JP2001197039A - System and device for optical transmission - Google Patents

Abstract

PROBLEM TO BE SOLVED: To improve the signal-to-noise power characteristics of an RF signal after transmission and to increase a transmission distance and the number of optical branches as to a system which transmits the RF signal by using an optical fiber. SOLUTION: This optical transmission device has an intensity/FM modulation laser 1 which performs light intensity modulation and light FM modulation with the RF signal at the same time, an FM signal down-converting laser 2, and an optical multiplexer 3. An optical reception device has an optical receiver 6, a photodetection power detector 7, an intensity/FM signal demultiplexer 8, an FM demodulator 9, and an RF signal selector 10. The RF signal selector 10 regenerated the RF signal from an intensity-modulated component above specific photodetection power according to the photodetection signal detected by the photodetection power detector 7 to obtain necessary signal-to-noise power characteristics. Below the photodetection power, on the other hand, the RF signal is sproduced from an FM modulated component. Consequently, the signal- to-noise power characteristics of the RF signal after transmission can be improved.

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 and an optical transmission apparatus for transmitting an RF signal via an optical fiber.
In particular, the present invention relates to an optical transmission system and an optical transmission device that can improve signal-to-noise power characteristics of an RF signal.

[0002]

2. Description of the Related Art Conventionally, as an example of an apparatus for optically transmitting an RF signal, an apparatus disclosed in Japanese Patent Publication No. 280,168 is known. FIG. 13 is a block diagram showing a configuration of an optical transmission device used for a conventional transmission method. As shown, the optical transmission device uses an optical transmission device 29 using a semiconductor laser 28 for optically modulating an optical carrier signal with an RF signal, and an optical receiver 30 for optically / electrically converting the transmitted optically modulated signal. The optical receiver 31 is provided. The optical transmitter 29 and the optical receiver 31 are connected via the optical fiber transmission line 5. According to the above configuration, the optical carrier signal of the semiconductor laser 28 is intensity-modulated by the RF signal, and
Is transmitted using The transmitted optical modulation signal is optically / electrically converted by the optical receiver 30 to reproduce the RF signal.

Next, as another example of a conventional apparatus for optically transmitting an RF signal, there is an optical transmission apparatus disclosed in Japanese Patent Application Laid-Open No. 8-274714. FIG. 14 is a block diagram showing a configuration of the optical transmission device. The optical transmitting device 34
FM modulator 3 for FM-modulating a carrier signal with an F signal
2. It has a semiconductor laser 33 for optically modulating the optical carrier signal with the FM modulation signal. The optical receiving device 37 includes an optical receiver 35 for performing optical / electrical conversion of the transmitted optical modulation signal,
It has an FM demodulator 36 for FM demodulating an electric signal. The modulated optical signal is transmitted via the optical fiber transmission line 5. According to the above configuration, the carrier signal is FM-modulated by the RF signal in the FM modulator 32, the optical carrier signal from the semiconductor laser 33 is intensity-modulated by the obtained FM signal, and transmitted using the optical fiber transmission line 5. You. The transmitted optical modulation signal is optically / electrically converted by the optical receiver 35, and the obtained FM electric signal is FM-demodulated by the FM demodulator 36, whereby the RF signal is reproduced.

[0004]

However, in such a conventional optical transmission device, the optical transmission device using intensity modulation increases the transmission distance and the number of optical branches in the optical transmission line, and the optical reception device has a problem. , The desired signal-to-noise power characteristic may not be obtained due to the influence of the light intensity noise. On the other hand, the optical transmission device using FM modulation has a problem that the signal-to-noise power characteristic is limited when the received light power is large due to phase fluctuations of the FM signal of the optical transmission device.

SUMMARY OF THE INVENTION The present invention has been made to solve the above-described problems, and it is possible to improve the signal-to-noise power characteristics of an RF signal after transmission, and to increase the transmission distance and the number of optical branches. It is intended to provide a device.

[0006]

In order to achieve the above object, an optical transmission system of the present invention is an optical transmission system for transmitting an RF signal using an optical fiber.
Signal to modulate the intensity of the optical carrier signal,
The optical carrier signal is FM-modulated by the F signal, the intensity-modulated light and the FM-modulated light are multiplexed and transmitted, and the optical receiving apparatus can obtain a required signal-to-noise power characteristic from the intensity-modulated component. When the received light power is equal to or higher than the received light power, the RF signal is reproduced from the intensity modulation component. On the other hand, when the light intensity noise is less than the received light power, the RF signal is reproduced by FM demodulating the FM modulation component. Features. According to the above-described method, since the intensity demodulation or the FM demodulation is performed with reference to a predetermined received power at which a required signal-to-noise power characteristic is obtained from the intensity modulation component, the signal-to-noise power characteristic of the transmitted RF signal is improved. And the transmission distance and the number of optical branches can be increased.

An optical transmission device according to the present invention is an optical transmission device for transmitting an RF signal using an optical fiber.
Intensity modulation means for intensity-modulating the optical carrier signal with a signal, FM modulation means for FM-modulating the optical carrier signal with an RF signal, and an optical multiplexer for multiplexing the intensity-modulated signal and the FM-modulated signal. An optical transmitting device, an intensity demodulating means for reproducing an RF signal from an intensity modulated component, and an F for reproducing an RF signal by FM demodulating the FM modulated component.
An optical receiver comprising: an M demodulator; and an RF signal selector that switches and outputs each RF signal demodulated by the intensity demodulator and the FM demodulator. According to the above device, the optical carrier signal is intensity-modulated and FM-modulated by the RF signal from the optical transmitting device and transmitted. The optical receiving device switches the intensity demodulation or the FM demodulation by the RF signal selecting means and outputs the signal. By performing the switching, demodulation corresponding to the transmission state can be performed, and the signal-to-noise power characteristics of the RF signal can be improved.

In the optical transmission device of the present invention, the optical receiving device is provided with a received light power detector for detecting the received light power of the transmitted optical modulation signal, and the RF signal selecting means includes:
Based on the value of the received light power detected by the received light power detector, the output is switched to the output of the intensity demodulation means in a state where the required signal-to-noise power characteristic is obtained from the intensity modulation component in a state of not less than the received light power. In a state where the influence of light intensity noise occurs, the output is switched to the output of the FM demodulation means. According to this configuration, the signal-to-noise power characteristic of the RF signal can be improved by switching between the case where the received power is detected and the RF signal is reproduced from the intensity modulation component and the case where the RF signal is reproduced from the FM modulation component. .

An optical transmission device according to the present invention is an optical transmission device for transmitting an RF signal using an optical fiber.
Intensity modulation means for intensity-modulating the optical carrier signal with a signal, FM modulation means for FM-modulating the optical carrier signal with an RF signal, and an optical multiplexer for multiplexing the intensity-modulated signal and the FM-modulated signal. An optical transmitting device, an intensity demodulating means for reproducing an RF signal from an intensity modulated component, and an F for reproducing an RF signal by FM demodulating the FM modulated component.
M demodulation means, and an RF for multiplexing and outputting each RF signal demodulated by the intensity demodulation means and the FM demodulation means at a predetermined multiplexing ratio.
And an optical receiving device having signal multiplexing means. According to the above configuration, the RF signal is transmitted from the optical transmission device.
The optical carrier signal is intensity-modulated and FM-modulated by the signal and transmitted. In the optical receiver, the RF signal multiplexing means combines the intensity demodulation or the FM demodulation at a predetermined combination ratio.
The signal-to-noise power characteristics of the RF signal can be improved without being affected by fluctuations in the transmission state, and the transmission distance and the number of optical branches can be increased.

[0010] In the optical transmission apparatus of the present invention, the optical receiving apparatus is provided with a received light power detector for detecting the received light power of the transmitted optical modulation signal, and the RF signal selecting means includes:
Based on the value of the received light power detected by the received light power detector, in a state where the required signal-to-noise power characteristic is obtained from the intensity modulated component or higher, the ratio of reproducing the RF signal from the intensity modulated component is increased. In a state where the influence of light intensity noise occurs below the received light power, the FM modulation component is FM-demodulated to increase the ratio of reproducing the RF signal and multiplex and output them. According to this configuration, the combination ratio can be adjusted optimally so that the signal-to-noise power characteristic after combining the RF signal reproduced from the intensity modulation component and the RF signal reproduced from the FM modulation component based on the received light power becomes the best. Become like

In the optical transmission apparatus of the present invention, a semiconductor laser is used for the intensity modulating means and the FM modulating means of the optical transmitting apparatus, and a characteristic that an output intensity changes according to an injection current of the semiconductor laser is used. In this configuration, intensity modulation and FM modulation using a chirp characteristic in which the oscillation frequency changes according to the injection current are performed simultaneously. According to the above configuration, since the semiconductor laser is used as the intensity modulating means and the FM modulating means of the optical transmission device, the intensity modulation according to the injection current of the semiconductor laser and the chirp characteristic in which the oscillation frequency changes according to the injection current are used. FM modulation can be performed simultaneously. Both intensity modulation and FM modulation can be performed using a semiconductor laser.

The optical transmission device of the present invention further comprises an optical intensity modulator as intensity modulation means of the optical transmission device;
An optical phase modulator as a modulating means is connected in cascade, an optical carrier signal from a semiconductor laser is FM-modulated with an RF signal using the optical phase modulator, and F-modulation is performed using the optical intensity modulator.
In this configuration, an M-modulated optical modulation signal is intensity-modulated by an RF signal. According to the above configuration, the optical phase modulator can FM-modulate the optical carrier signal from the semiconductor laser with the RF signal,
The optical modulation signal FM-modulated by the optical intensity modulator can be intensity-modulated by the RF signal.

[0013] In the optical transmission apparatus according to the present invention, the optical receiving apparatus may further include an FM signal down-conversion laser for down-converting an FM modulation component into an electric signal, and an optical modulation signal transmitted from the optical transmitting apparatus. An optical multiplexer for multiplexing with the optical signal of the FM signal down-conversion laser; and an optical receiver for optically / electrically converting the optical signal after the multiplexing, and an optical modulation signal sent from the optical transmitter. The configuration is such that an FM electric signal having an intermediate frequency equal to the difference frequency of the optical signal from the FM signal down-convert laser is generated. According to this configuration, the optical modulation signal transmitted from the optical transmitter and the F
It becomes possible to generate an FM electric signal having the difference frequency of the optical signal from the M signal down-convert laser as an intermediate wave. In addition, an FM signal down-conversion laser is not required on the optical transmitter side.

In the optical transmission apparatus according to the present invention, the optical transmission apparatus may further comprise an FM signal down-convert laser, an optical modulation signal modulated by the intensity modulator and the FM modulator, and an optical signal of the FM signal down-convert laser. An optical multiplexer for multiplexing and transmitting a signal, wherein the optical receiving device includes an optical receiver for performing optical / electrical conversion of the transmitted optical modulation signal and an optical signal from the FM down-convert laser. , An FM electrical signal having an intermediate frequency equal to the difference frequency between the optical modulation signal and the optical signal from the FM signal down-convert laser. According to this configuration, the optical transmitter can generate an FM electrical signal having an intermediate frequency equal to the difference frequency between the optical modulation signal and the optical signal from the FM signal down-convert laser. Further, since the FM down-conversion laser is provided only in one optical transmission device in the entire system, the economy can be further improved.

[0015]

(Embodiment 1) FIG. 1 is a block diagram showing a configuration of an optical transmission apparatus according to Embodiment 1 of the present invention. The optical transmission device includes an optical transmission device 4 and an optical reception device 11 and is connected via an optical fiber transmission line 5. The optical transmitter 4 includes an intensity / FM modulated laser 1, FM
A signal down-convert laser 2 and an optical multiplexer 3 are provided. The optical receiver 11 includes an optical receiver 6, a received light power detector 7, an intensity / FM signal splitter 8, an FM demodulator 9, an RF
A signal selector 10 is provided.

Next, the transmission operation by the optical transmission device will be described. In the optical transmission device 4, the RF signal input to the intensity / FM modulation laser 1 outputs the optical carrier signal after intensity modulation and FM modulation. This strength / FM
The modulation laser 1 is configured to simultaneously perform intensity modulation using a characteristic in which the output intensity changes according to an injection current to a semiconductor laser to be used and FM modulation using a chirp characteristic in which an oscillation frequency changes according to the injection current. It has become. The output of the intensity / FM modulation laser 1 and the optical signal from the FM signal down-convert laser 2 are multiplexed by the optical multiplexer 3 and optically transmitted through the optical fiber transmission line 5.

In the optical receiver 11, the transmitted optical modulation signal and the optical signal of the FM down-convert laser 2 are optically / electrically converted by the optical receiver 6. Then, the RF signal is reproduced from the intensity modulation component, and the FM electric signal having the difference frequency between the optical modulation signal and the optical signal of the FM signal down-convert laser 2 as an intermediate frequency is obtained. The RF signal and the FM electric signal reproduced from these intensity modulation components are represented by an intensity / F
The signal is divided by the M signal splitter 8. FIG. 2 is a characteristic diagram showing a demultiplexing operation by the intensity / FM signal demultiplexer 8. The FM signal after the demultiplexing is FM-demodulated by the FM demodulator 9 and RF-demodulated.
The signal is reproduced.

FIG. 3 shows the relationship between the received light power of the optical carrier signal and the CNR (carrier power to noise power ratio) of the demodulated RF signal in the optical receiver when the RF signal is optically transmitted using intensity modulation and FM modulation. It shows an example of the relationship. As shown in FIG. 3, in the FM modulation, a good CN is obtained even when the received light power is small and the influence of the light intensity noise is large due to the FM wide band gain.
An R characteristic is obtained. When the received light power is large, FM
In the modulation, the CNR is limited due to phase fluctuation of the light source, whereas in the intensity modulation, a good CNR characteristic is obtained.

Therefore, in the first embodiment, the received light power is detected by the received light power detector 7 and the RF signal reproduced from the intensity modulation component by the RF signal selector 10 and F
By selecting and outputting the RF signal reproduced from the M modulation component based on the detected received light power, it is possible to obtain good signal-to-noise power characteristics.

As described above, according to the first embodiment, in the optical receiving apparatus in which a sufficient received light power is obtained and a desired signal-to-noise power characteristic is obtained with the intensity modulation component, R
At the receiving point where the received light power is small and the desired signal-to-noise power characteristic cannot be obtained with the intensity modulation component, the F signal is reproduced from the FM modulation component. For example, as shown in FIG.
If the received light power is small starting from the received light power A at which the received light characteristic line of the modulation intersects, the RF modulation component
The signal is reproduced, and when the received light power is large, the RF signal is reproduced from the intensity modulation component. As a result, the signal-to-noise power characteristic can be improved, and the transmission distance and the number of optical branches can be increased.

(Embodiment 2) FIG. 4 is a block diagram showing a configuration of an optical transmission apparatus according to Embodiment 2 of the present invention. Note that the same parts as those described in the first embodiment are denoted by the same reference numerals, and detailed description thereof will be omitted. The second embodiment has a configuration in which the RF signal reproduced from the intensity modulation component and the RF signal reproduced from the FM modulation component in the optical receiving device 13 are phase-synchronized and combined to improve the signal quality.

The optical receiver 13 includes an optical receiver 6, a received light power detector 7, an intensity / FM signal splitter 8, an FM demodulator 9,
An F signal multiplexer 12 is provided.

Next, the operation of the above configuration will be described. In the optical transmitter 4, the intensity / FM modulation laser 1
The RF signal input to the optical signal is subjected to intensity modulation and FM modulation of the optical carrier signal, is multiplexed with the optical signal from the FM signal down-convert laser 2 by the optical multiplexer 3, and is optically transmitted through the optical fiber transmission line 5. .

In the optical receiver 13, the transmitted optical modulation signal and the optical signal of the FM down-convert laser 2 are optically / electrically converted by the optical receiver 6 to reproduce the RF signal from the intensity modulation component, and to perform optical modulation. An FM electrical signal having an intermediate frequency equal to the difference frequency between the signal and the optical signal from the FM signal down-convert laser 2 is obtained. An RF signal and an FM electric signal reproduced from these intensity modulation components are separated by an intensity / FM signal splitter 8 (a characteristic diagram is shown in FIG. 2), and the FM signal is FM-demodulated by an FM demodulator 9 and RF-demodulated. The signal is reproduced.

Next, the RF signal reproduced from the intensity modulation component and the RF signal reproduced from the FM modulation component are synthesized by the RF signal synthesizer 12. This RF signal synthesizer 12
The RF signal reproduced from the intensity modulation component and the RF signal reproduced from the FM modulation component by the received light power detected by the received light power detector 7 are set so that the signal-to-noise power characteristic of the combined RF signal is maximized. The combining is performed by optimizing the combining ratio.

FIG. 5 is a diagram showing the synthesis ratio of these.
(A) is a characteristic diagram of received light power vs. CNR, and (b) is a characteristic diagram of received light power vs. composition ratio. As shown in these figures, F
In the region of the received light power where the CNR of the RF signal reproduced from the M modulation component is good (the region of the received light power lower than the received light power A), the synthesis ratio of the RF signal reproduced from the FM modulation component is increased. On the other hand, the received light power increases and the CNR of the RF signal reproduced from the FM modulation component is saturated by the phase fluctuations of the intensity / FM modulation laser and the FM signal down-convert laser, and the CNR of the RF signal reproduced from the intensity modulation component is reduced. In the region of the received light power which is improved (the region of the received light power higher than the received light power A), the synthesis ratio of the RF signal reproduced from the intensity modulation component is increased. Then, at the received light power A in which the CNR of the RF signal reproduced from the FM modulation component and the RF signal reproduced from the intensity modulation component are equal, the combining ratio is set to 0.5: 0.5.

By the above combination, the RF signal reproduced from the FM modulation component and the R signal reproduced from the intensity modulation component
At the received light power A where the CNR of the F signal is equal, the RF signal is amplitude-added and increased by 6 dB, whereas the noise is power-added and increased by 3 dB, so that the CNR is improved by 3 dB.

As described above, according to the second embodiment, the RF signal reproduced from the intensity modulation component and the F
The signal-to-noise power characteristic is compared with that of the first embodiment by optimizing the combining ratio so as to maximize the signal-to-noise characteristic of the RF signal after combining the RF signal reproduced from the M-modulated component. Can be further improved.

(Embodiment 3) FIG. 6 is a block diagram showing a configuration of an optical transmission apparatus according to Embodiment 3 of the present invention. Note that the same parts as those described in the above embodiment are denoted by the same reference numerals, and detailed description thereof will be omitted. The third embodiment has a configuration in which the optical receiver has the FM signal down-conversion laser that the optical transmitter has in the first embodiment.

In the figure, an optical transmitter 14 is provided with an intensity / FM modulation laser. The optical receiver 19 includes an optical demultiplexer 15, an optical receiver 16, an optical multiplexer 17, an FM signal down-convert laser 18, an optical receiver 6, an intensity / FM signal demultiplexer 8, an FM demodulator 9, an RF signal It comprises a selector 10 and a received light power detector 7.

Next, the operation of the above configuration will be described. In the optical transmission device 14, the RF signal input to the intensity / FM modulation laser 1 intensity-modulates and FM-modulates the optical carrier signal, and the obtained optical modulation signal is optically transmitted through the optical fiber transmission line 5. You. In the optical modulation signal transmitted to the optical receiving device 19, the light for detecting the light reception level is split by the optical splitter 15. Then, the light of the main signal is multiplexed with the optical signal from the FM signal down-convert laser 18 by the optical multiplexer 17, and is optically / electrically converted by the optical receiver 6. Thus, the RF signal is reproduced from the intensity modulation component, and the transmitted optical modulation signal and the FM signal down-convert laser 18
, An FM electric signal having the intermediate frequency of the difference frequency of the optical signal from the optical signal is obtained.

The intensity / FM signal splitter 8 splits an RF signal and an FM electric signal reproduced from these intensity modulation components (a characteristic diagram is shown in FIG. 5). The FM signal is FM-demodulated by the FM demodulator 9 to reproduce an RF signal. here,
The optical power of the optical signal from the FM signal down-convert laser 18 is increased. FIG. 7 is a diagram showing received light power versus CNR characteristics. As shown, the CNR characteristic of the reproduced RF signal after the FM demodulation is governed by the phase noise of the intensity / FM modulated laser and the FM signal down-converted laser and the relative intensity noise (RIN) of the FM signal down-converted laser. The level is improved for any received power.

Then, the RF signal selector 10 outputs an RF signal switched according to the signal quality of the RF signal reproduced from the intensity modulation component and the RF signal reproduced from the FM modulation component at each reception point. For example, when the received light power is low with respect to the received light power A in FIG. 7, the RF signal is reproduced from the FM modulation component, and when the received light power is high, the RF signal is reproduced from the intensity modulation component.

As described above, according to the third embodiment, the optical power of the FM signal down-conversion laser 18 provided in the optical receiver is increased, and the FM power when the received optical power of the transmitted optical modulation signal is small is reduced. By improving the signal-to-noise power characteristic of the RF signal reproduced from the modulation component and switching and outputting the RF signal reproduced from the intensity modulation component and the RF signal reproduced from the FM modulation component according to the received light power, Thus, the signal-to-noise power characteristic can be greatly improved for any received power.

(Embodiment 4) FIG. 8 is a block diagram showing a configuration of an optical transmission apparatus according to Embodiment 4 of the present invention. Note that the same parts as those described in the above embodiment are denoted by the same reference numerals, and detailed description thereof will be omitted. In the fourth embodiment, in the optical receiver described in the third embodiment, the RF signal reproduced from the intensity modulation component and the RF signal reproduced from the FM modulation component are phase-synchronized and multiplexed to improve the signal quality. It is a configuration aiming at.

The optical receiver 20 includes an optical splitter 15, an optical receiver 16, an optical multiplexer 17, an FM signal down-convert laser 18, an optical receiver 6, an intensity / FM signal splitter 8, and an FM demodulator 9. , An RF signal multiplexer 12 and a received light power detector 7.

Next, the operation of the above configuration will be described. In the optical transmitter 14, the RF signal input to the intensity / FM modulation laser 1 modulates the intensity of the optical carrier signal with the intensity of the optical carrier signal.
The M-modulated optical modulation signal is applied to an optical fiber transmission line 5.
The light is transmitted via

In the optical receiver 20, the transmitted optical modulation signal is split by the optical splitter 15 into light for detecting the received light power.
The light of the main signal is multiplexed with the optical signal from the FM signal down-convert laser 18 by the optical multiplexer 17, and
Is subjected to optical / electrical conversion by the
While the F signal is reproduced, an FM electric signal having an intermediate frequency of the difference frequency between the transmitted optical modulation signal and the optical signal from the FM signal down-convert laser 18 is obtained. The RF signal and the FM electric signal reproduced from these intensity modulation components are split by an intensity / FM signal splitter 8 (a characteristic diagram is shown in FIG. 5), and the FM signal is FM-demodulated by an FM demodulator 9. The RF signal is reproduced.

FIG. 9A is a characteristic diagram of received light power versus CNR,
(B) is a characteristic diagram of the received light power / combination ratio. Where F
By increasing the optical power of the optical signal from the M signal down-convert laser 18, the FM signal is converted as shown in FIG.
The CNR characteristic of the reproduced RF signal after demodulation is represented by intensity / F
Any received power is improved to a level governed by the phase noise of the M-modulated laser and the FM signal downconvert laser 18 and the relative intensity noise (RIN) of the FM signal downconvert laser 18. Then, the RF signal synthesized by the RF signal synthesizer 12 from the intensity modulation component at each reception point is obtained.
The combining ratio is optimized so that the signal quality of the combined RF signal is maximized based on the signal quality of the RF signal reproduced from the signal and the FM modulation component, and then combined.

That is, as shown in FIG. 9B, in the region of the received light power where the CNR of the RF signal reproduced from the FM modulation component is good (the region of the received light power lower than the received light power A),
The synthesis ratio of the RF signal reproduced from the M modulation component is increased. On the other hand, the received light power is increased and the CNR of the RF signal reproduced from the FM modulation component is equal to the intensity / FM modulation laser and FM.
The CN of the RF signal which is saturated by the phase noise of the signal down-convert laser 18 and is reproduced from the intensity modulation component
In a region of the received light power where R is improved (a region of the received light power higher than the received light power A), the synthesis ratio of the RF signal reproduced from the intensity modulation component is increased. Then, at the received light power A where the CNR of the RF signal reproduced from the FM modulation component and the RF signal reproduced from the intensity modulation component are equal, the combining ratio is 0.5:
Set to 0.5.

As described above, according to the fourth embodiment, the optical power of the FM signal down-conversion laser provided in the optical receiver is increased, and the FM power when the received light power of the transmitted optical modulation signal is small is increased. The signal-to-noise power characteristic of the RF signal reproduced from the modulation component is improved, and the RF signal reproduced from the intensity modulation component and the RF signal reproduced from the FM modulation component are combined according to the received light power. By optimizing the combining ratio so as to maximize the signal-to-noise power characteristic and multiplexing, the signal-to-noise power characteristic can be significantly improved for any received light power.

(Embodiment 5) FIG. 10 is a block diagram showing a configuration of an optical transmission apparatus according to Embodiment 5 of the present invention. Note that the same parts as those described in the above embodiment are denoted by the same reference numerals, and detailed description thereof will be omitted. As shown in the figure, an optical receiving device 21 for intensity demodulation and an optical receiving device 22 for FM demodulation are provided on the receiving device side.

The light receiving device 21 for intensity demodulation is
It consists of. Further, the FM demodulation optical receiver 22 includes an optical multiplexer 17, an FM signal down-convert laser 18, an optical receiver 6, and an FM demodulator 9.

Next, the operation of the above configuration will be described. The optical transmission device 14 intensity-modulates and FM-modulates the optical carrier signal of the RF signal input to the intensity / FM modulation laser 1, and optically transmits the RF signal via the optical fiber transmission line 5. In the optical receiver 21 for intensity demodulation, the transmitted optical modulation signal is optically / electrically converted by the optical receiver 6 to reproduce an RF signal. In the FM demodulation optical receiver 22, the transmitted optical modulation signal is multiplexed with the optical signal from the FM signal down-conversion laser 18 by the optical multiplexer 17, and is optically / electrically converted by the optical receiver 6. An FM electric signal having an intermediate frequency equal to the difference frequency between the transmitted optical modulation signal and the optical signal from the FM signal down-convert laser is obtained. The FM electric signal is FM-demodulated by the FM demodulator 9 to reproduce the RF signal.

As shown in FIG. 3, in the FM modulation, the received light power is small due to the FM broadband gain, and good CNR characteristics can be obtained even when the influence of light intensity noise is large. In the intensity modulation, when the received light power is large, a good CNR characteristic can be obtained. That is, at the receiving point where the transmission distance is long, the number of optical branches is large, and the received light power is small, good CNR characteristics are obtained by using the optical receiver for FM demodulation, and the number of parts is small at the receiving point where sufficient received light power is obtained. Sufficient CNR characteristics can be obtained even with a less economical optical demodulation optical receiver.

As described above, according to the fifth embodiment, in the optical receiving device capable of sufficiently obtaining the received light power, the intensity modulation component is used to reduce R
The F signal is reproduced. On the other hand, the FDM demodulation optical receiver 22 is used for an optical receiver in which the received light power is small and a desired signal-to-noise power characteristic cannot be obtained with the intensity modulation component.
An RF signal is reproduced from the M modulation component. As described above, an intensity demodulation optical receiver that reproduces an RF signal from an inexpensive intensity modulation component in accordance with the received light power at a reception point, and an FM that can obtain a desired signal-to-noise power characteristic even at a low received power. By properly using the FM demodulation optical receiver for reproducing the RF signal from the modulation component, the transmission distance and the number of optical branches can be increased, and the cost of the entire system can be reduced. Further, since the dynamic range of the entire system is divided between an intensity demodulation optical receiver for reproducing an RF signal from an intensity modulation component and an FM demodulation optical receiver for reproducing an RF signal from an FM modulation component, the individual optical receivers are divided. In this case, a small dynamic range of light reception can be used, and the cost can be reduced.

(Embodiment 6) FIG. 11 is a block diagram showing a configuration of an optical transmission apparatus according to Embodiment 6 of the present invention. Note that the same parts as those described in the above embodiment are denoted by the same reference numerals, and detailed description thereof will be omitted. The sixth embodiment has a configuration in which the optical transmitter has the FM signal down-convert laser that the optical receiver has in the fifth embodiment.

The optical transmitter 4 comprises an intensity / FM modulation laser 1, an FM signal down-convert laser 2, and an optical multiplexer 3. On the receiving device side, an intensity demodulating optical receiving device 2
1 and an FM demodulation optical receiver 23 are provided.

The light receiving device 21 for intensity demodulation comprises the light receiver 6. The FM demodulation optical receiver 23 includes the optical receiver 6 and the FM demodulator 9.

Next, the operation of the above configuration will be described. In the optical transmitter 4, the RF signal input to the intensity / FM modulation laser 1 is intensity-modulated and FM-modulated on the optical carrier signal, and the obtained optical modulation signal is combined with the optical signal from the FM signal down-convert laser 2. The optical signals are multiplexed by the optical multiplexer 3 and optically transmitted through the optical fiber transmission line 5.

In the intensity demodulation optical receiver 21, the transmitted optical modulation signal is optically / electrically converted by the optical receiver 6, and the RF signal is reproduced from the intensity modulation component. In the FM demodulation optical receiver 23, the optical modulation signal and the optical signal from the FM signal down-convert laser 2 are optically / electrically converted by the optical receiver 6, so that the optical modulation signal and the FM signal down-convert laser 2 An FM electrical signal having the intermediate frequency of the difference frequency of the optical signal is obtained. The FM electric signal is FM-demodulated by the FM demodulator 9 to reproduce the RF signal.

As described above, according to the sixth embodiment, in the optical receiving apparatus capable of sufficiently obtaining the received light power, the intensity demodulated component is converted to the R by using the intensity demodulating optical receiving apparatus 21 having only the optical receiver 6.
Reproduce the F signal. On the other hand, an optical receiving device which has a small received light power and cannot obtain a desired signal-to-noise power characteristic with an intensity modulation component has an FM demodulator 9 and an FM demodulation optical receiving device 23 for reproducing an RF signal from the FM modulation component Is used.

As described above, the intensity demodulation optical receiver 21 for reproducing the RF signal from the inexpensive intensity modulation component in accordance with the received light power at the receiving point, and the desired signal-to-noise power characteristic even at low received power. By properly using the FM demodulation optical receiving device 23 that reproduces an RF signal from the FM modulation component obtained, the transmission distance and the number of optical branches can be increased, and the cost of the entire system can be reduced. Furthermore, the dynamic range of the entire system can be changed from the intensity modulation component to R
Intensity demodulation optical receiver 21 for reproducing the F signal, and FM demodulation optical receiver 2 for reproducing the RF signal from the FM modulation component
3, the dynamic range of light reception in each optical receiving device may be small, and the FM signal down-converting laser may be provided in the optical transmitting device 4 instead of in each of the FM demodulating optical receiving devices 23. Accordingly, since only one FM signal down-conversion laser is used in the entire system, economy can be further improved.

(Embodiment 7) FIG. 12 is a block diagram showing a configuration of an optical transmission apparatus according to Embodiment 7 of the present invention. Note that the same parts as those described in the above embodiment are denoted by the same reference numerals, and detailed description thereof will be omitted. In the seventh embodiment, the F / F modulation laser 1 performed by the intensity / FM modulation laser 1 in the optical receiver described in the first embodiment is used.
M phase modulation and intensity modulation are performed by the optical phase modulator 25 and the light intensity modulator 2
6 is changed to the configuration performed in FIG.

The optical transmitter 27 comprises a semiconductor laser 24, an optical phase modulator 25, an optical intensity modulator 26, an FM signal down-convert laser 2, and an optical multiplexer 3.
The optical receiver 11 includes an optical receiver 6, a received light power detector 7, an intensity / FM signal splitter 8, an FM demodulator 9, and an RF signal selector 10.

Next, the operation of the above configuration will be described. In the optical transmitter 27, the optical carrier signal output from the semiconductor laser 24 is converted into an RF signal by the optical phase modulator 25 as an F signal.
The M-modulated, FM-modulated optically modulated signal is intensity-modulated by an RF signal by an optical intensity modulator 26, multiplexed with an optical signal from an FM signal down-conversion laser 2 by an optical multiplexer 3, and The light is transmitted via

In the optical receiver 11, the transmitted optical modulation signal and the optical signal of the FM down-convert laser are optically / electrically converted by the optical receiver 6, and the RF signal is reproduced from the intensity modulation component. With the difference frequency between the optical signal of the down-converted laser and the FM signal as the intermediate frequency
An electric signal is obtained. The RF signal and the FM electric signal reproduced from these intensity modulation components are separated by an intensity / FM signal splitter 8 having characteristics as shown in FIG. 2, and the FM signal is FM-demodulated by an FM demodulator 9 and RF-demodulated. The signal is reproduced.

As can be seen from FIG. 3, in the FM modulation, a good CNR characteristic can be obtained by the FM broadband gain even when the received light power is small and the influence of the light intensity noise is large. Also, when the received light power is large, the CNR is limited due to the phase fluctuation of the light source in the FM modulation, whereas a good CNR characteristic is obtained in the intensity modulation. Therefore, the received light power is detected by the received light power detector 7 and the RF signal selector 10 is selected according to the detected power.
By selecting and outputting the RF signal reproduced from the intensity modulation component and the RF signal reproduced from the FM modulation component, good signal-to-noise power characteristics can be obtained.

As described above, according to the present embodiment, in the optical receiving apparatus in which sufficient received light power is obtained and a desired signal-to-noise power characteristic is obtained with the intensity modulation component, R
At the receiving point where the received light power is small and the desired signal-to-noise power characteristic cannot be obtained with the intensity modulation component, the signal-to-noise power characteristic is improved by reproducing the RF signal from the FM modulation component. And the transmission distance and the number of optical branches can be increased.

The RF signals described in the above embodiments include, for example, radio signals and frequency-multiplexed multi-channel video signals. By transmitting a wireless signal using an optical fiber, the wireless signal transmission characteristics of the optical transmission section can be improved, and the transmission / reception dynamic range of the wireless signal in the wireless section can be expanded. Further, by transmitting a frequency-multiplexed multi-channel video signal using an optical fiber, the video signal can be transmitted over a wide range and with high quality.

[0061]

According to the optical transmission system of the present invention, an optical carrier signal is intensity-modulated and FM-modulated by an RF signal on the transmitting side and multiplexed and transmitted, and the receiving side is subjected to intensity demodulation or demodulation based on a predetermined received light power. Since the FM demodulation is performed, the signal-to-noise power characteristics of the RF signal after transmission can be improved, and the transmission distance and the number of optical branches can be increased.

The optical transmission device according to the present invention uses an RF
RF signal selecting means of an optical receiving apparatus, comprising: an intensity modulating means for intensity modulating an optical carrier signal by a signal; an FM modulating means for FM modulating; and an optical multiplexer for multiplexing these. Can be used to select intensity demodulation or FM demodulation.
By switching M demodulation, the signal-to-noise power characteristic of the RF signal can be improved, and the transmission distance and the number of optical branches can be increased. Then, since two different modulations by the RF signal are superimposed on one optical carrier signal and transmitted,
The modulation method can be properly used according to the transmission distance, and the transmission characteristics can be improved.

According to a third aspect of the present invention, the light receiving device is provided with a received light power detector for detecting the received light power of the transmitted light modulated signal, and the RF signal selecting means sets the predetermined received light power as a reference. The configuration is such that intensity demodulation or FM demodulation is performed. Even if received light power fluctuates, intensity demodulation and FM demodulation can be switched correspondingly, and the signal-to-noise power characteristics of an RF signal can be constantly improved.

[Brief description of the drawings]

FIG. 1 is a block diagram illustrating a configuration of an optical transmission device according to a first embodiment of the present invention.

FIG. 2 is a diagram illustrating characteristics of a duplexer that splits an RF signal and an FM signal reproduced from an intensity modulation component.

FIG. 3 is a diagram illustrating an example of a relationship between received light power and CNR of an RF signal when an RF signal is optically transmitted using intensity modulation and FM modulation.

FIG. 4 is a block diagram showing a configuration of an optical transmission device according to a second embodiment of the present invention.

FIG. 5 is a diagram showing a synthesis ratio of an RF signal reproduced from an intensity modulation component and an RF signal reproduced from an FM modulation component.

FIG. 6 is a block diagram illustrating a configuration of an optical transmission device according to a third embodiment of the present invention.

FIG. 7 is a diagram showing a synthesis ratio of an RF signal reproduced from an intensity signal component and an RF signal reproduced from an FM signal component;

FIG. 8 is a block diagram showing a configuration of an optical transmission device according to a fourth embodiment of the present invention.

FIG. 9 is a diagram illustrating an example of a relationship between received light power and CNR of an RF signal when an RF signal is optically transmitted using intensity modulation and FM modulation.

FIG. 10 is a block diagram illustrating a configuration of an optical transmission device according to a fifth embodiment of the present invention.

FIG. 11 is a block diagram showing a configuration of an optical transmission device according to a sixth embodiment of the present invention.

FIG. 12 is a block diagram illustrating a configuration of an optical transmission device according to a seventh embodiment of the present invention.

FIG. 13 is a block diagram showing one configuration of a conventional optical transmission device.

FIG. 14 is a block diagram showing one configuration of a conventional optical transmission device.

[Explanation of symbols]

 Reference Signs List 1 intensity / FM modulation laser 2 FM signal down-convert laser 3 optical multiplexer 4, 14 optical transmitter 5 optical fiber transmission line 6, 16 optical receiver 7 received power detector 8 intensity / FM signal demultiplexer 9 FM demodulator REFERENCE SIGNS LIST 10 RF signal selector 11, 13, 19, 20 Optical receiving device 12 RF multiplexer 15 Optical demultiplexer 17 Optical multiplexer 18 FM signal down-converting laser 21 Intensity demodulating optical receiving device 22 FM demodulating optical receiving device 23 FM demodulation optical receiver 24 Semiconductor laser 25 Optical phase modulator 26 Optical intensity modulator

Continued on the front page (51) Int.Cl. ⁷ Identification symbol FI Theme coat II (reference) H04B 10/06

Claims (9)

[Claims] In an optical transmission system for transmitting an RF signal using an optical fiber, an optical transmitter modulates an intensity of an optical carrier signal by an RF signal, and modulates an optical carrier signal by an RF signal. The modulated light and the FM-modulated light are multiplexed and transmitted, and the optical receiver receives the RF signal from the intensity modulation component when the received light power is equal to or higher than a predetermined received power at which a required signal-to-noise power characteristic can be obtained from the intensity modulation component. The signal is reproduced. On the other hand, in a state where the influence of light intensity noise occurs below the received light power, the FM modulation component is
An optical transmission system characterized by reproducing an RF signal by demodulation. 2. An optical transmission apparatus for transmitting an RF signal using an optical fiber, comprising: an intensity modulator for intensity-modulating the optical carrier signal with the RF signal; an FM modulator for FM-modulating the optical carrier signal with the RF signal; An optical multiplexer that combines the intensity-modulated signal and the FM-modulated signal; an optical transmission device having: an intensity demodulator that reproduces an RF signal from the intensity-modulated component; and FM-demodulating the FM-modulated component. FM demodulation means for reproducing an RF signal according to the following: each RF demodulated by the intensity demodulation means and FM demodulation means
An optical transmission device comprising: an optical reception device having an RF signal selection unit for switching and outputting a signal. 3. The light receiving device is provided with a received light power detector for detecting the received light power of the transmitted optical modulation signal, and the RF signal selecting means is configured to detect the received light power detected by the received light power detector. Based on the value, the output is switched to the intensity demodulation means in a state where the required signal-to-noise power characteristic is obtained from the intensity modulation component when the received signal power is equal to or higher than the received power. 3. The optical transmission apparatus according to claim 2, wherein the output is switched to the output of the means. 4. An optical transmission apparatus for transmitting an RF signal using an optical fiber, comprising: an intensity modulator for intensity-modulating the optical carrier signal with the RF signal; an FM modulator for FM-modulating the optical carrier signal with the RF signal; An optical transmitter having an optical multiplexer for multiplexing the intensity-modulated signal and the FM-modulated signal; an intensity demodulator for reproducing an RF signal from the intensity-modulated component; and FM-demodulating the FM-modulated component. FM demodulation means for reproducing an RF signal according to the following: each RF demodulated by the intensity demodulation means and FM demodulation means
An optical transmission device comprising: an optical receiving device having RF signal multiplexing means for multiplexing and outputting signals at a predetermined multiplexing ratio. 5. The light receiving device is provided with a received light power detector for detecting a received light power of the transmitted light modulation signal, and the RF signal multiplexing means includes a received light power detected by the received light power detector. Based on the value of, the ratio of reproducing the RF signal from the intensity modulation component is increased in a state where the required signal-to-noise power characteristic is obtained from the intensity modulation component when the power is equal to or higher than the received power. In an affected state, F
4. The optical transmission apparatus according to claim 3, wherein the M-modulation component is FM-demodulated to increase the ratio of reproducing the RF signal and multiplex and output the RF signal. 6. The intensity modulating means and FM of the optical transmission device.
A semiconductor laser is used as the modulation means, and intensity modulation using a characteristic in which the output intensity changes according to the injection current of the semiconductor laser and FM modulation using a chirp characteristic in which the oscillation frequency changes according to the injection current are simultaneously performed. The optical transmission device according to claim 2, wherein the optical transmission device is configured to perform the operation. 7. An optical intensity modulator as an intensity modulator of the optical transmitter and an optical phase modulator as the FM modulator are connected in cascade, and light from a semiconductor laser is used by using the optical phase modulator. 6. The optical transmission device according to claim 2, wherein a carrier signal is FM-modulated with an RF signal, and the optical modulation signal FM-modulated using the optical intensity modulator is intensity-modulated with an RF signal. 8. An optical signal receiving apparatus comprising: an FM signal down-conversion laser for down-converting an FM modulation component into an electric signal; and an FM signal down-conversion laser transmitting the optical modulation signal sent from the optical transmitting apparatus.
An optical multiplexer for multiplexing with the optical signal of the signal down-conversion laser; and an optical receiver for optically / electrically converting the multiplexed optical signal, wherein the optical modulation signal and the FM transmitted from the optical transmitting device are provided. 6. The optical transmission device according to claim 2, wherein an FM electric signal having a difference frequency of an optical signal from the signal down-convert laser as an intermediate frequency is generated. 9. The optical transmitter, further comprising: transmitting an FM signal down-converted laser; an optical modulation signal modulated by the intensity modulator and the FM modulator; and an optical signal of the FM signal down-converted laser. An optical multiplexer, wherein the optical receiver includes an optical receiver for performing optical / electrical conversion of the transmitted optical modulation signal and an optical signal from the FM down-convert laser,
8. The optical transmission device according to claim 2, wherein an FM electric signal having a difference frequency between the optical modulation signal and the optical signal from the FM signal down-convert laser as an intermediate frequency is generated.