Optical transmission circuit

Description: TECHNICAL FIELD [Detailed description of the invention]

【0001】

[Technical field to which the invention belongs]

According to the present invention, in an optical transmission circuit used in a communication device using an optical fiber, an optical output waveform with small distortion and jitter can be obtained at high speed drive with low voltage and low power consumption, and high distance transmission is possible. It relates to a performance optical transmission circuit.

【0002】

[Conventional technology]

Figure 3 shows the configuration of an optical transmission circuit that is often used in the past. The optical transmission circuit is composed of a semiconductor laser diode (LD) 29, a current switch unit consisting of transistors 27 and 28, a constant current source 30 for modulation current control, and a constant current source 31 for bias current control. ..

【0003】

The current of the laser diode 29 is turned on and off by the current switch section to generate an optical signal, and its intensity is adjusted by the constant current source 30. Further, in order to turn on and off the laser diode 29 at high speed, a bias current of about the threshold current of the laser diode 29 is required, and this is generated by the constant current source 31. With such a configuration, an optical signal can be generated and transmitted according to the signal input.

【0004】

[Problems to be Solved by the Invention]

However, when the circuit of FIG. 3 is operated with a power supply voltage of 2 to 3 V, the operating voltage Vf of the laser diode 29 is generally 1 to 2 V, so that the transistor 28 is saturated and high-speed operation becomes difficult. In such a case, there is a method in which the coil 34 as shown in FIG. 4 is used as a load and the modulation current of the laser diode 29 is supplied by AC coupling using the capacitor 35.

【0005】

When the laser diode 29 is modulated at high speed, the lead inductance of the laser diode 29, the lead inductance of the transistors 27 and 28, the parasitic capacitance of the coil 34, the inductance of the wiring for connecting these, the parasitic capacitance, and the impedance mismatch Due to the reflection caused by the laser diode 29, distortion, jitter, and ringing occur in the modulated current waveform and the optical output waveform of the laser diode 29, which makes signal transmission difficult. In this case, waveform shaping is performed by adding a resistor 36 for damping or impedance matching.

【0006】

However, even with this, ringing may not be suppressed, or the waveform may be too dull and transmission may be difficult. Further, the residual ringing increases the wavelength chirp, which makes long-distance transmission difficult due to the influence of fiber dispersion when the optical fiber is transmitted.

【0007】

An object of the present invention is to eliminate the above-mentioned drawbacks of the prior art, to drive a laser diode at a low voltage and at a high speed, reduce optical output waveform jitter and ringing, and transmit a low distortion and low chirp optical signal. The purpose of the present invention is to provide an optical transmission circuit that enables long-distance transmission.

【0008】

[Means for solving problems]

In order to achieve the above object, in the optical transmission circuit according to the invention of claim 1, a coil is connected as a load of a collector or a drain of a transistor, and a resistor and a filter including a series circuit of a resistor and a capacitor are connected in parallel to the coil. Is connected, and as an AC coupling load, a laser diode is connected via a resistor, and a capacitor is connected in parallel with the resistor connected to the laser diode.

【0009】

Further, in the optical transmission circuit according to the invention of claim 3, a resistor is connected as a load of a collector or a drain of a transistor, a filter composed of a series circuit of a resistor and a capacitor is connected in parallel therewith, and a laser diode is a resistor. It is characterized in that a capacitor is connected in parallel with the resistor connected to the laser diode.

【0010】

In the optical transmission circuit of the present invention, it is preferable that the transistor is one of a pair of differential transistors and a capacitor is arranged between the collector or the drain of the pair of differential transistors (claims 2 and 4). ..

【0011】

<Gist of the Invention> The gist of the present invention is to add a resistor in parallel to the coil of the optical transmission circuit described above, a filter composed of a resistor and a capacitor, and a capacitor in parallel with the resistor for damping or impedance matching described above. The point is to achieve the above-mentioned object by adding.

【0012】

The resistance placed in parallel with the coil prevents resonance due to the parasitic capacitance of the coil, and the impedance that becomes the load of the collector of the transistor is specified by the inductance of the lead of the coil and LD (laser diode) and the inductance of the circuit wiring. It suppresses the rise in frequency and flattens it, and reduces the transmission signal pattern jitter and the ringing that occurs depending on the signal pattern. A low-pass filter consisting of a resistor and a capacitor blunts the rise and fall of the LD drive current waveform to reduce LD ringing. Capacitors placed in parallel with resistors for damping or impedance matching correct impedance deviations at high frequencies due to parasitic elements (capacitors, inductance) due to wiring patterns, LD leads, etc., and cause waveform disturbance due to impedance mismatching. Suppress.

【0013】

BEST MODE FOR CARRYING OUT THE INVENTION

Hereinafter, the present invention will be described based on the illustrated embodiment.

【0014】

FIG. 1 shows an optical transmission circuit according to an embodiment of the present invention. This optical transmission circuit is composed of resistors 1, 6, 8, 10, capacitors 3, 7, 9, 11, laser diodes 12, and constant current sources 13 and 14.

【0015】

In the optical transmission circuit of FIG. 1, a resistor 1 is inserted in the collector circuit of one transistor 4 of the differential transistors pairs 4 and 5, and a coil 2 is connected to this as a load of the collector of the other transistor 5. A resistor 6 is connected in parallel, and a low-pass filter consisting of a series circuit of a resistor 8 and a capacitor 7 is connected in parallel. Further, as an AC coupling load by the capacitor 9, a laser diode 12 is connected via a resistor 10, and a capacitor 11 is connected in parallel with the resistor 10 connected to the laser diode 12. The capacitor 3 is arranged between the collectors of the differential transistor pairs 4 and 5.

【0016】

The emitters of the differential transistor pairs 4 and 5 are connected to the constant current source 13, and the base thereof is connected to the input terminal. Further, a constant current source 14 is connected to the cathode of the laser diode 12.

【0017】

The current flowing through the transistor 5 changes according to the input signal applied to the base of the differential transistors pairs 4 and 5, the DC component mainly flows through the load coil 2, and the AC component passes through the resistor 10 and the capacitor 11. It flows through the laser diode 12, and this becomes the modulation current of the laser diode 12. The bias current required for light emission of the laser diode 12 is supplied from the constant current source 14.

【0018】

The resistor 6 arranged in parallel with the coil 2 prevents resonance due to the parasitic capacitance of the coil 2, and also has an impedance that becomes a load on the collector of the transistor 5 due to the inductance of the leads of the coil 2 and the laser diode 12 and the inductance of the circuit wiring. Is suppressed from rising at a specific frequency and flattened, and transmission signal pattern jitter and ringing that occurs depending on the signal pattern are reduced. The filter including the parallel circuit of the resistor 8 and the capacitor 7 and the capacitor 3 between the collectors blunt the rise and fall of the drive current waveform of the laser diode 12 to reduce the ringing of the laser diode 12. The capacitor 11 arranged in parallel with the resistor 10 for damping or impedance matching corrects the impedance deviation at high frequencies due to parasitic elements (capacitor, inductance) due to the wiring pattern, leads of the laser diode 12, etc., and is due to impedance mismatching. Suppresses disturbance of signal waveform.

【0019】

By adding resistors 6, 8 and capacitors 7, 3 and 11 as described above, ringing and jitter of the drive current waveform of the laser diode 12 can be reduced, thereby transmitting low distortion and low chirp optical signals. be able to. Therefore, long-distance transmission can be performed without lowering the extinction ratio of the optical signal.

【0020】

<Other Embodiments> Fig. 2 shows other embodiments. When the operating voltage of the laser diode LD is small and the power supply voltage has a margin, the modulation current of the laser diode LD can be directly driven at high speed by a transistor. In that case, as shown in FIG. 2, the optical transmission circuit is composed of resistors 15, 17, 19, 22, capacitors 16, 18, 23, a laser diode 24, and constant current sources 25, 26.

【0021】

In the optical transmission circuit of FIG. 2, a resistor 15 is inserted in the collector circuit of one transistor 4 of the differential transistors pairs 20 and 21, and a resistor 17 is connected as a load for the collector of the other transistor 21 in parallel therewith. A low-pass filter consisting of a series circuit of a resistor 19 and a capacitor 18 is connected to this. Further, as a load for the collector of the transistor 21, a laser diode 24 is connected via a resistor 22, and a capacitor 23 is connected in parallel with the resistor 22 connected to the laser diode 24. The capacitor 16 is arranged between the collectors of the differential transistors pairs 20 and 21.

【0022】

The emitters of the differential transistor pairs 20 and 21 are connected to the constant current source 25, and the base thereof is connected to the input terminal. Further, a constant current source 26 is connected to the cathode of the laser diode 24.

【0023】

The resistor 17 is flattened by suppressing the impedance that becomes the load of the collector of the transistor 21 from rising at a specific frequency due to the inductance of the lead of the laser diode 24 and the inductance of the circuit wiring, and depends on the transmission signal pattern jitter and the signal pattern. To reduce the ringing that occurs. The filter including the resistor 19 and the capacitor 18 and the capacitor 16 between the collectors blunt the rising and falling edges of the drive current waveform of the laser diode 24 to reduce the ringing of the laser diode 24. The capacitor 23 arranged in parallel with the resistor 22 for damping or impedance matching corrects the impedance deviation at high frequencies due to parasitic elements (capacitor, inductance) due to the wiring pattern, leads of the laser diode 24, etc., and is due to impedance mismatching. Suppresses disturbance of signal waveform.

【0024】

By adding resistors 17, 19 and capacitors 18, 16 and 23 as described above, ringing and jitter of the drive current waveform of the laser diode 24 can be reduced, thereby transmitting low distortion and low chirp optical signals. be able to. Therefore, long-distance transmission can be performed without lowering the extinction ratio of the optical signal.

【0025】

In the above embodiment, the capacitors 3 and 16 and the capacitors 11 and 23 may be substituted by the capacitance parasitic on the wiring or the lead, and in that case, it is not necessary to mount them.

【0026】

Further, in the above embodiment, the case where the differential transistor pair is composed of a transistor having a normal base-emitter collector is taken as an example, but it can also be composed of a field-effect transistor having a gate / drain source.

【0027】

The optical transmission circuit of the present invention can be applied not only as a transmitter for high-speed digital optical communication but also as a transmitter for high-frequency analog optical communication.

【0028】

[Effect of the invention]

As described above, according to the present invention, a resistor is arranged in parallel with the coil of the load of the transistor, or the resistor is provided in a form in which the coil is omitted. Therefore, this resistor prevents resonance due to the parasitic capacitance of the coil. At the same time, due to the lead inductance of the coil and laser diode and the inductance of the circuit wiring, the impedance that becomes the load of the transistor collector is suppressed from rising at a specific frequency and flattened, depending on the transmission signal pattern jitter and signal pattern. The ringing that occurs can be reduced.

【0029】

Further, since the low-pass filter composed of the resistor and the capacitor is provided, the ringing of the laser diode can be reduced by blunting the rising and falling edges of the LD drive current waveform.

【0030】

In addition, since a capacitor is connected in parallel with a resistor for damping or impedance matching, impedance deviation at high frequencies due to parasitic elements (capacitors, inductance) due to wiring patterns and laser diode leads is corrected, and waveforms due to impedance mismatching are corrected. Disturbance can be suppressed.

【0031】

Therefore, by using the optical transmission circuit of the present invention, high-speed operation and long-distance transmission at a lower voltage become possible. Therefore, power consumption can be significantly reduced without sacrificing speed. By suppressing ringing, the electromagnetic waves radiated from the optical transmission circuit are reduced, and countermeasures against electromagnetic wave radiation become easy. In addition, since signal interference with other circuits can be reduced, high-density mounting becomes possible, and the optical transmitter or device can be miniaturized. Since it can be placed close to an optical receiver that amplifies a minute signal, an optical transceiver can be easily configured. Interference measures such as shields can also be reduced, making it possible to reduce costs and increase sensitivity. Since the extinction ratio can be set high, the sensitivity of the optical receiver can be further improved. The fact that the extinction ratio can be set high means that even if the extinction ratio changes due to fluctuations in the temperature power supply voltage and sample variation of the laser diode LD, transmission will not be hindered. This leads to cost reduction.

[Simple explanation of drawings]

[Figure 1]

It is a figure which shows one Embodiment of the optical transmission circuit of this invention.

[Figure 2]

It is a figure which shows the other embodiment of the optical transmission circuit of this invention.

[Fig. 3]

It is a figure which shows the conventional laser diode drive circuit.

[Fig. 4]

It is a figure which showed the improved form of the conventional laser diode drive circuit.

[Explanation of symbols]

2 coil 3, 7, 9, 11 capacitors 4, 5 transistors 12 laser diode 13, 14 constant current source 15, 17, 19, 22 resistors 16, 18, 23 capacitors 20, 21 transistors 24 laser diode 25, 26 constant current source