Optical communications – Receiver – Heterodyne
Reexamination Certificate
2000-08-30
2004-09-28
Sedighian, M. R. (Department: 2633)
Optical communications
Receiver
Heterodyne
C398S140000, C398S195000, C398S209000, C398S213000, C398S162000
Reexamination Certificate
active
06798994
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to an optical communication system, and more particularly to an optical communication system feasible for the satellite communication architecture today.
DESCRIPTION OF THE RELATED ART
Nowadays, microwave is the popular carrier wave for signal transmission used in the current satellite communication system. Taking an example of the Ka band, the message bandwidth of the Ka band applied in practical system is limited up to 155 Mbit/s. In addition, there will be a great deal of power consumed in the microwave transmission. Therefore, the signal has to be transmitted in packets as transmitting a large quantity of digital data. Accordingly, conventional microwave satellite signal transmission will cost considerable time and a large expenditure. With the rapid increase of the transmitted data quantity, the microwave communication gradually can not meet with the high-capacity data transmission requirements in the future. In particular, the bandwidth of optical fiber communication backbone on the ground has reached 40 Gbit/s, which is far larger than the bandwidth of microwave communication. Thus, if the coming satellite communication is intended to be combined with the ground optical fiber communication system for the purpose of constructing the global high speed communication backbone network, it is in an urgent need to enlarge the data transmission quantity. It would be apparent that using laser light as the carrier signal in a satellite communication system will become an inevitable trend for carrying large quantity data.
With relation to the optical communication techniques of the prior art, S. Arnon and N. S. Kopeika presented a design about an adaptive optical transmitter and receiver for space communication through thin clouds in order to overcome the interference caused by the cloud, in “Applied Optics”, Volume 36, No. 9, 1997, p. 1987-1993. Such a design requires that the information about cloud's variation must be acquired in advance so as to adjust the optical transmitter and receiver. Unfortunately, the acquisition of the information about cloud would be a very tough task. Moreover, the design is focused on the effect of weather, but other factors such as transmitter, satellite, receiver and so on are not taken into consideration.
S. Arnon, S. Rotman and N. S. Kopeika proposed a method for minimizing the bit error rate resulting from the satellite vibration, in “IEEE Transaction on Aerospace and Electronic Systems”, Volume 34, No. 2, 1998, p. 590-596. The method calculates the bit error rate of the demodulated signal in a calculating unit for adjusting the system parameter and minimizing the bit error rate. However, this proposal did not explore the weather issues and frequency drifting effect, and hence it is not applicable to the practical communications emerging between satellite and the earth.
In 1995, Van Deventer proposes a new scheme of optical mixing device having one photodetector for a heterodyne receiver in order to eliminate the optical polarization phenomenon, in U.S. Pat. No. 5,457,563. This scheme attaches two additional optical mixers to the transmitting end and separates the signal into two packets for transmitting these two packets through two different transmission paths. Subsequently, a photodetector and a heterodyne receiver are attached to the receiving end in order to demodulate the signal. Because these two packets are emitted from the same signal source, the optical polarization phenomenon can be eliminated by means of the signal time delay resulting from applying two different transmission paths. Consequently, this scheme is effective on condition the laser is in stable state.
Horiuchi et al. disclose a coherent optical receiver having optically amplified local oscillator signal, in U.S. Pat. No. 5,463,461. The coherent optical receiver is characterized in that an optical amplifier is employed to amplify a local oscillating laser's power and a heterodyne receiver consisting of a combination of an optical coupler and a photodetector is used to amplify the received weak signal. Hereinafter, an electrical signal amplifier stage is attached to the output end of the photodetector. The advantage of this invention is that the advantage of using a high-power laser can be obtained without boosting laser's emitting power. Nevertheless, this invention does not deal with the drawback of laser's unstable frequency, and thus it will be uneasy to demodulate the modulated signal accurately.
Taneya et al. disclose a system for emitting a beam in the air, in U.S. Pat. No. 5,457,561. The system utilizes a transmitting device to mix the local oscillating laser signal with the emitted signal and then the mixed signal is propagated to a frequency discriminating circuit to generate a differential frequency. Thereafter, a control circuit and a wavelength variable laser element are employed to modulate the emitted signal. Because the local oscillating laser at the receiving end is the same as the laser element for modulating the emitted signal, the modulated signal can be accurately demodulated. Because the system utilizes laser's wavelength to modulate the signal, it will not be usable on the condition of the laser's frequency drifting. Besides, the system is designed for the purpose of short-distance communication, it is also not suitable for satellite communication.
However, the oscillator laser light frequency drifting is very sensitive to the temperature variation. Besides, while the signal is propagating from the transmitting end to the receiving end, it will encounter lots of impacts from the atmosphere, and thus the optical receiver of a conventional coherent optical communication system is not easy to lock the received signal during the demodulation process. That would seriously affect the demodulation ability.
It is therefore tried by the applicant to deal with the above situations encountered by the prior art.
SUMMARY OF THE INVENTION
It is an object of the present invention to develop an optical communication system for minimizing the bit error rate.
It is another object of the present invention to develop an optical communication system for improving the data transmission quantity.
It is another further object of the present invention to develop an optical communication system for resolving the problem of laser frequency drifting.
It is still an object of the present invention to develop a method for communication with an optical signal.
In accordance with a first aspect of the present invention, the optical communication system includes an optical transmitting device for emitting an optical signal carrying an input data as a carrier signal, a first reference optical signal generator for emitting a first reference optical signal, wherein the first reference optical signal has a first beat frequency difference with the frequency of the carrier signal, an optical receiving device receiving the carrier signal and the first reference optical signal through an optical transmission path for retrieving the input data, and a second reference optical signal generator for emitting a second reference optical signal to the optical receiving device, wherein the frequency of the second reference optical signal can be adjusted according to the first beat frequency difference and a second frequency difference which is a frequency difference between the second reference optical signal frequency generated in the second reference optical signal generator and the carrier signal.
At the transmitting end of the optical communication system according to the present invention, the aforesaid optical transmitting device includes an electrical signal modulating device for modulating the input data to an electrical signal, a laser signal generator electrically connected with the electrical signal modulating device for transforming the electrical signal into a laser signal, and a 2×2 optical coupler optically connected with the electrical signal modulating device and the laser signal generator for
Tsao Shyh-Lin
Yu Hao-Chih
National Science Council
Sedighian M. R.
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