JP2623975B2 - Light emitting element drive circuit - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a light-emitting element driving circuit of an optical transmission apparatus used for an optical communication system or optical information processing, and more particularly, to a light-emitting element used in an ultra-high-speed optical communication system having a transmission speed exceeding Gb / s. The present invention relates to an element driving circuit.

[0002]

2. Description of the Related Art Conventionally, as shown in FIG. 3 or FIG. 4, this type of light emitting element driving circuit has a differential light emitting structure in which, for example, the emitters of two transistors 11 and 12 are connected to a constant current source 13 in common. It has an element driving unit 1, its output terminal is connected to the cathode of the light emitting element 2, and a DC bias circuit consisting of an inductor 4 and a transistor 3 for blocking a high-frequency signal is connected. In this case, the light emitting element 2 uses a semiconductor laser or a light emitting diode.

In such a light emitting element driving circuit, an integrated circuit made of silicon or gallium arsenide is generally used as the light emitting element driving section 1 as a circuit body, and an open collector (silicon integrated circuit) is used as an output form. ) Or an open drain (gallium arsenide integrated circuit) circuit is generally used. On the other hand, a bias circuit for supplying a DC bias current to the light emitting element 2 has a collector of the transistor 3 or a drain of the gallium arsenide field effect transistor connected to an output terminal as shown in FIG. It is configured to be. When the DC bias circuit is not included in the integrated circuit, an inductor 4 for blocking a high-frequency signal and a bias transistor 3 are connected to an output terminal as shown in FIG. In addition,
In the figure, 9 is a resistor inserted between the biasing transistor 3 and its driving power supply (not shown), and 14 is a collector resistor of the transistor 11.

[0004]

As described above, in the conventional light emitting element driving circuit, when the driving section 1 incorporates a DC bias circuit in the same chip, the transistor 3 is directly connected to the output terminal. Become. Generally, the DC bias current of the light emitting element 2 needs to be supplied up to about 100 mA in consideration of a change in the threshold current of the light emitting element 2 due to a change in the operating temperature. Therefore, when a DC bias circuit is built in the integrated circuit, a relatively large transistor is required as the transistor 3. For this reason, the impedance seen from the collector side of the bias transistor 3 is connected to the output terminal in parallel with the light emitting element 2 as a load of the light emitting element drive circuit 1 (which becomes a capacitive load because the transistor size increases). become. Therefore, in the operation region where the transmission speed exceeds Gb / s, there are problems such as limiting the band of the output pulse and generating ringing in the waveform in combination with the parasitic inductance at the connection with the light emitting element 2. This has been a major obstacle in achieving stable operation in the / s band.

Conventionally, as one method for solving this problem, as shown in FIG. 4, a DC bias circuit is provided outside the light emitting element driving section 1, that is, the circuit main body 1, and the connection with the output terminal is made via the inductance 4. In this method, the components of the DC bias circuit need to be mounted between the output terminal of the light emitting element driving unit 1 and the light emitting element 2, so that the light emitting element driving circuit and the light emitting element 2, the connection length becomes long, and it is difficult to obtain sufficient performance for operation in the Gb / s band. In view of the above, it is an object of the present invention to provide a light emitting element driving circuit that solves the above-mentioned conventional problems.

[0006]

In order to achieve the above object, a light emitting element driving circuit according to the present invention comprises a light emitting element driving section for supplying a pulse current to the light emitting element and a predetermined circuit connected to an output terminal of the light emitting element driving section. It has a microstrip line with characteristic impedance of, and has a DC bias circuit consisting of a transistor connected in series with an inductance that blocks high-frequency signals at one end of the microstrip line, and a termination at the connection between the microstrip line and the inductance. It has a circuit in which a resistor and a capacitor are connected in series.

[0007]

According to the present invention, by connecting the microstrip line to the output terminal of the light emitting element driving section, the DC bias circuit of the light emitting element can be mounted at a position distant from the light emitting element. Long connection length. Further, by shunting one end of the terminating resistor to the reference potential with a capacitor, it is possible to eliminate a shunt loss for a DC bias current flowing through the bias transistor.

[0008]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described below with reference to the embodiments shown in the drawings. FIG. 1 is a circuit diagram showing a state in which a light emitting device driving circuit and a light emitting device according to one embodiment of the present invention are connected. Here, the light emitting element driving circuit of the present embodiment denoted by reference numeral 7 has the same light emitting element driving section 1 as the conventional example in which the emitters of the two transistors 11 and 12 are connected to the constant current source 13, for example. The terminal is connected to the cathode of the light emitting element 2, and the output terminal of the light emitting element driving unit 1 and the light emitting element 2
Is connected to a microstrip line 6 having a predetermined characteristic impedance. The other end of the microstrip line 6 is grounded to a reference potential by the terminating resistor 5 and the capacitor 8, and the microstrip line 6 and the terminating resistor 5
Is connected to a DC bias circuit composed of an inductance 4 and a transistor 3. The same reference numerals in the drawings indicate the same or corresponding parts.

In the configuration of the above embodiment, the pulse current 21 output from the light emitting element drive circuit 7 (see FIG. 2A)
Is supplied to the light emitting element 2 connected to the output terminal and is converted into an optical signal. On the other hand, part of the pulse current is split into the microstrip line 6. After the divided pulse current propagates through the microstrip line 6, an amount determined by the ratio of the characteristic impedance of the terminating resistor 5 to the microstrip line 6 is reflected as a reflected wave 22 in the opposite phase and again at the output terminal of the light emitting element drive circuit 7. Return (see FIG. 2 (b)). Therefore, the reflected wave and the original pulse current are merged and converted into a pulse current waveform 23 having overshoot and undershoot at the rising and falling portions of the pulse as shown in FIG.

In general, the light emitting element 2 continues to emit light in response to the fall of the pulse current until the carriers accumulated inside the light emitting element are released, so that the light emitting element 2 tends to exhibit a swell-like response. For this reason, it is necessary to take a method of forcibly releasing the accumulated carriers in the light emitting element by giving the pulse current an undershoot. In the present embodiment, it is possible to add an undershoot to the pulse current as shown in FIG. 2C, which has the effect of improving the response of the optical waveform.

In this embodiment, a silicon integrated circuit is used as the light emitting element driving circuit 7. The microstrip line 6 for connecting the DC bias circuit was formed as a part of a wiring pattern on a ceramic package on which the integrated circuit was mounted. As described above, by forming the microstrip line as a part of the wiring pattern of the ceramic package, the DC bias circuit of the light emitting element 2 can be mounted at a position away from the light emitting element 2, and the light emitting element 2 It is possible to supply a DC bias without increasing the connection length of the light emitting element drive circuit 7.

In this embodiment, the characteristic impedance of the microstrip line 6 is selected to be about 10 times the differential resistance of the light emitting element 2 and the value of the terminating resistance 5 is set to minimize the shunt loss of the pulse current. It was selected to have a reflectance of 10%. By doing so, it becomes possible to obtain an optical waveform having a good rising response characteristic.

In this embodiment, since one end of the terminating resistor 5 is grounded to the reference potential by the capacitor 8,
It is possible to adopt a configuration in which a shunt loss is eliminated for a DC bias current flowing through the transistor 4.

[0014]

As described above, according to the present invention, a microstrip line having a predetermined characteristic impedance is connected to the output terminal of the light emitting element driving section, and the other end of the microstrip line is connected to a terminating resistor and a capacitor. By connecting the series circuit and the DC bias circuit including the inductance and the transistor, the pulse response characteristics of the light emitting element can be improved without shunt loss of the DC bias current. This has the effect of providing a light emitting element drive circuit that operates stably in the Gb / s band.

[Brief description of the drawings]

FIG. 1 is a circuit diagram illustrating a light emitting device driving circuit according to an embodiment of the present invention.

FIG. 2 is an explanatory diagram showing an operation outline of the light emitting element drive circuit of the present embodiment.

FIG. 3 is a circuit diagram showing a conventional light emitting element drive circuit.

FIG. 4 is a circuit diagram showing another light emitting element drive circuit of a conventional example.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Light emitting element drive part 2 Light emitting element 3 Transistor 4 Inductance 5 Termination resistance 6 Microstrip line 8 Capacitor

──────────────────────────────────────────────────続 き Continued on the front page (51) Int.Cl. ⁶ Identification code Agency reference number FI Technical display location H04B 10/26 10/28

Claims (3)

(57) [Claims] In a light emitting element driving circuit for supplying a pulse current to a light emitting element, a microstrip line having a predetermined characteristic impedance is connected to an output terminal of the light emitting element driving section, and a high-frequency signal is connected to one end of the microstrip line. A light emitting element drive circuit comprising: an inductance for cutting off a current, and a transistor for supplying a DC bias current to the light emitting element connected in series, and a series circuit of a terminating resistor and a capacitor connected to a connection between the microstrip line and the inductance. 2. The light-emitting element driving circuit according to claim 1, wherein the light-emitting element driving circuit comprises an integrated circuit of silicon or gallium arsenide, and outputs the output with an open collector or an open drain. 3. The light emitting element drive circuit according to claim 1, wherein the microstrip line is made of a ceramic member and is formed as a part of a wiring pattern of a package on which the integrated circuit is mounted.