Phase calibration for optical phased arrays (OPAs) is a key process to compensate for the phase deviation and retrieve the initial working state. Conventional calibration approaches based on iterative optimization algorithms are tedious and time-consuming. The essential difficulty of such a problem is to inversely solve for the phase error distribution among OPA elements from the far-field pattern of an OPA. Deep-learning-based technology might offer an alternative approach without explicitly knowing the inverse solution. However, we find that the phase ambiguities, including conjugate ambiguity and periodic ambiguity, severely deter the accuracy and efficacy of deep-learning-based calibration. Device-physics-based analysis reveals the causes of the phase ambiguities, which can be resolved by creating a tailored artificial neural network with phase-masked far-field patterns in a conjugate pair and constructing a periodic continuity-preserving loss function. Through the ambiguity-resolved neural network, we can extract phase error distribution in an OPA and calibrate the device in a rapid, noniterative manner from the measured far-field patterns. The proposed approach is experimentally verified. Pure main-beam profiles with > 12    dB sidelobe suppression ratios are observed. This approach can help overcome a crucial bottleneck for the further advance of OPAs in a variety of applications such as lidar.

This work presents a design approach of multi-segment directional couplers with ultra-broadband flat spectra and benign fabrication tolerance on the silicon nitride platform. Using particle swarm optimization, we optimize design parameters of multiple coupling regions and asymmetric decoupling regions in the multi-segment couplers, and synthesize optimized structures for the intended power splitting ratio over optical telecommunication O, S, E, and C bands. To efficiently model the device with many structural parameters, each part of the fundamental structure is separately modelled by the most efficient method, including effective index method, coupled mode theory, and transfer matrix method to construct the high-dimensional design space. By choosing a proper evaluation function, the optimized couplers achieve flat spectra with less than ±2% fluctuation over ~300 nm spectrum for 50%/50%,30%/70% and 10%/90% splitting ratios, which is well verified by 3D FDTD. We also discuss performance degradation caused by fabrication variations and offer a general strategy to enhance fabrication tolerance for the broadband optical couplers with asymmetric decoupling regions.

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A theoretical model of X-ray fluorescence yield in perfect crystal by high frequency ultrasonic vibrations is supposed. The possibilities of the X-ray standing wave method are used basing on the registration of the angular dependence of the intensity of a secondary radiation yield [1,2].The calculations are based upon the presentation of vibrating crystal as a set of crystalline slides with different interplanar distances [3]. In the present paper the TDS is considered in crystal disturbed by high frequency ultrasonic vibrations. In this case additional X-ray reflexes (satellites) are formed out which can be used for structure investigations. By varying the incident angle one can excite all the satellites each after another and detect the variation in X-ray fluorescence yield. Theoretical analysis is made both in the case of the crystal matrix and the layer of impurity atoms.This work was supported by Russian Fund for Fundamental Research (N97-02-17966) [1] Kovalchuk M.V., Kohn V.G. "X-ray standing waves -new method of studying the structure of crystals", Sov. Phys. Usp, (1986), 29: 426-446. [2] Bedzyk, M.J., Materlik G. "Two beam dynamical diffraction solution of the phase problem: A determination with X-ray standing-wave fields", Phys. Rev. B (1985), 31: 6456-6463. [3] Kovalchuk M.V., Nosik V.L. "On the theory of X-ray standing waves in vibrating crystals" Nucl.