Methods of writing apodized fiber gratings and associated...

Details Methods of writing apodized fiber gratings and associated... Methods of writing apodized fiber gratings and associated...

2000-07-27

2003-09-02

Sanghavi, Hemang (Department: 2874)

Optical waveguides

With optical coupler

Input/output coupler

Reexamination Certificate

active

06614959

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention is directed to forming an optical waveguide grating having a refractive index variation, more particularly to an apodized fiber grating, by varying a fringe contrast of an interference pattern used to expose the waveguide along the waveguide.
2. Description of Related Art
A fiber grating is a modulation of the refractive index of a fiber. An interference pattern of ultraviolet (UV) radiation forms this modulation by locally increasing the index of the fiber core. This interference pattern, and thus the resulting modulation, generally has both a DC and an AC component. The AC component has a submicron periodicity and causes the grating to reflect light at a particular wavelength. The DC component is generally unwanted and causes the grating to be “chirped”, i.e., the grating wavelength varies along the length of the grating.
For wavelength division multiplexing (WDM) applications, fiber gratings are ideally “purely apodized”, i.e., the AC component has a smoothly varying envelope, e.g., a Gaussian envelope, and the DC component is uniform along the whole length of the grating. FIG.
1
A illustrates the index modulation of a grating having an AC component with a Gaussian envelope and a DC component which is also a Gaussian function.
FIG. 1B
illustrates the index modulation of a grating having an AC component with a Gaussian envelope and a DC component which is constant.
The Gaussian variation or chirp of the DC component of the grating with the index modulation shown in
FIG. 1A
results in a grating reflection spectrum, shown in
FIG. 2A
, which is strongly asymmetric, with sidelobes on the short wavelength side. In contrast, the grating having the uniform DC component shown in
FIG. 1B
has a symmetric reflection spectrum and very low sidelobes. This symmetric wavelength response results in improved filtering performance.
Several methods have been proposed to produce purely apodized gratings. The simplest method of apodizing a grating involves spatially varying the intensity of a beam being used to write to a fiber. This is typically achieved using an interferometer or a phase mask, as discussed below. One such method of spatially varying a beam intensity to form an apodized grating is a double exposure method, in which a fiber is exposed twice to raise the average index of the fiber. This double exposure may be performed either sequentially, as set forth in U.S. Pat. No. 5,309,260 to Mizrahi et al., entitled “Method for Forming Distributed Bragg Reflectors in Optical Media” or simultaneously, as set forth in H. Singh and M. Zippin, “Apodized Fiber Gratings for DWDM Applications using Uniform Phase Mask”, European Conference on Optical Communications Proceedings, 1998. In either scenario, the second exposure does not interfere with the first exposure, i.e., produce fringes, but merely alters the refractive index profile.
Another method for making a purely apodized grating is the use of an apodized phase mask to create the grating, as discussed, for example, in L. E. Erickson et al. “Fabrication of a Variable Diffraction Efficiency Phase Mask by Multiple Dose Ion Implantation,” J. of Vac. Sci. Tech. B 13(6), pp.2940-3, November 1995. This results in a constant average index of refraction along the grating, while the index modulation at the ends of the grating approaches zero. This apodized phase mask approach lacks flexibility, since a different phase mask is required for every different grating wavelength or apodization profile.
Yet another method for making a purely apodized grating involves moving the phase mask and the fiber relative to one another during scanning by a beam. In this techniques, apodization is achieved by dithering the relative phase between the phase mask and the fiber at the edges of the grating, as set forth, for example in R. Kashyap et al., Electronics Letters, Vol. 32 (15), pp. 1394-6, 1996. While this technique alone is limited to the length of available phase masks, longer gratings can be made by scanning several such masks, while trimming the discontinuities between sections, as disclosed in M. I. Cole et al., Electronics Letters, Vol. 31 (17), pp. 1488-9, 1995. Alternatively, longer gratings may be created by significantly overlapping the footprint of the writing light beam with previous lines to average the writing process, as set forth in PCT Application Number PCT/GB97/02099 to Laming et al., entitled “Fabricating Optical Waveguide Gratings” published on Feb. 26, 1998. While this technique is flexible, it is difficult to implement and requires expensive, accurate equipment.
SUMMARY OF THE INVENTION
The present invention is therefore directed to a method of creating apodized gratings which substantially overcomes one or more of the problems due to the limitations and disadvantages of the related art.
The above and other objects may be realized by exposing a fiber to ultraviolet (UV) interference patterns with a uniform average intensity, but with a varying fringe contrast.
At least one of the above and other objects may be realized by a method of fabricating an optical waveguide grating having a refractive index variation including providing a photosensitive optical waveguide, splitting input light into two beams, supplying the two beams to the photosensitive optical waveguide, exposing the photosensitive optical waveguide to an interference pattern formed by the supplying of the two beams, and varying a fringe contrast of the interference pattern along the photosensitive optical waveguide.
The varying may include altering a relative polarization between beams used to generate the interference pattern. The altering may include rotating a polarization of one of the beams used to generate the interference pattern.
The varying may include altering relative intensities of beams used to generate the interference pattern. The altering may include providing a mask having openings with sizes varying with position for each of the beams. The providing a mask may include creating a pair of masks includes generating a pair of masks designed to provide a substantially constant total intensity on the optical waveguide. The exposing may include providing a beam of uniform intensity to each of the masks.
The varying may include rotating a polarization of an input beam and using a polarizing beam splitter to split the input beam into beams used to generate the interference pattern. The varying may further include controlling said rotating such that the relative intensities of the beams used to generate the interference patter are related as follows:
I
1
(
z
)=½(
1±{square root over (1−
C
(
z
)
2
)})
I
2
(
z
)=½(
1∓{square root over (1−
C
(
z
)
2
)})
where z is a position along the waveguide, I (z) is the intensity of a beam used to generate the interference pattern at a position z, and C(z) is a desired contrast function.
The varying includes rotating a polarization of an input beam, deflecting a portion of the input beam having a first polarization from a portion of the input beam having a second polarization orthogonal to the first polarization, and amplitude splitting the input beam into the beams used to generate the interference pattern. The varying may further include controlling the rotating such that the relative intensities of the beams used to generate the interference patter are related as follows:
I
1
(
z
)=½(
1±{square root over (1−
C
(
z
)
2
)})
I
2
(
z
)=½(
1∓{square root over (1−
C
(
z
)
2
)})
where z is a position along the waveguide, I (z) is the intensity of a beam used to generate the interference pattern at a position z, and C(z) is a desired contrast function.
The exposing may include scanning a beam along the photosensitive optical waveguide. The varying may include altering a relative polarization between beams used to generate the interference pattern simultaneously with the scanning to generate a desired grating. The varying may include altering relative

Affiliated with

Also associated with

Rating

Methods of writing apodized fiber gratings and associated... does not yet have a rating. At this time, there are no reviews or comments for this patent.

If you have personal experience with Methods of writing apodized fiber gratings and associated..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Methods of writing apodized fiber gratings and associated... will most certainly appreciate the feedback.

Rate now

Comments { 0 }

Profile ID: LFUS-PAI-O-3003415