JPH10145303A - Optical transmission system - Google Patents

Abstract

PROBLEM TO BE SOLVED: To optimize a CNR (signal to carrier ratio) and a distortion characteristic depending on a transmission distance in the optical transmission of an analog signal. SOLUTION: In the optical transmission system, where a main signal transmitter side equipment 10 sends an optical main signal to a main signal receiver 20 through an optical transmission medium, the main signal receiver 20 uses a level detector 204 and a light-receiving level calculation section 205 to detect a light-receiving level of the received optical main signal and to calculate a control variable for the main signal transmitter side equipment 10, so that the CNR characteristic and the distortion characteristic of the main signal transmission system within a prescribed standard based on the light receiving level. The calculated control variable is sent from the main signal receiver side equipment 20 to the main signal transmitter side equipment 10, and the main signal transmitter side equipment 10 adjusts the optical modulation of the optical main signal according to the control variable.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to an optical transmission system, and more particularly, to an optical transmission system having an optical modulation degree adjusting function.

[0002]

2. Description of the Related Art Various methods have been proposed for maintaining the transmission quality of a transmitted main signal at a required level when a main signal light is transmitted from a transmission side and received at a reception side.

[0003] For example, an optical transmitter disclosed in Japanese Patent Laid-Open No. 7-31580 discloses an analog transmitter by setting each optical modulation factor of an FM signal and a QPSK signal to be transmitted to a value that satisfies the transmission quality of the FM signal. The transmission quality when simultaneously transmitting the modulated and digitally modulated signals is ensured.

[0004]

However, in the above-mentioned conventional optical transmitter, only the optical modulation factor is set to an optimum value on the transmitting side, and the level actually received on the receiving side is taken into consideration. is not. For this reason, when the transmission loss varies depending on the distance of the transmission section, it is necessary to reset the optimum optical modulation degree for each transmission section. For example, if the distance of a certain optical transmission section is long, the signal-to-noise ratio characteristic (hereinafter, CNR)
It is called a characteristic. ) Deteriorates, it is necessary to increase the light modulation degree. However, if the light modulation degree is too high, the distortion characteristics of the light receiving circuit and the like will deteriorate when the transmission distance is short. Therefore, according to the conventional technique, it is impossible to set the optical modulation factor to optimize both the CNR characteristic and the distortion characteristic.

Further, the conventional technique cannot deal with a case where a transmission loss fluctuates due to some cause in a transmission section.

An object of the present invention is to provide an optical transmission system capable of always obtaining optimum CNR characteristics and distortion characteristics irrespective of fluctuations in transmission distance and transmission loss.

[0007]

An optical transmission system according to the present invention is an optical transmission system for transmitting an optical main signal from an apparatus for transmitting a main signal to an apparatus for receiving a main signal through an optical transmission medium.
Further, transmission means for transmitting an optical signal from the main signal receiving side device to the main signal transmitting side device is provided, and the main signal receiving side device detects a light receiving level of an optical main signal received through the optical transmission medium. Light receiving level detecting means, and a signal-to-noise ratio (CNR) of a main signal transmission system based on the light receiving level
Calculating means for calculating a control amount in the main signal transmitting side device so that both characteristics and distortion characteristics are within a predetermined standard range; and transmitting the control amount to the main signal transmitting side device by the transmitting means. Means, and the main signal transmitting side device has an optical modulation degree adjusting means for changing an optical modulation degree of the optical main signal according to the control amount received from the transmission means.

The main signal receiving side device detects the light receiving level of the received optical signal, determines the control amount so that the receiving state is optimal, and controls the optical modulation degree of the main signal transmitting side device according to the control amount. Therefore, optimal CNR characteristics and distortion characteristics can be obtained regardless of the distance between the main signal transmitting side device and the main signal receiving side device. Further, since the optimum optical modulation factor is set based on the light receiving level, the fluctuation of the transmission loss can always be dealt with with the optimum characteristics.

[0009]

FIG. 1 is a block diagram showing a first preferred embodiment of an optical transmission system according to the present invention. Here, for simplicity of description, only the transmitting and receiving units of the opposing transmitting side apparatus and receiving side apparatus in the optical transmission system are illustrated.

The main signal transmitting device 10 is connected to the main signal receiving device 20 via a transmission medium such as an optical fiber. The transmission section of the main signal transmitting side device 10 is driven by the variable gain amplifier 101 for variably amplifying the transmission main signal, which is a multi-wave analog signal modulated in the previous stage, to adjust the degree of light modulation, and the variable gain amplifier 101 And a laser light source 102 such as a laser diode (LD). The transmitting unit of the main signal transmitting side device 10 further includes an optical modulation degree receiving unit and an optical modulation control unit. The optical modulation degree receiving section receives a photodiode (P) for receiving an optical signal from the main signal receiving side device 20.
D) and the like, and a receiving circuit 104 for detecting a control signal from a signal photoelectrically converted by the light receiving element 103
The optical modulation control unit includes a gain controller 105 that adjusts the amplification of the variable gain amplifier 101 according to the received control signal.

The receiving section of the main signal receiving side device 20 receives a light receiving element 201 such as a photodiode for receiving the optical main signal received from the main signal transmitting side device 10 and a light receiving current IPH photoelectrically converted by the light receiving element 201. And a receiving circuit 202 for amplification. The receiving main signal output from the receiving unit is split into two by the splitter 203, and one is sent to the subsequent stage as a receiving main signal, and the other is sent to the data processing unit for adjusting the degree of optical modulation. The data processing unit includes a level detector 204 that detects a power level LD of the received main signal, a light reception level calculation unit 205 that calculates a light reception level LRCV received by the light receiving element 201 from a known value and the detected power level. A characteristic calculating unit 206 for calculating a CNR characteristic and a distortion characteristic from the received light level, and an optimal modulation factor calculating unit for calculating an optimal optical modulation factor DMOP such that both the calculated CNR characteristic and the distortion characteristic fall within a standard range. 207. The calculated optimum optical modulation degree DMOP is
8 and is converted into an optical signal by a laser light source 209 such as a laser diode.
3 is sent.

The resistance R [Ω] and the loss PLR of the receiving section
[W], the photoelectric conversion efficiency EPD of the light receiving element 201, the gain GPD of the receiving unit, and the loss PLB [W] of the branching unit 203.
Is a known value, is stored in a memory (not shown), and is used for calculating the light receiving level as described below. Also, the calculation formulas of the CNR characteristics and the respective distortion characteristics of the variable gain amplifier 101, the laser light source 102, the light receiving element 201, and the receiving circuit 202 are stored in the memory in advance, and are given to the characteristic calculation unit 206.

The light receiving level calculation unit 205 receives the power level LD [W] detected from the level detector 204 and calculates the light receiving current IPH of the light receiving element 201 according to (Equation 1).
[A] is calculated.

[0014]

L = IPH2R-PLR-PLB where R is the resistance of the receiver, PLR is the loss of the light receiver, PLB
Is the loss of the branching unit 203, which is given in advance to a memory (not shown).

By transforming (Equation 1), (Equation 2) is obtained of the light receiving current IPH.

[0016]

IPH = {(LD + PLR + PLB) / R} 1/2 Then, the calculated light receiving current IPH is substituted into (Equation 3) to calculate the light receiving level LRCV.

[0017]

## EQU3 ## where, e is the charge of electrons, h is Planck's constant, f is the frequency of light, EPD is the photoelectric conversion efficiency of the light receiving element 201, and GPD is light receiving. The unit gain.

By transforming equation (3), the received light level LRCV
(Equation 4) is obtained.

[0019]

LRCV = IPHhf / (e * EPD * GPD) The light receiving level LRCV calculated as described above is input, and the characteristic calculating unit 206 firstly uses the given characteristic of the light receiving unit to perform the light modulation degree DM. CNR [dB] with respect to (Equation 5)
Is calculated from

[0020]

## EQU5 ## CNR = (DM × LRCV × EPD / √2) 2 / (N
z + Ns + Nh) Here, Nz is the noise characteristic of the laser light source 102 and the optical transmission line, Ns is the shot noise of the light receiving element 201, and Nh is the thermal noise.

Further, the characteristic calculator 206 calculates the distortion characteristics of the entire circuit with respect to the degree of light modulation by summing the distortion characteristics of the respective devices given in advance. That is, the distortion characteristic of the entire circuit = (distortion characteristic of variable gain amplifier 101) +++ (distortion characteristic of laser light source 102) + (distortion characteristic of light receiving unit) +... , In general, the distortion characteristic of each device = (IP of each device 3−input or output value of each device) × 2. However, the distortion characteristic in this case is 2
IP3 is the intermodulation third-order distortion characteristic when the input or output value of each device is 0 dBm.
Whether it is an input value or an output value depends on the type of device. Since the input value or the output value of each device changes with the light receiving level or the light modulation, it is understood that increasing the light modulation deteriorates the distortion characteristics. Therefore, CNR
The characteristics deteriorate when the light modulation degree is reduced, and the distortion characteristics deteriorate when the light modulation degree is increased.

The optimum modulation factor calculation unit 207 includes a characteristic calculation unit 2
The CNR characteristics and distortion characteristics for the received light level LRCV input from 06 are compared with these CNR and distortion specification ranges, and the optimal optical modulation factor DMOP is determined so that both the CNR characteristics and distortion characteristics are within the specification ranges. I do.

The thus obtained optimum light modulation degree data DM
The OP is converted into an optical signal by the transmission circuit 208 and the laser light source 209, and the light receiving element 10 of the main signal transmission side device 10
3 is transmitted by the reverse line. This optical signal is converted into the optimum optical modulation factor data DMOP by the light receiving element 103 and the receiving circuit 104 of the main signal transmitting side device 10 and output to the gain controller 105. Gain controller 10
5 generates a control voltage corresponding to the optimum optical modulation factor data DMOP, and outputs the control voltage to the variable gain amplifier 101. By feeding back the optimum optical modulation factor data from the main signal receiving side device 20 in this way, the amplification of the variable gain amplifier 101 can be adjusted so that the transmitted optical main signal always has the optimum optical modulation factor.

In other words, based on the level LD detected by the level detector 204, the data processing unit 20
5 to 207 are light receiving units 201 and 202 given in advance.
The light receiving level LRCV is calculated on the basis of the above characteristics, and the CNR characteristic and the distortion characteristic are balanced in consideration of the noise characteristic and the distortion characteristic of the main signal light transmission system to calculate the optimum optical modulation factor. The calculated optimal optical modulation factor is placed on the reverse line and returned to the main signal transmitting side device 10, whereby the variable gain amplifier 101
Is controlled so as to obtain the optimum modulation factor. This makes it possible to realize optical transmission in which the CNR characteristics and the distortion characteristics are always balanced irrespective of the transmission distance between the transmission side and the reception side and irrespective of the loss fluctuation in the transmission path.

FIG. 2 is a block diagram showing a second embodiment of the present invention. In the second embodiment, an uplink and a downlink are provided between the center node and the local node,
The optical main signal is transmitted on each line, and the optimum modulation degree generated on the receiving side is transmitted to the transmitting side by multiplexing it on the uplink or downlink. Since the basic operation is the same as that of the first embodiment, the description will focus on the differences from the second embodiment.

In the figure, at the local node,
The transmission main signal, which is a multi-wave analog signal modulated in the previous stage, is variably amplified by a variable gain amplifier 301 for adjusting the degree of light modulation, and is provided with a laser light source 3 such as a laser diode (LD).
02 to the center node through the uplink.
At this time, a downlink control signal DMOP-D, which will be described later, is multiplexed with the laser light source 302. On the other hand, the optical main signal from the downlink is received by a light receiving element 303 such as a photodiode (PD).
After being received by the receiving circuit 304 and amplified by the AGC variable amplifier 305, the signal is split by the splitter 306 into a downlink main signal and an uplink control signal DMOP-U.
The uplink control signal DMOP-U is input to the gain controller 307 as described above, and the gain of the variable gain amplifier 301 is adjusted so as to obtain the optimum optical modulation factor. For the downlink main signal branched by the branching unit 306, as described above, the optimal modulation factor is calculated by the level detector 308 and the data processing unit 309, and the downlink control signal DMOP-D
Is generated and multiplexed with the upstream main signal by the laser light source 302.

At the center node, an optical main signal from the uplink is transmitted to a light receiving element 40 such as a photodiode (PD).
1 and received by the receiving circuit 402, amplified by the AGC variable amplifier 403, and then split by the splitter 404 into an uplink main signal and a downlink control signal DMOP-D. The downlink control signal DMOP-D is input to the gain controller 409 as described above, and the gain of the variable gain amplifier 410 is adjusted so as to obtain the optimum optical modulation factor. For the uplink main signal branched by the branching unit 404, as described above, the optimum modulation factor is calculated by the level detector 405 and the data processing unit 406, and the uplink control signal DM
An OP-U is generated and multiplexed with a downstream main signal by a laser light source 408 such as a laser diode (LD). On the other hand, the downlink transmission main signal, which is a multi-wave analog signal modulated in the previous stage,
The signal is variably amplified by a variable gain amplifier 410 that adjusts the degree of light modulation, and transmitted from a laser light source 408 to a local node via a downlink.

As described above, the optimum modulation degree is transmitted bidirectionally using the uplink and downlink, regardless of the transmission distance between the transmission side and the reception side, and regardless of the loss fluctuation in the transmission path. Thus, it is possible to realize optical transmission in which CNR characteristics and distortion characteristics are always balanced.

The variable gain amplifier 305 uses the power level of the downstream main signal detected by the level detector 308.
Of the variable gain amplifier 403 using the power level of the upstream main signal detected by the level detector 405.
Is controlled. This automatic gain control corrects the level of the main signal whose optical modulation has been adjusted by the variable gain amplifiers 301 and 410.

FIG. 3 is a schematic configuration diagram showing a loop system according to a third embodiment of the present invention. Here, a loop network is formed by only one-way lines, and each node is connected to the variable gain amplifier 4 via a processing unit that requires a main signal output from the branching unit 404 of the center node shown in FIG.
It is configured to provide 10 inputs. Therefore, for example, the optical main signal output from the node N1 in FIG. 3 is multiplexed with the optimal modulation factor data for the preceding node NM, and the optimal modulation factor data passes through the nodes N2 to NM-1, Upon reaching the node NM, the optical modulation factor adjustment is performed as described above.

[Brief description of the drawings]

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

FIG. 2 is a block diagram showing a second embodiment of the present invention.

FIG. 3 is a schematic configuration diagram illustrating a loop system according to a third embodiment of the present invention.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 10 Main signal transmission side apparatus 20 Main signal reception side apparatus 101 Variable gain amplifier 102 Laser diode 103 Photodiode 104 Receiving circuit 105 Gain controller 201 Photodiode 202 Receiving circuit 203 Branching device 204 Level detector 205 Light receiving level calculation part 206 CNR characteristic and Distortion characteristic calculation unit 207 Optimal modulation factor calculation unit 208 Transmission circuit 209 Laser diode

──────────────────────────────────────────────────の Continued on the front page (51) Int.Cl. ⁶ Identification code FI H04B 10/18

Claims (7)

[Claims] 1. An optical transmission system for transmitting an optical main signal from a main signal transmitting device to a main signal receiving device through an optical transmission medium, wherein the optical signal is transmitted from the main signal receiving device to the main signal transmitting device. A main signal receiving side device, a light receiving level detecting means for detecting a light receiving level of an optical main signal received through the optical transmission medium, and a signal to noise of a main signal transmission system based on the light receiving level. Calculating means for calculating a control amount in the main signal transmitting side device so that both the ratio (CNR) characteristic and distortion characteristic are within predetermined specification ranges; and transmitting the control amount to the main signal transmitting side device by the transmitting means. Transmission means for transmitting to the main signal transmission side device, the main signal transmission side device has an optical modulation degree adjustment means for changing the optical modulation degree of the optical main signal according to the control amount received from the transmission means, An optical transmission system, characterized in that: 2. The arithmetic unit includes: a characteristic generation unit configured to generate a CNR characteristic and a distortion characteristic of a main signal transmission system with respect to the light receiving level; and a control unit that controls the CNR characteristic and the distortion characteristic to fall within the predetermined standard range. The optical transmission system according to claim 1, further comprising: control amount calculation means for calculating the control amount in the main signal transmission side device. 3. The light modulation degree adjusting means includes: a light emitting means for generating the optical main signal in accordance with a transmission signal; a variable amplifier for variably amplifying the transmission signal; and a variable amplifier in accordance with the control amount received from the transmission means. The optical transmission system according to claim 1, further comprising: a gain controller that changes an amplification degree of the amplifier. 4. The optical transmission medium includes the transmission means,
3. The optical transmission system according to claim 1, further comprising a main signal line for transmitting the optical main signal and a control line for transmitting the control amount in an opposite direction. 5. The transmission means transmits another optical main signal,
The optical transmission system according to claim 1, wherein the control amount is multiplexed with the other optical main signal and transmitted. 6. A light receiving unit for receiving an optical main signal from the optical transmission medium and converting the received optical main signal into an electric main signal, a branching unit for branching a control main signal from the electric main signal. A level detecting means for detecting a power level of the control main signal, and a calculating means for calculating the light receiving level based on the power level and a predetermined characteristic data of the main signal transmission system. The optical transmission system according to claim 1, wherein: 7. An optical modulation degree control method in an optical transmission system for transmitting an optical main signal from a main signal transmitting side device to a main signal receiving side device through an optical transmission medium, wherein the main signal receiving side device is connected to the main signal transmitting side device. A light receiving level of the optical main signal received from the device through the optical transmission medium is detected, and a signal-to-noise ratio (CNR) characteristic and a distortion characteristic of the main signal transmission system are both within a predetermined standard range based on the light receiving level. The optical modulation degree is determined so that the optical modulation degree is transmitted to the main signal transmitting side device, and the main signal transmitting side device is configured to transmit the optical modulation degree according to the optical modulation degree received from the main signal receiving side device. An optical modulation degree control method, comprising: changing an optical modulation degree of a signal.