Optical waveguides – With optical coupler – Input/output coupler
Reexamination Certificate
2002-01-22
2004-07-20
Healy, Brian (Department: 2874)
Optical waveguides
With optical coupler
Input/output coupler
C385S129000, C385S130000, C385S131000
Reexamination Certificate
active
06766080
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an optical waveguide type filter in which a part in a longitudinal direction of an optical waveguide such as an optical fiber is formed with a periodically perturbation part having a periodic change in refractive index and the like, a method of making the same, and an optical fiber amplifier using the optical waveguide type filter.
2. Related Background Art
A long period type tilted optical fiber grating in which a part of an optical fiber is formed with a periodically perturbation part for refractive index having a relatively long period has been known from A. M. Vengsarkar, et al., “Long-Period Fiber Grating as Band-Rejection Filters,” JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 14, NO. 1, pp. 58-65, and the like.
On the other hand, a tilted optical fiber grating in which a part of an optical fiber is formed with a periodically perturbation part having a relatively short period such that a line perpendicular to a level plane thereof is tilted with respect to the optical axis of the optical fiber has been known from literatures such as R. Kashyap, et al., “WIDEBAND GAIN FLATTENED ERBIUM FIBRE AMPLIFIER USING A PHOTOSENSITIVE FIBRE BLAZED GRATING,” ELECTRONICS LETTERS, Vol. 29, No. 2, pp. 154-156; and T. Erdogan, et al., “Radiation-mode coupling loss in tileted [sic] fiber phase graitings [sic],” OPTICS LETTERS, Vol. 20, No. 18, pp. 1838-1840.
Both of the long period type optical fiber grating and tilted optical fiber grating are based on an optical waveguide, and function as a wavelength selection type loss filter in a wavelength band of 1.5 &mgr;m and the like. When compared with an optical component having the same function, e.g., etalon, it is advantageous in that it can easily be connected to an optical waveguide such as an optical fiber while its insertion loss is low.
A long period type optical fiber grating is one in which a part of an optical fiber having a photosensitive dopant is irradiated with ultraviolet rays by use of a mask plate having a masking period of several hundreds of micrometers, so as to form the optical fiber with a refractive index perturbation part having a period of several hundreds of micrometers.
When a coating layer is disposed on the optical fiber in the long period type optical fiber grating, a leaky mode, which is formed on the whole cladding and is essential for exhibiting a function as a wavelength selection type loss filter, changes/disappears, thereby altering its cutoff spectrum. Therefore, the coating layer is hard to provide. When no coating layer is provided, however, the fear of damaging and breaking the optical fiber increases so much that its handling is not be easy. Also, the cutoff center wavelength in the long period type optical fiber grating is greatly influenced by its core/cladding refractive index difference. Since the core/cladding refractive index difference greatly varies depending on temperature, the cutoff center wavelength will vary if temperature changes.
On the other hand, the tilted optical fiber grating is considered to be in a mode more preferable as a wavelength selection type loss filter, since it does not have such demerits of the long period type optical fiber grating.
FIGS. 1A
,
1
B and
1
C are views showing an example of tilted optical fiber grating (hereinafter referred to as “tilted FG”), in which
FIG. 1A
is a longitudinal sectional view,
FIG. 1B
is a lateral sectional view, and
FIG. 1C
is a perspective view. In
FIGS. 1A
,
1
B and
1
C,
1
is an optical fiber,
2
is a core,
3
is a cladding,
4
is a periodically perturbation part for refractive index, A is a line passing a given point O on the optical axis in the periodically perturbation part and being perpendicular to a level plane L passing the given point, X is the optical axis, Y is a deflection angle direction, L is a level plane, M is a plane, also referred to as a deflection angle plane, formed between the line A passing the given point O on the optical axis in the periodically perturbation part and being perpendicular to the level plane passing the given point and the optical axis X, O is the given point, and &thgr; is the angle of inclination.
This tilted FG is one in which a part in a longitudinal direction of the optical fiber
1
comprising the core
2
and cladding
3
is formed with a part whose refractive index is periodically changed, i.e., periodically perturbation part
4
. A plane yielding a fixed refractive index in the periodically perturbation part
4
, i.e., the level plane L, is tilted from a plane perpendicular to the optical axis X of the optical fiber
1
. Also, the line A perpendicular to the level plane L passing a given point O in the periodically perturbation part of the tilted FG is tilted with respect to the optical axis X, whereby the line A and the optical axis X form the angle of inclination &thgr;.
Within the deflection angle plane M, a direction passing the point O and being perpendicular to the optical axis X is defined as the deflection angle direction Y. Therefore, all of the optical axis X, line A, and deflection angle direction Y are located within the polarization angle plane M.
In the conventionally known tilted FG, all the level planes L are parallel to each other even when the position of given point O varies within the periodically perturbation part
4
. Therefore, even when the position of given point O in the periodically perturbation part
4
changes, the deflection angle plane M is fixed, and the deflection angle directions Y are always parallel to each other and oriented in the same direction.
Such a tilted FG is made as follows.
FIGS. 2A and 2B
are views showing a major part of a manufacturing method, in which
FIG. 2A
is a perspective view, whereas
FIG. 2B
is a side view. In
FIGS. 2A
,
2
B and
2
C,
5
is a phase grating mask,
6
is a grating surface,
7
is an excimer laser, and
8
is an ultraviolet ray. The optical fiber
1
including a photosensitive dopant such as germanium in the core
2
is arranged parallel to the phase grating mask
5
formed with the grating surface
6
made of several thousands to several tens of thousands of groove-like recesses/projections usually having a pitch of about 1 &mgr;m therebetween, and is irradiated with the ultraviolet rays
8
by way of the phase grating mask
5
by using the excimer laser
7
. As a consequence, interference fringes of the ultraviolet rays
8
are generated by the grating surface
6
of the phase grating mask
5
, and the optical fiber
1
is irradiated with the interference fringes. As the ultraviolet light source, not only the excimer laser but also argon lasers may be used.
Since the refractive index of the core
2
in the optical fiber
1
varies depending on whether the ultraviolet rays are strong or weak, the interference fringes of ultraviolet rays form the periodically perturbation part
4
for refractive index in the optical fiber
1
. A normal optical fiber grating whose optical axis is perpendicular to the level planes of the periodically perturbation part
4
is obtained when the direction of grooves in the grating surface
6
is oriented in a direction perpendicular to the optical axis X of the optical fiber
1
, whereas a so-called tilted FG in which a line perpendicular to the level planes of the periodically perturbation part
4
is tilted with respect to the optical axis is obtained when the direction of grooves of the grating surface
6
is tilted by about 5 degrees with respect to a direction perpendicular to the optical axis X of the optical fiber
1
.
As mentioned above, using the tilted FG as a wavelength selection type loss filter is advantageous in that fluctuations in the cutoff center wavelength with respect to temperature changes are smaller than those in the long period optical fiber grating, and that its handling is easy since the coating layer can be provided on the optical fiber.
In the periodically perturbation part of the tilted FG in accordance with the prior art, however, the deflec
Ohmura Masaki
Shigehara Masakazu
Healy Brian
Lin Tina M
Sumitomo Electric Industries Ltd.
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