Optical fiber wireless transmission system, optical transmitter, optical receiver, and optical fiber deterioration monitoring device

Abstract

This optical fiber wireless transmission system comprises: an optical transmitter (10) that converts a wireless signal and a monitoring swept-frequency signal in a frequency band different from the frequency band of the wireless signal into an optical signal having an intensity and outputs the optical signal as transmitted signal light; an optical receiver (20) that receives the transmitted signal light from the optical transmitter (10) via an optical fiber (40); and an optical fiber deterioration monitoring device (30) that converts backscattered light of the transmitted signal light from the optical transmitter (10) caused by the optical fiber into an electrical signal, detects the monitoring swept-frequency signal as a reference signal, and performs arithmetic processing to obtain an impulse response.

Description

Radio-on-fiber transmission system, optical transmitter, optical receiver, and optical fiber degradation monitoring device

[0001] The present disclosure relates to a radio-on-fiber transmission system, an optical transmitter, an optical receiver, and an optical fiber degradation monitoring device.

[0002] A method for monitoring optical fiber degradation in a radio-on-fiber transmission system that includes an optical transmitter (OLT) and an optical receiver (ONU) connected by an optical fiber is described in Non-Patent Document 1. The method for monitoring optical fiber degradation proposed in Non-Patent Document 1 for a subcarrier multiplexing and wavelength division multiplexing passive optical network (SCM / WDM PON) involves coupling a pulse (an optical time domain reflectometry (OTDR) pulse) to a subcarrier for data transmission, modulating the pulse onto an optical carrier, transmitting the modulated pulse to the optical fiber, receiving the backscattered light with an OTDR receiver, and performing signal processing and detecting the OTDR pulse to monitor defects in the optical fiber.

[0003] D. Villafani Caballero et al., “SCM / WDM-PON with in-service baseband embedded OTDR monitoring.”, Optics Communications, Netherlands, February 1, 2015, Vol. 356, pp. 250-255

[0004] The method of monitoring the deterioration of optical fibers shown in Non-Patent Document 1 combines an OTDR pulse for monitoring the deterioration of optical fibers with a subcarrier for data transmission, which causes a sudden increase in the optical intensity of the subcarrier for data transmission due to the OTDR pulse, resulting in nonlinear distortion of the subcarrier for data transmission.

[0005] The present disclosure has been made in consideration of the above-mentioned points, and aims to provide a radio-on-fiber transmission system that transmits a transmission signal optical signal from an optical transmitter to an optical receiver via an optical fiber, and that is capable of suppressing nonlinear distortion of the transmission signal and monitoring degradation of the optical fiber.

[0006] The radio-on-fiber transmission system according to the present disclosure includes an optical transmitter that converts a radio signal and a supervisory frequency sweep signal in a frequency band different from the frequency band of the radio signal into an optical signal with intensity and outputs the converted signal light as a transmission signal light, an optical receiver that receives the transmission signal light from the optical transmitter via an optical fiber, and an optical fiber degradation monitoring device that converts backscattered light of the transmission signal light from the optical transmitter by the optical fiber into an electrical signal, detects the monitoring frequency sweep signal using a reference signal, and performs arithmetic processing to obtain an impulse response.

[0007] According to the present disclosure, in order to monitor the degradation of the optical fiber using backscattered light, a monitoring frequency sweep signal is superimposed on a radio signal and converted into a transmission signal light, thereby suppressing nonlinear distortion of the radio signal in the transmission signal light received by the receiver, and obtaining information for monitoring the degradation of the optical fiber by detecting the monitoring signal in the backscattered light of the transmission signal light using the monitoring frequency sweep signal as a reference signal.

[0008] 1 is a diagram illustrating a configuration example of a radio-on-fiber transmission system according to a first embodiment, and FIG. 2 is a diagram illustrating the relationship between the spectral intensity of a transmission signal and a supervisory sweep signal with respect to frequency in the radio-on-fiber transmission system according to the first embodiment.

[0009] First Embodiment A radio-on-fiber transmission system according to the first embodiment will be described with reference to Figures 1 and 2. The radio-on-fiber transmission system according to the first embodiment includes an optical transmitter 10 and an optical receiver 20, and is a radio-on-fiber transmission system in which the optical transmitter 10 and the optical receiver 20 are connected by an optical fiber 40. The radio-on-fiber transmission system also includes an optical fiber degradation monitoring device 30 for monitoring degradation of the optical fiber 40.

[0010] The optical fiber degradation monitoring device 30 in the radio-on-fiber transmission system according to the first embodiment may be used as a monitoring device for monitoring degradation of optical fibers in each optical network in an optical network in which each wavelength transmits eight subcarrier channels and 64-quadrature amplitude modulation (64QAM (Quadrature Amplitude Modulation)) is used, as shown in, for example, Non-Patent Document 1.

[0011] 1, an example of a radio-on-fiber transmission system according to the first embodiment will be described in which a single transmission signal light from an optical transmitter 10 is received by an optical receiver 20 via an optical fiber 40, and an optical fiber degradation monitoring device 30 uses backscattered light to obtain monitor information for monitoring degradation of the optical fiber 40. In an optical network in which a signal light is divided into a plurality of wavelengths and transmitted for each wavelength via an optical fiber, the optical fiber degradation monitoring device 30 may monitor degradation of each optical fiber for each wavelength.

[0012] 1, the dashed-dotted arrows connecting the components indicate a data communication system, the solid lines connecting the components indicate an electrical signal transmission system, and the dashed arrows connecting the components indicate an optical transmission system. As shown in FIG. 1, the radio-on-fiber transmission system according to the first embodiment includes an optical transmitter 10, an optical receiver 20, an optical fiber degradation monitoring device 30, and an optical fiber 40 connecting the optical transmitter 10 and the optical receiver 20.

[0013] The optical transmitter 10 outputs a transmission signal, which is a superposition of a radio signal and a supervisory frequency sweep signal, as a transmission signal light converted into an optical signal with intensity. The radio signal is a frequency-modulated signal in which data, which is the information to be sent, is superimposed on the supervisory frequency sweep signal.

[0014] As shown in Fig. 2, the frequency of the supervisory frequency sweep signal is a frequency band different from the frequency band of the radio signal, and in the first embodiment, it is a frequency lower than the minimum frequency of the radio signal. Since the frequency of the supervisory frequency sweep signal is a frequency band different from the frequency band of the radio signal, and in the first embodiment, the maximum frequency of the supervisory frequency sweep signal is a frequency lower than the minimum frequency of the radio signal, the radio signal and the supervisory frequency sweep signal can be separated in frequency. In Fig. 2, the horizontal axis represents frequency and the vertical axis represents spectral intensity. The supervisory frequency sweep signal may be a signal in which a sine wave is superimposed on the frequency sweep signal.

[0015] The transmission signal light is a frequency-modulated optical signal whose frequency changes over time. The optical signal obtained by converting a radio signal into an optical signal is a radio over fiber (ROF) main signal obtained by intensity-modulating the optical signal with the radio signal and frequency-swept with a signal frequency sweep signal. The transmission signal light output from the optical transmitter 10 to the optical fiber 40 is an optical signal obtained by superimposing the ROF main signal and the supervisory signal light converted into an optical signal whose optical intensity is frequency-swept with a supervisory frequency sweep signal.

[0016] The optical transmitter 10 comprises a radio signal generator 11, a sweep signal generator 12, a splitter 13, an adder 14, an electrical-to-optical signal converter (hereinafter referred to as an E / O converter) 15, and an optical directional coupler 16. The radio signal generator 11 generates and outputs a frequency-modulated radio signal in which data is superimposed on a signal frequency sweep signal and the frequency changes over time.

[0017] The sweep signal generator 12 outputs a supervisory frequency sweep signal having a frequency band different from the frequency band of the radio signal, which in the first embodiment is a frequency lower than the minimum frequency of the radio signal. The supervisory frequency sweep signal is used as a reference signal for detecting a supervisory signal obtained by converting backscattered light from the optical fiber 40 of the transmitted signal light from the optical transmitter 10 into an electrical signal in the optical fiber degradation monitoring device 30.

[0018] The sweep signal generator 12 has an oscillation frequency controller 12a and a frequency sweeper 12b. The oscillation frequency controller 12a outputs a control signal consisting of digital data to the frequency sweeper 12b, which controls the oscillation frequency of the monitoring frequency sweep signal output from the frequency sweeper 12b. The oscillation frequency controller 12a outputs frequency information consisting of digital data set for the monitoring frequency sweep signal in the control signal for the frequency sweeper 12b to the optical fiber degradation monitoring device 30 in order to obtain an impulse response.

[0019] The frequency sweeper 12b is controlled by a control signal from the oscillation frequency controller 12a and outputs a supervisory frequency sweep signal, which is an electrical signal. The supervisory frequency sweep signal starts sweeping when the signal frequency sweep signal in the radio signal starts sweeping, repeatedly sweeps until the sweep of the signal frequency sweep signal ends, and ends when the sweep of the signal frequency sweep signal ends. The supervisory frequency sweep signal from the frequency sweeper 12b may be a frequency-modulated sweep signal in which a sine wave is superimposed on the frequency sweep signal.

[0020] The splitter 13 splits the supervisory frequency sweep signal from the frequency sweeper 12b as a reference signal for detecting the supervisory signal. The supervisory frequency sweep signal split by the splitter 13 is output to the optical fiber degradation monitoring device 30.

[0021] The adder 14 adds, i.e., superimposes, the radio signal from the radio signal generator 11 and the monitoring frequency sweep signal from the sweep signal generator 12 input via the splitter 13, and outputs the result as a transmission signal. The electrical signal / optical signal converter (hereinafter referred to as E / O converter) 15 converts the transmission signal from the adder 14 into an optical signal having optical intensity, i.e., an intensity-modulated optical signal, and outputs it as a transmission signal light to the optical fiber 40 connected to the optical receiver 20.

[0022] The optical directional coupler 16 is disposed immediately after the E / O converter 15, and transmits the transmission signal light from the E / O converter 15 to the optical fiber 40, and outputs backscattered light having an optical intensity due to scattered light and reflected light from the optical fiber 40 in the transmission signal light to the optical fiber degradation monitoring device 30. The backscattered light from the optical directional coupler 16 contains frequency components due to the monitoring signal light in the transmission signal light. The backscattered light from the optical directional coupler 16 indicates the return loss due to the optical fiber 40.

[0023] The optical receiver 20 receives the transmitted signal light from the optical transmitter 10 via the optical fiber 40. The transmitted signal light from the optical transmitter 10 received by the optical receiver 20 includes a supervisory signal light in addition to the ROF main signal, but the supervisory signal light is obtained by converting a supervisory frequency sweep signal into an optical signal, and therefore the supervisory signal light is equivalent to pulsed light being frequency resolved and transmitted sequentially, so that the optical intensity can be suppressed and nonlinear distortion of the ROF main signal, which becomes a wireless signal due to the supervisory signal light, can be suppressed.

[0024] The optical receiver 20 includes a first optical signal / electrical signal converter (hereinafter referred to as O / E converter) 21 and a frequency separator 22. The first O / E converter 21 receives the transmission signal light from the E / O converter 15 in the optical transmitter 10 via the optical directional coupler 16 and the optical fiber 40, and converts the optical intensity of the received transmission signal light into a transmission signal, which is an electrical signal.

[0025] The transmission signal converted by the first O / E converter 21 is substantially the same as the transmission signal obtained by adding the radio signal and the supervisory frequency sweep signal from the adder 14. The frequency separator 22 blocks the frequency corresponding to the supervisory frequency sweep signal from the transmission signal from the first O / E converter 21 and outputs the signal to the transmitting antenna 23 as a radio transmission signal. That is, since the radio signal and the supervisory frequency sweep signal have different frequency bands, the frequency separator 22 frequency-separates the transmission signal from the first O / E converter 21 into the radio signal and the supervisory frequency sweep signal and outputs only the radio signal to the transmitting antenna 23. In the first embodiment, the frequency separator 22 is a high-pass filter (HPF).

[0026] The optical fiber degradation monitoring device 30 converts backscattered light from the optical fiber 40 of the transmitted signal light from the optical transmitter 10 into an electric signal, detects the converted electric signal using the supervisory frequency sweep signal from the optical transmitter 10 as a reference signal, and performs arithmetic processing on the supervisory frequency sweep signal in the detected supervisory signal to obtain an impulse response. The optical fiber degradation monitoring device 30 uses a method similar to the OTDR method of indirectly determining the optical loss in the optical fiber 40 and monitoring the degradation of the optical fiber by measuring the supervisory signal light based on the supervisory frequency sweep signal in the backscattered light from the optical fiber 40 of the transmitted signal light from the optical transmitter 10.

[0027] The optical fiber degradation monitoring device 30 includes a second optical signal / electrical signal converter (hereinafter referred to as O / E converter) 31, a homodyne detector 32, and an inverse fast fourier transform calculator (hereinafter referred to as IFFT calculator) 33. The second O / E converter 31 receives backscattered light from an optical fiber 40 to which the transmitted signal light from the optical directional coupler 16 in the optical transmitter 10 is output, and converts the received backscattered light into a monitoring signal.

[0028] The supervisory signal is a signal obtained by converting the optical intensity of the received backscattered light into an electrical signal. That is, the supervisory signal is a signal obtained by converting the optical intensity of the backscattered light due to the ROF main signal and the backscattered light due to the supervisory signal light into an electrical signal, and therefore has frequency components of the radio signal and the supervisory frequency sweep signal.

[0029] The homodyne detector 32 homodyne-detects the supervisory signal from the second O / E converter 31 using the supervisory frequency sweep signal from the splitter 13 in the optical transmitter 10 as a reference signal, and obtains a frequency response to the supervisory frequency sweep signal. The homodyne detector 32 measures the return loss due to the optical fiber 40 using the supervisory frequency sweep signal and the frequency component of the supervisory frequency sweep signal in the supervisory signal. The measured return loss indicates a transfer function for the reflection response.

[0030] The IFFT calculation unit 33 obtains monitor information by performing an inverse Fourier transform on the frequency response obtained by the homodyne detector 32. That is, the IFFT calculation unit 33 obtains an impulse response by performing an inverse Fourier transform on the amplitude and phase information of the frequency response obtained by the homodyne detector 32 based on the amplitude and phase information of the set frequency for the monitoring frequency sweep signal from the oscillation frequency controller 12 a in the optical transmitter 10, and obtains monitor information based on the signal waveform for the impulse response.

[0031] The signal waveform indicated by the monitor information is the same as the signal waveform obtained by an OTDR technique using a pulse wave, and the monitor information obtained by the IFFT calculation unit 33 can be used to monitor deterioration of the optical fiber 40. The IFFT calculation unit 33 is configured by a microcomputer or the like.

[0032] Next, the operation of the radio-on-fiber transmission system according to the first embodiment will be described. The radio signal generator 11 generates and outputs a radio signal. The oscillation frequency controller 12a in the sweep signal generator 12 supplies a control signal to the frequency sweeper 12b so that the frequency sweeper 12b outputs a supervisory frequency sweep signal in synchronization with the radio signal from the radio signal generator 11.

[0033] The radio signal from the radio signal generator 11 and the supervisory frequency sweep signal from the sweep signal generator 12 input via the splitter 13 are added by the adder 14, and a transmission signal in which the supervisory frequency sweep signal is superimposed on the radio signal is output from the adder 14. The transmission signal output from the adder 14 is converted into an intensity-modulated optical signal by the E / O converter 15, and output as a transmission signal light to the optical fiber 40 connected to the optical receiver 20 via the optical directional coupler 16.

[0034] The transmission signal light received by the optical receiver 20 via the optical fiber 40 is converted into a transmission signal, which is an electrical signal, by the first O / E converter 21. The transmission signal converted by the first O / E converter 21 is passed through a frequency separator 22, which blocks the frequency corresponding to the supervisory frequency sweep signal and passes the frequency band corresponding to the radio signal, and is output to the transmitting antenna 23 as a radio transmission signal corresponding to the radio signal. The radio transmission signal is converted into a transmission radio wave by the transmitting antenna 23 and radiated from the transmitting antenna 23 into the air.

[0035] On the other hand, the optical fiber degradation monitoring device 30 receives the backscattered light from the optical directional coupler 16 and converts the received backscattered light into an electrical monitoring signal in the second O / E converter 31. The monitoring signal from the second O / E converter 31 is homodyne detected by the homodyne detector 32 using the monitoring frequency sweep signal from the splitter 13 in the optical transmitter 10 as a reference signal, and the homodyne detector 32 obtains a frequency response.

[0036] The amplitude and phase information of the frequency response obtained by the homodyne detector 32 is inverse Fourier transformed by the IFFT calculation unit 33 based on the amplitude and phase information of the set frequency for the monitoring frequency sweep signal from the oscillation frequency controller 12a in the optical transmitter 10, and is obtained as an impulse response by the IFFT calculation unit 33. The impulse response obtained by the IFFT calculation unit 33 is used as monitor information, and the signal waveform indicated by the monitor information is displayed on a display device such as a display (not shown).

[0037] In the radio-on-fiber transmission system according to the first embodiment, the optical transmitter 10 superimposes a supervisory frequency sweep signal, which has a frequency band different from the frequency band of the radio signal, on a radio signal, converts the supervising frequency sweep signal into an intensity-modulated optical signal, and outputs the converted signal light to the optical fiber 40 connected to the optical receiver 20. The optical fiber degradation monitoring device 30 converts backscattered light of the transmission signal light by the optical fiber 40 into an electrical signal, detects the signal using the supervisory frequency sweep signal as a reference signal, and performs arithmetic processing to obtain an impulse response. This makes it possible to suppress nonlinear distortion of the transmission signal caused by the optical receiver 20 due to the use of the supervisory frequency sweep signal, and to monitor the degradation of the optical fiber 40.

[0038] Furthermore, by setting the frequency of the supervisory frequency sweep signal to a frequency different from the frequency band of the radio signal, particularly by setting the frequency of the supervisory frequency sweep signal to a frequency lower than the minimum frequency of the radio signal, interference between the radio signal and the supervisory frequency sweep signal can be prevented.

[0039] Furthermore, the optical receiver 20 in the radio-on-fiber transmission system according to the first embodiment is equipped with a frequency separator that blocks the frequency of the supervisory frequency sweep signal from the transmission signal obtained by converting the transmission signal light from the optical transmitter 10 into an electrical signal and outputs the signal to the transmitting antenna 23 as a wireless transmission signal. Therefore, the supervisory frequency sweep signal does not affect the transmission radio waves radiated from the transmitting antenna 23.

[0040] Any of the components of the embodiments may be modified or omitted.

[0041] The optical fiber degradation monitoring device in a radio-on-fiber transmission system according to the present disclosure can be applied to, for example, an optical fiber degradation monitoring device in a subcarrier multiplexing and wavelength division multiplexing passive optical network.

[0042] 10 Optical transmitter, 11 Radio signal generator, 12 Sweep signal generator, 13 Splitter, 14 Adder, 15 Electrical signal-to-optical signal converter, 16 Optical directional coupler, 20 Optical receiver, 21 First optical signal-to-electrical signal converter, 22 Frequency separator, 30 Optical fiber degradation monitoring device, 31 Second optical signal-to-electrical signal converter, 32 Homodyne detector, 33 Inverse Fourier transform calculator, 40 Optical fiber.

Claims

1. A radio-on-fiber transmission system comprising: an optical transmitter that converts a radio signal and a supervisory frequency sweep signal in a frequency band different from the frequency band of the radio signal into an optical signal with intensity and outputs it as a transmission signal light; an optical receiver that receives the transmission signal light from the optical transmitter via an optical fiber; and an optical fiber degradation monitoring device that converts backscattered light of the transmission signal light from the optical transmitter by the optical fiber into an electrical signal, detects the supervisory frequency sweep signal using a reference signal, and performs arithmetic processing to obtain an impulse response.

2. The radio-on-fiber transmission system according to claim 1, wherein the detection in said optical fiber degradation monitoring device is homodyne detection, and the impulse response obtained by arithmetic processing in said optical fiber degradation monitoring device is an impulse response calculated by performing an inverse Fourier transform on the frequency response obtained by said homodyne detection.

3. An optical transmitter comprising: a radio signal generator that outputs a radio signal; a sweep signal generator that outputs a supervisory frequency sweep signal in a frequency band different from the frequency band of the radio signal; an adder that adds the radio signal from the radio signal generator and the supervisory frequency sweep signal from the sweep signal generator and outputs the result as a transmission signal; and an electrical-to-optical converter that converts the transmission signal from the adder into an optical signal having intensity and outputs the transmission signal to an optical fiber as a transmission signal; and an optical receiver comprising: a first optical-to-electrical converter that receives the transmission signal light from the electrical-to-optical converter in the optical transmitter via the optical fiber and converts the received transmission signal light into a transmission signal; and a frequency separator that blocks the frequency corresponding to the supervisory frequency sweep signal from the transmission signal from the first optical-to-electrical converter and outputs the result as a radio transmission signal to a transmitting antenna. a second optical signal-electrical signal converter that receives backscattered light from an optical fiber to which a transmission signal light from the optical transmitter is output and converts the received backscattered light into a monitoring signal; a homodyne detector that homodyne detects the monitoring signal from the second optical signal-electrical signal converter using a monitoring frequency sweep signal from the optical transmitter as a reference signal and obtains a frequency response to the monitoring frequency sweep signal; and an inverse Fourier transform calculator that inverse Fourier transforms the frequency response obtained by the homodyne detector and obtains monitor information.

4. A radio-on-fiber transmission system according to any one of claims 1 to 3, wherein the frequency of the supervisory frequency sweep signal is lower than the lowest frequency of the radio signal.

5. An optical transmitter in a radio-on-fiber transmission system comprising an optical transmitter, an optical receiver, and an optical fiber degradation monitoring device, comprising: a radio signal generator that outputs a radio signal; a sweep signal generator that outputs a monitoring frequency sweep signal of a frequency band different from the frequency band of the radio signal, which is a reference signal for detecting a monitoring signal obtained by converting backscattered light of the transmission signal light from the optical transmitter into an electrical signal in the optical fiber degradation monitoring device; an adder that adds the radio signal from the radio signal generator and the monitoring frequency sweep signal from the sweep signal generator and outputs the result as a transmission signal; and an electrical signal-to-optical signal converter that converts the transmission signal from the adder into an optical signal with intensity and outputs it to the optical fiber connected to the optical receiver as the transmission signal light.

6. An optical transmitter according to claim 5, wherein the frequency of said supervisory frequency sweep signal is less than the lowest frequency of said transmission signal.

7. An optical receiver in a radio-on-fiber transmission system comprising an optical transmitter, an optical receiver, and an optical fiber degradation monitoring device, the optical receiver comprising: a first optical-to-electrical converter that receives, via an optical fiber, a transmission signal obtained by converting a transmission signal to an optical signal having intensity and to which a supervisory frequency sweep signal of a frequency band different from the frequency band of the radio signal has been added, the supervisory frequency sweep signal being a reference signal for detecting the radio signal from the optical transmitter and the supervisory signal obtained by converting backscattered light of the transmission signal light from the optical transmitter in the optical fiber degradation monitoring device into an electrical signal, and converts the received transmission signal light into a transmission signal; and a frequency separator that blocks the frequency of the supervisory frequency sweep signal from the transmission signal from the first optical-to-electrical converter and outputs it to a transmitting antenna as a radio transmission signal.

8. An optical receiver according to claim 7, wherein the frequency of said supervisory frequency sweep signal is lower than the lowest frequency of said radio signal, and said frequency separator is a high-pass filter that blocks the frequency of said supervisory frequency sweep signal.

9. An optical fiber degradation monitoring device in a radio-on-fiber transmission system comprising an optical transmitter, an optical receiver, and an optical fiber degradation monitoring device, comprising: a second optical signal-to-electrical signal converter that receives backscattered light from an optical fiber to which a transmission signal obtained by converting a transmission signal, obtained by adding a radio signal from the optical transmitter and a monitoring frequency sweep signal of a frequency band different from the frequency band of the radio signal, into an optical signal having intensity, and converts the received backscattered light into a monitoring signal; a homodyne detector that homodyne detects the monitoring signal from the second optical signal-to-electrical signal converter using the monitoring frequency sweep signal from the optical transmitter as a reference signal, to obtain a frequency response to the monitoring frequency sweep signal; and an inverse Fourier transform calculator that inverse Fourier transforms the frequency response obtained by the homodyne detector to obtain monitor information.

10. A radio-over-fiber transmission system comprising: an optical transmitter according to claim 5 or 6; an optical receiver according to claim 7 or 8; and an optical fiber degradation monitoring device according to claim 9.

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