SEMICONDUCTOR LASER

Abstract: An inner stripe with self-aligned saturable-absorber structure semiconductor laser comprises a semiconductor substrate (56) of a first conductivity type; a semiconductor current-blocking layer (CBL) (58) of a second conductivity type formed on the substrate (56) and having a stepped chan­ nel composed of an optical-guiding channel (66) and current-­ restricting channel (64) which is grooved at the bottom of the optical-guiding channel (66) through the CBL (58) into the substrate (56); a semiconductor cladding layer (68) of a first conductivity type formed over the CBL (58) to bury the stepped channel; a semiconductor active layer (70) of a first or second conductivity type formed on the cladding layer (68); and a semiconductor cladding layer (72) of a second conductivity type formed on the active layer (70), whereby laser noises can be markedly suppressed through wide tem­ perature fluctuations and optical feedback range.

Description: This invention relates to the i nner s tripe with s elf-aligned s aturable-absorber (IS³) laser, which is characterized by its low noise operation. Semiconductor lasers are employed in home-use optical-data processing systems such as digital audio disks (DAD) and video disks (VD). Semiconductor lasers enable the reduced size and cost and the marketability of these systems. However, the noise characteristics of semiconductor lasers has required improvement before they could be ideally utilized in these systems. Noise suppression is essential for the VD players, which read analog signals, while DAD players, which read digital signals, have higher noise allowances. Noise suppression, however, also becomes critical in DAD players as their sizes decrease. Relative-intensity-noise (RIN) levels of less than 5x10⁻¹⁴ Hz⁻¹ and 1x10⁻¹¹ Hz⁻¹ are required in analog and digital playback systems, respectively. RIN is defined as RIN = (ΔP/P)²/Δf , where P is the average output power and ΔP is the noise power as measured in band Δf around the center frequency fo. Single-longitudinal-mode index-guided lasers suffer from mode-hopping noise, which is caused by ambient temperature fluctuations, and optical feedback noise, which is induced by the light reflecting back to the emitting facet. Thus, RIN is so high (∼ 10⁻¹¹ Hz⁻¹ around fo = 200 KHz) that single-longitudinal-mode lasers are generally not used even in digital playback systems. On the other hand, multi-longitudinal-mode gain-guided lasers are less sensitive to the temperature fluctuations and the light feedback because not all the longitudinal modes but a few are sensitive at a same time. Therefore, gain-guided lasers are preferably used in digital playback systems such as DAD players. However, competition among their longitudinal modes increases RIN as high as 1x10⁻¹³ Hz. Therefore, gain-guided lasers cannot be used for analog playback systems such as VD players. In Fig. 1, the oxide-stripe laser is depicted as an example of gain-guided laser. The oxide-stripe laser comprises an n-type GaAs substrate 2, an n-type Aℓ x Ga 1-x As cladding layer 4, an n- or p-type Aℓ y Ga 1-y As (0 ≦ y < x ≦ 1) active layer 6, a p-type Aℓ x Ga 1-x As cladding layer 8, a p-type GaAs ohmic layer 10, a SiO₂ insulating layer 12 and metal electrodes 14 and 16. A pumping current is injected into the active layer 6 through the stripe window opened in the insulating layer 12. Thus, optical gain is restricted along the junction plane and single filament operation is realized. The oxide-stripe laser, as well as other gain-guided lasers, has rather long astigmatic distance of about 20 µm or more. Therefore, some compensation optics are required when gain-guided lasers are utilized in optical playback systems. Fig. 2 shows the channeled substrate planar (CSP) laser, which is disclosed in Japanese Patent Publication (Kokoku) No. 54-5273, as an example of index-guided lasers. The CSP laser comprises an n-type GaAs substrate 2 provided with an etched channel 18, an n-type Aℓ x Ga 1-x As cladding layer 4, an n- or p-type Aℓ y Ga 1-y As (0 ≦ y < x ≦ 1) active layer 6, a p-type Aℓ x Ga 1-x As cladding layer 8, a p-type GaAs ohmic layer 10, a SiO₂ insulating layer 12, and metal electrodes 14 and 16. In the region outside the channel 18, the n-type cladding layer 4 is thin enough for the optical field to penetrate the lossy substrate 2 and, therefore, an effective refractive index difference is provided. Thus, the optical field is confined to the channel region. The CSP laser, as well as other index-guided lasers, has short astigmatic distance of less than 5 µm. Therefore, no compensation optics are necessary, using the CSP laser in optical playback systems. However, the CSP laser exhibits so high value of RIN that it is hardly used, as described earlier. Japanese Patent Disclosure (Kokai) No. 57-159084 discloses a V-channeled-substrate inner-stripe (VSIS) laser in which an n-type GaAs layer grown on p-type GaAs substrate acts as a current-blocking layer (CBL). The basic structure of this prior art is shown in Fig. 3. The VSIS laser comprises a p-type GaAs substrate 18, an n-type GaAs layer 20, which act as a CBL, a channel 34 etched into the CBL 20 to reach the substrate 18, a p-type Aℓ x Ga 1-x As cladding layer 22, a p- or n-type Aℓ y Ga 1-y As (0 ≦ y < x ≦ 1) active layer 24, an n-type Aℓ x Ga 1-x As cladding layer 26, an n-type GaAs ohmic layer 28, and metal electrodes 30 and 32. The index-guiding mechanism of the VSIS laser is similar to that of the CSP laser. In the VSIS laser, however, the current is also restricted in the channel 34 because of CBL, while it isn't in the CSP laser. As a result, the VSIS laser has an intermediate characteristics between a gain-guided laser and an index-guided laser: multi-longitudinal-mode operation at low power level and rather short astigmatic distance (5 ∼ 15 µm). Owing to these favorable characteristics, the VSIS laser is preferably used in DAD players. However, the multi-longitudinal-mode VSIS laser generally exhibits higher value of RIN than required in analog playback systems such as VD players. Prior art document EP-A3-0 149 563 which forms state of the art in respect of the designated Contracting States DE and GB according to Article 54(3) EPC discloses a semiconductor laser which includes a current blocking layer formed on a substrate, a first cladding layer formed on the current blocking layer, an active layer formed on the first cladding layer, and a second cladding layer formed on the active layer. An indent is provided in the current blocking layer near the center of a laser element, and a V-shaped groove is formed in the current blocking layer across the indent. A gain guide structure is provided where the indent is formed, and an index guide structure is arranged near a cleaved facet. This semiconductor laser ensures a stable transverse mode laser emission, and the influence of the return beam reflected from the disc surface and mode competition noise are minimized. Further, prior art documents Patent Abstracts of Japan, Volume 6, No. 140 (E121) (1081), 29 July 1982, and JP-A-5 763 885 describe a semiconductor laser device in which a first layer has an opening in the middle thereof, and a further second layer has a larger opening. These first and second layers are provided on a substrate. On the substrate and both, first and second layers are laminated in turn a clad layer, a third layer, a further clad layer and a gap layer. Finally, a stripe electrode and a further electrode are formed on the gap layer and the substrate, respectively. Thus, this known semiconductor laser device comprises a two-step channel together with a stripe electrode, wherein the channel is formed by first and second layers. Finally, prior art document IEEE JOURNAL OF QUANTUM ELECTRONICS, Volume QE-19, No. 6, June 1983, pages 1009-1015, New York, US; S. YAMAMOTO et al.: "680nm CW operation ..." discloses a VSIS laser structure in which a V-shaped groove is provided in a current block layer to reach the surface region of a substrate. It is an object of the present invention to provide a semiconductor laser which operates with a value of RIN low enough and with a astigmatic distance short enough to be utilized in analog playback systems such as VD players. To solve this object the present invention provides a semiconductor laser as stated in claim 1. The optical-guiding channel has preferably a width of 3.8 to 4.2 µm and a depth of 0.3 to 0.8 µm. The current-restricting channel has preferably width of 0.5 to 1.5 µm. In the semiconductor laser of this invention, deformation-preventing layers can be inserted in the CBL so as to prevent the shoulders of the stepped channel from sagging during crystal growth. When the CBL is made of GaAs, the deformation-preventing layers can be made of Al x Ga 1-x As (0.1 < x < 1) abd is preferred to be less than 0.1 µm thick and not to be exposed at the CBL surface. The deformation-preventing layers should be formed such that they are exposed on side-walls of the optical-guiding channel and current-restricting channel, preferably near the shoulders of them. In the semiconductor laser provided by this invention, the current injected into the active layer is reduced at the edges of the optical-guiding channel adequately to make these active layer portions act as saturable-absorbers. Thus, the saturable-absorbers are self-aligned on the optical-guiding channel edges and cause the self-pulse modulation of the laser oscillation. The self-pulse modulation makes coherent length of the emitted light reduce markedly. Thus, is provided the i nner s tripe with s elf-aligned s aturable-absorber (IS³) laser, which is insensitive to the feedback light and temperature fluctuations. This invention can be more fully understood from the following detailed description when taken in conjunction with the accompanying drawings, in which: Fig. 1 is a cross-sectional view showing a conventional oxide-stripe type gain-guided laser; Fig. 2 is a cross-sectional view showing a conventional CSP type index-guided laser; Fig. 3 shows a cross-sectional view showing a conventional VSIS laser; Fig. 4 is an explanatory diagram for the operating mechanisms of the conventional VSIS laser; Fig. 5 is an explanatory diagram for the operating mechanisms of an innovative IS³ laser; Figs. 6 to 10 show cross-sectional views showing a manufacturing process of the semiconductor laser according to the embodiment of the present invention; Fig. 11 shows a cross-sectional view showing a semiconductor laser according to another embodiment of the present invention; Fig. 12 shows a typical current-light characteristics of the semiconductor laser shown in Fig. 11; Fig. 13 shows a typical emission spectrum at 5 mW output (denoted by A in Fig. 12); Fig. 14 shows a cross-sectional view of a laser according to another embodiment of the present invention. The operating mechanisms of a IS³ laser, which uses a stepped channeled substrate, will be explained below in comparison with that of a conventional VSIS laser, which uses a V-channeled substrate. Figs. 4 and 5 show channel portions of the VSIS laser and the IS³ laser, respectively. In Figs. 4 and 5, reference numerals, 36, 38, 40, 42 and 44 show a semiconductor substrate, a current-blocking-layer (CBL), a first cladding layer, an active layer, and a second cladding layer, respectively. The broken lines 46 in Figs. 4 and 5 show that the current, injected through the stripe window opened in the CBL 38, spreads as it flows towards the active layer 42. Using the stepped channel, the current width at its bottleneck and the optical guide width can be varied independently, while they cannot be, using the V-channel. Moreover, using the stepped channel, the bottle-neck of the current can be located much closer to the active layer 42, resulting in narrower current width in the active layer 42. Thus, the current injected into the active layer can be intentionally reduced at the edge regions (denoted by II in Fig. 5) of the optical guide. In Figs. 4 and 5, gain distributions (48 and 52, respectively) and optical density distributions (50 and 54, respectively) are shown. By the way, it is well known that the active layer of a semiconductor laser acts as an saturable-absorber when the injected current is reduced. Therefore, the saturable-absorbers are self-aligned at the edges of the wave guide when the stepped channel is used. It is also well known that self-pulse modulation arises and makes the laser insensitive to the optical feedback when the capacity of the saturable-absorbers is adequately adjusted. Therefore, the laser with such a low noise as to be used in VD players can be made more reproducibly using the stepped channel than using the V-channel. An experimental result revealed that the broader becomes the region II, the higher becomes the maximum light power at which the self-pulse modulation ceases. Hence, the current-restricting channel 64 is preferred to be much narrower than the optical-guiding channel 66, while the optical-guiding channel 66 has to be narrower than the critical width for fundamental transverse mode operation. Consequently, accordingly to a main embodiment of the invention, the optical-guiding channel 66 has a width of 3 to 5 µm and the current-restricting channel 64 has a width of less than 2 µm. As to the depth of the optical-guiding channel 66, it is 0.2 to 1.0 µm. If it is greater than 1.0 µm, the current spreads substantially in the active layer 42 and the region II vanishes effectively. On the other hand, if the depth of the optical-guiding channel 66 is less than 0.2 µm, the channel cannot guide the optical field. Reactive ion etching (RIE) is capable of fine pattern delineation with vertical etched walls. Therefore, RIE is effective technique to groove the stepped channel of the present invention. The present invention will be explained below in connection with examples. Example 1 The IS³ laser of the present invention was manufactured as shown in Figs. 6 to 10: (1) As shown in Fig. 6, about 0.7 µm thick n-type GaAs layer (CBL) 58 was grown on the p-type GaAs substrate 56. Then, about 0.25 µm thick SiO₂ layer 60 was sputtered on the CBL 58. A photoresist layer 62 with a stripe window having a width u was made by photolithography; (2) The SiO₂ layer 60 was selectively etched by buffered fluoric acid through the stripe window of the width u. The width w of the resultant stripe window opened in the SiO₂ layer 60 was controlled by the etching time. (See Fig. 7); (3) The CBL 58 was etched by RIE, as shown in Fig. 8, using the photoresist layer 62 as a mask and a channel 63 was formed. The channel width was equal to the stripe window width u, while the channel depth was controlled by the etching time; (4) After removing the photoresist layer 62, the CBL 58 was etched again by RIE as shown in Fig. 9. This time, the SiO₂ layer 60 was used as a mask. Thus, a stepped channel composed of a current-restricting channel 64 and a wave-guiding channel 66 was formed to reach the substrate 56. The width of the wave-guiding channel 66 was equal to the stripe window width w, while the depth of that was controlled by the etching time. On the other hand, the sizes of the current-restricting channel 64 were same as those of the channel 63; (5) After removing the SiO₂ layer 60, double-hetero (DH) structure with n-type GaAs ohmic layer 74 was grown by liquid phase epitaxy (LPE) on the channeled substrate as shown in Fig. 10. The DH structure was composed of a p-type Aℓ 0.45 Ga 0.55 As cladding layer 68, a p-type Aℓ 0.13 Ga 0.87 As active layer 70 and an n-type Aℓ 0.45 Ga 0.55 As cladding layer 72. Finally, metal electrodes 76 and 78 were formed on the rear and front surfaces, respectively. Example 2 As shown in Fig. 11, the fabrication procedure of the present example was same as that of the former example except that Aℓ 0.3 Ga 0.7 As deformation-preventing layers 82a and 82b were inserted so as to be exposed at the side-walls of the optical-guiding channel 66 and the current-restricting channel 64, respectively. During the LPE of the DH structure, GaAs tends to migrate from the channel shoulders to the channel corners because surface energy is higher at shoulders than at corners. As a result, reproducibility of the channel sizes were poor without the deformation-preventing layers 82a and 82b. With these layers, the stepped channel was free from this deformation, which is generally called "mass-transfer", "melt-back" or "etch-back". Fig. 12 shows a typical current-light characteristics of the IS³ laser of the present invention. As shown in Fig. 13, the IS³ laser of the present invention exhibits multi-longitudinal-mode operation. With the IS³ laser, each longitudinal mode spectrum is broad, and the whole spectrum seems to be composed of overlapping spectra having differing mode spacing. This unique spectrum and self-pulse modulation observed in the IS³ laser suggests that repetitive Q-switching is caused because of the self-aligned saturable-absorbers. Considering the well-known phenomena that the band gap and refractive index vary when the carrier density in the active layer varies, the IS³ laser spectrum can be described as being derived from time domain variations in the carrier density during the repetitive Q-switching. Relative intensity noise (RIN) of the IS³ laser was observed less than 1x10⁻¹⁴ Hz⁻¹ across a 10°C to 60°C temperature range and with optical feedback of up to 1%. This low noise property is attributed to the short coherent length derived from the self-pulse modulation and satisfies the noise specification for analog playback systems such as VD players. The IS³ laser was observed to operate in a fundamental transverse mode with an astigmatic distance of less than 15 µm at output power of up to 10 mW. Example 3 Fig. 14 shows another example of the present invention. It is same as the Example 1 shown in Fig. 10 except that the active layer 106 is bent down towards the stepped channel. This structure was easily made by setting the crystal growth time of the p-type cladding layer 104 shorter than that in the Example 1. In this structure, the current width in the active layer 106 is reduced because the active layer 106 is bent down so as to be located closer to the current-restricting channel 114. Moreover, a bent active layer forms an optical guide by itself and, therefore, the optical guide width is also reduced less than the width of the optical-guiding channel 116. Thus, with the bent active layer, the IS³ structure is fabricated reproducibly even if the stepped channel is deformed during the LPE. Although AℓGaAs and GaAs have been used as semiconductor materials in the examples, this invention is not restricted to those. Any semiconductor materials may be used so long as they can be used for a laser diode. As to the masking materials for RIE, not only the photoresist and SiO₂, which were used in the examples, but also other dielectric materials such as Aℓ₂O₃ and Si₃N₄ can be used. As described above in detail, the present invention provides the IS³ ( i nner s tripe with s elf-aligned s aturable-absorbers) structure. The laser with this structure operates with low noise as well as a small astigmatic distance. Thus, using this invention, it is possible to manufacture semiconductor lasers suitable for analog playback systems, which require very low noise lasers with a small astigmatic distance.

Claims: A semiconductor laser comprising: a semiconductor substrate (56) of a first conductivity type; a semiconductor current-blocking layer (58) of a second conductivity type formed on the substrate (56); a first semiconductor cladding layer (68) of a first conductivity type formed over the current-blocking layer (58); a semiconductor active layer (70) of a first or second conductivity type formed on the first cladding layer (68); and a second semiconductor cladding layer (72) of a second conductivity type formed on the active layer (70), wherein: said current-blocking layer (58) has a stepped channel therein composed of a first channel (66) and a second channel (64) which is grooved at the bottom of the first channel (66), and said first cladding layer (68) buries the stepped channel, characterized in that said first channel is formed by an optical-guiding channel (66) and said second channel is formed by a current-restricting channel (64) which is grooved at the bottom of the optical-guiding channel (66) through the current-blocking layer (58) into the substrate (56), said optical-guiding channel (66) is 3 to 5 µm in width and 0.2 to 1.0 µm in depth, and said current-restricting channel (64) has a width of less than 2 µm. A semiconductor laser according to claim 1, characterized in that said optical-guiding channel (66) is 3.8 to 4.2 µm in width and 0.3 to 0.8 µm in depth. A semiconductor laser according to claim 1 or 2, characterized in that said current-restricting channel (64) has a width of 0.5 to 1.5 µm. A semiconductor laser according to claim 1, characterized in that deformation-preventing layers (82a, 82b) are formed so as to be exposed at the side-walls of said optical-guiding channel (66) and said curent-restricting channel (64). A semiconductor laser according to claim 4, characterized in that said substrate (56) and said current-blocking layer (58) are made of GaAs and said deformation-preventing layers (82a, 82b) are made of Al x Ga 1-x As where 0.1 < x < 1. A semiconductor laser according to claim 1, characterized in that said first and second cladding layers (68, 72) and said active layer (70) are made of AlGaAs and said substrate (56) and said current-blocking layer (58) are made of GaAs. A semiconductor laser according to claim 1, characterized in that said active layer (106) is bent down towards said stepped channel.

Inventors: KURIHARA, HARUKI C/O PATENT DIVISION, MATSUMOTO, KENJI C/O PATENT DIVISION, SUZUKI, KAZUO C/O PATENT DIVISION, TAMURA, HIDEO C/O PATENT DIVISION
Applicants: KABUSHIKI KAISHA TOSHIBA
Application year: 1985
Main classification: H01S3
IPC codes: H01S5