JPH08167759A - Distributed feedback semiconductor laser and hologram scanner using it - Google Patents

Abstract

PURPOSE: To provide a distributed feedback semiconductor laser which is difficult to produce crystal defects, and easy to make the oscillation wavelength coincide with the peak of the gain curve of semiconductor material. CONSTITUTION: A diffraction grating to be a resonator is obtained by forming a semiconductor epitaxial substrate having a substrate 12, a lower clad layer 13, an activated layer 14. an upper clad layer 15, and a contact layer 16, and forming regular recessions 20 from the contact layer 16 over to the clad layer 15. When the combination of the clad layer and the activated layer is InP-In GaAsP, it is suitable for the oscillation of a laser beam of an infrared wavelength band. In the case of InGaAlP-InGaP, it is suitable for the oscillation of a laser beam of a visible red band. Crystal defects are difficult to produce since there are no crystal-grown films on the diffraction grating. It is possible to reduce threshold current, since the interval Λ of the recessions 20 can be set by adjusting it to the gain curve characteristic of an epitaxial substrate whose all layers are laminated.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a distributed feedback semiconductor laser capable of stable longitudinal single mode oscillation, and a hologram scanner using the distributed feedback semiconductor laser in the visible red band.

[0002]

2. Description of the Related Art Application fields of semiconductor lasers include optical memory, optical communication, optical applied measurement, and hologram scanner. For example, in the field of optical communication, a semiconductor laser in the infrared wavelength band is preferable in relation to the waveguiding characteristics of an optical fiber, and in the field of optical applied measurement and hologram scanner, use of a semiconductor laser in the visible red band is preferable. It is supposed to be. As a conventional semiconductor laser, a Fabry-Perot type is generally used. However, the Fabry-Perot type semiconductor laser has the drawback that the oscillation wavelength λ1 varies due to mode hopping and that the oscillation wavelength varies depending on the operating temperature, which makes longitudinal single mode oscillation impossible during high-speed modulation. ing.

Therefore, distributed feedback semiconductor lasers are drawing attention, for example, Y. Itaya et.al., Electron. Lett., Vo.
The contents are disclosed in l.18, No.23 P.1006 (1982). The conventional distributed feedback semiconductor laser has a structure as shown in FIG. In the conventional distributed feedback semiconductor laser shown in FIG. 6, the oscillation wavelength λ1 is in the infrared wavelength band,
The basic structure has an n-type InP (indium-phosphorus) substrate 2 having an electrode 1 such as Au (gold) on the lower surface. An n-type InP clad layer 3 is formed on the substrate 2.
And an InGaAsP (indium-gallium-arsenic-phosphorus) active layer 4, an InGaAsP diffraction grating layer 5, a p-type InP clad layer 6, and an InGaAsP contact layer 7 are laminated. An electrode 8 made of Au or the like is laminated on the upper surface. And
The diffraction grating 9 is formed on the diffraction grating layer 5. Active layer 4
The light in the diffraction grating layer 5 is distributed and fed back by the diffraction grating 9 and resonates to emit a laser beam in a longitudinal single mode. The oscillation wavelength λ1 of the laser light is determined by the period of the diffraction grating 9.

[0004]

In the manufacturing process of this type of semiconductor laser, MOCVD (Metal Organic Chemical) is used.
Vapor Deposition) equipment is used to grow each layer epitaxially. In the conventional example shown in FIG. 6, since the diffraction grating 9 is embedded in the vicinity of the active layer, the cladding layer 6 and the like are crystal-grown on the diffraction grating 9. In the manufacturing method in which the epitaxial crystal is grown on the uneven diffraction grating 9, crystal defects are likely to occur in the layer on the diffraction grating 9, and the reliability and the yield are reduced.

As shown in FIG. 7, in the distributed feedback semiconductor laser, an oscillation wavelength λ1 determined by the peak of the gain curve (a) which depends on the physical properties of the semiconductor material and the thickness of each layer, and the period of the diffraction grating 9. Matching and at the operating temperature is a desirable state for reducing the threshold current that causes oscillation. However, in the structure in which the diffraction grating 9 is embedded as shown in FIG. 6, it is difficult to measure or control the period of the unevenness of the diffraction grating 9 during the crystal growth of each layer, and all layers are stacked. It is only possible to evaluate the peak (a) of the gain curve. Therefore, the relationship between the gain curve characteristics and the oscillation wavelength can be grasped for the first time as a result of the stacking of all layers, and it is difficult to match the peak (a) of the gain curve with the oscillation wavelength λ1 at the operating temperature. Therefore, it is difficult to reduce the threshold current.

In the conventional distributed feedback semiconductor laser described above, the semiconductor material forming the cladding layer is InP and the semiconductor material forming the active layer is InGaAsP. From the relationship with the gain curve resulting from the combination of the semiconductor materials, the oscillation wavelength λ1 is in the infrared wavelength band of about 1.3 μm. The laser light in the infrared wavelength band has excellent waveguiding characteristics in the optical fiber, which is suitable for application to optical communication equipment.

On the other hand, there is a hologram scanner as an application field of a semiconductor laser. In this hologram scanner, a semiconductor laser is diffracted by a hologram disc composed of a diffraction grating, and this hologram disc is rotated by a motor to scan (scan) the diffracted laser light on a photosensitive drum.

However, the photosensitive drum used in the hologram scanner uses a silicon-based photosensitive material. This photosensitive drum is not very sensitive in the infrared wavelength band, but rather is highly sensitive in the visible red band.
However, there is no distributed feedback semiconductor laser in the visible red band at present.

Therefore, a hologram scanner using a Fabry-Perot type semiconductor laser emitting a laser beam in the visible red band has been studied. However, as described above, in the Fabry-Perot type semiconductor laser, the oscillation wavelength fluctuates due to the temperature change, so that the diffraction angle of the laser light by the hologram disk fluctuates, and the laser beam cannot be accurately scanned on the photosensitive drum. Become.

According to the present invention, crystal defects such as a cladding layer due to the formation of regular irregularities (diffraction grating) are less likely to occur, and irregularities (diffraction grating) having a period matched to a gain curve peculiar to a semiconductor are formed. It is an object of the present invention to provide a distributed feedback semiconductor laser capable of reducing the threshold current.

Another object of the present invention is to make it possible to construct a distributed feedback semiconductor laser capable of oscillating a laser beam in the visible red band so that crystal defects hardly occur and the threshold current is low. Further, it is an object of the present invention to provide a hologram scanner having a high scanning accuracy of a laser beam by using this distributed feedback semiconductor laser.

[0012]

A distributed feedback semiconductor laser according to the present invention comprises a substrate, a lower cladding layer, an active layer,
An upper clad layer, a contact layer, and an electrode are laminated, and regular recesses are formed from the contact layer to the upper clad layer or from the contact layer to the upper clad layer and the active layer. The above-mentioned regular recesses or irregularities, that is, the diffraction grating,
In the portion of the insulating layer where the window is not formed, for example, the unevenness is formed so as to be arranged in one row on one side of the window of the insulating layer, or the unevenness is arranged on both sides of the window of the insulating layer.

When the oscillation wavelength is in the infrared wavelength band, the semiconductor material forming the cladding layer is InP, the semiconductor material forming the active layer is InGaAsP, and when the oscillation wavelength is in the visible red band, the cladding layer is formed. The semiconductor material used is InGaAlP, and the semiconductor material forming the active layer is In
It becomes GaP.

The oscillation wavelength is determined by the arrangement period of the recesses, and the arrangement period is determined at the point where the peak of the gain curve due to each semiconductor material and the oscillation wavelength coincide.

The semiconductor materials are InP and InGaAsP.
In the case of the combination of the above, it is preferable that d / Λ is 0.3 or more, where the forming period of the recesses is Λ and the depth of the recesses is d, and the semiconductor material is InGaAlP and InGa.
In the case of a combination of P, d / Λ is preferably 1.7 or more, where Λ is the formation period of the recesses and d is the depth of the recesses.

Although the recess may be formed up to the active layer as described above, 0.01 is provided between the bottom of the recess and the active layer.
It is preferable to leave a distance of μm or more.

Since the combination of InGaAlP and InGaP has an oscillation wavelength in the visible red band, a hologram scanner using this distributed feedback semiconductor laser and a hologram disk for scanning the laser beam emitted from this semiconductor laser. Can be configured.

[0018]

In the distributed feedback semiconductor laser of the present invention, regular concave portions are formed in the contact layer and upper cladding layer to form a diffraction grating, or regular concave portions are formed in the contact layer, upper cladding layer and active layer. And the diffraction grating is formed. Therefore, since there is no epitaxially grown clad layer or the like on the diffraction grating, crystal defects do not occur in the layer on the diffraction grating as in the conventional case, and reliability and yield are improved.

Further, after the lamination of the lower clad layer, the active layer, the upper clad layer and the contact layer is completed, the regular concave portions are processed by, for example, dry etching or a combination of dry etching and wet etching.
Therefore, when the lamination of the lower clad layer, the active layer, the upper clad layer and the contact layer is completed, the semiconductor epitaxial substrate can be evaluated, that is, the gain curve unique to the completed semiconductor epitaxial substrate can be evaluated. Therefore, it is easy to match the oscillation wavelength determined by the regular period of the concave portion serving as the diffraction grating with the peak of the gain curve, and a semiconductor laser with a low threshold current can be obtained.

Further, in this distributed feedback semiconductor laser, by forming the cladding layer of InGaAlP and the active layer of InGaP, the oscillation wavelength can be in the visible red band. Using this visible red band distributed feedback semiconductor laser and hologram disk,
By configuring the hologram scanner, it is possible to use vertical single mode laser light in which the oscillation wavelength does not fluctuate due to the mode hopping of the semiconductor laser, and the fluctuation of the diffraction angle on the hologram disc does not occur. Further, when the photosensitive drum is scanned by the laser beam diffracted by the hologram disk, since it is light in the visible red band, the wavelength sensitivity characteristic of the photosensitive drum made of a silicon material is satisfied, and the photosensitive drum is exposed with high sensitivity. be able to.

In the distributed feedback laser, a resonator is formed by a diffraction grating formed close to the active layer, and the strength of this resonance action (feedback action) is affected by the shape of the diffraction grating. A recess formed in the contact layer and the upper cladding layer,
Alternatively, when the depth of the recesses formed in the contact layer, the upper clad layer and the active layer is d, and the array period of the recesses is Λ, d / Λ is optimal to obtain sufficient resonance (feedback). It is necessary to set the relationship. The semiconductor material is In
In the case of oscillating in the infrared wavelength band by the combination of p and InGaAsP, d / Λ is preferably 0.3 or more, more preferably 0.5 or more. When the semiconductor material is a combination of InGaAlP and InGaP and oscillates in the visible red band, d / Λ is preferably 1.7 or more, and more preferably 2 or more.

Further, the regular concave portions forming the diffraction grating may extend to the active layer. However, in the method of etching regular recesses, for example, in the dry etching method, when the regular recesses are formed, a work-affected layer is formed in the active layer in the portion of the insulating layer under the window where the recess is not formed. May be formed. Therefore, it is preferable to leave a distance of at least 0.01 μm or more between the bottom of the recess and the active layer.

[0023]

Embodiments of the present invention will be described below. 1 is a partially broken perspective view showing a schematic structure of a distributed feedback semiconductor laser of the present invention, and FIG. 2 is a sectional view taken along line II-II of FIG. The distributed feedback semiconductor laser shown in FIG.
An electrode 11 is formed on the lower surface of the. A lower clad layer 13, an active layer 14, and an upper clad layer 15 are laminated on the substrate 12, and a contact layer 16 is formed on the entire upper surface of the upper clad layer 15. Further contact layer 1
6 has an insulating layer 17 formed thereon.
A window 19 of an insulating layer having a predetermined width is opened at the center of. An electrode 18 is formed on the upper surface of the insulating layer 17 and the upper surface of the contact layer 16 in the window 19 of the insulating layer.
Oscillation occurs in the active layer 14 within the width dimension of the window 19 of the insulating layer. This oscillation region, that is, the resonance (feedback) region is indicated by the symbol (c).

The semiconductor epitaxial substrate composed of the above layers is manufactured by epitaxial crystal growth using a MOCVD apparatus or the like. In this semiconductor epitaxial substrate, regular recesses 20, 20, ... Are formed from the contact layer 16 to the cladding layer 15.
The middle of the concave portions 20, 20, ... Is the relative convex portions 21, 21,
, And the concave portions 20, 20, ... And the convex portions 21, 21 ,. This diffraction grating is formed in a portion where the insulating layer window 19 is not formed (a portion other than the oscillation region) and in the vicinity of the active layer 14. In the embodiment of FIG. 1, the insulating layer window 19 is formed. Diffraction gratings are formed in two rows on both sides. However, the diffraction grating may be formed in only one row on one side of the window 19 of the insulating layer.

In the embodiment shown in FIGS. 1 and 2, the recess 2
0, 20, ... Are the contact layer 16 and the upper cladding layer 1
5 and the bottom of the recess is separated from the active layer 14, but the bottom of the recess may extend into the active layer 14. The recesses 20, 20, ... Are processed by dry etching or a combination of dry etching and wet etching on the epitaxial substrate on which the lamination is completed. In the example shown in FIG. 2, the concave portion 20 and the convex portion 21 have a rectangular cross section, but the boundary line between the concave portion 20 and the convex portion 21 is continuous like a wave like the diffraction grating 9 shown in FIG. It may have a shape.

AC driving power of a predetermined frequency is applied to the electrode 1 on the lower surface.
1 and electrodes 18 are provided. In the oscillation region (c) of the active layer 14, the diffraction grating composed of the concave portions 20 and the convex portions 21 serves as a resonator to cause resonance (feedback), and longitudinal single mode laser light having a resonance wavelength λ1 is emitted. 1 and 2
In, a waveguide layer may be formed between the active layer 14 and the upper cladding layer 15.

(First Embodiment) When the oscillation wavelength λ1 is 1.3 μm in the infrared wavelength band, the substrate 12 is an n-type InP substrate.
(Indium-phosphorus), the lower cladding layer 13 is an n-type I
nP, the active layer 14 is InGaAsP (indium-gallium-arsenic-phosphorus), and the upper cladding layer 15 is p-type In.
P. The contact layer is p-type InGaAsP, the electrodes 11 and 18 are mainly made of Au, and the underlying layer is provided on this.

The oscillation wavelength λ1 is determined by the period Λ of the diffraction grating (the concave portion 20 and the convex portion 21). In addition, the semiconductor material In
Gain curve peak (a) of a semiconductor epitaxial substrate in which P and InGaAsP are combined (see FIG. 7)
Corresponds to the oscillation wavelength λ1 = 1.3 μm. Here, the relationship between the oscillation wavelength λ1 and the period Λ of the diffraction grating is expressed by the following mathematical expression 1. In Equation 1, N is an equivalent refractive index determined by the refractive index of the semiconductor material, and m is an order and is a natural number.

[0029]

[Equation 1]

In the first embodiment, the equivalent refractive index N is 3.
36, the oscillation wavelength λ1 is 1.3 μm. When the order m is 1, Λ is 0.19 μm. Therefore, the period Λ of the regular concave portions 20 (diffraction grating) in the first embodiment is 0.19.
μm.

(Second Embodiment) When the oscillation wavelength λ1 is 0.68 μm in the visible red band, the substrate 12 is an n-type G
aAs (gallium-arsenic), the lower cladding layer 13 is n-type InGaAlP (indium-gallium-aluminum-phosphorus), the active layer 14 is InGaP (indium-gallium-phosphorus), and the upper cladding layer 15 is p-type InGa.
It is AlP. The contact layer 16 is p-type GaA.
s, the electrode 11 and the electrode 18 are mainly made of Au. With this combination of semiconductor materials, the peak (a) of the gain curve can be adjusted to the oscillation wavelength λ1 = 0.68 μm. Further, in the equation 1, the equivalent refractive index N is 3.
44. When the order m is 1, Λ becomes 0.1 μm, which makes it difficult to regularly process the concave portions 20. Therefore, in this embodiment, the order m is 3 and Λ is 0.3 μm.

Next, as described above, the period Λ of the regular recesses 20 is determined by the oscillation wavelength λ1.
The intensity of optical resonance (feedback) inside is affected by the depth d of the recess 20, that is, the ratio of d / Λ. 3 and 4, the horizontal axis represents d / Λ and the vertical axis represents the coupling constant k, which shows the change in the resonance (feedback) strength in the diffraction grating with respect to the ratio of d / Λ. ing. FIG. 3 relates to the distributed feedback semiconductor laser described in the first embodiment, and FIG. 4 relates to the distributed feedback semiconductor laser described in the second embodiment.

In the distributed feedback semiconductor laser, the coupling constant k
Is preferably 5 (/ cm) or more, more preferably 10
(/ Cm) or more. Applying this to FIG. 3, d
/ Λ is 0.3 or more or 0.5 or more. That is, when the clad layer is InP and the active layer is InGaAsP, or in the equation 1, the concave portion 2 whose order m is 1
When 0 (diffraction grating) is formed, d / Λ is preferably 0.3 or more, and more preferably 0.5 or more. In the first embodiment, since Λ is 0.19 μm, the depth d of the recess 20 is preferably 0.06 μm or more, and more preferably 0.1 μm or more.

Here, the cladding layer 15 and the contact layer 1
6 has a film thickness t of 0.7 μm, the depth d is 0.
When it is not less than 06 μm, the distance δ between the bottom of the recess 20 and the active layer 14 is not more than 0.64 μm, and the depth d is 0.1 μm.
When it is m or more, the distance δ becomes 0.6 μm or less.

The preferred range of the coupling constant k is 5 (/ cm)
If the above or 10 (/ cm) or more is applied to FIG. 4,
d / Λ is 1.7 or more or 2 or more. That is, when the clad layer is InGaAlP and the active layer is InGaP, or when the concave portion 20 (diffraction grating) is formed such that the order m is 3 in Expression 1, d / Λ is preferably 1.7 or more, and It is preferably 2 or more. In the second embodiment, since Λ is 0.3 μm, the depth d of the recess 20 is
Is preferably 0.51 μm or more, more preferably 0.
It is 6 μm or more.

In the second embodiment, if the film thickness t of the cladding layer 15 and the contact layer 16 is 0.7 μm,
When the depth d is 0.51 μm or more, the distance δ is 0.1
When the depth d is 9 μm or less and the depth d is 0.6 μm or more, the distance δ is 0.1 μm or less.

As shown in FIGS. 3 and 4, in order to increase the resonance (return strength) of light, it is necessary to increase the depth dimension d of the recess 20. In the first embodiment shown in FIG. 3, even if the depth d is increased in order to increase the coupling constant k, there is a margin in the distance δ between the bottom of the recess 20 and the active layer 14. However, in the second embodiment shown in FIG. 4, that is, when the semiconductor material is a combination of InGaAlP and InGaP or the order m in Equation 1 is 3, there is no margin in the distance δ, and for example, the coupling constant k is 15 When d / Λ is set to 2.5 in order to increase the size to around (/ cm), the depth d becomes 0.75 μm. Contact layer 16 and clad layer 1
If the film thickness t of 5 is 0.7 μm, the recess 20 extends to the active layer 14.

In the embodiment shown in FIG. 1, even if the recess 20 reaches the active layer 14, the oscillation region (C) is not affected.
However, depending on the processing means such as dry etching,
When forming the recess 20, the active layer 14 (oscillation region (C))
The work-affected layer may remain in the active layer). Therefore, in order to prevent the active layer 14 from being affected by the etching method, it is preferable to leave the distance δ between the bottom of the recess 20 and the active layer 14 at least 0.01 μm or more.

In the first and second embodiments, d
By selecting / Λ within the above-mentioned preferable range, a high intensity laser beam in the longitudinal single mode can be obtained. The first embodiment that emits laser light in the infrared wavelength band is suitable for fields such as optical communication. In addition, the second that emits laser light in the visible red band
The embodiment is suitable for a hologram scanner using a photosensitive drum having a high visible sensitivity in the visible red band.

FIG. 5 shows a configuration example of the hologram scanner. Reference numeral 31 is a light emitting device using the distributed feedback semiconductor laser shown in the second embodiment. This light emitting device 31
The laser light in the visible red band emitted in the vertical single mode is diffracted by the hologram lens 32 which is a diffraction grating. The hologram disk 33, which is a diffraction grating, is rotationally driven by a motor 34. The laser light diffracted by the hologram lens 32 is diffracted by the hologram disk 33 and scans the photosensitive drum 35 in the axial direction.

If the photosensitive drum 35 is made of, for example, a photosensitive material such as a silicon material and has a high visible sensitivity in the visible red band, the photosensitive accuracy of the photosensitive drum 35 can be increased. Further, in the distributed feedback semiconductor laser shown in the second embodiment, since the oscillation wavelength does not change, the scanning accuracy of the laser light with respect to the photosensitive drum 35 becomes high.

[0042]

As described above, according to the present invention, a recess is formed in a contact layer and a clad layer or a recess is formed in a contact layer, a clad layer and an active layer in a semiconductor epitaxial substrate in which each layer is laminated. As a diffraction grating. Since it is not necessary to grow a clad layer or the like on the diffraction grating, crystal defects do not occur in the clad layer or the like, and reliability and yield are improved. Further, the period of the diffraction grating can be easily set according to the peak of the gain curve of the semiconductor epitaxial substrate, so that the threshold current can be reduced.

InP and InGaA are used as semiconductor materials.
By selecting sP, it becomes possible to emit vertical single mode laser light in the infrared wavelength band, and In
By selecting GaAlP and InGaP, it becomes possible to emit vertical single mode laser light in the visible red band.

Further, strong resonance can be realized by setting the ratio of the period and the depth of the regular recesses to a preferable range.

Further, by constructing a hologram scanner using vertical single mode laser light in the visible red band, it is possible to perform highly accurate scanning without wavelength fluctuation.

[Brief description of drawings]

FIG. 1 is a perspective view having a partial cross section of a distributed feedback semiconductor laser of the present invention,

2 is a vertical sectional view taken along line II-II of the semiconductor laser of FIG.

FIG. 3 is a diagram showing a relation of a coupling constant with respect to a ratio of a period and a depth of a recess of the semiconductor laser of the first embodiment,

FIG. 4 is a diagram showing a relationship of a coupling constant with respect to a ratio between a period and a depth of a recess of the semiconductor laser of the second embodiment,

FIG. 5 is a perspective view showing a hologram scanner using the semiconductor laser of the second embodiment,

FIG. 6 is a sectional view showing the structure of a conventional distributed feedback semiconductor laser;

FIG. 7 is a diagram showing a relationship between a gain curve of a semiconductor laser and an oscillation wavelength,

[Explanation of symbols]

 11 Electrode 12 Substrate 13 Lower Cladding Layer 14 Active Layer 15 Upper Cladding Layer 16 Contact Layer 17 Insulating Layer 18 Electrode 19 Insulating Layer Window 20 Regular Recess 21 Convex

Claims (7)

[Claims] 1. A lower clad layer, an active layer, an upper clad layer, a contact layer, and an electrode are laminated on a substrate, and are regularly formed from the contact layer to the upper clad layer or from the contact layer to the upper clad layer and the active layer. A distributed feedback semiconductor laser, which is characterized in that various concave portions are formed. 2. The semiconductor material forming the cladding layer is In
The distributed feedback semiconductor laser according to claim 1, wherein the semiconductor material of P that forms the active layer is InGaAsP. 3. The semiconductor material forming the cladding layer is In
In GaAlP, the semiconductor material forming the active layer is InGa
The distributed feedback semiconductor laser according to claim 1, wherein P is P. 4. The distributed feedback semiconductor laser according to claim 2, wherein d / Λ is 0.3 or more, where Λ is the formation period of the recesses and d is the depth of the recesses. 5. The distributed feedback semiconductor laser according to claim 3, wherein d / Λ is 1.7 or more, where Λ is the formation period of the recesses and d is the depth of the recesses. 6. A gap between the bottom of the recess and the active layer is 0.01 μm.
The distributed feedback semiconductor laser according to claim 4, wherein a distance of m or more is left. 7. A hologram scanner comprising the distributed feedback semiconductor laser according to claim 3, and a hologram disk for scanning laser light emitted from the semiconductor laser.