The properties of ferroelectric films are known to degrade when subjected to hydrogen in forming gas anneals. Earlier studies have attributed this degradation to the loss of oxygen from these films during these anneals. In this study, we show that though oxygen is lost during forming gas annealing, hydrogen incorporation is the primary mechanism for the degradation of ferroelectric properties. Raman spectra obtained from the forming gas-annealed films show evidence of polar hydroxil [OH−] bonds in the films. The most probable site for hydrogen ions is discussed based on ionic radii, crystal structure, electrical properties, and Raman spectra. We propose that the hydrogen ion is bonded with one of the apical oxygen ions and prevents the Ti ion from switching. Pyroelectric measurements on forming gas-annealed capacitors confirm that the capacitors no longer possess spontaneous polarization.

Effect of hydrogen on pb(zr, ti)o3-based ferroelectric capacitors

Damage evolution in ferroelectric PZT induced by bipolar electric cycling

An optical switch (10) includes fused coupler (19) having first and second ends and a coupling region therebetween. There are first and fourth optical ports at the first end of the coupler and second and third optical ports at the second end of the coupler (19). A clamping element (20) rigidly clamps the first end of the coupler, the second end being free. A magnetic sleeve (52) surrounds the coupler (19) near the free end. A first stop block (30) is located on one side of the sleeve. A second stop block (40) is located on the opposite side of the sleeve from the first stop block (30). There is a circuit for generating a repelling magnetic field which causes the sleeve to move into contact with the second stop block (40). In the first position, the first port is optically connected with the third port and the fourth port is optically connected with the second port. In the second position the fourth port is optically connected with the third port. In a preferred embodiment of the invention, the first stop block (30) comprises a first flat stop and a first V-groove stop (45). Similarly, the second stop block (40) may comprise a second flat stop and a second V-groove stop (45). The optical switch (10) may include a sensor element (22) for detecting the first magnetic field or the second magnetic field, thereby detecting the state of the optical switch (10).

High reliability fiber coupled optical switch

A chirped pulse amplification system includes one or more polarization compensator configured to compensate for polarization altering elements with the chirped pulse amplification system. The polarization compensator is responsive to a sensor configured to provide feedback to the polarization compensator. In some embodiments, the chirped pulse amplification system further includes a controller configured to automatically adjust the polarization compensator responsive to the sensor. The sensor is optionally a power sensor.

Automated polarization correction

We report the first observation of spatial one-dimensional photorefractive screening solitons in centrosymmetric media and compare the experimental results with recent theoretical predictions. We find good qualitative agreement with theory.

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One-dimensional steady-state photorefractive spatial solitons in centrosymmetric paraelectric potassium lithium tantalate niobate.

Advantageous methodologies are disclosed for embedding periodic patterns in optical waveguide elements such as optical fibers. Polarization independence in an elongated waist region of a coupler can be established by measuring polarization characteristics during fusion and elongation, and controlling the heating and stretching to impart a cross-sectional shape, such as a hybrid dumbbell-ellipsoid produces a polarization insensitive drop wavelength. Alternatively, or additionally, polarization dependence can be minimized by angular deformation of the elements along its light transmissive axis. In addition, an element of relatively low photosensitivity is held in an hydrogen or deuterium environment pressurized to about 1000 to 5000 psi. While the environment is pressurized, a scanning UV beam is transmitted through a photomask and impinges on the coupler waist. In writing the grating, the in-diffused gas is constantly replenished, enabling the grating to grow. Prior to writing the periodic pattern dimensional variations in the element which can affect spectral bandwidth are sensed by writing a test pattern in the element and then locally measuring the spectral properties of the test pattern progressively along the element and adjusting the local level of background index of refraction so that the modal index of refraction is substantially constant, minimizing imperfections in the precision of the wavelength pattern. The scanning writing beam, which can be of substantially larger cross-sectional dimensions than the waveguide element (which may be in the 4-10 micron range) is caused to track on the element despite positional imprecision and temporal shifting by using fluorescence induced in the elements to provide an error signal for positive correction. To apodize the grating in accordance with a selected function, a constant power beam is directed through a rotating half-wave plate and into a polarizing beam splitter, where it is divided into two beams having oppositely varying d.c. amplitude characteristics. One beam is varied by a periodic pattern, and the other beam is free of a periodic pattern. Alternately, a scanner toggles the constant intensity beam between the two beam paths in rapid succession, varying the duty cycle of toggling as the beams scan along the coupler waist to produce the desired apodization profile of the grating.

Methods of fabricating grating assisted coupler devices

An add/drop filter for optical wave energy incorporates a Bragg grating in a very narrow waist region defined by merged lengths of elongated optical fibers. Light is propagated into the waist region via adiabatically tapered fibers and is transformed from two longitudinally adjacent fibers into two orthogonal modes within the air-glass waveguide of the waist and reflected off the grating from one fiber into the other. The geometry of the waist region is such that the reflected drop wavelength is polarization independent, without lossy peaks in the wavelength band of interest. Additionally, back reflection are shifted out of the wavelength band of interest. High strength gratings are written by photosensitizing the waist region fibers by constantly in-diffusing pressurized hydrogen or deuterium. For narrow spectral bandwidth gratings, dimensional variations must be minimized or compensated, and the grating is apodized by both a.c. and d.c. variations in writing beams at a net constant power. A coupler device employing these precisely is arranged in a support structure which facilitates writing, testing and adjustment of gratings, and also provides temperature compensation and wavelength adjustment.

Grating assisted coupler devices

We report the f ixing of photorefractive holographic gratings with high eff iciency in a sample of K1-yLiyTa1-xNbxO3 doped with Cu, V, and Ti. Holograms are thermally fixed through the screening of a photorefractive space-charge field by a nonphotoactive species at elevated temperatures. Fixed holograms are revealed by illumination at lower temperatures. Diffraction efficiencies of 25% in a 0.54-cm-thick sample are measured. Holograms undergo thermal decay with a 0.67-eV activation energy.

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Fixing of photorefractive volume holograms in K(1-y)Li(y)Ta(1-x)O(3).

Systems devices and methods in accordance with the invention impart high strength index of refraction patterns to photosensitive optical devices, such as Bragg gratings written in optical fibers. A length of small diameter fiber retaining photosensitivity is fabricated by flame elongation of an optical fiber precursor having dopant containing cladding, using a diffuse, low velocity inverted flame that does not introduce water, OH or H2 into the fiber. By varying the flame velocity during each scan the fiber is diminished to a small, uniform diameter, waist region. Photosensitivity is preserved and enhanced by exposure of the prepared waist region to scanning actinic illumination within an in-diffusing environment of pressurizing hydrogen or deuterium, and controlling the exposure to optimize the photo-induced index change. The photochemistry is balanced to provide a strong index of refraction grating by adjusting illumination instensity and scanning velocity to arrive at a low fraction of a selected photoabsorbing component of the dopant.

Fabrication of photosensitive strong grating couplers

An optical interleaver assembly (10) for modifying the transmission characteristics of an input optical beam. The interleaver assembly including a first polarization beam splitter device (18) for separating the input beam into first and second orthogonally polarized output beams, an optical delay device (36, 37) having a first non-birefringent optical delay line (36) and a second non-birefringent optical delay line (37) for providing a time delay difference between the first and second polarized output beams and a second polarization beam splitter (56) for combining the first and second delayed beams into a single beam. The first and second optical delay lines are positioned between the first and second polarization beam splitters and consist of glass.

Xiaolin Tong

Papers

Methods of fabricating grating assisted coupler devices

Grating assisted coupler devices

Fixing of photorefractive volume holograms in K(1-y)Li(y)Ta(1-x)O(3).

Fabrication of photosensitive strong grating couplers

Interleaver filters employing non-birefringent elements