Broadband efficient circulating type lithium niobate film phase modulator and implementation method thereof

Abstract

The invention discloses a broadband efficient circulating type lithium niobate film phase modulator and an implementation method thereof. The phase modulator comprises a substrate layer, a buried oxide layer, an X-cut lithium niobate layer, a silicon dioxide upper cladding and a periodic capacitance loading traveling wave electrode structure which are sequentially stacked from bottom to top. The X-cut lithium niobate layer contains a lithium niobate film waveguide formed through an etching technology, and the lithium niobate film waveguide comprises an input end waveguide, a first loop waveguide, a second loop waveguide, a third loop waveguide and an output end waveguide which are connected in sequence. The invention provides a design method of a broadband high-efficiency circulating type lithium niobate film phase modulator, circulating type electro-optical modulation is realized, the speed of a light group entering an electro-optical modulation area every time is matched with the speed of a radio frequency phase by reasonably setting the time delay of a loop waveguide, and the phase modulation efficiency is improved. The half-wave voltage at the target frequency can be reduced to one fourth of the half-wave voltage of a traditional phase modulator.

Description

Technical Field

[0001] This invention belongs to the field of lithium niobate thin-film electro-optic modulator technology, specifically relating to a broadband, high-efficiency cyclic lithium niobate thin-film phase modulator and its implementation method. Background Technology

[0002] On-chip optical frequency combs and ultrashort optical pulse sources are key components of numerous photonic systems, including optical communication, optical sampling analog-to-digital converters, microwave photonics, optical atomic clocks, lidar, broadband spectrum generation, and spectroscopy. Currently, on-chip ultrashort optical pulse generation methods are mainly divided into three categories: resonant cavity-based semiconductor mode-locked lasers, soliton optical frequency combs, and electro-optic modulation-based ultrashort optical pulse sources.

[0003] On-chip mode-locked lasers mostly employ passive mode-locking with saturable absorbers or active mode-locking with electro-optic modulation, which generally suffers from poor tunability, low energy conversion efficiency, high noise, and weak stability. Currently, research on soliton frequency combs focuses on the dispersion design of microcavity waveguides, soliton frequency comb envelope flattening, and process improvements for high-Q resonators, with little research on the time-domain pulse characteristics of soliton frequency combs. Electro-optic modulation-based ultrashort optical pulse sources generate optical pulses by electro-optically modulating continuous-wave lasers and utilizing the time-lensing characteristics of phase modulators, exhibiting excellent characteristics such as tunable operating wavelength and repetition frequency, and highly flat spectra. However, limited by the large half-wave voltage of traditional bulk modulators, the realization of such ultrashort optical pulse sources typically requires cascading an intensity modulator and multiple phase modulators, while also requiring multiple electrical amplifiers and phase shifters to precisely control the microwave power and phase of each modulator. Therefore, traditional ultrashort optical pulse sources based on electro-optic modulation suffer from high power consumption, large size, and high cost.

[0004] With the development of lithium niobate thin-film integrated photonic platforms, optical waveguides with strong optical field confinement capabilities and low loss have been realized, and lithium niobate thin-film electro-optic modulators with low half-wave voltage and high electro-optic bandwidth have been successfully developed. In 2018, Marko Loncar's research group at Harvard University proposed the first CMOS-driven lithium niobate thin-film electro-optic modulator, achieving a high bandwidth and low power consumption of 1.4V driving voltage and an electro-optic bandwidth of up to 45GHz. In 2022, Cai Xinlun's research group at Sun Yat-sen University, based on a quartz substrate-based lithium niobate thin-film capacitor-loaded modulator, achieved a half-wave voltage of 1V and an electro-optic bandwidth of 110GHz, completing the verification of single-wavelength 1.96Tb / s dual-polarization IQ modulation coherent optical communication. These advanced electro-optic modulation technologies provide key technical support for realizing efficient and compact on-chip cavity-free ultrashort optical pulse sources. In 2022, Marko Loncar's research group at Harvard University achieved the world's first on-chip electro-optic modulated ultrashort optical pulse source using a lithium niobate thin-film electro-optic modulator, generating ultrashort optical pulses with a repetition frequency of 30 GHz, a pulse width of 503 fs, and a timing jitter of 19 fs. However, in this work, the incident light only undergoes modulation once in each electro-optic modulation region, still requiring relatively high RF drive power; moreover, the 3 dB electro-optic modulation bandwidth in the paper is only 45 GHz, which cannot support the generation of optical pulses with higher repetition frequencies. Summary of the Invention

[0005] This invention aims to address the shortcomings of existing technologies and provides the following solutions: A broadband, high-efficiency cyclic lithium niobate thin-film phase modulator includes: a substrate layer, a buried oxide layer, an X-cut lithium niobate layer, a silicon dioxide upper cladding layer, and a periodic capacitor-loaded traveling wave electrode structure, which are stacked sequentially from bottom to top. The X-cut lithium niobate layer contains a lithium niobate thin film waveguide formed by etching technology. The lithium niobate thin film waveguide includes an input waveguide, a loop waveguide 1, a loop waveguide 2, a loop waveguide 3, and an output waveguide connected in sequence.

[0006] Preferably, the periodic capacitively loaded traveling wave electrode structure includes: a first ground electrode, a signal electrode, and a second ground electrode; A first electric field exists between the first ground electrode and the signal electrode, and a second electric field exists between the signal electrode and the second ground electrode, with the first and second electric fields in opposite directions.

[0007] Preferably, the first ground electrode consists of a first strip-shaped grounding main electrode and periodic micro T-electrodes connected to each other; The second ground electrode consists of an interconnected second strip-shaped grounding main electrode and the periodic micro T-electrode; The periodic micro T-electrodes connected to the first strip grounded main electrode and the second strip grounded main electrode are respectively located on the side near the upper arm electro-optic modulation region waveguide and the lower arm electro-optic modulation region waveguide; The signal electrode consists of an interconnected bar-shaped main signal electrode and the periodic micro T-electrode, but the periodic micro T-electrode is located on both sides of the bar-shaped main signal electrode. All of the periodic micro T-electrodes described herein have the same structure.

[0008] Preferably, the loop waveguide includes a first mode converter, an upper arm electro-optic modulation region waveguide, a second mode converter, and a loop delay waveguide connected in sequence. The loop waveguide includes the upper arm electro-optic modulation region waveguide, the second mode converter, the cross waveguide, the loop delay waveguide, and the third mode converter connected in sequence. The loop three-waveguide includes a lower arm electro-optic modulation region waveguide, a fourth mode converter, a loop three-delay waveguide, and a third mode converter connected in sequence.

[0009] Preferably, the first mode converter, the second mode converter, the third mode converter, and the fourth mode converter are all fast adiabatic mode converters; the fast adiabatic mode converter includes a lower arm S-bend waveguide, a lower arm fast adiabatic waveguide, a lower arm tapered waveguide, and an upper arm multimode waveguide.

[0010] Preferably, the cross waveguide is a multimode interference cross waveguide, including a first tapered waveguide in the Y direction, a multimode waveguide in the Y direction, a second tapered waveguide in the Y direction, a first tapered waveguide in the Z direction, a multimode waveguide in the Z direction, and a second tapered waveguide in the Z direction, and is completely symmetrical about the central intersection point in the Y and Z directions.

[0011] This invention also provides a method for implementing a broadband, high-efficiency cyclic lithium niobate thin-film phase modulator, the method comprising the following steps: The first TE0 mode optical signal is input through the input waveguide; The first TE0 mode optical signal output from the input waveguide is input to the upper arm electro-optic modulation region waveguide through the first mode converter for the first electro-optic phase modulation. The modulated first TE0 mode optical signal output from the upper arm electro-optic modulation region waveguide is input to the loop delay waveguide after passing through the second mode converter. After passing through the loop delay waveguide, it is input to the first mode converter again and converted into the first TE1 mode optical signal. The first TE1 mode optical signal output from the first mode converter is input into the upper arm electro-optic modulation region waveguide for a second electro-optic phase modulation. The modulated first TE1 mode optical signal output from the upper arm electro-optic modulation region waveguide is converted into a second TE0 mode optical signal after passing through the second mode converter and input into the cross waveguide. The second TE0 mode optical signal output from the cross waveguide is input into the third mode converter after passing through the loop two-delay waveguide. The second TE0 mode optical signal output from the third mode converter is input into the lower arm electro-optic modulation region waveguide for a third electro-optic phase modulation. The modulated second TE0 mode optical signal output from the lower arm electro-optic modulation region waveguide is input into the loop three-delay waveguide after passing through the fourth mode converter. The modulated second TE0 mode optical signal output from the loop three-delay waveguide is converted into a second TE1 mode optical signal by the third mode converter. The second TE1 mode optical signal output from the third mode converter is input into the lower arm electro-optic modulation region waveguide for fourth electro-optic phase modulation. The modulated second TE1 mode optical signal output from the lower arm electro-optic modulation region waveguide is converted into a third TE0 mode optical signal by the fourth mode converter and input into the cross waveguide. The third TE0 mode optical signal output from the cross waveguide is output from the output waveguide to the cyclic lithium niobate thin film phase modulator. The total length of the first, second, and third loop waveguides is set so that the optical group velocity is consistent with the radio frequency phase velocity each time it enters the electro-optic modulation process. This makes the cyclic lithium niobate thin film phase modulator equivalent to four cascaded conventional phase modulators, that is, the half-wave voltage drop of the cyclic lithium niobate thin film phase modulator is one-quarter of that of the conventional phase modulator.

[0012] Preferably, in order to achieve the target radio frequency f target At this point, the half-wave voltage of the cyclic lithium niobate thin-film phase modulator is reduced to one-quarter of that of a conventional phase modulator, and the total delay of the first loop waveguide, the second loop waveguide, and the third loop waveguide is reduced. τ 1. τ 2 and τ 3 is: in, n, k, q Represents positive integers. N + Represents the set of positive integers; Waveguide lengths of the first, second, and third waveguides of the loop L i for: in, c Represents the speed of light. i Indicates the waveguide index. i =1,2,3 ng This represents the group refractive index of the waveguide.

[0013] Compared with the prior art, the beneficial effects of the present invention are as follows: (1) This invention proposes a design method for a broadband and efficient cyclic lithium niobate thin film phase modulator to achieve cyclic electro-optic modulation. By reasonably setting the delay of the loop waveguide, the group velocity of the light entering the electro-optic modulation area is matched with the radio frequency phase velocity, and the half-wave voltage at the target frequency can be reduced to one-quarter of the half-wave voltage of the traditional phase modulator.

[0014] (2) The broadband, high-efficiency cyclic lithium niobate thin-film phase modulator proposed in this invention employs periodically capacitively loaded traveling-wave electrodes. This structure, without increasing the electrode spacing, alters the current distribution pattern in the conductor through periodically loaded micro-T electrodes. In traditional coplanar waveguide strip electrode structures, current concentrates in the edge region of the strip electrode, resulting in uneven current density distribution and significant microwave transmission loss. However, the periodically capacitively loaded traveling-wave electrode structure, by introducing additional capacitive coupling, prevents current from flowing near the edge region of the strip main electrode, allowing the current to be evenly distributed in the wider main electrode structure, thereby increasing the effective conductive area of ​​the conductor and significantly reducing microwave transmission loss. Furthermore, reducing the spacing between the electrode and the waveguide to the submicron level not only achieves higher modulation efficiency but also enables more precise electro-optical velocity matching, successfully breaking through the traditional trade-off between bandwidth and modulation efficiency in the modulation region. This is beneficial for achieving low half-wave voltage while maintaining a large electro-optical bandwidth.

[0015] (3) The broadband and efficient cyclic lithium niobate thin film phase modulator proposed in this invention adopts a fast adiabatic mode converter to efficiently realize the conversion from TE0 mode to TE1 mode. It has the advantages of small size, low insertion loss, large bandwidth and large fabrication tolerance. Attached Figure Description

[0016] To more clearly illustrate the technical solution of the present invention, the drawings used in the embodiments are briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0017] Figure 1 This is a top view of a broadband, high-efficiency cyclic lithium niobate thin-film phase modulator in an embodiment of the present invention. Figure 2This is a schematic diagram of the cross-section of a broadband, high-efficiency cyclic lithium niobate thin-film phase modulator in an embodiment of the present invention; Figure 3 This is a partially enlarged top view of the periodically capacitively loaded traveling wave electrode in an embodiment of the present invention; Figure 4 This is a top view of the fast adiabatic mode converter in an embodiment of the present invention. Figure 5 This is a top view of the multimode interferometric cross waveguide in an embodiment of the present invention. Figure 6 This is a parameter diagram of the fast adiabatic mode converter in an embodiment of the present invention, wherein (a) represents the adiabatic parameters. c ( y (a) is a schematic diagram, and (b) is the taper function. y ( w 1) Schematic diagram; Figure 7 The transmission spectrum and electric field intensity propagation diagram of the fast adiabatic mode converter in this embodiment of the invention are shown, where (a) is a schematic diagram of the transmission spectrum and (b) is a schematic diagram of the electric field intensity propagation. Figure 8 The transmission spectrum and electric field intensity propagation diagrams of the multimode interferometric cross waveguide in this embodiment of the invention are shown, where (a) is a schematic diagram of the transmission spectrum and (b) is a schematic diagram of the electric field intensity propagation. Figure 9 The diagrams show the radio frequency effective refractive index, radio frequency loss, impedance, and electro-optic response of a broadband high-efficiency cyclic lithium niobate thin film phase modulator in this embodiment of the invention. (a) is a schematic diagram of the radio frequency effective refractive index, (b) is a schematic diagram of the radio frequency loss, (c) is a schematic diagram of the impedance, and (d) is a schematic diagram of the electro-optic response. Figure 10 This is a schematic diagram showing the relationship between the ratio of the half-wave voltage of the broadband high-efficiency cyclic lithium niobate thin-film phase modulator in this embodiment of the invention and the half-wave voltage of a conventional phase modulator with the same electro-optic modulation region structure, and the radio frequency. Explanation of reference numerals in the attached figures: 101. Input waveguide; 102. First mode converter; 103. Upper arm electro-optic modulation region waveguide; 104. Second mode converter; 105. Loop one delay waveguide; 106. Cross waveguide; 107. Loop two delay waveguide; 108. Third mode converter; 109. Lower arm electro-optic modulation region waveguide; 1010. Fourth mode converter; 1011. Loop three delay waveguide; 1012. Output waveguide; 201. First ground electrode; 202. Signal electrode; 203. Second ground electrode; 204. First strip 205. Second strip grounding main electrode; 206. Second strip grounding main electrode; 207. Periodic miniature T-electrode; 208. Air hole; 301. Lower arm S-bend waveguide; 302. Lower arm fast thermal insulation waveguide; 303. Lower arm tapered waveguide; 304. Upper arm multimode waveguide; 401. First tapered waveguide in the Y direction; 402. Multimode waveguide in the Y direction; 403. Second tapered waveguide in the Y direction; 404. First tapered waveguide in the Z direction; 405. Multimode waveguide in the Z direction; 406. Second tapered waveguide in the Z direction. Detailed Implementation

[0018] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0019] To make the above-mentioned objects, features and advantages of the present invention more apparent and understandable, the present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments.

[0020] Example 1 In this embodiment, as Figure 1 , Figure 2 As shown, a broadband, high-efficiency cyclic lithium niobate thin-film phase modulator includes: a substrate layer, a buried oxide layer, an X-cut lithium niobate layer, a silicon dioxide upper cladding layer, and a periodically capacitively loaded traveling wave electrode structure, which are stacked sequentially from bottom to top.

[0021] The X-cut lithium niobate layer contains a lithium niobate thin film waveguide formed by etching technology. The lithium niobate thin film waveguide includes an input waveguide 101, a loop waveguide 1 (represented by a short white dashed line), a loop waveguide 2 (represented by a solid white line), a loop waveguide 3 (represented by a long white dashed line), and an output waveguide 1012 connected in sequence.

[0022] like Figure 3As shown, the periodic capacitively loaded traveling wave electrode structure includes: a first ground electrode 201, a signal electrode 202, and a second ground electrode 203; a first electric field exists between the first ground electrode 201 and the signal electrode 202, and a second electric field exists between the signal electrode 202 and the second ground electrode 203, with the first and second electric fields having opposite directions.

[0023] The first ground electrode 201 consists of a first strip-shaped ground main electrode 204 and a periodic miniature T-electrode 207 connected to each other; the second ground electrode 203 consists of a second strip-shaped ground main electrode 206 and a periodic miniature T-electrode 207 connected to each other; the periodic miniature T-electrode 207 connected to the first strip-shaped ground main electrode 204 and the second strip-shaped ground main electrode 206 are respectively located on one side near the upper arm electro-optic modulation region waveguide 103 and the lower arm electro-optic modulation region waveguide 109; the signal electrode 202 consists of a strip-shaped signal main electrode 205 and a periodic miniature T-electrode 207 connected to each other, but there are periodic miniature T-electrodes 207 on both sides of the strip-shaped signal main electrode 205; all periodic miniature T-electrodes 207 have the same structure.

[0024] The first loop waveguide comprises a first mode converter 102, an upper arm electro-optic modulation region waveguide 103, a second mode converter 104, and a first loop delay waveguide 105 connected in sequence; the second loop waveguide comprises an upper arm electro-optic modulation region waveguide 103, a second mode converter 104, a cross waveguide 106, a second loop delay waveguide 107, and a third mode converter 108 connected in sequence; the third loop waveguide comprises a lower arm electro-optic modulation region waveguide 109, a fourth mode converter 1010, a third loop delay waveguide 1011, and a third mode converter 108 connected in sequence.

[0025] Preferably, the process of optical signal propagation in lithium niobate thin film waveguide is as follows: the first TEO mode optical signal output from the input waveguide 101 is input into the upper arm electro-optic modulation region waveguide 103 through the first mode converter 102 for the first electro-optic phase modulation; the modulated first TEO mode optical signal output from the upper arm electro-optic modulation region waveguide 103 is input into the loop delay waveguide 105 after passing through the second mode converter 104; after passing through the loop delay waveguide 105, it is input into the first mode converter 102 again and converted into the first TEO mode optical signal. The first TE1 mode optical signal output from the first mode converter 102 is input into the upper arm electro-optic modulation region waveguide 103 for a second electro-optic phase modulation. The modulated first TE1 mode optical signal output from the upper arm electro-optic modulation region waveguide 103 is converted into a second TE0 mode optical signal after passing through the second mode converter 104 and input into the cross waveguide 106. The second TE0 mode optical signal output from the cross waveguide 106 is input into the third mode converter 108 after passing through the loop second delay waveguide 107. The second TE0 mode optical signal output from the third mode converter 108 is input into the lower arm electro-optic modulation region waveguide 109 for a third electro-optic phase modulation. The modulated second TE0 mode optical signal output from the lower arm electro-optic modulation region waveguide 109 is input into the loop three-delay waveguide 1011 after passing through the fourth mode converter 1010. The modulated second TE0 mode optical signal output from the loop three-delay waveguide 1011 is converted into a second TE1 mode optical signal by the third mode converter 108. The second TE1 mode optical signal output from the third mode converter 108 is input into the lower arm electro-optic modulation region waveguide 109 for the fourth electro-optic phase modulation. The modulated second TE1 mode optical signal output from the lower arm electro-optic modulation region waveguide 109 is converted into a third TE0 mode optical signal by the fourth mode converter 1010 and input into the cross waveguide 106. The third TE0 mode optical signal output from the cross waveguide 106 is output to the cyclic lithium niobate thin film phase modulator through the output waveguide.

[0026] In this embodiment, because the substrate is made of silicon, which has a high dielectric constant (11.7), the effective refractive index of the microwave signal is relatively high (3.1), while the group refractive index of the light wave in the lithium niobate waveguide is 2.2~2.3. When the speeds of the two are mismatched, the modulation efficiency will decrease sharply with increasing frequency, severely limiting the modulation bandwidth. Therefore, in order to reduce the effective refractive index of the microwave signal, it is necessary to hollow out the silicon substrate through air holes 208 located between the periodic micro T electrodes 207 to form air trenches, which is beneficial to achieve speed matching between microwaves and light waves.

[0027] The first mode converter 102, the second mode converter 104, the third mode converter 108, and the fourth mode converter 1010 all employ fast adiabatic mode converters; the fast adiabatic mode converter includes a lower arm S-bend waveguide 301, a lower arm fast adiabatic waveguide 302, a lower arm tapered waveguide 303, and an upper arm multimode waveguide 304, such as Figure 4 As shown, the propagation process of the optical signal in the fast adiabatic mode converter is as follows: If the TE0 mode optical signal is input from port 2, it passes through the lower arm S-bend waveguide 301 and then enters the mode conversion area. The energy is gradually converted from the lower arm fast adiabatic waveguide 302 to the upper arm multimode waveguide 304 in TE1 mode, and finally outputs at port 3; conversely, if the TE0 mode optical signal is input from port 1, it passes through the upper arm multimode waveguide 304 in the mode conversion area and finally outputs directly from port 3.

[0028] The cross waveguide 106 is a multimode interference type cross waveguide 106, including a first tapered waveguide 401 in the Y direction, a multimode waveguide 402 in the Y direction, a second tapered waveguide 403 in the Y direction, a first tapered waveguide 404 in the Z direction, a multimode waveguide 405 in the Z direction, and a second tapered waveguide 406 in the Z direction, and is completely symmetrical about the central intersection point in the Y and Z directions, such as... Figure 5 As shown. Taking the Y-direction as an example, the process of optical signal propagation in the multimode interferometric cross-waveguide 106 is as follows: the TE0 mode optical signal is input from port 1, passes through the first tapered waveguide 401 in the Y direction, and is then input into the multimode waveguide 402 in the Y direction, exciting higher-order modes; according to the principle of multimode interference and self-imaging effect, when the light propagates to a specific length, each order mode undergoes constructive interference, periodically reproducing the input light field at a specific position, forming a self-image; if the self-image point is exactly at the waveguide cross-point and port 2, the TE0 mode optical signal can be directly output from port 2.

[0029] Example 2 In this embodiment, the propagation direction of the optical signal is set along the Y direction of the lithium niobate crystal. The ridge height and plate thickness of the lithium niobate thin film waveguide are both 200 nm, and the angle between the waveguide sidewall and the horizontal plane is 60°. The thickness of the silicon dioxide buried oxide layer is 3 μm, and its refractive index is 1.457. The thickness of the upper cladding layer is 1 μm, and its refractive index is 1.455. The total thickness of the silicon substrate is 700 μm, and the air hollowing depth is 35 μm. The width and length of the upper arm electro-optic modulation region waveguide 103 and the lower arm electro-optic modulation region waveguide 109 are 2.4 μm and 1 cm, respectively.

[0030] This embodiment employs a periodic capacitively loaded traveling wave electrode. The thickness and length of the first strip-shaped grounding main electrode 204, the strip-shaped signal main electrode 205, and the second strip-shaped grounding main electrode 206 are set to 1.1 μm and 1 cm, respectively. The widths of the first strip-shaped grounding main electrode 204, the strip-shaped signal main electrode 205, and the second strip-shaped grounding main electrode 206 are 150 μm, 85 μm, and 150 μm, respectively. The thickness of the periodic micro-T electrode 207 is 0.2 μm. gap , hw , hl , r , h , c , t , s The value is (2.7, 9, 39, 47, 14, 3, 5, 2) μm, as shown in the figure. Figure 3 As shown.

[0031] In this embodiment, the waveguide widths of loop delay waveguide 105, loop delay waveguide 107, and loop delay waveguide 1011 are set to 1 μm, and the total delay of loop delay waveguide 1011, loop delay waveguide 105, loop delay waveguide 107, and loop delay waveguide 1011 is set to 1 μm. τ 1. τ 2 and τ 3 equals 200ps, 216.7ps, and 200ps, the target radio frequency. f target It is 30GHz.

[0032] This embodiment employs a fast adiabatic mode converter, and sets ( gap 1, gap 2, w 1max , w 1min , w 1_terminate , w 2, L 1, L 2, L 3) is (5, 0.5, 1.4, 0.6, 0.2, 2.4, 38, 50, 38) μm, such as Figure 4 As shown. It should be noted that, in order to achieve device thermal insulation with a shorter length, the lower arm fast thermal insulation waveguide 302 does not use a traditional linear tapered waveguide. The width of the lower arm fast thermal insulation waveguide 302 exhibits a non-linear variation throughout the entire tapered region, as shown... Figure 6 Taper function in (b) y ( w As shown in Figure 1), insulation parameters are allocated as needed based on the rapid insulation theory. c ( y ), at the starting point and the end point c ( y The width of the lower arm rapid thermally insulating waveguide 302 changes rapidly in smaller locations, especially in strongly coupled regions. c ( y Larger positional changes are slow, making the overall situation... c ( y It presents a stable value, thus avoiding cross-effects, such as Figure 6 adiabatic parameters in (a) c ( y As shown in the figure; Furthermore, the transport spectrum and electric field propagation map of the fast adiabatic mode converter were calculated using a Lumerical Finite-Difference Time-Domain numerical simulator, and the results are as follows: Figure 7 As shown; from Figure 7 As can be seen in (a), within the wavelength range of 1400~1700nm, the 0.5-dB bandwidth of the fast adiabatic mode converter is 262nm, and the crosstalk within the 0.5-dB bandwidth is less than −14.84dB; Figure 7 (b) shows the electric field intensity propagation diagram of the fast adiabatic mode converter at a wavelength of λ=1550nm. It can be seen that the energy of the TE0 mode input to the fast adiabatic mode converter from port 2 is converted to the TE1 mode in the mode conversion region and output from port 3; the energy of the TE0 mode input to the fast adiabatic mode converter from port 1 is directly output from port 3.

[0033] This embodiment uses a multimode interferometric cross waveguide 106, and sets ( L 1, L 2, L 3, L 4, w 1, w 2, w 3, w 4) is (8, 29, 26, 7, 3.45, 1, 3.45, 1) μm, as shown in the figure. Figure 5 As shown; furthermore, the transmission spectrum and electric field propagation diagram of the multimode interferometric cross-waveguide 106 were calculated using the Lumerical Finite-Difference Time-Domain numerical simulator, and the results are as follows. Figure 8 As shown; from Figure 8 As can be seen in (a), the loss of the multimode interferometric cross waveguide 106 is less than 0.2dB in the wavelength range of 1400~1700nm; Figure 8 (b) shows the electric field intensity propagation diagram of the multimode interference cross waveguide 106 at a wavelength of λ=1550nm. It can be seen that the energy of the TE0 mode input from port 1 forms a self-image at the waveguide crossover point and port 2, and is output from port 2; the energy of the TE0 mode input from port 3 forms a self-image at the waveguide crossover point and port 4, and is output from port 4.

[0034] Next, the effective refractive index of the cyclic lithium niobate thin-film phase modulator was calculated. n m RF loss, impedance, and electro-optic response, such as Figure 9 As shown; from Figure 9 As can be seen in (a), the effective refractive index of radio frequency in the 100GHz range is... n m Approximately equal to the group refractive index of light n g That is, speed matching between light and radio frequency, which is beneficial for achieving a larger electro-optical bandwidth; from Figure 9 As can be seen from (b), the RF loss is less than 3dB / cm in the 100GHz range; Figure 9 As can be seen from (c), the impedance is approximately 40Ω in the 100GHz range, indicating a 10Ω impedance mismatch; from Figure 9 As can be seen in (d), although the 10Ω impedance mismatch causes slight oscillations in the electro-optic bandwidth curve, it does not lead to a sharp decrease in the electro-optic bandwidth. The electro-optic bandwidth of the cyclic lithium niobate thin film phase modulator is still greater than 100GHz.

[0035] Calculations show that, at a wavelength of λ=1550nm and a radio frequency of 30GHz, the half-wave voltage-length product and optical loss in the electro-optic modulation region are 1.26V∙cm and 0.028dB / cm, respectively. Figure 10 This represents the ratio of the half-wave voltage of a cyclic lithium niobate thin-film phase modulator to the half-wave voltage of a conventional phase modulator with the same electro-optic modulation region structure at different radio frequency frequencies. It should be noted that at a radio frequency of 30 GHz, optimal speed matching can be achieved each time the optical signal cycles to the electro-optic modulation region, reducing the half-wave voltage of the cyclic lithium niobate thin-film phase modulator to one-quarter that of the conventional phase modulator with the same electro-optic modulation region structure. Figure 10 As shown.

[0036] Example 3 In this embodiment, a method for implementing a broadband, high-efficiency cyclic lithium niobate thin-film phase modulator includes the following steps: S1. The first TE0 mode optical signal is input through the input waveguide 101; S2. The first TE0 mode optical signal output from the input waveguide 101 is input to the upper arm electro-optic modulation region waveguide 103 through the first mode converter 102 for the first electro-optic phase modulation. The modulated first TE0 mode optical signal output from the upper arm electro-optic modulation region waveguide 103 is input to the loop delay waveguide 105 after passing through the second mode converter 104. After passing through the loop delay waveguide 105, it is input to the first mode converter 102 again and converted into the first TE1 mode optical signal. S3. The first TE1 mode optical signal output from the first mode converter 102 is input into the upper arm electro-optic modulation region waveguide 103 for a second electro-optic phase modulation. The modulated first TE1 mode optical signal output from the upper arm electro-optic modulation region waveguide 103 is converted into a second TE0 mode optical signal after passing through the second mode converter 104 and input into the cross waveguide 106. The second TE0 mode optical signal output from the cross waveguide 106 is input into the third mode converter 108 after passing through the loop second delay waveguide 107. S4. The second TE0 mode optical signal output from the third mode converter 108 is input into the lower arm electro-optic modulation region waveguide 109 for a third electro-optic phase modulation. The modulated second TE0 mode optical signal output from the lower arm electro-optic modulation region waveguide 109 is input into the loop three-delay waveguide 1011 after passing through the fourth mode converter 1010. The modulated second TE0 mode optical signal output from the loop three-delay waveguide 1011 is converted into a second TE1 mode optical signal by the third mode converter 108. S5. The second TE1 mode optical signal output from the third mode converter 108 is input into the lower arm electro-optic modulation region waveguide 109 for the fourth electro-optic phase modulation. The modulated second TE1 mode optical signal output from the lower arm electro-optic modulation region waveguide 109 is converted into a third TE0 mode optical signal by the fourth mode converter 1010 and input into the cross waveguide 106. The third TE0 mode optical signal output from the cross waveguide 106 is output to the cyclic lithium niobate thin film phase modulator through the output waveguide. S6. Set the total length of the first, second, and third loop waveguides so that the optical group velocity is consistent with the radio frequency phase velocity each time it enters the electro-optic modulation process, making the cyclic lithium niobate thin film phase modulator equivalent to four cascaded traditional phase modulators, that is, the half-wave voltage drop of the cyclic lithium niobate thin film phase modulator is one-quarter of that of the traditional phase modulator.

[0037] In order to achieve the target radio frequency f target At this point, the half-wave voltage of the cyclic lithium niobate thin-film phase modulator is reduced to one-quarter of that of the conventional phase modulator, and the total delay of the first, second, and third loop waveguides is reduced. τ 1. τ 2 and τ 3 is: in, n, k, q Represents positive integers. N + Represents the set of positive integers; waveguide lengths of loop waveguide one, loop waveguide two, and loop waveguide three. L i for: in, c Represents the speed of light. i Indicates the waveguide index. i =1,2,3 ng This represents the group refractive index of the waveguide.

[0038] The embodiments described above are merely preferred embodiments of the present invention and are not intended to limit the scope of the present invention. Various modifications and improvements made to the technical solutions of the present invention by those skilled in the art without departing from the spirit of the present invention should fall within the protection scope defined by the claims of the present invention.

Claims

1. A broadband, high-efficiency cyclic lithium niobate thin-film phase modulator, characterized in that, include: The substrate layer, buried oxide layer, X-cut lithium niobate layer, silicon dioxide cladding layer and periodic capacitively loaded traveling wave electrode structure are stacked sequentially from bottom to top. The X-cut lithium niobate layer contains a lithium niobate thin film waveguide formed by etching technology. The lithium niobate thin film waveguide includes an input waveguide, a loop waveguide 1, a loop waveguide 2, a loop waveguide 3, and an output waveguide connected in sequence.

2. The broadband, high-efficiency cyclic lithium niobate thin-film phase modulator according to claim 1, characterized in that, The periodic capacitively loaded traveling wave electrode structure includes: a first ground electrode, a signal electrode, and a second ground electrode; A first electric field exists between the first ground electrode and the signal electrode, and a second electric field exists between the signal electrode and the second ground electrode, with the first and second electric fields in opposite directions.

3. The broadband, high-efficiency cyclic lithium niobate thin-film phase modulator according to claim 2, characterized in that, The first ground electrode consists of an interconnected first strip-shaped grounding main electrode and periodic miniature T-electrodes; The second ground electrode consists of an interconnected second strip-shaped grounding main electrode and the periodic micro T-electrode; The periodic micro T-electrodes connected to the first strip grounded main electrode and the second strip grounded main electrode are respectively located on the side near the upper arm electro-optic modulation region waveguide and the lower arm electro-optic modulation region waveguide; The signal electrode consists of an interconnected bar-shaped main signal electrode and the periodic micro T-electrode, but the periodic micro T-electrode is located on both sides of the bar-shaped main signal electrode. All of the periodic micro T-electrodes described herein have the same structure.

4. The broadband, high-efficiency cyclic lithium niobate thin-film phase modulator according to claim 1, characterized in that, The loop waveguide includes a first mode converter, an upper arm electro-optic modulation region waveguide, a second mode converter, and a loop delay waveguide connected in sequence. The loop waveguide includes the upper arm electro-optic modulation region waveguide, the second mode converter, the cross waveguide, the loop delay waveguide, and the third mode converter connected in sequence. The loop three-waveguide includes a lower arm electro-optic modulation region waveguide, a fourth mode converter, a loop three-delay waveguide, and a third mode converter connected in sequence.

5. The broadband, high-efficiency cyclic lithium niobate thin-film phase modulator according to claim 4, characterized in that, The first mode converter, the second mode converter, the third mode converter, and the fourth mode converter all employ fast adiabatic mode converters; the fast adiabatic mode converter includes a lower arm S-bend waveguide, a lower arm fast adiabatic waveguide, a lower arm tapered waveguide, and an upper arm multimode waveguide.

6. The broadband, high-efficiency cyclic lithium niobate thin-film phase modulator according to claim 4, characterized in that, The cross waveguide is a multimode interference type cross waveguide, including a first tapered waveguide in the Y direction, a multimode waveguide in the Y direction, a second tapered waveguide in the Y direction, a first tapered waveguide in the Z direction, a multimode waveguide in the Z direction, and a second tapered waveguide in the Z direction, and is completely symmetrical about the central intersection point in the Y and Z directions.

7. A method for implementing a broadband, high-efficiency cyclic lithium niobate thin-film phase modulator, said method being used to implement the phase modulator according to any one of claims 1-6, characterized in that, Includes the following steps: The first TE0 mode optical signal is input through the input waveguide; The first TE0 mode optical signal output from the input waveguide is input to the upper arm electro-optic modulation region waveguide through the first mode converter for the first electro-optic phase modulation. The modulated first TE0 mode optical signal output from the upper arm electro-optic modulation region waveguide is input to the loop delay waveguide after passing through the second mode converter. After passing through the loop delay waveguide, it is input to the first mode converter again and converted into the first TE1 mode optical signal. The first TE1 mode optical signal output from the first mode converter is input into the upper arm electro-optic modulation region waveguide for a second electro-optic phase modulation. The modulated first TE1 mode optical signal output from the upper arm electro-optic modulation region waveguide is converted into a second TE0 mode optical signal after passing through the second mode converter and input into the cross waveguide. The second TE0 mode optical signal output from the cross waveguide is input into the third mode converter after passing through the loop two-delay waveguide. The second TE0 mode optical signal output from the third mode converter is input into the lower arm electro-optic modulation region waveguide for a third electro-optic phase modulation. The modulated second TE0 mode optical signal output from the lower arm electro-optic modulation region waveguide is input into the loop three-delay waveguide after passing through the fourth mode converter. The modulated second TE0 mode optical signal output from the loop three-delay waveguide is converted into a second TE1 mode optical signal by the third mode converter. The second TE1 mode optical signal output from the third mode converter is input into the lower arm electro-optic modulation region waveguide for fourth electro-optic phase modulation. The modulated second TE1 mode optical signal output from the lower arm electro-optic modulation region waveguide is converted into a third TE0 mode optical signal by the fourth mode converter and input into the cross waveguide. The third TE0 mode optical signal output from the cross waveguide is output from the output waveguide to the cyclic lithium niobate thin film phase modulator. The total length of the first, second, and third loop waveguides is set so that the optical group velocity is consistent with the radio frequency phase velocity each time it enters the electro-optic modulation process. This makes the cyclic lithium niobate thin film phase modulator equivalent to four cascaded conventional phase modulators, that is, the half-wave voltage drop of the cyclic lithium niobate thin film phase modulator is one-quarter of that of the conventional phase modulator.

8. The method for implementing the broadband, high-efficiency cyclic lithium niobate thin-film phase modulator according to claim 7, characterized in that, In order to achieve the target radio frequency f target At this point, the half-wave voltage of the cyclic lithium niobate thin-film phase modulator is reduced to one-quarter of that of a conventional phase modulator, and the total delay of the first loop waveguide, the second loop waveguide, and the third loop waveguide is reduced. τ 1. τ 2 and τ 3 is: in, n, k, q Represents positive integers. N + Represents the set of positive integers; Waveguide lengths of the first, second, and third waveguides of the loop L i for: in, c Represents the speed of light. i Indicates the waveguide index. i =1,2,3 n g This represents the group refractive index of the waveguide.

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