JP2003332986A - Optical module and optical communication system - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an optical module which inputs a burst signal and is capable of correcting a waveform distortion based on a feedforward system, and also to provide an optical communication system using the same. <P>SOLUTION: An optical receiver which is applied to the optical module includes a light-receiving element 1, preamplifier 2, discriminator 3, level detecting circuit 4, and duty adjusting circuit 5. When the preamplifier 2 is of a transimpedance-type, a distortion occurs in an output signal of the preamplifier 2 when receiving a signal with a large amplitude level. The waveform distortion is corrected by the circuit 5 on the basis of a monitoring result of the signal waveform of the circuit 4, so that an output signal having no waveform distortion can be obtained based on the feedforward system. <P>COPYRIGHT: (C)2004,JPO

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical module, and more particularly to an optical module to which a burst signal is input, which is applied to an optical module for correcting waveform distortion by a feedforward method and an optical communication system using the same. Regarding effective technology.

[0002]

2. Description of the Related Art According to a study made by the present inventor, the following techniques can be considered for an optical module.

For example, as an example of an optical communication system using an optical module, a PON (Passive) is generally used.
There is an optical subscriber system called an optical network. In this system, an optical fiber extending from a station is branched by a star coupler provided on the way and connected to each subscriber's house.

In such a system, the upstream signal transmitted from each subscriber's house is time-division multiplexed by using a star coupler, so that it becomes a burst signal in which there is no signal interval between signals. . In this burst signal, the signal level received by the station differs for each signal transmitted from each subscriber's home due to the difference in distance between the station and each subscriber's home. Therefore, the optical module of the station employs a technique for suppressing duty deterioration caused by a difference in signal level.

As a technique for suppressing such duty deterioration, for example, a technique described in JP-A-10-126349 can be cited. In this publication,
ATC (Auto Threshold Cond) including photodiode, preamplifier, and threshold voltage adjustment circuit
control) circuit, a duty monitoring circuit (AC / DC conversion circuit, comparator) connected to the output of the ATC circuit, and the like.
There is disclosed a technique of suppressing deterioration of duty by feeding back to a threshold voltage adjusting circuit in the circuit.

[0006]

DISCLOSURE OF THE INVENTION By the way, as a result of the inventor's examination of the above-mentioned optical module, the following has been clarified.

For example, the above-mentioned Japanese Patent Laid-Open No. 10-126349.
The technique disclosed in the publication is a method of feeding back the monitoring result by the duty monitoring circuit connected to the output of the ATC circuit to the threshold voltage adjusting circuit in the ATC circuit.
This is different from the feedforward method, which is one of the features of the present invention.

In an optical module, a transimpedance type amplifier is generally used as a preamplifier. This transimpedance type amplifier is caused by the difference in the distance between the station and each subscriber's house, and the level of the input signal becomes large especially when the distance from the subscriber's house is short.
As described above, when the input signal is large, the problem occurs that the output signal is distorted.

Therefore, the inventor of the present invention provides a duty adjusting circuit in the subsequent stage of the preamplifier in order to prevent the output signal from being distorted even when the input signal to the preamplifier is large. , We have come up with the idea of adopting the feedforward method to correct the waveform distortion of the output signal output from the preamplifier.

Therefore, an object of the present invention is to provide an optical module capable of correcting waveform distortion by a feedforward method in an optical module to which a burst signal is input, and an optical communication system using the same. .

The above and other objects and novel features of the present invention will be apparent from the description of this specification and the accompanying drawings.

[0012]

Among the inventions disclosed in the present application, a brief description will be given to the outline of typical ones.
It is as follows.

That is, the optical module according to the present invention is
A light receiving element that converts the optical signal of the burst signal into a current signal, a preamplifier that converts the current signal converted by this light receiving element into a voltage signal, and a waveform of the voltage signal converted by the preamplifier that is connected after this preamplifier And a correction circuit for correcting the distortion by the feedforward method. As a result, in the optical module to which the burst signal is input, the correction circuit can correct the waveform distortion of the output signal output from the preamplifier by the feedforward method.

Specifically, in this optical module, the correction circuit includes a level detection circuit for detecting a signal amplitude level by using an output signal of the preamplifier as an input signal, and a preamplifier based on a detection result of the signal amplitude level by the level detection circuit. By having a duty adjusting circuit that adjusts the duty ratio of the output signal output from, the signal amplitude level of the output signal of the preamplifier is detected, and the duty ratio of the output signal output from the preamplifier is detected based on this detection result. You will be able to adjust.

In particular, the optical module has a star coupler connected to the optical module and a plurality of transmitters branched and connected from the star coupler outside the optical module, and each of the plurality of transmitters and the optical module. Even if the level of the optical signal received by the optical module varies depending on the distance between the optical signal and the signal, the waveform distortion can be corrected in accordance with the level of the optical signal. For example, waveform distortion occurs when a signal with a large amplitude level is input,
Also in this case, a signal waveform without waveform distortion can be obtained as in the case where the amplitude level is small.

The optical communication system according to the present invention includes an OLT for transmitting and receiving optical signals, a star coupler connected to the OLT via an optical fiber, and a branch from the star coupler for connection via the optical fiber. In the configuration having a plurality of ONUs for transmitting and receiving optical signals, the optical module in the OLT has the above-described light receiving element, preamplifier, and correction circuit. As a result, in the optical module in the OLT, the waveform distortion can be corrected by the feed-forward method, as in the optical module, and the optical module receives light according to the distance between each of the plurality of ONUs and the OLT. Even when the levels of the optical signals to be processed are different, the waveform distortion can be corrected corresponding to the level of the optical signals.

[0017]

BEST MODE FOR CARRYING OUT THE INVENTION Embodiments of the present invention will be described in detail below with reference to the drawings. In all the drawings for explaining the embodiments, members having the same function are designated by the same reference numerals, and the repeated description thereof will be omitted.

An example of the configuration of an optical receiver according to an embodiment of the present invention will be described with reference to FIG. In addition, an example of a waveform with / without distortion will be described with reference to FIG. FIG. 1 is a configuration diagram of an optical receiver according to the present embodiment, and FIG. 2 is an explanatory diagram of waveforms with and without distortion.

The optical receiver of the present embodiment is applied to, for example, an optical module, and as shown in FIG. 1, an example is a light receiving element 1, a preamplifier 2, a discriminator 3, a level detecting circuit 4, a duty adjusting circuit 5 and the like. Composed of. In particular, the discriminator 3, the level detection circuit 4, and the duty adjustment circuit 5 are
It is included in the TC circuit 6.

The light receiving element 1 is an element for converting an optical signal of a burst signal into a current signal, and is composed of, for example, a photodiode, and the cathode side is connected to the power supply voltage VCC and the anode side is connected to the preamplifier 2. The light receiving element 1 receives an optical signal, converts the optical signal into a current signal (electrical signal), and outputs the current signal to the preamplifier 2.

The preamplifier 2 is an amplifier that converts the current signal converted by the light receiving element 1 into a voltage signal, and is composed of, for example, a transimpedance type amplifier, and the input side is the anode side of the light receiving element 1 and the output side is in the ATC circuit 6. Are connected to the discriminator 3 and the level detection circuit 4, respectively. In the preamplifier 2, the current signal converted by the light receiving element 1 is input, and the current signal is converted into a voltage signal and output to the discriminator 3 and the level detection circuit 4.

The discriminator 3 is a circuit for converting the voltage signal from the preamplifier 2 into a digital signal, and its input side is connected to the preamplifier 2 and its output side is connected to the duty adjusting circuit 5. In the discriminator 3, the voltage signal from the preamplifier 2 is input, the voltage signal is divided by the threshold voltage, converted into a digital signal of High level / Low level, and output to the duty adjustment circuit 5.

The level detection circuit 4 is a circuit for detecting the signal amplitude level using the output signal of the preamplifier 2 as an input signal, and the input side is connected to the preamplifier 2 and the output side is connected to the duty adjustment circuit 5. The level detection circuit 4 receives the voltage signal output from the preamplifier 2 as input, detects the signal amplitude level of this voltage signal, and outputs the detection result to the duty adjustment circuit 5.

The duty adjustment circuit 5 is a circuit for adjusting the duty ratio of the output signal output from the preamplifier 2 based on the detection result of the signal amplitude level by the level detection circuit 4, and the input side is used by the discriminator 3 for correction. The input side is connected to the level detection circuit 4, and the output side is connected to the outside of the ATC circuit 6. In the duty adjustment circuit 5, the digital signal output from the discriminator 3 and the level detection circuit 4
The detection result output from the ATC circuit 6 is input, the duty ratio of the digital signal is adjusted based on the detection result, and the signal having no waveform distortion corrected by this adjustment is output to the outside of the ATC circuit 6.

In the structure of such an optical receiver, in particular, the level detection circuit 4 and the duty adjustment circuit 5 are
It is connected to the subsequent stage of the preamplifier 2 and operates as a correction circuit for correcting the waveform distortion of the voltage signal converted by the preamplifier 2 by the feedforward method.

In the operation of this optical receiver, an optical signal is converted into a current signal by the light receiving element 1 and a voltage signal by the preamplifier 2, and further, the discriminator 3 in the subsequent stage is operated at a high level / Lo.
The digital signal is divided into w levels. And
This digital signal is output to the duty adjustment circuit 5.

On the other hand, the level detection circuit 4 includes the preamplifier 2
Is used as an input signal and the signal amplitude level of this input signal is monitored and output to the duty adjustment circuit 5. The duty adjustment circuit 5 determines the amount of waveform distortion correction based on the result of monitoring the signal amplitude level of the level detection circuit 4, and adjusts the duty ratio of the pulse of the digital signal classified into the High / Low level by the discriminator 3. To do.

As a result, when the preamplifier 2 is composed of a transimpedance type amplifier, the output signal of the preamplifier 2 is distorted when a signal with a large amplitude level is input, but the result of monitoring the signal amplitude of the level detection circuit 4 is shown. Thus, by correcting the waveform distortion by the duty adjusting circuit 5, it is possible to obtain an output signal having no waveform distortion by the feedforward method.

The reason why the output distortion increases as the signal amplitude increases can be considered as follows. For example, when the input of the optical signal becomes large, the current converted by the light receiving element 1 also increases, and when the transimpedance is large, the signal amplitude becomes too large and the waveform is limited, so that distortion occurs. Conceivable.

For example, as shown in FIG.
When the amplitude is increased on the High level side with reference to the w level side, if the amplitude is limited, distortion occurs as if the section on the High level side is extended. Therefore,
By monitoring the signal amplitude as described above, the magnitude of the distortion at the output of the preamplifier 2 is grasped, and the duty adjusting circuit 5 corrects the amount corresponding to the magnitude of the distortion, so that there is no waveform distortion. An output signal is obtained.

Specifically, for example, High level / L
Consider a case where a signal having an ow level of 50% is input and the transimpedance is large, so that the signal amplitude of the output of the preamplifier 2 at the time of a large signal is insufficient. In this case, if the distortion is such that the High level section becomes longer and the Low level section becomes shorter, the duty adjustment circuit 5 performs correction so that the High level section becomes shorter. In other words, the distortion can be canceled by using the duty adjustment circuit 5 to make a correction in the direction opposite to the distortion generated in the transimpedance type amplifier.

Next, an example of the configuration and operation of the duty adjusting circuit and the configuration of the threshold value generating circuit in the optical receiver of the present embodiment will be described with reference to FIGS. FIG. 3 is a configuration diagram of the duty adjustment circuit, FIG. 4 is a waveform diagram of the operation of each part of the duty adjustment circuit, and FIG. 5 is a circuit diagram of the threshold value generation circuit.

The duty adjusting circuit 5 is, for example, as shown in FIG. 3, a threshold value generating circuit 11 having an output signal from the level detecting circuit 4 as an input signal, and an LPF having an output signal from the discriminator 3 as an input signal. (Low Pass Filter
er) 12, a threshold value generation circuit 11, and a comparator 13 that receives the output from the LPF 12 as an input signal. The output of the comparator 13 becomes the output signal of the optical receiver.

In the duty adjusting circuit 5 having such a configuration, the output signal (High / Low rectangular wave) 14b of the discriminator 3 is fed to the LPF 1 as shown in an example in FIG.
A signal 14c having a waveform whose rising and falling are blunted by passing 2 is obtained. Then, in the comparator 13 in the next stage, by changing the set value of the reference level 14c of the threshold for the blunted signal 14c in the threshold generation circuit 11, the output signal 14
The duty ratio of e can be adjusted.

That is, the signal 1 output from the discriminator 3
Although the waveform of 4b is longer in the High level section than in the Low level section, the waveform of the output signal 14e passing through the LPF 12 and the comparator 13 has the same length in the High level section and the Low level section. (Duty ratio = 50%). The reference level 14c to the comparator 13 at this time is generated by the threshold value generation circuit 11 based on the output 14a of the level detection circuit 4.

The threshold value generation circuit 11 will be described in detail with reference to resistors R1, R2, NPN as shown in FIG.
Type bipolar transistors Q1, Q2, current sources I1, I
2. Compensation reference voltage circuit 21 and the like.

The resistor R1 has one end connected to the power supply potential VCC and the other end connected to the ground potential through the current source I1. One end of the resistor R2 is connected to the power supply potential VCC,
The other end is connected to the collector of the NPN bipolar transistor Q2, and the emitter is connected to the ground potential through the common current source I2. The other end of the resistor R1 is NP
The collector of the N-type bipolar transistor Q1 is connected, and the emitter is connected to the ground potential through the common current source I2. The output signal 22a of the correction reference voltage circuit 21 is input to the base of the NPN bipolar transistor Q1, and the output signal 14a from the external level detection circuit 4 is input to the base of the NPN bipolar transistor Q2. Controlled. The output signal 14c of the threshold value generation circuit 11 is taken out from the other end of the resistor R1.

The threshold value generation circuit 1 configured as described above
1, when the output signal 14a from the level detection circuit 4 is a voltage sufficiently lower than the output signal 22a from the correction reference voltage circuit 21 (small signal amplitude), the NPN bipolar transistor Q1 is ON, and the NPN bipolar transistor is ON. The transistor Q2 is turned off, and the voltage of the output signal 14c of the threshold value generation circuit 11 becomes VCC-R1 × (I1 + I2).

This VCC-R1 × (I1 + I2)
In the equation, VCC is the voltage value of the power supply potential VCC, R1
Is the resistance value of the resistor R1, I1 is the current value of the current source I1, I2
Indicates the current value of the current source I2.

On the contrary, the output signal 1 from the level detection circuit 4
When 4a is a voltage sufficiently higher than the output signal 22a from the correction reference voltage circuit 21 (large signal amplitude), the NPN type bipolar transistor Q1 is turned off, the NPN type bipolar transistor Q2 is turned on, and the threshold value generation circuit is turned on. The voltage of the output signal 14c of 11 becomes VCC-R1 × I1.

The output signal 1 from the level detection circuit 4
When 4a is a voltage between the above two states, the voltage of the output signal 14c of the threshold generation circuit 11 outputs a voltage determined by the ratio of the currents flowing through the NPN bipolar transistors Q1 and Q2.

Therefore, the distortion generated in the preamplifier 2 is
If it increases as the signal level increases,
With the threshold value generation circuit 11 as described above, the threshold value can be adjusted according to the signal level, so that a sufficient correction effect can be obtained. From the above, by providing the level detecting circuit 4 and the duty adjusting circuit 5, the waveform distortion generated in the preamplifier 2 can be corrected.

Next, an example of the configuration of another optical receiver in the present embodiment will be described with reference to FIG. FIG. 6 is a block diagram of another optical receiver.

The optical receiver shown in FIG. 6 is different from the optical receiver shown in FIG. 1 in the following points. That is, although the level detection circuit 4 is arranged outside the discriminator 3 in FIG.
As described above, the level detection circuit 4 may be arranged inside the discriminator 3. The level detection circuit 4 may also be used as a circuit already used as another function inside the discriminator 3.

In the optical receiver having such a structure, the purpose of the level detection circuit 4 is to detect the signal amplitude level at the output point of the preamplifier 2, and therefore, the same effect as that of the configuration of FIG. 1 can be obtained. You can

Next, an example of the configuration of the burst optical communication system using the optical receiver of this embodiment will be described with reference to FIGS. FIG. 7 is a block diagram of a burst optical communication system,
FIG. 8 is a block diagram of a burst optical communication system to which video distribution is added.

The burst optical communication system is shown in FIG.
, An OLT (O
optical line terminal 31, a star coupler 33 connected to the OLT 31 through an optical fiber 32, an ONU (Optical Network U) installed at a plurality of subscribers' houses branching from the star coupler 33 and connected through an optical fiber 32.
nit) 34 and the like. Here, ONU3
This is a case where three subscriber homes having 4a to 34c are considered, and each subscriber home is a PC (Personal Comp).
Uter) 35a-35c.

In the burst optical communication system configured as described above, signals are wavelength division multiplexed (WDM: Wavelength Division) using different wavelengths in the upstream (direction to OLT) and the downstream (direction to ONU).
Multiplex) technology is applied. Therefore, the WDM 36 is provided in the OLT 31 and each ONU is
34a to 34c also include WDMs 37a to 37c,
The upstream signal and the downstream signal are separated.

The optical signal transmitted from the OLT 31 is transmitted through the optical fiber 32 and the star coupler 33 to each ONU 34.
a to 34c. Conversely, each ONU 34a
The optical signals transmitted from ~ 34c are received by the OLT 31 via the optical fiber 32 and the star coupler 33. As a result, the OLT 31 and each ONU 34a to 34c
Two-way communication is realized between the two.

In a burst optical communication system to which video distribution is added, for example, as shown in an example in FIG. 8, in addition to the upstream / downstream signals shown in FIG. 7, the wavelength (only downstream) for video distribution is increased. A WDM 41 is added in the front stage to the inside of the station, and WDMs 42a to 42c are added in the front stage to each subscriber's house. Further, V-OLT (Video-OL) for video distribution is added.
T) 43, V-ONU (Video-ON) for video reception
U) 44a to 44c, STB (Set Top Bo)
x) 45a to 45c, TV (TeleVision) 4
6a-46c etc. are added and comprised.

If there is such a video distribution system,
WDM 41, 42 on the optical transmission path by the optical fiber 32
Since a to 42c are added, each ONU 34a to
The attenuation of the optical signal sent from 34c increases,
The optical receiver inside the LT 31 is required to have higher sensitivity characteristics. The inside of the OLT 31 will be described below.

Next, an example of the configuration of the OLT in the burst optical communication system of this embodiment will be described with reference to FIG. FIG. 9 shows a configuration diagram of the OLT.

The OLT 31 includes an optical module 51 and a SerDes (Seri) as shown in FIG.
ALIZER / DESERIALIZER) & CPU
(Central Processing Unit)
& MAC (Media Access Control)
The optical receiver, which is a feature of the present invention, is provided inside the optical module 51.

The optical module 51 includes an optical receiver including a light receiving element 1, a preamplifier 2, a discriminator 3, a level detecting circuit 4 and a duty adjusting circuit 5, an optical transmitter including an LD 53 and an LD driver 54, and an optical signal. The WDM 36 for wavelength division multiplexing is used. The optical receiver is as described above. The LD 53 of the optical transmitter is a laser diode and is driven by the LD driver 54.

In SerDes & CPU & MAC 52, SerDes is a serial / parallel conversion, CPU is an operation control, and MAC is a functional block in charge of an interface with a host device.

Next, with reference to FIGS. 10 to 13, an example of reproduction of an optical signal in the OLT in the burst optical communication system of the present embodiment will be described. 10 is a waveform diagram of an optical signal input to the OLT, FIG. 11 is a waveform diagram of a burst signal corresponding to a large level, FIG. 12 is a waveform diagram of a burst signal corresponding to a medium level, and FIG. 13 is a diagram corresponding to a small level. Waveform diagrams of burst signals are shown respectively.

In the burst optical communication system, the distance between the OLT 31 installed in the station and each ONU 34 installed in each subscriber's house varies, and even if the upstream signal level received by the OLT 31 is significantly different, the accuracy is improved. It is necessary to reproduce the signal well.

For example, in the upstream signal received by the OLT 31, as shown in an example in FIG. 10, burst signals divided into non-signal intervals called guard time are classified into large, medium and small for each burst. The level is input to the OLT 31. Then, the optical module 51 inside the OLT 31
Will be played. The waveforms corresponding to the large, medium, and small levels are as shown in FIGS. 11 to 13.

FIG. 11 shows an example of a waveform corresponding to a large level. Only in this case, a signal exceeding the amplitude upper limit value of the preamplifier 2 is assumed, and distortion is observed in the output waveform of the preamplifier 2. Therefore, in the output of the discriminator 3,
It is reproduced with distortion as it is, and the duty adjusting circuit 5 operates in the subsequent stage to generate a waveform 14d whose rising / falling is blunted by the internal LPF 12, and further based on the threshold value of the threshold value generating circuit 11. The waveform of the output signal of the duty adjustment circuit 5 which has been determined is corrected for distortion.

That is, since the output waveform of the preamplifier 2 is distorted, the output waveform of the discriminator 3 is High.
Although the level section is longer than the Low level section, the duty adjusting circuit 5 works so that the output waveform of the duty adjusting circuit 5 has the same length in the High level section and the Low level section. As a result, the waveform distortion is corrected. ing.

FIG. 12 shows an example of a waveform corresponding to the medium level and FIG. 13 shows an example of the waveform corresponding to the small level. In these cases, it is assumed that the preamplifier 2 is not distorted, and therefore the duty adjusting circuit 5 is used. Then, distortion correction is not applied.

That is, since the output waveform of the preamplifier 2 is not distorted, the output waveform of the discriminator 3 is High.
The level section and the Low level section have the same length, and therefore the output waveform of the duty adjusting circuit 5 also has the same length as the High level section and the Low level section.

Next, an example of the signal in the burst basic cell will be described with reference to FIGS. FIG. 14 is a waveform diagram of the signal in the burst basic cell, FIG. 15 is a waveform diagram of the synchronizing signal, and FIG. 16 is an explanatory diagram of the data signal retiming.

In the burst basic cell, for example, as shown in FIG. 14, the signal division of each burst basic cell is divided by a guard time which is a no signal section. Immediately after the guard time in each burst basic cell, there is a High / Low level continuous signal (= synchronization signal) for extracting the synchronization timing, and after that, the data signal exists.

The OLT 31 performs synchronization timing extraction (retiming extraction) using this synchronization signal for each burst basic cell, determines the timing for punching out the subsequent data signal, and accurately outputs the data signal at the timing of the OLT 31. Is synchronized with. These functions are performed by the SerDes & CPU & MAC which is the latter stage circuit of the optical module 51.
It is realized by 52.

The synchronizing signal is a continuous signal of High / Low level, as shown in FIG.
Here, for convenience, the first High level is changed to the next Lo level.
A description will be given by numbering the waveforms so that the w level and the subsequent High level become.

Since the number of bits of this synchronizing signal is an area unrelated to the original data, it is desirable that the number of bits is small for efficient data communication. Therefore, the reproduction of the synchronization signal is required to be performed in the shortest time for each burst signal at the output of the optical module 51.

For example, at the shortest, it is ideal to output from without distortion, but since is used to determine the threshold level of the discriminator 3, the output waveform of the discriminator 3 must be after that. it's not correct. Therefore, by mounting the function of correcting the distortion by the feedforward method as in the present invention, it becomes possible to correct the waveform distortion and obtain a stable output thereafter.

The state of the data signal retiming is as follows.
For example, the process is performed as shown in FIG. That is, the data signal has a signal after the data signal is cut out as shown in FIG. 16 according to the data cutting point (retiming).

Although the invention made by the present inventor has been specifically described based on the embodiments, the present invention is not limited to the above embodiments, and various modifications can be made without departing from the scope of the invention. It goes without saying that it is possible.

For example, the present invention is suitable for an optical receiver used in a burst optical communication system which requires a particularly wide dynamic range, but is also applicable to general circuits for correcting the signal distortion generated in the front stage circuit by the rear stage circuit. It can be widely applied.

[0072]

The effects obtained by the typical ones of the inventions disclosed in the present application will be briefly described as follows.
It is as follows.

(1) In an optical module to which a burst signal is input, a waveform distortion of the voltage signal converted by this preamplifier is fed-forward system after the preamplifier which converts the current signal converted by the light receiving element into a voltage signal. By having a correction circuit that corrects the waveform, the waveform distortion of the output signal output from the preamplifier can be corrected by the feedforward method.

(2) Since the correction circuit has a level detection circuit for the output signal of the preamplifier and a duty adjustment circuit for the output signal of the preamplifier based on the detection result of this level detection circuit, the signal of the output signal of the preamplifier is obtained. It is possible to detect the amplitude level and adjust the duty ratio of the output signal output from the preamplifier based on the detection result.

(3) A star coupler connected to the optical module and a plurality of transmitters branched from the star coupler are connected to the outside of the optical module. Even if the level of the optical signal received by the optical module differs depending on the distance to the module, the waveform distortion can be corrected in accordance with the level of the optical signal.

(4) Also in an optical communication system having an OLT including an optical module, a star coupler connected to the OLT through an optical fiber, and a plurality of ONUs branched from the star coupler and connected through an optical fiber. Similarly to the above (1) to (3), it is possible to correct the waveform distortion by the feedforward method, and particularly it is possible to cope with the large / small level of the optical signal.

[Brief description of drawings]

FIG. 1 is a configuration diagram showing an optical receiver according to an embodiment of the present invention.

FIG. 2 is an explanatory diagram showing waveforms with and without distortion in the optical receiver according to the embodiment of the present invention.

FIG. 3 is a configuration diagram showing a duty adjustment circuit in the optical receiver according to the embodiment of the present invention.

FIG. 4 is a waveform diagram showing the operation of each part of the duty adjustment circuit in the optical receiver according to the embodiment of the present invention.

FIG. 5 is a circuit diagram showing a threshold generation circuit in the optical receiver according to the embodiment of the present invention.

FIG. 6 is a configuration diagram showing another optical receiver in the optical receiver according to the embodiment of the present invention.

FIG. 7 is a configuration diagram showing a burst optical communication system using an optical receiver according to an embodiment of the present invention.

FIG. 8 is a configuration diagram showing a burst optical communication system to which video distribution is added in an embodiment of the present invention.

FIG. 9 is a configuration diagram showing an OLT in the burst optical communication system according to the embodiment of the present invention.

FIG. 10 is a waveform diagram showing an optical signal input to the OLT in the burst optical communication system according to the embodiment of the present invention.

FIG. 11 is a waveform diagram showing a burst signal corresponding to a large level in the burst optical communication system according to the embodiment of the present invention.

FIG. 12 is a waveform diagram showing a burst signal corresponding to a medium level in the burst optical communication system according to the embodiment of the present invention.

FIG. 13 is a waveform diagram showing a burst signal corresponding to a small level in the burst optical communication system according to the embodiment of the present invention.

FIG. 14 is a waveform diagram showing signals in a burst basic cell in the burst optical communication system according to the embodiment of the present invention.

FIG. 15 is a waveform diagram showing a synchronization signal in the burst optical communication system according to the embodiment of the present invention.

FIG. 16 is an explanatory diagram showing data signal retiming in the burst optical communication system according to the embodiment of the present invention.

[Explanation of symbols]

1 Light receiving element 2 preamplifier 3 discriminator 4 level detection circuit 5 Duty adjustment circuit 6 ATC circuit 11 Threshold generation circuit 12 LPF 13 Comparator 21 Reference voltage circuit for correction 31 OLT 32 optical fiber 33 star coupler 34a-34c ONU 35a-35c PC 36, 37a-37c WDM 41, 42a to 42c WDM 43 V-OLT 44a-44c V-ONU 45a-45c STB 46a-46c TV 51 Optical module 52 SerDes & CPU & MAC 53 LD 54 LD driver R1, R2 resistance Q1, Q2 NPN type bipolar transistor I1, I2 current source

Continued front page    (72) Inventor Masakazu Ishibashi             5-22-1 Kamimizuhonmachi, Kodaira-shi, Tokyo Stock             Ceremony Company Hitachi Cho-LS System             Within (72) Inventor Satoru Fukuchi             5-22-1 Kamimizuhonmachi, Kodaira-shi, Tokyo Stock             Ceremony Company Hitachi Cho-LS System             Within (72) Inventor Takao Okazaki             3 shares at 6-16 Shinmachi, Ome City, Tokyo             Hitachi Device Development Center (72) Inventor Yoshitaka Abe             3 shares at 6-16 Shinmachi, Ome City, Tokyo             Hitachi Device Development Center F term (reference) 5K102 AA01 AA52 AB10 AD12 AL07                       KA40 MB14 MB15 MC29 MD02                       MD03 PH49 RD01 RD02 RD05

Claims (5)

[Claims] 1. A light-receiving element for converting an optical signal of a burst signal into a current signal, a preamplifier for converting the current signal converted by the light-receiving element into a voltage signal, and a pre-amplifier which is connected to the latter stage of the preamplifier and converted by the preamplifier. An optical module having a correction circuit for correcting the waveform distortion of the generated voltage signal by a feedforward method. 2. The optical module according to claim 1, wherein the correction circuit detects a signal amplitude level using the output signal of the preamplifier as an input signal, and a detection result of the signal amplitude level by the level detection circuit. And a duty adjustment circuit that adjusts the duty ratio of the output signal output from the preamplifier based on the above. 3. The optical module according to claim 1, further comprising a star coupler connected to the optical module and a plurality of transmitters branched from the star coupler and connected to the outside of the optical module. An optical module, in which the level of an optical signal received by the optical module differs depending on the distance between each of the plurality of transmitters and the optical module. 4. An OLT including an optical module for transmitting and receiving an optical signal, a star coupler connected to the OLT through an optical fiber, and a plurality of units branching from the star coupler and connected through an optical fiber for transmitting and receiving an optical signal. The optical module includes a light receiving element for converting an optical signal of a burst signal into a current signal, a preamplifier for converting the current signal converted by the light receiving element into a voltage signal, and a post-stage of the preamplifier. An optical communication system, comprising: a correction circuit connected to correct the waveform distortion of the voltage signal converted by the preamplifier by a feedforward method. 5. The optical communication system according to claim 4, wherein a level of an optical signal received by the optical module is different depending on a distance between each of the plurality of ONUs and the OLT. Communications system.