Optical modulator integrated laser device and optical modulation circuit

Abstract

To provide an optical modulator integrated laser element capable of avoiding narrowing of a band of a high frequency signal while reducing reflection of the high frequency signal.SOLUTION: The optical modulator integrated laser element includes a laser portion that outputs laser light, an optical modulation portion having a modulation electrode to which one of differential signals is input, and a dummy element portion to which the other of the differential signals is input. The optical modulation unit is connected between one of the differential signals and a reference potential. The dummy element portion is connected between the other of the differential signals and a reference potential, and includes a resistance component and a capacitance component connected in parallel.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present disclosure relates to an optical modulator integrated laser device and an optical modulation circuit. [Background technology]

[0002] Patent Document 1 discloses a high-frequency circuit in which a signal line transmitting a high-frequency signal is connected to a capacitive element by a first bonding wire, and the capacitive element is connected to a termination resistor for impedance matching by a second bonding wire. Patent Document 2 discloses a driving circuit and a driving method for a semiconductor laser module equipped with an electroabsorption optical modulator. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Laid-Open No. 2001-308130 [Patent Document 2] Japanese Patent Application Laid-Open No. 2004-061556 Summary of the Invention [Problem to be solved by the invention]

[0004] In conventional optical modulator-integrated laser devices that integrate a semiconductor laser and an electroabsorption optical modulator, reflection of the high-frequency signal occurs due to a load impedance (e.g., 33 Ω) that differs from the characteristic impedance (e.g., 50 Ω) of the transmission line for transmitting the high-frequency signal supplied to the optical modulator. Since reflection of the high-frequency signal leads to signal loss, it is desirable to reduce it. High-frequency signal reflection can be reduced by bringing the load impedance closer to the characteristic impedance. However, this narrows the bandwidth of the high-frequency signal, making it difficult to ensure a sufficient bandwidth for the high-frequency signal. Thus, in conventional optical modulator-integrated laser devices, it is difficult to avoid the trade-off between reflection of the high-frequency signal and ensuring a sufficient bandwidth.

[0005] An object of the present disclosure is to provide an optical modulator integrated laser element and an optical modulation circuit that can reduce the reflection of high-frequency signals while avoiding narrowing of the band of the high-frequency signals. [Means for solving the problem]

[0006] An optical modulator-integrated laser device according to an embodiment of the present disclosure includes a laser section that outputs laser light, an optical modulation section having modulation electrodes to which one of the differential signals is input, and a dummy element section to which the other of the differential signals is input. The optical modulation section is connected between one of the differential signals and a reference potential. The dummy element section is connected between the other of the differential signals and the reference potential and includes a resistance component and a capacitance component connected in parallel. [Effects of the Invention]

[0007] According to the present disclosure, it is possible to provide an optical modulator integrated laser element and an optical modulation circuit that can reduce the reflection of high-frequency signals while preventing the band of the high-frequency signals from becoming narrow. [Brief explanation of the drawings]

[0008] [Figure 1] FIG. 1 is a circuit diagram showing the configuration of an optical modulator integrated laser device according to an embodiment of the present disclosure. [Figure 2] FIG. 2 is a perspective view showing the appearance of the optical modulator integrated laser device. [Figure 3] FIG. 3 is a diagram showing a cross section passing through the second signal pad, resistor, electrode, and modulation electrode shown in FIG. [Figure 4] FIG. 4 is a cross-sectional view taken along line III-III shown in FIG. [Figure 5] FIG. 5 is a cross-sectional view taken along line IV-IV shown in FIG. [Figure 6] FIG. 6 is a diagram showing a cross section taken along line VV shown in FIG. [Figure 7] FIG. 7 is a cross-sectional view taken along line VI-VI shown in FIG. [Figure 8]FIG. 8 is a cross-sectional view taken along line VII-VII shown in FIG. [Figure 9] FIG. 9 is a perspective view showing an optical modulation circuit. [Figure 10] FIG. 10 is a plan view showing the optical modulation circuit. [Figure 11] FIG. 11 is a circuit diagram when the dummy element portion is provided away from the semiconductor substrate. [Figure 12] FIG. 12 is a circuit diagram of an optical modulator integrated laser device. [Figure 13] FIG. 13 is a graph showing the relationship between frequency and Sdd11. [Figure 14] FIG. 14 is a graph showing the relationship between frequency and Sdd11. [Figure 15] FIG. 15 is a table showing examples of combinations of the impedance of the optical modulation section and the impedance of the dummy element section. [Figure 16] FIG. 16 is a graph showing the relationship between the ratio of the impedance of the optical modulation section to the impedance of the dummy element section and the reflection coefficient at 1 GHz. [Figure 17] FIG. 17 is a circuit diagram showing the configuration of a modified example. DETAILED DESCRIPTION OF THE INVENTION

[0009] [Description of the embodiments of the present disclosure] First, the details of the embodiments of the present disclosure will be listed and described. [1] An optical modulator-integrated laser device according to an embodiment of the present disclosure includes a laser section that outputs laser light, an optical modulation section having modulation electrodes to which one of the differential signals is input, and a dummy element section to which the other of the differential signals is input. The optical modulation section is connected between one of the differential signals and a reference potential. The dummy element section is connected between the other of the differential signals and the reference potential and includes a resistive component and a capacitive component connected in parallel. In this optical modulator-integrated laser device, the other of the differential signals is input to the dummy element section. This combines the load impedance of a circuit formed by the optical modulation section and termination resistor to which one of the differential signals is input and the load impedance of the dummy element section to which the other of the differential signals is input, allowing the overall load impedance to approach the characteristic impedance of the transmission line. This reduces reflection of high-frequency signals. In addition, it is possible to ensure a high-frequency signal bandwidth as wide as that of conventional optical modulators. Therefore, the optical modulator integrated laser element [1] above can reduce the reflection of high frequency signals while avoiding narrowing of the band of the high frequency signals, thereby improving the trade-off between the reflection of high frequency signals and ensuring the band.

[0010] [2] In the optical modulator-integrated laser device described above in [1], the capacitance component of the dummy element portion may be a capacitor element formed by sandwiching a dielectric between a pair of electrodes. For example, the capacitance component of the dummy element portion can be obtained by such a configuration.

[0011] [3] In the optical modulator-integrated laser device described above in [1], the capacitance component of the dummy element section may have a region of the same semiconductor laminate structure as the optical modulator section connected to both ends of the dummy element section. For example, with such a configuration, the capacitance component of the dummy element section can be obtained.

[0012] [4] In the optical modulator-integrated laser device according to the above [1] to [3], the impedance Zp of the optical modulation section and the impedance Zn of the dummy element section may satisfy 1≦Zn / Zp≦3. In this case, reflection of high-frequency signals can be further reduced.

[0013] [5] An optical modulation circuit according to an embodiment of the present disclosure includes a pair of transmission lines for transmitting differential signals, an optical modulation unit to which one of the differential signals is input, and a dummy element unit to which the other of the differential signals is input. The optical modulation unit is connected between one of the transmission lines and a reference potential. The dummy element unit is connected between the other of the differential signals and the reference potential and includes a resistive component and a capacitive component connected in parallel. This optical modulator-integrated laser element can reduce the reflection of high-frequency signals while avoiding narrowing of the high-frequency signal bandwidth, thereby improving the trade-off between high-frequency signal reflection and bandwidth maintenance.

[0014] [6] In the optical modulation circuit of the above [5], the capacitance component of the dummy element section may be a capacitor element formed by sandwiching a dielectric between a pair of electrodes. For example, the capacitance component of the dummy element section can be obtained by such a configuration.

[0015] [7] In the optical modulation circuit of [5] above, the capacitance component of the dummy element section may have a region of the same semiconductor laminate structure as the optical modulation section connected to both ends of the dummy element section. For example, with such a configuration, the capacitance component of the dummy element section can be obtained.

[0016] [8] In the optical modulation circuits [5] to [7] above, the impedance Zp of the optical modulation section and the impedance Zn of the dummy element section may satisfy 1≦Zn / Zp≦3. In this case, reflection of high-frequency signals can be further reduced. [Details of the embodiments of the present disclosure]

[0017] Specific examples of the present disclosure will be described below with reference to the drawings. Note that the present invention is not limited to these examples, but is defined by the claims, and is intended to include all modifications within the meaning and scope equivalent to the claims. In the following description, the same elements in the description of the drawings will be given the same reference numerals, and duplicate explanations will be omitted. [First embodiment]

[0018] FIG. 1 is a circuit diagram showing the configuration of an electro-absorption modulator integrated laser diode (EML) 1 according to an embodiment of the present disclosure. The EML 1 of this embodiment includes a laser unit 2 that outputs laser light, an optical modulator (optical modulation unit 3) that modulates the laser light, and a dummy element unit 4 that simulates the characteristics of the optical modulator. Pads 26 and 27 are connected to the anode of the laser unit 2. A first signal pad 32 is connected to the anode of the optical modulation unit 3. A second signal pad 42 is connected to a first end of the dummy element unit 4. The cathode of the laser unit 2, the cathode of the optical modulation unit 3, and a second end of the dummy element unit 4 are electrically connected to each other and set to a reference potential. A termination resistor 16 is connected in parallel with the optical modulation unit 3. The dummy element unit 4 of this embodiment includes a resistor 43 and a capacitor 48 connected in parallel to each other. The resistor 43 corresponds to the resistive component of this disclosure. The capacitor 48 corresponds to the capacitive component of this disclosure. The impedance Zp of the optical modulation section 3 and the impedance Zn of the dummy element section 4 satisfy the relationship 1≦Zn / Zp≦3, for example.

[0019] FIG. 2 is a perspective view showing the appearance of the EML1. As shown in FIG. 2, the EML1 includes a semiconductor substrate 5. The semiconductor substrate 5 has a conductivity type, such as n-type. The laser section 2, optical modulation section 3, and dummy element section 4 are provided on the same semiconductor substrate 5. The planar shape of the semiconductor substrate 5 is a rectangle whose length is in the optical waveguide direction. The laser section 2 and optical modulation section 3 are aligned along the optical waveguide direction. An optical waveguide 61 extends from a first end to a second end of the semiconductor substrate 5 in the optical waveguide direction. The laser section 2 includes a portion of the optical waveguide 61 closer to the first end. The optical modulation section 3 includes a portion of the optical waveguide 61 closer to the second end. The dummy element section 4 is disposed between the laser section 2 and the optical modulation section 3.

[0020] FIG. 3 shows a cross section passing through the second signal pad 42, resistor 43, electrode 46, and modulating electrode 31 shown in FIG. 2. FIG. 4 shows a cross section taken along line III-III in FIG. 2 (a cross section passing through the modulating electrode 31 and the first signal pad 32). FIG. 5 shows a cross section taken along line IV-IV in FIG. 2 (a cross section passing through the electrode 25 and the second signal pad 42). FIG. 6 shows a cross section taken along line VV in FIG. 2 (a cross section passing through the electrode 46 and modulating electrode 31). FIG. 7 shows a cross section taken along line VI-VI in FIG. 2 (a cross section passing through the resistor 43). FIG. 8 shows a cross section taken along line VII-VII in FIG. 2 (a cross section passing through the pad 26, electrode 25, and pad 27). The configurations of the laser section 2, the optical modulation section 3, and the dummy element section 4 will be described below with reference to FIGS. 2 to 8.

[0021] As described above, the laser section 2 has a portion of the optical waveguide 61. The optical waveguide 61 is provided on the primary surface 5a of the semiconductor substrate 5 and includes a lower cladding layer 21, an active layer 22, an upper cladding layer 23, and a contact layer 24. The lower cladding layer 21, the active layer 22, the upper cladding layer 23, and the contact layer 24 are stacked in this order from the primary surface 5a. The lower cladding layer 21, the active layer 22, the upper cladding layer 23, and the contact layer 24 have a mesa structure confined on both sides by semi-insulating layers 6 provided on the primary surface 5a. The lower cladding layer 21, the active layer 22, the upper cladding layer 23, and the contact layer 24 mainly contain, for example, InP-based semiconductors.

[0022] The laser section 2 further includes an electrode 25, a pad 26, and a pad 27. The electrode 25 includes a layer that forms ohmic contact with the contact layer 24 and a wiring layer provided on the layer. The wiring layer is, for example, a gold (Au) layer. The electrode 25 is provided on an insulating film 7 formed on the semi-insulating layer 6 and contacts the contact layer 24 through an opening formed in the insulating film 7. The upper surface of the electrode 25 is covered with an insulating film 8. The insulating films 7 and 8 contain a silicon compound such as SiO2. The pads 26 and 27 are provided on the insulating film 7 formed on the semi-insulating layer 6 and are provided on both sides of the electrode 25 in a direction intersecting the optical waveguide direction. The pads 26 and 27 contain, for example, gold (Au). The upper surfaces of the pads 26 and 27 are exposed through openings formed in the insulating film 8.

[0023] In the laser unit 2, a bias current is supplied to the electrode 25 through the pad 26 or the pad 27. A back electrode (not shown) set to a reference potential is provided on the back surface 5b of the semiconductor substrate 5. When a bias current flows between the electrode 25 and the back surface electrode, light is generated in the active layer 22. The light resonates in the optical waveguide 61 along the optical waveguiding direction, and is output to the optical modulation unit 3 as laser light.

[0024] As described above, the optical modulation section 3 has another part of the optical waveguide 61. The configuration of the optical waveguide 61 in the optical modulation section 3 is the same as the configuration of the optical waveguide 61 in the laser section 2. The optical modulation section 3 further has a modulation electrode 31 (modulation electrode), a first signal pad 32, and a dielectric layer 33. The modulation electrode 31 includes a layer that forms ohmic contact with the contact layer 24 and a wiring layer provided on the layer. The wiring layer is, for example, a gold (Au) layer. The modulation electrode 31 is provided on an insulating film 7 formed on the semi-insulating layer 6, and is in contact with the contact layer 24 through an opening formed in the insulating film 7. The upper surface of the modulation electrode 31 is covered with an insulating film 8.

[0025] The first signal pad 32 is provided on an insulating film 7 formed on the semi-insulating layer 6, and is provided on one side of the modulation electrode 31 in a direction intersecting the optical waveguiding direction. The first signal pad 32 contains, for example, gold (Au). The upper surface of the first signal pad 32 is exposed through an opening formed in the insulating film 8. The dielectric layer 33 is provided between the semi-insulating layer 6 and the first signal pad 32, and is surrounded by the insulating film 7. The dielectric layer 33 has a lower dielectric constant than the insulating film 7. The dielectric layer 33 is made of, for example, BCB (Benzo-Cyclo-Butene). A structure in which the dielectric layer 33 and the semi-insulating layer 6 are arranged between the first signal pad 32 and the semiconductor substrate 5 functions as a capacitor.

[0026] In the optical modulation unit 3, the positive-phase signal of the differential signals serving as modulation signals is input to the first signal pad 32. As a result, the modulation signal is supplied to the modulation electrode 31 through the first signal pad 32. When the modulation signal is applied between the modulation electrode 31 and the rear electrode, the laser light guided within the optical waveguide 61 is modulated. The modulated laser light is output to the outside of the EML1.

[0027] The dummy element section 4 is located away from the optical waveguide 61 and is not optically coupled to the laser section 2. The dummy element section 4 adjusts the impedance of the optical modulator section 3 to approximate the impedance of the transmission line transmitting the differential signal (an impedance adjustment circuit for achieving both bandwidth and reflection characteristics). The equivalent circuit (simulating the optical modulator) constituting the dummy element section 4 includes a capacitance component and a resistor connected in parallel thereto (which functions to simulate the optical absorption current flowing through the modulator). The capacitance component is a capacitor element formed by sandwiching a dielectric between a pair of electrodes. The dummy element section 4 includes a dielectric layer 41, a second signal pad 42, a resistor 43, wiring 44, wiring 45, and an electrode 46. The electrode 46 includes an ohmic metal layer 47 that forms ohmic contact with the semiconductor substrate 5 and a wiring layer provided on the ohmic metal layer 47. The ohmic metal layer 47 is, for example, a TiW layer. The wiring layer is, for example, a gold (Au) layer. The electrode 46 is in contact with the semiconductor substrate 5 through an opening formed in the semi-insulating layer 6 and the insulating film 7. The upper surface of the electrode 46 is covered with the insulating film 8. The electrode 46 extends the potential (reference potential) of the back electrode up to the insulating film 7.

[0028] The second signal pad 42 is provided on an insulating film 7 formed on the semi-insulating layer 6, and is provided on one side of the optical waveguide 61 in a direction intersecting the optical waveguiding direction. The second signal pad 42 contains, for example, gold (Au). The upper surface of the second signal pad 42 is exposed through an opening formed in the insulating film 8. The dielectric layer 41 is provided between the semi-insulating layer 6 and the second signal pad 42, and is surrounded by the insulating film 7. The dielectric layer 41 has a lower dielectric constant than the insulating film 7. The dielectric layer 41 is made of, for example, BCB. A structure in which the dielectric layer 41 and the semi-insulating layer 6 are arranged between the second signal pad 42 and the semiconductor substrate 5 functions as a capacitor 48.

[0029] The resistor 43 is a film resistor formed on the insulating film 7. The resistor 43 is, for example, a NiCrSi film. A first end of the resistor 43 is connected to the second signal pad 42 via a wiring 44. A second end of the resistor 43 is connected to the electrode 46 via a wiring 45. As a result, the resistor 43 is connected in parallel with the capacitor 48 and is also connected to the second signal pad 42. The wiring 44 and 45 contain, for example, gold (Au). As shown in FIG. 2 , the resistor 43 is aligned with the second signal pad 42 in the optical waveguide direction. The resistor 43 is also aligned with the electrode 46 in a direction intersecting the optical waveguide direction. The upper surface of the resistor 43 is covered with the insulating film 8.

[0030] In the dummy element section 4, of the differential signals serving as the modulation signals, the reverse phase signal is input to the second signal pad 42. As a result, the modulation signal is applied to the resistor 43 through the second signal pad 42.

[0031] FIG. 9 is a perspective view showing an optical modulation circuit 10 including an EML 1. FIG. 10 is a plan view showing the optical modulation circuit 10. As shown in FIGS. 9 and 10, the optical modulation circuit 10 includes an EML 1, a carrier 11 having a main surface 11a, a capacitor 14, a capacitor 15, and a termination resistor 16. The carrier 11 is made of AlN, for example. The main surface 11a of the carrier 11 is provided with a signal line 121, a signal line 122, a ground pattern 123, and a pattern wiring 124. The ground pattern 123 is set to a reference potential. The back electrode of the EML 1 is conductively connected to the ground pattern 123.

[0032] The pad 26 of the laser unit 2 is connected to a first electrode of the capacitor 14 by a bonding wire 131. A second electrode of the capacitor 14 is conductively bonded to the ground pattern 123. The capacitor 14 functions as a bypass capacitor. A bias voltage is applied to the pad 26 via the bonding wire 131.

[0033] Ground patterns 123 are arranged on both sides of the signal line 121, and the signal line 121 and the ground patterns 123 form a transmission line (coplanar line). The tip of the signal line 121 is connected to a first signal pad 32 of the optical modulation unit 3 by a bonding wire 132. A positive phase signal of the differential signal is input to the first signal pad 32 via the signal line 121 and the bonding wire 132. The first signal pad 32 is further connected to a first electrode of the capacitor 15 via a bonding wire 133. A second electrode of the capacitor 15 is connected to the ground pattern 123 via a termination resistor 16.

[0034] Ground patterns 123 are arranged on both sides of the signal line 122, and a transmission line (coplanar line) is formed by the signal line 122 and the ground patterns 123. The tip of the signal line 122 is connected to a second signal pad 42 of the dummy element portion 4 by a bonding wire 134. A reverse-phase signal of the differential signal is input to the second signal pad 42 via the signal line 122 and the bonding wire 134.

[0035] The effects achieved by the EML1 according to the present embodiment described above will now be described. In a conventional single-phase drive EML, for example, when the characteristic impedance of the transmission line is 50 Ω, the termination resistance value is often also set to 50 Ω. However, considering the impedance of the EML (e.g., 100 Ω), the combined load impedance ZL of the EML and termination resistance is ZL = 1 / (1 / 100 + 1 / 50) = 33 Ω, resulting in reflection of high-frequency signals. The reflection coefficient Γ is defined as Γ = (Zo - ZL) / (Zo + ZL) using the characteristic impedance Zo of the transmission line and the load impedance ZL. The reflection coefficient Γ (ignoring the reactance component at low frequencies) of a conventional single-phase drive EML is ZL = 33 Ω, so Γ = (50 - 33) / (50 + 33) = 0.2.

[0036] To address this issue, if the termination resistance value is set to 100 Ω, the combined load impedance of the EML and termination resistance will be 50 Ω, which will be equal to the characteristic impedance of the transmission line, thereby suppressing the reflection of high-frequency signals.

[0037] However, increasing the termination resistance value narrows the bandwidth allowed for high-frequency signals for the following reasons. Let Ro be the combined resistance of the output impedance of the driver circuit driving the EML, the impedance of the EML, and the termination resistance, and Co be the combined capacitance of the capacitance of the EML and the parasitic capacitance of the carrier. The bandwidth fc is defined as fc = 1 / (2π × Co × Ro). Let Zo be the characteristic impedance of the transmission line, Ract be the resistance of the EML, and Rt be the termination resistance. Therefore, 1 / Ro = 1 / Zo + 1 / Ract + 1 / Rt. Therefore, if the termination resistance Rt is 50Ω, then Ro = 1 / (1 / 50 + 1 / 100 + 1 / 50) = 20Ω. Therefore, the bandwidth fc of the EML is fc = 1 / (2π × Co × 20). In contrast, if the termination resistance value Rt is 100Ω, Zo = 1 / (1 / 50 + 1 / 100 + 1 / 100) = 25Ω, so the EML bandwidth fc is fc = 1 / (2π × Co × 25). In this way, if the termination resistance value is doubled, the EML bandwidth fc becomes 0.8 times (the bandwidth becomes narrower).

[0038] The EML 1 according to this embodiment includes a laser unit 2 that outputs laser light, an optical modulation unit 3 having a modulation electrode 31 to which one of the differential signals is input, and a dummy element unit 4 to which the other of the differential signals is input. The optical modulation unit 3 is connected between one of the differential signals and a reference potential. The dummy element unit 4 is connected between the other of the differential signals and the reference potential and includes a resistance component and a capacitance component connected in parallel. In this EML 1, the other of the differential signals (the negative-phase signal) is input to the dummy element unit 4 via a second signal pad 42. This combines the load impedance of the circuit formed by the optical modulation unit 3 and the termination resistor 16 to which one of the differential signals (the positive-phase signal) is input and the load impedance of the dummy element unit 4 to which the other of the differential signals (the negative-phase signal) is input, thereby enabling the overall load impedance to approach the characteristic impedance of the transmission line. This reduces the reflection of high-frequency signals. For example, if the characteristic impedance Zo of the transmission line and the load impedance ZL are both adjusted to 100Ω, the reflection coefficient Γ becomes Γ=(100−100) / (100+100)=0.

[0039] In addition, it is possible to ensure a high-frequency signal bandwidth equivalent to that of conventional optical modulators. For example, if the termination resistance Rt is 50 Ω, Zo = 1 / (1 / 50 + 1 / 100 + 1 / 50) = 20 Ω, and fc = 1 / (2π × Co × 20), thereby ensuring the same bandwidth as a conventional single-phase drive EML. Therefore, the EML1 of this embodiment can reduce high-frequency signal reflection while avoiding a narrowing of the high-frequency signal bandwidth, thereby improving the trade-off between high-frequency signal reflection and bandwidth assurance.

[0040] As in this embodiment, the capacitance component of the dummy element portion 4 may be a capacitor 48 configured by sandwiching a dielectric layer 41 between a pair of electrodes. For example, with such a configuration, the capacitance component of the dummy element portion 4 can be obtained.

[0041] As in this embodiment, the electrode (rear electrode) on the opposite side to the modulation electrode 31 of the optical modulation section 3 and the end (electrode 46) on the opposite side to the end connected to the second signal pad 42 of the dummy element section 4 may be set to the reference potential. Even in this case, it is possible to reduce the reflection of the high frequency signal and avoid narrowing of the band of the high frequency signal.

[0042] As in this embodiment, the end (electrode 46) opposite to the end connected to the second signal pad 42 of the dummy element section 4 may be connected to the electrode (rear electrode) opposite to the modulation electrode 31 of the optical modulation section 3. Even in this case, it is possible to reduce the reflection of the high frequency signal and avoid narrowing of the band of the high frequency signal.

[0043] As in this embodiment, the laser section 2, the optical modulation section 3, and the dummy element section 4 may be provided on the same semiconductor substrate 5. This can contribute to the miniaturization of the EML 1. In addition, providing the dummy element section 4 on the same semiconductor substrate 5 as the optical modulation section 3 provides the following effects.

[0044] FIG. 11 shows a circuit diagram for a comparative example in which the dummy element section 4 is disposed away from the semiconductor substrate 5. As shown in the figure, in this case, the end of the dummy element section 4 opposite the end connected to the second signal pad 42 is separated from the electrode (rear electrode) opposite the modulation electrode 31 of the optical modulation section 3. This increases the distance L of the wiring 9 (e.g., part of the ground pattern 123) between them, making them unable to be considered to be at the same potential. Furthermore, even if they are at the same potential, if the distance L is increased, a portion that is not a differential line will be generated, and crosstalk will occur in proportion to the distance. Furthermore, if the distance L is increased, the effect of reducing the reflection of high-frequency signals will be reduced.

[0045] However, if the distance L is within a certain range, the above problem hardly occurs. In other words, the distance L should be sufficiently short compared to the wavelength of the modulated signal. In one example, the distance L should be at least 1 / 20 of the wavelength, which is the distance L at which the wavelength of the modulated signal can be ignored (the distance that can be handled by a lumped constant circuit). For example, when the effective dielectric constant εeff When the modulated signal has a wavelength of 1 mm and a frequency of 100 GHz, the distance L is set to 50 μm or less, which is 1 / 20 of the wavelength. For example, in 112 GBaud PAM4 transmission, when a coplanar line is provided on an AlN carrier 11 on which EML1 is mounted, the effective dielectric constant ε eff is approximately 6. Since the Nyquist frequency of 112 GBaud PAM4 is 56 GHz, the allowable distance L is 110 μm. Therefore, the distance L is substantially smaller than the chip size of EML1. For this reason, it is desirable to provide the dummy element portion 4 inside the chip, i.e., on the semiconductor substrate 5.

[0046] In addition to satisfying the above-mentioned condition of the distance L, it is desirable that the impedance of the wiring 9 is sufficiently low. Qualitatively, it is desirable that the impedance of the wiring 9 is large enough to ignore the modulation in the optical modulation section 3 when a modulation signal is input to the dummy element section 4. The impedance of the wiring 9 is, for example, 2 Ω or less.

[0047] Here, Figure 12(a) shows a circuit diagram in which the electrode opposite the modulation electrode 31 of the optical modulation unit 3 and the end of the dummy element unit 4 opposite the end connected to the second signal pad 42 are each individually connected to a reference potential line. Figure 12(b) shows a circuit diagram in which the electrode opposite the modulation electrode 31 of the optical modulation unit 3 and the end of the dummy element unit 4 opposite the end connected to the second signal pad 42 are both connected to each other and floating. When the impedance Zn of the dummy element unit 4 is equal to the impedance Zp of the optical modulation unit 3, the magnitude of the reflection of the high-frequency signal as seen from the input side is equal. For example, when the capacitance of the optical modulation unit 3 and the capacitor 48 is 0.2 pF and the capacitances of the termination resistor 16 and the resistor 43 are 50 Ω, the S parameter (Sdd11), which indicates the magnitude of the reflection of the high-frequency signal, is 30.1 dB in both cases. The reflection coefficient Γ is also 0.03 in both cases. Fig. 13(a) is a graph showing the relationship between frequency (GHz) and Sdd11 in the circuit shown in Fig. 12(a). Fig. 13(b) is a graph showing the relationship between frequency (GHz) and Sdd11 in the circuit shown in Fig. 12(b).

[0048] Furthermore, even if the impedance Zn of the dummy element unit 4 is different from the impedance Zp of the optical modulation unit 3, the magnitude of the reflection of the high-frequency signal is approximately equal. For example, when the capacitances of the optical modulation unit 3 and the capacitor 48 are 0.2 pF, the termination resistor 16 is 33 Ω, and the resistor 43 is 67 Ω, the S parameter (Sdd11), which indicates the magnitude of the reflection of the high-frequency signal, is 27.3 dB for the circuit shown in FIG. 12(a) and 29.1 dB for the circuit shown in FIG. 12(b). The reflection coefficient Γ is 0.04 in both cases. FIG. 14(a) is a graph showing the relationship between frequency (GHz) and Sdd11 for the circuit shown in FIG. 12(a). FIG. 14(b) is a graph showing the relationship between frequency (GHz) and Sdd11 for the circuit shown in FIG. 12(b).

[0049] FIG. 15 is a table showing examples of combinations of the impedance Zp of the optical modulation section 3 and the impedance Zn of the dummy element section 4. FIG. 16 is a graph showing the relationship between the ratio (Zp / Zn) and the reflection coefficient Γ at 1 GHz. In FIG. 16, plot P1 shows values ​​for the circuit shown in FIG. 12(a), and plot P2 shows values ​​for the circuit shown in FIG. 12(b). As shown in FIG. 16, in the circuit shown in FIG. 12(a), when the ratio (Zp / Zn) satisfies 1≦Zn / Zp≦3, the reflection coefficient Γ is less than 0.1 and is sufficiently reduced. In addition, in the circuit shown in FIG. 12(b), the reflection coefficient Γ is less than 0.1 and is sufficiently reduced regardless of the ratio (Zp / Zn). 12(a), that is, when both the electrode opposite to the modulation electrode 31 of the optical modulation section 3 and the end opposite to the end connected to the second signal pad 42 of the dummy element section 4 are connected to the reference potential line individually, it is desirable that the ratio (Zp / Zn) satisfies 1≦Zn / Zp≦3. This makes it possible to further reduce the reflection of high frequency signals.

[0050] 2 to 10, the end of the dummy element section 4 opposite to the end connected to the second signal pad 42 is connected to the opposite side of the modulation electrode 31 of the optical modulation section 3 via a back electrode. The back electrode is connected to an absolute reference potential provided outside the optical modulation circuit 10 via the ground pattern 123, so there is a slight impedance between the back electrode and the absolute reference potential. Therefore, it can be said that the actual configuration is close to the circuit configuration shown in FIG. 12(b).

[0051] [Variations] FIG. 17 is a circuit diagram showing the configuration of an EML 1A as a modification of the above embodiment. The EML 1A of this modification differs from the above embodiment in the configuration of the dummy element section, but is otherwise identical to the above embodiment. The EML 1A of this modification includes a dummy element section 4A instead of the dummy element section 4 of the above embodiment. The dummy element section 4A includes a dummy optical modulation section 49 and a resistor 43 instead of the resistor 43 and capacitor 48. The capacitance component of the dummy optical modulation section 49 is a dummy region having regions with the same semiconductor stack structure as the optical modulation section connected to both ends thereof, and is not optically coupled to the laser section 2. A modulation electrode (dummy modulation electrode) having the same configuration as the modulation electrode 31 is in contact with the dummy optical modulation section 49. The modulation electrode is connected to a second signal pad 42. As a result, the dummy optical modulation section 49 receives a signal with a phase opposite to that of the modulation signal. The dummy optical modulation section 49 functions as a reverse-biased diode, and the resistor 43 connected in parallel with this diode simulates the optical modulation section 3. In this way, the capacitance component of the dummy element section 4A may have regions of the same semiconductor laminate structure as the optical modulation section 3 connected to both ends of the dummy element section 4A. For example, with this configuration, the capacitance component of the dummy element section 4A can be obtained.

[0052] The optical modulator integrated laser element and optical modulation circuit according to the present disclosure are not limited to the above-described embodiments, and various other modifications are possible. For example, the above-described embodiments and modifications illustrate a case in which the dummy element section includes a resistor and a capacitor connected in parallel with each other, and a case in which the dummy element section has an optical modulation section that does not modulate laser light. The configuration of the dummy element section is not limited to these, and various configurations can be applied as long as they simulate the characteristics of the optical modulator. [Explanation of symbols]

[0053] 1,1A... Optical Modulator Integrated Laser Device (EML) 2...Laser section 3...Optical modulation section 4, 4A...Dummy element section 5...Semiconductor substrate 5a…main surface 5b...back side 6...Semi-insulating layer 7, 8...Insulating film 9...Wiring 10...Optical modulation circuit 11. Career 11a…main surface 14,15...Capacitor 16...Termination resistor 21...Lower cladding layer 22…Active layer 23...Upper clad layer 24...Contact layer 25...Electrode 26,27...Pad 31...Modulation electrode 32...First signal pad 33, 41...Dielectric layers 42...Second signal pad 43...Resistance 44,45...Wiring 46...Electrode 47...Ohmic metal layer 48...Capacitor 49...Dummy optical modulation section 61...Optical waveguide 121,122...Signal lines 123...Grand pattern 124...Pattern wiring 131, 132, 133, 134...Bonding wire L…Distance P1, P2...plot

Claims

1. a laser unit that outputs laser light; an optical modulation unit having a modulation electrode to which one of the differential signals is input; a dummy element portion to which the other of the differential signals is input, the optical modulation unit is connected between one of the differential signals and a reference potential, The dummy element portion is connected between the other of the differential signals and the reference potential, and includes a resistance component and a capacitance component connected in parallel.

2. 2. The optical modulator integrated laser device according to claim 1, wherein the capacitance component of the dummy element portion is a capacitor element formed by sandwiching a dielectric between a pair of electrodes.

3. 2. The optical modulator integrated laser element according to claim 1, wherein the capacitance component of the dummy element section comprises a region having the same semiconductor laminate structure as the optical modulation section connected to both ends of the dummy element section.

4. 4. The optical modulator integrated laser element according to claim 1, wherein an impedance Zp of said optical modulation section and an impedance Zn of said dummy element section satisfy 1≦Zn / Zp≦3.

5. a pair of transmission lines for transmitting differential signals; an optical modulation unit to which one of the differential signals is input; a dummy element portion to which the other of the differential signals is input; Equipped with the optical modulation unit is connected between one end of the transmission line and a reference potential, the dummy element portion is connected between the other of the differential signals and the reference potential, and an optical modulation circuit including a resistive component and a capacitive component connected in parallel.

6. 6. The optical modulation circuit according to claim 5, wherein the capacitance component of the dummy element portion is a capacitor element formed by sandwiching a dielectric between a pair of electrodes.

7. 6. The optical modulation circuit according to claim 5, wherein the capacitance component of the dummy element section comprises a region having the same semiconductor laminated structure as the optical modulation section connected to both ends of the dummy element section.

8. 8. The optical modulation circuit according to claim 5, wherein an impedance Zp of said optical modulation section and an impedance Zn of said dummy element section satisfy 1≦Zn / Zp≦3.

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