Mach-Zehnder type optical modulator and optical transmission device

Abstract

In the present invention, n-type diffusion prevention layers (11b, 30b) are provided on a semi-insulating or p-type substrate (10). A p-type cladding layer (12b, 13b) is provided on the n-type diffusion prevention layer (11b, 30b). The first and second quantum well active layers (14b, 14c) are provided on the p-type cladding layers (12b, 13b) so as to be separated from each other. First and second n-type cladding layers (15b, 15c) are provided on the first and second quantum well active layers (14b, 14c), respectively. The first and second traveling wave electrodes (20a, 20b) are connected to the first and second n-type cladding layers (15b, 15c), respectively. The DC bias electrode (27) is connected to the p-type cladding layers (12b, 13b) and is not directly connected to the n-type diffusion prevention layers (11b, 30b).

Description

Technical Field

[0001] The present disclosure relates to a Mach-Zehnder optical modulator and an optical transmitting device. Background Art

[0002] The expansion of cloud services has increased the communication business. In addition, assuming the popularization of IoT (Internet of Things) and the fifth-generation mobile communication system called 5G, AI (Artificial Intelligence) that processes huge amounts of data has also penetrated into society. As a result, there may be an increasing demand for higher speed and higher capacity optical communication systems. In response to these requirements, the multi-value technology of digital coherent communication technology that can give signals not only to the intensity of light but also to the phase or polarization wave is continuously developed. The Mach-Zehnder modulator that can control the amplitude and phase of light separately and generate a zero-chirp optical modulated signal is used in the multi-value optical modulator. In addition, in order to increase the signal capacity per unit time, the response speed required of the modulator is increasing and continuous. It is expected that an optical modulator that can input a modulated electrical signal of 64GBaud or 96GBaud or more with low loss and generate a high-speed modulated optical signal through electro-optical interaction is used.

[0003] In order to realize the above optical modulator, Mach-Zehnder optical modulators with traveling wave electrodes have been actively developed. The traveling wave electrodes interact with the light waves propagating in the optical waveguide. By adopting a high-frequency line structure that optimizes the traveling wave electrodes in a manner driven by differential signals, it can be driven by a differential driver with high power efficiency.

[0004] In the above-mentioned traveling wave electrode, as a first requirement, the characteristic impedance of the driver, the terminal resistance, and the impedance of the traveling wave electrode are matched. As a second requirement, the propagation speed of the microwave propagating in the electrode is matched with the propagation speed of the light traveling in the waveguide provided near the electrode. As a third requirement, the loss of the microwave propagating in the electrode is reduced.

[0005] If the first impedance matching is not obtained, reflection occurs when the electric signal is input from the driver to the traveling wave electrode, resulting in power loss. In addition, in the terminal resistance part, the electric signal reflected by the impedance mismatch recedes in the traveling wave electrode, and the light and the receding wave also interact with each other to cause degradation of the modulation waveform. If the second speed matching is not obtained, a phase shift occurs with the modulated light that is modulated based on the electric field amplitude while propagating together with the microwave. In particular, the higher the wavelength of the microwave, the more significant the phase shift, resulting in degradation of the modulation band. The third low loss can also be achieved by increasing the electro-optical interaction (refractive index change) per unit length, resulting in shortening the modulator length used to obtain the required phase rotation amount, but more essentially by reducing the semiconductor resistance between the traveling wave electrodes. In particular, at high frequencies, the losses caused by the semiconductor resistance and contact resistance between the traveling wave electrodes dominate, and the reduction of these resistances is important for improving the modulation band.

[0006] A Mach-Zehnder optical modulator is disclosed, wherein a lower n-type cladding layer, a p-type carrier blocking layer, a core layer, and an upper n-type cladding layer are sequentially stacked on a substrate, and a DC bias is applied to the lower n-type cladding layer (for example, see Patent Document 1). Figure 7 , 8 ). Thus, when the lower cladding layer is of n-type, the lower semiconductor resistance becomes small.

[0007] Patent Document 1: International Publication No. 2016 / 194369

[0008] However, in a Mach-Zehnder optical modulator in which a p-type cladding layer, an active layer, and an n-type cladding layer are sequentially stacked on a substrate and a DC bias is applied to the p-type cladding layer, the lower cladding layer is p-type, so the lower semiconductor resistance increases, and there is a problem that the modulation band cannot be increased. Summary of the invention

[0009] The present disclosure has been made to solve the above-mentioned problems, and its object is to provide a Mach-Zehnder optical modulator and an optical transmission device that can reduce the lower semiconductor resistance and increase the modulation band even when the lower cladding layer is of p-type.

[0010] The Mach-Zehnder optical modulator disclosed in the present invention is characterized in that it comprises: a semi-insulating or p-type substrate; an n-type anti-diffusion layer, which is arranged on the substrate; a p-type cladding layer, which is arranged on the n-type anti-diffusion layer; a first and a second quantum well active layer, which are arranged separately from each other on the p-type cladding layer; a first and a second n-type cladding layer, which are respectively arranged on the first and the second quantum well active layers; a first and a second traveling wave electrode, which are respectively connected to the first and the second n-type cladding layers; and a DC bias electrode, which is connected to the p-type cladding layer and is not directly connected to the n-type anti-diffusion layer, the DC bias electrode, the p-type cladding layer, the first quantum well active layer, the first n-type cladding layer and the first traveling wave electrode constitute a first optical modulation unit, and the DC bias electrode, the p-type cladding layer, the second quantum well active layer, the second n-type cladding layer and the second traveling wave electrode constitute a second optical modulation unit.

[0011] Another Mach-Zehnder type optical modulator disclosed in the present invention is characterized in that it comprises: a p-type semiconductor substrate with a thickness of more than 10 μm; a p-type cladding layer, which is arranged on the p-type semiconductor substrate; first and second quantum well active layers, which are arranged separately from each other on the p-type cladding layer; first and second n-type cladding layers, which are respectively arranged on the first and second quantum well active layers; first and second traveling wave electrodes, which are respectively connected to the first and second n-type cladding layers; and a DC bias electrode, which is connected to the p-type cladding layer, the DC bias electrode, the p-type cladding layer, the first quantum well active layer, the first n-type cladding layer and the first traveling wave electrode constitute a first optical modulation unit, and the DC bias electrode, the p-type cladding layer, the second quantum well active layer, the second n-type cladding layer and the second traveling wave electrode constitute a second optical modulation unit.

[0012] In the Mach-Zehnder type optical modulator disclosed in the present invention, the DC bias electrode is connected to the p-type cladding layer, so the lower cladding layer is p-type. In addition, an n-type diffusion prevention layer is provided between the substrate and the p-type cladding layer, so an n-type semiconductor resistor is connected in parallel with the lower semiconductor resistor composed of a p-type semiconductor. Therefore, even in the case where the lower cladding layer is p-type, the lower semiconductor resistance can be reduced. As a result, the semiconductor resistance between the two optical modulation units is greatly reduced, so that the loss of microwaves propagating in the traveling wave electrode can be reduced, and the modulation band can be greatly improved.

[0013] In another Mach-Zehnder optical modulator of the present disclosure, the thickness of the p-type semiconductor substrate is 10 μm or more, thereby reducing the resistance of the underlying semiconductor and further increasing the modulation bandwidth. BRIEF DESCRIPTION OF THE DRAWINGS

[0014] Figure 1This is a diagram showing an optical transmission device according to Embodiment 1.

[0015] Figure 2 1 is a cross-sectional view showing a refractive index modulation region of the Mach-Zehnder optical modulator according to the first embodiment.

[0016] Figure 3 This is a cross-sectional view showing the periphery of a DC bias electrode of the Mach-Zehnder optical modulator according to the first embodiment.

[0017] Figure 4 This is a diagram showing the calculation result of the frequency dependency of the modulation band in the first embodiment.

[0018] Figure 5 This is a cross-sectional view showing a modified example of the refractive index modulation region of the Mach-Zehnder optical modulator according to the first embodiment.

[0019] Figure 6 This is a diagram showing the results of calculating the modulation band by changing the thickness of the n-type diffusion prevention layer in the first embodiment.

[0020] Figure 7 1 is a cross-sectional view showing a refractive index modulation region of a Mach-Zehnder optical modulator according to the second embodiment.

[0021] Figure 8 This is a diagram showing the results of calculating the modulation band by changing the thickness of the n-type diffusion prevention layer in the second embodiment.

[0022] Fig. 9 1 is a cross-sectional view showing a refractive index modulation region of a Mach-Zehnder optical modulator according to a third embodiment.

[0023] Fig.10 This is a diagram showing the calculation result of the frequency dependency of the modulation band in the third embodiment.

[0024] Fig.11 1 is a cross-sectional view showing a refractive index modulation region of a Mach-Zehnder optical modulator according to a fourth embodiment.

[0025] Fig.12 This is a diagram showing the calculation result of the acceptor diffusion length dependency of the modulation band in the fourth embodiment.

[0026] Fig.13 This is a diagram showing the calculation result of the acceptor diffusion length dependency of the modulation band in the fourth embodiment.

[0027] Fig.14 This is a diagram showing the calculation result of the acceptor diffusion length dependency of the modulation band in the fourth embodiment.

[0028] Fig.15 This is a diagram showing the calculation result of the frequency dependency of the modulation band in the fourth embodiment.

[0029] Fig.16 This is a diagram showing the calculation result of the frequency dependency of the modulation band in the fourth embodiment.

[0030] Fig.17 This is a diagram showing the calculation result of the frequency dependency of the modulation band in the fourth embodiment.

[0031] Fig.18 This is a diagram showing the calculation result of the acceptor diffusion length dependency of the modulation band in the fourth embodiment.

[0032] Fig.19 This is a diagram showing the calculation result of the acceptor diffusion length dependency of the modulation band in the fourth embodiment.

[0033] Fig. 20 This is a diagram showing the calculation result of the frequency dependency of the modulation band in the fourth embodiment.

[0034] Fig.21 This is a diagram showing the calculation result of the frequency dependency of the modulation band in the fourth embodiment. DETAILED DESCRIPTION

[0035] The Mach-Zehnder optical modulator and the optical transmission device according to the embodiments will be described with reference to the drawings. The same reference numerals are used for the same or corresponding components, and repeated description may be omitted.

[0036] Implementation method 1.

[0037] Figure 1 1 is a diagram showing an optical transmitter according to Embodiment 1. Only one optical transmitter is shown here, but a structure in which a plurality of the same optical transmitters are arranged may be used. An IQ signal generator 102 generates a differential modulation signal. A driver 103 amplifies the differential modulation signal. A gain control unit 104 controls the gain of the driver 103. A Mach-Zehnder optical modulator 106 modulates light from a light source 105 such as a laser according to a signal from the driver 103, and outputs the optical modulation signal to an optical output waveguide 108.

[0038] The Mach-Zehnder optical modulator 106 includes a splitter 109 , arm waveguides 21 a and 21 b , optical modulation units 22 a and 22 b , phase adjustment units 110 a and 110 b , phase adjustment bias control units 111 a and 111 b , a combiner 112 , and an optical output monitor 113 .

[0039] The arm waveguide 21a, the optical modulator 22a, and the phase adjuster 110a form the first arm. The arm waveguide 21b, the optical modulator 22b, and the phase adjuster 110b form the second arm. The splitter 109 splits the light output from the light source 105 and inputs it into the first arm and the second arm.

[0040] The optical modulator 22a changes the refractive index of the waveguide through which the optical signal passes based on the I-channel modulation signal input from the IQ signal generator 102 and the bias signal input from the modulation bias control unit 107. Thus, the optical modulator 22a modulates the phase of the optical signal to generate a first optical modulated signal. The optical modulator 22b changes the refractive index of the waveguide based on the I-channel modulation signal input from the IQ signal generator 102 and the bias signal input from the modulation bias control unit 107. Thus, the optical modulator 22b modulates the phase of the optical signal to generate a second optical modulated signal. In addition, the multi-value signal input from the IQ signal generator 102 may be any modulation method.

[0041] The phase adjustment unit 110a adjusts the phase of the first optical modulation signal at a phase rotation rate determined based on a bias signal from the phase adjustment bias control unit 111a. The phase adjustment unit 110b adjusts the phase of the second optical modulation signal at a phase rotation rate determined based on a bias signal from the phase adjustment bias control unit 111b. The combiner 112 combines the output signal of the phase adjustment unit 110a and the output signal of the phase adjustment unit 110b.

[0042] The optical output monitor 113 monitors the output signal of the combiner 112. The phase adjustment bias control units 111a and 111b adjust the bias applied to the phase adjustment units 110a and 110b based on the detection signal of the optical output monitor 113. Specifically, the phase adjustment units 110a and 110b adjust the phase so that the phase difference of the optical signals output from the phase adjustment units 110a and 110b becomes π / 2. In this way, the light output from the first arm and the second arm is combined and output to the outside via the optical output waveguide 108.

[0043] The optical modulation units 22a and 22b are composed of compound semiconductors such as lithium niobate (LiNbO3) or indium phosphide (InP). When lithium niobate is used, the refractive index in the waveguide is changed by the Pockels effect to perform phase modulation. In this case, there are advantages of small light absorption and temperature dependence, but high-frequency characteristics become a problem. On the other hand, when compound semiconductors are used, phase modulation can be performed by the quantum confined Stark effect. In this case, excellent high-frequency characteristics can be obtained based on high mobility. In addition, the integration of compound semiconductors is also excellent, and the miniaturization of the Mach-Zehnder type optical modulator 106 can be achieved. Not only the optical modulation units 22a and 22b, but also the splitter 109, arm waveguides 21a and 21b, the combiner 112, the optical output monitor 113, the light source 105, etc. can be integrally formed on the compound semiconductor substrate. By setting a structure that uses an optical waveguide to connect the components, a small and integrated optical transmission device can be formed. In addition, instead of the light modulation units 22a and 22b, light modulator elements other than the Mach-Zehnder type may be used.

[0044] Figure 2 1 is a cross-sectional view showing a refractive index modulation region of a Mach-Zehnder optical modulator according to Embodiment 1. The refractive index modulation region is a region where the refractive index of the core layer is modulated by applying a voltage to the quantum well active layer to cause an electro-optical effect. Figure 1 The optical modulation units 22a and 22b correspond to each other.

[0045] The substrate 10 is a semi-insulating semiconductor substrate composed of, for example, Fe-InP, but may also be a p-type semiconductor substrate. On the substrate 10, n-type diffusion prevention layers 11a, 11b, and 11c composed of n-type semiconductor are provided separately from each other. The n-type diffusion prevention layers 11a, 11b, and 11c are composed of, for example, a semiconductor substrate having a carrier concentration of 2.0E+18cm -3 The p-type InGaAs layers 12a, 12b, 12c made of a p-type semiconductor are provided on the n-type diffusion prevention layers 11a, 11b, 11c, respectively. The carrier concentration of the p-type InGaAs layers 12a, 12b, 12c is, for example, 1.5E+19cm -3 .

[0046] P-type cladding layers 13a, 13b, 13c made of p-type semiconductor are provided on the p-type InGaAs layers 12a, 12b, 12c, respectively. The p-type cladding layers 13a, 13b, 13c are made of, for example, a semiconductor having a carrier concentration of 2.0E+18 cm -3The p-type InP is composed of a p-type InP. A quantum well active layer 14a is provided on the p-type cladding layer 13a. Quantum well active layers 14b and 14c are provided separately from each other on the p-type cladding layer 13b. A quantum well active layer 14d is provided on the p-type cladding layer 13c. The quantum well active layers 14a, 14b, 14c, and 14d are composed of, for example, AlGaInAs mixed crystals.

[0047] N-type cladding layers 15a, 15b, 15c, 15d made of n-type semiconductor are provided on the quantum well active layers 14a, 14b, 14c, 14d, respectively. The n-type cladding layers 15a, 15b, 15c, 15d are made of, for example, a semiconductor with a carrier concentration of 2.0E+18cm -3 The n-type contact layers 16a, 16b, 16c, 16d made of n-type semiconductor are provided on the n-type cladding layers 15a, 15b, 15c, 15d, respectively. The n-type contact layers 16a, 16b, 16c, 16d are made of, for example, a semiconductor with a carrier concentration of 1.0E+19cm -3 It is composed of n-type InGaAs.

[0048] The arm waveguide 21a includes a substrate 10 and an n-type diffusion prevention layer 11b, a p-type InGaAs layer 12b, a p-type cladding layer 13b, a quantum well active layer 14b, an n-type cladding layer 15b, and an n-type contact layer 16b stacked in sequence on the substrate 10. The arm waveguide 21b includes a substrate 10 and an n-type diffusion prevention layer 11b, a p-type InGaAs layer 12b, a p-type cladding layer 13b, a quantum well active layer 14c, an n-type cladding layer 15c, and an n-type contact layer 16c stacked in sequence on the substrate 10. The arm waveguides 21a and 21b are separated from each other on the p-type cladding layer 13b.

[0049] The side surfaces of the quantum well active layers 14b, 14c, the n-type cladding layers 15b, 15c, and the n-type contact layers 16b, 16c are covered by semi-insulating semiconductor buried layers 17a, 17b, 17c, 17d and insulating films 18a, 18b, 18c, 18d. Ground electrodes 19a, 19b are provided on the n-type contact layers 16a, 16d, respectively, via the semiconductor buried layers 17a, 17d and insulating films 18a, 18d. Traveling wave electrodes 20a, 20b are provided on the n-type contact layers 16b, 16c, respectively, and are electrically connected.

[0050] Figure 3 2 is a cross-sectional view showing the periphery of a DC bias electrode of the Mach-Zehnder optical modulator according to Embodiment 1. The DC bias electrode 27 is bonded to the p-type InGaAs layer 12 b . Therefore, the DC bias electrode 27 is connected to the p-type cladding layer 13 b via the p-type InGaAs layer 12 b .

[0051] The DC bias electrode 27, the arm waveguide 21a, and the traveling wave electrode 20a constitute the light modulator 22a. Similarly, the DC bias electrode 27, the arm waveguide 21b, and the traveling wave electrode 20b constitute the light modulator 22b. The light modulators 22a and 22b are separated from each other above the p-type cladding layer 13b. On the other hand, the p-type cladding layer 13b of the light modulator 22a and the p-type cladding layer 13b of the light modulator 22b are not separated from each other.

[0052] A predetermined DC bias is applied between the DC bias electrode 27 and the traveling wave electrodes 20a and 20b, and an electric field in the opposite direction is applied to the quantum well active layers 14b and 14c. Furthermore, a high-frequency differential modulation signal output from the driver 103 is applied to the traveling wave electrodes 20a and 20b. Thus, the Mach-Zehnder type optical modulator is driven as a modulator. In addition, in order to prevent the increase of leakage current and ensure the long-term electrical reliability of the optical modulator, the DC bias electrode 27 is not directly connected to the n-type diffusion prevention layers 11a, 11b, and 11c.

[0053] In order to speed up the modulation operation, impedance matching and speed matching between light and microwaves need to be satisfied. As an example, the thickness of the traveling wave electrodes 20a and 20b is about 14μm, the width is about 4.4μm, and the distance between the centers of the electrodes is about 11μm. The width of the quantum well active layer 14b, 14c, the n-type cladding layer 15b, 15c, and the n-type contact layer 16b, 16c is about 11μm. The thickness of the p-type InGaAs layer 12b is about 0.8μm. The thickness of the p-type cladding layer 13b is about 2.0μm. The thickness of the quantum well active layer 14b, 14c is about 1.0μm. The thickness of the n-type cladding layer 15b, 15c is about 1.2μm, and the thickness of the n-type contact layer 16b, 16c is about 0.3μm. The dielectric constant of the quantum well active layer 14b, 14c and the semi-insulating semiconductor buried layer 17a, 17b, 17c, 17d is about 14. Under this condition, the differential impedance can be matched to 65Ω, and the propagation speed of microwaves can be matched with the propagation speed of light guided in a waveguide with a refractive index of 3.5. In addition, not limited to the above parameters, the thickness and width of the traveling wave electrode and the semiconductor layer, the spacing between the traveling wave electrodes, and the spacing between the traveling wave electrode and the ground electrode also affect the impedance matching and the propagation speed matching.

[0054] When further improving the modulation frequency band, it is effective to shorten the length of the traveling wave electrode structure. The arm waveguides 21a and 21b are composed of compound semiconductors such as lithium niobate (LiNbO3) or indium phosphide (InP). When lithium niobate is used, the refractive index in the waveguide is changed by the Pockels effect to perform phase modulation. In this case, there are advantages of small light absorption and temperature dependence, but high-frequency characteristics become a problem. On the other hand, when compound semiconductors are used, phase modulation can be performed by the quantum confined Stark effect. In this case, excellent high-frequency characteristics can be obtained based on high mobility.

[0055] The surface orientation of the substrate 10 is <100>. According to the constraints of the re-growth shape after the waveguide is formed, the extension direction of the p-type cladding layer 13b, the quantum well active layer 14b, 14c, and the n-type cladding layer 15b, 15c is preferably <011>. In the case of this extension direction, the p-type cladding layer, the quantum well active layer, and the n-type cladding layer are stacked in sequence on the substrate 10, so that it is foreseeable that the phase modulation efficiency will be improved by the multiplication of the quantum confined Stark effect and the Pockels effect. Therefore, it is expected that the half-wavelength voltage characteristic, which is one of the main characteristics of the Mach-Zehnder optical modulator and is in a trade-off relationship with the modulation band, will be reduced, and it is expected that the modulation band will be improved by shortening the length of the traveling wave electrode.

[0056] In addition, reducing the main resistance component is also effective for improving the frequency band. Therefore, the thickness and width of the traveling wave electrode and the semiconductor layer, the spacing of the traveling wave electrodes, etc. are generally configured so that various resistance components become minimum after satisfying the impedance matching and propagation speed matching. The main resistance components in the above structure are the electrode resistance of the traveling wave electrodes 20a, 20b, the upper semiconductor resistance of the n-type contact layer 16b, 16c and the n-type cladding layer 15b, 15c, and the lower semiconductor resistance of the p-type cladding layer 13b and the p-type InGaAs layer 12b. With respect to the passage path of the modulation signal, the electrode resistance is connected in series, and the upper and lower semiconductor resistances are connected in parallel.

[0057] Since the light modulators 22a and 22b formed separately are connected by the p-type InGaAs layer 12b and the p-type cladding layer 13b, the lower semiconductor resistance of the p-type InGaAs layer 12b and the p-type cladding layer 13b dominates the resistance value between the light modulators 22a and 22b. Therefore, it is considered that reducing the electrode resistance and the lower semiconductor resistance is effective in improving the modulation band in the Mach-Zehnder optical modulator.

[0058] Therefore, in the present embodiment, the n-type diffusion prevention layer 11 is provided between the substrate 10 and the p-type InGaAs layer 12 b and the p-type cladding layer 13 b . Figure 41 is a diagram showing the result of calculating the frequency dependency of the modulation band in Embodiment 1. Electromagnetic field analysis confirmed that the modulation band can be improved by providing the n-type diffusion prevention layer 11 .

[0059] As described above, in this embodiment, the DC bias electrode 27 is connected to the p-type cladding layer 13b, so the lower cladding layer is p-type. In addition, an n-type diffusion prevention layer 11 is provided between the substrate 10 and the p-type cladding layer 13b, so an n-type semiconductor resistor is connected in parallel with the lower semiconductor resistor composed of the p-type InGaAs layer 12b and the p-type cladding layer 13b. Therefore, the lower semiconductor resistor can be reduced even when the lower cladding layer is p-type. As a result, the semiconductor resistance between the two optical modulation units 22a and 22b is greatly reduced, so that the loss of microwaves propagating in the traveling wave electrodes 20a and 20b can be reduced, and the modulation band can be greatly improved.

[0060] In addition, without using a planar waveguide or a capacitor-loaded traveling wave electrode structure, the modulation bandwidth can be increased with a very simple optical modulation unit structure, so the miniaturization of the Mach-Zehnder optical modulator and the optical transmitter can also be expected. In addition, the n-type diffusion prevention layers 11a, 11b, 11c can prevent the diffusion of impurities between the p-type InGaAs layers 12a, 12b, 12c and the substrate 10.

[0061] Figure 5 FIG. 2 is a cross-sectional view showing a modified example of the refractive index modulation region of the Mach-Zehnder optical modulator of Embodiment 1. Figure 2 The n-type diffusion prevention layers 11a, 11b, and 11c are shared as the n-type diffusion prevention layer 11. Figure 2 The p-type InGaAs layers 12a, 12b, and 12c are shared as the p-type InGaAs layer 12. Figure 2 The p-type cladding layers 13a, 13b, and 13c are shared as the p-type cladding layer 13. The same effect can be obtained in this structure. In addition, the common mode impedance can be reduced by sharing. By matching the common mode impedance of the desired differential driver, the frequency band degradation caused by the common mode resonance can also be suppressed.

[0062] Furthermore, by increasing the thickness of the n-type diffusion prevention layers 11 a , 11 b , and 11 c , the resistance of the lower semiconductor layer can be reduced, thereby further increasing the modulation bandwidth. Figure 6 The graph shows the results of calculating the modulation band by changing the thickness of the n-type diffusion prevention layer in Embodiment 1. The n-type diffusion prevention layers 11a, 11b, and 11c have a carrier concentration of 2.0E+18 cm -3The modulation band can be further increased by making the thickness of the n-type diffusion prevention layers 11a, 11b, and 11c 0.05 μm or more. In addition, the n-type diffusion prevention layers 11a, 11b, and 11c are shared, the p-type InGaAs layers 12a, 12b, and 12c are shared, and the p-type cladding layers 13a, 13b, and 13c are shared. Figure 5 The same effect can be obtained in the structure of -3 ~2.0E+19cm -3 The n-type diffusion prevention layers 11a, 11b, and 11c composed of n-type InP can also obtain the same effect.

[0063] Implementation method 2.

[0064] Figure 7 1 is a cross-sectional view showing the refractive index modulation region of the Mach-Zehnder optical modulator of Embodiment 2. This is different from Embodiment 1 in that n-type InGaAs layers 30a, 30b, 30c are provided on the n-type diffusion prevention layers 11a, 11b, 11c, respectively. The n-type diffusion prevention layers 11a, 11b, 11c are formed of, for example, a carrier concentration of 2.0E+18cm -3 The carrier concentration of the n-type InGaAs layers 30a, 30b, and 30c is, for example, 1.0E+19cm -3 .

[0065] The arm waveguide 21a includes a substrate 10 and an n-type diffusion prevention layer 11b, an n-type InGaAs layer 30b, a p-type InGaAs layer 12b, a p-type cladding layer 13b, a quantum well active layer 14b, an n-type cladding layer 15b, and an n-type contact layer 16b formed in sequence on the substrate 10. The arm waveguide 21b includes a substrate 10 and an n-type diffusion prevention layer 11b, an n-type InGaAs layer 30b, a p-type InGaAs layer 12b, a p-type cladding layer 13b, a quantum well active layer 14c, an n-type cladding layer 15c, and an n-type contact layer 16c formed in sequence on the substrate 10. The other structures are the same as those in the first embodiment.

[0066] In the first embodiment, there is a concern that the interface between the n-type diffusion prevention layer 11b and the p-type InGaAs layer 12b may be depleted, which may be an important factor that degrades the modulation band. In contrast, in the present embodiment, n-type InGaAs layers 30a, 30b, and 30c with high carrier concentration are formed on the n-type diffusion prevention layers 11a, 11b, and 11c as n-type diffusion prevention layers. The band gap of n-type InGaAs is very small, so if the p-type InGaAs layer 12b and the n-type InGaAs layer 30b are joined, the depletion layer can be made thinner, and the capacitance based on the tunnel effect can be reduced. As a result, the semiconductor resistance between the two optical modulation units is greatly reduced, so that the loss of microwaves propagating in the traveling wave electrode can be reduced, and the modulation band can be greatly improved.

[0067] Figure 8 This is a diagram showing the results of calculating the modulation band by changing the thickness of the n-type diffusion prevention layer in Embodiment 2. The thickness of the n-type diffusion prevention layers 11a, 11b, and 11c is set to 2.0 μm, and the thickness of the n-type InGaAs layers 30a, 30b, and 30c is changed. The carrier concentration of the n-type InGaAs layers 30a, 30b, and 30c is 1.0E+19 cm -3 Electromagnetic field analysis confirmed that the modulation band can be further increased by forming the n-type InGaAs layers 30 a , 30 b , and 30 c with a film thickness of 0.05 μm or more.

[0068] In addition, the same effect can be obtained in a structure in which the n-type diffusion prevention layers 11a, 11b, 11c are shared, the n-type InGaAs layers 30a, 30b, 30c are shared, the p-type InGaAs layers 12a, 12b, 12c are shared, and the p-type cladding layers 13a, 13b, 13c are shared.

[0069] Implementation method 3.

[0070] Fig. 9 is a cross-sectional view showing a refractive index modulation region of a Mach-Zehnder optical modulator according to Embodiment 3. In this embodiment, a substrate 10 which is a semi-insulating semiconductor substrate is replaced with a substrate having a carrier concentration of 2.0E+18 cm -3 The p-type semiconductor substrate 40 is composed of p-type InP. In addition, there are no n-type diffusion prevention layers 11a, 11b, and 11c.

[0071] The arm waveguide 21a includes a p-type semiconductor substrate 40 and a p-type InGaAs layer 12b, a p-type cladding layer 13b, a quantum well active layer 14b, an n-type cladding layer 15b, and an n-type contact layer 16b sequentially stacked on the p-type semiconductor substrate 40. The arm waveguide 21b includes a p-type semiconductor substrate 40 and a p-type InGaAs layer 12b, a p-type cladding layer 13b, a quantum well active layer 14c, an n-type cladding layer 15c, and an n-type contact layer 16c sequentially stacked on the p-type semiconductor substrate 40. The other structures are the same as those in the first embodiment.

[0072] Fig.10 : This is a diagram showing the result of calculating the frequency dependence of the modulation band of Embodiment 3. Electromagnetic field analysis confirmed that the modulation band can be further improved by making the thickness of the p-type semiconductor substrate 40 greater than 10 μm. Therefore, in this embodiment, the thickness of the p-type semiconductor substrate 40 is made greater than 10 μm. As a result, the lower semiconductor resistance is reduced, and microwave loss can be suppressed, so the modulation band is improved.

[0073] In addition, of course, in the structures of the first and second embodiments, instead of the substrate 10, for example, a substrate having a doping concentration of 2.0E+18 cm -3 The same effect can be obtained by using a p-type semiconductor substrate 40 composed of p-type InP.

[0074] Implementation method 4.

[0075] Fig.11 1 is a cross-sectional view showing the refractive index modulation region of the Mach-Zehnder optical modulator of Embodiment 4. In this embodiment, diffusion prevention layers 50a and 50b made of mixed crystal materials containing As, such as AlGaInAs and InGaAsP, are respectively provided between the quantum well active layers 14b and 14c and the p-type cladding layer 13b. The other structures are the same as those of Embodiment 1.

[0076] By providing the diffusion prevention layers 50a and 50b, the carrier transmission distance based on the reduction of the acceptor diffusion length can be shortened. In addition, the doping concentration of the p-type cladding layer 13b can be increased to reduce the resistance. Therefore, the frequency band can be improved. In addition, the composition of the mixed crystal material containing As can also be selected to interpolate the refractive index difference between the quantum well active layer and the p-type cladding layer.

[0077] Figure 12-14 This is a diagram showing the calculation result of the acceptor diffusion length dependency of the modulation band in the fourth embodiment. Figure 15-17 1 is a diagram showing the result of calculating the frequency dependence of the modulation band in Embodiment 4. Fig.12 , 15 In the case of Fig.13 ,16 In the case of Fig.14 , 17 In either case, the doping concentration of the p-type cladding layer is 1.0E+17cm -3 The frequency at which the modulation band becomes -3 dB decreases as the acceptor diffusion length of the p-type cladding layer indicated by the dotted line increases. Therefore, it is determined that the modulation band deteriorates as the acceptor diffusion length increases.

[0078] Therefore, in this embodiment, a diffusion prevention layer 50a is provided between the p-type cladding layer 13b and the quantum well active layer 14b, and a diffusion prevention layer 50b is provided between the p-type cladding layer 13b and the quantum well active layer 14c. This can reduce the acceptor diffusion length, so that the doping concentration of the p-type cladding layer can be set higher.

[0079] Fig.18 , 19 This is a diagram showing the calculation result of the acceptor diffusion length dependency of the modulation band in the fourth embodiment. Fig. 20 , 21 Graph showing the result of calculating the frequency dependence of the modulation band in Embodiment 4. In any case, the acceptor diffusion length is 100 nm. Fig.18 , 20 In the example, the doping concentration of the p-type cladding layer is 1.0E+16cm -3 ,exist Fig.19 , 21 In the example, the doping concentration of the p-type cladding layer is 1.0E+18cm -3 It was determined that the modulation band could be increased by increasing the doping concentration of the p-type cladding layer 13 b by two orders of magnitude.

[0080] In this embodiment, a diffusion prevention layer 50a is provided between the p-type cladding layer 13b and the quantum well active layer 14b, and a diffusion prevention layer 50b is provided between the p-type cladding layer 13b and the quantum well active layer 14c. Thus, it is possible to suppress the acceptor diffusion of the p-type cladding layer 13b to the quantum well active layers 14b and 14c. Thus, it is possible to set the doping concentration of the p-type cladding layer to a higher level, thereby increasing the modulation band.

[0081] Furthermore, it is needless to say that the diffusion prevention layers 50a and 50b of this embodiment can also be applied to any of the Mach-Zehnder optical modulators of Embodiments 1 to 3. Furthermore, the technical features described in each embodiment can be used in combination as appropriate.

[0082] Description of Reference Numerals

[0083] 10…substrate; 11b…n-type anti-diffusion layer; 12b…p-type InGaAs layer; 13b…p-type cladding layer; 14b, 14c…quantum well active layer; 15b, 15c…n-type cladding layer; 20a, 20b…traveling wave electrode; 22a, 22b…optical modulation unit; 27…DC bias electrode; 30b…n-type InGaAs layer; 40…p-type semiconductor substrate; 50a, 50b…anti-diffusion layer; 106…Mach-Zehnder optical modulator.

Claims

1. A Mach-Zehnder optical modulator, characterized in that: have: Semi-insulating or p-type substrate; An n-type anti-diffusion layer is disposed on the substrate; A p-type cladding layer, which is disposed on the n-type anti-diffusion layer; First and second quantum well active layers are disposed separately from each other on the p-type cladding layer; First and second n-type cladding layers, which are respectively disposed on the first and second quantum well active layers; first and second traveling wave electrodes, which are connected to the first and second n-type cladding layers respectively; and A DC bias electrode is connected to the p-type cladding layer and is not directly connected to the n-type anti-diffusion layer. The DC bias electrode, the p-type cladding layer, the first quantum well active layer, the first n-type cladding layer, and the first traveling wave electrode constitute a first light modulation unit. The DC bias electrode, the p-type cladding layer, the second quantum well active layer, the second n-type cladding layer, and the second traveling wave electrode constitute a second light modulation unit.

2. The Mach-Zehnder optical modulator according to claim 1, characterized in that: The p-type cladding layer comprises a p-type InGaAs layer, The n-type diffusion prevention layer includes an n-type InGaAs layer bonded to the p-type InGaAs layer.

3. A Mach-Zehnder optical modulator, characterized in that: have: A p-type semiconductor substrate having a thickness of 10 μm or more; A p-type cladding layer is disposed on the p-type semiconductor substrate; First and second quantum well active layers are disposed separately from each other on the p-type cladding layer; First and second n-type cladding layers, which are respectively disposed on the first and second quantum well active layers; first and second traveling wave electrodes, which are connected to the first and second n-type cladding layers respectively; and A DC bias electrode connected to the p-type cladding layer, The DC bias electrode, the p-type cladding layer, the first quantum well active layer, the first n-type cladding layer, and the first traveling wave electrode constitute a first light modulation unit. The DC bias electrode, the p-type cladding layer, the second quantum well active layer, the second n-type cladding layer, and the second traveling wave electrode constitute a second light modulation unit.

4. The Mach-Zehnder optical modulator according to any one of claims 1 to 3, characterized in that: It also includes a diffusion prevention layer, which is provided between the p-type cladding layer and the first and second quantum well active layers and is composed of a mixed crystal material containing As.

5. The Mach-Zehnder optical modulator according to any one of claims 1 to 4, characterized in that: The p-type cladding layer of the first light modulation section and the p-type cladding layer of the second light modulation section are not separated from each other.

6. An optical transmission device, characterized in that: A Mach-Zehnder optical modulator according to any one of claims 1 to 5.

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