JP2006253266A - Circuit and method for driving light emitting device and optical transmission device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To solve the problem that when the light output intensity of a light emitting device decreases with the passage of time, modulation current Imod is kept as it were before the decrease of the light output intensity, even if the light output intensity of the light emitting device is kept constant by APC circuit, which causes the amplitude of a superimposed signal to become relatively small and reduces the amplitude of light output of the light emitting device. <P>SOLUTION: In a light emitting device driving circuit including the APC circuit 40, three current sources 26, 27, and 28 which are connected in parallel are arranged as sources of modulation current. An arbitrary temperature characteristic of the light emitting device is set with respect to the modulation current Imod by the ratio of temperature characteristics of the current sources 26 and 27. When the light output intensity of the light emitting device 10 decreases and then the light emitting device 10 is controlled by the APC circuit 40, in such a direction that bias current Ibias may increase; the magnitude of the modulation current Imod which adds modulation to the light emitting device 10 is controlled (compensated) by the current source 28 according to the bias current Ibias. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a light emitting element driving circuit, a light emitting element driving method, and an optical transmitter.

  Usually, in the light emitting element drive circuit, the light output intensity of the light emitting element is monitored by the light receiving element so that the light output intensity (light emitting intensity) of the light emitting element is always constant, and feedback control is performed on the bias current of the light emitting element. An APC (Auto Power Control) circuit for performing the above is provided. By the action of the APC circuit, it is possible to compensate for the deterioration of the light output intensity due to the temperature characteristics of the light emitting element and the change with time, and to obtain a stable light output intensity.

  In addition, a modulation circuit that superimposes a signal such as a clock or data on the light emitting element is configured independently of the APC circuit. This modulation circuit is configured to have a constant intensity without having temperature characteristics, or is configured to compensate for the temperature characteristics of the differential efficiency and differential resistance of the light emitting element (see, for example, Patent Document 1). ).

JP-A-3-133187

  FIG. 5 is a circuit diagram showing a configuration example of a light emitting element driving circuit according to a conventional example. In FIG. 5, the modulation circuit 200 is AC coupled to the light emitting element 100 via a capacitor C. The modulation circuit 200 includes a buffer 201 and a differential pair transistor 202 and 203 whose emitters are connected in common and whose bases are respectively connected to the positive phase output terminal and the negative phase output terminal of the buffer 201, and the differential pair transistor 202. , 203, resistors 204 and 205 connected between the collector and the power supply VCC, respectively, and two modulation current sources 206 connected in parallel between the emitter common connection node of the differential pair transistors 202 and 203 and the ground. , 207.

  The modulation circuit 200 buffers the input signal with the buffer 201 and drives the light emitting element 100 with the output amplitude of the modulation current sources 206 and 207. In this example, a current source having no temperature characteristic (0 ppm) is used as one modulation current source 206, and a current source having a temperature characteristic of 5000 ppm, for example, is used as the other modulation current source 207. An arbitrary temperature characteristic can be set for the modulation current Imod by the ratio of the temperature characteristics of the current sources 206 and 207.

  In the vicinity of the light emitting element 100, a light receiving element 300 is arranged as a monitor element. The light receiving element 300 monitors the light output intensity of the light emitting element 100 and outputs a monitor signal proportional to the light output intensity to the APC circuit 400. The APC circuit 400 compares the output voltage of the light receiving element 300 with the bias current source 401 connected in series to the light emitting element 100 via the inductor L with the APC set voltage, and according to the difference voltage, the bias current source The bias current source 401 is controlled so that the output of the light receiving element 300 becomes constant. The differential amplifier 402 controls the bias current Ibias 401.

  By the operation of the modulation circuit 200 and the APC circuit 300 described above, the temperature characteristic of the light emitting element as shown in FIG. 6 is compensated, and the light output intensity (light emission intensity) and the amplitude of the superimposed signal are constant over a wide temperature characteristic. Can be kept in.

  By the way, as shown in FIG. 7, the light output intensity of the light emitting element is deteriorated due to a change with time. At this time, in the above-described conventional technique, the APC circuit 400 controls the bias current source 401 in a direction in which the bias current Ibias increases, so that the light output intensity of the light emitting element 100 is kept constant.

  However, the modulation current Imod remains the magnitude before the deterioration of the light emitting element 100. For this reason, the amplitude of the superimposed signal becomes relatively small, and as a result, the amplitude of the light output of the light emitting element 100 decreases. In particular, in the case of optical communication, a shortage of light output from the light emitting element causes a communication error. Therefore, a technique for keeping the light output amplitude of the light emitting element constant even after the light output intensity of the light emitting element is deteriorated is required.

  Accordingly, an object of the present invention is to provide a light emitting element driving circuit, a light emitting element driving method, and an optical transmission device capable of keeping the light output amplitude of the light emitting element constant even after the light output intensity of the light emitting element is deteriorated. And

  In order to achieve the above object, in the present invention, a modulation current is supplied to the light emitting element in accordance with an input signal, and the bias current of the light emitting element is controlled so that the light output intensity of the light emitting element is constant, When the bias current is controlled to increase when the light output intensity of the light emitting element is deteriorated, the modulation current is controlled in accordance with the bias current.

  In the above configuration, when the light output intensity of the light emitting element is deteriorated and the bias current is controlled to increase, light emission is controlled by controlling the magnitude of the modulation current that modulates the light emitting element according to the bias current. Even if the light output intensity of the element deteriorates, it is possible to prevent the amplitude of the superimposed signal from becoming relatively small.

  According to the present invention, even if the light output intensity of the light emitting element is deteriorated, the amplitude of the superimposed signal can be prevented from becoming relatively small. Therefore, the light output amplitude can be reduced even after the light output intensity of the light emitting element is deteriorated. Can be kept constant.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.

[First Embodiment]
FIG. 1 is a circuit diagram showing a configuration of a light emitting element driving circuit according to the first embodiment of the present invention. In FIG. 1, a light emitting element 100 such as a laser diode can be directly modulated by changing a current equal to or higher than a threshold current Ith of an excitation current. Accordingly, the operating point of the light emitting element 100 is determined by an appropriate bias current Ibias, and a signal such as a clock or data is superimposed on the bias current Ibias as amplitude modulation by the modulation circuit 20, so that the change in current is almost linear. Output modulated output light proportionally proportionally.

  The modulation circuit 20 includes a differential pair transistors 22 and 23 each having a buffer 21 and an emitter connected in common and each base connected to the positive phase output terminal and the negative phase output terminal of the buffer 21, respectively. , 23 and resistors 24 and 25 respectively connected between the power supply VCC and three modulation current sources 26 connected in parallel between the emitter common connection node of the differential pair transistors 22 and 23 and the ground. 27, 28, and the circuit output end, that is, the collector of the transistor 23 is AC coupled to the cathode of the light emitting element 10 via the capacitor C.

  The light receiving element 30 is, for example, a photodiode, and is disposed as a monitor element in the vicinity of the light emitting element 10. The light receiving element 30 monitors the light output intensity of the light emitting element 10 and sends a monitor signal proportional to the light output intensity to the APC circuit 40. Output. The APC circuit 40 compares the output voltage of the light receiving element 30 with the bias current source 41 connected in series to the light emitting element 10 via the inductor L, and the bias current source according to the difference voltage. The bias current source 41 is controlled such that the output of the light receiving element 30 is constant. The differential amplifier 42 controls the bias current Ibias 41.

  In the light emitting element driving circuit having the above configuration, in this embodiment, in addition to the current source 26 having no temperature characteristic (0 ppm) and the current source 27 having a temperature characteristic of 5000 ppm, for example, as a modulation current source for passing the modulation current Imod. And a current source 28 for supplying a current proportional to the bias current Ibias, specifically, a current 1 / n times the bias current Ibias. The current source 28 forms a current mirror circuit with the bias current source 41, thereby constituting a modulation current control means for controlling the modulation current Imod according to the bias current Ibias.

  When the bias current source 401 is controlled in the direction in which the bias current Ibias is increased by the APC circuit 40, the current source 28 is deteriorated in light output intensity of the light emitting element 10 due to the temporal change of the light emitting element 10. The modulation current Imod is controlled according to the bias current Ibias. Specifically, an appropriate current proportional to the bias current Ibias (in this example, a current 1 / n times) is added to the modulation current Imod. By the action of the current source 28, even if the light output intensity of the light emitting element 10 is deteriorated, the amplitude of the superimposed signal can be prevented from becoming relatively small. Therefore, even after the light emitting element 10 is deteriorated, the light emitting element 10 It is possible to keep the light output amplitude of the constant.

  Incidentally, even during normal control by the APC circuit 40 (hereinafter referred to as “APC control”), the current value of the bias current Ibias changes in accordance with the fluctuation of the light output intensity of the light emitting element 10. The amount of change at that time is slight and finally converges to a constant value corresponding to the APC set voltage. Therefore, the current source 28 controls the modulation current Imod according to the change of the bias current Ibias even during the APC control. This modulation current Imod also finally has a current value corresponding to the bias current Ibias having a constant value. Will converge to.

  On the other hand, when the light output intensity of the light emitting element 10 deteriorates due to the change of the light emitting element 10 with time, the APC circuit 40 controls the bias current source 41 in the direction in which the bias current Ibias increases, and the bias current By changing the convergence value of Ibias, the light output intensity of the light emitting element 10 is kept constant. The current source 28 sets the modulation current Imod to a current value (in this example, 1 / n times) corresponding to the convergence value of the bias current Ibias after being changed by the control by the APC circuit 40.

  Thus, in the light emitting element driving circuit having the APC circuit 40, three current sources 26, 27, and 28 connected in parallel to each other are prepared as modulation current sources, and the ratio of the temperature characteristics of the current sources 26 and 27 is determined. Arbitrary temperature characteristics are set for the modulation current Imod, and the light output intensity of the light emitting element 10 is deteriorated. When the APC circuit 40 controls the bias current Ibias to increase, the light emitting element 10 is modulated. By controlling (compensating) the magnitude of the modulation current Imod to be applied according to the bias current Ibias by the current source 28, the temperature characteristic of the light emitting element as shown in FIG. 6 is compensated, and the light output intensity over a wide temperature characteristic. The amplitude of the signal superimposed with (light emission intensity) can be kept constant, and the light output amplitude can be kept constant even after the light emitting element 10 is deteriorated. That.

  In this embodiment, the case where the current source 27 having the temperature characteristic is provided as an example of the modulation current source has been described as an example. However, the current source 27 is not essential. However, as in the present embodiment, the current source 27 having temperature characteristics is connected in parallel to the current source 26 having no temperature characteristics, and the modulation current Imod is controlled by the ratio of the temperature characteristics of the current sources 26 and 27. Therefore, it is advantageous to adopt a configuration in which an arbitrary temperature characteristic is set because the temperature characteristic of the light emitting element as shown in FIG. 6 can be compensated.

[Second Embodiment]
FIG. 2 is a circuit diagram showing a configuration of a light emitting element driving circuit according to the second embodiment of the present invention. In FIG. 2, the same parts as those in FIG.

  In the light emitting element driving circuit according to the first embodiment, three current sources 26, 27, and 28 connected in parallel to each other are prepared as modulation current sources, and the modulation current is determined according to the ratio of the temperature characteristics of the current sources 26 and 27. An arbitrary temperature characteristic is set for Imod, and an appropriate current proportional to the bias current Ibias is added to the modulation current Imod by the current source 28.

  In contrast, in the light emitting element driving circuit according to the present embodiment, a single current source 29 is used as a modulation current source, and a modulation current setting circuit 51 for setting a modulation current Imod flowing through the current source 29 is provided. The modulation current setting circuit 51 is configured to set the magnitude of the modulation current Imod based on the modulation current setting voltage and add an appropriate current proportional to the bias current Ibias to the modulation current Imod. The rest of the configuration is the same as that of the light emitting element driving circuit according to the first embodiment, and a description thereof will be omitted because it is redundant.

  The modulation current setting circuit 51 includes an adder, a multiplier, a weighting circuit, and the like that are analog circuits. Further, in the light emitting element driving circuit according to the present embodiment, the modulation current setting circuit 51 uses the atmospheric temperature detected by the temperature sensor 52 as a parameter and sets the modulation current Imod in consideration of the atmospheric temperature. However, this is not a mandatory requirement. However, it is advantageous to set the modulation current Imod in consideration of the ambient temperature because the temperature characteristics of the light emitting element as shown in FIG. 6 can be compensated.

  As described above, the single current source 29 is used as the modulation current source, and the magnitude of the modulation current Imod is set under the control of the modulation current setting circuit 51, and the light output intensity of the light emitting element 10 is deteriorated. When the bias current Ibias is controlled to increase by the APC circuit 40, the magnitude of the modulation current Imod that modulates the light emitting element 10 is controlled in accordance with the bias current Ibias, thereby enabling the first embodiment. In addition to obtaining the same effects as those of the light emitting element driving circuit according to the present invention, there is an advantage that the circuit configuration can be simplified correspondingly because only one modulation current source is required.

[Third Embodiment]
FIG. 3 is a circuit diagram showing a configuration of a light emitting element driving circuit according to the third embodiment of the present invention. In FIG. 3, the same parts as those in FIG.

  In the light emitting element driving circuit according to the second embodiment, the modulation current source is a single current source 29, the magnitude of the modulation current Imod is set by the modulation current setting circuit 51, and a bias is applied when the light emitting element 10 deteriorates. Although the magnitude of the modulation current Imod is compensated according to the current Ibias, in the light emitting element driving circuit according to the present embodiment, a memory is used instead of the modulation current setting circuit 51 configured by an adder, a multiplier, a weighting circuit, and the like. A configuration is employed in which an optimum value of the modulation current Imod is set using the unit 53 with reference to a lookup table (LUT) stored in the memory unit 53 based on the modulation current setting voltage and the bias current Ibias.

  It is also possible to set the optimum value of the modulation current Imod by providing the memory unit 53 with a calculation function and performing an appropriate calculation as necessary. In the light emitting element driving circuit according to the present embodiment, as in the case of the light emitting element driving circuit according to the second embodiment, the ambient temperature detected by the temperature sensor 52 is also used as a parameter, and the ambient temperature is also taken into account. The optimum value of the modulation current Imod is set, but this is not an essential requirement. However, it is advantageous to set the modulation current Imod in consideration of the ambient temperature because the temperature characteristics of the light emitting element as shown in FIG. 6 can be compensated.

  Thus, the optimum value of the modulation current Imod is set with reference to the lookup table stored in the memory unit 53, the light output intensity of the light emitting element 10 is deteriorated, and the bias current Ibias is increased by the APC circuit 40. By controlling the magnitude of the modulation current Imod that modulates the light emitting element 10 according to the bias current Ibias, the same as in the case of the light emitting element driving circuit according to the second embodiment. In addition to obtaining the effects, only one memory unit 53 is required instead of the modulation current setting circuit 51 constituted by an analog circuit adder, multiplier, weighting circuit, and the like. Compared to such a light emitting element driving circuit, there is an advantage that the circuit configuration can be simplified.

[Application example]
The light emitting element driving circuits according to the first, second, and third embodiments described above can be used as, for example, a light emitting element driving circuit of an optical transmission device in an optical communication device.

  FIG. 4 is a block diagram showing an outline of the configuration of an optical communication apparatus to which the present invention is applied. As shown in FIG. 4, the present optical communication device includes an optical transmission device 60, an optical transmission medium 70, and an optical reception device 80.

  The optical transmission device 60 includes a light emitting element 61 such as a laser diode and a light emitting element driving circuit 62 that drives (modulates) the light emitting element 61 according to a clock or data signal. The optical transmission medium 70 is made of, for example, an optical fiber, and transmits an optical signal output from the light emitting element 61 to the optical receiver 80 side. The optical receiver 80 includes a light receiving element 81 such as a photodiode that receives an optical signal transmitted by the optical transmission medium 70, and a demodulation circuit 82 that demodulates the light reception output of the light receiving element 81.

  The light emitting element driving circuit according to the first, second, or third embodiment described above is used as the light emitting element driving circuit 62 of the optical transmission device 60 in the optical communication apparatus having the above configuration. In optical communication, a decrease in optical output amplitude causes a large communication error. By using the light emitting element driving circuit according to the first, second, or third embodiment as the light emitting element driving circuit 62 of the optical transmission device 60, These light-emitting element driving circuits have the advantage that the light output amplitude can be kept constant even after deterioration of the light-emitting element, and the quality of the optical signal can be assured, even if the light output intensity of the light-emitting element 61 deteriorates with time. Reliability can be improved because communication errors can be reduced.

  Here, the case where the present invention is applied to an optical transmission apparatus in an optical communication apparatus has been described as an example. However, the present invention is not limited to this application example, and a light emitting element driving circuit including a modulation circuit and an APC circuit is provided. Applicable to all devices to be used.

1 is a circuit diagram showing a configuration of a light emitting element driving circuit according to a first embodiment of the present invention. It is a circuit diagram which shows the structure of the light emitting element drive circuit which concerns on 2nd Embodiment of this invention. It is a circuit diagram which shows the structure of the light emitting element drive circuit which concerns on 3rd Embodiment of this invention. It is a block diagram which shows the outline of a structure of the optical communication apparatus with which this invention is applied. It is a circuit diagram which shows the structure of the light emitting element drive circuit which concerns on a prior art example. It is a figure which shows the temperature characteristic of a light emitting element. It is a figure which shows the time-dependent change of a light emitting element.

Explanation of symbols

  DESCRIPTION OF SYMBOLS 10,61 ... Light emitting element, 20 ... Modulation circuit, 21 ... Buffer, 22, 23 ... Differential pair transistor, 26, 27, 28, 29 ... Modulation current source, 30, 81 ... Light receiving element, 40 ... APC circuit, 41 DESCRIPTION OF SYMBOLS ... Bias current source, 51 ... Modulation current setting circuit, 52 ... Temperature sensor, 53 ... Memory part, 60 ... Optical transmitter, 62 ... Light-emitting element drive or color, 70 ... Optical transmission medium, 80 ... Optical receiver

Claims (9)

Modulation means for causing a modulation current to flow through the light emitting element in response to an input signal;
Light output control means for controlling the bias current of the light emitting element so that the light output intensity of the light emitting element is constant;
Modulation current control means for controlling the magnitude of the modulation current according to the bias current when the bias output is controlled by the light output control means when the light output intensity of the light emitting element is deteriorated; A light emitting element driving circuit comprising: The modulation current control means is a second current source that is connected in parallel to the first current source that supplies the modulation current, and that forms a current mirror circuit together with the bias current source that supplies the bias current. The light-emitting element driving circuit according to claim 1. The light emitting element drive circuit according to claim 2, further comprising a third current source having a temperature characteristic connected in parallel to the first current source. The light emitting element drive circuit according to claim 2, wherein the modulation current control unit sets the magnitude of the modulation current by performing a calculation based on a predetermined set voltage and the bias current. The light emitting element drive circuit according to claim 4, wherein the modulation current control means sets the magnitude of the modulation current using an ambient temperature as a parameter. The light emitting element drive circuit according to claim 2, wherein the modulation current control means sets the magnitude of the modulation current with reference to a lookup table based on a predetermined set voltage and the bias current. The light emitting element drive circuit according to claim 6, wherein the modulation current control unit sets the magnitude of the modulation current using an ambient temperature as a parameter. A modulation current is supplied to the light emitting element in accordance with the input signal, and the bias current of the light emitting element is controlled so that the light output intensity of the light emitting element is constant,
When the bias current is controlled to increase when the light output intensity of the light emitting element is deteriorated, the magnitude of the modulation current is controlled according to the bias current. A light emitting element;
A modulation circuit for supplying a modulation current to the light emitting element in response to an input signal;
A light output control circuit for controlling the bias current of the light emitting element so that the light output intensity of the light emitting element is constant;
A modulation current control circuit for controlling the magnitude of the modulation current according to the bias current when the bias output is controlled by the light output control circuit when the light output intensity of the light emitting element is deteriorated; An optical transmission device comprising:

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