JPH0387087A - Semiconductor laser device - Google Patents

Abstract

PURPOSE:To obtain a wide range for changing wavelength by a method wherein, in a distributed feedback type laser having a region in which refractive index is perturbed in the vicinity of an active region along the axial direction of a laser resonator, the magnitude of perturbation in the vicinity of one end or both ends of the resonator is made small as compared with the perturbation of the other parts. CONSTITUTION:A diffraction grating 2 which has a period of 240nm and a quarter wavelength shift structure in the vicinity of the central part is partly formed on the surface of an N-type InP substrate 1. By low pressure vapor growth method, the following are laminated and grown on the grating: an N- type InGaAsP guide layer 3, an InGaAs/InGaAsP multi-quantum well active layer 4 and a P-type InP clad layer 5. An N electrode 6 and P electrodes 71-73 are vapor-deposited, and an antireflection film 10 is formed by sputtering. Thereby a laser device is made to oscillate with a stable longitudinal single mode. The wavelength width is changed to be, e.g. 5.0nm by adjusting currents applied to the electrodes 71-73. FM modulation characteristics which are flat in the range of 1kHz-100GHz are obtained by modulating the current applied to the electrode 72 or 73.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a semiconductor laser device whose oscillation wavelength can be freely changed while keeping the spectral linewidth narrow.

[Conventional technology]

Regarding semiconductor lasers that can freely change the oscillation wavelength while keeping the spectral linewidth narrow, see IEICE Spring National Conference (1989), C-394.
is discussed. In other words, three electrodes are provided on the side near the active region of a long-cavity 174-wavelength-shifted distributed feedback semiconductor laser, and the amount of phase shift is controlled by changing the ratio of current flowing through the center and both ends, thereby changing the oscillation wavelength. The idea is to change freely.

[Problem to be solved by the invention]

In principle, the above conventional technology has a narrow wavelength tuning width of 1 to 2n.
The movable width of m was the limit.

The purpose of the present invention is to maintain a narrow spectral linewidth while
The object of the present invention is to obtain a semiconductor laser device having a wide wavelength tuning range.

[Means for Solving the Problem] The above object is to provide a distributed feedback semiconductor laser device having a region in which the refractive index fluctuates near the active region along the axial direction of the laser resonator. This is achieved by making the magnitude of the vibration near both ends smaller than the magnitude of the vibration in other parts, and by reducing the coupling coefficient near the end face of one or both ends.

[Effect]

In a distributed feedback semiconductor laser device, by making the coupling coefficient near the end face smaller than in other parts, the wavelength interval between the main mode and side mode can be widened, and the stability of single mode oscillation can be further improved. I can do it. Thereby, it is possible to obtain a semiconductor laser device having a wide wavelength tuning range while keeping the spectral linewidth narrow. 7th
The figure shows the calculated axial light intensity distribution of the 4-wavelength shift distribution feedback type semiconductor laser device. In this way, the light intensity of the main mode (zero-order mode) is concentrated near the center of the resonator, and on the contrary, the light intensity of the side modes (±first-order mode) is concentrated near the end faces. Therefore, when the coupling coefficient near the end face is reduced, the coupling coefficient felt by the side mode becomes particularly small, making it difficult for the side mode to oscillate and widening the wavelength interval. As a result, relatively stable main mode oscillation can be obtained. Next, FIG. 8 shows the results of calculating the axial light intensity distribution when optical amplification regions are provided in series in the same waveguide on the right side of the semiconductor laser device. As described above, by providing an optical amplification region connected to a distributed feedback semiconductor laser device, it is possible to increase the optical output emitted from its end face. The above calculation shows the case where all regions are uniformly excited, but
Even in the case of non-uniform excitation, the optical output can be similarly increased by the optical amplification region, so it is possible to change the oscillation wavelength while maintaining a large optical output.

Next, it will be explained with reference to FIG. 9 that excellent longitudinal mode stability is achieved even when the optical amplification region as described above is provided. FIG. 9 shows the results of calculating the ease of oscillation of each longitudinal mode using light wave coupling theory. The horizontal axis indicates the deviation δβ=27c between the Bragg wavelength λB determined by the period of the diffraction grating and the oscillation wavelength λ. The vertical axis indicates the normalized threshold gain αL. Calculations were made for two types: the 174-wavelength shift distributed feedback semiconductor laser device shown in FIG. 7 and the 1/4 wavelength shift distributed feedback semiconductor laser device with an optical amplifier shown in FIG. Both are in oscillation mode, that is, α
The mode with the smallest thL oscillates at the Bragg wavelength.

Furthermore, the normalized threshold gain difference ΔαthL between the above-mentioned oscillation mode and the second mode, which represents the stability of the oscillation mode, is 0.7K in the case without an optical amplifier, but 0.94 in the case with an optical amplifier. The longitudinal mode stability was further improved by adding the above optical amplifier. Furthermore, the wavelength interval between the oscillation mode and the next mode is widened by adding an optical amplifier, and the wavelength variable width is also widened.

Furthermore, by increasing the number of electrodes, the wavelength of the main mode can be freely moved between the two side modes, so the wavelength tuning range can be expanded by an amount corresponding to the widening of the wavelength interval between the two side modes.

〔Example〕

Next, embodiments of the present invention will be described with reference to the drawings. 1st
The figure is a sectional view showing a first embodiment of a semiconductor laser device according to the present invention, FIG. 2 is a sectional view showing a second embodiment of the invention, and FIG. 3 is a sectional view showing a third embodiment of the invention.
FIG. 4 is a sectional view showing the fourth embodiment of the present invention, and FIG.
The figure is a sectional view showing a fifth embodiment of the invention, and FIG. 6 is a block diagram showing an embodiment of the invention.

First Embodiment In FIG. 1 showing the first embodiment of the present invention, n-type InP
A diffraction grating 2 is partially created on the surface of a substrate 1. The period of the above-mentioned diffraction grating 2 is 240 nm, and 1/
It has a four-wavelength shift structure 9. On the diffraction grating 2, an n-type InGaAsP guide layer 3. InGaAs/I
An nGaAsP multi-quantum well active layer 4 and a p-type InP cladding layer 5 were sequentially grown into multiple layers by low pressure vapor deposition. Thereafter, an n-electrode 6 and p-electrodes 71, 72, 73 were formed by vapor deposition, and an antireflection film 10 was applied by sputtering, thereby forming a structure of a semiconductor laser device. The semiconductor laser device oscillates in a stable longitudinal single mode. Then, the three P electrodes 71.
By adjusting the current applied to each of 72 and 73, a wavelength tuning width of 5.0 nm was obtained. Moreover, within the above range, the spectral linewidth could be kept below IMHz. Furthermore, by modulating the current applied to the p-electrode 72 or the electrode 73, it was possible to obtain flat FM modulation characteristics in the frequency band of 1 kHz to 10 GHz.

Second Embodiment Next, a second embodiment of the present invention will be described with reference to FIG. This embodiment differs from the first embodiment in that a shallow diffraction grating 21 is also provided in the region where there was no diffraction grating in the first embodiment. This semiconductor laser device oscillates in a stable longitudinal single mode. and three p-electrodes 71.72.7
A wavelength tuning width of 5 nm was obtained by adjusting the currents applied to each of the above. Furthermore, within this range, the spectral linewidth could be kept below IMHz. In addition, the p electrode 72
Alternatively, by adjusting the current applied to 73, 1k
It was possible to obtain flat FM modulation characteristics in the frequency band from Hz to 10 GHz.

Third Embodiment Next, a third embodiment of the present invention will be described with reference to FIG. The third embodiment differs from the first embodiment in that a diffraction grating 22 is provided near the center of the diffraction grating 2 and has a slightly shorter period than other parts. The semiconductor laser device described above also oscillates in a stable longitudinal single mode. Then, by adjusting the currents applied to the three p-electrodes 71, 72, and 73, a wavelength tuning width of 5 nm was obtained. Also.

Within this range, the spectral linewidth could be kept below IMHz. In addition, by modulating the current applied to the p-electrode 72 or 73, the frequency of 1kHz to 10GHz can be
It was possible to obtain flat FM modulation characteristics in the frequency band of .

Fourth example A fourth embodiment of the present invention will be described using FIG. 4.

This embodiment differs from the first embodiment in that a uniform diffraction grating 2 is used as the diffraction grating, and the antireflection film 10 is covered only on the end face near the area where the diffraction grating is not present. The semiconductor laser device described above also oscillates in a stable longitudinal single mode. Then, by adjusting the currents applied to the three p-electrodes 71, 72, and 73, a wavelength tuning width of 5 nm was obtained. Furthermore, within this range, the spectral linewidth could be kept below IMHz. Furthermore, by modulating the current applied to the p-electrode 72 or 73, flat FM modulation characteristics could be obtained in a frequency band of 1 to 10 GHz.

Fifth Embodiment Next, a fifth embodiment of the present invention will be explained with reference to FIG. are different. This semiconductor laser device oscillated in a stable longitudinal single mode, and by varying the ratio of current applied to the p-electrodes 71 and 72, a wavelength tuning width of 5 nm was obtained. Moreover, within this range, the spectral linewidth could be kept below IM&. Furthermore, by modulating the current applied to the p-electrode 17 or 72, flat FM modulation characteristics could be obtained within the range of 1 kHz to 10 GHz.

Although each of the above embodiments has been described using an InP-based semiconductor laser device as an example, the present invention is also applicable to semiconductor laser devices made of any material.

Furthermore, the present invention is applicable to any semiconductor laser device having any transverse mode control structure.

Sixth Embodiment Next, a sixth embodiment of the present invention will be described with reference to FIG. FIG. 6 is a block diagram showing an optical multiplexing coherent communication system. Light emitted from nine semiconductor laser devices 11, 12, etc. each having a different oscillation wavelength is coupled into one optical fiber, and the receiving side This system uses a variable local light source 4 to select a specific wavelength and demodulate the signal.

In this system, by using the semiconductor laser device of the present invention as the local light source 14, it was possible to perform transmission at 2 Gb/s in 50 channels.

〔Effect of the invention〕

As described above, the semiconductor laser device according to the present invention is a distributed feedback semiconductor laser device having a region in which the refractive index fluctuates near the active region along the axial direction of the laser cavity. Since the magnitude of the vibration in the vicinity is smaller than the magnitude of vibration in other parts, it is possible to obtain a semiconductor laser device with a wide wavelength tunable width while keeping the spectral linewidth narrow. It is useful as a local light source in optical communication systems.

[Brief explanation of drawings]

FIG. 1 is a cross-sectional view showing a first embodiment of a semiconductor laser device according to the present invention, FIG. 2 is a cross-sectional view showing a second embodiment according to the present invention, and FIG. 3 is a cross-sectional view showing a third embodiment according to the present invention. figure,
FIG. 4 is a sectional view showing a fourth embodiment according to the invention, FIG. 5 is a sectional view showing a fifth embodiment according to the invention, FIG. 6 is a block diagram showing a sixth embodiment according to the invention, and FIG. 8 is a diagram showing a detailed explanation of the present invention, FIG. 8 is a diagram showing a light intensity distribution of a quarter-wavelength shift distribution feedback type semiconductor laser device equipped with an optical amplifier, and FIG. FIG. 3 is a diagram showing mode stability. 2... Region where the refractive index fluctuates 9... 174 Wavelength shift structure 10... Non-reflection film 71, 72, 73... P-type electrode

Claims (1)

[Claims] 1. In a distributed feedback semiconductor laser device having a region in which the refractive index fluctuates near the active region along the axial direction of the laser resonator, the vibration near one end or both ends of the resonator A semiconductor laser device characterized in that the size of vibration is smaller than the size of vibration in other parts. 2. The semiconductor laser device according to claim 1, wherein the vicinity of one end or both ends of the resonator is a region in which there is no fluctuation in the refractive index. 3. The semiconductor laser device according to claim 1 or 2, wherein the fluctuation of the refractive index has periodicity. 4. The semiconductor laser according to any one of claims 1 to 3, wherein the fluctuation of the refractive index has one or more regions in which the periodic phase changes discontinuously. Device. 5. The semiconductor laser device according to any one of claims 1 to 4, wherein the fluctuation of the refractive index has two or more regions with different periods. 6. The semiconductor laser device according to claim 1 or 2, wherein the resonator has one or both end surfaces coated with anti-reflection coating. 7. The resonator according to claim 1, 2, or 6, wherein the resonator has two or more electrically isolated electrodes on the side closer to the active region. Semiconductor laser equipment. 8. A coherent communication system using the semiconductor laser device according to any one of claims 1 to 7.