JP2006269781A - Semiconductor light emitting device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To reduce a reflection factor on an outgoing end surface by minimizing a spot size at an optical waveguide while securing a sufficient gain. <P>SOLUTION: This semiconductor light emitting element 10 has a window structure where a window area E2 is formed between at least one element end face and the tip of the optical wave guide having a gain on the element end face, and the optical waveguide is formed with a spot size reduction area E3 arranged so as to contact with the connection surface of the window area E2 and the optical waveguide. The spot size reduction area E3 is formed with a width minimizing the spot size of the optical waveguide on a connection surface with the window area E2 so that a waveguide width may be reduced toward the window area E2. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a semiconductor light-emitting device having a window structure in which an active layer region in an optical waveguide is interrupted in the vicinity of an end surface, and has a window structure formed on at least one of the end surfaces. The present invention relates to a semiconductor light emitting device capable of reducing the reflectance.

  A semiconductor light-emitting device uses a phenomenon in which an energy difference when electrons and holes are combined is emitted as light. Light-emitting diodes (LEDs) and superluminescents are produced by differences in the waveguide structure. It is classified into a diode (Superluminescent Diode: SLD), a laser diode (Laser Diode), and the like. All have excellent characteristics such as small size, high reliability, low price, and low power consumption. Therefore, LED is used for various lighting and display devices, SLD is used for various measurement light sources, and LD is used for optical fiber communication and measurement. Widely used in light sources and processing equipment.

  Of the above, the LD usually has two end faces formed by cleaving, and this acts as a reflecting mirror to cause light to reciprocate between them, causing laser oscillation. If an external cavity type (External Cavity: EC) LD is configured to oscillate by lowering and extracting light to the outside and optically coupling with an arbitrary reflecting mirror, the wavelength can be arbitrarily set by using a movable diffraction grating or the like for the reflecting mirror. It can be controlled and is useful as a measurement light source when measuring the wavelength dependence of an optical component. In addition, the light-emitting element that does not oscillate by reducing the reflectance of one end face operates as an SLD that emits incoherent light.

  Therefore, in the light emitting element for EC-LD (hereinafter simply referred to as EC-LD) and SLD, it is desirable to reduce the end face reflectance as much as possible so as not to cause laser oscillation. Usually, a non-reflective (AR) coating with a dielectric multilayer film is applied, but with this alone, the reflectance can only be reduced to about 0.1%. Moreover, since the reflectance of the AR coating is highly wavelength-dependent, even if an EC-LD having a wavelength variable width exceeding 100 nm and an SLD having a wide emission spectrum half-value width exceeding 100 nm are all realized, It was not possible to obtain a low reflectance at a wavelength of. If the reflectance is not lowered sufficiently, the EC-LD may become unstable in operation or generate a wavelength band that does not oscillate. In SLD, periodicity of emission spectrum called ripple appears.

  Therefore, in order to further reduce the reflectance at the light emitting end face of the light source, for example, a semiconductor light emitting element having a window structure disclosed in Patent Document 1 below is known.

As shown in FIG. 3, the semiconductor light emitting device 51 disclosed in Patent Document 1 is formed along a longitudinal direction on the upper surface of the semiconductor substrate 52 and the semiconductor substrate 52, and is a first cladding layer of the first conductivity type. And a mesa stripe portion 53 having a trapezoidal cross-sectional shape composed of the active layer and the second conductivity type second cladding layer, and a window between the mesa stripe portion 53 and at least one end of the semiconductor substrate 52 in the longitudinal direction. It is formed with the region 54a or 54b interposed. Further, the end surfaces 55a and 55b on the one end region side in the mesa stripe portion are formed to be inclined at an acute angle as shown in the figure, and non-reflective films 57a and 57b are formed on the end surfaces 56a and 56b.
Japanese Patent Laid-Open No. 2003-17809

  By the way, in the semiconductor light emitting device having this type of window structure, (1) the window length of the window structure, (2) the spot size of light in the optical waveguide (the electric field of light when viewed in a cross section perpendicular to the optical waveguide direction) The distribution has a characteristic that the reflectance at the light emitting end face is determined.

  First, regarding (1), the longer the window length of the window structure is, the more light is emitted from the light emitting end face, so that the reflectance at the light emitting end face can be lowered.

  Regarding (2), the reflectance at the light exit end face can be lowered as the spot size of light in the optical waveguide is reduced.

  Specifically, FIG. 4 shows the relationship between the spot size and the reflectance at the light emitting end face when the light emitting end face is cleaved and the window region length is 25 μm and 40 μm. As is apparent from FIG. 4, it can be seen that the reflectance at the light emitting end face is lower when the window region length is 40 μm than when the window region length is 25 μm. In addition, even when the window region length is the same, it can be seen that the smaller the spot size, the lower the reflectance at the light exit end face.

  Thus, the longer the window region length, the lower the reflectivity at the light emitting end face, but the longer the window region length, the longer the device length and the larger the overall structure of the device. was there.

  In addition, the spot size of light in the above-described optical waveguide is determined by the active layer width of the semiconductor light emitting device, and the spot size is minimum when the active layer width is a certain value, and the value of the active layer width at this time is the center. The spot size is increased regardless of whether the active layer width is increased or decreased. FIG. 5 shows the relationship between the active layer width and the spot size. In this example, the spot size is minimized when the active layer width is 1.8 μm. However, when the active layer width was set to 1.8 μm, the maximum gain could not be obtained when the electric field distribution of light was in a single mode when viewed in a cross section perpendicular to the optical waveguide direction. For this reason, in the conventional semiconductor light emitting device, the active layer width is set to a constant width so that the maximum gain can be obtained when the light spot size in the optical waveguide is single mode (in the example of FIG. Active layer width shown 2.0 μm).

  However, if the active layer width is set so as to obtain the maximum gain, the light spot size in the optical waveguide is not minimized, and the reflectance at the light emitting end face cannot be sufficiently reduced to a desired value. It was. If the active layer width is set so that the spot size of light in the optical waveguide is minimized, the reflectance at the light emitting end face can be reduced, but the maximum gain cannot be obtained.

  Therefore, the present invention has been made in view of the above problems, and an object of the present invention is to provide a semiconductor light emitting device capable of reducing the reflectance at the light emitting end face while ensuring a sufficient gain. is there.

In order to achieve the above object, in the semiconductor light emitting device according to claim 1, the window region E2 is formed on at least one device end surface between the device end surface and the tip of the optical waveguide having a gain. In the semiconductor light emitting device 10 having a window structure,
The optical waveguide includes a spot size reduction region E3 disposed so as to be in contact with a connection surface between the window region and the optical waveguide.

Moreover, the semiconductor light-emitting device according to claim 2 is the semiconductor light-emitting device according to claim 1,
The spot size reduction region E3 is characterized in that the waveguide width decreases toward the window region E2.

  According to the semiconductor light emitting device of the present invention, since the active layer is formed with a width that minimizes the spot size of the optical waveguide at the connection surface with the window region, it is possible to transmit light in the optical waveguide while ensuring a sufficient gain. The reflectance at the light exit end face can be reduced by minimizing the spot size.

  Hereinafter, embodiments of the present invention will be specifically described with reference to the accompanying drawings. FIG. 1 is a schematic configuration diagram showing an example of a semiconductor light emitting device according to the present invention. FIG. 1 (a) is a top view of the device, FIG. 1 (b) is a cross-sectional view along line AA in FIG. (C) is a sectional view taken along line BB of (a), and FIG. 2 is a schematic plan view showing another example of the semiconductor light emitting device according to the present invention.

  First, a schematic configuration of the semiconductor light emitting device of this example will be described with reference to FIGS.

  As shown in FIGS. 1A to 1C, in the semiconductor light emitting device 10 of this example, a mesa structure is formed on an n-InP substrate 1 as a semiconductor substrate, and an active layer 2 is formed in the mesa and on the upper surface thereof. The p-InP clad layer 3 is sequentially laminated as a clad layer. In the semiconductor light emitting device 10, the active layer region E1 composed of the active layer 2 extends a predetermined length from the reflection end face 10a toward the light exit end face 10b, is interrupted in the vicinity of the light exit end face 10b, and has a window between the light exit end face 10b. Region E2 is formed. By adopting the window structure having the window region E2, the reflectance of about 30% when the light emitting surface 10b without the window structure is a cleaved surface can be reduced to about 1/4.

  Further, as shown in FIGS. 1B and 1C, the p-InP current blocking layer 4 and the n-InP current blocking layer 5 are embedded as embedded layers on both sides of the mesa and the window region E2. The current path is narrowed and a stripe type optical waveguide is formed.

  The reflection end surface 10a may be a surface having a predetermined reflectivity, and is composed of, for example, a cleavage surface, or a low reflection surface such as HR or LR. The light emitting end face 10b is provided with an AR-coated non-reflective film made of a dielectric multilayer film for the purpose of reducing reflection at the end face. Thereby, the reflectance of about 30% when the light emitting surface 10b is a cleavage plane can be reduced to about 0.1%.

  Here, in the element configuration of this example, in order to further reduce the reflectance at the light emitting end face 10b, the active layer region E1 extends from the reflecting end face 10a to the window region E2 side by a predetermined length with a width H1, A tapered spot size reduction region E3 is formed from the midway position so as to have a width H2 (<H1) on the connection surface with the window region E2. The width H2 of the connection surface with the window region E2 in the tapered spot size reduction region E3 is formed so that the spot size of the optical waveguide at the connection surface with the window region E2 is reduced. Since the reflectance at the exit end face is determined by the spot size at the connection surface between the window region and the optical waveguide and the length of the window region, the spot size is reduced depending on the width H2 of the connection surface depending on the length of the window region. However, it is most preferable that the spot size be minimized.

  Referring to FIG. 5 as an example, specific values are shown. The width H2 of the spot size reduction region E3 is set to 1.8 μm at which the spot size on the connection surface with the window region E2 is minimized. In addition, the width H1 of the active layer region E1 serving as a boundary between the single mode and the multimode in FIG. 5 varies depending on the layer structure of the element (for example, the layer structure and thickness of the active layer 2, the layer structure of the cladding layer, etc.). 5, the width at the boundary between the single mode and the multimode is also changed around 2.0 μm as a boundary, but in this example, the active layer region is set to a value that becomes a single mode (for example, about 2.0 to 2.2 μm). A width H1 of E1 is set.

  The width H1 of the active layer region E1 is preferably set to a width that can compensate for the gain that is reduced by the spot size reduction region E3. As a result, while ensuring a desired gain, the spot size on the connection surface with the window region E2 can be minimized to reduce the reflectance at the light emitting end face 10b. Further, the length of the window region E2 is set to such a dimension that light emitted from the active layer 2 is not diffusely reflected by the upper surface side electrode due to the thin p-side layer thickness, for example, 25 to 40 μm, preferably 30 to 35 μm. To do.

  In the semiconductor light emitting device 10 of this example configured as described above, if the width H1 of the active layer region E1 is 2.0 μm and the width H2 of the spot size reduction region E3 is 1.8 μm, for example, the active layer region E1 When compared with a conventional window structure semiconductor light emitting device having a width H1 of 2.0 μm, the gain is slightly reduced, but the spot size of the optical waveguide at the connection surface with the window region E2 is minimized, so that the light emission The reflectance at the end face 10b can be reduced. In addition, if the width H1 of the active layer region E1 is set to be larger than 2.0 μm in the single mode state, a decrease in gain in the spot size minimizing region E3 can be compensated, and a desired gain is secured. The reflectance at the light emitting end face 10b can be reduced.

On the other hand, as one of the techniques for increasing the output of InP-based light emitting devices, InGaAsP is used as an n-type cladding layer as described in Patent Document 2 below in order to suppress loss due to valence band absorption occurring in the p-type cladding layer. The quaternary clad structure used is known.
Japanese Patent No. 3525257

  When such a structure is used, the average refractive index of the cladding is increased, the refractive index difference from the active layer being the core is reduced, and the cut-off width of the transverse mode is considerably widened. That is, the width H1 of the active layer region E1 can be expanded to about 4 to 6 μm, and a large gain can be obtained. By implementing the present invention, the window layer structure can sufficiently reduce the reflectivity reduction effect by narrowing the active layer width to a width that minimizes the spot size at the connection surface with the window region. EC-LD and SLD with high output and low ripple are realized.

  Incidentally, the semiconductor light emitting element 10 described above has a structure having a window structure only on one end face side, but may have a structure having a window structure on both end face sides. For example, in the case of a semiconductor light emitting element used in an optical amplifier or the like, as shown in FIG. 2, the semiconductor light emitting element 10 has window regions E2 (E2a, E2b) on both end face sides of the element. At that time, spot size reduced regions E3 (E3a, E3b) are formed at both ends of the active layer region E1, respectively. Further, window regions E2 (E2a, E2b) are formed between the spot size reduction region E2 and the light emitting end faces 10b, 10b, respectively. Even in the case of such a semiconductor light emitting device having a window structure on both end face sides, the light emitting end faces 10b and 10b are made by minimizing the spot size on the connection surface with the window region E2 while ensuring the desired gain as described above. Can be reduced, and the same effect as in the case of one window region E2 can be obtained.

  Thus, the semiconductor light emitting device of this example has the window region E2 on at least one end face side, and the spot size in the optical waveguide of the laser light emitted from the active layer region E1 is connected to the window region E2. The active layer 2 is formed so as to have a spot size reduced region E3 at the tip of the active layer region E1 so as to be minimized on the surface. And the light radiate | emitted from the active layer area | region E1 is finally output from the light-projection end surface 10b through the window area | region E2 as single mode light with the minimum spot size in the spot size reduction area | region E3. Therefore, by forming the active layer 2 by appropriately setting the width H1 of the active layer region E1 and the width H2 of the spot size minimizing region E3, a connection surface to the window region while sufficiently securing a desired gain. In this case, the reflectance at the light exit end face can be reduced by minimizing the spot size.

  As mentioned above, although the best form was demonstrated using this invention, this invention is not limited with the description and drawing by this form. That is, it is a matter of course that all other forms, examples, operation techniques, and the like made by those skilled in the art based on this form are included in the scope of the present invention.

(A) It is a top view which shows an example of the semiconductor light-emitting device based on this invention. (B) It is the sectional view on the AA line of (a). (C) It is BB sectional drawing of (a). It is a schematic plan view which shows the other example of the semiconductor light-emitting device based on this invention. It is a perspective view which shows schematic structure of the conventional semiconductor light-emitting device disclosed by patent document 1. FIG. It is a figure which shows the relationship between the spot size when a window area | region length is changed, and the reflectance in case a light-projection end surface is a cleavage state. It is a figure which shows the relationship between an active layer width and spot size.

Explanation of symbols

1 n-InP substrate (semiconductor substrate)
2 active layer 3 p-InP clad layer 4 p-InP current blocking layer 5 n-InP current blocking layer 10 semiconductor light emitting element 10a reflecting end face 10b light emitting end face E1 active layer area E2 (E2a, E2b) window area E3 (E3a, E3b) Spot size reduced region H1 Active layer width H2 Width of connection surface with window region in spot size reduced region

Claims (2)

In the semiconductor light emitting device (10) having a window structure, the window region (E2) is formed on at least one element end surface between the element end surface and the tip of the optical waveguide having a gain.
The semiconductor light emitting element, wherein the optical waveguide includes a spot size reduction region (E3) disposed so as to be in contact with a connection surface between the window region and the optical waveguide. The semiconductor light emitting element according to claim 1, wherein the spot size reduction region (E3) has a waveguide width that decreases toward the window region (E2).

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