Optical waveguides – With optical coupler – Input/output coupler
Reexamination Certificate
2000-08-24
2002-08-27
Palmer, Phan T. H. (Department: 2874)
Optical waveguides
With optical coupler
Input/output coupler
C385S010000, C385S027000, C430S321000
Reexamination Certificate
active
06442312
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to an optical filter comprising a Bragg grating formed in an optical fiber, to a method and apparatus for manufacturing such a filter, and to a fiber holder and phase mask used in the manufacturing process.
In-fiber Bragg gratings, also known simply as fiber Bragg gratings or FBGs, are useful in the field of optical communication as optical filters for such purposes as wavelength-division multiplexing and dispersion compensation. U.S. Pat. No. 5,367,588 describes a method of manufacturing an in-fiber Bragg grating by exposing a photosensitive optical fiber to ultraviolet light through a phase grating mask. The phase grating mask comprises a quartz glass plate, which is transparent to ultraviolet light, having a periodic relief pattern of parallel corrugations on one surface. The corrugations have the form of, for example, parallel channels with a rectangular cross section. Diffraction in the phase mask modulates the intensity of the emerging ultraviolet light with a periodicity determined by the grating spacing or pitch.
The photosensitive optical fiber is placed in contact or near-contact with the phase grating mask, in a direction orthogonal to the corrugations. Exposure to the ultraviolet light changes the refractive index of the core of the fiber, imprinting an index modulation in the fiber core with the same periodicity as that of the phase grating mask. This index modulation constitutes the Bragg grating.
A chirped Bragg grating can be formed by modulating the grating pitch of the phase grating mask. An apodized Bragg grating can be formed by modulating the strength of the ultraviolet light along the length of the optical fiber.
The phase grating mask can be fabricated by reactive ion etching of a fused quartz substrate, as described, for example, on page 567 of Electronics Letters, Vol. 29, No. 6 (Mar. 18, 1993).
Filter performance parameters such as the reflection bandwidth and the top flatness of the reflection spectrum are known to depend on the length of the imprinted grating. When an in-fiber Bragg grating is used for dispersion compensation, for example, the reflection bandwidth &Dgr;&lgr; is given by the following formula, in which L is the length of the Bragg grating, c is the speed of light, and D is the dispersion value.
&Dgr;&lgr;=2
L
/(
cD
)
This formula indicates that for a given dispersion D, the reflection bandwidth &Dgr;&lgr; increases in proportion to the grating length L.
Long in-fiber Bragg gratings are not easily fabricated with a phase grating mask of the type described above, however, because the size of the phase grating mask is limited by the need to form the phase grating mask itself in a vacuum chamber. A step-and-repeat process can be carried out by moving the fiber past the phase grating mask, but this process is time-consuming and requires extremely accurate alignment from one step to the next. For these reasons, the length of in-fiber Bragg gratings formed by use of conventional phase grating masks has been limited to a maximum of about one hundred millimeters (100 mm).
The limited length of the conventional phase grating mask is thus an obstacle to the attainment of wide reflection bandwidths and other desirable filter characteristics. The limited length is also an obstacle to effective apodization of the in-fiber Bragg grating.
A further obstacle to the use of long in-fiber Bragg gratings is the need to package the fiber containing the grating in such a way as to protect the grating from temperature variations and other external effects. Conventional packaging processes cannot easily be applied to long lengths of fiber.
SUMMARY OF THE INVENTION
It is accordingly an object of the present invention to manufacture an optical filter having an in-fiber Bragg grating longer than one hundred millimeters.
Another object is to provide apparatus for manufacturing such an optical filter.
Another object is to provide a fiber holder useful in the manufacture of such an optical filter.
Another object is to provide a phase mask useful in the manufacture of such an optical filter.
Another object of the invention is to manufacture an optical filter having a highly accurate in-fiber Bragg grating.
Another object is to manufacture an optical filter having an apodized in-fiber Bragg grating longer than one hundred millimeters.
Another object is to manufacture an optical filter having a chirped in-fiber Bragg grating longer than one hundred millimeters.
Another object is to provide an efficiently packaged optical filter.
Another object is to provide a compactly packaged optical filter.
Another object is to provide a well-protected optical filter.
The invented method of manufacturing an optical filter comprises the steps of:
securing an optical fiber having a photosensitive core in a flat spiral arrangement on the upper surface of a fiber holder;
placing a phase mask parallel to the upper surface of the fiber holder, the lower surface of the phase mask having a spiral diffraction grating facing the optical fiber; and
exposing the optical fiber to ultraviolet light through the phase mask, thereby creating an in-fiber Bragg grating.
The step of exposing is preferably carried out by rotating the fiber holder and phase mask while radially scanning the phase mask with an ultraviolet beam, the scanning being synchronized with the rotation so that the beam follows the spiral arrangement of the optical fiber.
The invented fiber holder comprises a flat plate having a spiral groove for holding the photosensitive optical fiber.
The invented phase mask comprises a plate transparent to ultraviolet light, having a spiral pattern of periodic pits on one surface.
The invented apparatus for manufacturing an optical filter comprises the invented fiber holder, the invented phase mask, a rotating stage supporting the fiber holder and phase mask, and an optical system for illuminating the photosensitive optical fiber with ultraviolet light through the phase mask.
The invented optical filter comprises the invented fiber holder, and an optical fiber with a periodically modulated refractive index, held in the spiral groove.
In-fiber Bragg gratings up to at least about four meters in length can be manufactured by the invented method, using a fiber holder and phase mask having the form of five-inch discs. The fiber holder and phase mask can be fabricated using equipment of the type conventionally used for processing semiconductor wafers.
An accurate in-fiber Bragg grating can be produced because a continuous manufacturing process is employed, rather than a step-and-repeat process.
An apodized in-fiber Bragg grating is formed by varying the amount of ultraviolet light to which the optical fiber is exposed according to position on the upper surface of the fiber holder. If the phase mask is scanned by an ultraviolet beam, the amount of ultraviolet light can be varied by using a pulsed light source and varying the pulse repetition rate. Alternatively, a variable optical attenuator can be employed, or the rotational speed of the fiber holder can be varied.
A chirped in-fiber Bragg grating is formed by dividing the phase mask concentrically into zones, and varying the spacing of the pits in the spiral diffraction grating from zone to zone.
The in-fiber Bragg grating can be efficiently packaged between the invented fiber holder and a cover. The cover may also have a spiral groove.
The cover can be formed by applying a protective layer to the fiber holder and optical fiber after formation of the in-fiber Bragg grating. The fiber holder can be formed by patterning a polymer layer disposed on a substrate, creating a spiral groove in the polymer layer. Alternatively, a polymer layer can be patterned to form a dummy fiber, a polymer protective layer can be applied around the dummy fiber, and then the dummy fiber can be removed, leaving a spiral groove in the polymer protective layer. A polymer protective underlayer may also be applied to the substrate. A compact, well-protected optical filter module can be manufactured in this way.
Nishiki Akihiko
Nomoto Tsutomu
Terao Yoshitaka
Frank Robert J.
Oki Electric Industry Co. Ltd.
Palmer Phan T. H.
Sartori Michael A.
Venable
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