Optical semiconductor device

Abstract

A semiconductor laser (LD) that emits a laser beam and an electro-absorption type optical modulator (EA) having a plurality of optical modulators (EA1, EA2) connected in series and sequentially modulating the laser beam are formed on a semiconductor substrate (1). The plurality of optical modulators (EA1, EA2) each have an optical waveguide (11) having at least an optical absorption layer (9), an upper surface electrode (10) formed on the optical waveguide (11), and a lower surface electrode (7) formed on a lower surface of the semiconductor substrate (1). The upper surface electrodes (10) of the plurality of optical modulators (EA1, EA2) are separated from each other. The upper surface electrodes (10) of adjacent optical modulators (EA1, EA2) are connected via an inductor (16).

Description

Technical Field

[0001] This disclosure relates to optical semiconductor devices. Background Technology

[0002] In recent years, EMLs (Electro-absorption Modulator Laser Diodes) have been used, which integrate a semiconductor laser and an electric field absorption modulator onto a single chip. Electric field absorption modulators with multiple electric field absorption regions have also been proposed (for example, see Patent Document 1). However, for multiple electric field absorption regions, there is only one upper surface electrode, therefore there is only one electric field absorption modulator. Therefore, the frequency band of the optical semiconductor device is determined by the capacitance of the electric field absorption modulator and the inductance of the wire bonded to the upper surface electrode of the electric field absorption modulator.

[0003] Patent Document 1: International Publication No. 2018 / 100634

[0004] If the length of the electric field absorption type optical modulator is shortened to reduce the capacitance in order to improve the bandwidth of the optical semiconductor device, then there is a problem of reducing the extinction ratio of the electric field absorption type optical modulator. Summary of the Invention

[0005] This disclosure was made to solve the aforementioned problems, and its purpose is to obtain an optical semiconductor device that can improve the frequency band without reducing the extinction ratio.

[0006] The optical semiconductor device disclosed herein is characterized by comprising: a semiconductor substrate; a semiconductor laser formed on the semiconductor substrate and emitting a laser beam; an electric field absorption type optical modulator formed on the semiconductor substrate, having a plurality of optical modulators connected in series and sequentially modulating the laser beam; and an inductor, wherein each of the plurality of optical modulators has: an optical waveguide having at least an optical absorption layer, an upper surface electrode formed on the optical waveguide, and a lower surface electrode formed on the lower surface of the semiconductor substrate, wherein the upper surface electrodes of the plurality of optical modulators are separated from each other, and the upper surface electrodes of adjacent optical modulators are connected via the inductor.

[0007] In this disclosure, the electric field absorption type optical modulator is separated into multiple optical modulators connected via inductors. Therefore, by reducing the capacitance of each optical modulator, the bandwidth can be improved. If the total length of the multiple optical modulators is the same as that of conventional optical modulators, the extinction ratio is also the same as conventional. Therefore, the bandwidth can be improved without reducing the extinction ratio. Attached Figure Description

[0008] Figure 1 This is a cross-sectional view showing the optical semiconductor device of Embodiment 1.

[0009] Figure 2 This is a top view of the optical semiconductor device according to Embodiment 1.

[0010] Figure 3 This is a circuit diagram showing the electric field absorption type optical modulator of Embodiment 1.

[0011] Figure 4 This is a cross-sectional view of a comparative example of an optical semiconductor device.

[0012] Figure 5 This is a top view of a comparative example optical semiconductor device.

[0013] Figure 6 This is a circuit diagram of a comparative example of an electric field absorption type optical modulator.

[0014] Figure 7 This is a graph showing the frequency characteristics of S21 in Embodiment 1 and the comparative example.

[0015] Figure 8 This is a diagram showing the electrical signals input to the first and second optical modulators and the optical signals output.

[0016] Figure 9 This is a top view showing the optical semiconductor device of Embodiment 2.

[0017] Figure 10 This is a circuit diagram of the electric field absorption type optical modulator of embodiment 2.

[0018] Figure 11 This is a top view showing the optical semiconductor device of Embodiment 3.

[0019] Figure 12 This is a diagram showing the extinction characteristics of the three optical modulators in Embodiment 4.

[0020] Figure 13 This is a diagram showing the extinction characteristics and light output waveform of the electric field absorption type optical modulator of Embodiment 4.

[0021] Figure 14 This is a diagram showing the modulation voltage of a four-level pulse amplitude modulation (PAM4) mode. Detailed Implementation

[0022] The optical semiconductor device of the embodiment will be described with reference to the accompanying drawings. The same or corresponding components are labeled with the same reference numerals, and sometimes repeated descriptions are omitted.

[0023] Implementation method 1.

[0024] Figure 1This is a cross-sectional view showing the optical semiconductor device of Embodiment 1. A semiconductor laser LD and an electric field absorption type optical modulator EA are formed on a semiconductor substrate 1. The semiconductor laser LD emits a laser beam. The electric field absorption type optical modulator EA has a first optical modulator EA1 and a second optical modulator EA2 connected in series and sequentially modulating the laser beam. The first optical modulator EA1 modulates the laser beam, and the second optical modulator EA2 modulates the output light of the first optical modulator EA1.

[0025] 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 sequentially formed on a semiconductor substrate 1, a laser electrode 6 formed on the p-type contact layer 5, and a lower surface electrode 7 formed on the lower surface of the semiconductor substrate 1. The semiconductor substrate 1 is an n-InP substrate. The n-type cladding layer 2 is composed of n-InP. The active layer 3 is composed of i-InGaAsP multiple quantum wells. The p-type cladding layer 4 is composed of p-InP. The p-type contact layer 5 is composed of p-InGaAsP. A diffraction grating 8 composed of p-InGaAsP is formed on the n-type cladding layer 2.

[0026] The first optical modulator EA1 and the second optical modulator EA2 each have an n-type cladding layer 2, a light absorption layer 9, a p-type cladding layer 4, and a p-type contact layer 5 sequentially formed on a semiconductor substrate 1, an upper surface electrode 10 formed on the p-type contact layer 5, and a lower surface electrode 7 formed on the lower surface of the semiconductor substrate 1. The light absorption layer 9 is composed of i-InGaAsP multiple quantum wells. The upper surface electrode 10 of the first optical modulator EA1 and the upper surface electrode 10 of the second optical modulator EA2 are separated from each other.

[0027] 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. Between the laser electrode 6 of the semiconductor laser LD, the upper surface electrode 10 of the first optical modulator EA1, and the upper surface electrode 10 of the second optical modulator EA2, the chip surface is covered by an insulating film 12 made of SiN.

[0028] Figure 2 This is a top view showing the optical semiconductor device of Embodiment 1. Upper surface electrodes 10 are formed on the optical waveguides WG of the first optical modulator EA1 and the second optical modulator EA2. The optical waveguides WG are formed by etching the n-type cladding 2, the light-absorbing layer 9, the p-type cladding 4, and the p-type contact layer 5 of the first optical modulator EA1 and the second optical modulator EA2. Therefore, the optical waveguides WG of the first optical modulator EA1 and the second optical modulator EA2 have at least the light-absorbing layer 9.

[0029] A first pad PD1, connected to the upper surface electrode 10 of the first optical modulator EA1, and a second pad PD2, connected to the upper surface electrode 10 of the second optical modulator EA2, are formed on the upper surface of the electric field absorption type optical modulator EA. When viewed from above, the first pad PD1 is positioned on one side of the optical waveguide WG, and the second pad PD2 is positioned on the other side of the optical waveguide WG. Wire 14 is bonded to the first pad PD1. One end of wire 15 is bonded to the second pad PD2.

[0030] The first wire 16 connects the first pad PD1 to the second pad PD2. Therefore, the upper surface electrode 10 of the adjacent first optical modulator EA1 and the upper surface electrode 10 of the second optical modulator EA2 are connected via the first wire 16, which serves as an inductor.

[0031] Figure 3 This is a circuit diagram of the electric field absorption type optical modulator of Embodiment 1. The input signal is input to the upper surface electrode 10 of the first optical modulator EA1 via wire 14 and the first pad PD1. The other end of wire 15 is grounded via the terminating resistor R. The inductance of wires 14, 15, and the first wire 16 is adjusted to achieve optimal impedance matching based on the capacitance of the first optical modulator EA1 and the second optical modulator EA2.

[0032] Next, the effects of this embodiment will be explained by comparing it with a comparative example. Figure 4 This is a cross-sectional view of a comparative example of an optical semiconductor device. Figure 5 This is a top view of a comparative example optical semiconductor device. Figure 6 This is the circuit diagram of a comparative example of an electric field absorption type optical modulator. In this comparative example, the electric field absorption type optical modulator EA contains only one optical modulator EA0. If the capacitance of the optical modulator EA0 is set to C, and the inductance of wires 14 and 15 is set to L, then the frequency fc at S21 = -3dB will be... Therefore, shortening the length of the optical modulator EA0 to reduce the capacitance C results in a larger fc, which improves the bandwidth. However, shortening the length of the optical modulator EA0 also reduces its extinction ratio.

[0033] In contrast, in this embodiment, the electric field absorption type optical modulator EA is separated into two optical modulators and connected via an inductor. Therefore, as... Figure 3 As shown, the capacitance of each optical modulator is reduced to 1 / 2×C, thus improving the bandwidth. Figure 7 This is a graph showing the frequency characteristics of S21 in Embodiment 1 and the Comparative Example. It can be seen that, compared with the Comparative Example, the frequency fc when S21 = -3dB is larger in Embodiment 1.

[0034] Figure 8This diagram illustrates the electrical signals input to the first and second optical modulators and the optical signals output. The optical signal modulated by the first optical modulator EA1 is incident on the second optical modulator EA2. An electrical signal with 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, 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 in the comparative example, and thus the extinction ratio is also the same as that in the comparative example. Therefore, according to this embodiment, the bandwidth can be improved without reducing the extinction ratio.

[0035] Furthermore, the rise and fall times of the optical signal incident on the second optical modulator EA2 are almost identical to the rise and fall times of the electrical signal applied to the second optical modulator EA2. Therefore, since the rise time Tr and fall time Tf of the output signal of the second optical modulator EA2 remain constant, the extinction ratio will not decrease.

[0036] Furthermore, when viewed from above, the first pad PD1 is positioned on one side of the optical waveguide WG, and the second pad PD2 is positioned on the other side of the optical waveguide WG. This ensures sufficient spacing between the pads, facilitating the bonding of the first wire 16. Additionally, the sufficient wire length of the first wire 16 facilitates impedance matching adjustments.

[0037] Implementation method 2.

[0038] Figure 9 This is a top view showing the optical semiconductor device of Embodiment 2. Figure 10 This is a circuit diagram of the electric field absorption type optical modulator of Embodiment 2. The electric field absorption type optical modulator EA has a first optical modulator EA1, a second optical modulator EA2, and a third optical modulator EA3 connected in series and modulating the laser beam in sequence. The third optical modulator EA3 modulates the output light of the second optical modulator EA2. A third pad PD3, connected to the upper surface electrode 10 of the third optical modulator EA3, is formed on the upper surface of the electric field absorption type optical modulator EA. A second wire 18 connects the second pad PD2 and the third pad PD3. One end of the wire 15 is bonded to the third pad PD3. Other structures are the same as in Embodiment 1.

[0039] In this embodiment, the electric field absorption type 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 is also the same as in the comparative example. Therefore, the bandwidth can be improved without reducing the extinction ratio.

[0040] Furthermore, when viewed from above, the first pad PD1 and the third pad PD3 are positioned on one side of the optical waveguide WG, while the second pad PD2 is positioned on the other side of the optical waveguide WG. This ensures that the distance between the pads is maintained, facilitating the bonding of the first wire 16 and the second wire 18. Additionally, ensuring the wire lengths of the first wire 16 and the second wire 18 facilitates impedance matching adjustments.

[0041] Implementation method 3.

[0042] Figure 11 This is a top view showing the optical semiconductor device of Embodiment 3. A first wiring pattern 19 connecting the upper surface electrode 10 of the first optical modulator EA1 to the upper surface electrode 10 of the second optical modulator EA2, and a second wiring pattern 20 connecting the upper surface electrode 10 of the second optical modulator EA2 to the upper surface electrode 10 of the third optical modulator EA3, are formed on the upper surface of the electric field absorption type optical modulator EA. Although the materials of the first wiring pattern 19 and the second wiring pattern 20 are the same as those of the upper surface electrode 10, they can also be made of different materials. In addition, although the upper surface electrode 10 is in contact with the p-type contact layer 5, the first wiring pattern 19 and the second wiring pattern 20 are formed on the insulating film 12.

[0043] In this embodiment, the first wire 16 and the second wire 18 of Embodiment 2 can be omitted, thus simplifying assembly. Furthermore, since the first wiring pattern 19 and the second wiring pattern 20 with lower capacitance are used instead of the second pad PD2 of Embodiment 2 with higher capacitance, the electrode capacitance can be reduced. Other structures and effects are the same as in Embodiment 2.

[0044] Implementation method 4.

[0045] In this embodiment, in the structure of embodiment 2 or 3, the extinction characteristics of the first to third optical modulators EA1, EA2, and EA3 are different from each other. Figure 12 This is a graph showing the extinction characteristics of the three optical modulators in Embodiment 4. The vertical axis represents the extinction ratio after normalizing the light output with 1 when the reverse bias voltage is 0V. Figure 13 This diagram illustrates the extinction characteristics and optical output waveform of the electric field absorption type optical modulator in Embodiment 4. By synthesizing the extinction characteristics of the first to third optical modulators EA1, EA2, and EA3, it is possible to obtain an optical modulator that is extremely difficult to achieve in a single construction. Figure 13 As shown, it has extinction characteristics with multiple inflection points, i.e. multiple steps.

[0046] The pulse signal of the four-stage pulse amplitude modulation method is input to the upper surface electrode 10 of the first optical modulator EA1 via the wire 14. Thus, the electric field absorption type optical modulator EA is driven by the four-stage pulse amplitude modulation method. Figure 14This is a diagram showing the modulation voltage of four-level pulse amplitude modulation (PAM4). Four-level pulse amplitude modulation is a method of modulating and transmitting a bit string consisting of "0" and "1" as a pulse signal with four voltage levels (e.g., "00", "01", "10", and "11").

[0047] The transition between high and low voltages in the output waveform of the light from the electric field absorption type optical modulator EA, i.e., the rise and fall, is significantly affected by the tilt of the extinction characteristics. The electric field absorption type optical modulator EA of this embodiment exhibits extreme extinction characteristics within its operating region, thus becoming... Figure 13 The light output waveform is shown in the right figure. This ensures the eye opening (mask margin) of each of the three overlapping waveforms.

[0048] As explained above, in this embodiment, the extinction characteristics of the multiple optical modulators are different from each other; therefore, the extinction ratio curve of the optical device can be controlled to have a shape with multiple steps suitable for the driving conditions. Furthermore, if the number of multiple optical modulators is 3, then as... Figure 13 As shown in the left figure, the extinction ratio curve can be controlled to a four-stage stepped shape suitable for a four-stage pulse amplitude modulation method. Therefore, communication quality can be improved and stabilized. As a result, the optical waveform quality at PAM4 signal input can be improved. Other structures and effects are the same as in embodiments 2 or 3.

[0049] Explanation of reference numerals in the attached figures

[0050] 1...Semiconductor substrate; 7...Lower surface electrode; 9...Light absorption layer; 10...Upper surface electrode; 16...First wire (inductor); 18...Second wire (inductor); 19...First wiring pattern (inductor); 20...Second wiring pattern (inductor); EA...Electrical field absorption type optical modulator; EA1...First optical modulator (multiple optical modulators); EA2...Second optical modulator (multiple optical modulators); EA3...Third optical modulator (multiple optical modulators); LD...Semiconductor laser; PD1...First pad; PD2...Second pad; PD3...Third pad; WG...Optical waveguide.

Claims

1. An optical semiconductor device, characterized in that, have: Semiconductor substrate; A semiconductor laser, which is formed on the semiconductor substrate and emits a laser beam; An electric field absorption type optical modulator is formed on the semiconductor substrate and has multiple optical modulators connected in series and sequentially modulating the laser beam; as well as Inductor The plurality of optical modulators each have: an optical waveguide having at least one light absorption layer, an upper surface electrode formed on the optical waveguide, and a lower surface electrode formed on the lower surface of the semiconductor substrate. The upper surface electrodes of the plurality of optical modulators are separated from each other. The upper surface electrodes of the adjacent optical modulators are connected via the inductor.

2. The optical semiconductor device according to claim 1, characterized in that, The plurality of optical modulators includes: a first optical modulator that modulates the laser beam, and a second optical modulator that modulates the output light of the first optical modulator. A first pad connected to the upper surface electrode of the first optical modulator and a second pad connected to the upper surface electrode of the second optical modulator are formed on the upper surface of the electric field absorption type optical modulator. The inductor has a first wire connecting the first pad and the second pad.

3. The optical semiconductor device according to claim 2, characterized in that, When viewed from above, the first pad is positioned on one side of the optical waveguide, and the second pad is positioned on the other side of the optical waveguide.

4. The optical semiconductor device according to claim 2, characterized in that, The plurality of optical modulators also include a third optical modulator that modulates the output light of the second optical modulator. A third pad, connected to the upper surface electrode of the third optical modulator, is formed on the upper surface of the electric field absorption type optical modulator. The inductor also has a second wire connecting the second pad to the third pad.

5. The optical semiconductor device according to claim 4, characterized in that, When viewed from above, the first pad and the third pad are positioned on one side of the optical waveguide, and the second pad is positioned on the other side of the optical waveguide.

6. The optical semiconductor device according to claim 1, characterized in that, The plurality of optical modulators includes: a first optical modulator that modulates the laser beam, 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 is formed on the upper surface of the electric field absorption type optical modulator, and the upper surface electrode of the first optical modulator is connected to the upper surface electrode of the second optical modulator; and A second wiring pattern is formed on the upper surface of the electric field absorption type optical modulator, and connects the upper surface electrode of the second optical modulator to the upper surface electrode of the third optical modulator.

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

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