JPH0454029A - Optical output stabilizing system - Google Patents

Abstract

PURPOSE:To make an optical output constant independently of the signal speed and pattern by providing a light emitting element for which the modulation current of a same amplitude but in opposite phase is injected in addition to a light emitting element for optical signal output to the system and controlling the modulation current so that the sum of optical outputs from both light emitting elements is constant. CONSTITUTION:A semiconductor laser LD1 for optical signal output is connected to one differential output of a differential current switch 1 composed of transistors(TRs) Q1-Q3 and a modulation current is injected to the laser in response to differential signals VIN<+>, VIN<->. A semiconductor laser LD2 is connected to the other differential output, and a modulation current whose amplitude is the same and whose phase is inverted with respect to the modulation current injected to the laser LD1 is injected to the laser LD2. Then optical outputs from the laser diodes LD1, LD2 are respectively received by monitoring photodiodes PD1, PD2 and the sum of the optical outputs is detected as a voltage signal through a resistor R. The detected voltage signal is compared with a reference voltage Vref at an error amplifier 2, and an error signal from the amplifier 2 controls the current of the TR Q3, that is, the amplitude of the modulation current.

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Field of Application] The present invention relates to a method for stabilizing the light output of a light emitting element.

[Prior Art] Generally, when transmitting an optical signal using a light emitting device such as a semiconductor laser, a modulated current A is injected into the light emitting device to transmit the modulated light, as shown in FIG. It is necessary to form an output signal B.

However, the relationship between the current ■ injected into the light-emitting element and the resulting optical output P is highly dependent on temperature.For example, in the case of a semiconductor laser, the relationship between the current The threshold value Ith of @flow I and the rate of change ΔP/ΔI (differential quantum efficiency) of optical output P with respect to injection current I change greatly.

In order to prevent the optical output signal from the light emitting element from fluctuating due to such temperature dependence, measures shown in FIG. 5 have conventionally been taken. That is, while the differential current switch 1 is switched to modulate the current injected into the semiconductor laser 101, a portion of the optical output from the laser [01 is detected by the monitoring photodiode PD1,
This detected current is converted into a voltage signal by a resistor R, the deviation between this voltage value and a reference voltage Vref is determined by an error amplifier 2, and the output of this amplifier 2 is used to control the transistor Q3 of the current switch 1. According to this feedback control system,
The current injected into the semiconductor laser LD1 is controlled so that the voltage value detected by the resistor R is always equal to the reference voltage V ref, making it possible to keep the optical output signal constant regardless of temperature fluctuations. .

However, with the above-mentioned measures, it is only possible to keep the average value of the optical output signal from the laser [01 constant, and the mark rate (
If the ratio of 1 to 0 contained in the transmission data string changes, the peak value of the optical output signal will change accordingly.

In order to suppress such fluctuations, measures shown in FIG. 6 have been conventionally taken. The overall operation is almost the same as in Fig. 5, but here, the peak detection times v! 13 is added to detect the peak value of the voltage signal from the resistor R, and the injection current is controlled so that this peak value becomes equal to the voltage V ref. By controlling the peak value of the voltage signal to be constant in this manner, even if there is a variation in the mark rate as described above, variation in the peak value of the optical output signal can be suppressed.

By the way, in the conventional example described above, as shown in FIG. 4, the injection current corresponding to the low level of the optical output signal B is sufficiently small compared to the threshold current 1th. When a light emitting element is modulated at high speed, the rise of the optical output signal B is delayed and the pulse width is distorted, and ringing occurs due to a transient response. The faster the modulation speed is, the more problematic the output signal waveform will be deteriorated due to this effect.

To avoid such problems, during high-speed modulation, the current injected into the light emitting element is normally biased, as shown in FIG. At this time, if the bias current 1B is sufficiently larger than the threshold current Ith, the ratio (extinction ratio) between the high level PH1 and the low level pt of the optical output signal becomes small, and the signal component contained in the signal becomes small. will decrease. Therefore, it is necessary to keep the bias current I8 constant at a value that is equal to or slightly larger than the threshold I current Ith.

However, as already mentioned, the threshold 1 current Ith changes greatly depending on the temperature of the light emitting element, and also has large variations depending on the individual element.
It is desirable to maintain the relationship between h and bias current IB.

Therefore, as a method of controlling the bias current IB so as to maintain this relationship, there is a conventional method shown in FIG. 8. In this method, the differential current switch 4 is switched,
While modulating current ID is injected into semiconductor laser 101, bias current IB is injected by transistor Q6. Then, a part of the optical output of the laser 101 is detected by the monitoring photodiode PD1, this is converted into a voltage signal by the resistor R, the lowest value of this voltage signal is detected by the bottom detection circuit 5, and this lowest value and Reference voltage V ref
The error amplifier 2 calculates the difference between the two, and the output of the amplifier 2 is used to control the bias current IB. According to this,
The low level PL of the optical output signal can be controlled to a constant value regardless of temperature fluctuations, and the extinction ratio of the optical output signal can be ensured.

“Problems to be Solved by the Invention” However, in both of the conventional systems shown in FIGS. 6 and 8, when the speed of the optical output signal is increased, the monitor photodiode PD1 and , the response speed of the peak detection circuit I#I3 and the bottom detection circuit 5 must be increased, and there is a problem in that it is difficult to deal with high-speed signals.

Further, in the method shown in FIG. 6, since the peak detection circuit 3 is required, there is a limit to the ability to maintain a high level for burst data (data in which the signal is intermittent in a long period). On the other hand, in the method shown in FIG. 8, since it is necessary to use the bottom detection circuit 5, if the high level of the optical output signal continues for a long period, there is a limit to maintaining the low level. There was a problem in that it was important to stabilize the optical output.

As described above, with conventional stabilization methods, there are restrictions on signal speed and data patterns, making it difficult to obtain stable optical output.

SUMMARY OF THE INVENTION An object of the present invention is to eliminate the drawbacks of the prior art described above and to provide a novel optical output stabilization method that can always keep optical output constant regardless of signal speed or pattern.

[Means for Solving the Problems] In order to achieve the above object, the optical output stabilization method of the present invention includes, in addition to the first light emitting element for outputting an optical signal into which a modulation current is injected, the light emitting element A second light emitting element is provided, into which a modulated current of the same amplitude and opposite phase is injected, and a portion of the optical output of the first and second light emitting elements is detected by the corresponding monitoring light receiving element, and these The amplitude of the current injected into the first light emitting element is controlled so that the sum of detection outputs is constant. Here, it is preferable that the light outputs of the first and second light emitting elements be detected by a single monitoring light receiving element.

Further, in addition to the first light emitting element for optical signal output into which the bias current and modulation current are injected, a second light emitting element into which only the same bias current as that of the first light emitting element is injected is provided, and the second light emitting element is provided. A portion of the light output of the second light emitting element is detected by a monitoring light receiving element, and the bias current injected into the first light emitting element is controlled so that the detected output is constant.

[Operation] According to the former method, the first and second light emitting elements are driven differentially with respect to each other, and the sum of the light outputs from these two light emitting elements is equal to the light output of the first light emitting element. It becomes a direct current equal to high level. Therefore, by electrically detecting the sum of the light outputs and controlling the amplitude of the modulation current so that this becomes constant, it is possible to control the mark rate, speed, pattern, etc. of the light output from the first light emitting element. First, the high level of the optical output can be kept constant. Furthermore, according to the latter method, the second light emitting element is driven with a current equal to the bias current injected into the first light emitting element, and the light output from the second light emitting element is equal to that of the first light emitting element. This becomes a direct current equal to the low level of the optical output. Therefore, by controlling the bias current so that the light output of the second light emitting element is constant, the low level of the light output can be maintained regardless of the speed or pattern of the light output from the first light emitting element. can be kept constant.

[Example] Hereinafter, an example of the present invention will be described based on the accompanying drawings.

In FIG. 1, l is a differential current switch consisting of transistors Q1 to Q3, and is interposed between a positive power supply voltage Vcc and a negative power supply voltage vee. One differential output of the differential current switch l is connected to a semiconductor laser [Dl
(first light emitting element) is connected, and the differential signal VIN
”, a modulation current is injected according to VIN-.

In this embodiment, a semiconductor laser E02 (second light emitting element) is connected to the other differential output of the differential current switch 1, and the amplitude of the modulated current injected into the laser 101 is Currents that are the same and have opposite phases are injected into the laser 102. The optical outputs from each laser [01LD2 are received by monitoring photodiodes PDI and PO2, respectively, and the sum of these optical outputs is detected as a voltage signal by a resistor R connected in series with each PDI and PO2. Ru. The voltage signal detected in this way is converted to the reference voltage V by the error amplifier 2.
ref and based on these deviations the amplitude of transistor Q3, ie the modulation current, is controlled. This control is performed until the voltage signal matches the reference voltage Vrctf.

If the semiconductor lasers [ports 1 and 102 are mounted so that their characteristics are aligned with each other and their temperatures are equal, the sum of the optical outputs from these lasers 101 and 102 is
The optical output signal from the laser 101 has a constant value, that is, a DC component, regardless of variations in the mark rate, pattern, etc.

Therefore, the sum of the optical outputs is calculated as described above from photodiode PD1. When detected by PD2 and resistor R,
The detected voltage signal can be directly introduced into the error amplifier 2 and compared with the reference voltage V ref, making it unnecessary to use a peak detection circuit or the like that was conventionally required.

Moreover, the magnitude of the sum of the optical outputs is approximately equal to the high level of the optical output signal, and controlling the sum of the optical outputs is equivalent to controlling the high level of the optical output signal. Therefore, the laser 101 . By controlling the amplitude of the current injected into LD2, the mark rate, speed,
Characteristic fluctuations due to temperature fluctuations can be compensated for without being affected by patterns, etc., and the high level of the optical output signal can be kept constant.

In addition, in the present invention, as in the above embodiment, the monitor photodiodes PD1 and PO2 are replaced with the semiconductor laser LD1.
．． It is not necessary to provide one for each LD2, and the number may be any number as long as the sum of the optical outputs of each laser LD1.102 can be detected. For example, as shown in FIG. 2, the rear output light of the laser LD1.102 may be detected by a common monitor photodiode PD3. According to this, only one monitor photodiode is required, so the configuration It has the advantage of being simplified. In addition, each laser 10
1 and 102 can be mounted close to each other on the same chip, making it easy to match their characteristics and achieve thermal coupling.

Next, another embodiment of the present invention will be described based on FIG.

In FIG. 3, 4 is a transistor Q4. The differential current switch Q5 is a constant current source 6, which is connected in series and interposed between the positive power supply voltage Vcc and the negative power supply voltage Vee. A semiconductor laser [port 1 (first light emitting element)] for outputting an optical signal is connected to one differential output of the differential current switch 4, and a differential signal VIN'', VIN
- modulation current IP is injected in accordance with -. A bias current IB is further supplied to the semiconductor laser LD1 by a transistor Q6 provided in parallel with the current switch 4 and the constant current source 6.

In this embodiment, a series circuit of a semiconductor laser 03 (second light emitting element) and a transistor Q7 is inserted between power supply voltages VCC and Vee, and the same bias current as the bias current 1B injected into the laser [formerly] is inserted. The current of the laser [
It is designed to be injected into the mouth 3. This laser 103
The optical output from the monitor photodiode PD4 and the resistor R are detected as a voltage signal. Then, the detected voltage signal is converted to the reference voltage Vref by the error amplifier 2.
and based on these deviations, transistor Q6
°Q7 is controlled together.

If semiconductor lasers 101 and [03 are mounted so that their characteristics are aligned with each other and their temperatures are equal, the optical output of semiconductor laser [03] is approximately the same as the low level of the optical output signal from laser [Dl]. be equal. Therefore, controlling the optical output from the laser [03 is equivalent to controlling the low level of the optical output signal. Furthermore, the optical output from the laser LD 3 is only a DC component, regardless of the pattern of the optical output signal.

The detected voltage signal can be directly introduced into the error amplifier 2, eliminating the need to use a bottom detection circuit or the like as in the conventional case, and simplifying the circuit configuration of the control system. Also, to keep the voltage signal constant, transistor Q6. Q
By controlling 7 and varying the bias current IB, it is not affected by the speed, pattern, etc. of the optical output signal.
The low level of the optical output signal can always be kept constant.

[Effects of the Invention] In summary, according to the present invention, the following excellent effects are achieved.

(1) By differentially driving the first and second light emitting elements and controlling the amplitude of the injected current so that the sum of their optical outputs is constant, temperature fluctuations can be avoided. Of course,
Regardless of the speed or pattern of the optical output signal, the signal can be stabilized to a constant amplitude. In addition, since the control system only needs to handle DC signals, a high-speed and complicated circuit such as a peak detection circuit is not required, and a low-speed and simple control system can be built horizontally.

(2) By injecting the same bias current as the first light emitting element into the second light emitting element and controlling the bias current so that the light output of this second light emitting element is constant, temperature fluctuations and signal The low level of the signal can be kept constant regardless of the speed, pattern, etc., and a low-speed and simple control system can be built horizontally, as described above.

(3) In this way, the optical output signal can be reliably stabilized.

[Brief explanation of drawings]

Fig. 1 is a circuit diagram showing an embodiment of the optical output stabilization method of the present invention, Fig. 2 is a circuit diagram showing another embodiment of the optical output stabilization method of the invention, and Fig. 3 is a circuit diagram showing an embodiment of the optical output stabilization method of the invention. FIG. 4 is a circuit diagram showing an example of a method for obtaining an optical output signal from a light emitting element, and FIG. 5 is a circuit diagram showing an example of a conventional optical output stabilization method. , Fig. 6 is a circuit diagram showing another example of the conventional optical output stabilization method, Fig. 7 is a diagram explaining another example of the method of obtaining an optical output signal from a light emitting element, and Fig. 8 is a circuit diagram showing another example of the conventional optical output stabilization method. FIG. 7 is a circuit diagram showing still another example of the output stabilization method. In the figure, 1.4 is a differential current switch, 2 is an error amplifier, 1
01 is a semiconductor laser for optical signal output (first light emitting element'
), 102, 103 are semiconductor lasers (second light emitting elements), and PDI, PO2, PO2, PO2 are monitor phototransistors (monitor light receiving elements). Patent applicant: Hitachi Cable Co., Ltd. Representative Patent Attorney Nobuo Kinutani PO4-Monitor output power switch (Fig. 2) Pρ3 ・Monitor switch (1-foot output) A-auto (Fig. 4, Fig. 5, Fig. 7)

Claims (1)

[Claims] 1. In addition to the first light emitting element for optical signal output into which a modulated current is injected, a second light emitting element into which a modulated current having the same amplitude and opposite phase as that of the light emitting element is injected. A part of the light output of the first and second light emitting elements is detected by the corresponding monitoring light receiving element, and the light is injected into the first light emitting element so that the sum of these detected outputs becomes constant. An optical output stabilization method characterized by controlling the amplitude of the current generated. 2. The optical output stabilization method according to claim 1, wherein the optical outputs of the first and second light emitting elements are detected by a single monitoring light receiving element. 3. In addition to the first light emitting element for optical signal output into which a bias current and modulation current are injected, a second light emitting element into which only the same bias current as that of the first light emitting element is injected; A part of the light output of the second light emitting element is detected by a monitoring light receiving element, and the detected output is kept constant.
An optical output stabilization method characterized by controlling a bias current injected into the first light emitting element.