Arroyo Optics, Inc.

Phil A Willems

Anthony Stephen Kewitsch

George A Rakuljic

Xiaolin Tong

Doruk Engin

Seth J Greenberg

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

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

Advantageous methodologies are disclosed for embedding periodic patterns in optical waveguide elements such as optical fibers. Polarization independence in an elongated waist region of an add/drop coupler can be established by measuring polarization characteristics during fusion and elongation, and controlling the elongation to impart a cross-sectional shape, such as a hybrid dumbbell-ellipsoid. Polarization dependence can also be minimized by angular deformation of the elements along the light transmissive axis. To write a pattern, an element having potential photosensitivity is markedly photosensitized in a hydrogen or deuterium environment pressurized to about 1000 to 5000 psi and a scanning UV beam that is transmitted through a photo mask and impinges on the coupler waist as the in-diffused gas is constantly replenished. Dimensional variations in the element which can affect spectral bandwidth are sensed by writing a preliminary test pattern in the element and then locally measuring the spectral properties of the test pattern and adjusting the local levels of background index of refractions so that the modal index of refraction is substantially constant along the length of the pattern. A relatively wide scanning writing beam tracks on a narrower waveguide element despite positional imprecision and temporal shifting by using a feedback signal derived from fluorescence induced in the element. To apodize the grating in accordance with a selected function, a constant power beam is directed through a rotating half-wave plate and divided by a polarizing beam splitter into two beams having oppositely varying DC amplitude characteristics. The two beams are separately varied to produce the desired apodization profile of the grating.

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Pasadena, CA, United States of America

Phil A Willems