Optical modulator

Abstract

An optical modulator (1) is provided with: a light incidence side waveguide (2); a light emission-side waveguide (3); a grating (4) provided on the light incidence side waveguide (2); a grating (5) provided on the light emission-side waveguide (3); and an optical resonator (6) provided between the grating (4) and the grating (5), the optical resonator (6) having an MMI (7) for branching light that has passed through the grating (4), parallel waveguides (8A, 8B) extending parallel to the MMI (7), and an MMI (11) for multiplexing light that has passed through the parallel waveguides (8A, 8B) and outputting the multiplexed light to the grating (5). The optical resonator (6) outputs, to the grating (5), light multiplexed so as not to impart a phase difference to the light passing through the parallel waveguides (8A, 8B), and outputs, to the grating (5), light multiplexed so as to impart a phase difference to the light passing through the parallel waveguides (8A, 8B) in an off state of the optical modulation, light multiplexed so as to impart a phase difference to the light passing through the parallel waveguides (8A, 8B).

Description

Technical Field

[0001] This disclosure relates to optical modulators. Background Technology

[0002] In a communication method known as co-packaged optics (hereinafter referred to as CPO), power consumption can be suppressed by minimizing the length of the high-frequency signal line located on the input side of the optical modulator. The optical modulator in CPO requires improved integration that enables miniaturization, non-hermetic operation that does not require hermetic sealing, and operation that does not require temperature adjustment.

[0003] On the other hand, for example, Mach-Zehnder (hereinafter referred to as MZ) type silicon photonic modulators can meet all the above requirements. However, in MZ type silicon photonic modulators, due to the presence of a phase shifter that utilizes the carrier plasma effect, there is a problem: poor modulation efficiency, and consequently, a smaller optical modulation width, i.e., a smaller dynamic extinction ratio.

[0004] As a conventional technology for solving the above-mentioned problems, there is, for example, the optical modulator described in Non-Patent Document 1. This optical modulator lengthens the phase shifter region that allows the light wave to propagate in order to increase the interaction length between the light wave and the phase shifter.

[0005] Existing technical documents

[0006] Non-patent literature

[0007] Non-patent literature 1: T. Baehr-Jones et al., “Ultralow drive voltage silicon traveling-wave modulator,” Opt. Express, vol. 20, 12014-12020, May 2012. Summary of the Invention

[0008] The problem that the invention aims to solve

[0009] However, in the conventional optical modulator described in Non-Patent Document 1, as the phase shifter becomes longer, the optical loss caused by the carrier plasma effect increases. Therefore, the traveling wave electrode used to apply the electrical signal to the optical modulator must also become longer according to the length of the phase shifter, resulting in the problem of frequency band degradation of high-frequency characteristics due to the loss of high frequencies in the traveling wave electrode.

[0010] The present disclosure addresses the aforementioned issues and aims to provide an optical modulator that can improve the dynamic extinction ratio without causing optical loss or high-frequency characteristic degradation.

[0011] Methods for solving problems

[0012] The optical modulator disclosed herein includes: a light incident waveguide for light incident; a light emitting waveguide for light emitting; a first semi-transparent mirror disposed on the light incident waveguide; a second semi-transparent mirror disposed on the light emitting waveguide; and an optical resonator disposed between the first and second semi-transparent mirrors. The optical resonator has a branching portion that branches off light passing through the first semi-transparent mirror, a parallel waveguide extending parallel to the branching portion and allowing light branched off from the branching portion to pass through, and a combining portion that combines the light passing through the parallel waveguide and outputs it to the second semi-transparent mirror. In the optical modulation open state, the optical resonator outputs light to the second semi-transparent mirror obtained by combining light in a manner that does not impart a phase difference to the light passing through the parallel waveguide. In the optical modulation closed state, the optical resonator outputs light to the second semi-transparent mirror obtained by combining light in a manner that imparts a phase difference to the light passing through the parallel waveguide.

[0013] Invention Effects

[0014] According to this disclosure, in the optical modulation-on state, the optical resonator outputs light to the second semi-transparent mirror by combining the light without imparting a phase difference to the light passing through the parallel waveguide. In the optical modulation-off state, the optical resonator outputs light to the second semi-transparent mirror by combining the light with the light passing through the parallel waveguide by imparting a phase difference to the light passing through the parallel waveguide. The optical resonator is disposed between the first and second semi-transparent mirrors. The optical resonator has a branch portion that branches the light passing through the first semi-transparent mirror, a parallel waveguide extending parallel to the branch portion, and a combining portion that combines the light passing through the parallel waveguide and outputs it to the second semi-transparent mirror.

[0015] With optical modulation enabled, the refractive index of the parallel waveguide is determined such that the phase difference of the light passing through the parallel waveguide is approximately zero, resulting in interference. If the wavelength satisfying the resonance condition determined by this refractive index and the length between the first and second semi-transparent mirrors is set as the operating wavelength, then the light passing through the parallel waveguide will resonate and tunnel in the optical resonator, passing through the light-incident waveguide without loss from the light-outceasing waveguide.

[0016] On the other hand, in the off state of optical modulation, the refractive index of the parallel waveguide is determined such that the light passing through the parallel waveguide has a phase difference. Therefore, corresponding to this phase difference, the power of the light passing from the incident waveguide to the exit waveguide is reduced. Furthermore, at this refractive index, the resonance condition is not satisfied between the first and second semi-transparent mirrors. Therefore, the light incident from the incident waveguide undergoes anti-resonance reflection, further reducing the power of the light passing through the exit waveguide.

[0017] Therefore, the power ratio and power difference of the light passing through the light incident waveguide to the light exit waveguide can be obtained between the on and off states of the optical modulation. Thus, the optical modulator of this disclosure can improve the dynamic extinction ratio without causing optical loss or frequency band degradation of high-frequency characteristics. Attached Figure Description

[0018] Figure 1 This is a schematic top view showing the structure of a conventional optical modulator.

[0019] Figure 2 This is a schematic top view showing the structure of the optical modulator of Embodiment 1.

[0020] Figure 3 It shows along Figure 2 An enlarged cross-sectional view of the section obtained by cutting the optical modulator of Embodiment 1 with the AA line.

[0021] Figure 4 This is a graph showing the wavelength dependence of the dynamic extinction ratio of the optical modulator.

[0022] Figure 5 This is a top view schematically showing the structure of a modified example of the optical modulator of Embodiment 1. Detailed Implementation

[0023] Implementation method 1.

[0024] First, use Figure 1 This paper explains the previous optical modulators and their problems.

[0025] Optical communication employs either intensity modulation (IM) or digital coherence (DC). For example, in data center optical communication, IMD is used for short-distance communication within the data center, while DC is used for long-distance communication between data centers. IMD utilizes MZ-type optical modulators. Furthermore, DC can also be implemented by arranging MZ-type optical modulators in a specific configuration.

[0026] Figure 1 This is a schematic top view showing the structure of a conventional optical modulator 100. Figure 1 In this design, the optical modulator 100 is an MZ-type optical modulator, having an incident waveguide 101, an exit waveguide 102, and an MZ interferometer 103. Furthermore, the MZ interferometer 103 includes a multimode interference waveguide (hereinafter referred to as MMI) 104, an MMI 105, parallel waveguides 106A and 106B, a phase shifter 107, and a delay line 108.

[0027] In the optical modulator 100, the refractive index of the light in parallel waveguides 106A and 106B is changed by phase shifter 107 and delay line 108, thereby causing a phase shift in the light passing through parallel waveguides 106A and 106B. In the optical modulator 100, a phase difference is assigned to the light passing through parallel waveguides 106A and 106B, causing MMI 105 to interfere with the light with different phases, thereby generating a "1" or "0" symbol. Regarding the optical modulator 100, the state generating a "1" symbol is the optical modulation on state, and the state generating a "0" symbol is the optical modulation off state.

[0028] exist Figure 1 As shown by the arrow, the light input to the light incident side waveguide 101 is branched by MMI 104, passes through parallel waveguides 106A, 106B, phase shifter 107 and delay line 108, is combined by MMI 105 and output to the light exit side waveguide 102.

[0029] In addition, Y-branch waveguides are sometimes used instead of MMI 104 and 105.

[0030] In addition, Figure 1 The diagram shows a configuration in which a phase shifter 107 is provided on both parallel waveguides 106A and 106B, but sometimes a phase shifter 107 is provided on either parallel waveguide 106A or parallel waveguide 106B.

[0031] In the optical modulator 100 used for intensity modulation optical communication, a phase difference of, for example, π / 2 is provided between parallel waveguides 106A and 106B. To provide this phase difference, a delay line 108 is provided in parallel waveguide 106B.

[0032] In addition, although Figure 1 While not explicitly described, heaters are sometimes used in place of delay line 108. For example, when a heater is installed in parallel waveguide 106B, the heater heats the core of parallel waveguide 106B. This causes a change in the refractive index of the light in parallel waveguide 106B, resulting in a phase shift in the light passing through parallel waveguide 106B. The change in refractive index of light caused by heating the core of the waveguide is called the thermo-optic effect. In the thermo-optic effect, there is almost no wavelength dependence on the amount of phase shift.

[0033] Furthermore, the light propagating from the incident side to the emitting side in the parallel waveguide 106B is delayed in phase as it propagates through the delay line 108. Unlike the heater, the phase shift of the light passing through the parallel waveguide 106B, utilizing the delay line 108, is more dependent on its wavelength. In other words, by changing the incident wavelength of the light passing through the parallel waveguide 106B, the phase difference of the light passing through the parallel waveguides 106A and 106B can be adjusted. Hereinafter, a phase shifter that stably imparts a phase difference of π / 2 between the light passing through the parallel waveguide 106A and the light passing through the parallel waveguide 106B, as described by the delay line 108 or the aforementioned heater, will be referred to as a DC phase shifter.

[0034] Phase shifter 107 is an RF phase shifter (high-frequency phase shifter) driven by a high-frequency signal. For example, phase shifter 107 is high-speed modulated by a high-frequency electrical signal in a frequency band of tens of GHz. Furthermore, phase shifter 107 is a diode formed by providing a pin junction or pn junction on the silicon substrate of optical modulator 100. Depending on the current or voltage flowing in phase shifter 107 (diode) formed on the silicon substrate, the refractive index of phase shifter 107 changes, thus enabling phase shifting of the light passing through phase shifter 107. Optical modulator 100 is adjusted in a manner that the on and off of the electrical signal input to phase shifter 107 corresponds to the on and off of the light.

[0035] Key characteristics of the optical modulator 100 include high-frequency characteristics, optical loss characteristics, and dynamic extinction ratio (or optical modulation amplitude). High-frequency characteristics are crucial for high-speed switching of the electrical signals driving the optical modulator 100, and bandwidth degradation must be suppressed as much as possible. Optical loss characteristics mainly arise from absorption losses in the materials constituting the phase shifter and must be minimized as much as possible.

[0036] In addition, it is important to maximize the dynamic extinction ratio, which is the power ratio of the light when the light modulation is on to the light modulation is off.

[0037] In addition, by increasing the dynamic extinction ratio, the power difference between the light power when the light modulation is on and the light power when the light modulation is off, i.e., the light modulation amplitude, also increases.

[0038] The optical modulator 100 is an MZ-type silicon optical modulator manufactured using silicon photonics. In the MZ-type silicon optical modulator, the carrier plasmon effect is used in the phase shifter 107. Therefore, as the phase shifter 107 becomes longer, the optical loss caused by the carrier plasmon effect increases. Consequently, the traveling wave electrode used to apply the electrical signal to the optical modulator must also become longer according to the length of the phase shifter 107, resulting in bandwidth degradation of high-frequency characteristics due to high-frequency losses in the traveling wave electrode.

[0039] In contrast, the optical modulator of Embodiment 1 has semi-transparent mirrors (hereinafter referred to as semi-transparent mirrors) arranged on the light incident waveguide and light exit waveguide of the MZ-type silicon optical modulator. This allows for an improvement in the dynamic extinction ratio without optical loss or bandwidth degradation of high-frequency characteristics. Simultaneously, the optical modulation amplitude can also be increased.

[0040] The following uses Figure 2 The optical modulator of Embodiment 1 will be described.

[0041] Figure 2 This is a schematic top view showing the structure of the optical modulator 1 in Embodiment 1. Figure 2 In this design, optical modulator 1 is the same as optical modulator 100, and is an MZ-type optical modulator, having an incident waveguide 2, an exit waveguide 3, a grating 4, a grating 5, and an optical resonator 6. Optical resonator 6 is an optical resonator with an MMI 7, parallel waveguides 8A and 8B, a phase shifter 9, a delay line 10, and an MMI 11.

[0042] The light incident waveguide 2 and the light exit waveguide 3 are optical waveguides formed on a silicon substrate. For example, in... Figure 2 As indicated by the arrow, waveguide 2 on the light incident side is the optical waveguide that supplies light. (As shown in...) Figure 2 As indicated by the arrow, the light-emitting waveguide 3 is the optical waveguide through which light passing through the grating 4, optical resonator 6, and grating 5 is emitted.

[0043] Grating 4 is disposed on the light incident side waveguide 2 and is a first semi-transparent mirror used to change the refractive index of light in the light incident side waveguide 2. For example, grating 4 changes the waveguide width so that the effective refractive index changes periodically relative to the propagation direction of light in the light incident side waveguide 2.

[0044] Grating 5 is disposed on the light-emitting side waveguide 3 and is a second semi-transparent mirror used to change the refractive index of light in the light-emitting side waveguide 3. Similar to grating 4, grating 5 changes the waveguide width, so that the effective refractive index changes periodically relative to the propagation direction of light in the light-emitting side waveguide 3.

[0045] MMI 7 is a branch section that branches the light passing through grating 4 into parallel waveguides 8A and 8B. Alternatively, the branch section can also be a Y-branch waveguide. MMI 11 is a combining section that combines the light passing through parallel waveguide 8A and parallel waveguide 8B and outputs it to grating 5. Alternatively, the combining section can also be a Y-branch waveguide.

[0046] Parallel waveguide 8A is an optical waveguide formed parallel to parallel waveguide 8B on the silicon substrate of optical modulator 1. Parallel waveguide 8B is an optical waveguide formed parallel to parallel waveguide 8A on the silicon substrate of optical modulator 1.

[0047] Phase shifter 9 is a phase shifter that shifts the phase of light passing through parallel waveguides 8A and 8B. Phase shifter 9 is an RF phase shifter driven by a high-frequency signal.

[0048] For example, the phase shifter 9 is a diode formed by forming a pin junction (carrier injection type) or a pn junction (carrier depletion type) on the silicon substrate of the optical modulator 1. By changing the refractive index of the phase shifter 9 according to the current or voltage flowing in the phase shifter 9 formed on the silicon substrate, the light passing through the phase shifter 9 can be phase-shifted. The optical modulator 1 is adjusted in a manner that the on and off of the electrical signal input to the phase shifter 9 corresponds to the on and off of the light.

[0049] Delay line 10 is a phase shifter that causes phase shifting of light passing through parallel waveguide 8B. Light propagating in parallel waveguide 8B from the incident side to the exit side is delayed in phase as it propagates through delay line 10. In delay line 10, the amount of phase shift of light passing through parallel waveguide 8B is strongly dependent on wavelength. In other words, by changing the incident wavelength of light passing through parallel waveguide 8B, the phase difference of light passing through parallel waveguides 8A and 8B can be adjusted. Delay line 10 is a DC phase shifter that stably imparts a phase difference of π / 2 between the light passing through parallel waveguide 8A and the light passing through parallel waveguide 8B.

[0050] In addition, although Figure 2 While not explicitly described, a heater is sometimes used in place of the delay line 10. For example, when a heater is installed in the parallel waveguide 8B, the heater heats the core of the parallel waveguide 8B. Through the thermo-optic effect, the refractive index of the light in the parallel waveguide 8B changes, causing a phase shift in the light passing through the parallel waveguide 8B.

[0051] Next, the operation of the optical modulator 1 will be explained.

[0052] The optical modulator 1 has an optical resonator 6 disposed between grating 4 and grating 5. When the optical modulation is in the on state (symbol "1"), the optical resonator 6 outputs light to grating 5 in a manner that combines the light passing through parallel waveguides 8A and 8B without imparting a phase difference. That is, in the optical modulator 1, the refractive index of the phase shifter 9 is determined such that the light passing through parallel waveguide 8A and the light passing through parallel waveguide 8B interfere with each other with a phase difference of approximately 0. At this time, the wavelength that is in a resonant state in the optical resonator 6 becomes the operating wavelength.

[0053] When optical modulation is on, optical resonator 6 satisfies the resonant wavelength condition. Therefore, resonant tunneling occurs inside optical resonator 6, and light is transmitted without loss from the light-incident waveguide 2 to the light-exiting waveguide 3. When optical modulation is on, in optical resonator 6, light reaches the light-exiting waveguide 3 with the same transmittance as optical modulator 100 without gratings 4 and 5.

[0054] When the optical modulation is off (symbol "0"), the optical resonator 6 outputs light to the grating 5 to combine the light that has passed through the parallel waveguides 8A and 8B by imparting a phase difference.

[0055] That is, in the optical modulator 1, the refractive index of the phase shifter 9 is determined such that the phase difference between the light passing through the parallel waveguide 8A and the light passing through the parallel waveguide 8B is approximately π, and they interfere with each other. At this time, in the operating wavelength of the optical resonator 6, it becomes an anti-resonance state.

[0056] This is because the refractive index of parallel waveguides 8A and 8B changes, and the resonant wavelength condition of optical resonator 6 changes.

[0057] When the optical modulation is off, the optical resonator 6 changes to an anti-resonant wavelength condition. Therefore, anti-resonant reflection occurs inside the optical resonator 6, reducing the transmittance of light from the incident waveguide 2 to the exit waveguide 3. That is, when the optical modulation is off, the transmittance of the optical resonator 6 further reduces the amount of anti-resonant reflection compared to the transmittance of the optical modulator 100 without gratings 4 and 5. Thus, the power ratio and power difference of light passing from the incident waveguide 2 to the exit waveguide 3 can be obtained between the on and off states of the optical modulation. Consequently, the optical modulator 1 can improve the dynamic extinction ratio and increase the optical modulation amplitude without optical loss or high-frequency bandgap degradation.

[0058] In optical modulator 1, a high dynamic extinction ratio and a high optical modulation amplitude can be obtained without lengthening the phase shifter to achieve a larger refractive index change. Therefore, the increase in optical loss is suppressed, as is the degradation of high-frequency characteristics in the frequency band. Furthermore, in optical modulator 1, even without a structure in the phase shifter 9 that reduces the group velocity of light, the dynamic extinction ratio and optical modulation amplitude can be improved. Thus, in optical modulator 1, optical loss is suppressed, and consequently, the degradation of the optical modulation waveform caused by inter-symbol interference due to the mismatch between the group velocity of light and the phase velocity of the high-frequency signal can be suppressed.

[0059] Furthermore, in the off state of optical modulation, the light returning to the light incident waveguide 2 from the optical modulator 1 via anti-resonant reflection is suppressed, for example, by the optical isolator of a laser source (hereinafter referred to as an external laser source) located outside the optical modulator 1. As a result, the noise caused by the return light returning to the laser element of the aforementioned external laser source is reduced in the optical modulator 1.

[0060] Next, the effect obtained by the optical modulator 1 will be explained quantitatively.

[0061] First, the structure of the phase shifter 9 in the optical modulator 1 will be explained.

[0062] Figure 3 It shows along Figure 2 The magnified cross-sectional view of the section obtained by cutting the optical modulator 1 with line AA shows the cross-section of the phase shifter 9. Figure 3 In this circuit, phase shifter 9 is a carrier depletion type RF phase shifter with a pn junction. The rib-shaped waveguide has a width W1 of 400 nm, and half of its width W2 is 200 nm. The position of width W2 is the center of the rib. It is assumed that there is a pn junction interface between p-type semiconductor 91 and n-type semiconductor 92 at the center of the rib.

[0063] p-type semiconductor 91 and n-type semiconductor 92 are formed on silicon oxide (SiO2) film 93 on the silicon-on-insulator substrate of optical modulator 1. A silicon oxide layer 94 is stacked on the rib formed by p-type semiconductor 91 and n-type semiconductor 92. Furthermore, the silicon film thickness D1 is set to 200 nm, and the half-etch depth D2 for forming the rib waveguide is set to 110 nm.

[0064] In phase shifter 9, the impurity density of p-type semiconductor 91 and n-type semiconductor 92 is set to 1×10⁻⁶. 18 cm -3 Furthermore, the length of phase shifter 9 is set to 2mm.

[0065] Furthermore, in optical modulator 1, Figure 2 The delay line 10 shown is set to a length of 11 nm relative to the wavelength, representing the free spectral interval.

[0066] Furthermore, regarding the driving conditions of optical modulator 1, it is assumed that the reverse bias voltage is 2V and the driving voltage amplitude is 1.6V.

[0067] The optical modulator 1 is configured to have an optical resonator 6 sandwiched between gratings 4 and 5, and gratings 4 and 5 have spectral transmittance that produces 0.02% ripple and transmission characteristics.

[0068] Figure 4 This is a graph showing the wavelength dependence of the dynamic extinction ratio of optical modulator 1 and optical modulator 100. Figure 4 In the diagram, the horizontal axis represents the wavelength of the incident light (nm), and the vertical axis represents the dynamic extinction ratio (dB) of optical modulator 1 and optical modulator 100. Characteristic B1, shown by the dashed line, illustrates... Figure 1 The wavelength-dependent characteristics of the dynamic extinction ratio of the conventional optical modulator 100 are shown. Furthermore, characteristic B2, shown by the solid line, illustrates the wavelength-dependent characteristics of the dynamic extinction ratio of the optical modulator 1.

[0069] As can be seen from characteristic B1, in the optical modulator 100, the shorter the wavelength of the light, the higher the dynamic extinction ratio. This is because the phase difference between the light generated by the delay line 108 and the light passing through the parallel waveguide 106A and the light passing through the parallel waveguide 106B deviates from the optimal π / 2. In other words, this is because the crossover point of the optical modulation waveform is less than 50% of the optimal value. In fact, the wavelength at which the crossover point coincides with 50%, i.e., 1538.63 nm, becomes the operating wavelength.

[0070] Furthermore, as shown by characteristic B2, in optical modulator 1, the shorter the wavelength of light, the higher the dynamic extinction ratio. This is because the crossover point is less than the optimal 50%.

[0071] On the other hand, in optical modulator 1, there exists a wavelength where the dynamic extinction ratio periodically increases relative to the wavelength. Therefore, for example, when 1538.63 nm is set as the operating wavelength, a higher dynamic extinction ratio and a higher optical modulation amplitude can be obtained. Assuming that the reflectivity of gratings 4 and 5, which act as semi-transparent mirrors, is higher, and the ripple generated in the transmittance of the spectrum of optical resonator 6 is larger, Figure 4 The periodic increase in dynamic extinction ratio shown further increases, resulting in a greater effect.

[0072] In addition, the optical modulator 1 has the following variations.

[0073] Figure 5 This is a schematic top view showing the structure of an optical modulator 1A, a modified example of an optical modulator 1. Figure 5 In the optical modulator 1A, a ring mirror 4A and a ring mirror 5A are used to replace the grating 4 and grating 5 in the optical modulator 1.

[0074] Circular mirror 4A is the first semi-transparent mirror located on the light-incident waveguide 2. Circular mirror 5A is the second semi-transparent mirror located on the light-exiting waveguide 3.

[0075] In the region where loop-connecting waveguides 12A and 12B are close together, a portion of the light power is combined with the adjacent waveguides, thus enabling ring mirror 4A to function as a transmissive mirror. Similarly, in the region where loop-connecting waveguides 13A and 13B are close together, a portion of the light power is combined with the adjacent waveguides, thus enabling ring mirror 5A to function as a transmissive mirror. By using ring mirrors 4A and 5A as transmissive mirrors, optical modulator 1A can achieve the same effect as optical modulator 1.

[0076] As described above, the optical modulator 1 of Embodiment 1 includes an incident waveguide 2, an exit waveguide 3, a grating 4 disposed on the incident waveguide 2, a grating 5 disposed on the exit waveguide 3, and an optical resonator 6. The optical resonator 6 is disposed between the grating 4 and the grating 5. The optical resonator 6 includes a micro-interference device (MMI) 7 that branches the light passing through the grating 4, parallel waveguides 8A and 8B extending parallel to the MMI 7, and an MMI 11 that combines the light passing through the parallel waveguides 8A and 8B and outputs it to the grating 5. Furthermore, in the optical modulation-on state, the optical resonator 6 outputs light to the grating 5 obtained by combining the light passing through the parallel waveguides 8A and 8B without imparting a phase difference. In the optical modulation-off state, the optical resonator 6 outputs light to the grating 5 obtained by combining the light passing through the parallel waveguides 8A and 8B with a phase difference.

[0077] With optical modulation in the on state, the refractive index of the light in parallel waveguides 8A and 8B is determined such that the phase difference of the light passing through parallel waveguides 8A and 8B is approximately 0, thus causing interference. If the wavelength satisfying the resonance condition determined by this refractive index and the length between grating 4 (or ring mirror 4A) and grating 5 (or ring mirror 5A) is set as the operating wavelength, then the light passing through parallel waveguides 8A and 8B will resonate and tunnel in the optical resonator 6, allowing the light to pass through without loss from the light-incident waveguide 2 to the light-exiting waveguide 3.

[0078] On the other hand, in the off state of optical modulation, the refractive index of the light in parallel waveguides 8A and 8B is determined such that a phase difference is imparted to the light passing through parallel waveguides 8A and 8B. Therefore, corresponding to the phase difference, the power of the light passing from the light-incident waveguide 2 to the light-emission waveguide 3 is reduced. Furthermore, at this refractive index, the resonance condition is not satisfied between grating 4 (or ring mirror 4A) and grating 5 (or ring mirror 5A). Therefore, the light incident from the light-incident waveguide 2 undergoes anti-resonant reflection, thereby further reducing the power of the light passing through the light-emission waveguide 3.

[0079] Therefore, the power ratio and power difference of the light passing from the light-incident waveguide 2 to the light-emission waveguide 3 can be obtained between the on and off states of the optical modulation. Thus, the optical modulator 1 can improve the dynamic extinction ratio without causing optical loss or frequency band degradation of high-frequency characteristics.

[0080] In the optical modulator 1 and optical modulator 1A of Embodiment 1, the first semi-transparent mirror and the second semi-transparent mirror are gratings 4 and 5 or ring mirrors 4A and 5A. Thus, the optical modulator 1 and optical modulator 1A can obtain the power ratio and power difference of the light passing from the light incident side waveguide 2 to the light emitting side waveguide 3 between the on and off states of optical modulation.

[0081] In the optical modulator 1 of Embodiment 1, the optical resonator 6 has a phase shifter that causes phase shifting of the light passing through the parallel waveguides 8A and 8B. Thus, the optical modulator 1 and the optical modulator 1A can obtain the power ratio and power difference of the light passing from the light incident side waveguide 2 to the light emitting side waveguide 3 between the on and off states of optical modulation.

[0082] In the optical modulator 1 of Embodiment 1, the phase shifter 9 includes a waveguide with a pin junction or a pn junction. Thus, the optical modulator 1 and the optical modulator 1A can obtain the power ratio and power difference of the light passing from the light incident side waveguide 2 to the light emitting side waveguide 3 between the on and off states of optical modulation.

[0083] In the optical modulator 1 of Embodiment 1, the phase shifter includes a delay line 10 or a heater disposed in the parallel waveguide 8B. Thus, the optical modulator 1 and the optical modulator 1A can obtain the power ratio and power difference of the light passing from the light incident side waveguide 2 to the light emitting side waveguide 3 between the on and off states of optical modulation.

[0084] Furthermore, it is possible to modify any structural element of the implementation method or omit any structural element of the implementation method.

[0085] Industrial availability

[0086] The optical modulator disclosed herein can be applied, for example, to optical communications in data centers.

[0087] Label Explanation

[0088] 1, 1A: Optical modulator; 2: Optical incident waveguide; 3: Optical exit waveguide; 4, 5: Grating; 4A, 5A: Ring mirror; 6: Optical resonator; 7, 11: MMI; 8A, 8B: Parallel waveguide; 9: Phase shifter; 10: Delay line; 100: Optical modulator; 101: Optical incident waveguide; 102: Optical exit waveguide; 103: MZ interferometer; 104, 105: MMI; 106A, 106B: Parallel waveguide; 107: Phase shifter; 108: Delay line.

Claims

1. An optical modulator, characterized in that, This optical modulator has the following characteristics: The light-incident waveguide supplies the light input. The light-emitting side waveguide supplies light out. A first semi-transparent mirror is disposed on the light incident side waveguide; A second semi-transparent mirror is disposed on the light-emitting side waveguide; as well as An optical resonator is disposed between the first semi-transparent mirror and the second semi-transparent mirror. The optical resonator has a branching section that allows light passing through the first semi-transparent mirror to branch off, a parallel waveguide extending parallel to the branching section and allowing light branched off from the branching section to pass through, and a combining section that combines the light passing through the parallel waveguide and outputs it to the second semi-transparent mirror. When optical modulation is enabled, the optical resonator outputs light to the second semi-transparent mirror in a manner that combines the light without imparting a phase difference to the light passing through the parallel waveguide. In the off state of optical modulation, the optical resonator outputs light to the second semi-transparent mirror to combine the light that has passed through the parallel waveguide by imparting a phase difference.

2. The optical modulator according to claim 1, characterized in that, The first and second semi-transparent mirrors are gratings or ring mirrors.

3. The optical modulator according to claim 1, characterized in that, The optical resonator has a phase shifter that shifts the phase of light passing through the parallel waveguide.

4. The optical modulator according to claim 3, characterized in that, The phase shifter includes a waveguide with a pin junction or a pn junction.

5. The optical modulator according to claim 3, characterized in that, The phase shifter includes a delay line or heater disposed on the parallel waveguide.

6. The optical modulator according to any one of claims 1 to 5, characterized in that, The branch section and the combining section are multimode interference waveguides or Y-branch waveguides.

Images