JP2570150B2 - Optical receiving method, optical receiving circuit, and optical repeater - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical receiving method for performing identification reproduction by receiving an optical signal, and more particularly to an optical reception method for adjusting an identification voltage used at the time of identification reproduction. Further, the present invention relates to an optical receiving circuit or an optical repeater having an optical signal discriminating / reproducing function, and particularly to an optical receiving circuit or an optical repeater suitable for implementing the above-described optical receiving method.

[0002]

2. Description of the Related Art Generally, in an optical cable system, optical repeaters are arranged on an optical cable at predetermined intervals in order to compensate for transmission loss of an optical signal.

FIG. 4 schematically shows a conventional optical repeater. An optical signal 2-1 arriving via an optical cable (not shown) is input to the light receiving point 1. The light receiving element 3 receives the optical signal 2-1, and an element such as an avalanche photodiode is used. The equalizing amplifier circuit 4 amplifies the detection output of the light receiving element 3 to a level suitable for the subsequent processing. The timing extraction circuit 5 extracts a clock from the output signal of the equalization amplification circuit 4. The identification reproduction circuit 6 operates according to the output clock of the timing extraction circuit 5, and identifies and reproduces a pulse from the output signal of the equalization amplification circuit 4. The laser driving circuit 7 drives the laser 8 based on the output signal of the identification reproducing circuit 6. The laser 8 performs an oscillating operation according to the driving instruction, and outputs an optical signal 2-2. An optical signal 2-2 is output from an output point 9 to an optical cable (not shown).

The operation of this conventional optical repeater will be briefly described. The optical signal 2-1 input from the light receiving point 1 is converted into an electric signal by the light receiving element 3. A digital signal corresponding to the optical signal 2-1 is obtained by amplifying the electric signal by the equalizing amplifier circuit 4. A clock is extracted from the digital signal by the timing extraction circuit 5.
The identification reproducing circuit 6 identifies the digital signal in synchronization with this clock. That is, a pulse is identified by judging the level of the signal level, and is output as a reproduction reception signal. As a result, a reproduced signal with an improved SN ratio (signal-to-noise ratio) can be obtained. The laser drive circuit 7 drives the laser 8 based on the reproduction reception signal. The laser 8 emits an optical signal 2-2 modulated with a reproduction signal. This optical signal 2-2 is
From the output point 9 it is sent to an optical cable (not shown).

[0005]

In such an optical repeater, the point is to reduce the error rate of the reproduced reception signal. Conventionally, a discrimination voltage, that is, a threshold for pulse recognition is set based on the quality of an optical signal such as an expected S / N ratio, and discrimination and reproduction of a pulse are performed using the discrimination voltage, whereby reproduction and reception Ensures signal quality.

However, the optimum value of the threshold value fluctuates due to a difference in signal waveform shape, noise deterioration or interference deterioration in an optical transmission line. For this reason,
If the fixed threshold value deviates from the optimum value, the error rate of the reproduction signal of the identification reproduction circuit 6 increases, which causes the code error rate characteristics of the optical repeater to deteriorate.

As described in JP-A-2-288640, a code error rate of a reproduced reception signal is calculated.
There is a technique for correcting the threshold value based on the calculation result. In this case, it is necessary to add a function of calculating a bit error rate in addition to the function of correcting the threshold value. The problem that performance and inexpensiveness are obstructed arises. Especially,
In the case of an optical repeater, the main use mode is that the optical repeater is disposed on an optical cable system, and a complicated operation is required for maintenance. Therefore, it is necessary to realize a high degree of reliability.

Therefore, an object of the present invention is to control the threshold value used in the identification and reproduction processing to an appropriate value without involving a code error rate operation, and to reduce the error rate caused by the variation of the optimum value of the reference level value. An object of the present invention is to provide an optical receiving technique capable of avoiding deterioration.

[0009]

An optical receiving method according to the present invention comprises the following steps. (1) Convert the received optical signal into an electric signal. (2) The electric signal of (1) is identified by a reference threshold to generate a pulse signal, and this pulse signal is used as a reproduction reception signal. (3) A low threshold value lower than the reference threshold value and a high threshold value higher than the reference threshold value are set in conjunction with the reference threshold value. (4) Two types of pulse signals are generated by discriminating the reproduced reception signal by a low threshold value and a high threshold value, and the reference threshold value is determined in accordance with a deviation between these two types of pulse signals.
To adjust.

[0010] An optical receiving circuit according to a second aspect includes the following means. (1) Light receiving means for receiving an optical signal and converting it into an electric signal. (2) Identification reproducing means for identifying the electric signal of (1) by a reference threshold value. The discrimination reproduction means outputs a pulse signal obtained from the discrimination result as a reproduction reception signal. (3) Low-threshold identification / reproduction means for performing identification on the reproduced reception signal using a low threshold lower than the reference threshold. The low-threshold identification reproducing means outputs a pulse signal obtained from the identification result as a low-threshold reproduction reception signal. (4) High-threshold discrimination / reproduction means for discriminating a reproduction reception signal by a high threshold higher than a reference threshold. The high-threshold identification reproducing means outputs a pulse signal obtained from the identification result as a high-threshold reproduction reception signal. (5) Smoothing comparison means for generating a smooth deviation component between the low-threshold reproduced reception signal and the high-threshold reproduction reception signal. The smoothing comparing means outputs the smoothed deviation component as a threshold adjustment instruction signal. It does not matter before and after the smoothing part and the comparison part in the smoothing comparison means. (6) Threshold generation means for generating a reference threshold, a low threshold, and a high threshold. This threshold value generation means adjusts each threshold value based on the threshold value adjustment instruction signal.

An optical repeater according to a third aspect includes the following means. (1) Light receiving means for receiving an optical signal from a receiving-side optical transmission line and converting it into an electric signal. (2) Identification reproducing means for identifying the electric signal of (1) by a reference threshold value. The discrimination reproduction means outputs a pulse signal obtained from the discrimination result as a reproduction reception signal. (3) Optical modulation means for performing optical modulation based on a reproduction reception signal to generate a reproduction optical signal. This optical modulation means outputs a reproduced optical signal to a transmission-side optical transmission line. (4) A low-threshold discrimination / reproduction means for discriminating the reproduction reception signal by a low threshold lower than the reference threshold. The low-threshold identification reproducing means outputs a pulse signal obtained from the identification result as a low-threshold reproduction reception signal. (5) High threshold discrimination / reproduction means for discriminating the reproduction reception signal by a high threshold higher than the reference threshold. The high-threshold identification reproducing means outputs a pulse signal obtained from the identification result as a high-threshold reproduction reception signal. (6) Smoothing comparing means for generating a smooth deviation component between the low-threshold reproduced reception signal and the high-threshold reproduction reception signal. The smoothing comparing means outputs the smoothed deviation component as a threshold adjustment instruction signal. It does not matter before and after the smoothing part and the comparison part in the smoothing comparison means. (7) Threshold generation means for generating a reference threshold, a low threshold, and a high threshold. This threshold value generation means adjusts each threshold value based on the threshold value adjustment instruction signal.

[0012]

In discriminating and reproducing the electric signal obtained by photoelectrically converting the received optical signal, the optimum value of the threshold value in the discriminating reproduction changes due to the fluctuation of the optical transmission environment as described above. In the optical receiving method according to the first aspect, the threshold value can be adjusted by following a change in the optimum value of the threshold value. That is, if the above-described threshold is referred to as a reference threshold, two types of pulse signals are generated by separately performing discrimination and reproduction using a low threshold and a high threshold lower or higher than the reference threshold. . If the low threshold value and the high threshold value are set in conjunction with the reference threshold value, if the reference threshold value deviates from the optimum value, the error rate of one of the above two types of pulse signals Deterioration becomes remarkable. Therefore, by adjusting the reference threshold value according to the difference between the two types of pulse signals, the reference threshold value can follow the optimum value.

In the optical receiving circuit according to the second aspect, the optical signal is photoelectrically converted by the light receiving means, and the identification reproduction is performed by the identification reproduction means. On the other hand, the low threshold discriminating and reproducing means and the high threshold discriminating and reproducing means also perform the discriminating and reproducing operation.
The low threshold discriminating / reproducing means uses a low threshold value lower than the reference threshold value, assuming that the threshold value used in the discriminating / reproducing means is called a reference threshold value. Conversely, the high threshold discriminating and reproducing means uses a high threshold higher than the reference threshold. The deviation between the pulse signals obtained by the low threshold discriminating / reproducing means and the high threshold discriminating / reproducing means serves as an index of the difference between the reference threshold value and the optimum value as described above. For example, the two pulse signals are smoothed and matched by the smoothing comparing means to generate a smoothing deviation component. Adjust the high threshold. Thus, the reference threshold follows the optimum value, and the low threshold and the high threshold are linked to the reference threshold.

In the optical repeater according to the third aspect, the optical signal is received from the optical transmission line on the receiving side by the light receiving means and the identification and reproduction means, and is identified and reproduced. The reproduced optical signal is transmitted by the optical modulation means. Output to By adding the same means as in claim 2 to this optical repeater, it is possible to make the reference threshold follow the optimum value.

[0015]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described in detail below with reference to embodiments.

FIG. 1 schematically shows an optical repeater according to one embodiment of the present invention. An optical cable (not shown) is provided at the light receiving point 1
The optical signal 2-1 that arrives via is input. The light receiving element 3 receives the optical signal 2-1, and an element such as an avalanche photodiode is used. The equalizing amplifier circuit 4 amplifies the detection output of the light receiving element 3 to a level suitable for the subsequent processing. Timing extraction circuit 5
Extracts a clock from the output signal of the equalizing amplifier circuit 4. The first identification and reproduction circuit 10 operates in accordance with the output clock of the timing extraction circuit 5, and identifies and reproduces a pulse from the output signal of the equalization amplification circuit 4. The laser drive circuit 7 drives the laser 8 based on the output signal of the first identification reproduction circuit 10. The laser 8 performs an oscillating operation according to the driving instruction,
-2 is output. Optical signal 2-2 from output point 9
Is output to an optical cable (not shown).

The second identification reproduction circuit 11 and the third identification reproduction circuit 12 have the same configuration as the first identification reproduction circuit. Each of the identification reproducing circuits 10 to 12 has a different identification voltage, that is, a different threshold voltage for pulse identification. The integration circuits 13 and 14 smooth the output signals of the second identification reproduction circuit 11 and the third identification reproduction circuit 12, respectively. Comparator 15 includes integrating circuits 13 and 1
4 and outputs the deviation thereof.

The identification voltage control circuit 16 generates an identification voltage adjustment instruction based on the output signal of the comparator 15. The identification voltage variable circuit 17 includes the identification reproduction circuits 10 to 1
2 outputs an identification voltage, and changes the identification voltage level in accordance with the above-described identification voltage adjustment instruction.

The operation of the optical repeater will be described. Light receiving point 1
The optical signal 2-1 input from is converted by the light receiving element 3 into an electric signal. At the equalization amplifier 4, by amplifying the electrical signal to obtain a digital signal D _t corresponding to the optical signal 2-1. Clock is extracted from the digital data signal D _t by the timing extraction circuit 5. First regenerating circuit 10 performs identification of the digital data signal D _t in synchronization with this clock. That is, the level of the level of the output signal of the equalizing amplifier circuit 4 is determined to identify the pulse, and the digital signal obtained as a result of this determination is output as a reproduced data signal. In determining whether the signal level is high or low, the identification voltage output from the identification voltage variable circuit 17 is used as a threshold. As a result, a reproduced data signal with an improved SN ratio can be obtained. Laser drive circuit 7
Drives the laser 8 based on the reproduced data signal. The laser 8 outputs an optical signal 2−2 modulated by a reproduced data signal.
Inject 2. This optical signal 2-2 is sent from an output point 9 to an optical cable (not shown).

On the other hand, assuming that the identification voltage output to the first identification reproducing circuit 10 is V _th ,
The voltage V _th is shifted in the forward and reverse directions by a predetermined offset value a (a> 0), and the obtained voltages V _th −a and V _th + a are respectively _supplied to the second identification reproduction circuit 11 and the third identification reproduction circuit 12. It is output as the identification voltage. Identification reproduction circuit 11
And 12 are smoothed by the integration circuits 13 and 14, respectively. When the output of the integrating circuit 13 and 14 and V _2, V ₃ respectively, the comparator 15 outputs the deviations F _B of both voltage (= V ₂ -V _3). The deviation ΔV increases when the identification voltage V _th deviates from the optimum value, as described later. The identification voltage adjustment circuit 16 calculates the deviation F
_An identification voltage adjustment instruction is output so that _B is suppressed to zero or a minimum value. The discrimination voltage variable circuit 17 changes the discrimination voltage _Vth in accordance with the discrimination voltage adjustment instruction and maintains it at an optimum value.

Here, the behavior of the above-described deviation ΔV when the identification voltage V _th deviates from the optimum value will be described with reference to FIGS. 2 and 3. 2 and 3 show the transmission data signal and the identification voltages V _th , V _{th at} the mark rate １ ／.
The relationship between _th -a and V _th + a is shown. If based on the identification regeneration by identifying voltage V _th, the regenerator according to the identification voltage V _th -a L (low) and a high probability of identifying erroneous H (high) level level. Conversely, in the identification reproduction with the identification voltage V _th + a, there is a high probability that the H level is erroneously identified as the L level.

As shown in FIG. 2, when the identification voltage V _th takes the optimum value, the probability of both misidentification of regenerating by regenerating and identification voltage V _th + a by identification voltage V _th -a decreases, The deviation ΔV takes a minimum value. Further, as shown in FIG. 3, for example, when the optimum value fluctuates and the identification voltage _Vth deviates from the optimum value in the lower direction, the erroneous identification probability of the identification reproduction by the identification voltage _Vth− a increases, and the integration circuit 13 while the output voltage V ₂ is increased, erroneous identification probability of regenerating by identifying voltage V _th + a is less not vary, the output voltage V ₃ of the integrating circuit 14 remains low. Therefore, the deviation ΔV increases with the fluctuation of the output voltage V ₂ as the main increase component. Similarly, when the identification voltage V _th deviates from the optimum value in the high direction,
The erroneous identification probability of the identification reproduction by the identification voltage V _th + a increases, and the output voltage V ₃ decreases, and the deviation ΔV increases using the decrease as the main increase component.

Therefore, by adjusting the identification voltage _Vth so as to keep the deviation ΔV low, the identification voltage _Vth can follow the optimum value. In particular, when the mark ratio is 1/2, when the deviation ΔV is suppressed to almost zero, the discrimination voltage V _th −
voltage V _th is the center of a the identification voltage V _th + a is equal to the optimum value. If the mark ratio is other than 1/2, the ratio between the offset value -a and the offset value + a may be changed according to the mark ratio. The level of the deviation ΔV has a positive correlation with “a”. The offset value a may be appropriately set in consideration of this.

Further, in the present embodiment, an example of adjusting the identification voltage V _th on the basis of the deviations ΔV of the output voltage V ₂ and V ₃ of the integrating circuit 13 and 14, the present invention is not limited thereto . For example, the following aspects can be taken. First, the integrating circuit output stage of the first regenerator circuit 10 is added, and voltages V ₁ and smoothing the reproduced data signal. The difference ΔV ₁ between the voltage V ₁ and the voltages V ₂ and V ₃ (= V ₂ −V
₁ ), ΔV ₂ (= V ₁ −V ₃ ) are obtained and input to the identification voltage control circuit 16. The identification voltage control circuit 16 adjusts the identification voltage _Vth in the high direction when the deviation ΔV ₁ increases, and adjusts the identification voltage _Vth in the low direction when the deviation ΔV ₂ increases.

Further, in this embodiment, an example of application to an optical repeater has been described. However, the present invention is not limited to this, and can be applied to an optical receiving circuit that performs an operation of identifying and reproducing an optical received signal.

[0026]

Effect of the Invention] As described above, according to the optical receiving method according to claim 1, deviations between the resulting pulse signal by performing a regenerating using a low threshold and high threshold
Adjusts the reference threshold value according to the above , the following effects can be obtained. (1) The threshold used for discrimination reproduction can be made to follow the optimum value based on the actual reception situation, and high-quality optical reception operation can be maintained even when the optical transmission environment changes. (2) A reference threshold value according to the difference between the two types of pulse signals
Therefore, the quality can be improved without complicating the procedure.

According to the optical receiving circuit of the second aspect, a smooth deviation component between the pulse signals obtained by the low threshold discriminating / reproducing means and the high threshold discriminating / reproducing means is generated, and the smooth deviation component is used. Adjust the threshold. Therefore, it can be realized by a configuration in which only a feedback loop including a pulse signal smoothing element, a signal matching element, and the like is added. Therefore, not only can the quality of the optical receiving operation be improved, but also the complexity of the circuit configuration can be avoided and the reliability and low cost of the circuit can be ensured.

If the present invention is applied to an optical repeater as described in claim 3, it is useful in a long-distance optical cable system where the optical transmission environment is liable to change. In addition, in view of the difficulty of maintenance of the optical repeater arranged in the cable system, it is extremely practical that reliability of the optical repeater can be ensured without complicating the circuit configuration.

[Brief description of the drawings]

FIG. 1 is a block diagram schematically showing a main part of an optical repeater according to one embodiment of the present invention.

FIG. 2 is an explanatory diagram showing a relationship between a transmission data signal and an identification voltage when the identification voltage _Vth takes an optimum value.

FIG. 3 is an explanatory diagram showing a relationship between a transmission data signal and an identification voltage when the identification voltage V _th deviates from an optimum value in a low direction.

FIG. 4 is a block diagram schematically showing a main part of a conventional optical repeater.

[Explanation of symbols]

 REFERENCE SIGNS LIST 3 light receiving element 4 equalizing amplifier circuit 5 timing extraction circuit 7 laser drive circuit 8 laser 10 first identification reproduction circuit 11 second identification reproduction circuit 12 third identification reproduction circuit 13, 14 integration circuit 15 comparator 16 identification voltage control circuit 17 Variable identification voltage circuit

──────────────────────────────────────────────────続 き Continued on the front page (51) Int.Cl. ⁶ Identification code Agency reference number FI Technical display location H04B 10/17 10/26 H04L 25/03

Claims (3)

(57) [Claims] 1. A received optical signal is converted into an electric signal, the electric signal is identified by a reference threshold as a reference, a pulse signal is generated, and the pulse signal is used as a reproduction reception signal. Setting a low threshold value lower than the reference threshold value and a high threshold value higher than the reference threshold value in conjunction with the reference threshold value; And generating two types of pulse signals, and adjusting the reference threshold value according to a deviation between the two types of pulse signals. 2. A light receiving means for receiving an optical signal and converting it into an electric signal, identifying the electric signal based on a reference threshold, and outputting a pulse signal obtained from the identification result as a reproduced reception signal. Reproducing means for performing discrimination by a low threshold value lower than a reference threshold value on the reproduced received signal, and outputting a pulse signal obtained from the discrimination result as a low threshold reproduced signal; Means for discriminating the reproduced reception signal by a high threshold higher than a reference threshold, and outputting a pulse signal obtained from the discrimination result as a high threshold reproduction reception signal. And a smoothing comparing means for generating a smooth deviation component between the low threshold reproduction reception signal and the high threshold reproduction reception signal, and outputting the smooth deviation component as a threshold adjustment instruction signal. Threshold value generating means for generating the reference threshold value, the low threshold value, and the high threshold value, and adjusting each threshold value based on the threshold value adjustment instruction signal. An optical receiving circuit characterized by the above-mentioned. 3. A light receiving means for receiving an optical signal from a receiving side optical transmission line and converting it into an electric signal, discriminating the electric signal by a reference threshold value, and reproducing a pulse signal obtained from the discrimination result. Identification reproducing means for outputting as a received signal, optical modulation means for performing optical modulation based on the reproduced received signal to generate a reproduced optical signal, and outputting the reproduced optical signal to a transmission-side optical transmission line; A low threshold discriminating / reproducing means for performing a discrimination by a low threshold lower than the reference threshold, and outputting a pulse signal obtained from the discrimination result as a low threshold reproduced / received signal; performs identification by a reference threshold higher high threshold against a high threshold regenerating means for outputting a pulse signal obtained from the identification result as a high threshold reproduced received signal, re low threshold Generates a smoothing error component between the received signal and the high threshold reproduced received signal, a smoothing comparing means for outputting the smoothed error component as a threshold adjustment instruction signal, the reference threshold, low threshold and high An optical repeater for generating a threshold value, comprising: threshold value generation means for adjusting each threshold value based on the threshold value adjustment instruction signal.