Surface emitting type semiconductor laser

Abstract: A surface emitting laser diode having a two-­ dimensionally extended light emitting region is disclosed in which the light emitting region includes a plurality of stripe regions (I) and gap regions (II) each formed between adjacent stripe regions, the stripe regions are optically coupled with each other through the gap regions, and an even-order diffraction grating (19, 29, 39, 49) is formed in at least a part of the light emit­ ting area. The laser diode having the above structure can emit a high-intensity, well-collimated laser beam.

Description: BACKGROUND OF THE INVENTION The present invention relates to a laser diode of the surface emitting type, and more particularly to a laser diode suitable for use in technical fields such as optical communication, optical recording or regeneration, optical interconnection and optical measurement. In other words, the present invention relates to a laser diode which can be used as the light sources of an optical communication system, a laser beam printer, an optical disk apparatus, an optical gyroscope, and others. A laser diode includes not only an edge emitting laser diode which is usually used, but also a surface emitting diode which emits a laser beam in a direction perpendicular to the upper surface of a diode chip. An example of the surface emitting laser diode is described in, for example, an article entitled "Surface Emitting GaAs/GaAlAs DFB-TJS Laser" by K. Mitsunaga et al. (The Institute of Electronics and Communication Engineers of Japan, Technical digest, OQE 86-152). In this laser diode, an optical guide layer having a second-order diffraction grating is formed between an active layer and an upper cladding layer, and a diffracted light from the grating due to the second-order Bragg reflection is subjected to distributed feedback, to generate laser oscillation. Simultaneously with the laser oscillation, a diffracted laser beam from the grating due to the first-order Bragg reflection is propagated in upward and downward directions perpendicular to the chip of the laser diode, and is taken out through a groove which is formed in a cap layer in accordance with the shape of a light emitting area. Accordingly, the groove is provided in the form of a stripe which is extended along the axial direction of a resonant cavity. The far-field pattern of a laser beam which is emitted in an upward direction perpendicular to the chip of the laser diode, is as follows. (1) As to the spread of the laser beam in a direction parallel to the axial direction of the resonant cavity, light rays with the same phase are emitted from an aperture corresponding to the length of resonant cavity (for example, aperture having a length of 300 µm), and hence the spread of the laser beam due to diffraction is very small. For example, the half-intensity angle of the intensity distribution of the laser beam is about 0.2°. That is, the laser beam is well collimated in the direction parallel to the axial direction of the resonant cavity. (2) As to the spread of the laser beam in a direction perpendicular to the axial direction of the resonant cavity, light existing in the active layer and optical guide layer are confined in a narrow region having a width of 2 to 3 µm, and hence the spread of the laser beam in the above direction due to diffraction is very large. For example, the half-intensity angle of the intensity distribution of the laser beam is about 12°. In other words, the laser beam has the form of a fan when viewed in a direction parallel to the axial direction of the resonant cavity. As mentioned above, the laser beam emitted in an upward direction perpendicular to the chip of the laser diode has the intensity distribution shown in Fig. 4A, that is, has asymmetric intensity distribution. That is, in a surface emitting laser diode having the above-mentioned structure, there arises a problem that the laser beam emitted from the surface emitting laser diode is well collimated in a direction parallel to the axial direction of the resonant cavity, but the spread of the laser beam in a direction perpendicular to the axial direction of the resonant cavity is large. JP-A-62-172780 discloses a semiconductor laser with the features included in the pre-characterizing part of claim 1. An outcoupler is formed by second-order gratings provided over the light emitting region. SUMMARY OF THE INVENTION It is an object of this invention to provide a surface emitting laser diode in which the stability of the laser oscillation having a single transverse mode is improved. This object is met by the laser diode characterised in claim 1. In accordance with the present invention, there is provided a surface emitting laser diode, in which an even-order diffraction grating is formed in a gap region existing between adjacent stripe regions. A diffracted wave from the grating due to Bragg reflection is emitted, as a laser beam, from the upper or lower surface of a laser chip. When the light intensity in the gap region is increased, the stability of the laser oscillation having a single transverse mode is improved. In order to improve the stability of such laser oscillation, it is desirable to provide electrodes for injecting currents into gap regions, at positions corresponding to the gap regions, thereby increasing the currents injected into the gap regions, or to make small the distance between adjacent stripe regions, thereby improving the optical coupling of stripe regions. BRIEF DESCRIPTION OF THE DRAWINGS The invention may take form in various parts and arrangements of parts or in various steps and arrangements of steps. The drawings are only for purposes of illustrating the preferred embodiments and are not to be construed as limiting the invention. Fig. 1 is a sectional view showing an embodiment of a semiconductor laser according to the present invention. Fig. 1B is a sectional view taken along the line IIB-IIB of Fig. 1A. Fig. 1C is a sectional view taken along the line IIC-IIC of Fig. 1A. Fig. 2A is a sectional view showing a further embodiment of a semiconductor laser according to the present invention which embodiment includes an MQW (multi-quantum well) active layer. Fig. 2B is a sectional view taken along the line IIIA-IIIA of Fig. 2A. Fig. 2C is a sectional view taken along the line IIIB-IIIB of Fig. 2B. Fig. 3A is a sectional view showing still another embodiment of a semiconductor laser according to the present invention which embodiment includes a V-grooved structure. Fig. 3B is a sectional view taken along the line IVB-IVB of Fig. 3A. Fig. 3C is a sectional view taken along the line IVC-IVC of Fig. 3A. Fig. 4A is a graph showing the intensity distribution of the laser beam emitted from a conventional semiconductor laser. Fig. 4B is a graph showing the intensity distribution of the laser beam emitted from a semiconductor laser according to the present invention. Fig. 5A is a perspective view showing the fundamental construction of a semiconductor laser according to the present invention. Fig. 5B is an enlarged view showing a part of an end surface of the semiconductor laser shown in Fig. 5A. Fig. 6 is a schematic view showing a modified version of the structure of Fig. 5B. DESCRIPTION OF THE PREFERRED EMBODIMENTS In order to attain an object which has been already described, a surface emitting laser diode according to the present invention has a large light emitting region, which is made up of a plurality of stripe regions and gap regions each provided between adjacent stripe regions. In a conventional semiconductor laser, a light emitting region has the form of a stripe which is long in the axial direction of a resonant cavity and is very short in a direction perpendicular to the axial direction of the resonant cavity. In a semiconductor laser according to the present invention, a plurality of stripe regions are formed side by side, to realize a large light emitting region with two dimension. Fig. 6A shows the fundamental construction of such a semiconductor laser, and shows the semiconductor laser viewed obliquely from above the semiconductor laser. In this semiconductor laser, a window for emitting a laser beam is provided on the upper surface of a semiconductor laser chip, and thus the laser beam is emitted in an upward direction. Referring to Fig. 6A, a laminated semiconductor structure 69 is formed on a substrate 61, and includes cladding layers 62 and 64 opposite in conductivity type to each other and an active layer 63 sandwiched between the cladding layers 62 and 64. Further, a negative or positive electrode 67 and a positive or negative electrode 68 are formed as shown in Fig. 6A. It is to be noted that the electrode 67 on the light emitting side is provided with a window corresponding to a light emitting region. However, the electrode 67 is not always required to have the window, but may be made of a transparent material. Further, the form of the window is not limited to that shown in Fig. 6A, but the window may have a desired form. Fig. 6B is an enlarged view showing a part of an end surface of the semiconductor laser of Fig. 6A. In Figs. 6A and 6B, like reference numerals designated like parts. Referring to Fig. 6B, all the regions designated by reference symbols I and II constitute the light emitting region of the semiconductor laser. That is, the light emitting region is made up of a plurality of stripe regions I and gap regions II each formed between adjacent stripe regions. A laser beam is taken out of the light emitting region in a direction perpendicular to a plane, on which the stripe regions I and the gap regions II are arranged. Each stripe region I includes an optical guide layer 65 for guiding light which is emitted from the active layer 63. The laser beam passing through a window which is provided on the upper surface of the semiconductor laser, includes light generating from the stripe regions I and light generating from the gap regions II. Hence, the stripe regions I and the gap regions II are luminous when viewed from outside. An important feature of a semiconductor laser according to the present invention resides in that 2N-th order diffraction gratings are formed in at least a part of the light emitting region. The diffraction gratings are formed in those surface areas 60 of the cladding layer 64 of Fig. 6B which are included in the gap regions II. When the 2N-th order diffraction grating (where N is an integer) is formed in the light emitting region, a laser beam can be emitted in the following manner. The Bragg reflection of laser light, that is, distributed feedback for laser light is caused by the 2N-th order Fourier component of the grating and the output coupling of laser light to upward or downward direction is caused by the Nth order Fourier component of the grating. According to the present invention, the light emitting region is two-dimensionally enlarged, and hence the spread of the laser beam due to diffraction is very small with two dimension. Further, when a plurality of stripe regions are formed in a light emitting region in accordance with the present invention, a plurality of transverse modes may occur. However, as to a fundamental transverse mode having the same phase of lasing mode in the stripe regions, the degree of optical coupling of the stripe regions is largest, and the light intensity in the gap regions is relatively large. Accordingly, when diffraction gratings for distributed feedback are formed only in the gap regions, the threshold gain for the above transverse mode is reduced, and thus the fundamental transverse mode is readily generated. When the fundamental transverse mode is taken out of the light emitting region by utilizing a lower-order Bragg reflection at the grating, a laser beam is emitted from an area of tens of microns by hundreds of microns, and thus a two-dimensionally collimated beam can be obtained. Further, when a current injected into each gap region is increased, a fundamental transverse mode having a large light intensity in the gap regions is generated, that is, the area of a light emitting portion having the same phase of lasing mode is two-dimensionally enlarged. The fundamental transverse mode having the same phase in the light emitting region can also be generated by making small the distance between adjacent stripe regions. In a case where a laser beam is taken out on the basis of Bragg reflection at the diffraction grating, it is necessary to make the electrode on the light emitting side of a conductive material permeable to the laser beam or to form a window corresponding to the light emitting region, in the above electrode. Further, as shown in Fig. 6, an electrode 77a corresponding to each gap region II and an electrode 77b corresponding to each stripe region may be formed in place of the electrode 67. In this case, a current injected into each gap portion can be made different from a current injected into each stripe region, and thus stable laser oscillation of a single transverse mode can be readily obtained. It is to be noted that in Figs. 6B and 7, like reference numerals designate like parts. Now, the present invention will be explained below in detail, on the basis of embodiments thereof. EMBODIMENT 1 Fig. 1A is a sectional view which shows an embodiment of a semiconductor laser according to the present invention. Fig. 1B is a sectional view taken along the line IIB-IIB of Fig. 1A, and Fig. 1C is a sectional view taken along the line IIC-IIC of Fig. 1A. Referring to Figs. 1A, 1B and 1C, an n-cladding layer 22, an active layer 23 and a p-cladding layer 24 are piled on an n-semiconductor substrate 21. Then, a p-optical guide layer 25 which is smaller in refractive index than the active layer 23 and is larger in refractive index than the p-cladding layer 24, is formed on the p-cladding layer 24. Then, those portions of the optical guide layer 25 which correspond to gap regions, are completely etched off so that the surface of the p-cladding layer 24 is exposed. Thus, a plurality of stripe regions (that is, four stripe regions) are formed of the optical guide layer 25. The width of each stripe region is in the range from 1 to 10 µm, and the distance between adjacent stripe regions is in the range from 1 to 8 µm. Then, a diffraction grating 29 for generating the fourth-order Bragg reflection is formed on the surface of the structure thus obtained, as shown in Figs. 1B and 1C. Thereafter, a p-cladding layer 20 for burying the optical guide layer 25 therein is grown, and a current path 26 is formed in the p-cladding layer 20 by diffusing a p-type impurity into the cladding layer 20. Further, a positive electrode 27 and a negative electrode 28 are formed as shown in Fig. 1A. In the present embodiment, the diffraction grating 29 is formed all over a light emitting region, though a desired function can be performed by forming the diffraction grating 29 only in the gap regions. The diffraction grating 29 in the stripe regions is formed on the optical guide layer 25 as shown in Fig. 1B, and hence is spaced apart from the active layer 23. Accordingly, the diffraction grating in the stripe regions has no effect on the distributed feedback of light which is generated in the active layer 23. In other words, the distributed feedback of the above light is carried out by the diffraction grating 29 in the gap regions (that is, the grating shown in Fig. 1C). Accordingly, an oscillation mode having a large light intensity in the gap regions is generated. Since laser oscillation is generated on the basis of the distributed feedback of a diffracted wave due to the fourth-order Bragg reflection, a laser beam is emitted in a direction perpendicular to the active layer, on the basis of the first- or second-order Bragg reflection at the diffraction grating. Fig. 4B shows the intensity distribution of the laser beam. In Fig. 4B, curve 53 indicates the intensity distribution in a direction perpendicular to the grooves of the diffraction grating, and has a half-intensity angle of 0.1 to 0.2°. This value corresponds to diffraction limit which is determined by the laser beam determined by the length of the resonant cavity. Further, curve 54 in Fig. 4B indicates the intensity distribution in a direction parallel to the grooves of the diffraction grating, and has a half-intensity angle of 0.5 to 1.5°. This value corresponds to diffraction limit which is determined by the laser beam determined by the total width of the stripe regions. As mentioned above, according to the present embodiment, a two-dimensionally collimated laser beam can be obtained. Further, the present embodiment is large in light emitting area, and thus can produce a light output of 500 to 1,000 mW. EMBODIMENT II Now, explanation will be made of a further embodiment of a semiconductor laser according to the present invention which embodiment is a GaAlAs laser having an MQW (multi-quantum well) active layer and a buried stripe structure, with reference to Figs. 2A, 2B and 2C. Fig. 2A is a sectional view showing the present embodiment. Fig. 2B is a sectional view taken along the line IIIB-IIIB of Fig. 2A, and Fig. 2C is a sectional view taken along the line IIIC-IIIC of Fig. 2A. Referring to Figs. 2A, 2B and 2C, an n-GaAlAs cladding layer 32, a GaAs/GaAlAs MQW active layer 33 and a p-GaAlAs cladding layer 34 are piled on an n-GaAs substrate 31. Then, a plurality of stripe regions are formed so that each stripe region has a width of 1 µm and the distance between adjacent stripe regions is equal to 1 µm. In the present embodiment, the number of stripe regions is equal to five (5), as shown in Fig. 2A. Thereafter, a p-GaAlAs layer 35 and a p-GaAlAs optical guide layer 36 are successively formed. A diffraction grating 39 having a grating constant of 240 nm is formed on the optical guide layer 36, and then an n-GaAlAs layer 30 is grown. Thereafter, a p-GaAs contact layer 3 is formed all over the surface of this wafer, and then a positive electrode 37 provided with a window and a negative electrode 38 are formed as shown in Fig. 2A. Finally, the structure thus obtained is cut off by cleavage so that the resonant cavity has a length of 100 to 600 µm. A semiconductor laser according to the present embodiment was fabricated, and it was confirmed that a well-collimated laser beam was emitted from the semiconductor laser in an upward direction. Further, the threshold current of the semiconductor laser was about one-half an ordinary threshold current, because the MQW active layer was used. The semiconductor laser was able to respond to a high frequency signal up to about 15 GHz by virtue of the quantum size effect. Further, in the semiconductor laser, a TE mode was readily generated, and moreover the stable plane of polarization was formed. EMBODIMENT III Fig. 3A is a sectional view showing still another embodiment of a semiconductor laser according to the present invention. Fig. 3B is a sectional view taken along the line IVB-IVB of Fig. 3A, and Fig. 3C is a sectional view taken along the line IVC-IVC of Fig. 3A. Referring to Figs. 3A and 3C, an n-InGaAsP optical guide layer 42 is grown on a p-InP substrate 41, and a second-order diffraction grating 49 is formed in the surface of the optical guide layer 42. Thereafter, a p-InP layer 45 is grown on the grating 49, and then three V-shaped grooves are formed as shown in Fig. 3A. Next, a p-InP layer 44, an InGaAsP active layer 43 and an n-InP layer 46 are successively grown by the liquid phase epitaxial growth method. Then, a negative electrode 48 provided with a window and a positive electrode 47 are formed. A semiconductor laser according to the present embodiment was fabricated, and it was confirmed that the semiconductor laser was able to emit a well-collimated laser beam as in the semiconductor lasers mentioned in the EMBODIMENT II. The present embodiment can emit a laser beam having a wavelength of 1.3 to 1.6 µm. Further, the present embodiment emits a thin laser beam, and hence can be readily coupled with an optical fiber for use in optical communication. Although the present invention has been described with reference to a surface emitting laser diode which has a light emitting portion on the upper surface of the diode, it is to be appreciated that the present invention is also applicable to other types of surface emitting laser diodes which include a laser diode having a light emitting portion on the lower surface of the diode. Specifically, it is to be appreciated that the present invention is applicable to all kinds of semiconductor lasers capable of generating CW (continuous wave) oscillation, and is independent of the conductivity type of a semiconductor substrate.

Claims: A semiconductor laser of the surface emitting type comprising: a substrate (21, 31, 41, 61); and a plurality of semiconductor layers (22, 23, 24, 25, 20; 32, 33, 34, 35, 36, 30, 3; 42, 45, 44, 43, 46; 62, 63, 64, 65, 66) stacked on the substrate, having an active layer (23, 33, 43, 63) which radiates light, wherein the semiconductor layers include a light emitting region which has a plurality of stripe regions (I) and gap regions (II) each placed between adjacent stripe regions, the stripe regions being laterally optically coupled to each other through the gap regions, and even-order first gratings (29, 39; 49) being provided in the light emitting region as an outcoupler, characterized in that the first gratings (29; 39; 49) are formed in said gap regions (II). The laser of claim 1, wherein the semiconductor layers further include second gratings (29) formed at the stripe regions (I), the distance between the first gratings (29; 39; 49) and the active layer (23, 33, 43, 63) being shorter than that between the second gratings (29) and the active layer (23, 33, 43, 63). The laser of claim 1 or 2, wherein the current injected into the gap regions (II) is larger than that of the stripe regions (I). The laser of any of claims 1 to 3, wherein the width of each of the gap regions (II) is smaller than that of each of the stripe regions (I).

Inventors: CHINONE NAOKI, OKAI MAKOTO, SAKANO SHINJI, TSUJI SHINJI, UOMI KAZUHISA HITACHI KOYASUDA, CHINONE, NAOKI, OKAI, MAKOTO, SAKANO, SHINJI, TSUJI, SHINJI, UOMI, KAZUHISA HITACHI KOYASUDAI APARTMENT D-401
Applicants: HITACHI LTD, HITACHI, LTD.
Application year: 1988
Main classification: H01S3
IPC codes: H01S5