JPS63122291A - Light output stabilizing circuit - Google Patents

Abstract

PURPOSE:To obtain a light output which is stable to dispersion of elements and a characteristic change to temperatures, by dividing this circuit into a high-speed differential amplification circuit for signal current amplification and an emitter follower circuit for supplying a bias current and a signal current and performing feedback control of a collector potential in the differential amplification circuit. CONSTITUTION:Assuming that light output of semiconductor laser 11 has increased, a current flowing across a light detecting element 12 is increased and also a voltage generated across a resistor 13 is raised. However, because a d-c polarity is inverted in an inversion d-c amplifier 15, an output voltage of a d-c adder 17 is decreased in comparison with the voltage which is obtained before the light output increases. Noises are removed from this voltage through an integrator 18 and this voltage is applied to a base of a transistor 19. This operation corresponds to the lowering of a transistor 19's emitter voltage VE. When the emitter voltage VE is lowered, an emitter current 16 is also decreased and so the light output is suppressed. The light signal of a constant output generated by this control is sent outside through an optical fiber 14.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention relates to an optical output stabilizing circuit, and more specifically, to an optical output stabilizing circuit suitable for operating an optical transmitter using a semiconductor laser as a light source, particularly at high frequencies. This invention relates to an optical output stabilization circuit.

[Conventional technology]

Optical communication using semiconductor lasers has already entered the practical stage #)L
A N (Local Area Network
) The current-optical output (knee L) characteristics of semiconductor lasers for transmitting optical signals, which have been put into practical use in many fields such as high-speed paths and ex-computers, have a light emission threshold current as shown in Figure 2. Moreover, this threshold current value varies greatly depending on the temperature from T1 to T3 as shown in the figure. For this reason, circuits that stabilize the optical output of semiconductor lasers have been studied. For example, there is a method described in Japanese Patent Application Laid-Open No. 60-25335. As shown in FIG. 3, transistors 21, 22, . A differential amplifier is configured including the resistor 23 and the semiconductor laser 20, and the semiconductor laser 20
The output of photodetector 262 is inverted and non-inverted DC amplifier 27.
An integrator 29, a signal current control circuit 24, a resistor 30, and a part of the laser light output of the semiconductor laser 20 is monitored by the photodetector element 26, and the monitored output is inverted and non-inverted DC amplifier 27. By providing a stabilizing circuit that applies the voltage to the base of the transistor 25 via the integrator 28 and changes the potential of the base, the bias current flowing to the semiconductor laser 20 can be controlled and a stable optical output can be obtained. In the above-mentioned conventional technology, as a load of the transistor 22 for signal current amplification,
The collector load of the bias current control transistor 25 is added together with the semiconductor laser 20, and in general, a transistor with a fast switching speed (high gain bandwidth product f!) has a low current value that can be passed through it, and for this reason, a large load (especially (capacitive load) cannot be driven at high speed. Therefore, the transistor 22 that performs differential amplification
This poses a problem when attempting to operate at high speed (higher speed than IGb/S).

The purpose of the present invention is to be able to obtain stable optical output despite variations in semiconductor laser elements and changes in characteristics due to temperature, drive the semiconductor laser at high speed, and enable ultra-high-speed digital optical transmission. An object of the present invention is to provide an optical output stabilizing circuit that achieves the following.

[Means for solving the problem] The above object is to separate a high-speed differential amplifier circuit for signal current amplification into a bias current supply and an emitter follower circuit for supplying a signal current, and to separate the bias current flowing into the semiconductor laser into This is achieved by feedback controlling the collector potential of the differential amplifier circuit.

(Function) The collector side load of the above-mentioned high-speed differential amplifier circuit is only the load resistance and the base capacitance of the next-stage emitter follower circuit.Generally, the base capacitance is smaller than the collector capacitance, and semiconductors with relatively large capacitance Since the laser passes through the emitter follower circuit, it does not directly serve as the collector load of the differential amplifier circuit.Therefore, high-speed switching characteristics can be achieved.

Furthermore, by controlling the collector potential of the differential amplifier circuit according to the optical output of the semiconductor laser, the base potential of the emitter follower circuit that is directly connected to it changes, and therefore the bias current flowing through the laser is controlled. The optical output of the laser can be stabilized.

[Example] An example of the present invention will be described below with reference to FIG. This optical output stabilizing circuit includes resistors 1, 2, 4jp and 5. Bypass condenser? 5.l'? Nzislis.

7. A high-speed differential amplifier for signal current amplification consisting of a current source 8, and an emitter follower circuit for bias current supply and signal current superimposition of the semiconductor laser 11 consisting of a transistor 9 directly connected to the transistor 7 and a resistor 10;
Photodetection element 12. The optical signal detected by the resistor 13 is converted into an inverting DC amplifier 15. Variable DC voltage source 16 and DC adder 1
In addition to the base of a transistor 19 which removes noise in step 7 and is connected to the differential amplifier in a direct current manner through an integrator 1, it also consists of a stabilizing circuit that changes the bias of the semiconductor laser 11. The ultra-high-speed digital signal is input to the emitter follower circuit via the high-speed differential amplifier, and the emitter voltage of the transistor 19 becomes v! 11, the current flowing through the current source 8 is expressed as x, and the width of change in the signal current flowing through the transistor 7 is expressed as x. Let the base voltage of transistor 9 be vB, and resistor 2
When the resistance value of is R2, vB is expressed as in equation (1).

vB=vII! -R,X (-±/L)
(1) Generally, the potential difference between the base and emitter of a transistor is a constant voltage, and this potential difference is about 1.6 V in the case of a silicon transistor.

If the emitter voltage of transistor 9 is VB, then Vj
=VB -R2X (-:h L ) -0,6
"..."...(2).

Here, the current flowing through the semiconductor laser 1 is approximately equal to the current flowing out from the emitter of the transistor 9. The current flowing from this emitter is E, and the resistance value of resistor 10 is R.
When IO is assumed, work (2) is expressed by the following equation (3). However, the power supply to this circuit system is VOO and vz
Let it be z.

Since the cono emitter current l1 is the bias current of the semiconductor laser 1, the optical output is determined as shown in the semiconductor laser bias current-optical output characteristic shown in FIG.

The current flowing through the photodetector element 12 is proportional to the optical output of the semiconductor laser 11, and the current information (optical output information) is transmitted through the resistor 1.
3) is converted into voltage information, DC amplified by the inverting DC amplifier 15, applied with the voltage of the variable DC voltage source 16 for adjusting the nominal optical output in the DC adder 17, and integrated for noise removal. 18 and is applied to the base of transistor 190. As described above, since there is a constant voltage between the base and emitter of the transistor, the emitter voltage vE of the transistor 19 is controlled by the bias current flowing through the semiconductor laser 11, that is, the optical output of the semiconductor laser 11.

The above equation (5) is as follows.The first half of equation (4), equation (5), is the bias current fRo when determining the average optical output of the semiconductor laser.・・・・・・・・・・・・・・・
(The phrase is the signal current change width.

Assuming that the optical output of the current conductor laser 1 increases, the current flowing through the photodetecting element 12 increases and the voltage generated in the resistor 13 also increases. However, since the DC visibility is inverted in the inverting DC amplifier 15, the voltage of the output of the DC adder 17 is reduced compared to before the optical output became large. This voltage is passed through an integrator 18 to remove noise and is applied to the transistor 9. This operation causes the emitter voltage of transistor 9 to be
! This corresponds to lowering O. When the emitter voltage VB decreases, the emitter current xfb decreases from equation (4),
As can be seen from FIG. 2 mentioned above, the optical output can be suppressed. Even when the optical output of the semiconductor laser 1 decreases, a similar operation is performed to increase the optical output. The optical signal, which has a constant output through this control, is transmitted through the optical fiber 4,
Sent to the outside. The above is an outline of the circuit operation for stabilizing the optical output of the laser.

Further, according to this circuit configuration, the signal current amplification section is a high-speed differential amplification circuit including transistors 6 and 7, and bias current supply to the semiconductor laser 11 and signal current superimposition are performed by an emitter follower including transistor 9. Since the emitter follower circuit has a voltage gain of 1, it can drive a heavy load such as a semiconductor laser having a relatively large capacity.

The collector of one transistor 70 constituting the differential amplifier is loaded only by the resistor 2 and the base capacitance of the transistor 9, and as in the conventional configuration shown in FIG. This is advantageous in terms of high-speed switching characteristics, compared to a circuit in which the load is a large-capacitance semiconductor laser 20 or a collector capacitance (generally larger than the base capacitance h) of the transistor 25. As described above, with this circuit configuration, an optical output stabilization circuit suitable for high-speed digital transmission can be realized.

〔Effect of the invention〕

As explained above, according to the present invention, stable optical output can be obtained regardless of variations in semiconductor laser elements and changes in ambient temperature, and signal current amplification is more efficient than in conventional circuit formats. Since the capacitive load on the differential amplifier for use in the present invention is reduced, an optical transmitter suitable for ultra-high-speed digital transmission can be realized.

[Brief explanation of the drawing]

FIG. 1 shows an embodiment of the present invention, and is a block diagram of an optical output stabilizing circuit for an ultrahigh-speed optical transmitter, and FIG. 2 shows bias current-optical output characteristics of a semiconductor laser. FIG. 3 is a block diagram of a conventional optical output stabilizing circuit. 1.2, 4, 5, 10, 13...resistance, 3...bypass capacitor, 6, 7, 9, 19...transistor, 8...
current source. DESCRIPTION OF SYMBOLS 11... Semiconductor laser, 12... Photodetection element, 14... Optical fiber, 15... Inverted DC amplifier 16... Variable DC voltage source, 17... DC adder, 18...・Integrator. Tsuta 1 Shu ViEVEt;-

Claims (1)

[Claims] 1. In driving a semiconductor laser by superimposing a signal current on a bias current, the semiconductor laser has a differential amplifier circuit for amplifying the signal current and an emitter follower circuit for supplying the bias current and superimposing the signal current. An optical output stabilizing circuit characterized in that a part of the optical output is detected, and the collector voltage of the differential amplifier circuit is feedback-controlled based on the detected optical output information.