optical modulator

Abstract

The present invention provides an optical modulator comprising: a waveguide formed of a semiconductor and through which light propagates; a first electrode arranged on the waveguide and electrically connected to the waveguide; and a second electrode separated from the waveguide and electrically connected to the waveguide, wherein, in the propagation direction of the light, the end of the second electrode on the side on which the light is incident is located at a later stage than the end of the first electrode on the side on which the light is incident.

Description

Technical Field

[0001] This application claims the benefit of priority based on Japanese Application No. 2019-152709, filed on August 23, 2019, and incorporates by reference all of the contents described in the Japanese Application.

[0002] The present invention relates to light modulators. Background Art

[0003] In the non-patent document M. Yuda, M. Fukuda and H. Miyazawa, Degradation mode semiconductor optical modulators, ELECTRONICS LETTERS, 1995, September 28th, Vol. 31, No. 20, pp. 1778-1779, an optical modulator formed of a semiconductor layer and modulating light has been developed. Summary of the Invention

[0004] An optical modulator operates by allowing light to enter it and applying a voltage to its electrodes. The semiconductor layer of the optical modulator absorbs light, generating a current. This light absorption current can sometimes damage the optical modulator. Therefore, an optical modulator that can minimize damage is desired.

[0005] The optical modulator disclosed herein comprises: a waveguide formed of a semiconductor and through which light propagates; a first electrode arranged on the waveguide and electrically connected to the waveguide; and a second electrode separated from the waveguide and electrically connected to the waveguide, wherein, in the propagation direction of the light, the end of the second electrode on the side on which the light is incident is located at a later stage than the end of the first electrode on the side on which the light is incident. BRIEF DESCRIPTION OF THE DRAWINGS

[0006] Figure 1A This is a top view of the optical modulator according to Example 1.

[0007] Figure 1B is an enlarged cross-sectional view of the arm waveguide.

[0008] Figure 2A This is an enlarged top view of the arm waveguide.

[0009] Figure 2B This is a graph showing the relationship between light absorption current density and position.

[0010] Figure 3A This is a top view of the optical modulator of Example 2.

[0011] Figure 3B It is a graph showing the evaluation results of the breakdown voltage. DETAILED DESCRIPTION

[0012] [Description of Embodiments of the Present Disclosure]

[0013] First, the contents of the embodiments of the present disclosure are listed for description.

[0014] One embodiment of the present disclosure is (1) an optical modulator comprising: a waveguide formed of a semiconductor and through which light propagates; a first electrode disposed on the waveguide and electrically connected to the waveguide; and a second electrode separated from the waveguide and electrically connected to the waveguide, wherein, in the direction of propagation of the light, the end of the second electrode on the side where the light is incident is located later than the end of the first electrode on the side where the light is incident. A voltage drop occurs until the current generated in the waveguide due to the incident light reaches the second electrode. Therefore, the voltage applied to the end of the first electrode decreases, which can suppress damage to the waveguide.

[0015] (2) Alternatively, the waveguide may include: a first semiconductor layer having a first conductivity type; a core layer provided on the first semiconductor layer; and a second semiconductor layer provided on the core layer and having a second conductivity type, wherein the first semiconductor layer extends outside the waveguide, the first electrode is electrically connected to the second semiconductor layer, and the second electrode is provided on the first semiconductor layer and electrically connected to the first semiconductor layer. This can suppress the destruction of the pn junction of the waveguide.

[0016] (3) The first semiconductor layer and the second semiconductor layer may include indium phosphide, and the core layer may include aluminum gallium indium arsenide. This can suppress the destruction of the pn junction of the waveguide.

[0017] (4) A distance from an end of the first electrode to an end of the second electrode in the propagation direction of the light may be 350 μm or more. This effectively suppresses the destruction of the waveguide.

[0018] (5) The first electrode and the second electrode may extend in the light propagation direction, and the length of the second electrode in the light propagation direction may be 200 μm or longer. This can suppress concentration of the electric field in the second electrode, thereby suppressing destruction.

[0019] [Details of the embodiments of the present disclosure]

[0020] Specific examples of the optical modulator according to the embodiment of the present disclosure will be described below with reference to the accompanying drawings. 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 of the claims.

[0021] [Example 1]

[0022] (Light Modulator)

[0023] Figure 1A This is a top view of an optical modulator 100 of Example 1. The optical modulator 100 is, for example, a Mach-Zehnder modulator formed from a GaAs-based semiconductor or an InP-based semiconductor. The optical modulator 100 includes a substrate 10, an input port 40, output ports 42, 44, 46, and 48, multiple waveguides, multiple couplers, and an antireflection film 54. The waveguides are arm waveguides having a mesa structure. The couplers are MMI (Multi-Mode Interferometer) couplers. The X-axis and Y-axis directions are the directions of the sides of the substrate 10, and the Z-axis direction is the direction in which the semiconductor layers are stacked. These directions are orthogonal to each other.

[0024] Substrate 10 is a semiconductor substrate formed from a compound semiconductor. Substrate 10 is rectangular and has four faces 10a to 10d. Faces 10a and 10b extend in the Y-axis direction and face each other in the X-axis direction. Faces 10c and 10d extend in the X-axis direction and face each other in the Y-axis direction. Faces 10a and 10b are, for example, 8 to 9 mm long, and faces 10c and 10d are, for example, 10 to 12 mm long.

[0025] An incident port 40, and exit ports 42, 44, 46, and 48 are provided on the surface 10a. On one side of the incident port 40 in the Y-axis direction, exit ports 42 and 46 are provided in order from near to far from the incident port 40. On the opposite side, exit ports 44 and 48 are provided in order from near to far from the incident port 40. An antireflection film 54 is provided on the surface 10b. The antireflection film 54 is formed of, for example, aluminum oxide (Al2O3) with a thickness of 0.22 μm, and suppresses reflection of light in the wavelength range of 1.53 μm to 1.57 μm. In addition, an antireflection film not shown is also provided on the surface 10a. In addition, on Figure 1A In the figure, electrodes are not shown.

[0026] Multiple waveguides, arm waveguides, and couplers are formed on substrate 10. As described later, an arm waveguide comprises, for example, multiple cladding layers and a core layer sandwiched between the cladding layers. Waveguides 41, 47, and 52a to 52d extend in the X-axis direction. Arm waveguides 34a and 34b have portions extending in the X-axis direction and portions extending in the Y-axis direction.

[0027] One end of waveguide 41 is optically coupled to input port 40, and the other end is optically coupled to one end of single-input, single-output coupler 45. One end of waveguide 47 is optically coupled to the other end of coupler 45, and the other end is optically coupled to one end of single-input, dual-output coupler 49. The other end of coupler 49 is optically coupled to waveguides 52a and 52b via coupler 50a, and to waveguides 52c and 52d via coupler 50b. Waveguide 52a is optically coupled to one end of two arm waveguides 34a and 34b via couplers. The other end of arm waveguide 34a is optically coupled to one end of a subsequent-stage coupler. Waveguides 52b through 52d are also optically coupled to two arm waveguides, respectively.

[0028] That is, multiple arm waveguides and couplers are further provided after waveguides 52a to 52d. A portion of arm waveguide 34 is bent and extends back from the +X direction to the -X direction. Output ports 42, 44, 46, and 48 are each optically coupled to one waveguide.

[0029] Figure 1B A contact layer 12 (first semiconductor layer), a lower cladding layer 13, a core layer 14, an upper cladding layer 16, and a contact layer 18 (second semiconductor layer) are sequentially stacked on a substrate 10.

[0030] The substrate 10 is, for example, a semiconductor substrate formed of semi-insulating indium phosphide (InP). The contact layer 12 and the lower cladding layer 13 are formed of, for example, n-type InP doped with silicon (Si). The thickness of the contact layer 12 is, for example, 500 nm, and the thickness of the lower cladding layer 13 is, for example, 800 nm. The core layer 14 is, for example, formed of 500 nm thick gallium indium arsenide phosphide (GaInAsP) and has a multiple quantum well (MQW) structure. The upper cladding layer 16 is, for example, formed of 1300 nm thick p-type InP doped with zinc (Zn). The contact layer 18 is, for example, formed of 200 nm thick p-type InGaAs doped with Zn.

[0031] The compound semiconductor layers (contact layer 12, lower cladding layer 13, core layer 14, upper cladding layer 16, and contact layer 18) on substrate 10 form arm waveguides 34a and 34b. Contact layer 12 extends continuously between arm waveguide 34a and arm waveguide 34b and to a position closer to the -Y side than arm waveguide 34b. Arm waveguides 34a and 34b are electrically connected via contact layer 12. Substrate 10 and contact layer 12 form a mesa, on which the arm waveguides 34a and 34b of the mesa structure are disposed. The two arm waveguides 34a and 34b are sometimes collectively referred to as an arm waveguide 34.

[0032] Resin layers 20 and 21, and insulating films 22, 23, 24, and 25 are provided on substrate 10. Insulating film 22 covers the upper surface of substrate 10 and the side surfaces and upper surface of arm waveguide 34. Resin layer 20 is provided on the upper surface of insulating film 22, burying the side surfaces of arm waveguides 34a and 34b. Insulating film 23 is provided on the upper surface of resin layer 20, and resin layer 21 is provided on the upper surface of insulating film 23. Insulating film 24 is provided on the upper surface of resin layer 21, and insulating film 25 is provided on the upper surface of insulating film 24.

[0033] An ohmic layer 28, a plating layer 26, and a wiring layer 27 are sequentially stacked on the arm waveguide 34, functioning as a modulation electrode 35 (first electrode). The ohmic layer 28 contacts the upper surface of the contact layer 18, the plating layer 26 contacts the upper surface of the ohmic layer 28, and the wiring layer 27 contacts the upper surface of the plating layer 26.

[0034] The ohmic layer 28 is, for example, a sequentially stacked layer of platinum (Pt) with a thickness of 30 nm, a titanium (Ti) layer with a thickness of 50 nm, a Pt layer with a thickness of 50 nm, and a gold (Au) layer with a thickness of 200 nm. The width of the ohmic layer 28 is, for example, 1 μm. The plating layer 26 and the wiring layer 27 are metal layers (TiW / Pt / Au) each formed by sequentially stacking a titanium tungsten (TiW) layer with a thickness of 50 nm, a Pt layer with a thickness of 50 nm, and an Au layer with a thickness of 50 nm. The plating layer 26 is, for example, 1 μm thick, and the wiring layer 27 is, for example, 4 μm thick. The modulation electrode 35 is electrically connected to the contact layer 18 and the upper cladding layer 16 of the arm waveguides 34 a and 34 b.

[0035] An electrode 36 (second electrode) is provided on the upper surface of the contact layer 12 at a position separated from the arm waveguides 34a and 34b. The electrode 36 includes an electrode 36a and an electrode 36b, which are sequentially stacked. Electrode 36a is formed, for example, from an alloy of Au, germanium (Ge), and nickel (Ni), and has a thickness of 200 nm. Electrode 36b is formed, for example, from a sequential stack of a 50 nm thick Ti layer, a 50 nm thick Pt layer, and a 900 nm thick Au layer. Electrode 36 is electrically connected to the contact layer 12.

[0036] Insulating film 25 covers insulating film 24 and wiring layer 27. Resin layers 20 and 21 are formed of, for example, BCB (benzocyclobutene). Resin layer 20 has a thickness of, for example, 2.5 μm, and resin layer 21 has a thickness of, for example, 3.5 μm. Insulating film 24 is, for example, a 0.3 μm thick silicon oxide (SiO2) film, and insulating films 22, 23, and 25 are, for example, 0.3 μm thick silicon oxynitride (SiON) films.

[0037] Figure 2A FIG is an enlarged top view of the arm waveguide 34. Figure 2A In the example, the coupler is omitted. Figure 2AAs shown, arm waveguides 34a and 34b are partially parallel. At this location, arm waveguides 34a and 34b, two modulation electrodes 35, and electrode 36 are parallel to each other and extend in the X-axis direction. Light L0 enters arm waveguides 34a and 34b from the -X side and propagates in the +X direction. The width W1 of arm waveguides 34a and 34b in the Y-axis direction is, for example, 1.5 μm. Contact layer 12 extends within the XY plane to a width wider than arm waveguide 34. Electrode 36 is provided above contact layer 12 and at a position separated from arm waveguides 34a and 34b.

[0038] like Figure 2A As shown, in the Y-axis direction, the distance D2 from the -Y-side end of the arm waveguide 34b to the end of the contact layer 12 is, for example, 1 μm. The length L1 of the modulation electrode 35 in the X-axis direction is, for example, 1500 μm. The length L2 of the electrode 36 in the X-axis direction is smaller than the length L1. The width W2 of the modulation electrode 35 is, for example, 5 μm, and the width W3 of the electrode 36 is, for example, 14 μm.

[0039] In the direction of light propagation, the side where the light arrives first is the front stage, and the side where the light arrives later is the back stage. Figure 2A In the figure, the front stage is the -X side, and the back stage is the +X side. The end of the modulation electrode 35 on the -X side (the incident side of the light L0) is referred to as end 35a, and the end of the electrode 36 on the -X side (the incident side of the light L0) is referred to as end 36c. End 36c is located at a position further back than end 35a. Figure 1A The distance from the incident port 40 to the end 36 c is greater than the distance from the incident port 40 to the end 35 a .

[0040] from Figure 1A Light incident on input port 40 propagates through waveguide 41, coupler 45, waveguide 47, coupler 49, and waveguides 52a to 52d, before entering the arm waveguides. Light propagating through arm waveguides 34a and 34b is, for example, the fundamental mode. Light propagating through the multiple arm waveguides is branched and combined in a subsequent coupler and then emitted from output ports 42, 44, 46, and 48.

[0041] Input light into Figure 1B Arm waveguides 34a and 34b are shown, and a DC reverse bias voltage of, for example, a few negative volts and an AC voltage of, for example, 1.5V with a frequency of 20GHz are applied between modulation electrode 35 and electrode 36. A high-frequency electrical signal (e.g., approximately 20GHz) flows between modulation electrode 35 and electrode 36. The refractive index of arm waveguides 34a and 34b changes due to the electrical signal, thereby changing the optical path length in arm waveguides 34a and 34b. As a result, the phase of the light propagating in arm waveguides 34a and 34b changes. As a result, the combined light can be modulated. Light can be modulated in the same manner in the other arm waveguides.

[0042] The electrode 36 is connected to the contact layer 12, which is an n-type semiconductor layer. The contact layer 12 and the lower cladding layer 13 are located below the core layer 14. The modulation electrode 35 is connected to the contact layer 18, which is a p-type semiconductor layer. The contact layer 18 and the upper cladding layer 16 are located above the core layer 14. Therefore, when a reverse bias is applied, an electric field is applied to the core layer 14 along the Z-axis direction. The core layer 14 absorbs light and generates electron-hole pairs, and these carriers flow toward the +Z side or the -Z side due to the electric field. Therefore, a current flows along the Z-axis direction in the arm waveguide 34. When the current density increases, the arm waveguide 34 may be short-circuited and damaged.

[0043] Figure 2B This graph shows the relationship between light absorption current density and position. The horizontal axis represents the position in the X-axis direction based on the end 35a of the modulation electrode 35, with larger values ​​indicating positions further away from the end 35a toward the +X side. The vertical axis represents the calculated current density generated by the core layer 14 of the arm waveguide 34 absorbing 1 mW of light. The solid line indicates that the light absorption coefficient α of the core layer 14 is 23 cm -1 For example, the dotted line indicates that the absorption coefficient α is 15cm -1 For example, the dotted line indicates that the absorption coefficient α is 7.7 cm -1 For example, the single-dot chain line indicates that the absorption coefficient α is 0.8 cm -1 example.

[0044] like Figure 2B As shown, the closer to the end 35a, the higher the current density, and the farther away from the end 35a, the lower the current density. The intensity of the light incident on the arm waveguide 34 becomes stronger the closer it is to the incident position of the arm waveguide 34, and attenuates as it propagates. If the light intensity is high, the current generated in the core layer 14 also becomes larger, and if the light intensity is weak, the current also becomes smaller. The end 35a of the modulation electrode 35 is close to the incident position of the arm waveguide 34, so the light intensity is high. Therefore, as Figure 2B As shown, the current density is high at a position close to the end 35a, and the current density is low at a position away from the end 35a.

[0045] By applying a high voltage and injecting strong light, the current increases, and the core layer 14 may be damaged. For example, when the intensity of light incident on the arm waveguide 34 reaches 4dBm or more, the current density near the end 35a exceeds 1kA, causing damage. Heat is generated in the arm waveguide 34 due to the current. Due to the heat, the core layer 14 easily absorbs light, the current increases, and thus generates more heat. Such positive feedback can easily cause damage. Due to the destruction of the core layer 14, the rectifying characteristics of the pn junction in the arm waveguide 34 disappear, resulting in a short-circuit state. Therefore, a large current flows even when a reverse voltage is applied, making modulation difficult.

[0046] To prevent damage, for example, it is sufficient to reduce the voltage applied to the modulation electrode 35. However, to modulate light, a reverse voltage of, for example, 10V or more is applied. Therefore, reducing the voltage is difficult. Alternatively, reducing the light intensity and thus the current density can also prevent damage. However, this requires the use of a large number of amplifiers to amplify the optical signals used in optical communication, resulting in an increased size of the device. Therefore, reducing the light intensity is difficult.

[0047] According to Example 1, Figure 2A As shown, in the direction of light propagation, end 36c of electrode 36 is located further back than end 35a of modulation electrode 35. The current generated in core layer 14 reaches end 36c of electrode 36 through contact layer 12. Due to the resistance component of contact layer 12 and the current, a voltage drop occurs within contact layer 12 in the X-axis direction. The potential of contact layer 12 at end 36c is equal to the potential applied to electrode 36, for example, ground potential (0V). In the region from end 36c toward the -X-axis side, the potential of contact layer 12 gradually decreases from end 36c toward the -X-axis due to the voltage drop. Meanwhile, the potential of contact layer 18 of arm waveguide 34, including the position at end 35a where modulation electrode 35 contacts arm waveguide 34, is equal to the reverse voltage applied to modulation electrode 35, for example, negative 10V. The voltage applied to the core layer 14 of arm waveguide 34 is proportional to the difference between the potentials of contact layer 12 and contact layer 18. The voltage drop in the contact layer 12 reduces the voltage applied to the core layer 14 of the arm waveguide 34, especially near the end 35a where the light intensity is high. This can suppress damage to the arm waveguide 34.

[0048] The voltage drop is expressed by the following equation: R is the resistance of the contact layer 12, Rs is the surface resistance of the contact layer 12, L is the length between the end 35a and the end 36c, ηλ(-dP / dx) / 1260 is the current generated by light absorption, W4 is the width of the contact layer 12 in the Y-axis direction, α is the absorption coefficient, η is the quantum efficiency, λ is the wavelength of light incident on the arm waveguide 34, and P0 is the intensity of the light.

[0049] [Formula 1]

[0050]

[0051] At λ=1550nm, P0=10mW, η=1, Rs=30Ω / sq, α=4cm -1 When W4 = 25 μm and L = 750 μm, the voltage drop is about 1.5 V. To suppress destruction, the voltage drop is preferably 1 V or more, and the distance L between the end portions is preferably 350 μm or more.

[0052] According to Example 1, the destruction of the core layer 14 is suppressed by the voltage drop. Therefore, it is not necessary to reduce the intensity of light incident on the arm waveguide 34, and it is not necessary to increase the number of amplifiers. The voltage only needs to be a magnitude sufficient for modulation.

[0053] The arm waveguide 34 includes an n-type contact layer 12 and a lower cladding layer 13, a core layer 14, a p-type upper cladding layer 16, and a contact layer 18. They form a pn junction. Figure 1B and 2A As shown, the contact layer 12 continuously extends below, between, and to the outside (-Y side) of the arm waveguides 34a and 34b. Electrode 36 is provided on the contact layer 12. As current flows through the contact layer 12 until it reaches the end 36c of the electrode 36, a voltage drop occurs, which can suppress damage to the core layer 14. As a result, damage to the pn junction of the arm waveguide 34 is suppressed, and the rectifying characteristics are maintained.

[0054] The contact layer 12 is made of n-InP, the upper cladding layer 16 is made of p-InP, and the core layer 14 is made of AlGaInAs. This suppresses damage and protects the pn junction therebetween. The arm waveguide 34 may also be made of other compound semiconductors.

[0055] [Example 2]

[0056] In Example 2, the ESD (Electrostatic Discharge) breakdown voltage is improved. Figure 3A FIG. 2 is a top view of the optical modulator 200 of Example 2. The same structure as that of Example 1 is omitted. Figure 3A As shown, the length of the electrode 36 is L3. By increasing the length L3, the concentration of the electric field in the electrode 36 is suppressed, thereby suppressing electrostatic damage.

[0057] In the optical modulator 200 , the length L1 of the modulation electrode 35 was 1500 μm, and the length L3 of the electrode 36 was set to 14 μm or 1400 μm. A pulse voltage was applied to examine the voltage at which destruction occurred. Figure 3B This is a graph showing the evaluation results of breakdown voltage. The horizontal axis represents the pulse voltage, and the vertical axis represents the multiple of the standard deviation related to destruction (the expected probability in a normal distribution). The triangle is an example of L3 = 14μm, and the circle is an example of L3 = 1400μm. Figure 3B As shown, in the example of L3 = 14 μm, the optical modulator is destroyed at a voltage of approximately 500 V. This is because the electric field generated by the modulation electrode 35 is concentrated on the electrode 36. On the other hand, in the example of L3 = 1400 μm, the optical modulator is destroyed at 2500 V.

[0058] According to Example 2, by increasing the length L3 of the electrode 36 in the X-axis direction, the concentration of the electric field in the electrode 36 is suppressed, so that the electric field is evenly distributed in the electrode 36. This can improve the breakdown voltage and suppress damage. In particular, the length L3 of the electrode 36 is preferably 200 μm or more. For example, the length L3 is preferably the same as the length L1 of the modulation electrode 35, or is 20% or more of the length L1. When the optical modulator 200 is assembled in a package, a voltage of several hundred volts may be applied due to static electricity. By increasing the breakdown voltage, damage caused by static electricity can be suppressed. By positioning the end 36 c at a later stage than the end 35 a as in Example 1, damage caused by current can be suppressed.

[0059] While the embodiments of the present disclosure have been described in detail above, the present invention is not limited to the specific embodiments involved, and various modifications and changes are possible within the scope of the gist of the present invention described in the claims.

Claims

1. An optical modulator comprising: a waveguide, formed of a semiconductor, in which the light propagates; a first electrode, disposed on the waveguide and electrically connected to the waveguide; as well as a second electrode, separated from the waveguide and the first electrode and connected to the waveguide, In the propagation direction of the light, the end of the second electrode on the incident side of the light is located at a later stage than the end of the first electrode on the incident side of the light. The waveguide includes: a first semiconductor layer having a first conductivity type; a first cladding layer; a core layer disposed on the first cladding layer; a second semiconductor layer having a second conductivity type; and a second cladding layer disposed between the core layer and the second semiconductor layer. The first semiconductor layer extends to the outside of the waveguide, The first electrode is electrically connected to the second semiconductor layer, The second electrode is disposed on the first semiconductor layer and is electrically connected to the first semiconductor layer. The first electrode contacts the upper surface of the second semiconductor layer and extends from the end of the first electrode toward the subsequent stage in the propagation direction of the light. The second electrode contacts the upper surface of the first semiconductor layer and extends from the end of the second electrode toward the subsequent stage in the propagation direction of the light. The length of the first electrode along the propagation direction of the light and the length of the second electrode along the propagation direction of the light are shorter than the length of the waveguide along the propagation direction of the light, The current generated in the core layer reaches the end of the second electrode on the light incident side through the first semiconductor layer. Due to the resistance component of the first semiconductor layer and the current, a voltage drop in the light propagation direction occurs inside the first semiconductor layer.

2. The optical modulator according to claim 1, wherein The first semiconductor layer and the second semiconductor layer include indium phosphide, The core layer includes aluminum gallium indium arsenide.

3. The optical modulator according to claim 1 or 2, wherein: A distance from an end portion of the first electrode to an end portion of the second electrode in the propagation direction of the light is 350 μm or more.

4. The optical modulator according to claim 1 or 2, wherein: The first electrode and the second electrode extend in the propagation direction of the light, The length of the second electrode in the light propagation direction is greater than or equal to 200 μm.

5. The optical modulator according to claim 1 or 2, wherein: The side surfaces and upper surface of the waveguide are covered by a first insulating film, The first resin layer is provided on the upper surface of the first insulating film and buries the side surface of the waveguide. The second electrode and the waveguide are located in the first resin layer.

6. The optical modulator according to claim 5, wherein A second insulating film is provided on the upper surface of the first resin layer.

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