Patent Application
20100290496
This application claims priority to Japanese Patent Application No. 2009-116824 filed on May 13, 2009, the disclosure of which including the specification, the drawings, and the claims is hereby incorporated by reference in its entirety.
The present disclosure relates to a nitride semiconductor laser device, and more particularly to a nitride semiconductor laser device having a ridge structure.
Blue semiconductor laser devices have been used as light sources for recording/reproduction to/from high-density optical disc systems such as a disc storage device and a Blu-ray Disc (registered trademark). As blue semiconductor laser devices, semiconductor laser devices using III-V group nitrides are used.
When used as the light source for an optical disc system, a blue semiconductor laser device is requested to exhibit properties excellent in linearity with no kink occurring in current-light output characteristics even at a high output of 300 mW or more that is required for quad-speed or faster recording. Moreover, to record/reproduce information to/from an optical disc with high precision, a far field pattern (FFP) suitable for the laser light condensing property of a lens is requested. To enhance the property of condensing laser light on an optical disc, the luminous diameter of a near field pattern (NFP) as the light distribution propagating in a waveguide is preferably small. Assume herein that the direction parallel to an active layer is the horizontal direction and the direction perpendicular to the active layer is the vertical direction. When the full width at half maximum of the NFP in the horizontal direction is small, for example, the full width at half maximum of the FFP is large. Hence, a horizontal spread angle of 8° or more is desired as the full width at half maximum of the FFP in the horizontal direction.
At present, as a blue semiconductor laser device, a ridge nitride semiconductor laser device having a stripe-shaped ridge structure extending along the length of the cavity is widely known. In this structure, normally, a current blocking layer made of a semiconductor or a dielectric is formed on the side faces of the ridge portion, and the difference (ΔN) in effective refractive index between the outside and inside of the ridge portion can be controlled using the distance (dp) between the current blocking layer in the regions outside the ridge portion and an active layer. The width of the ridge portion may also be used as a parameter, to control the full width at half maximum of laser light propagating in the waveguide. In this way, using the ridge structure, the light distribution can be controlled with the ridge width and dp, to obtain a desired light distribution. Hence, this structure has been widely used as a structure of semiconductor laser devices for optical discs.
To increase the horizontal spread angle of the FFP, it is effective to increase ΔN to strengthen confinement of the light distribution within the ridge portion, or reduce the width of the ridge portion to reduce the width of the light distribution directly. However, when ΔN is increased, laser oscillation in a horizontal high-order transverse mode occurs, causing a kink in the current-light output characteristics. To prevent this, the width of the ridge portion may be reduced to cut off the horizontal high-order transverse mode. However, reduction in ridge width will lead to increase in device resistance because the drive current for the semiconductor laser device is narrowed by the ridge portion before being fed into the active layer. Increase in device resistance also results in increase in drive voltage. Hence, reducing the width of the ridge portion is undesirable because this causes heat generation due to increased power consumption and limitation of the supply voltage for a laser drive circuit.
A nitride semiconductor laser device that can enhance the kink level without reducing the width of the ridge portion is described in Japanese Patent Publication No. 2002-314197.
As shown in
With the structure described above, the amount of increase in waveguide loss in the horizontal high-order transverse mode can be made larger than that in the horizontal fundamental transverse mode, so that the oscillation in the horizontal high-order transverse mode can be suppressed. Hence, occurrence of a kink in the current-light output characteristics can be suppressed without reducing the ridge width.
However, the configuration shown in
To address the above problem, a nitride semiconductor laser device that can suppress increase in waveguide loss in a horizontal fundamental transverse mode is described in Japanese Patent Publication No. 2003-198065.
As shown in
With the structure of the nitride semiconductor laser device of the second conventional example shown in
When amorphous silicon (Si) is used for the light absorption layer 322, the distance between each side face of the ridge portion and the light absorption layer 322 may be set at 0.3 μm, to obtain the greatest effect of suppressing oscillation in the horizontal high-order transverse mode.
However, the nitride semiconductor laser device of the second conventional example shown in
Amorphous silicon (Si) is often used as a light absorbent material for blue laser light for the reason of easiness of fabrication. The thermal expansion coefficient of amorphous Si is 0.6×10−6/k or less, and that of GaN as a nitride material is 5.6×10−6/k. Therefore, in the device fabrication process, when the temperature is lowered from a high-temperature state during formation of the light absorption layer 322 to room temperature, a stress may be generated in the light absorption layer 322 and a portion of the nitride material constituting the p-type cladding layer 319 near the ridge bottom due to the difference in thermal expansion coefficient between the two layers.
The nitride material has a wurtzite (WZ) crystal structure. Therefore, if a stress is generated in the crystal distorting the crystal structure, the symmetry of an array of III-group atoms and V-group atoms is lost, and as a result, a large electric field is generated inside the crystal due to the piezoelectric effect. With this internal electric field, the atom arrangement is further deformed, changing the electron orbit in the crystal and hence greatly changing the dielectric constant. The change in dielectric constant leads to change in refractive index. The influence of the stress-caused crystal distortion on the change in refractive index in the nitride material is greater than that in a semiconductor material having a zincblende (ZB) crystal structure typified by gallium arsenide (GaAs).
When amorphous Si is used for the light absorption layer 322, the refractive index of a portion of the p-type cladding layer 319 located between a region of the current blocking layer 321 near the ridge bottom and the active layer 316 increases. As a result, the light distribution deforms to spread to the outside of the ridge portion due to the stress-caused change in refractive index. This increases the waveguide loss in the horizontal fundamental transverse mode by the optical absorption layer 322, leading to decrease in slope efficiency and increase in operating current value.
In view of the above, in the configuration shown in
As described above, the nitride semiconductor laser device of the second conventional example fails to improve the kink level and obtain a desired FFP horizontal spread angle characteristic as wide as 8° or more without causing increase in waveguide loss in the horizontal fundamental transverse mode and increase in device resistance.
In view of the above problem, it is an object of the present invention to provide a nitride semiconductor laser device in which a stress generated in a cladding layer is reduced to improve the slope efficiency and the kink level and hence a FFP with no disturbance is presented.
To attain the object described above, a nitride semiconductor laser device according to the present invention is configured to have a third current blocking layer larger in thermal expansion coefficient than gallium arsenide, which is formed on a first current blocking layer transparent to light generated from an active layer and a second current blocking layer as an optical absorption layer.
Specifically, the nitride semiconductor laser device of the present invention includes: an active layer made of a nitride formed on a semiconductor substrate; a stripe-shaped ridge waveguide including a cladding layer having a ridge structure in its upper portion, formed on the active layer; a first current blocking layer transparent to light generated from the active layer, formed at least on a side face of the ridge waveguide; a second current blocking layer having light absorbency, formed on a flat portion of the cladding layer on each side of the ridge waveguide at a position spaced from the side face of the ridge waveguide, and a third current blocking layer formed on the first current blocking layer and the second current blocking layer, wherein ηg>η1, ηg>η2, and ηg<η3 are satisfied where η1, η2, and η3 are respectively the heat expansion coefficients of the first, second, and third current blocking layers, and ηg is the heat expansion coefficient of gallium nitride.
According to the nitride semiconductor laser device of the present invention, the stress generated in the cladding layer made of a nitride material formed between the first current blocking layer and the active layer can be reduced. As a result, the change in the refractive index of the cladding layer having the ridge portion can be reduced. Hence, variations in FFP caused by variations in fabrication process steps for the first current blocking layer and the second current blocking layer can be suppressed. A desired light distribution can therefore be achieved with high precision. Moreover, the light distribution can be prevented from largely spreading to the outside of the ridge portion under the influence of a distortion of the crystal structure. Accordingly, since the waveguide loss in the horizontal fundamental transverse mode can be prevented from increasing, stable and high slope efficiency can be obtained.
In the nitride semiconductor laser device of the present invention, preferably, the first current blocking layer is not formed on a top portion of the side face of the ridge waveguide.
With the above configuration, the thickness of the first current blocking layer formed on each side face of the ridge portion in the direction normal to the substrate is reduced, and this can reduce the stress exerted on the cladding layer at each bottom edge of the ridge portion caused by the difference in heat expansion coefficient between the first current blocking layer and the nitride material constituting the waveguide. As a result, the change in refractive index caused by the stress exerted on the cladding layer at each bottom edge of the ridge portion can be reduced, and hence variations in FFP caused by variations in fabrication process steps for the first current blocking layer can be further suppressed. A desired light distribution can therefore be achieved with further high precision. Moreover, the light distribution can be prevented from largely spreading to the outside of the ridge portion under the influence of a distortion of the crystal structure.
In the nitride semiconductor laser device of the present invention, preferably, Rf<Rr and A1f<A1r are satisfied where Rf and Rr are respectively the end facet reflectances on the front end facet side and rear end facet side of the cavity, and A1f and A1r are respectively the distances from the side face of the ridge waveguide to the second block layer at the front end facet and the rear end facet.
With the above configuration, since the distance between each side face of the ridge waveguide and the second current blocking layer is small on the front end facet side where the light density is high, the waveguide loss in the horizontal high-order transverse mode increases. Furthermore, scattered light from the waveguide can be absorbed by the second current blocking layer on the front end facet side. As a result, a good unimodial FFP can be obtained, and occurrence of a kink in the current-light output characteristics can be suppressed.
In the case described above, preferably, Wf>Wr is satisfied where Wf and Wr are respectively the widths of the ridge waveguide on the front end facet side and rear end facet side of the cavity.
With the above configuration, since a larger amount of current can be injected on the front end facet side where the light density is high, the efficiency of converting the current to light improves. As the width of the ridge portion is smaller, the spread of the light distribution to the outside of the ridge portion is larger. Therefore, the distance between the second current blocking layer and each bottom edge of the ridge portion on the rear end facet side where the ridge width is small is made larger than the distance thereof on the front end facet side. With this configuration, it is possible to increase the difference in waveguide loss between the horizontal fundamental transverse mode and the horizontal high-order transverse mode while reducing the light absorption loss in the horizontal fundamental transverse mode by the second current blocking layer 19. Hence, the emission efficiency and the kink level can be improved.
In the nitride semiconductor laser device of the present invention, the second current blocking layer is preferably formed on the first current blocking layer.
With the above configuration, the waveguide loss in the horizontal fundamental transverse mode incurred by the second current blocking layer can be controlled with high precision.
In the nitride semiconductor laser device of the present invention, the second current blocking layer is preferably formed on the cladding layer to be in contact with the cladding layer.
With the above configuration, the waveguide loss in the horizontal high-order transverse mode by the second current blocking layer can be increased. As a result, oscillation in the horizontal high-order transverse mode can be efficiently suppressed.
In the nitride semiconductor laser device of the present invention, the second current blocking layer is preferably formed on at least one of cavity end facets formed at both ends of the ridge waveguide.
With the above configuration, it is possible to increase the waveguide loss in the horizontal high-order transverse mode while suppressing increase in waveguide loss in the horizontal fundamental transverse mode. Hence, the oscillation threshold current value can be reduced, the slope efficiency can be improved, and the kink level in the current-light output characteristics can be improved. Also, with formation of the light absorption region on the front end facet side from which laser light is extracted, scattered light and spontaneously emitted light from the waveguide are absorbed on the front end facet side and removed. Therefore, a good FFP shape can be obtained. Moreover, laser oscillation in the horizontal high-order mode can be suppressed without reducing the width of the ridge portion. Therefore, occurrence of a kink in the current-light output characteristics can be prevented without causing increase in device resistance.
As described above, in the nitride semiconductor laser device of the present invention, it is possible to increase the waveguide loss in the horizontal high-order transverse mode while suppressing increase in waveguide loss in the horizontal fundamental transverse mode. This reduces the oscillation threshold current value and improves the slope efficiency. Accordingly, the kink level in the current-light output characteristics can be improved, and hence a good FFP shape can be obtained.
Nitride semiconductor laser devices according to the present disclosure will be described with reference to the drawings.
First, the first comparative example in this disclosure will be described.
As shown in
As shown in
The waveguide losses can be reduced by increasing d1 as described above. To improve the kink level in the current-light output characteristics, however, it is necessary to suppress oscillation in the horizontal high-order transverse mode. To attain this, the waveguide loss α1 in the horizontal high-order transverse mode may be increased, to suppress oscillation in the high-order transverse mode. However, if d1 is simply reduced to increase α1, α0 will also increase. Hence, in oscillation in the fundamental transverse mode, the oscillation threshold current value will increase, reducing the slope efficiency in the current-light output characteristics. As a result, the operating current value will increase, causing degradation in reliability. Accordingly, to improve the kink level without causing decrease in slope efficiency, it is necessary to increase α1 while reducing α0 as much as possible.
The cavity length of a high-output nitride semiconductor laser device is set at 700 μm or more, and the front and rear end facets of the cavity are coated with a dielectric film to give reflectances of 10% or less and 90% or less, respectively, thereby ensuring high slope efficiency. The mirror loss αm of a semiconductor laser device is expressed by
αm=(½L)Loge(1/RfRr) (1)
where Rf, Rr and L are respectively the front reflectance, the rear reflectance, and the cavity length. At this time, the slope efficiency Se of the semiconductor laser device is expressed by
Se=ηdηi{αm/(αi+αm)} (2)
where ηd, ηi, and αi are respectively the light extraction efficiency from the front end facet, the internal quantum efficiency, and the waveguide loss. The waveguide loss αi is given by α0 for the horizontal fundamental transverse mode and α1 for the horizontal high-order transverse mode. From Equation (1), the mirror loss αm of the semiconductor laser device of which the front and rear end facets are coated with a dielectric film to give reflectances of 10% and 90%, respectively, is 17.2 cm−1 when the cavity length L is 700 μm. The mirror loss αm decreases with increase of the cavity length L: it is 15 cm−1 when the cavity length L is 800 μm, 12 cm−1 when the cavity length L is 1000 μm, and 8 cm−1 when the cavity length L is 1500 μm.
From Equation (2), it is found that the slope efficiency Se is proportional to the ratio of the mirror loss αm to the entire loss of the cavity (=αi+αm). Hence, the slope efficiency Se will decrease unless the device is fabricated so that the waveguide loss αi is small compared with the mirror loss αm. As described above, the mirror loss αm of a high-output nitride blue laser device is roughly in the range of 17.2 cm−1 to 8 cm−1 when the cavity length L is in the range of 700 μm to 1500 μm. In particular, the cavity length L is set in the range of 800 μm to 1000 μm for a high-output nitride semiconductor laser device having an output of about 300 mW to about 400 mW, and the mirror loss αm in this cavity length range is roughly in the range of 15 cm−1 to 12 cm−1. Accordingly, unless the waveguide loss α0 in the horizontal fundamental transverse mode is set at 15 cm−1 or less, the increase in waveguide loss will increase the amount of decrease of the slope efficiency Se. To set α0 at 15 cm−1 or less, d1 should be 0.081 μm or more as shown in
When a semiconductor laser is subjected to high-output operation, implanted carriers are lost due to emission recombination under strong stimulated emission in an active layer portion in the center of a near field pattern (NFP) where the light density is high. Therefore, the operating carrier density in the active layer portion corresponding to the center of the light distribution is relatively lower than that in its flanking regions, causing spatial hole burning of carriers in which a horizontally concave shape is formed. When spatial hole burning of carriers occurs, the overlap between the shape of the active layer carrier distribution and the light distribution corresponding to the high-order transverse mode increases, and this increases the mode gain of the horizontal high-order transverse mode. Also, when the device is subjected to high-temperature, high-output operation, the temperature of the device rises increasing the refractive index of the ridge portion. This increases ΔN, and hence the horizontal high-order transverse mode becomes less easy to be cut off. Therefore, if Δα is small, oscillation in the horizontal high-order transverse mode will occur in high-temperature, high-output operation, generating a kink. Accordingly, in the configuration shown in
When d1 is reduced to increase the waveguide loss α1 in the horizontal high-order transverse mode for suppressing oscillation in the horizontal high-order transverse mode, α0 also increases, and this reduces the slope efficiency.
For the reason described above, the double-layer current block structure as shown in
The first illustrative embodiment will be described with reference to
As shown in
Unlike the first comparative example shown in
As shown in
As shown in
As a result of decrease in ΔN, the NFP in the horizontal fundamental transverse mode changes to spread in the horizontal direction, and α0 increases under the influence of the light absorption loss by the second current blocking layer 19 having light absorbency, thereby reducing the slope efficiency. Moreover, as the NFP spreads horizontally, the full width at half maximum of the horizontal spread angle of the FFP changes to decrease. If this decreases by about 2° compared with the case of having no change in refractive index, a desired FFP will no more be obtainable. Accordingly, in the second comparative example shown in
In contrast to the above, in the nitride semiconductor laser device of the first illustrative embodiment, in which the third current blocking layer 22 is formed, it is found from
As shown in
As shown in
As shown in
As described earlier, α0 must be 15 cm−1 or less to obtain high slope efficiency, and Δα must be 10 cm−1 or more to prevent occurrence of a kink. When A1 is large, the light distribution is free from the influence of light absorption by the second current blocking layer 19. Therefore, the waveguide losses α0 and α1 are both small, and hence the effect of formation of the second current blocking layer 19 is lost. When A1 is small, α0 increases, reducing the slope efficiency. Accordingly, the value of A1 must be set to satisfy the requirements that α0 is 15 cm−1 or less and Δα is 10 cm−1 or more.
As shown in
As shown in
As shown in
As shown in
In the first illustrative embodiment, by setting A1 at any of the above ranges, high slope efficiency, high kink level due to suppression of oscillation in the horizontal high-order transverse mode, and a wide FFP horizontal spread angle of 8° or more can be achieved. In this embodiment, α0 of 15 cm−1 and Δα of 11 cm−1 are obtained by setting d1 at 0.05 μm and A1 at 1 μm.
In the first illustrative embodiment, SiO2 was used for the first current blocking layer 18, amorphous Si for the second current blocking layer 19, and ZrO2 for the third current blocking layer 22. The materials are not limited to these dielectric materials, but silicon nitride (SiN), titanium dioxide (TiO2), tantalum dioxide (TaO2), aluminum oxide (Al2O3), and the like may be used. The current blocking layers are just required to satisfy ηg>η1, ηg>η2, and ηg<η3 where η1, η2, and η3 are respectively the heat expansion coefficients of the first, second, and third current blocking layers 18, 19, and 22, and ηg is the heat expansion coefficient of gallium nitride, and the second current blocking layer 19 is required to have absorbency for laser light. The first current blocking layer 18 may not be formed on the top portion of each side face of the p-type cladding layer 16. This structure will reduce the thickness of the first current blocking layer formed on each side face of the ridge portion in the direction normal to the substrate, and hence permit further reduction of the stress on the cladding layer at each bottom edge of the ridge portion generated by the difference in heat expansion coefficient between the first current blocking layer and the nitride material constituting the waveguide.
As shown in
In a nitride semiconductor laser device, the band gap energy of the p-type cladding layer 16 constituting the waveguide and the semiconductor layer constituting the semiconductor substrate 11 is larger than that of the active layer made of InGaN. Therefore, laser light scattered from the waveguide due to microscopic asperities on the side faces of the ridge portion is hardly absorbed by semiconductor layers other than the active layer, but is reflected by the waveguide and the electrodes to be output from the front end facet. Under the influence of such scattered light, the shape of the FFP may be disturbed and a desired unimodial FFP pattern may not be obtained. To prevent this, scattered light as a cause of FFP disturbance should just be absorbed by the second current blocking layer 19 at the front end facet that is the laser light output side and removed. In this way, a unimodial FFP can be obtained.
In this variation, by setting A1f at a value in the range of 0.5 μm to 2 μm, scattered light components from the waveguide, as a cause of FFP disturbance, can be removed by the second current blocking layer 19 near the front end facet.
In the first variation of the first illustrative embodiment, by reducing the distance between the ridge portion and the second current blocking layer 19 on the front end facet side where the light density is high, the waveguide loss in the horizontal high-order transverse mode can be increased, and furthermore scattered light from the waveguide can be absorbed by the second current blocking layer 19 at the front end facet. As a result, a good unimodial FFP can be obtained. Also, high slope efficiency can be obtained in the current-light output characteristics, and hence occurrence of a kink can be suppressed.
As shown in
In this variation, by setting A1f at a value in the range of 0.5 μm to 2 μm, scattered light components from the waveguide, as a cause of FFP disturbance, can be removed by the second current blocking layer 19 near the front end facet. As Lf is larger, the effect of removing scattered light increases. However, if it is excessively large, the waveguide loss in the horizontal fundamental transverse mode will increase. Hence, Lf may be set at a value in the range of 5 μm to 50 μm to be 10% or less of the cavity length, to prevent redundant increase in waveguide loss in the horizontal fundamental transverse mode, whereby decrease in slope efficiency can be suppressed.
In the second variation of the first illustrative embodiment, by reducing the distance between each side face of the ridge waveguide and the light absorption film on the front end facet side where the light density is high, the waveguide loss in the horizontal high-order transverse mode can be increased, and furthermore scattered light from the waveguide can be absorbed by the second current blocking layer 19 at the front end facet. As a result, a good unimodial FFP can be obtained. Also, high slope efficiency can be obtained in the current-light output characteristics, and hence occurrence of a kink can be suppressed.
As shown in
In this variation, by setting A1f at a value in the range of 0.5 μm to 2 μm, scattered light components from the waveguide, as a cause of FFP disturbance, can be removed by the second current blocking layer 19 near the front end facet.
In this variation, since Wf is larger than Wr, a larger amount of current can be injected on the front end facet side where the light density is high. Hence, the efficiency of converting the current to light improves. Also, by setting A1r to be larger than A1f, it is possible to increase the difference in waveguide loss between the horizontal fundamental transverse mode and the horizontal high-order transverse mode while reducing the light absorption loss in the horizontal fundamental transverse mode by the second current blocking layer 19. In addition, by reducing A1f, scattered light from the waveguide can be absorbed by the second current blocking layer 19 on the front end facet side where the light density is high.
In such a stripe-shaped laser device, guided light tends to scatter when the ridge width changes along the cavity length. To remove scattered light, it is advisable to set A1f at a value less than 0.5 μm. However, if A1f is excessively reduced, the waveguide loss in the horizontal fundamental transverse mode will increase. In consideration of this, the angle θ1 at which the ridge width changes may be set to be as small as 2° or less. With this setting, radiation scattering of laser light from the side faces of the ridge portion and disturbance of the FFP shape can be suppressed. In this variation, θ1 is set at 0.3°, to suppress scattering of light from the side faces of the ridge portion.
In the third variation of the first illustrative embodiment, a good unimodial FFP can be obtained. Also, high slope efficiency can be obtained in the current-light output characteristics, and hence occurrence of a kink can be suppressed.
As shown in
In this variation, by setting A1f at a value in the range of 0.5 μm to 2 μm, scattered light components from the waveguide, as a cause of FFP disturbance, can be removed by the second current blocking layer 19 near the front end facet. As Lf is larger, the effect of removing scattered light increases. However, if it is excessively large, the waveguide loss in the horizontal fundamental transverse mode will increase. Hence, Lf may be set at a value in the range of 5 μm to 50 μm to be 10% or less of the cavity length, to prevent redundant increase in waveguide loss in the horizontal fundamental transverse mode, whereby decrease in slope efficiency can be suppressed.
In this variation, since Wf is larger than Wr, a larger amount of current can be injected on the front end facet side where the light density is high. Hence, the efficiency of converting the current to light improves. A smaller ridge width will increase spread of the light distribution to the outside of the ridge portion. Therefore, by setting A1r to be larger than A1f, it is possible to increase the difference in waveguide loss between the horizontal fundamental transverse mode and the horizontal high-order transverse mode while reducing the light absorption loss in the horizontal fundamental transverse mode by the second current blocking layer 19. In addition, by reducing A1f, scattered light from the waveguide can be absorbed by the second current blocking layer 19 on the front end facet side where the light density is high.
In such a stripe-shaped laser device, guided light tends to scatter when the ridge width changes along the cavity length. To remove scattered light, it is advisable to set A1f at a value less than 0.5 μm. However, if A1f is excessively reduced, the waveguide loss in the horizontal fundamental transverse mode will increase. In consideration of this, the angle θ1 at which the ridge width changes may be set to be as small as 2° or less. With this setting, radiation scattering of laser light from the side faces of the ridge portion and disturbance of the FFP shape can be suppressed. In this variation, θ1 is set at 0.3°, to suppress scattering of light from the side faces of the ridge portion.
In the fourth variation of the first illustrative embodiment, a good unimodial FFP can be obtained. Also, high slope efficiency can be obtained in the current-light output characteristics, and hence occurrence of a kink can be suppressed.
As shown in
In this variation, by setting A1f at a value in the range of 0.5 μm to 2 μm, scattered light components from the waveguide, as a cause of FFP disturbance, can be removed by the second current blocking layer 19 near the front end facet.
In this variation, since Wf is larger than Wr, a larger amount of current can be injected on the front end facet side where the light density is high. Hence, the efficiency of converting the current to light improves.
In such a stripe-shaped laser device, guided light tends to scatter when the ridge width changes along the cavity length. In consideration of this, A1r and A1f are set to be the same. With this setting, scattered light generated near the region covering the distance L1 from the front end facet and the region covering the distance L3 from the rear end facet can be absorbed by the second current blocking layer 19 and removed. To remove scattered light, it is advisable to set A1f at a value less than 0.5 μm. However, if A1f is excessively reduced, the waveguide loss in the horizontal fundamental transverse mode will increase. In consideration of this, the angle θ1 at which the width of the ridge portion changes may be set to be as small as 2° or less. With this setting, radiation scattering of laser light from the side faces of the ridge portion and disturbance of the FFP shape can be suppressed. In this variation, θ1 is set at 0.3°, to suppress scattering of light from the side faces of the ridge portion.
In the fifth variation of the first illustrative embodiment, a good unimodial FFP can be obtained. Also, high slope efficiency can be obtained in the current-light output characteristics, and hence occurrence of a kink can be suppressed.
As shown in
In this variation, by setting A1f at a value in the range of 0.5 μm to 2 μm, scattered light components from the waveguide, as a cause of FFP disturbance, can be removed by the second current blocking layer 19 near the front end facet.
In this variation, since Wf is larger than Wr, a larger amount of current can be injected on the front end facet side where the light density is high. Hence, the efficiency of converting the current to light improves.
In this variation, as Lf is larger, the effect of removing scattered light increases. However, if it is excessively large, the waveguide loss in the horizontal fundamental transverse mode will increase. Hence, Lf may be set at a value in the range of 5 μm to 50 μm to be 10% or less of the cavity length, to prevent redundant increase in waveguide loss in the horizontal fundamental transverse mode, whereby decrease in slope efficiency can be suppressed.
In such a stripe-shaped laser device, guided light tends to scatter when the ridge width changes along the cavity length. In consideration of this, the distance from the second current blocking layer 19 to the ridge portion is kept fixed except for the region Lf. With this setting, scattered light occurring near the region covering the distance L1 from the front end facet and the region covering the distance L3 from the rear end facet can be absorbed by the second current blocking layer 19 and removed. To remove scattered light, it is advisable to set A1f at a value less than 0.5 μm. However, if A1f is excessively reduced, the waveguide loss in the horizontal fundamental transverse mode will increase. In consideration of this, the angle θ1 at which the width of the ridge portion changes may be set to be as small as 2° or less. With this setting, radiation scattering of laser light from the side faces of the ridge portion and disturbance of the FFP shape can be suppressed. In this variation, θ1 is set at 0.3°, to suppress scattering of light from the side faces of the ridge portion.
In the sixth variation of the first illustrative embodiment, a good unimodial FFP can be obtained. Also, high slope efficiency can be obtained in the current-light output characteristics, and hence occurrence of a kink can be suppressed.
As shown in
In this variation, by setting A1f at a value in the range of 0.5 μm to 2 μm, scattered light components from the waveguide, as a cause of FFP disturbance, can be removed by the second current blocking layer 19 near the front end facet. Also, since the second current blocking layer 19 is formed only in a region on the front end facet side where the light density is high, only the waveguide loss in the horizontal high-order transverse mode can be increased without increasing the waveguide loss in the horizontal fundamental transverse mode more than necessary.
In this variation, A1f may be further reduced in a region covering distance Lf from the front end facet in the cavity length direction. In this case, the effect of removing scattered light can be further increased. As Lf is larger, the effect of removing scattered light increases. However, if it is excessively large, the waveguide loss in the horizontal fundamental transverse mode will increase. Hence, Lf may be set at a value in the range of 5 μm to 50 μm to be 10% or less of the cavity length, to prevent redundant increase in waveguide loss in the horizontal fundamental transverse mode, whereby decrease in slope efficiency can be suppressed.
In the seventh variation of the first illustrative embodiment, a good unimodial FFP can be obtained. Also, high slope efficiency can be obtained in the current-light output characteristics, and hence occurrence of a kink can be suppressed.
As shown in
In this variation, by setting A1f at a value in the range of 0.5 μm to 2 μm, scattered light components from the waveguide, as a cause of FFP disturbance, can be removed by the second current blocking layer 19 near the front end facet. By setting A1f in the region near the front end facet to be smaller than A1r, the effect of removing scattered light can be increased. Also, since the second current blocking layer 19 is formed only in the region on the front end facet side where the light density is high, only the waveguide loss in the horizontal high-order transverse mode can be increased without increasing the waveguide loss in the horizontal fundamental transverse mode more than necessary.
In this variation, A1f may be further reduced in a region covering distance Lf from the front end facet in the cavity length direction. In this case, the effect of removing scattered light can be further increased. As Lf is larger, the effect of removing scattered light increases. However, if it is excessively large, the waveguide loss in the horizontal fundamental transverse mode will increase. Hence, Lf may be set at a value in the range of 5 μm to 50 μm to be 10% or less of the cavity length, to prevent redundant increase in waveguide loss in the horizontal fundamental transverse mode, whereby decrease in the slope efficiency can be suppressed.
In such a stripe-shaped laser device, guided light tends to scatter when the ridge width changes along the cavity length. To remove scattered light, it is advisable to set A1f at a value less than 0.5 μm. However, if A1f is excessively reduced, the waveguide loss in the horizontal fundamental transverse mode will increase. In consideration of this, the angle θ1 at which the ridge width changes may be set to be as small as 2° or less. With this setting, radiation scattering of laser light from the side faces of the ridge portion and disturbance of the FFP shape can be suppressed. In this variation, θ1 is set at 0.3°, to suppress scattering of light from the side faces of the ridge portion.
In the eighth variation of the first illustrative embodiment, a good unimodial FFP can be obtained. Also, high slope efficiency can be obtained in the current-light output characteristics, and hence occurrence of a kink can be suppressed.
As shown in
In the ninth variation of the first illustrative embodiment, the following effect is obtained in addition to the effect described in the seventh variation. Scattered light reflected from the rear end facet to be returned to the inside of the cavity can be absorbed by the second current blocking layer 19 near the rear end facet and removed. Therefore, generation of noise in laser light intensity, which may occur when scattered light from the waveguide is reflected from the rear end facet and absorbed into the active layer, can be prevented. Also, since the second current blocking layer 19 is formed only in the region near the rear end facet in addition to the region near the front end facet, decrease in slope efficiency can be suppressed.
As shown in
In the tenth variation of the first illustrative embodiment, the following effect is obtained in addition to the effect described in the eighth variation. Scattered light reflected from the rear end facet to be returned to the inside of the cavity can be absorbed by the second current blocking layer 19 near the rear end facet and removed. Therefore, generation of noise in laser light intensity, which may occur when scattered light from the waveguide is reflected from the rear end facet and absorbed into the active layer, can be prevented. Also, since the second current blocking layer 19 is formed only in the region near the rear end facet in addition to the region near the front end facet, decrease in slope efficiency can be suppressed.
As shown in
In the second illustrative embodiment, in which the second current blocking layer 19 is formed right on the p-type cladding layer 16, both α0 and α1 are large when A1 is small as shown in
In the nitride semiconductor laser device of the second illustrative embodiment, Δα can be increased, and hence the kink level can be further improved.
As shown in
The nitride semiconductor laser device of the first comparative example does not have the third current blocking layer 22 that can compensate the difference in heat expansion coefficient between the first and second current blocking layers 18 and 19 and the nitride material. Therefore, to obtain a FFP horizontal spread angle of 9.5°, it is necessary to reduce the width of the ridge portion from 1.4 μm to 1 μm, or increase ΔN by about 1.5 times from 5×10−3 to 8×10−3, the value obtained when no consideration is given to a change in refractive index due to a stress caused by the difference in heat expansion coefficient. Hence, in the first comparative example, when the device is subjected to high-output operation, the series resistance increases with the decrease in ridge width, increasing power consumption. As a result, the emission efficiency decreases due to self-heating of the device. Otherwise, with the increase in ΔN, the horizontal high-order transverse mode becomes less easy to be cut off, and this lowers the kink level. In contrast to the above, both the nitride semiconductor laser devices of the first illustrative embodiment and the second illustrative embodiment can obtain a FFP full width at half maximum of 9.5° in the horizontal direction and 19° in the vertical direction (not shown).
In the second illustrative embodiment, SiO2 was used for the first current blocking layer 18, amorphous Si for the second current blocking layer 19, and ZrO2 for the third current blocking layer 22. The materials are not limited to these dielectric materials, but silicon nitride (SiN), titanium dioxide (TiO2), tantalum dioxide (TaO2), aluminum oxide (Al2O3), and the like may be used. The current blocking layers are just required to satisfy ηg>η1, ηg>η2, and ηg<η3 where η1, η2, and η3 are respectively the heat expansion coefficients of the first, second, and third current blocking layers 18, 19, and 22, and ηg is the heat expansion coefficient of gallium nitride, and the second current blocking layer 19 is required to have absorbency for laser light. The first current blocking layer 18 may not be formed on the top portion of each side face of the p-type cladding layer 16. This structure will reduce the thickness of the first current blocking layer formed on each side face of the ridge portion in the direction normal to the substrate, and hence permit further reduction of the stress exerted on the cladding layer at each bottom edge of the ridge portion caused by the difference in heat expansion coefficient between the first current blocking layer and the nitride material constituting the waveguide.
The variations of the first illustrative embodiment described above can also be applied to the second illustrative embodiment.
As described above, in the nitride semiconductor laser device of the present disclosure, the oscillation threshold current value can be reduced, the slope efficiency can be improved, and the kink level in the current-light output characteristics can be improved, obtaining a FFP having no disturbance. Hence, the present disclosure is particularly useful in nitride semiconductor laser devices having a ridge structure.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2009-116824 | May 2009 | JP | national |
