JPS6290991A - Controlling method for spectral width of semiconductor laser element - Google Patents

Abstract

PURPOSE:To maintain the spectral width of a laser element at the minimum by obtaining information of the spectral width of the laser from the variation of the output light intensity of the laser element for the variation in the implanted current to a phase control region of the laser element, thereby controlling the current to the phase control region. CONSTITUTION:Part of the output light 6 of a phase control distribution feedback semiconductor laser (DFB-LD) 1 is branched by a half mirror 7, and the light intensity is detected by a monitoring photodetector 8. The amplitude of the implanting current from the second bias circuit 5 to a phase control region 3 is detected by a monitoring circuit 9. When the two detection outputs are matched to the levels and input to a differential amplifier 10, an error signal 11 which becomes a zero output at the minimum of the width is obtained. The signal 11 is fed back to the second bias circuit 5 to control the implanted current to the region 3, thereby always maintaining the width of the phase control DFB-LD1 at the minimum point.

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to a control method for constantly operating a semiconductor laser with a narrow spectral linewidth.

(Prior art) In recent years, the development of distributed feedback semiconductor lasers (DFB-LD) and distributed Bragg reflection semiconductor lasers (DBR-LD), which oscillate in a single-axis mode, has been progressing, and these lasers are capable of producing coherent light using a gold layer as a light source. Transmission methods are being studied.

In order to realize this coherent optical transmission system, it is necessary that the light source oscillates in a single-axis mode and that its spectral purity is high. By the way, in a DFB-LD, the oscillation condition is strongly influenced by the condition of the diffraction grating phase at the reflection end face, and depending on the phase condition, 2
The axis mode oscillates, and the oscillation axis mode becomes unstable. Therefore, a phase control type DFB-LD is being developed as a method for operating in a stable state where DFB-LD=i. For example, [Kitamura et al.'s paper "DFB-D C-PBH containing a phase control mechanism" shown in Proceedings of the National Conference of the Institute of Electronics and Communication Engineers in 1988 J1024.
See LDJ.

FIG. 2 is a diagram showing the element structure of a phase control type DFB-LD. This phase control type DFB-LDx is equipped with a DFB @ region 2 and a phase control region 3, and by controlling the current injected into the phase control region 3, υ isometrically The aim is to fully control the phase conditions of the grating and to stably oscillate the LD in a single-axis mode with a narrow spectrum width.

(Problems to be Solved by the Invention) In this phase control type DFB-LD, depending on the magnitude of the current injected into the phase control region, the LD may stably oscillate in a single axis mode, but it may oscillate stably in a biaxial mode. In some cases, stable oscillation occurs. Therefore, in order to operate the LD stably, the magnitude of the current injected into the phase control region must always be controlled to a value that maintains stable single-axis mode oscillation of the LD.

An object of the present invention is to provide a complete method for controlling phase-controlled DFB-LD odor flow.

(Means for Solving the Problems) The method for controlling the spectral width of a ten-dimensional laser device according to the present invention has a configuration in which a laser region having a gold film on a diffraction grating along an active layer and light optically coupled to the active layer. An integrated semiconductor laser device consisting of a phase control region with a gold guide layer is used as a light source, and the spectral width of this laser device is determined from the change in the output light intensity of this laser device with respect to the change in the current injected into the phase control region of this laser device. The spectral width of this laser element is kept at a minimum value by controlling the current injected into the phase control region.

(Principle of the Invention) Fig. 3 shows a phase control type DFB-L in which a high reflection gold coating film 21t is formed on the reflective end face of the phase control region 30 used in the present invention, and a non-reflection coating film 20 is entirely formed on the end face of the DFB region 2.
It is a perspective view of the element structure of D. The present invention will be explained in detail by taking this end face high reflection type phase control type DFB-LD as an example.

FIG. 4 is a ratio characteristic diagram showing changes in threshold gain depending on the phase conditions of the reflective end face of this phase-controlled DFB-LD. When the current injected into the phase control region 3 is changed, the phase of the reflective end face changes isometrically, the threshold gain changes along the characteristic line in Figure 4, and the mode changes in the broken line area in the figure. Causes jump. The point where this threshold gain is minimum, that is, the isometric antigain ratio is maximum and the oscillation spectrum width is also minimum.

FIG. 5 is a diagram showing the phase control current versus light output characteristic of this phase control DFB-LD. In this example, the active layer remains in the phase control region 3, and therefore the optical output increases as the phase control current increases. Furthermore, a kink is seen in the optical output corresponding to the mode jump shown in FIG. The point indicated by a black circle in FIG. 5 is the point where the oscillation spectrum width is minimum, and the oscillation spectrum width takes a very large value before and after the mode jump. Therefore, by controlling the phase control current to a midpoint between mode jumps, that is, near this black circle point, the oscillation spectrum width can always be kept at a small value.

As an example of this method, the following method can be considered.

As shown in FIG. 5, the optical output shows a kink corresponding to mode skipping, but the phase control current changes linearly. Therefore, consider taking the difference depending on the level of the optical output monitor and the injection current monitor. FIG. 6 is a ratio characteristic diagram showing the difference output between the two. By appropriately adjusting the levels of the optical output monitor and the injection current monitor, the difference output between the two can be made into an error signal whose output becomes zero at the minimum spectral width. By feeding back the error signal obtained here to the current injected into the phase control region, the oscillation spectrum @ of the phase control type DFB-LD can always be kept at a minimum.

(Embodiment) FIG. 1 is a block diagram illustrating a first embodiment of the present invention. DFB area 29 of high reflection type phase control DFB-LD1 with gold coating 21 fully applied to the phase control head mounting end face
A bias 1E current is applied to the phase control region 3 by first and second bias circuits 4 and 5, respectively. A part of the output light 6 of this phase control DFB-LDI is transmitted to a half mirror 7.
The light intensity is detected by a monitoring photodetector 8. ``Furthermore, the magnitude of the current injected from the second bias circuit 5 into the phase control region 3 is also detected by the monitor circuit 9.

When these two detection outputs are inputted into the differential amplifier circuit 10 after matching their respective levels, an error signal 11 is obtained which has a zero output at the minimum point of the spectrum width. By feeding back this error signal 11 to the second bias circuit 5, the injection current to the phase control region 3 is fully controlled, and the phase control DF
The spectral width of B-LDI is always kept at the minimum point.

In this embodiment, the stable point of the current injected into the phase control region 3 is always the minimum point of the spectral width, and even when the current injected into the phase control region 3 is changed for wavelength tuning etc. was kept at a minimum point.

FIG. 7 is a block diagram of a second embodiment of the present invention, and FIG. 8 is a diagram showing the element structure of a phase-controlled DFB-LD with high reflection on the end face used in this embodiment. Phase control of this embodiment I)F
An active layer is not formed in the phase control region 3 of the B-LD 1, and it is made only of a waveguide layer. Therefore, the current injected into the phase control region 3 only changes the equivalent end facet phase, and the optical output does not increase due to an increase in the injected current, and the change in the optical output corresponds to the state of the end facet phase. There is only change.

FIG. 9 is a characteristic diagram of the current injected into the phase control region 3 of the phase control DFB-LD1 of this embodiment versus the light output.

In the figure, the point where the optical output becomes extremely thick corresponds to the point where the threshold gain becomes minimum, and the oscillation spectrum width becomes almost minimum at this point.

In this embodiment, the entire spectral width is kept at a minimum state by the following method. The current injected into the phase control region 3 is:
In addition to the bias current from the second bias circuit 5, a modulation signal from the oscillator 12 is included, and the phase control DFB-LDI
This means that the optical output is minutely modulated. For example, when the injection current is biased at point A in FIG. 9, the output light 6 is also modulated in phase with the modulation signal from the oscillator 12.

On the other hand, when the injected current is biased at point C in FIG. 9, the output light 6 is modulated in phase opposite to the modulation signal from the oscillator 12. Furthermore, when the injected current is biased at point B in FIG. 9, the output light 6 fluctuates at a frequency that is twice that of the modulation signal. The fluctuation of this output light 6 is detected by a photodetector 8, and the detection signal and the modulation signal from the oscillator 12 are synchronously detected by the Wonokuin amplifier 13 to generate the control signal 1.
4 is obtained. By controlling the bias current from the second bias circuit 5 using this control signal 14.

The optical output 6 of the phase control DFB-LDI is always kept at the maximum point, and at the same time the spectral width is always kept narrow. The other configurations are similar to the first embodiment.

In this example as well, even if the operating conditions of the phase-controlled DFB-LDI changed, the oscillation spectrum width could always be kept at a narrow value.

In addition to the embodiments described above, various modifications of the present invention are possible. For example, as a phase control type DFB-LD, there is also a phase control type DFB-L with both ends open.
It is also possible to use D, etc., and the control method may be determined according to the optical output characteristics and spectral width characteristics of each.

Furthermore, in the second embodiment, the control signal 14'1DF
The end face phase may be equivalently controlled by feeding back the current injected into the B region 2 and changing the oscillation wavelength of the DPB-LD.

(Effects of the Invention) As explained above, according to the present invention, the output of the DFB-LD can always be kept in a stable state of single-axis mode oscillation, and the spectral width at that time can also be kept at a narrow value. I can do it.

[Brief explanation of drawings]

FIG. 1 is a block diagram completely explaining the first embodiment of the present invention;
Fig. 2 is a perspective view showing the element structure of a general phase control type DFB-LD, and Fig. 3 is a phase control type DFB-LD in which a gold high-reflection coating film is provided on the reflective end face of the phase control region. ! A perspective view of the il control type DFB-LD, Figure 4 is a characteristic diagram showing the change in threshold gain of the end face high reflection type phase control DI"B-LD depending on the phase condition of the reflective end face, and Figure 5 is a characteristic diagram showing the change in the threshold gain of the end face high reflection type phase control DI"B-LD depending on the phase condition of the reflective end face. Reflective phase control DFB-LD
FIG. 6 is a characteristic diagram showing the difference output between the optical output and the injection current level, FIG. 7 is a block diagram of the second embodiment of the present invention, and FIG. FIG. 9 is a perspective view of the end face high reflection type phase control DFB-LD of the second embodiment, and FIG. 9 is the end face high reflection type phase control DFB-LD used in the second embodiment.
FIG. 3 is an injection current vs. optical output characteristic diagram. 1... End face high reflection type phase control DFB-LD, 2
．． ...DFB area, 3... Phase control area, 4
, 5...Bias circuit, 7...Half mirror 18...Photodetector, 9...Monitor circuit, 10...Differential amplifier circuit , 12...
Oscillator, 13...Lock-in amplifier. Agent Patent Attorney Susumu Uchihara Is he blind?

Claims (1)

[Claims] An integrated semiconductor laser device consisting of a laser region provided with a diffraction grating along an active layer and a phase control region having a light guide layer optically coupled to the active layer is used as a light source, and the phase control of this laser device is performed. Information about the spectral width of this laser element is obtained from changes in the output light intensity of this laser element with respect to changes in the current injected into the phase control region, and the spectral width of this laser element is minimized by controlling the current injected into the phase control region. A method for controlling the spectral width of a semiconductor laser device, characterized by maintaining the spectral width at a certain value.