Optical waveguides – With optical coupler – Input/output coupler
Reexamination Certificate
2001-11-09
2004-10-19
Ullah, Akm Enayet (Department: 2874)
Optical waveguides
With optical coupler
Input/output coupler
C385S024000
Reexamination Certificate
active
06807339
ABSTRACT:
TECHNICAL FIELD
The present invention relates generally to systems for wavelength division multiplexing, and more particularly to applying particular forms of optical gratings to multiplex, de-multiplex, interleave, and de-interleave multiple light wavelengths.
BACKGROUND ART
Optical technology is progressing rapidly. Growing needs, particularly in the telecommunications industry, are driving this progress and there is currently a major impetus to improve existing optical systems and to develop new ones. Unfortunately, several major components still are not completely meeting manufacturing yield, field reliability, and operating capacity requirements. These failings have resulted in high costs in existing systems and are limiting the adoption of future systems. One such component is the optical grating.
FIGS. 1
a-b
(background art) depict two variations of traditional gratings. As can be seen, the shape of the groove can vary.
FIG. 1
a
shows square steps and
FIG. 1
b
shows blazed triangles, but other shapes are also possible, e.g., sinusoidal shaped grooves, and the physics is essentially the same.
Such “traditional gratings” were initially made of glass with grooves, and a few are still produced in this manner today. This, however, has a number of disadvantages. For instance, the density of the grooves is limited by the capability of the ruling engine, and the quality of the grooves produced tends to decrease as elements of the ruling engine wear from usage. Production of this type of gratings is time consuming and difficult, and the cost of such gratings is therefore high.
Molded and holographic gratings were invented later on, and their production cost is significantly lower than for glass gratings. Unfortunately, although suitable for many applications, these gratings tend to deteriorate in harsh environments. For example, in fiber optic communications, all optical components must operate for long periods of time in temperatures ranging from sub-zero to over eighty degrees Centigrade, and in humidity ranging from zero to 100 percent (see e.g., GR468-CORE, Generic Reliability Assurance Requirements for Optoelectronic Devices Used In Telecommunications Equipment).
As can also be seen in
FIG. 1
a-b
, traditional gratings have the property that light has to shine on the grating surface from above. This limits the useful diffraction effect of such gratings to only one dimension, and multiple units need to be assembled if multiple dimensions (axes of direction) are required.
One example of an application where the need to work with multiple wavelengths and axes is common, and growing, is wavelength division multiplexing and de-multiplexing (collectively, WDM) in fiber optic communications. The use of traditional gratings in WDM usually requires either adhesives or mechanical fixtures to keep the assembly intact. Alignment is also needed to make sure that the gratings diffract light in the proper directions. The resulting assemblies formed with such traditional gratings thus tend to be significantly larger than the optical fibers being worked with and mechanical connectors are needed for connection. All of these considerations, and others, increase the cost in a fiber optic communications system.
A relatively recent invention is the fiber Bragg grating. The fiber Bragg grating is a periodic perturbation in the refractive index which runs lengthwise in the core of a fiber waveguide. Based on the grating period, a Bragg grating reflects light within a narrow spectral band and transmits all other wavelengths which are present but outside that band. This makes Bragg gratings useful for light signal redirection, and they are now being widely used in WDM.
The typical fiber Bragg grating today is a germanium-doped optical fiber that has been exposed to ultraviolet (UV) light under a phase shift mask or grating pattern. The unmasked doped sections undergo a permanent change to a slightly higher refractive index after such exposure, resulting in an interlayer or a grating having two alternating different refractive indexes. This permits characteristic and useful partial reflection to then occur when a laser beam transmits through each interlayer. The reflected beam portions form a constructive interference pattern if the period of the exposed grating meets the condition:
2
*&Lgr;n
eff
=&lgr;
where &Lgr; is the grating spacing, n
eff
is the effective index of refraction between the unchanged and the changed indexes, and &lgr; is the laser light wavelength.
FIG. 2
(background art) shows the structure of a conventional fiber Bragg grating
1
according to the prior art. A grating region
2
includes an interlayer
3
having two periodically alternating different refractive indexes. As a laser beam
4
passes through the interlayer
3
partial reflection occurs, in the characteristic manner described above, forming a reflected beam
5
and a passed beam
6
. The reflected beam
5
thus produced will include a narrow range of wavelengths. For example, if the reflected beam
5
is that being worked with in an application, this separated narrow band of wavelengths may carry data which has been superimposed by modulation. The reflected beam
5
is stylistically shown in
FIG. 2
as a plurality of parts with incidence angles purposely skewed to distinguish the reflected beam
5
from the laser beam
4
. Since the reflected beam
5
is merely directed back in the direction of the original laser beam
4
, additional structure is usually also needed to separate it for actual use.
Unfortunately, as already noted, conventional fiber Bragg gratings and the processes used to make them have a number of problems which it is desirable to overcome. For example, the fibers usually have to be exposed one-by-one, severely limiting mass-production. Specialized handling during manufacturing is generally necessary because the dosage of the UV exposure determines the quality of the grating produced. The orientation of the fiber is also critical, and best results are achieved when the fiber is oriented in exactly the same direction as the phase shift mask. The desired period of the Bragg grating will be deviated from if the fiber is not precisely aligned, and accomplishing this, in turn, introduces mechanical problems. Thus, merely the way that the fiber work piece is held during manufacturing may produce stresses that can cause birefringes to form in the fiber and reduce the efficiency of the end product grating.
Once in use, conventional fiber Bragg gratings may again require special handling. The thermal expansion coefficient of the base optical fiber is often significant enough that changing environmental conditions can cause the fiber to either expand or shrink to the extent that the period of the grating and its center wavelength shift.
From the preceding discussion of traditional and fiber Bragg gratings it can be appreciated that there is a need for optical gratings which are better suited to the growing range of grating applications. Two such applications are multiplexing and de-multiplexing. Fiber Bragg gratings have been widely used for these applications, despite the severe problems that come with them. In particular, handling large numbers of light wavelengths and ranges of light wavelengths has been quite problematical with fiber Bragg gratings. Firstly, without complex additional structure, fiber gratings do not direct the light beams carrying multiplexed and especially demultiplexed wavelengths where they are usually desired. For example, the basic fiber Bragg grating merely reflects a separated wavelength back in the very same direction as the input beam from which it is being separated. Secondly, applying multiple wavelength handling characteristics and “chirping” to handle wavelength ranges in fiber gratings is difficult, with the difficulty increasing at a non-linear rate as additional wavelengths and ranges are provided for. Thirdly, as can be appreciated from the above discussion, constructing and maintaining assemblies of multiple traditional or fiber Bragg gratings to handle l
Tsai John C.
Wang David W.
Connelly-Cushwa Michelle R.
Fibera, Inc.
Intellectual Property Law Offices
Roberta Raymond E.
Ullah Akm Enayet
LandOfFree
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