Optical: systems and elements – Deflection using a moving element – Using a periodically moving element
Reexamination Certificate
1998-10-22
2003-01-14
Chan, Jason (Department: 2633)
Optical: systems and elements
Deflection using a moving element
Using a periodically moving element
C359S199200
Reexamination Certificate
active
06507424
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an optical space communication apparatus which performs communication between two points remote from each other by propagating a beam of a light signal modulated by a transmitted signal, in the atmosphere.
2. Related Background Art
In general, communication making use of the light signal permits transmission of large-capacity information at high speed and, particularly, optical space communication using the free space as a transmission line has advantages of higher portability and easier installation of communication channel than wire communications using optical fibers and the like. Conventional communication apparatuses employ automatic tracking (autotracking) for controlling the angle of emission of the light beam so as to prevent the light signal from dropping off the apparatus, in order to improve the reliability of optical space communication.
FIG. 1
is a diagram showing the structure of an optical space communication apparatus having a tracking function in a conventional example, in which a lens
1
for transmission and reception is placed at a position opposite a party apparatus and in which a lens
2
and a movable mirror
3
are located on the optical path behind the lens
1
. A polarization beam splitter
4
, a lens
5
, and a light-emitting device
6
comprised of a semiconductor laser light source or the like are arranged in the direction of incidence of the movable mirror
3
. A beam splitting mirror
7
, a lens
8
, and a position detector
9
are arranged in the direction of reflection of the polarization beam splitter
4
, and a lens
10
and a light receiving element
11
are arranged in the direction of reflection of the beam splitting mirror
7
.
The output terminal of a multiplexer
12
is connected to the light-emitting device
6
, the output of a transmitted signal input terminal
13
is connected through an amplifier
14
to the multiplexer
12
, and the output of an oscillator
15
is also connected to the multiplexer
12
. Further, the output of the light receiving element
11
is connected through an amplifier
16
to a received signal output terminal
17
.
The output of position detector
9
is connected to a tracking control circuit
18
and the output of the tracking control circuit
18
is connected to the movable mirror
3
through two actuators
19
,
20
for varying the angle of the movable mirror
3
. A collimation scope
21
for an inspector to verify collimation of axis by vision is provided nearly in parallel with the optical axis of the movable mirror
3
.
On the occasion of transmission, the transmitted signal is input via the transmitted signal input terminal
13
, amplified by the amplifier
14
, and further multiplexed with a signal from the oscillator
15
in the multiplexer
12
. Thereafter, the resultant signal is outputted to the light-emitting device
6
. The light-emitting device
6
intensity-modulates its emitting light according to the input signal to convert it into a light signal. The beam from the light-emitting device
6
is guided through the lens
5
to the polarization beam splitter
4
. Since this beam is polarized in parallel with the plane of the drawing, it is transmitted by the polarization beam splitter
4
as it is. Then the beam is reflected to the left by the movable mirror
3
and then passes through the lenses
2
,
1
to be emitted toward the party apparatus.
On the occasion of reception, a light beam from the party apparatus is incident from the left into the lens
1
, passes through the lens
2
to be reflected downward by the movable mirror
3
, and then reaches the polarization beam splitter
4
. Since this beam is polarized normally to the plane of the drawing, it is reflected to the right by the bond face of the polarization beam splitter
4
to be split into two directions by the beam splitting mirror
7
. The beam reflected by the beam splitting mirror
7
passes through the lens
10
and then is received by the light receiving element
11
to be converted into an electric signal. Thereafter, the electric signal is amplified to an appropriate level by the amplifier
16
and the amplified signal is outputted from the received signal output terminal
17
.
On the other hand, the beam transmitted through the beam splitting mirror
7
is converged by the lens
8
to be received as a spot image S by the position detector
9
. The position detector
9
detects the position of the spot image S and outputs it as a position signal to the tracking control circuit
18
. The tracking control circuit
18
computes an angle of the light beam from the party apparatus with respect to the optical path of the present apparatus, based on this position signal, and makes driving signals for the actuators
19
,
20
. The actuators
19
,
20
adjust the angle of the movable mirror
3
so that the spot image S falls on the center of the position detector
9
.
With this adjustment, the position of the light-emitting device
6
is also adjusted, whereby the optical path of the emitted beam comes to agree with that of the incident beam. Therefore, the light beam is accurately emitted toward the party apparatus. If the apparatus is inclined during communication to shift the optical path of the received light to deviate the position of the spot image S on the position detector
9
from the center, then the movable mirror
3
will be moved immediately to successively correct the optical path of incidence of the light beam so as to receive the spot image S at the center of the position detector
9
, thereby preventing the incident light beam from dropping off the apparatus.
Here, the position detector
9
selectively detects only an ac pilot signal of a specific frequency in order to avoid influence of background light around the party apparatus, i.e., in order to prevent the position detection signal from being drawn to a strong background light so as to cause an error. This pilot signal is generated from the oscillator
15
and is multiplexed with the transmitted signal in the multiplexer
12
to be transmitted to the party apparatus.
FIG. 2
is a front elevation of a position detector
9
divided into four sections. The four segmental photodetecting elements
9
a
to
9
d
obtain the position of the spot image S by comparison of their outputs.
FIG. 3
shows a two-dimensional light position detector
9
e,
in which a vertical position of the spot light S on this position detector
9
e
is detected by a difference between input voltages at the positive input terminal and at the negative input terminal of amplifier
22
and a horizontal position thereof by a difference between input voltages at the positive input terminal and at the negative input terminal of amplifier
23
. In either case of FIG.
2
and
FIG. 3
, the background light, in addition to the spot light S, is also incident on the position detector
9
a
to
9
e,
and components of frequencies other than that of the pilot signal are removed by an electric circuit such as a filter or a frequency selective amplifier at the rear end of the position detector and are thus not detected.
The tracking function stated above does not work unless the light beam from the party apparatus arrives at a level that can be received and unless the spot image S impinges on a part of the position detector
9
. In initial adjustment during installation of the apparatus, an operator fixes the movable mirror
3
at an initial position near the middle point and manually adjusts the direction of the apparatus while observing the party apparatus through the collimation scope
21
.
The optical space communication apparatus of the conventional example described above, however, has the following problems, because it uses the pilot signal in order to avoid the influence of the background light.
(1) The apparatus has to incorporate an oscillator
15
and a multiplexer
12
of the pilot signal and the receiving section has to include a circuit for extracting only the pilot signal. Therefore, the cost is high.
(2)
Canon Kabushiki Kaisha
Chan Jason
Leung Christina Y
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