Optical waveguides – With optical coupler – Input/output coupler
Reexamination Certificate
2000-05-26
2004-06-15
Font, Frank G. (Department: 2877)
Optical waveguides
With optical coupler
Input/output coupler
C385S010000, C385S128000
Reexamination Certificate
active
06751380
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a fiber grating in which a diffraction grating (grating) exhibiting a periodical index difference is written in a core of an optical fiber, and a method and an apparatus for fabricating the same.
BACKGROUND ART
A conventionally known fiber grating of this type includes a grating written in a core of an optical fiber by a two-beam interference method or a phase mask method (for example, as disclosed in Japanese Laid-Open Patent Publication No. 6-235808, Japanese Laid-Open Patent Publication No. 7-140311 and Japanese Patent No. 2521708). In such a fiber grating, a fused quartz (core) doped with germanium (Ge) is irradiated with a coherent UV laser beam so as to generate (write) a Bragg grating by causing photo-induced refractive index change in an irradiated portion.
The optical fiber used as the target for writing is generally obtained by coating a core and a cladding with a coat layer of a UV curable resin or the like that is cured by absorbing UV. The coat layer formed on a write target portion is generally removed in writing the grating through the UV irradiation in employing either of the two-beam interference method and the phase mask method, and the portion from which the coat layer is removed is coated again after completing writing the grating.
When the coat layer is removed, however, the outer face of a non-coated fiber (the outer face of the cladding) is exposed to air, and there is a fear of degradation of the non-coated fiber proceeded through the exposure to air during the writing work, which can degrade the transmitting characteristic. In addition, the coat layer of the write target portion is removed not by mechanical means but by a chemical treatment for dissolving it with, for example, a chemical, so as not to damage the non-coated fiber. Therefore, the removal of the coat layer is troublesome, which is a factor in degrading efficiency in mass processing for writing the grating.
On the other hand, in order to effectively write the grating by UV irradiation through the coat layer without removing the coat layer, sensitivity of the core of the optical fiber, that is, the target for writing, to the photo-induced refractive index change (photosensitivity) may be possibly increased. As means for increasing the photo sensitivity, namely, means for causing comparatively large photo-induced refractive index change, it has been proposed that the core corresponding to the target for writing is doped with Ge in a higher concentration (a concentration for attaining a relative refractive index difference between the core and the cladding of, for example, 1.5 through 2.0%) than a general concentration (a concentration for attaining the relative refractive index difference of, for example, 0.9%), or that the core is loaded with hydrogen at a high pressure after doping it with Ge in a general concentration (as described in the transactions of the Institute of Electronics, Information and Communication Engineers, Vol. J79-C-1, No. 11, p. 415, November 1996).
When a fiber grating is fabricated by using a core doped with Ge in a higher concentration, however, the following problem occurs in connecting this fiber grating between general optical fibers so as to be used as a filter or the like: Since the core of the optical fiber connected (fused) to the fiber grating has the general specifications doped with Ge in the general concentration, the cores cannot be matched and a difference in the concentration of the doped Ge increases connection loss. On the other hand, when a core loaded with hydrogen at a high pressure is to be used for fabricating a fiber grating, the loaded hydrogen is diffused with time, and the core returns to the original state prior to the hydrogen loading in a comparatively short period of time (for example, several days). Therefore, the time period for forming the grating by the UV irradiation is limited to a rather short period, and additionally, it is difficult to control the wavelength in the UV irradiation because it is necessary to write the grating in consideration of the diffusion of hydrogen.
Furthermore, when a fiber grating where a grating has been written is expanded or shrunken due to an influence of temperature change or an externally applied tensile force, the reflective wavelength of the grating is shifted. Therefore, a fiber grating is required to have a mechanical strength characteristic and a stable temperature characteristic that it cannot be expanded/shrunken by temperature change.
The present invention was devised in consideration of these circumstances, and an object is providing a fiber grating in which a grating can be easily written without causing degradation of the transmitting characteristic. In addition, another object is providing a highly reliable fiber grating by stabilizing not only the transmitting characteristic but also the temperature characteristic.
Another object of the invention is providing a fiber grating and a method of fabricating the same in which the transmitting characteristic and the mechanical strength characteristic can be consistent with each other without spoiling improvement of productivity.
DISCLOSURE OF THE INVENTION
The fiber grating of this invention includes a core where a grating is written, a cladding for covering the core, and a UV transmitting resin layer for coating the outer face of the cladding, and the grating is written in the core by irradiating the core through the resin layer with UV of a specific wavelength band.
Since the coat layer covering the core and the cladding is made from a UV transmitting resin in this fiber grating, even when UV is irradiated through the coat layer, the UV transmits the coat layer to effectively irradiate the core, so that the grating can be written in the core. Accordingly, the grating can be written without removing the coat layer. Therefore, it is possible to avoid degradation of the transmitting characteristic accompanied by the removal of the coat layer as well as to easily fabricate the fiber grating with a process for removing the coat layer omitted.
The coat layer of the UV transmitting resin has a characteristic of transmitting at least UV of a specific wavelength band used for writing the grating. In this case, a preferred coat layer of a UV transmitting resin is concretely specified as follows: The preferred coat layer transmits UV of the specific wavelength band used for writing the grating, for example, a wavelength band of 250 nm through 270 nm. The UV transmitting resin may have a specific wavelength band (transmitting band) within a range between 250 nm and 350 nm. As a light source of the UV having such a wavelength band, one having high spatial coherency, such as solid laser, is preferably used.
The coat layer has a characteristic of curing by absorbing UV in a shorter wavelength band or a longer wavelength band than the specific wavelength band used for writing the grating.
In using the coat layer of this invention, the grating can be written by UV irradiation through the coat layer because it transmits UV in the specific wavelength band, and at the same time, the coat layer absorbs UV and cures in the formation thereof so as to work as a protecting coat of the optical fiber.
The core is preferably co-doped with at least Sn addition to Ge in an amount equivalent to that in the core of an optical fiber to be connected. The amount of Ge to be doped is preferably that for attaining a relative refractive index difference between the cladding and the core of approximately 0.9%, while the concentration of Sn is preferably 10000 through 15000 ppm.
The co-doped Sn (tin) stationary increases the photo-induced refractive index change of the core as compared with a core doped with Ge alone in a general concentration. As a result, the reflectance attained by irradiating with the UV can be increased as compared with that of the core doped with Ge alone in the general concentration. Specifically, a specific wavelength (Bragg wavelength) &lgr;B reflected by a grating is represented by the following formula (1
Imamura Kazuo
Nakai Tadahiko
Sudo Yasuhide
Font Frank G.
Kianni Kevin C
Mitsubishi Cable Industries Ltd.
Nixon & Peabody LLP
LandOfFree
Fiber grating and method and apparatus for fabricating the same does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Fiber grating and method and apparatus for fabricating the same, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Fiber grating and method and apparatus for fabricating the same will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3336274