Title: Variable optical attenuator
Abstract: A variable optical attenuator is constituted of input/output optical systems, a birefringent member provided at output sides of the input/output optical systems, a liquid-crystal member capable of individually varying polarizing states of input beams exiting the birefringent member, and a reflection member which reflects light passing through the liquid-crystal member, to thereby cause the light to return to an output lens of the input/output optical systems by way of the liquid-crystal member and the birefringent member. Thus, there can be provided a variable optical attenuator which is more compact and less expensive than a related-art variable optical attenuator.
Patent Number: 6,978,076 Issued on 12/20/2005 to Kishida, et al.
| Inventors:
|
Kishida; Toshiya (Kawasaki, JP);
Yamane; Takashi (Kawasaki, JP);
Kunikane; Tatsuro (Kawasaki, JP)
|
| Assignee:
|
Fujitsu Limited (Kawasaki, JP)
|
| Appl. No.:
|
727671 |
| Filed:
|
December 5, 2003 |
Foreign Application Priority Data
| Jan 20, 2003[JP] | 2003-011424 |
| Current U.S. Class: |
385/140; 385/14 |
| Intern'l Class: |
G02B 006/00 |
| Field of Search: |
385/140,14
|
References Cited [Referenced By]
U.S. Patent Documents
| 5727109 | Mar., 1998 | Pan et al.
| |
| 6055104 | Apr., 2000 | Cheng.
| |
| 6282361 | Aug., 2001 | Nishimura et al.
| |
| 6781736 | Aug., 2004 | Hoyt et al.
| |
| Foreign Patent Documents |
| 63201815 | Aug., 1988 | JP.
| |
| 2000/-180803 | Jun., 2000 | JP.
| |
Primary Examiner: Lee; John D.
Assistant Examiner: Stein; James D.
Attorney, Agent or Firm: Staas & Halsey LLP
Claims
1. A variable optical attenuator comprising:
an input/output optical system to which are connected an input optical fiber
and an output optical fiber and which has an input lens for taking light having
entered by way of said input optical fiber as input light and an output lens for
gathering output light to be coupled to said output optical fiber, to thereby couple
said output light to said output optical fiber;
a birefringent device provided on an output side of said input/output optical system;
a liquid crystal device capable of changing polarizing states of said input light
exiting said birefringent device; and
a reflection device which reflects light passing through said liquid-crystal
device so that the light returns to said output lens of said input/output optical
system by way of said liquid-crystal device and said birefringent device,
wherein said input/output optical system, said birefringent device, said liquid-crystal
device, and said reflection device are integrated together.
2. The variable optical attenuator according to claim 1, wherein said input/output
optical system comprises
a fiber array block, in which a plurality of said input optical fibers are arranged
and connected in the form of an array and a plurality of said output optical fibers
are arranged and connected in the form of an array in the same direction as that
in which the input optical fibers are arranged; and
a lens array block, in which a plurality of said input lenses are arranged in
the form of an array in accordance with the arrangement of said input optical fibers
in said input array fiber block and in which a plurality of said output lenses
are arranged in the form of an array in accordance with the arrangement of said
output optical fibers in said output array fiber block.
3. The variable optical attenuator according to claim 2, wherein a pitch between
said input optical fibers and a pitch between said output optical fibers are set
so as to become greater than a pitch between said input lenses and a pitch between
said output lenses.
4. The variable optical attenuator according to claim 2, wherein said input/output
optical system has
a prism unit which is interposed between said fiber array block and said lens
array block and which reflects a portion of incident light in a direction crossing
the direction of an optical axis; and
a light-receiving unit for monitoring input and output light which receives the
light reflected from said prism unit.
5. The variable optical attenuator according to claim 4, wherein said light-receiving
unit is formed from a photodiode array, in which a plurality of photodiodes, each
photodiode having a P electrode on one surface thereof and an N electrode on the
other surface thereof, are arranged in an array pattern on a conductive transparent
substrate such that said other surfaces come into contact with said transparent
substrate; and wherein a common terminal of said N electrodes of said respective
photodiodes are provided on said transparent substrate.
6. The variable optical attenuator according to claim 4, wherein said light-receiving
unit is formed from a photodiode array, in which a plurality of photodiodes, each
having a P electrode on one surface thereof and an N electrode formed around said
P electrodes, are arranged in the form of an array on a transparent substrate.
7. The variable optical attenuator according to claim 2 wherein said input/output
optical system has
a prism unit which is interposed between said fiber array block and said lens
array block and which reflects a portion of incident light in a direction crossing
the direction of an optical axis; and
a light-receiving unit for monitoring input and output light which receives the
light reflected from said prism unit.
8. The variable optical attenuator according to claim 7, wherein said light-receiving
unit is formed from a photodiode array, in which a plurality of photodiodes, each
photodiode having a P electrode on one surface thereof and an N electrode on the
other surface thereof, are arranged in an array pattern on a conductive transparent
substrate such that said other surfaces come into contact with said transparent
substrate; and wherein a common terminal of said N electrodes of said respective
photodiodes is provided on said transparent substrate.
9. The variable optical attenuator according to claim 7, wherein said light-receiving
unit is formed from a photodiode array, in which a plurality of photodiodes, each
having a P electrode on one surface thereof and an N electrode formed around said
P electrode, are arranged in the form of an array on a transparent substrate.
10. The variable optical attenuator according to claim 1, wherein said reflection
device is formed from a coupler film which permits transmission of a portion of
the light exiting the liquid-crystal device; and
an input light monitor light-receiving unit for receiving the light having passed
through said coupler film is provided on the surface of said coupler film.
11. The variable optical attenuator according to claim 10, wherein said light-receiving
unit is formed from a photodiode array, in which a plurality of photodiodes, each
photodiode having a P electrode on one surface thereof and an N electrode on the
other surface thereof, are arranged in an array pattern on a conductive transparent
substrate such that said other surfaces come into contact with said transparent
substrate; and wherein a common terminal of said N electrodes of said respective
photodiodes is provided on said transparent substrate.
12. The variable optical attenuator according to claim 10, wherein said light-receiving
unit is formed from a photodiode array, in which a plurality of photodiodes, each
having a P electrode on one surface thereof and an N electrode formed around said
P electrode, are arranged in the form of an array on a transparent substrate.
13. The variable optical attenuator according to claim 1, wherein said input/output
optical system is provided with an output light monitor light-receiving unit for
receiving the light that is not coupled to said output optical fiber as a result
of a variation in the polarizing state of said liquid-crystal device from the light
reflected from said reflection device.
14. The variable optical attenuator according to claim 13, wherein said light-receiving
unit is formed from a photodiode array, in which a plurality of photodiodes, each
photodiode having a P electrode on one surface thereof and an N electrode on the
other surface thereof, are arranged in an array pattern on a conductive transparent
substrate such that said other surfaces come into contact with said transparent
substrate; and wherein a common terminal of said N electrodes of said respective
photodiodes is provided on said transparent substrate.
15. The variable optical attenuator according to claim 9, wherein said light-receiving
unit is formed from a photodiode array, in which a plurality of photodiodes, each
having a P electrode on one surface thereof and an N electrode formed around said
P electrode, are arranged in the form of an array on a transparent substrate.
16. The variable optical attenuator according to claim 1, wherein said liquid-crystal
device has a plurality of sets, each set comprising liquid crystal and electrodes
to be used for applying an electric field to said liquid crystal, for controlling
a polarizing state of said liquid-crystal device for light exiting said input optical fiber.
17. The variable optical attenuator according to claim 1, wherein said liquid-crystal
device has a plurality of sets, each set comprising liquid crystal and electrodes
to be used for applying an electric field to said liquid crystal, for controlling
polarizing states of the liquid-crystal device for different respective polarizing
components of said input light separated by said birefringent device.
18. The variable optical attenuator according to claim 1, wherein said liquid-crystal
device is formed from liquid-crystal molecules and glass plates to be used for
sandwiching said liquid-crystal molecules, and said reflection device is formed
on the surface of one of said glass plates.
19. A variable optical attenuator comprising:
an input optical system to which an input optical fiber is connected and which
has an input lens that take light exiting said input optical fiber as input light;
a first birefringent device provided on an output side of said input optical system;
a liquid-crystal device capable of varying the polarizing states of the input
light exiting said first birefringent device;
a second birefringent device provided on an output side of said liquid-crystal
device; and
an output optical system to which an output optical fiber is connected and which
has an output lens for gathering output light exiting said second birefringent
device and coupling the gathered output light to an output optical fiber,
wherein said input optical system, said first liquid-crystal device, said liquid-crystal
device, said second birefringent device, and said output optical system are integrated together.
20. The variable optical attenuator according to claim 19, wherein said input
optical system comprises
an input fiber array block in which a plurality of said input optical fibers
are arranged and connected in the form of an array; and an input lens array block
in which a plurality of said input lenses are arranged in the form of an array
according to the arrangement of said input optical fibers provided in said input
fiber array block; and
wherein said output optical system comprises
an output fiber array block in which a plurality of said output optical fibers
are arranged and connected in the form of an array; and an output lens array block
in which a plurality of said output lenses are arranged in the form of an array
according to the arrangement of said output optical fibers provided in said output
fiber array block.
21. The variable optical attenuator according to claim 19, wherein said liquid-crystal
device has a plurality of sets, each set comprising liquid crystal and electrodes
to be used for applying an electric field to said liquid crystal, for controlling
a polarizing state of light exiting from said input optical fiber.
22. The variable optical attenuator according to claim 19, wherein said liquid-crystal
device has a plurality of sets, each set comprising liquid crystal and electrodes
to be used for applying an electric field to said liquid crystal, for controlling
polarizing states of different polarizing components of said input light separated
by said first birefringent device on a per-polarizing-component basis.
23. A variable optical attenuator comprising:
an input/output optical system to which are connected an input optical fiber
and a an output optical fiber and which has an input lens for taking light having
entered by way of said input optical fiber as input light and an output lens for
gathering output light to be coupled to said output optical fiber, to thereby couple
said output light to said output/optical fiber;
a birefringent device provided on an output side of said input/output optical system;
a liquid crystal device capable of changing polarizing states of said input light
exiting said birefringent device; and
a reflection device which reflects light passing through said liquid-crystal
device so that the light returns to said output lens of said input/output optical
system by way of said liquid-crystal device and said birefringent device,
wherein said input/output optical system comprises
a fiber array block, in which a plurality of said input optical fibers are arranged
and connected in the form of an array and a plurality of said output optical fibers
are arranged and connected in the form of an array in the same direction as that
in which the input optical fibers are arranged,
a lens array block, in which a plurality of said input lenses are arranged in
the form of an array in accordance with the arrangement of said input optical fibers
in said input array fiber block and in which a plurality of said output lenses
are arranged in the form of an array in accordance with the arrangement of said
output optical fibers in said output array fiber block,
a prism unit which is interposed between said fiber array block and said lens
array block and which reflects a portion of incident light in a direction crossing
the direction of an optical axis, and
a light-receiving unit for monitoring input and output light which receives the
light reflected from said prism unit.
24. The variable optical attenuator according to claim 23, wherein said light-receiving
unit is formed from a photodiode array, in which a plurality of photodiodes, each
photodiode having a P electrode on one surface thereof and an N electrode on the
other surface thereof, are arranged in an array pattern on a conductive transparent
substrate such that said other surfaces come into contact with said transparent
substrate; and wherein a common terminal of said N electrodes of said respective
photodiodes is provided on said transparent substrate.
25. The variable optical attenuator according to claim 23, wherein said light-receiving
unit is formed from a photodiode array, in which a plurality of photodiodes, each
having a P electrode on one surface thereof and an N electrode formed around said
P electrode, are arranged in the form of an array on a transparent substrate.
26. A variable optical attenuator comprising:
an input/output optical system to which are connected an input optical fiber
and an output optical fiber and which has an input lens for taking light having
entered by way of said input optical fiber as input light and an output lens for
gathering output light to be coupled to said output optical fiber, to thereby couple
said output light to said output optical fiber;
a birefringent device provided on an output side of said input/output optical system;
a liquid crystal device capable of changing polarizing states of said input light
exiting said birefringent device; and
a reflection device which reflects light passing through said liquid-crystal
device so that the light returns to said output lens of said input/output optical
system by way of said liquid-crystal device and said birefringent device, wherein
said reflection device is formed from a coupler film which permits transmission
of a portion of the light exiting the liquid-crystal device, and
an input light monitor light-receiving unit for receiving the light having passed
through said coupler film is provided on the surface of said coupler film.
27. The variable optical attenuator according to claim 26, wherein said light-receiving
unit is formed from a photodiode array, in which a plurality of photodiodes, each
photodiode having a P electrode on one surface thereof and an N electrode on the
other surface thereof, are arranged in an array pattern on a conductive transparent
substrate such that said other surfaces come into contact with said transparent
substrate; and wherein a common terminal of said N electrodes of said respective
photodiodes is provided on said transparent substrate.
28. The variable optical attenuator according to claim 26, wherein said light-receiving
unit is formed from a photodiode array, in which a plurality of photodiodes, each
having a P electrode on one surface thereof and an N electrode formed around said
P electrode, are arranged in the form of an array on a transparent substrate.
29. A variable optical attenuator comprising:
an input/output optical system to which are connected an input optical fiber
and an output optical fiber and which has an input lens for taking light having
entered by way of said input optical fiber as input light and an output lens for
gathering output light to be coupled to said output optical fiber, to thereby couple
said output light to said output optical fiber;
a birefringent device provided on an output side of said input/output optical system;
a liquid crystal device capable of changing polarizing states of said input light
exiting said birefringent device; and
a reflection device which reflects light passing through said liquid-crystal
device so that the light returns to said output lens of said input/output optical
system by way of said liquid-crystal device and said birefringent device,
wherein said input/output optical system is provided with an output light monitor
light-receiving unit for receiving the light that is not coupled to said output
optical fiber as a result of a variation in the polarizing state of said liquid-crystal
device from the light reflected from said reflection device.
30. The variable optical attenuator according to claim 29, wherein said light-receiving
unit is formed from a photodiode array, in which a plurality of photodiodes, each
photodiode having a P electrode on one surface thereof and an N electrode on the
other surface thereof, are arranged in an array pattern on a conductive transparent
substrate such that said other surfaces come into contact with said transparent
substrate; and wherein a common terminal of said N electrodes of said respective
photodiodes is provided on said transparent substrate.
31. The variable optical attenuator according to claim 25, wherein said light-receiving
unit is formed from a photodiode array, in which a plurality of photodiodes, each
having a P electrode on one surface thereof and an N electrode formed around said
P electrode, are arranged in the form of an array on a transparent substrate.
32. A variable optical attenuator comprising:
an input/output optical system to which are connected an input optical fiber
and an output optical fiber and which has an input lens for taking light having
entered by way of said input optical fiber as input light and an output lens for
gathering output light to be coupled to said output optical fiber, to thereby couple
said output light to said output optical fiber;
a birefringent device provided on an output side of said input/output optical system;
a liquid crystal device capable of changing polarizing states of said input light
exiting said birefringent device; and
a reflection device which reflects light passing through said liquid-crystal
device so that the light returns to said output lens of said input/output optical
system by way of said liquid-crystal device and said birefringent device,
wherein said liquid-crystal device is formed from liquid-crystal molecules and
glass plates to be used for sandwiching said liquid-crystal molecules, and said
reflection device is formed on the surface of one of said glass plates.
33. An apparatus comprising:
a birefringent device receiving an input light which propagates through the birefringent
device and is thereby separated into polarized components which exit the birefringent device;
a liquid crystal device changing polarization states of the polarized components
exiting the birefringent device, so that the polarized components having the changed
polarization states exit the liquid crystal device as light output from the liquid
crystal device; and
a reflection device reflecting the light output from the liquid crystal device
back to the liquid crystal device so that the reflect light passes through the
liquid crystal device and the birefringent device and thereby exits the birefringent
device; and
an input/output optical system guiding the input light from an input fiber to
the birefringent device and guiding the reflected light exiting the birefringent
device to an output fiber so that the birefringent device, the liquid crystal device,
the reflection device and the input/output optical system thereby operate together
as a variable optical attenuator to attenuate the input light,
wherein the birefringent device, the liquid crystal device, the reflection device
and the input/output optical system are integrated together.
34. An apparatus comprising:
an input/output optical system;
a birefringent device;
a liquid crystal device; and
a reflection device,
wherein the input/output optical system, the birefringent device, the liquid
crystal device, the reflection device are integrated together and arranged in order
so that an input light is guided from an input fiber to the birefringent device
by the input/output optical system, then passes through the birefringent device,
then passes through the liquid crystal device and is then reflected by the reflection
device so that the reflected light passes through the liquid crystal device and
then through the birefringent device and is then guided from the birefringent device
to an output fiber by the input/output optical system,
the apparatus thereby operating as a variable optical attenuator.
Description
BACKGROUND OF THE INVENTION
(1) Field of the Invention
The present invention relates to a variable optical attenuator, and more particularly,
to a variable optical attenuator capable of changing output optical power by means
of varying the magnitude of optical coupling existing between input and output
optical fibers through control of the polarizing state of light.
(2) Description of the Related Art
In association with an increase in the traffic over the Internet, the need to
increase the capacity of optical communication has recently become urgent. One
of the measures for increasing the capacity of optical communication is to increase
a bit rate, and another measure is to employ wavelength division multiplexing (WDM).
Prompt realization of an optical device which constitutes such a system is desired.
Here, WDM transmission is a technique for transmitting a plurality of wavelengths
over a single optical transmission line (e.g., an optical fiber), wherein data
are transferred at respective wavelengths, to thereby increase the capacity of
communication. However, when data are transmitted through the optical fiber, propagation
loss differs from one wavelength to another, and after transmission over a long
distance changes arise in optical levels of the respective wavelengths.
When a branch device or an erbium-doped fiber (EDF) amplifier is used in the
optical transmission line, this phenomenon becomes more noticeable. For this reason,
optical levels at respective wavelengths must be made constant before optical transmission
is performed. A solution for this is a technique (called "pre-emphasis") for controlling
an optical output achieved at the time of transmission beforehand such that an
optical level achieved after transmission becomes constant, through use of a variable
optical attenuator (hereinafter also called an "optical attenuator"), or the like,
which controls levels of individual wavelengths. However, under the assumption
that WDM transmission would be performed, optical levels must be set for respective
wavelengths (channels). Hence, there must be provided an optical attenuator capable
of varying optical power on a per-channel basis.
However, under present circumstance, there are many cases where optical
attenuators are provided on a per-channel basis, thereby rendering devices, such
as optical repeaters, bulky and incurring a cost hike. A technique described in
Patent Publication 1 has hitherto been proposed as a measure for making the device
compact. Specifically, as shown in FIGS. 16A and 16B, development has been pursued
to constitute, as a single device, an optical attenuator capable of varying individual
optical power levels of a plurality of channels through use of an optical waveguide
device of planar type (or a planar lightwave circuit: PLC)
100. FIG. 16A
is a top view of the optical attenuator, and FIG. 16B is a side view of the optical attenuator.
In the optical attenuator shown in FIGS. 16A and 16B, tape fibers (each being
formed into a tape by stranding a plurality of optical fibers)
200 are connected
to mutually-opposing input and output sections of the PLC
100 within a package
(housing)
400. A desired voltage is applied, by way of electrical terminals
300, to electrodes provided in equal number to channels within the PCL
100,
thereby changing the refractive index of a waveguide on a per-channel basis in
order to change optical power.
Patent Publication 2 describes a conventional "handwritten input display device"
which enables handwritten input and display of an image and a character by means
of utilizing a phenomenon of changing a polarizing state of light through control
of arrangement of liquid-crystal molecules.
- [Patent Publication 1]JP-A-2000-180803
- [Patent Publication 2]JP-A-63-201815
However, the above-described planar lightwave device
100 usually
requires micromachining of a quartz substrate through reactive ion etching (RIE)
or like processing, thus incurring costs. Further, sufficient miniaturization of
the lightwave device cannot be said to have been achieved, for reasons of a limitation
on the micromachining technique.
SUMMARY OF THE INVENTION
The invention has been conceived in view of the problem and aims at providing
a variable optical attenuator which is more compact and less expensive than a conventional
variable optical attenuator.
To achieve the object, the variable optical attenuator of the invention is characterized
by comprising the following elements.
(1) an input/output optical system to which are connected a plurality of input
optical fibers and a plurality of output optical fibers and which has a plurality
of input lenses for taking beams having entered by way of the input optical fibers
as input beams and a plurality of output lenses for gathering output beams to be
coupled to the output optical fibers, to thereby couple the output beams to the
output optical fibers;
(2) a birefringent device provided on an output side of the input/output optical system;
(3) a liquid crystal device capable of changing polarizing states of the input
beams exiting the birefringent device; and
(4) a reflection device which reflects beams passing through the liquid-crystal
device so as to return the beams to the output lens of the input/output optical
system by way of the liquid-crystal device and the birefringent device.
Here, the input/output optical system, the birefringent device, the liquid-crystal
device, and the reflection device are preferably integrated together.
The input/output optical system preferably comprises a fiber array block, in
which a plurality of the input optical fibers are arranged and connected in the
form of an array and a plurality of the output optical fibers are arranged and
connected in the form of an array and in the same direction as that in which the
input optical fibers are arranged; and a lens array block, in which a plurality
of the input lenses are arranged in the form of an array in accordance with the
arrangement of the input optical fibers in the input array fiber block and in which
a plurality of the output lenses are arranged in the form of an array in accordance
with the arrangement of the output optical fibers in the output array fiber block.
The liquid-crystal device may preferably have a plurality of sets, each set comprising
liquid crystal and electrodes to be used for applying an electric field to the
liquid crystal, for controlling polarizing states of different polarizing components
of the input light separated by the birefringent device on a per-polarizing-component basis.
A variable optical attenuator according to another embodiment of the invention
has the following devices:
(1) an input optical system to which a plurality of input optical fibers are
connected and which has a plurality of input lenses taking beams exiting from the
input optical fibers as input beams;
(2) a first birefringent device provided on an output side of the input optical system;
(3) a liquid-crystal device capable of varying polarizing state of input beams
exiting the first birefringent device;
(4) a second birefringent device provided on an output side of the liquid-crystal
device; and
(5) an output optical system to which a plurality of output optical fibers are
connected and which has a plurality of output lenses for gathering output light
exiting the second birefringent device and coupling the gathered output light to
an output optical fiber.
The variable optical attenuator of the invention yields the following advantages:
(1) Input beams are caused to reciprocally pass through the birefringent device
and the liquid-crystal device between a plurality of input optical fibers and a
plurality of output optical fibers, both being connected to the input/output optical
system, through use of the reflection device. Polarizing states of the respective
input beams are controlled by means of the liquid-crystal device. The quantity
of light coupled to the output optical fiber can be changed freely for respective
input beams; that is, on a per-channel basis. A variable optical attenuator compatible
with multiple channels can be realized in the form of a compact and inexpensive
variable optical attenuator while suppressing an increase in the size of the attenuator
and an increase in the area occupied by the attenuator, which would otherwise be
caused if the number of channels were increased.
(2) Here, if the input/output optical system, the birefringent device, the liquid-crystal
device, and the reflection device are integrated together, the variable optical
attenuator can be made much more compact.
(3) Under the assumption that the respective input optical fibers and the respective
output optical fibers are arranged and connected in the form of an array by means
of a fiber array block and that the respective input and output lenses are arranged
in the form of an array according to the arrangement of the optical fibers by means
of the lens array block, even when the number of channels has been increased, the
attenuator can be collectively configured by forming individual devices into an
array. Hence, the cost of the optical attenuator array per channel can be significantly
reduced as compared with the related-art optical attenuator array, by means of
significantly curtailing the number of components.
(4) Further, if a pitch between the input optical fibers and a pitch between
the output optical fibers are set so as to become greater than a pitch between
the input lenses and a pitch between the output lenses, an improvement in polarization
extinction ratio can be expected. Hence, occurrence of interference between channels
can be inhibited.
(5) Under the assumption that the reflection device is formed from a coupler
film which permits transmission of a portion of the light exiting the liquid-crystal
device and that an input light monitor light-receiving unit for receiving the light
having passed through the coupler film is provided on the surface of the coupler
film. The power of input light can be monitored, and hence there can be realized
a compact, inexpensive variable optical attenuator capable of incorporating an
optical monitor function that is indispensable as an optical output variable component.
(6) Under the assumption that there is further provided an output light monitor
light-receiving unit for receiving the light not coupled to the output optical
fiber as a result of a variation in the polarizing state of the liquid-crystal
device from among the beams reflected from the reflection device, the quantity
of light attenuation can be monitored. Similarly, there can be realized a compact,
inexpensive variable optical attenuator capable of incorporating an optical monitor
function that is indispensable as an optical output variable component.
(7) Under that assumption that, in order to control the polarizing states of
the liquid-crystal device for each beam exiting the input optical fiber or for
different respective polarizing components of the input light separated by the
birefringent device, the liquid-crystal device is constituted by comprising a plurality
of sets, each set consisting of a piece of liquid crystal and electrodes to be
used for applying an electric field to the liquid crystal, the polarizing state
of the liquid-crystal device can be controlled on a per-channel basis or for respective
polarizing components of different channels, the quantity of light attenuation
can be controlled more precisely, and hence an improvement in polarization extinction
ratio can be expected.
(8) Further, under the assumption that the liquid-crystal device is formed by
comprising liquid-crystal molecules and glass plates to be used for sandwiching
the liquid-crystal molecules, and the reflection device is formed on the surface
of one of the glass plates, the liquid-crystal device and the reflection device
can be integrated together, and hence the variable optical attenuator can be downsized further.
(9) Under the assumption that a prism unit—which reflects a portion of
incident light in a direction crossing the direction of an optical axis—is
interposed between the fiber array block and the lens array block and that a light-receiving
unit for monitoring input and output light which receives the light reflected from
the prism unit is provided, the power of input light and/or output light can be
monitored. Even in this case, there can be realized a compact, inexpensive variable
optical attenuator capable of incorporating an optical monitor function that is
indispensable as an optical output variable component.
(10) Further, under the assumption that the light-receiving unit is formed from
a photodiode array—in which a plurality of photodiodes, each photodiode having
a P electrode on one surface thereof and an N electrode on the other surface thereof,
are arranged in an array pattern on a conductive transparent substrate such that
the other surfaces come into contact with the transparent substrate—and that
a common terminal of the N electrodes of the respective photodiodes are provided
on the transparent substrate, there is no necessity for providing an N electrode
terminal on a per-N-electrode basis. Hence, the number of wiring units is curtailed,
thereby improving efficiency. An attempt can be made to downsize the variable optical
attenuator by a great extent.
(11) Under the assumption that the light-receiving unit is formed from a photodiode
array, in which a plurality of photodiodes, each having a P electrode on one surface
thereof and an N electrode formed around the P electrodes, are arranged in the
form of an array on a transparent substrate, a limitation imposed on the materials
which can be used for the transparent substrate are mitigated, thereby broadening
the range of choice of materials. Therefore, the variable optical attenuator can
be made further inexpensive.
(12) Even when the input optical system and the output optical system are constituted
individually without use of a reflection device, the variable optical attenuator
enables a free change in the amount of light coupled to the output optical fiber
on a per-channel basis. Hence, the variable optical attenuator can be realized
less expensively than a conventional variable optical attenuator.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1A is a schematic plan view showing the basic configuration of a variable
optical attenuator employed as a first embodiment of the invention in conjunction
with a lightwave;
FIG. 1B is a schematic side view of the variable optical attenuator shown in
FIG. 1A;
FIG. 2 is a schematic perspective view showing the variable optical attenuator
shown in FIGS. 1A and 1B with portions of the attenuator being made transparent;
FIG. 3 is a schematic diagram for describing the principle on which a liquid-crystal
element of the embodiment operates;
FIG. 4 is a schematic diagram for describing the principle on which a liquid-crystal
element of the embodiment operates;
FIG. 5 is a schematic diagram for describing the principle on which a liquid-crystal
element of the embodiment operates;
FIG. 6 is a schematic diagram for describing the principle on which a liquid-crystal
element of the embodiment operates;
FIG. 7A is a schematic plan view showing the configuration of the principal
section of the liquid-crystal element of the embodiment;
FIG. 7B is a side view of the principal section when viewed in the direction
A shown in FIG. 7A;
FIG. 8 is a schematic plan view showing the configuration of a variable optical
attenuator array for describing a specific example of the variable optical attenuator
of the embodiment;
FIG. 9A is a schematic top view showing an example overview of a variable optical
attenuator array of the embodiment;
FIG. 9B is a schematic side view showing an example overview of a variable optical
attenuator array of the embodiment;
FIG. 10 is a schematic plan view showing a first modification of the variable
optical attenuator array of the embodiment;
FIG. 11 is a schematic plan view showing a second modification of the variable
optical attenuator array of the embodiment;
FIG. 12 is a schematic side view showing a third modification of the variable
optical attenuator array of the embodiment;
FIGS. 13A to 13C are views for describing a first configuration of a photodiode
(PD) according to any of the embodiments;
FIGS. 14A to 14C are views for describing a second configuration of a photodiode
(PD) according to any of the embodiments;
FIG. 15 is a schematic plan view showing the basic configuration of a variable
optical attenuator employed as a second embodiment of the invention in conjunction
with an optical path;
FIG. 16A is a schematic plan view showing the configuration of a variable optical
attenuator using a related-art planar lightwave circuit (PLC); and
FIG. 16B is a schematic side view of the variable optical attenuator shown in
FIG. 16A.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
Embodiments of the invention will be described hereinbelow by reference
to the drawings.
[A] Description of the First Embodiment
(A1) Description of the Basic Configuration
FIG. 1A is a schematic plan view showing the basic configuration of a variable
optical attenuator (hereinafter also called an "optical attenuator") according
to a first embodiment of the invention, along with a lightwave; FIG. 1B is a schematic
side view of the variable optical attenuator shown in FIG. 1A; and FIG. 2 is a
schematic perspective view showing the variable optical attenuator shown in FIGS.
1A and 1B with portions of the attenuator being made transparent.
As shown in FIGS. 1A,
1B, and
2, the optical attenuator of the
embodiment
is basically constituted of a fiber array block (a fiber-arrayed precision device)
2, a lens array block (a lens-arrayed precision device)
3, a birefringent
crystal
4, a liquid-crystal element (a liquid-crystal device)
5,
and a reflection element (reflection device)
6. The fiber array block
2,
the lens array block
3, the birefringent crystal
4, the liquid-crystal
element
5, and the reflection element
6 are integrally arranged without
any space therebetween such that input planes of light and output planes of light
remain in contact with each other.
Here, an input light fiber
1a and an output light fiber
1b
are connected to the fiber array block (hereinafter also called merely "fiber
block")
2 in the same direction (e.g., the direction of the Z axis shown
in FIG. 1B). An input lens
3a and an output lens
3b,
which are arranged in the direction of the Z axis such that the optical axes of
the lenses are aligned with the optical axes of the respective optical fibers
1a,
1b, are provided on the lens array block (hereinafter also called
simply a "lens block")
3. A collimator lens or a light-gathering lens, which
converts input light into collimated light, can be employed as the input lens
3a
and the output lens
3b.
The fiber block
2 is also equipped with an input waveguide (input port)
2a for causing the light originating from the core of the input optical
fiber
1a to propagate to and enter the input lens
3a of
the lens block
3, and an output waveguide (output port)
2b for
causing the light originating from the output lens
3b to propagate
to and enter the core of the output optical fiber
1b.
Specifically, the fiber block
2 and the lens block
3
constitute an input/output optical system. In the lens block
3, the input
lens
3a performs the function of converging into collimated light
the light that has entered by way of the input port
2a. The output
lens
3b performs the function of gathering the light reflected from
the reflection element
6, which will be described later, and coupling the
thus-converged light to the output port
3b. As shown in FIG. 1B,
when a gap existing between the input lens
3a and the output lens
3b (i.e., an input/output lens gap) G is taken as 0.25 mm (=250 μm),
the input and output optical fibers
1a,
1b are fixed
such that a gap existing between the optical fibers in the direction of the Z axis
(i.e., an input/output fiber gap) "g" assumes a value of about 0.3 mm (300 μm).
A rutile plate (another crystal may also be usable) which is cut so as to assume
an optical axis at an angle of 45°, for example, is used as the birefringent
crystal (birefringent member)
4. As shown in FIGS. 1A and 2, if such a rutile
plate is used, the light that has entered by way of the input lens
3a
will be separated into polarized components (an ordinary beam
41 and
an extraordinary beam
42) (in the direction of the Y axis), which are polarized
orthogonal to each other, while propagating through the rutile plate in the direction
of the X axis. In FIG. 1A, the thickness "d" of the rutile plate (i.e., the thickness
of the rutile plate in the direction of the X axis) is set to 2.5 mm such that
a distance S between the ordinary beam
41 and the extraordinary beam
42
(i.e., a distance between points of reflection in the direction of the Y axis on
the reflection element
6), which are separated from each other, assumes
a value of 0.25 mm (250 μm).
The liquid-crystal element
5 can change polarizing states of the respective
beams (beams) exiting the birefringent crystal
4 (i.e., for the normal beam
41 and the extraordinary beam
42, respectively). The liquid-crystal
element
5 has a structure in which liquid crystal
53 is sandwiched
between two glass plates
51,
52. There is utilized a phenomenon of
a beam having passed through the liquid-crystal element
5 being converted
from a linearly-polarized beam to an elliptically-polarized beam, by means of application
of an arbitrary electric field between the glass plates
51,
52 so
as to change the birefringence of the liquid-crystal element
5. If such
a phenomenon can be utilized, the liquid-crystal element
5 may be a commonly-used
liquid-crystal element of nematic type or a liquid-crystal element of another type
(smectic type).
For instance, the structure of the liquid-crystal element
5 of a twisted
nematic (TN) type will be described by reference to "Principle of a Liquid-Crystal
Display" (see the URL http://www.sharp.co.jp/products/lcd/tech/s2
—1.html
on the Internet, Sharp Corporation). As schematically shown in FIGS. 3 and 4, the
liquid-crystal element
5 has a structure in which molecules
53′
of the liquid crystal
53 are sandwiched between the glass plates (orientation
films)
51,
52 having trenches engraved therein in given directions
while orientations of the trenches of the glass plates are offset from each other
by 90°.
By means of such a structure, molecules
53′ of the liquid crystal
53 (hereinafter denoted as "liquid-crystal molecules
53′)
having a loose regularity in the direction of a major axis in a natural state are
arranged along the trenches of the respective glass plates
51,
52.
Further, the liquid-crystal molecules
53′ remaining in contact with
the glass plate
51 and the liquid-crystal molecules
53′ remaining
in contact with the glass plate
52 are twisted from each other by 90°
between the glass plates
51,
52.
Light travels along a gap between the liquid-crystal molecules
53′.
Hence, when the arrangements of the liquid-crystal molecules
53′
are twisted, and the light also travels along a twisted path, as schematically
shown in FIG. 5 (i.e., a linearly-polarized beam is converted into an elliptically-polarized
beam). As schematically shown in FIG. 6, when a voltage is applied between the
glass plates
51,
52, the arrangement of the liquid-crystal molecules
53′ is changed (i.e., aligned along the electric field) in accordance
with the voltage. Hence, light travels in straight lines (i.e., a linearly-polarized
beam travels in unmodified form).
On the basis of the above-described principle, the liquid-crystal element
5
can consecutively change the polarizing state of an input beam in accordance with
a voltage (i.e., an electric field) applied from the outside. Here, in order to
independently change (control) the polarizing state of the ordinary beam
41
and that of the extraordinary beam
42 on a per-beam basis in the same manner
as mentioned previously, the liquid-crystal element
5 is configured in,
e.g., a manner shown in FIGS. 7A and 7B.
FIG. 7A is a schematic plan view showing the configuration of the principal
section of the liquid-crystal element
5 of the embodiment; and FIG. 7B is
a side view of the principal section when viewed in the direction A shown in FIG.
7A. As shown in FIGS. 7A and 7B, the liquid-crystal
53 partitioned by sealing
material
54 constitutes a set in conjunction with transparent (translucent)
electrodes
55a,
55b to be used for applying a voltage
(electric field) to the liquid-crystal
53. The set is arranged between the
glass plates
51,
52 for the ordinary beam
41 and the extraordinary
beam
42 (i.e., for different respective polarization components) independently.
For example, an indium-tin oxide (ITO) electrode can be used for the transparent
electrodes
55a,
55b.
However, the set consisting of the liquid crystal
53 and the transparent
electrodes
55a,
55b is not necessarily provided for
the ordinary beam
41 and the extraordinary beam
42, respectively.
It may be the case that only sets equal in number to input beams—which are
not yet separated from each other (i.e., input ports)—are provided as common
sets for the ordinary beam
41 and the extraordinary beam
42. However,
providing separate sets for the ordinary beam
41 and the extraordinary beam
42 is preferable, because the quantity of light attenuation can be controlled
more precisely. Hence, an improvement in polarization extinction ratio can be expected.
The reflection element
6 reflects the light having passed through the
liquid-crystal element
5, to thereby introduce the light again into the
liquid-crystal element
5 and the birefringent crystal
4. In the embodiment,
the reflection element is formed as a total reflection film formed on the plane
of light exit of the liquid-crystal element
5 (i.e., the back of the glass
plate
52). The total reflection film may be a multilayer dielectric film
or a metal film (Al, Au or the like). Here, the reflection element
6 may
be provided as an individual device on a stage subsequent to the liquid-crystal
element
5. As mentioned above, integrating the reflection element
6
with the liquid-crystal element
5 through formation of a reflection film
is advantageous for miniaturization of a variable optical attenuator.
The basic operation of the optical attenuator of the embodiment having the foregoing
configuration will now be described. First, the light exiting the upper input optical
fiber
1a enters the input lens
3a provided in the direction
of the optical axis after having passed through the input port
2a,
as well as into the birefringent crystal
4 after having been converted into
collimated light by the input lens
3a.
The light having entered the birefringent crystal
4 is divided into the
ordinary beam
41 and the extraordinary beam
42, and the thus-divided
beams enter the liquid-crystal element
5. The liquid-crystal element
5
is provided with the pieces of liquid crystal
53 and the transparent electrodes
55a,
55b, which are provided for the respective beams
as mentioned previously. The pieces of liquid crystal
53 and the transparent
electrodes
55a,
55b can be controlled independently.
Hence, the polarizing state of the ordinary beam
41 and that of the extraordinary
beam
42, both beams having entered the liquid-crystal element
5,
are independently controlled by the corresponding pieces of liquid crystal
53.
As a result, the light having passed through the liquid-crystal element
5
is converted from, e.g., linearly-polarized light into elliptically-polarized light
(i.e., a state in which the linearly-polarized light component is merged with a
vertically-polarized light component), by means of birefringence of the liquid
crystal
53, and enters the reflection element
6 formed on the back
of the liquid-crystal element
5.
The light reflected from the reflection element
6 again enters the liquid-crystal
element
5. By means of birefringence of a corresponding piece of liquid
crystal
53, a change similar to that mentioned previously arises in the
polarizing state of light, and the light enters the birefringent crystal
4.
Of the beams having entered the birefringent crystal
4, only a component
which is identical in polarizing state with the light having entered the birefringent
crystal
4 by way of the input lens
3 is finally coupled with the
lower output port
2b by way of the output lens
3b.
The light is then output to the output optical fiber
1b. As shown
in FIG. 1A, other components (beams)
43,
44 do not return to and
are not coupled with the output port
2b.
Therefore, the arrangement of the liquid-crystal molecules
53′
is controlled through control of the voltage applied to the two electrodes
55a,
55b provided for the respective pieces of liquid crystal
53.
Thereby, the polarizing state of the light that travels back and forth within the
birefringent crystal
4 and passes through the liquid-crystal element
5
is controlled for each beam input to the liquid-crystal element
5. As a
result, the quantity of light coupled to the output port
2b (i.e.,
the output optical fiber
1b) can be changed freely on a per-channel
basis. Thus, the optical output power can be changed on a per-channel basis.
(A2) Description of a Specific Example
A variable optical attenuator array will now be described hereinbelow as a specific
example of the invention on the premise that the array has the foregoing basic configuration.
FIG. 8 is a schematic top view showing the configuration of a variable optical
attenuator array of the embodiment. The variable optical attenuator array shown
in FIG. 8 has a structure in which a multicore tape fiber
10 (including
12 cores)—into which a plurality of input optical fibers
1a (twelve
input optical fibers in FIG. 8) are aggregated in the form of a tape—is connected
to an upper layer section of the fiber block
2 as an input tape fiber.
Although not shown in FIG. 8, an analogous multicore tape fiber (including
twelve cores) is connected to a lower layer section of the fiber block
2
as an output tape fiber. Specifically, in the present embodiment, the tape fibers
are fixed to the fiber block
2 so as to be stacked on top of each other
in two layers in a vertical direction (i.e., a direction identical with the direction
of the Z axis shown in FIG. 2) with desired accuracy. An epoxy-based optical adhesive
or the like, for instance, is used for fixing the multicore tape fibers (hereinafter
also called simply "tape fibers").
The input ports
2a—which are equal in number with the cores
of the tape fiber
10 (i.e., twelve input ports)—are arranged into
an array within an X-Y plane of the upper layer section of the fiber block
2
at an interval between fiber cores of the input tape fiber
10 (e.g., a pitch
of 250 μm). Similarly, the twelve output ports
2b are arranged
into an array within the X-Y plane of the lower layer section at the pitch between
the fiber cores.
Twelve input lenses
3a are arranged within the X-Y plane of
an upper layer section of the lens block
3 so as to coincide with the optical
axes of the respective input ports
2a. Twelve output lenses
3b
are arranged within the X-Y plane of a lower layer section of the lens block
3 so as to coincide with the optical axes of the respective output ports
2b.
Specifically, a total of 24 (2×12) ports are arranged into an
array within a Y-Z plane in the fiber block
2. Similarly, a total of 24
(2×12) lenses are arranged into an array within the Y-Z plane in the lens
block
3 in agreement with the arrangement of the ports in the fiber block
2 (i.e., the arrangement of the input and output optical fibers
1a,
1b).
The thickness "d" of the birefringent crystal
4 is set to 1 mm such that
a distance S between the ordinary beam
41 and the extraordinary beam
42
assumes a value of about 0.1 mm (100 μm).
As mentioned previously by reference to FIGS. 7A and 7B, the set consisting of
the liquid crystal
53 and the transparent electrodes
55a,
55b, the liquid crystal being partitioned by the sealing material
54, is provided in the liquid-crystal element
5 for the respective
ordinary and extraordinary beams
41,
42 of the light having entered
by way of the respective input ports
2a (i.e., a total of 24 sets).
Even in this case, the only requirement for the liquid-crystal element
5
is to use a single glass plate
51 (or
52). The glass plate
52
can be readily formed into an array by means of forming electrodes in one glass
plate
52, each electrode having a width corresponding to the size of a beam
(about 200 μm). The set consisting of the liquid crystal
53 and the
transparent electrodes
55a,
55b may be provided for
each input port so as to be common to the ordinary beam
41 and the extraordinary
beam
42.
As mentioned above, the variable optical attenuator array compatible with multiple
channels (12 channels) can be implemented in the form of a compact, inexpensive
variable optical attenuator array while inhibiting an increase in the size of the
array and the area occupied by the same, which would otherwise be caused by an
increase in the number of channels. Even when the number of channels has been increased,
the attenuator can be collectively configured by forming individual members into
an array. Hence, the price of the optical attenuator array per channel can be significantly
reduced when compared with the related-art optical attenuator array.
In particular, the variable optical attenuator is formed as a single piece by
arranging the fiber block
2, the lens block
3, the birefringent crystal
4, the liquid-crystal element
5, and the reflection element
6
without any space therebetween. When compared with a related-art atte
