JPH07131440A - Analog optical fiber transmission system - Google Patents

Abstract

PURPOSE:To automatically reduce the increase of noise caused by the deviation of the emission wavelength due to the ambient temperature variance and the secular change. CONSTITUTION:The wavelength multiplexed optical signal of an optical fiber 14 is branched and inputted to a wavelength control circuit 21 by an optical coupler 23, and the input optical signal is converted into an electric signal by the circuit 21, and the noise level on the outside of the transmission RF signal band of this electric signal is measured, and a wavelength control signal is so made that the noise level is reduced, and a current control circuit 22 of one of terminals 111 to 11n is controlled to control the emission wavelength of an electrooptic transducer 12, and thus, emission wavelengths of terminals 111 to 11n are controlled so that the noise is not reduced.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention is applied to communication between an antenna of a base station and the inside of a station building in which the antenna is mounted in mobile communication, for example, and converts high frequency electric signals of a plurality of terminals into an electro-optical converter. The present invention relates to an analog optical fiber transmission system for converting optical signals having wavelengths different from each other and coupling and transmitting the optical signals to one optical fiber by an optical star coupler.

[0002]

2. Description of the Related Art Conventional analog optical fiber transmission system
Is shown in FIG. Multiple terminals 11 ₁~ 11_nsmell
These radio frequency (RF) signals are each electro-optically converted.
In the converter 12, optical signals of light intensity modulated waves having different wavelengths from each other
Are converted into optical signals by the optical star coupler 13.
An optical fiber 14 is coupled and incident on one end of one optical fiber 14.
It is transmitted as a wavelength division multiplexed signal by the bus 14. On the receiving side
The optical signal transmitted by the optical fiber 14 is photodiode.
The optical-electrical converter 15 like
No. 11 and each terminal₁~ 11_nBelieve in RF
No. is demodulated.

For example, the base station antenna in mobile communication is provided in at least three directions, and in the case of diversity reception, one antenna is provided for each, that is, a total of six antennas are provided on the roof of the station building. Although installed, the RF signals received by these 6 antennas are sent to the terminal 11 ₁
It has been proposed that each of the optical signals be converted into an optical signal in 11 to 11 _n and transmitted to the inside of the station through the optical fiber 14. In this case, since a plurality of RF signals can be transmitted by one optical fiber, the capacity is large as compared with the case of the coaxial cable, and economical transmission is possible.

In such an analog optical fiber transmission system
In each terminal 11₁~ 11 _nElectrical-to-optical converter
The 12 emission wavelengths are offset so that they are different from each other.
It The amount of offset wavelength is due to the wavelength difference.
Noise is set outside the RF signal transmission band
To do. For example, it transmits an RF signal of about 1 to 2 GHz.
In this case, set the emission wavelength offset amount to about 1 nm.
And the beat frequency becomes about 100 GHz, and the RF signal
It can be set outside the transmission band of. Such wavelength offset
Do not use an optical filter, etc.
In addition, the upstream RF signal can be transmitted by one optical fiber.

[0005]

As described above, in the conventional analog optical fiber transmission system, the emission wavelengths of the electro-optical converters are offset from each other to reduce the beat noise in the transmission band. However, the emission wavelength of the electro-optical converter 12 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.

As an example, as shown in FIG. 6A, the wavelength of the laser diode (LD) as the electro-optical converter 12 of the terminal 11 ₁ is λ1, the wavelength of the electro-optical converter 12 of the terminal 11 ₂ is λ2, and the terminal Consider the case where the wavelength of the electro-optical converter 12 of 11 ₃ is λ3. When the wavelength of the electro-optical converter 12 of the terminal 11 ₂ is λ2, the wavelength is shorter than the originally set wavelength λ2 ′, so that it overlaps with the spectrum of the optical signal from the terminal 11 ₁ and the RF signal is transmitted. The beat noise in the band increases, and the S / N characteristics deteriorate significantly. For example, when the RF transmission band is 2 GHz or less, λ2-λ1 <0.01
When it becomes nm, beat noise influences the transmission band. Further, since the spectrum of the optical signal of the electro-optical converter 12 has phase noise, it has a distribution broader than the central wavelength, and even if the emission wavelength fluctuates λ2-λ1> 0.01 nm or more, the RF It will affect the transmission band. As shown in FIG. 6A, terminal 1
When the wavelength difference between the wavelength λ2 of 1 ₂ and the wavelength λ1 of the terminal 11 ₁ becomes smaller, the beat noise level in the transmission band of the RF signal increases as shown in FIG. 6B.

The present invention provides an analog optical fiber transmission system having means for reducing an increase in beat noise caused by a wavelength variation of an electro-optical converter which occurs when a large capacity transmission is performed by wavelength multiplexing. It is in.

[0008]

According to the present invention, the transmission signal of the optical fiber is branched by the optical coupler and supplied to the wavelength control circuit, and the noise level in the optical signal is measured by the wavelength control circuit. A wavelength control signal for each terminal is generated so that its noise level is reduced. The wavelength control signal is supplied to the drive current control circuit of the corresponding terminal, and the drive current control circuit controls the drive current of the electro-optical converter of the terminal according to the wavelength control signal to control the emission wavelength. It

In the invention of claim 2, a temperature control circuit is provided in place of the drive current control circuit in the invention of claim 1,
The temperature of the electro-optical converter at the terminal is controlled according to the inputted wavelength control signal to control the emission wavelength.

[0010]

FIG. 1 shows an embodiment of the invention of claim 1 and FIG.
The same symbols are attached to the portions corresponding to. In the present invention, the wavelength control circuit 21 is provided on the side of the terminals 11 _{1 to} 11 _n , and the wavelength control signal generated by the wavelength control circuit 21 for each terminal is controlled by the drive currents provided to the terminals 11 _{1 to} 11 _n. It is supplied to the corresponding one of the circuits 22. Also, the optical star coupler 1 of the optical fiber 14
The optical coupler 23 is provided on the third side, and the optical fiber 14
A part of the optical signal transmitted by the optical fiber is branched and the wavelength control circuit 2
1 is supplied.

The wavelength control circuit 21 measures the noise level of the input optical signal, and generates a wavelength control signal so as to reduce the noise level based on the measured value. For example, as shown in FIG. 2A, the optical signal from the optical coupler 23 is the optical-electrical converter 2
4 is converted into an RF signal and demodulated, and the RF signal is input to the bandpass filter 25 to block the band of the transmission RF signal as shown in FIG. 2B, and a part of the band outside the band of the transmission RF signal is bandpassed. The noise picked up from the filter 25 is detected by the noise detector 26, and the level of this detected noise is measured by the processor 27. From the measured noise level, the processor 27 will determine the emission wavelength of either terminal from the measured noise level.
It is determined how much to offset (shift), a wavelength control signal is created according to the offset amount, and the drive current control circuit 22 of the terminal corresponding to the wavelength control signal is generated.
Supply to.

A laser diode, for example, is provided as the electro-optical converter 12 in each of the terminals 11 _{1 to} 11 _n , and the drive current control circuit 22 responds to the input wavelength control signal by the laser diode (electric-optical) of its own station. Converter) 1
2 is changed, and the emission wavelength of the laser diode 12 is changed accordingly. Here, the amount of change in the drive current is set to a level that does not significantly affect the emission intensity. The control of the drive current is performed, for example, as shown in FIG.
It suffices to control the bias current to be applied to 2.

The processor 27 of the wavelength control circuit 21 sends a signal
An example of the wavelength control process will be described with reference to the flowchart of FIG.
First, the noise level outside the band of the transmitted RF signal (hereinafter simply referred to as noise
Measured as (level) (S₁), I-th terminal
Shift the wavelength of the 11i laser diode 12 by + Δλ.
(S₂). Measure the noise level at this time
(S₃), The measured level is
Check the change state (S _Four), If increasing
Is the laser diode 12 of the i-th terminal 11i
To the original wavelength and shift by -Δλ
(S_Five). In this state, measure the noise level (S₆), So
Of the noise level immediately before (S₇), The increasing place
In the case of i-th terminal 11i laser diode 12
To the original wavelength (S₈), Then increment i by 1
Step S₂Return to and transfer control to the next terminal
(S₉). Step S_FourThe noise level changed
If not, step S₈Move to i-th terminal
Return the wavelength of the 11i laser diode 12 to the original
If the sound level is decreasing, step S₉Move on to. Su
Step S₇If the measurement noise level does not change in
If step S₈Move on to step S₉
Move on to.

In this way, each terminal 11 _{1 to} 11
By controlling the wavelength of the laser diode 12 with respect to _n and controlling so that the noise level does not change or decreases at that time, the wavelengths of the laser diodes 12 of all the terminals 11 _{1 to} 11 _n are changed. The noise does not change even if it changes by a certain amount, and the wavelength converges to a wavelength at which the noise is minimized. This condition is terminal 11 ₁ to 1
The wavelength difference between the emission wavelengths of the laser diodes 12 of 1 _n is large, and the frequency of the difference wavelength does not fall within the frequency band of the transmission RF signal, and is not affected by the mutual interference of the optical signals.

Next, referring to FIG. 4, an embodiment of the invention according to claim 2 will be described by giving the same reference numerals to the portions corresponding to those in FIG. In this case, the laser diode current control circuit 2 in FIG.
A laser diode temperature control circuit 31 is provided instead of 2.
The emission wavelength of the laser diode 12 is controlled by changing the temperature of the laser diode 12 according to the wavelength control signal.
This temperature control can be easily realized by using, for example, a Peltier element. That is, the temperature is controlled by changing the current flowing through the Peltier element. Alternatively, it may be controlled by using a heating element used in a constant temperature bath or the like. The control procedure and the like may be the same as in the case of FIG.

The present invention is applicable not only to mobile communication but also to an analog optical fiber transmission system for wavelength division multiplexing transmission.

[0017]

As described above, according to the present invention, wavelength-multiplexed optical signals can be collectively converted into RF electric signals, and moreover, the emission wavelength fluctuates due to changes in ambient temperature and changes over time, resulting in wavelength interference. When the influence occurs, the emission wavelength is automatically controlled so as to eliminate the influence, and it is possible to provide an analog optical fiber transmission system having an extremely high S / N.

[Brief description of drawings]

FIG. 1 is a block diagram showing an embodiment of the invention of claim 1;

2A is a block diagram showing a specific example of the wavelength control circuit 21, FIG. 2B is a diagram showing a relationship example between a transmission RF signal band and a detection noise band, and C is a circuit showing a specific example of the electro-optical converter 12. FIG. It is a figure.

FIG. 3 is a flow chart showing an example of a control procedure of an emission wavelength of each terminal in the wavelength control circuit 21.

FIG. 4 is a block diagram showing an embodiment of the invention of claim 2;

FIG. 5 is a block diagram showing a conventional analog optical fiber transmission system.

FIG. 6A is a diagram showing a state where the emission wavelength of the terminal is deviated from a set value, and B is a diagram showing a relationship between the emission wavelength shift and a noise level.

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Claims (2)

[Claims] 1. An analog optical system in which high-frequency electrical signals from a plurality of terminals are converted into optical signals having mutually different wavelengths by an electro-optical converter, and these optical signals are coupled and transmitted to one optical fiber by an optical star coupler. In a fiber transmission system, an optical coupler that splits the optical signal transmitted by the optical fiber and the noise level in the optical signal split by the optical coupler are measured, and the noise level of each terminal is reduced so that the noise level is reduced. A wavelength control circuit that generates a wavelength control signal, provided in each of the terminals, and controls the drive current of the electro-optical converter of the terminal according to the wavelength control signal input from the wavelength control circuit, An analog optical fiber transmission system comprising: a drive current control circuit that controls the emission wavelength. 2. An analog optical signal in which high-frequency electrical signals from a plurality of terminals are converted into optical signals having mutually different wavelengths by an electro-optical converter, and these optical signals are coupled and transmitted to one optical fiber by an optical star coupler. In a fiber transmission system, an optical coupler that splits the optical signal transmitted by the optical fiber and the noise level in the optical signal split by the optical coupler are measured, and the noise level of each terminal is reduced so that the noise level is reduced. A wavelength control circuit for generating a wavelength control signal, and provided in each of the terminals, according to the wavelength control signal input from the wavelength control circuit, to control the temperature of the electro-optical converter of the terminal, An analog optical fiber transmission system comprising: a temperature control circuit for controlling the emission wavelength, and a temperature control circuit.