JP2007241142A - Compact bias tee - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide the bias tee of an optical modulator capable of obtaining an optical signal of good quality even when a circuit driving the optical modulator is made compact. <P>SOLUTION: A high-pass filter comprising a capacitor 13 and a coil uses only a high-frequency coil and an intermediate-frequency coil, and a large-sized low-frequency coil is removed. A low-frequency variation detecting part 20 is provided instead which detects the low-frequency component of a driving signal from a driving circuit 10. The low-frequency variation detecting part 20 is a low-pass filter comprising a resistance 21 and a capacitor, and variation of the low-frequency component is input to the forward rotation terminal of an operational amplifier 21. When the low-frequency component of the driving signal decreases, a current flowing to the resistance 24 decreases and a bias voltage applied to an EA modulator 11 rises by a decrement in voltage drop at the resistance 24. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a bias tee in a drive circuit that drives an optical modulator.

  In the peripheral circuit of an electro-absorption external modulator (hereinafter referred to as EA modulator) used for high-speed optical modulation, the low current used to cut off the low band at the bias tee at the connection between the EA modulator and the drive circuit. Since the area coil (large inductance) is large in size, it becomes a problem when it is desired to reduce the mounting area.

FIG. 4 is a diagram showing a configuration of a conventional EA modulator, a drive circuit, and a bias tee.
A drive voltage is applied from the drive circuit 10 to the EA modulator 11. The drive voltage from the drive circuit 10 is applied to the EA modulator 11 after the DC component is removed by passing through a high-pass filter comprising a high-frequency coil, a mid-frequency coil, a low-frequency coil and a capacitor 13. Is done. A DC voltage generated by the EA bias control circuit 12 is applied as the bias voltage to the EA modulator 11. The high frequency coil has a small inductance and a small size. The mid-range coil has a medium inductance and a medium size. The low frequency coil has a large inductance and a large size.

FIG. 5 is a diagram for explaining the reason for using a plurality of coils in the high-pass filter.
Since it is desired to apply a drive signal from which the DC component has been removed to the EA modulator 11, in order to remove a component having a frequency lower than the signal band, a high-pass filter is formed by a coil and a capacitor, and the drive signal is passed therethrough. However, since each coil L1 (coil for high frequency band), L2 (coil for mid frequency band), and L3 (coil for low frequency band) has a Q value as an operating characteristic, an ideal high-pass characteristic cannot be shown. That is, the high-frequency coil L1, the mid-frequency coil L2, and the low-frequency coil L3 can each transmit only a part of the signal band. Therefore, in order to pass the entire signal band of the drive signal, it is necessary to prepare two or three coils having different cutoff frequencies and form a high-pass filter. However, the size of the low frequency coil is larger than that of the high frequency coil, which is about three times longer and about 27 times larger than the high frequency coil. Therefore, using a low-frequency coil is an obstacle to miniaturizing the device itself.

In order to reduce the size of the device, when the bias tee circuit is configured with only the high-frequency coil (small inductance) and the mid-frequency coil (medium inductance), the low frequency included in the signal as shown in the following operating principle Sag occurs in the component. With the occurrence of sag,
(1) Extinction ratio deterioration, pulse mask standard margin deterioration, and bit error rate (hereinafter referred to as BER) deterioration on the receiver side due to a decrease in eye opening (increase in interference) of the modulation signal.
(2) Cross point (Duty) fluctuation in the eye diagram. Also, BER degradation on the receiver side due to cross point fluctuation. (When the receiver side is cross point tracking control system)
(3) Problems such as deterioration of chromatic dispersion tolerance occur.

As a conventional optical modulator drive circuit, there is one as disclosed in Japanese Patent Application Laid-Open No. H10-228707. Patent Document 1 discloses a technique for driving an optical modulator with low power by removing a direct current component from the drive signal of the optical modulator.
JP-A-5-264934

6-10 is a figure explaining the subject of a prior art.
FIG. 6 is a diagram illustrating the configuration of the drive circuit, the EA modulator, and the bias tee from which the low-frequency coil is removed.

6, the same components as those in FIG. 4 are denoted by the same reference numerals.
FIG. 7A shows the voltage and current waveforms of each part when the signal changes from a signal with a mark rate of 50% to a continuous state of the same sign (logic 1 code) at a certain time a. The numbers in parentheses in FIG. 6 indicate that the signal waveform at that point is represented in the corresponding graphs in FIGS. 7 (a) and 7 (b). The operation of each part is as follows.

  It is assumed that the drive signal (1) from the drive circuit 10 changes to a continuous state of 1 after time a. The waveform (2) before AC coupling input to the bias tee 14 cannot maintain a HIGH level for one continuation having a frequency component lower than the low frequency cutoff frequency of the bias tee 14, and sag occurs after time a. Since the conventional EA bias control circuit 12 does not have a function of recognizing sag, the constant current (3) is continuously supplied so that the DC level of the EA bias is constant, and the control voltage is also constant. After the bias tee AC coupling, the waveform having a sag from (2) is input to the EA modulator 11 as it is, but since the EA bias control circuit 12 does not perform any particular correction, the signal (5) is (2). ) Remains the same waveform.

  FIG. 7B shows an electric signal-optical signal conversion process in the EA modulator 11 and an optical waveform after modulation. The sag of the electrical signal remaining in (5) falls directly to the extinction side (electrostatic absorption increase side) even in the EA emission (extinction) characteristic curve, and as a result, the sag also remains in the optical waveform. When the optical output waveform is indicated by an eye pattern (6) ', the sag causes interference on the HIGH side of the eye and reduces the eye opening.

As a result, the following new problem arises, and it is impossible to configure a bias tee circuit using only the high-frequency and middle-frequency coils (small inductance, medium) in the conventional circuit.
(1) Reduction of eye opening of optical output waveform (increased interference)
・ Degradation of extinction ratio ・ Degradation of pulse mask standard margin ・ Deterioration of BER at receiver side Also, voltage and current of each part when signal mark ratio changes from 50% to >> 50% after a certain time a The waveform is shown in FIG. Further, FIG. 9 shows a supplementary diagram of changes in the DC level and the bias point.

  It is assumed that the signal from the drive circuit 10 changes to mark ratio >> 50% after time a (1). At this time, the DC level of the signal waveform is higher than when the mark rate is 50%. When the fluctuation of the DC level is a change at a frequency component whose frequency component is lower than the low cutoff frequency of the bias tee, the DC level before the bias tee AC coupling is corrected by the bias voltage supplied from the coil and input to the bias tee. The waveform before AC coupling is biased toward the LOW level after time a, and sag occurs (2).

As shown in FIG. 9, the position of the bias point (the center point between the HIGH level and the LOW level) is lowered by the sag, but the DC level (average voltage) does not change.
Since the conventional EA bias control circuit 12 does not have a function of recognizing a sag, a constant current is continuously applied to keep the DC level of the EA bias constant (3), and the control voltage is also constant. After the bias tee AC coupling, the waveform having a sag from (5) is inputted as it is to the EA modulator 11, but since the EA bias control circuit 12 does not perform any particular correction and makes the DC level constant (4 ), The waveform is the same as (2).

  FIG. 10 shows an electric signal-optical signal conversion process in the EA modulator 11 and a modulated optical waveform. The sag of the electrical signal remaining in (5) falls directly to the extinction side (electrostatic absorption increase side) even in the EA emission (extinction) characteristic curve, and as a result, the sag also remains in the optical waveform (6). When the optical output waveform is represented by an eye pattern ((6) '), the sag causes interference on the HIGH side of the eye, thereby reducing the eye opening. Further, the fluctuation of the cross point (Duty) and the fluctuation of Chirp occur as the bias point of the waveform moves.

As a result, the following new problem arises, and it is impossible to configure a bias tee circuit using only the high-frequency and middle-frequency coils (small inductance, medium) in the conventional circuit.
(1) Reduction of eye opening of optical output waveform (increased interference)
-Degradation of extinction ratio-Degradation of pulse mask standard margin-BER degradation on receiver side (2) Cross point (Duty) fluctuates When identification threshold control method on receiver side is DC feedback method (cross point tracking), There is a possibility that BER degradation occurs due to fluctuation of the identification threshold.
(3) Chirp variation Generally, when a bias point variation occurs in an EA modulator, a Chirp parameter (α parameter) varies. For example, when the EA bias increases, the α parameter increases, and the waveform compression after + dispersion (SMF transmission) becomes intense, causing BER degradation on the receiver side.

When the mark ratio of the waveform is << 50%, in (1) to (6), the waveform is shifted to the HIGH level side, contrary to FIGS. Arise.
An object of the present invention is to provide a bias tee of an optical modulator capable of obtaining a high-quality optical signal even if a circuit for driving the optical modulator is miniaturized.

The bias tee of the present invention comprises a high-pass filter means provided with a coil that cuts the low frequency side of the signal band of the drive signal of the optical modulator,
The high-pass filter means detects a low-frequency component of the drive signal to be cut;
Bias means for varying a bias voltage to the optical modulator so as to compensate for a sag of a drive signal applied to the optical modulator as the low-frequency component fluctuates;
It is characterized by providing.

  According to the present invention, it is possible to provide a bias tee for a drive circuit of an optical modulator that is small and can generate a high-quality optical signal.

  1-3 is a figure explaining embodiment of this invention. In FIG. 1, the same components as those in FIG. 6 are denoted by the same reference numerals. In the description of the present embodiment, a portion surrounded by a line is defined as a bias tee 14a, and a lower half thereof is defined as an EA bias control circuit 12a.

  In the embodiment of the present invention, the DC component of the drive circuit output (before AC coupling of the bias tee 14a) is detected by only a small component, and the DC bias supply circuit output of the EA modulator is corrected, thereby correcting the sag. Compensation (expansion of low cutoff frequency) is possible.

  The frequency component to be corrected can be limited on the high frequency side by the high frequency cutoff frequency of the DC component (low frequency fluctuation) detection circuit and the DC bias supply circuit of the EA modulator. Works as usual and causes no problems.

By the low frequency component compensation using the configuration of the embodiment of the invention, sag is improved,
(1) Compensation for reduction of eye opening (increased interference) (2) Compensation for cross point (Duty) fluctuation (3) Compensation for deterioration of dispersion tolerance

  In FIG. 1, the drive signal output from the drive circuit 10 is input to the low-frequency fluctuation detection unit 20 before passing through a high-pass filter composed of high-frequency and mid-frequency coils and a capacitor 13. The low-frequency fluctuation detection unit 20 is, for example, a low-pass filter composed of a capacitor and a resistor, can be configured to be small in size, and allows a low-frequency component removed by the high-pass filter to pass through. The low frequency component detected by the low frequency fluctuation detection unit 20 in this way is input to the normal terminal of the operational amplifier 21. In FIG. 6 of the prior art, since this configuration is not provided, the voltage input to the normal rotation terminal and the inverting terminal of the operational amplifier 21 is constant, and thus the bias voltage applied to the EA modulator 11 is also constant. However, in the configuration of FIG. 1, the operation of the EA bias control circuit 12 a can be changed depending on the magnitude of the low frequency component by inputting the low frequency component of the drive signal to the normal terminal of the operational amplifier 21.

  Specifically, when the voltage value of the low-frequency component detected by the low-frequency fluctuation detection unit 20 becomes small, the voltage value input to the normal rotation terminal of the operational amplifier 21 also becomes small. Then, the voltage value of the inverting terminal of the operational amplifier 21 follows and decreases due to negative feedback. Then, the value of the current flowing through the resistor 24 is reduced, and the voltage drop at the resistor 24 is reduced. Therefore, the voltage applied to the EA modulator 11 connected via the coil 25 and the transistor 23 functioning as a closed switch increases. As a result, the decrease in the DC component applied to the EA modulator 11 due to the occurrence of the sag is compensated by the increase in the bias voltage caused by this decrease. It should be noted that how much the change in the low frequency component detected by the low frequency fluctuation detecting unit 20 is converted into the change in the bias voltage can be determined by appropriately setting the resistance values of the resistors 21 and 22. .

  Fig. 2 (a) shows the voltage and current waveforms of each part when the drive signal changes from a signal with a mark ratio = 50% to a continuous state of the same sign (1) at a certain time a. The electric signal-optical signal conversion process and the optical waveform after modulation are shown in FIG. 2B, and the voltage of each part when the signal mark ratio changes from 50% to >> 50% after a certain time a. FIG. 3A shows the current waveform, and FIG. 3B shows the electric signal-optical signal conversion process in the EA modulator and the modulated optical waveform.

  In FIG. 2A, it is assumed that the drive signal (1) from the drive circuit 10 changes to a continuous state of 1 after time a. The waveform (2) before AC coupling input to the bias tee 14a cannot maintain a HIGH level for one continuous frequency component lower than the low-frequency cutoff frequency of the bias tee 14, and sag occurs after time a. A low-frequency fluctuation (voltage drop) before the bias tee AC coupling is detected by the low-frequency fluctuation detecting unit 20 and input to the operational amplifier normal rotation input of the EA bias control circuit 12a. The operational amplifier operates as a normal rotation amplifier, changes the low-frequency fluctuation in the same direction from (3) to (3) ″ ″, and reduces the voltage of (3) ″ ″. (3) The current (4) flowing through the resistor 24 decreases due to the voltage drop of ″ ″ ″. Due to the current decrease in (4), the drop amount from GND decreases, and the bias voltage increases (5). The sag of the waveform of the drive signal can be canceled by increasing the voltage from the EA bias control circuit 12a, thereby compensating for the sag (6).

  Therefore, as shown in FIG. 2B, since the sag of the drive signal (6) is compensated, the optical output (7) output based on the EA emission characteristics is not affected by the sag. , (7) ′, no reduction in eye opening occurs.

As the above effect,
(1) Compensation for reduction of eye opening (increased interference) of optical output waveform, compensation for deterioration of extinction ratio, compensation for degradation of pulse mask standard margin, compensation for BER degradation on receiver side.

FIG. 3 shows a case where the mark ratio of the drive signal is greater than 50%.
In FIG. 3A, it is assumed that the drive signal (1) from the drive circuit 10 changes to a state where the mark rate is greater than 50% after time a. The waveform (2) before AC coupling input to the bias tee 14a cannot hold a HIGH level for a drive signal having a frequency component lower than the low-frequency cutoff frequency of the bias tee 14 and a mark ratio exceeding 50%, and after time a. Sag occurs. A low-frequency fluctuation (voltage drop) before the bias tee AC coupling is detected by the low-frequency fluctuation detecting unit 20 and input to the operational amplifier normal rotation input of the EA bias control circuit 12a. The operational amplifier operates as a normal amplifier, changes the low frequency fluctuation in the same direction from (3) to (3) ′ ″, and lowers the voltage of (3) ′ ″. (3) The current (4) flowing through the resistor 24 decreases due to the voltage drop of '''. Due to the current decrease in (4), the drop amount from GND decreases, and the bias voltage increases (5). The sag of the waveform of the drive signal can be canceled by increasing the voltage from the EA bias control circuit 12a, and the sag can be compensated (6).

  Therefore, as shown in FIG. 3B, since the sag of the drive signal (6) is compensated, the optical output (7) output based on the EA emission characteristics is not affected by the sag. , (7) ′, no reduction in eye opening occurs.

As the above effect,
(1) Compensation for reduction of eye opening (increased interference) in optical output waveform ・ Compensation for degradation of extinction ratio ・ Compensation for degradation of pulse mask standard margin ・ Compensation for BER degradation on receiver side (2) Crosspoint (Duty) Compensation for fluctuation (compensation for BER degradation on the receiver side)
(3) Compensation for Chirp variation (compensation for degradation of dispersion tolerance)
Is possible.

It is FIG. (1) explaining embodiment of this invention. It is FIG. (2) explaining embodiment of this invention. It is FIG. (3) explaining embodiment of this invention. It is a figure which shows the structure of the conventional EA modulator, a drive circuit, and a bias tee. It is a figure explaining the reason for using two or more coils for a high pass filter. It is FIG. (1) explaining the subject of a prior art. It is FIG. (2) explaining the subject of a prior art. It is FIG. (3) explaining the subject of a prior art. It is FIG. (4) explaining the subject of a prior art. It is FIG. (5) explaining the subject of a prior art.

Explanation of symbols

DESCRIPTION OF SYMBOLS 10 Drive circuit 11 EA modulator 12, 12a Bias control circuit 13 Capacitor 14, 14a Bias tee 20 Low range fluctuation detection part 21, 22, 24 Resistance 23 Transistor 25 Coil

Claims (4)

High-pass filter means provided with a coil that cuts the low band side of the signal band of the drive signal of the optical modulator;
The high-pass filter means detects a low-frequency component of the drive signal to be cut;
Bias means for varying a bias voltage to the optical modulator so as to compensate for a sag of a drive signal applied to the optical modulator as the low-frequency component fluctuates;
A bias tee characterized by comprising.   2. The bias tee according to claim 1, wherein the coil of the high-pass filter means is reduced in size by cutting a low-frequency side of a signal band.   The bias tee according to claim 1, wherein the low-frequency component detection means is a low-pass filter.   The bias means comprises an operational amplifier in which the low-frequency component detected by the low-frequency component detection means is connected to the normal terminal, the output is connected to the base of the transistor, and the inverting terminal is connected to the emitter of the transistor. 1. Bias tee according to 1.

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