We propose a two-step trapezoidal-pattern phase-shifting method for 3-D surface-shape measurements. Shape measurements by trapezoidal phase-shifting methods require high-quality trapezoidal patterns. Furthermore, most of the video projectors are nonlinear, making it difficult to generate high quality phase without nonlinearity calibration and correction. To overcome the limitations, we propose a method for synthesizing trapezoidal intensity fringes as a way to solve the problems caused by projector/camera gamma nonlinearity. The fringe generation technique consists of projecting and acquiring a temporal sequence of strictly binary color patterns (Gray code), whose (adequately weighted) average leads to trapezoidal fringe patterns with the required number of bits, which allows a reliable three-dimensional profile reconstruction using phase-shifting methods. Validation experiments are presented.
To characterize and predict the dynamics of the nonlinear polarization rotation (NPR) in semiconductor optical
amplifiers (SOA) an experimental method based on the frequency response technique and a model based on the
density matrix and effective index formalisms are presented. Particularly, the frequency response of the NPR is
obtained by determining, for each studied frequency, the angular displacement, at the Poincare Sphere, that separates
the initial and final points of the polarization evolution of the output beam.
This communication addresses, theoretically and experimentally, the effect of the cross polarization modulation (XPolM)
phenomenon on the performance of a wavelength converter based on a four wave mixing process (FWM) developed in a
semiconductor optical amplifier (SOA). The main studied parameter for evaluating the performance of the wavelength
converter is the conversion efficiency, whose degradation, provoked by the XPolM, is analyzed when the four wave
mixing process is developed with input signals linearly co-polarized at 0, 45, 90, and -45 degrees. The analysis is based
on the theoretical and experimental determinations of the polarization mismatch of the pump and probe signals. Finally,
it is demonstrated that classic models lacking the inclusion of the XPolM effect can reflect a significant error in the
estimation of the conversion efficiency of a wavelength converter based on FWM.
In this communication we demonstrate the implementation of an external cavity fiber grating laser with a side mode suppression ratio higher than 40 dB utilizing a Fabry-Perot laser diode with a facet containing an antireflection coating which thickness is controlled by a technique allowing the direct observation of the laser resonance behavior during the application process.
In this communication we present all-optical AND, NOT XOR and NOR gates with two and three input logic signals using the cross-polarization modulation (XPolM) effect in a semiconductor optical amplifier (SOA). The gates utilize only a SOA and they need neither an inversion stage nor an additional synchronized clock. The XpolM effect is discussed as well as its repercussion on the polarization-state of a probe beam crossing a SOA perturbed by the power of a control beam. Based on this discussion, we present the design criteria allowing the implementation of the all-optical logic gates with two and three input logic signals. Finally the gates are experimentally demonstrated with input logic signals modulated with a 2.5 Gb/s NRZ code with a pseudo-random data sequence of 223-1 length.
In this communication, we demonstrate experimentally a new design for all-optical AND and NAND gates operating in the GHz regime using the cross-polarization modulation effect in a semiconductor optical amplifier. The efficiency of this effect was estimated by measuring the conversion coefficients indicating the TE to TM mode conversion and vice versa when the amplifier is perturbed with a wavelength tunable control beam. The all-optical gates here described differ from other ones developed before using semiconductor optical amplifiers by their ability to operate on non-degenerate input signals with an output signal wavelength independent of the input signals wavelength.
In this communication we present a NOT XOR gate using the cross-polarization modulation (XPolM) effect in a semiconductor optical amplifier (SOA). The gate utilizes only a SOA and it needs neither an inversion stage nor an additional synchronized clock. We demonstrate that the XPolM effect can rotate 73.5C the polarization-state of a CW beam when the power of a control beam changes from 0 to 300 (mu) W. The control and CW beams are introduced into the amplifier with a linear-polarization near to the unperturbed amplifier TM axis where the waveguide eigenmodes modification has a strong participation on the XPolM effect.
In this paper, we demonstrate experimentally the fulfillment of al-optical AND and NAND gates using the cross- polarization modulator effect in a semiconductor optical amplifier. The efficiency of this effect was estimated by measuring the conversion coefficients indicating the TE to TM mode conversion and vice versa when the amplifier is perturbed with a wavelength tunable control beam.
In this paper we present a NOT XOR gate using the cross- polarization modulation effect in a semiconductor optical amplifier. The gate utilizes only a SOA and does not need an inversion stage and an additional synchronized clock.
In a helically wound single-mode fiber the mode degeneracy is lifted by the geometric form and the photo-elastic effect. Fiber bending induces linear birefringence' and the presence of torsion is responsible for circular birefringence.2 Considering the geometrical properties of a helix and using Jones calculus, it has been shown3 that a helically wound fiber can be described as the combination of two distributed homogeneous retarders: a linear retarder and a circular retarder. Since linear and circular retardation can be easily followed on the Poincaré sphere, we use Mueller calculus to describe the polarization optics of helical fiber structures.4 Because of the strong absorption and the subsequent emission that active fibers present within the amplification band, the signal becomes depolarized and it is not possible to characterize the birefringence properties of active fibers at these wavelengths. In this work, the birefringence performance of active fibers is evaluated in the neighborhood of the amplification band, using signals with a high signal to noise ratio at the active fiber output. We present the birefringence characterization of a helically wound erbium fiber. This helical fiber structure was built with a commercial fiber. Since the birefringence parameters we measured agree with the values predicted by the theoretical model, we propose that we can make use of this model to design the helix structure and, to select the input polarization states of the pump and the signal that can be used to control the polarization evolution of the amplified signal as it propagates along the fiber.
The polarization optical properties of single mode fibers are important in those applications based on the use of coherent polarized light, such as fiber optic interferometric sensors and coherent communication systems. Various birefringence mechanisms and combinations of them have been developed to control the polarization evolution of light along the fiber. Since 1977 when A.Papp and H.Harms1 suggested the application of helical core fibers for this purpose, their polarization properties have been studied by several authors.24 In I 984 J.N. Ross2 showed that helically wound monomode fibers behave as the combination of a distributed linear retarder and a distributed circular retarder. Since linear and circular retardation can be easily followed on the Poincaré sphere, this representation of polarized light results adequate for helical fibers. In this work the non-destructive evaluation of the equivalent optical activity and the equivalent linear retardation of a fiber helix are performed using the trajectory described on the Poincaré sphere when the orientation of the linearly polarized light at the input rotates 360°. The results we obtain are compared with the values determined for the linear and circular retardation using Ross model and an input circular or linear polarization. We present preliminary results obtained for the birefringence characterization of two helically wound fibers built with .-l4m and —27m of a telecommunications fiber ( I 550nm).
Several models have been developed to describe the interaction between neighboring ions in heavily doped erbium fibers. All of them are based on the anomalous saturation of the pump power. In this work we present experimental results of the anomalous behavior of the gain saturation in an EDFA, induced by the signal's power. This additional information shows us that energy migration is also induced by phonons.
In this work we present an analysis of the birefringence effects induced by external fields on passive fibers and, on the basis of this information, we propose an explanation for some effects observed in active fibers. To modify the passive fiber birefringence we have used external forces. The photo-elastic description of the birefringence induced by simple stresses is verified for short circuit cylindrical samples of silica glass (step-index and GRIN). Phase and polarization changes of light transmitted through these samples were studied using holographic interferometry and polarimetry. Using these results, a proposal is made to understand the polarization sensitivity of EDFAs. In the case of active fibers its birefringence was altered changing its geometrical shape. The amplifying performance of an EDFA built with a helically-wound active fiber was compared with that of an EDFA built with an irregularly wound erbium fiber.
The polarization dependent performance of EDFA has been explained in terms of polarization hole burning, considering that the erbium sites in the silica matrix preserve its random nature. Although typical EDFAs are built with an erbium fiber longer than 10 m (hence the active fiber is necessarily wound) the influence of the bending has not been included in the investigation of this effect. To verify the relevance of bending induced birefringence, a helically- wound active fiber has been used to build an EDFA. The characterization of this helically-wound optical amplifier has shown that the polarization dependent performance was enhanced. The stronger link between the states of polarization of the signal wave, the pump wave and the active fiber birefringence axes was used to show that a selective excitation of the erbium ions by a polarized pump and a selective de-excitation of those ions by a polarized signal are the origin of the polarization dependent performance of EDFAs.
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