Fiber grating and fiber optic devices using the same

Details Fiber grating and fiber optic devices using the same Fiber grating and fiber optic devices using the same

2000-01-10

2002-08-06

Bovernick, Rodney (Department: 2874)

Optical waveguides

With optical coupler

Input/output coupler

C385S007000, C385S028000, C385S039000

Reexamination Certificate

active

06430342

ABSTRACT:

TECHNICAL FIELD
The present invention relates to a fiber optic element, more specifically to a fiber grating that couples a light mode propagating along a fiber into another mode by a plurality of microbends formed in the fiber.
The present invention also relates to optical devices, more specifically to fiber optic devices, such as a fiber-optic filter, a fiber-optic polarizer, a fiber-optic wavelength tunable bandpass filter, a fiber-optic frequency shifter, using the above fiber grating which has asymmetric mode-coupling characteristics.
BACKGROUND ART
Recently, increasing use is made of fiber Bragg gratings in various fiber-optic applications such as telecommunications, fiber sensors and lasers. The fiber Bragg grating consists of a periodic stack of regions of higher and lower refractive index along an optical fiber. Gratings are made by exposing the core of a fiber to an interference pattern of strong laser light. It has the property of reflecting light within a narrow band of wavelengths and transmitting all wavelengths outside of that band. The central reflected wavelength is equal to twice the period of the grating, multiplied by the fiber refractive index. For example, a grating reflecting at 1560 nm would have a period of about 535 nm. Most of the fiber Bragg gratings have periods of a few 100 nanometers.
On the other hand, a long period fiber grating has a period of a few 100 microns. The long period fiber grating couples a specific wavelength light, propagating along the core of the grating, into a cladding mode of the same propagating direction. The long period fiber grating can act as a band-rejection filter since the coupled cladding mode can easily be stripped. These long period fiber gratings have the advantages of easy fabricating, reduced fabricating cost and compact size. They will therefore be useful in many applications including the gain-flattening filter of optical amplifiers.
Hereinafter, the conventional methods for fabricating these long period fiber gratings will be explained in brief as follows:
[Method Using the Photosensitivity of Optical Fibers]
FIG. 1
shows the cross section of a conventional fiber grating that is fabricated using the photosensitivity of a single-mode optical fiber. In principle, this method is the same as the conventional method for fabricating fiber Bragg gratings. However, this method should employ a specific optical fiber including a fiber core with photosensitivity enhanced by doping therein Germanium(Ge) or the like.
Referring to
FIG. 1
, the side of a single-mode optical fiber is exposed to the light
10
of an excimer laser. The molecular structure of the exposed portions
30
in the fiber core
20
is deformed, thereby the portions
30
have higher refractive index. Thus, by irradiating the fiber with uniformly spaced laser light along the fiber axis, a single-mode fiber grating
40
with a periodically varying refractive index can be obtained. This grating couples a specific wavelength light, propagating along the core of the grating, into a cladding mode. Therefore, this grating can act as a filter.
FIG. 2
shows the cross section of another conventional fiber grating that is fabricated using the photosensitivity of a two-mode optical fiber. The two-mode fiber grating
40
′ is also fabricated by the same manner as that of the single-mode fiber grating. The fiber grating
40
′ can couple the fundamental LP
01
mode into the second-order LP
11
mode, since the regions
30
′ of higher refractive index are asymmetrically formed along the fiber axis.
However, the fiber gratings fabricated by this method have a disadvantage that the gratings are erased with the passage of time. In addition, it is difficult to make shorter fiber gratings because they have low mode coupling efficiency.
[Method Using the Thermal Expansion of Fiber Core]
These fiber gratings are fabricated using the thermal diffusion of the dopants in the fiber core. When the core is strongly heated, the core expansion is induced by the thermal diffusion of the dopants.
FIG. 3
shows the procedure of fabricating such a fiber grating. Referring to
FIG. 3
, the core
22
of an optical fiber is locally heated to form a core portion
24
with a larger radius by the light
12
from a high power laser. The light
22
is periodically scanned along the fiber axis. For efficient local heating, a convex lens C focusing the light
12
can be used together with the high power laser. Instead of the laser heating method, electric arc method may be used.
However, the fiber gratings fabricated by this method have a disadvantage that special optical fibers doped with an element of low molecular weight such as nitrogen should be used to enhance the thermal expansion effect of the core.
[Method Using the Index Change Due to the Stress Removal]
In fabricating an optical fiber, if the fiber is cooled in a state that tensile force is applied to the fiber, stress will exist in the core of the fabricated fiber because of the difference of cooling speed between the core and cladding. The stress can be removed by reheating the fiber, raising the refractive index of the core. Fiber gratings can be fabricated using the above phenomenon. That is, heating an optical fiber locally using a high power laser or an electric arc can induce the refractive index change.
However, this method should be applied to an optical fiber with a core made of pure silica that is not doped with germanium or the like.
[Method Using the Periodic Deformations of Fiber Core]
It is well-known that closely spaced microbends in the fiber core, which are introduced using two deformers with teeth thereon, can couple a core mode into a cladding mode or other core modes. In this case, the symmetric core mode LP
01
can be coupled into asymmetric modes such as LP
11
, LP
21
and LP
31
since asymmetric deformations are introduced along the fiber axis.
A schematic illustration of this fiber grating is shown in FIG.
4
. Referring to
FIG. 4
, an optical fiber
60
is inserted between two deformers
50
with periodic teeth thereon. The fiber
60
is bent to form microbends by the pressure F applied to the deformers
50
. However, the fiber gratings fabricated by this method exhibit unstable performance characteristics depending upon the pressure applied to the deformers.
Another method was therefore suggested that could obtain better stability in the periodic deformations.
FIG. 5
shows the procedure of introducing periodic deformations in the fiber core by another method. Referring to
FIG. 5
, grooves G made by a CO
2
laser are spaced apart by an equal spacing. The grooves G are heated by the electric arc A of electrodes
70
vertically disposed on both sides of the optical fiber. The heated groove is melt to deform the fiber core due to surface tension as shown in the left side of the electrodes
70
. This method base on the physical deformation are applicable to almost all types of optical fibers, but a high power laser is required to make grooves on the fiber. Additionally, the grooves made on the fiber-weaken the overall strength of the completed grating to resist torsion, bending and the like loads. As described above, the conventional fiber gratings have the disadvantages of poor characteristics and complexities in the fabrication process.
DISCLOSURE OF INVENTION
It is therefore an object of the present invention to provide an improved fiber grating which can be fabricated by simple process.
Another object is to provide a variety of improved optical devices realized by using the above fiber grating.
In order to accomplish the aforementioned object, the present invention provides a fiber grating for inducing a coupling between different light modes, comprising: a length of an optical fiber; and a plurality of stepped microbends formed along the length of the optical fiber, each of the microbends being stress relieved.
The microbends may be spaced apart by a periodic distance substantially equal to a beat length of the different modes to be coupled and th

Affiliated with

Also associated with

Rating

Fiber grating and fiber optic devices using 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 fiber optic devices using the same, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Fiber grating and fiber optic devices using the same will most certainly appreciate the feedback.

Rate now

Comments { 0 }

Profile ID: LFUS-PAI-O-2956504