JP2003198486A - Bidirectional light transmission system, light transmitter, and light receiver - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide bidirectional light transmission in a single wavelength in a light transmission system which transmits an optical signal with a passive optical fiber transmission line through the use of light transmitter and a plurality of light receivers. <P>SOLUTION: The light transmitter 1 inputs an outgoing signal such as a multi-channel video signal, etc., multiplexed on a frequency axis to a semi-conductor laser 3 and, outputs the optical signal with the light frequency modulated therein (outgoing FM modulation light). The light receivers 2 multiplex the outgoing FM modulation light outputted from the light transmitter with the output of the semi-conductor laser 15 which modulates the light frequency by an incoming signal input. The receivers obtain an FM signal with a frequency being a difference between the two light frequencies after heterodyne-detection. When the FM signal is FM-demodulated by an FM demodulator 18, a signal is obtained, which is provided by frequency-multiplexing the outgoing signal such as the multi-channel video signal that is inputted to the optical transmitter with an incoming signal that is inputted to the light receivers. <P>COPYRIGHT: (C)2003,JPO

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical transmission system, an optical transmission device, and an optical reception device which perform bidirectional communication using a passive double star (PDS) type optical fiber network, and more specifically, to both. Like a CATV system for terminals, a common signal is transmitted from a transmitting device placed on the station side to a plurality of receiving devices on the subscriber side, and different signals are respectively transmitted from the receiving device on the subscriber side to the transmitting device on the station side. Transmission system.

[0002]

2. Description of the Related Art The configuration of an optical CATV system using a general passive double star type optical fiber network is shown in FIG.
Shown in 1. This optical CATV system is a system for transmitting multi-channel analog and digital video signals multiplexed on the frequency axis. In the optical transmitter 101, the multi-channel video signal multiplexed on the frequency axis is transmitted by the semiconductor laser 103. It is converted into an optical signal and sent to the PDS type optical fiber network 110. This optical signal is branched into N by the star coupler 105 on the optical fiber path,
The branched optical signals are transmitted to the respective optical receiving devices 102 through the optical fibers. Optical receiver 10
In 2, the optical signal is photoelectrically converted by the photoelectric converter 104 to obtain the original multi-channel video signal.

However, this system has only one-way downlink signal transmission function, and cannot perform telephone service and Internet service which are provided by the conventional CATV system using a coaxial cable. Therefore, in order to solve this, a PDS type optical fiber network is used for the above-mentioned CATV.
The optical wavelength of the system should be set to 1.5 μm, and the two-way communication service system must be separately provided in the 1.3 μm band to realize the wavelength division multiplexing of the two systems (for example, NTT Review Vol.9). No.6 Nov.1997 pp104-1
12 etc.).

[0004]

However, in the above-mentioned conventional example, both wavelength bands of 1.3 μm / 1.5 μm must be used in order to realize bidirectionality of the CATV system, and the same fiber network will be used in the future. Makes it impossible to do another service.

Therefore, an object of the present invention is to provide bidirectional C using only a single wavelength such as 1.5 μm band (or 1.3 μm band).
An object of the present invention is to provide a bidirectional optical transmission system, an optical transmitter, and an optical receiver capable of ATV service.

[0006]

In order to achieve the above object, the present invention provides a bidirectional optical transmission system in which an optical transmitter and a plurality of optical receivers transmit an optical signal through a passive optical fiber transmission line. An optical transmission device, wherein a first semiconductor laser that FM-modulates an optical frequency of an optical signal according to a downstream original signal transmitted to the optical reception device, and a downstream FM optical signal output from the first semiconductor laser To the optical fiber transmission line, and outputs a first optical circulator that branches the upstream FM optical signal from the plurality of optical receiving devices received from the optical fiber transmission line, and a local optical signal of a fixed wavelength. A local laser; a first optical coupler for multiplexing the upstream FM optical signal branched by the first circulator and the local optical signal output by the local laser; A first optoelectric converter for obtaining a heterodyne beat signal corresponding to a difference in optical frequency between the upstream FM optical signal and the local optical signal from the optical signal multiplexed by the first optical coupler; From the heterodyne beat signal output by the opto-electric converter, the upstream FM modulation transmitted by each optical receiving device at a pass frequency corresponding to the difference between the wavelength of the local laser and the optical frequency output by each of the plurality of optical receiving devices. A plurality of band pass filters for obtaining signals, and a plurality of first FM demodulations for FM demodulating the upstream FM modulated signals output by each of the plurality of band pass filters and reproducing the upstream original signals transmitted by each optical receiving device. And a container.

Further, in order to achieve the above object, the present invention provides the optical receiver or the optical receiver in a bidirectional optical transmission system in which an optical transmitter and a plurality of optical receivers transmit an optical signal through a passive optical fiber transmission line. The optical receiving device according to claim 1, wherein a second semiconductor laser that FM-modulates an optical frequency of an optical signal according to an upstream original signal transmitted to the optical transmitting device, and the second semiconductor laser. A second optical coupler for branching the output FM optical signal into two, and a downlink FM optical signal of the optical transmitter transmitted from the optical fiber transmission line are passed by the second optical coupler. A second optical circulator that sends out one of the branched upstream FM optical signals to the optical fiber transmission line, and a downstream FM optical signal that has passed through the second optical circulator and the first optical coupler A third optical coupler that combines the other upstream FM optical signal and an optical signal that is multiplexed by the third optical coupler are used as a heterodyne beat signal with the downstream FM optical signal and the upstream FM optical signal. A second opto-electric converter that obtains an electric FM signal corresponding to the difference between the two optical frequencies, and FM demodulation of the FM signal output from the second opto-electric converter to obtain a downlink original signal and an uplink original signal. A second FM demodulator for reproducing a signal and a high-pass filter for extracting only a downlink original signal from the output of the FM demodulator are provided. With the above configuration, bidirectional optical transmission can be performed with a single wavelength by using N optical receiving devices with respect to the optical transmitting device by using the PDS type optical fiber network. Further optical transmitter,
Since both of the optical receivers perform reception by heterodyne detection when receiving an optical signal, highly sensitive reception is possible and the transmission distance and the number of branches of the star coupler can be increased.

The present invention also provides the optical transmitter according to claim 1, wherein the downlink F output from the first semiconductor laser is output.
A fourth optical coupler for branching the M optical signal into two and outputting one of the downlink FM optical signals to the first optical circulator;
A fifth optical coupler that branches the local optical signal output by the local laser into two and outputs one to the first optical coupler, and the other downlink FM optical signal that is branched by the fourth optical coupler. And a sixth optical coupler that multiplexes the other local optical signal branched by the fifth optical coupler, and an optical signal output from the sixth optical coupler, which is input to the sixth optical coupler. A third opto-electrical converter that outputs a heterodyne beat signal corresponding to the frequency of the difference between the two optical signals, and an optical frequency of the first semiconductor laser based on the heterodyne beat signal that is output by the third opto-electrical converter. A frequency discriminator that detects a frequency difference between the optical frequencies of the local laser and a frequency difference difference between the optical frequencies of the first semiconductor laser and the local laser detected by the frequency discriminator is zero. The so is properly constant first
And a temperature control circuit for controlling the temperature of the semiconductor laser. Since the optical frequency of the first semiconductor laser can be made constant by adding the above configuration, the frequency of the heterodyne beat signal obtained at the output of the third opto-electric converter of the optical receiving device also becomes constant, and stable reception is achieved. Is possible.

The present invention is also the optical receiving device according to claim 2, wherein the direct current component is extracted from the output of the second FM demodulator so that the optical frequencies of the downstream FM optical signal and the upstream FM optical signal are detected. Low-pass filter for detecting difference, downlink FM optical signal and uplink FM detected by the low-pal filter
And a temperature control circuit for controlling the temperature of the second semiconductor laser so that the difference between the optical frequencies of the optical signals becomes constant. Since the optical frequency of the second semiconductor laser can be made constant by adding the above configuration, the frequency of the heterodyne beat signal obtained at the output of the second optoelectric converter of the optical transmitter becomes constant and stable reception can be achieved. It will be possible. Further, with the above configuration, the first and second
The difference between the optical frequencies of the semiconductor laser and the local laser is constant. Therefore, the second of the optical transmitter
Since the frequency of the heterodyne beat signal obtained at the output of the opto-electrical converter and the frequency of the heterodyne beat signal obtained at the output of the third opto-electrical converter of the optical receiving device can be always constant, both the optical transmitting device and the optical receiving device can It becomes possible to receive a good optical signal.

The present invention is also characterized in that, in the optical transmitter according to claim 1 or 3, a seventh optical coupler is provided instead of the first optical circulator. The present invention also provides the optical receiving device according to claim 2 or 4,
An eighth optical coupler is provided in place of the second optical circulator. With these configurations, a bidirectional optical transmission system can be realized, and the system cost can be reduced because it is cheaper than the optical circulator.

According to a fifth aspect of the present invention, in the optical transmitter according to the fifth aspect, the first optical isolator is inserted between the first semiconductor laser and the seventh optical coupler. According to a sixth aspect of the present invention, in the optical receiving device according to the sixth aspect, a second optical isolator is inserted between the second semiconductor laser and the eighth optical coupler. With the above configuration, when an optical coupler is used instead of the optical circulator, it is possible to prevent the transmitted optical signal from being input to the semiconductor laser and causing a malfunction in the operation of the semiconductor laser.

According to the present invention, in the optical transmitter according to any one of claims 1, 3, 5 or 7, instead of the first FM demodulator, a level detection circuit in which an output voltage changes according to an input level amplitude. Is provided. With the above configuration,
By performing FM demodulation with the attenuation slope depending on the frequency of the bandpass filter and the level detection circuit, it is possible to receive the signal transmitted from the optical receiving device, as in the case of using the FM demodulator.

According to the present invention, in the optical transmitter according to any one of claims 1, 3, 5, 7 or 9, the local optical signal output by the local laser is branched into two, and one of them is the first optical signal. A ninth optical coupler for outputting to the coupler, a wavelength detector for detecting the wavelength of the other local optical signal branched by the ninth optical coupler, and an optical wavelength of the local optical signal detected by the wavelength detector is constant. And a second temperature control circuit for controlling the temperature of the local laser so that With the above configuration, the optical frequency of the optical output of the local laser is stabilized, so that the difference in optical frequency between the optical signal sent from the optical receiving device and the optical output of the local laser can be stabilized, and the second photoelectric The frequency of the heterodyne beat signal generated by the converter can be stabilized, and good reception is possible.

The present invention also provides claims 1, 3, 5, 7, 9
Alternatively, in the optical transmission device according to the tenth aspect, all or a part of the first, fourth, fifth, sixth, seventh, ninth optical coupler and the first optical circulator are integrated into an optical waveguide. It is characterized by being configured. The present invention also provides the optical receiving device according to claim 2, 4 or 6, wherein:
All or part of the third and eighth optical couplers and the second optical circulator are integrated to form an optical waveguide. By constructing the optical coupler and optical circulator with optical waveguides, it is possible to integrate them and downsize the optical transmitter and the optical receiver.

The present invention also provides claims 1, 3, 5, 7,
The optical transmission device according to claim 9, 10 or 11, and
A bidirectional optical transmission system having the optical receiving device according to item 4, 6 or 8.

According to the present invention, in the bidirectional optical transmission system according to claim 13, an original downlink signal sent by the optical transmitter to the optical receiver and an upstream signal sent by the optical receiver to the optical transmitter. It is characterized in that the two original signals do not overlap on the frequency axis. When the optical signal received by the optical receiving device and the optical signal output by the optical receiving device are multiplexed by the above method, heterodyne detection is performed by the third opto-electric converter, and the signal is FM demodulated, FM demodulation is performed. In the received signal, the original signal sent by the optical transmitter to the optical receiver and the original signal sent by the optical receiver to the optical transmitter do not interfere on the frequency axis, and the optical transmitter transmits the optical signal. The downlink signal sent to the receiving device can be reproduced without deterioration.

The present invention is also the bidirectional optical transmission system according to claim 13 or 14, wherein the optical frequency of the local optical signal output by the local laser and the upstream FM optical signal closest to the optical frequency of the local optical signal. Wavelengths of the respective optical signals are set so that the absolute value of the difference between the maximum value and the minimum value of the optical frequencies of the upstream FM optical signals of the plurality of optical receivers becomes smaller than the absolute value of the difference between the optical frequencies. It is characterized by According to the above method, the local laser light and the optical signal sent from the optical receiving device are combined to generate the first signal.
The signal obtained from the difference in frequency between the optical signal output by the local laser and the optical signal output by the optical receiving device in the signal obtained by the heterodyne detection by the optical-electrical converter of A signal generated due to a difference in frequency between optical signals does not interfere with each other on the frequency axis, and the upstream original signal sent from the optical receiving apparatus to the optical transmitting apparatus can be reproduced without deterioration.

The present invention is also the bidirectional optical transmission system according to any one of claims 13 to 15, wherein the downlink original signal is a carrier signal by an analog video signal.
It is characterized in that it is an M-modulated signal or a signal in which a carrier signal is QAM-modulated by a digital video signal is multiplexed on the frequency axis. A bidirectional CATV service or the like can be realized by the above method.

The present invention is also the bidirectional optical transmission system according to any one of claims 13 to 16, wherein the upstream original signal is a baseband digital signal. The above method enables data transmission from the optical receiving device to the optical transmitting device.

[0020]

BEST MODE FOR CARRYING OUT THE INVENTION <First Embodiment> An embodiment of the present invention will be described below with reference to the drawings. Figure 1
1 is a block diagram showing a first embodiment of a bidirectional optical transmission system according to the present invention, FIG. 2 is an explanatory diagram showing a downlink signal in the system of FIG. 1, and FIG. 3 is an explanation showing an upstream signal in the system of FIG. It is a figure.

In the system shown in FIG. 1, the N optical receiving devices (1) to (N) 2 are passive, with respect to the optical transmitting device 1.
They are connected by a double star (PDS) type optical fiber network 12. In the optical transmitter 1, by inputting a downstream signal such as a multi-channel video signal multiplexed on the frequency axis to the semiconductor laser 3, an optical signal (downstream FM modulated light) whose optical frequency is modulated is output. The downlink FM modulated light passes through the optical circulator 11 and is transmitted to the PDS type optical fiber network 12 as an output of the optical transmitter 1. In addition,
By configuring the optical transmission lines in the optical transmitter 1 and the optical receiver 2 and the optical elements such as the optical circulator 11 by the optical waveguide, the optical transmitter 1 and the optical receiver 2 can be integrated and downsized. .

The downlink FM modulated light output from the optical transmitter 1 is input to the N optical transmitters 2 through the star coupler 26 of the PDS type optical fiber network 12. In each optical receiver 2, the input downlink FM modulated light passes through the optical circulator 13 and is input to one of the optical couplers 14. In each of the optical receivers (1) to (N) 2, the semiconductor laser 15 modulated by the upstream signal input (1) to (N), respectively.
The upstream FM-modulated light of 1 is branched by the optical coupler 16, one of which is input to the other of the optical coupler 14 to be combined with the downstream FM-modulated light, and the other is passed through the optical circulator 13 to the PDS type optical fiber network 12. Is output.

The downstream FM-modulated light and the upstream FM-modulated light multiplexed by the optical coupler 14 are heterodyne-detected by the optical-electrical converter 17 to output a signal having a frequency difference between the two optical frequencies. The signal obtained here is an electrical signal that is FM-modulated by combining the downlink signal and the uplink signal. When this FM signal is FM-demodulated by the FM demodulator 18, a signal in which a downlink signal such as a multi-channel video signal input to the optical transmitter 1 and an upstream signal input to the optical receiver 2 are frequency-multiplexed is obtained. . Therefore, it is necessary to make the frequency bands of the downlink signal and the uplink signal different in advance. Therefore, the high-pass filter 25 extracts only the downlink signal from the signals output from the FM demodulator 18, and outputs the downlink signal such as a multi-channel video signal as the output of the optical receiver 2.

From the optical receiver 2 to the PDS type optical fiber network 1
The upstream FM-modulated light output to 2 is combined with the FM-modulated light output from the other optical receiving device 2 by the star coupler 26 on the PDS type optical fiber network 12 and sent to the optical transmitting device 1. In the optical transmitter 1, the light is input to one of the optical couplers 21 by the optical circulator 11. Optical coupler 21
Then, a total of N + 1 optical signals of the upstream FM-modulated light from the N optical receiving devices 2 and the local optical signals output by the local laser 5 are combined, and then these optical signals are converted into optoelectric converters 2.
2, the heterodyne detection is performed, and a heterodyne beat signal having a frequency generated by the combination of the optical frequency differences of N + 1 waves is obtained.

Among them, each heterodyne beat signal generated by the combination of the optical frequency difference between the upstream FM modulated light output from each optical receiving device 2 and the local optical signal is respectively passed through the band pass filters (1) to (N) 23. Extract. Here, the extracted signal is a signal having a frequency difference of the optical frequency of the local light having a constant wavelength from one FM-modulated light, and therefore each of them is passed through the FM demodulators (1) to (N) 24. Therefore, when FM demodulation is performed, the optical receiving devices (1) to (N) 2
It is possible to obtain the respective upstream signals (1) to (N) input to the. That is, the bandpass filter 23 and the FM demodulator 24 are provided for each of the frequencies of the difference between the upstream FM-modulated light output by the optical transmitter 2 and the local optical signal output by the local laser 5 to provide N optical signals. Each uplink signal transmitted from the receiver 2 can be received.

FIG. 2 shows an example of a spectrum of an electric signal and an optical signal in the process in which the downlink signal input to the optical transmitter 1 is transmitted and the downlink signal is output again by the optical receiver 2. FIG. 2A shows 90 MHz input to the optical transmitter 1.
2B shows a multi-channel video signal multiplexed on the frequency axis of ˜800 MHz, and FIG. 2B shows an optical signal (downstream FM modulated light) in which this video signal is modulated by the semiconductor laser 3 with a width of about 1 to 4 GHz. Indicates. This downlink FM modulated light is transmitted through the PDS type optical fiber network 12 and input to the optical receiver 2.

On the other hand, in the optical receiver 2, the baseband digital signal of the upstream signal as shown in FIG. 2C is similarly output by the semiconductor laser 15 as shown in FIG. An optical signal (upstream FM modulated light) modulated by the width is output. A part of it is branched by the optical coupler 16,
Downlink FM signal input to the optical receiver 2 and the optical coupler 14
When combined at, the spectrum becomes as shown in FIG.

When the two optical signals are heterodyne-detected by the photoelectric converter 17, an electric signal corresponding to the difference between the optical frequencies of the two optical signals is obtained as shown in FIG. 2 (f).
Since the two optical signals input at this time are both FM-modulated, an FM signal obtained by adding two FM-modulating components is obtained. When this FM signal is demodulated by the FM demodulator 18, a signal in which the down signal and the up signal are multiplexed on the frequency axis is obtained as shown in FIG. Then Fig. 2
By extracting only the high frequency downlink signal of the signal shown in (g), the optical receiving apparatus 2 can obtain the downlink signal as shown in FIG. 2 (h).

FIG. 3 shows an example of a spectrum of an electric signal and an optical signal in the process in which the upstream signal input to the optical receiver 2 is transmitted and the upstream signal is output again by the optical transmitter 1. As shown in FIG. 3A, the upstream signal input to the optical receiving device 2 is an optical signal (the optical frequency of which is modulated by the semiconductor laser 15 as shown in FIG. Upstream FM modulated light). This upstream FM modulated light is PD
As shown in FIG. 3 (c), the total N waves transmitted from the S-type optical fiber network 12 and output from the other optical receiving device 2 are combined with FM-modulated lights having different central wavelengths. Such N-wave upstream FM modulated light is input to the optical transmitter 1.

In the optical transmitter 1, this and FIG.
When the optical coupler 21 combines the local signal light output from the local laser 5 having a constant wavelength as shown in (d),
The spectrum is as shown in FIG. Then
When this is heterodyne-detected by the optical-electrical converter 22, an electric signal is obtained at the frequency of the difference between the respective optical frequencies shown in FIG. 3E, and as shown in FIG. A signal in combination with the local signal light or a combination of the upstream FM modulated lights can be obtained. Here, the required signal is a combination of the local signal light and the upstream FM modulated light, and the signal obtained by this combination is obtained by converting the frequency modulation of each upstream FM modulated light into an electrical FM signal. Therefore, the electric FM signal is extracted by the bandpass filter 23 as shown in FIG. 3 (g), and then demodulated by the FM demodulator 24 as shown in FIG. The signal is obtained.

As described above, in the bidirectional optical transmission system of the present invention, bidirectional optical transmission can be realized in the PDS type optical fiber network 12 with only a single wavelength band. Furthermore, since both the optical transmitter 1 and the optical receiver 2 perform reception by heterodyne detection when receiving an optical signal, highly sensitive reception is possible, and the transmission distance and the number of branches of the star coupler can be increased.

<Second Embodiment> FIG. 4 shows a second embodiment of the present invention.
2 shows a bidirectional optical transmission system of the embodiment. The bidirectional optical transmission system of the second embodiment is different from the optical transmitter 1 of the bidirectional optical transmission system of the first embodiment in that the semiconductor laser 3 is provided between the semiconductor laser 3 and the optical circulator 11. Optical coupler 4 that branches a part of the output optical signal
And an optical coupler 6 that branches a part of the optical signal output by the local laser 5 between the local laser 5 and the optical coupler 21.
And an optical coupler 7 that multiplexes the optical signals output by the optical coupler 4 and the optical coupler 6, and from the optical signal output by the optical coupler 7 to the frequency of the difference between the two optical signals input to the optical coupler 7. An optoelectric converter 8 that outputs a corresponding heterodyne beat signal, and a frequency discriminator that detects the frequency of the difference between the optical frequency of the semiconductor laser 3 and the optical frequency of the local laser 5 from the heterodyne beat signal output by the optoelectric converter 8. 9
And a temperature control circuit 10 for controlling the temperature of the semiconductor laser 3 so that the frequency of the difference between the optical frequency of the semiconductor laser 3 detected by the frequency discriminator 9 and the optical frequency of the local laser 5 becomes zero or constant. ing. Optical receiver 2
Has the same configuration as that of the first embodiment.

The above configuration is added to the optical transmitter 1 so that the difference between the optical center frequencies of the downlink FM modulated light output from the semiconductor laser 3 and the local signal light output from the local laser 5 is controlled to be constant. As a result, the center frequency of the FM signal in the heterodyne beat signal generated by the output of the optoelectric converter 17 of the optical receiving device 2 also becomes constant, and stable reception becomes possible.

<Third Embodiment> FIG. 5 shows a third embodiment of the present invention.
2 shows a bidirectional optical transmission system of the embodiment. In the bidirectional optical transmission system of the third embodiment, the optical receiver 2 of the bidirectional optical transmission system of the first embodiment is
A low-pass filter 19 for detecting a difference in optical frequency between the optical signal transmitted from the optical transmitter 1 and the optical signal output by the semiconductor laser 15 by extracting a DC component from the output of the M demodulator 18, and a low-pass filter 19 The temperature control circuit 20 for controlling the temperature of the semiconductor laser 15 is added so that the difference between the optical frequency of the optical signal transmitted from the optical transmitter 1 detected in step 1 and the optical frequency of the optical signal output by the semiconductor laser 15 becomes constant. There is. The optical transmitter 1 has the same configuration as that of the first embodiment.

Since the optical frequency of the upstream FM modulated light output from the semiconductor laser 15 can be made constant by adding the above configuration to the optical receiving device 2, it is generated by the output of the photoelectric converter 22 of the optical transmitting device 1. FM in heterodyne beat signal
The center frequency of the signal is also constant and stable reception is possible.

<Fourth Embodiment> FIG. 6 shows a fourth embodiment of the present invention.
2 shows a bidirectional optical transmission system of the embodiment. In the bidirectional optical transmission system of the fourth embodiment, the semiconductor laser 3 outputs between the semiconductor laser 3 and the optical circulator 11 in the optical transmitter 1 of the bidirectional optical transmission system of the first embodiment. Optical coupler 4 for branching a part of optical signal
And an optical coupler 6 that branches a part of the optical signal output by the local laser 5 between the local laser 5 and the optical coupler 21.
And the optical coupler 7 that combines the optical signals output by the optical coupler 4 and the optical coupler 6, and the optical signal output by the optical coupler 7 corresponds to the frequency of the difference between the two optical signals input to the optical coupler 7. Optical-electrical converter 8 for outputting a heterodyne beat signal
A frequency discriminator 9 for detecting the difference frequency between the optical frequency of the semiconductor laser 3 and the optical frequency of the local laser 5 from the heterodyne beat signal output from the opto-electric converter 8, and the semiconductor laser 3 detected by the frequency discriminator 9. A temperature control circuit for controlling the temperature of the semiconductor laser 3 is added so that the frequency of the difference between the optical frequency of 1 and the optical frequency of the local laser 5 becomes zero or constant.

Further, in the optical receiver 2, the FM demodulator 18
A low-pass filter 19 for detecting a difference between the optical frequencies of the downlink FM-modulated light transmitted from the optical transmitter 1 and the uplink FM-modulated light output from the semiconductor laser 15 by extracting a DC component from the output of The temperature control circuit 20 for controlling the temperature of the semiconductor laser 15 is added so that the difference between the optical frequency of the optical signal transmitted from the optical transmitter 1 detected in step 1 and the optical frequency of the optical signal output by the semiconductor laser 15 becomes constant. There is. That is, the optical transmitter 1 of the second embodiment and the optical receiver 2 of the third embodiment are combined.

By adding the above configuration to the optical transmitter 1 and the optical receiver 2, the difference between the optical frequencies of the semiconductor lasers 3, 15 and the local laser 5 becomes constant.
Therefore, the frequency of the heterodyne beat signal obtained by the opto-electric converter 22 of the optical transmitter 1 and the frequency of the heterodyne beat signal obtained by the opto-electric converter 17 of the optical receiver 2 can be kept constant, so that the optical transmitter 1, Both optical receivers 2 can receive good optical signals.

<Fifth Embodiment> FIG. 7 shows a fifth embodiment of the present invention.
2 shows a bidirectional optical transmission system of the embodiment. In the bidirectional optical transmission system of the fifth embodiment, an optical coupler 27 is used instead of the optical circulator 11 in the optical transmitter 1 of the bidirectional optical transmission system of the first embodiment, and an optical receiver 27 in the optical receiver 2. An optical coupler 28 is provided instead of the circulator 13. A bidirectional optical transmission system can also be realized by these configurations, and further, the optical couplers 27, 2
Since 8 is cheaper than the optical circulators 11 and 13, the system cost can be reduced.

<Sixth Embodiment> FIG. 8 shows a sixth embodiment of the present invention.
2 shows a bidirectional optical transmission system of the embodiment. In the bidirectional optical transmission system of the sixth embodiment, in the optical transmitter 1 of the bidirectional optical transmission system of the fifth embodiment,
An optical isolator 2 is provided between the optical coupler 27 and the semiconductor laser 3.
The optical receiving device 2 is characterized in that the optical isolator 30 is inserted between the optical coupler 28 and the semiconductor laser 15.

With the above structure, the optical circulator 1
When optical couplers 27 and 28 are used instead of 1 and 13,
It is possible to prevent the transmitted optical signal from being input to the semiconductor laser 3 and the semiconductor laser 15 and causing a malfunction in the operation of the semiconductor lasers 3 and 15.

<Seventh Embodiment> FIG. 9 shows a seventh embodiment of the present invention.
2 shows a bidirectional optical transmission system of the embodiment. In the bidirectional optical transmission system according to the seventh embodiment, the FM demodulators (1) to (N) 24 in the optical transmitter 1 of the bidirectional optical transmission system according to the first to sixth embodiments are input level amplitude. Level detection circuits (1) to (N) whose output voltage changes according to
31 instead of 31.

With the above structure, the bandpass filter 23
By performing FM demodulation with the attenuation slope according to the frequency and the level detection circuit 31, it is possible to receive the signal transmitted from the optical receiving device 2 as in the case of using the FM demodulator 24.

<Eighth Embodiment> FIG. 10 shows a bidirectional optical transmission system according to an eighth embodiment of the present invention. In the bidirectional optical transmission system of the eighth embodiment, in the bidirectional optical transmission system of the first to seventh embodiments, the optical transmitter 1 branches a part of the optical signal output by the local laser 5. The optical coupler 21, the wavelength detector 32 that monitors the wavelength of the optical signal output by the local laser 5 branched by the optical coupler 21, and the local laser 5 detected by the wavelength detector 32.
Is added to the temperature control circuit 33 for controlling the temperature of the local laser 5 so that the light wavelength is constant.

Since the optical frequency of the optical output of the local laser 5 is stabilized by the above configuration, the difference in optical frequency between the optical signal sent from the optical receiving device 2 and the optical output of the local laser 5 can be stabilized, The frequency of the heterodyne beat signal generated by the optoelectric converter 22 can be stabilized, and good reception is possible.

Further, according to the present invention, the local laser 5
From the absolute value of the difference between the optical frequency of the local optical signal and the optical frequency of the closest upstream FM optical signal that is output from the optical frequency of the plurality of optical receiving devices 2 By setting the wavelengths of the respective optical signals so that the absolute value of the difference between the value and the minimum value becomes small, the local laser light and the optical signal sent from the optical receiving device 2 are combined to generate the first optical signal. In the signal obtained by the heterodyne detection by the electric converter 22, the signal obtained from the difference in frequency between the optical signal output by the local laser 5 and the optical signal output by the optical receiving device 2 and the optical receiving device 2 The signal generated due to the difference in frequency between the output optical signals does not interfere on the frequency axis, and the upstream original signal sent to the optical transmitter 1 by the optical receiver 2 can be reproduced without deterioration.

Further, according to the present invention, as the downlink original signal, a signal obtained by AM-modulating a carrier signal with an analog video signal or a QA signal with a carrier signal with a digital video signal.
By using a signal obtained by multiplexing the M-modulated signal on the frequency axis, a bidirectional CATV service or the like can be realized. Furthermore, by using a baseband digital signal as the upstream original signal, data transmission from the optical receiver 2 to the optical transmitter 1 becomes possible.

[0048]

As described above, according to the present invention, P
It is possible to realize a bidirectional optical transmission system with only a single wavelength band in the DS type optical fiber network. Further optical transmitter,
Since both of the optical receivers perform reception by heterodyne detection when receiving an optical signal, highly sensitive reception is possible and the transmission distance and the number of branches of the star coupler can be increased.

[Brief description of drawings]

FIG. 1 is a block diagram showing a first embodiment of a bidirectional optical transmission system according to the present invention.

2 is an explanatory diagram showing a spectrum of a downlink signal in the system of FIG.

FIG. 3 is an explanatory diagram showing a spectrum of an upstream signal in the system of FIG.

FIG. 4 is a block diagram showing a bidirectional optical transmission system according to a second embodiment of the present invention.

FIG. 5 is a block diagram showing a bidirectional optical transmission system according to a third embodiment of the present invention.

FIG. 6 is a block diagram showing a bidirectional optical transmission system according to a fourth embodiment of the present invention.

FIG. 7 is a block diagram showing a bidirectional optical transmission system according to a fifth embodiment of the present invention.

FIG. 8 is a block diagram showing a bidirectional optical transmission system according to a sixth embodiment of the present invention.

FIG. 9 is a block diagram showing a bidirectional optical transmission system according to a seventh embodiment of the present invention.

FIG. 10 is a block diagram showing a bidirectional optical transmission system according to an eighth embodiment of the present invention.

FIG. 11 is a block diagram showing a PDS optical transmission system.

[Explanation of symbols]

1 Optical transmitter 2 Optical receiver 3,15 Semiconductor laser 4, 6, 7, 14, 16, 21, 27, 28 Optical coupler 5 local laser 8, 17, 22 photoelectric converter 9 frequency discriminator 10, 20, 33 Temperature control circuit 11,13 Optical circulator 12,110 PDS type optical fiber network 18, 24 FM demodulator 19 Low-pass filter 23 bandpass filter 25 high pass filter 26, 105 Star coupler 29,30 Optical Isolator 31 Level detection circuit 32 wavelength detector

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Claims (17)

[Claims] 1. An optical transmitter in a bidirectional optical transmission system, wherein an optical transmitter and a plurality of optical receivers transmit an optical signal through a passive optical fiber transmission line, wherein the optical frequency of the optical signal is the optical signal. A first semiconductor laser that performs FM optical modulation according to a downlink original signal transmitted to a receiving device, and a downlink FM optical signal output from the first semiconductor laser is output to the optical fiber transmission line and the optical fiber transmission is performed. A first optical circulator for branching the upstream FM optical signals from the plurality of optical receivers received from the optical path, a local laser for outputting a local optical signal of a fixed wavelength, and an upstream F for branching by the first circulator.
A first optical coupler for multiplexing the M optical signal and the local optical signal output by the local laser; and an optical signal multiplexed by the first optical coupler, from the upstream FM optical signal and the local optical signal. A first opto-electrical converter that obtains a heterodyne beat signal corresponding to the difference in optical frequencies, and a wavelength of the local laser and the plurality of optical receivers from the heterodyne beat signal output by the first opto-electrical converter. A plurality of band pass filters for obtaining the upstream FM modulated signals transmitted by the respective optical receiving devices at a pass frequency corresponding to the difference between the optical frequencies output by the respective devices, and an upstream FM output by each of the plurality of band pass filters. An optical transmitter of a bidirectional optical transmission system, comprising: a plurality of first FM demodulators that demodulate a modulated signal by FM and reproduce the upstream original signal transmitted by each optical receiver. 2. The optical receiver according to claim 1, or the optical receiver in a bidirectional optical transmission system in which an optical transmitter and a plurality of optical receivers transmit an optical signal through a passive optical fiber transmission line. A second semiconductor laser that FM-modulates the optical frequency of the optical signal in accordance with the upstream original signal that is transmitted to the optical transmitter, and the upstream FM optical signal that is output by the second semiconductor laser.
A second optical coupler branched into two, and one upstream FM light branched by the second optical coupler while allowing a downstream FM optical signal of the optical transmitter transmitted from the optical fiber transmission line to pass therethrough. A second optical circulator for sending a signal to the optical fiber transmission line, a downstream FM optical signal that has passed through the second optical circulator, and the other upstream FM branched by the first optical coupler.
A third optical coupler that multiplexes an optical signal, and a difference between two optical frequencies as a heterodyne beat signal in the downlink FM optical signal and the upstream FM optical signal from the optical signal multiplexed by the third optical coupler. A second opto-electrical converter for obtaining an electric FM signal corresponding to the frequency, and a second opto-electrical demodulator for FM demodulating the FM signal output by the second opto-electrical converter to regenerate a down original signal and an up original signal. 2. An optical receiver comprising: the FM demodulator and the high-pass filter that extracts only the downlink original signal from the output of the FM demodulator. 3. The optical transmitter according to claim 1, wherein the downlink FM optical signal output from the first semiconductor laser is 2
A fourth optical coupler that branches into two, and outputs one of the downlink FM optical signals to the first optical circulator; and a local optical signal that is output from the local laser into two, and one of the first optical circulator and the first optical circulator. A fifth optical coupler for outputting to the optical coupler, a sixth downstream FM optical signal branched by the fourth optical coupler, and a sixth local optical signal multiplexed by the other local optical signal branched by the fifth optical coupler. And an optical optocoupler for outputting a heterodyne beat signal corresponding to the frequency of the difference between the two optical signals input to the sixth optical coupler from the optical signal output by the sixth optical coupler. A frequency discriminator for detecting a frequency difference between an optical frequency of the first semiconductor laser and an optical frequency of the local laser from a heterodyne beat signal output from the third photoelectric converter, and the frequency discriminator. And a temperature control circuit for controlling the temperature of the first semiconductor laser so that the frequency of the difference between the optical frequency of the first semiconductor laser detected by and the optical frequency of the local laser becomes zero or constant. An optical transmission device characterized by the above. 4. The optical receiving device according to claim 2, wherein a DC component is extracted from the output of the second FM demodulator to detect a difference in optical frequency between the downlink FM optical signal and the uplink FM optical signal. A low-pass filter for controlling the second FM signal so that the difference between the optical frequencies of the downstream FM optical signal and the upstream FM optical signal detected by the low-pal filter becomes constant.
And a temperature control circuit for controlling the temperature of the semiconductor laser. 5. The optical transmission device according to claim 1, wherein a seventh optical coupler is provided instead of the first optical circulator. 6. The optical receiver according to claim 2, wherein an eighth optical coupler is provided instead of the second optical circulator. 7. The optical transmission device according to claim 5, wherein a first optical isolator is inserted between the first semiconductor laser and the seventh optical coupler. 8. The optical receiving device according to claim 6, wherein a second optical isolator is inserted between the second semiconductor laser and the eighth optical coupler. 9. The optical transmitter according to claim 1, 3, 5 or 7, wherein a level detection circuit whose output voltage changes according to an input level amplitude is provided instead of the first FM demodulator. An optical transmission device characterized by the above. 10. The optical transmitter according to claim 1, 3, 5, 7 or 9, wherein a local optical signal output by the local laser is branched into two and one is output to the first optical coupler. A ninth optical coupler, a wavelength detector for detecting the wavelength of the other local optical signal branched by the ninth optical coupler, and an optical wavelength of the local optical signal detected by the wavelength detector to be constant. And a second temperature control circuit for controlling the temperature of the local laser. 11. The optical transmitter according to claim 1, 3, 5, 7, 9 or 10, wherein the first, fourth, fifth, sixth, seventh and ninth optical couplers and the first optical coupler are provided. An optical transmission device characterized in that all or part of the optical circulator of (1) is integrated into an optical waveguide. 12. The optical receiver according to claim 2, 4 or 6, wherein all or part of the second, third and eighth optical couplers and the second optical circulator are integrated into an optical waveguide. An optical receiving device characterized by being configured. 13. Bidirectional light comprising: the optical transmitter according to claim 1, 3, 5, 7, 9, 10 or 11, and the optical receiver according to claim 2, 4, 6, or 8. Transmission system. 14. A downlink original signal sent by the optical transmitter to the optical receiver and an upstream original signal sent by the optical receiver to the optical transmitter are not overlapped on a frequency axis. The bidirectional optical transmission system according to claim 13, which is characterized in that. 15. The absolute value of the difference between the optical frequency of the local optical signal output by the local laser and the optical frequency of the closest upstream FM optical signal to the optical frequency of the local optical signal, The bidirectional light according to claim 13 or 14, wherein the wavelength of each optical signal is set so that the absolute value of the difference between the maximum value and the minimum value of the optical frequency of the upstream FM optical signal becomes small. Transmission system. 16. The downlink original signal is a signal in which a carrier signal is AM-modulated by an analog video signal or a signal in which a carrier signal is QAM-modulated by a digital video signal is multiplexed on a frequency axis. Item 1
The bidirectional optical transmission system according to any one of 3 to 15. 17. The bidirectional optical transmission system according to claim 13, wherein the upstream original signal is a baseband digital signal.