Lithium niobate electro-optical polarization modulator based on adiabatic coupling structure and modulation method

Abstract

The invention discloses a lithium niobate electro-optical polarization modulator based on an adiabatic coupling structure and a modulation method thereof. The modulator comprises a first adiabatic coupling polarization beam splitting rotator, an electro-optical phase modulator and a second adiabatic coupling polarization beam splitting rotator, wherein each adiabatic coupling polarization beam splitting rotator comprises an input waveguide, an adiabatic coupler and a separation waveguide. According to the modulation method, transmission is carried out through a straight-through end in a first adiabatic coupling type polarization beam splitting rotator, and TE0 is kept unchanged; when the input light is TM0, the input light is converted into TE0 through the first adiabatic coupling type polarization beam splitting rotator and is transmitted through the cross end. And after phase modulation is carried out by the electro-optic phase modulator and beam combination is carried out by the second adiabatic coupling type polarization beam splitting rotator, output light is remapped into TE0 and TM0, and the polarization state is determined by the relative phase difference, so that dynamic regulation and control of the polarization state are realized.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of integrated photonics, and particularly relates to a lithium niobate electro-optic polarization modulator and a modulation method based on an adiabatic coupling structure, which is suitable for systems with strict requirements for dynamic polarization state regulation, such as coherent optical communication, quantum information processing and polarization encoding sensing. BACKGROUND

[0002] As a basic property of electromagnetic waves, the polarization state plays a key role in the encoding, processing and storage of optical information. Polarization modulation devices, as core optical elements for controlling the polarization state, have an irreplaceable role in the field of optical fiber communication and sensing, and their performance directly affects the performance of the entire optical system. Traditional polarization modulation technology mainly relies on large-size optical elements based on wave plates or fiber ring structures, and such schemes generally have the disadvantages of slow response speed and limited modulation precision. Under this background, the development of on-chip polarization modulation technology has become an important research direction. By constructing micro / nano-scale polarization modulation devices, lightweight and high integration of the device structure can be achieved, which not only breaks through the physical limitations of traditional devices, but also provides key device support for technological innovation in frontier fields such as quantum communication and biomedical imaging.

[0003] The thin film lithium niobate (LNOI) platform has excellent electro-optic coefficients (r33≈30 pm / V) and low propagation loss (<0.3 dB / cm) characteristics, providing an ideal carrier for developing high-performance integrated photonics devices. In particular, the electro-optic modulator based on LNOI has achieved breakthrough progress, with a modulation energy consumption as low as tens of fJ / bit, which is significantly better than traditional silicon-based schemes. However, the strong birefringence effect (Δn≈0.08@1550nm) inherent in LNOI material leads to significant polarization sensitivity of the device performance: the phase modulation efficiency difference between TE / TM polarization modes exceeds 60%, and the mode crosstalk caused by the mismatch of propagation constants, which seriously restricts its application in polarization-independent systems. Given the superiority of thin film lithium niobate material and the necessity of polarization state regulation, it is urgent to realize polarization modulation devices based on thin film lithium niobate. SUMMARY

[0004] To solve the above problems in the prior art, the application provides a lithium niobate electro-optic polarization modulator and a modulation method based on an adiabatic coupling structure.

[0005] The technical problem to be solved by the application is solved by the following technical scheme:

[0006] The first aspect of the application relates to a lithium niobate electro-optic polarization modulator based on adiabatic coupling structure, comprising a first adiabatic coupling polarization rotation beam splitter, an electro-optic phase modulator and a second adiabatic coupling polarization rotation beam splitter connected in sequence. The first polarization rotation beam splitter and the second polarization rotation beam splitter are structurally identical and are arranged in left-right mirror symmetry.

[0007] The first polarization rotation beam splitter comprises an input waveguide, an adiabatic coupler, a separation waveguide and a mode field adaptation waveguide connected in sequence.

[0008] The adiabatic coupler is composed of parallel straight waveguides with constant spacing and an adiabatic tapered waveguide, wherein the width of the adiabatic tapered waveguide increases along the light propagation direction, forming a linear tapered waveguide, and high-efficiency conversion from TM0 to TE0 is achieved through adiabatic design.

[0009] The separation waveguide comprises a pair of asymmetric S-shaped curved waveguides with different widths, forming two output paths of straight-through end and cross end;

[0010] The mode field adaptation waveguide connects the cross-arm output end of the separation waveguide, so that the waveguide width gradually changes to the same as the straight-through end, ensuring that the modulation light field distributions of the two output paths are consistent.

[0011] The electro-optic phase modulator comprises double parallel modulation waveguides and push-pull traveling wave electrodes.

[0012] The straight-through end and the cross end of the first adiabatic coupling polarization beam rotation are connected to the electro-optic phase modulator, ensuring that the optical signals participating in modulation are all TE0, so as to realize efficient electro-optic modulation.

[0013] The second adiabatic coupling polarization beam rotation is a mirror-symmetrical structure of the first adiabatic coupling polarization beam rotation, which is used to efficiently convert the TE0 transmitted by the cross end after phase modulation into TM0; at the same time, the TE0 of the straight-through end and the TM0 converted by the cross end are combined and output.

[0014] Further, the modulation method using the lithium niobate electro-optic polarization modulator based on adiabatic coupling structure of the application, characterized in that:

[0015] When the input light is TE0, it is output through the straight-through end in the first adiabatic coupling polarization beam rotation, keeping TE0 unchanged;

[0016] When the input light is TM0, it is converted into TE0 by the first adiabatic coupling polarization beam rotation and transmitted from the cross end;

[0017] After phase modulation by the electro-optic phase modulator, the TE0 at the cross end is recovered to TM0 by the second adiabatic coupling polarization beam splitter rotator, and is combined with the TE0 at the straight-through end to be output, and finally is remapped to TE0 and TM0, and the polarization state is determined by the relative phase difference, so that the dynamic control of the polarization state is realized.

[0018] The polarization state and phase change of the optical signal in the modulation process can be represented as follows:

[0019] Wherein represents the phase difference caused by the intrinsic structural birefringence of lithium niobate; represents the phase modulation amount generated by the single TE0 under the action of the electro-optic phase modulator, and the relationship with the applied voltage can be represented as:

[0020] Wherein is the half-wave voltage of the electro-optic phase modulator.

[0021] After the combination, the output light is remapped to TE0 and TM0, and the relative phase difference generated by the combination is .

[0022] When the output light is 45° linearly polarized light.

[0023] When the output light is right-handed circularly polarized light.

[0024] When the output light is 135° linearly polarized light.

[0025] When the output light is left-handed circularly polarized light.

[0026] By adjusting the voltage applied to the electro-optic phase modulator, the single-arm phase modulation amount can be controlled, so that the relative phase difference of the two light beams after the combination is changed, and the dynamic control of the polarization state of the output light is realized.

[0027] Compared with the prior art, the technical scheme of the present application has the following beneficial effects:

[0028] (1) The present application adopts an adiabatic coupling structure as a polarization rotation beam splitter, which can effectively separate TE0 and TM0, and convert TM0 to TE0. The adiabatic coupler not only has a simple structure, but also has low wavelength dependence. The electro-optic polarization modulator designed based on the structure shows good modulation characteristics in a wide wavelength range.

[0029] (2) Based on the unique advantages of X-cut thin film lithium niobate, this invention combines its maximum electro-optic coefficient direction (Z-axis) with TEO to form the optimal combination for achieving efficient electro-optic modulation; it can give full play to the electro-optic properties of the material, thereby achieving a significant reduction in driving voltage and a substantial improvement in modulation linearity with the highest efficiency, thus meeting the core requirements of high-speed optical communication and real-time polarization coding for high-performance modulators.

[0030] (3) The device involved in this invention is compatible with the mainstream process of thin-film lithium niobate and supports monolithic integration with photonic components such as intensity modulators and wavelength division multiplexers, laying the foundation for building a multifunctional polarization-controlled electro-optic chip. Attached Figure Description

[0031] Figure 1 This is a schematic diagram of a lithium niobate electro-optic polarization modulator structure based on an adiabatic coupling structure according to an embodiment of the present invention.

[0032] Figure 2 This is a schematic diagram of the effective refractive index of the mode as a function of the ridge waveguide width in an embodiment of the present invention.

[0033] Figure 3 This is a cross-sectional view of a lithium niobate electro-optic polarization modulator based on an adiabatic coupling structure according to an embodiment of the present invention;

[0034] Figure 4 This is a schematic diagram illustrating the working principle of a lithium niobate electro-optic polarization modulator structure based on an adiabatic coupling structure according to an embodiment of the present invention.

[0035] Figure 5 shows the optical field transmission diagram of 1550nm for the thermally coupled polarization rotating beam splitter according to an embodiment of the present invention when TE0 and TM0 are inputs.

[0036] Figure 6 This is a transmission loss diagram of the adiabatic coupled polarization rotating beam splitter of this invention in the wavelength range of 1400nm-1700nm.

[0037] Figure 7 This is a process flow diagram of an embodiment of the present invention. Detailed Implementation

[0038] The present invention will now be further described and illustrated in detail with reference to the accompanying drawings and embodiments.

[0039] This invention proposes a lithium niobate electro-optic polarization modulator structure based on an adiabatic coupling structure, as shown in the three-dimensional structural schematic diagram below. Figure 1 As shown, it includes a first thermally adiabatic coupled polarization rotating beam splitter 110, an electro-optic phase modulator 120, and a second thermally adiabatic coupled polarization rotating beam splitter 130.

[0040] The first polarization-rotating beam splitter 110 and the second polarization-rotating beam splitter 130 include an input waveguide 111, an adiabatic coupler 112, a split waveguide 113 and a mode field adaptor waveguide 114.

[0041] The electro-optic phase modulator 120 includes a dual-parallel modulated waveguide 121 and a push-pull traveling wave electrode 122.

[0042] Figure 2 A curve diagram for the mode effective refractive index changing with the ridge waveguide width is shown. The results show that when the waveguide width is in the range of 0.3-0.6 μm, the effective refractive index of TE0 in the width range is equal to the effective refractive index of TM0 at the width of 0.85 μm, thereby realizing the phase matching of the two; and the TE0 at the width of 0.85 μm cannot realize the phase matching with any other mode in the width range of 0.3-0.6 μm.

[0043] Therefore, the input waveguide 111 can adopt a straight waveguide with a width of 0.75-0.95 μm; the adiabatic coupler 112 includes parallel straight waveguides with a fixed pitch and an adiabatic tapered waveguide, wherein the linearly changing widths of the two ends of the adiabatic tapered waveguide can be set to 0.25-0.35 μm and 0.55-0.75 μm respectively, to realize TM0→TE0 mode conversion; the width of the straight-through port of the split waveguide 113 is consistent with the input waveguide and can be set to 0.75-0.95 μm, and the width of the cross arm is set to 0.55-0.75 μm to suppress mode crosstalk; the mode field adaptor waveguide 114 is located at the end of the cross port, and the waveguide widths of the two ends can be set to 0.55-0.75 μm and 0.75-0.95 μm to realize optical field distribution matching.

[0044] The lithium niobate electro-optic polarization modulator structure based on the adiabatic coupling structure proposed in the embodiment has the working principle as shown in Figure 3 The first polarization-rotating beam splitter 110 and the second polarization-rotating beam splitter 130 are located in the passive region, and the surface is air-covered to reduce mode conversion loss; the electro-optic phase modulator 120 is located in the active region, and a 1 μm silicon dioxide buffer layer is deposited at the lithium niobate waveguide to reduce metal absorption loss.

[0045] Further, the lithium niobate waveguide adopts an X-cut 400 nm thin film with an etching depth of 250 nm.

[0046] The working principle of the lithium niobate electro-optic polarization modulator structure based on the adiabatic coupling structure proposed in the embodiment is as shown in Figure 4As shown: when the input light is TE0, it is transmitted through the through port in the first adiabatic coupling type polarization beam splitting rotator 110, keeping TE0 unchanged; when the input light is TM0, it is coupled and converted to TE0 output through the cross port of the first adiabatic coupling type polarization beam splitting rotator 110. After phase modulation by the electro-optic phase modulator 120, the output light is re-mapped to TE0 and TM0 after beam combination by the second adiabatic coupling type polarization beam splitting rotator 130, and the polarization state is determined by the relative phase difference, realizing dynamic regulation of the polarization state.

[0047] Figure 5 The transmission characteristics of the adiabatic coupling polarization rotating beam splitter 110 at 1550 nm are shown when the input is TE0 and TM0. The simulation results show that when the input is TE0, the energy is localized in the through port for stable transmission, verifying the low-loss maintenance capability of the through port for the original polarization state; when the input is TM0, mode selective coupling occurs through the adiabatic coupler, and polarization conversion from TM0 to TE0 is completed when transmitted through the cross port, to realize polarization state separation and rotation.

[0048] Figure 6 The transmission characteristics of the adiabatic coupling polarization rotating beam splitter 110 at 1550 nm are shown when the input is TE0 and TM0. The simulation results show that when the input is TE0, the energy is localized in the through port for stable transmission, verifying the low-loss maintenance capability of the through port for the original polarization state; when the input is TM0, mode selective coupling occurs through the adiabatic coupler, and polarization conversion from TM0 to TE0 is completed when transmitted through the cross port, to realize polarization state separation and rotation.

[0049] Figure 7 The preparation flowchart of the lithium niobate electro-optic polarization modulator based on the adiabatic coupling structure is shown, including:

[0050] Ⅰ. Substrate pretreatment: ultrasonic cleaning (acetone, isopropyl alcohol for 15 minutes each) and nitrogen blowing dry of the lithium niobate chip in sequence;

[0051] Ⅱ. Waveguide mask preparation: using electron beam lithography technology to form a waveguide pattern mask layer on the surface of the chip treated in step I;

[0052] Ⅲ. Chrome mask transfer: depositing a chromium layer by electron beam evaporation process, and then ultrasonic peeling off the redundant area to complete the formation of the chromium hard mask of the waveguide structure;

[0053] IV. Waveguide etching: Inductively coupled plasma reactive ion etching is used to etch lithium niobate to a depth of 300 nm, the chromium mask is removed and cleaned to obtain a lithium niobate waveguide structure comprising an input waveguide (111), an adiabatic coupler (112), a separation waveguide (113), a mode field adaptation waveguide (114) and a double parallel modulation waveguide (121);

[0054] V. Buffer layer deposition: A 1 μm thick silicon dioxide buffer layer is grown on the surface of the etched waveguide using plasma enhanced chemical vapor deposition;

[0055] VI. Cladding mask lithography: A cladding mask pattern is defined on the silicon dioxide buffer layer by ultraviolet lithography;

[0056] VII. Cladding etching: ICP-RIE is used to selectively etch the exposed area of the silicon dioxide, and the buffer layer in the electrode area is retained after cleaning;

[0057] VIII. Electrode mask preparation: An electrode mask pattern is formed on the surface of the chip after step VII by electron beam lithography;

[0058] IX. Electrode shaping: After electron beam evaporation of the gold layer, ultrasonic peeling is performed to obtain a push-pull modulation electrode;

[0059] The above-described embodiments are only the preferred specific embodiments of the present application, and the usual changes and replacements made by those skilled in the art within the scope of the technical solutions of the present application should be included in the protection scope of the present application.

Claims

1. A lithium niobate electro-optic polarization modulator based on an adiabatic coupling structure, characterized in that: The first adiabatic coupling polarization beam splitting rotator, the electro-optical phase modulator and the second adiabatic coupling polarization beam splitting rotator are sequentially connected; input light is subjected to polarization separation and mode conversion by the first adiabatic coupling polarization beam splitting rotator to obtain two branch light signals, the two branch light signals enter the electro-optical phase modulator to be modulated, and polarization rotation and beam combining are completed by the second adiabatic coupling polarization beam splitting rotator to be output. The first adiabatic coupling polarization beam splitting rotator and the second adiabatic coupling polarization beam splitting rotator comprise an input waveguide, an adiabatic coupler, a separation waveguide and a mode field adaptation waveguide connected in sequence. The first adiabatic coupling polarization beam splitting rotator and the second adiabatic coupling polarization beam splitting rotator comprise an input end and two output ends, wherein the input end is used for receiving an input light signal; the output ends comprise a straight-through end and a cross end, the straight-through end is used for transmitting TE0, and the cross end completes conversion of TM0 to TE0 and outputs.

2. The lithium niobate electro-optic polarization modulator based on adiabatic coupling structure according to claim 1, characterized in that: The input waveguide of the first adiabatic coupling polarization beam splitting rotator is a single-mode straight waveguide, which can realize low-loss transmission of TE0 and TM0.

3. The lithium niobate electro-optic polarization modulator based on adiabatic coupling structure as claimed in claim 1, wherein: The adiabatic coupler of the first adiabatic coupling polarization beam splitting rotator is composed of parallel straight waveguides with constant spacing and an adiabatic tapered waveguide, wherein the width of the tapered waveguide gradually increases from the starting end along the light transmission direction, and high-efficiency conversion of TM0 to TE0 is realized through adiabatic condition design; input TM0 is converted to TE0 and coupled to the output of the adiabatic tapered waveguide, while input TE0 is directly transmitted along the straight waveguide.

4. The lithium niobate electro-optic polarization modulator based on adiabatic coupling structure according to claim 1, characterized in that: The separation waveguide of the first adiabatic coupling polarization beam splitting rotator is composed of a pair of asymmetric S-bend waveguides with different widths, corresponding to the two output paths of the straight-through end and the cross end respectively; wherein the straight-through end is connected with the straight waveguide of the coupling waveguide to receive TE0, and the cross end is connected with the adiabatic tapered waveguide of the coupling waveguide to receive TE0 converted from TM0.

5. The lithium niobate electro-optic polarization modulator based on adiabatic coupling structure as claimed in claim 1, wherein: The cross end of the separation waveguide of the first adiabatic coupling polarization beam splitting rotator is connected with the mode field adaptation waveguide, so that the waveguide width gradually changes to the same as that of the straight-through end, to ensure that the modulation light field distributions of the two output paths are consistent.

6. The lithium niobate electro-optic polarization modulator based on adiabatic coupling structure according to claim 1, characterized in that: The output end of the first adiabatic coupling polarization beam splitting rotator is connected with the electro-optical phase modulator, which is used for modulating the two TE0 to realize high-efficiency electro-optical modulation; wherein the modulation light of the cross end is TE0 converted from TM0, and the modulation light of the straight-through end is TE0.

7. The lithium niobate electro-optic polarization modulator based on adiabatic coupling structure according to claim 1, characterized in that: The second adiabatic coupling polarization beam splitting rotator is a mirror-symmetrical structure of the first adiabatic coupling polarization beam splitting rotator, which is used for efficiently converting the TE0 transmitted by the cross end after phase modulation to TM0, so that the TE0 of the straight-through end is combined with the TM0 converted by the cross end to be output.

8. A method of modulating a lithium niobate electro-optic polarization modulator based on an adiabatic coupling structure as claimed in any one of claims 1 to 7, characterized in that, When the input light is TE0, it is transmitted through the straight-through end in the first adiabatic coupling polarization beam splitting rotator without change; when the input light is TM0, it is converted to TE0 in the first adiabatic coupling polarization beam splitting rotator and is transmitted from the cross end. After phase modulation by the electro-optic phase modulator, the output light is remapped as TE0 and TM0 after being combined by the second adiabatic coupling type polarization beam splitting rotator, and the polarization state is determined by the relative phase difference, so as to realize dynamic control of the polarization state.

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