The Karhunen-Loeve transform is applied to the coarsely sampled impulse response generated by an FBG cascade in order to calculate the temperature change suffered by the FBGs. Thanks to a dispersive media, the wavelength change performed by the temperature change produces a delay shift in the sample generated by an FBG, delay shift which is recorded in the eigenvalues calculated by the KLT routine, letting to measure the temperature variation. Although the FBGs samples are represented only by four points, a continuous temperature measurement can be performed thanks to the KLT algorithm. This means a three order reduction in the number of points giving this method a low computational complexity. Simulations are performed to validate the interrogation technique and estimate performance and an experimental example is provided to demonstrate real operation.
We report on an incoherent OFDR interrogator of FBG arrays based on the concept of dispersive wavelength to time delay
mapping. The system is specifically designed to show stability to environmental thermal variations by the incorporation
of a composite dispersive delay and weak broadband reflectors for delay and dispersion monitoring. Dispersion is imparted
by the combination of a fiber coil and an athermally-packaged chirped fiber Bragg grating for dispersion compensation.
Using differential measurements over a single acquisition trace, the values of Bragg wavelengths and dispersion are
determined from the delays experienced by the FBGs and by additional reference wavelengths reflected in the broadband
reflectors. The results show maximum deviations of 20 pm and 0.2 ps/nm with respect to OSA measurements of Bragg
wavelengths and nominal dispersion values, respectively.
An optical fiber sensor composed of six standard FBGs in cascade is interrogated by use of a technique based on wavelength to delay mapping. A microwave-modulated optical signal enters the FBG cascade and, after being sliced and reflected, experiences dispersion in standard single-mode fiber. The Bragg wavelengths of the FBGs are mapped into the delays that determine the peaks in the system’s electrical impulse response. The Bragg wavelength shifts are calculated from the difference of the delays undergone by FBGs samples. A resolution of 9.2 pm in Bragg wavelength shift is demonstrated.
A novel interferometer interrogation concept based on Microwave Photonics is proposed and demonstrated. The system is conceptually equivalent to the double-interferometer interrogation technique in low-coherence interferometry and similar to a Microwave Photonics filter based on a fiber-optics dispersive link driven by a sliced broadband source, where the resonance in its frequency response determines the interferometer’s optical path difference. In a proof-ofconcept experiment, we use the interferometer as a displacement sensor, attaining a resolution of 63 μm over a measurement range of 3.7 mm using conventional 20-nm C-band EDFA sources and a radio-frequency bandwidth of 7 GHz.
We propose the use of Dammann gratings commonly applied to problems in diffractive optics as a flexible tool for practical design of devices which perform multiple nonlinear optical frequency conversion processes, and which exhibit a bandpass grid-like quasiphase matched response in the wavevector mismatch space (Δk). By designing a periodic structure where several inversions in the sign of the nonlinear figure of merit take place at strategic locations within a fundamental period, baseband grid-like responses can be achieved in nonlinear materials consisting of sharp quasihase matching (QPM) peaks equally spaced in the wavevector mismatch space around Δk=0. With this design tool, the relative efficiency of the individual peaks in the grid can be arbitrarily set (equalization), including targeted suppression of specific nonlinear interactions in the grid (channel suppression). The baseband gratings so designed are then reallocated in Δk space by superimposing the baseband design on a suitable square carrier to achieve a targeted band-pass response. We present a simple procedure for the design of both baseband and bandpass Dammann gratings with examples of equalization and suppression in the context of nonlinear optics based on optimization algorithms. The tolerance of the designs under random variations of the inversion points and design quality parameters are also discussed.
The power spectrum density of the intensity of jittery but coherent trains of linearly chirped Gaussian pulses after a high-dispersion line with arbitrary first (β2) and second order(β3) dispersion is computed in the small-signal
approximation. Before the dispersive line the timing jitter of the input train causes noise sidebands around the harmonics of the train. The noise bandwidth of these jitter sidebands depends on the pulse-to-pulse correlation. The result of the propagation in a dispersive line is a multiplicative factor in the noise spectral density. This term depends on the dispersive characteristics of the line and the pulse parameters but not on the timing jitter's correlation. The structure of this new factor is peaked, resulting in narrowband noise patterns at specific locations of the spectrum. The bandwidth of the dispersion-induced noise patterns is in general broader than the timing jitter's bandwidth. When the lines are Talbot dispersive devices, i. e., are designed to multiply the repetition rate of the train), jitter noise around the harmonics of the output train is left untouched. Therefore the jitter structure of the multiplied train is inherited from the initial train. More general RF spectral patterns, depending on the pulse-to-pulse jitter correlation, are also analyzed.
KEYWORDS: Zone plates, Diffraction, Near field diffraction, Signal attenuation, Laser optics, Constructive interference, GRIN lenses, Electroluminescence, Coded apertures, Cameras
There exists a well-known analogy between the paraxial or one-dimensional Fresnel diffraction and the propagation of pulses in linear dispersive medium with negligible attenuation. Under this analogy, the envelope of a pulse is equivalent to the distribution of complex amplitude of the light in diffraction. In this context, we study the propagation of a train of identical Gaussian chirped pulses arranged in time in the same way as the Fresnel zones of a phase zone plate, in a highly dispersive guiding medium. From this study we find that the input train focuses in an only pulse, for certain values of total dispersion. We establish the focusing condition and characterize the output signal through its width and peak intensity.
We present a hybrid optical device for measurement of refractive index variation in a selfoc lens by the Talbot effect. The intrinsic nature of the device that combines a linear grating and an inhomogeneous medium permits to measure changes in the refractive index along the axis as a function of the position shift of the first self-image. Possible changes in the temperature during lens fabrication can be extracted from refractive index variation. The hybrid device has a refractive index sensitivity of ±7 x 10-5 per nanometer of position shift of the first self-image.
In this work we present a method for the determination of the Jones matrix of a liquid crystal display (LCD), independent of the knowledge of the orientation of the liquid crystal director. The method is based on the measurement of the LCD transmittance for various polarization configurations. Once the Jones matrix has been determined, it is demonstrated to predict with high accuracy the transmission of the LCD for other polarization configurations.
A generalization of the fractional Talbot effect to the case of a tapered gradient-index medium for uniform illumination is considered. A unit cell of the fractional Talbot image contains the superposition of unit cell images of the periodic object.
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