Optical waveguides – With optical coupler – Input/output coupler
Reexamination Certificate
1999-10-14
2001-07-31
Spyrou, Cassandra (Department: 2872)
Optical waveguides
With optical coupler
Input/output coupler
C359S559000, C359S566000, C359S570000, C359S577000, C359S900000
Reexamination Certificate
active
06269208
ABSTRACT:
TECHNICAL FIELD
The invention relates to optical gratings manufactured in photosensitive media of optical waveguides by patterned exposures to actinic radiation. The patterns which include alternating bands of light are generally produced by interference fringes and can have periods of less than one micron.
BACKGROUND
Interference patterns for making optical waveguide gratings, particularly fiber Bragg gratings, can be produced using an interferometer or a phase mask. Interferometers divide a coherent beam of light into two separate beams that are angularly recombined at the waveguides for producing a desired interference pattern. Phase masks, which are themselves diffraction gratings, divide a similarly coherent beam into different diffraction orders that are recombined at the waveguides for producing a similar interference pattern.
For purposes of manufacturing, phase masks are often preferred because interferometers can be less stable and difficult to use in production environments. Phase masks are more stable but have less flexibility for adjusting the period of their resulting interference patterns. The interference period between two collimated beams is a function of the wavelength of the interfering beams and the angle at which the beams are combined. Any change in the wavelength of a beam divided by a phase mask also changes the diffraction angles through which the divided beams are recombined, so a different phase mask is often needed for each desired interference pattern.
A few techniques have been developed to adjust the effect of photoinduced waveguide patterns produced by phase masks. Bragg wavelengths (center wavelengths) of the resulting grating responses are a function of both the grating period and the average refractive index of the waveguides. Small adjustments to the Bragg wavelength have been made by pre-straining waveguides (i.e., optical fibers) and by illuminating phase masks with converging or diverging beams.
The former technique is described in a paper entitled “Tuning Bragg Wavelength by Writing Gratings on Prestrained Fibers” by Quin Zhang et al., published in Photonics Technology Letters, Vol., 6, No. 7, July 1994. A photosensitive fiber is exposed to an interference pattern produced by a phase mask while under strain. When the strain is relieved, the Bragg wavelength is down-shifted with respect to a similarly exposed unstrained fiber. Only a limited amount of strain can be tolerated by fibers and other waveguides, so the amount of adjustment by this technique is limited.
A paper entitled “Magnification of Mask Fabricated Fibre Bragg Gratings” by J. D. Prohaska et al., published in Electronics Letters, Vol. 29, No. 18, Sep. 2, 1993, proposes to illuminate phase masks with converging or diverging beams to adjust the magnification of interference patterns incident to photosensitive fibers. The power of a converging or diverging lens, the distance between the lens and the phase mask, and the distance between the phase mask and fiber can be changed to adjust the magnification of the interference pattern within the Fresnel near field of the light passing through the phase mask. However, only small changes in periodicity are practical because interference patterns produced at a distance from the phase mask are limited by spatial coherence of the illuminating beam.
U.S. Pat. No. 5,327,515 to Anderson et al. mounts a lens between a phase mask and a photosensitive fiber to project an image of a interference pattern formed at the phase mask onto the fiber. The lens projection system can be arranged to provide magnification or reduction of the interference pattern projected onto the fiber. However, like the known interferometer arrangements, issues of stability and alignment render this technique less practical in a production environment.
SUMMARY OF INVENTION
Our invention provides more flexibility in the manufacture of optical waveguide gratings with phase masks. Adjustments can be made in grating period and grating length, as well as apodization profiles. Small changes can be made to the gratings for purposes of tuning to compensate for other design variations, or large changes can be made to manufacture gratings with different specifications. One grating can be written over another with the same phase mask, which is particularly useful for making compound sensors. Grating chirp can also be controlled to support more complex spectral responses.
Our preferred embodiment includes a spatial filter that removes spatially incoherent light from a beam of light (i.e., actinic radiation). A phase mask divides the filtered beam of light into two interfering beams that form an interference pattern with an average fringe period along a waveguide. A focusing system directs the beam of light approaching the spatial filter as a converging beam and further directs the filtered beam of light as a noncollimated beam impinging upon the phase mask. A waveguide support positions the waveguide at a distance from the phase mask to adjust the average fringe period of the interference pattern formed along the waveguide.
The spatial filter filters incoherent light from at least a first of two orthogonal directions transverse to an axis of beam propagation. This is the same direction in which the waveguide is oriented. The focusing system converges the beam of light in the first orthogonal direction through a first focal line located at the spatial filter. Approaching the waveguide, the focusing system diverges or converges the filtered beam in the first orthogonal direction. By locating a diverging or converging element between the spatial filter and the phase mask, the effective center of curvature of the beam impinging on the phase mask can be varied. A diverging beam impinging on the phase mask in the first orthogonal direction increases the average fringe period formed along the waveguide, and a converging beam impinging on the phase mask in the same first orthogonal direction decreases the average fringe period. To minimize optical components requiring alignment, the diverging beam can have a center of curvature on the first focal line, which is located at the spatial filter.
The focusing system preferably provides separate control over beam shape in the two orthogonal directions. In the first direction, which is filtered to enhance spatial coherence, the beam is shaped to influence the magnitude of a change in the fringe period associated with a given spacing between the phase mask and waveguide. In the second direction, a separate focusing optic can be used to concentrate light energy on the waveguide. For example, the filtering system can be arranged to diverge the filtered beam along the first orthogonal direction approaching the phase mask for increasing the average fringe period and to converge the same beam along the second orthogonal direction approaching the phase mask for concentrating more light energy on the waveguide.
The spatial filter permits the phase mask and the waveguide to be separated through larger distances while still producing an interference pattern with good fringe contrast. A similar fringe period can be obtained at more than one distance between the phase mask and the waveguide by adjusting the rate of convergence or divergence of the filtered beam. Other variables affected by the distance separating the phase mask and the waveguide include the length of overlap between interfering beams emerging from the phase mask and the intensity profile of the recombined beams. The length of overlap controls the length of grating written into the waveguide. The intensity profile affects apodization issues for obtaining the desired spectral response.
The waveguide can also be tilted in an axial plane that includes the first orthogonal direction for producing a linear chirp in the grating, evident as a grating period that varies from one end of the grating to the other. Shorter focal lengths of the diverging or converging beam upon the phase mask (e.g., a shorter distance between the first focal line at the spatial filter and the phase mask) can produce a qu
Bhatia Vikram
Modavis Robert A.
Corning Incorporated
Jr. John Juba
Redman Mary Y.
Short Svetlana
Spyrou Cassandra
LandOfFree
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