Optical Modulator

Abstract

In order to efficiently operate an optical fiber communication network, the function of transmission / reception of control signals is essential in addition to the function of transmission of large-capacity data. Methods for the transmission / reception of control signals include a method in which a control signal is superimposed, on signal light carrying a data signal, at a low frequency that has no effect on the signal light. However, an increase in power consumption associated with heating during modulation of the signal light and a pattern effect due to the required long time for cooling have been problems. In order to solve these problems, the present disclosure proposes: an optical modulator comprising a planar lightwave circuit (PLC) structure which is provided with a Mach-Zehnder interferometer (MZI) that is formed on a substrate and a cladding that is formed so as to cover the periphery of the MZI, and said optical modulator further comprising a heat insulation groove structure which is formed in the vicinity of at least one side surface of a waveguide core, wherein the depth of the heat insulation groove structure is within the range of the thickness of the waveguide core; and a method for driving the optical modulator.

Description

[Technical field]

[0001] The present disclosure relates to an optical modulator, and more specifically, to an optical modulator that is a waveguide-type optical device that superimposes information on light, such as that used in optical communication networks, and that can suppress both power consumption and pattern effects. [Background technology]

[0002] With the spread of the Internet, the demand for data communication networks has exploded, and the importance of optical fiber communication networks for transmitting large amounts of data has increased. In order to operate an optical fiber communication network efficiently, in addition to the function of transmitting large amounts of data, the function of transmitting and receiving control signals is essential. The transmission and reception of this control signal can be performed by providing a communication network for control signals separately from the optical fiber communication network for data transmission, or by using a part of the bandwidth of the optical fiber communication network for data transmission to transmit and receive control signals. In addition, there is a method of superimposing and transmitting a control signal at a low frequency that does not affect the signal light carrying the data signal. This method is advantageous in terms of cost and bandwidth utilization efficiency because it does not require a dedicated communication network and the bandwidth for data transmission is not consumed by the transmission of the control signal. And, as a simple method for generating a low-frequency control signal that does not affect such signal light for data transmission, a method of further modulating the signal light carrying the data signal using a low-speed optical modulator is known. Hereinafter, this low-speed optical modulator and its driving method will be described.

[0003] FIG. 1 is a diagram illustrating the configuration of a low-speed optical modulator 10 according to the prior art. The optical modulator 10 has a two-input, two-output (hereinafter referred to as 2×2) Mach-Zehnder interferometer (hereinafter referred to as MZI) type circuit structure. Since the operating speed of the optical modulator 10 can be low, for example, a planar lightwave circuit (hereinafter referred to as PLC) using the thermo-optic effect can be applied to the circuit constituting the optical modulator 10. As an example, the optical modulator 10 shown in FIG. 1 is a quartz-based PLC formed by depositing quartz glass on a silicon (Si) substrate.

[0004] The optical modulator 10 includes an optical splitter 11 that splits an input signal light, an optical coupler 12 that multiplexes the signal lights split by the optical splitter 11, causes interference, and outputs the multiplexed signal light, a waveguide core 13 that connects the optical splitter 11 and the optical coupler 12, a heater 14 installed directly above the waveguide core 13, an input port 15 installed on the input side of the signal light in the optical splitter 11, and an output port 16 installed on the output side of the signal light in the optical coupler 12. The waveguide core 13 includes a first waveguide core 13a and a second waveguide core 13b. The heater 14 includes a first heater 14a installed on the first waveguide core 13a, and a second heater 14b installed on the second waveguide core 13b. Furthermore, the input port 15 includes a first input port 15a and a second input port 15b, and the output port 16 includes a first output port 16a and a second output port 16b. As described above, the optical modulator 10 is a silica-based PLC, and the materials of the waveguide core 13 and the clad in which the waveguide core 13 is embedded are both silica-based materials, and the refractive index of the waveguide is increased so that the signal light propagates through the waveguide.

[0005] Here, the signal light is input from the first input port 15a, and the first and second waveguide cores 13a and 13b have the same length. When no power is supplied to the heater 14, the signal light is designed to be output from the second output port 16b. In such an optical modulator 10, when a voltage is applied to at least one of the first heater 14a or the second heater 14b, the heater 14 generates heat and the waveguide core 13 is heated. Then, the refractive index of the waveguide core 13 changes due to the thermo-optic effect, and the optical path length of the waveguide core 13 changes accordingly, so that the intensity of the signal light output from the second output port 16b decreases. When the amount of change in the optical path length reaches λ / 2 with respect to the wavelength λ of the signal light, the intensity of the signal light output from the second output port 16b becomes 0. Alternatively, the first waveguide core 13a and the second waveguide core 13b may be designed to have an optical path length difference of λ / 2 in advance, so that when power is not supplied to the heater 14, the signal light is output from the first output port 16a. Even in this case, the refractive index of the waveguide core 13 is changed by the thermo-optic effect, and the intensity of the signal light output from the first output port 16a is reduced. In this way, the signal light is modulated by changing the optical path length of the waveguide core 13.

[0006] 2 is a diagram showing the relationship between time and the voltage applied to the heater 14 (applied voltage waveform) when the optical modulator 10 is driven. FIG. 2(a) shows the voltage waveform applied to the first heater 14a, and FIG. 2(b) shows the voltage waveform applied to the second heater 14b. In order to modulate the signal light, it is necessary to change the optical path length difference once in the waveguide core 13, maintain a constant optical path length difference thereafter, and return the optical path length difference to its original state. Here, the time to change the optical path length difference is T1, the applied voltage at T1 is V1, the voltage required to maintain a constant value after the optical path length difference has changed is V2, the time to return the optical path length difference is T2, and the applied voltage at T2 is V3.

[0007] FIG. 3 is a flowchart showing a method for driving the optical modulator 10 according to the prior art. The method for driving the optical modulator 10 for modulating the signal light includes step 31 of applying a voltage V1 sufficiently larger than V2 to the first heater 14a for a time T1 at T1, step 32 of repeatedly applying a pulsed voltage with a period T sufficiently shorter than the thermal response time constant of the first waveguide core 13a, and step 33 of applying a voltage V3 to the second heater 14b for a time T2 while setting the applied voltage to the first heater 14a to 0. The reason for applying a voltage V1 higher than V2 in step 31 is to instantaneously apply a large amount of heat to the first waveguide core 13a, thereby allowing the first waveguide core 13a to reach a temperature corresponding to a predetermined optical path length difference in a short time. The pulsed voltage in step 32 has a pulse waveform whose time average value corresponds to the voltage V2, and its duty is (V2 / V1). 2 By applying a voltage of such a pulse waveform, heating is performed with the same amount of work as when V2 is applied at a constant voltage, so a predetermined optical path length is maintained. Furthermore, in step 33, a voltage V3 higher than V2 is applied for a time T2. This is intended to equalize the amount of change in the optical path length of the first waveguide core 13a and the second waveguide core 13b in a short time, and the optical path length difference can be restored in a shorter time than cooling by natural heat dissipation alone (for example, see Non-Patent Document 1). Thereafter, the state in which the optical output level is restored by natural heat dissipation while maintaining the amount of change in the optical path length of the first waveguide core 13a and the second waveguide core 13b equal is maintained.

[0008] By modulating the signal light in this way, it is possible to generate a low-frequency control signal that does not affect the signal light for data transmission. However, in a quartz-based PLC that does not incorporate any special features, most of the heat supplied from the heater 14 is diffused in the in-plane direction and does not heat the waveguide core 13, which causes an issue of increased power consumption. As a technique for solving this issue, a technique for introducing a heat insulating groove structure has been proposed (for example, see Non-Patent Document 2).

[0009] FIG. 4 is a diagram showing the configuration of an optical modulator 40 having a heat insulating groove structure 41 according to the prior art. FIG. 4(a) shows a top view of the optical modulator 40, and FIG. 4(b) shows a cross-sectional view of the optical modulator 40 taken along the IVb-IVb cross-sectional line in FIG. 4(a) from the input side of the signal light. The basic circuit configuration of the optical modulator 40 is the same as that of the optical modulator 10 shown in FIG. 1. However, the optical modulator 40 differs from the optical modulator 10 shown in FIG. 1 in that it has a heat insulating groove structure 41 near both side surfaces of the waveguide core 13. As shown in FIG. 4(b), this heat insulating groove structure 41 has a depth that reaches from the surface of the cladding 42 to the surface of the substrate 43. With this configuration, the heat supplied from the heater 14 is prevented from propagating in the in-plane direction (horizontal to the upper surface of the substrate 43) by the heat insulating groove structure 41, and is guided in the depth direction (perpendicular to the upper surface of the substrate 43). This is because the thermal conductivity of air is lower than that of quartz glass. Therefore, the waveguide core 13 can be efficiently heated with less power, and the amount of power consumed by the heater 14 can be reduced.

[0010] However, the optical modulator 40 having the heat insulating groove structure 41 has a problem that it takes a long time to cool the waveguide core 13 in step 33. This is because the heat insulating groove structure 41 blocks the heat dissipation path in the in-plane direction, limiting the heat dissipation path to only the thickness direction. As a result, for example, when the optical modulator 40 generates an alternating signal of "0" and "1", a phenomenon may occur in which the waveguide core 13 is heated again before the temperature drops completely. This causes a problem that a phenomenon called a pattern effect occurs, in which the waveform changes depending on the bit string generated, and the generated waveform is disturbed. [Prior art documents] [Non-patent literature]

[0011] [Non-Patent Document 1] H. Matsuura et al., “Accelerating switching speed of thermo-optic MZI silicon-photonic switches with “turbo pulse” in PWM control,”2017 Optical Fiber Communications Conference and Exhibition. [Non-Patent Document 2] S. Sohma, T. Goh, H. Okazaki, M. Okuno, and A. Sugita, "Low switching power silica-based super high delta thermo-optic switch with heat insulating grooves," Electron. Lett., Vol. 38, No. 3, pp. 127-128, 2002. Summary of the Invention

[0012] The present disclosure has been made in consideration of the above-mentioned problems, and an object of the present disclosure is to provide an optical modulator formed from a silica-based PLC, which can achieve both reduced power consumption during operation and a reduced cooling time for the waveguide core.

[0013] In response to the above-mentioned problems, the present disclosure provides an optical modulator having a planar lightwave circuit (PLC) structure including a two-input, two-output Mach-Zehnder interferometer (MZI) formed on a substrate, and a cladding formed to cover the periphery of the MZI, in which the MZI includes an optical splitter that splits an input signal light, an optical coupler that combines the signal lights split by the optical splitter, causes interference, and outputs the combined signal light, a first waveguide core and a second waveguide core that connect the optical splitter and the optical coupler, and a second waveguide core that connects the first waveguide core and the second waveguide core to the first waveguide core. Provided is an optical modulator comprising a first heater installed directly above a first waveguide core and a second heater installed directly above a second waveguide core, and further comprising a heat insulating groove structure having an opening and a bottom formed near at least one side of the first heater of the first waveguide core and the second heater of the second waveguide core, the bottom of the heat insulating groove structure being located between the substrate side positions of the first waveguide core and the second waveguide core and the first heater and the second heater side positions. [Brief description of the drawings]

[0014] [Figure 1] FIG. 1 illustrates a configuration of a low-speed optical modulator according to the prior art. [Diagram 2] FIG. 13 is a diagram showing the relationship between time and the voltage applied to the heater (applied voltage waveform) when the optical modulator is driven. [Diagram 3] 1 is a flowchart showing a method for driving a low-speed optical modulator according to the prior art. [Figure 4] FIG. 1 is a diagram showing the configuration of an optical modulator having a heat insulating groove structure according to the prior art. [Diagram 5] 1 is a diagram showing a configuration of an optical modulator according to an embodiment of the present disclosure. [Figure 6] 11 is a diagram showing the relationship between time and the voltage applied to a heater (applied voltage waveform) and the temperature history in a waveguide core when the optical modulator according to the embodiment of the present disclosure is driven. FIG. [Figure 7] 4 is a flowchart illustrating a method for driving an optical modulator according to an embodiment of the present disclosure. [Figure 8]1A and 1B are diagrams showing the cooling behavior of the waveguide core in an optical modulator according to the prior art and in the optical modulator according to the present embodiment. [Figure 9] 11 is a diagram showing the relationship between time and the voltage applied to a heater (applied voltage waveform) and the temperature history in a waveguide core when the optical modulator according to the embodiment of the present disclosure is driven. FIG. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0015] Hereinafter, an embodiment of the present disclosure will be described in detail with reference to the drawings. The embodiment of the present disclosure is a silica-based PCL optical modulator having a heat insulating groove structure, which is different from the conventional technology in that the heat insulating groove structure is designed to have a depth that achieves both power saving and high heat dissipation.

[0016] (First embodiment) FIG. 5 shows a configuration of an optical modulator 50 according to an embodiment of the present disclosure. FIG. 5(a) shows a top view of the optical modulator 50, and FIG. 5(b) shows a cross-sectional view of the optical modulator 50 at the position of the Vb-Vb cross-sectional line in FIG. 5(a), observed from the input side of the signal light. The optical modulator 50 according to an embodiment of the present disclosure has a basic circuit configuration using a 2×2 MZI type silica-based PLC as shown in FIG. 1, and further includes a heat insulating groove structure 51 formed near at least one side surface of the waveguide core 13 and having a bottom surface within the thickness range of the waveguide core 13. Note that the heater 14 in this embodiment is a thin film heater, and the material of the substrate 43 is Si (silicon).

[0017] In the optical modulator 50 of this embodiment, the first waveguide core 13a and the second waveguide core 13b have the same length. The optical modulator 50 is designed so that when a signal light is input from the first input port 15a and the voltage applied to the heater 14 is zero, the signal light is output from the second output port 16b. The optical modulator 50 of this embodiment configured in this way is driven to heat the waveguide core 13 by applying a voltage to the heater 14, thereby modulating the signal light.

[0018] 6 is a diagram showing the relationship between time and the voltage applied to the heater 14 (applied voltage waveform) and the temperature history in the waveguide core 13 when the optical modulator 50 according to the embodiment of the present disclosure is driven. FIG. 6(a) shows the applied voltage waveform to the first heater 14a, FIG. 6(b) shows the temperature history of the first waveguide core 13a, FIG. 6(c) shows the applied voltage waveform to the second heater 14b, and FIG. 6(d) shows the temperature history of the second waveguide core 13b. As in FIG. 2, in this embodiment, the time to reduce the optical output level in the waveguide core 13 is T1, the voltage applied at T1 is V1, the voltage required to maintain the optical output level after the reduction is V2, the time to increase the optical output level is T2, and the voltage applied at T2 is V3.

[0019] FIG. 7 is a flowchart showing a method for driving the optical modulator 50 according to an embodiment of the present disclosure. The method for driving the optical modulator 50 in this embodiment includes step 71 of applying a voltage V1 higher than V2 to the first heater 14a for a time T1, step 72 of applying V2 to the first heater 14a at a constant voltage, step 73 of setting the applied power to the first heater 14a to 0, and step 74 of applying a voltage V3 to the second heater 14b for a time T2. The reason for applying a voltage V1 higher than V2 in step 71 is to instantaneously apply a large amount of heat to the first waveguide core 13a to reach a temperature that forms an optical path length difference corresponding to a predetermined optical output level in a short time. For comparison, the temperature history in the case where V2 is continuously applied in step 71 is shown by a broken line in FIG. 6(b), and as can be seen from this figure, applying V1 in step 71 as in this embodiment allows the predetermined temperature to be reached in a shorter time. Moreover, by keeping the applied voltage in step 72 constant at voltage V2, a predetermined optical path length difference is maintained. Furthermore, in step 74, a voltage V3 higher than V2 is applied for a time T2. This aims to return the optical path length difference between the first waveguide core 13a and the second waveguide core 13b to the value before step 71 in a short time, and the optical path length difference, i.e., the optical output level, can be restored in a shorter time than cooling by natural heat dissipation alone (for example, see Non-Patent Document 1). Thereafter, the original state of the optical output level is maintained by natural heat dissipation while maintaining the optical path length difference between the first waveguide core 13a and the second waveguide core 13b.

[0020] When step 71 is repeated after step 73, it is desirable that the parameters of the applied voltage are unchanged, that is, the optical output level is reduced by the combination of voltage V1 and time T1. That is, it is desirable that first waveguide core 13a and second waveguide core 13b are sufficiently cooled.

[0021] In this way, when the optical modulator 50 according to the present embodiment is driven by the above-mentioned method, the cooling rate of the waveguide core 13 in step 73 can be improved compared to the conventional technology. This is because the depth of the heat insulating groove structure 51 of the optical modulator 50 is within the range of the thickness of the waveguide core 13. Comparing the optical modulator 50 according to the present embodiment with the optical modulator 10 according to the conventional technology, in the optical modulator 50, the depth of the heat insulating groove structure 51 is formed to the depth where the waveguide core 13 is arranged, so that the heat supplied from the heater 14 is guided in the thickness direction as described above. Therefore, the waveguide core 13 is efficiently heated and can be heated to a predetermined temperature with less power than the optical modulator 10. On the other hand, comparing the optical modulator 50 according to the present embodiment with the optical modulator 40 according to the conventional technology, in the optical modulator 50, the bottom surface of the heat insulating groove structure 51 does not reach the surface of the substrate 43, so that an area where heat is transferred in the in-plane direction of the cladding 42 remains. Therefore, the path for dissipating heat to the substrate 43 during cooling is wider than that of the optical modulator 40, and the time required for cooling can be shorter than that of the optical modulator 40.

[0022] 8 is a diagram showing the cooling behavior of the waveguide core 13 in the optical modulator 10 and 40 according to the conventional technology, and the optical modulator 50 according to this embodiment. This diagram shows the temperature history over time when the waveguide core 13 installed in each is heated to an initial temperature of 335 K and then cooled by cutting off the heat source. Although not shown, in this embodiment, the power consumption Q of the heater 14 required to heat up to the initial temperature of 335 K is also evaluated at the same time.

[0023] Comparing the power consumption Q of the heater 14 required to heat up to an initial temperature of 335K, assuming that the optical modulator 10 is 100%, the optical modulator 40 is 64% and the optical modulator 50 is 89%. That is, the optical modulator 40 and the optical modulator 50 can reduce the power consumption of the heater 14 during heating in step 71 compared to the optical modulator 10. On the other hand, as shown in FIG. 8, it can be seen that the cooling speed of the optical modulator 10 and the optical modulator 50 is faster than that of the optical modulator 40. For example, when cooling from an initial temperature of 335K to 300K, the time required to cool down to 304K was 0.42 ms when the optical modulator 40 was used, 0.33 ms when the optical modulator 50 was used, and 0.30 ms when the optical modulator 10 was used. That is, the optical modulators 10 and 50 can cool the waveguide core 13 in a short time during cooling in step 73 compared to the optical modulator 40, thereby enhancing the effect of suppressing the pattern effect.

[0024] From the above, the optical modulator 50 according to this embodiment has improved heating and cooling rates during operation, and has the effect of suppressing both the amount of power consumption and the pattern effect.

[0025] The lengths of the first waveguide core 13a and the second waveguide core 13b may be set in advance to an optical path length difference of λ / 2 with respect to the wavelength λ of the signal light. In this case, when the signal light is input from the first input port 15a, if the voltage applied to the heater 14 is 0, the signal light is output from the first output port 16a.

[0026] In addition, since the heat insulating groove structure 51 described in this embodiment defines the depth (position of the bottom surface) based on the thickness of the waveguide core 13, it may be assumed that the effect of suppressing both the power consumption and the pattern effect as described above is not achieved depending on the arrangement of the waveguide core 13. In other words, when the waveguide core 13 is arranged in a position close to the substrate 43, the depth of the heat insulating groove structure 51 becomes very deep, so that the cooling time may be required to be the same as that of the optical modulator 40. Conversely, when the waveguide core 13 is arranged in a position close to the heater 14, the depth of the heat insulating groove structure 51 becomes very shallow, so that the power consumption for heating may be required to be the same as that of the optical modulator 10. However, it is not realistic in terms of the optical design of the element for the depth position of the waveguide core 13 to be close to the substrate 43 or the heater 14. The reason for this is that the signal light propagating in the waveguide core 13 interferes not only with the cladding 42 but also with the substrate 43 and the heater 14, causing the signal light to be disturbed. Therefore, since it is not realistic for the depth of the insulating groove structure to be extremely shallow or extremely deep, by defining the depth of the insulating groove structure 51 by the depth of the waveguide core 13, it is possible to efficiently suppress both the power consumption and the pattern effect.

[0027] Second embodiment A second embodiment of the present disclosure will be described in detail below with reference to the drawings. This embodiment is an example in which the voltage applied to maintain the optical output level is pulsed in the method for driving the optical modulator 50 in the first embodiment.

[0028] In this embodiment, as in the first embodiment, an optical modulator 50 as shown in Fig. 5 is used. As described above, the optical modulator 50 has a heat insulating groove structure 51 whose bottom surface is located within the thickness range of the waveguide core 13, and is characterized by being more effective in suppressing both the power consumption and the pattern effect than the optical modulators 10 and 40 according to the conventional techniques.

[0029] 9 is a diagram showing the relationship between time and the voltage applied to the heater 14 (applied voltage waveform) and the temperature history in the waveguide core 13 when the optical modulator 50 is driven according to an embodiment of the present disclosure. FIG. 9(a) shows the applied voltage waveform to the first heater 14a, FIG. 9(b) shows the temperature history of the first waveguide core 13a, FIG. 9(c) shows the applied voltage waveform to the second heater 14b, and FIG. 9(d) shows the temperature history of the second waveguide core 13b. As in FIG. 2 and FIG. 6, in this embodiment, the time to reduce the optical output level is T1, the applied voltage at T1 is V1, the voltage required to maintain a constant value after the optical output level has been reduced is V2, the time to increase the optical output level is T2, and the applied voltage at T2 is V3.

[0030] The optical modulator 50 according to this embodiment configured as described above is driven by the method shown in Fig. 3. As described above, this method applies a pulsed voltage V1 in step 32, which maintains the optical output level of the waveguide core 13 at a predetermined value. The pulsed voltage V1 has a pulse waveform whose time average value corresponds to the voltage V2, and its duty is (V2 / V1). 2 By applying a voltage with such a pulse waveform, heating is performed with the same amount of work as when V2 is applied at a constant voltage, so a specified optical path length is maintained. Driving in this way reduces the number of applied voltages to two, V1 and V3, making it possible to simplify the drive circuit configuration (driver circuit configuration). Furthermore, by selecting T2 so that V3=V1, the number of applied voltages can be reduced to only one, V1, making it possible to further simplify the drive circuit configuration.

[0031] When the optical modulator 50 is driven in this manner, both the heating rate and the cooling rate are improved, as in the first embodiment, and the effect of suppressing both the power consumption and the pattern effect is achieved. [Industrial Applicability]

[0032] As an optical modulator that can reduce both power consumption and pattern effects more than optical modulators based on conventional technology, it is expected to be applied to optical fiber communication networks, etc.

Claims

1. An optical modulator having a silica-based planar lightwave circuit (PLC) structure including a two-input, two-output Mach-Zehnder interferometer (MZI) formed on a substrate and a cladding formed to cover the periphery of the MZI, The MZI is an optical splitter for splitting an input signal light; an optical coupler that multiplexes the signal lights split by the optical splitter, causes interference, and outputs the multiplexed signal lights; a first waveguide core and a second waveguide core connecting the optical splitter and the optical coupler; a first heater disposed directly above the first waveguide core and a second heater disposed directly above the second waveguide core; Equipped with a heat insulating groove structure having an opening and a bottom formed near at least one side surface of the first heater of the first waveguide core and the second heater of the second waveguide core; a position of the bottom of the heat insulating groove structure in a thickness direction is between a position on the substrate side and a position on the first heater side and a position on the second heater side in the thickness direction of the first waveguide core and the second waveguide core, The material of the substrate is silicon (Si); Optical modulator.

2. 2. The optical modulator of claim 1, wherein the first waveguide core and the second waveguide core have equal lengths.

3. 2. The optical modulator according to claim 1, wherein a difference in optical path length between said first waveguide core and said second waveguide core is designed to be half the wavelength of said signal light.

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