Note: Descriptions are shown in the official language in which they were submitted.
1295S02
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COLLIMATOR LENS FOR OPTICAL FIBER
1 BACKGROUND OF THE INVENTION
The present invention relates to a collimator lens
for an optical fiber, which is adapted to convert
divergent lights radiated from a light emitting end face
of the optical fiber into a parallel luminous flux.
Utilized as light transmittive optical fiber
sensor, light reflective type optical fiber sensor and
light bifurcation device are a device including an optical
fiber and a collimator lens attached to a light emitting
end face of the optical fiber to convert divergent lights
radiated from the light emitting end face of the optical
fiber into a parallel luminous flux. In the device, the
collimator lens is mounted on the light emitting end face
of the optical fiber.
According to the collimator lens used in the
conventional device, as shown in Fig. 1, a convex lens 22
formed of glass or transparent plastic material is
disposed so as to position a light emitting end face of
an optical fiber 21 at a focal point 23 of the convex lens
22.
Another type of the conventional device is shown
in Fig. 2, wherein a rod lens 32 is disposed which has
convergent distribution of refractive index, and a light
emitting end face of an optical a fiber 31 is positioned
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1 at one end face of the rod lens 32. The latter type is
disclosed in Japanese patent application laid open No. 59-
38706. In these devices, parallel light 33 is provided
at the end of the collimator lens, such as the convex lens
s 22 and the rod lens 32.
In the collimator for the optical fiber employing
the convex lens 22 formed of transparent material such as
glass and plastic material as a collimator lens shown in
the firstly described conventional device with reference
to Fig. ,1, it is necessary to provide sufficient
efficiency to convert divergent light beam radiated from
the optical fiber end into parallel light beam by way of
the collimator lens 22. For this purpose, the collimator
lens 22 should be a convex lens subjected to highly
lS precise machining. Accordingly, the resultant lens becomes
costly. Further, it is also necessary to precisely
control an angle of arrangement of the highly processed
collimator lens, the position of the focal point thereof,
the position of the light emitting end face of the optical
fiber, and angular positional relationship therebetween.
If these relative arrangement is not precisely provided,
it would be impossible to provide parallel light beams.
Further , according to the second type of the
conventional optical fiber collimator shown in Fig. 2,
wherein employed is the rod lens 32 as a collimator lens
J.
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1 having convergent type distribution of refractive index,
the rod lens has a diameter not more than about 2mm,
otherwise the rod lens does not provide precise refractive
index distribution. Therefore, the second type is not
available for wide utility. Moreover, since the rod lens
having convergent refractive index distribution has small
diameter, high technique is required for axial alignment
between the rod lens axis and the light axis at the light
emitting end face of the optical fiber. If the axes are
10 . offset from each other, it would be impossible to provide
precise parallel light, moreover, the conventional rod
lens cannot necessarily give the enough numerical aperture
(NA) to be adapted to the high NA optical fiber such as
plastic optical fiber so that the convertion from the
divergent light to a parallel luminous flux connot be
performed sufficiently.
SUMMARY OF THE INVENTION
It is therefore, an object of the present
invention to overcome the above-described prior art
disadvantages and drawbacks and to provide an improved
collimator for an optical fiber.
Another object of the present invention is to
provide such collimator capable of easily performing axial
alignment between a collimator lens axis and an optical
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1 axis defined by the light radiation from an end face of an
optical fiber.
Still another object of the present invention is
to provide the collimator lens which can be easily
produced.
In accordance with the present invention, provided
is a cylindrical member formed of an optically transparent
material, which has refractive index n1, effective radius
R, and effective axial length L. The cylindrical member
has one circular end provided with Presnel lens pattern
having a focal length F, and the other circular end face
. provided with a connecting portion connectable with a
light emitting end face of an optical fiber. The focal
length F, the effective axial length L, the effective
radius R and the refractive index nl are so arranged as to
be satisfied with the following formula (I) and (II):
F/L = tan {sin-l(NA/n1)}/tan {sin-1(NA)}
R 2 L-tan {sin-1(NA/n1)} ----(II
in which NA is a numerical aperture of an optical fiber.
BRIEF DESCRIPTION OF THE DRAWINGS
Fig. 1 is a schematic view showing one example of
a conventional collimator lens coupled with an optical
fiber, and shows a light path passing through the
conventional system;
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.
1 Fig. 2 is a schematic view showing another example
of a conventional collimator lens coupled with an optical
fiber, and shows a light path passing through the second
conventional system;
Fig. 3 is a schematic view showing a combination
of a collimator lens according to the present invention
and an optical fiber assembled thereto;
Fig. 4 is an explanatory diagram showing operation
of the optical fiber collimator lens according to the
present invention;
Figs. 5 and 6 shown examples of Fresnel lens
patterns formed at one end face of the collimator lens
according to the present invention;
Fig. 7 is a schematic diagram showing an apparatus
for measuring a characteristics of an optical fiber
collimator lens according to the present invention;
figs. 8(a) and 8(9) are graphical representations
showing optical transmission characteristics of collimator
lenses which are measured by the apparatus shown in Fig.
7; and
Figs. 9(a) and 9(b) are schematic views showing
light bifurcation devices which use optical fiber
collimators according to the present invention.
1295S02
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1 DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
A collimator lens for an optical fiber according
to the present invention will be described with reference
to the accompanying drawings.
Fig. 4 is a schematic cross-sectional view showing
a collimator lens 41 of the present invention coupled with
an optical fiber 42. In Fig. 4, ~c designates a light
spreading angle in which light emitted from a light
emitting end face of the optical fiber 42 spreadingly pass
through the collimator lens 41. In case the optical fiber
has a numerical aperture NA, and the collimator lens is
formed of an optically transparent material having
refractive index nl, the light spreading angle ~c meets
with the following equation (III):
~c = sin-l(NA/nl) ----(III)
In this case, in order to provide parallel
luminous flux 43 from the tip end face of the collimator
lens the effective axial length L and effective radius R
of ~he collimator lens must be satisfied with the
following formula (II):
R 2 L~tan~c = L~tan{sin-1(NA/n1)3 -----(II)
If R is smaller than L tan~c, some of the light
proceeding through the collimator is reflected on the
inside wall before reaching to the end face. Such
collimator lens hardly produce parallel luminous flux from
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1 the light radiated from the optical fiber. Therefore, the
radius R of the collimator lens must be satisfied with the
above formula (II).
Further, the collimator lens should have a
circular end face 44 formed with a Fresnel lens pattern
having a positive focal length F in order to produce
parallel luminous flux from the circular end face having
an effective radius R, the parallel luminous flux being
converted from light propataged through the collimator
lens with the spreading angle ~c from the light emitting
end face of the optical fiber.
In this case, as shown in Fig. 4, the following
equation (IV) must be satisfied by the focal length F,
effective axial length L, and the light spreading angle
~max, when the light is emitted from the end face of the
optical fiber 32 into a cylindrical collimator lens 41
having Fresnel lens pattern and the focal length F.
F = L-tan~c/tan~max ----(IV)
In the equation (IV), L designates the effective
axial length of the collimator lens. This length is
measured from the optical fiber end face disposed at one
end of the collimator lens to the other end face thereof
provided with the Fresnel lens pattern.
~max can be represented by the following equation
(V):
~ .
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1 ~max = sin-l(NA) ----(V)
Therefore, the equation ~IV) can be converted into
the following equation (VI):
F = L-tan {sin-l(NA/nl)}/tan{sin-l(NA)}----(YI)
The optical fiber coupled to the collimator lens
according to the present invention and having numerical
aperture NA is selected from the group consisting of
polymethylmethacrylate core optical fiber having numerical
aperture NA of from 0.45 to 0.55, polystylene core optical
fiber having NA of from 0.53 to 0.58, polycarbonate core
optical fiber having NA of from 0.70 to 0.80, etc.
Further, the collimator lens having refractive
index nl according to the present invention is selected
from the group consisting of acrylic group resin having
refractive index nl of from 1.47 to 1.50, polystylene
group resin having nl of from 1.50 to 1.58, silicon group
resin having nl of from 1.35 to 1.60 and fluorine group
resin having nl of from 1.30 to 1.42.
When using the collimator lens for optical fiber
according to the present invention, the divergent luminous
flux emitted from the optical fiber end face can be
converted into parallel luminous flux, which can be
transmitted through an atmosphere for an increased
distance with high directivity.
!~
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g
1 When the divergent luminous flux emitted from the
optical fiber end is converged by means of the convex-
shaped collimator lens, the luminous flux is transmitted
through, in order, the optical fiber, air, convex-shaped
collimator lens, and air, so that Fresnel reflection may
be induced upon light passing through each phase boundary.
As a result, a several percentages of light amount may be
reduced as a loss. However, the boundary area which
generates disadvantageous Fresnel reflection can be
greatly reduced by using the collimator lens of the
present invention and by filling matching oil such as
silicone oil into the boundary face between the fiber end
and the collimator lens. As a result, obtained is the
collimator lens having reduced amount of optical
transmission loss.
As described above, the collimator lens for the
optical fiber according to the present invention can
convert luminous flux radiated from the optical fiber end
~ar~ int~ para~e'~ ~uminous f ~ux ~hieh ran ~e ~f f ec~ y
transmitted for a long distance with reduced amount of
transmission loss. Therefore, the collimator lens of the
present invention provides such advantages and is
available as devices for optical communication such as an
optical bifurcation device and a wave length devider, and
other optical elements.
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1 On example according to the present invention will
be described.
EXAMPLE
Prepared were three types of optical fibers having
NA of 0.5 for optical transmission comprising a core
formed of polymethylmethacrylate and a clad formed of
fluorine resin. A first fiber had an outer diameter of
lmm(ESKA EH 4001; Trademark of Mitsubishi Rayon Co.,
Ltd.), the second fiber had an outer diameter of 500~m
(ESKA EH 2001, Trademark of Mitsubishi Rayon Co., Ltd.),
and the third fiber had an outer diameter of 250~um (ESKA
EH 10, Trademark of Mitsubishi Rayon Co., ltd.,). Each of
the three optical fibers was subjected to cable
processing.
Polymethylmethacrylate having refractive index n
of 1.492 was used as the raw material of the collimator
lens. Two types of collimator lenses were prepared, each
having effective radius of 5mm and 10mm. Light spreading
angle ~c radiated from the fiber end into the collimator
lens was 20 degrees as is apparent from equation (III).
Further, from the formula (II), the effective axial
lengths L of the two collimator lenses were L = 13.74mm
with respect to the lens having the radius of 5mm, and L =
27.48mm with respect to the lens having the radius of
10mm. Furthermore, focal lengths F were;
.~
: `~
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1 F = 8.64mm for radius Smm lens
F = 16.93mm for radius lOmm lens
as is apparent from equation (IV).
Prepared were two cylindrical rods formed of
polymethylmethacrlate having radii of 5mm and lOmm. Each
of end faces of the rods was subjected to machining by
numerial control lathe, so that fresnel lenses having
focal lengths (F) of 8.64mm and 16.93mm were formed. The
pitches of the frensnel lenses pattern were about lOO~m.
The Fresnel patterns were measured by needle touching type
surface roughness tester, and resultant measuri~g patterns
are shown in Figs. 5 and 6.
As shown in Fig. 3, each tip end of the three
types of optical fibers 13 was connected with optical
fiher connector 12. A tip end portion 14 of each optical
fiber was extended from the connector and was inserted
into each coupling portion 15 of the collimator lens. A
matching li~uid of silicone oil for refractive index
distribution control was coated over the tip end portion
14 and it was inserted into the coupling portion 15 to
thereby provide an optical fiber element coupling with the
collimator lens.
Prepared were two independent optical fiber
elements coupled with the collimator lenses. As
shown in Fig. 7, free end faces (fresnel lens sides) were
lZ95502
1 confronted with each other and spaced away by a distance
e. One optical fiber end of one optical fiber element was
connected to a ~e-Ne laser source, and one optical fiber
end of the other optical fiber element was connected to an
optical power meter. By changing the distance e, light
attenuation amount were measured in the light radiated
from the collimator lens face (Presnel lens side). The
results are shown in Figs. 8(a) to 8(9-) and Table 1 below.
As shown in Fig. 8(a), light transmittable
distance was only several cm, if collimator lens according
to the present invention was not coupled to the fiber
element but optical fiber ends were merely confronted with
each other for optical tranmission. On the other hand, as
shown in Figs. 8(b) to 8(g), if the optical fiber element
is provided with the collimator lens of the present
invention, the light transmittable distance became very
long exceeding 50cm. Such optical fiber element provided
with the collimator lens was able to be used as light
bifurcation devices as shown in Figs. 9(a) and 9(b). The
light bifurcation element includes an optical tranmission
optical fiber 91, a collimator lens 92 connected to the
light emitting end face of the fiber, half mirrors 94,
collimator lenses 93 for receiving fiburcated luminous
flux, and light receiving optical fibers 95. These
collimator lenses 92 and 93 are in accordance with the
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1 present invention. If desired, an optical filter 96 may
be disposed in front of the collimator 93 of light
receiving elements, as shown in Fig. 9(b).
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5502
1 While the invention has been described in detail
with reference to specific embodiment thereof, it will be
aparent to one skilled in the art that various changes and
modification can be made therein without departing from
the spirit and scope of the invention.
~.,~, i
