摘要 :
When photoelastic modulators (PEMs) are used with lasers as light sources, modulated interference effects may appear as spurious signals at the fundamental and harmonic frequencies of the PEM. They are correlated with the modulato...
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When photoelastic modulators (PEMs) are used with lasers as light sources, modulated interference effects may appear as spurious signals at the fundamental and harmonic frequencies of the PEM. They are correlated with the modulator reference signal and are at precisely the same frequencies as the polarization modulation effects being studied. This modulated interference does not appear to be a problem with any light source other than lasers. The modulated interference effects arise because of interference between light reflected at the surfaces of the modulator optical element and the primary beam and relative motion of these two surfaces synchronized with the modulator oscillations. Interference in modulator optical elements, which is similar to multiple beam interference in thin films, is examined. Criteria for estimating the strength of the modulated interference are presented and a simple test for the presence of modulated interference in an optical system that includes a PEM is described. Several strategies are presented for reducing or eliminating these troublesome effects including (1) careful positioning of the PEM, (2) use of antireflective (AR) coatings, and (3) techniques that physically separate the primary beam from multiple reflected beams. Data are given for suppression of the modulated interference by using AR coatings and for one method of beam separation.
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A new calibration method for a photoelastic modulator is proposed. The calibration includes a coarse calibration and a fine calibration. In the coarse calibration, the peak retardation of the photoelastic modulator is set near 1.8...
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A new calibration method for a photoelastic modulator is proposed. The calibration includes a coarse calibration and a fine calibration. In the coarse calibration, the peak retardation of the photoelastic modulator is set near 1.841 rad. In the fine calibration, the value of the zeroth Bessel function is obtained. The zeroth Bessel function is approximated as a linear equation to directly calculate the peak retardation. In experiments, the usefulness of the calibration method is verified and the calibration error is less than 0.014 rad. The calibration is immune to the intensity fluctuation of the light source and independent of the circuit parameters. The method specially suits the calibration of a photoelastic modulator with a peak retardation of less than a half-wavelength.
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A polarization modulator based on splitting with a Savcirt plate and rotation of an analyzer for a moire system with grating imaging is presented, and its modulation principle is analyzed. The polarization modulator is simple and ...
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A polarization modulator based on splitting with a Savcirt plate and rotation of an analyzer for a moire system with grating imaging is presented, and its modulation principle is analyzed. The polarization modulator is simple and achromatic. It is composed of a polarizer, a Savart plate, and an analyzer. The polarizer and the Savart plate are placed in front of the index grating to split the image of the scale grating in the moire system. The analyzer is placed behind the grating and rotated to realize the modulation of the moire signal. The analyzer can be rotated either continually with high speed or step by step with low speed to form different modulation modes. The polarization modulator makes the moire system insensitive to the change of initial intensity. In experiments, we verified the usefulness of the polarization modulator.
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A polar modulator for wireless RF transmitters nonlinearly transforms complex-valued Cartesian baseband modulating signal into amplitude and phase components of the polar coordinate representation before they are recombined in a p...
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A polar modulator for wireless RF transmitters nonlinearly transforms complex-valued Cartesian baseband modulating signal into amplitude and phase components of the polar coordinate representation before they are recombined in a power amplifier. The resulting explosion in the bandwidth requirements of the polar components can so far be only tolerated for narrowband transmitters, such as EDGE of the 2G cellular. To enable polar topology for wideband transmitters, we propose a technique that alters the signal trajectory such that it avoids crossing (and proximity) of the constellation origin. The resulting substantial decrease of the polar modulator bandwidth is traded off against slight increase of in-band modulation distortion and adjacent channel leakage. We illustrate effectiveness of this method using wideband CDMA (WCDMA) of the 3G cellular. The technique is first mathematically analyzed for various tradeoffs followed by high-level modeling and simulation results. Since the technique is fully contained in the digital domain, its performance effects on the entire RF transmitter can be accurately simulated. A digital architecture to implement the proposed technique is also presented.
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Channeled spectropolarimetry is a snapshot scheme for the full polarization spectra measurement. It has the advantages of static, compact, lightweight, and easy to implement. However, while the broadband polarization spectra can b...
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Channeled spectropolarimetry is a snapshot scheme for the full polarization spectra measurement. It has the advantages of static, compact, lightweight, and easy to implement. However, while the broadband polarization spectra can be precisely reconstructed using the regular methods, such as the method based on the Fourier transform, the reconstruction of narrowband polarization spectra always suffers from errors caused by the crosstalk among channels in the optical path difference domain. We present an efficient narrowband full polarization spectra reconstruction method for channeled spectropolarimetry. The line shape of the intensity spectrum is estimated by the envelop curve of the measured high frequency modulated spectrum. The full polarization spectra can be reconstructed by the least-square method under the assumption that polarization states change slowly with wavelength for narrowband polarization spectra. The feasibility of the reconstruction method has been verified by the simulation and experimental results.
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In this paper, we compare by numerical simulations different schemes to emulate the interaction between polarization mode dispersion (PMD) and nonlinear effects in 100 Gb/s polarization-division multiplexing quadrature phase shift...
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In this paper, we compare by numerical simulations different schemes to emulate the interaction between polarization mode dispersion (PMD) and nonlinear effects in 100 Gb/s polarization-division multiplexing quadrature phase shift keying (PDM-QPSK) transmission systems in laboratory experiments. We find that the correct choice of emulation scheme strongly depends on the characteristics of the line and, in general, emulating the PMD at the transmitter doesn't ensure a correct reproduction of the PMD/nonlinear effects interaction.
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The effects of surface morphology on polarization switching in thin ferroelectric films areinvestigated using a real-space, time-dependent Ginzburg–Landau model that incorporateselectrostatic interactions. We consider a two-dimen...
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The effects of surface morphology on polarization switching in thin ferroelectric films areinvestigated using a real-space, time-dependent Ginzburg–Landau model that incorporateselectrostatic interactions. We consider a two-dimensional uni-axial ferroelectric film with athickness that varies sinusoidally. Polarization switching, starting from a single domain remnantstate, is simulated for several surface modulation amplitudes and wavelengths. We demonstratethat surface heterogeneities produce inhomogeneities in the electric field within the film. Theseinhomogeneities become preferential sites for easy nucleation of reverse domains. This has aprofound effect on the external field necessary to switch the polarization. Increasing the surfaceundulation amplitude significantly reduces the coercive field compared to the ideal flat film,even for very small amplitude modulations in the thickness. Although surface roughnessdecreases the field required to form reverse domains, it also hinders subsequent domain wallmigration. In fact, for very high amplitude and small wavelength surface morphologies,complete switching to a single domain state becomes impossible. This is because the domainwalls become trapped near the peaks in the modulated surface. The technological implicationsof the present results for utilization of surface roughness and for surface morphology design arediscussed.
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Active polarizers that consist of parallel metal wires are fabricated by silicon micromachining techniques. If wavelength of incident radiation is larger than the period of the wire grid, the wire grid reflects incident radiation ...
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Active polarizers that consist of parallel metal wires are fabricated by silicon micromachining techniques. If wavelength of incident radiation is larger than the period of the wire grid, the wire grid reflects incident radiation with the electric field parallel to the grid line, while transmits the other. Polarization of infrared light can be succussfully modulated by changing the period of the grid or changing the geometrical configuration. Two kinds of silicon structures are fabricated and demonstrated. On of them is electrostatic driven structure composed of mechanically flexible grid. The period of the grid can be modulated by itself with electrostatic actuation.
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We propose and demonstrate a photonic scheme to generate and transmit dual-band dual-chirp microwave waveforms based on a single dual-polarization dual-parallel Mach-Zehnder modulator (DP-DPMZM), which structure is simple and comp...
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We propose and demonstrate a photonic scheme to generate and transmit dual-band dual-chirp microwave waveforms based on a single dual-polarization dual-parallel Mach-Zehnder modulator (DP-DPMZM), which structure is simple and compact. The center frequencies of the generated dual-band dual-chirp signals are frequency-doubling and can be adjusted flexibly and independently to meet the practical requirement of radar systems. Moreover, chromatic dispersion-induced power fading (CDIP) can be eliminated by appropriately adjusting the direct current biases of the modulator to achieve carrier-suppressed single-sideband (CS-SSB) modulation. The proposed method is theoretically analyzed and experimentally proved. Dual-band dual-chirp signals with center frequencies of 12 GHz, 17 GHz, and 8 GHz, 14 GHz are successfully generated. In addition, the signal-to-noise ratios (SNRs) and pulse compression performance of the generated signals are evaluated. The proposed scheme is suitable for dual-band radar systems to conduct target detection, Doppler blind speed elimination, and one-to-multi base station transmission.
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Using the Jones formalism, it is shown that electro-optic modulators used for phase modulation generate a modulation of the output polarization induced by the difference of phase modulation depth along the crystallographic axes of...
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Using the Jones formalism, it is shown that electro-optic modulators used for phase modulation generate a modulation of the output polarization induced by the difference of phase modulation depth along the crystallographic axes of the modulator. We study two consequences of this polarization modulation in the fiber setups, limiting the performance of high sensitivity measurement devices. The first one is its partial conversion into a residual amplitude modulation (RAM) within any component presenting polarization dependent losses (PDL). The second one is a new effect that consists of the distortion of the signal detected at the output of a fiber cavity. The theoretical expressions of the detected signals are computed in each case.
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