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Description  |
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BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an alignment adjusting system for use in
an optical system of an optical transceiver, in particular, which is used
for intersatellite communications.
2. Description of the Related Art
Conventionally, optical communications using a beam of laser light have the
following features:
(a) since the wavelength of the laser light is smaller than one thousandth
that of the microwave, antennas and transceivers can be downsized;
(b) since a wider frequency band can be used, communications with a lager
capacity can be performed in a higher speed; and
(c) since sharp laser beams are used, mutual interference among them can be
neglected.
However, to make the best use of the above features, it is necessary to
direct sharper laser beam to an antenna of the destination station with
higher accuracy. For example, in an optical communication between a
geostationary satellite and a low-earth-orbiting satellite, it is
necessary to provide an optical antenna having an aperture diameter
aperture of about 20 cm. When a conventionally used semiconductor laser
light with a wavelength of 0.83 .mu.m is used, the half-width of a beam of
transmitting laser light becomes approximately 5 micro-radian. In this
case, pointing and tracking with an accuracy of 1 micro-radian or less is
required to maintain a stable optical communication. Further, for an
optical transceiver to be provided in a low-earth-orbiting satellite, it
is necessary to provide a wider directivity range over 2.pi. steradian or
more in solid angle in the sphere.
In a conventional pointing and tracking system, in order to simultaneously
obtain a wider angle range and a higher tracking accuracy, there has been
used a control system having a double feedback loop, comprising:
(a) a coarse tracking system having a relatively slow response speed but
having a wider field of view, wherein the coarse tracking system comprises
a CCD sensor, and two-axes gimbals for controlling an optical antenna with
two directions; and
(b) a fine tracking system having a narrower field of view but having a
higher speed response, wherein the fine tracking system comprises a
quadrant detector sensor (hereinafter, referred to as a QD sensor)
composed of four photodiode cells divided into four quadrants, and a fine
pointing mirror module (hereinafter, referred to as an FPM).
Besides, a conventional center-fed cassegrain type optical antenna
comprises a main mirror consisting of a concave parabolic mirror, and a
submirror consisting of a convex hyperbolic mirror. In the center-fed
cassegrain type optical antenna, after a beam of transmitting laser light
outputted from an optical transceiver is led to the submirror through a
hole formed in the center of the main mirror, the beam of laser light is
reflected by the submirror, and then is reflected by the main mirror.
Thereafter, the beam of transmitting laser light is transmitted to an
optical antenna of a destination station.
Further, in a conventional pointing, acquisition and tracking control
system, the initial acquisition is performed by receiving a beacon light
having a wider field of view transmitted from a satellite of a destination
station. In this case, a CCD sensor is used as the acquisition sensor, and
then an error from the tracking center of the luminescent spot of the
beacon light which is outputted from the CCD sensor is detected.
Thereafter, based on the detected error, the two-axes gimbals mechanically
connected with the optical antenna are controlled to be driven, so that
the optical antenna is directed to the satellite of the destination
station. The beacon light is then captured within the field of view of the
QD sensor, and thereafter, tracking errors in two directions perpendicular
to each other are detected based on a relative ratio of amounts of light
incident on the respective photodiode cells of the QD sensor. Based on the
tracking errors, the driving mechanism of the FPM is controlled to be
driven so that a beam of laser light transmitted from the satellite of the
destination station is captured within the field of view of a receiving
photodiode sensor (hereinafter, photodiode sensor will be referred to as a
PD sensor). Further, a point-ahead mirror module (hereinafter, referred to
as a PAM) is used to correct a point-ahead angle of a beam of transmitting
laser light so as to be substantially zero, namely, to accurately
illuminate the destination station, which will be described in detail
later. Then, a beam of laser light is transmitted in the corrected
direction toward the optical antenna of the destination station, and then
the optical communication is started.
To maintain the tracking accuracy under severe environments such as a large
vibration at the launching of the satellite, a large variation in the
ambient temperature, so that optical communication can be carried out, it
is necessary to provide 1 .mu.m or less alignment accuracy for arrangement
of devices of the optical system including mirrors and lenses. For this
reason, conventionally, by using Invar (trademark: Ni-based alloy) or
Zerodur (trademark: glass ceramic material), each of which has a small
thermal expansion coefficient and has established performance results as a
material constituting precision optical equipment, such design has been
implemented that arrangement of optical devices will not change due to
change in temperature.
However, there have been such problems that the above-mentioned Invar has a
relatively large specific gravity while the above-mentioned Zerodur
encounters difficulty in processing the same into a complex shape.
Suited for structural materials for optical transceiver to be provided in
the satellite are light-weight metal materials with a specific gravity as
small as possible and a thermal conductivity as large as possible, such as
Al, Mg, Be or the like. However, the light-weight metal materials, in
general, have relatively large thermal expansion coefficients,
respectively, and therefore, it is necessary to provide an alignment
adjustment mechanism for compensating for alignment errors in equipment
arrangement due to thermal expansion in order to achieve optical
communications. However, there has been provided no proper method for
adjusting alignment of the optical transceiver after launching of the
satellite. Even if such adjustment is made possible in some way, severe
demand for accuracy would cause the optical transceiver provided
comprising an optical antenna to be relatively large sized because of the
provision of the alignment adjustment mechanism, disadvantageously.
SUMMARY OF THE INVENTION
An essential object of the present invention is therefore to provide an
alignment adjusting system for use in an optical system of an optical
transceiver, having a smaller size and a smaller weight than those of the
conventional apparatuses, and being capable of adjusting the arrangement
of the optical system with higher accuracy.
In order to achieve the aforementioned objective, according to one aspect
of the present invention, there is provided an alignment adjusting system
for use in an optical system in an optical transceiver, comprising:
an optical antenna having a main mirror and a submirror, both of which
being provided around an optical axis of said optical antenna and away
from each other by a predetermined interval, said optical antenna
transmitting a beam of transmitting light outputted from said optical
transceiver to an optical antenna of a destination station after
sequentially reflecting said beam of transmitting light by said submirror
and said main mirror;
an optical coupling means for optically coupling said optical antenna to
said optical transceiver;
a return-reflection mirror for return-reflecting a part of said beam of
transmitting light, said return-reflection mirror being provided in either
one of said main mirror and said submirror;
first detection means for detecting a defocusing amount of said beam of
transmitting light in said optical antenna, in response to said beam of
transmitting light return-reflected by said return-reflection mirror;
first moving means for moving said submirror in a direction parallel to the
optical axis of said optical antenna; and
first control means for controlling said first moving means so that the
defocusing amount of said beam of transmitting light becomes substantially
zero, in response to a defocusing amount of said beam of transmitting
light in said optical antenna which is detected by said first detection
means.
By the arrangement of the above-mentioned invention, when the reflection
mirror for return-reflecting a beam of transmitting light is provided in
the main mirror, the defocusing amount of the beam of transmitting laser
light can be corrected so as to be substantially zero over the optical
system from the optical transceiver through the optical coupling or
connection means to the submirror and the main mirror of the optical
antenna. On the other hand, when the reflection mirror for
return-reflecting a beam of transmitting light is provided in the
submirror, the defocusing amount of the beam of transmitting laser light
can be corrected so as to be substantially zero over the optical system
from the transceiver through the optical coupling or connection means to
the submirror of the optical antenna. As a result, the size of the
alignment adjusting system for use in the optical system in the optical
transceiver of the present invention can be made smaller and the weight
thereof can be made smaller than those of the conventional apparatuses,
and further the present invention is capable of adjusting arrangement of
the optical system located from the optical transceiver through the
optical coupling or connection means to the submirror or the main mirror
of the optical antenna with a higher accuracy. The provision of the
alignment adjusting system allows the main mirror and the submirror to be
made of light-weight metal materials each having a relatively smaller
specific gravity and a relatively larger thermal conductivity, such as Al,
Mg, Be or the like. Accordingly, the weight of the satellite in which the
optical transceiver is provided can be advantageously and remarkably
reduced.
Further, in the above-mentioned alignment adjusting system, said optical
antenna sequentially reflects a beam of received light received from said
optical antenna of said destination station by said main mirror and said
submirror, and thereafter, outputs said beam of reflected light to said
optical transceiver through said optical coupling means; and
said alignment adjusting system further comprising:
second detection means for detecting a deviation amount of said beam of
transmitting light from the optical axis of said optical antenna, in
response to said beam of transmitting light return-reflected by said
return-reflection mirror;
third detection means for detecting a deviation amount of said beam of
received light from the optical axis of said optical antenna in response
to said beam of received light reflected by said submirror;
second moving means for moving the optical axis of said beam of
transmitting light transmitting in said optical coupling means, said
second moving means being provided in said optical coupling means; and
second control means for controlling said second moving means so that the
deviation amount of said beam of transmitting light from the optical axis
of said optical antenna coincides with the deviation amount of said beam
of received light from the optical axis of said optical antenna, in
response to the deviation amount of said beam of transmitting light from
the optical axis of said optical antenna which is detected by said second
detection means and the deviation amount of said beam of received light
from the optical axis of said optical antenna which is detected by said
third detection means.
By the arrangement of the above-mentioned invention, when the reflection
mirror for return-reflecting a beam of transmitting laser light is
provided in the main mirror, the deviation amount of the beam of
transmitting light from the optical axis of the optical system can be
corrected so that the optical axis coincides with that of the beam of
received light, over the optical system from the optical transceiver
through the optical coupling or connection means to the submirror and the
main mirror of the optical antenna. On the other hand, when the reflection
mirror for return-reflecting a beam of transmitting light is provided in
the submirror, the deviation amount of the beam of transmitting laser
light from the optical axis of the optical system can be corrected so that
the optical axis of the beam of transmitting light coincides with that of
the beam of received light, over the optical system from the optical
transceiver through the optical coupling or connection means to the
submirror of the optical antenna. Accordingly, the size of the alignment
adjusting system for use in the optical system in the optical transceiver
of the present invention can be made smaller and the weight thereof can be
made smaller than those of the conventional apparatuses, and further the
present invention is capable of adjusting arrangement of the optical
system from the optical transceiver through the optical coupling or
connection means to the submirror or the main mirror of the optical
antenna with a higher accuracy.
Further, in the above-mentioned alignment adjusting system, said main
mirror is either one of a ring-shaped concave parabolic mirror and a
ring-shaped concave nearly-parabolic hyperbolic mirror;
said return-reflection mirror is either one of a ring-shaped concave
spherical mirror and a ring-shaped concave nearly-spherical elliptical
mirror, said return-reflection mirror being formed integrally together
with said main mirror and concentrically with said main mirror around the
optical axis of said optical antenna; and
said main mirror and said return-reflection mirror are arranged so that a
focal point of said main mirror coincides with the center of curvature of
said return-reflection mirror.
Still further, in the above-mentioned alignment adjusting system, said main
mirror is provided on an outer periphery of a ring-shaped body, while said
return-reflection mirror is provided on an inner periphery of said
ring-shaped body, and
said return-reflection mirror has an optical through hole for passing said
beam of transmitting light and said beam of received light therethrough,
said optical through hole having a center coinciding with the optical axis
of said optical antenna, said optical through hole being formed so as to
be concentric with said main mirror around the optical axis of said
optical antenna.
Still more further, in the above-mentioned alignment adjusting system, said
submirror is a convex hyperbolic mirror, and
said return-reflection mirror is either one of a concave spherical mirror
and a concave nearly-spherical elliptical mirror which is formed
integrally and concentrically with said submirror around the optical axis
of said optical antenna.
BRIEF DESCRIPTION OF THE DRAWINGS
These and other objects and features of the present invention will become
clear from the following description taken in conjunction with the
preferred embodiments thereof with reference to the accompanying drawings
throughout which like parts are designated by like reference numerals, and
in which:
FIG. 1 is a schematic block diagram showing an optical transceiver together
with an optical antenna and an adjusting system for an optical system of
the optical transceiver of a preferred embodiment according to the present
invention;
FIG. 2 is a longitudinal cross-sectional view showing details of the
optical antenna shown in FIG. 1;
FIG. 3 is a front view of an optical detection surface of a CCD sensor
shown in FIG. 1;
FIG. 4 is a front view of an optical detection surface of a QD sensor shown
in FIG. 1;
FIG. 5 is a longitudinal cross-sectional view of a convex parabolic mirror
showing an action thereof;
FIG. 6 is a longitudinal cross-sectional view of a convex spherical mirror
showing an action thereof;
FIG. 7 is a longitudinal cross-sectional view of a main mirror of an
optical antenna of the present invention comprising a convex parabolic
mirror and a convex spherical mirror;
FIG. 8 is a flowchart of a control flow showing an alignment adjusting
process which is executed by the adjusting system for the optical system
of the optical transceiver shown in FIG. 1; and
FIG. 9 is a graph showing a relative sensitivity on a reciprocal of a
wavefront radius in the preferred embodiment according to the present
invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
A preferred embodiment according to the present invention is described
hereinbelow with the accompanying drawings.
FIG. 1, showing a preferred embodiment of the present invention, is a
schematic block diagram of an optical transceiver comprising an optical
antenna 10 and an alignment adjusting system for an optical system for use
in optical communications.
The center-fed cassegrain type optical antenna 10 of the present preferred
embodiment, as shown in FIG. 2, comprises the followings:
(a) a main mirror or reflector 1 comprising a ring-shaped concave parabolic
mirror 1a for transmitting and receiving beams of lights, and a
ring-shaped concave spherical return-reflection mirror or retro-reflector
1b for turn-around-reflecting, retro-reflecting or return-reflecting a
beam of transmitting laser light, both of which are provided
concentrically around an optical axis CL of the beam of laser light, the
parabolic mirror 1a being provided on the outer periphery of the main
mirror 1, the concave spherical return-reflection mirror 1b being provided
on the inner periphery of the main mirror 1 and having an optical through
hole 1h in the center of the concave spherical return-reflection mirror
1b, and both of the parabolic mirror 1a and the spherical
return-reflection mirror 1b being formed integrally with each other; and
(b) a submirror or subreflector 2 made of a convex hyperbolic mirror
provided at a position shifted from the coincident position of the focal
point Fp of the parabolic mirror 1a and the center of curvature Rc of the
spherical return-reflection mirror 1b (for reference characters Fp and Rc,
See FIG. 7) toward the main mirror 1 around the optical axis CL.
In the center-fed cassegrain type optical antenna 10, a beam of
transmitting laser light outputted from the optical transceiver is led to
the submirror 2 through the optical through hole 1h, and then is
sequentially reflected by the submirror 2 and the parabolic mirror 1a of
the main mirror 1. Thereafter, a beam of transmitting laser light is
transmitted to an optical antenna of a destination station. On the other
hand, a beam of transmitting laser light which has been reflected by the
submirror 2 is reflected by the spherical return-reflection mirror 1b of
the main mirror 1, so as to be returned toward the submirror 2, and then
is reflected again by the submirror 2 so as to be incident toward the
optical transceiver. Then, based on the beam of returned transmitting
laser light (hereinafter, referred to as a beam of returned transmitting
laser light), an alignment adjustment for the optical system in the
optical transceiver is performed.
In the present preferred embodiment, it is noted that the wavelength of the
beam of the transmitting light outputted from a laser diode 37 is slightly
different from the wavelength of the beam of received light which is
transmitted from the optical transceiver of the destination station and is
received by the optical antenna 10 in such a degree that they can be
separated by respective optical filters provided in a CCD sensor 30 and a
QD sensor (quadrant detector sensor) 32.
Referring to FIG. 2, in the center-fed cassegrain type optical antenna 10,
the ring-shaped parabolic mirror 1a for transmitting and receiving beams
of laser lights is provided on the outer periphery of the main mirror 1 so
as to oppose to the submirror 2, namely, so as to be formed on the surface
of the main mirror 1 located on the side toward the direction of reception
and transmission of the optical antenna 10. Further, the ring-shaped
concave spherical return-reflection mirror 1b for return-reflecting a beam
of transmitting laser light, in which an optical through hole 1h of a
circular shape is formed having the center the optical axis CL passes
through, is provided on the inner periphery of the main mirror 1 so as to
oppose to the submirror 2, namely, so as to be formed on the surface of
the main mirror 1 located on the side toward the direction of reception
and transmission of the optical antenna 10.
In the optical antenna 10, the position of the focal point Fp of the
parabolic mirror 1a and the position of the center of curvature Rc of the
spherical return-reflection mirror 1b are located at a point (referred to
as a coincident point Fp=Rc hereinafter) so as to coincide with each
other. Further, the submirror 2 made of the convex hyperbolic mirror is
provided at a position shifted from the coincident position Fp=Rc on the
side toward a concave reflection surface of the main mirror 1, for
example, See FIG. 7. In the present preferred embodiment, the centers of
curvatures of the mirrors 1a and 1b are so set as to be different from
each other, and further, a ratio fp/fc of the focal length fp of the
parabolic mirror 1a to the focal length fc of the spherical
return-reflection mirror 1b is preferably set to a value slightly smaller
than 2.
As each of the materials for the main mirror 1 and the submirror 2, optical
glass is preferably used, such as the Zerodur or the like, having a
relatively smaller thermal expansion coefficient. However, when the
alignment adjusting system according to the present invention is provided,
this allows the use of light-weight metal materials, such as Al, Mg, Be or
the like, having a relatively smaller specific gravity and a relatively
larger thermal conductivity, in the application of mounting the alignment
adjusting system onto a satellite.
Further, if the two mirrors, that is parabolic mirror and spherical
return-reflection mirror, are processed simultaneously by an ultra
high-precision machining, mis-alignment between the two mirrors is
negligibly small and this good alignment accuracy will be maintained after
these mirrors are launched in the space because the above-mentioned larger
thermal conductivity will minimize the thermal deformation of the mirrors.
In general, the shape of the mirror surface of the optical antenna 10 can
be approximated by an axially symmetrical quadratic surface. Assuming that
the radius at any given position of the mirror surface from the optical
axis is .rho., the curvature of the center of the quadratic surface is C,
and the deformation parameter representing the shape of the quadratic
surface is K, then the depth of mirror surface Sag from the intersection
(vertex) between the quadratic surface and the optical axis of the mirror
surface can be represented by the following equation (1):
Sag=(C.multidot..rho..sup.2) / {1+(1-C.sup.2.rho..sup.2 K).sup.1/2
}+a.sub.6.rho..sup.6 +a.sub.8.rho..sup.8 +a.sub.10.rho..sup.10 +. . .
where a.sub.6 to a.sub.10 are coefficients representing higher-order
aspherical surfaces, and further the deformation parameter K has the
following meanings:
(a) If K<0, then the mirror surface of the main mirror 1 becomes a
hyperbolic surface;
(b) If K=0, then the mirror surface of the main mirror 1 becomes a
parabolic surface;
(c) If 0<K<1, then the mirror surface of the main mirror 1 becomes an
elliptical surface;
(d) If K=1, then the mirror surface of the main mirror 1 becomes a
spherical surface; and
(e) If K>1, then the mirror surface of the main mirror 1 becomes an
elliptical surface.
For example, when the main mirror 1 is provided comprising only a parabolic
mirror 51 having a focal length fp of 30 as shown in FIG. 5, like a
conventional optical antenna, a beam of transmitting light transmitted
from the focal point Fp toward the parabolic mirror 51 is reflected by the
parabolic mirror 51, and then, the transmitting light becomes collimated
parallel light, which is then transmitted to the optical antenna of the
destination station.
On the other hand, when the main mirror 1 is provided comprising only a
spherical mirror 52 with a focal length fc of 30 as shown in FIG. 6, a
beam of transmitting light transmitted from the center of curvature Rc of
the spherical mirror 52 toward the spherical mirror 52 is reflected by the
spherical mirror 52, and then, it is returned to the center of curvature
Rc along the same optical path.
In the present preferred embodiment, as shown in FIG. 7, the main mirror 1
has not only a parabolic mirror 51 provided on the outer periphery side of
the main mirror 1 but also a spherical mirror 52a provided on the inner
periphery side of the main mirror 1. In the preferred embodiment of FIG.
7, the focal length fp of the parabolic mirror 51 is equal to 30, and the
focal length fp thereof is set to be slightly larger than the focal length
fc of the spherical mirror 52a equal to 15. In this case, a beam of
transmitting light transmitted from the coincident position Fp=Rc of the
focal point Fp and the center of curvature Rc toward the parabolic mirror
51 and the spherical mirror 52a is reflected by the parabolic mirror 51,
and then, the transmitting light becomes a collimated parallel light,
which is then transmitted toward the optical antenna of the destination
station. The transmitting light is further reflected by the spherical
mirror 52a, and thereafter, it is returned to the center of curvature Rc
along the same optical path. The returned transmitting light is used as a
reference optical signal for the alignment adjustment for the optical
system in the optical transceiver, as described in detail later.
Further, in the optical antenna 10, a submirror driving mechanism 2d which
has been known to those skilled in the art is provided on the rear surface
of the submirror 2 being opposite to the mirror surface of the submirror
2. In response to a control signal from an MPU (microprocessing unit) 20,
the submirror driving mechanism 2d moves the submirror 2 in a direction
parallel to the optical axis CL coincident with the center axis of the
optical antenna 10 which is the Z-axis direction of FIGS. 1 and 2
(hereinafter, referred to as a Z-axis direction of the optical antenna
10), and further moves the submirror 2 in two axial directions orthogonal
to each other and perpendicular to the optical axis CL, wherein the two
axial directions have an origin of the mirror center of the submirror 2.
One of the two axial directions is, as shown in FIGS. 1 and 2, a X-axis
direction (referred to as an X-axis direction hereinafter) perpendicular
to the paper surface of the drawings, and another one thereof is a Y-axis
direction (referred to as a Y-axis direction hereinafter) defined on the
paper surface of the drawings.
Further, in the optical antenna 10, two-axes gimbals 1d are provided, the
angle of which is changed in response to a control signal from the MPU 20,
with the two axes in two directions orthogonal to each other which
respectively correspond to the X-axis direction and the Y-axis direction
of the optical antenna 10 and are parallel to the X-axis direction and the
Y-axis direction thereof. Still further, the optical antenna 10 is
provided with a hood 3 having a cylindrical shape concentric with the
optical axis CL of the optical antenna 10, wherein the hood 3 is formed so
as to cover the inner mirror surface of the main mirror 1 and the entire
submirror 2, for the purpose of preventing unnecessary signal light from
being incident into the inside of the optical antenna 10.
As shown in FIG. 1, the optical system for optically connecting the optical
transceiver and the optical antenna 10 comprises the followings:
(a) an eyepiece 4 composed of a single-convex lens 4 and a double-convex
lens 4b;
(b) an FPM (fine pointing mirror module) 11;
(c) three beam splitters 12, 13 and 14; and
(d) a PAM (point-ahead mirror module) 15.
The FPM 11 is provided for reflecting a beam of transmitting laser light
and a beam of received light so that a beam of returned transmitting light
sent from the eyepiece 4 is incident onto a reflecting mirror of the FPM
11 at an incident angle of 45.degree. and then goes out at an outgoing
angle of 45.degree. so as to be incident into the beam splitter 12. In the
FPM 11, a direction passing through the intersection between the mirror
surface of the reflection mirror of the FPM 11 and the optical axis CL and
being perpendicular to the paper surface of FIG. 1 is defined as a Y-axis
of the FPM 11, while a direction located on the mirror surface of the
reflection mirror of the FPM 11 and perpendicular to the Y-axis is defined
as an X-axis of the FPM 11. The FPM 11 comprises a driving mechanism (not
shown) for rotating the mirror surface of the reflection mirror thereof
around the X-axis and Y-axis of the FPM 11 in response to a control signal
sent from the MPU 20.
Further, the PAM 15 is provided for reflecting a beam of transmitting light
so that a beam of transmitting light generated and transmitted from the
laser diode 37 is incident onto a reflection mirror of the PAM 15 at an
incident angle of 45.degree. and then goes out at an outgoing angle of
45.degree. so as to be incident into the beam splitter 14. In the PAM 15,
a direction passing through the intersection between the mirror surface of
the reflection mirror of the PAM 15 and the optical axis CL and being
perpendicular to the paper surface of FIG. 1 is defined as a Y-axis of the
PAM 15, while a direction located on the mirror surface of the reflection
mirror of the PAM 15 and perpendicular to the Y-axis is defined as an
X-axis of the PAM 15. The PAM 15 comprises a driving mechanism (not shown)
for rotating the mirror surface of the reflection mirror thereof around
the X-axis and Y-axis of the PAM 15 in response to a control signal sent
from the MPU 20.
In the optical antenna 10, a beam of receiving light received by the
optical antenna 10 is reflected by the parabolic mirror 1a of the main
mirror 1 and then is directed toward the submirror 2, which then reflects
the beam of received light. The beam of received light reflected by the
submirror 2 transmits toward the eyepiece 4 approximately in a direction
parallel to the optical axis CL.
The beam of received light and the above-mentioned beam of returned
transmitting light are incident onto the FPM 11 through the eyepiece 4,
and then are reflected by the reflection mirror of the FPM 11. Thereafter,
these lights are incident into the beam splitter 12. The beam splitter 12
divides these incident lights into two beams of lights, one of which
(referred to as a beam of first light hereinafter) passes through the beam
splitter 12 as it is, and another one of which (referred to as a beam of
second light hereinafter) is reflected and extracted. A beam of second
light reflected by the beam splitter 12 is incident into the CCD sensor
30, while a beam of first light passing through the beam splitter 12 is
incident into the next beam splitter 13.
Then the beam splitter 13 divides the incident light into two beams of
lights, one of which (referred to as a beam of third light hereinafter)
passes through the beam splitter 13 as it is, and another one of which
(referred to as a beam of fourth light hereinafter) is reflected and
extracted. A beam of fourth light reflected by the beam splitter 13 is
incident into the QD sensor 32, while a beam of third light passing
through the beam splitter 12 is incident into the next beam splitter 14.
Further, the beam splitter 14 then reflects the beam of light incident
from the beam splitter 13, and makes the beam of reflected light be
incident into a PD sensor (photodiode sensor) 34.
On the other hand, a modulator 36 modulates the transmitting light having a
predetermined transmitting wavelength by a predetermined modulation method
such as an intensity modulation according to a transmission base band
signal inputted from an external apparatus, and then a beam of modulated
transmitting light is outputted from the laser diode 37. The beam of
modulated transmitting light is reflected by the reflection mirror of the
PAM 15, and thereafter, passes sequentially through the three beam
splitters 14, 13 and 12, then is incident onto the reflection mirror of
the FPM 11. Thereafter, the beam of transmitting light is reflected by the
reflection mirror of the FPM 11, and then is projected through the
eyepiece 4 onto the submirror 2 of the optical antenna 10.
The CCD sensor 30, as shown in FIG. 3, has a square-shaped photo-detector
plane consisting of an X-axis and a Y-axis corresponding to the X-axis and
the Y-axis of the optical antenna 10, respectively. The photo-detector
plane of the CCD sensor 30 comprises a plurality of photo-detector cells
located at predetermined equal intervals so as to be parallel to the
X-axis and the Y-axis, wherein each photo-detector cell is labeled with X
and Y coordinates on a plane in a shape of a predetermined grating. The
CCD sensor 30 further comprises two band-pass filters (not shown), one of
which passes therethrough only the above-mentioned beam of returned
transmitting light among the incident lights, and another one of which
passes therethrough only the above-mentioned beam of received light among
the incident light, namely, the beam of returned transmitting light is
separated from the beam of received light. Thereafter, the beam of
separated returned transmitting light is detected by the respective
photo-detector cell and then is converted into an electric signal, which
is outputted to a CCD processing circuit 31, while the beam of separated
received light is detected by the respective photo-detector cell and then
is converted into an electric signal, which is outputted to the CCD
processing circuit 31.
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