U.S. patent application number 10/901219 was filed with the patent office on 2005-09-29 for wavelength dispersion compensating apparatus.
This patent application is currently assigned to Fujitsu Limited. Invention is credited to Maruyama, Shinji, Mitamura, Nobuaki.
Application Number | 20050213215 10/901219 |
Document ID | / |
Family ID | 34989506 |
Filed Date | 2005-09-29 |
United States Patent
Application |
20050213215 |
Kind Code |
A1 |
Mitamura, Nobuaki ; et
al. |
September 29, 2005 |
WAVELENGTH DISPERSION COMPENSATING APPARATUS
Abstract
A wavelength dispersion compensating apparatus of the invention
comprises: a VIPA plate capable to output incident lights at
different angles according to wavelengths; a variable dispersion
diffraction grating which can angularly disperse the lights of
respective wavelengths output from the VIPA plate, in a direction
substantially perpendicular to a direction of angular dispersion in
the VIPA plate and also capable to change an amount of the angular
dispersion; a light return apparatus which condenses the output
lights from the variable dispersion diffraction grating and
reflects them by a mirror, to return them to the VIPA plate side;
and a stage rotation mechanism which rotates a movable stage on
which the lens and the mirror are mounted, according to a
diffraction angle in the variable dispersion diffraction grating,
so as to enable wavelength dispersion and wavelength dispersion
slope to be given to a WDM light, to be changed independently. As a
result, it becomes possible to compensate for, over a wide
wavelength band, the wavelength dispersion and wavelength
dispersion slope of the WDM light, which are propagated through an
optical fiber to be accumulated.
Inventors: |
Mitamura, Nobuaki;
(Yokohama, JP) ; Maruyama, Shinji; (Yokohama,
JP) |
Correspondence
Address: |
STAAS & HALSEY LLP
SUITE 700
1201 NEW YORK AVENUE, N.W.
WASHINGTON
DC
20005
US
|
Assignee: |
Fujitsu Limited
Kawasaki
JP
|
Family ID: |
34989506 |
Appl. No.: |
10/901219 |
Filed: |
July 29, 2004 |
Current U.S.
Class: |
359/589 ;
359/558 |
Current CPC
Class: |
G02B 6/29358 20130101;
H04B 10/25133 20130101; G02B 6/12007 20130101; G02B 6/29311
20130101; G02B 6/29394 20130101; G02B 6/29395 20130101 |
Class at
Publication: |
359/589 ;
359/558 |
International
Class: |
G02B 005/18 |
Foreign Application Data
Date |
Code |
Application Number |
Mar 25, 2004 |
JP |
2004-89740 |
Claims
What is claimed is:
1. A wavelength dispersion compensating apparatus, comprising: an
optical component having two mutually parallel reflective surfaces,
in which a light condensed in the one-dimensional direction is
incident between said reflective surfaces, and a part of said
incident light is transmitted through one of said reflective
surfaces while said incident light being multiple-reflected on the
reflective surfaces, so that lights dispersed, due to interference
by said transmitted light, in a substantially linear direction at
different angles according to wavelengths are output; a lens which
condenses the lights of respective wavelengths output from the
optical component; a mirror having a reflective surface which
reflects the lights which have passed through the lens to be
condensed, in which said reflective surface has a shape capable to
give constant wavelength dispersion to the lights of respective
wavelengths output from said optical component, in a direction
substantially parallel to a direction of angular dispersion in said
optical component, and also capable to give different wavelength
dispersion to the lights of respective wavelengths output from said
optical component, in a direction substantially perpendicular to
the direction of angular dispersion in said optical component; and
a slide mechanism which slides said mirror in the direction
substantially perpendicular to the direction of angular dispersion
in said optical component, to vary the wavelength dispersion to be
given to the lights of respective wavelengths output from said
optical component according to a sliding amount of said mirror,
wherein said wavelength dispersion compensating apparatus further
comprises: a variable dispersion diffraction grating disposed
between said optical component and said lens, which is capable to
disperse the lights output from said optical component in different
angles according to wavelengths, and also capable to change an
amount of the angular dispersion, in the direction substantially
perpendicular to the direction of angular dispersion in said
optical component; and a rotation mechanism which rotates said lens
and said mirror integrally about a reference point on said variable
dispersion diffraction grating according to a diffraction angle in
said variable dispersion diffraction grating, and wherein
wavelength dispersion slope is varied independently of the
wavelength dispersion, according to an angular dispersion amount of
said variable dispersion diffraction grating and a rotation amount
of said rotation mechanism.
2. A wavelength dispersion compensating apparatus according to
claim 1, wherein a distance from said variable dispersion
diffraction grating to said lens is made substantially equal to a
distance from said lens to said mirror.
3. A wavelength dispersion compensating apparatus according to
claim 1, wherein in place of said lens, there are provided: a first
cylindrical lens disposed between said optical component and said
variable dispersion diffraction grating, which condenses the lights
of respective wavelengths output from said optical component, in a
direction substantially parallel to the direction of angular
dispersion in said optical component; and a second cylindrical lens
disposed between said variable dispersion diffraction grating and
said mirror, which condenses the lights of respective wavelengths
output from said variable dispersion diffraction grating in the
direction substantially perpendicular to the direction of angular
dispersion in said optical component, and said rotation mechanism
rotates said second cylindrical lens and said mirror integrally
about the reference point on said variable dispersion diffraction
grating, according to the diffraction angle in said variable
dispersion diffraction grating.
4. A wavelength dispersion compensating apparatus according to
claim 3, wherein a distance from said variable dispersion
diffraction grating to said second cylindrical lens is made
substantially equal to a distance from said second cylindrical lens
to said mirror.
5. A wavelength dispersion compensating apparatus according to
claim 1, wherein a position of said lens is changed between said
optical component and said variable dispersion diffraction grating,
and the lights of respective wavelengths output from said optical
component are given to said mirror after passing in sequence
through said lens and said variable dispersion diffraction grating,
and said rotation mechanism rotates said mirror about the reference
point on said variable dispersion diffraction grating, according to
the diffraction angle in said variable dispersion diffraction
grating.
6. A wavelength dispersion compensating apparatus according to
claim 5, wherein said mirror has a reflective surface of a concave
shape along an arc, which is centered on the reference point on
said variable dispersion diffraction grating and passes through a
center of said reflective surface, in the direction substantially
perpendicular to the direction of angular dispersion in said
optical component, and is rotated by said rotation mechanism,
instead of being slid by said slide mechanism.
7. A wavelength dispersion compensating apparatus according to
claim 1, wherein said variable dispersion diffraction grating
includes: a flat plate formed from an acousto-optic material; an
electrode formed on a surface of said flat plate; and a drive power
source which supplies a high frequency signal to said electrode, to
generate a surface acoustic wave in said flat plate, and changes a
period of the diffraction grating formed on said flat plate
according to a frequency of the high frequency signal supplied from
said drive power source to said electrode.
8. A wavelength dispersion compensating apparatus according to
claim 1, wherein said variable dispersion diffraction grating
includes: a flat plate formed from a photo-refractive material; and
a variable wavelength twin-beam interferometer which generates an
interference fringe of light on said flat plate, and changes a
period of the diffraction grating formed on said flat plate
according to wavelengths and an intersection angle of two optical
beams in said twin-beam interferometer.
9. A wavelength dispersion compensating apparatus according to
claim 1, wherein said optical component is formed on a waveguide
substrate, and lights having been propagated through said
waveguide, are incident between said reflective surfaces, and the
lights transmitted through one of said reflective surfaces are
propagated through a slab waveguide, to interfere with each
other.
10. A wavelength dispersion compensating apparatus according to
claim 9, wherein said variable dispersion diffraction grating is
formed on a portion corresponding to the slab waveguide on said
waveguide substrate, and said lens condenses the lights of
respective wavelengths emitted at different angles according to
wavelengths, from a surface of said waveguide substrate by said
variable dispersion diffraction grating, to give the condensed
lights to said mirror.
Description
BACKGROUND OF THE INVENTION
[0001] (1) Field of the Invention
[0002] The present invention relates to a wavelength dispersion
(chromatic dispersion) compensating apparatus for compensating for
wavelength dispersion and wavelength dispersion slope occurring in
signal lights of respective wavelengths transmitted on optical
fibers, in an optical communication of a wavelength division
multiplexing (WDM) system.
[0003] (2) Description of the Related Art
[0004] In a conventional optical fiber communication system for
transmitting information using light, a transmitter sends out an
optical pulse via an optical fiber to a receiver. However,
wavelength dispersion, also known as "chromatic dispersion",
occurring in the optical fiber deteriorates the signal quality in
the system.
[0005] Specifically, due to wavelength dispersion characteristics
of the optical fiber, propagation speed of a signal light in the
optical fiber is dependent on a wavelength of the signal light. For
example, when an optical pulse having a long wavelength (for
example, an optical pulse of a wavelength indicating red color) is
propagated at a speed higher than an optical pulse having a short
wavelength (for example, an optical pulse of a wavelength
indicating blue color), the wavelength dispersion in the signal
light is called normal dispersion. Conversely, when an optical
pulse having a short wavelength (for example, a blue pulse) is
propagated at a speed higher than an optical pulse having a long
wavelength (for example, a red pulse), the wavelength dispersion in
the signal light is called abnormal dispersion.
[0006] Accordingly, in the case where a signal light contains a red
pulse and a blue pulse when sent out from a transmitter, the signal
light is separated into the red and blue pulses while being
propagated through the optical fiber, and then each separated pulse
is received by a light receiver at dfferent times.
[0007] As another example of optical pulse transmission, in the
case where a signal light having wavelength components which are
consecutive from blue to red is transmitted, the respective
components are propagated through the optical fiber at different
speeds, and thus the time-width of pulse waveform of the signal
light is extended inside the optical fiber, resulting in the
distortion of pulse waveform. Since all pulses include components
within a limited wavelength range, this wavelength dispersion is
extremely common in optical fiber communications systems.
[0008] Particularly in a high-speed optical fiber communication
system, it is necessary to compensate for the wavelength dispersion
in order to obtain high transmission performance.
[0009] In order to compensate for this wavelength dispersion, the
optical fiber communication system needs to be provided with a
"reciprocal dispersion component" which gives wavelength dispersion
reciprocal to the wavelength dispersion occurring in the optical
fiber to the optical pulse. In the conventional apparatuses, there
exists the one capable to be used as this reciprocal dispersion
component. For example, a dispersion compensating fiber has a
specific cross-sectional refractive index distribution, and is
capable to give wavelength dispersion reciprocal to wavelength
dispersion occurring in a normal transmission path fiber to the
optical pulse. Therefore, it can be used as the reciprocal
dispersion component.
[0010] However, the dispersion compensating fiber is expensive in
manufacturing cost, and it is necessary to make the fiber length
thereof relatively long in order to sufficiently compensate for the
wavelength dispersion occurring in the transmission path fiber. For
example, to completely compensate for the wavelength dispersion
occurred in the transmission path fiber of 100 km, a dispersion
compensating fiber of between 20 km and 30 km is required.
Therefore, there are caused problems of a large optical loss, and a
large size.
[0011] Furthermore, in addition to the above dispersion
compensating fiber, a chirped fiber grating can be used as the
reciprocal dispersion component to compensate for the wavelength
dispersion. A fiber grating is formed with, in the core thereof, a
grating structure whose refractive index is changed at a
half-wavelength period, using a phenomenon in which the refractive
index of core-doped germanium oxide is changed with the ultraviolet
beam irradiation. The chirped fiber grating is designed such that,
by gradually changing the grating intervals in a longitudinal
direction of the above fiber grating to reflect long wavelength
components at long distances so that the long wavelength components
are propagated for distances longer than propagation distances of
short wavelength components. Accordingly, the chirped fiber grating
can also give the reciprocal dispersion to the optical pulse.
[0012] However, since the chirped fiber grating has a reflective
band of very narrow wavelength width, it is difficult to realize a
sufficient reflective band for compensating for wavelength
dispersion of a light containing a large number of wavelengths such
as a WDM light. It is possible to connect in cascade multiple
chirped fiber gratings to realize a reflective band corresponding
to the WDM light. However, there is a problem in that a system
applying such a reciprocal dispersion component is expensive.
[0013] As one of conventional techniques to resolve these problems,
there has been proposed an apparatus in which, for example, a
device called a virtually imaged phased array (hereafter referred
to as `VIPA`) as shown in FIG. 18 is utilized to compensate for the
wavelength dispersion occurring in the WDM light (refer to Japanese
Unexamined Patent Publication No. 2002-258207).
[0014] This apparatus includes a VIPA plate 110 which demultiplexes
the WDM light into a plurality of optical beams capable to be
spatially discriminated from each other (for example, traveling
directions of optical beams being different from each other),
according to wavelengths, and a light return apparatus which
reflects a light output from the VIPA plate 110 to return it to the
VIPA plate 110 again. The VIPA plate 110 includes a transparent
member 111 having parallel first and second planes 112 and 113. The
first plane 112 of the transparent member 111 has a characteristic
to reflect a light at the reflectance of approximately 100% except
for a transmission area 114 formed on a part thereof, and the light
passes through the transmission area 114, to be input to and output
from the transparent member 111. The second plane 113 of the
transparent member 111 has a characteristic to reflect a light at
the reflectance lower than 100%. The light having passed through
the transparent area 114 to be input to the transparent member 111
is multiple-reflected repeatedly between the first and second
planes 112 and 113. At this time, a few percent of the light is
transmitted through the second plane 113 to be emitted to the
outside of the transparent member 111. The lights transmitted
through the transparent member 111 interfere mutually and generate
a plurality of optical beams capable to be spatially discriminated,
traveling directions of which are different from each other,
according to wavelengths. The VIPA plate 110 is a device with
angular dispersion, since the output lights can be discriminated
from each other according to traveling angles thereof. The light
return apparatus reflects the output light from the VIPA plate 110,
to return it to the VIPA plate 110. The light reflected by the
light return apparatus is transmitted through the second plane 113
to be input to the transparent member 111, and is
multiple-reflected repeatedly between the first and second planes
112 and 113, to be output to an input path from the transparent
area 114.
[0015] Furthermore, the above VIPA plate 110 has the same
wavelength as the wavelength of the input light, and has a function
of generating a plurality of output lights having different orders
of interference. The light return apparatus is provided with a
structure in which the output light of one order of interference is
returned to the VIPA plate 110, but the output lights of other
orders of interference are not returned to the VIPA plate 110.
Thus, only the light corresponding to one order of interference
passes through the VIPA plate 110, to be output to the input
path.
[0016] Moreover, the above light return apparatus is provided with
a lens 160 and a mirror 170, as a specific configuration thereof.
The lens 160 has a function of condensing the lights output from
the VIPA plate 110 to the different directions according to the
wavelengths, onto different positions on the surface of the mirror
170, and also orienting the lights reflected by the mirror 170 to
the VIPA plate 110. The mirror 170 is located such that the light
traveling from the VIPA plate 110 to the lens 160, and the light
returning from the lens 160 to the VIPA plate 110 are propagated in
parallel and opposite directions, and are prevented from being
overlapped with each other. As a result, the lights of respective
wavelengths reflected by the light return apparatus are propagated
for different distances, so that the wavelength dispersion of the
WDM light is compensated for.
[0017] As described in the above, the apparatus using the VIPA
plate 110 has the angular dispersion function similar to a
diffraction grating, and is capable to compensate for the
wavelength dispersion occurring in WDM light. In particular, a
VIPA-type wavelength dispersion compensating apparatus has a
feature capable to generate considerable angular dispersion, and
accordingly, can readily provide a practical reciprocal dispersion
component.
[0018] A practical reciprocal dispersion component for use in a WDM
transmission system is required to serve the following special
needs.
[0019] A wavelength dispersion characteristic of an optical fiber
generally in practical use is not constant depending on wavelength
as shown in FIG. 19 for example, and frequently has a slightly
positive inclination (wavelength dispersion is increased as the
wavelength becomes longer). Such an inclination of wavelength
dispersion is referred to as wavelength dispersion slope, or second
order wavelength dispersion. Specifically, in a typical 1.3 .mu.m
zero-dispersion single mode fiber (SMF) as shown by the dotted line
in FIG. 19, for a light of wavelength 1550 nm, the wavelength
dispersion per 1 km is +16.79 ps/nm/km, while the wavelength
dispersion slope per 1 km is 0.057 ps/nm.sup.2/km. In the case
where the necessary wavelength bandwidth is 35 nm for example, a
variation in wavelength dispersion of approximately +2 ps/nm occurs
within such a wavelength band.
[0020] The solid line in FIG. 19 indicates a characteristic of
E-LEAF optical fiber manufactured by Corning Inc. In this E-LEAF
optical fiber, for the light of wavelength 1550 nm, the wavelength
dispersion is 3.852 ps/nm/km, and the dispersion slope is 0.083
ps/nm.sup.2/km. On the other hand, the broken line in FIG. 19
indicates a characteristic of TW-RS optical fiber manufactured by
Lucent Inc., and for the light of 1550 nm wavelength, the
wavelength dispersion is 4.219 ps/nm/km, and the dispersion slope
is 0.045 ps/nm.sup.2/km. Furthermore, the respective wavelength
dispersion characteristics in FIG. 19 are practically not linear,
and strictly speaking, the inclinations (wavelength dispersion
slope) of the wavelength dispersion are not constant. However, such
third-order wavelength dispersion can be neglected since it
presents very few problems at a transmission speed of approximately
40 Gb/s.
[0021] Here, if the wavelength dispersion in the optical fiber
transmission path is considered in practice, as shown in FIG. 19,
the wavelength dispersion and wavelength dispersion slope per unit
length, are determined depending on the type of optical fiber used
as the transmission path, and next, the actual wavelength
dispersion and wavelength dispersion slope are determined depending
on the length of the optical fiber (transmission distance). In the
case where such actual wavelength dispersion in the optical fiber
transmission path is compensated for with the reciprocal dispersion
component, it is desirable to set the wavelength dispersion to be
variable within a certain range, as a characteristic of the
reciprocal dispersion component. This is because the types and
transmission distances of the optical fiber are in infinite variety
depending on the transmission speed and wavelength band of the
transmission system, the timing at which the optical fiber was
installed, and the conditions of the installation site.
[0022] Moreover, in the case of WDM transmission, it is
insufficient even if only the wavelength dispersion can be
compensated as described above, and the wavelength dispersion slope
also becomes problematic. This is because, even if the dispersion
can be compensated with a wavelength of a given signal channel, if
the wavelength dispersion of the reciprocal dispersion component is
constant, the wavelength dispersion cannot be compensated
completely with a wavelength of a different signal channel. It is
therefore desirable that the characteristic of the reciprocal
dispersion component for WDM transmission has the wavelength
dispersion slope. Furthermore, as described above, since the
transmission distances are in infinite variety, and the wavelength
dispersion slope is varied together with the wavelength dispersion
in proportion to the length of the optical fiber, it is desirable
that the wavelength dispersion slope is also set to be variable
within a certain range.
[0023] However, a value of the wavelength dispersion slope to be
given, is not determined uniquely with a wavelength dispersion
value. This is because, not only the wavelength dispersion but also
the wavelength dispersion slope are varied if the type of optical
fiber is changed, as is apparent from FIG. 19. That is to say, in
the case of WDM transmission, in order to compensate for the
wavelength dispersion in the optical fiber transmission path by the
reciprocal dispersion component, it is most desirable to set the
wavelength dispersion and the wavelength dispersion slope to be
variable independently within certain ranges.
[0024] However, although the wavelength dispersion can be set to be
variable within a required range by the conventional reciprocal
dispersion component as described above, it cannot have been
realized that the wavelength dispersion and the wavelength
dispersion slope are varied independently, as described above.
[0025] Specifically, for the dispersion compensating fiber, since
it is possible to design an index profile having the reciprocal
dispersion slope, a dispersion compensating fiber having the
required wavelength dispersion and wavelength dispersion slope can
be realized. However, in order to vary the wavelength dispersion
and the wavelength dispersion slope independently, a dispersion
compensating fiber having a variety of index profiles, and a
variety of lengths, is necessary. Therefore, such a dispersion
compensating fiber is not practical. Moreover, as described above,
such a dispersion compensating fiber has problems of high cost,
large loss, large size and the like.
[0026] Furthermore, in the chirped fiber grating, as with the
dispersion compensating fiber, if the chirp design of chirped fiber
grating is optimized, the reciprocal dispersion slope can be given.
However, in order to change the value thereof, a variety of index
profiles and a variety of lengths, are necessary. Therefore, such a
chirped fiber grating is not practical. Even if the temperature is
changed to vary the wavelength dispersion and the wavelength
dispersion slope, since the wavelength dispersion slope value is
determined uniquely with the wavelength dispersion value, the
wavelength dispersion and the wavelength dispersion slope cannot be
varied independently. Additionally, as described above, it is also
hard for the chirped fiber grating to obtain the sufficient
wavelength bandwidth for compensating for the light having a large
number of wavelengths such as WDM light.
[0027] Moreover, in the reciprocal dispersion components using
conventional diffraction gratings, there is a possibility of
varying the wavelength dispersion and the wavelength dispersion
slope independently to a certain extent depending on how the
diffraction gratings are combined. However, since there is a limit
in the angular dispersion obtainable within practical dimensions of
typical diffraction gratings other than the VIPA, it is difficult
to give the sufficiently large angular dispersion capable to
compensate for the wavelength dispersion of relatively large value,
which occurs in the optical fiber communication system. Therefore,
such a reciprocal dispersion component is not practical.
SUMMARY OF THE INVENTION
[0028] The present invention has been accomplished in view of the
above problems, with an object of providing a wavelength dispersion
compensating apparatus capable of generating an arbitrary
wavelength dispersion and wavelength dispersion slope, to
compensate for wavelength dispersion and wavelength dispersion
slope of a WDM light, which have been propagated through an optical
fiber to be accumulated, over a wide wavelength band.
[0029] In order to achieve the aforementioned object, a wavelength
dispersion compensating apparatus of the present invention
comprises: an optical component having two mutually parallel
reflective surfaces, in which a light condensed in the
one-dimensional direction is incident between the reflective
surfaces, and a part of the incident light is transmitted through
one of the reflective surfaces while the incident light being
multiple-reflected on the reflective surfaces, so that lights
dispersed, due to interference by the transmitted light, in a
substantially linear direction at different angles according to
wavelengths are output; a lens which condenses the lights of
respective wavelengths output from the optical component; a mirror
having a reflective surface which reflects the lights which have
passed through the lens to be condensed, in which the reflective
surface has a shape capable to give constant wavelength dispersion
to the lights of respective wavelengths output from the optical
component, in a direction substantially parallel to a direction of
angular dispersion in the optical component, and also capable to
give different wavelength dispersion to the lights of respective
wavelengths output from the optical component, in a direction
substantially perpendicular to the direction of angular dispersion
in the optical component; and a slide mechanism which slides the
mirror in the direction substantially perpendicular to the
direction of angular dispersion in the optical component, to vary
the wavelength dispersion to be given to the lights of respective
wavelengths output from the optical component according to a
sliding amount of the mirror. Moreover, the wavelength dispersion
compensating apparatus comprises: a variable dispersion diffraction
grating disposed between the optical component and the lens, which
is capable to disperse the lights output from the optical component
in different angles according to wavelengths, and also capable to
change an amount of angular dispersion, in the direction
substantially perpendicular to the direction of angular dispersion
in the optical component; and a rotation mechanism which rotates
the lens and the mirror integrally about a reference point on the
variable dispersion diffraction grating according to a diffraction
angle in the variable dispersion diffraction grating, wherein
wavelength dispersion slope is varied independently of the
wavelength dispersion, according to an angular dispersion amount of
the variable dispersion diffraction grating and a rotation amount
of the rotation mechanism.
[0030] In the wavelength dispersion compensating apparatus of the
above configuration, the optical component corresponds to the
conventional VIPA, and the variable dispersion diffraction grating
generating variable angular dispersion is disposed between the
optical component and the lens. In this variable dispersion
diffraction grating, the lights output from the optical component
are angular-dispersed in the direction substantially perpendicular
to the direction of angular dispersion in the optical component.
Therefore, by changing the angular dispersion amount, and also by
rotating the lens and mirror integrally according to the
diffraction angle of the variable dispersion diffraction grating,
variable wavelength dispersion slope is given to the optical
signals of respective wavelengths output from the optical
component. Since this wavelength dispersion slope can be varied
independently of the wavelength dispersion which is varied with the
movement of the mirror via the slide mechanism, arbitrary
wavelength dispersion and wavelength dispersion slope can be given
to the lights of respective wavelengths.
[0031] According to the wavelength dispersion compensating
apparatus of the present invention, since the wavelength dispersion
and the wavelength dispersion slope to be given to the input light
can be varied independently, it becomes possible to reliably
compensate for wavelength dispersion and wavelength dispersion
slope of a WDM light which has been propagated through an optical
fiber to be accumulated, over a wide wavelength band.
[0032] Other objects, features, and advantages of the present
invention will become apparent from the following description of
the embodiments, in conjunction with the appended drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0033] FIG. 1 is a perspective view showing a configuration of a
wavelength dispersion compensating apparatus according to a first
embodiment of the present invention.
[0034] FIG. 2 is a diagram showing a model for explaining the
principle of operation of a VIPA.
[0035] FIG. 3 is a diagram showing an example of an optical path
returned by a light return apparatus.
[0036] FIG. 4 is a top plan view of the configuration shown in FIG.
1.
[0037] FIG. 5 is a diagram showing an enlarged view of a portion in
the vicinity of the variable dispersion diffraction grating and the
light return apparatus in FIG. 4.
[0038] FIG. 6 is a diagram showing an example of wavelength
dispersion and wavelength dispersion slope realized by the
wavelength dispersion compensating apparatus of the first
embodiment.
[0039] FIG. 7 is a top plan view showing a configuration of a
wavelength dispersion compensating apparatus according to a second
embodiment of the present invention.
[0040] FIG. 8 is a perspective view showing a configuration of a
wavelength dispersion compensating apparatus according to a third
embodiment of the present invention.
[0041] FIG. 9 is a top plan view of the configuration shown in FIG.
8.
[0042] FIG. 10 is a top plan view showing a constitutional example
in the case where a photo-refractive type variable dispersion
diffraction grating is used in the third embodiment.
[0043] FIG. 11 is a perspective view showing a configuration of a
wavelength dispersion compensating apparatus according to a fourth
embodiment of the present invention.
[0044] FIG. 12 is a top plan view of the configuration shown in
FIG. 11.
[0045] FIG. 13 is a top plan view showing a constitutional example
in the case where the photo-refractive variable dispersion
diffraction grating is used in the fourth embodiment.
[0046] FIG. 14 is a top plan view showing a configuration of a
wavelength dispersion compensating apparatus according to a fifth
embodiment of the present invention.
[0047] FIG. 15 is a perspective view showing a configuration of a
waveguide-type VIPA plate in the fifth embodiment.
[0048] FIG. 16 is a perspective view showing configurations of a
waveguide-type VIPA plate and a variable dispersion diffraction
grating used in a wavelength dispersion compensating apparatus
according to a sixth embodiment of the present invention.
[0049] FIG. 17 is a diagram exemplarily showing a state of channel
light output from a surface of a substrate in the sixth
embodiment.
[0050] FIG. 18 is a perspective view showing a constitutional
example of a configuration of a conventional VIPA-type wavelength
dispersion compensating apparatus.
[0051] FIG. 19 is a diagram exemplarily showing wavelength
dispersion and wavelength dispersion slope of various types of
optical fiber.
DETAILED DESCRIPTION OF THE INVENTION
[0052] There will be described embodiments for implementing a
signal light interruption detecting method of the present invention
and an optical amplifier using the same, with reference to the
accompanying drawings. The same reference numerals denote the same
or equivalent parts in all drawings.
[0053] FIG. 1 is a perspective view showing a configuration of a
wavelength dispersion (chromatic dispersion) compensating apparatus
according to a first embodiment of the present invention.
[0054] In FIG. 1, the wavelength dispersion compensating apparatus
of the present embodiment comprises for example: a VIPA plate 10;
an optical system consisting of an optical fiber 1, a collimate
lens 2, and a cylindrical lens 3, which permits a WDM light
condensed on one segment to be incident on a transmission area 14
of the VIPA plate 10; a variable dispersion diffraction grating 20
which is given with a light multi-reflected by the VIPA plate 10
and emitted from one of planes of the VIPA plate 10; and a light
return apparatus 30 which reflects the light which has passed
through the variable dispersion diffraction grating 20 and returns
it to the VIPA plate 10 via the variable dispersion diffraction
grating 20.
[0055] As with the conventional configuration shown in the above
described FIG. 18, the VIPA plate 10 has a transparent member 11 of
a thin glass plate or the like provided with parallel planes
opposed to each other, a reflective film 12 formed on one of the
parallel planes of the transparent member 11, and a reflective film
13 and a transmission area 14 formed on the other parallel plane.
In this VIPA plate 10, an optical axis of a light incident on the
transparent area 14 is inclined to a perpendicular incident angle
by a required angle. The reflective film 12 formed on the parallel
pate on the opposite side to the transparent area 14 has the
reflectance less than 100% (preferably approximately 98%) to the
WDM light incident from the transparent area 14, and is formed over
the entirety of the one plane of the transparent member 11.
Moreover, the reflective film 13 opposed to the reflective film 12
has the reflectance of approximately 100% to the WDM light incident
from the transparent area 14, and is formed on a part of the other
plane of the transparent member 11. A part of the other plane of
the transparent member 11 on which the reflective film 13 is not
formed, forms the transparent area 14 transparent to the WDM
light.
[0056] The optical fiber 1 is, for example, a single mode fiber or
the like, with one end (omitted in the figure) connected to an
optical circulator (refer to FIG. 18), and the other end positioned
in the vicinity of the collimate lens 2. The collimate lens 2 is a
typical lens for converting an optical beam emitted from the other
end of the optical fiber 1 into parallel lights, to give the
parallel lights to the cylindrical lens 3. The cylindrical lens 3
is a line focal lens which condenses the parallel lights from the
collimate lens 2 onto one segment. The cylindrical lens 3 may be
substituted with a line focal lens such as a refractive index
distribution lens or the like.
[0057] The variable dispersion diffraction grating 20, which is
disposed between the VIPA plate 10 and the light return apparatus
30, is a well-known optical device capable to disperse lights of
respective wavelengths at different output angles and also capable
to change the angular dispersion, in a substantially perpendicular
direction to a direction of angular dispersion of the VIPA plate
10. Here, a diffraction grating utilizing, for example, an
acousto-optic effect, is used as the variable dispersion
diffraction grating 20. This acousto-optic type variable dispersion
diffraction grating 20 is structured such that an electrode (not
shown in the figure) is formed on a thin flat plate 21 made from an
acousto-optic material, and a high frequency signal output from a
drive power source 22 is supplied to the electrode, to generate
surface acoustic waves, and a diffraction grating is formed
depending on a variation of the refractive index of the material
due to the surface acoustic waves. In this acousto-optic type
variable dispersion diffraction grating 20, a frequency of the high
frequency signal to be applied to the electrode from the drive
power source 22 is changed, to change a period of the diffraction
grating, thereby enabling the angular dispersion to be variable.
Specifically, the higher the drive frequency from the drive power
source 22 is, the shorter the period of the diffraction grating
becomes and the greater the angular dispersion becomes. However, as
the angular dispersion becomes greater, the diffraction angle also
becomes greater. Note, the above described acousto-optic type
variable dispersion diffraction grating 20 is also disclosed in
Japanese Unexamined Patent Publication No. 6-50844.
[0058] The light return apparatus 30 is provided with, for example,
a focusing lens 31, a three-dimensional mirror 32, and a movable
stage 33. The focusing lens 31 is a typical lens for condensing a
diffracted light having passed through the variable dispersion
diffraction grating 20 on a single point for each wavelength. In
order to arbitrarily vary the wavelength dispersion given by the
present apparatus as described below, the three-dimensional mirror
32 has a three-dimensional structure in which a cross-sectional
shape of a reflective surface thereof is gradually changed from a
convex face to a flat face, and subsequently to a concave face, in
the direction perpendicular to the direction of angular dispersion
of the VIPA plate 10. The three-dimensional mirror 32 is provided
with a mirror slide mechanism M1 to slide the three-dimensional
mirror 32 in the direction perpendicular to the direction of
angular dispersion of the VIPA plate 10. The focusing lens 31 and
the three-dimensional mirror 32 (and the mirror slide mechanism M1)
are mounted on the movable stage 33. This movable stage 33 is
provided with a stage rotation mechanism M2 to rotate the focusing
lens 31 and the three-dimensional mirror 32 integrally according to
the setting of the variable dispersion diffraction grating 20.
Note, the mirror slide mechanism M1 and the stage rotation
mechanism M2 are described in detail below.
[0059] Next, an operation of the first embodiment will be
described.
[0060] In the wavelength dispersion compensating apparatus of the
above configuration, the WDM light emitted from the optical fiber 1
is converted into parallel lights by the collimate lens 2, and then
condensed on one segment by the cylindrical lens 3, to be incident
between the opposing parallel planes of the VIPA plate 10. This
light incident on the VIPA plate 10 is multi-reflected repeatedly
between the reflective films 12 and 13 formed on the parallel
planes of the VIPA plate 10. At this time, every time the incident
light is reflected on the reflective film 12, a few % of the light
is transmitted through the reflective surface to be emitted outside
the VIPA plate 110.
[0061] In consideration of a model shown in FIG. 2 for example,
behavior of the light multi-reflected by the VIPA plate 10 is
similar to that of an Echelon grating known as a stepped
diffraction grating. Therefore, the VIPA plate 10 can be considered
as a virtual diffraction grating. Further, in consideration of an
interference condition in the VIPA plate 10, as shown on the right
side in FIG. 2, the emitted light interferes under a condition in
which with an optical axis thereof as a reference, a shorter
wavelength is above the optical axis and a longer wavelength is
below the optical axis, and therefore, among a plurality of optical
signals contained in the WDM light, optical signals on the shorter
wavelength side are emitted above the optical axis and optical
signals on the longer wavelength side are emitted below the optical
axis.
[0062] As shown in FIG. 3 for example, the light condensed on the
reflective surface of the three-dimensional mirror 32 disposed on
the rear side of the focusing lens 31, is returned at a reflection
angle which is determined according to the cross-sectional shape of
the three-dimensional mirror 32 at a condensed position, and then
output from the optical fiber 1 after passing through the focusing
lens 31, the VIPA plate 10, the cylindrical lens 3, and the
collimate lens 2 in this order, in a direction opposite to a path
at incidence. Specifically, in the case where the cross-sectional
shape of the three-dimensional mirror 32 is convex as shown in FIG.
3, the light of the short wavelength side returns to an upper side
beam waist, and the optical path thereof becomes longer in
comparison to that of the light of the long wavelength side,
resulting in an increase of a delay. In this case, therefore, the
present wavelength dispersion compensating apparatus generates
negative dispersion. Conversely, while not shown in the figure, in
the case where the cross-section of the three-dimensional mirror 32
is concave, positive dispersion is generated. Since the
configuration of the dispersion compensation apparatus using the
VIPA plate 10 in the present embodiment is such that the output
light returns on the same path as the input light, as with the
conventional configuration shown in the above described FIG. 18,
the light input/output to/from the optical fiber 1 can be handled
in-line using an optical circulator.
[0063] Here, the wavelength dispersion compensation utilizing the
VIPA plate 10 will be described in detail.
[0064] Since the VIPA plate 10 used in the present embodiment is
designed to satisfy the relationships of the following equations
(1) and (2), in order to simultaneously compensate for wavelength
dispersion of channel lights of respective wavelengths.
2.multidot.n.multidot.t.multidot.cos .theta.=m.multidot..lambda.
(1)
FSR=c/(2.multidot.n.multidot.t.multidot.cos .theta.) (2)
[0065] In the above equations, "n" represents the refractive index
of the transparent member 11, "t" represents the physical thickness
of the transparent member 11, .theta. represents an inclination
angle of the VIPA plate 10 (the inclination angle to an angle of
the VIPA plate 10 at which the incident light is perpendicularly
incident; refer to FIG. 2), FSR represents an interval of the
central wavelengths of respective channels, and "c" represents a
speed of light.
[0066] Under the above condition (referred to as the "WDM matching
FSR thickness" condition), since all wavelength components
corresponding to the central wavelengths of the respective channels
are emitted from the VIPA plate 10 at the same angle as shown in
FIG. 3, the focusing lens 31 is able to condense the wavelength
components corresponding to the central wavelengths of the
respective channels (intermediate-wavelength light) on a point 32a
on the three-dimensional mirror 32, the wavelength components
corresponding to long wavelength elements of respective channels
(long-wavelength light) on a point 32b on the three-dimensional
mirror 32, and the wavelength components corresponding to short
wavelength elements of respective channels (short-wavelength light)
on a point 32c on the three-dimensional mirror 32. Therefore, the
wavelength dispersion can be compensated for all channels of the
WDM light using the VIPA plate 10.
[0067] For example, if the VIPA plate 10 thickness t=1 mm and the
refractive index n=1.5, all wavelengths at 100 GHz intervals
satisfy the "WDM matching FSR thickness" condition (FSR=100 GHz).
As a result, the VIPA plate 10 is able to give the same wavelength
dispersion simultaneously to all channels of the WDM light at 100
GHz intervals.
[0068] Furthermore, a value of wavelength dispersion to be given to
all channels of the above WDM light can be changed, by sliding the
three-dimensional mirror 32 by the mirror slide mechanism M1. That
is to say, since the cross-sectional shape of the three-dimensional
mirror 32 has the three-dimensional structure in which the shape is
gradually changed from the convex face to the flat face, and
subsequently to the concave face, in the direction perpendicular to
the direction of angular dispersion of the VIPA plate 10, by
sliding this three-dimensional mirror 32 in the direction
perpendicular to the direction of angular dispersion of the VIPA
plate 10, it is possible to change the cross-sectional shape of the
three-dimensional mirror 32 that receives the angularly dispersed
lights from the VIPA plate 10 to the convex or concave shape. Thus,
it is possible to give different wavelength dispersion to the WDM
light according to the cross-sectional shape of the
three-dimensional mirror 32. As a specific example, the mirror
slide mechanism M1 can be constructed of mainly a linear slider,
and a motor or the like.
[0069] As described above, by designing the thickness of the VIPA
plate 10 to satisfy the "WDM matching FSR thickness" condition, it
becomes possible to compensate for the wavelength dispersion
simultaneously for all channels of the WDM light. And also, by
sliding the three-dimensional mirror 32 by the mirror slide
mechanism M1, it becomes possible to vary the wavelength
dispersion. The operation described hereinabove is similar to that
of the wavelength dispersion compensating apparatus using the
conventional VIPA plate.
[0070] Further, as shown below, in the present embodiment, the
wavelength dispersion and wavelength dispersion slope are made
variable independently by the variable dispersion diffraction
grating 20 and the stage rotation mechanism M2. This will be
described below in detail.
[0071] Generally, for the diffraction grating, the relationship
expressed in the following equation (3) is established.
sin .alpha..+-.sin .beta.=N.multidot.m.multidot..lambda. (3)
[0072] In the above equation, .alpha. represents an angle between
the incident light and the normal of the diffraction grating,
.beta. represents an angle between the diffracted light and the
normal of the diffraction grating, N represents the number of
grooves per 1 mm in the diffraction grating (the reciprocal of
diffraction grating period "s"), "m" represents the order of
diffraction (m=.+-.1, .+-.2, . . . ), and .lambda. represents the
wavelength.
[0073] With the incident angle .alpha. constant, if both sides of
equation (3) are differentiated by .lambda., then the following
equation (4) can be obtained.
d.beta./d.lambda.=N.multidot.m/cos .beta. (4)
[0074] In the above equation, d.beta./d.lambda. represents a
diffraction angle change d.beta. to a wavelength change d.lambda.,
and is referred to as the angular dispersion (or angle dispersion).
Here, for example, in order to diffract the light in the order of 1
.mu.m in wavelength, grooves in the diffraction grating are formed
in the order of 1 .mu.m in period, and therefore, if N=1000 and the
diffracted light of m=first order is used, N.multidot.m=1000>cos
.beta., and cos .beta. can be considered to be approximately
constant. Therefore, if C is a constant, the above equation (4) is
expressed as shown in the equation (5).
d.beta./d.lambda.=N.multidot.C (5)
[0075] From the equation (5), it is seen that as the number of
grooves N in the diffraction grating is increased, that is to say,
as the period "s" of the diffraction grating becomes smaller, the
angular dispersion becomes larger.
[0076] If the equation (3) is transformed, the following equation
(6) is established.
.beta.=sin.sup.-1(.vertline.N.multidot.m.multidot..lambda.-sin
.alpha..vertline.) (6)
[0077] From the equation (6), it is necessary to pay an attention
to the fact that when the incident angle .alpha. is made constant,
the number of grooves N in the diffraction grating is increased,
that is to say, the diffraction angle .beta. becomes larger as the
period "s" of the diffraction grating becomes smaller.
[0078] In the present embodiment, as described above, the
diffraction grating utilizing the acousto-optic effect is used as
the variable dispersion diffraction grating 20. In such an
acousto-optic type variable dispersion diffraction grating 20, in
the case where the high frequency signal is not supplied to the
electrode, that is to say, when the dispersion of the diffraction
grating becomes zero, as shown by the dotted line in a top plan
view of FIG. 4 when viewing the configuration in FIG. 1 from above
for example, the channel lights of respective wavelengths are
condensed on the same position on the reflective surface of the
three-dimensional mirror 32. This condition is similar to that in
the conventional configuration shown in FIG. 18.
[0079] Here, if the high frequency signal is supplied to the
electrode of the acousto-optic-type variable dispersion diffraction
grating 20, to generate the angular dispersion with the diffraction
grating, output angles of the channel lights of respective
wavelengths differ from each other in the direction substantially
perpendicular (horizontal direction in the perspective view in FIG.
1) to the direction of angular dispersion in the VIPA plate 10
(vertical direction in the perspective view in FIG. 1). Therefore,
as shown by the solid line in FIG. 4, the positions of the channel
lights condensed on the reflective surface of the three-dimensional
mirror 32 via the focusing lens 31 are displaced in transverse. At
this time, centerline directions of optical beams obtained by
condensing the channel lights of respective wavelengths are
respectively bent by the focusing lens 31, to be the same in all
channel lights. Specifically, among the channel lights of
respective wavelengths contained in the WDM light, the channel
lights of intermediate wavelengths are condensed on a position as
shown by the solid line in FIG. 4, while the channel lights of
short wavelengths are condensed on a position displaced to the
right as shown by the broken line in FIG. 4, and the channel lights
of longer wavelengths are condensed on a position displaced to the
left as shown by the double-dashed line in FIG. 4. The displacement
in transverse of the position on which the lights are condensed on
the reflective surface of the three-dimensional mirror 32 in this
manner implies that the different wavelength dispersion is given
for each channel light of each wavelength, and means that the
wavelength dispersion slope occurs. Moreover, the angular
dispersion in the variable dispersion diffraction grating 20 can be
varied, by changing the frequency of the drive signal for the
acousto-optic type variable dispersion diffraction grating 20.
Thus, it becomes possible to change an amount of transverse
displacement of each channel light of each channel condensed on the
reflective surface of the three-dimensional mirror 32, and also to
vary the wavelength dispersion slope.
[0080] However, as described above, with an increase of the angular
dispersion in the variable dispersion diffraction grating 20, the
diffraction angle in the variable dispersion diffraction grating 20
is also increased, so the optical paths through which the channel
lights are propagated, are also greatly displaced. Therefore, it is
necessary to rotate the focusing lens 31 and the three-dimensional
mirror 32 integrally about a reference point 0 on the diffraction
grating 20 according to the optical path (diffraction angle) as
shown in an enlarged view in FIG. 5. In the present embodiment,
therefore, the stage rotation mechanism M2 is provided to rotate
the movable stage 33 on which the focusing lens 31 and the
three-dimensional mirror 32 (and the mirror slide mechanism M1) are
mounted, according to the frequency of the high frequency signal
given to the variable dispersion diffraction grating 20. The
reference point 0 on the diffraction grating 20 corresponds to a
central position at which the power distribution of the WDM light
incident on the diffraction grating 20 from the VIPA plate 10
becomes a maximum.
[0081] Summarizing the operation of the present embodiment as
described above, in the case where the wavelength dispersion slope
is varied by the present wavelength dispersion apparatus, the
configuration may be such that the drive frequency for the
acousto-optic type variable dispersion diffraction grating 20 is
changed, to vary the angular dispersion in a transverse direction,
and also the movable stage 33 is rotated to an optimum position by
the stage rotation mechanism M2 according to the diffraction angle
changed by the variable dispersion diffraction grating 20.
Furthermore, in order to change the wavelength dispersion value
while holding the wavelength dispersion slope, the
three-dimensional mirror 32 may be slid by the mirror slide
mechanism M1 as in the conventional case, with the drive frequency
for the variable dispersion diffraction grating 20 and the position
of the movable stage 33 unchanged. As a result, in the present
embodiment, it becomes possible to vary the wavelength dispersion
and the wavelength dispersion slope independently.
[0082] Moreover, in the configuration of the present embodiment, as
shown in FIG. 5, for example, in the case where the centerline of
the beam having passed through the focusing lens 31 is not incident
approximately perpendicularly on the reflective surface of the
three-dimensional mirror 32, the reflected beam is displaced in
transverse, and accordingly, a loss occurs. In order to suppress
such a loss to be a minimum, it is desirable design an optical
system such that a distance D1 from the variable dispersion
diffraction grating 20 to the focusing lens 31 is approximately
equal to a distance D2 from the focusing lens 31 to the
three-dimensional mirror 32 (D1.apprxeq.D2=f), that is, is
approximately equal to a focal distance f of the focusing lens
31.
[0083] An example of the wavelength dispersion and the wavelength
dispersion slope realized by the wavelength dispersion compensating
apparatus of the above present embodiment is shown in FIG. 6. Here,
for example, for three types of optical path fiber; E-LEAF shown in
(A) (manufactured by Corning Inc.: dispersion; 3.852 ps/nm/km,
dispersion slope; 0.083 ps/nm.sup.2/km), TW-RS shown in (B)
(manufactured by Lucent Inc.: dispersion; 4.219 ps/nm/kr,
dispersion slope; 0.045 ps/nm.sup.2/km), and SMF shown in (C)
(dispersion; 16.79 ps/nm/km, dispersion slope; 0.057
ps/nm.sup.2/km), there is shown results of optimization of the
wavelength dispersion compensating apparatus so that the wavelength
dispersion and the wavelength dispersion slope occurring in the
case of assuming that each transmission distance is 80 km, are
capable to be compensated. As a result, it is understood that
wavelength dispersion and wavelength dispersion slope completely
opposite to the wavelength dispersion and the wavelength dispersion
slope in different types of optical fiber can be realized by the
present wavelength dispersion compensating apparatus. Consequently,
if the present wavelength dispersion compensating apparatus
optimized according to the type of optical fiber used for the
transmission path is used for the transmission of the WDM light, it
becomes possible to obtain a satisfactory eye-opening for received
waveform of the channel light of each waveform, even in the case of
the transmission of WDM light at ultra-high speed at 40
Gbits/second or the like.
[0084] A second embodiment of the present invention will be
described.
[0085] FIG. 7 is a top plan view showing a configuration of a
wavelength dispersion compensating apparatus of the second
embodiment.
[0086] In FIG. 7, the configuration of the present embodiment
differs from that of the first embodiment in that a
photo-refractive type variable dispersion diffraction grating 40 is
provided in place of the acousto-optic type variable dispersion
diffraction grating 20 used in the first embodiment. Other
components are similar to those in the first embodiment.
[0087] The photo-refractive type of variable dispersion diffraction
grating 40 is a well-known diffraction grating disclosed in
Japanese Unexamined Patent Publication No. 2001-324731. That is to
say, the photo-refractive type diffraction grating is configured
such that interference infringes are formed on a surface of a thin
plate 41 of a photo-refractive material, with a wavelength control
laser light which is efficiently absorbed by the photo-refractive
material, to form a diffracting grating depending on a variation of
refractive index of the photo-refractive material occurring
according to the optical intensity of these interference fringes.
As shown in FIG. 7, a twin-beam interferometer or the like can be
utilized in order to form the interference fringes. In this
twin-beam interferometer, output lights from a light source 42 are
made parallel lights by a collimate lens 43, to be branched into
two optical paths by a half mirror 44, and one of the optical paths
is returned by a half mirror 45 to have a difference from the other
optical path, and then, the two optical paths are recombined to
form the interference fringes.
[0088] Here, the spacing (period) "s" of the interference fringes
can be expressed by the following equation (7), where .gamma.
represents an intersection angle of the two optical paths, and k
represents a wavelength of the output from the light source 42.
s=.lambda./sin .gamma. (7)
[0089] As is seen from the equation (7), the period "s" of the
interference fringes can be changed by the wavelength .lambda. and
the intersection angle .gamma. of the two optical paths, and in
order to reduce the period "s" of the interference fringes (that is
to say, in order to increase the number of grooves), the wavelength
.lambda. may be shortened, or the intersection angle .gamma. of the
two optical paths may be increased. In order to change the
wavelength .lambda. in practice, a variable wavelength laser light
source may be used as the light source 42, for example.
Furthermore, in order to change the intersection angle .gamma. of
the two optical paths, the mirror 45 of the one optical path may be
rotated to move by the mirror rotation mechanism M3 to change an
emission angle of the reflected light from the mirror 45, as shown
by the dotted line in FIG. 7.
[0090] In this manner, in the photo-refractive type variable
dispersion diffraction grating 40, the period "s" of the
diffraction grating can be changed to make the angular dispersion
variable, by changing the wavelength .lambda. of the light source
42 in the twin-beam interferometer, or by rotating to move the
mirror 45 to change the intersection angle .gamma. of the two
optical paths. As described above, as the wavelength .lambda.
becomes shorter, or as the intersection angle of the two optical
paths .gamma. becomes greater, the period "s" of the diffraction
grating is reduced and the angular dispersion is increased.
However, for a same reason as with the first embodiment, as the
angular dispersion in the variable dispersion diffraction grating
40 becomes greater, a diffraction angle also becomes greater, and
propagation directions of channel lights of respective wavelengths
output from the variable dispersion diffraction grating 40 are also
greatly displaced. As with the first embodiment, therefore, it is
necessary to rotate the movable stage 33 by the stage rotation
mechanism M2 according to the diffraction angle in the variable
dispersion diffraction grating 40.
[0091] Accordingly, in the present wavelength dispersion
compensating apparatus, to change the wavelength dispersion slope,
the angular dispersion of the photorefractive type variable
dispersion diffraction grating 40 may be varied by changing the
wavelength .lambda. of the light source 42 in the twin-beam
interferometer, or by rotating to move the mirror 45 to change the
intersection angle .gamma. of the two optical paths, and also the
movable stage 33 may be rotated to an optimum position by the stage
rotation mechanism 27 according to the diffraction angle in the
variable dispersion diffraction grating 40. Furthermore, in order
to change the wavelength dispersion value while holding the
wavelength dispersion slope, the three-dimensional mirror 32 is
slid by the mirror slide mechanism M1 as in the conventional case,
with the setting of the twin-beam interferometer and the position
of the stage 33 unchanged.
[0092] As described above, also in the wavelength dispersion
compensating apparatus of the second embodiment using the
photo-refractive type variable dispersion diffraction grating 40,
the wavelength dispersion and the wavelength dispersion slope can
also be varied independently, and a similar effect to that in the
first embodiment can be obtained.
[0093] A third embodiment of the present invention will be
described.
[0094] FIG. 8 is a perspective view showing a configuration of a
wavelength dispersion compensating apparatus according to the third
embodiment of the present invention. Moreover, FIG. 9 is a top plan
view when viewing the configuration shown in FIG. 8 from above.
[0095] In FIG. 8 and FIG. 9, the configuration of the present
embodiment differs from that of the first embodiment in that two
cylindrical lenses 34 and 35 are provided in place of the single
focusing lens 31 which has condensed the output light from the VIPA
plate 10 on the reflective surface of the three-dimensional mirror
32 in the first embodiment. Other components are similar to those
of the first embodiment.
[0096] The cylindrical lens 34 is a line focal lens, which is
disposed, for example, between the VIPA plate 10 and the variable
dispersion diffraction grating 20, and condenses the output light
from the VIPA plate 10 into the same direction as the direction of
angular dispersion (a direction perpendicular to the paper in the
top plan view in FIG. 9) in the VIPA plate 10. On the other hand,
the cylindrical lens 35 is a line focal lens, which is mounted on
the movable stage 33 between the variable dispersion diffraction
grating 20 and the three-dimensional mirror 32, and condenses the
output light from the variable dispersion diffraction grating 20 in
the same direction as the direction of angular dispersion in the
variable dispersion diffraction grating 20 (direction parallel to
the paper in the top plan view in FIG. 9). Here, the cylindrical
lens 34 is referred to as a "vertical cylindrical lens", and the
cylindrical lens 35 is referred to as a "horizontal cylindrical
lens" corresponding to their condensing direction.
[0097] In the configuration of the aforementioned first embodiment,
the occurrence of loss is suppressed to a minimum by making the
distance D1 from the variable dispersion diffraction grating 20 to
the focusing lens 31, and the distance D2 from the focusing lens 31
to the three-dimensional mirror 32 (that is to say, the focal
distance f), approximately equal to each other. In this case, if
the focal distance f of the focusing lens 31 is lengthened, the
size of the wavelength dispersion compensating apparatus becomes
greater. On the other hand, in the case where the focal distance f
of the focusing lens 31 is made shorter, to reduce the size of the
wavelength dispersion compensating apparatus, it is necessary to
make the concavity and convexity of the reflective surface of the
three-dimensional mirror 32 to be greater, to ensure that the
amount of wavelength dispersion occurring in the VIPA plate 10 is
approximately equal to that prior to the reduction of the focal
distance f of the focusing lens 31, resulting in a problem that
such a three-dimensional mirror 32 is not readily manufactured.
[0098] In order to resolve this problem, in the configuration of
the present embodiment, it is possible to extend the distance
between the vertical cylindrical lens 34, which has an affect on
the shape of the reflective surface of the three-dimensional mirror
32, and the three-dimensional mirror 32, without increasing the
size of the wavelength dispersion compensating apparatus. The
variable dispersion diffraction grating 20 and the horizontal
cylindrical lens 35 are disposed so that the distance from the
variable dispersion diffraction grating 20 to the horizontal
cylindrical lens 35, and the distance from the horizontal
cylindrical lens 35 to the three-dimensional mirror 32, are
approximately equal to each other, on the optical path between the
vertical cylindrical lens 34 and the three-dimensional mirror 32.
Then, the movable stage on which the horizontal cylindrical lens 35
and the three-dimensional mirror 32 (and the mirror slide mechanism
M1) are mounted, is rotated to the optimum position by the stage
rotation mechanism M2 about the reference point O of the variable
dispersion diffraction grating 20, according to the diffraction
angle in the variable dispersion diffraction grating 20. Thus,
similar function and effect to those in the first embodiment can be
achieved, and also it becomes possible to miniaturize the
wavelength dispersion compensating apparatus.
[0099] The above third embodiment shows an example in which the
vertical cylindrical lens 34 and the horizontal cylindrical lens 35
are used in the configuration of the first embodiment using the
acousto-optic type variable dispersion diffraction grating 20. In a
similar manner, it is also possible to use the vertical cylindrical
lens 34 and the horizontal cylindrical lens 35 in the configuration
of the second embodiment using the photo-refractive type variable
dispersion diffraction grating 40. A top plan view in FIG. 10 shows
a constitutional example of the wavelength dispersion compensating
apparatus in this case.
[0100] A fourth embodiment of the present invention will be
described.
[0101] FI.L 11 is a perspective view showing a configuration of a
wavelength dispersion compensating apparatus of the fourth
embodiment of the present invention. FIG. 12 is a top plan view
when viewing the configuration in FIG. 11 from above.
[0102] As shown in the figures, the wavelength dispersion
compensating apparatus of the present embodiment is an application
example in which, for example, in the configuration of the first
embodiment, the positioning of the variable dispersion diffraction
grating 20 is changed from the position between the VIPA plate 10
and the focusing lens 31, to the position between the focusing lens
31 and the three-dimensional mirror 32, and the reflective surface
of the three-dimensional mirror 32 is curved as described below, so
that the mirror slide mechanism M1 and the stage rotation mechanism
M2 can be common. Other components are similar to those of the
first embodiment.
[0103] In the wavelength dispersion compensating apparatus of the
above configuration, the channel lights of respective wavelengths
output from the VIPA plate 10 are angularly dispersed in the
direction perpendicular to the direction of angular dispersion in
the VIPA plate 10 by the variable dispersion diffraction grating
20, before being condensed on the reflective surface of the
three-dimensional mirror 32 by the focusing lens 31, and thus the
position on which the lights are condensed on the reflective
surface of the three-dimensional mirror 32 is displaced according
to wavelengths in a transverse direction in the top plan view in
FIG. 12. Furthermore, at the same time, the centerline directions
(angles) of focused beams of respective wavelengths differ from
each other according to respective wavelengths. Therefore, if the
consideration is made on the case of using the three-dimensional
mirror 32 having a reflective surface shape similar to that in the
first embodiment, there occur wavelengths at which the centerlines
of the focused beams are not incident perpendicularly on the
reflective surface of the three-dimensional mirror 32, so that, at
these wavelengths, the reflected beams are displaced in transverse
to occur losses. That is to say, the wavelength dependence of the
losses occurs.
[0104] In the present embodiment, therefore, the shape of the
entire reflective surface is curved so that the three-dimensional
mirror 32 is concaved along an arc, which is centered on the
reference point O of the variable dispersion diffraction grating 20
and passes through the center of the reflective surface (a face
averaging the concave-convex shape). Thus, even if the angular
dispersion occurs in the variable dispersion diffraction grating
20, and the centerline directions (angles) of the focused beams of
respective wavelengths differ from each other according to
wavelengths, since each focused beam is incident approximately
perpendicularly on the reflective surface of the three-dimensional
mirror 32, the reflected beam is not displaced in transverse, and
also a loss does not occur, so that the wavelength dependence of
the loss does not occur.
[0105] In the case where the three-dimensional mirror 32 whose
entire reflective surface having a concave-convex shape is curved
in a concave shape, is used, in order to vary the wavelength
dispersion, it is necessary to rotate the three-dimensional mirror
32 along the arc, which is centered on the reference point O of the
variable dispersion diffraction grating 20 and passes through the
center of the reflective surface, and not to slide the
three-dimensional mirror 32 in a straight line as in the first
embodiment. Moreover, also in the case where the wavelength
dispersion slope is varied, it is necessary to rotate the
three-dimensional mirror 32 to the optimum position according to a
change in the diffraction angle in the variable dispersion
diffraction grating 20. However, since this rotation is also the
rotation along an arc, which is centered on the reference point O
of the variable dispersion diffraction grating 20 and passes
through the center of the reflective surface of the
three-dimensional mirror 32, the rotation operation is exactly the
same as the case where the wavelength dispersion is varied.
Accordingly, it becomes possible to vary the wavelength dispersion
and wavelength dispersion slope independently utilizing the common
stage rotation mechanism M2. Thus, the configuration of the
wavelength dispersion compensating apparatus can be simplified,
miniaturized, and reduced in cost.
[0106] In the above fourth embodiment, the mirror slide mechanism
M1 and the stage rotation mechanism M2 are commonly used in the
configuration of the first embodiment using the acousto-optic type
variable dispersion diffraction grating 20. In a similar manner, it
is possible to use commonly the mirror slide mechanism M1 and the
stage rotation mechanism M2 in the configuration of the second
embodiment using the photo-refractive type variable dispersion
diffraction grating 40. A top plan view in FIG. 13 shows a
constitutional example of the wavelength dispersion compensating
apparatus in this case.
[0107] A fifth embodiment of the present invention will be
described.
[0108] FIG. 14 is a top plan view showing a configuration of a
wavelength dispersion compensating apparatus of the fifth
embodiment of the present invention. Furthermore, FIG. 15 is a
perspective view showing a configuration of a waveguide type VIPA
plate used in the configuration in FIG. 14.
[0109] As shown in the figures, in the wavelength dispersion
compensating apparatus of the present embodiment, the bulk type
configuration of the VIPA plate 10 is replaced with a waveguide
type configuration. Specifically, the optical fiber 1, the
collimate lens 2, and the cylindrical lens 3 for guiding the WDM
light to the VIPA plate 10 in the first embodiment are replaced by
a single waveguide 52 formed on a substrate 50. The VIPA plate 10
is then disposed on the substrate 50, which is positioned on an end
portion of the waveguide 52. The configuration of the VIPA plate 10
disposed on the substrate 50 is similar to the bulk type VIPA plate
10 used in the first embodiment. The lights multi-reflected by the
VIPA plate 10 are propagated through the slab waveguide 52 formed
on the surface of the substrate 50, to interfere with each other,
and are emitted from an end face of the substrate 50 at different
angles for each wavelength. The lights emitted from the substrate
50 pass through the cylindrical lens 53, to become parallel lights
angularly dispersed in the direction parallel to the surface of the
substrate 50. The light of each wavelength output from this
cylindrical lens 53 becomes the same condition as that of the light
output from the bulk type VIPA plate 10 in the first embodiment,
and is given to the variable dispersion diffraction grating 20, to
be angularly dispersed in a direction substantially perpendicular
to the surface of the substrate 50, and subsequently, in a manner
similar to the first embodiment, the light is condensed by the
focusing lens 31 to be reflected by the three-dimensional mirror
32. Thus, similar function and effect to those in the first
embodiment can be achieved even by the wavelength dispersion
compensating apparatus using the waveguide type VIPA plate 10.
[0110] A sixth embodiment of the present invention will be
described.
[0111] FIG. 16 is a perspective view showing a configuration of a
waveguide type VIPA plate and variable dispersion diffraction
grating used in a wavelength dispersion compensating apparatus of
the sixth embodiment of the present invention.
[0112] As shown in FIG. 16, the wavelength dispersion compensating
apparatus of the present embodiment is an application example in
which the variable dispersion diffraction grating 20 is also formed
on the substrate 50 on which the VIPA plate 10 is formed, in the
configuration of the above fifth embodiment. Specifically, a
material having an acousto-optic effect is used for the substrate
50, and an interdigital transducer 61 is disposed on the portion
where the slab waveguide 52 is formed in the surface of the
substrate 50. Then, a high frequency signal output from a drive
power source (not shown in figures) is supplied to the interdigital
transducer 61, to generate a surface acoustic wave, so that a
diffraction grating is formed on the slab waveguide 52 depending on
a variation of the refractive index of the material due to the
surface acoustic wave. Note, it is preferable that an absorbent
body 62 is disposed between the VIPA plate 10 and the interdigil
transducer 61 for preventing the propagation of the surface
acoustic wave to the VIPA plate 10 side.
[0113] In the above configuration, the channel lights of respective
wavelengths, which have been emitted from the VIPA plate 10 and
angularly dispersed, are output from the surface of the substrate
50 at different angles for each wavelength by the diffraction
grating due to the acousto-optic effect. FIG. 17 is a diagram
exemplarily showing states of channel lights of wavelengths
.lambda..sub.A through .lambda..sub.C output from the surface of
the substrate 50, in which (A) is a side view of the substrate 50
when viewing from a direction A of FIG. 16, and (B) is a side view
of the substrate 50 when viewing from a direction B of FIG. 16. In
this manner, the channel lights of respective wavelengths which
have been angularly dispersed in a direction parallel to the
surface of the substrate 50 in the VIPA plate 10 and the slab
waveguide 52, are further angularly dispersed in a direction
substantially perpendicular to the surface of the substrate 50 in
the variable dispersion diffraction grating 20 formed on the
substrate 50, to be output to the outside of the substrate.
Accordingly, the focusing lens 31 is arranged above the substrate
50 according to the diffraction angle in the variable dispersion
diffraction grating 20, and the output lights from the substrate 50
are condensed on the reflective surface of the three-dimensional
mirror 32, so that an optical system similar to that in the fifth
embodiment is formed. Thus, similar function and effect as in the
fifth embodiment is achieved, and also the configuration of the
apparatus can be simplified. Therefore, it becomes possible to
achieve the miniaturization and cost reduction of the wavelength
dispersion compensating apparatus.
[0114] In the above fifth and sixth embodiments, examples have been
shown in which the waveguide type configuration is applied to the
first embodiment. However, similarly, it is also possible to apply
the waveguide type configuration to the second through fourth
embodiments.
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