JPS60102788A - Distributed feedback semiconductor laser - Google Patents

Abstract

PURPOSE:To enable to realize a stable single-wavelength operation in the titled laser having an active layer and a roughness, which is made to periodically differ the refractive index of laser beams along the proceeding direction of the laser beams, by a method wherein a periodic variation is given to the laser gain by forming the roughness inclusive of the active layer in the structure of the laser. CONSTITUTION:A DFB laser is constituted in a structure, wherein an active layer 3 was parted by the periodic roughness of a diffraction grating 5. In the structure inclusive of the active layer 3 having been parted like this, when carriers at the P-N junction part were injected in the parted active layers 3 having the smallest forbidden band width, the laser gain, which can be seen in the longitudinal direction of the DFB laser, is made to periodically change, and the imaginary component of the refractive index also can be made to periodically change as well. As a result, in the spectrum of the DFB laser with a refractive index, whose imaginary component can be made to periodically change, the central mode coincides with the Bragg wavelength and the threshold values of other modes become ones higher than that of the central mode.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to improvements in distributed feedback semiconductor lasers.

Distributed feedback semiconductor lasers (hereinafter abbreviated as rDFB lasers) can easily operate at a single wavelength even during high-speed modulation.
It is expected to be used as a light source for long-distance, high-capacity optical fiber communications. As shown in FIG. 1(a), in a conventional DFB laser, periodic unevenness (diffraction grating 5) is provided on a waveguide layer 2 adjacent to an active layer 3 on a substrate 1, and the refraction given by the periodic unevenness is The rate change caused distributed light feedback, resulting in laser oscillation. Note that 4 is a buffer layer. At this time, only light having a wavelength near the Bragg wavelength determined by the period of refractive index change and the average value of the refractive index is selectively fed back, so it is easy to obtain single wavelength operation with the DFB laser.

However, the conventional DFB laser as shown in FIG. 1(a) has an end face 6 consisting of a valve opening face in order to effectively take out the output light. It has been theoretically pointed out that there is a possibility of two-wavelength oscillation as shown by the broken line in FIG. 1(b), and it has also been experimentally confirmed.

Therefore, in order to obtain stable single wavelength operation, it is necessary to
) The end face must be formed at a specific position of the diffraction grating 5 where the mode shown by the solid line can be obtained, and considering that the period of the diffraction grating is 2000 to 5 ooo By forming end faces, it is almost impossible to obtain a device that operates at a single wavelength with good reproducibility. It's a size.
Although it is possible to achieve the optimal position of the end face by scraping the end face little by little by etching, etc., this method is not effective as it would damage the element body and electrodes since the chip must be scraped off after the chip is fabricated.

The present invention was made in view of the drawbacks of the prior art described above, and by periodically changing the laser gain,
An object of the present invention is to provide a DFB laser that enables stable single wavelength operation.

This will be explained in detail below using the drawings.

Figure 2 shows an embodiment of the present invention, in the longitudinal direction (direction of light travel).
A cross-sectional view of the is shown. In the same figure, 1 is n-type InP
The substrate 3 is divided by the concavo-convex period of the diffraction grating 5.
nGaAsP active layer, 5 is a diffraction grating 7 is a p-type InP layer divided by 50 periods of the diffraction grating, 8 is p-type InGaA
sP layer or p-type InP layer, 9 is p-type InP layer, ]0
are InGa and AsP layers, 11 and 12 are electrodes, 13.
14 is a buried InP layer, and 16 is a Zn diffusion region. Note that layer 15 has the same composition as p-type InP layer 9 .

As is clear from FIG. 2, in the conventional DFB laser shown in FIG. 1, the single active layer 3 was uniformly flat in the longitudinal direction, whereas in the present invention, the active layer 3 is It has a structure in which layer 3 is divided. In the structure of the active layer 3 divided in this way, carriers at the pn junction are injected into the divided active layer 3 having the smallest forbidden band width, so that the laser gain seen in the longitudinal direction of the DFB laser becomes periodic. The imaginary part of the refractive index can also be changed periodically. Note that DF with periodic changes in the imaginary part of the refractive index
In the spectrum of the B laser, as shown by the solid line in FIG. 3, theory has already clarified that the central mode coincides with the Bragg wavelength, and that the other modes have higher thresholds than the central mode.

Therefore, in the present invention, the active layer 3 is divided by periodic unevenness provided along the direction of light propagation, the laser gain is changed periodically, and the imaginary part of the refractive index changes periodically. The DFB has a stable lifetime at the Bragg wavelength determined by the period of the diffraction grating 5 and the average refractive index.
It is possible to realize a laser. Also, InGaAsP
When a p-type InP layer is used instead of layer 80, there is also a periodic change in the real part of the refractive index within the DFB region, and the spectrum shifts as shown by the broken line in Figure 3, but stable single wavelength operation is not possible. Similarly obtained.

The fabrication of this example is as easy as that of a conventional DFB laser, and if a buried structure is assumed, fabrication can be performed with three crystal growths. That is, in the first growth, the InGaAsP active layer 3 is grown at 500X% on the n-type InP substrate 1.
Ill-type InP layers 7 are sequentially grown to a thickness of 500 to 1000×. Formation of such a uniform thin film is possible using VPE method, MO-C
This can be done by the VD method or the MBE method. next,
When a first-order diffraction grating is used by interference exposure and etching, periodic 2500 i irregularities are formed. At this time, the depth of the diffraction grating 5 is approximately 1000X, and the active layer 3 and the p-type InP layer 7 are separated. In the second growth, a p-type InP layer or an InGaA layer is formed on the diffraction grating 5.
After growing the sP waveguide layer 8 and the p-type InP layer 9, embedding processing is performed, and in the third growth, embedding growth is performed. 5- After growing these, an element is manufactured by performing Zn diffusion in the DFB region to form electrodes. In addition, in this embodiment, even more stable single-wavelength operation can be obtained by making the reflectance of both end faces as close to zero as possible.
-140177) is provided as an example of a non-excitation region.

Next, other embodiments of the present invention will be described.

In the above description, the active layer 3 is divided at the open portions of the diffraction grating for ease of fabrication, but the same effect can be obtained even if the active layer 3 is divided at any point with the same period in the periodic uneven portion.

Furthermore, they do not necessarily have to be completely separated, and although the oscillation threshold changes even if the layer thickness is changed, the spectral characteristics are the same, and stable single wavelength operation is possible. Also, regarding the material, I mentioned InGaAsP system, but
It goes without saying that it can be applied to AtGaAs systems, AtInGaAs systems, etc., and can also be applied to any structure in which transverse mode is controlled.

As described above, according to the present invention, the thickness of the active layer is changed by changing the thickness of the active layer at the same period as that of the diffraction grating.
B laser can be provided. Further, since the DF'B laser according to the present invention can be easily manufactured, it can be realized with good reproducibility and can be used as a light source for long-distance, large-capacity optical fiber communication, and its effects are extremely large.

[Brief explanation of drawings]

FIG. 1(a) is a cross-sectional view of a conventional DFB laser in which a layer adjacent to the active layer is provided with unevenness, and FIG. 1(b) is a cross-sectional view of the DFB laser shown in FIG. 1(a).
A characteristic diagram showing how the spectrum is affected by the position of the end face in an FB laser. FIG. 2 is a cross-sectional view of a DFB laser in which a single active layer is periodically divided as an example according to the present invention. FIG. 3 is a cross-sectional view of a DFB laser according to the present invention. FIG. 2 is a characteristic diagram showing a spectrum of a DFB laser. 1... n-type InP substrate, 2... waveguide layer, 3...
・InGaA, sP active layer, 4... buffer layer, 5.
... Diffraction grating, 6... End face, 7... P-type InP layer,
8-p-type InP or InGaAsP layer, 9...p
type InP cladding layer, 10... InGaAsP cap layer, 11, 1.2... electrode, 13, 14,
1.5...Buried InP layer, ``6...Zn diffusion region. Patent applicant International Telegraph and Telephone Co., Ltd. Agent Otsuka 1 external person

Claims (1)

[Claims] A distributed feedback semiconductor laser having at least an active layer and irregularities that periodically vary the refractive index of light along the traveling direction of light, characterized in that the irregularities are formed including the active layer. Feedback type semiconductor laser.