Optical Modulator

Abstract

To provide a light modulator capable of reducing optical loss when shifting the phase of light.SOLUTION: A light modulator 10 includes: a p-type Si layer 13; a gate insulating film 14; a gate layer 15 comprising at least an n-type III-V group semiconductor; and a voltage application unit 19 that applies a gate voltage, where the Si layer 13 is as the ground and the gate layer 15 is at a predetermined potential, between the Si layer 13 and the gate layer 15. The voltage application unit 19 shifts the phase of the light propagating through an optical propagation path by changing the applied gate voltage from either a negative voltage value or a positive voltage value to the other.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to an optical modulator that changes the phase of light. [Background technology]

[0002] Patent Document 1 discloses an optical modulator that shifts (i.e., changes) the phase of propagating light by changing the refractive index through voltage application. This optical modulator includes a p-type Si layer, a gate insulating film provided on and in contact with the Si layer, and a gate layer made of an n-type III-V group semiconductor provided on and in contact with the gate insulating film. A gate voltage is applied between the Si layer and the gate layer, with the Si layer at 0 V and the gate layer at a predetermined potential. This causes the optical modulator to change the phase of the propagating light. Patent Document 1 discloses the use of 0 V and negative voltage values ​​as the gate voltage.

[0003] It is also known that in the optical modulator having the above structure, 0V and positive voltage values ​​are used as the gate voltage. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Patent No. 6870813 Summary of the Invention [Problem to be solved by the invention]

[0005] In the optical modulator with the above structure, the optical loss is large when the phase of light is shifted by 2π using a gate voltage between 0V and a negative voltage value, i.e., a gate voltage in the negative direction. Similarly, in the optical modulator with the above structure, the optical loss is large when the phase of light is shifted by 2π using a gate voltage between 0V and a positive voltage value, i.e., a gate voltage in the positive direction. A large optical loss means that the attenuation of the optical intensity of the output light from the optical modulator relative to the optical intensity of the input light to the optical modulator is large. Note that the optical loss is large not only when the phase of light is shifted by 2π, but also when the phase of light is shifted within a range of more than 0 and less than 2π.

[0006] In view of the above, it is an object of the present invention to provide an optical modulator capable of reducing optical loss when shifting the phase of light. [Means for solving the problem]

[0007] In order to achieve the above object, according to the invention described in claim 1, The optical modulator is a p-type Si layer (13) that constitutes a part of the optical propagation path; a gate insulating film (14) provided on and in contact with the Si layer; a gate layer (15) that forms another part of the optical propagation path, is provided on and in contact with the gate insulating film, and is made of at least an n-type III-V group semiconductor; a voltage application unit (19) that applies a gate voltage between the Si layer and the gate layer, where one of the Si layer and the gate layer is at ground and the other of the Si layer and the gate layer is at an arbitrary potential; The voltage application unit can shift the phase of light propagating through the optical propagation path by 2π by changing the gate voltage to be applied from one of a minimum gate voltage, which is a negative voltage value, to the other of a maximum gate voltage, which is a positive voltage value, and can shift the phase of light propagating through the optical propagation path within a range greater than 0 and less than 2π by changing the gate voltage to be applied from one of a first positive voltage value and a second negative voltage value, which are within a range greater than the minimum value and less than the maximum value, to the other.

[0008] This makes it possible to reduce optical loss when shifting the phase of light, compared to when gate voltages are used on only one side, either the negative or positive direction.

[0009] The reference symbols in parentheses attached to each component indicate an example of the correspondence between the component and the specific components described in the embodiments described below. [Brief explanation of the drawings]

[0010] [Figure 1] FIG. 2 is a schematic diagram showing a cross-sectional configuration of the optical modulator according to the first embodiment. [Figure 2] 3 is a schematic diagram showing a cross-sectional configuration of the optical modulator according to the first embodiment, illustrating a state when a gate voltage having a negative voltage value is applied. FIG. [Figure 3] 3 is a schematic diagram showing a cross-sectional configuration of the optical modulator according to the first embodiment, illustrating a state when a gate voltage having a positive voltage value is applied. FIG. [Figure 4] FIG. 10 is a diagram showing the relationship between the gate voltage and the amount of change in refractive index when negative voltage values ​​V-2π and 0 are used as gate voltages for shifting the phase of light by 2π in the optical modulator of Comparative Example 1. [Figure 5] FIG. 10 is a diagram showing the relationship between the gate voltage and the amount of change in optical loss when negative voltage values ​​V-2π and 0 are used as gate voltages for shifting the phase of light by 2π in the optical modulator of Comparative Example 1. [Figure 6] FIG. 10 is a diagram showing the relationship between the gate voltage and the amount of change in refractive index when 0 and a positive voltage value V2π are used as the gate voltage for shifting the phase of light by 2π in the optical modulator of Comparative Example 2. [Figure 7] FIG. 10 is a diagram showing the relationship between the gate voltage and the amount of change in optical loss when 0 and a positive voltage value V2π are used as the gate voltage for shifting the phase of light by 2π in the optical modulator of Comparative Example 2. [Figure 8]FIG. 10 is a diagram showing the relationship between the gate voltage and the amount of change in optical loss when a negative voltage value Vφ0-2π and a positive voltage value Vφ0 are used as the gate voltage for shifting the phase of light by 2π in the optical modulator of the first embodiment. [Figure 9] This figure shows a comparison between the average modulation loss <αEA> when driven using only a positive voltage and the average modulation loss <αCP+EA> when driven using voltages in both positive and negative directions. [Figure 10] FIG. 10 is a diagram showing the relationship between Vg and φ when a gate voltage of a negative voltage value is applied. [Figure 11] FIG. 10 is a diagram showing the relationship between Vg and αφ when a gate voltage of a negative voltage value is applied. [Figure 12] FIG. 10 is a diagram showing the relationship between αφ and φ when a gate voltage of a negative voltage value is applied. [Figure 13] FIG. 10 is a diagram showing the relationship between Vg and φ when a gate voltage of a positive voltage value is applied. [Figure 14] FIG. 10 is a diagram showing the relationship between Vg and αφ when a gate voltage of a positive voltage value is applied. [Figure 15] FIG. 10 is a diagram showing the relationship between αφ and φ when a gate voltage of a positive voltage value is applied. [Figure 16] 10 is a diagram showing the input light intensity I0, the output light intensity Iφ, and the amount of change in optical loss αφ of an optical modulator. [Figure 17] FIG. 10 is a diagram showing the relationship between αφ and φ in Comparative Example 1. [Figure 18] FIG. 10 is a diagram showing the relationship between αφ and φ in Comparative Example 2. [Figure 19] FIG. 10 is a diagram showing the relationship between αφ and φ in the first embodiment. [Figure 20] FIG. 1 is a diagram showing the relationship between <αCP> and απ(CP). [Figure 21] FIG. 12 is a diagram showing formula (12-1). [Figure 22] FIG. 1 is a diagram showing formula (13-1). [Figure 23] FIG. 10 is a diagram showing a specific example of the minimum value of the gate voltage that allows loss reduction. [Figure 24]FIG. 10 is a diagram showing a specific example of the maximum value of the gate voltage at which loss can be reduced. [Figure 25] FIG. 10 is a schematic diagram showing a cross-sectional configuration of an optical modulator according to a second embodiment. [Figure 26] FIG. 10 is a schematic diagram showing a cross-sectional configuration of an optical modulator according to a third embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0011] Hereinafter, embodiments of the present invention will be described with reference to the accompanying drawings. In the following embodiments, parts that are identical or equivalent to each other will be denoted by the same reference numerals.

[0012] (First embodiment) 1, the optical modulator 10 of this embodiment is a MOS type. Specifically, the optical modulator 10 includes a substrate 11, a first clad layer 12, a Si (i.e., silicon) layer 13, a gate insulating film 14, a gate layer 15, a first electrode portion 16, a second electrode portion 17, and a second clad layer 18.

[0013] The substrate 11 is made of Si, a semiconductor material. The first cladding layer 12 is provided on and in contact with the substrate 11. The first cladding layer 12 is made of SiO2 (i.e., silicon oxide), an insulating material. The substrate 11 and the first cladding layer 12 may be made of other materials.

[0014] The Si layer 13 is provided on and in contact with the first cladding layer 12. The Si layer 13 is a p-type semiconductor layer that is mainly made of Si and is doped with p-type conductivity impurities.

[0015] The Si layer 13 has a convex rib 131 and a pair of terraces 132 and 133. The rib 131 extends in the vertical direction of the drawing. The vertical direction of the drawing is the height direction of the rib 131. The pair of terraces 132 and 133 are arranged on both sides of the rib 131 in the width direction of the rib 131, spaced apart from the rib 131. In other words, grooves 134 and 135 are formed between the rib 131 and each of the pair of terraces 132 and 133. The width direction of the rib 131 is the left-right direction of the drawing, and is perpendicular to both the extension direction of the rib 131 and the height direction of the rib 131.

[0016] The height of the rib 131 is, for example, 100 nm to 150 nm. The length of the rib 131 in the width direction is, for example, 400 nm to 1000 nm. The thickness of the portion of the Si layer 13 below the rib 131 (i.e., the thickness of the Si layer 13 from the position of the upper surface of the first cladding layer 12 to the positions of the bottom surfaces of the grooves 134 and 135) is, for example, 70 nm to 120 nm.

[0017] The interiors of the grooves 134, 135 are spaces 136, 137 in which air exists. The spaces 136, 137 are low-refractive-index portions having a refractive index lower than that of the Si layer 13. The interiors of the grooves 134, 135 may be filled with a material having a refractive index lower than that of the Si layer 13, such as SiO2. In this case, the material having a refractive index lower than that of the Si layer 13 is the low-refractive-index portion.

[0018] The gate insulating film 14 is provided on and in contact with the Si layer 13. The gate insulating film 14 is disposed across the rib 131, the spaces 136 and 137, and the pair of terraces 132 and 133. The gate insulating film 14 is in contact with the upper surface of the rib 131 and the upper surfaces of the pair of terraces 132 and 133. The gate insulating film 14 is made of an insulating material, Al2O2 (i.e., aluminum oxide). The gate insulating film 14 may also be made of other materials, such as SiO2 and HfO2 (i.e., hafnium oxide). The thickness of the gate insulating film 14 is, for example, 3 nm to 10 nm.

[0019] The gate layer 15 is made of a single layer of an n-type III-V semiconductor doped with n-type conductivity impurities. The III-V semiconductor is InGaAsP. The III-V semiconductor is not limited to InGaAsP, but may be InP, InGaAs, InAlAs, GaAs, AlGaAs, GaSb, AlGaSb, InSb, InGaSb, etc. The thickness of the gate layer 15 is, for example, 100 nm to 200 nm.

[0020] The first electrode unit 16 is electrically connected to the gate layer 15. Specifically, the first electrode unit 16 is provided on and in contact with the gate layer 15. The first electrode unit 16 is disposed on one of the pair of terraces 132, 133, namely, on one of the terraces 132. That is, the first electrode unit 16 is disposed at a position spaced apart in the width direction of the rib 131 from a position directly above the rib 131. The first electrode unit 16 is made of a metal material. Although not shown, the first electrode unit 16 may include a contact unit that forms ohmic contact with the gate layer 15 and a metal unit that is electrically connected to the contact unit and is made of a metal material.

[0021] The second electrode portion 17 is electrically connected to the Si layer 13. Specifically, the second electrode portion 17 is provided on and in contact with the Si layer 13. The second electrode portion 17 is disposed on the other terrace 133 of the pair of terraces 132, 133. That is, the second electrode portion 17 is disposed on the opposite side of the rib 131 from the first electrode portion 16 in the width direction of the rib 131, at a position away from the position directly above the rib 131. The second electrode portion 17 is made of a metal material. Although not shown, the second electrode portion 17 may include a contact portion that forms ohmic contact with the Si layer and a metal portion that is electrically connected to the contact portion and is made of a metal material.

[0022] The second cladding layer 18 is provided on and in contact with the gate layer 15. The second cladding layer 18 covers the gate layer 15 while leaving the first electrode portion 16 and the second electrode portion 17 exposed. The second cladding layer 18 is made of SiO2, which is an insulating material. The second cladding layer 18 may also be made of other materials.

[0023] The region indicated by the dashed circle in Fig. 1 is the optical propagation path. As shown in Fig. 1, the rib 131 of the Si layer 13 constitutes one part of the optical propagation path. The portion of the gate layer 15 located above the rib 131 with the gate insulating film 14 interposed therebetween constitutes another part of the optical propagation path. The length of the Si layer 13 and the gate layer 15 in the direction perpendicular to the drawing is the length of the optical propagation path, i.e., the length of the optical modulator 10.

[0024] The optical modulator 10 includes a voltage application unit 19 that applies a gate voltage (i.e., a drive voltage). The voltage application unit 19 is electrically connected to each of the first electrode unit 16 and the second electrode unit 17. The voltage application unit 19 applies the gate voltage between the first electrode unit 16 and the second electrode unit 17. That is, the voltage application unit 19 applies a gate voltage between the gate layer 15 and the Si layer 13, where the Si layer 13 is at GND (i.e., ground) and the gate layer 15 is at an arbitrary potential. The Si layer 13 is set to a reference potential, i.e., 0V.

[0025] A positive and negative power supply is used as the voltage application unit 19. The voltage application unit 19 applies a gate voltage with a minimum value V g_min and the maximum value V g_max The minimum value of the gate voltage is a negative voltage value. The maximum value of the gate voltage is a positive voltage value.

[0026] The voltage application unit 19 applies a gate voltage to change the refractive index of light propagating through the optical propagation path, thereby changing the phase of the light propagating through the optical propagation path. The voltage application unit 19 shifts the phase of the light propagating through the optical propagation path by 2π by changing the gate voltage it applies from one of a minimum value and a maximum value to the other. At this time, the minimum value and maximum value of the gate voltage are set to magnitudes that can shift the phase of the light propagating through the optical propagation path by 2π. Furthermore, the voltage application unit 19 shifts the phase of the light propagating through the optical propagation path within a range greater than 0 and less than 2π by changing the gate voltage it applies from one of a positive first voltage value and a negative second voltage value that are within a range greater than the minimum value and less than the maximum value to the other.

[0027] FIG. 2 shows the case where a negative gate voltage is applied, i.e., forward bias driving, in which the p-type Si layer 13 has a higher potential than the n-type gate layer 15. In this case, electrons C1 as carriers accumulate in the region of the gate layer 15 on the gate insulating film 14 side. Also, holes C2 as carriers accumulate in the region of the Si layer 13 on the gate insulating film 14 side. The interaction between these carriers and light, i.e., the carrier plasma effect, changes the refractive index of the light propagation path. The change in refractive index changes the phase of the light. Hereinafter, the carrier plasma effect will also be referred to as CP.

[0028] FIG. 3 shows the case where a positive gate voltage is applied, i.e., the case of reverse bias driving where the p-type Si layer 13 has a lower potential than the n-type gate layer 15. In this case, an electric field is generated between the gate layer 15 and the Si layer 13, with the gate insulating film 14 sandwiched between them. The refractive index of the light propagation path changes due to the electroabsorption effect. Hereinafter, the electroabsorption effect will also be referred to as EA.

[0029] Here, this embodiment will be compared with Comparative Examples 1 and 2. Unlike this embodiment, Comparative Example 1 uses a gate voltage when shifting the phase of light by 2π, which is the minimum negative voltage V -2π A negative voltage between V and the maximum voltage of 0V produces a phase change from -2π to 0.-2π is the voltage value for generating a phase change of −2π. The other configurations of the optical modulator of Comparative Example 1 are the same as those of this embodiment.

[0030] As shown in Figure 4, the gate voltage V g When a gate voltage V is applied, g As the absolute value of increases, that is, as the gate voltage V g As the voltage increases, the refractive index change amount n φ The horizontal axis of Figure 4 is the gate voltage V g The vertical axis of Fig. 4 shows the refractive index change amount n required when the phase changes from 0 to φ. φ Shows.

[0031] As shown in Figure 5, the gate voltage V g When a gate voltage V is applied, g The higher the voltage, the greater the optical loss change α φ The horizontal axis of Figure 5 is the gate voltage V g The vertical axis of FIG. 5 represents the amount of change in optical loss when the phase is changed, α φ Therefore, in Comparative Example 1, the average modulation loss when the phase of the light is shifted by 2π is CP 〉 is large.

[0032] In Comparative Example 2, unlike this embodiment, the gate voltage when shifting the phase of light by 2π is 0V and the maximum positive voltage value V 2π A positive voltage between V and V produces a phase change of 0 to 2π. 2π is the voltage value for generating a phase change of 2π. The other configurations of the optical modulator of Comparative Example 2 are the same as those of this embodiment.

[0033] As shown in Figure 6, the gate voltage V g When a gate voltage V is applied, g As the absolute value of increases, that is, as the voltage increases, the refractive index change amount n φ The horizontal axis of Figure 6 is the gate voltage V g The vertical axis of FIG. 6 represents the refractive index change amount nφ Shows.

[0034] As shown in Figure 7, the gate voltage V g When a gate voltage V is applied, g The higher the voltage, the greater the optical loss change amount α φ The horizontal axis of Figure 7 is the gate voltage V g The vertical axis of FIG. 7 represents the amount of change in optical loss α φ Therefore, in Comparative Example 2, the average modulation loss when the phase of the light is shifted by 2π is EA 〉 is large.

[0035] In contrast, in this embodiment, the gate voltage V g As a result, the negative voltage value V in Figure 8 φ0-2π and a positive voltage value V φ0 By using the above, a phase change of φ0 occurs from (φ0-2π). Here, φ0 is the offset amount from the phase change of 2π in Comparative Example 1. According to this, the phase change in the high voltage region in Comparative Examples 1 and 2 is compensated for in the low voltage region in the opposite direction, and the average modulation loss when the phase of the light is shifted by 2π is reduced to 〈α CP+EA 〉 is the average modulation loss 〈α CP 〉 and the average modulation loss 〈α EA 〉 can be made smaller.

[0036] In this way, driving using both positive and negative voltages has the effect of reducing optical loss more than driving using only one of the positive and negative voltages. By using a first positive voltage value and a second negative voltage value as gate voltages that are greater than the minimum value and less than the maximum value of the gate voltage, the same effect can be obtained even when the phase of light is shifted within a range greater than 0 and less than 2π.

[0037] The average modulation loss <α> of this embodiment when the phase of light is shifted by 2π is expressed by the following equation.

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[0038] As shown in Figure 9, even when driving with both positive and negative voltages, the average modulation loss <α CP+EA 〉 is the average modulation loss when driven using only positive voltage〈α EA Although not shown, even when driving using voltages in both positive and negative directions, the average modulation loss <α CP+EA 〉 is the average modulation loss when driven using only negative voltages〈α CP 〉 may be larger than

[0039] Even when driving with both positive and negative voltages, the loss function α φ The offset phase φ0, which can reduce losses compared to driving using only one voltage in the positive and negative directions, changes depending on the phase of the light. Therefore, the minimum value of the gate voltage when shifting the phase of the light by 2π is V g_min The maximum value is V g_max Then, it is preferable that each of them satisfies the following conditions.

[0040] The gate voltage required to shift the phase of light by π is V π and the modulation efficiency when the length of the optical propagation path is L is V π L. The modulation efficiency when the gate voltage is positive is (V π L) EA When the gate voltage is set to a positive value and the optical phase is shifted by π, the loss change is α π(EA) The modulation efficiency when the gate voltage is negative (V π L) CP When the gate voltage is set to a negative value and the phase of the light is shifted by π, the loss change is α π(CP) is.

[0041] α π(EA) >α π(CP) When V g_min is a value that satisfies equation (1a). g_max is a value that satisfies equation (1b).

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[0042] α π(EA) <α π(CP) When V g_min is a value that satisfies equation (2a). g_max is a value that satisfies equation (2b).

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[0043] In addition, (V π L) EA , α π(EA) , (V π L) CP , α π(CP) The values ​​of the above are measured in advance or calculated by numerical calculation in the optical modulator having the structure shown in FIG.

[0044] Each of the formulas (1a), (1b), (2a), and (2b) is derived through the following steps 1 to 5.

[0045] In STEP 1, φ and α φ The function of forward bias driving, i.e., V g When Vg<0, light is modulated by the carrier plasma effect (i.e., CP). The relationship between Vg and φ at this time is shown in FIG. 10, as disclosed in Non-Patent Document 1. The relationship between Vg and φ at this time is expressed by the following equations (3-6) and (3-7) based on the following equations (3-1) to (3-5). ·Non-patent document 1: [1] JH. Han, F. Boeuf, J. Fujikata, S. Takahashi, S. Takagi, and M. Takenaka, Nat. Photon. 11, 486-490 (2017)

[0046]

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[0047] q=CV In this equation, q is the amount of charge, C is the capacitance of the gate insulating film, and V is the gate voltage. From this equation,

[0048]

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[0049] V g Vg and α when <0 φ The relationship between these is shown in FIG. 11 as disclosed in Non-Patent Document 2, and is expressed by the following equation (4-4) from the following equations (4-1) to (4-3). ·Non-patent document 2: Q. Li, C. Ho, S. Takagi, and M. Takenaka, “Optical Phase Modulators Based on Reverse-Biased III-V / Si Hybrid Metal-Oxide-Semiconductor Capacitors,” IEEE photon. Tech. let., 32(6) (2020)

[0050]

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[0051] From q=CV,

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[0052] V g α when <0 φ The relationship between and φ is shown in FIG. 12, and is expressed by the following equation (5) from equations (3-6), (3-7) and (4-4).

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[0053] Reverse bias drive, i.e., V g When Vg is greater than 0, light is modulated by the electroabsorption effect (i.e., EA). The relationship between Vg and φ at this time is shown in FIG. 13, as disclosed in Non-Patent Document 3. The relationship between Vg and φ at this time is expressed by the following equations (6-6) and (6-7) based on the following equations (6-1) to (6-5). ·Non-patent document 3: A. Alping and LA Coldren, “Electrorefraction in GaAs and InGaAsP and its application to phase modulators,” J. Appl. Phys., vol. 61, no. 7, pp. 2430-2433, Apr. 1987.

[0054]

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[0055] V g Vg and α when >0 φ The relationship between these is shown in FIG. 14 as disclosed in Non-Patent Document 4, and is expressed by the following equation (7-4) based on the following equations (7-1) to (7-3). ·Non-patent document 4: [4] W.

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[0056] V g α when >0 φ The relationship between and φ is shown in FIG. 15, and is expressed by the following equation (8) from equations (6-6), (6-7) and (7-4).

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[0057] In STEP 2, the average optical loss value <α φ First, the optical loss change amount α φ [dB] is the input / output intensity ratio (I φ / I0).

[0058] As shown in FIG. 16, the input light intensity of the optical modulator 10 is I0, and the output light intensity of the optical modulator 10 is I φ and the change in optical loss of the optical modulator 10 is α φ [dB]. In this case, α φ [dB] and (I φ / I0) is shown by the following formula:

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[0059] 17, in Comparative Example 1 in which the phase of light is changed by 2π only by CP, the phase range is −2π to 0. The input / output intensity ratio at this time is expressed by equation (9-1) from equation (5).

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[0060] 18, in Comparative Example 2 in which the phase of light is changed by 2π only by EA, the phase range is from 0 to 2π. The input / output intensity ratio at this time is expressed by Equation (10-1) from Equation (8).

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[0061] 19, in this embodiment, where the phase of light is changed by 2π using both CP and EA, the phase range is from (φ0-2π) to φ0. The input / output intensity ratio at this time is expressed by equation (11-1) from equations (5) and (8).

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[0062] Next, the average value of the input / output intensity ratio is calculated.

[0063] Here, the general formula for the average value 〈y〉 of y=f(x) is given by the following formula:

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[0064] The average value of the input / output intensity ratio in Comparative Example 1 shown in FIG. 17 is expressed by equation (9-2). CP / I0〉 is the average input / output intensity ratio when the phase of the light is changed from -2π to 0 at the CP.

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[0065] The average value of the input / output intensity ratio in Comparative Example 2 shown in FIG. 18 is expressed by the formula (10-2). EA / I0〉 is the average input / output intensity ratio when the phase of the light is changed from 0 to 2π in the EA.

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[0066] The average value of the input / output intensity ratio in this embodiment shown in FIG. 19 is expressed by equation (11-2). CP+EA (φ0) / I0〉 is the average input / output intensity ratio when the phase of the light is changed from 0 to φ0 in the EA and from (φ0-2π) to 0 in the CP.

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[0067] The average input / output intensity ratio is then converted into an average optical loss value.

[0068] The average optical loss in Comparative Example 1 shown in FIG. 17 is expressed by equation (9-3): CP 〉 is the average modulation loss of the CP.

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[0069] The average optical loss in the comparative example shown in FIG. 18 is expressed by equation (10-3): EA〉 is the average modulation loss of the EA.

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[0070] The average optical loss in this embodiment shown in FIG. 19 is expressed by equation (11-3). CP+EA (φ0)〉 is the average value of the modulation loss when the phase of the light is changed from 0 to φ0 in the EA and when the phase of the light is changed from (φ0-2π) to 0 in the CP.

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[0071] The loss level in the optical modulator of this embodiment is 1 dB or less. π(CP) = 0.23 dB, then 〈α CP 〉=0.228 dB. Therefore, as shown in FIG. CP 〉?α π(CP) Similarly, α π(EA) = 0.6 dB, then 〈α EA 〉=0.586dB. Therefore, 〈α EA 〉?α π(EA) It can be thought of as such.

[0072] In STEP 3, a conditional expression for φ0 that can reduce losses is derived.

[0073] (Conditional Expression 1) Condition 1 is: <α EA 〉>〈α CP 〉, that is, 〈α CP 〉>〈α CP+EA (φ0)〉, which is a comparison with the carrier plasma effect alone. CP 〉<〈I CP+EA (φ0)〉, the following equation (12-1) is derived.

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[0074] Equation (12-1) is shown in FIG. 21. As shown in FIG. 21, one of the solutions for D=0 is φ0=0. The φ0 that can reduce the loss is in the range where D is smaller than 0 in the graph shown in FIG. 21. Therefore, <α EA 〉>〈α CP >, φ0 that can reduce the loss is shown in the following equation (12-2).

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[0075] (Conditional expression 2) Condition 2 is: <α CP 〉>〈α EA 〉, and 〈α EA 〉>〈α CP+EA (φ0)〉, so this is a comparison with the electroabsorption effect alone. <I EA 〉<〈I CP+EA (φ0)〉, the following equation (13-1) is derived.

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[0076] Equation (13-1) is shown in FIG. 22. As shown in FIG. 22, one of the solutions for D=0 is φ0=2π. The φ0 that can reduce the loss is in the range where D is smaller than 0 in the graph shown in FIG. 22. Therefore, <α CP 〉>〈α EA >, φ0 that can reduce losses is shown in the following equation (13-2).

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[0077] In STEP 4, the phase range and voltage range in which the loss can be reduced are derived.

[0078] The minimum value of the phase range for changing the phase by 2π is φ min The maximum value is φ max Then, φ min =(φ0-2π), φmax =φ0, where φ min <0, φ max > 0. Therefore, using equations (12-2) and (13-2), it can be expressed as follows:

[0079] (Conditional Expression 1) 〈α EA 〉>〈α CP φ in the case of min Conditions

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[0080] (Conditional expression 2) 〈α CP 〉>〈α EA φ in the case of min Conditions

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[0081] Next, the phase change φ min , φ max V corresponding to g ,V g_min , V g_max Then, by substituting equation (3-7) when φ<0 and equation (6-7) when φ>0 into equations (14-1), (14-2), (15-1), and (15-2), equations (1a), (1b), (2a), and (2b) are derived.

[0082] As stated in paragraph 0072, EA〉 is the loss change amount α when the phase is shifted by π π(EA) It is almost equal to 〈α CP 〉 is the loss change amount α when the phase is shifted by π π(CP) Therefore, the conditional expression 1, EA 〉>〈α CP 〉, then α π(EA) >α π(CP) It can be considered that the condition is the same as the case of CP 〉>〈α EA 〉, then α π(CP) >α π(EA) It can be considered the same as in the case of

[0083] where V g_min and V g_max According to the above Non-Patent Document 1, the loss change amount and modulation efficiency when the gate voltage is negative are π(CP) =0.23dB, V π L (CP) =0.047Vcm. According to the above Non-Patent Document 2, the loss change amount and modulation efficiency when the gate voltage is positive are π(EA) =0.6dB, V π L (EA) = 0.4Vcm. Also, assume that the length of the optical modulator is L = 500 μm. In this case, α π(EA) >α π(CP) Therefore, by solving equation (1a) and equation (1b), V g_min and V g_max is calculated.

[0084] The left side of equation (1a) is D 1a D 1a is expressed by the following formula:

number

[0085] D 1a The graph of is shown in Figure 23. In Figure 23, D 1a The Vg range that satisfies >0 is -1.9V <Vg<-0.8Vである。

[0086] The left side of equation (1b) is D 1b D 1b is expressed by the following formula:

number

[0087] D 1b The graph of is shown in Figure 24. In Figure 24, D 1b The Vg range that satisfies >0 is 0V <Vg<8.5Vである。

[0088] Therefore, the minimum gate voltage V when shifting the phase by 2π can reduce losses. g_min and the maximum value V g_max is -1.9V <V g_min <-0.8V, 0V <V g_max <8.5V.

[0089] (Second embodiment) As shown in FIG. 25 , in this embodiment, unlike the first embodiment, the gate layer 15 is composed of multiple layers of n-type III-V group semiconductors. Specifically, the gate layer 15 has a stacked structure of one quantum well (i.e., QW) layer 151 and one barrier layer 152. QW is an abbreviation for quantum well. The quantum well layer 151 is provided on and in contact with the gate insulating film 14. The barrier layer 152 has a function of confining carriers in the quantum well layer 151. The quantum well layer 151 has a smaller band gap energy than the barrier layer 152.

[0090] The quantum well layer 151 is made of n-type InGaAsP. The barrier layer 152 is made of n-type InP. The quantum well layer 151 has a thickness of 20 nm or less. The barrier layer 152 has a thickness of more than 20 nm.

[0091] As described above, the gate layer 15 has the quantum well layer 151 in contact with the gate insulating film 14. As a result, the bandgap wavelength of InGaAsP approaches the wavelength of light, and the effective mass decreases, enhancing the carrier plasma effect when a negative gate voltage is applied. This reduces optical loss when a negative gate voltage is applied.

[0092] Other configurations of the optical modulator 10 are the same as those of the first embodiment. In this embodiment, too, the same effects as those of the first embodiment can be obtained by driving using voltages in both positive and negative directions.

[0093] (Third embodiment) As shown in FIG. 26, in this embodiment, unlike the first embodiment, the gate layer 15 is made up of multiple layers of n-type III-V group semiconductors.

[0094] Specifically, the gate layer 15 has a multiple quantum well (i.e., MQW) structure including a quantum well layer 153a in contact with the gate insulating film 14. MQW is an abbreviation for multi quantum well. The MQW structure is a structure in which multiple quantum well layers 153 and multiple barrier layers 154 are alternately stacked. In other words, the gate layer 15 has a lowermost quantum well layer 153a in contact with the gate insulating film 14, an uppermost barrier layer 154a farthest from the gate insulating film 14, and multiple intermediate quantum well layers 153b and multiple intermediate barrier layers 154b located between the lowermost quantum well layer 153a and the uppermost barrier layer 154a.

[0095] Each of the multiple quantum well layers 153 has a thickness of 20 nm or less. Each of the multiple intermediate barrier layers 154b has a thickness of 20 nm or less. The uppermost barrier layer 154a has a thickness greater than 20 nm. Each of the multiple quantum well layers 153 is made of n-type InGaAsP. Each of the multiple barrier layers 154 is made of n-type InP. The other configurations of the optical modulator 10 of this embodiment are the same as those of the first embodiment.

[0096] Here, specific examples of the composition, thickness, and doping concentration of each layer constituting the optical modulator 10 of this embodiment are shown in Table 1. The number of quantum well layers 153b among the multiple intermediate layers is 10. The number of barrier layers 154b among the multiple intermediate layers is 10.

[0097] [Table 1] As shown in Table 1, the composition of each quantum well layer 153a, 153b is n-In 0.66 Ga 0.34 As 0.73 P 0.27 This is because x and y in the composition formula below are set to satisfy the following condition so as to achieve lattice matching with InP. In 1-x Ga x As y P 1-y The lattice matching conditions with InP are as follows: x=0.1896y / (0.4176-0.0125y) The relationship with the target bandgap wavelength is as follows: Eg=1.35-0.72y+0.12y 2 [eV] =1.24 / λ g =1.24 / 1.40μm

[0098] Each quantum well layer 153 of the gate layer 15 may be made of a material other than n-type InGaAsP as long as it is an n-type III-V semiconductor. Each barrier layer 154 of the gate layer 15 may be made of a material other than n-type InP as long as it is an n-type III-V semiconductor.

[0099] This embodiment also achieves the same effects as the first embodiment by using voltages in both positive and negative directions.

[0100] Furthermore, the optical modulator 10 of this embodiment is not limited to being driven using both positive and negative voltages. It may also be driven using only one of the positive and negative voltages, as in Comparative Examples 1 and 2 described in the first embodiment. In this case, according to this embodiment, the gate layer 15 has a quantum well layer 153a in contact with the gate insulating film 14. Therefore, the bandgap wavelength of InGaAsP approaches the wavelength of light, reducing the effective mass and enhancing the carrier plasma effect when a negative gate voltage is applied. This allows for greater loss reduction when driven using a negative gate voltage of 0 or less, compared to when the gate layer 15 is a single layer. Furthermore, according to this embodiment, the gate layer 15 has a multiple quantum well structure. Therefore, when a positive gate voltage is applied, the modulation efficiency is higher than when the gate layer 15 is a single layer, under the same gate voltage conditions. This allows for greater optical loss reduction when driven using a positive gate voltage of 0 or more, compared to when the gate layer 15 is a single layer.

[0101] (Other embodiments) (1) In the first embodiment, the gate layer 15 is composed of a single layer of an n-type III-V semiconductor. However, the gate layer 15 may be composed of multiple layers, each with a thickness greater than 20 nm. For example, the gate layer 15 is composed of two layers: a first layer composed of n-type InGaAsP and a second layer composed of n-type InP. The thickness of the first layer is 50 to 120 nm. The thickness of the second layer is 50 to 80 nm. Furthermore, the gate layer 15 only needs to be composed of at least an n-type III-V semiconductor, and may also include portions composed of other materials.

[0102] (2) In each of the above-described embodiments, the voltage application unit 19 applies a gate voltage in which the Si layer 13 is at GND and the gate layer 15 is at an arbitrary potential. However, the voltage application unit 19 may also apply a gate voltage in which the gate layer 15 is at GND and the Si layer 13 is at an arbitrary potential. In short, the voltage application unit 19 applies a gate voltage in which one of the Si layer 13 and the gate layer 15 is at GND and the other of the Si layer 13 and the gate layer 15 is at an arbitrary potential.

[0103] (3) The present invention is not limited to the above-described embodiments and can be modified as appropriate within the scope of the claims, including various modifications and modifications within the scope of equivalents. Furthermore, the above-described embodiments are not unrelated to each other and can be combined as appropriate unless the combination is clearly impossible. It goes without saying that the elements constituting the embodiments are not necessarily essential unless specifically stated as essential or clearly considered essential in principle. Furthermore, when the numbers, values, amounts, ranges, etc. of the components of the embodiments are mentioned in the above-described embodiments, they are not limited to the specific numbers unless specifically stated as essential or clearly limited to a specific number in principle. Furthermore, when the materials, shapes, positional relationships, etc. of the components are mentioned in the above-described embodiments, they are not limited to the specific materials, shapes, positional relationships, etc., unless specifically stated or clearly limited to a specific material, shape, positional relationship, etc. in principle. [Explanation of symbols]

[0104] 13 Si layer 14 Gate insulating film 15 Gate Layer 19 Voltage application section

Claims

1. An optical modulator that changes the phase of light, a p-type Si layer (13) that constitutes a part of the optical propagation path; a gate insulating film (14) provided on and in contact with the Si layer; a gate layer (15) that constitutes another part of the optical propagation path, is provided on and in contact with the gate insulating film, and is made of at least an n-type III-V group semiconductor; a voltage application unit (19) that applies a gate voltage between the Si layer and the gate layer, where one of the Si layer and the gate layer is ground and the other of the Si layer and the gate layer is at an arbitrary potential; The voltage application unit is capable of shifting the phase of light propagating through the optical propagation path by 2π by changing the gate voltage to be applied from one of a minimum value of the gate voltage, which is a negative voltage value, to the other of a maximum value of the gate voltage, which is a positive voltage value, and is capable of shifting the phase of light propagating through the optical propagation path within a range of greater than 0 and less than 2π by changing the gate voltage to be applied from one of a first positive voltage value and a second negative voltage value, which are within a range of greater than the minimum value and less than the maximum value, to the other.

2. The value of the gate voltage required to shift the phase of light by π is V π and the modulation efficiency when the length of the optical propagation path is L is V π L, The modulation efficiency when the gate voltage is positive is (V π L) ＥＡ The gate voltage is set to a positive value, and the loss change amount when the phase of the light is shifted by π is α π（ＥＡ） and The modulation efficiency when the gate voltage is negative is (V π L) ＣＰ The gate voltage is set to a negative value, and the loss change amount when the phase of the light is shifted by π is α π（ＣＰ） and The minimum value is V ｇ＿ｍｉｎ and the maximum value is V ｇ＿ｍａｘ If α π（ＥＡ） >α π（ＣＰ） When V ｇ＿ｍｉｎ is a value that satisfies the formula (1a), and V ｇ＿ｍａｘ is a value that satisfies formula (1b), [Equation 1] [Equation 2] α π（ＥＡ） <α π（ＣＰ） When V ｇ＿ｍｉｎ is a value that satisfies equation (2a), and V ｇ＿ｍａｘ 2. The optical modulator according to claim 1, wherein is a value that satisfies formula (2b). [Equation 3] [Equation 4]

3. 3. The optical modulator according to claim 1, wherein the gate layer has a multiple quantum well structure including a quantum well layer (153a) in contact with the gate insulating film.

4. An optical modulator, a p-type Si layer (13) that constitutes a part of the optical propagation path; a gate insulating film (14) provided on and in contact with the Si layer; a gate layer (15) that constitutes another part of the optical propagation path, is provided on and in contact with the gate insulating film, and is made of at least an n-type III-V group semiconductor; a voltage application unit (19) that applies a gate voltage between the Si layer and the gate layer, The optical modulator, wherein the gate layer has a multiple quantum well structure including a quantum well layer (153a) in contact with the gate insulating film.

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