JPS5810882A - Distributed feedback type semiconductor laser - Google Patents

Abstract

PURPOSE:To obtain stable single longitudinal mode oscillation, by the constitution wherein an optical waveguide is intersected with light emitting surfaces at an angle other than a right angle, and suppressing the oscillation in a Fabry- Perot resonating mode. CONSTITUTION:Grooves are formed in an N type InP substrate 11 in a stripe shape. On one surface of both sides of the groove, a diffraction grating made of periodic concave and convex parts is formed so as to cross the direction of the groove at a right angle. On said grating, an N type layer 12, a nondoped P1-z layer 13, a P type layer 14, a P type InP layer 15, and an SiO2 film 16 are provided. Said layers are held between metal electrodes 17 and 18. In this laser, the grooves formed on the substrate and the waveguide 19 are intersected with the light emitting surface 20 and 21 at an angle other than the right angle. In this constitution, the oscillation threshold value in the Fabry-Perot resonating mode is increased, and the operation in the Fabry-Perot resonating mode when the operation is performed in the distributed feedback oscillating mode can be suppressed.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an improvement in a distributed feedback semiconductor laser that oscillates in a single longitudinal mode.

Large capacity semiconductor lasers, especially optical fibers.

When used as a light source for long-distance optical transmission, it is desirable that a semiconductor laser oscillate in a fundamental transverse mode and a single longitudinal mode. Many proposals have been made regarding structures for fundamental transverse mode oscillation, and stable transverse mode oscillation has been practically obtained. For example, buried heterostructures in which the active region is surrounded by another material with a low refractive index, grooves formed in the substrate to change the thickness distribution of the active layer or layers adjacent to the active layer,
A structure in which the active layer has an equivalent refractive index distribution to form an optical waveguide (e.g. plano-convex waveguide structure, channelAds
ubatrate pIAnar structure, etc.). For these transverse mode controlled lasers, e.g.
Applied Physics” Volume 48, No. 5, pp. 466-470 (19
79), or by G. H. B. Thompson” p.
hyaica of Sem1conductorIa
ser Devices”, John VAtay
or 5ona Publishing (1980), pp. 289-304.

On the other hand, sufficient stability has not yet been achieved in unifying the longitudinal mode. What is considered to be the best conventional method is to form a diffraction grating consisting of periodic irregularities in the light propagation direction in the active region or in the vicinity of the active region, and to couple light with this, the pitch of the diffraction grating can be adjusted. It has a structure that oscillates at a constant wavelength determined by the light propagation speed inside the optical waveguide.

This structure is roughly divided into two types: a distributed Bragg reflector structure that uses a diffraction grating as a reflector that reflects only light of a specific wavelength, and a distributed feedback structure that attempts to obtain feedback for light of a specific wavelength. Ru. The present invention relates to the latter type of distributed feedback laser.

In order to unify the longitudinal mode in this distributed feedback laser, it is necessary to devise a method to prevent oscillation at wavelengths other than those determined by the diffraction grating.

Here, an example of a conventional distributed feedback laser is InGaAs.
This will be explained using a P-based laser. Figure 1.

FIG. 2 is a perspective view and a plan view showing an example of a conventional distributed feedback laser, and FIG. 3 is a sectional view taken along line AA' in FIG. In the figure, 1 is an n-type InP substrate with periodic unevenness (diffraction grating), 2 is non-doped In1-, Ga, As8P.
1-, layer (0,42s≦r≦Q, 50s, and 0 (s
≦1), 3 is p-type InP layer, 4 is n-type Int-p Ga
p A8qP1-4 layer (0,42q≦p≦0.50q
, and 0 (q≦1), 5° 6 is a metal electrode, 7 is a p-type impurity diffusion region, and 8 and 9 are end faces formed by the interbones of the wafer.

In this structure, when a positive voltage is applied to the electrode 5 and a negative voltage is applied to the electrode 6, the current flows concentrated in the region where the impurity is diffused, and the In1 r Gar A! ls Pl
The s layer 2 becomes a light emitting region.

This distributed feedback laser attempts to obtain single-longitudinal mode oscillation by utilizing the fact that a diffraction grating provided in a waveguide that serves as a light emitting region selectively reflects only light of a specific wavelength. be. That is, when light is incident parallel to the lattice plane, the lattice spacing is A.

When the wavelength in the waveguide is λ, A=, λp ・・・・・・・・・・・・
(1) Only the same wavelength is reflected (this is called Bragg reflection). This reflected light causes a feedback effect and causes laser oscillation at that wavelength.

However, if the p-type impurity diffusion region 7 is formed over the entire area between the end faces 8 and 9, unnecessary light having a wavelength that is not reflected by the diffraction grating will be repeatedly reflected between the end faces 8 and 9. This forms a so-called Fabry-Perot resonator, which oscillates at wavelengths other than those determined by the diffraction grating.

Conventionally, the following measures have been taken to suppress the oscillation due to the above-mentioned Fapry-Perot resonance mode. One of them is the end faces 8 and 9, as shown in Fig. 1, Fig. 2, and Fig. 3.
A region 10 in which impurities are not diffused is formed in between. By doing this, no current flows through the partial region 10 of the In1-rGarAsBP+-B layer 2, and light of wavelengths that are not reflected by the diffraction grating is attenuated in this region 10, and is not reflected by the end face 9 or is reflected. It is possible to prevent oscillation due to the Fabry-Perot resonance mode by receiving attenuation to such an extent that no feedback effect occurs even when the oscillation occurs. In another example, the surface facing the light exit surface (for example, if the light exit surface is 8 in FIGS. 1, 2, and 3, the end surface 9) is cut with a wire cutter instead of between the bones. There have also been reports of suppressing oscillation due to the Fabry-Perot resonance mode by roughening the posterior interosseous surface so that it does not function as a mirror surface for reflecting light.

However, with the former structure in which a light absorption region is formed, it is difficult to distinguish between the light emitting region and the light absorption region when cutting out the laser chip, resulting in a lack of workability. There are drawbacks such as a decrease in the number of devices that can be obtained from a single wafer, resulting in poor yield. Furthermore, the latter structure in which the end face is roughened may introduce lattice defects from this face, and thus it is not appropriate to use this technique for high reliability devices. One object of the present invention is to provide a distributed feedback semiconductor laser having a stripe structure capable of obtaining stable single-longitudinal mode oscillation in order to solve the above-mentioned drawbacks of the prior art. The structure is such that the light exit surface intersects at an angle other than perpendicular to suppress oscillation due to the 7 appli-Perot resonance mode.

The present invention will be explained in detail below.

Although the present invention can be applied to any distributed feedback semiconductor laser that oscillates in the transverse fundamental mode, the description will focus on a heterostructure distributed feedback laser using a grooved substrate on which a diffraction grating is formed. This will be explained by giving examples. Fourth
5, 6, and 7 are views showing embodiments of the present invention, in which FIG. 4 is a perspective view, FIG. 5 is a plan view, and FIG.
7 is a cross-sectional view taken along the line C-C'' in FIG. A diffraction grating made of periodic unevenness is formed on both sides of the groove in an InP type substrate so as to be perpendicular to the direction of the groove.

12 is an n-type In l-7GazA8y Pl-y layer (0
,42y≦X≦o, soy, 0<y<1),
13 is a non-doped In1-wGawAs2p, -z layer (
0,42z≦W≦0.50z, and 0 (z layer 1),
14 is a p-type In1 uGauAsyPty layer (0,4
2v≦U≦Q, 50v, 0 (v(1), 15 is p
16 is a 5i02 film, and 17 and 18 are metal electrodes. Layer 14 is an active layer, and has a forbidden band width smaller than that of layers 13 and 15. A 5i02 film is removed in a stripe pattern directly above the groove of the substrate, so that current can flow. It has become. In this laser, the grooves formed in the substrate 11 create a difference in effective refractive index between the inside and outside of the stripe, so that it functions as an optical waveguide.

The feature of the present invention is that the optical waveguide 19 and the light exit surface 20°2
1 intersects at an angle other than perpendicular. In other words, in a typical Fabry-Perot laser, the optical waveguide and the light exit surface are formed so as to intersect perpendicularly, and a part of the light is reflected at the exit surface, thereby forming a pair of opposing optical resonators. Ta. However, if the optical waveguide and the light exit surface are made to intersect at an angle other than perpendicular as shown in FIG. 5, the effective reflectance at this end surface changes. - As an example, wavelength 1.55
An example of an InGaAsP laser that oscillates at μm will be explained. Assuming that the width of the optical waveguide is 3 μm and the equal refractive index of the inner and outer sides of the optical waveguide is 3.33:3.32, respectively,
This laser satisfies the transverse fundamental mode condition. The light reflected by the light exit surface is converted into not only fundamental mode light but also higher-order mode light, but since this optical waveguide satisfies the condition that only the transverse fundamental mode propagates, only the fundamental mode is converted. All you have to do is consider the reflectance.

Calculation of the fundamental mode reflectance when the reflecting surface is tilted is described, for example, in Proceedings of the 1981 National Conference of the Institute of Electronics and Communication Engineers, 870.

That is, as shown in Fig. 8, the light propagating in the -2 direction along the Z-axis in the transverse fundamental mode is expressed as uo((trans)ej/l!...
・・・・・・・・・・・・・・・(2), and the light after being reflected by the light exit surface is aOuQ (x)e−'β8+Σbrvr(x)e ”
” --・(3) Here, β and β are the propagation constants of the fundamental mode and higher-order mode.If the reflectance with respect to the vertical end face is rQ, then the angle θ is tilted as shown in Figure 8. The light in equation (2) immediately after reflection on the surface is z=Q and r
o−uoω, ej2β°−“)−x ・・・・・・
It can be expressed as (4). (3)
,, From equation (4), when z=Q, rouo(kaj・2β
・(-〇)・x= ao uo (x)+Σbr vf
i (5) is derived and rearranged using the orthogonality of the fundamental mode and higher-order mode, the relative reflectance R/Ro for the fundamental mode is R/Ro = (: (/J(x)am(2β( 1=-θ
)x)dx)2+ (/J(dgin(2β(-〇)
x) dx) 21)/Cfu revision x) dx]
It is given by (6).

For the example of the semiconductor laser described above, Figure 9 shows the relative reflectance for the transverse fundamental mode as a function of the inclination angle θ as a result of calculation based on equation (6). The time when the optical waveguide intersects perpendicularly is θ=
0, and the reflectance is normalized to the value when θ=0. From this figure, the angle θ is the relative reflectance of 1%. Take a larger angle of inclination than For example, if θ=5°, the reflectance for the fundamental mode is only 0.3% of that when the optical waveguide and the light exit surface intersect perpendicularly. In this way, the oscillation threshold of the Fabry-Perot resonance mode increases and can be made sufficiently larger than the threshold of laser oscillation using Bragg reflection by the diffraction grating, and the Fabry-Perot resonance mode increases when operating in the distributed feedback oscillation mode. It is possible to prevent it from operating in oscillation mode. This angle θ needs to be smaller than the total reflection angle, but especially when it is made equal to the Brewster angle θ8, the T
There is also the advantage that the reflection of E waves becomes zero, and the light inside the laser can be extracted effectively. In addition, since this structure has two light emitting surfaces, it becomes easy to extract the optical output for output monitoring, which was difficult with conventional distributed feedback lasers.

The above explanation was about a laser formed on a grooved substrate.
The reflectivity for the fundamental mode varies depending on the two dimensions of the laser structure. Find the reflectance for the transverse fundamental mode for each laser, and find the angle θ at which the reflectance is 1%. If the angle of inclination is larger, it can be applied to any type of laser structure.

Next, the manufacturing method will be briefly explained. The (110) direction or (1
10) Grooves are formed so as to form an angle θ with the direction, and layers 12, 13, 1 are formed on this substrate by epitaxial growth.
4, 15 are grown sequentially, then KCVD method Nyori 5i0
Two films 16 are formed. Next, using photo-etching technology, the 5i02 film in the portion directly above the groove of the substrate 11 is removed in stripes. Thereafter, the substrate side is polished to a wafer thickness of about 100 μm, metal electrodes 17 and 18 are formed, and then a laser is produced by interosseous cutting.

In this embodiment, a (001) plane is used, but a substrate having another plane index such as a (111) plane or a (111) plane may also be used.

The present invention has been explained above by taking up a laser using an InGaAsP-based grooved substrate, but the present invention can be applied to any waveguide structure for transverse mode control.
It is applicable to lasers using any materials such as As-based, AAGaSbAs-based, InGaAsSb-based, etc.

As explained in detail above, in the distributed feedback laser of the present invention, the optical waveguide and the light output surface are configured to intersect at an angle other than perpendicular to suppress Fabry-Bello resonance, thereby suppressing the output surface due to the inter-bone space. It becomes possible to fabricate the structure, and there is no need to form a light absorption region. For this reason, it has the advantages of better workability compared to conventional ones, as well as the ability to extract a large number of laser chimps from a single wafer, and the ability to easily extract optical output for output monitoring. Its industrial value is extremely poor.

[Brief explanation of the drawing]

Figures 1 and 2 are a perspective view and a plan view of a conventional distributed feedback laser, Figure 3 is a sectional view taken along line AA' in Figure 2, Figures 4 and 5.
The figures are a perspective view and a plan view of an embodiment of the present invention, FIGS. 6 and 7 are cross-sectional views taken along lines B-B' and C-C' in FIG. FIG. 9, which is a schematic diagram for explaining a calculation example, is a characteristic diagram showing the relative reflectance for the transverse fundamental mode when the inclination angle of the optical waveguide is changed. 1.=n-type InP substrate, 2...non-doped Inl
, GarAs! P1-8 layer, 3-p-type InP layer, 4-
n-type In1-pGapAsqPl-9 layer, 5,6...
・Metal electrode, 7...p-type impurity diffusion region, 8.9...
・End face along the (110) plane or (ITo) plane, 10...
- Light absorption region, 11... n-type InP substrate, 12-n-type In1 xGaxI'hBy Pl -y layer, 13
...Non-doped Inl-WGawAs2Pt-z layer,
14-p type In1-u GauAsv Pl v layer, 1
5--p-type InP layer, 16 =-5i02 film, 17
, 18... Metal electrode, 19... Optical waveguide, 20,
21...Light exit surface. Patent Applicant: International Telegraph and Telephone Corporation Representative: Inuzuka, 1 external person, 1st mouth wash, 3rd figure, 4th figure, figure 5' (Figure 6 + Figure 7, Figure 18)

Claims (1)

[Claims] A diffraction grating is formed with periodic unevenness in a direction perpendicular to the optical waveguide, and the optical waveguide and the light exit surface are configured to intersect at an angle other than perpendicular so that the light exit surface does not form a 77 Preperot resonator. A distributed feedback semiconductor laser characterized by: