Optical communications – Transmitter and receiver system – Including alignment between transmitter and receiver
Reexamination Certificate
2000-06-05
2003-09-09
Pascal, Leslie (Department: 2633)
Optical communications
Transmitter and receiver system
Including alignment between transmitter and receiver
C398S122000, C398S124000, C398S129000, C398S183000
Reexamination Certificate
active
06616352
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to an optical transmission apparatus adapted to emit and receive a light beam modulated as a function of the information it carries. The present invention also relates to a bidirectional optical space transmission system comprising a pair of such optical transmission apparatus that are arranged at the opposite ends of a transmission path having a predetermined distance in order to bidirectionally transmit information.
2. Related Background Art
In optical space transmission systems comprising a pair of optical transmission apparatus arranged at the opposite ends of a transmission path, a signal to be transmitted is modulated into an optical signal by the transmission apparatus operating as signal sender and then a light beam carrying the optical signal is emitted to the other transmission apparatus operating as signal receiver. The emitted light beam is transmitted through the atmosphere and received by the transmission apparatus operating as signal receiver at the other end of the transmission path, which apparatus then demodulates the optical signal transmitted from the signal sender to complete the signal transmission through the atmosphere.
However, the transmission path of the light beam emitted from the optical transmission apparatus can be fluctuated by atmospheric disturbances. Additionally, the optical transmission apparatus can become deformed, if slightly, due to its temperature changes particularly when the apparatus is arranged on the roof of a building. Then, the direction in which it emits the light beam can be slightly shifted. If the transmission path of the light beam is shifted due to any of such external causes before the light beam gets to the signal receiver, the energy level of the signal received by the optical transmission apparatus operating as signal receiver is reduced and the signal transmission can be interrupted in worst cases.
Currently, such problems are avoided by making the light beam coming from the signal sender show a large beam diameter at the receiving apparatus and/or by using a high optical output level at the sending apparatus so that the receiving apparatus can receive the signal with a sufficient energy level.
However, the energy level of the optical signal transmitted from an optical transmission apparatus is limited in order to protect people from adverse effects of light beams and hence there may be occasions where the optical transmission apparatus cannot emit a light beam with a satisfactory energy level. In view of these circumstances, there have been devised bidirectional optical space transmission systems having an automatic tracking feature. With such a system, the optical transmission apparatus operating as signal sender is provided with a function of correcting the error, if any, in the angle of emitting the light beam so that the light beam may reliably get to the effective reception area of optical transmission apparatus operating as signal receiver if the diameter of the light beam is minimized at the receiving apparatus.
FIG. 1
of the accompanying drawings schematically illustrates the configuration of a known optical transmission apparatus used in a bidirectional optical space transmission system. Referring to
FIG. 1
, when transmitting a signal, the optical transmission apparatus multiplexes a pilot signal (auxiliary signal) output from its pilot signal generator
2
and a main signal input to its main signal input section
1
from an external signal source such as a computer by means of its multiplexing section
3
. The pilot signal is designed to correct the error in the angle with which the optical transmission apparatus emits a light beam from it for the main signal. A sinusoidal wave signal that is a narrow band signal is typically used for such a pilot signal.
The signal produced from the multiplexing section
3
is then transformed into an optical signal by electrooptic converting section
4
and the light beam carrying the optical signal is collimated by collimator lens
5
before it is transmitted to the optical transmission apparatus operating as signal receiver by way of beam splitter
6
, light beam transmission angle modifying section
7
and a group of lenses
8
.
The optical transmission apparatus operating as signal receiver has also the configuration illustrated in FIG.
1
. The light beam carrying the optical signal and transmitted from the signal sender is taken into the apparatus by means of a group of lenses
8
. The light beam is then reflected by beam splitter
6
by way of light beam transmission angle modifying section
7
and its optical path is separated from the optical path of the light beam emitted from electrooptic converting section
4
. Then, the main signal and the pilot signal carried by the light beam that is reflected by the beam splitter
6
are then separated from each other by beam splitter
9
and the main signal is transmitted through the beam splitter
9
and converted into an electric signal by signal receiving section
10
, which electric signal is then output from main signal output section
11
, while the pilot signal is reflected by the beam splitter
9
and detected by light beam transmission angle error detecting section
12
. Then, light beam transmission angle modifying section
7
is driven by angle of optical axis regulating drive control section
13
to correct the angle of emission of the light beam being transmitted at the start of and during the operation of the system.
Thus, in the known bidirectional optical space transmission system, the optical axis of the signal transmitting section and that of the signal receiving section are made to agree with each in each optical transmission apparatus so that the apparatus may detect and correct the relative angular error between the optical axis of the light beam it receives from the signal transmitting apparatus and that of its own signal receiving section. With such a regulating operation conducted at the opposite ends of the signal transmission path, each of the optical transmission apparatus can emit a light beam for signal transmission with its optical axis agreeing with that of the light beam emitted from the apparatus at the other end of the transmission path to reliably realize a bidirectional optical space transmission scheme.
In a bidirectional optical space transmission system adapted to regulate the angle of emission of the light beam being transmitted, the optical transmission apparatus transmitting a signal generally multiplexes a pilot signal and a main signal. Since the pilot signal is a narrow band signal if compared with the main signal, it can be detected with a high S/N ratio if the signal strength is weak. In other words, if the optical signal becomes weak and the main signal can no longer provide a required signal quality, the pilot signal can operate properly and maintain its quality control function even when its signal level is by far lower than that of the main signal. Additionally, the influence of background light can be reduced by detecting the angular error not directly by way of the light beam but by way of the pilot signal.
However, in known bidirectional optical space transmission systems having a configuration as described above, there arises a problem as pointed out below when a plurality of optical transmission apparatus are arranged adjacently in parallel at each of the opposite ends of the transmission path so that pairs of apparatus may exchange information independently between the opposite ends of the transmission path. If the power supply breaks down and is restored or the optical axis of the signal transmitting section and that of the signal receiving section are turned disagreeing with each other by violent vibrations or for some other cause, the apparatus can try to correct the relative angular error between the optical axis of its own signal receiving section and that of the light beam of a wrong signal transmitting apparatus at the other end of the transmission path because the apparatus does
Moritomo Kazuo
Myojo Toshihiko
Shigeta Junji
Takahashi Yasuhiro
Pascal Leslie
Singh Dalzid
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