JP4665741B2 - Optical transmitter - Google Patents

Description

  The present invention relates to an optical transmitter that transmits an optical signal in optical communication.

  In an optical transmitter used in optical communication, a monitor built in the same casing as an LD is usually used to make the optical output of an optical signal output from a built-in laser diode (hereinafter referred to as LD) constant. The LD drive current is controlled so that the current output from the photo diode (hereinafter referred to as PD) is constant. Recently, in order to increase the degree of freedom in controlling the LD drive current, the PD current is converted into a digital signal by an ADC (A / D converter) and output to the CPU, and the LD drive is performed using software stored on the CPU. A method for controlling current is used. The configuration of a general optical transmitter in this system is shown in FIG. In the optical transmitter 910 shown in the figure, a detection resistor 912 is connected in series to the PD 911, and a PD current flowing through the detection resistor 912 is IV converted and output to the CPU 914 as a monitor value via the ADC 913.

  The problem with such a conventional optical transmitter is that the individual variation of the PD current in a steady state is very large. When LDs with the same optical output are targeted, there may be a difference of 20 times in the PD current when the LD outputs appropriate light due to individual differences (manufacturing variation) of optical transmitters. Since such a difference in the PD current directly leads to a monitor error, the following measures are generally taken. The detection resistors are exchanged according to variations in individual PD currents (Countermeasure 1). Also, the detection resistor connected to the PD is replaced with a variable resistor as shown in FIG. 6A, or a plurality of detection resistors can be switched in parallel as shown in FIG. 6B. This absorbs the variation in the PD current (measure 2). Further, as shown in FIG. 7A, the detection resistor is fixedly connected, and a variable gain amplifier (PGA) is inserted in the subsequent stage, or as shown in FIG. 7B. In addition, a plurality of amplifiers having different amplification factors are connected in parallel so as to be switchable, thereby absorbing variations in PD current (measure 3).

On the other hand, Patent Document 1 below discloses a photodetection circuit using a logarithmic amplifier (log amplifier). In the photodetection circuit 920, the PD current is directly input to the log amplifier 922, and the change in the input voltage of the ADC 913 is reduced by logarithmically converting the PD current in a wide range (see FIG. 8). In this case, even if the PD current variation reaches 20 times, the ADC input voltage variation becomes log (20) = approximately 1.3 times smaller.
JP 2004-200355 A

  In the conventional method for dealing with the individual variations in the PD current described above, there is a tendency as follows. In the method according to the above countermeasure 1, it is necessary to replace the resistor with an appropriate characteristic for each LD, so that the manufacturing efficiency is lowered, and replacement of a component mounted by solder is preferable in terms of reliability. Absent. In the methods according to the above countermeasures 2 and 3, the circuit scale becomes relatively large. In particular, when it is desired to realize a monitoring circuit with only an analog circuit without using an ADC or a CPU, an increase in circuit scale becomes significant. Further, when a variable resistor such as a digital potentiometer, a variable gain amplifier, or the like is used, the cost increases.

  In addition, in the above-described photodetection circuit using the log amplifier, the monitor error may increase due to the large variation in the gain of the log amplifier and the fluctuation of the gain due to the temperature change. As a result, the light output intensity is stabilized. Measurement is difficult. By incorporating a compensation circuit into the IC included in the photodetection circuit, it is possible to suppress variations in the amplification factor of the log amplifier and fluctuations due to temperature changes, but in this case, the cost increases due to the complexity of the circuit configuration. It becomes.

  Therefore, an object of the present invention is to provide an optical transmitter that enables stable monitoring of optical output intensity and realizes cost reduction and scale reduction.

  In order to solve the above problems, an optical transmitter according to the present invention is based on a semiconductor laser diode that generates an optical signal, a light receiving element that converts the optical signal of the semiconductor laser diode into a current, and a current output from the light receiving element. In an optical transmitter including a monitor circuit for monitoring the intensity of an optical signal, the monitor circuit includes a current source circuit, a first input terminal to which one of the light receiving element and the current source circuit is connected, and a light receiving circuit. A second input terminal to which the other of the element and the current source circuit is connected, an output terminal for outputting a monitor signal of the intensity of the optical signal, an emitter is connected to the first input terminal, and the base is the second input terminal. The first transistor connected to the input terminal, the base is connected to the emitter of the first transistor, the emitter is connected to the constant voltage source, and the collector is connected to the base of the first transistor. The second transistor, the emitter is connected to the emitter of the second transistor, the base and the collector are connected to each other, the emitter is connected to the collector of the third transistor, and the base is the second transistor. A fourth transistor having a collector connected to the output terminal and a resistive load element connected to the collector of the fourth transistor.

  In such an optical transmitter, a light receiving element that receives an optical signal from a semiconductor laser diode and a monitor circuit for measuring a current output from the light receiving element are provided. Is converted into a monitor signal corresponding to the intensity of the optical signal and output. Here, even when a variation in the current of the light receiving element or a temperature change occurs due to the monitor circuit having a simple configuration including the first to fourth transistors, the current source circuit, and the resistive load element, it is caused by them. This is output as a monitor signal with suppressed variation. As a result, it is possible to stably monitor the optical signal intensity with a small circuit scale that can be easily integrated, and to reduce the cost of the apparatus.

  Alternatively, the optical transmitter of the present invention includes a semiconductor laser diode that generates an optical signal, a light receiving element that converts the optical signal of the semiconductor laser diode into a current, and an intensity of the optical signal based on a current output from the light receiving element. In an optical transmitter including a monitor circuit for monitoring, the monitor circuit includes a current source circuit, a first input terminal to which one of the light receiving element and the current source circuit is connected, and the light receiving element and the current source circuit. A second input terminal connected to the other, an output terminal for outputting a monitor signal of the intensity of the optical signal, a source connected to the first input terminal, and a gate connected to the second input terminal. A first FET, a gate connected to the source of the first FET, a source connected to the constant voltage source, a drain connected to the gate of the first FET, and a source connected to the second FET. FET A third FET connected to the source and having the gate and drain connected to each other, a source connected to the drain of the third FET, a gate connected to the drain of the second FET, and a drain connected to the output terminal And a resistive load element connected to the drain of the fourth FET.

  In such an optical transmitter, a light receiving element that receives an optical signal from a semiconductor laser diode and a monitor circuit for measuring a current output from the light receiving element are provided. Is converted into a monitor signal corresponding to the intensity of the optical signal and output. Here, even when a variation in the current of the light receiving element or a temperature change occurs due to the monitor circuit having a simple configuration including the first to fourth FETs, the constant voltage source, the current source circuit, and the resistive load element. , And output as a monitor signal in which variations due to them are suppressed. As a result, it is possible to stably monitor the optical signal intensity with a small circuit scale that can be easily integrated, and to reduce the cost of the apparatus.

  The current source circuit is connected to the second resistive load element and the second resistive load element formed by the same integrated circuit manufacturing process as the resistive load element on the same integrated circuit as the resistive load element. And a voltage source circuit that outputs a voltage having a temperature change inversely proportional to the temperature change of the resistance value of the second resistive load element. By providing such a current source circuit, integration of the entire monitor circuit is further facilitated, and at the same time, the fluctuation of the monitor signal due to the temperature change is canceled, and the fluctuation of the monitor value of the light output intensity due to the temperature change is effectively obtained. Can be reduced.

  According to the optical transmitter of the present invention, it is possible to stably monitor the optical output intensity, and it is possible to realize cost reduction and downsizing.

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

FIG. 1 is a diagram showing a configuration of an optical transmitter 10 according to a preferred embodiment of the present invention. An optical transmitter 10 shown in FIG. 1 is a device that is used for optical communication and transmits an optical signal to an optical transmission cable or the like. A semiconductor laser diode (hereinafter referred to as LD) 11 that generates an optical signal O _out, and an LD 11 receiving an output optical signal _{O out} emitted from the photodiode which converts the current _{I Pd} corresponding to the intensity (hereinafter, PD hereinafter) 12, an output optical signal O on the basis of the current _{I Pd} output from PD12 a monitor circuit 13 for monitoring the intensity of the _out, is constituted by a drive current source circuit 14 for generating a drive current supplied to the LD11 in accordance with the control signal S _C that is output from the monitor circuit 13.

Monitor circuit 13 includes a conversion circuit 15 connected to the anode terminal of the PD 12, the conversion circuit 15 A / D converter for converting the monitor signal _{V out} is the output analog signal to a digital signal _{S d} (hereinafter, referred to as ADC ) 16, and the CPU17 current I _Pd output from PD12 on the basis of the digital signal S _d to control the drive current of the drive current source circuit 14 to be constant is formed by connecting in series. From this CPU 17, the control signal S _C that controls the drive current of the drive current source circuit 14 is output, is input to the drive current source circuit 14. Hereinafter, the configuration of the conversion circuit 15 will be described in detail.

  FIG. 2 is a circuit diagram showing the main part of the optical transmitter 10 including the conversion circuit 15. As shown in the figure, the conversion circuit 15 includes transistors 18a, 18b, 18c, and 18d that are four PNP bipolar transistors having the same characteristics, a current source circuit 19, and a resistance element (resistive load element) 20. And has two input terminals 21 and 22 and one output terminal 23.

More specifically, in the conversion circuit 15, the anode of the PD 12 is connected to the input terminal 21, and a current I _Pd corresponding to the light reception intensity at the PD 12 is input to the input terminal 21. On the other hand, the current source circuit 19 is connected to the input terminal 22, the current I _c flowing from the input terminal 22 by the current source circuit 19 to ground is produced. Further, the input of the ADC 16 is connected to the output terminal 23.

  The electrical connection relationship of each element in the conversion circuit 15 will be described. The input terminal 21 is connected to the emitter of the transistor 18a, the base is connected to the input terminal 22, and the collector is connected to the ground. The base of the transistor 18b is connected to the emitter of the transistor 18a and the input terminal 21, the emitter of the transistor 18b is connected to the constant voltage source 24, and the collector of the transistor 18b is connected to the base and input terminal 22 of the transistor 18a. It is connected. Further, the emitter of the transistor 18c is connected to the emitter of the transistor 18b and the constant voltage source 24, and the base and collector of the transistor 18c are connected to each other. The emitter of the transistor 18d is connected to the base and collector of the transistor 18c. The base of the transistor 18d is connected to the collector of the transistor 18b, the base of the transistor 18a and the input terminal 22, and the collector of the transistor 18d is connected to the output terminal 23. Each is connected. One terminal of the resistance element 20 is connected to the output terminal 23, and the other terminal of the resistance element 20 is connected to the ground.

  Hereinafter, the operation of the conversion circuit 15 configured as described above will be described.

  Assume that the base-emitter voltages of the transistors 18a, 18b, 18c, and 18d are Vbe1, Vbe2, Vbe3, and Vbe4, respectively, and the collector currents are Ic1, Ic2, Ic3, and Ic4. The transistors 18c and 18d have the same emitter current, so Vbe3 and Vbe4 are equal.

Here, when the DC current amplification factor hfe is infinite, the current I _Pd output from the PD 12, the current I _c generated by the current source circuit 19, and the current I _mon flowing through the resistance element 20 are expressed by the following formula (1 ) To (3).
I _pd = Ic1≈I _S · exp (−Vbe1 / Vt) (1)
_{I c = Ic2 ≒ I S ·} exp (-Vbe2 / Vt) ... (2)
I _mon = Ic3 = Ic4≈I _S · exp (−Vbe4 / Vt) (3)
Above formula (1) ~ (3), I S is a constant, Vt is, Vt = kT / e (k : Boltzmann constant, T: absolute temperature, e: elementary charge) represents the number that is calculated by .

Since the relationship of Vbe4 + Vbe3 = Vbe1 + Vbe2 is established, the following formulas (4) to (6) are derived using the formulas (1) to (3).
I _pd ≈I _S · exp {− (2 Vbe 4 −Vbe 2) / Vt} (4)
I _pd · I _S · exp (−Vbe ² / Vt) ≈I _S ² • exp (−2 Vbe 4 / Vt) (5)
I _pd · I _c = I _mon ² (6)

That is, a current I _mon that satisfies the relationship of the above formula (6) flows through the resistance element 20. Here, when the current source circuit 19 is a constant current source, I _mon = (I _pd · I _c ) ^0.5 = a · I _pd ^0.5 (a is a constant), and I _pd regardless of the temperature. A current proportional to the square root of the current flows through the resistance element 20, and the range of the monitor signal _Vout that is a voltage signal output from the output terminal 23 with respect to the input I _pd , in other words, the maximum value of the monitor signal _Vout . And the minimum ratio can be compressed.

For _{example, I pd} is 50Myuei～1,000myuei, is 20 times the ratio of maximum and minimum values of the current that is output from the PD 12, when the current _{I c} generated by the current source circuit 19 is 1,000μA The change in the current I _mon is 223.6 μA to 1,000 μA, and the ratio between the maximum value and the minimum value is suppressed to 5 times or less.

Further, according to such a conversion circuit 15, a measurement error in the output light monitor is also reduced. Now, the input range of the subsequent ADC16 and 0 to 1 V, the error between 10mV measurement, when the resistance _R mon = 1 k [Omega resistive element 20, the conventional linear circuit of Figure 5 is the ADC16 when the current _I Pd = _1000μA The current having the maximum error is I _mon = (1000 mV−10 mV) / 1 kΩ = 990 μA, the measurement error is 1%, and the current having the maximum error measured by the ADC 16 when the current I _Pd = 50 μA is I _mon = (50 mV-10 mV) / 1 kΩ = 40 μA, the measurement error reaches 20%. On the other hand, in the conversion circuit 15, when the current I _c in the current source circuit 19 is set to a value satisfying (1000 μA · I _c ) ^0.5 · 1 kΩ = 1000 mV, the maximum measured by the ADC 16 when the current I _Pd = 1000 μA. The current with error is I _mon = {(1000 mV−10 mV) / 1 kΩ} ² / I _c = 980 μA, the measurement error is 2%, and the current with the maximum error measured by the ADC 16 when the current I _Pd = 50 μA is , I _mon = {(223.6 mV-10 mV) / 1 kΩ} ² / I _c = 45.6 μA, the measurement error is 9%. Thus, in the conversion circuit 15, the measurement error when the input is small can be reduced to about half compared to the conventional case.

In the above-described conversion circuit, the influence of the transistor characteristic variation and the direct current amplification factor hfe are ignored. However, even if these influences are taken into consideration, I _mon is a monotonically increasing function close to I _pd ^0.5. Determined by. Therefore, even in such a case, the output monotonously increases with good reproducibility with respect to the input, and the deviation due to the temperature change and the element characteristic variation is reduced to the deviation calculated by the square root. There is no loss of usefulness as a control circuit.

  Next, a preferred configuration example of the current source circuit 19 in the conversion circuit 15 will be described.

The configuration of the current source circuit 19 shown in FIG. 3 is for reducing the influence of the temperature change of the resistance element on the monitor signal _Vout input to the ADC. In general, in an IC, it is easy to generate a constant voltage with little influence of temperature change by a band gap circuit or the like, but current is affected by temperature change of a resistance element. In the conventional circuit of FIG. 5, the temperature change ΔV _out of the monitor signal V _{out with} respect to the temperature change ΔR _{mon of the} resistance is expressed by the following equation (7):
ΔV _out / V _out = ΔR _mon / R _mon (7)
The change in resistance is received as is.

On the other hand, in the conversion circuit 15 including the current source circuit 19 shown in FIG. 3, the influence of the temperature change of the resistance can be reduced with a simple configuration. That is, the current source circuit 19 includes a voltage source circuit 25 that outputs the reference voltage V _ref , a resistance element (resistive load element) 26 having one terminal connected to the ground, an output of the voltage source circuit 25, and a resistance element And a current generation unit 27 that generates a current I _c that flows through the resistance element 26 so that a voltage drop in the resistance element 26 is equal to the reference voltage V _ref of the voltage source circuit 25. . Here, the current source circuit 19 is formed on the same integrated circuit as the integrated circuit on which the conversion circuit 15 is mounted, and the resistance element 26 is formed by the same formation process as that of the resistance element 20, and changes in temperature. Characteristics such as characteristics are almost the same.

Now, assuming that the reference voltage V _ref is constant regardless of the temperature change and the resistance value of the resistance element 26 is r, the monitor signal V _out is expressed by the following formula (8);
V _out = R _mon · {(V _ref · I _pd ) / r} ^0.5 (8)
Given in. Further, since the resistance value r and _{R mon} is proportional to the temperature change, the temperature change [Delta] V _out of the monitor signal _{V out,} the following formula (9);
ΔV _out / V _out ≈ΔR _mon / (2R _mon ) (9)
Thus, the effect of temperature change is ½.

The voltage source circuit 25 preferably generates the reference voltage V _ref so as to have a temperature change that is inversely proportional to the temperature change of the resistance element 20 or the resistance element 26. Specifically, when R _mon is proportional to the temperature change, the voltage source circuit 25 generates the reference voltage V _ref so as to be inversely proportional to the temperature change. In this way, the influence of the temperature change of the resistance element on the monitor signal _Vout is offset. Generally, a voltage source circuit can obtain an arbitrary temperature coefficient up to several percent if the generated voltage is removed from the bandgap voltage. Therefore, such a voltage source circuit can be obtained by measuring the temperature coefficient of a resistance element to be used in advance. The circuit 25 can be realized.

  Hereinafter, the function and effect of the optical transmitter 10 described above will be described.

The optical transmitter 10 includes a PD 12 that receives the optical signal O _out from the LD 11 and a monitor circuit 13 for measuring the current I _pd output from the PD 12. Current I _pd is converted into a monitor signal V _out corresponding to the intensity of the optical signal and output. Here, even if variations or temperature changes in the PD current occur due to the monitor circuit 13 having a simple configuration including the transistors 18a, 18b, 18c, 18d, the current source circuit 19, and the resistance element 20, This is output as a monitor signal in which variations due to the suppression are suppressed. As a result, it is possible to monitor an optical signal intensity stably with a large input dynamic range and a small circuit scale that can be easily integrated, and to reduce the cost of the apparatus.

  Here, when all the circuits of the optical transmitter operate with a positive power supply, if you look into a special configuration, normal ADCs and operational amplifiers will malfunction if the input voltage is 100 to 200 mV or less. It is usual to prepare a current source circuit and a voltage source circuit for increasing the minimum input voltage. However, this increases the circuit scale and also increases the monitor error due to the input voltage shift circuit. On the other hand, in the conversion circuit 15 in the optical transmitter 10, since the gain increases when the input voltage is small, an input voltage shift circuit is not required, so that the circuit scale is not unnecessarily increased and the input voltage is not increased. There is no increase in monitoring error due to the shift circuit.

  Further, in the optical transmitter 10, since it is not necessary to replace circuit elements such as resistors for dealing with variations in the PD current, the optical transmitter 10 is more reliable than the conventional method, and is also more cost-effective and adjustable than the conventional method. It is advantageous. Furthermore, the optical transmitter 10 does not use a special element and has a small circuit scale, so that integration is easy. On the other hand, conventional methods using digital potentiometers and variable gain amplifiers require cumbersome control for range adjustment and increase costs.

  In addition, this invention is not limited to embodiment mentioned above. For example, in the optical transmitter 10, the current source circuit 19 and the PD 12 may be interchanged. In this case, when the potential of the case of the subassembly in which the LD 11 and the PD 12 are accommodated is GND, the lead pins in the sub assembly can be reduced by dropping the cathode of the PD 12 into the case.

  In the conversion circuit 15, a P-type voltage effect transistor (hereinafter referred to as FET) may be used as the transistor. FIG. 4 is a circuit diagram of a conversion circuit 115 which is a modification of the present invention in such a case. The conversion circuit 115 includes four transistors 118a, 118b, 118c, and 118d that are P-type FETs. More specifically, the input terminal 21 is connected to the source of the transistor 118a, the gate thereof is connected to the input terminal 22, and the drain thereof is connected to the ground. The gate of the transistor 118b is connected to the source and input terminal 21 of the transistor 118a, the source of the transistor 118b is connected to the constant voltage source 24, and the drain of the transistor 118b is connected to the gate and input terminal 22 of the transistor 118a. It is connected. Further, the source of the transistor 118c is connected to the source of the transistor 118b and the constant voltage source 24, and the gate and the drain of the transistor 118c are connected to each other. The source of the transistor 118d is connected to the gate and drain of the transistor 118c, the gate of the transistor 118d is connected to the drain of the transistor 118b, the gate of the transistor 118a, and the input terminal 22, and the drain of the transistor 118d is connected to the output terminal 23. Each is connected.

  Further, as the four transistors in the conversion circuit 15, an NPN bipolar transistor or an N-type FET may be used. However, the configuration of the conversion circuit in this case is a configuration in which the circuit polarity in FIG. Furthermore, one or more current mirror circuits may be provided between the PD 12 and the transistors 18a and 18b due to restrictions from other circuits.

It is a figure which shows the structure of the optical transmitter which is preferable one Embodiment of this invention. FIG. 2 is a circuit diagram illustrating a main part of an optical transmitter including the conversion circuit of FIG. 1. FIG. 3 is a circuit diagram of a current source circuit 19 in FIG. 2. It is a circuit diagram which shows the principal part of the optical transmitter containing the conversion circuit which is a modification of this invention. It is a figure which shows the structure of the conventional optical transmitter. It is a figure which shows the structure of the conventional optical transmitter. It is a figure which shows the structure of the conventional optical transmitter. It is a figure which shows the structure of the conventional optical transmitter.

Explanation of symbols

DESCRIPTION OF SYMBOLS 10 ... Optical transmitter, 11 ... LD (semiconductor laser diode), 12 ... PD (light receiving element), 13 ... Monitor circuit, 15, 115 ... Conversion circuit, 18a, 18b, 18c, 18d, 118a, 118b, 118c, 118d ... Transistor, 19 ... Current source circuit, 20 ... Resistive element (resistive load element), 21, 22 ... Input terminal, 23 ... Output terminal, 24 ... Constant voltage source, 25 ... Voltage source circuit, 26 ... Resistor element (first) 2 resistive load elements).

Claims (3)

A semiconductor laser diode that generates an optical signal; a light receiving element that converts the optical signal of the semiconductor laser diode into a current; and a monitor circuit that monitors the intensity of the optical signal based on the current output from the light receiving element. An optical transmitter comprising:
The monitor circuit is
A current source circuit;
A first input terminal to which one of the light receiving element and the current source circuit is connected;
A second input terminal to which the other of the light receiving element and the current source circuit is connected;
An output terminal for outputting a monitor signal of the intensity of the optical signal;
A first transistor having an emitter connected to the first input terminal and a base connected to the second input terminal;
A second transistor having a base connected to the emitter of the first transistor, an emitter connected to a constant voltage source, and a collector connected to the base of the first transistor;
A third transistor having an emitter connected to the emitter of the second transistor and a base and a collector connected to each other;
A fourth transistor having an emitter connected to the collector of the third transistor, a base connected to the collector of the second transistor, and a collector connected to the output terminal;
A resistive load element connected to the collector of the fourth transistor;
An optical transmitter comprising: A semiconductor laser diode that generates an optical signal; a light receiving element that converts the optical signal of the semiconductor laser diode into a current; and a monitor circuit that monitors the intensity of the optical signal based on the current output from the light receiving element. An optical transmitter comprising:
The monitor circuit is
A current source circuit;
A first input terminal to which one of the light receiving element and the current source circuit is connected;
A second input terminal to which the other of the light receiving element and the current source circuit is connected;
An output terminal for outputting a monitor signal of the intensity of the optical signal;
A first FET having a source connected to the first input terminal and a gate connected to the second input terminal;
A second FET having a gate connected to the source of the first FET, a source connected to a constant voltage source, and a drain connected to the gate of the first FET;
A third FET having a source connected to the source of the second FET and a gate and a drain connected to each other;
A fourth FET having a source connected to the drain of the third FET, a gate connected to the drain of the second FET, and a drain connected to the output terminal;
A resistive load element connected to the drain of the fourth FET;
An optical transmitter comprising: The current source circuit is:
A second resistive load element formed by the same integrated circuit manufacturing process as the resistive load element on the same integrated circuit as the resistive load element;
A voltage source circuit connected to the second resistive load element and outputting a voltage having a temperature change inversely proportional to the temperature change of the resistance value of the second resistive load element;
The optical transmitter according to claim 1, further comprising:

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