Optical semiconductor device

Abstract

According to the present invention, a Fabry-Perot laser (1) and an electro-absorption modulator (2) are formed on a semiconductor substrate (9). The Fabry-Perot laser (1) has an active layer (11), and generates laser light. The electro-absorption modulator (2) has an absorption layer (14) connected to the active layer (11), and modulates the laser light. At a connection portion (3) formed by the Fabry-Perot laser (1) and the electro-absorption modulator (2), the mesa width sharply changes from that of the Fabry-Perot laser (1) to that of the electro-absorption modulator (2).

Description

Optical semiconductor device

[0001] The present disclosure relates to optical semiconductor devices.

[0002] In recent years, the speed of optical communications has been increasing rapidly. In particular, data centers and AI networks require a large number of high-speed optical modulators for relatively short-distance communications of approximately 2 km or less. Conventional ultra-high-speed modulators include electro-absorption (EA) modulators and distributed Bragg reflector (DFB) lasers.

[0003] The parameter that most affects the characteristics of an EA modulator-integrated DFB laser is the difference Δλ between the absorption edge wavelength of the EA modulator and the DFB laser oscillation wavelength. The temperature dependence of the absorption edge wavelength of an EA modulator is approximately 0.5 nm / °C, due to changes in the energy band caused by factors such as thermal expansion of the semiconductor. On the other hand, the temperature dependence of the oscillation wavelength of a DFB laser is approximately 0.1 nm / °C. This is because the oscillation wavelength is determined by the period of the diffraction grating, but the amount of thermal expansion and changes in refractive index due to temperature are smaller than changes in the energy band. Therefore, because the temperature dependence of Δλ is large for EA modulator-integrated DFB lasers, a temperature regulator is often used.

[0004] On the other hand, the temperature dependence of the oscillation wavelength of a Fabry-Perot (FP) laser, which does not use a diffraction grating, is determined by the energy band and threshold, and is approximately 0.5 nm / °C. Since this value is almost the same as the temperature dependence of the absorption edge wavelength of an EA modulator, the temperature dependence of Δλ is small, and the temperature dependence of the characteristics is also small. Therefore, an EA modulator-integrated FP laser does not require a temperature controller.

[0005] However, to operate it as an FP laser, it is necessary to provide a reflective structure that reflects the laser light at the connection between the FP laser and the EA modulator. To address this issue, it has been proposed to provide a groove between the FP laser and the EA modulator (see, for example, Patent Document 1).

[0006] Japanese Patent Application Publication No. 2005-019533

[0007] However, light propagates from the portion of the waveguide where the groove divides it. To couple this light back into the waveguide, it is necessary to control the mode diameter on the laser side, add a focusing function such as SSC to the modulator side, and ensure that the light is incident on the focusing portion at the focal length. It is difficult to achieve both this focusing design and reflectivity design. Furthermore, when processing semiconductors, variations occur in the processing depth. This is also influenced by variations in the thickness of the grown epitaxial layer. Furthermore, it is difficult to measure the depth simultaneously with processing. Therefore, controlling reflectivity by changing the groove depth is not easy in manufacturing.

[0008] The present disclosure has been made to solve the above-mentioned problems, and its object is to obtain an optical semiconductor device whose characteristics have little temperature dependency and which can be easily manufactured.

[0009] The optical semiconductor device according to the present disclosure comprises a semiconductor substrate, a Fabry-Perot laser formed on the semiconductor substrate, having an active layer, and generating laser light, and an electroabsorption modulator formed on the semiconductor substrate, having an absorption layer connected to the active layer, and modulating the laser light, wherein the mesa width of the Fabry-Perot laser changes abruptly to the mesa width of the electroabsorption modulator at a connection between the Fabry-Perot laser and the electroabsorption modulator.

[0010] This disclosure makes it possible to realize a reflective structure that reflects laser light at the connection between the Fabry-Perot laser and the electroabsorption modulator. By feeding back light through the reflective structure, the laser can operate as a Fabry-Perot laser with low temperature dependency in its characteristics. Furthermore, since the mesa width is easily controlled, it is also easy to control the reflectivity by controlling the mesa width. As a result, an optical semiconductor device with low temperature dependency in its characteristics and easy to manufacture can be obtained.

[0011] 1 is a plan view showing an optical semiconductor device according to a first embodiment; a cross-sectional view along the resonance direction of the optical semiconductor device according to the first embodiment; a perspective view showing an optical semiconductor device according to the first embodiment; a cross-sectional view showing an FP laser; a cross-sectional view showing an EA modulator; a plan view of the optical semiconductor device according to the first embodiment cut at the height of the core layer; a plan view of a modified optical semiconductor device according to the first embodiment; a plan view of the optical semiconductor device according to the second embodiment cut at the height of the core layer; a graph showing a change in the overlap integral of an optical mode when the width of the absorption layer of the EA modulator is changed; a plan view of the optical semiconductor device according to the third embodiment cut at the height of the core layer; a plan view of the modified optical semiconductor device according to the third embodiment cut at the height of the core layer; a graph showing a change in the overlap integral of an optical mode when the width of the active layer of the FP laser is changed; a perspective view showing an optical semiconductor device according to a fourth embodiment; a plan view of the optical semiconductor device according to the fourth embodiment cut at the height of the core layer; a plan view of the modified optical semiconductor device according to the fourth embodiment cut at the height of the core layer; a cross-sectional view of the modified optical semiconductor device according to the fourth embodiment cut at the height of the core layer; a cross-sectional view of the optical semiconductor device according to the fifth embodiment; a cross-sectional view of the optical semiconductor device according to the sixth embodiment; a cross-sectional view of the modified optical semiconductor device according to the sixth embodiment. 1 is a diagram showing a change in the effective refractive index when the core width is changed; FIG. 2 is a diagram showing a change in the overlap integral of an optical mode when the core width is changed; FIG. 3 is a diagram showing a change in the overlap integral of an optical mode when the thickness of the core layer of an FP laser is changed; and FIG. 4 is a diagram showing a change in the overlap integral of an optical mode when the refractive index of the core layer of an EA modulator is changed.

[0012] An optical semiconductor device according to an embodiment will be described with reference to the drawings. The same or corresponding components are designated by the same reference numerals, and repeated description may be omitted.

[0013] 1 is a plan view showing an optical semiconductor device according to a first embodiment. A Fabry-Perot (FP) laser 1 and an electro-absorption (EA) modulator 2 are integrated on a single chip and connected to each other at a connection 3. The FP laser 1 and the EA modulator 2 each have a mesa structure formed by a groove 4. An electrode 5 is formed on the top surface of the FP laser 1. An electrode 6 is formed on the top surface of the EA modulator 2. The FP laser 1 generates laser light without using a diffraction grating by using the space between the connection 3 and the rear end face 7 of the chip as a Fabry-Perot resonator. The EA modulator 2 modulates the laser light. The modulated laser light is emitted from the front end face 8 of the chip into an optical fiber or the like.

[0014] If the laser light is directly modulated by the current supplied to the FP laser 1, the high-frequency characteristics will be degraded due to relaxation oscillation. On the other hand, the EA modulator 2 does not suffer from the band degradation that is unique to lasers. Therefore, by modulating the laser light generated by the FP laser 1 with the EA modulator 2, high-speed operation is possible.

[0015] FIG. 2 is a cross-sectional view of the optical semiconductor device according to the first embodiment taken along the resonance direction. FIG. 3 is a perspective view showing the optical semiconductor device according to the first embodiment. An FP laser 1 and an EA modulator 2 are formed on a semiconductor substrate 9. The FP laser 1 has an n-type cladding layer 10, an active layer 11, a p-type cladding layer 12, and a p-type contact layer 13, which are formed in this order on the semiconductor substrate 9. An electrode 5 is formed on the p-type contact layer 13. The EA modulator 2 has an n-type cladding layer 10, an absorption layer 14, a p-type cladding layer 12, and a p-type contact layer 13, which are formed in this order on the semiconductor substrate 9. An electrode 6 is formed on the p-type contact layer 13. An electrode 15 is formed on the entire lower surface of the semiconductor substrate 9. The absorption layer 14 is directly connected to the active layer 11 at the connection portion 3 without any other layers interposed therebetween. That is, the active layer 11 and the absorption layer 14 are butt-jointed.

[0016] The semiconductor substrate 9 is made of, for example, n-InP. The active layer 11 is made of, for example, InGaAsP, AlGaInAs, etc. The absorption layer 14 is made of, for example, InGaAsP, AlGaInAs, etc. The n-type cladding layer 10 and the p-type cladding layer 12 are made of InP, and the p-type contact layer 13 is made of, for example, InGaAs, InGaAsP, etc.

[0017] 4 is a cross-sectional view of an FP laser. The FP laser 1 has a buried structure in which the sides of a ridge structure including an active layer 11 are buried with a current blocking layer 16. The current blocking layer 16 is made of a semi-insulating semiconductor such as Fe-InP. This buried structure is formed into a mesa structure by a groove 4. The sides of the mesa structure are covered with an insulating film 17. The insulating film 17 is made of, for example, SiN, SiO, or the like.

[0018] The mesa width of the FP laser 1 is the sum of the widths of the ridge structure and the buried structure. The widths of the left and right current blocking layers 16 are each 3 μm. The refractive index of the active layer 11 is 3.5, the width is 1.5 μm, and the thickness is 200 nm. The refractive index of the n-type cladding layer 10 and the p-type cladding layer 12 are 3.2, and the thickness is 2 μm.

[0019] 5 is a cross-sectional view showing an EA modulator. The EA modulator 2 has a high mesa structure including an absorption layer 14. The mesa width of the EA modulator 2 is the width of the high mesa structure. The sides of the high mesa structure are covered with an insulating film 17. The refractive index of the absorption layer 14 is 3.4, the width is 1.5 μm, and the thickness is 200 nm. The refractive index of the insulating film 17 is 2.0, and the thickness is 0.5 μm.

[0020] 6 is a plan view of the optical semiconductor device according to the first embodiment, cut at the height of the core layer. This view corresponds to the plan view of the device cut along I-II in FIG. 2. The mesa width of the FP laser 1 is 8 μm. The mesa width of the EA modulator 2 is 1.5 μm. The mesa widths of the FP laser 1 and EA modulator 2 are constant and do not change until the connection part 3. Therefore, at the connection part 3, there is a sharp change from the mesa width of the FP laser 1 to the mesa width of the EA modulator 2.

[0021] 7 is a plan view showing a modified example of the optical semiconductor device according to the first embodiment. A spot size converter 18 is formed on the output side of the EA modulator 2. The spot size converter 18 converts the spot size of the emitted laser light to match the size of the optical fiber. This allows the laser light to be introduced into the optical fiber with high efficiency.

[0022] As described above, in this embodiment, the mesa width of the FP laser 1 changes sharply to that of the EA modulator 2 at the connection 3 between the FP laser 1 and the EA modulator 2. This increases the difference in effective refractive index, enabling a reflective structure that reflects laser light. By feeding back light through the reflective structure, the laser can function as a Fabry-Perot laser with low temperature dependency. Furthermore, the mesa width can be controlled by the width of the transfer mask used during mesa processing. The width of the transfer resist can be measured before processing, and if the desired width is not achieved, the transfer can be repeated. Therefore, since the mesa width is easily controlled, the reflectivity can also be easily controlled by controlling the mesa width. As a result, an optical semiconductor device with low temperature dependency and easy manufacturing can be obtained.

[0023] Second Embodiment FIG. 8 is a plan view of an optical semiconductor device according to a second embodiment, cut at the height of the core layer. The waveguide width of the active layer 11 of the FP laser 1 is constant. On the other hand, the waveguide width of the absorption layer 14 of the EA modulator 2 changes toward the connection 3 between the FP laser 1 and the EA modulator 2. Specifically, the absorption layer 14 of the EA modulator 2 has a straight portion 14a with a constant waveguide width and a tapered portion 14b disposed between the straight portion 14a and the connection 3, whose waveguide width narrows toward the connection 3. For example, the length of the tapered portion 14b is 30 μm in the resonator direction, and the waveguide width narrows from 1.5 μm to 1.0 μm toward the connection 3. Therefore, the waveguide width of the absorption layer 14 at the connection 3 is narrower than the waveguide width of the active layer 11.

[0024] 9 shows the change in the overlap integral of the optical modes when the width of the absorption layer of the EA modulator is changed. The width of the active layer 11 of the FP laser 1 is constant at 1500 nm. Changing the width of the absorption layer 14 of the EA modulator 2 changes the overlap integral of the FP laser 1 and the EA modulator 2. A portion of the component (1 - overlap integral) returns to the FP laser 1, thereby obtaining the optical amplification factor required for laser oscillation.

[0025] As described above, in this embodiment, by making the waveguide width of the active layer 11 and the waveguide width of the absorption layer 14 different at the connection portion 3, a reflective structure that reflects laser light can be realized. By feeding back light using the reflective structure, it is possible to operate as a Fabry-Perot laser with small temperature dependency in its characteristics. Furthermore, since the dimensional control of the waveguide width is easy, it is also easy to control the reflectivity by controlling the dimensional control of the waveguide width. As a result, an optical semiconductor device with small temperature dependency in its characteristics and easy to manufacture can be obtained.

[0026] Third Embodiment FIG. 10 is a plan view of an optical semiconductor device according to a third embodiment, cut at the height of the core layer. The waveguide width of the absorption layer 14 of the EA modulator 2 is constant. On the other hand, the waveguide width of the active layer 11 of the FP laser 1 changes toward the connection 3 between the FP laser 1 and the EA modulator 2. Specifically, the active layer 11 of the FP laser 1 has a straight portion 11a with a constant waveguide width and a tapered portion 11b disposed between the straight portion 11a and the connection 3, whose waveguide width increases toward the connection 3. For example, the length of the tapered portion 11b is 30 μm in the resonator direction, and the waveguide width increases from 1.5 μm to 3.0 μm toward the connection 3. Therefore, the waveguide width of the absorption layer 14 at the connection 3 is narrower than the waveguide width of the active layer 11.

[0027] 11 is a plan view of a modified optical semiconductor device according to the third embodiment, cut at the height of the core layer. The active layer 11 of the FP laser 1 has a straight portion 11a with a constant waveguide width and a tapered portion 11b disposed between the straight portion 11a and the connection portion 3, in which the waveguide width narrows toward the connection portion 3. For example, the length of the tapered portion 11b is 30 μm in the resonator direction, and the waveguide width narrows from 1.5 μm to 1.0 μm toward the connection portion 3. Therefore, the waveguide width of the absorption layer 14 at the connection portion 3 is wider than the waveguide width of the active layer 11.

[0028] 12 is a diagram showing the change in the overlap integral of the optical modes when the width of the active layer of the FP laser is changed. The width of the absorption layer 14 of the EA modulator 2 is constant at 1500 nm. Changing the width of the active layer 11 of the FP laser 1 changes the overlap integral of the FP laser 1 and the EA modulator 2. A portion of the component (1 - overlap integral) returns to the FP laser 1, thereby generating gain for laser oscillation.

[0029] As described above, in this embodiment, by making the waveguide width of the active layer 11 and the waveguide width of the absorption layer 14 different at the connection portion 3, a reflective structure that reflects laser light can be realized. By feeding back light using the reflective structure, it is possible to operate as a Fabry-Perot laser with small temperature dependency in its characteristics. Furthermore, since the dimensional control of the waveguide width is easy, it is also easy to control the reflectivity by controlling the dimensional control of the waveguide width. As a result, an optical semiconductor device with small temperature dependency in its characteristics and easy to manufacture can be obtained.

[0030] Fourth Embodiment Fig. 13 is a perspective view showing an optical semiconductor device according to a fourth embodiment. Fig. 14 is a plan view of the optical semiconductor device according to the fourth embodiment, cut at the height of the core layer. The FP laser 1 has a buried structure in which the sides of a ridge structure including an active layer 11 are buried with a current blocking layer 16. On the other hand, the EA modulator 2 has a high mesa structure including an absorption layer 14. A portion of the front end face of the FP laser 1 is connected to the EA modulator 2. Because the mesa width of the FP laser 1 is wider than the mesa width of the EA modulator 2, there is a portion of the front end face of the FP laser 1 that is not connected to the EA modulator 2. A reflective film 19 that reflects laser light is formed on this portion of the front end face.

[0031] By reflecting a part of the optical mode of the FP laser 1 by the reflective film 19 and returning it to the FP laser 1, it is possible to reduce the mirror loss of the FP laser 1 and obtain the gain necessary for laser oscillation. In addition, the reflective film 19 is easy to form. As a result, an optical semiconductor device whose characteristics have little temperature dependency and can be easily manufactured can be obtained.

[0032] 15 is a plan view of a modified optical semiconductor device according to the fourth embodiment, cut at the height of the core layer. A reflective film 19 is also formed between the active layer 11 and the absorption layer 14. When the reflective film 19 is not formed between the active layer 11 and the absorption layer 14 as shown in FIG. 14, manufacturing is easy and the waveguide structure is maintained. On the other hand, when the reflective film 19 is formed between the active layer 11 and the absorption layer 14 as shown in FIG. 15, manufacturing becomes more difficult, but a large reflectance can be obtained.

[0033] 16 is a cross-sectional view showing an optical semiconductor device according to embodiment 5. At the connection portion 3 between the FP laser 1 and the EA modulator 2, a waveguide layer 20 is formed between the active layer 11 of the FP laser 1 and the absorption layer 14 of the EA modulator 2. The waveguide layer 20 is made of a semiconductor such as InP or an insulating film such as SiO, and has a refractive index different from that of the active layer 11 and the absorption layer 14.

[0034] The effective refractive index of the active layer 11 of the FP laser 1 is 3.371, and the effective refractive index of the insulating waveguide layer 20 is 1.636. The overlap integral is 95.142%. Due to the large difference in effective refractive indices, 12% of the optical mode in the core layer is also reflected. By adjusting the refractive index of the waveguide layer 20, the reflectivity can be easily controlled. Furthermore, the waveguide layer 20 is easy to form. As a result, an optical semiconductor device with low temperature dependency and easy manufacturing can be obtained. The waveguide layer 20 may also have a structure in which multiple layers with different refractive indices are stacked. Also, in FIG. 16, a groove is formed on the waveguide layer 20. Alternatively, a p-type cladding layer 12 made of InP may be formed on the waveguide layer 20, or a cladding layer made of a semiconductor material other than InP may be formed.

[0035] 17 is a cross-sectional view showing an optical semiconductor device according to a sixth embodiment. A low-reflectivity film 21 is formed on the front end surface of the chip from which laser light is emitted. A high-reflectivity film 22 is formed on the rear end surface of the chip opposite the front end surface. The low-reflectivity film 21 and the high-reflectivity film 22 are, for example, formed by alternately laminating Si layers and SiO layers. The low-reflectivity film 21 has a lower reflectivity for laser light than the high-reflectivity film 22. As a result, the laser light modulated by the EA modulator 2 does not return to the FP laser 1, stabilizing the operation of the FP laser 1 and improving its modulation characteristics. Other configurations and effects are the same as those of the first embodiment.

[0036] 18 is a cross-sectional view showing a modified example of the optical semiconductor device according to the sixth embodiment. A spot size converter 18 is formed on the output side of the EA modulator 2. In this case, the same effect can be obtained by forming a low-reflectivity film 21 and a high-reflectivity film 22. Note that the low-reflectivity film 21 and the high-reflectivity film 22 may also be formed in the second to fifth embodiments.

[0037] In typical FP lasers, the end faces are often not coated, and the cleaved end faces are used as they are as the reflective structure. In this case, the reflectivity of both the front and rear end faces is 30%. If a coating film with a reflectivity of 90% or more is formed on the rear end face, the reflectivity that results in the same mirror loss as above is 10%. Therefore, it is preferable that the reflectivity of the laser light in the reflective structure of the connection part 3 in embodiments 1 to 6 is 10% or more. Furthermore, if the reflectivity of the reflective structure of the connection part 3 has wavelength selectivity, the temperature dependence of the oscillation wavelength of the FP laser 1 will differ from the temperature dependence of the EA modulator 2, resulting in poor temperature characteristics. Therefore, it is preferable that the reflective structure of the connection part 3 has low wavelength selectivity.

[0038] Figure 19 shows the change in effective refractive index when the core width is changed. The current blocking layer 16 on the ridge side of the FP laser 1 is made of InP, and the insulating film 17 on the ridge side of the EA modulator 2 is made of SiO. Even if the core width is the same, there is a difference in effective refractive index. The difference in effective refractive index correlates with the overlap integral of the FP laser 1 and EA modulator 2. A portion of the component (1 - overlap integral) returns to the FP laser 1, thereby generating gain for laser oscillation. Therefore, the reflectivity can be adjusted by controlling the refractive index of the material on the ridge side.

[0039] Figure 20 shows the change in the overlap integral of the optical modes when the core width is changed. The figures show the case where the ridge sidewall material connects semiconductor waveguides, and the case where the ridge sidewall material connects a semiconductor waveguide and an insulating film waveguide. Having different ridge sidewall materials is more effective in increasing the gain for laser oscillation. Therefore, to increase the reflectivity at the connection 3 between the FP laser 1 and the EA modulator 2, it is preferable that the material covering the ridge sidewall of the FP laser 1 and the material covering the ridge sidewall of the EA modulator 2 are different.

[0040] 21 is a diagram showing the change in the overlap integral of the optical modes when the thickness of the core layer of the FP laser is changed. When the thickness of the active layer 11 of the FP laser 1 is changed relative to the thickness of the absorption layer 14 of the EA modulator 2, which is 300 nm, the overlap integral of the FP laser 1 and the EA modulator 2 changes. Therefore, to increase the reflectivity at the connection 3 between the FP laser 1 and the EA modulator 2, it is preferable that the thickness of the active layer 11 of the FP laser 1 and the thickness of the absorption layer 14 of the EA modulator 2 are different.

[0041] 22 is a diagram showing the change in the overlap integral of the optical modes when the refractive index of the core layer of the EA modulator is changed. When the refractive index of the absorption layer 14 of the EA modulator 2 is changed relative to the refractive index of the active layer 11 of the FP laser 1, which is 3.56, the overlap integral of the FP laser 1 and the EA modulator 2 changes. Therefore, to increase the reflectance at the connection portion 3 between the FP laser 1 and the EA modulator 2, it is preferable that the refractive index of the active layer 11 of the FP laser 1 and the refractive index of the absorption layer 14 of the EA modulator 2 are different.

[0042] REFERENCE SIGNS LIST 1 FP laser, 2 EA modulator, 3 connection portion, 9 semiconductor substrate, 11 active layer, 11a, 14a straight portion, 11b, 14b tapered portion, 14 absorption layer, 16 current blocking layer, 19 reflective film, 20 waveguide layer, 21 low reflectivity film, 22 high reflectivity film

Claims

1. An optical semiconductor device comprising: a semiconductor substrate; a Fabry-Perot laser formed on said semiconductor substrate, having an active layer, for generating laser light; and an electroabsorption modulator formed on said semiconductor substrate, having an absorption layer connected to said active layer, for modulating said laser light, wherein the mesa width of said Fabry-Perot laser changes abruptly to the mesa width of said electroabsorption modulator at the junction between said Fabry-Perot laser and said electroabsorption modulator.

2. The optical semiconductor device according to claim 1, wherein the mesa width of the Fabry-Perot laser and the mesa width of the electroabsorption modulator are both constant and do not change up to the connecting portion.

3. The optical semiconductor device according to claim 1 or 2, characterized in that the Fabry-Perot laser has a buried structure in which the sides of a ridge structure including the active layer are buried with a current blocking layer, the electroabsorption modulator has a high mesa structure including the absorption layer, the mesa width of the Fabry-Perot laser is the width of the buried structure, and the mesa width of the electroabsorption modulator is the width of the high mesa structure.

4. An optical semiconductor device comprising: a semiconductor substrate; a Fabry-Perot laser formed on the semiconductor substrate, having an active layer, for generating laser light; and an electroabsorption modulator formed on the semiconductor substrate, having an absorption layer connected to the active layer, for modulating the laser light, wherein the waveguide width of the active layer or the absorption layer changes toward a junction between the Fabry-Perot laser and the electroabsorption modulator, and the waveguide width of the active layer and the waveguide width of the absorption layer are different at the junction.

5. The optical semiconductor device according to claim 4, wherein the absorption layer has a straight portion with a constant waveguide width and a tapered portion disposed between the straight portion and the connection portion and in which the waveguide width narrows toward the connection portion, and the waveguide width of the absorption layer at the connection portion is narrower than the waveguide width of the active layer.

6. The optical semiconductor device according to claim 4, wherein the active layer has a straight section with a constant waveguide width and a tapered section disposed between the straight section and the connection section and in which the waveguide width increases toward the connection section, and wherein the waveguide width of the absorption layer at the connection section is narrower than the waveguide width of the active layer.

7. The optical semiconductor device according to claim 4, wherein the active layer has a straight section with a constant waveguide width and a tapered section disposed between the straight section and the connection section and in which the waveguide width narrows toward the connection section, and wherein the waveguide width of the absorption layer at the connection section is wider than the waveguide width of the active layer.

8. An optical semiconductor device according to claim 3, wherein a portion of the front end face of said Fabry-Perot laser is connected to said electroabsorption modulator, and a reflective film for reflecting said laser light is formed on said front end face.

9. The optical semiconductor device according to claim 8, wherein the reflective film is also formed between the active layer and the absorption layer.

10. An optical semiconductor device comprising: a semiconductor substrate; a Fabry-Perot laser formed on the semiconductor substrate, having an active layer, and generating laser light; an electroabsorption modulator formed on the semiconductor substrate, having an absorption layer connected to the active layer, and modulating the laser light; and a waveguide layer formed between the active layer and the absorption layer at a connection between the Fabry-Perot laser and the electroabsorption modulator, and having a refractive index different from that of the active layer and the absorption layer.

11. An optical semiconductor device according to any one of claims 1 to 10, further comprising a low-reflectivity film formed on the front end face of the chip from which the laser light is emitted, and a high-reflectivity film formed on the rear end face of the chip opposite to the front end face of the chip, wherein the low-reflectivity film has a lower reflectivity for the laser light than the high-reflectivity film.

12. The optical semiconductor device according to any one of claims 1 to 11, wherein the reflectance of the laser beam at the connection portion is 10% or more.

13. An optical semiconductor device according to any one of claims 1 to 12, wherein the material covering the ridge side surfaces of the Fabry-Perot laser and the material covering the ridge side surfaces of the electroabsorption modulator are different.

14. An optical semiconductor device according to any one of claims 1 to 13, wherein the thickness of the active layer of the Fabry-Perot laser is different from the thickness of the absorption layer of the electroabsorption modulator.

15. An optical semiconductor device according to any one of claims 1 to 14, wherein the refractive index of the active layer of the Fabry-Perot laser is different from the refractive index of the absorption layer of the electroabsorption modulator.

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