JP2837209B2 - Optical receiver and optical communication system including the optical receiver - Google Patents

Description

The present invention relates to an optical communication system including an optical amplifier and an optical receiver as a component of the system in an intensity modulation / direct detection system, and a waveform generated when the optical amplifier is used in a gain saturated state. The main object of the present invention is to prevent an increase in the bit error rate due to deterioration. For example, a photodetector that photoelectrically converts received intensity-modulated light and outputs a signal of a level corresponding to the light intensity, For an input signal, an input level smaller than a predetermined limit level is an output level corresponding to the input level, and an input level higher than the limit level is a fixed level corresponding to the input of the limit level. A limiter circuit that is an output level, and the limit level is controlled to a predetermined level between a maximum level and a minimum level when an input signal to the limiter circuit is marked. And a limit level control circuit to be controlled.

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical communication system including an optical amplifier in an intensity modulation / direct detection system and an optical receiver that is a component of the system.

In recent years, in order to improve the performance of an optical communication system, introduction of an optical amplifier that directly amplifies an optical signal without converting it into an electric signal into an optical communication system has been considered. But,
When the input light intensity of the optical amplifier is excessive, a gain saturation state occurs, which deteriorates a transmission waveform. In particular, when multistage relaying is performed using a plurality of optical amplifiers, waveform distortion may be accumulated and the code error rate may increase. For this reason, in an optical communication system including an optical amplifier, it is necessary to effectively prevent the waveform deterioration and prevent the code error rate from increasing.

2. Description of the Related Art FIG. 10 shows an example of a conventional optical receiver used in an optical communication system to which an intensity modulation / direct detection system is applied. The received intensity-modulated light is photoelectrically converted by the photodetector 101, amplified by the pre-amplifier 102, the variable gain AGC amplifier 103, and the post-amplifier 104, and then input to the discriminator 105. The discriminator 105 reproduces the transmission information based on the set discrimination level and the clock signal from the clock extraction circuit 106. An AGC circuit 107 detects a peak value of a signal input to the discriminator 105, and performs feedback control of a multiplication factor of the light receiver 101 and / or an amplification factor of the AGC circuit 103 so that the detected value becomes constant. .

Problems to be Solved by the Invention In an optical communication system comprising the optical receiver shown in FIG. 10 as a component, when an optical amplifier for relaying or the like is provided in an optical transmission line, reception by a pattern effect is performed. The bit error rate in the optical receiver may increase due to signal waveform deterioration. This will be described in detail.

FIG. 11 shows input / output characteristics of an optical semiconductor amplifier (semiconductor laser type optical amplifier). FIG. 6 shows a relation between a steady input light intensity and an output light intensity as an input / output characteristic curve. The gain is constant in a region where the input light intensity is low, but the gain is saturated when the input light intensity becomes excessive. When the optical amplifier is used in a gain-saturated state and the carrier lifetime (inversion distribution lifetime) of the optical amplifier is about the same as one time slot time of a transmission signal, a step response waveform as shown in FIG. Deterioration will occur.
That is, at the rise (A) immediately after the continuous space, the signal is amplified with a non-saturation gain larger than the saturation gain (P _A ),
When the mark falls (B) after the mark continuation, the signal is amplified with a saturation gain (P _B ), and the waveform is distorted.

When an optical fiber amplifier using an Er-doped fiber or the like is used as the optical amplifier, its population inversion lifetime is 15
Since the time is as long as about ms, this waveform distortion can be neglected in high-speed transmission of about several Gb / s, but when the optical amplifier is an optical semiconductor amplifier, the population inversion lifetime is about several ns. Therefore, the waveform distortion cannot be ignored in high-speed transmission of about several Gb / s. That is, since the form of the distorted waveform is different depending on the continuation of the mark and the space immediately before the waveform, when a large number of optical semiconductor amplifiers are connected in series,
The eye distortion is remarkably deteriorated due to the accumulation of the waveform distortion.

FIG. 13 shows an example of an eye pattern after multi-stage relay. Eleventh
FIG. 10 illustrates a result of simulating an eye pattern for a 1 Gb / s transmission signal by connecting optical semiconductor amplifiers having the input / output characteristics illustrated in FIG. 10 at 10 stages with input light intensity set to −17 dBm. . When the eye pattern is deteriorated in this way, when the AGC control based on the peak detection value is performed as described with reference to FIG. 10, the AGC control does not follow the level fluctuation within one time slot of the transmission signal. Depending on the setting of the discrimination level of the discriminator, the bit error rate will be greatly increased.

The present invention has been created in view of such circumstances,
It is an object of the present invention to prevent an increase in a bit error rate due to waveform deterioration that occurs when an optical amplifier is used in a gain saturated state.

Means for Solving the Problems FIG. 1 is a diagram showing a basic configuration of the present invention. The optical communication system shown in FIG. 1 includes an optical transmitter 4 that outputs intensity-modulated light, an optical transmission line 5 that transmits light from the optical transmitter 4, and a light that is transmitted by the optical transmission line 5. It comprises a single or a plurality of optical amplifiers 6 for amplification and an optical receiver 7 to which light from the optical transmission line 5 is input.

A first configuration of an optical receiver according to the present invention includes a photodetector 1 that photoelectrically converts received intensity-modulated light and outputs a signal of a level corresponding to the light intensity, a limiter circuit 2, and a limiter circuit for setting a limit level described later. And a limit level control circuit 3 for controlling the level of the input signal to a predetermined level between the maximum and minimum levels when the input signal is marked.

In the limiter circuit 2, as shown in FIG. 2, a signal from the light receiver 1 (not limited to an output signal of the light receiver 1).
For the input level, for an input level smaller than the predetermined limit level _RL , an output level corresponding to this input level (for example, an output level proportional to the input level), and for an input level higher than the limit level _RL, In other words, the output level becomes a constant output level corresponding to the input of the limit level _RL .

As shown in FIG. 3, the second configuration of the optical receiver according to the present invention includes, in addition to the first configuration, a limiter circuit 2 and a limit level control circuit 3 that function with respect to the output signal of the limiter circuit 2. The limit level in each limiter circuit 2 is sequentially reduced from the light receiver 1 side.

The optical communication system of the present invention is the same as the system shown in FIG. 1, except that the optical receiver 7 has the above-described first or second configuration.

The peak value of the waveform in which the waveform distortion caused by the pattern effect has occurred is the accumulation of the maximum amplitude amplified by the non-saturation gain (which is not linearly accumulated due to the coupling loss of the optical amplifier) _RA and the steady state. a level between the cumulative R _B of minimum amplitude amplified by the saturated gain sought state. Therefore, by controlling the limit level of the limiter circuit 2 to a predetermined level between the maximum level and the minimum level when the input signal to the limiter circuit 2 is marked, it is possible to improve the eye pattern and prevent the code error rate from increasing. become able to. The description of this operation is for a case where a plurality of optical amplifiers are used. However, even when a single optical amplifier is used, an increase in the bit error rate is prevented according to an operation similar to this operation. Is done.

According to the second configuration, since the limit levels in the respective limiter circuits are sequentially reduced from the side of the light receiver, the degree of improvement of the finally obtained eye pattern increases.

Examples Examples of the present invention will be described below.

FIG. 4 is a block diagram of an optical receiver showing a first embodiment of the present invention, and this optical receiver corresponds to the first configuration.
Parts that are substantially the same as those in the conventional configuration (FIG. 10) are denoted by the same reference numerals, and parts other than those indispensable for explaining the features of the invention (AGC circuit, etc.) are shown. The description is omitted (the same applies to other embodiments).

A signal having a level corresponding to the light intensity photoelectrically converted by the light receiver 101 is amplified by the amplifier 102, input to the limiter circuit 11, shaped into a waveform, and output. In this embodiment, the limit level control circuit includes an average value detection circuit 12 for detecting an average level of an output signal of the amplifier 102, and a DC amplifier 13 having an amplification factor of 2 for amplifying a DC output of the detection circuit 12. It is configured. The average value detection circuit 12 is composed of an average value output type AC-DC conversion circuit.

In general, in a system to which the intensity modulation / direct detection method is applied, a scrambler circuit is usually adopted on the transmission side for the purpose of facilitating clock extraction and the like. And the mark rate
1/2. Therefore, the limit level obtained by detecting the average value of the intensity modulation signal and doubling the detection value is a rough average level between the maximum and minimum levels at the time of marking of the intensity modulation signal, and is deteriorated by the pattern effect. The waveform can be shaped to improve the bit error rate.

FIG. 5 is a block diagram of an optical receiver showing a second embodiment of the present invention, and this optical receiver also corresponds to the first configuration.
In this embodiment, the limit level control circuit includes the amplifier 10
An amplifier 14 having an amplification factor of 2 for amplifying the output signal of 2;
And an average value detection circuit 15 for detecting the average level of the output signal of the amplifier. With this configuration, as in the previous embodiment, the code error rate can be improved through the function of waveform shaping. In this embodiment, since the intensity modulation signal, that is, the AC signal is amplified before the average value is detected, the portion where the DC drift should be considered can be reduced.

FIG. 6 is a block diagram of an optical receiver showing a third embodiment of the present invention, and this optical receiver corresponds to the second configuration.
In this embodiment, in addition to the configuration of the first embodiment shown in FIG. 4, another set of a limiter circuit 11 and an average value detection circuit 12 are provided.
And a DC amplifier 13.

The operation in the third embodiment will be described with reference to FIG. FIGS. 7 (a), (b) and (c) show signal waveforms (voltage amplitude waveforms in this example) at points in FIG. 6, respectively. In that corresponding to the output of the amplifier 102, since the waveform deterioration due to the use of optical amplifiers appears as, the waveform is assumed that the amplitude of the time mark is changed from V ₁ to V _2. The average level V ₃ is detected by the first stage of the average value detecting circuit 12, the DC signal is V ₄ is amplified by a DC amplifier _{13 (V 4 = 2V 3)} . First-stage limiter circuit
In the point corresponding to the output end of the 11, because the limiter circuit 11 to limit level as V ₄ is functioning, the wave height at the mark is in the range of V ₂ ~V _4. The waveform shaping operation so far is the same as that in the first embodiment. Then 2
Is detected mean level V ₅ by the mean value detecting circuit 12 of the stage, the V ₆ corresponding to twice acts limiter circuit 11 of the second stage as a limit level, the waveform at the point corresponding to its output , The wave height at the time of marking is within the range of V _{2 to} V ₆ . As is clear from this description, there is a relationship of V ₂ <V ₆ <V ₄ between the first-stage limit level V ₄ , the second-stage limit level V ₆ , and the minimum level V ₂ at the time of marking. the limiter circuit 11 and the like by connecting two stages, it is possible to more complete waveform shaping close the limit level to the minimum level V ₂ at the time mark. By further multistage limiter circuit 11 and the like, it is possible to focus the limit level to the minimum level V ₂ at the time mark.

FIG. 8 is a block diagram of an optical receiver showing a fourth embodiment of the present invention, and this optical receiver corresponds to the first configuration.
In this embodiment, when m and n are natural numbers, the limit control circuit includes a first amplifier 16 having an amplification factor of 2 m / (m + n) for amplifying an input signal to the limiter circuit 11; Amplification rate to amplify output signal of 2n / (m + n)
A second amplifier 17 and first and second amplifiers 16 and 17
And an average value detection circuit 19 for detecting the average level of the output signal of the adder 18.

Although the limit level in the first embodiment is a roughly average level between the maximum and minimum levels at the time of marking, according to the configuration of the fourth embodiment, the output signal of the limiter circuit 11 is also used for setting the limit level. Therefore, as compared with the case of the first embodiment, the limit level can be made closer to the minimum level at the time of marking.

By configuring the limiter circuit 11 and the limit level control circuit in the fourth embodiment in multiple stages, an optical receiver having a second configuration is provided. In this case, the limiter circuit 11 is compared with the optical receiver having the second configuration described in the third embodiment.
If the number of stages is the same, the limit level can be made closer to the minimum level at the time of marking. Also, the number of limiter circuits required for focusing the limit level can be reduced.

For reference, FIG. 9 shows an example in which the eye pattern is improved in the embodiment. Even if the eye pattern is deteriorated as shown in FIG. 13, by implementing the present invention, the eye pattern can be improved as shown in FIG. This eliminates the possibility that the rate will deteriorate.

Effects of the Invention As described above, according to the present invention, it is possible to prevent an increase in the bit error rate caused by waveform deterioration that occurs when an optical amplifier is used in a gain-saturated state. Therefore, according to the present invention, the optical semiconductor amplifier can be used in the gain saturation region, which contributes to the improvement of the performance of the optical communication system to which the intensity modulation / direct detection method is applied in terms of transmission distance and the like. But big.

[Brief description of the drawings]

FIG. 1 is a diagram showing a basic configuration of the present invention, FIG. 2 is a diagram for explaining the function of a limiter circuit, FIG. 3 is a diagram showing another basic configuration of the present invention, FIG. 4 to FIG. Is a block diagram of an optical receiver showing first to third embodiments of the present invention, FIG. 7 is an explanatory diagram of waveform shaping in the third embodiment and the like, and FIG. 8 shows a fourth embodiment of the present invention. FIG. 9 is a block diagram of an optical receiver, FIG. 9 is a diagram showing an example in which an eye pattern is improved in the embodiment, FIG. 10 is an explanatory diagram of a conventional optical receiver, and FIG. 11 is an input / output characteristic of an optical semiconductor amplifier. FIG. 12 is a diagram showing a step response waveform for input light that causes gain saturation, and FIG. 13 is a diagram showing an example of an eye pattern after multistage relay. 1,101 ... receiver, 2,11 ... limiter circuit, 3 ... limit level control circuit, 4 ... optical transmitter, 5 ... optical transmission line, 6 ... optical amplifier, 7 ... optical receiver.

──────────────────────────────────────────────────の Continued on the front page (51) Int.Cl. ⁶ identification code FI H04L 25/03 (58) Investigated field (Int.Cl. ⁶ , DB name) H04B 10/06 H04B 10/158

Claims (6)

(57) [Claims] 1. A photodetector that photoelectrically converts received intensity-modulated light and outputs a signal having a level corresponding to the light intensity, a plurality of limiter circuits cascaded to an output of the photodetector, and the plurality of limiters Means for controlling a limit level of each of the circuits, wherein each of the plurality of limiter circuits has an output level corresponding to the input level for an input level smaller than the limit level and an input level higher than the limit level. The level is a constant output level corresponding to the input of the limit level. The controlling means sets the limit level between a maximum level and a minimum level when the input signal to the limiter circuit is marked. The limit levels in the plurality of limiter circuits are controlled to decrease in order from the side of the light receiver. Optical receiver according to claim Rukoto. 2. The optical receiver according to claim 1, wherein said controlling means comprises: an average value detection circuit for detecting an average level of an input signal to each of said plurality of limiter circuits; and said average value detection circuit. And a DC amplifier having an amplification factor of 2 for amplifying the DC output of the optical receiver. 3. The optical receiver according to claim 1, wherein said controlling means comprises: an amplifier having an amplification factor of 2 for amplifying an input signal to each of said plurality of limiter circuits; and an output signal of said amplifier. And an average value detection circuit for detecting an average level of the optical receiver. 4. The optical receiver according to claim 1, wherein, when m and n are natural numbers, the controlling means has an amplification factor of amplifying an input signal to each of the plurality of limiter circuits of 2 m. / (M + n), and an amplification factor for amplifying the output signal of each limiter circuit is 2n /
(M + n), an adder that adds the output signals of the first and second amplifiers, and an average value detection circuit that detects an average level of the output signal of the adder. Features an optical receiver. 5. A photodetector for photoelectrically converting received intensity-modulated light and outputting a signal having a level corresponding to the light intensity, and an input level smaller than a predetermined limit level for a signal input from the photodetector. And a limiter circuit that has an output level corresponding to the input level, and has an output level corresponding to the input of the limit level when the input level is equal to or higher than the limit level. A limit level control circuit for controlling the input signal to a predetermined level between a maximum level and a minimum level at the time of marking the input signal to the limiter circuit when m and n are natural numbers. Amplification rate to amplify signal is 2m /
A first amplifier of (m + n), and an amplification factor for amplifying an output signal of the limiter circuit is 2n /
A second amplifier that is (m + n); an adder that adds the output signals of the first and second amplifiers; and an average value detection circuit that detects an average level of the output signal of the adder. An optical receiver, characterized in that: 6. An optical transmitter for outputting intensity-modulated light, an optical transmission line for transmitting light from the optical transmitter, and a single or a plurality of optical amplifiers for amplifying light transmitted by the optical transmission line. An optical communication system comprising: an optical receiver according to claim 1, to which light from the optical transmission path is input.