JP4790306B2 - Laser diode drive circuit - Google Patents

Description

  The present invention relates to a laser diode drive circuit for driving a laser diode.

  When an optical signal is input to an optical semiconductor element such as a laser diode, and a modulation operation is performed to output an optical signal, the drive modulated based on data to be transmitted (transmission data) The current is supplied from the laser diode drive circuit to the laser diode. For example, when the change rate (transmission rate) of the drive current input to the laser diode becomes about 10 Gbit / s, the current flows through the laser diode at the rising portion. It is known that vibration occurs and causes waveform deterioration. Such waveform deterioration due to the nature of the laser diode is one of the causes for defining the upper limit of the transmission speed, and various studies have been made as one of the problems that must be solved in order to realize high-speed transmission. Has been made.

  Conventionally, there are publications that disclose techniques for suppressing relaxation oscillation in this type of laser diode drive circuit (for example, Patent Documents 1 and 2).

  For example, in the prior art disclosed in Patent Document 1 below, a capacitor is connected between the input terminal of the differential amplifier circuit and the ground, and the capacitor is charged at the rising edge of the input signal to suppress the relaxation oscillation. . Further, for example, in the prior art disclosed in Patent Document 2 below, a buffer circuit is provided in the input stage of the laser diode driving circuit, and the rising waveform of the input signal is blunted to suppress relaxation oscillation.

Japanese Patent No. 2910279 JP-A-4-94581

  However, in the prior art disclosed in Patent Document 1, the waveform of the rising portion of the input signal is blunted, and at the same time, the waveform of the falling portion is also blunted. There was a problem such as.

  In the prior art disclosed in Patent Document 2, the shape of the output waveform of the buffer circuit provided to suppress the relaxation oscillation is determined by the transistor size and transistor characteristics, and is adjusted from the outside. There was a problem that it was not possible.

  The present invention has been made in view of the above, and while suppressing the relaxation oscillation by dulling the waveform of the rising portion while maintaining the steep waveform of the falling portion of the drive signal output to the laser diode, It is a first object of the present invention to provide a laser diode driving circuit in which such a driving signal waveform can be adjusted according to transistor size and transistor characteristics.

  A second object of the present invention is to provide a laser diode drive circuit having an external adjustment function for outputting the drive signal waveform as described above after the laser diode drive circuit is configured.

  In order to solve the above-described problems and achieve the object, a laser diode driving circuit according to the present invention includes a laser diode driving circuit that generates and outputs a laser diode driving signal to be applied to the laser diode based on an input signal. The drive circuit has a pair of differential input terminals, a differential amplifier circuit that drives a laser diode based on the laser diode drive signal applied to the differential input terminal, and one of the differential amplifier circuits A feedback amplification circuit that generates and outputs a laser diode drive signal to be applied to the differential input terminal; and a switch circuit that transmits the input signal to the feedback amplification circuit. Based on the input signal, the laser diode drive signal The signal waveform at the falling part is more than the signal waveform at the rising part of the laser diode drive signal. Wherein the time constant of the switching circuit is switching control so that Shun.

  According to the laser diode drive circuit of the present invention, the switch circuit for transmitting the input signal to the subsequent amplifier is provided, and the signal waveform at the falling portion of the transmitted laser diode drive signal is the rising portion of the laser diode drive signal. Since the time constant of the switch circuit is controlled so as to be steeper than the signal waveform at, the waveform of the rising portion can be blunted while maintaining the steep waveform of the falling portion of the laser diode drive signal, The driving signal waveform adjusted according to the transistor size and transistor characteristics can be output.

  Embodiments of a laser diode drive circuit according to the present invention will be described below in detail with reference to the drawings. In addition, this invention is not limited by this embodiment.

Embodiment 1 FIG.
FIG. 1 is a diagram showing a basic configuration of a laser diode drive circuit according to a first embodiment of the present invention. The laser diode drive circuit shown in FIG. 1 has a pair of differential input ends, and drives a laser diode (hereinafter referred to as “LD”) 13 based on a laser diode drive signal applied to the differential input ends. The differential amplifier circuit 9, the feedback amplifier circuit 7 that generates and outputs a laser diode drive signal applied to one differential input terminal of the differential amplifier circuit 9, and the input signal (for example, a modulation signal applied to the laser diode) are fed back. A switch circuit 6 for transmitting to the amplifier circuit 7 and an inverting terminal and a non-inverting terminal. For example, an output signal of a non-inverting output terminal based on an input signal is applied to the switching circuit 6, and an output of an inverting output terminal based on the input signal A buffer circuit 5 for applying a signal to the other differential input terminal of the differential amplifier circuit 9 is provided.

  In the differential amplifier circuit 9, one end (for example, cathode end) of the LD 13 is connected to one end (for example, drain end) of the FET 11 as a load of a field effect transistor (hereinafter abbreviated as “FET”) 11. The other end (for example, the source end) of the FET 12 and one end (for example, the source end) of the FET 12 are commonly connected, and one end of the current source 14 is connected to the common connection point. The output terminal of the feedback amplifier circuit 7 is connected to the control terminal (for example, the gate terminal) of the FET 11 forming one differential input terminal of the differential amplifier circuit 9, and the other differential input terminal of the differential amplifier circuit 9 is connected. The inverting output terminal of the buffer circuit 5 is connected to the control terminal (for example, the gate terminal) of the FET 12 forming the circuit. The power source Vcc is connected to a connection point between the anode end of the LD 13 and the other end (eg, collector end) of the FET 12.

  2 to 5 shown below and the conductivity types of the FETs 11 and 12 constituting the differential amplifier circuit 9 shown in FIGS. 1 to 5 are not particularly specified, but the corresponding figures are used. It is assumed that a conductive FET conforming to the explanation of the operation is connected.

  Returning to FIG. 1, the feedback amplifier circuit 7 includes an amplifier 19 having a resistor 17 and an electromotive force 18 as a feedback circuit, and the output terminal of the switch circuit 6 is connected to the input terminal of the feedback amplifier circuit 7. Has been.

  The switch circuit 6 includes a switch means 16 and a capacitor (capacitance element) 20 connected in parallel to both ends of the switch means 16, and the non-inverting output terminal of the buffer circuit 5 is connected to the input terminal of the switch circuit 6. It is configured to be.

  The buffer circuit 5 includes two input terminals which are an in-phase signal input terminal (S) and a negative-phase signal input terminal (S ′), and a non-inverted signal and an inverted signal based on the input signals input to these input terminals. And two output terminals (a non-inverted output terminal and an inverted output terminal) that output to the predetermined terminal described above. Note that these two input terminals do not necessarily have to be two, and it is also possible to configure so that two outputs (non-inverted output and inverted output) are generated based on an input signal input to one input terminal. It is.

  Next, the operation of the laser diode drive circuit shown in FIG. 1 will be described. In the figure, a switch circuit 6 is a circuit that opens when a rising edge of an input waveform is detected and shorts when a falling edge is detected. Now, assuming that the input waveform rises, the switch means 16 of the switch circuit 6 is in an open state, so the voltage equation of the feedback loop is expressed by the following equation.

V _O (t) −V _I (t) = R · i (t) (1)

  On the other hand, the relationship between the input and output voltages is expressed by the following equation when the amplification degree of the amplifier 19 is A.

V _O (t) = A · V _I (t) (2)

  If the input impedance of the amplifier 19 is large, the current i (t) hardly flows into the amplifier 19, so the following equation is established.

i (t) = C {dV _I (t) / dt} (3)

From the above formulas (1) to (3), the output voltage V _O (t) can be expressed by the following formula.

V _O (t) = [E / (1-A)] · [1-exp (− (1-A) t / (CR)] (4)

  Here, if the amplification degree A of the amplifier 19 is close to 1, CR / (1-A) >> t is established. Therefore, if formula (4) is expanded and arranged, it can be expressed by the following formula.

V _O (t) = [E / (CR)] · t (5)

  Equation (5) means that the rise of the output voltage is determined by the CR product. Therefore, the rise time of the current waveform flowing through the LD 13 is also determined by the CR product.

On the other hand, consider the case where the switch circuit 6 detects the falling edge of the input waveform, contrary to the previous case. Since the resistance of the switch means 16 when the switch is short-circuited is extremely small, the charge of the capacitor 20 is rapidly discharged through the switch means 16. Therefore, the input potential of the amplifier 19 decreases rapidly, and the output voltage V _O (t) also decreases rapidly, so that the fall time of the current waveform flowing through the LD 13 becomes steep.

  FIG. 2 is a diagram showing the configuration of the laser diode drive circuit according to the first embodiment of the present invention. The circuit configuration more specifically represents a part of the principle configuration of the laser diode drive circuit shown in FIG. It is shown as an example.

  In FIG. 2, the switch circuit 6 is realized by a transistor 21, a capacitor 20, a resistor 24, a diode 25, and a resistor 17. Further, the feedback amplifier circuit 7 is realized by the transistor 22, the resistor 27, and the resistor 17. On the other hand, an output buffer circuit 8 configured as an emitter follower circuit having a transistor 28 and a resistor 29 is inserted between the feedback amplifier circuit 7 and the differential amplifier circuit 9, and the output terminal of the output buffer circuit 8 has a difference. The dynamic amplifying circuit 9 is configured to be connected to the control terminal (gate terminal) of the FET 11 which is one differential input terminal.

  The resistor 17 functions as both a feedback resistor of the feedback amplifier circuit 7 and a first time constant setting unit (to be described later) of the switch circuit 6. The capacitor 23 and the diode 25 are elements that embody the electromotive force 18.

  The power source Vcc is connected to the control terminal (for example, the base terminal) of the transistor 21 via the resistor 24, and the output terminal (for example, the collector terminal, simultaneously the transistor of the feedback amplifier circuit 7 via the diode 25 and the resistor 17). 22 is connected to the control terminal (for example, the base terminal) of the transistor 22 and is connected to the output terminal of the transistor 22 (for example, the emitter terminal and the control terminal (for example, the base terminal) of the transistor 28 of the output buffer circuit 8) via the diode 25 and the capacitor 23. In addition to being connected, the transistor 28 is directly connected to a non-output terminal (for example, collector terminal) which is not a control terminal.

  Next, the operation of the laser diode drive circuit shown in FIG. 2 will be described. In the circuit configuration shown in FIG. 2, since the input signal is inverted in the switch circuit 6, the polarity of the laser diode drive signal input to the differential amplifier circuit 9 to which the LD 13 is connected is reversed unlike FIG. Become. However, from the viewpoint of passing a current through the LD 13, when the laser diode drive signal falls, the current of the LD 13 is drawn (that is, the current of the LD 13 rises), and conversely, when the laser diode drive signal rises, the LD 13 The relationship that the flow of the current that has been flowing through stops (that is, the current of the LD 13 falls) does not change. Therefore, in the following description, for the sake of consistency with the description of FIG. 1, the case where the potential of the laser diode drive signal drops is described as “rise”, and conversely, the case where the potential of the laser diode drive signal rises is “falling” ".

  In FIG. 2, when no input signal is input, the base current Ib1 of the transistor 21 is supplied through the resistor 24, and the transistor 21 is saturated. At this time, the potential Vce21 at the connection point (Q point) between the capacitor 20 and the resistor 17 is a potential difference between the emitter and the collector when the transistor 21 is saturated, and is about 0.3V. On the other hand, since the current Ic flows from the power source Vcc through the transistor 22 and the resistor 27 toward the power source “−Vee”, the potential difference between the emitter and base of the transistor 22 becomes a potential difference when the PN junction is turned on, and is about 0.7V. become. Therefore, the potential at the emitter end (point S) of the transistor 22 is about -0.4 V (0.3-0.7). Therefore, the transistor 28 to the output buffer circuit 8 does not conduct, the output voltage to the differential amplifier circuit 9 becomes 0V, and no current flows through the LD 13.

  Further, assuming that the rising voltage of the diode 25 is Vf and the potential at the S point is Vs, the voltage between the terminals of the capacitor 23 is Vcc−Vf−Vs≈Vcc−Vf. Now, assuming that the capacitance of the capacitor 23 is sufficiently large, the voltage decrease due to the discharge for a very short time can be ignored, and it can be considered as a constant voltage source.

  Next, consider the case where the input potential drops from the steady state. When the drop of the input potential becomes 0.7V or more, this potential is transmitted to the base end of the transistor 21 via the capacitor 26, and the transistor 21 is cut off. Then, the capacitor 20 receives electric charge from the capacitor 23 via the resistor 17, and the potential at the point Q rises. Since the transistor 22 constitutes an emitter follower circuit, the potential at the point S also rises. Therefore, the output potential of the output buffer circuit 8 also rises and current starts to flow through the LD 13. The rise time tr of the current flowing in the LD 13 (hereinafter referred to as “LD current”) is determined by the CR product (ie, time constant) of the resistance value (R) of the resistor 17 and the capacitance value (C) of the capacitor 20. That is, the resistor 17 and the capacitor 20 have a first time for setting a time constant (first time constant) that determines the rising characteristics of the LD current waveform, that is, the steepness of the signal waveform at the rising portion of the laser diode drive signal. Acts as a constant setting means.

  On the other hand, consider a case where the input potential rises from the above state. When the increase in input potential is 0.7 V or more, the transistor 21 is saturated again. A current flows through the transistor 21, and the Q point decreases toward Vce21 (about 0.3V). At this time, the electric charge accumulated in the capacitor 20 is discharged through the transistor 21. However, since the ON resistance of the transistor is very small, the electric charge of the capacitor 20 is rapidly discharged, and the potentials at the Q point and the S point also drop rapidly. Therefore, the output potential of the output buffer circuit 8 also decreases rapidly, and the current flowing through the laser diode also decreases rapidly. Thus, the fall time tf of the LD current waveform is determined by the CR product (time constant) of the capacitance (C) of the capacitor 20 and the ON resistance value (Ron) of the transistor 21. That is, the transistor 21 (ON resistance) and the capacitor 20 set the time constant (second time constant) that determines the falling characteristics of the LD current waveform, that is, the steepness of the signal waveform at the falling portion of the laser diode drive signal. Therefore, it acts as a second time constant setting means.

  In addition, between the resistance value (R) of the resistor 17 which is one of the first time constant setting means and the ON resistance value (Ron) of the transistor 21 which is one of the second time constant setting means. , R >> Ron, the rise time tr of the LD current becomes slow, and conversely, the fall time tf of the LD current becomes steep.

  In this way, if the capacitor 20, the resistor 17 and the transistor 21 that act as the first and second time constant setting means as described above are selected according to the characteristics of the LD 13 to be used, a desired value can be obtained. A laser diode drive signal having characteristics can be output.

  As described above, according to the laser diode drive circuit of this embodiment, based on the input signal, the signal waveform at the falling portion of the laser diode drive signal is more than the signal waveform at the rise portion of the laser diode drive signal. Since the switching circuit is controlled so that the time constant of the switch circuit is steep, the waveform of the rising part is dulled while the steep waveform of the falling part of the drive signal output to the laser diode is maintained. It is possible to provide a laser diode driving circuit that can suppress and adjust such a driving signal waveform according to the transistor size and transistor characteristics.

  In this embodiment, the resistor constituting the first time constant setting means of the switch circuit 6 and the feedback resistor of the feedback amplifier circuit 7 are configured to be shared. These may be provided separately.

Embodiment 2. FIG.
FIG. 3 is a diagram showing a configuration of a laser diode drive circuit according to the second embodiment of the present invention. In the laser diode drive circuit shown in FIG. 1, the capacitor 20 is a variable capacitance element in the configuration of the first embodiment. Other configurations are the same as or equivalent to those of the first embodiment shown in FIG. 2, and these components are denoted by the same reference numerals.

  In the case of the laser diode drive circuit shown in FIG. 2, when the circuit is integrated, the rise time tr and the fall time tf of the laser diode drive current are respectively determined. Therefore, a detailed transient response model of the laser diode may be required to reliably suppress the relaxation oscillation. In addition, if the laser diode to be used is changed, the required rise time is different, so that the circuit may not be shared.

  Therefore, the laser diode drive circuit according to this embodiment is configured so that the capacitor 20 is a variable capacitance element and includes an external control terminal Ctr for variably controlling the capacitance value of the capacitor 20. With such a configuration, the capacitance value of the capacitor 20 can be varied based on an external signal, the rise time can be arbitrarily set, and circuit design can be performed without a detailed laser diode transient response model. be able to. In addition, laser diodes having different characteristics can be handled with a single circuit, and the circuit can be shared.

  As described above, according to the laser diode drive circuit of this embodiment, since the external adjustment function for controlling the laser diode drive signal waveform is provided, after the laser diode drive circuit is configured, However, the rise time of the current waveform flowing in various laser diodes can be arbitrarily set, and the circuit can be shared even when laser diodes having different characteristics are used.

Embodiment 3 FIG.
FIG. 4 is a diagram showing the configuration of the laser diode drive circuit according to the third embodiment of the present invention. The laser diode drive circuit shown in the figure is configured without using the negative power source -Vee and the resistor 27 by connecting the current source 31 to the collector terminal of the transistor 22 in the configuration of the first embodiment. Other configurations are the same as or equivalent to those of the first embodiment shown in FIG. 2, and these components are denoted by the same reference numerals.

  Next, the operation of the laser diode drive circuit shown in FIG. 4 will be described. In the circuit shown in the figure, a collector current Ic flows from the power supply Vcc through the transistor 22. In this case, the voltage across the terminals of the capacitor 23 is determined by the rising voltage Vf of the diode 25, the collector current Ic, and the static characteristics of the transistor 22. On the other hand, the voltage between the terminals of the capacitor 23 acts as an electromotive force that dominates the charging time of the capacitor 20, so that the current between the current source 31 is controlled to change the voltage between the terminals of the capacitor 23, and LD The rise time tr of the current can be varied.

  As described above, according to the laser diode driving circuit of this embodiment, the power source -Vee and the resistor 27 are replaced with the current source 31 in order to realize the same function as that of the first embodiment. Therefore, it can be configured with only one type of power supply voltage (Vcc), and the circuit can be configured compactly.

Embodiment 4 FIG.
FIG. 5 is a diagram showing the configuration of the laser diode drive circuit according to the fourth embodiment of the present invention. In the laser diode drive circuit shown in FIG. 3, the capacitor 20 is a variable capacitance element in the configuration of the third embodiment. In addition, about another structure, it is the same as that of Embodiment 1 shown in FIG. 4, or is equivalent, The same code | symbol is attached | subjected and shown to these components.

  In the laser diode driving circuit shown in FIG. 5, the capacitance value of the capacitor 20 can be variably controlled based on an external signal, and the voltage across the capacitor 23 can be changed by controlling the current drawn by the current source 31. The rise time tr of the LD current can be varied. In addition, since both variable control functions are used, the adjustment range of the rise time in various laser diodes is widened and the adjustment becomes easy.

  As described above, according to the laser diode driving circuit of this embodiment, the configuration of the third embodiment further includes an external adjustment function for controlling the laser diode driving signal waveform. In addition to the effects of the third embodiment, the adjustment of the rise time in various laser diodes can be facilitated.

  As described above, the laser diode drive circuit according to the present invention is useful as a laser diode drive circuit that suppresses relaxation oscillation and minimizes waveform deterioration. In particular, the current waveform of the current flowing through the laser diode is converted to an external signal. It is suitable as a laser diode driving circuit that can be adjusted based on the above.

It is a figure which shows the fundamental structure of the laser-diode drive circuit concerning Embodiment 1 of this invention. It is a figure which shows the structure of the laser diode drive circuit concerning Embodiment 1 of this invention. It is a figure which shows the structure of the laser-diode drive circuit concerning Embodiment 2 of this invention. It is a figure which shows the structure of the laser-diode drive circuit concerning Embodiment 3 of this invention. It is a figure which shows the structure of the laser-diode drive circuit concerning Embodiment 4 of this invention.

Explanation of symbols

DESCRIPTION OF SYMBOLS 5 Buffer circuit 6 Switch circuit 7 Feedback amplifier circuit 8 Output buffer circuit 9 Differential amplifier circuit 14,31 Current source 16 Switch means 17,24,27,29 Resistor 18 Electromotive force 19 Amplifier 20,23,26 Capacitor 21,22 28 transistor 25 diode

Claims (8)

In a laser diode drive circuit that generates and outputs a laser diode drive signal applied to a laser diode based on an input signal,
The laser diode drive circuit is
A differential amplifier circuit that has a pair of differential input ends and drives a laser diode based on the laser diode drive signal applied to the differential input ends;
A feedback amplifier circuit for generating and outputting a laser diode drive signal applied to one differential input terminal of the differential amplifier circuit;
A switch circuit for transmitting the input signal to a feedback amplifier circuit;
With
Based on the input signal, the time constant of the switch circuit is switched and controlled so that the signal waveform at the falling portion of the laser diode driving signal is steeper than the signal waveform at the rising portion of the laser diode driving signal. A laser diode driving circuit. The switch circuit is
A switching element for transmitting a change in the input signal to the feedback amplifier circuit;
First time constant setting means for setting a first time constant for determining a rising characteristic of a signal waveform of the laser diode drive signal;
Second time constant setting means for setting a second time constant for determining a falling characteristic of the signal waveform;
With
2. The second time constant set by the second time constant setting means is smaller than the first time constant set by the first time constant setting means. Laser diode drive circuit.   3. The laser diode drive circuit according to claim 1, wherein the feedback amplifier circuit includes an emitter follower circuit (source follower circuit) including a feedback circuit including a resistance element and an electromotive force. 4. The switch circuit includes a variable capacitance element that functions as both the first time constant setting means and the second time constant setting means, an external control terminal for variably controlling the capacitance value of the variable capacitance element,
The laser diode drive circuit according to claim 1, further comprising: The switch circuit includes a capacitive element that functions as both the first time constant setting unit and the second time constant setting unit,
4. The laser diode driving circuit according to claim 1, wherein the feedback amplifier circuit includes a current source having one end connected to its own emitter terminal (source terminal). The switch circuit includes a variable capacitance element that functions as both the first time constant setting means and the second time constant setting means, an external control terminal for variably controlling the capacitance value of the variable capacitance element, Comprising
4. The laser diode driving circuit according to claim 1, wherein the feedback amplifier circuit includes a current source having one end connected to its own emitter terminal (source terminal).   The first time constant is set based on a capacitance value of the capacitive element or the variable capacitive element and a resistance value of the feedback resistor, and the second time constant is set to the capacitive element or the variable capacitive element. The laser diode driving circuit according to claim 2, wherein the laser diode driving circuit is set based on a capacitance value of the switching element and a resistance value of the switching element when the switching element is turned on. An output buffer circuit that outputs the laser diode drive signal to one differential input terminal of the differential amplifier circuit using an output signal from the feedback amplifier circuit as an input signal;
8. The laser diode driving circuit according to claim 7, wherein the output buffer circuit includes an emitter follower circuit (source follower circuit).

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