Ultra-short film lithium niobate phase modulator for sensing and working method thereof

Abstract

The present invention discloses an ultra-short thin-film lithium niobate phase modulator for sensing and a working method thereof, belonging to the field of integrated optical devices, including an input step-type tapered waveguide, a first sub-wavelength grating reflector, a Fabry-Perot resonant cavity, a second sub-wavelength grating reflector and an output step-type tapered waveguide which are sequentially integrated along the propagation order of the optical path; thereby realizing that the light input from the outside propagates back and forth inside the Fabry-Perot resonant cavity between the first sub-wavelength grating reflector and the second sub-wavelength grating reflector to receive modulation. The present invention adopts the above-mentioned ultra-short thin-film lithium niobate phase modulator for sensing and a working method thereof, and uses a grating reflector to make the modulated signal light resonate in the Fabry-Perot resonant cavity, which plays the role of folding the optical path, so that the effective optical path of the light propagating in the modulation area is much greater than the actual length of the device, thereby greatly reducing the overall size of the device while ensuring that the modulation efficiency is almost unchanged, and at the same time reducing the modulation half-wave voltage.

Description

Technical Field

[0001] The present invention relates to the field of integrated optical devices, and in particular to an ultra-short thin-film lithium niobate phase modulator for sensing and a working method thereof. Background Art

[0002] The field of integrated optics is the main direction of future development of optical communications, sensing and other industries. The integrated optical technology derived from waveguide optics theory such as optical waveguide mode analysis can realize the separation of the original optical system, thereby realizing the miniaturization and integration of optical components, facilitating mass production, thereby reducing production costs and enhancing the core competitiveness of enterprises.

[0003] At present, the integrated optical devices that have been studied in depth include passive devices such as on-chip spot converters, couplers, polarizers, rotators, resonant rings, isolators, and active devices such as lasers, detectors, and modulators. Among them, the modulator is the core component that loads and transmits information on the optical signal, and its modulation rate, modulation efficiency and other parameters directly affect the communication rate. Unlike modulators in the communication field that mostly use intensity modulation and focus on modulation rate, modulators in the sensing field mostly use phase modulation methods, and modulation efficiency has become the most critical parameter restricting its performance. How to further reduce the modulation half-wave voltage and shorten the overall size of the modulation area is a research hotspot for integrated phase modulators for sensing.

[0004] All functional devices in the field of integrated optics are based on optical waveguides to realize their functions and use optical waveguides to achieve optical interconnection. Among the many waveguide core materials used, lithium niobate material has a relatively high electro-optic coefficient and excellent linear electro-optic effect, so it is widely used in the manufacture of optical modulators. With the advancement of processing technology, the thin-film lithium niobate ridge waveguide structure prepared by electron beam lithography, inductively coupled reactive ion etching and other methods has a stronger binding ability for the light field, thereby achieving lower light transmission loss and reducing the metal's absorption loss of light, so that the modulating metal electrode can be closer to the waveguide, greatly improving the modulation efficiency.

[0005] However, the current mainstream on-chip thin-film lithium niobate modulator structure still uses the most basic waveguide structure and adds metal electrodes on both sides of the waveguide. This makes the on-chip modulator still larger in size than other integrated optical devices, making it difficult to further reduce the size of the integrated optical chip. Although modulators that use curved waveguides to fold the optical path and thus shorten the length have been widely studied, they have not been able to truly shorten the length of the modulation area. In addition, due to the constraints of modulation efficiency, the modulation half-wave voltage is also difficult to further reduce. Summary of the invention

[0006] In order to solve the above problems, the present invention provides an ultra-short thin film lithium niobate phase modulator for sensing and a working method thereof, which utilizes a grating reflector to make the modulated signal light resonate in the Fabry-Perot cavity, thereby playing the role of folding the optical path, so that the effective optical path of the light propagating in the modulation area is much larger than the actual length of the device, thereby greatly reducing the overall size of the device while ensuring that the modulation efficiency remains almost unchanged, and at the same time reducing the modulation half-wave voltage (the overall length is only about 300μm, and can be applied to both O band and C band at the same time).

[0007] To achieve the above-mentioned object, the present invention provides an ultra-short thin-film lithium niobate phase modulator for sensing, comprising an input step-type tapered waveguide, a first sub-wavelength grating reflector, a Fabry-Perot resonant cavity, a second sub-wavelength grating reflector and an output step-type tapered waveguide which are sequentially integrated in the order of propagation along an optical path, the input step-type tapered waveguide, the first sub-wavelength grating reflector, the Fabry-Perot resonant cavity, the second sub-wavelength grating reflector and the output step-type tapered waveguide are optically interconnected via input and output thin-film lithium niobate ridge waveguides, and metal electrodes are arranged on the surfaces of the first sub-wavelength grating reflector, the Fabry-Perot resonant cavity and the second sub-wavelength grating reflector;

[0008] Thus, it is achieved that the external input light propagates back and forth in the Fabry-Perot resonant cavity between the first sub-wavelength grating reflector and the second sub-wavelength grating reflector to receive modulation.

[0009] Preferably, the input step-type tapered waveguide, the first sub-wavelength grating reflector, the Fabry-Perot resonant cavity, the second sub-wavelength grating reflector and the output step-type tapered waveguide are all made of functional thin-film lithium niobate ridge waveguide;

[0010] The functional thin-film lithium niobate ridge waveguide and the input-output thin-film lithium niobate ridge waveguide both include a silicon dioxide upper cladding layer, a thin-film lithium niobate ridge waveguide core layer, a silicon dioxide lower cladding layer and a silicon substrate layer arranged in sequence from top to bottom;

[0011] A silicon dioxide intermediate layer is grown between the thin-film lithium niobate ridge waveguide core layer and the silicon dioxide upper cladding layer of the Fabry-Perot resonant cavity, and metal electrodes are covered on the silicon dioxide intermediate layer. The metal electrodes are symmetrically arranged on both sides of the thin-film lithium niobate ridge waveguide core layer.

[0012] Preferably, the waveguide widths at the centers of the first sub-wavelength grating reflector and the second sub-wavelength grating reflector are smaller than the waveguide widths at the input and output ends, and the centers of the first sub-wavelength grating reflector and the second sub-wavelength grating reflector are connected to the waveguides at the input and output ends via two sections of tapered waveguides.

[0013] Preferably, the first sub-wavelength grating reflector and the second sub-wavelength grating reflector are both composed of Bragg gratings arranged alternately according to long gratings and short grating periods, and the total number of periods of the Bragg gratings is greater than 100;

[0014] The reflectivity of the first sub-wavelength grating reflector and the second sub-wavelength grating reflector to the light of O band and C band are both greater than 99%.

[0015] Preferably, the long grating extends beyond the length of the input and output thin film lithium niobate ridge wave , the length of the short grating extending out from the input and output thin film lithium niobate ridge wave ;

[0016] Grating period between long grating and short grating , the grating period from short grating to long grating .

[0017] Preferably, the input step-type tapered waveguide and the output step-type tapered waveguide are both coupled to the pigtail;

[0018] Both the input step-type tapered waveguide and the output step-type tapered waveguide taper the rib layer of the functional thin-film lithium niobate ridge waveguide three times with successively decreasing heights, and taper the slab layer of the functional thin-film lithium niobate ridge waveguide once with the same height, and the minimum width of each taper is 200nm.

[0019] Preferably, when the rib layer of the functional thin-film lithium niobate ridge waveguide is tapered, the height of the input step-type tapered waveguide and the output step-type tapered waveguide is successively reduced by 70 nm, and the length of each tapered waveguide is not less than 150 μm;

[0020] When the input step-type tapered waveguide and the output step-type tapered waveguide are tapered on the slab layer of the functional thin-film lithium niobate ridge waveguide, the height is 210nm and the length is not less than 300μm;

[0021] The mode field diameter of the light spot after beam expansion of both the input step-type tapered waveguide and the output step-type tapered waveguide is not less than 4 μm.

[0022] The working method of the ultra-short thin-film lithium niobate phase modulator for sensing comprises the following steps:

[0023] The external input light enters the input step-type tapered waveguide through the pigtail coupling, and then enters the Fabry-Perot resonant cavity after being transmitted through the input step-type tapered waveguide and the first sub-wavelength grating reflector in sequence, and then propagates back and forth in the Fabry-Perot resonant cavity by using the first sub-wavelength grating reflector and the second sub-wavelength grating reflector;

[0024] The input light resonates in the Fabry-Perot cavity until the energy in the Fabry-Perot cavity reaches a saturation state, and the output light is coupled into the pigtail output through the second sub-wavelength grating reflector and the output step-type tapered waveguide in sequence.

[0025] Preferably, in the resonant state, a voltage signal is applied to the metal electrodes on both sides of the Fabry-Perot resonant cavity to change the effective refractive index of the propagation mode in the Fabry-Perot resonant cavity, and the electrical signal information is loaded on the phase change of the optical signal, so that the optical path of the light transmitted in the Fabry-Perot resonant cavity is greater than the cavity length of the Fabry-Perot resonant cavity.

[0026] The present invention has the following beneficial effects:

[0027] 1. Design phase modulator using thin film lithium niobate material and apply it to the field of sensing, broadening the application prospects of thin film lithium niobate;

[0028] 2. While ensuring that the modulation efficiency remains almost unchanged, the modulator size is shortened, the modulation half-wave voltage is reduced, and the overall modulation performance is improved;

[0029] 3. It is compatible with the currently more mature CMOS process (complementary metal oxide semiconductor), has a process basis for mass production, facilitates production scale, and reduces production costs.

[0030] The technical solution of the present invention is further described in detail below through the accompanying drawings and embodiments. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] Figure 1 This is a schematic structural diagram of an ultra-short thin-film lithium niobate phase modulator for sensing of the present invention;

[0032] Figure 2 A cross-sectional view of a functional thin-film lithium niobate waveguide structure at a Fabry-Perot resonant cavity of an ultra-short thin-film lithium niobate phase modulator for sensing according to the present invention;

[0033] Figure 3 It is a schematic structural diagram of a first sub-wavelength grating reflector and a second sub-wavelength grating reflector of an ultra-short thin-film lithium niobate phase modulator for sensing of the present invention;

[0034] Figure 4 A step-type tapered waveguide of an ultra-short thin-film lithium niobate phase modulator for sensing of the present invention is a top view of a tapered rib layer of a functional thin-film lithium niobate ridge waveguide;

[0035] Figure 5 The present invention is a side view of a stepped tapered waveguide of an ultra-short thin-film lithium niobate phase modulator for sensing, which is a tapered side view of a rib layer of a functional thin-film lithium niobate ridge waveguide. DETAILED DESCRIPTION

[0036] In order to make the purpose, technical scheme and advantages disclosed in the embodiments of the present invention clearer, the embodiments of the present invention are further described in detail in conjunction with the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are only used to explain the embodiments of the present invention and are not intended to limit the embodiments of the present invention. Based on the embodiments in this application, all other embodiments obtained by ordinary technicians in this field without making creative work are within the scope of protection of this application. Examples of the embodiments are shown in the accompanying drawings, where the same or similar reference numerals throughout represent the same or similar elements or elements with the same or similar functions.

[0037] It should be noted that the terms "include", "have" and any variations thereof are intended to cover non-exclusive inclusions. For example, a process, method, system, product or server comprising a series of steps or units is not necessarily limited to those steps or units explicitly listed, but may include other steps or units not explicitly listed or inherent to these processes, methods, products or devices.

[0038] Like reference numerals and letters denote similar items in the following drawings, and thus, once an item is defined in one drawing, further definition and explanation thereof is not required in subsequent drawings.

[0039] In the description of the present invention, it should be noted that the terms "upper", "lower", "inside", "outside", etc. indicate directions or positional relationships based on the directions or positional relationships shown in the accompanying drawings, or are directions or positional relationships in which the product of the invention is usually placed when in use. They are only for the convenience of describing the present invention and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific direction, be constructed and operated in a specific direction, and therefore should not be understood as a limitation on the present invention.

[0040] In the description of the present invention, it is also necessary to explain that, unless otherwise clearly specified and limited, the terms "setting", "installation" and "connection" should be understood in a broad sense, for example, it can be a fixed connection, a detachable connection, or an integral connection; it can be a mechanical connection or an electrical connection; it can be a direct connection, or it can be an indirect connection through an intermediate medium, or it can be the internal communication of two components. For ordinary technicians in this field, the specific meanings of the above terms in the present invention can be understood according to specific circumstances.

[0041] like Figure 1-Figure 5As shown, the ultra-short thin-film lithium niobate phase modulator for sensing described in the present invention comprises an input step-type tapered waveguide, a first sub-wavelength grating reflector, a Fabry-Perot resonant cavity, a second sub-wavelength grating reflector and an output step-type tapered waveguide which are sequentially integrated in the order of propagation along the optical path, wherein the input step-type tapered waveguide and the output step-type tapered waveguide are used to reduce or expand the mode field diameter of light during transmission, reduce the coupling loss of light between the waveguide and the device pigtail, thereby reducing the transmission loss, the input step-type tapered waveguide, the first sub-wavelength grating reflector, the Fabry-Perot resonant cavity, the second sub-wavelength grating reflector and the output step-type tapered waveguide are optically interconnected via input and output thin-film lithium niobate ridge waveguides, and metal electrodes are provided on the surfaces of the first sub-wavelength grating reflector, the Fabry-Perot resonant cavity and the second sub-wavelength grating reflector; thereby, light input from the outside is transmitted back and forth in the Fabry-Perot resonant cavity between the first sub-wavelength grating reflector and the second sub-wavelength grating reflector to be modulated.

[0042] Specifically, the input step-type tapered waveguide, the first sub-wavelength grating reflector, the Fabry-Perot resonant cavity, the second sub-wavelength grating reflector and the output step-type tapered waveguide are all made of functional thin-film lithium niobate ridge waveguide; the functional thin-film lithium niobate ridge waveguide and the input and output thin-film lithium niobate ridge waveguides all include a silicon dioxide upper cladding, a thin-film lithium niobate ridge waveguide core layer, a silicon dioxide lower cladding and a silicon substrate layer arranged in sequence from top to bottom; a silicon dioxide intermediate layer is grown between the thin-film lithium niobate ridge waveguide core layer and the silicon dioxide upper cladding of the Fabry-Perot resonant cavity, and the silicon dioxide intermediate layer is covered with metal electrodes, which are symmetrically arranged on both sides of the thin-film lithium niobate ridge waveguide core layer.

[0043] Through the above structure, the Fabry-Perot resonant cavity can meet the resonance conditions of O-band and C-band light, and at the same time meet ,in are the effective refractive indices of TE0 mode at 1310nm and 1550nm respectively, are two integers, which respectively represent the optical path of light in the cavity and the central wavelength of the two bands. In this embodiment, the change in the effective refractive index of the mode field caused by applying a modulation voltage to the electrode has a negligible effect on the resonance condition of the Fabry-Perot resonant cavity.

[0044] The waveguide widths at the centers of the first sub-wavelength grating reflector and the second sub-wavelength grating reflector are smaller than the waveguide widths at the input and output ends. The narrower the central waveguide width is, the higher the single-period grating reflectivity is. The centers of the first sub-wavelength grating reflector and the second sub-wavelength grating reflector are connected to the waveguides at the input and output ends via two sections of tapered waveguides.

[0045] The first sub-wavelength grating reflector and the second sub-wavelength grating reflector are both composed of Bragg gratings arranged alternately according to long grating and short grating periods, and in order to meet the requirement of high reflectivity, the total number of periods of the Bragg grating is greater than 100; the reflectivity of the first sub-wavelength grating reflector and the second sub-wavelength grating reflector to light in the O-band and C-band is greater than 99%, which can further improve the grating reflectivity and increase the reflection spectrum width while reducing the transmission loss.

[0046] The long grating extends the length of the input and output thin-film lithium niobate ridge waves , the length of the short grating extending out from the input and output thin film lithium niobate ridge wave ; Grating period between long grating and short grating , the grating period from short grating to long grating .

[0047] The input step-type tapered waveguide and the output step-type tapered waveguide are both coupled with the pigtail, and the step-type tapered waveguide and the device pigtail are matched with the mode field to reduce the loss of light during transmission; the input step-type tapered waveguide and the output step-type tapered waveguide taper the rib layer of the functional thin-film lithium niobate ridge waveguide three times with decreasing heights, and taper the slab layer of the functional thin-film lithium niobate ridge waveguide once with the same height, and the minimum width of each taper is 200nm.

[0048] When the rib layer of the functional thin film lithium niobate ridge waveguide is tapered, the height of the input step-type tapered waveguide and the output step-type tapered waveguide is reduced by 70nm successively, and the length of each tapered waveguide is not less than 150μm; when the slab layer of the functional thin film lithium niobate ridge waveguide is tapered, the height of the input step-type tapered waveguide and the output step-type tapered waveguide is 210nm, and the length is not less than 300μm; the mode field diameter of the light spot after beam expansion of both the input step-type tapered waveguide and the output step-type tapered waveguide is not less than 4μm.

[0049] The working method of the ultra-short thin-film lithium niobate phase modulator for sensing comprises the following steps:

[0050] The light input from the outside enters the input step-type tapered waveguide through the pigtail coupling. During the transmission process, the mode field diameter gradually decreases. A small part of the light is transmitted through the input step-type tapered waveguide and the first sub-wavelength grating reflector in turn, and then enters the Fabry-Perot resonant cavity. It is then propagated back and forth in the Fabry-Perot resonant cavity by using the first sub-wavelength grating reflector and the second sub-wavelength grating reflector.

[0051] The input light resonates in the Fabry-Perot cavity until the energy in the Fabry-Perot cavity reaches a saturation state, and the output light is coupled into the pigtail output through the second sub-wavelength grating reflector and the output step-type tapered waveguide in sequence.

[0052] Among them, in the resonant state, a voltage signal is applied to the metal electrodes on both sides of the Fabry-Perot cavity to change the effective refractive index of the propagation mode in the Fabry-Perot cavity, and the electrical signal information is loaded on the phase change of the optical signal, so that the optical path of light transmitted in the Fabry-Perot cavity is greater than the cavity length of the Fabry-Perot cavity. Therefore, efficient phase modulation can be achieved using a shorter cavity length while maintaining a lower half-wave voltage to meet the needs of phase modulators for sensing.

[0053] Therefore, the present invention adopts the above-mentioned ultra-short thin film lithium niobate phase modulator for sensing and its working method, and uses a grating reflector to make the modulated signal light resonate in the Fabry-Perot cavity, which plays the role of folding the optical path, so that the effective optical path of light propagating in the modulation area is much larger than the actual length of the device, thereby greatly reducing the overall size of the device while ensuring that the modulation efficiency remains almost unchanged, and at the same time reducing the modulation half-wave voltage.

[0054] Finally, it should be noted that the above embodiments are only used to illustrate the technical solution of the present invention rather than to limit it. Although the present invention has been described in detail with reference to the preferred embodiments, those skilled in the art should understand that they can still modify or replace the technical solution of the present invention with equivalents, and these modifications or equivalent replacements cannot cause the modified technical solution to deviate from the spirit and scope of the technical solution of the present invention.

Claims

1. An ultra-short thin-film lithium niobate phase modulator for sensing, which can be used in both O-band and C-band, characterized in that: The invention comprises an input step-type tapered waveguide, a first sub-wavelength grating reflector, a Fabry-Perot resonant cavity, a second sub-wavelength grating reflector and an output step-type tapered waveguide which are sequentially integrated in the order of propagation along an optical path, wherein the input step-type tapered waveguide, the first sub-wavelength grating reflector, the Fabry-Perot resonant cavity, the second sub-wavelength grating reflector and the output step-type tapered waveguide are optically interconnected via input and output thin-film lithium niobate ridge waveguides, and metal electrodes are arranged on the surfaces of the first sub-wavelength grating reflector, the Fabry-Perot resonant cavity and the second sub-wavelength grating reflector; Thus, the external input light is modulated by propagating back and forth in the Fabry-Perot resonant cavity between the first sub-wavelength grating reflector and the second sub-wavelength grating reflector; The input step-type tapered waveguide, the first sub-wavelength grating reflector, the Fabry-Perot resonant cavity, the second sub-wavelength grating reflector and the output step-type tapered waveguide are all made of functional thin-film lithium niobate ridge waveguide; The functional thin-film lithium niobate ridge waveguide and the input-output thin-film lithium niobate ridge waveguide both include a silicon dioxide upper cladding layer, a thin-film lithium niobate ridge waveguide core layer, a silicon dioxide lower cladding layer and a silicon substrate layer arranged in sequence from top to bottom; A silicon dioxide intermediate layer is grown between the thin-film lithium niobate ridge waveguide core layer and the silicon dioxide upper cladding layer of the Fabry-Perot resonant cavity, and a metal electrode is covered on the silicon dioxide intermediate layer. The metal electrodes are symmetrically arranged on both sides of the thin-film lithium niobate ridge waveguide core layer. The waveguide widths at the centers of the first sub-wavelength grating reflector and the second sub-wavelength grating reflector are both smaller than the waveguide widths at the input and output ends, and the centers of the first sub-wavelength grating reflector and the second sub-wavelength grating reflector are connected to the waveguides at the input and output ends via two sections of tapered waveguides; The first sub-wavelength grating reflector and the second sub-wavelength grating reflector are both composed of Bragg gratings arranged alternately according to long gratings and short grating periods, and the total number of periods of the Bragg gratings is greater than 100; The reflectivity of the first sub-wavelength grating reflector and the second sub-wavelength grating reflector to light in the O-band and the C-band are both greater than 99%; The input step-type tapered waveguide and the output step-type tapered waveguide are both coupled with the pigtail fiber; Both the input step-type tapered waveguide and the output step-type tapered waveguide taper the rib layer of the functional thin-film lithium niobate ridge waveguide three times with successively decreasing heights, and taper the slab layer of the functional thin-film lithium niobate ridge waveguide once with the same height, and the minimum width of each taper is 200nm.

2. The ultra-short film lithium niobate phase modulator for sensing according to claim 1, characterized in that: The long grating extends the length of the input and output thin-film lithium niobate ridge waves , the short grating extends the length of the input and output thin film lithium niobate ridge wave ; Grating period between long grating and short grating , the grating period from short grating to long grating .

3. The ultra-short film lithium niobate phase modulator for sensing according to claim 1, characterized in that: When the rib layer of the functional thin-film lithium niobate ridge waveguide is tapered, the height of the input step-type tapered waveguide and the output step-type tapered waveguide is successively reduced by 70nm, and the length of each tapered waveguide is not less than 150μm; When the input step-type tapered waveguide and the output step-type tapered waveguide are tapered on the slab layer of the functional thin-film lithium niobate ridge waveguide, the height is 210nm and the length is not less than 300μm; The mode field diameters of the light spots after beam expansion of the input step-type tapered waveguide and the output step-type tapered waveguide are no less than 4 μm.

4. A method for operating an ultra-short thin film lithium niobate phase modulator for sensing as claimed in any one of claims 1 to 3, characterized in that: The following steps are involved: The external input light enters the input step-type tapered waveguide through the pigtail coupling, and then enters the Fabry-Perot resonant cavity after being transmitted through the input step-type tapered waveguide and the first sub-wavelength grating reflector in sequence, and then propagates back and forth in the Fabry-Perot resonant cavity by using the first sub-wavelength grating reflector and the second sub-wavelength grating reflector; The input light resonates in the Fabry-Perot cavity until the energy in the Fabry-Perot cavity reaches a saturation state, and the output light is coupled into the pigtail output through the second sub-wavelength grating reflector and the output step-type tapered waveguide in sequence.

5. The working method of the ultra-short film lithium niobate phase modulator for sensing according to claim 4, characterized in that: In the resonant state, a voltage signal is applied to the metal electrodes on both sides of the Fabry-Perot resonant cavity to change the effective refractive index of the propagation mode in the Fabry-Perot resonant cavity, and the electrical signal information is loaded on the phase change of the optical signal, so that the optical path of light transmitted in the Fabry-Perot resonant cavity is greater than the cavity length of the Fabry-Perot resonant cavity.

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