Patent Application
20060258026
The present invention relates to a semiconductor laser device, in particular, a multi-wavelength semiconductor laser device made of a plurality of semiconductor laser elements that oscillate at mutually different wavelengths, and a manufacturing method thereof.
In an existing monolithic two-wavelength laser made of two semiconductor laser elements, high reflectance coatings at faces thereof are each formed as a film having a different structure by making use of a mask. In the case of a multi-layered film most suitable for a first wavelength being formed, by covering a facet of the second semiconductor laser element of the second wavelength with a mask, on the contrary, in the case a multi-layered film most suitable for a second wavelength being formed, by covering a facet of the first semiconductor laser element of the first wavelength with a mask, a high reflectance coating of the multi-layered film most suitable for each is separately formed. A distance between light emitting spots of two wavelengths of the monolithic two-wavelength laser is one hundred and several tens micrometers or less. Therefore, according to an existing method in which films having different structures are separately formed by use of the mask, high positioning accuracy is necessary, resulting in poor productivity.
Furthermore, in order to enable to more improve the productivity of two-wavelength semiconductor laser devices, a resonator having a multi-layered reflective film that can be used in two wavelength ranges has been proposed (JP-A-2001-57413). This two-wavelength semiconductor laser device has a structure of a multi-layered reflective film that has a film thickness that an optical length to an average wavelength λ of two wavelengths is (¼+j)×λ(j=0, 1, 2, . . . ). The reason for adapting the film thickness to an average wavelength of two wavelengths is that in a multi-layered film of which wavelength is adapted to one wavelength range, the reflectance to the other wavelength is extremely deteriorated.
However, in such two-wavelength semiconductor laser device, since an average wavelength k is deviated from both wavelengths, it is difficult to obtain high reflectance close to 100% for both wavelengths. In particular, in the case of a two-wavelength semiconductor laser device having relatively largely different wavelength ranges such as a 405 nm range and a 650 nm range being manufactured, differences of the respective wavelengths from the average wavelength become large. Therefore, it is furthermore difficult to dispose high reflectance multi-layered films to both wavelengths.
In order to solve the above-mentioned problems, the present invention has an object, for example, to provide a multi-wavelength semiconductor laser device that has high reflectance multi-layered films that can be collectively formed with the same structure on two or more semiconductor laser element facets and a manufacturing method thereof.
According to the present invention, there is provided a multi-wavelength semiconductor laser device comprising:
a plurality of semiconductor laser elements that oscillate at wavelengths that are different from each other,
wherein the plurality of semiconductor laser elements have a reflective film that is deposited on at least one of a front facet and a backside facet thereof and has a multilayer structure common to the semiconductor laser elements,
wherein the reflective film includes first and second reflective regions disposed in a film thickness direction thereof, the first reflective region having a first predetermined reflectance to a first wavelength oscillated at a first semiconductor laser element of the semiconductor laser elements; and the second reflective region having a second predetermined reflectance to a second wavelength that is oscillated at a second semiconductor laser element other than the first semiconductor laser element and different from the first wavelength.
Furthermore according to the present invention, there is provided a method of manufacturing a multi-wavelength semiconductor laser device that includes a plurality of semiconductor laser elements each of which oscillates at a wavelength different from each other, comprising the steps of:
forming a laser bar for a plurality of semiconductor laser elements that oscillate at different wavelengths;
forming, over all facets of the laser bar corresponding to the plurality of semiconductor laser elements, a first reflective region that includes a plurality of dielectric films each of which is different in the refractive index from adjacently deposited ones and has a first predetermined reflectance to a first wavelength; and
forming, on the first reflective region, a second reflective region that includes a plurality of dielectric films each of which is different in the refractive index from adjacently deposited ones and has a second predetermined reflectance to a second wavelength that is oscillated by a second semiconductor laser element other than the first semiconductor laser element and different from the first wavelength.
In the following, modes for carrying out the present invention will be detailed with reference to attached drawings.
<Two-Wavelength Semiconductor Laser Device>
The first light-emitting element 2 includes a laser part 5 having a ridge stripe waveguide 6, and the second light-emitting element 3 includes a laser part 9 that is formed on an n-GaAs substrate 13 and has a ridge stripe waveguide 10.
Furthermore, an ohmic electrode P1 is formed on the laser part 5, an ohmic electrode P2 is formed on the n-GaAs substrate 13, and an ohmic electrode P3 is formed on an exposed portion 4R of the fusing metal layer 4.
When a driving current is supplied through the ohmic electrodes P1 and P3, 405 nm band laser light is emitted from the first light-emitting element 2. And when a driving current is supplied through the ohmic electrodes P2 and P3, 650 nm band laser light is emitted from the second light-emitting element 3.
The laser part 5 of the first light-emitting element 2 has a multilayer structure in which a bottom layer 5b, an n-type cladding layer 5c, an n-type guide layer 5d, an active layer 5e, an electron barrier layer 5f, a p-type guide layer 5g, a p-type cladding layer 5h and a p-type contact layer 5i are sequentially stacked in this order, wherein the p-type contact layer 5i and the p-type cladding layer 5h are partially etched and removed, and thereby the above ridge waveguide 6 is formed in a stripe shape that extends from this side of paper space toward a back side thereof.
Except for the p-type contact layer 5i of the ridge waveguide 6, on the whole surface of the p-type cladding layer 5h, an insulating layer 7 is formed, and further, on the whole surface of the p-type contact layer 5i and the insulating layer 7, an ohmic electrode layer 8 is formed.
Accordingly, the ridge waveguide 6 is electrically connected through the p-type contact layer 5i and the ohmic electrode layer 8 to the fusing metal layer 4.
More specifically, the bottom layer 5b is made of Si-doped n-type GaN and formed into a thickness in the range of substantially 5 to 15 μm. The n-type cladding layer 5c is made of n-type Al0.08Ga0.92N and formed into a thickness of substantially 0.8 μm. The n-type guide layer 5d is made of n-type GaN and formed into a thickness of substantially 0.2 μm.
The active layer 5e has a thickness of substantially several tens nanometers and a multiple quantum well structure that includes a well layer and a barrier layer that are made of InxGa1-xN (0≦x) different in the composition from one another, such as In0.08Ga0.92N and In0.01Ga0.99N. The electron barrier layer 5f is made of AlGaN and has a thickness of substantially 0.02 μm. The p-type guide layer 5g is made of Mg-doped p-type GaN and has a thickness of substantially 0.02 μm.
The p-type cladding layer 5h is made of p-type Al0.08Ga0.92N and has a thickness of substantially 0.4 μm. The p-type contact layer 5i is made of p-type GaN and has a thickness of substantially 0.1 μm.
Furthermore, the ohmic electrode layer 8 is made of any one of Pd, Pt, Au or Ni or an alloy thereof, and the insulating layer 7 is made of SiO2 or the like.
Still furthermore, the fusing metal layer 4 is made of an alloy that is generated by fusing a fusing metal layer 4a made of Au and a fusing metal layer 4b made of Sn.
The laser part 9 of the second light-emitting element 3 has a multilayer structure in which on an n-type GaAs substrate 13, a n-type buffer layer 9a, an n-type cladding layer 9b, an active layer 9c, a p-type cladding layer 9d, a smoothing layer 9e and a p-type contact layer 9f are deposited in this order.
Subsequently, by removing the p-type contact layer 9f and the smoothing layer 9e and a part of the p-type cladding layer 9d by means of the etching or the like, the ridge waveguide 10 is formed in a stripe shape that extends from this side of the paper space to a back side thereof. Furthermore, after the ridge waveguide 10 is formed, with a region where the laser part 9 is to be formed and the ridge waveguide 10 masked, a non-masked region is etched to a relatively deep portion of the n-type GaAs substrate 13, and thereby the laser part 9 having a projected sectional shape as shown in
An entire surface of the laser part 9 and the n-type GaAs substrate 13 except for the p-type contact layer 9f is covered with an insulating layer 11, and furthermore an ohmic electrode layer 12 is deposited on an entire surface of the p-type contact layer 9f and the insulating layer 11, and thereby the p-type contact layer 9f is electrically connected to the ohmic electrode layer 12 and further connected electrically through the ohmic electrode layer 12 to the fusing metal layer 4.
More specifically, the buffer layer 9a is made of Si-doped n-type GaAs and formed into a thickness of substantially 0.5 μm. The n-type cladding layer 9b is made of n-type Al0.35Ga0.15 In0.5P and formed into a thickness of substantially 1.2 μm.
The active layer 9c has a thickness of substantially ten and several nanometers and a strained quantum well structure made of GaInP and AlGaInP. The p-type cladding layer 9d is made of Zn-doped p-type Al0.35Ga0.15 In0.5P and has a thickness of substantially 1.2 μm. The smoothing layer 9e is made of p-type Ga0.51In0.49P and has a thickness of substantially 0.05 μm. The p-type contact layer 9f is made of p-type GaAs and has a thickness of substantially 0.2 μm.
Furthermore, the ohmic electrode layer 12 is made of any one of Ti, Pt, Cr, Au or Au—Zn or an alloy thereof, and the insulating layer 7 is made of SiO2 or the like.
In manufacturing the two-wavelength semiconductor laser provided with the first and second light-emitting elements 2, 3, shown in
The first light-emitting element 2 is manufactured in such a way that, on a sapphire substrate, layers from the bottom layer 5b to the p-type contact layer 5i are deposited by means of MOCVD process or the like, followed by etching to form the laser part 5 and the ridge waveguide 6, and on the entire surface thereof, the insulating layer 7, the ohmic electrode layer 8 and the fusing metal layer 4 are sequentially deposited.
The second light-emitting element 3 is manufactured in such a way that, on the n-type GaAs substrate 13, layers from the n-type buffer layer 9a to the p-type contact layer 9f are deposited by means of MOCVD process or the like, followed by etching to form the laser part 9 and the ridge waveguide 10, and over the entire surface thereof, the insulating layer 11, the ohmic electrode layer 12 and the fusing metal layer 4 are sequentially deposited.
Wafers of the first and second light-emitting elements 2 and 3 are bonded through the respective fusing metal layers 4.
The sapphire substrate of the first light-emitting element 2 is removed by use of laser lift-off process. That is, UV light that transmits sapphire and is absorbed by GaN is irradiated from the back surface of the sapphire substrate to decompose GaN in the vicinity of the interface of sapphire and GaN, and thereby the sapphire substrate is peeled. At this time, in a region other than the laser part 9, the first light-emitting element 2 is not fused. Therefore, in the region, the exposure portion 4R of the fusing metal layer is formed.
By cleaving the GaAs substrate 13, a resonator facet is formed. The cleaved substrate i.e., a laser bar carrying semiconductor laser elements that oscillate at different wavelengths is formed.
As shown in
<Multi-Layered Film of Backside Facet>
A structure of the multi-layered film (reflective film) of the backside facet in the two-wavelength semiconductor laser device is in common for the two semiconductor laser elements and, as shown in
Each of the dielectric films 1Di1, . . . 1Dik of the first reflective region 121 has a predetermined optical thickness (refractive index×film thickness) to the first wavelength λ1, and the predetermined optical thicknesses are each set at (2p+1)/4×λ1 (wherein p=0, 1, 2, . . . ). Each of the dielectric films 2Di1, . . . 2Dil of the second reflective region 122 has a predetermined optical thickness to the second wavelength λ2, and the predetermined optical thicknesses are each set at (2q+1)/4×λ2 (wherein q=0, 1, 2, . . . ). In order to obtain refractive index steps, each of the dielectric films is set differently in the refractive index from adjacently ones. Therefore, the respective film thicknesses become different from one another.
In the above structure, the phases of lights reflected by the first reflective region 121 and the second reflective region 122 are not always coincident with each other. Accordingly, the multi-layered film 12 may further contain a phase adjustment region 123 between the reflective regions 121 and 122. Since light radiated from the semiconductor has two wavelengths λ1, λ2, within a range that makes the respective phase differences of both wavelengths smaller, an optical thickness of the phase adjustment region 123 is determined. The phase adjustment region 123 is made of a single layer dielectric film that is either higher or lower in the refractive index than both dielectric films in the first and second reflective regions 121, 122 that come into contact therewith.
Another method of determining the optical thickness of the multi-layered film 12 is carried out as follows. That is, in a system where the first reflective region is present on the semiconductor and there exists infinite phase adjustment region thereon, the first reflective region is constituted of a multi-layered film the reflectance of which, seen from the semiconductor, becomes substantially the same as the first predetermined reflectance R1 to the first wavelength λ1.
The second reflective region is constituted of such a multi-layered film, in a system where the second reflective region is present on the phase adjustment region, the reflectance of which, seen from the phase adjustment region, becomes substantially the same as the second predetermined reflectance R2 to the second wavelength λ2.
As to the optical thickness of the phase adjustment region, the film thickness d3 and the refractive index n3 defining the optical thickness are determined such that the reflectance of a reflective film as a whole becomes the first predetermined reflectance R1 to the first wavelength λ1 and the second predetermined reflectance R2 to the second wavelength λ2.
The method of determining a multi-layered film according to the invention will be explained based on a flowchart shown in
As a first step (S1), in order that for all of the first and second reflective regions as well as the phase adjustment region, a high refractive index material and a low refractive index material are alternately disposed, dielectrics materials of the respective layers are determined. In k layers of dielectrics in the first reflective region, their refractive indices are denoted as n11, n12, . . . n1k to the first wavelength and m11, m12, . . . m1k to the second wavelength, and their thicknesses are in terms of d11, d12, . . . d1k. In l layers of dielectrics in the second reflective region, their refractive indices are denoted as n21, n22, . . . n2l to the first wavelength and m21, m22, . . . m2l to the second wavelength, and their thicknesses are in terms of d21, d22, . . . d2l. In the phase adjustment region, its refractive index is denoted as n3 to the first wavelength and m3 to the second wavelength, and its thickness is in terms of d3.
In the next place, as a second step (S2), film thicknesses d21, d22, . . . d2l of the respective layers in the second reflective region 122 are determined so that the optical thicknesses become (2p+1)/4×λ2 (wherein p=0, 1, 2, . . . ) to the second wavelength λ2.
Next, as a third step (S3), the reflectances R12, R22 of the second reflective region to the first and second wavelengths λ1, λ2 seen from the single layer phase adjustment region (refractive indices to the first and second wavelengths: n3 and m3) are calculated, respectively. In the third step, as shown in
Subsequently, as a fourth step (S4) shown in
In the next place, as a fifth step (S5) shown in
Subsequently, as a sixth step (S6), the ref lectances R1 (x), R2(x) of the multi-layered film as a whole for each wavelength seen from the semiconductor to the first and second wavelengths λ1, λ2, (reflectances to the first and second wavelengths: nsem1 and msem1) respectively, are calculated with the obtained reflectances R13(x), R23(x), and thereby the energy reflectances F1(x)=R1(x)×R1(x)*, F2(x)=R2(x)×R2(x)* are obtained. Here, the R1(x)* and R2(x)* denote complex conjugate values of the R1(x), R2(x), respectively. In the sixth step, as shown in
In the next place, as a seventh step (S7) shown in
Subsequently, as an eighth step (S8) shown in
According to the present invention, even when two or more wavelengths are relatively largely different from each other, a multi-layered film structure that can be high in the reflectance to a plurality of wavelengths can be attained.
With the first and second wavelengths λ1 and λ2 set at 405 nm and 650 nm respectively, a two-wavelength semiconductor laser device is designed and manufactured.
In
The first reflective region 121 that is in contact with the semiconductor is designed so as to be high in the reflectance to the wavelength 405 nm. Specifically, a dielectrics that is low in the refractive index to the wavelength 405 nm and a dielectrics that is high in the refractive index thereto are alternately deposited with film thicknesses such that the respective optical thicknesses (refractive index×film thickness) become (2p+1)/4×405 nm (wherein, p=0, 1, 2, . . . ). For instance, when SiO2 and TiO2, respectively, are used as a low refractive index dielectrics material and a high refractive index material, the refractive indices to wavelength 405 nm, respectively, are 1.47 and 2.98. Accordingly, film thicknesses where the optical thicknesses become 101 nm at p=0 in the above equation become 69 nm and 34 nm, respectively. Accordingly, as shown in
The film thickness x of a single layer of the phase adjustment region 123 that uses SiO2 as the dielectrics is determined based on the flowchart shown in
In
As another embodiment, in a two-wavelength semiconductor laser device according to a configuration example of multi-layered film (III), similarly to the above embodiment, the multi-layered film has a structure that includes a first reflective region in contact with the semiconductor and made of dielectric films of 2.5 pairs of SiO2 (69 nm)/TiO2 (34 nm), that is, 5 layers. A second reflective region deposited more separated from the semiconductor than the first reflective region and made of dielectric films of two pairs of SiO2 (112 nm)/TiO2 (63 nm), that is, 4 layers. And a phase adjustment region TiO2 (75 nm) between both reflective regions.
The values of reflectance to the wavelengths 405 nm and 650 nm in the configuration examples (I), (II) and (III) of the above multi-layered films are summarized in Tables 1, 2 and 3.
In the above embodiments, cases where SiO2 and TiO2 are used as the materials of the multi-layered films are explained. However, materials such as Al2O3, ZrO2, Si3N4, Ta2O5 and so on may be used in appropriate combinations.
A reflective region of a quarter-wave film to a short wavelength and a reflective region of a quarter-wave film to a long wavelength may be deposited in either order. However, in the first reflective region, when the low reflectance is set to the second wavelength (wavelength 650 nm in the above embodiment), over a wide range of the film thickness x of the phase adjustment region, the reflectance of the multi-layered film 12 to the second wavelength can be made higher. The dielectrics multi-layered film is easy in designing the reflectance high to short wavelengths with low to long wavelengths. Accordingly, as to the order of deposition of the first reflective region and the second reflective region, it is preferable to form a quarter-wave film to the short wavelength in the first reflective region and a quarter-wave film to the long wavelength in the second reflective region. A case where as to an order of deposition of the first reflective region and the second reflective region, a quarter-wave film to the short wavelength is formed in the first reflective region and a quarter-wave film to the long wavelength is formed in the second reflective region was explained. In another embodiment, even if the second wavelength is reflected by the first reflective region, as far as the phase difference of reflected light from the first reflective region and reflected light from the second reflective region can be sufficiently adjusted in the phase adjustment region, high reflectance can be obtained even to the second wavelength. Accordingly, the reflective region of a quarter-wave film to a short wavelength and the reflective region of a quarter-wave film to a long wavelength may be deposited in whatever order.
Furthermore, in the above embodiments, the phase adjustment region is disposed between the first reflective region and the second reflective region. However, depending on the number of layers in the first and second reflective regions, in some cases, when the film thickness of the phase adjustment region is zero, the phase difference of both wavelengths becomes smaller. In this case, the multi-layered film is formed with the first and second reflective regions in direct contact.
Still furthermore, the two-wavelength semiconductor laser device has been explained above, however, the present invention can be applied also to multi-wavelength semiconductor laser devices having three or more semiconductor laser elements.
According to a multi-wavelength laser of the invention, multi-layered films at facets of a resonator may be obtained with high reflectance to all of a plurality of wavelengths. Therefore, the characteristics of all laser elements can be improved. Furthermore, since the film formation can be collectively performed to all of the laser elements, the productivity can be also improved.
Furthermore, even in the case where a distance between light emitting spots is so small (for instance 50 μm or less) that a conventional method in which films of different structures are separately formed by use of masks can be hardly applied, according to the invention, a multi-layered film that has high reflectance to both wavelengths can be easily manufactured. Still furthermore, even in the case where difference of two wavelength ranges is so large (for instance, 405 nm band and 650 nm band) that a conventional method in which a film is designed so as to adapt to an average wavelength of two wavelengths can be applied with difficulty, the multi-layered film according to the invention can obtain sufficient characteristics.
The two-wavelength semiconductor laser device shown in
| Number | Date | Country | Kind |
|---|---|---|---|
| 2003-65186 | Mar 2003 | JP | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/JP04/02351 | 2/27/2004 | WO | 6/27/2006 |
