A folded phase modulator with ultra-low half-wave voltage and high bandwidth utilization

Abstract

This invention discloses a folded phase modulator with ultra-low half-wave voltage and high bandwidth utilization, comprising an input optical waveguide, an optical waveguide in the modulation region, an optical waveguide in the waveguide folding region, a waveguide cross structure, a loop waveguide, an output optical waveguide, signal electrodes, and a ground electrode. In the proposed folded loop phase modulator, the optical waveguide passes through one electrode gap of the GSG traveling wave electrode to achieve the first stage of electro-optic modulation, and then enters another electrode gap through the loop waveguide to achieve the second stage of phase modulation. This maximizes the interaction between the electric and optical fields, reducing the half-wave voltage by half. Simultaneously, the folded structure design significantly shortens the length of the loop waveguide, increasing the modulator's usable bandwidth. This invention achieves a phase modulator with ultra-low half-wave voltage and high bandwidth utilization; furthermore, the folded structure significantly shortens the modulator's lateral length, improving on-chip area utilization and facilitating modulator packaging.

Description

Technical Field

[0001] This invention belongs to the field of electro-optic modulator technology, and particularly relates to a folded phase modulator with ultra-low half-wave voltage and high bandwidth utilization. Background Technology

[0002] Electro-optic phase modulators are key components in modern communications, microwave photonics, and quantum systems, crucial for applications such as optical wireless systems, optical frequency comb generation, and microwave photonic radar. Future photonic systems require phase modulators with characteristics such as CMOS-compatible drive voltages, large electro-optic bandwidth, low optical insertion loss, high extinction ratio, and compatibility with large-scale manufacturing. Lithium niobate (LNiO) has become one of the most versatile and attractive materials in photonics due to its excellent electro-optic, nonlinear, and acousto-optic properties, and most commercial electro-optic modulators are based on LNiO. However, the optical waveguides of commercial LNiO modulators are typically based on titanium diffusion or proton exchange processes, and these commercial devices often exhibit weak optical mode confinement, large device footprint, and low modulation efficiency. For example, a typical commercial phase modulator operating at wavelengths of 1525–1605 nm has a half-wave voltage of 7.5 V at a 30 GHz radio frequency.

[0003] Emerging thin-film lithium niobate platforms offer superior performance for phase modulators. Thin-film lithium niobate modulators feature strong optical mode confinement, low half-wave voltage, large electro-optic bandwidth, and relatively mature fabrication technology, making them an effective solution for the future development of integrated optical paths. On-chip modulators typically employ a traveling-wave electrode arrangement using a GSG. In traditional on-chip phase modulators, the optical waveguide only passes through one electrode gap of the GSG, leaving the other gap underutilized. Therefore, the half-wave voltage of an on-chip phase modulator is twice that of an intensity modulator of the same length. For example, an on-chip phase modulator with a modulation length of 1.4 cm exhibits a half-wave voltage of 5 V at a 40 GHz radio frequency (Mengyue Xu, et al. “Flat optical frequency comb generator based on integrated lithium niobate modulators,” Journal of Lightwave Technology, 40,2 (2022): 339-345.), while a dual-channel phase modulator with a modulation length of 2 cm shows a low half-wave voltage of 4.5 V at 40 GHz (Tianhao Ren, et al. “An integrated low-voltage broadband lithium niobate phase modulator,” IEEE Photonics Technology Letters, 31,11 (2019): 889-892.). Although the half-wave voltage at low frequencies can be reduced by increasing the length of the modulation electrode, a longer electrode length increases RF losses, thereby reducing the electro-optic bandwidth of the modulator and causing the half-wave voltage to rise at high frequencies. Therefore, achieving a phase modulator with ultra-low half-wave voltage remains a challenge. Summary of the Invention

[0004] To address the technical problems mentioned in the background and to achieve phase modulation with ultra-low half-wave voltage and high bandwidth utilization, this invention provides a folded phase modulator with ultra-low half-wave voltage and high bandwidth utilization.

[0005] The present invention discloses a folded phase modulator with ultra-low half-wave voltage and high bandwidth utilization, comprising an input optical waveguide, an optical waveguide in the modulation region, an optical waveguide in the waveguide folding region, a waveguide cross structure, a loop waveguide, an output optical waveguide, a signal electrode, a first ground electrode, and a second ground electrode.

[0006] The modulation region includes a first straight waveguide, a second straight waveguide, a third straight waveguide, a fourth straight waveguide, and a straight electrode. The microwave signal modulates the optical signal in this region. The waveguide folding region includes a first 135° bent waveguide, a second 135° bent waveguide, a first 45° bent waveguide, a second 45° bent waveguide, and a waveguide cross structure. The optical waveguide is folded in this region, and the optical signal passes through the electrode gap with the same electric field direction to ensure the continuous accumulation of the modulation phase. The waveguide cross structure ensures that the two optical signals do not interfere with each other in the cross-folded optical waveguide.

[0007] The signal electrode consists of two straight electrodes and one curved electrode. The microwave signal is first transmitted from left to right in the first straight electrode, and the optical signal is modulated in the first modulation region. Then it enters the curved electrode to achieve one folding of the electrode, and finally enters the second straight electrode and is transmitted from right to left, that is, the optical signal is modulated in the second modulation region. The first ground electrode consists of two straight electrodes and one curved electrode, and the second ground electrode consists of only one straight electrode.

[0008] The optical signal enters the folded phase modulator through the input optical waveguide; the optical signal sequentially passes through the first straight waveguide, the first 135° bent optical waveguide, the waveguide cross structure, the first 45° bent optical waveguide, and the second straight waveguide to achieve the first stage of phase modulation. The two straight waveguides are located in the two electrode gaps with the same electrode direction, thus ensuring the continuous accumulation of the modulated phase; then the optical signal enters the third electrode gap after passing through the loop waveguide; the optical signal sequentially passes through the third straight waveguide, the second 45° bent optical waveguide, the waveguide cross structure, the second 135° bent optical waveguide, and the fourth straight waveguide to complete the second stage of phase modulation. The two straight waveguides are located in the two electrode gaps with the same electrode direction, thus ensuring the continuous accumulation of the modulated phase; finally, the optical signal is output from the phase modulator through the output optical waveguide.

[0009] Furthermore, the thin-film lithium niobate wafer structure of the modulator, from bottom to top, consists of a silicon substrate, a SiO2 buried layer, a lithium niobate planar layer, a lithium niobate waveguide, and a metal electrode.

[0010] Furthermore, in the waveguide folded region, since the waveguide and the metal electrode are on the same plane, in order to avoid direct contact between the optical waveguide and the metal electrode in the waveguide folded region, it is necessary to grow a SiO2 cladding layer or photoresist isolation layer structure with a thickness greater than 1μm in this region.

[0011] Furthermore, the signal electrode, the first ground electrode, and the second ground electrode can be cuboid electrode structures.

[0012] Furthermore, the signal electrode, the first ground electrode, and the second ground electrode can be T-shaped electrode structures, with the signal electrode extending into a first T-shaped microstructure electrode and a second T-shaped microstructure electrode, the first ground electrode extending into a third T-shaped microstructure electrode, and the second ground electrode extending into a fourth T-shaped microstructure electrode.

[0013] Furthermore, the width of the T-shaped microstructure electrode is 45 μm and the gap between the two T-shaped microstructures is 5 μm.

[0014] The beneficial technical effects of this invention are as follows:

[0015] 1. This invention enhances the interaction between the electric field and the optical field through cyclic modulation, reducing the half-wave voltage of the phase modulator by half, and realizing an ultra-low half-wave voltage phase modulator.

[0016] 2. This invention utilizes a folded structure to minimize the length of the loop waveguide, thereby increasing the modulation period of the modulator and realizing a phase modulator with high bandwidth utilization.

[0017] 3. The present invention utilizes a folded structure to significantly shorten the lateral length of the modulator, improve the utilization rate of the on-chip area, and facilitate the packaging of the modulator. Attached Figure Description

[0018] Figure 1 This is a schematic diagram of the folded cyclic phase modulator with a cuboid electrode structure according to the present invention.

[0019] Figure 2 This is a schematic diagram of the folded cyclic phase modulator with a T-shaped electrode structure according to the present invention.

[0020] Figure 3 The graph shows the half-wave voltage measurement results of a folded cyclic phase modulator with a modulation length of 2 cm.

[0021] Figure 4 A magnified view of the half-wave voltage measurement results of a folded cyclic phase modulator with a modulation length of 2 cm.

[0022] Figure 5 A summary graph of the measurements of the lowest half-wave voltage and available bandwidth for each modulation cycle.

[0023] Figure 6 The electro-optic bandwidth measurement results of a folded cyclic phase modulator with a modulation length of 2 cm are shown in the figure. Detailed Implementation

[0024] The present invention will now be described in further detail with reference to the accompanying drawings and specific embodiments.

[0025] In traditional on-chip phase modulators, the optical waveguide achieves electro-optic modulation only through one electrode gap of the GSG electrodes, while the other electrode gap is not fully utilized. By employing cyclic modulation, an ultra-low half-wave voltage phase modulator can be achieved. The principle of the cyclic phase modulator is as follows: the optical waveguide passes through one electrode gap of the GSG electrodes to achieve the first stage of electro-optic modulation. Since effective modulation only occurs when the RF transmission direction is aligned with the optical transmission direction, the optical signal needs to return to the input through the loop waveguide and then enter the other electrode gap to achieve the second stage of phase modulation. This maximizes the interaction between the electric and optical fields, theoretically reducing the half-wave voltage by half. This achieves an ultra-low half-wave voltage phase modulator.

[0026] However, because they share the same signal electrode, the optical signal entering the electrode gap a second time is out of phase with the radio frequency signal in the signal electrode, leading to periodic modulation. This means that not all radio frequency frequencies will be effectively modulated. The usable electro-optic bandwidth within each modulation cycle is defined as the 3dB power bandwidth, i.e., the lowest half-wave voltage. The modulation period is closely related to the round-trip time (RTD) of the optical signal, which is caused by the waveguide and loop waveguide in the first-stage phase modulation. A larger RTD results in a smaller modulation period, thus reducing the usable bandwidth within each modulation period. Therefore, to achieve a large bandwidth utilization, the modulation period must be increased, i.e., the length of the RTD must be reduced. Therefore, this invention proposes a phase modulator that utilizes a folded structure to minimize the length of the loop waveguide to achieve a large bandwidth utilization.

[0027] First embodiment:

[0028] Figure 1A schematic diagram of a cuboid electrode structure folded cyclic phase modulator is shown. Its overall structure includes an input optical waveguide 101, an optical waveguide in the modulation region, an optical waveguide in the waveguide folding region, a waveguide cross structure 102, a loop waveguide 1015, an output optical waveguide 103, a signal electrode 201, a first ground electrode 202, and a second ground electrode 203. The modulation region 301 includes a first straight waveguide 1011, a second straight waveguide 1014, a third straight waveguide 1016, a fourth straight waveguide 1019, and straight electrodes. Microwave signals modulate optical signals in this region. The waveguide folding region 302 includes a first 135° bent waveguide 1012, a second 135° bent waveguide 1018, a first 45° bent waveguide 1013, a second 45° bent waveguide 1017, and a waveguide cross structure 102. The optical waveguide is folded in this region, and the optical signal passes through the electrode gap with the same electric field direction to ensure the continuous accumulation of the modulation phase. The waveguide cross structure 102 ensures that the two optical signals do not interfere with each other in the cross-folded optical waveguide. It should be noted that since the waveguide and the metal electrode are on the same plane, in order to avoid direct contact between the optical waveguide and the metal electrode in the waveguide folding region 302, it is necessary to grow an isolation layer structure such as a SiO2 cladding or optical adhesive with a thickness greater than 1μm in this region to reduce the absorption loss of light by the metal. Figure 1 The illustration shows a cross-sectional view of a thin-film lithium niobate modulator. The thin-film lithium niobate wafer structure, from bottom to top, consists of a silicon substrate 401, a SiO2 buried layer 402, a lithium niobate planar layer 403, a lithium niobate waveguide, and a metal electrode.

[0029] The optical signal enters the folded phase modulator through the input optical waveguide 101; the optical signal sequentially passes through the first straight waveguide 1011, the first 135° bent optical waveguide 1012, the waveguide cross structure 102, the first 45° bent optical waveguide 1013, and the second straight waveguide 1014 to achieve the first stage of phase modulation. The first straight waveguide 1011 is located in the electrode gap between the signal electrode 201 and the first ground electrode 202, and the second straight waveguide 1014 is located in the electrode gap between the signal electrode 201 and the second ground electrode 203. The electric field directions of the two electrode gaps are the same, thus ensuring the continuous accumulation of the modulated phase. Subsequently, the optical signal passes through the loop waveguide 1011. After 15, the optical signal enters the third electrode gap; the optical signal passes sequentially through the third straight waveguide 1016, the second 45° bent optical waveguide 1017, the waveguide cross structure 102, the second 135° bent optical waveguide 1018, and the fourth straight waveguide 1019 to complete the second-stage phase modulation. The third straight waveguide 1016 is located in the electrode gap between the signal electrode 201 and the second ground electrode 203, and the fourth straight waveguide 1019 is located in the electrode gap between the signal electrode 201 and the first ground electrode 202. The electric field directions of the two electrode gaps are the same, thus ensuring the continuous accumulation of the modulated phase; finally, the optical signal is output from the phase modulator through the output optical waveguide 103.

[0030] When the optical signal propagates in the straight waveguide of modulation region 301, electro-optic modulation is achieved. There are four modulation segments: the first straight waveguide 1011, the second straight waveguide 1014, the third straight waveguide 1016, and the fourth straight waveguide 1019. In the waveguide folding region 302, the optical waveguide is folded and connected to the modulation optical waveguide with the same electric field direction. The curved waveguides of the two-stage phase modulators in this region are symmetrically distributed and have the same optical delay.

[0031] The metal electrodes employ a traveling wave electrode design. The signal electrode 201 comprises two straight electrodes and one curved electrode. The microwave signal first propagates from left to right in the first straight signal electrode, modulating the optical signal in the first modulation region. It then enters the curved signal electrode, achieving one fold, and finally enters the second straight signal electrode, propagating from right to left, modulating the optical signal in the second modulation region. Throughout the modulation process, the propagation directions of the microwave and optical signals must remain consistent to achieve effective modulation. The first ground electrode 202 comprises two straight electrodes and one curved electrode, achieving one fold. The second ground electrode 203 comprises only one straight electrode. The electric field directions in the electrode gaps between the signal electrode and the two ground electrodes are different. In the waveguide folding region 302, the optical delay should be equal to the microwave delay; that is, the optical delay caused by the 135° curved waveguide, the 45° curved waveguide, and the waveguide intersection structure should be equal to the microwave delay caused by the curved electrode in this region, thus ensuring speed matching between the modulator's optical and microwave waves.

[0032] Second embodiment:

[0033] Figure 2A schematic diagram of a folded cyclic phase modulator with a T-shaped electrode structure is shown. Its overall structure includes an input optical waveguide 101, an optical waveguide in the modulation region, an optical waveguide in the waveguide folding region, a waveguide cross structure 102, a loop waveguide 1015, an output optical waveguide 103, a signal electrode 201, a first T-shaped microstructure electrode 2011 and a second T-shaped microstructure electrode 2012 extending from the signal electrode, a first ground electrode 202, a second ground electrode 203, a third T-shaped microstructure electrode 2021 extending from the first ground electrode 202, and a fourth T-shaped microstructure electrode 2031 extending from the second ground electrode 203. The modulation region 301 includes a first straight waveguide 1011, a second straight waveguide 1014, a third straight waveguide 1016, a fourth straight waveguide 1019, and electrodes. Microwave signals modulate optical signals in this region. The waveguide folding region 302 includes a first 135° bent waveguide 1012, a second 135° bent waveguide 1018, a first 45° bent waveguide 1013, a second 45° bent waveguide 1017, and a waveguide cross structure 102. The optical waveguide is folded in this region, and the optical signal passes through the electrode gap with the same electric field direction to ensure the continuous accumulation of the modulation phase. The waveguide cross structure 102 ensures that the two optical signals do not interfere with each other in the cross-folded optical waveguide. It should be noted that since the waveguide and the metal electrode are on the same plane, in order to avoid direct contact between the optical waveguide and the metal electrode in the waveguide folding region 302, it is necessary to grow an isolation layer structure such as a SiO2 cladding or optical adhesive with a thickness greater than 1μm in this region to reduce the absorption loss of light by the metal. Figure 2 The illustration shows a cross-sectional view of a thin-film lithium niobate modulator. The thin-film lithium niobate wafer structure, from bottom to top, consists of a silicon substrate 401, a SiO2 buried layer 402, a lithium niobate planar layer 403, a lithium niobate waveguide, a metal electrode, and its extended T-shaped electrode microstructure.

[0034] Compared with the first embodiment, the optical waveguide structure is basically the same, except that the cuboid electrode is replaced by a combination of cuboid electrode and T-shaped microstructure electrode. Using T-shaped electrode as microwave transmission line can effectively reduce the radio frequency loss of the electrode and help improve the overall electro-optic bandwidth of the modulator.

[0035] The modulation principle of the second embodiment is basically the same as that of the first embodiment. The optical signal enters the folded phase modulator through the input optical waveguide 101; the optical signal sequentially passes through the first straight waveguide 1011, the first 135° bent optical waveguide 1012, the waveguide cross structure 102, the first 45° bent optical waveguide 1013, and the second straight waveguide 1014 to achieve the first stage of phase modulation. The first straight waveguide 1011 is located in the electrode gap between the signal electrode 201 and the first ground electrode 202, and the second straight waveguide 1014 is located in the electrode gap between the signal electrode 201 and the second ground electrode 203. The electric field directions of the two electrode gaps are the same, thus ensuring the continuous accumulation of the modulated phase. Subsequently, the optical signal passes through the loop waveguide 1011... After 15, the optical signal enters the third electrode gap; the optical signal passes sequentially through the third straight waveguide 1016, the second 45° bent optical waveguide 1017, the waveguide cross structure 102, the second 135° bent optical waveguide 1018, and the fourth straight waveguide 1019 to complete the second-stage phase modulation. The third straight waveguide 1016 is located in the electrode gap between the signal electrode 201 and the second ground electrode 203, and the fourth straight waveguide 1019 is located in the electrode gap between the signal electrode 201 and the first ground electrode 202. The electric field directions of the two electrode gaps are the same, thus ensuring the continuous accumulation of the modulated phase; finally, the optical signal is output from the phase modulator through the output optical waveguide 103.

[0036] When the optical signal propagates in the straight waveguide of modulation region 301, electro-optic modulation is achieved. There are four modulation segments: the first straight waveguide 1011, the second straight waveguide 1014, the third straight waveguide 1016, and the fourth straight waveguide 1019. In the waveguide folding region 302, the optical waveguide is folded and connected to the modulation optical waveguide with the same electric field direction. The curved waveguides of the two-stage phase modulators in this region are symmetrically distributed and have the same optical delay.

[0037] The metal electrodes employ a traveling-wave electrode design. T-shaped microstructure electrodes extending from the electrodes reduce current congestion at the electrode edges, thereby reducing conductor losses. The arrangement of the T-shaped microelectrodes in the signal and ground electrodes is identical. The width of the T-shaped electrodes is 45 μm, and the gap between two adjacent T-shaped microstructures is 5 μm. This design ensures that the cutoff frequency of the microwave signal is much greater than the electro-optic bandwidth of the modulator. The signal electrode 201 includes two straight electrodes and one curved electrode. The microwave signal first propagates from left to right in the first straight signal electrode, modulating the optical signal in the first modulation region. It then enters the curved signal electrode, achieving one fold, and finally enters the second straight signal electrode, propagating from right to left, thus modulating the optical signal in the second modulation region. Throughout the modulation process, the propagation directions of the microwave and optical signals must remain consistent to achieve effective modulation. The first ground electrode 202 includes two straight electrodes and one curved electrode, achieving one fold. The second ground electrode 203 consists of only one straight electrode. The electric field directions in the electrode gaps between the signal electrode and the two ground electrodes are different. In the waveguide folding region 302, the delay of the optical wave should be equal to the delay of the microwave. That is, the optical delay caused by the 135° bent waveguide, the 45° bent waveguide and the waveguide cross structure should be equal to the microwave delay caused by the bent electrode in this region, so as to ensure the speed matching of the optical wave and the microwave in the modulator.

[0038] A practical measurement was performed on a folded cyclic phase modulator with a modulation length of 2 cm. Since this device is a phase modulator, its half-wave voltage was measured using a spectroscopic method. By applying microwave signals of different radio frequency frequencies, the half-wave voltage was calculated based on the variation of the optical carrier power and the power of the applied radio frequency signal. The radio frequency was measured in 100 MHz steps. The measurement results for the half-wave voltage of the folded cyclic phase modulator with a total modulation length of 2 cm are shown below. Figure 3 As shown. Due to the limitations of spectrometer resolution, the testing range for radio frequency is 3.5 GHz to 40 GHz. Figure 3 It can be seen that seven modulation cycles can be observed in the radio frequency range of 3.5 GHz to 40 GHz. Due to the resolution limitations of the spectrometer, the complete first modulation cycle could not be measured; magnified views of the subsequent six cycles are shown below. Figure 4 As shown, the available bandwidth in each cycle is greater than 3 GHz. The measurement results of the lowest half-wave voltage and available bandwidth in each modulation cycle are summarized as follows: Figure 5 As shown, the minimum half-wave voltage and available bandwidth for the last six cycles are 2.0V, 2.1V, 2.4V, 2.7V, 3.0V, and 3.0V, respectively, at 3GHz, 3GHz, 3GHz, 3.1GHz, 3.2GHz, and 3GHz. With increasing RF frequency, the minimum half-wave voltage value within each modulation cycle increases from 2.0V to 3.0V.

[0039] The electro-optic response of a folded cyclic phase modulator was measured using a method that filters out a single sideband. The measurement results reflect the modulator's response to radio frequency (RF) signals of different frequencies. The optical signal modulated by the phase modulator is passed through a bandpass filter to remove one of the modulated optical sidebands, ensuring that the optical signal entering the photodetector can generate an RF signal. Due to limitations in the steepness of the bandpass filter's slope, the electro-optic response was measured at RF frequencies from 5 GHz to 40 GHz. The measurement results are as follows: Figure 6 As shown, the available electro-optic bandwidths in each cycle are 2.89 GHz, 3.05 GHz, 2.97 GHz, 3.01 GHz, 2.89 GHz and 2.97 GHz, which can meet the bandwidth requirements of most experiments.

Claims

1. A folded phase modulator with ultra-low half-wave voltage and high bandwidth utilization, characterized in that, It includes an input optical waveguide (101), an optical waveguide with a modulation region (301), an optical waveguide with a waveguide folding region, a waveguide cross structure (102), a loop waveguide (1015), an output optical waveguide (103), a signal electrode (201), a first ground electrode (202), and a second ground electrode (203). The modulation region (301) includes a first straight waveguide (1011), a second straight waveguide (1014), a third straight waveguide (1016), a fourth straight waveguide (1019), and a straight electrode. The microwave signal modulates the optical signal in this region. The waveguide folding region (302) includes a first 135° bent waveguide (1012), a second 135° bent waveguide (1018), a first 45° bent waveguide (1013), a second 45° bent waveguide (1017), and a waveguide cross structure (102). The optical waveguide is folded in this region, and the optical signal passes through an electrode gap with the same electric field direction to ensure the continuous accumulation of the modulation phase. The waveguide cross structure (102) ensures that the two optical signals do not interfere with each other in the cross-folded optical waveguide. The signal electrode (201) includes two straight electrodes and one curved electrode. The microwave signal is first transmitted from left to right in the first straight electrode, and the optical signal is modulated in the first modulation region. Then it enters the curved electrode to achieve one folding of the electrode, and finally enters the second straight electrode and is transmitted from right to left, that is, the optical signal is modulated in the second modulation region. The first ground electrode (202) includes two straight electrodes and one curved electrode, and the second ground electrode (203) includes only one straight electrode. The optical signal enters the folded phase modulator through the input optical waveguide (101); the optical signal sequentially passes through the first straight waveguide (1011), the first 135° bent optical waveguide (1012), the waveguide cross structure (102), the first 45° bent waveguide (1013), and the second straight waveguide (1014) to achieve the first stage of phase modulation. The first straight waveguide (1011) is located in the electrode gap between the signal electrode (201) and the first ground electrode (202), and the second straight waveguide (1014) is located in the electrode gap between the signal electrode (201) and the second ground electrode (203). The electric field directions of the two electrode gaps are the same, thus ensuring the continuous accumulation of the modulated phase; subsequently, the optical signal passes through the loop waveguide (101) 5) Then it enters the third electrode gap; the optical signal passes through the third straight waveguide (1016), the second 45° bent optical waveguide (1017), the waveguide cross structure (102), the second 135° bent optical waveguide (1018) and the fourth straight waveguide (1019) in sequence to complete the second stage phase modulation. The third straight waveguide (1016) is located in the electrode gap between the signal electrode (201) and the second ground electrode (203), and the fourth straight waveguide (1019) is located in the electrode gap between the signal electrode (201) and the first ground electrode (202). The electric field directions of the two electrode gaps are the same, thus ensuring the continuous accumulation of the modulation phase; finally, the optical signal is output from the phase modulator through the output optical waveguide (103).

2. The folded phase modulator with ultra-low half-wave voltage and high bandwidth utilization according to claim 1, characterized in that, The thin-film lithium niobate wafer structure of the modulator consists of, from bottom to top, a silicon substrate (401), a SiO2 buried layer (402), a lithium niobate planar layer (403), a lithium niobate waveguide, and a metal electrode.

3. The folded phase modulator with ultra-low half-wave voltage and high bandwidth utilization according to claim 1, characterized in that, In the waveguide folded region (302), since the waveguide and the metal electrode are on the same plane, in order to avoid the optical waveguide in the waveguide folded region (302) from directly contacting the metal electrode, it is necessary to grow a SiO2 cladding layer or optical adhesive isolation layer structure with a thickness greater than 1μm in this region.

4. The folded phase modulator with ultra-low half-wave voltage and high bandwidth utilization according to claim 1, characterized in that, The signal electrode (201), the first ground electrode (202), and the second ground electrode (203) are rectangular parallelepiped structures.

5. A folded phase modulator with ultra-low half-wave voltage and high bandwidth utilization according to claim 1, characterized in that, The electrode consists of a signal electrode (201), a first ground electrode (202), a second ground electrode (203), and T-shaped electrodes extending from them. The signal electrode (201) extends into a first T-shaped microstructure electrode (2011) and a second T-shaped microstructure electrode (2012). The first ground electrode (202) extends into a third T-shaped microstructure electrode (2021). The second ground electrode (203) extends into a fourth T-shaped microstructure electrode (2031).

6. A folded phase modulator with ultra-low half-wave voltage and high bandwidth utilization according to claim 5, characterized in that, The width of the T-shaped microstructure electrode is 45 μm and the gap between the two T-shaped microstructures is 5 μm.

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