Optical semiconductor device

Abstract

In the present invention, the following are formed on a semiconductor substrate (1): a semiconductor laser (LD) which emits laser light; and an electro-absorption optical modulator (EA) which has a plurality of optical modulators (EA1, EA2) that are connected in series and sequentially modulate laser light. Each of the plurality of optical modulators (EA1, EA2) includes: an optical waveguide (11) having at least a light absorption layer (9); an upper surface electrode (10) formed on the optical waveguide (11); and a lower surface electrode (7) formed on the lower surface of the semiconductor substrate (1). The upper surface electrodes (10) of the plurality of optical modulators (EA1, EA2) are spaced apart from each other. The upper surface electrodes (10) of the adjacent optical modulators (EA1, EA2) are connected via an inductor (16).

Description

Optical semiconductor device

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

[0002] In recent years, electro-absorption modulator laser diodes (EMLs), which integrate a semiconductor laser and an electro-absorption modulator on a single chip, have been used. Electro-absorption modulators with multiple electro-absorption regions have also been proposed (see, for example, Patent Document 1). However, since there is only one top electrode for the multiple electro-absorption regions, there is only one electro-absorption modulator. Therefore, the bandwidth of the optical semiconductor device is determined by the capacitance of the electro-absorption modulator and the inductance of the wire bonded to the top electrode of the electro-absorption modulator.

[0003] International Publication No. 2018 / 100634

[0004] When an attempt is made to reduce the capacitance by shortening the length of the electroabsorption optical modulator in order to improve the bandwidth of the optical semiconductor device, the extinction ratio of the electroabsorption optical modulator decreases.

[0005] The present disclosure has been made to solve the above-mentioned problems, and its object is to provide an optical semiconductor device that can improve the bandwidth without reducing the extinction ratio.

[0006] The optical semiconductor device according to the present disclosure comprises a semiconductor substrate, a semiconductor laser formed on the semiconductor substrate and emitting laser light, an electro-absorption optical modulator formed on the semiconductor substrate and having a plurality of optical modulators connected in series to sequentially modulate the laser light, and an inductor, wherein each of the plurality of optical modulators has an optical waveguide having at least a light absorption layer, a top electrode formed on the optical waveguide, and a bottom electrode formed on the bottom surface of the semiconductor substrate, the top electrodes of the plurality of optical modulators being spaced apart from each other, and the top electrodes of adjacent optical modulators being connected via the inductor.

[0007] In this disclosure, the electroabsorption optical modulator is separated into multiple optical modulators, which are connected via inductors. This reduces the capacitance of each optical modulator, improving the bandwidth. If the total length of the multiple optical modulators is the same as that of a conventional optical modulator, the extinction ratio will also be the same as that of a conventional optical modulator. This allows the bandwidth to be improved without reducing the extinction ratio.

[0008] 1 is a cross-sectional view showing an optical semiconductor device according to a first embodiment; FIG. 2 is a top view showing an optical semiconductor device according to the first embodiment; FIG. 3 is a circuit diagram of an electro-absorption optical modulator according to the first embodiment; FIG. 4 is a cross-sectional view showing an optical semiconductor device according to a comparative example; FIG. 5 is a top view showing an optical semiconductor device according to the comparative example; FIG. 6 is a circuit diagram of an electro-absorption optical modulator according to the comparative example; FIG. 7 is a diagram showing frequency characteristics of S21 of the first embodiment and the comparative example; FIG. 8 is a diagram showing electrical signals input to and optical signals output from first and second optical modulators; FIG. 9 is a top view showing an optical semiconductor device according to a second embodiment; FIG. 10 is a circuit diagram of an electro-absorption optical modulator according to the second embodiment; FIG. 11 is a top view showing an optical semiconductor device according to a third embodiment; FIG. 12 is a diagram showing extinction characteristics of three optical modulators according to a fourth embodiment; FIG. 13 is a diagram showing the extinction characteristics and optical output waveform of an electro-absorption optical modulator according to the fourth embodiment; FIG. 14 is a diagram showing modulation voltage of a four-level pulse amplitude modulation method (PAM4).

[0009] 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.

[0010] First Embodiment. Figure 1 is a cross-sectional view showing an optical semiconductor device according to a first embodiment. A semiconductor laser LD and an electro-absorption modulator EA are formed on a single semiconductor substrate 1. The semiconductor laser LD emits laser light. The electro-absorption modulator EA has a first optical modulator EA1 and a second optical modulator EA2 connected in series to sequentially modulate the laser light. The first optical modulator EA1 modulates the laser light, and the second optical modulator EA2 modulates the output light of the first optical modulator EA1.

[0011] The semiconductor laser LD has an n-type cladding layer 2, an active layer 3, a p-type cladding layer 4, and a p-type contact layer 5 formed in this order on a semiconductor substrate 1, a laser electrode 6 formed on the p-type contact layer 5, and a bottom electrode 7 formed on the bottom surface of the semiconductor substrate 1. The semiconductor substrate 1 is an n-InP substrate. The n-type cladding layer 2 is made of n-InP. The active layer 3 is made of i-InGaAsP multiple quantum wells. The p-type cladding layer 4 is made of p-InP. The p-type contact layer 5 is made of p-InGaAsP. A diffraction grating 8 made of p-InGaAsP is formed in the n-type cladding layer 2.

[0012] Each of the first optical modulator EA1 and the second optical modulator EA2 has an n-type cladding layer 2, an optical absorption layer 9, a p-type cladding layer 4, and a p-type contact layer 5 formed in this order on a semiconductor substrate 1, an upper electrode 10 formed on the p-type contact layer 5, and a lower electrode 7 formed on the lower surface of the semiconductor substrate 1. The optical absorption layer 9 is made of i-InGaAsP multiple quantum well. The upper electrode 10 of the first optical modulator EA1 and the upper electrode 10 of the second optical modulator EA2 are spaced apart from each other.

[0013] A transparent waveguide 11 made of i-InGaAsP connects the active layer 3 of the semiconductor laser LD, the light absorption layer 9 of the first optical modulator EA1, and the light absorption layer 9 of the second optical modulator EA2. The chip surface is covered with an insulating film 12 made of SiN between the laser electrode 6 of the semiconductor laser LD, the top electrode 10 of the first optical modulator EA1, and the top electrode 10 of the second optical modulator EA2.

[0014] 2 is a top view showing the optical semiconductor device according to the first embodiment. A top electrode 10 is formed on the optical waveguide WG of the first optical modulator EA1 and the second optical modulator EA2. The optical waveguide WG is formed by etching the n-type cladding layer 2, the optical absorption layer 9, the p-type cladding layer 4, and the p-type contact layer 5 of the first optical modulator EA1 and the second optical modulator EA2. Therefore, the optical waveguide WG of the first optical modulator EA1 and the second optical modulator EA2 has at least the optical absorption layer 9.

[0015] A first pad PD1 connected to the top electrode 10 of the first optical modulator EA1 and a second pad PD2 connected to the top electrode 10 of the second optical modulator EA2 are formed on the top surface of the electro-absorption optical modulator EA. In a plan view, the first pad PD1 is arranged on one side of the optical waveguide WG, and the second pad PD2 is arranged on the other side of the optical waveguide WG. A wire 14 is bonded to the first pad PD1. One end of a wire 15 is bonded to the second pad PD2.

[0016] A first wire 16 connects the first pad PD1 and the second pad PD2. Therefore, the top electrodes 10 of the adjacent first optical modulator EA1 and second optical modulator EA2 are connected via the first wire 16, which serves as an inductor.

[0017] 3 is a circuit diagram of the electro-absorption optical modulator according to the first embodiment. An input signal is input to the top electrode 10 of the first optical modulator EA1 via the wire 14 and the first pad PD1. The other end of the wire 15 is grounded via a termination resistor R. The inductances of the wire 14, wire 15, and first wire 16 are adjusted to optimize impedance matching according to the capacitances of the first optical modulator EA1 and the second optical modulator EA2.

[0018] Next, the effects of this embodiment will be described in comparison with a comparative example. FIG. 4 is a cross-sectional view showing an optical semiconductor device according to the comparative example. FIG. 5 is a top view showing an optical semiconductor device according to the comparative example. FIG. 6 is a circuit diagram of an electro-absorption optical modulator according to the comparative example. In the comparative example, the electro-absorption optical modulator EA includes only one optical modulator EA0. If the capacitance of the optical modulator EA0 is C and the inductance of the wires 14 and 15 is L, the frequency fc when S21 = -3 dB is calculated as fc = 1 / (2π√LC). Therefore, if the length of the optical modulator EA0 is shortened to reduce the capacitance C, fc increases, improving the bandwidth. However, shortening the length of the optical modulator EA0 reduces the extinction ratio of the optical modulator EA0.

[0019] In contrast, in this embodiment, the electro-absorption optical modulator EA is separated into two optical modulators, which are connected via an inductor. Therefore, as shown in FIG. 3, the capacitance of each optical modulator is reduced to 1 / 2×C, thereby improving the bandwidth. FIG. 7 shows the frequency characteristics of S21 in the first embodiment and the comparative example. It can be seen that the frequency fc when S21 = -3 dB in the first embodiment is higher than in the comparative example.

[0020] 8 is a diagram showing the electrical signals input to and output from the first and second optical modulators. An optical signal modulated by the first optical modulator EA1 is input to the second optical modulator EA2. An electrical signal having the same voltage waveform as the electrical signal applied to the first optical modulator EA1 is applied to the second optical modulator EA2 via the first wire 16. Therefore, if the total length of the first optical modulator EA1 and the second optical modulator EA2 is the same as that of the optical modulator EA0 of the comparative example, the extinction ratio will also be the same as that of the comparative example. Therefore, this embodiment can improve the bandwidth without reducing the extinction ratio.

[0021] Furthermore, the rise and fall timing of the optical signal input to the second optical modulator EA2 and the rise and fall timing of the electrical signal applied to the second optical modulator EA2 are approximately the same, so the rise time Tr and fall time Tf of the output signal from the second optical modulator EA2 do not become long, and the extinction ratio does not decrease.

[0022] In addition, in a plan view, the first pad PD1 is disposed on one side of the optical waveguide WG, and the second pad PD2 is disposed on the other side of the optical waveguide WG. This ensures a sufficient distance between the pads, making it easy to bond the first wire 16. Furthermore, the wire length of the first wire 16 can be ensured, making it easy to adjust impedance matching.

[0023] Second Embodiment. Figure 9 is a top view showing an optical semiconductor device according to a second embodiment. Figure 10 is a circuit diagram of an electro-absorption optical modulator according to the second embodiment. The electro-absorption optical modulator EA has a first optical modulator EA1, a second optical modulator EA2, and a third optical modulator EA3 connected in series to sequentially modulate laser light. The third optical modulator EA3 modulates the output light of the second optical modulator EA2. A third pad PD3 connected to the top electrode 10 of the third optical modulator EA3 is formed on the top surface of the electro-absorption optical modulator EA. A second wire 18 connects the second pad PD2 and the third pad PD3. One end of a wire 15 is bonded to the third pad PD3. The other configurations are the same as those of the first embodiment.

[0024] In this embodiment, the electro-absorption optical modulator EA is separated into three optical modulators. Therefore, the capacitance of each optical modulator is reduced to 1 / 3 × C, thereby improving the bandwidth. Furthermore, if the total length of the three optical modulators is the same as that of the optical modulator EA0 in the comparative example, the extinction ratio will also be the same as that in the comparative example. Therefore, the bandwidth can be improved without reducing the extinction ratio.

[0025] In addition, in a plan view, the first pad PD1 and the third pad PD3 are arranged on one side of the optical waveguide WG, and the second pad PD2 is arranged on the other side of the optical waveguide WG. This ensures a sufficient distance between the pads, making it easy to bond the first wire 16 and the second wire 18. Furthermore, the wire lengths of the first wire 16 and the second wire 18 can be ensured, making it easy to adjust impedance matching.

[0026] Third Embodiment. Figure 11 is a top view showing an optical semiconductor device according to a third embodiment. A first wiring pattern 19 connecting the top electrode 10 of the first optical modulator EA1 to the top electrode 10 of the second optical modulator EA2, and a second wiring pattern 20 connecting the top electrode 10 of the second optical modulator EA2 to the top electrode 10 of the third optical modulator EA3 are formed on the top surface of the electro-absorption optical modulator EA. The material of the first wiring pattern 19 and the second wiring pattern 20 is the same as that of the top electrode 10, but may be a different material. Note that the top electrode 10 is in contact with the p-type contact layer 5, but the first wiring pattern 19 and the second wiring pattern 20 are formed on an insulating film 12.

[0027] In this embodiment, the first wire 16 and the second wire 18 of the second embodiment can be omitted, making assembly easier. Also, instead of the second pad PD2 of the second embodiment, which has a large capacitance, the first wiring pattern 19 and the second wiring pattern 20, which have a small capacitance, are used, making it possible to reduce the electrode capacitance. Other configurations and effects are the same as those of the second embodiment.

[0028] Embodiment 4 In this embodiment, the extinction characteristics of the first to third optical modulators EA1, EA2, and EA3 in the configuration of embodiment 2 or 3 are different from one another. FIG. 12 is a diagram showing the extinction characteristics of three optical modulators according to embodiment 4. The vertical axis represents the extinction ratio when the optical output when the reverse bias is 0 V is normalized to 1. FIG. 13 is a diagram showing the extinction characteristic and optical output waveform of the electro-absorption optical modulator according to embodiment 4. By combining the extinction characteristics of the first to third optical modulators EA1, EA2, and EA3, it is possible to obtain an extinction characteristic having multiple inflection points, i.e., multiple steps, as shown in FIG. 13, which is extremely difficult to achieve with a single optical modulator structure.

[0029] A four-level pulse amplitude modulation (PAM4) pulse signal is input to the top electrode 10 of the first optical modulator EA1 via the wire 14. This drives the electro-absorption optical modulator EA using the four-level pulse amplitude modulation (PAM4). FIG. 14 shows the modulation voltage of the four-level pulse amplitude modulation (PAM4). The four-level PAM4 is a method of modulating and transmitting a bit string consisting of "0" and "1" as a pulse signal with four voltage levels (for example, four values: "00," "01," "10," and "11").

[0030] The transition between high and low voltages of the optical output waveform output from the electro-absorption optical modulator EA, i.e., the rising and falling edges, is greatly affected by the slope of the extinction characteristics. The electro-absorption optical modulator EA of this embodiment has extreme extinction characteristics in the operating region, resulting in an optical output waveform like the one shown on the right in Figure 13. This ensures the eye opening (mask margin) for each of the three overlapping waveforms.

[0031] As described above, in this embodiment, the extinction characteristics of the multiple optical modulators are different from one another, so the extinction ratio curve of the optical device can be controlled to have multiple steps suitable for the driving conditions. Furthermore, if the number of multiple optical modulators is three, the extinction ratio curve can be controlled to a four-step staircase suitable for the four-level pulse amplitude modulation method, as shown in the left diagram of FIG. 13. Therefore, communication quality can be improved and stabilized. As a result, the optical waveform quality when a PAM4 signal is input can be improved. Other configurations and effects are the same as those of the second or third embodiment.

[0032] 1 semiconductor substrate, 7 lower electrode, 9 light absorption layer, 10 upper electrode, 16 first wire (inductor), 18 second wire (inductor), 19 first wiring pattern (inductor), 20 second wiring pattern (inductor), EA electroabsorption optical modulator, EA1 first optical modulator (plurality of optical modulators), EA2 second optical modulator (plurality of optical modulators), EA3 third optical modulator (plurality of optical modulators), LD semiconductor laser, PD1 first pad, PD2 second pad, PD3 third pad, WG optical waveguide

Claims

1. A semiconductor device comprising: a semiconductor substrate; a semiconductor laser formed on the semiconductor substrate and emitting laser light; an electroabsorption type optical modulator formed on the semiconductor substrate and having a plurality of optical modulators connected in series to modulate the laser light in sequence; and an inductor, wherein each of the plurality of optical modulators has an optical waveguide having at least an optical absorption layer, an upper electrode formed on the optical waveguide, and a lower electrode formed on a lower surface of the semiconductor substrate, the upper electrodes of the plurality of optical modulators are spaced apart from each other, and the upper electrodes of adjacent optical modulators are connected via the inductor.

2. The semiconductor optical device according to claim 1, wherein the plurality of optical modulators include a first optical modulator that modulates the laser light and a second optical modulator that modulates output light of the first optical modulator, a first pad connected to the upper electrode of the first optical modulator and a second pad connected to the upper electrode of the second optical modulator are formed on an upper surface of the electroabsorption type optical modulator, and the inductor has a first wire connecting the first pad and the second pad.

3. The semiconductor optical device according to claim 2, wherein in a plan view, the first pad is disposed on one side of the optical waveguide, and the second pad is disposed on the other side of the optical waveguide.

4. The semiconductor optical device according to claim 2, wherein the plurality of optical modulators further include a third optical modulator that modulates output light of the second optical modulator, a third pad connected to the upper electrode of the third optical modulator is formed on an upper surface of the electroabsorption type optical modulator, and the inductor further has a second wire connecting the second pad and the third pad.

5. The semiconductor optical device according to claim 4, wherein in a plan view, the first pad and the third pad are disposed on one side of the optical waveguide, and the second pad is disposed on the other side of the optical waveguide.

6. The plurality of optical modulators include a first optical modulator that modulates the laser light, a second optical modulator that modulates the output light of the first optical modulator, and a third optical modulator that modulates the output light of the second optical modulator. The inductor has a first wiring pattern formed on the upper surface of the electroabsorption type optical modulator and connecting the upper surface electrode of the first optical modulator and the upper surface electrode of the second optical modulator, and a second wiring pattern formed on the upper surface of the electroabsorption type optical modulator and connecting the upper surface electrode of the second optical modulator and the upper surface electrode of the third optical modulator. The optical semiconductor device according to claim 1, characterized in that it has the above.

7. The optical semiconductor device according to any one of claims 1 to 6, characterized in that the extinction characteristics of the plurality of optical modulators are different from each other.

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