Optical fiber grating and manufacturing method therefor

Details Optical fiber grating and manufacturing method therefor Optical fiber grating and manufacturing method therefor

1998-06-02

2001-10-02

Sikder, Mohammad Y. (Department: 2872)

Optical waveguides

With optical coupler

Input/output coupler

C385S039000

Reexamination Certificate

active

06298183

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an optical fiber grating and to a manufacturing method therefor. More particularly, it relates to an optical fiber grating which is low cost and exhibits small changes over time, and to a manufacturing method therefor. The present specification is based on Japanese Patent Application No. Hei 9-247292, filed in Japan, the contents of which are incorporated herein by reference.
2. Description of the Related Art
An optical waveguide grating has a spatially periodic perturbation of waveguide structure, formed in a longitudinal direction of an optical fiber or of a Planar Lightwave Circuit (PLC).
This optical wageguide grating is a device which is capable of making loss of light in a specified wavelength band, by generating couplings between the mutual specified modes. Then, having such a characteristic, it may be utilized as coupling-type devices for the elimination of light for specified wavelength band and as coupling-type devices between specified modes.
The optical waveguide grating may be classified into a reflecting type and a radiating type, according to the relationship between the coupling modes.
Here, let the direction of incidence of light for the optical waveguide be the positive direction, and a direction opposite to it, the negative direction.
A reflecting-type optical waveguide grating has been imparted with a characteristic for reflecting light of specified wavelength, by coupling a mode which propagates through the core in the positive direction and a mode which propagates through the core in the negative direction.
A radiating-type optical waveguide grating has been imparted with a characteristic for radiating by coupling a mode which propagates through the core and a mode which propagates through the cladding, so as to obtain the characteristic by having the light of specified wavelength which is radiated to the outside of the waveguide and then attenuated.
Now, the periodic perturbation In the waveguide structure for an optical waveguide grating may be formed by changing the index of refraction for the core or the core diameter.
The most general method of manufacturing an optical waveguide grating is one in which the index of refraction of the core is made to change by a phtotorefractive effect (sometimes also called the “photosensitive effect”).
The photorefractive effect is a phenomenon whereby, for example, when silica glass with germanium as the dopant is irradiated with ultraviolet rays in the neighborhood of wavelength 240 nm, an increase in the index of refraction is observed for the silica glass.
Specific description of the optical fiber will be given concretely as an example as follows.
FIG. 14
is a diagrammatic configuration drawing which describes the manufacturing process for a conventional optical fiber grating.
In the drawing, reference symbol
11
represents an optical fiber, and this optical fiber
11
is composed of the central part thereof, core
11
a
, and a cladding
11
b
which is provided on the outer circumference of this core
11
a.
This optical fiber
11
is, for example, an optical fiber which act as a single mode at a wavelength of 1.55 &mgr;m (the “single-mode optical fiber”).
The core
11
a
is made of silica glass added germanium as a dopant. Germanium is normally added as germanium dioxide to the silica glass.
In this example, the core
11
a
is made of silica glass containing 5 wt % germanium dioxide, and the cladding
11
b
is composed of silica glass the purity of which is effectively at about a level in which the dopant can be ignored (hereinafter, referred to as “the pure silica glass”).
Hereinafter, pure silica glass or silica glass added with a dopant, may at times be referred to as the “the silica based glass”.
The reference symbol
12
is a phase mask, and this phase mask
12
is made of silica glass. A plurality of gratings
12
a
is formed at specified intervals on the one side.
The grating part
13
may be formed in the following manner: namely, an ultraviolet laser beam of wavelength 240 nm from an ultraviolet laser generator (not shown) is irradiated on the side surface of the optical fiber
11
via the phase mask
12
.
As ultraviolet ray laser generator, KrF excimer laser and the like is used.
Whereupon, an interference fringe is generated from plus first-order diffracted light and minus first- order by the gratings
12
a
of the phase mask
12
from the irradiation of the ultraviolet ray laser beam. Then, the index of refraction for the part of the core
11
a
in which this interference fringe has been generated changes, and as a result, the relative refractive index-difference between the core
11
a
and the cladding
11
b
changes.
In this manner, a periodic change in the index of refraction for the core
11
a
is formed along the longitudinal direction of the optical fiber
11
. Then, a grating part
13
which is formed with a periodic change for the relative refractive-index difference between the core
11
a
and the cladding
11
b
is obtained.
Here, what determines the characteristic as to the optical fiber grating being either a radiating type or a reflecting type is the grating period, representing the period of the change in the index of refraction of the core
11
a
(the period of the relative refractive index-difference between the core
11
a
and the cladding
11
b
).
Now, assume that the propagation constant of the mode, which propagates in the optical fiber is &bgr;1, and the propagation constant of the mode to be coupled, is &bgr;2. Then the difference in the propagation constants between these coupling modes, &Dgr;&bgr;, is represented by the following equation (1):
&Dgr;&bgr;=&bgr;1−&bgr;2  Equation (1).
Now, grating period &Lgr; is given by the following equation (2), where:
&Lgr;=2&pgr;/&Dgr;&bgr;  Equation (2).
Here, the propagation constants &bgr;1 and &bgr;2 for light are taken as being positive in the direction of the incidence of light and negative in the direction opposite to that of the direction of incidence.
The approximate values taken by &bgr;1 and &bgr;2 are roughly equal to 2 &pgr; divided by the wavelength of light propagating in the optical fiber. The orders of magnitude of the values are roughly equal to the wavelength of light in a vacuum divided by the index of refraction of the optical fiber.
For example, the orders of magnitude of the various values as a guide are set as follows:
Wavelength of light (in vacuum): 1.55 &mgr;m.
Index of refraction of optical fiber: approximately 1.5
Wavelength of light in optical fiber (wavelength in the guide): approximately 1 &mgr;m. &bgr;1 and &bgr;2: approximately 2 &pgr; rad/&mgr;m.
When the grating period &Lgr; is short, the optical fiber acts as a reflecting type, and when the grating period is long, the optical fiber grating acts as a radiating type.
For this reason, there are cases in which the reflecting-type optical fiber grating will be called the “short-period optical fiber grating”, and the radiating-type optical fiber grating will be called the “long-period optical fiber grating”.
For example, when the grating period &Lgr; is 0.5 &mgr;m, the optical fiber grating used is the reflecting type. That is, a certain mode of light incident from one end of this optical fiber grating (optical fiber
11
) couples with the other mode, which proceeds in the core
11
a
in a direction opposite to that of the incident light (the negative direction) and turns into a reflected light.
This reflected light suffers a loss in the outgoing light, so that it may be used as a device for imparting a loss in a specified wavelength band.
The value of the grating period &Lgr; of 0.5 &mgr;m, at this time, corresponds to approximately one half of the wavelength of light in the optical fiber (wavelength in the guide) which has been indicated as the aforementioned guide. By imparting disturbances of such a short period in the longitudinal direction of the optical fiber
11
, an indication is made that the light is made to be reflect

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