Analysis of a reset-free polarization controller for fast automaticpolarization stabilization in fiber-optic transmission systems

We analyze the operation of a reset-free polarization controller comprising three endlessly rotatable wave plates: a first quarter-wave plate followed by a half-wave plate and a second quarter-wave plate, which is rotated synchronously with the first quarter-wave plate. We show that for any arbitrary angular offset between the two quarter-wave plates; the controller allows continuous, reset-free transformations from any varying general input state of polarization into any general output state. An integrated-optic realization of this scheme on z-propagation LiNbO₃ offers control speeds that well exceed the speed of natural polarization fluctuations in standard single-mode fibers, thus allowing fast automatic polarization stabilization in fiber-optic transmission systems     

Ray contribution masks for structure adaptive sinogram filtering

The patient dose in computed tomography (CT) imaging is linked to measurement noise. Various noise-reduction techniques have been developed that adapt structure preserving filters like anisotropic diffusion or bilateral filters to CT noise properties. We introduce a structure adaptive sinogram (SAS) filter that incorporates the specific properties of the CT measurement process. It uses a point-based forward projector to generate a local structure representation called ray contribution mask (RCM). The similarities between neighboring RCMs are used in an enhanced variant of the bilateral filtering concept, where the photometric similarity is replaced with the structural similarity. We evaluate the performance in four different scenarios: The robustness against reconstruction artifacts is demonstrated by a scan of a high-resolution-phantom. Without changing the modulation transfer function (MTF) nor introducing artifacts, the SAS filter reduces the noise level by 13.6%. The image sharpness and noise reduction capabilities are visually assessed on in vivo patient scans and quantitatively evaluated on a simulated phantom. Unlike a standard bilateral filter, the SAS filter preserves edge information and high-frequency components of organ textures well. It shows a homogeneous noise reduction behavior throughout the whole frequency range. The last scenario uses a simulated edge phantom to estimate the filter MTF for various contrasts: the noise reduction for the simple edge phantom exceeds 80%. For low contrasts at 55 Hounsfield units (HU), the mid-frequency range is slightly attenuated, at higher contrasts of approximately 100 HU and above, the MTF is fully preserved.     

Compact electro-optic polarization scramblers for opticallyamplified lightwave systems

Fast polarization scramblers have become important components in optically amplified transoceanic communication systems to eliminate anisotropic gain saturation (polarization hole burning) in the erbium-doped fiber amplifiers by depolarizing the launched optical carrier. This paper reviews the operation and implementation of compact electro-optic polarization scramblers on x-cut and z-cut lithium niobate. These integrated-optic devices employ low-loss single-mode waveguides and allow polarization modulation at variable frequencies ranging from a few 100 Hz to more than 10 GHz. We describe simple linear phase retarders that depolarize light in a fixed, well-maintained polarization state as well as cascaded multi-stage scramblers that are capable of depolarizing arbitrarily polarized light. These scramblers can be operated over a broad optical bandwidth of more than 40 nm with less than 5% residual degree of polarization and are further wavelength-tunable over more than 100 nm     

Tolerance of Duobinary and NRZ-OOK Signals to Second-Order Polarization-Mode Dispersion

The sensitivity of tightly filtered duobinary and conventional nonreturn-to zero on-off-keyed (NRZ-OOK) signals to second-order polarization-mode dispersion (SO-PMD) is assessed through extensive numerical simulations. On average, duobinary signals are significantly more sensitive to SO-PMD than to ordinary fiber chromatic dispersion (CD), whereas NRZ-OOK signals are more tolerant to SO-PMD than to CD. This seemingly contradicting behavior is explained by the difference in the frequency dependence of SO-PMD and CD. In addition, duobinary signals suffer from residual differential group delay, even after first-order PMD compensation, which is introduced between signal components that are transmitted in the launched principal state of polarization (PSP) and those that are coupled from the launched to the orthogonal PSP.     

Mirror-folded polarization-independent wavelength filter

A 1.55- mu m polarization-independent optical bandpass filter with a 3-dB bandwidth of 6 AA and a tuning range of at least 100 AA is demonstrated. The photonic circuit of the filter consists of a waveguide TE-TM mode splitter, two parallel electrooptic TE-to-TM, TM-to-TE mode converters as the wavelength selective element, and a reflective mirror at the far end of the crystal. The circuit can be viewed as a mirror-folded version of the polarization-independent filter. It is shown that by passing the light two times through the narrowband mode converters, the filter bandwidth can be reduced by about a factor of two compared to a conventional single-pass filter of the same length. A novel tunable mode converter structure is also demonstrated that allows coherent in-phase mode conversion during the two passes through the mirror-folded filter.<>     

Electro-optically tunable, narrowband Ti:LiNbO3 wavelength filter

We report a tunable optical transmission filter for 1.5?n with a bandwidth of 12 ? and a tuning range of at least 80 ?. The filter is based on wavelength-dependent polarisation conversion in a birefringent waveguide and employs a periodic electrode structure with interleaved birefringence tuning electrodes.     

Integrated-optic polarization transformer for reset-free endlesspolarization control

An electrooptic polarization transformer which employs a multiple cascade of alternating two-way TE-TM polarization-converter and TE/TM phase-shifter sections in a birefringent crystal, such as LiNbO₃ , and requires only three independent drive voltages is described. It is capable of transforming any arbitrarily varying input polarization into any desired output polarization without ever requiring a reset cycle. It accomplishes this by producing adjustable elliptical birefringence of constant total phase retardation in a single-mode waveguide     

Improved Numerical Model for the Average Transfer Matrix of First- and Second-Order PMD

An improved model for the average transfer matrix of first- and second-order polarization-mode dispersion (F&SO-PMD) in optical fibers is derived from numerical simulations of the frequency dependence of the differential phase shifts and cross coupling between signal components that are transmitted in two principal states of polarization (PSPs). The mean differential phase shifts and cross-coupling phases are calculated for various given values of the differential group delay (DGD) and the second-order PMD parameter that characterizes the coupling between PSPs. It is found that the mean differential phase delays are equal to the DGDs only over a narrow optical bandwidth, beyond which they decrease with increasing frequency offset and in some cases even reverse sign. Similarly, the mean cross-coupling phases increase less rapidly with frequency than assumed in other popular models and always approach an asymptotic value, at which half of the optical power is coupled from one PSP to the other. Moreover, it is shown that these mean differential phase shifts and cross-coupling phases define a transfer matrix for F&SO-PMD that nicely predicts the average eye-opening penalties in return-to-zero-formatted digital optical signals that are transmitted in one of the two PSPs. These predictions are particularly accurate when the average polarization-dependent chromatic dispersion (PCD) is included in the PSP phases. Additional simulations of F&SO-PMD compensation reveal that the signal impairments caused by PCD, on average, are substantially smaller than those introduced by the cross coupling between PSPs     

Integrated-optical single-sideband modulator and phase shifter

A scheme of spatially weighted coupling between two waveguide modes is proposed in which the weights form an electrooptically induced periodic coupling structure that can be moved at variable speeds forward or backward along the guide. The device can operate as an optical phase shifter with unlimited phase range, as a single-sideband amplitude modulator or frequency shifter, or as a polarization transformer.