JP5666415B2 - Driving circuit of electroabsorption optical modulator - Google Patents

Description

  The present invention relates to a technique for preventing a drive signal waveform from being deteriorated due to a change in DC voltage of a drive signal applied to an electroabsorption optical modulator.

  Currently, in an optical communication system, an electro absorption type optical modulator (hereinafter referred to as an EA modulator) using an electroabsorption effect of a semiconductor element is used as a modulator capable of performing optical modulation at high speed. ing.

  The electroabsorption effect is an effect in which the band structure of a semiconductor to which an electric field is applied changes and the amount of light absorption changes. Specifically, the EA modulation semiconductor receives continuous light from a laser diode or the like. When a reverse bias voltage is applied to the element, the light absorption amount of the semiconductor element increases and light is not transmitted (OFF state). When the reverse bias voltage is stopped, the light absorption amount of the semiconductor element decreases and light is not transmitted. Since it is transmitted (ON state), an optical signal whose intensity is modulated can be output by supplying a drive signal whose level changes in the range from the reverse bias voltage to zero bias to the semiconductor element.

  As a drive circuit for driving the EA modulator using the electroabsorption effect at high speed, conventionally, as shown in FIG. 6, a modulation data signal d is amplified by a preamplifier 11, and the amplified signal d ′ is amplified. Is further amplified by the driver circuit 12 and supplied to the EA modulator 1.

  The modulation characteristic of the EA modulator 1 is a characteristic in which the bias voltage is inclined within a negative (reverse bias) range as shown in FIG. 7, for example, and the amplitude of −Va to −Vb corresponds to the modulation characteristic. By supplying the drive signal D to the EA modulator 1, the modulated light P whose intensity changes between the light level and the dark level is output.

  The modulation characteristic of the EA modulator 1 is shifted (left and right in FIG. 7) due to the influence of the ambient temperature and the like, thereby changing the level of light and darkness of the modulated light P. In particular, the modulation characteristic is bias voltage. If the light is shifted in the positive direction, the decrease in the light level becomes remarkable and the desired extinction ratio cannot be obtained.

  Further, there is a problem that it is difficult to obtain a uniform light output characteristic with the same drive signal because there is a solid difference due to manufacturing variations of the EA modulator.

For this purpose, conventionally, the direct current voltage V _DC (operating point voltage) of the drive signal D is varied in accordance with individual differences due to temperature variation of the modulation characteristics and manufacturing variations, and adjusted so that the level of the modulated light becomes constant. ing.

For example, in the example of FIG. 6, the output terminal of the final stage transistor TR of the driver circuit 12 (in this case, the connection point between the collector and the load resistor RL) is connected to the variable power source 14 via the resistor Ra. The DC voltage _VDC of the drive signal D is changed (In the figure, Re is an emitter resistance).

  A technique for adjusting the direct current component of the drive signal input to the EA modulator 1 is disclosed in, for example, the following Patent Document 1.

JP 2004-61556 A

  In Patent Document 1, the AC component and the DC voltage of the drive signal are applied to the EA modulator via a bias T circuit including a capacitor for passing AC and a coil for passing DC, so that the circuit is complicated. At the same time, when the data signal to be handled is the NRZ system, if the data of the same level continues, the waveform is degraded due to the C coupling of the bias T circuit.

  On the other hand, in the direct connection method in which the voltage of the specific terminal (collector) for signal output of the final stage transistor TR of the driver circuit 12 is directly supplied to the EA modulator 1 as described above, the waveform deterioration due to C coupling is in principle. There is an advantage that does not occur.

  However, as described above, the method of changing only the voltage at the specific terminal for signal output of the final stage transistor TR of the driver circuit 12 from the outside is the operating point of the final stage transistor TR (in this case, the collector-emitter voltage). As a result, the waveform of the drive signal D is degraded.

8 and 9 are diagrams showing this state. FIG. 8 is a diagram showing a load curve at the time of signal D output on a graph of a typical I _C -V _CE characteristic of the transistor TR. Like a load curve of B around the operating point Pb, If the collector-emitter voltage draws an appropriate trajectory, the drive signal waveform (eye pattern) does not significantly deteriorate as shown in FIG. 9B, but as shown in FIG. When the load curve is on the low voltage side, it enters the saturation region, the jitter of the waveform of the drive signal increases as shown in FIG. 9A, and the load curve becomes on the high voltage side as shown in FIG. If so, an undershoot occurs as shown in FIG.

  For this reason, the variable range of the DC voltage at the specific terminal of the driver circuit 12 is greatly limited, and there is a problem that the variable control of the operating point of the EA modulator 1 cannot be performed sufficiently.

  An object of the present invention is to provide a drive circuit that solves this problem and prevents the deterioration of the drive signal waveform associated with variable DC voltage of the drive signal supplied to the EA modulator.

In order to achieve the above object, a drive circuit for an electroabsorption optical modulator according to claim 1 of the present invention comprises:
A drive circuit (21) that receives and amplifies the modulation data signal and outputs a drive signal for modulating and driving the electroabsorption optical modulator (1) from a connection point between the specific terminal of the final stage transistor and the load resistor. In
An operating point of the final stage transistor is changed in conjunction with a DC voltage of the specific terminal of the final stage transistor and a DC voltage of another terminal that determines the operating point of the final stage transistor together with the specific terminal in the same direction. And a voltage interlocking variable means (30) for changing the DC voltage of the drive signal output to the electroabsorption optical modulator while suppressing the fluctuation of the electric field.

  As described above, the drive circuit for the electroabsorption optical modulator of the present invention makes the DC voltage of the specific terminal for output of the final stage transistor of the driver circuit and the DC voltage of the other terminals interlocked and variably set in the same direction. In addition, since it has voltage interlocking variable means for changing the DC voltage of the drive signal applied to the electroabsorption optical modulator while suppressing the fluctuation of the operating point of the final stage transistor, it accompanies the variable DC voltage of the drive signal. Deterioration of the drive signal waveform is prevented, and modulated light without waveform deterioration can be output.

Configuration diagram of a single drive type embodiment of the present invention The figure which shows the circuit example of the principal part of embodiment Configuration diagram of differential drive type embodiment of the present invention The figure which shows embodiment using a distributed amplifier circuit The figure which shows embodiment using a differential cascode type amplifier circuit The figure which shows the conventional drive circuit The figure which shows the example of the modulation | alteration characteristic of an EA modulator The figure which shows the operating point of the last stage transistor of the drive circuit The figure which shows the example of the drive signal waveform corresponding to an operating point

Hereinafter, embodiments of the present invention will be described with reference to the drawings.
FIG. 1 is an overall configuration diagram of an EA modulator 20 using a drive circuit to which the present invention is applied.

  The EA modulator 20 is composed of the EA modulator 1 and the drive circuit 21 described above. As described above, the EA modulator 1 uses the electroabsorption effect on the light of the semiconductor element. For example, the EA modulator 1 receives the continuous light Pin output from a semiconductor laser or the like, and receives the level of the drive signal D from the drive circuit 21. In response to this, it is turned on (transmitted) and turned off (blocked), and the modulated light Pout is output.

  The drive circuit 21 includes a preamplifier 22, a driver circuit 25, and voltage interlocking variable means 30. Here, the preamplifier 22 and the driver circuit 25 are illustrated separately, but these may be integrated circuits formed on the same semiconductor substrate.

  The preamplifier 22 amplifies the input data signal d and inputs the amplified data signal d ′ to the signal driver circuit 25.

  The driver circuit 25 further amplifies the output signal d ′ of the preamplifier 22 and outputs a single-ended drive signal D from a connection point between the specific terminal (in this case, the collector terminal) of the final stage transistor TR and the load resistor RL. As described above, the drive signal D has an amplitude corresponding to the modulation characteristic of the EA modulator 1 (Re in the figure is an emitter resistance).

The voltage interlocking variable means 30 includes the DC voltage V _C at the specific terminal (collector terminal) of the final stage transistor of the drive circuit 21, that is, the final stage transistor TR of the driver circuit 25, and the final stage together with the specific terminal (collector terminal). a DC voltage V _E of the other terminal for determining the operating point of the transistor TR (in this case emitter terminal) by approximately the same voltage interlock variable in the same direction, the operating point of the final stage transistor TR of (collector-emitter voltage) The DC voltage of the drive signal D is changed while suppressing the fluctuation. Incidentally, the bipolar transistor, because the DC voltage V _BE between the base and emitter during operation is constant, here indirectly by varying the DC voltage V _E of the emitter by varying the base direct voltage.

  In FIG. 1, as an easily understandable configuration of the voltage interlocking variable means 30, a resistor Ra connected to the collector of the final stage transistor TR and a resistor Rb connected to the base are connected to a fixed power source -V '. Are connected to VR1 and VR2 via voltage buffers BUF1 and BUF2 having low output impedance, and interlocked variable resistors VR1 and VR2 are connected to a control system (not shown) (for example, EA modulator 1). The control system is configured to be interlocked and variable by automatic control or manual operation using a control system that suppresses fluctuations in the operating point.

  The configuration of the voltage interlocking variable means 30 is arbitrary. For example, as shown in FIG. 2, one end of resistors Ra and Rb is connected to each other, and a voltage is connected between the connection point and a voltage variabler (which may be configured by a semiconductor) VR3. A configuration in which the collector DC voltage and the base DC voltage of the final stage transistor TR are linked and varied by automatic control or manual operation of the voltage variable VR3 by connecting via the buffer BUF3.

In the driving circuit 21 constructed in this manner, varying the collector current voltage V _C of the final-stage transistor TR in order to change the DC voltage of the drive signal D, therewith interlocked, the base direct voltage V _B is also the same direction the same voltage change to it by the base-current voltage V _bE partial lower emitter DC voltage V _E of the emitter also, since the same voltage change in the same direction, the operating point of the final stage transistor TR (the collector-emitter voltage) 8 can always be maintained in the state of B in FIG. 8, and the drive signal D without waveform deterioration can be supplied to the EA modulator 1.

  Although the above embodiment is an example of a single drive type, FIG. 3 shows an example of a circuit of a differential drive type. In the differential drive type, the differentially input data signals d (+) and d (−) are input to the differential type preamplifier 22 and amplified, and the output signals d (+) ′ and d (− ) ′ Is input to an emitter follower including transistors TR1 and TR2 and resistors R1 and R2 of the differential driver circuit 25. The emitters of these transistors TR1 and TR2 are connected to the bases of two terminal transistors TR3 and TR4 that are differentially connected, and these terminal transistors TR3 and TR4 amplify the data signals output from both emitter followers. .

  Load resistors RL1 and RL2 are connected to the collectors of the final stage transistors TR3 and TR4, and the emitters are connected to a common current source I. Each base receives a signal whose phase is inverted, and its input and phase are inverted. Generated signal to the collector. Here, since the drive signal D is single-ended, the drive signal D is output from the collector of one final stage transistor TR3.

Here, as described above, the resistors Ra1 and Ra2 for changing the DC voltage of the drive signal D are connected to the collectors of the final stage transistors TR3 and TR4 of the driver circuit 25. Also, the same resistors Rb1 and Rb2 are connected to the bases of the transistors TR1 and TR2 on the input side, and these resistors are connected to the above-mentioned interlocking voltage variable devices VR1 and VR2 or the common voltage variable device VR3. Te, by interlocking the variable or variable voltage variable device VR3 interlocking voltage changer VR1, VR2, the collector DC voltage _{V C} of the final stage transistor TR3, TR4 are changed, the transistors TR1, TR2 the base DC voltage _{V B} and the emitter DC voltage V _E is the same voltage change in the same direction. As a result, the base DC voltage and the emitter DC voltage of the final stage transistors TR3 and TR4 also change in the same direction in the same direction. TR3, it is possible to change the collector current voltage V _C of the TR4, waveform deterioration does not occur by the operating point change.

Here, in order to hold the balance of the circuit, not only the final stage transistor TR3 of the drive signal output, by varying the collector current voltage V _C also the final stage transistor TR4 which forms a pair therewith, accordingly had also in conjunction base direct voltage V _B Te, when the unbalance of the circuit is not a problem, skip resistor Ra2 or Rb2, based DC voltage of the final stage transistor TR4 V _B or collector DC voltage V _C It is good also as a circuit which does not change (fixed).

  Here, the preamplifier 22 is provided in front of the driver circuit 25. However, if the DC level and amplitude of the input data signal correspond to the driver circuit 25 having the above configuration, the preamplifier can be omitted.

  In the above embodiment, the final stage transistor is a bipolar type. However, the present invention can be applied even when a field effect transistor (FET) is used. In the case of a grounded source amplifier using an FET, the operating point is the drain-source voltage, and waveform deterioration occurs due to fluctuations in the operating point. Therefore, the source DC voltage is varied in conjunction with the variation of the drain DC voltage. As a result, the DC voltage of the drive signal can be varied while suppressing fluctuations in the operating point.

  In the above embodiment, the drive signal D amplified by the single driver circuit 25 is given to the EA modulator 1. However, the drive signal having the amplitude necessary for driving the EA modulator 1 by the single driver circuit 25. 4 cannot be obtained, a distributed (or traveling wave) amplifier circuit as shown in FIG. 4 may be used.

  In this distributed amplifier circuit, the output signal of the preamplifier 22 is input to a plurality of driver circuits 25 (1) to 25 (N) provided at a predetermined interval L via a differential transmission path 40. A signal amplified by the driver circuit is in-phase multiplexed in the output-side transmission line 50, so that a broadband signal can be amplified to a high output (in the figure, reference numerals 40a and 40b are a pair of main lines).

In this case, the termination resistors Rz1 and Rz2 of the differential transmission line 40 to which the output of the preamplifier 22 is supplied are also used as the voltage variable resistor Rb, and one end of the output-side transmission line 50 is connected to one end of the output-side transmission line 50. The resistor Ra is connected to change the DC voltage of the drive signal D, and the base DC voltage V _B of the final stage transistors TR (1) to TR (N) of the driver circuits 25 (1) to 25 (N) is also changed. Similarly to the above, the operating point is changed in the same direction and the operating point is maintained in an appropriate state. In the case of this circuit, the load resistance of the final stage transistors TR (1) to TR (N) of the driver circuits 25 (1) to 25 (N) is the termination resistance connected to the other end of the transmission line 50 on the output side. Rz3.

  In the above embodiment, the example in which the drive signal is output from the collector of the final stage transistor of the grounded emitter circuit has been described. However, the present invention can also be applied to a grounded collector type, a grounded drain type, or a cascode type.

  FIG. 5 shows a configuration example of the differential cascode driver circuit 25. In this circuit, a grounded base circuit using transistors TR5 and TR6 is inserted between the collectors of the transistors TR3 and TR4 and the load resistors RL1 and RL2 of the differential driver circuit described above. It is known that a wider band characteristic can be obtained than the configuration of the ground circuit alone.

In the case of the differential cascode driver circuit 25, a resistor Ra1 connected to the collector terminal of at least one of the two final stage transistors TR5 and TR6 (here, the transistor TR5) and the base terminals of the two final stage transistors TR5 and TR6. of a resistor Rb3 which is connected to the connection point, connected to a voltage interlock adjusting means 30 described above, the same manner, interlocking the variably final stage transistor TR5, TR6 base DC voltage V _B of the DC voltage of the drive signal D To prevent waveform deterioration due to operating point fluctuations. In the case of this circuit, the base DC voltage of the transistors TR3 and TR4 is also linked to the voltages of the final stage transistors TR5 and TR6 via the resistors Rb1 and Rb2 connected to the voltage interlocking variable means 30, Although the fluctuation of the operating point of the entire cascode circuit is suppressed, it is possible to omit the interlocking variation of the base voltages of the transistors TR3 and TR4.

  DESCRIPTION OF SYMBOLS 1 ... EA modulator, 20 ... EA modulator, 21 ... Drive circuit, 22 ... Preamplifier, 25 ... Driver circuit, 30 ... Voltage interlocking variable means, 40 ... Transmission path, 50 ... Transmission path

Claims (1)

A drive circuit (21) that receives and amplifies the modulation data signal and outputs a drive signal for modulating and driving the electroabsorption optical modulator (1) from a connection point between the specific terminal of the final stage transistor and the load resistor. In
An operating point of the final stage transistor is changed in conjunction with a DC voltage of the specific terminal of the final stage transistor and a DC voltage of another terminal that determines the operating point of the final stage transistor together with the specific terminal in the same direction. A drive circuit for an electroabsorption optical modulator comprising voltage interlocking variable means (30) for changing a DC voltage of the drive signal output to the electroabsorption optical modulator while suppressing fluctuations .

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