Optical waveguides – With disengagable mechanical connector – Structure surrounding optical fiber-to-fiber connection
Reexamination Certificate
2000-07-29
2002-11-26
Abrams, Neil (Department: 2839)
Optical waveguides
With disengagable mechanical connector
Structure surrounding optical fiber-to-fiber connection
Reexamination Certificate
active
06485191
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to an optical module suitably used in an optical communication apparatus, sensor or the like, and to a fiber stub type optical device mountable in an optical module and having a built-in optoisolator for blocking reflected and returned light from the outside, and to a method for producing a fiber stub type optical device.
In a semiconductor laser diode (hereinafter, “LD”), which is one of semiconductor devices, used as a light source in the optical communication, if emitted light is reflected and returned to an active layer of the LD, an oscillating state of the LD is disturbed. This causes a variation in emission power and a wavelength deviation, thereby deteriorating signals.
In order to prevent such a problem, the LD is normally mounted in the same package as an optoisolator for transmitting light only in one direction to thereby construct an LD module which is one type of the optical module.
Particularly, analog signals are likely to be deteriorated by the reflected and returned light, and the higher the density thereof, the more they are likely to be influenced by the reflected and returned light. Accordingly, optoisolators have become essential elements as analog transmission data via, e.g., CATV increases and requires larger capacity and higher speed.
The operation of a general optoisolator is described. The optoisolator is, as shown in
FIGS. 26A and 26B
, comprised of a Faraday rotator
19
c
and two polarizers
19
a
,
19
b
at the opposite sides of the Faraday rotator
19
c
. As shown in
FIG. 26A
, a forward propagating light
22
incident on the first polarizer
19
a
becomes a linearly polarized light of a specific polarization direction (see
23
a
in FIG.
26
A). This forward propagating light
22
has its polarization direction
24
rotated by 45° to the right with respect to the propagating direction of the light in
FIG. 26A
by the Faraday rotator
19
c
, is then incident on the second polarizer
19
b
having the polarization direction
23
b
rotated by 45° to the right from the polarization direction
23
a
of the first polarizer
19
a
with respect to the propagating direction of the light, and emerges out of the optoisolator while maintaining its polarization direction.
On the other hand, as shown in
FIG. 26B
, backward propagating light
25
is made into a linearly polarized light by the second polarizer
19
b
, and has its polarization direction
24
rotated by 45° in the same direction as in
FIG. 26A
by the Faraday rotator
19
c
, with the result that this light is blocked by the first polarizer
19
a
by forming an angle of 90° with respect to the polarization direction
23
a
of the first polarizer
19
a.
Next, an example of the conventional LD module is described. As shown in
FIG. 27
, an LD module J
1
is constructed such that at least an LD
15
, lenses
6
a
,
6
b
, an optoisolator
2
, and one end of a single-mode fiber
4
are accommodated in a package
18
. Identified by
16
in
FIG. 27
is a light detector, by
17
a Peltier cooler and by
32
a rubber boot for protecting an optical fiber margin.
In this LD module J
1
, the light emitted from the LD
15
is collimated by the lens
6
a
, transmits through the optoisolator
2
, and is gathered by the lens
6
b
to be incident on the single-mode fiber
4
. The package is used to isolate the respective parts from external environments. Ball lenses, biconvex lenses, aspheric lenses, gradient-index lenses (hereinafter, “GRIN lenses”) and the like are used as the lenses
6
a
,
6
b.
Further, in order to miniaturize the entire module and facilitate an alignment, there has been proposed an optical device J
2
which is a combination of an optical fiber and an optoisolator without using lenses as shown in
FIG. 28
(see Japanese Unexamined Patent Publication No. 9-105886). In this optical device J
2
, a core enlarged fiber
4
obtained by enlarging a core diameter of an optical fiber is used, the elements (polarizers
19
a
,
19
b
, Faraday rotator
19
c
) of the optoisolator are separately arranged and while a specified element (Faraday rotator
19
c
) is arranged in a groove
7
formed obliquely to an optic axis in order to prevent reflection.
In order to miniaturize the entire optical module and facilitate an alignment, there has been also proposed an optical device J
3
in which an optoisolator is mounted on a fiber stub similarly using a core enlarged fiber
5
without using lenses as shown in
FIG. 29
(see Japanese Unexamined Patent Publication No. 10-68909).
This optical device J
3
uses the core enlarged fiber
5
obtained by enlarging a core diameter of an optical fiber in order to improve optical coupling, and the optoisolator
2
is obliquely inserted with respect to an optic axis in order to prevent reflection. The optical device J
3
is constructed by fitting an optoisolator
2
and a cylindrical magnet
30
surrounding the optoisolator
2
in a ferrule
3
holding a fiber
9
having a spherical end in its axial center and fixedly mounting an entire assembly in a sleeve
13
.
The optical device J
3
is free from radial displacement since the ferrule
3
is coaxially mounted with high precision. Further, the module provided with the optoisolator can be easily assembled by operation steps similar to those for a module having no optoisolator. However, in this prior art, how the ferrule
3
is processed, how the optoisolator
2
and the magnet
30
are assembled and fixed are unclear.
The core enlarged fiber used in the optical devices J
2
, J
3
and the like is formed by locally heating a general single-mode fiber. Specifically, the single-mode fiber is heated to diffuse dopants such as germanium in the core, thereby enlarging a diffusion area of the dopants and making a specific refractive index difference smaller.
If a diameter of the core increases with the specific refractive index difference between the core of the optical fiber and the cladding thereof unchanged, a single-mode condition breaks and a multimode is excited. However, in the case of the core enlarged fiber, the enlargement of the core and a reduction of the specific refractive index difference simultaneously occur due to the diffusion of the dopants caused by heat and accordingly r×(D)
½
is automatically maintained at constant value. Here, r denotes a radius of the core of the optical fiber, D a specific refractive index difference between the core and the cladding, and r×D
½
an amount in proportion to normalized frequency. The single-mode condition is maintained if this value r×(D)
½
is constant.
FIG. 30
shows optical coupling characteristics of the core enlarged fiber. A horizontal axis represents a fiber spacing which is a spacing between the divided sections of the core enlarged fiber (width of a groove formed in a core enlarged portion) and a vertical axis represents an optical coupling loss (diffraction loss). Here, w denotes a mode field diameter and corresponds to the respective curves. It is assumed that wavelength is 1.31 &mgr;m generally used in optical communication and the groove (clearance between fibers) is filled with air (refractive index n=1).
This graph shows that the larger the mode field diameter w, the smaller the diffraction loss. For example, in the case of the mode field diameter w of 10 &mgr;m (i.e., the core is not enlarged), when the fiber spacing is 70 &mgr;m, the diffraction loss is above 1 dB. Contrary to this, in the case of the mode field diameter w of 40 &mgr;m, even when the fiber spacing is 800 &mgr;m, the diffraction loss is below 1 dB. This clearly shows an improvement in coupling characteristic.
However, in the module as shown in
FIG. 27
, the optoisolator
2
, the lenses
6
a
,
6
b
and other parts are aligned after being separately fixedly mounted in the holder. Thus, this module disadvantageously requires many parts and a cumbersome adjustment and results in a large size.
Although the core enlarged fibers are used in the examples shown in
FIGS. 28 and 29
, the conventional
Abrams Neil
Duverne J. F.
Hogan & Hartson LLP
Kyocera Corporation
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