Optical modulator and optical transmitter

Abstract

An EA modulator 10 comprises: a semiconductor substrate 11; a mesa waveguide structure 10A including an n-type clad layer 11b, a core layer 12 having an absorption layer, and a p-type clad layer 13, which are laminated sequentially on the upper surface 11a of the semiconductor substrate 11; and a mesa embedding layer 15 disposed on the upper surface 11a of the semiconductor substrate 11 so as to embed the mesa waveguide structure 10A. The mesa waveguide structure 10A transitions from an embedded mesa structure 10B embedded in the mesa embedding layer 15 to an isolated mesa structure 10C not embedded in an embedded semiconductor 16 in a light propagation direction from an entry end to an emission end.

Description

Optical modulator and optical transmitter

[0001] The present disclosure relates to an optical modulator and an optical transmitter.

[0002] In recent years, the demand for increased communication capacity due to the increase in communication traffic has necessitated high-speed optical signal transmission. In the intensity modulation method, which modulates the intensity of light, electro-absorption (EA) modulators, which control the amount of light absorption depending on the bias voltage applied to a multi-quantum well (MQL), have been used to achieve high-speed signal transmission exceeding 50 gigabaud. However, even higher speeds are required for optical interface applications in data centers.

[0003] Hideo Arimoto, Masataka Shirai, Shinji Tsuji, "Measurement of Carrier Emission Time in a 40 Gbit / s Optical Modulator", IEICE Technical Report: IEICE Technical Report 101 452 101(452) 2001.11.22 pp.83-88

[0004] In EA modulators, photocarriers are generated when light is absorbed, and current flows even when a reverse bias is applied. However, holes have lower mobility than electrons in semiconductors and tend to accumulate in multiple quantum wells. It is known that accumulated holes reduce the operating bandwidth of EA modulators, and there is a need to suppress hole accumulation (see Non-Patent Document 1). In EA modulators, the light intensity is higher on the input side than on the output side, generating more photocarriers and accumulating holes. Therefore, there is a need to suppress rapid light absorption and achieve a more uniform photocarrier concentration throughout the EA modulator.

[0005] The present disclosure has been proposed in consideration of the above-described circumstances, and aims to provide an optical modulator that can suppress rapid light absorption, achieve a more uniform photocarrier concentration throughout the EA modulator, and suppress deterioration of the operating band, as well as an optical transmitter including such an optical modulator.

[0006] In order to solve the above-described problems, the optical modulator according to the present disclosure includes a semiconductor substrate, a mesa waveguide structure including an n-type cladding layer, a core layer having an absorption layer, and a p-type cladding layer, which are stacked in this order on the upper surface of the semiconductor substrate, and a mesa burying layer arranged on the upper surface of the semiconductor substrate so as to bury the mesa waveguide structure, and the mesa waveguide structure transitions from a buried mesa structure buried in the mesa burying layer to an isolated mesa structure not buried in the mesa burying layer in the propagation direction of light from its input end to its output end.

[0007] An optical transmitter according to the present disclosure includes an electro-absorption modulator according to the present disclosure, and a laser diode including another mesa waveguide structure arranged adjacent to the mesa waveguide structure on an upper surface of a semiconductor substrate common to the electro-absorption modulator so as to extend the mesa waveguide structure from the incident end to the upstream side in the propagation direction of light, and in which the core of the other mesa waveguide structure has a light-emitting layer instead of an absorption layer.

[0008] According to the present disclosure, rapid light absorption can be suppressed to achieve a more uniform photocarrier concentration throughout the EA modulator, thereby suppressing degradation of the operating band.

[0009] 1 is a cross-sectional view of an EA modulator. 2 is a cross-sectional view of an EA modulator. 3 is a cross-sectional view of an EA modulator. 4 is a flowchart of a method for manufacturing an EA modulator. 5 is a process flow diagram of a method for manufacturing an EA modulator. 6 is a process flow diagram of a method for manufacturing an EA modulator. 7 is a process flow diagram of a method for manufacturing an EA modulator. 8 is a process flow diagram of a method for manufacturing an EA modulator. 9 is a process flow diagram of a method for manufacturing an EA modulator. 10 is a graph showing simulation results of an optical confinement factor. 11 is a graph showing the relationship between an effective refractive index and a width of a mesa waveguide structure. 12 is a graph showing a distribution of an effective refractive index. 13 is a graph showing a distribution of an optical confinement factor. 14 is a graph showing a distribution of a photocarrier concentration. 15 is a graph showing a distribution of an optical power. 16 is a cross-sectional view of an EADFB laser. 17 is a left side view of an EADFB laser. 18 is a flowchart of a method for manufacturing an EADFB laser. 19 is a process flow diagram of an EADFB laser. 19 is a process flow diagram of an EADFB laser. 1 is a process flow diagram for an EADFB laser; FIG. 2 is a process flow diagram for an EADFB laser; FIG. 3 is a process flow diagram for an EADFB laser; FIG. 4 is a process flow diagram for an EADFB laser; FIG. 5 is a process flow diagram for an EADFB laser; FIG. 6 is a process flow diagram for an EADFB laser; FIG.

[0010] Embodiments of an optical modulator and an optical transmitter according to the present disclosure will be described in detail below with reference to the drawings. Figures 1, 2A, and 2B are cross-sectional views of an electro-absorption (EA) modulator 10 according to the present embodiment. Figure 1 shows a cross section of the EA modulator 10 taken along section line II shown in Figures 2A and 2B. Figures 2A and 2B show cross sections of the EA modulator 10 taken along section lines IIA-IIA and IIB-IIB, respectively, shown in Figure 1.

[0011] In the EA modulator 10, a mesa waveguide structure 10A is disposed on an upper surface 11a of a semiconductor substrate 11 made of n-type InP, and is formed by sequentially stacking an n-type cladding layer 11b made of n-type InP, a core layer 12 having an absorption layer, a p-type cladding layer 13 made of p-type InP, and a contact layer 14 made of InGaAs, which are integral with the semiconductor substrate 11. The mesa waveguide structure 10A is disposed approximately at the center in the short direction of the upper surface 11a, which has a substantially rectangular shape in a plan view, and extends in the long direction from one short side to the opposing other short side.

[0012] Here, in the mesa waveguide structure 10A, the z-axis is defined as the light propagation direction along the optical axis from the input end where the incident light L1 is incident to the output end where the output light L2 is emitted, with the input end being the origin of the z-coordinate. An xyz Cartesian coordinate system is defined such that the x-axis extends within the upper surface 11a of the semiconductor substrate 11, and the y-axis extends in the height direction from the upper surface 11a. In the z-direction, the width Wmesa(z) of the mesa waveguide structure 10A is uniform, and the height is also uniform. Hereinafter, the light propagation direction along the optical axis may also be referred to as the z-direction.

[0013] In the mesa waveguide structure 10A, the n-type cladding layer 11b is formed to protrude from the upper surface 11a of the semiconductor substrate 11. The core layer 12 has an absorption layer with a multiple quantum well (MQW) structure made of InGaAlAs or InGaAsP. An n-side electrode layer 18 is formed on the lower surface of the semiconductor substrate 11, which is integral with the n-type cladding layer 11b, and a p-side electrode layer 19 is formed on the upper surface of the p-type cladding layer 13, with a contact layer 14 interposed therebetween.

[0014] In the EA modulator 10, a semi-insulating mesa burying layer 15 made of InP doped with Fe is disposed on the top surface 11a of the semiconductor substrate 11 so as to bury the mesa waveguide structure 10A. The mesa burying layer 15 is adjacent to both side surfaces of the mesa waveguide structure 10A and reaches the height of the mesa waveguide structure 10A. The mesa burying layer 15 is formed a predetermined distance in the z direction from the incident end of the mesa waveguide structure 10A where z = 0, and its width Wbh(z) gradually decreases in a tapered manner as it extends in the z direction. Specifically, on the top surface 11a of the semiconductor substrate 11, which has a substantially rectangular shape in plan view, the mesa burying layer 15 extends from both side surfaces of the mesa waveguide structure 10A along one short side where the incident end of the mesa waveguide structure 10A where z = 0 is located, reaching the long side of the mesa waveguide structure 10A, and its width Wbh(z) reaches a maximum value Wbh(0). The width Wbh(z) decreases linearly from a maximum value Wbh(0) to z, and becomes 0 at a position z1 approximately in the center of the upper surface 11a in the longitudinal direction, i.e., Wbh(z1)=0.

[0015] In the EA modulator 10, the structure of the portion in the z direction where the mesa burying layer 15 exists and the mesa waveguide structure 10A is buried in the mesa burying layer 15 will be referred to as the buried mesa structure 10B, and the structure of the portion in which the mesa waveguide structure 10A is exposed and not buried in the mesa burying layer 15 will be referred to as the isolated mesa structure 10C. The EA modulator 10 is configured such that the buried mesa structure 10B is connected to the isolated mesa structure 10C in the z direction. In other words, the EA modulator 10 has a composite structure that transitions from the buried mesa structure 10B to the isolated mesa structure 10C in the z direction.

[0016] Such an EA modulator 10 may have a length in the z direction of 100 μm, a width Wmesa of the mesa waveguide structure 10A of 0.7 μm, and a width Wbh of the tapered mesa buried layer 15 of 1 μm at the incident end at z=0, decreasing linearly with z to 0 at z=50 μm.

[0017] Fig. 3 is a flowchart of a method for manufacturing the EA modulator 10. The method for manufacturing the EA modulator 10 will be described with reference to Fig. 3. Reference will also be made to the process flow diagrams of Fig. 4A to Fig. 4G. These process flow diagrams are cross-sectional views taken along the cutting line IIA-IIA in Fig. 1, similar to Fig. 2A.

[0018] In step S11, as shown in FIG. 4A, a semiconductor substrate 11 is prepared. The semiconductor substrate 11 is made of n-type InP. In step S12, as shown in FIG. 4B, a core layer 12 is formed. The core layer 12 has an absorption layer with a multiple quantum well (MQW) structure made of InGaAlAs or InGaAsP. The core layer 12 is grown on the upper surface of the semiconductor substrate 11 by metal organic chemical vapor deposition (MOCVD). The thickness of the core layer 12 may be approximately 0.8 μm.

[0019] 4C, the p-type cladding layer 13 and the contact layer 14 are formed. The p-type cladding layer 13 is made of p-type InP, and the contact layer 14 is made of InGaAs. The p-type cladding layer 13 and the contact layer 14 are grown in this order on the upper surface of the core layer 12 by MOCVD. The thicknesses of the p-type cladding layer 13 and the contact layer 14 may be approximately 2 μm.

[0020] In step S14, a mesa waveguide structure 10A is formed as shown in FIG. 4D . The mesa waveguide structure 10A is formed by sequentially stacking an n-type cladding layer 11b, a core layer 12, a p-type cladding layer 13, and a contact layer 14 on the upper surface 11a of the semiconductor substrate 11. Therefore, the upper surface of the contact layer 14, which is the top layer of the stack shown in FIG. 4C , is masked and dry etching is performed to leave the mesa waveguide structure 10A. The mesa waveguide structure 10A may have a height of approximately 4 μm. By dry etching, the semiconductor substrate 11 is dug down except for the masked region where the mesa waveguide structure 10A is formed, forming the upper surface 11a. However, the portion constituting the n-type cladding layer 11b of the mesa waveguide structure 10A remains unduly dug down and protrudes in the height direction from the upper surface 11a. Therefore, the n-type cladding layer 11b is integrally formed with the semiconductor substrate 11.

[0021] In step S15, as shown in FIG. 4E, a mesa burying layer 15 is formed. The mesa burying layer 15 is a semi-insulating semiconductor made of InP doped with Fe. With the mesa waveguide structure 10A still masked, it is formed by burying and regrowing by MOCVD on the upper surface 11a of the semiconductor substrate 11 except for the region where the mesa waveguide structure 10A is formed. In step S16, an isolated mesa structure 10C is formed. With the mask remaining on the mesa waveguide structure 10A buried in the mesa burying layer 15, a mask is formed on the upper surface of the mesa burying layer 15 so that the mesa burying layer 15 has a shape that gradually tapers in width in the z direction, and the unmasked region of the mesa burying layer 15 is removed by dry etching. As a result, a buried mesa structure 10B is formed on the upper surface 11a of the semiconductor substrate 11, in which the width of the mesa burying layer 15 gradually tapers in the z direction, and an isolated mesa structure 10C is also formed that is not buried in the mesa burying layer 15 and is exposed. The isolated mesa structure 10C may have a height of about 4 μm.

[0022] In step S17, as shown in FIG. 4G, a p-side electrode layer 19 is formed on the upper surface of the mesa waveguide structure 10A. The p-side electrode layer 19 is formed by evaporating metal on the upper surface of the contact layer 14 of the mesa waveguide structure 10A. In step S18, the lower surface of the semiconductor substrate 11 is polished to thin the semiconductor substrate 11. The semiconductor substrate 11 may be polished to a thickness of 150 μm.

[0023] In step S19, an n-side electrode layer 18 is formed on the lower surface of the semiconductor substrate 11. The n-side electrode layer 18 is formed by evaporating metal on the lower surface of the semiconductor substrate 11. In step S20, an antireflection (AR) film is formed on each of the input end and output end of the core layer 12 of the mesa waveguide structure 10A. Through this series of steps, the EA modulator 10 as shown in the cross-sectional view of FIG. 2A is manufactured.

[0024] FIG. 5 is a graph showing the results of a simulation of the optical confinement factors of the buried mesa structure 10B and the isolated mesa structure 10C of the EA modulator 10, using the width W of the mesa waveguide structure 10A as a variable (see FIG. 1 for W). In this simulation, for convenience, it was assumed that the width of the mesa buried layer 15 in the buried mesa structure 10B was uniform in the z direction rather than tapering in the z direction. In FIG. 5, curve a represents the data for the isolated mesa structure 10C, and curve b represents the data for the buried mesa structure 10C. In FIG. 5, when the width W of the mesa waveguide structure 10A is 0.7 μm, the optical confinement factor of the isolated mesa structure 10C is 0.19, while the optical confinement factor of the buried mesa structure 10B is 0.15. It can be observed that the optical confinement factor of the buried mesa structure 10B is approximately 0.8 times that of the isolated mesa structure 10C. The optical confinement coefficient represents the overlap between the optical propagation mode and the core region, which is the absorption layer. Therefore, it is clear that the buried mesa structure 10B, which has a smaller optical confinement coefficient, can weaken optical absorption and suppress carrier generation.

[0025] 6 is a graph showing the relationship between the effective refractive index and the width W of the mesa waveguide structure 10A in the buried mesa structure 10B and the isolated mesa structure 10C of the EA modulator 10. Here, too, it is assumed that the width of the mesa buried layer 15 in the buried mesa structure 10B is uniform in the z direction. In FIG. 6, curve a shows the data for the isolated mesa structure 10C, and curve b shows the data for the buried mesa structure 10B. In the effective refractive index shown in FIG. 6, the effective refractive index differs between the buried mesa structure 10B and the isolated mesa structure 10C, as shown by the gap between the buried mesa structure 10B and the isolated mesa structure 10C indicated by the arrows. Therefore, when these waveguides are butt-joined, reflection may occur.

[0026] FIG. 7 is a graph showing the distribution of the effective refractive index in the z direction for the buried mesa structure 10B, in which the width Wbh of the mesa burying layer 15 tapers off as the EA modulator 10 moves in the z direction (see FIG. 2A for Wbh). Here, the position in the z direction is expressed as a relative distance z / L based on the length L of the EA modulator 10 in the z direction. L is 50 μm. The mesa burying layer 15 is formed in the z direction from z=0 at the incident end to 50 μm, and the width Wbh tapers off from 1 μm and decreases uniformly, reaching 0 at L=50 μm. In the EA modulator 10, by providing the mesa burying layer 15 with this tapered structure, the effective refractive index can be continuously converted from that of the buried mesa structure 10B to that of the isolated mesa structure 10C, as shown in FIG. 7 .

[0027] Fig. 8 is a graph showing the distribution of the optical confinement factor in the z direction based on the distribution of the effective refractive index in Fig. 7. In Fig. 8 as well, the position in the z direction is expressed as a relative distance z / L based on the length L of the EA modulator 10 in the z direction. As shown in Fig. 7, the effective refractive index is continuously changed from that corresponding to the buried mesa structure 10B to that corresponding to the isolated mesa structure 10C, so that the optical confinement factor can be continuously changed in the z direction.

[0028] 9 is a graph showing the distribution of the photocarrier concentration in the z direction in the EA modulator 10. The photocarrier concentration is such that the absorption coefficient is 1100 cm when a reverse bias of −3 V is applied to the EA modulator 10. －１Since the above equation is true, the results were calculated from the change in input light intensity in the light propagation direction when a reverse bias is applied. Here, calculations were performed assuming other waveguide structures in addition to the waveguide structure of the EA modulator 10. Data a represents the waveguide structure of the EA modulator 10, in which the mesa waveguide structure 10A has a composite structure of a buried mesa structure 10B and an isolated mesa structure 10C. Data b represents the EA modulator 10 configured with a uniform isolated mesa structure 10C instead of the waveguide structure. Data c represents the EA modulator 10 configured with a uniform buried mesa structure 10B instead of the waveguide structure. The photocarrier concentrations in FIG. 9 are expressed as relative photocarrier concentrations, with the photocarrier concentration at the incident end for the isolated mesa structure 10C of data b being set to 1. The isolated mesa structure 10C of data b has the highest photocarrier concentration at the incident end, and the buried mesa structure 10B of data c can suppress the photocarrier concentration at the incident end by up to 80% of that of the isolated mesa structure 10C of data b. This is because the light confinement of the buried mesa structure 10B is lower than that of the isolated mesa structure 10C.

[0029] 10 is a graph showing the distribution of optical power in the z-direction in the EA modulator 10. In Fig. 10, as in Fig. 9, data a represents the waveguide structure of the EA modulator 10 having a composite structure of the buried mesa structure 10B and the isolated mesa structure 10C, while data b and c represent the EA modulator 10 configured with the isolated mesa structure 10C and the buried mesa structure 10B, respectively, instead of the waveguide structure. Comparing the optical power at the output end, it can be seen that the buried mesa structure 10B shown in data c has an output power that is about 2 dB higher than that of the isolated mesa structure 10C shown in data b, and has poorer extinction characteristics.

[0030] The EA modulator 10 shown in data a has a composite structure of a buried mesa structure 10B and an isolated mesa structure 10C. As a result, in the EA modulator 10, the generation of photocarriers at the input end of Fig. 9 can be reduced by about 80% compared to the isolated mesa structure 10C shown in data b, similar to the buried mesa structure 10B shown in data c, and the extinction characteristics at the output end of Fig. 10 can also be improved by about 1 dB compared to the buried mesa structure 10B shown in data c.

[0031] In this way, in the EA modulator 10, by forming the mesa buried layer 15 in a tapered shape that gradually decreases in width in the z direction, it is possible to continuously convert the effective refractive index between the buried mesa structure 10B and the isolated mesa structure 10C. Therefore, by continuously changing the optical confinement factor in the z direction, it is possible to suppress reflection due to changes in the refractive index.

[0032] The EA modulator 10 also has a composite structure consisting of a buried mesa structure 10B and an isolated mesa structure 10C. This reduces the generation of photocarriers at the input end of the EA modulator 10. It also improves the extinction characteristics at the output end of the EA modulator 10.

[0033] In the EA modulator 10, reducing the generation of photocarriers at the input end allows for a more uniform photocarrier concentration throughout the entire EA modulator 10. This reduces hole accumulation in the multiple quantum well (MQW) and suppresses degradation of the operating bandwidth. Furthermore, improving the extinction characteristics allows for an improved signal-to-noise ratio.

[0034] Next, the optical transmitter of this embodiment will be described. The optical transmitter of this embodiment is an electro absorption modulator integrated with distributed feedback (EADFB) laser 30, which is configured by integrating an LD section 30A consisting of a distributed feedback (DFB) laser diode (LD) on the input side of an EA modulation section 30B having a configuration similar to that of the EA modulator 10 of this embodiment.

[0035] 11 and 12 are cross-sectional views of the EADFB laser 30. Fig. 11 shows a cross section of the EADFB laser 30 taken along the cutting line XI-XI shown in Fig. 12. Fig. 12 shows a cross section of the EADFB laser 30 taken along the cutting line XII-XII shown in Fig. 11. Fig. 13 is a left side view of the EADFB laser 30.

[0036] In the LD section 30A of the EADFB laser 30, a mesa waveguide structure 20A is disposed on the upper surface 31a of a semiconductor substrate 31 made of n-type InP, which is also used for the EA modulation section 30B. The mesa waveguide structure 20A is formed by sequentially stacking an n-type cladding layer 31b made of n-type InP integral with the semiconductor substrate 31, a core layer 22 having an emission layer, a p-type cladding layer 33 made of p-type InP, which is also used for the EA modulation section 30B, and a contact layer 24 made of InGaAs. The mesa waveguide structure 20A is disposed approximately at the center of the LD section 30A in the short direction on the upper surface 31a of the semiconductor substrate 31, which has a substantially rectangular shape in plan view, and extends from one short side to the opposite short side in the long direction. In this way, the mesa waveguide structure 20A is disposed adjacent to the mesa waveguide structure 10A of the EA modulation section 30B so as to extend the mesa waveguide structure 10A upstream in the light propagation direction along the optical axis.

[0037] In the mesa waveguide structure 20A, the z-axis is set to the propagation direction of light from the reflecting end to the emitting end along the optical axis of the core layer 22. An xyz Cartesian coordinate system is set so that the x-axis extends within the upper surface 31a of the semiconductor substrate 31 and the y-axis extends in the height direction from the upper surface 31a. The width and height of the mesa waveguide structure 20A are uniform in the z-direction.

[0038] In the mesa waveguide structure 20A, the n-type cladding layer 31b is integral with the semiconductor substrate 31 so as to protrude from the upper surface 31a of the semiconductor substrate 31. The core layer 22 has an InGaAlAs or InGaAsP light-emitting layer with a multiple quantum well (MQW) structure. An n-side electrode layer 38, which is also used in the EA modulation section 30B, is formed on the lower surface of the semiconductor substrate 31 integral with the n-type cladding layer 31b, and a p-side electrode layer 29 is formed on the upper surface of the p-type cladding layer 33 with a contact layer 24 interposed therebetween. The contact layer 24 and the p-side electrode layer 29 are separated from the contact layer 14 and p-side electrode layer 19 of the EA modulation section 30B by a separation groove 37.

[0039] In the LD section 30A, a mesa burying layer 35, which is common to the EA modulation section 30B, is disposed on the upper surface 31a of the semiconductor substrate 31 so as to bury the mesa waveguide structure 20A. The mesa burying layer 35 is adjacent to both side surfaces of the mesa waveguide structure 20A and reaches the height of the mesa waveguide structure 20A, and is formed on the upper surface 31a of the semiconductor substrate 31 in the LD section 30A except for the region where the mesa waveguide structure 20A is disposed. The mesa burying layer 35 is made of a semi-insulating semiconductor such as InP doped with Fe.

[0040] In the EA modulation section 30B, a mesa waveguide structure 10A is disposed on an upper surface 31a of a semiconductor substrate 31 common to the LD section 30A made of n-type InP, and the mesa waveguide structure 10A is formed by sequentially stacking an n-type cladding layer 31b made of n-type InP integral with the semiconductor substrate 31, a core layer 12 having an absorption layer, a p-type cladding layer 33 made of p-type InP common to the LD section 30A, and a contact layer 14 made of InGaAs. The mesa waveguide structure 10A is adjacent to and on the same optical axis as the mesa waveguide structure 20A of the LD section 30A, and the output end of the mesa waveguide structure 20A of the LD section 30A serves as the input end of the mesa waveguide structure 10A. The mesa waveguide structure 10A is disposed approximately at the center in the short direction of the EA modulation section 30B on the upper surface 31a of the semiconductor substrate 31, which has a substantially rectangular shape in a plan view, and extends in the long direction from one short side to the opposing other short side.

[0041] Here, in the mesa waveguide structure 10A, the z-axis extends in the propagation direction of light from the incident end toward the output end where the output light L3 is emitted. The incident end of the mesa waveguide structure 10A may be set as the origin of the z-coordinate. In the EA modulation section 30B, the x-axis extends within the upper surface 31a of the semiconductor substrate 31, and the y-axis extends in the height direction from the upper surface 31a. The width and height of the mesa waveguide structure 10A are uniform in the z-direction.

[0042] In the mesa waveguide structure 10A, the n-type cladding layer 31b is formed to protrude from the upper surface 31a of the semiconductor substrate 31. The core layer 12 has an absorption layer with a multiple quantum well (MQW) structure made of InGaAlAs or InGaAsP. An n-side electrode layer 38, which is common to the LD section 30A, is formed on the lower surface of the semiconductor substrate 31 integrally formed with the n-type cladding layer 31b. A p-side electrode layer 19 is formed on the upper surface of the p-type cladding layer 13 with a contact layer 14 interposed therebetween. The contact layer 14 and the p-side electrode layer 19 are separated from the contact layer 24 and the p-side electrode layer 29 of the LD 30A by a separation groove 37.

[0043] In the EA modulation section 30B, a semi-insulating mesa burying layer 35 made of InP doped with Fe is disposed on the upper surface 31a of the semiconductor substrate 31 so as to bury the mesa waveguide structure 10A. The mesa burying layer 35 is adjacent to both side surfaces of the mesa waveguide structure 10A and reaches the height of the mesa waveguide structure 10A. The mesa burying layer 35 is formed a predetermined distance in the z direction from the incident end of the mesa waveguide structure 10A where z = 0, and its width Wbh(z) gradually decreases in a tapered manner as it extends in the z direction. Specifically, in the EA modulation section 30B on the upper surface 31a of the semiconductor substrate 31, which has a substantially rectangular shape in plan view, the mesa burying layer 35 extends from both side surfaces of the mesa waveguide structure 10A along one short side where the incident end of the mesa waveguide structure 10A where z = 0 is located, reaching the long side, and its width Wbh(z) reaches a maximum value Wbh(0). The width Wbh(z) decreases linearly from the maximum value Wbh(0) to z, and becomes 0 at a position z1 that is approximately the center of the EA modulation section 30B in the longitudinal direction on the upper surface 31a of the semiconductor substrate 31, i.e., Wbh(z1)=0.

[0044] In the EA modulation section 30B, the structure of the portion where the mesa waveguide structure 10A is buried in the mesa burying layer 35 in the z direction will be referred to as the buried mesa structure 10B, and the structure of the portion where the mesa waveguide structure 10A is exposed and not buried in the mesa burying layer 35 will be referred to as the isolated mesa structure 10C. The EA modulation section 30B is configured such that the buried mesa structure 10B is connected to the isolated mesa structure 10C in the z direction. In other words, the EA modulation section 30B has a composite structure that transitions from the buried mesa structure 10B to the isolated mesa structure 10C in the z direction.

[0045] In the EADFB laser 30, the length of the LD section 30A in the z direction may be 300 μm, and the length of the EA modulation section 30B may be 100 μm. As in the above-described EA modulator 10, the EA modulation section 30B may have a width Wmesa of 0.7 μm in the mesa waveguide structure 10A, and a width Wbh of the tapered mesa buried layer 15, Wbh(0) at z=0 at the incident end, which decreases linearly with z, and the width Wbh(z) may be 0 at z=50 μm.

[0046] Fig. 14 is a flowchart of a method for manufacturing the EADFB laser 30. The method for manufacturing the EADFB laser 30 will be described with reference to Fig. 14. Reference will also be made to the process flow diagrams of Figs. 15A to 15H. These process flow diagrams are cross-sectional views taken along the line XII-XII in Fig. 11, similar to Fig. 12.

[0047] In step S21, as shown in Fig. 15A, a semiconductor substrate 31 is prepared. The semiconductor substrate 31 is made of n-type InP. In step S22, as shown in Fig. 15B, a core layer 22 is formed. The core layer 22 has a light-emitting layer with a multiple quantum well structure made of InGaAlAs or InGaAsP, and is grown on the upper surface of the semiconductor substrate 31 by MOCVD. The thickness of the core layer 22 may be approximately 0.8 µm.

[0048] In step S23, as shown in Fig. 15C, a diffraction grating 22a is formed on the upper surface of the core layer 22 of the LD section 30A. In step S24, as shown in Fig. 15D, the core layer 22 formed in the EA modulation section 30B is removed.

[0049] 15E, the core layer 12 is formed in the EA modulation section 30B. The core layer 12 has an absorption layer with a multiple quantum well structure made of InGaAlAs or InGaAsP, and is grown by MOCVD on the upper surface of the semiconductor substrate 31. The thickness of the core layer 12 may be approximately 0.8 μm.

[0050] In step S26, a common p-type cladding layer 33 and a common contact layer are formed for the LD section 30A and the EA modulation section 30B. The common p-type cladding layer 33 and the common contact layer are formed on the upper surfaces of the core layer 22 of the LD section 30A and the core layer 12 of the EA modulation section 30B. In step S27, a separation groove 37 is formed in the common contact layer to separate the contact layer 24 of the LD section 30A from the contact layer 14 of the EA modulation section 30B. Figure 15G shows the p-type cladding layer 33, the contact layer 24 of the LD section 30A, and the contact layer 14 of the EA modulation section 30B formed in steps S26 and S27. The p-type cladding layer 33 is made of p-type InP and grown by MOCVD. The thickness of the p-type cladding layer 33, the contact layer 24 of the LD section 30A, and the contact layer 14 of the EA modulation section 30B may be approximately 2 μm.

[0051] In step S28, the mesa waveguide structure 20A of the LD section 30A and the mesa waveguide structure 10A of the EA modulation section 30B are formed. The formation of these mesa waveguide structures 20A and 10A is similar to step S14 for the EA modulator 10 described with reference to FIG. 4D. Specifically, the upper surfaces of the contact layers 14 and 24, which are the uppermost layers of the stack shown in FIG. 15G, are masked and dry-etched to leave the mesa waveguide structures 20A and 10A. The mesa waveguide structures 20A and 10A may have a height of approximately 4 μm. By dry-etching, the semiconductor substrate 31 is dug down except for the masked areas where the mesa waveguide structures 20A and 10A are formed, forming the upper surface 31a. However, the portion constituting the n-type cladding layer 31b of the mesa waveguide structures 20A and 10A remains unduly dug down and protrudes in the height direction from the upper surface 31a. Therefore, the n-type cladding layer 31 b is integral with the semiconductor substrate 31 .

[0052] In step S29, the mesa burying layer 35 is formed. The formation of the mesa burying layer 35 is similar to step S15 of the manufacturing method of the EA modulator 10 described with reference to Fig. 4E. More specifically, the mesa burying layer 35 is made of semi-insulating InP doped with Fe, and is formed by burying and regrowth by MOCVD on the upper surface 31a of the semiconductor substrate 31 except for the regions where the mesa waveguide structures 20A and 10A are formed, with the mesa waveguide structures 20A and 10A remaining masked.

[0053] In step S30, an isolated mesa structure 10C of the EA modulation section 30B is formed. The formation of the isolated mesa structure 10C is similar to step S16 of the manufacturing method of the EA modulator 10 described with reference to FIG. 4F . Specifically, while leaving the mask on the mesa waveguide structures 20A, 10A buried in the mesa burying layer 35, a mask is formed on the upper surface of the mesa burying layer 35 of the EA modulation section 30B so that the width of the mesa burying layer 35 gradually decreases in a tapered manner in the z direction, and the mesa burying layer 35 in the unmasked area is removed by dry etching. As a result, a buried mesa structure 10B in which the width of the mesa burying layer 35 gradually decreases in a tapered manner in the z direction is formed on the upper surface 31a of the semiconductor substrate 31 of the EA modulation section 30B, and an isolated mesa structure 10C that is not buried in the mesa burying layer 35 and is exposed is also formed.

[0054] 15H, in step S31, the p-side electrode layer 29 of the LD section 30A and the p-side electrode layer 19 of the EA modulation section 30B are formed on the contact layer 24 of the LD section 30A and the contact layer 14 of the EA modulation section 30B, respectively. These p-side electrode layers 29, 19 are formed by evaporating metal on the upper surfaces of the contact layers 24, 14.

[0055] In step S32, the lower surface of the semiconductor substrate 31 is ground to thin the semiconductor substrate 31. In step S33, an n-side electrode layer 38 is formed on the lower surface of the semiconductor substrate 31. The n-side electrode layer 38 is formed by evaporating metal on the lower surface of the semiconductor substrate 31. In step S34, a high-reflection film (HR) is formed on the reflecting end of the core layer 22 of the mesa waveguide structure 20A of the LD section 30A, and an anti-reflection (AR) film is formed on the emitting end of the core layer 12 of the mesa waveguide structure 10A of the EA modulation section 30B. Through this series of steps, the EADFB laser 30 shown in the cross-sectional view of FIG. 12 is manufactured.

[0056] The EADFB laser 30 has a constant current source connected to the p-side electrode layer 29 of the LD section 30A, and a bias Tee that applies a superimposed modulation signal and DC bias to the p-side electrode layer 19 of the EA modulation section 30B. The n-side electrode layer 38 is connected to ground. A current of approximately 60 mA is injected into the LD section 30A by the constant current source, and a modulation voltage signal with an amplitude of 1 V is applied to the EA modulation section 30B, superimposed on a reverse bias of -1.5 V. The modulation signal is a 50 Gbit / s NRZ signal generated by a pulse pattern generator. As a result, a modulated signal output of 4 dBm and an extinction ratio of 7 dB were obtained.

[0057] The EADFB laser 30 also has an EA modulation section 30B with a configuration similar to that of the EA modulator 10. Therefore, similar to the EA modulator 10, reflection due to changes in refractive index can be suppressed by continuously changing the effective refractive index. Furthermore, by reducing the generation of photocarriers at the input end, it is possible to achieve a more uniform photocarrier concentration throughout the EA modulation section 30B. This suppresses hole accumulation in the multiple quantum well (MQW) and reduces degradation of the operating bandwidth. Furthermore, by improving the extinction characteristics of the EA modulation section 30B, the signal-to-noise ratio can be improved.

[0058] Furthermore, the EADFB laser 30 has the LD section 30A and the EA modulation section 30B monolithically formed on a common semiconductor substrate 31. This allows the laser light generated by the LD section 30A to be effectively utilized by the EA modulation section 30B. Furthermore, the EADFB laser 30 can be made smaller, and can also be monolithically formed together with other optical elements.

[0059] In this embodiment, the EA modulator 10 and the EADFB laser 30 are constructed using InP, but they may also be constructed using other III-V group compound semiconductors such as GaAs, GaN, and InGaAs.

[0060] REFERENCE SIGNS LIST 10 EA modulator 10A mesa waveguide structure 10B buried mesa structure 10C isolated mesa structure 11 semiconductor substrate 11a upper surface 11b n-type cladding layer 12 core layer 13 p-type cladding layer 14 contact layer 15 mesa buried layer 20A mesa waveguide structure 22 core layer 22a diffraction grating 30 EADFB laser 30A LD section 30B EA modulation section 31b n-type cladding layer 33 p-type cladding layer

Claims

1. An electroabsorption modulator comprising: a semiconductor substrate; a mesa waveguide structure including an n-type cladding layer, a core layer having an absorption layer, and a p-type cladding layer stacked in this order on an upper surface of the semiconductor substrate; and a mesa embedding layer disposed on the upper surface of the semiconductor substrate so as to embed the mesa waveguide structure, wherein the mesa waveguide structure transitions from a buried mesa structure buried in the mesa embedding layer to an isolated mesa structure not buried in the mesa embedding layer in the direction of light propagation from its input end to its output end.

2. The electroabsorption modulator according to claim 1, wherein said mesa burying layer is made of a semi-insulating semiconductor.

3. The electroabsorption modulator according to claim 1, wherein the width of said mesa buried layer in said buried mesa structure gradually decreases in the direction of propagation of said light.

4. The electroabsorption modulator according to claim 1, wherein said n-type cladding layer is integral with said semiconductor substrate.

5. An optical transmitter comprising: an electroabsorption modulator as claimed in claim 1; and another mesa waveguide structure arranged adjacent to said electroabsorption modulator on the upper surface of a semiconductor substrate common to said electroabsorption modulator so as to extend said mesa waveguide structure from said input end to the upstream side in the propagation direction of said light, said other mesa waveguide structure having a core of a laser diode having a light emitting layer in place of said absorption layer.

6. The optical transmitter according to claim 5, wherein the core layer of said other waveguide structure has a diffraction grating formed on an upper surface thereof.

7. An optical transmitter according to claim 5, wherein the p-type cladding layer included in said other waveguide structure is separated from the p-type cladding layer of said waveguide structure.

Images