Optical modulators and optical transmitters

Abstract

Provided are an EA modulator (50) and an EA-DFB laser, the EA modulator including a low-dielectric material (51) between a signal electrode pad (17) and a GND electrode pad (16) and having a structure such that the occurrence of stepping is suppressed in mounting of the EA modulator. The EA modulator (50) according to the present disclosure is an electroabsorption-type optical modulator in which a low-dielectric material (51) is arranged between a signal electrode pad (17) and a GND electrode pad (16), the low-dielectric material (51) comprising a notch (511) that is recessed to the inside in a direction perpendicular to an optical axis direction.

Description

Technical Field

[0001] The present disclosure relates to an optical modulator and an optical transmitter, and more specifically, to an EA modulator and an EA-DFB laser equipped with the same.

Background Art

[0002] A distributed feedback laser (hereinafter referred to as a DFB laser) has a narrow oscillation linewidth controlled by a diffraction grating. An EA-DFB laser in which this DFB laser and an electro-absorption type external modulator (hereinafter referred to as an EA modulator) are integrally integrated has attracted attention as a device for an optical transmitter suitable for optical communication because it generates high-speed signal light.

[0003] In recent years, due to the increase in communication traffic, an increase in transmission capacity is required, and for optical transmitters, a further increase in modulation speed is required. Since the EA-DFB laser has a high modulation speed for intensity modulation and a small change in refractive index during modulation, it is promising as a device for an optical transmitter that realizes such high-speed modulation. In recent years, in order to reduce the cost of communication infrastructure facilities, an extension of the transmission distance is also required. However, since the EA-DFB laser has a feature of small chirp (fluctuation), it is expected as an optical transmitter for medium- and long-distance transmission. Furthermore, in a semiconductor laser, there is a problem that the output power may decrease as the temperature of the active layer rises. However, if the EA-DFB laser has a buried waveguide structure with high heat dissipation efficiency, it is also possible to obtain high output power. Thus, the EA-DFB laser has attracted attention as a promising device for an optical transmitter that meets the needs of optical communication in recent years, such as medium- and long-distance transmission, high speed, and high output.

[0004] Figure 1 is a conceptual diagram showing the structure of a conventional EA modulator 10, where (a) is a top view and (b) is a cross-sectional view along the Ib-Ib cross-section. In an EA-DFB laser, the EA modulator 10 that modulates the intensity of the signal light includes, as shown in Figure 1, an EA absorption layer 11 that modulates intensity by controlling the absorption of the signal light, a p-type semiconductor 12 installed on the upper surface of the EA absorption layer in the z direction, an n-type semiconductor 13 installed on the lower surface of the EA absorption layer in the z direction, insulating layers 15a and b installed on both sides of the mesa region 14, which is the waveguide of the signal light, in the y direction, a GND electrode pad 16 installed on the lower surface of the n-type semiconductor 13 in the z direction and functioning as an electrode, a signal electrode pad 17 installed on the upper surface of the p-type semiconductor 12 and the insulating layer 15 in the z direction and functioning as an electrode, and bonding wires 18 for electrically connecting the signal electrode pad 17 to other devices. Then, a periodic voltage that repeatedly switches on and off is applied from the high-frequency power supply 19 between the GND electrode pad 16 and the signal electrode pad 17, and the intensity of the signal light is modulated by controlling the absorption wavelength of the signal light in accordance with the on / off state of this voltage.

[0005] Furthermore, for example, indium phosphide (InP) can be used as the base material for the p-type semiconductor 12, the n-type semiconductor 13, and the insulating layers 15a and 15b.

[0006] Furthermore, the EA absorption layer 11 can be made of, for example, InGaAsP or InGaAlAs having a Multi Quantum Well (MQW) structure. In this case, when a voltage is applied between the GND electrode pad 16 and the signal electrode pad 17, a reverse bias is applied to the EA absorption layer 11. As a result, a shift in the absorption wavelength determined by the band gap occurs due to the quantum confinement Stark effect, and the amount of signal light absorbed can be efficiently controlled.

[0007] In an EA absorption layer 11 having such an MQW structure, the response speed of signal light control is known to be determined by the CR time constant in the EA absorption layer 11. This capacitance component C has two components: one attributable to the EA absorption layer 11 and one attributable to the space between the electrodes (GND electrode pad 16 and signal electrode pad 17). Attempts to improve the response speed by reducing the capacitance component attributable to the space between the electrodes are already known.

[0008] Generally, since the signal electrode pad 17 is connected to the bonding wire 18, it is necessary to secure a certain area, and capacitance occurs between the GND electrode pad 16 and the signal electrode pad 17. To reduce this capacitance component, a low dielectric constant material 21 is placed between the GND electrode pad 16 and the signal electrode pad 17, as shown in Figure 2. Here, insulating layers 15c and 15d are placed on both sides of the low dielectric constant material 21 in the y direction. The EA modulator 20, having such a configuration, has a reduced capacitive component between the GND electrode pad 16 and the signal electrode pad 17 compared to the EA modulator 10, and the response speed of signal light absorption control is improved. Benzocyclobutene (hereinafter referred to as BCB) can be used as the low dielectric constant material 21.

[0009] However, EA modulators 20 containing such low dielectric constant materials 21 have the challenge of low yield in the mounting process.

[0010] Figure 3 is a flowchart illustrating a conventional method for mounting an EA modulator containing a low dielectric constant material below the signal electrode pads. The mounting process 30 for an EA modulator containing a low dielectric constant material below the signal electrode pads includes: forming a waveguide core containing an EA absorption layer having an MQW structure on an n-type semiconductor substrate (step 31); forming a p-type semiconductor on the upper surface of the waveguide core (step 32); removing the remaining p-type semiconductor, waveguide core, and n-type semiconductor, leaving the mesa region (step 33); filling the removed region with an insulating material to form an insulating layer (step 34); forming a groove in the z-direction in a part of the insulating layer to a depth that reaches the n-type semiconductor (step 35); filling the formed groove with a low dielectric constant material (step 36); and forming a GND electrode pad and a signal electrode pad (step 37).

[0011] In such an implementation process 30, for example, as shown in Figure 4, a step in the height direction (z direction) occurs between the low dielectric constant material 21 and the insulating layer 15b, and the horizontal alignment of the EA modulator 20 in the xy plane is not ensured, and it is tilted. Then, when the signal electrode pad 17 is formed by vapor deposition in step 37, a gap (step) may occur on the end face of the signal electrode pad 17 formed on the upper surface of the low dielectric constant material 21, on the side that is in contact with the insulating layer 15c. This is because, due to the aforementioned step and tilt, the end face becomes a "shadow" of the insulating layer 15c, and the incidence of flying particles that form the signal electrode pad 17 during vapor deposition is hindered.

[0012] In Figure 4, the step in the z-direction is depicted as being convex on the insulating layer 15c. However, a similar step can occur even if the low dielectric constant material 21 is convex. In this case, the step occurs on the signal electrode pad 17 formed on the upper surface of the insulating layer 15c.

[0013] Thus, conventional EA modulators containing low dielectric constant materials had the problem of step breakage occurring during the formation of signal electrode pads, which in turn led to a decrease in yield. [Prior art documents] [Non-patent literature]

[0014] [Non-Patent Document 1] T. Shindo et al., “25-Gbit / s 100-km Transmission using 1358-nm-wavelength SOA Assisted Extended Reach EADFB Laser (AXEL) for 25 Gbit / s-class PON”, 2021 Optical Fiber Communications Conference and Exhibition (OFC), pp.1-3 (2021) [Overview of the project]

[0015] This disclosure has been made in view of the above-mentioned problems, and its purpose is to provide an EA modulator and an EA-DFB laser having a structure that suppresses the occurrence of step breaks when mounting an EA modulator that includes a low dielectric constant material between the signal electrode pad and the GND electrode pad.

[0016] To address the above-mentioned challenges, this disclosure provides an electric field absorption type optical modulator in which a low dielectric constant material is placed between a signal electrode pad and a GND electrode pad, wherein the low dielectric constant material is perpendicular to the optical axis direction. Exchange The present invention provides an optical modulator having an inwardly concave notch in the direction of the optical modulation. [Brief explanation of the drawing]

[0017] [Figure 1] This diagram conceptually shows the structure of a conventional EA modulator 10, with (a) being a top view and (b) being a cross-sectional view along the Ib-Ib cross-section. [Figure 2] This diagram conceptually shows the structure of an EA modulator 20 containing a low dielectric constant material 21 according to prior art, where (a) is a top view and (b) is a cross-sectional view along the IIb-IIb cross-section. [Figure 3]It is a flowchart illustrating a mounting process 30 of an EA modulator including a low dielectric constant material below a signal electrode pad according to the prior art. [Figure 4] It is a diagram specifically showing the mounting when a step in the height direction (z direction) occurs between the low dielectric constant material 21 and the insulating layer 15b and the EA modulator 20 is inclined. (a) shows an overall view, and (b) shows an enlarged view of part VI-VI. [Figure 5] It is a diagram conceptually showing the structure of an EA modulator 50 according to the first embodiment of the present disclosure. (a) shows a top view, and (b) shows a cross-sectional view taken along the cross-section line Vb-Vb. [Figure 6] It is a diagram conceptually showing the shape of a form in which the corner of the notch 511 has a curvature in the EA modulator 50 according to the first embodiment of the present disclosure. [Figure 7] FIG. 7 is a diagram conceptually showing the structure of an EA modulator 70 according to the second embodiment of the present disclosure. (a) shows a top view, and (b) shows a cross-sectional view taken along the cross-section line VIIb-VIIb.

Mode for Carrying Out the Invention

[0018] Hereinafter, various embodiments of the present disclosure will be described in detail with reference to the drawings. The same or similar reference numerals indicate the same or similar elements, and redundant descriptions may be omitted. The materials and numerical values are for illustrative purposes and are not intended to limit the technical scope of the present disclosure. The following description is an example, and as long as it does not deviate from the gist of an embodiment of the present disclosure, some configurations may be omitted, modified, or implemented with additional configurations.

[0019] (First Embodiment) Hereinafter, the first embodiment of the present disclosure will be described in detail with reference to the drawings. The EA modulator in this embodiment relates to a form having a structure in which a low dielectric constant material installed below a signal electrode pad has a notch that is concave inward.

[0020] FIG. 5 is a diagram conceptually showing the structure of an EA modulator 50 according to the first embodiment of the present disclosure, where (a) shows a top view and (b) shows a cross-sectional view taken along the Vb-Vb cross-section line. As shown in FIG. 5, the EA modulator 50 according to the present embodiment has the same configuration as the EA modulator 20 according to the prior art, and a low dielectric constant material 51 further includes a bridge-shaped notch 511 that is concave inward in the direction perpendicular to the optical axis direction (y direction). In other words, the insulating layer 15 includes a notch 511 in which an insulating layer 15c protruding outward is embedded in the direction perpendicular to the optical axis direction (y direction). And a signal electrode pad 17 is formed on the upper surface in the z direction of this notch 511. Exchange The low dielectric constant material 51 having this notch 511 can be easily realized by forming a groove in a shape that leaves the portion of the notch 511 in advance in step 35 of the mounting process 30 shown in FIG. 3. That is, the shape of the low dielectric constant material 51 of the EA modulator 50 is a structure that can be easily achieved by the mounting process according to the prior art. Exchange

[0021]

[0022] Exchange

[0023] The EA modulator 50 having such a structure has a structure in which the low dielectric constant material 51 has three surfaces perpendicular to each other at the portion of the notch 511. In this case, in the formation of the signal electrode pad 17 by vapor deposition, except when the EA modulator 50 is tilted clockwise with respect to the x axis in FIG. 5(b), at least one surface is always exposed to the flying particles incident from the upper part in the z direction, and the vapor deposition metal also adheres to the surface. As a result, the pad surfaces including the step are connected via the vertical surface of the step, so the occurrence of step cuts is suppressed.

[0023] In fact, when 20,000 EA modulators 50 were mounted using the mounting process 30, no step breaks occurred in any of them. In contrast, when 20,000 conventional EA modulators 20 of the same dimensions were mounted in the same manner, 450 step breaks occurred. That is, the step break rate in 20,000 trials was 2.25% for the conventional EA modulator, while it was 0% for the EA modulator according to this disclosure. Here, BCB is applied to the low dielectric constant material 51 and low dielectric constant material 21, and the dimensions in the xy plane are 5 μm × 25 μm.

[0024] From the above, it can be said that the EA modulator according to this embodiment (for example, EA modulator 50) has been demonstrated to have a structure that achieves a higher yield compared to the conventional EA modulator (for example, EA modulator 20). It should be noted that by introducing the notch 511, the area of ​​the low dielectric constant material 51 in the xy plane is reduced compared to the low dielectric constant material 21, and as a result, a decrease in response speed due to the increase in capacitance may occur. However, even with the introduction of the notch 511, the area reduction rate is expected to be only about 4%, and it is considered that the contribution to the decrease in response speed is negligible.

[0025] Furthermore, although the notch 511 of the low dielectric constant material 51 is described as a rectangle with right angles in its corners in this embodiment, it is not limited to this, and a similar effect can be achieved even with a structure in which the corners have curvature, such as as shown in Figure 6.

[0026] Furthermore, the EA modulator 50 according to this embodiment may be integrated with a DFB laser to constitute an EA-DFB laser. As described above, the response speed of the EA modulator 50 is equivalent to that of the conventional EA modulator 20, so the EA-DFB laser becomes an optical transmitter with performance equivalent to that of the conventional technology.

[0027] (Second embodiment) A second embodiment of the present disclosure will be described in detail below with reference to the drawings. The EA modulator in this embodiment relates to a configuration in which the low dielectric constant material installed below the signal electrode pad is divided into two parts.

[0028] Figure 7 is a conceptual diagram showing the structure of the EA modulator 70 according to the second embodiment of this disclosure, where (a) is a top view and (b) is a cross-sectional view along the VIIb-VIIb cross-sectional line. As shown in Figure 7, the EA modulator 70 according to this embodiment has the same configuration as the prior art EA modulator 20, and has a structure in which the low dielectric constant material 61 is divided into a first low dielectric constant material 71a and a second low dielectric constant material 71b in the x-direction. In other words, the EA modulator 70 has a structure in which the notch 511 in the first embodiment penetrates in the y-direction. To put it another way, the low dielectric constant material 51 in the first embodiment penetrates in the y-direction. Exchange It is divided into two sections in the direction of the object, and is positioned with a gap corresponding to a notch.

[0029] An EA modulator 70 having such a structure has two parallel surfaces. Furthermore, since the electrode on the notch 511 is always connected to the mesa region 14 of the EA even when a step cut occurs in the pad, conductivity is ensured as long as the bonding wire 18 is in contact with the notch 511. Therefore, similar to the first embodiment, it is possible to suppress the occurrence of step cuts in the formation of the signal electrode pad 17, and furthermore, it is possible to increase the conductivity probability between the GND electrode pad 16 and the signal electrode pad 17.

[0030] Furthermore, compared to the EA modulator 70 described in the first embodiment, the EA modulator 70 has a reduced area of ​​low dielectric constant material, which may decrease its response speed. However, on the other hand, because the notch 511 has a structure that penetrates in the y direction, that portion does not have a surface parallel to the xz plane. Therefore, it is possible to reliably suppress at least step cuts parallel to the xz plane.

[0031] In addition, similar to the first embodiment, the EA modulator 70 according to this embodiment may also be integrated with the DFB laser to constitute an EA-DFB laser. [Industrial applicability]

[0032] As described above, the EA modulator and DFB laser disclosed herein have a structure that can suppress step cutting in the mounting process and achieve high yield, and are therefore expected to be applied as devices for optical transmitters in optical communications.

Claims

1. An electric field absorption type optical modulator comprising a low dielectric constant material disposed between a signal electrode pad and a GND electrode pad and embedded in one of the insulating layers formed on either side of a mesa region, wherein the low dielectric constant material has an inwardly concave notch in a direction perpendicular to the optical axis direction, The notch has a structure in which the insulating material constituting the insulating layer is embedded. The upper surface of the notch is located on the same plane as the upper surface of the portion of the insulating layer in which the low dielectric constant material is not embedded. The signal electrode pad is formed so as to be in contact with the low dielectric constant material and the notch, and so as to straddle the interface between the low dielectric constant material and the notch. Optical modulator.

2. An electric field absorption type optical modulator comprising a low dielectric constant material disposed between a signal electrode pad and a GND electrode pad, and embedded in one of the insulating layers formed on either side of a mesa region, wherein the low dielectric constant material is divided into two parts in a direction perpendicular to the optical axis direction and arranged with a gap between them, The gap has a structure in which the insulating material constituting the insulating layer is embedded. The upper surface of the gap is located on the same plane as the upper surface of the portion of the insulating layer in which the low dielectric constant material is not embedded. The signal electrode pad is formed so as to be in contact with the low dielectric constant material and the gap, and so as to straddle the interface between the low dielectric constant material and the gap, in an optical modulator.

3. The optical modulator according to claim 1 or 2, The EA-DFB laser coupled to the optical modulator and An optical transmitter equipped with [a specific feature].

4. A method for manufacturing an electric field absorption type optical modulator, Forming a waveguide core including an EA absorption layer having an MQW structure on an n-type semiconductor substrate, Forming a p-type semiconductor on the upper surface of the waveguide core, Removing the p-type semiconductor, the waveguide core, and the n-type semiconductor other than the mesa region, The removed portion is filled with insulating material to form an insulating layer, In a portion of the insulating layer, a groove is formed having a depth that reaches the n-type semiconductor, wherein the groove has a notch that is concave inward in a direction perpendicular to the optical axis, and the upper surface of the notch is located on the same plane as the upper surface of the portion of the insulating layer in which the groove is not formed. The groove is filled with a low dielectric constant material, The method involves forming a GND electrode pad and a signal electrode pad, wherein the signal electrode pad is formed to be in contact with the low dielectric constant material and the notch, and to straddle the interface between the low dielectric constant material and the notch. A manufacturing method that includes [the necessary equipment / features].

5. A method for manufacturing an electric field absorption type optical modulator, Forming a waveguide core including an EA absorption layer having an MQW structure on an n-type semiconductor substrate, Forming a p-type semiconductor on the upper surface of the waveguide core, Removing the p-type semiconductor, the waveguide core, and the n-type semiconductor other than the mesa region, The removed portion is filled with insulating material to form an insulating layer, In a portion of the insulating layer, two grooves are formed having a depth that reaches the n-type semiconductor, wherein the two grooves are arranged with a gap between them in a direction perpendicular to the optical axis, and the upper surface of the gap lies on the same plane as the upper surface of the portion of the insulating layer in which the two grooves are not formed. The groove is filled with a low dielectric constant material, The method involves forming a GND electrode pad and a signal electrode pad, wherein the signal electrode pad is formed so as to be in contact with the low dielectric constant material and the gap, and so as to straddle the interface between the low dielectric constant material and the gap. A manufacturing method that includes [the necessary equipment / features].

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