles across an observation of an alphanumeric character to enable humans to accurately read it. This has serious implications for the resolution needed for a surveillance camera to present a "readable" image to a human. We present the theory based both on an extension of the Ratches/Johnson criteria and Fourier analysis. The theory is supported by empirical data based on user identification of random English letters and Arabic numerals. The results strongly indicate that accurate readability (defined as 90% correctness or better) can be accomplished with approximately 2.8 cycles across a block letter. This appears to suggest a lower requirement than that generally accepted for unknown target identification. However, this is consistent due to the limited dataset: there are only 36 alphanumeric characters, and the observer intuitively possesses this a priori knowledge. Moreover, the ability to read an alphanumeric is a steep function of the resolution at a certain number of cycles, which lies between 2 and 3 cycles. The authors define a new level of discrimination, "read,"which lies between the classical "recognize" and "identify." The probability of correct reading can be expressed similarly to that of Detection, Recognition, and Identification by using a postscript such as "Read₉₀."

Techniques based on angle scanning reflectometry near the critical angle can provide high-resolution optical measurements and testing of transparent, absorbing, and turbid liquid samples. Until recently, such techniques were practical for laboratory instruments only. In this work we propose and analyze the design of a relatively simple, compact, and robust optical probe based on angle scanning reflectometry near the critical angle. The optical probe can be used as a high-resolution refractometer, permitting the measurement of the real and imaginary parts of the refractive index, or as an optical sensor to monitor physical and chemical processes in real time within complex liquids. The design offers easy handling, portability, and an adequate balance between size and stability. We study the optical characteristics of the instrument and present the results of an experimental prototype of the device.

A Nd:glass regenerative amplifier has been set up to generate the pumping pulse with variable pulse width for an optical parametric chirped-pulse amplification (OPCPA) laser system. Each pulse of the pulse train from a cw self-mode-locking femtosecond Ti:sapphire oscillator is stretched to ≈ 300 ps at 1062 nm to be split equally and injected into a nonlinear crystal and the Nd:glass regenerative amplifier, as the chirped signal pulse train and the seed pulse train of the pumping laser system, respectively. By adjusting the cavity length of the regenerative amplifier directly, the width of amplified pulse could be varied continuously from ≈ 300 ps to ≈ 3 ns. The chirped signal pulse for the OPCPA laser system and the seed pulse for the pumping laser system come from the same oscillator, so that the time jitter between the signal pulse and the pumping pulse in optical parametric amplification stages could be <10 ps.

The role of active arrayed-waveguide gratings (AWGs) in future dynamic wavelength-division multiplexing (WDM) networking and routing is investigated. Simulations are done using the beam propagation method (BPM). The results indicate that active AWGs can be used in multiple-channel control with an SNR up to 19 dB and dynamic dispersion compensation of up to 160 ps/nm.

We present a dual-core fiber that can function as a wavelength selector for applications in dense wavelength division multiplexing (WDM) systems. The fiber consists of an erbium-doped core and an undoped core. The principle of operation relies on the large nonlinear effect of the Er-doped core when it is pumped by a 980-nm laser diode. The selection of wavelength is achieved by adjusting the pump power launched into the doped core. Wavelengths with spacing of 1 nm can be selected with cross talks as low as –28 dB.

Based on comprehensive computer modeling and numerical simulation of dense wavelength-division multiplexing (DWDM) systems, it is demonstrated that by judicious use of erbium-doped fiber amplifiers (EDFAs) and distributed Raman amplifiers (DRAs), the Q factor and optical signal-to-noise ratio (OSNR) are improved compared with EDFA alone. This hybrid combination yields a 13.5-dB improvement in OSNR and a peak Q factor of 12.1 for the hybrid versus a peak of 8.1 when EDFA is used alone.