JPH07235904A - Analog optical fiber transmitting device - Google Patents

Abstract

PURPOSE:To decrease the beating noises caused by the wavelength variance of an electro-optical transducer by controlling the wavelength of the electro- optical transducer which transduces the radio signals into the optical signals in an access terminal. CONSTITUTION:A wavelength control circuit 14 measures the noise levels based on the output of a photoelectric transducer 13 and controls the wavelength of an electro-optical transducer 7 based on the measured noise levels in order to reduce these nosie levels. The up-RF signal of each of access terminals 5-1, 5-2... 5-n is turned into a light intensity modulation wave by the transducer 7. These modulation waves are connected together into a single piece of optical fiber 9 by the photocouplers 8-1, 8-2... 8-n. Therefore the intensity of the optical signal is modulated on the up-fiber 9 by the RF signal of each access terminal. Thus a synthetic signal of optical signals is obtained. The synthesized RF signals can be collectively demodulated by a photoelectric transducer 10 such as a photodiode, etc., at the output terminal of the fiber 9. As a result, the RF signals can be transmitted via a single optical fiber and also the beating noises can always be decreased.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical fiber transmission device for converting a radio signal existing in a plurality of areas into an optical signal and transmitting the optical signal through one optical fiber. INDUSTRIAL APPLICABILITY The present invention is used for transmitting a radio signal from a place where radio waves do not reach, that is, a dead zone such as a tunnel or an underground mall.

[0002]

2. Description of the Related Art In a large underground mall or a long tunnel, it is difficult to receive a radio signal transmitted from the inside (hereinafter referred to as "uplink signal") at one place. Therefore, wireless signals are received at a plurality of points, converted into optical signals, respectively, and coupled to a common upstream optical fiber for transmission. In this case, in order to avoid interference, each wireless signal receiving point converts into an optical signal of a different wavelength, and at the output port of the upstream optical fiber, the transmitted optical signal is wavelength-divided and then converted into an electrical signal. It is possible. However, such a configuration requires an expensive optical demultiplexer and a large number of opto-electrical converters, resulting in an expensive device.

As a technique for solving such a problem, Japanese Patent Application No. 4-296007 (unpublished at the time of filing of the present application, hereinafter referred to as “first application”) discloses that beat noise due to wavelength difference causes transmission of an RF signal. It is shown that the emission wavelength at each wireless signal reception point is set to be out of the band. This technique will be described with reference to the drawings.

FIG. 10 is a block diagram showing a conventional analog optical fiber transmission device. The downstream RF signal is input from the input port 1, converted into an optical signal by the electro-optical converter 2, and enters the downstream optical fiber 3. Optical couplers 4-1 to 4-n are arranged in the downstream optical fiber 3, and branch optical signals to access terminals 5-1, 5-2, ... 5-n.
Supply to. This access terminal 5-1, 5-2,
... 5-n is arranged in a large underground mall or in a long tunnel, and an optical-electrical converter 6 reproduces an RF signal from the branched optical signals of the optical couplers 4-1 to 4-n and outputs it. As the photoelectric converter 6, for example, a photodiode is used.
On the other hand, the upstream RF signal input to the access terminals 5-1, 5-2, ... 5-n is converted into a light intensity modulated wave by the electro-optical converter 7, and the optical couplers 8-1, 8-2 ,. 8-
It is coupled to one upstream optical fiber 9 by n. Therefore, the intensity of the optical signal on the upstream optical fiber 9 is a combined signal of the upstream RF signals in each of the access terminals 5-1, 5-2, ... 5-n. This combined signal is converted into an electric signal by the photoelectric converter 10 and output to the output port 11 as an upstream RF signal. As the photoelectric converter 10, for example, a photodiode is used.

In such a configuration, the emission wavelengths of the electro-optical converters 7 of the access terminals 5-1, 5-2, ... 5-n are offset so as to be different from each other. The wavelength amount to be offset is set so that the beat noise caused by the wavelength difference is outside the transmission band of the RF signal. For example, when transmitting an RF signal of about 1 to 2 GHz, if the offset amount of the emission wavelength is set to about 1 nm, the beat frequency becomes about 100 GHz, which can be set outside the transmission band of the RF signal. With such a wavelength offset, it is possible to reproduce the RF signals that are collectively synthesized by one photoelectric converter 10. Therefore, upstream and downstream RF can be used without using an optical filter or optical demultiplexer.
Each signal can be transmitted by one optical fiber, and a wireless zone for mobile communication can be constructed economically and easily in a dead zone such as an underground mall or in a tunnel.

FIG. 11 is a diagram showing the relationship between two emission wavelengths and beat noise. Here, as an example, a case where a laser diode with good coherency is used as an electro-optical converter, and the wavelength of the first access terminal is λ1 and the wavelength of the second access terminal is λ2 will be described. As shown by the broken line in FIG. 11, when the peak of the beat noise is in the RF signal transmission band, the noise in this band remarkably increases. On the other hand, when the peak of the beat noise exists outside the RF signal transmission band,
There is little noise in the transmission band. The peak of this beat noise is, for example, about 1 to 2 GHz when λ2-λ1 <0.01 nm. The peak of the beat noise level moves to a lower frequency as the wavelength difference between the two access terminals becomes smaller. If you adjust the emission wavelength so that the wavelength difference between each access terminal is the largest,
Beat noise is minimized.

[0007]

As described above, in the analog optical fiber transmission device disclosed in the previous application, the emission wavelengths of the electro-optical converters are offset from each other to reduce beat noise in the transmission band. Was. However, the emission wavelength of the electro-optical converter changes from the initial set value due to changes over time and changes in ambient temperature. Therefore, beat noise may increase during operation, and the signal power to noise power ratio (S / N) may significantly decrease.

An object of the present invention is to solve the above problems and to provide an analog optical fiber transmission device in which the decrease in the signal power to noise power ratio due to changes over time or changes in ambient temperature is small.

[0009]

SUMMARY OF THE INVENTION An analog optical fiber transmission apparatus according to the present invention comprises a plurality of access terminals cascaded with respect to one optical fiber, and an optical signal output from each of the plurality of access terminals. And an optical coupling means for coupling in the same transmission direction, and the access terminal includes an electro-optical converter for converting a high-frequency electric signal received as a radio wave into an optical signal analog-modulated by the signal. The converter is an analog optical fiber transmission device in which the respective emission wavelengths are set differently from each other so that the beat noise caused by the wavelength difference between them is outside the transmission band of the high frequency electric signal,
The optical branching means for branching the output of the corresponding optical coupling means from the optical fiber corresponding to at least the second and subsequent transmission directions of the access terminal, and the beat noise level of the branched optical signal are detected. And a control means for controlling the emission wavelength of the electro-optical converter in the access terminal based on the detection level.

Further, means for individually setting the start and stop of the control of the emission wavelength of the electro-optical converter in each access terminal is provided, and the emission wavelength of each access terminal is controlled so that the beat noise is reduced in the shortest time. The start and stop of the control can be set appropriately.

[0011]

The peak frequency of beat noise moves to a lower frequency as the wavelength difference between access terminals becomes smaller. If the emission wavelength is adjusted so that the wavelength difference between the access terminals is maximized, the beat noise is minimized. Therefore, the wavelength of the electro-optical converter of each access terminal is automatically adjusted to an appropriate value so that the beat noise is reduced. That is, in each access terminal, the optical transmission signal is extracted, the noise level is measured to detect the variation of the emission wavelength, and the emission wavelength is controlled to reduce the noise.

[0012]

1 is a block diagram showing an analog optical fiber transmission device according to a first embodiment of the present invention. The apparatus of this embodiment has a plurality of access terminals 5-1, 5-2, ... 5-n cascaded to the upstream optical fiber 9 and a plurality of access terminals 5-1, 5-2 ,. A plurality of access terminals 5-1 and 5-2 are provided, which are provided with optical couplers 8-1, 8-2, ... 8-n for coupling the respective output optical signals of −n to the upstream optical fiber 9 in the same transmission direction. , ... 5
Each -n includes an electro-optical converter 7 that converts a high-frequency electric signal (RF signal) received as a radio wave into an optical signal that is analog-modulated by the signal, and the electro-optical converter 7 has a wavelength difference between them. The respective emission wavelengths are set differently from each other so that the resulting beat noise is out of the transmission band of the high frequency electric signal. The feature of this embodiment is that a plurality of access terminals 5-1 and 5-
Optical couplers 12-1 and 12 that branch the outputs of the corresponding optical couplers 8-1, 8-2, ... From the upstream optical fiber 9 in correspondence with at least the second and subsequent transmission directions of 2 ,.
-2, ..., The beat noise level of the branched optical signal is detected, and the opto-electric converter as a control means for controlling the emission wavelength of the electro-optical converter 7 in the access terminal based on the detected level. 13 and the wavelength control circuit 14 are provided.

The wavelength control circuit 14 measures the noise level from the output of the photoelectric converter 13, and based on this measured value,
The emission wavelength of the electro-optical converter 7 is controlled so that the noise level is reduced. When a laser diode is used as the electro-optical converter 7, the drive current or the temperature is controlled in order to control the emission wavelength. The amount of change when the emission wavelength is changed by the drive current is such that it does not significantly affect the emission intensity.

The upstream RF signal of each access terminal is
It is converted into a light intensity modulated wave by the electro-optical converter 7, and is coupled to one upstream optical fiber 9 by the optical couplers 8-1, 8-2, ... 8-n. Therefore, the intensity of the optical signal on the upstream optical fiber 9 is a composite signal of the optical signals modulated by the RF signals at the respective access terminals, and at the output end of the upstream optical fiber 9, photoelectric conversion such as a photodiode is performed. The combined RF signal can be demodulated collectively by the device 10. With such a configuration, the RF signal can be transmitted by one optical fiber, and the beat noise can be constantly suppressed.

In this embodiment, an example in which separate optical couplers (8-1 and 12-1, 8-2 and 12-2, ...) Are used for optical signal synthesis and optical signal monitoring, respectively. 1 2 inputs 2
The present invention can be similarly implemented using an output directional coupler.

FIG. 2 shows a structural example of the electro-optical converter 7, and FIG. 3 shows another structural example of the electro-optical converter 7. In addition, FIG. 4 shows a configuration example of the photoelectric converter 13. Here, it is assumed that the light modulation method is light intensity modulation as in the conventional case. The electro-optical converter shown in FIG. 2 is a direct modulation type, and an input RF signal is superimposed on a bias current and applied to the laser diode 21.
The electro-optical converter shown in FIG. 3 is an external modulation type, and the output light of the laser diode 31 is intensity-modulated by the intensity modulator 32. As the electro-optical converter 13, for example, a photodiode 41 is used.

FIG. 5 shows a configuration example of the wavelength control circuit 14.
This circuit includes a bandpass filter 51 and a noise detector 52.
And a processor 53. Here, for example, the wavelength control circuit 14 in the access terminal 5-2 will be described as an example. Photoelectric converter 1 in this access terminal 5-2
In 3, the composite signal of the optical signals from the access terminals 5-2, ... 5-n is input and converted into an RF signal. The bandpass filter 51 has the opto-electric converter 13
The output RF signal component is blocked. The noise detector 52 measures beat noise due to the access terminals 5-2, ... 5-n. From the measurement result, the processor 53 determines how much the emission wavelength should be offset, and controls the emission wavelength by the drive current or the temperature.

FIG. 6 shows the relationship between the band of the transmitted RF signal and the band for detecting noise. The pass frequency of the band pass filter 51 is set outside the band of the RF signal to be transmitted. Here, only the case where the electro-optical converter 7 has good coherency will be described. When the measurement band of the noise level is set to a frequency very close to the RF signal transmission band, the smaller the noise level, the smaller the influence of the beat noise on the RF signal. On the other hand, when the measurement band of the noise level is set to a frequency much higher than the RF signal transmission band, the greater the beat noise in this band, the smaller the influence of the beat noise in the RF signal band. .

FIG. 7 shows an example of the flow of wavelength control processing by the processor 53. After measuring the noise level, the wavelength of the electro-optical converter 7 is changed by Δλ, and the noise level is measured again and compared with the previous measured level. When the noise level has decreased, the wavelength is returned to the original value, and Δλ is changed in the opposite direction, and the same processing is performed. When the noise level increases, Δλ is further changed and the same processing is performed to converge the wavelength to the maximum.

FIG. 8 is a block diagram showing an analog optical fiber transmission device according to the second embodiment of the present invention. The device of this embodiment has an electro-optical converter 1 in each access terminal.
The terminal control circuit 15 is provided as means for individually setting the start and stop of the emission wavelength control of No. 4, and the start and stop of the emission wavelength control at each access terminal are appropriately performed so that beat noise is reduced in the shortest time. The setting is different from the first embodiment.

Here, a case where five access terminals are provided, that is, n = 5 will be described as an example. When the access terminals 5-1 to 5-5 are turned on at the same time,
At Access Terminal 5-4, Access Terminal 5-
4, 5-5 beat noise, Access Terminal 5-3
In access terminal 5-3, 5-4, 5-5, beat noise of access terminal 5-2. In access terminal 5-2, beat noise of access terminal 5-2, 5-3, 5-4, 5-5, access terminal 5 At -1, beat noises of the access terminals 5-1, 5-2, 5-3, 5-4 and 5-5 occur. Therefore, there are many combinations of beat frequencies, and it may take a long time to reduce beat noise. The terminal control circuit 15 is used to shorten the time for reducing this noise. That is, the terminal control circuit 15 uses the access terminals 5-4 to 5-5.
A control signal is sent to each wavelength control circuit 14 so that wavelength control is performed in the order of -3, 5-2, and 5-1. If wavelength control is performed in this order, it is possible to converge in a short time.

FIG. 9 is a flow chart showing an example of the wavelength control processing procedure of the terminal control circuit 15. In the initial state, the wavelength control circuit of each access terminal is inactive. First, the wavelength control operation of the wavelength control circuit of the (n-1) th terminal is started. Wait for a certain period of time until the wavelength control is completed, and stop the operation of the wavelength control circuit of the terminal. After that, similarly, the operation of the wavelength control circuit of the access terminal is controlled in the order of n-2th and n-3th.

[0023]

As described above, the analog optical fiber transmission device of the present invention controls the wavelength of the electro-optical converter in each access terminal to thereby produce beat noise when converted into an electric signal. Can be reduced and an extremely high signal power to noise power ratio can be realized.

[Brief description of drawings]

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

FIG. 2 is a circuit diagram showing a configuration example of an electro-optical converter.

FIG. 3 is a block diagram showing another configuration example of the electro-optical converter.

FIG. 4 is a circuit diagram showing a configuration example of an optoelectric converter.

FIG. 5 is a block diagram showing a configuration example of a wavelength control circuit.

FIG. 6 is a diagram showing a relationship between a band of an RF signal to be transmitted and a band for detecting noise.

FIG. 7 is a diagram showing an example of a flow of wavelength control processing.

FIG. 8 is a block diagram showing an analog optical fiber transmission device according to a second embodiment of the present invention.

FIG. 9 is a flowchart showing an example of a wavelength control processing procedure of the terminal control circuit.

FIG. 10 is a block diagram showing a conventional analog optical fiber transmission device.

FIG. 11 is a diagram showing a relationship between two emission wavelengths and beat noise.

[Explanation of Codes] 1 Input Port 2 Electro-Optical Converter 3 Downstream Optical Fiber 4-1 to 4-n Optical Coupler 5-1, 5-2, ... 5-n Access Terminal 6 Opto-electric Converter 7 Electro-optical Converter 8-1, 8-2, ... 8-n Optical coupler 9 Upstream optical fiber 10 Photoelectric converter 11 Output port 12-1, 12-2, ... Optical coupler 13 Photoelectric converter 14 Wavelength control circuit 15 Terminal control circuit 21, 31 Laser diode 32 Intensity modulator 51 Bandpass filter 52 Noise detector 53 Processor

Continuation of front page (51) Int.Cl. ⁶ Identification number Office reference number FI Technical indication location H04Q 7/22 7/24 7/26 7/30 7605-5K H04Q 7/04 A

Claims (2)

[Claims] 1. A plurality of access terminals cascaded with respect to one optical fiber, and optical coupling means for coupling output optical signals of the plurality of access terminals to the optical fiber in the same transmission direction. The plurality of access terminals each include an electro-optical converter that converts a high-frequency electric signal received as a radio wave into an optical signal analog-modulated by the signal, and the electro-optical converter is caused by a difference in wavelength between each other. So that the beat noise is out of the transmission band of the high frequency electric signal,
In an analog optical fiber transmission device in which the respective emission wavelengths are set to be different from each other, the output of the corresponding optical coupling means corresponds to at least the second and subsequent transmission directions of the plurality of access terminals. Optical branching means for branching from
An analog optical fiber transmission comprising: a beat noise level of the branched optical signal; and a control means for controlling the emission wavelength of the electro-optical converter in the access terminal based on the detected level. apparatus. 2. The analog optical fiber transmission device according to claim 1, further comprising means for individually setting start and stop of control of an emission wavelength of the electro-optical converter in each of the plurality of access terminals.