JP2009278379A - Optical receiver - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To keep a bit error rate during optical signal reception well even when wideband noise occurs on an optical transmission path. <P>SOLUTION: This optical receiver 1 is provided with: a PD 2 generating a current signal I<SB>in</SB>according to input light intensity; a preamplifier 3 converting the current signal I<SB>in</SB>from the PD 2 to a voltage signal V<SB>in</SB>not more than a predetermined amplitude value by dynamically changing a current-voltage conversion gain; a post amplifier 4 comparing the voltage signal V<SB>in</SB>from the preamplifier 3 with a threshold level to amplify the difference thereof; and a threshold control part 5 dynamically changing the threshold level by following the change of the current-voltage conversion gain. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to an optical receiver, and more particularly to an optical receiver with improved reception characteristics.

Conventionally, a WDM transmission system has been put into practical use with an increase in transmission capacity of optical communication. An optical receiver that is used in the WDM transmission system or the like and that reduces tailing in bit error characteristics is known.
JP 2000-134160 A

  However, when a conventional optical receiver receives a light reception signal amplified in multiple stages in an optical transmission line, the light reception signal includes broadband noise, and the gain band of the preamplifier in the optical receiver is the magnitude of the light reception power. As a result, there is a case where the bit error rate is lowered (bottomed) when the received light power is high.

  Specifically, in a DWDM optical transmission system, a plurality of optical amplifiers are generally connected in cascade along the transmission path. A high frequency noise component is superimposed on the output light of the optical amplifier. On the other hand, a variable gain trans-impedance amplifier (v-TIA) with variable transimpedance (current-voltage conversion gain) is mounted in the optical receiver in order to ensure the dynamic range. When this variable gain amplifier is used as a preamplifier, the transimpedance value is set small in order to reduce the gain of v-TIA when the optical input increases. Therefore, the v-TIA band is automatically expanded, and the influence of the high frequency noise of the optical amplifier superimposed on the transmission line tends to increase.

  FIG. 7A shows the output of the variable gain amplifier when the optical input is small, and shows a state where the transimpedance is at a predetermined value. Even in this case, when the optical input is on (the upper level in FIG. 7A), vibration due to noise is seen due to the influence of noise. When the optical input intensity is further increased from this state (see FIG. 7B), the transimpedance remains at a predetermined value, and the output amplitude of the v-TIA increases with the increase of the optical input. At the same time, the noise that appears on the on-level side also increases. When the optical input is further increased, the transimpedance is adjusted so that the output of v-TIA is kept constant (see FIG. 7C). In this case, since the transimpedance is reduced to make the output constant, the v-TIA band is widened, and the noise component on the high frequency side appearing in the output of the v-TIA is increased. In particular, the on-level includes a large amount of high-frequency noise components due to the optical amplifier inserted in the transmission line, so that the influence thereof becomes large. The output waveform of the v-TIA is affected by noise at the on-level. The vibration width becomes wider.

  The output of the v-TIA is discriminated using a threshold for discriminating data, and it is common to set the threshold at the center of the on level and off level (one-dot chain line portion in FIG. 7). However, in the above system, since a larger noise is included on the on-level side, the S / N ratio on the on-side of the preamplifier output deteriorates as the optical input increases, and the bit error rate increases as the optical input intensity increases. There has been a problem that increases.

  Therefore, the present invention has been made in view of such problems, and is an optical device capable of maintaining a good bit error rate when receiving an optical signal even when broadband noise occurs on an optical transmission line. An object is to provide a receiver.

  In order to solve the above problems, an optical receiver of the present invention includes a photoelectric conversion unit that generates a current signal according to input light intensity, and a current from the photoelectric conversion unit by dynamically changing a current-voltage conversion gain. A current-voltage conversion unit that converts the signal into a voltage signal having a predetermined amplitude value or less; and a differential amplifier that compares the voltage signal from the current-voltage conversion unit with a threshold level and amplifies the difference. It is dynamically changed following the change of the voltage conversion gain.

  According to such an optical receiver, the current signal corresponding to the input light intensity from the photoelectric conversion unit is converted into a voltage signal having a predetermined amplitude or less by changing the current-voltage conversion gain by the current-voltage conversion unit, and the difference The difference between the voltage signal and the threshold level is amplified and output by the dynamic amplifier. At that time, the threshold level dynamically changes following the change of the current-voltage conversion gain. As a result, even if the influence of noise superimposed on the optical transmission line at the output of the current-voltage converter becomes larger due to the change in the current-voltage conversion gain, the band of the current-voltage converter is expanded, Since the threshold level for discrimination can be adjusted, the bit error rate at the time of optical signal reception can be reduced.

  The threshold level is preferably controlled so as to decrease as the current-voltage conversion gain decreases. In this case, even if the influence of noise on the on-level of the output of the current-voltage converter is increased due to the decrease in the current-voltage conversion gain, the band of the current-voltage converter is expanded, The bit error rate can be effectively reduced by lowering the threshold level.

  It is also preferable to further include a control unit that holds a series of threshold level values corresponding to the input light intensity and outputs one of the series of threshold level values as the threshold level. By adopting such a configuration, it is possible to set an optimal threshold level corresponding to the input light intensity in advance and perform data discrimination based on the threshold, so that the bit error rate can be reduced more reliably.

  Furthermore, it is also preferable to receive an optical signal propagating through an optical transmission line through which the optical amplifier is interposed. In this way, even when the influence of noise generated by an optical amplifier used in a long-distance transmission system becomes large, the bit error rate at the time of receiving an optical signal can be reduced.

  According to the optical receiver of the present invention, it is possible to maintain a good bit error rate when receiving an optical signal even when broadband noise occurs on the optical transmission line.

  DESCRIPTION OF EMBODIMENTS Hereinafter, preferred embodiments of an optical receiver according to the present invention will be described in detail with reference to the drawings. In the description of the drawings, the same or corresponding parts are denoted by the same reference numerals, and redundant description is omitted.

  FIG. 1 is a block diagram showing a schematic configuration of an optical receiver 1 according to a preferred embodiment of the present invention. The optical receiver 1 shown in the figure is preferably used particularly in a DWDM optical transmission system in which optical amplifiers are connected in cascade in the middle of a transmission path. The optical receiver 1 includes a photodiode (photoelectric conversion unit, hereinafter referred to as PD) 2, a preamplifier (current / voltage conversion unit) 3, a post amplifier (differential amplifier) 4, a threshold control unit 5, and an optical input monitor. And circuit 6.

The PD 2 receives input light from the optical transmission line and generates a current signal I _in corresponding to the intensity thereof. The preamplifier 3 converts the current signal I _in into a voltage signal V _in . The post amplifier 4 as the voltage signal V _in and the threshold voltage complementary to determine the data contained in the voltage signal by comparing the output V _{out +,} it generates a V _out-.

FIG. 2 is a block diagram specifically showing the configuration of the preamplifier 3. As shown in the figure, the preamplifier 3 has an amplifier 8 in which a feedback resistor 7 is connected between the input and output. The output of the amplifier 8 includes a control unit 9 that variably controls the resistance value of the feedback resistor 7. Is connected. The current signal I _in input from the PD 2 is converted to a voltage signal V _in with a current-voltage conversion gain determined by the resistance value of the feedback resistor 7 and output. That is, the amplitude of the voltage signal V _in is determined by the magnitude of the current signal I _in and the feedback resistor 7.

The control unit 9 monitors the amplitude of the voltage signal V _in, reduces the value of the feedback resistor when the amplitude of the input current I _in so as not to exceed a predetermined value is increased.

FIG. 3 is a graph showing the dependency of the output amplitude C ₁ of the preamplifier 3, the gain band C ₂ , and the resistance value C ₃ of the feedback resistor 7 on the received light power. Output amplitude C ₁ is controlled divided into a constant gain region A and the gain variable domain B.

That is, in the small gain constant region A light power is always conversion gain for the current signal I _in is small from the PD2 resistance value C ₃ of the feedback resistor 7 so as to maximize is maintained at the maximum value. Therefore, the output amplitude C ₁ increases in proportion to the current signal I _in , that is, the received light power. Further, the band C _{2 of the} preamplifier 3 is constant regardless of the magnitude of the current signal I _{in because} the GB product (gain bandwidth product) is constant.

In contrast, when a transition is made to the gain variation region B by receiving power is increased, the control unit 9 of the amplifier to lower the resistance value C ₃ of the feedback resistor 7 in order to keep the amplitude C ₁ of the voltage signal V _in constant Control to reduce the gain. In this case, the gain band C ₂ of the preamplifier 3 from GB product constant relationship in proportion to the received light power will be spread.

Returning to FIG. 1, the post-amplifier 4, to determine the data by comparing the voltage signal V _in and the threshold level outputted from the preamplifier 3, and outputs the resulting output signal V _{out +,} as V _out-.

The threshold level used for comparison in the post-amplifier 4 is controlled in separate modes in the constant gain region A and the gain fluctuation region B. As described with reference to FIG. 3, the output V _in of the preamplifier 3 in the constant gain region A has an amplitude C ₁ that increases as the received light power increases, and therefore a threshold level C ₄ for data discrimination. It is controlled to increase in proportion to the increase in output amplitude C ₁ (see FIG. 3).

On the other hand, in the gain fluctuation region B, optical noise is usually superimposed on the received light signal. Since this optical noise is present in a wide band across all signal frequencies, the gain of the preamplifier is reduced at the same time. effect of high-frequency components of relatively optical noise by band C ₂ spreads come conspicuous. Since this optical noise is propagated by being superimposed on the optical signal, it appears particularly on the on-level side of the received light signal, and as a result, the noise component on the on-level side of the output signal of the preamplifier increases. In view of its threshold level C ₄ in the gain variation region B, by being controlled so as to decrease with an increase in light power (see FIG. 3), the on-level side of the voltage signal V _in noise Is set smaller than the average level.

  In order to realize the above-described threshold level control, the optical receiver 1 further includes a threshold control unit 5 and an optical input monitor circuit 6.

The optical input monitor circuit 6 includes a current mirror circuit 10 and a resistance element 11, generates a monitor current corresponding to the current signal I _in of the PD 2 by the current mirror circuit 10, and converts the monitor current to the monitor voltage V by the resistor 11. Convert to _mon . The monitor voltage V _mon is input to the threshold control unit 5.

The threshold control unit 5 is configured by a CPU incorporating an EEPROM or the like, and holds in advance a table for storing a series of threshold levels for optimal data discrimination with respect to received light power corresponding to the monitor voltage _Vmon . Then, the threshold control unit 5 reads the threshold level for data discrimination corresponding to the monitor voltage V _mon input from the optical input monitor circuit 6 from the table, and inputs this to the post amplifier 4 as the threshold voltage V _th . . As a result, the post-amplifier 4 is controlled to perform data discrimination at an optimum threshold level with respect to the received light power.

The table held by the threshold control unit 5 is generated in advance so that the relationship between the received light power and the data determination threshold satisfies a predetermined relationship as shown in FIG. For example, the two linear approximation formulas in the constant gain region A and the gain fluctuation region B in FIG. 3 are retained, and the two approximation formulas are switched and used in accordance with the received light power (monitor voltage V _mon ). It is possible to generate a threshold value for data discrimination. For example, the table is generated by measuring a threshold voltage V _th that minimizes the data error rate when an optical signal is input in advance in the manufacturing stage of the optical receiver 1 while inputting a plurality of optical signal levels. This is realized by approximating the threshold voltage _Vth using a straight line or a curve.

According to the optical receiver 1 described above, the current signal I _in corresponding to the input light intensity from the PD 2 is converted by the preamplifier 3 into the voltage signal V _in having a predetermined amplitude or less while changing the current-voltage conversion gain. , its voltage signal _{V in} and the threshold level by the post-amplifier 4 are compared, the result differential signal _V out _{+, V out-} is generated and output. At that time, the threshold level dynamically changes following the change of the current-voltage conversion gain. In particular, this receiver is characterized in that when the optical input intensity is in the gain fluctuation region B, the threshold level decreases with the optical input intensity. Thus, a current - spreading gain bandwidth of the preamplifier 3 by the change of the voltage conversion gain, in the case where the influence of the high-frequency component of the noise due to optical characteristics of the optical transmission line at the output V _in the pre-amplifier 3 is increased Even in this case, the threshold level for data discrimination can be lowered accordingly, and the bit error rate of the optical receiver can be reduced.

FIG. 4A shows the output of the preamplifier 3 when the optical input is small, and shows a state where the transimpedance is maximum. Furthermore, as shown in FIG. 4B, the transimpedance is maintained at the maximum value even when the optical input intensity is increased in a range of a predetermined value or less. At this time, the threshold level C ₄ increases in proportion to the output amplitude. Furthermore, as shown in FIG. 4C, when the optical input intensity increases beyond a predetermined value, the output amplitude of the preamplifier is kept constant while being superimposed on the on-level on the optical transmission line. since a characteristic as the oscillation width is widened due to the influence of optical noise, the threshold level C ₄ for data determination is lower with an increase in the optical input intensity.

  Thus, even when the gain band of the preamplifier 3 is widened and the influence of noise on the output on level becomes large, the bit error rate is effectively reduced by lowering the threshold level for data discrimination accordingly. Can be made smaller.

  Further, the threshold control unit 5 can set the optimum data discrimination threshold level corresponding to the input light intensity in the table data in advance and perform data discrimination based on the threshold value. The rate can be reduced.

In addition, this invention is not limited to embodiment mentioned above. For example, a feedback unit 12 may be provided between the input and output of the postamplifier 4 as in the optical receiver 101 which is a modification of the present invention shown in FIG. The feedback unit 12 is used to cancel the deviation of the cross point of the voltage signal _{V in} caused by changes in the optical input, resistor output voltage signal _V out of the post-amplifier 4 _+, a _{V out-} into a voltage signal _{V in} Feedback through. According to the optical receiver 101, the automatic gain region A, the threshold level C ₄ which has been controlled by the variation of the optical input level to a constant value, as well as the optical input and the first embodiment in the gain variation region B it is possible to obtain an optimum data determination threshold to decrease the threshold level C ₄ in response to an increase in level (Fig. 6).

1 is a block diagram showing a schematic configuration of an optical receiver according to a preferred embodiment of the present invention. FIG. 2 is a block diagram specifically illustrating a configuration of the preamplifier in FIG. 1. 3 is a graph showing the received power dependence of the output amplitude, gain band, and resistance value of a feedback resistor of the preamplifier of FIG. 1. It is a figure which shows the output waveform of the preamplifier of FIG. It is a block diagram which shows schematic structure of the optical receiver concerning the modification of this invention. 6 is a graph showing dependency of received light power on a gain band and a threshold level of the preamplifier in FIG. 5. It is a figure which shows the output waveform of the conventional variable gain amplifier.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1,101 ... Optical receiver, 2 ... Photodiode (photoelectric conversion part), 3 ... Preamplifier (current voltage conversion part), 4 ... Post amplifier (differential amplifier), 5 ... Threshold control part, _Iin ... Current signal , Vin ... voltage _{signal, V} out _{+, V out- ...} comparison signal.

Claims (4)

A photoelectric conversion unit that generates a current signal according to the input light intensity;
A current-voltage conversion unit that converts a current signal from the photoelectric conversion unit into a voltage signal having a predetermined amplitude value or less by dynamically changing a current-voltage conversion gain;
A differential amplifier that compares the voltage signal from the current-voltage converter and a threshold level and amplifies the difference;
The threshold level is dynamically changed following the change of the current-voltage conversion gain,
An optical receiver. The threshold level is controlled to decrease as the current-voltage conversion gain decreases.
The optical receiver according to claim 1. A controller that holds a series of threshold level values corresponding to the input light intensity and outputs one of the series of threshold level values as the threshold level;
The optical receiver according to claim 1 or 2.   The optical receiver according to any one of claims 1 to 3, which receives an optical signal propagating through an optical transmission line through which an optical amplifier is interposed.

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