JPS5978588A - Drive circuit for light emitting element - Google Patents

Abstract

PURPOSE:To contrive ratios of a peak value of a light emitting element to a means value, the peak value to the minimum value, and the mean value to the minimum value become fixed values by a method wherein a bias current and a pulse current are so controlled that a ratio of light extinction becomes constant. CONSTITUTION:A bias control current 15 so controls a bias circuit 16 that the peak value detected by a peak detection circuit 18 becomes constant, and the bias current is supplied from the circuit 16 to a semiconductor laser 13. A pulse current control circuit 20 determines the peak value detected in a circuit 18 by a variable attenuator 19 as 1/K1, the value is compared in the circuit 20 with the mean value detected in a mean value detection circuit 17, and then a current amplification circuit 12 is so controlled that the mean value becomes smaller. Thereby, a pulse current value from the circuit 12 increases, and the ratio of extinction is maintained nearly constant.

Description

DETAILED DESCRIPTION OF THE INVENTION Technical Field of the Invention The present invention relates to a light emitting element drive circuit that stabilizes and drives output light of a light emitting element whose dark value current and differential quantum efficiency vary with temperature.

Prior Art and Problems As shown in Figure 1, the current-light output characteristics of a conventional general semiconductor laser are as follows: As the temperature rises, the dark value current increases to +th,
It increases from Ith2. However, the differential quantum efficiency hardly changes. Therefore, when driving with a pulse current, the magnitude of the pulse current 1p is kept constant, and the bias current is set to Ib+, Jb2 near the threshold current depending on the temperature.
If controlled as follows, the optical output level and extinction ratio will be approximately constant.

A conventional driving circuit for such a light emitting element has a configuration shown in FIG. 2, for example. In the same figure, data DT is input to the data terminal 1, and clock terminal C
K is a flip-flop to which a clock signal CLK is input; 2 is a current amplification circuit; 3 is a semiconductor laser as a light emitting element; 4 is a photodetector for detecting the output light of the semiconductor laser; 5 is a bias control circuit; 6 The other example is a bias circuit. Data D is input to the semiconductor laser 3 by a flip-flop I.
The pulse current from the current amplification circuit 2, which controls T in synchronization with the clock signal CLK, is supplied superimposed on the bias current from the bias circuit 6, and the bias current, for example, Ib in FIG. is supplied from the bias circuit 6 and a pulse current 1p is supplied from the current amplification circuit 2 in response to the data DT, and when the temperature rises,
Even if the pulse current 1p is supplied to the semiconductor laser 3, its optical output level decreases. This is detected by the photodetector 4, and the bias control circuit 5 identifies the detection output of the photodetector 4 with respect to the output of the flip-flop 1, and controls the bias circuit 6 so that a predetermined optical output is obtained. The bias current is increased, for example, as Ib2 in FIG. Thereby, the light output level is maintained approximately constant.

However, as shown in Fig. 3, for a light emitting element whose differential quantum efficiency changes with temperature, the bias current is controlled according to the temperature, that is, by detecting the optical output level, as in the drive circuit shown in Fig. 2. Only by doing so, it becomes impossible to obtain the desired optical output and extinction ratio. That is, as the temperature rises, the characteristics change from the curve ta+(bl), and the bias current changes to Ib
Even if , is increased from Ib2 to Ib2, the optical output has the waveform shown from A to B, and even if the optical output level is maintained almost constant, the extinction ratio becomes small, resulting in a disadvantage that transmission errors occur.

Examples of light emitting elements whose differential quantum efficiency changes depending on temperature include V, SB (V -groove
clSubstrate Buried hetero
Structure) Semiconductor lasers with the name laser are known. This VSB laser is, for example, an n-In
A cladding layer, an active layer, and a cladding layer are formed by forming a V groove in a P substrate, and the active layer is buried in the groove.The dark value current is extremely small, e.g.
~20mA, and has good waveform response, so for example 4
It has advantages such as easy modulation at 00 Mb/S. When such a semiconductor laser is driven by a conventional general drive circuit as shown in FIG. 2, there is a drawback that the extinction ratio becomes small due to temperature changes.

Purpose of the Invention The object of the present invention is to make it possible to drive a light emitting element whose differential quantum efficiency changes depending on temperature while maintaining a substantially constant optical output level and extinction ratio. be. Examples will be described in detail below.

Embodiment of the invention FIG. 4 is a block diagram of main parts of an embodiment of the invention, and 11
12 is a current amplification circuit whose amplification factor is controlled; 13 is a semiconductor laser as a light emitting element; 14 is a semiconductor laser; 15 is a bias control circuit; 1 is a photodetector for detecting the optical output of
6 is a bias circuit, 17 is an average value detection circuit, 18 is a peak detection circuit, 19 is a variable attenuator, and 2o is a pulse current control circuit. To the bias current from the bias circuit 16,
A pulse current according to data from the current amplification circuit 12 is superimposed and supplied to the semiconductor laser 13.
The light of No. 3 is detected by the photodetector 14. The detection output of the photodetector 14 is transmitted to an average value detection circuit 17 and a peak detection circuit 18.
The output of the peak detection circuit 18 is applied to the variable attenuator 19 and the bias control circuit 15. The bias control circuit 15 controls the bias circuit 16 so that the peak value detected by the peak detection circuit 18 is constant, and supplies a bias current to the semiconductor laser 13 from the bias circuit 16.

The pulse current control circuit 20 receives the peak value via the variable attenuator 19 and the average value detected by the average value detection circuit 17, and identifies whether (peak value)/(average value)≦1. , the current amplification circuit 12 is controlled to increase the pulse current value under these conditions. That is, the peak value detected by the peak detection circuit 18 is set to 1/1 by the variable attenuator 19, and the pulse current control circuit 20 compares it with the average value detected by the average value detection circuit 17, and the average value is smaller. The current amplification circuit 12 is controlled so that

As a result, the pulse current value from the current amplifier circuit 12 increases, and the extinction ratio is maintained substantially constant.

FIG. 5 is an explanatory diagram of the operation. When the current-light output characteristic at temperature T1 is shown by curve a, the bias current is IB
,, Assuming that the desired optical output can be obtained by setting the pulse current to IP, when the temperature rises to 1゛2 and the characteristic curves, the differential quantum efficiency of curve a and curve If the difference is small, change the bias current to IP2
(IBI<l82), and the pulse current is IP2-IP
, the desired light output can be obtained. That is, in the pulse current control circuit 20, since the ratio between the peak value and the average value is in a predetermined relationship, the pulse current from the current amplifier circuit 12 remains unchanged, and the bias control circuit 15 maintains the bias current IB. , the peak value decreases, so control is performed to increase it to IP2.

If the temperature further rises to temperature T3 and the characteristic curve becomes C, the bias control circuit 15 will control the bias current to increase to IP3. In that case, if the pulse current is set to 1p3=+p2 as shown by the dotted line, the optical output will be as shown by the dotted line, and its extinction ratio will decrease.

That is, even if the peak value is constant, the average value increases and the extinction ratio decreases. Therefore, the pulse current control circuit 20 controls the current amplifier circuit 12 to increase the pulse current to IP4. In that case, the bias current is controlled so that the peak value is constant, so the bias current is I B 4
(< I B 3 ). Therefore, the optical output maintains a predetermined peak value and average value.

In other words, by controlling so that (peak value)/(average value) > ki, it is possible to keep the peak value of the optical output constant and to keep the extinction ratio constant.

It is also possible to control the current amplification circuit 12 by detecting the ratio between the average value and the minimum value of the optical output. That is, in FIG. 5, when the temperature T3 of the characteristic curve C is reached, when the semiconductor laser 13 is driven with a pulse current shown by the dotted line, the optical output becomes as shown by the dotted line, and the minimum value of the optical output increases. Therefore, the minimum value is detected, the average value of the previous output is detected, and the pulse current control circuit 20 amplifies the current so that the condition of (average value)/(minimum value)>k2 is satisfied. circuit 12
It would be a good idea to control it. In the configuration shown in Figure 4,
Reference numeral 17 is a minimum value detection circuit, reference numeral 18 is an average value detection circuit, and their detection outputs are sent to the pulse current control circuit 2.
0 is compared, and the current amplification circuit 12 is controlled based on the comparison result. Note that the lowest value detection circuit is the photodetector 1
This can be realized by a configuration in which the detection output of No. 4 is inverted and the peak value of the inverted signal is detected.

The current amplification circuit 12 also detects the ratio between the peak value and the minimum value.
can also be controlled. In this case, (peak value)/
(Lowest value)> The current amplification circuit 12 is controlled so as to satisfy the condition of k3. That is, in FIG. 5, when the semiconductor laser 13 is driven with a pulse current indicated by a dotted line,
The optical output shown by the dotted line is obtained, and the minimum value becomes large, so the above-mentioned conditions are no longer satisfied, and this is detected and the current amplification circuit 12 is controlled to increase the pulse current like IP4, so that the peak value remains constant. It is only necessary to control the bias current as shown in l84 so that it becomes .

In this case, this can be realized by using the reference numeral 17 as a minimum value detection circuit in FIG.

The aforementioned conditions of (peak value)/(average value)>kl, (average value)/(minimum value)>k2 condition, and (peak value)/(minimum value)>k3 condition are such that the extinction ratio is higher than a predetermined value. The temperature control of the noisy current and the pulse current is as shown in FIG. curve 1
a shows the bias current or pulsed current, curve 1b shows the pulsed current or noisy current, and the temperature Ta! 22, both the bias current and the pulse current are controlled to increase as the temperature rises. In the first embodiment, the pulse current is kept constant below the temperature Ta, the bias current is controlled so that the peak value remains constant as the temperature rises, and when the temperature rises above the temperature Ta, the pulse current is also controlled to increase. This shows the case when

As described in detail, the present invention detects the optical output of a light emitting element such as a semiconductor laser and calculates its peak value even when the light emitting element has a characteristic that the differential quantum efficiency changes with respect to temperature. and a peak detection circuit that controls the bias current and the pulse current so that the ratio between the average value, the peak value and the minimum value, and the average value and the minimum value becomes a predetermined value, that is, the extinction ratio is constant. 17, average value detection circuit 18, variable attenuator 19,
Since it is equipped with circuits such as the pulse current control circuit 20, a desired optical output can be obtained. Therefore, there is an advantage that semiconductor lasers with various characteristics can be stably driven.

[Brief explanation of drawings]

Figure 1 is an explanatory diagram of the characteristics of a light emitting element whose differential quantum efficiency hardly changes with temperature. Figure 2 is a block diagram of the main parts of a conventional drive circuit. Figure 3 is a light emitting element whose differential quantum efficiency changes with temperature. FIG. 4 is a block diagram of the main part of the embodiment of the present invention, FIG. 5 is an explanatory diagram of the operation of the embodiment of the present invention, and FIG. 6 is the control characteristics of bias current and pulse current due to temperature change. FIG. 11 is a flip-flop to which data DT is input to the data terminal and clock signal CLK is input to the clock terminal CK; 12 is a current amplification circuit whose amplification factor is controlled; 13
1 is a semiconductor laser as a light emitting element, 14 is a photodetector that detects the optical output of the semiconductor laser, 15 is a bias control circuit, 16 is a bias circuit, 17 is an average value detection circuit, 18 is a peak detection circuit, and 19 is a variable attenuation. 20 is a pulse current control circuit. Patent applicant Fujitsu Ltd. Representative Patent Attorney Gobe Tamamushi and 3 others Figure 5 Figure 6 Design a, M degree

Claims (1)

[Claims] A current amplification circuit that supplies a pulse current to the light emitting element, a bias circuit that supplies a bias current to the light emitting element, a bias control circuit that controls the bias circuit, a photodetector that detects the light output of the light emitting element, and the photodetection. so that the extinction ratio of the light output of the light emitting element is constant according to the detection output of the device,
A light emitting element drive circuit comprising a circuit that controls the current amplification circuit and the bias control circuit to control pulse current and bias current.