JPH0551196B2 - - Google Patents

Description

[Detailed Description of the Invention] <Industrial Application Field> The present invention relates to improvements for stabilizing the oscillation mode of a semiconductor laser and improving the waveform of emitted laser light, and is particularly effective for high-power semiconductor lasers. Regarding.

<Prior Art> In general, a semiconductor laser has an active layer 11 with a refractive index of na, as shown in FIG.
A pair of cladding layers 1 sandwiching the top and bottom, each having a refractive index nc smaller than the refractive index na of the active layer.
Both end surfaces 13, 13 of the active layer 11, which are composed of the active layer 2 and 12 and form an optical resonator, are generally formed by cleaving.

However, when increasing the output of such a semiconductor laser 10, one method for achieving this is to increase the output of the active layer 1.
There is a method of increasing the thickness of the laser beam 1 and increasing the area of the laser emitting end faces 13, 13.

<Problems to be Solved by the Invention> It is true that increasing the laser beam output by expanding the area of the output end faces 13, 13 as described above is one of the simplest methods. It can be evaluated as

However, when such a method is used, high-order standing waves are allowed to exist in the thick active layer 11 in order to enlarge the emission end faces 13, 13, and therefore the emission laser beam is emitted from a far distance. The drawbacks have been that the visual field image becomes multimodal and that the oscillation mode becomes unstable.

That is, for example, in the case of a conventional double heterojunction laser (DH laser), the optical resonance mode in the active layer 11 is incident on the optical resonator or the output end face 13 at an angle of incidence θ, as shown in FIG. can be thought of as the superposition of two incident plane waves W1 and W2 (in this book, "incident angle" is defined as the angle between the surface on which a light wave is incident and the light wave). do).

Therefore, in accordance with the above-mentioned request for higher output, the thickness of the active layer 11 is adjusted to the wavelength (for example, 0.3 μm) in the optical medium.
If you try to make it sufficiently thick compared to the 7th
As schematically shown on the right side of the output end face 13 in the figure, the phase φ of the output light is reversed in positive and negative directions in the x direction along the output end face.

Since the light from this exit surface becomes aligned in phase as a plane wave in an oblique direction, its far-field image is shown schematically at the rightmost end in Figure 7.
Ir becomes multimodal.

In particular, the two illustrated plane waves W1 and W2 have a critical angle at which total reflection occurs between the active layer 11 and the cladding layer 12 (at the GaAs/Al _0.3 Ga _0.7 As interface, the inclination from the interface is approximately 30°). It is possible to exist if the standing wave condition is satisfied in the following range,
When the active layer 11 becomes thicker, the standing wave condition is satisfied at a large number of incident angles, and as a result, the thicker the active layer 11 is made to achieve higher output, the more unstable the oscillation mode becomes.

The present invention was made in order to eliminate the drawbacks that are particularly common in conventional high-output lasers. Basically, even if high output is achieved by thickening the active layer, the laser emission shape The present invention aims to provide a semiconductor laser with a structure that can be expected to improve the oscillation mode and stabilize the oscillation mode.

<Means for solving the problem> As mentioned above, in most of the semiconductor lasers that have been developed so far, the emission end face is formed by cleavage, and is a face that stands perpendicular to the active layer 11. was. As for the optical resonance mode within the active layer, attention has been paid only to the normal (regular) waveguide mode in which direct reflection is repeated between a pair of end faces. However, because of this, when the thickness of the active layer was increased in order to meet the demand for higher output, a problem occurred in which multiple standing numbers were superimposed.

Therefore, in the present invention, unlike in the past, we focus on plane waves that propagate while repeating total reflection at the interface between the active layer and the cladding layer, and select this as the output laser beam to achieve stable oscillation. In order to obtain mode and unimodal perspective images, we propose the following configuration.

First, in a semiconductor laser in which an active layer is sandwiched between a pair of clad layers from above and below, and both end faces of the active layer are used as optical resonators, at least one end face of the active layer is set in the longitudinal direction of the active layer. It is formed obliquely at an angle less than perpendicular to the optical axis along.

However, it is not enough to simply form at least one end face obliquely at an arbitrary angle; among the multiple plane waves that may exist within the active layer, all plane waves are A plane wave that propagates through repeated reflections and is incident perpendicularly to the above-mentioned oblique end face is selectively resonated, and the resonant laser is perpendicular to the above-mentioned oblique end face and oblique to the above-mentioned optical axis. The oblique angle value of the end face is determined so as to emit light.

<Operations and Effects> In the semiconductor laser of the present invention configured as described above, among the plurality of plane waves that may exist in the active layer, the waves are repeatedly reflected at a predetermined angle of incidence at the interface with the cladding layer. Only plane waves that propagate and are incident perpendicularly to the obliquely formed end face of the active layer are allowed to resonate, and other plane waves are absorbed, for example, through the cladding layer.

By the way, the normal waveguide mode (regular waveguide mode) that has been targeted in conventional lasers
On the other hand, the waveguide mode focused on in the present invention, that is, the waveguide mode that propagates while repeating total reflection at a predetermined angle of incidence at the interface between the active layer and the cladding layer, can be called a "pseudo waveguide mode." . In addition,
In the above, the expression "perpendicular to the end surface" includes the case where the end surface is slightly deviated from perpendicular within a reasonable range that is recognized as following the construction principle of the present invention.

Therefore, the phase of the laser beam at the end face of the active layer (output end face) becomes uniform, the oscillation mode becomes stable, and the far-field pattern becomes unimodal.

Furthermore, in contrast to the conventional method, in which cleavage must be used to form a surface that stands perpendicular to the active layer, two-dimensional integration is extremely difficult. When the active layer is intentionally formed obliquely on the end face of the active layer, focusing on the pseudo waveguide mode as in the present invention, the processing accuracy is high, there is little risk of damaging the processed member, and the molded surface Existing wet etching processes can also be used to smooth the surface, thereby facilitating two-dimensional integration of multiple elements on a suitable substrate.

In particular, the semiconductor laser of the present invention has an emitting laser
Since the beam is directed in an oblique direction, even when achieving such two-dimensional integration, the laser beam emitted from one semiconductor laser will not be physically blocked by another adjacent semiconductor laser. The distance between semiconductor lasers can be sufficiently shortened, and high integration can be achieved.

<Embodiment> FIG. 1 shows a principle diagram of the present invention and a basic embodiment of a semiconductor laser 10 constructed in accordance with the present invention. Regarding the reference numerals in the drawings, the same reference numerals are given to those functionally corresponding to those in the conventional semiconductor laser shown in FIG. End face or optical resonator is code 1
Changed to 5.

In the semiconductor laser 10 of the present invention, as only one of them is shown in FIG. It is formed as an oblique surface with an angle θa less than perpendicular to the axis Az along the length direction.

When both end surfaces 15, 15 are both oblique surfaces, the semiconductor laser of the present invention has a trapezoidal shape when viewed as a whole.

In this way, among the plane waves guided in the active layer 11, for example, only the upward plane wave W1 that is incident perpendicularly to the active layer end face 15 satisfies the resonance condition, and the downward plane wave W2 is shown by an imaginary line. As a result, it will be absorbed through the cladding layer 12, or at least the resonance condition will no longer be satisfied.

Therefore, as shown in FIG. 1, the phase φ at the active layer end face 15, that is, the laser emission end face 15, becomes uniform, and the far-field image Ir also becomes a single peak with a peak at the center. Stabilize.

In the end, in the semiconductor laser of the present invention, only a specific plane wave following a quasi-guide mode corresponding to the emission single-plane slope θa can be allowed to resonate under the resonance condition, and therefore it is possible to increase the output power of the semiconductor laser. Even if the active layer is made thicker, single longitudinal mode oscillation may be possible.

However, even in the semiconductor laser 10 of the present invention having an optical resonator formed by the oblique end face 15, if we also consider the waveguide path in the active layer, as shown in FIG. Total reflection mirror 15' due to the boundary between the active layer and the cladding layer,
Since it can be seen as a collection of..., it naturally does not have the function of converging light. Equivalently the second
As shown in FIG. B, the plane mirrors 15, 15 are simply opposed to each other.

Therefore, in order to reduce the loss of the optical resonator, it is necessary to make the luminous flux sufficiently large compared to the wavelength inside the tube, for example, 1μ.
It is desirable to set the thickness to about 3 μm.

However, if necessary, it is effective to curve the single surfaces 15'', 15'' as shown in FIG. 2C, since the light can be converged.

When the active layer end face is formed obliquely as in the present invention, the slope θa is the same as that between the active layer 11 and the cladding layer 12.
is limited by the critical angle θimax at the boundary with

That is, the incident angle θi of the plane wave W1, which satisfies the resonance condition in FIG. 1, with respect to the cladding layer 12 is limited by the maximum value θimax at which total reflection is observed. Of course, if the plane wave W1 is perpendicularly incident on the oblique end surface 15 as in the case shown, θa=90°−θi
It is.

However, for example, when GaAs and Al _0.3 Ga _0.7 As are used as the active layer 11 and cladding layer 12, respectively, the above-mentioned critical angle is about 30°, so the lower limit of the slope θa of the active layer end face 15 is about 70°. However,
Since light is not effectively confined near the critical angle, the practical upper limit of the incident angle θi is about 20°, and therefore the lower limit of the end face inclination θa is about 80°.

However, in the case of the semiconductor laser of the present invention, since the end faces of the active layer are not formed vertically as in the conventional case, there is no necessity of using a cleavage process.

In other words, it is possible to adopt an appropriate etching process. In particular, when using the existing wet etching method, the etching rate of the crystal varies depending on the orientation of the crystal. It can be easily cut out.

This method is not only simple, but also has an extremely low risk of damaging the cut surfaces, so it can significantly improve yield compared to the cleavage method.

Furthermore, the important task of two-dimensional integration of semiconductor lasers on a suitable mechanical support substrate, which could not be achieved using the cleavage method, can be achieved.

That is, as shown in a schematic construction example in FIG. 3, on almost the entire surface of a suitable support substrate 20, a series of clad layers, active layers,
Global laser structure layer 21 consisting of a cladding layer
After forming the semiconductor laser 10, it is possible to perform etching according to a desired pattern to cut out a plurality of individual semiconductor lasers 10, . . . of the present invention.
In such a case, the active layer end faces 15, 15 of these individual semiconductor lasers 10 may be formed according to the idea of the present invention.
It is formed diagonally.

Furthermore, since the distance between adjacent semiconductor lasers 10, 10 can be made sufficiently close to each other in a device having an oblique emission end face 15 as in the present invention, it can be sufficiently increased if two-dimensional integration density is required. Collateral effects also occur.

This is because the output laser beams from each semiconductor laser 10 are emitted obliquely upward as shown in the third figure, so even if the adjacent semiconductor laser 10 is located nearby, its height is This is because it can be easily surpassed.

If necessary, the adjacent semiconductor lasers 10, 10 may be shifted in the direction perpendicular to the plane of the drawing.
It is also possible to prevent the emitted laser beams from intersecting each other in space.

In addition, in the gist of the present invention, only the end face 15 of the active layer 11 is formed diagonally, but when the etching method is actually applied, it goes without saying that the upper and lower cruds are formed diagonally. layer 1
The end faces 2 and 12 are also cut diagonally in series.

Another embodiment of the invention is shown in FIG.

The basic structure is of course the same as that shown in FIG. 1, but the refractive index distribution of the cladding layers 12, 12 has been devised so that it decreases as it moves away from the active layer 11 in the thickness direction. ing.

There is no problem as the technology to do this has already been established in the field of optical waveguides, but as a result,
A function similar to a type of self-locking lens occurs, allowing the angle of incidence of the waveguide defined by the active layer to be increased.

FIG. 5 shows still another embodiment of the present invention, in which the cladding layers 12, 12 are composed of heteromultilayer films.

That is, the cladding layers 12, 12 formed above and below the active layer 11 are alternately laminated and overlapped with 10 to 20 layers each, forming a first layer 12-1 and a second layer 12-2. This embodiment has a multilayer structure.

Specifically, for example, when the active layer 11 is made of GaAs, the first and second layers 12-1 and 12-2 are
AlGaAs/GaAs or Al _0.3 Ga _0.7 As/Al _0.1
It can be Ga _0.9 As, etc. In that case,
If the film thickness of each layer is set so that the wave number component in the thickness direction is about 1/4 wavelength, for example about 100 nm, the phases of the reflected waves in each layer of the multilayer film will match, resulting in a high Reflectance can be obtained at relatively deep angles of incidence.

Figures 6A and 6B show the results of the multilayer film when light is incident from the GaAs active layer into the Al _0.3 Ga _0.7 As/Al _0.1 Ga _0.9 As multilayer cladding layer at angles of 30° and 25° with respect to the interface. It shows the number of superimpositions and reflectance.

When the active layer thickness is 2 μm and the laser length is 100 μm, reflections are repeated about 100 times in one round trip of the optical path, but in the case of a normal DH laser, the reflectance of the cleavage plane is 0.32 and resonance The device loss [1n (1/R)] is
It is 1.14.

The condition for the reflection loss due to the hetero multilayer film to be less than half of the resonator loss due to end face reflection is that the resonator loss for two reflections of the multilayer film is 1/100 or less.

Therefore, when reading the number of superimpositions that satisfy this condition, in the case of 30° incidence in Figure 6A, 15 times (film thickness
4.5μm), and in the case of 25° incidence in Figure 6B,
10 times (film thickness 4 μm).

The imaginary lines in Figure 6 indicate the reflectance when the light beam is incident from the GaAs active layer to the Al _0.3 Ga _0.7 As cladding layer at each incident angle. The reflectances are low at 0.146 and 0.247, respectively, so it can be considered that a satisfactory optical waveguide has been formed, and it can be seen that reflectances that pose no practical problems can only be achieved using a semiconductor hetero multilayer film. .

Of course, as with the excitation mechanism of existing semiconductor lasers, p-type and n-type conductivity types may be selected for the cladding layer, and with regard to such an operation mechanism, according to the present invention, the excitation current density is not reduced. By increasing the thickness of the active layer, it is possible to achieve high output.

[Brief explanation of drawings]

FIG. 1 is a schematic diagram of the principle or basic implementation side of the semiconductor laser of the present invention, FIG. 2 is an explanatory diagram of the optical resonator of the semiconductor laser of the present invention, and FIG. 3 is a diagram of the second embodiment of the semiconductor laser of the present invention. Diagram of dimensional integration,
4 and 5 are schematic configuration diagrams of other embodiments of the present invention, FIG. 6 is a characteristic diagram of the cladding layer of the embodiment shown in FIG. 5, and FIG. 7 is a diagram showing the active layer end face or optical 1 is a schematic configuration diagram of a conventional semiconductor laser that uses a cleavage plane in a resonator. In the figure, 10 is the semiconductor laser as a whole, 11
12 is an active layer, 12 is a cladding layer, 13 is an active layer end face formed vertically by conventional cleavage, and 15 is an active layer end face or optical resonator formed obliquely according to the present invention.

Claims (1)

[Scope of Claims] 1. In a semiconductor laser in which an active layer is sandwiched from above and below by a pair of cladding layers, and both end faces of the active layer are used as optical resonators; at least one end face of the above-mentioned both end faces of the active layer. with respect to the optical axis along the length direction of the active layer,
among the plurality of plane waves that may be present in the active layer;
At the interface with the cladding layer, a plane wave that propagates while repeating total reflection at a predetermined incident angle and is incident perpendicularly to the oblique end face is selectively resonated. A semiconductor laser characterized in that an oblique angle value of the end face is determined so that the resonant laser beam is emitted in a direction oblique to an optical axis. 2. The semiconductor laser according to claim 1, wherein the cladding layer is composed of a semiconductor heteromultilayer film.