JPH0821759B2 - Semiconductor laser array - Google Patents

Description

TECHNICAL FIELD The present invention relates to a semiconductor laser light source having high output and sharp directivity. The present invention is suitable for use as a light source for optical communication, optical storage and reproduction, optical calculation and the like.

[Conventional technology]

Semiconductor lasers are small, lightweight, highly reliable, and
Since it can be easily oscillated by current injection, it has already been used in the field of optical transmission and the field of storing and reproducing information using an optical disk. However, in order to extend the range of application to longer-distance optical transmission, writable optical disks, and other fields, it is necessary to improve the output power, which is currently about 10 mV, by one digit or more. As an element for obtaining such a high output power, a phase-locking type semiconductor laser array has been proposed in which a large number of semiconductor lasers are arranged in parallel and periodically, and the light inside thereof is mutually coupled to oscillate. . By using such a semiconductor laser array, a high output and an optical output having a sharp directivity can be obtained.

FIG. 4 is a perspective view of the first conventional semiconductor laser array. This semiconductor laser array is called a refractive index modulation type.

This semiconductor laser array has a layered structure including an n-type cladding layer 1, an active layer 2 and a p-type cladding layer 3.
The p-type cladding layer 3 is provided with parallel and periodic elongated projections. Here, for convenience of description, the direction in which the convex portions are arranged is defined as the x direction, the direction in which the layered structure is formed is defined as the y direction, and the length direction of the convex portions is defined as the z direction.

FIG. 5 shows the refractive index period of this semiconductor laser array.

The convex portion provided on the p-type clad layer 3 changes the refractive index of the region in the vicinity thereof, so that the convex portion has a direction perpendicular to the optical axis, that is, x.
Spatially modulate the refractive index in the direction. This change in the refractive index defines the light emitting region in the active layer 2, and thus defines each laser resonator. The array of convex portions is set so that lights propagating in the respective regions interfere with each other in the x direction and a plurality of laser resonators oscillate in synchronization.

When this semiconductor laser array is operated, the laser resonators oscillate in synchronization with each other and output light in the z direction.
At this time, since the phases of the emitted lights of the laser resonators match each other, the emitted lights of the semiconductor laser array show a sharp directivity.

FIG. 6 shows an end face structure of a second conventional semiconductor laser array. As the distributed feedback type semiconductor laser array, a gain modulation type is known in addition to the above-described refractive index modulation type. The conventional example shown in FIG. 6 is a gain modulation type semiconductor laser array.

The gain modulation type semiconductor laser array has the same layer structure as the refractive index modulation type in that it has the layer structure of the n-type clad layer 1, the active layer 2 and the p-type clad layer 3. The electrodes 6 are arranged at equal intervals and in parallel, not at the convex portions. The gain is modulated by injecting a bias current from the electrode 6 to the active layer 2 formed on the entire surface. Therefore, the electrode 6 defines the light emitting region 4.

In the case of the gain modulation type as well, the far-field pattern becomes bimodal as in the refractive index modulation type.

FIG. 7 shows a far-field image of a conventional refractive index modulation type semiconductor laser array.

Phase-locked semiconductor laser arrays generally exhibit a bimodal far-field pattern. There are several logical interpretations, but as one of them, the logic of lateral distributed feedback semiconductor laser is known by Suemne et al. Of Faculty of Engineering, Hiroshima University. This logic is based on "Room Tempered Operation of a Transverse Destributed Feedback Cavity Laser," Electronics Letters, Vol. 18, 1982, p. 745 ("Room Temperature
operation of a transverse−distributed−feedback c
avity laser ", Electronics Letters, Vol.18, 1982, p.74
5). According to this theory, assuming that the spatial period of the laser resonator is Λ and the wavelength of light oscillated in the laser resonator in vacuum is λ, the angle θ of the emitted beam is
It is given by θ = ± sin ⁻¹ (λ / 2Λ) with respect to the normal line standing on the emitting surface of the semiconductor laser array, that is, the z direction. In this direction, of the wave number vector of light propagating in the laser resonator, the component perpendicular to the optical axis satisfies the Bragg condition due to the periodic refractive index distribution.

[Problems to be solved by the invention]

An object of the present invention is to provide a semiconductor laser array that emits a single beam from an end face.

[Means for solving problems]

The semiconductor laser array of the present invention is a semiconductor laser array comprising a plurality of laser resonators arranged in parallel and periodically on the same semiconductor substrate and interfering with each other to oscillate. Are arranged in a positional relationship in which a phase discontinuity of approximately π / 2 occurs in the period of the array near the center of the array.

[Work]

The theory of Suemne et al. Described above is equivalent to the theory for the oscillation spectrum of a normal distributed feedback semiconductor laser in which the periodicity of the refractive index is provided in the optical axis direction. The difference is whether they appear as a spatial spectrum or a temporal spectrum.

It is known that the spectrum of an ordinary distributed feedback semiconductor laser becomes bimodal when the reflection at the end face of the laser cavity is negligible. In order to make such an oscillation spectrum unimodal, a method of discontinuing the phase of the periodic modulation of the refractive index has been proposed and is being prototyped as an actual device. For example, Utaka et al. Of KDD Research Laboratories "DFB laser with λ / 4-shifted grating", ed.
Near the center of the region, the phase of the diffraction grating is antisymmetric and
It is explained that a distributed feedback semiconductor laser shifted by an amount equivalent to 1/4 oscillates only at the Bragg wavelength. Then, such a λ / 4-shift DFB laser is already in the stage of practical application.

On the other hand, the present invention applies a method of matching the oscillation spectrum with the Bragg wavelength to make it single because of the equivalence of the spatial spectrum and the temporal spectrum, and a single laser perpendicular to the laser end face is applied. To obtain a perfect beam.

The light wave oscillating in the phase-locked semiconductor laser array is
Vertical direction (direction in which light is emitted in the optical axis direction of the laser resonator)
Can be decomposed into a horizontal direction and a vertical direction, and the vertical direction oscillates at a wavelength that satisfies the Fabry-Perot type resonance condition as in a normal cleave laser. On the other hand, the component in the horizontal direction is a kind of DFB in which the stripe groove between the laser cavities serves as a spatial grating.
It satisfies the same oscillation conditions as a laser (distributed feedback semiconductor laser). The oscillation conditions in the horizontal direction given by the stripe grooves at equal intervals give two different frequencies as in the initial DFB laser in which a distributed grating having a uniform spatial frequency in the vertical direction was formed. This corresponds to the appearance of two longitudinal modes having substantially equal threshold values in the conventional DFB laser, and the appearance of two transverse modes, that is, a bimodal emission beam in the phase-locked laser.

A single mode with a DFB laser is λ / 4 as described above.
Achieved by a shift DFB laser. The present invention introduces this principle into the transverse oscillation condition, and the phase of the laser resonator constituting the phase-locked laser is equal to λ / 4 in the central portion, that is, π / in the cycle of the arrangement of the laser resonators. 2
Only, the phase discontinuity is provided. This makes it possible to realize a single peak emission mode.

〔Example〕

FIG. 1 shows a perspective view of a semiconductor laser array according to a first embodiment of the present invention, and FIG. 2 shows an emitting end face.

This semiconductor laser array has a layered structure including an n-type clad layer 1, an active layer 2 and a p-type clad layer 3, as in the conventional example. The p-type cladding layer 3 is provided with elongated protrusions parallel to each other with a period Λ, which differs from the conventional example in that the periodicity of the protrusions is discontinuous in the central portion. Although not shown, the electrodes are front electrodes.

The width of the discontinuity of the convex portion is 1/4 of its period Λ, that is, π /
There are two phases. When the width of the convex portions is equal to the interval between the convex portions, the width of the central convex portion of the array or the distance between the convex portions is increased by 1.5 times.

Here, for the sake of explanation, the same coordinate system as the conventional example is used.
That is, the direction in which the protrusions are arranged is the x direction, the direction in which the layered structure is formed is the y direction, and the length direction of the protrusions is the z direction.

The convex portion provided on the p-type clad layer 3 changes the refractive index of the region in the vicinity thereof, so that the convex portion has a direction perpendicular to the optical axis, that is, x.
Spatially modulate the refractive index in the direction. This change in the refractive index defines the light emitting region 4 in the active layer 2 and thus the individual laser resonator. The array of convex portions is set so that lights propagating in the respective regions interfere with each other in the x direction and a plurality of laser resonators oscillate in synchronization.

Each laser resonator oscillates in synchronism with each other, and the light emitting region 4
To output light in the z direction. Since the phases of the light emitted from the respective laser resonators are the same, the light emitted from the semiconductor laser array exhibits a sharp directivity. At this time, since the periodicity of the laser resonator is deviated by π / 2 phase at the central portion, a single-peaked emitted light can be obtained.

When controlling the oscillation spectrum of a normal distributed feedback semiconductor laser, it is necessary to consider the reflected light from the laser end face. However, in the case of the laser array, since light is absorbed outside the light emitting region 4, it is not necessary to consider reflected light.

FIG. 3 shows an end face of a semiconductor laser device according to a second embodiment of the present invention. In this embodiment, the present invention is applied to a quantum well type semiconductor laser array.

This semiconductor laser array has a layered structure including an n-type clad layer 1, an active layer 2 ′ having a quantum well structure, and a p-type clad layer 3, and is disordered by impurity diffusion to define a light emitting region 4. Area 5 is provided. The disordered regions 5 are formed so that the arrangement period of the light emitting regions 4 is shifted by 1/4 of the period Λ, that is, π / 2 phase, in the central portion of the arrangement.

In this embodiment, the widths of the light emitting regions 4 and their intervals (the width of the disordered region 5) are equal, and the width of the central light emitting region 4'is 1.5 times the width of the other light emitting regions. The present invention can be similarly implemented when the widths of the light emitting regions 4 and the intervals thereof are different, and the present invention can be similarly implemented even when the intervals of the light emitting regions 4 are widened instead of the width of the light emitting regions 4 '.

The present invention can be applied to a gain modulation type semiconductor laser array, but it is difficult to control the beam shape because loss occurs between the light emitting regions 4.

In the above embodiments, the case where the convex portion or the disordered region 5 is provided in the n-type clad layer 1 has been described on the assumption that the p-type substrate is used, but the present invention is the same even when the n-type substrate is used. Can be carried out.

〔The invention's effect〕

As described above, the semiconductor laser array of the present invention can emit a single output beam with high output. INDUSTRIAL APPLICABILITY The semiconductor laser array of the present invention can be used for optical communication including air propagation, writing on an optical disk or a magneto-optical disk, a light source device for an optical arithmetic device, a laser knife for a form, a laser display light source, and the like.

[Brief description of drawings]

FIG. 1 is a perspective view of a semiconductor laser array according to an embodiment of the present invention. Figure 2 is a structural drawing of the end face. FIG. 3 is a structural diagram of an end face of a semiconductor laser array according to a second embodiment of the present invention. FIG. 4 is a perspective view of a first conventional semiconductor laser array. FIG. 5 is a diagram showing a refractive index distribution. FIG. 6 is a structural diagram of an end face of a second conventional semiconductor laser array. FIG. 7 is a view showing a far field image of a conventional refractive index modulation type semiconductor laser array. 1 ... n-type clad layer, 2, 2 '... active layer, 3 ... p
Type cladding layer, 4, 4 '... light emitting region, 5 ... disordered region, 6 ... electrode.

Claims (1)

[Claims] 1. A semiconductor laser array comprising a plurality of laser resonators arranged in parallel and periodically on the same semiconductor substrate and oscillated by mutual interference of light inside each other. The semiconductor laser array is arranged in a positional relationship in which a phase discontinuity of approximately π / 2 occurs in the array period near the center of the array.