DISTRIBUTED-FEEDBACK SEMICONDUCTOR LASER

Abstract: The distributed-feedback (DFB) semiconductor laser diode comprises an active layer (12) emitting light rays upon injection of an electric current; and an optical waveguide (20) adjoining the active layer, for guiding the light rays together with the active layer (12). The optical waveguide (20) comprises a first semiconductor layer (13) formed in the vicinity of the active layer (12) and having a greater bandgap than that of the active layer (12), whose thickness varies in a corrugation of a prescribed cycle in the direction of the active layer (12), and a second semiconductor layer (14) having a bandgap smaller than that of the first semiconductor layer (13), but greater than that of the active layer (12), and so formed as to confirm the corrugation of the first semiconduc­ tor layer (13). First and second cladding layers adjoin the active layer (12) and the optical waveguide (20), respectively, and have bandgaps greater than those of the active layer (12) and of the second semiconductor layer (14). First and second electrodes inject an electric current into the active layer (12) in which a PN junction is formed by forward biasing. Light rays emitted from the active layer (12) interact with the diffraction grating formed by the boundary of the corruga­ tion to receive positive feedback, resulting in laser oscillation in a single longitudinal mode.

Description: The present invention relates to a single longitudinal mode semiconductor laser, and more particularly to a semiconductor laser having a distributed feedback structure. Optical fiber communication has now reached a stage of practical application along with the reduction of losses in optical fibers and the improvement of semiconductor laser performance. The attraction of optical fiber communication lies above all in the long-distance large-capacity transmission it permits. Optical fibers, however, vary in transmission velocity with the wavelength (wavelength dispersion), and semiconductor lasers used as light sources, since they oscillate in multiaxial modes (in a plurality of modes having different wavelengths) during high-speed modulation, are susceptible to signal waveform disturbance in large-capacity transmission over a long distance. To solve these problems, many attempts are under way to develop a single longitudinal mode semiconductor laser oscillating in a single longitudinal mode even during high-speed modulation. Single longitudinal mode semiconductor lasers include distributed-feedback semiconductor lasers (DFB lasers). A DFB laser usually has a structure in which a grating is formed in an optical guide layer provided in the vicinity of an active layer and, utilizing periodic variations of the refractive index resulting from the presence of the grating, oscillates in a single longitudinal mode around the Bragg wavelength of this grating. In order to achieve satisfactory single longitudinal mode oscillation, a DFB laser requires a efficient coupling of the grating and the optical field. Therefore, the most suitable position where the grating is to be formed is an area in which there is a strong optical field. In other words, as light rays propagate through the active layer and the optical guide layer, their intensity is the greatest in or around the area between the active and optical guide layers, and it is desirable to form the grating in this area. However, in most of the previously produced DFB lasers, the grating is formed on the side of the optical guide layer farther from the active layer, where the optical field is relatively weak. (See, for example, T. Matsuoka et al., "CW Operation of DFB-BH GaInAsP/InP Lasers in 1.5 µm Wavelength Region", ELECTRONICS LETTERS, Vol. 18 (1982), No. 1, pp. 27-28.) Accordingly, the index K which represents the coupling efficiency between the grating and the optical field is no greater than 40 cm⁻¹ or so, resulting in the problem that increasing the injected current would invite oscillation in the Fabry-Perot mode instead of oscillation in a single wavelength. US-A-4025 939 describes a DFB semiconductor laser with the features of the preamble of claim 1. An object of the present invention, therefore, is to provide a DFB laser having a periodic refractive index distribution in the strongest part of the optical field and capable of satisfactory oscillation in a single longitudinal mode. This object is solved with the features of claim 1. A semiconductor laser according to the invention has a stratified structure in which a semiconductor layer having a lower refractive index than those of an active layer and an optical guide layer is buried with a periodic thickness distribution between the active layer and the optical guide layer. In semiconductor materials, generally, the greater the bandgap energy, the shorter the light emitting wavelength corresponding to that particular bandgap, and the wider the bandgap, the smaller the refractive index. In this specification, the magnitude of the bandgap energy or the bandgap wavelength will be referred to as required in place of the level of the refractive index. FIGS. 1a and 1b are longitudinal sections illustrating the production process of a first preferred embodiment of the present invention; FIG. 1c is a longitudinal section, along the laser oscillation axis, of the first preferred embodiment of the invention; FIGS. 2a and 2b are longitudinal sections illustrating the production process of a distributed feedback laser diode (DFB LD), according to a second preferred embodiment of the invention; and FIG. 2c is a longitudinal section of the DFB LD of the second preferred embodiment. Referring to FIG. 1c, a DFB LD according to the present invention comprises an active layer 12, emitting light rays upon injection of an electric current, an optical guide 20 for guiding the light rays oozing out of the thin active layer 12, a cladding layer 15, and a contact layer 16, all formed over a substrate 11. Over the bottom of the substrate 11 and the upper face of the contact layer 16 are formed a negative electrode 18 and a positive electrode 17, respectively. Every layer other than the electrodes is composed of a semiconductor material. The optical waveguide 20 comprises a low refractive index layer 13, made of a material having a lower refractive index than that of the active layer 12, and a high refractive index layer 14, made of a material having a refractive index lower than that of the active layer 12 but higher than that of the low refractive index layer 13. The low refractive index layer 13 consists of a first semiconductor layer 13, whose thickness periodically varies in a corrugation along the direction of the active layer 12. The high refractive index layer 14 consists of a second semiconductor layer 14 whose thickness periodically varies to confirm the corrugation of the first semiconductor layer 13. The first and second semiconductor layers constitute a diffraction grating on the boundary of the corrugation. As the electrodes 17 and 18 are biased forward, the light rays oozing out of the active layer interact with the diffraction index, and oscillate at a Bragg wavelength determined by the pitch of the corrugation. The cladding layer 15 has a smaller refractive index than that of second semiconductor layer 14 and, together with the substrate 11 which has a smaller refractive index than that of the active layer 12 and functions as a cladding region, confining the light rays. Now will be described the production process of the DFB LD with reference to FIGS. 1a and 1b. Referring first to FIG. 1a, over the n-InP (bandgap wavelength λg = 0.92 micron) substrate 11 are epitaxially grown the non-doped InGaAsP (λg = 1.3 microns) active layer 12 and the p-InP (λg = 0.92 micron) first semiconductor layer 13 to thicknesses of 0.1 micron and 0.2 micron, respectively. Referring next to FIG. 1b, a corrugation of 4000 Å (1Å = 0,1 nm in pitch and 1500 Å in trough depth is formed in the p-InP semiconductor layer or low refractive index layer 13 by photolithography using an interferential exposure technique and chemical etching. At this step, the etching may be so accomplished that the troughs of the corrugation reach the active layer 12. Suitable etching liquids include Br + CH₃OH and HCl. Use of an HCl-based etchant as etching liquid would provide the advantage of etching only the p-InP layer 13 but not the non-doped InGaAsP active layer 12. The p-InGaAsP (λg = 1.2 microns) second semiconductor layer or high refractive index layer 14, the p-InP cladding layer 15 and the p⁺-InGaAsP contact layer 16 are so epitaxially grown to the respective thicknesses of 0.2 micron, 3 microns and 1 micron as to bury the corrugated p-InP first semiconductor layer 13 into this structure as illustrated in FIG. 1c, which shows the complete form of the DFB LD. The thickness of the second semiconductor layer 14 is measured from the middle level between the peaks and troughs of the corrugation. Further is formed the p-electrode 17 of AuZn alloy over the p⁺-InGaAsP contact layer 16, and the n-electrode 18 of AuGeNi alloy underneath the n-InP substrate 11. In the DFB LD thereby produced, the p-InP first semiconductor layer 13, buried periodically between the active layer 12, which has the strongest optical field, and the second semiconductor layer 14, provides an extensive periodic distribution of refractive index, so that a satisfactory coupling is attained between the optical field and the grating, resulting in a satisfactory single longitudinal mode oscillation at or around the wavelength of 1.3 microns. The first semiconductor layer 13 and/or the second semiconductor layer 14 is or are interrupted at the peaks of the periodic corrugation. Further, although the non-doped InGaAsP active layer 12 and the p-InP first semiconductor layer 13 are in contact with each other in this embodiment, there may as well be, sandwiched between them, a p-InGaAsP third semiconductor layer of less than 1.3 microns in bandgap wavelength. Still further, the first semiconductor layer 13 may consist of p-InGaAsP having a smaller refractive index than that of the optical waveguide 14. If first semiconductor layer 13 is periodically interrupted, it may consist of an n-type semiconductor. If the first semiconductor layer consists of an n-type semiconductor, the current will be injected through the interruptions of the periodic corrugation of this layer, so that a periodic gain distribution will be added to the periodic refractive index distribution, resulting in oscillation in an even more stable single longitudinal mode. FIGS. 2a, 2b and 2c are sectional views illustrating the production process of a DFB laser, which is a second preferred embodiment of the present invention. In the description hereunder, like reference numerals are used to denote the identical parts of the LD. As is evident from FIG. 2c, this second embodiment differs from the first in that an optical guide 20 is formed beneath an active layer 12 herein. In FIG. 2a, an n-InGaAsP second semiconductor layer or high refractive index layer 14 of 1.2 microns in bandgap wavelength is epitaxially grown to a thickness of 0.2 micron over an n-InP substrate 11. Referring next to FIG. 2b, a 4000 Å grating is formed in the high refractive index layer or second semiconductor layer 14 by photolithography using an interferential exposure technique and chemical etching. Referring now to FIG. 2c, a low refractive index layer or n-InP first semiconductor layer 13 is so grown as to flatly fill the troughs of a corrugation formed on the surface of the high refractive index layer 14. On this occasion, the first layer 13 may be periodically interrupted by the peaks of the corrugation formed on the second layer 14. Further, after forming the non-doped InGaAsP active layer 12 of 1.3 microns in bandgap wavelength, a p-InP cladding layer 15 and a p⁺-InGaAsP contact layer 16 to the respective thicknesses of 0.1 micron, 3 microns and 1 micron, there are formed a p-electrode 17 and an n-electrode 18 (FIG. 2c). The DFB laser thereby produced provides satisfactory oscillation in a single longitudinal mode in the vicinity of 1.3 microns in wavelength because, as in the first preferred embodiment, the n-InP layer 13 periodically buried in the strongest optical field gives an extensive periodic distribution of refractive index. Although the n-InP first semiconductor layer 13 and the non-doped InGaAsP active layer 12 are in contact with each other in this embodiment, there may as well be, sandwiched between them, an n-InGaAsP third semiconductor layer of less than 1.3 microns in bandgap wavelength. Further, the first semiconductor layer 13 may be interrupted, as stated above, if only it is periodic. The low refractive index layer 13 in the second preferred embodiment of the present invention is supposed to consist of InP, but it may be composed of n-InGaAsP having a smaller refractive index than that of the high refractive index layer 14. Further, if the low refractive index layer 13 is periodically interrupted, this layer 13 may be a p-semiconductor. In this instance, the current will be injected through the layer 13 burried in an interrupted manner, so that a periodic gain distribution will be added to the periodic refractive index distribution, resulting in oscillation in an even more stable single longitudinal mode. The material is not limited to InP-based ones, but may as well be something else, such as AlGaAs-based ones. Although the preferred embodiments of the present invention described herein are DFB semiconductor lasers having an oscillation wavelength of or around 1.3 microns, the pitch of the low refractive index layer 13 to be buried may be 2000 Å, half the aforementioned 4000 Å. Or if about 4700 Å or 2350 Å is selected for this pitch and 1.55 microns for the bandgap wavelength of the active layer, there will be provided a DFB laser oscillating at a wavelength of or around 1.55 microns. A DFB laser according to the first or second preferred embodiment of the present invention has a coupling efficiency of 70 to 80 cm⁻¹ between the optical field and the grating, and is likely to provide a high light output of more than 30 mW. To sum up the remarkable features of the present invention, there is achieved an extensive periodic distribution of refractive index by burying in the strongest optical field in a multi-layered structure, comprising an active layer 12 and an optical waveguide 14 and having a light-propagating function, a semiconductor layer having a lower refractive index than that of the optical waveguide, and as a result is provided a DFB laser oscillating in a stable single longitudinal mode.

Claims: A distributed-feedback (DFB) semiconductor laser diode comprising: an active layer (12) emitting light upon injection of an electric current; an optical waveguide (20), adjoining said active layer (12), for guiding the light together with said active layer (12), said optical waveguide (20) comprising a first semiconductor layer (13) of a first conductivity type adjoining said active layer (12) and having a greater bandgap than that of said active layer, and a second semiconductor layer (14) of the first conductivity type adjoining said first semiconductor layer and having a bandgap smaller than that of said first semiconductor layer (13) but greater than that of said active layer (12), said optical waveguide (20) having a corrugation pattern of a prescribed pitch in the direction of said active layer (12); a first cladding layer (15) of the first conductivity type and a second cladding layer of a second conductivity type, said first and second cladding layers (11) adjoining said optical waveguide (20) and said active layer (12), respectively, and having bandgaps greater than those of said active layer and of said second semiconductor layer; wherein said corrugation pattern is formed on said first semiconductor layer (13), said second semiconductor layer (14) is formed to conform to the corrugation pattern of said first semiconductor layer (13), and the light emitted from said active layer (12) interacts with the diffraction grating formed by the boundary of said corrugation pattern and causes said diode to lase, characterized in that the first and/or the second semiconductor layer(s) (13,14) is/are interrupted at the peaks of the periodic corrugation. A laser diode as recited in Claim 1, wherein a third semiconductor layer is sandwiched between the active layer (12) and the first semiconductor layer (13). A laser diode as recited in Claims 1 or 2, wherein said active layer (12) is formed on said optical waveguide (20). A laser diode as recited in Claims 1 or 2, wherein said optical waveguide (20) is formed on said active layer (12).

Inventors: YAMAGUCHI, MASAYUKI
Applicants: NEC CORPORATION
Application year: 1985
Main classification: H01S3
IPC codes: H01S5