JPS5942475B2 - Semiconductor laser light modulation circuit - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a semiconductor laser light modulation circuit, and more particularly to a semiconductor laser light modulation circuit that controls a DC bias level of an injected current.

Semiconductor lasers generate light by injecting current. The current-light output characteristics (hereinafter referred to as I-L characteristics) of a semiconductor laser have temperature dependence, and change depending on the deterioration of the device. FIG. 1 is a graph showing the IL characteristics of a semiconductor laser.

In the figure, curve A shows the I-L characteristic under normal conditions, and curve B shows the characteristic when the element temperature of the semiconductor laser increases for some reason or the semiconductor laser itself deteriorates. For this reason,
c in Fig. 1 when DC bias current 10 is superimposed on the signal current.
When a modulation current such as
become that way. In other words, the modulation characteristics are subject to changes due to changes in the element temperature of the semiconductor laser or deterioration of the semiconductor laser, so that it is unsuitable as a light source for optical communications as it is. In order to eliminate the above drawbacks, a method has been used in which the DC bias current 10 is controlled so that the average value or peak value of the output light is constant.

However, when modulating the output light of a semiconductor laser with a television video signal, the method of keeping the average value of the output light constant causes the DC level to change depending on the brightness of the screen, resulting in sudden changes in the brightness of the screen. If there is a change, the received light will cause out-of-sync or fluctuations in brightness on the TV screen played back.

On the other hand, a method of controlling the peak value is used in the case of binary pulse transmission.

To apply this method to video signal transmission, if optical modulation is performed by making the synchronization side of the video signal correspond to the greater optical output side, the DC level of the synchronization pulse will always be fixed, and the above drawbacks will be avoided. Does not occur. However, since the I-L characteristic of a semiconductor laser is curved in the low current range, in order to perform transmission with little distortion, the curved part of the I-L characteristic of the semiconductor laser must be adjusted for any voltage of the video signal. It is necessary to select the injection current at the synchronous pulse position to be sufficiently high, such as 12 shown in FIG. 2, so as not to reach this point.

However, in this case, the percentage of time when the injection current takes a large value is very large, which adversely affects the life of the semiconductor laser. SUMMARY OF THE INVENTION An object of the present invention is to provide an optical modulation circuit for a semiconductor laser that can lower the DC bias level of the semiconductor laser and perform optical modulation without impairing image quality. In order to achieve the above object, the optical modulation circuit according to the present invention modulates the current injected into the semiconductor laser with a polarity such that the sync pulse side of the video signal is a low current and the white level is a high current, and corresponds to the sync pulse. The output of the semiconductor laser is detected, and the DC bias level of the injection current is controlled based on the detection level. According to the above configuration, the injection current can be clamped to the peak level of the synchronization pulse, the loss can be suppressed, and high-quality image transmission is possible, so that the object of the present invention is completely achieved. .

The present invention will be described in more detail below with reference to the drawings and the like.
FIG. 3 shows an embodiment of the optical modulation circuit according to the present invention.

14 is a voltage-current converter which converts the video signal voltage applied to the terminal 18 into a current 13;

14 is a DC bias current which is injected into the semiconductor laser 11 together with 13 to generate lights Ll and L2.

The light L1 is supplied to the communication path and transmitted to the receiving end as communication light. Light L, is emitted from the other end of the semiconductor laser, and its intensity is proportional to light L1. Light L2 is from photodetector 1
2 is converted into electric current. On the other hand, the synchronization separation circuit 16 has almost the same configuration as that commonly used in television receivers, and outputs a pulse at the same time as the synchronization pulse of the video signal arrives, and the gate circuit 15 uses this pulse to output a pulse. The output of the photodetector 12 is sampled and held, and a current 16 proportional to the light intensity of the portion corresponding to the synchronous pulse of the light L2 is supplied to the differential current amplifier 17, and a reference current 1 is supplied to the differential current amplifier 17.
The difference from 5 is amplified to 14. Here it is.

However, μ is the gain of the differential current amplifier 17 and is a positive number. If the conversion coefficient from the semiconductor laser injection current (13+14) to 6 is β, it becomes (13+I4)β-16,
This can be obtained by substituting this into equation (1).

If μβ is chosen to be a sufficiently large value, I6+I5, and the optical output at the time of the synchronization pulse is kept constant even if the characteristics of the semiconductor laser change, that is, even if β changes. From the viewpoint of the life of the semiconductor laser, the smaller the better, but from the viewpoint of the distortion of the modulated waveform of the output light, it cannot be reduced very much.

However, the linearity of the synchronization pulse portion does not affect the image quality, and expansion and contraction of the amplitude is allowed as long as it does not interfere with the synchronization operation on the receiving side. Therefore, as shown in Figure 4, there is no problem even if the value at the time of the synchronizing pulse of I, +I4 is selected at point D, which has a lower current than point E, the lowest operating point where the linearity of the IL characteristic is maintained. Does not occur. Furthermore, even if point E is selected, it is clear that the average value of the injection current will be smaller than in the method shown in FIG. Detection of the optical output at the time of the synchronization pulse is possible with the configuration shown in FIG. 5 in addition to the configuration described above.

A current proportional to the intensity of the light L3 is obtained by the photodetector 101, and then this current is passed through the resistor 102 to obtain the voltage Vll. This voltage Vll changes as shown in FIG. 6 in accordance with the injection current of the semiconductor laser. The diode 103 is forward biased to Vl2 by the bias power supply 104, which is larger than the value of Vll at the time of the synchronization pulse, Vl3. Therefore, the diode 103 and the bias power supply 1
04, a capacitor 105, and a resistor 106, the voltage V,4 at the output terminal 107 of the peak detection circuit is clamped to the voltage (Vl3-Vl2) as shown in FIG. In the figure, the change in Vl4 is shown in an enlarged manner, but in reality, the product of the values of the resistor 106 and the capacitor 105 is large, so Vl4 hardly changes and takes a constant value of approximately Vl3Vl2. It is clear that if the above-described circuit is substituted for the gate circuit 15 and synchronous separation circuit 16 in FIG. 3, and the voltage and current are applied to the differential current amplifier 17 with varying voltages, the same operation as in FIG. 3 can be obtained. However, the polarities of 5 and 16 need to be reversed. As explained above, according to the present invention, the level of the synchronization pulse of the video signal can be clamped at a desired position in the low current range of the semiconductor laser, so that the low current operating range of the semiconductor laser can be effectively utilized. Compared to the case where the synchronous pulse portion is also set in a relatively high current range, the life of the semiconductor laser can be extended, and the DC level of the output light can be fixed.

Various modifications can be made to the embodiments described in detail above within the scope of the present invention.

For example, in the previous explanation, the optical output used for control was taken out from the end face of the semiconductor laser opposite to the end face that outputs the communication light, but a part of the communication output light is taken out by a half mirror or other means. It is also possible to take out and use it as a control light output.

The scope of the present invention extends to all of the claims, including such modifications.

[Brief explanation of the drawings] Figure 1 is a graph to explain that the modulated optical output changes when the characteristics of a semiconductor laser change, and Figure 2 is a conventional method for controlling the peak value of optical output. FIG. 3 is a circuit diagram showing an embodiment of the present invention, and FIG. 4 is a graph for explaining the operation of the circuit shown in FIG. 3. 5 is a circuit diagram showing another configuration example of the light intensity detector used in the circuit shown in FIG. 3, and FIG. 6 is a graph for explaining the operation of the circuit shown in FIG. 5. be. L1...Light for communication supplied to the communication path, L2
...Light for photodetection proportional to L, 11...
... Semiconductor laser, 12 ... Photodetector, 14
...Voltage-current converter, 15...Gate circuit, 16...Synchronization separation circuit, 17...
・Differential amplifier, 18...Modulation input terminal, 101
...Photodetector, 102,106...Resistor, 103...Diode, 104...
・Power supply, 105...Capacitor, 107...
...Output terminal.

Claims (1)

[Claims] 1 Modulate the injection current of the semiconductor laser with a polarity such that the synchronization pulse side of the video signal is a low current and the white level is a high current, detect the semiconductor laser output corresponding to the synchronization pulse, and according to this detection level , a semiconductor laser light modulation circuit configured to control a DC bias level of the injection current.