JP3016462B2 - Analog optical fiber transmission system - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention is applied to, for example, communication between an antenna of a base station in a mobile communication system and a building in which the antenna is mounted, and converts an electric signal from a plurality of terminals into an electro-optical converter. The present invention relates to an analog optical fiber transmission system in which optical signals of different wavelengths are converted into optical signals, and these optical signals are coupled and transmitted 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
Each of these radio frequency (RF) signals is
Optical signals of light intensity modulated waves having different wavelengths in the converter 12
These optical signals are converted by the optical star coupler 13 into
The light is coupled into one end of one optical fiber 14 and
The signal is transmitted as a wavelength multiplexed signal by the bus 14. On the receiving side
The optical signal transmitted by the optical fiber 14 is a photodiode
RF-electrical converter 15 such as
Is converted into a terminal and each terminal 11₁~ 11_nRF
The signal is demodulated.

For example, base station antennas for mobile communication are 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 installed on the rooftop of a station building. Although installed, terminals 11 ₁ receive RF signals received by these six antennas.
It has been proposed that the optical signals are converted into optical signals at .about.11 _n and transmitted through the optical fiber 14 into the office. In this case, since a plurality of RF signals can be transmitted by one optical fiber, the capacity is larger than that of a coaxial cable, and economical transmission is possible.

In such an analog optical fiber transmission system,
In each terminal 11₁~ 11 _nElectro-optical converter
Twelve emission wavelengths are offset so as to be different from each other.
You. The amount of wavelength to be offset is determined by the
Noise is set outside the transmission band of the RF signal
I do. For example, transmitting an RF signal of about 1 to 2 GHz
In this case, the emission wavelength offset amount is set to about 1 nm.
And the beat frequency becomes about 100 GHz, and the RF signal
Out of the transmission band. Such offset of wavelength
Do not use optical filters on the receiving side
In addition, an upstream RF signal can be transmitted with one optical fiber.

[0005]

As described above, in the conventional analog optical fiber transmission system, the emission wavelengths of the respective 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 an initial set value due to a change with time or a change in ambient temperature. For this reason, beat noise may increase during operation, and the signal power to noise power ratio (S / N) may significantly decrease.

[0006] As an example, the terminal 11 ₁ of electrically as shown in FIG. 6A - wavelength of the laser diode (LD) as an optical transducer 12 .lambda.1, terminal 11 ₂ of the electric - light wavelength converter 12 is .lambda.2, terminal 11 _third electrical - wavelength optical transducer 12 is considered for the case where a [lambda] 3. Terminal 11 _second electrical - case of the wavelength of the light transducer 12 .lambda.2, because of the shorter wavelength than the wavelength .lambda.2 'being originally set overlaps with the spectrum of the optical signal from the terminal 11 _1, the transmission of RF signals The beat noise in the band increases, and the S / N characteristic is significantly deteriorated. For example, when the RF transmission band is 2 GHz or less, λ2−λ1 <0.01
When it becomes nm, beat noise affects the transmission band. Further, since the spectrum of the optical signal of the electro-optical converter 12 has phase noise, it has a distribution wider than the center wavelength, and even if the emission wavelength fluctuates more than λ2-λ1> 0.01 nm, the RF It will affect within the transmission band. As shown in FIG. 6A, terminal 1
And 1 ₂ of the wavelength .lambda.2, the wavelength difference between the wavelength λ1 of the terminal 11 ₁ is reduced, the beat noise level in the transmission band of the RF signal is increased as shown in Figure 6B.

An object of the present invention is to provide an analog optical fiber transmission system having means for reducing an increase in beat noise caused by fluctuations in the wavelength of an electro-optical converter, which occurs when performing large-capacity transmission by wavelength multiplexing. It is in.

[0008]

According to the first aspect of the present invention, a transmission optical signal of an optical fiber is split by an optical coupler,
Transmission optical signal split by optical coupler to wavelength control circuit
And convert this signal with a wavelength control circuit with an optical-electrical converter.
Each high-frequency electric signal in the electric signal obtained by conversion and conversion
Out-of-band noise level and reduce the noise level
Generate a wavelength control signal for each terminal
You. Each terminal has a wavelength control input from the wavelength control circuit.
Driving the electro-optical converter of the terminal in response to a control signal
The current is controlled to control the emission wavelength.

According to a second aspect of the present invention, a temperature control circuit is provided in place of the drive current control circuit according to the first aspect of the present invention,
The temperature of the electro-optical converter at the terminal is controlled according to the input wavelength control signal, and the emission wavelength is controlled.

[0010]

FIG. 1 shows an embodiment of the invention of claim 1, and FIG.
The same reference numerals are given to portions corresponding to. The present invention wavelength control circuit 21 is provided to the terminal 11 ₁ to 11 _n side, the wavelength control signal in the wavelength control circuit 21 is generated for each terminal is, the drive current control respectively provided to the terminal 11 ₁ to 11 _n It is supplied to a corresponding one of the circuits 22. The optical star coupler 1 of the optical fiber 14
An optical coupler 23 is provided on the third side, and an optical fiber 14 is provided.
A part of the optical signal transmitted by the wavelength control circuit 2 is branched.
1 is supplied.

The wavelength control circuit 21 measures the noise level of the input optical signal, and generates a wavelength control signal based on the measured value so as to reduce the noise level. For example, as shown in FIG. 2A, the optical signal from the optical coupler 23 is
4, the RF signal is converted and demodulated into an RF signal, and the RF signal is input to a band-pass filter 25 to block the band of the transmission RF signal as shown in FIG. 2B. The noise extracted from the filter 25 is detected by a noise detector 26, and the level of the detected noise is measured by a processor 27. As will be described later, the processor 27 determines the emission wavelength of any terminal from the measured noise level.
A determination is made as to how much the offset (shift) is made, a wavelength control signal is generated in accordance with the offset amount, and the wavelength control signal is applied to the drive current control circuit 22 of the corresponding terminal
Supply to

[0012] Electrical In each terminal 11 ₁ to 11 _n - is provided an optical transducer 12 and to, for example, a laser diode, the driving current control circuit 22 of own station in accordance with the input wavelength control signal laser diode (electro - optical Converter) 1
2, the drive current of the laser diode 12 is changed, whereby the emission wavelength of the laser diode 12 is changed. Here, the amount of change in the drive current is set to such an extent that the emission intensity is not significantly affected. The drive current is controlled, for example, as shown in FIG.
2 may be controlled.

The processor 27 of the wavelength control circuit 21
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 transmission RF signal (hereinafter simply referred to as noise
Level) (S)₁), I-th terminal
Shifts the wavelength of the laser diode 12i by + Δλ.
(S_Two). Measure the noise level at this time
(S_Three), This measured level is
Change state (S _Four), If it ’s growing
Is the laser diode 12 of the i-th terminal 11i.
Is returned to the original wavelength and further shifted by -Δλ
(S_Five). In this state, the noise level is measured (S₆)
Is compared with the noise level immediately before (S₇), An increasing place
The laser diode 12 at the i-th terminal 11i
To the original wavelength (S₈), Then add i to +1
Tep S_TwoReturn to control for the next terminal
(S₉). Step S_FourThe noise level changes at
If not, step S₈Move to the i-th terminal
Return the wavelength of the laser diode 11i to its original
Step S if the sound level is decreasing₉Move on to S
Tep S₇If the measurement noise level does not change at
If step S₈To step S.₉
Move on to

Thus, each of the terminals 11 _{1 to} 11
and controls the wavelength of the laser diode 12 for _n, then the or noise level does not change, by performing sequentially repeatedly be controlled to decrease, the wavelength of the laser diode 12 of all terminals 11 ₁ to 11 _n Even if it changes by a certain amount, the noise does not change, and it converges to a wavelength at which the noise is minimized. In this state, terminals 11 ₁ to 1
The wavelength difference between the emission wavelengths of the 1 _n laser diodes 12 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 interference between optical signals.

Next, referring to FIG. 4, an embodiment of the present invention according to claim 2 is shown by attaching the same reference numerals to parts corresponding to those in FIG. In this case, the laser diode current control circuit 2 shown in FIG.
A laser diode temperature control circuit 31 is provided in place 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 using, for example, a Peltier element. That is, the temperature is controlled by changing the current flowing through the Peltier element. Alternatively, the control may be performed by using a heating element used in a thermostat or the like. The control procedure and others may be the same as in the case of FIG.

The present invention can be applied not only to mobile communication but also to an analog optical fiber transmission system for wavelength multiplex transmission.

[0017]

As described above, according to the present invention, a wavelength multiplexed optical signal can be converted into an RF electric signal in a lump, and furthermore, the emission wavelength fluctuates due to ambient temperature fluctuation and aging, and wavelength interference can be reduced. When the influence occurs, the emission wavelength is automatically controlled so as to eliminate the influence, and an analog optical fiber transmission system having an extremely high S / N can be provided.

[Brief description of the drawings]

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

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

FIG. 3 is a flowchart showing an example of a procedure for controlling the 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 shifted from a set value, and FIG. 6B is a diagram showing a relationship between the emission wavelength shift and a noise level.

────────────────────────────────────────────────── ─── Continued on the front page (51) Int.Cl. ⁷ Identification symbol FI H04B 10/18 H04J 14/02 (56) References JP-A-63-253739 (JP, A) JP-A-64-18132 (JP) , A) JP-A-6-153255 (JP, A) JP-A-4-48832 (JP, A) Makoto Shibuya, et al. "Wide area surveillance information transmission system-aiming at video information service at any point in city-" IEICE Technical Report, CS92-25, Vol. 92, No. 78, p. 89-96 (Issued May 29, 1992) (58) Fields investigated (Int. Cl. ⁷ , DB name) H04B 10/00-10/28 H04J 14/00-14/08

Claims (2)

(57) [Claims] 1. A electric each high-frequency electrical signals of a plurality of terminals - each by the optical converter to convert the mutually different wavelengths of the optical signal, and wavelength-multiplexed into a single optical fiber by respective optical signals optical star coupler Te, in the analog optical fiber transmission system for transmitting a transmitted optical signal, an optical coupler for branching the transmitted optical signal of the optical fiber, the transmission optical signals branched by the optical coupler light - electrical variable
Each high-frequency electricity in the obtained electrical signal
A wavelength control circuit that generates a wavelength control signal for each of the terminals and measures the noise level outside the band of the signal and reduces the noise level.The wavelength control circuit is provided in each of the terminals and is input from the wavelength control circuit. A drive current control circuit for controlling a drive current of the electro-optical converter of the terminal according to the wavelength control signal to control an emission wavelength thereof. . Wherein electrical respective high-frequency electrical signals of a plurality of terminals - each by the optical converter to convert the mutually different wavelengths of the optical signal, and wavelength-multiplexed into a single optical fiber by respective optical signals optical star coupler Te, in the analog optical fiber transmission system for transmitting a transmitted optical signal, an optical coupler for branching the transmitted optical signal of the optical fiber, the transmission optical signals branched by the optical coupler light - electrical variable
Each high-frequency electricity in the obtained electrical signal
A wavelength control circuit that generates a wavelength control signal for each of the terminals and measures the noise level outside the band of the signal and reduces the noise level.The wavelength control circuit is provided in each of the terminals and is input from the wavelength control circuit. An analog optical fiber transmission system, comprising: a temperature control circuit that controls the temperature of the electro-optical converter at the terminal according to the wavelength control signal and controls the emission wavelength.