On-chip terahertz electro-optical sampling device based on thin-film lithium niobate and preparation method of on-chip terahertz electro-optical sampling device

Abstract

The invention provides an on-chip terahertz electro-optical sampling device based on thin-film lithium niobate and a preparation method thereof, the device comprises a device supporting layer, a terahertz chip layer, an optical fiber fixing layer and a single-mode polarization maintaining optical fiber from bottom to top, and the layers are cured and packaged through ultraviolet glue; the thickness of the device supporting layer is 500 [mu] m-5 mm, the device supporting layer is made of polyethylene or glass and used for supporting a device, the terahertz chip layer is located at the top of the device supporting layer, made of thin-film lithium niobate and used for preparing a terahertz electro-optical sampling chip, and the optical fiber fixing layer is located at the top of the terahertz chip layer, made of polyethylene or glass and used for fixing the terahertz electro-optical sampling chip. The packaging structure is used for packaging polarization-maintaining optical fibers. And the sensitivity, the bandwidth and the application range of the terahertz detection technology can be obviously improved. Light beam design is simple, debugging steps are simple, and sensitivity is high.

Description

Technical Field

[0001] This invention relates to the field of terahertz optoelectronic measurement technology, and in particular to an on-chip terahertz electro-optic sampling device based on thin-film lithium niobate and its fabrication method. Background Technology

[0002] Terahertz electromagnetic waves typically range in frequency from 0.1 THz to 10 THz, with wavelengths ranging from approximately 0.03 mm to 3 mm. They lie between the microwave and infrared bands, possessing characteristics such as high frequency, wide bandwidth, strong coherence, good penetration, low photon energy, and strong anti-interference capabilities. This makes them promising for applications in radar, target imaging, electronic jamming and countermeasures, and non-destructive testing, and they represent a crucial technological direction for future 6G communication development. However, the short-pulse characteristics (femtosecond to picosecond range) and complex transmission properties of terahertz electromagnetic waves place extremely high demands on their measurement techniques. Currently, commonly used terahertz electromagnetic wave measurement methods are mainly based on two types: free-space electro-optic sampling (EOS) and photoconductive sampling (PCS). EOS utilizes the Pockels effect of an electro-optic crystal to couple the terahertz field with the probe laser, converting the modulation of the probe laser's polarization state by the terahertz electric field into an intensity difference, thereby detecting the terahertz electric field signal. PCS, on the other hand, utilizes the transient photocurrent generated by a photoconductive antenna under laser excitation to directly detect the terahertz electric field signal. However, due to limitations imposed by the photoconductive antenna's resonant frequency, photocurrent saturation effect, thermal noise, and carrier lifetime, PCS exhibits poor performance in terms of bandwidth, dynamic range, and signal-to-noise ratio. Traditional EOS methods generally rely on complex beam designs, involve cumbersome debugging procedures, have a large system footprint, and are susceptible to environmental interference, exhibiting significant instability and limitations, making it difficult to meet the demands of portable applications and the trend towards integration. Summary of the Invention

[0003] This invention provides an on-chip terahertz electro-optic sampling device based on thin-film lithium niobate, which comprises four parts from bottom to top: a device support layer, a terahertz chip layer, an optical fiber fixing layer, and a single-mode polarization-maintaining optical fiber. The layers are encapsulated by UV adhesive curing.

[0004] The device support layer has a thickness of 500μm to 5mm and is made of polyethylene or glass. It is used to support the device. The terahertz chip layer is located on top of the device support layer and is made of thin-film lithium niobate. It is used to prepare the terahertz electro-optic sampling chip. The fiber fixing layer is located on top of the terahertz chip layer and is made of polyethylene or glass. It is used to encapsulate the polarization-maintaining fiber.

[0005] Optionally, the terahertz chip layer includes, from bottom to top, a substrate layer, a buried insulator layer, and a lithium niobate waveguide thin film layer;

[0006] The substrate layer has a thickness of 200μm to 500μm and is made of lithium niobate or silicon. It is used to support the device structure. The buried insulator layer is located on top of the substrate layer and has a thickness of 1μm to 4μm. The lithium niobate waveguide thin film layer is located on top of the buried insulator layer and has a thickness of 300nm to 900nm. It is used to fabricate a terahertz electro-optic sampling chip.

[0007] Optionally, the buried insulating layer is SiO2 or Si3N4, used to fix the lithium niobate waveguide thin film layer and the substrate layer.

[0008] Optionally, the terahertz electro-optic sampling chip includes two parts: an on-chip optical waveguide and an on-chip traveling wave electrode, wherein an insulating layer covers the surface between the on-chip optical waveguide and the on-chip traveling wave electrode.

[0009] Optionally, the on-chip optical waveguide is a ridge waveguide structure, which includes, in sequence, an optical input grating, a transmission straight waveguide, a 3dB beam splitter, vertically parallel waveguide interference arms, and an optical output grating.

[0010] Optionally, the transmission line type of the on-chip traveling wave electrode is a coplanar waveguide, with a rectangular or T-shaped structure, placed on both sides of the waveguide interference arms that are parallel to each other on the on-chip optical waveguide.

[0011] Optionally, the insulating layer is SiO2 or Si3N4.

[0012] Optionally, the transverse dimension of the optical fiber fixing layer is the same as that of the thin-film lithium niobate layer, and the thickness is 1.5mm-3mm.

[0013] This invention provides a method for fabricating the aforementioned thin-film lithium niobate on-chip terahertz electro-optic sampling device, comprising the following steps:

[0014] The thin-film lithium niobate substrate is cleaned and dried for later use. A chromium layer of a predetermined thickness is deposited on the surface of the thin-film lithium niobate substrate using an electron beam evaporator as a mask. The desired on-chip optical waveguide pattern structure is fabricated on the mask using electron beam lithography. The thin-film lithium niobate on-chip optical waveguide is obtained using SF6 and Cr gas. The remaining chromium mask is removed. Finally, a SiO2 insulating layer of a certain thickness is deposited on top of the optical waveguide layer to complete the fabrication of the on-chip optical waveguide of the electro-optic sampling chip.

[0015] A Ti / Au seed layer with a preset thickness is deposited on the surface of the thin-film lithium niobate substrate. The pattern of the coplanar waveguide traveling wave electrode is exposed and developed on the seed layer using contact lithography. The thin-film lithium niobate substrate, after being soft-baked and hardened, is placed in an electroplating solution. The seed layer of the thin-film lithium niobate substrate to be plated is connected to the negative terminal of the power supply, and the anode plate placed in the electroplating bath is connected to the positive terminal of the power supply. By setting the electroplating current and the electrolysis current, the fabrication of the traveling wave electrode on the electro-optic sampling chip is completed.

[0016] UV adhesive was applied to the upper surface of the device support layer and attached to the lower surface of the thin-film lithium niobate. After the position was fixed, UV lamp was used for curing and encapsulation. Then, UV adhesive was applied to the four edges of the upper surface of the thin-film lithium niobate for later use.

[0017] The optical input / output polarization-maintaining fiber is pre-placed in the reserved coupling hole of the fiber fixing layer. A dual six-axis coupling platform is used to couple the fiber fixing layer and the terahertz chip layer with the pre-fixed polarization-maintaining fiber. When the laser light incident from the optical input polarization-maintaining fiber enters the optical waveguide on the electro-optic sampling chip through the optical input coupling grating, it is output through the optical output coupling grating and received by the optical output polarization-maintaining fiber. The relative position of the polarization-maintaining fiber and the fiber fixing layer is first solidified by an ultraviolet lamp, and then the fiber fixing layer and the terahertz chip layer are solidified.

[0018] The present invention also provides a terahertz signal measurement system based on any of the aforementioned thin-film lithium niobate on-chip terahertz electro-optic sampling devices, characterized in that it includes: an optical link and a terahertz link, wherein the optical link includes a repetition-locked femtosecond laser, an on-chip terahertz electro-optic sampling device, a photodetector, and a signal acquisition module connected in sequence; the terahertz link includes a terahertz signal generator composed of a microwave signal source, a spread spectrum module, and a gain horn antenna, an on-chip terahertz electro-optic sampling device, and a trigger signal source.

[0019] Other features and advantages of the invention will be set forth in the following description, and will be apparent in part from the description, or may be learned by practicing the invention. The objects and other advantages of the invention may be realized and obtained by means of the structures particularly pointed out in the written description, claims, and accompanying drawings. Attached Figure Description

[0020] The accompanying drawings, which are included to provide a further understanding of the invention and form part of this invention, illustrate exemplary embodiments of the invention and are used to explain the invention, but do not constitute an undue limitation of the invention. In the drawings:

[0021] Figure 1 A schematic diagram of the structure of an on-chip terahertz electro-optic sampling device based on thin-film lithium niobate provided by the present invention;

[0022] Figure 2 A schematic diagram of the structure of a terahertz electro-optic sampling chip in an on-chip terahertz electro-optic sampling device based on thin-film lithium niobate provided by the present invention;

[0023] Figure 3 A schematic cross-sectional view of the terahertz electro-optic sampling chip in an on-chip terahertz electro-optic sampling device based on thin-film lithium niobate provided by the present invention.

[0024] Figure 4 The schematic diagram of the on-chip coplanar waveguide traveling wave electrode of the terahertz electro-optic sampling chip in the on-chip terahertz electro-optic sampling device based on thin-film lithium niobate provided by the present invention is shown in the top view. (a) is a rectangular metal electrode, and (b) is a T-shaped metal electrode.

[0025] Figure 5 The above-view and cross-sectional view of the fiber fixing layer in an on-chip terahertz electro-optic sampling device based on thin-film lithium niobate are provided for the present invention.

[0026] Figure 6 A schematic diagram of a terahertz signal measurement system based on an on-chip terahertz electro-optic sampling device using thin-film lithium niobate, provided for this invention;

[0027] Figure 7 This is a schematic diagram illustrating a method for fabricating an on-chip terahertz electro-optic sampling device based on thin-film lithium niobate, as provided by the present invention. Detailed Implementation

[0028] To address the technical challenges of complex beam design and cumbersome debugging procedures in terahertz electromagnetic wave measurement methods, this paper provides an on-chip terahertz electro-optic sampling device based on thin-film lithium niobate and its fabrication method.

[0029] The preferred embodiments of the present invention will be described below with reference to the accompanying drawings. It should be understood that the preferred embodiments described herein are for illustration and explanation only and are not intended to limit the present invention. Furthermore, the embodiments and features in the embodiments of the present invention can be combined with each other without conflict.

[0030] It should be noted that the terms "first," "second," etc., in the specification, claims, and accompanying drawings of the embodiments of the present invention are used to distinguish similar objects and are not necessarily used to describe a specific order or sequence. It should be understood that such data can be interchanged where appropriate so that the embodiments described herein can be implemented in a sequence other than that illustrated or described herein.

[0031] Currently, commonly used terahertz electromagnetic wave measurement methods are mainly based on two techniques: free-space electro-optic sampling (EOS) and photoconductive sampling (PCS). EOS utilizes the Pockels effect of an electro-optic crystal to couple the terahertz field with the probe laser, converting the modulation of the probe laser's polarization state by the terahertz electric field into an intensity difference, thereby detecting the terahertz electric field signal. PCS, on the other hand, utilizes the transient photocurrent generated by a photoconductive antenna under laser excitation to directly detect the terahertz electric field signal. However, due to limitations imposed by the photoconductive antenna's resonant frequency, photocurrent saturation effect, thermal noise, and carrier lifetime, PCS's bandwidth, dynamic range, and signal-to-noise ratio are inferior to EOS. Furthermore, thanks to its unique working principle and balanced optical design, EOS exhibits higher sensitivity than PCS, making it more suitable for measuring low-intensity terahertz electric field signals.

[0032] Lithium niobate (LiNbO3, LN), as a synthetically produced negative uniaxial electro-optic crystal, possesses a wide transparency window (0.45 μm–5.5 μm), a moderate refractive index, a large linear electro-optic coefficient (~30.8 pm / V), good temperature stability, and a low thermo-optic coefficient (~1.8 × 10⁻⁶). -4 K -1 For decades, lithium niobate (LN) has been a classic material for terahertz detection due to its excellent physical and chemical properties. Thin-film lithium niobate (TFLN), through innovative material structure, not only retains the inherent excellent optoelectronic properties of LN electro-optic crystals, but also, because the upper medium of the TFLN-based optical waveguide is air and the lower medium is a low-refractive-index material with a refractive index difference of at least 0.7, it can better confine light waves in a smaller optical waveguide, enabling integrated design of devices and modules. This further promotes the miniaturization and chip-based development of terahertz measurement technology, achieving technological breakthroughs in sensitivity, bandwidth, and power consumption.

[0033] This invention provides an on-chip terahertz electro-optic sampling device based on TFLN and its fabrication method. Through innovative material properties and integrated optical path design, the sensitivity, bandwidth, and application range of terahertz detection technology can be significantly improved. The beam design is simple, the debugging steps are straightforward, and the sensitivity is high.

[0034] This invention belongs to the field of microwave photonic devices and terahertz optoelectronic measurement technology, and particularly relates to an on-chip terahertz electro-optic sampling device based on thin-film lithium niobate and its fabrication method.

[0035] This invention provides an on-chip terahertz electro-optic sampling device based on a TFLN (Transient Fluorescent Alternating Current). Utilizing a patch antenna directly as the traveling-wave electrode of the on-chip terahertz electro-optic sampling device, it can efficiently couple free-space terahertz signals onto the substrate material, achieving direct conversion with optical signals within the waveguide. Furthermore, the on-chip terahertz electro-optic sampling device proposed in this invention employs vertical coupling technology to achieve efficient coupling with optical fibers, effectively improving the device's integration density and long-term stability. This invention also provides a method for fabricating the on-chip terahertz electro-optic sampling device based on a TFLN, using dry etching to effectively control the morphology of the TFLN on-chip waveguide structure, avoiding problems such as side etching, demonstrating significant technical advantages.

[0036] This invention provides an on-chip terahertz electro-optic sampling device based on thin-film lithium niobate. This on-chip terahertz electro-optic sampling device integrates patch electrodes with a thin-film lithium niobate optical waveguide structure. A patch antenna within the coplanar waveguide structure serves as the traveling wave electrode of the on-chip terahertz electro-optic sampling device, efficiently coupling free-space terahertz signals onto the thin-film lithium niobate substrate, achieving direct conversion with optical signals within the waveguide. The electro-optic sampling system built using this on-chip terahertz electro-optic sampling device based on thin-film lithium niobate provides advantages such as wide bandwidth, strong anti-interference capability, portability, and integration, enabling efficient detection of terahertz waves and demonstrating significant technical advantages.

[0037] This invention provides an on-chip terahertz electro-optic sampling device based on thin-film lithium niobate, which comprises four parts from bottom to top: a device support layer, a terahertz chip layer, an optical fiber fixing layer, and a single-mode polarization-maintaining optical fiber. The layers are encapsulated by UV adhesive curing.

[0038] The device support layer has a thickness of 500μm to 5mm and is made of polyethylene or glass. It is used to support the device. The terahertz chip layer is located on top of the device support layer and is made of thin-film lithium niobate. It is used to prepare the terahertz electro-optic sampling chip. The fiber fixing layer is located on top of the terahertz chip layer and is made of polyethylene or glass. It is used to encapsulate the polarization-maintaining fiber.

[0039] The material of the device support layer can be high-density polyethylene, which is mainly used to support the device, and the material of the optical fiber fixing layer can be high-density polyethylene.

[0040] Optionally, the terahertz chip layer includes, from bottom to top, a substrate layer, a buried insulator layer, and a lithium niobate waveguide thin film layer;

[0041] The substrate layer has a thickness of 200μm to 500μm and is made of lithium niobate or silicon. It is used to support the device structure. The buried insulator layer is located on top of the substrate layer and has a thickness of 1μm to 4μm. The lithium niobate waveguide thin film layer is located on top of the buried insulator layer and has a thickness of 300nm to 900nm. It is used to fabricate a terahertz electro-optic sampling chip.

[0042] Optionally, the buried insulating layer is SiO2 or Si3N4, used to fix the lithium niobate waveguide thin film layer and the substrate layer.

[0043] The buried insulating layer is made of materials with low dielectric constant, such as SiO2 or Si3N4, and has low absorption loss for terahertz waves. It is mainly used to fix and connect the lithium niobate waveguide thin film layer and the substrate layer, and the bonding of the composite epitaxial wafer is achieved through a smart-cut process.

[0044] Optionally, the terahertz electro-optic sampling chip includes two parts: an on-chip optical waveguide and an on-chip traveling wave electrode, wherein an insulating layer covers the surface between the on-chip optical waveguide and the on-chip traveling wave electrode.

[0045] An insulating layer is placed between the on-chip optical waveguide and the on-chip traveling wave electrode to achieve phase velocity matching.

[0046] Optionally, the on-chip optical waveguide is a ridge waveguide structure, which includes, in sequence, an optical input grating, a transmission straight waveguide, a 3dB beam splitter, vertically parallel waveguide interference arms, and an optical output grating.

[0047] Optionally, from left to right, it may include an optical input grating, a transmission straight waveguide, a 3dB beam splitter, vertically parallel waveguide interferometer arms, and an optical output grating, wherein the 3dB beam splitter may be a Y-shaped branch with rising cosine bending, a 1×2 multimode interference coupler, or a directional coupler structure.

[0048] Optionally, the transmission line type of the on-chip traveling wave electrode is a coplanar waveguide, with a rectangular or T-shaped structure, placed on both sides of the waveguide interference arms that are parallel to each other on the on-chip optical waveguide.

[0049] Optionally, the insulating layer is SiO2 or Si3N4.

[0050] The insulating isolation layer is located on the on-chip optical waveguide structure and on the on-chip traveling wave electrode structure. It is mainly made of low dielectric constant material, and its purpose is to reduce the effective refractive index of microwaves. It can be a terahertz low absorption loss material such as SiO2 or Si3N4.

[0051] Optionally, the transverse dimension of the optical fiber fixing layer is the same as that of the thin-film lithium niobate layer, and the thickness is 1.5mm-3mm.

[0052] The optical fiber fixing layer has pre-drilled optical input / output fiber coupling holes with a diameter of approximately 1.5 mm and an angle of 8° with the vertical direction. The optical input fiber coupling hole is located on the lower surface of the optical fiber fixing layer at the same position as the optical input grating on the thin-film lithium niobate sheet, and the optical output fiber coupling hole is located on the lower surface of the optical fiber fixing layer at the same position as the optical output grating on the thin-film lithium niobate sheet.

[0053] The present invention also provides a method for fabricating the aforementioned thin-film lithium niobate on-chip terahertz electro-optic sampling devices, comprising the following steps:

[0054] S01. Control the cleaning and drying of the thin-film lithium niobate substrate for later use. Use an electron beam evaporator to deposit a chromium layer of a predetermined thickness on the surface of the thin-film lithium niobate substrate as a mask. Use electron beam lithography to fabricate the desired on-chip optical waveguide pattern on the mask. Use SF6 and Cr gas to obtain the thin-film lithium niobate on-chip optical waveguide. Remove the remaining chromium mask. Finally, deposit a certain thickness of SiO2 insulating isolation layer on top of the optical waveguide layer to complete the fabrication of the on-chip optical waveguide of the electro-optic sampling chip.

[0055] The aforementioned preset thickness is 100nm, which can be specifically prepared by using SF6 and Cr gas with a dry etching process to obtain a thin-film lithium niobate optical waveguide.

[0056] S02. Deposit a Ti / Au seed layer with a preset thickness on the surface of the thin-film lithium niobate substrate; use contact lithography to expose and develop the pattern of the coplanar waveguide traveling wave electrode on the seed layer; place the thin-film lithium niobate substrate after soft baking and hardening into the electroplating solution, wherein the seed layer of the thin-film lithium niobate substrate to be plated is connected to the negative terminal of the power supply, and the anode plate placed in the electroplating bath is connected to the positive terminal of the power supply; by setting the electroplating current and the electrolysis current, the fabrication of the traveling wave electrode on the electro-optic sampling chip is completed.

[0057] Specifically, a Ti / Au seed layer with a thickness of about 100nm can be deposited on the substrate surface using magnetron sputtering to ensure overall conductivity of the substrate surface.

[0058] S03. Control the coating of UV adhesive on the upper surface of the device support layer and attach it to the lower surface of the thin film lithium niobate. After fixing the position, use a UV lamp for curing and encapsulation. Then, coat the edges of the upper surface of the thin film lithium niobate with UV adhesive for later use.

[0059] S04. The optical input / output polarization-maintaining fiber is pre-placed in the reserved coupling hole of the fiber fixing layer. The fiber fixing layer and the terahertz chip layer of the pre-fixed polarization-maintaining fiber are coupled using a dual six-axis coupling platform. When the laser incident from the optical input polarization-maintaining fiber enters the optical waveguide on the electro-optic sampling chip through the optical input coupling grating, it is output through the optical output coupling grating. After being received by the optical output polarization-maintaining fiber with high coupling efficiency, the relative position of the polarization-maintaining fiber and the fiber fixing layer is first solidified by an ultraviolet lamp, and then the fiber fixing layer and the terahertz chip layer are solidified.

[0060] This invention also provides a terahertz signal measurement system based on the aforementioned thin-film lithium niobate on-chip terahertz electro-optic sampling device, comprising: an optical link and a terahertz link, wherein the optical link includes a frequency-locked femtosecond laser, an on-chip terahertz electro-optic sampling device, a photodetector, and a signal acquisition module connected sequentially; the terahertz link includes a terahertz signal generator composed of a microwave signal source, a spread spectrum module, and a gain horn antenna, an on-chip terahertz electro-optic sampling device, and a trigger signal source. To ensure a common time base for all instruments, a small rubidium clock is used to provide a reference clock for the measurement system, and is connected to the terahertz signal generator, trigger signal source, signal acquisition and analysis module, and frequency-locked femtosecond laser via a frequency beam splitter.

[0061] The present invention will be further described below with reference to the accompanying drawings:

[0062] Figure 1 The schematic diagram of an on-chip terahertz electro-optic sampling device based on thin-film lithium niobate provided by this invention includes four parts from bottom to top: a device support layer, a terahertz chip layer, an optical fiber fixing layer, and a single-mode polarization-maintaining fiber. Each layer is encapsulated by UV adhesive curing. The device support layer mainly supports the entire device and can be made of high-density polyethylene or glass, with a thickness of 500 μm to 5 mm. The terahertz chip layer, located on top of the device support layer, is made of thin-film lithium niobate and is mainly used to fabricate the terahertz electro-optic sampling chip. The optical fiber fixing layer, located on top of the terahertz chip layer, has the same horizontal dimensions as the thin-film lithium niobate layer and a thickness of 1.5 mm to 3 mm. It has a pre-drilled optical input / output fiber coupling hole with an angle of 8° to the vertical direction and a diameter of approximately 1.5 mm. The labels in the figure represent: 1. Device support layer; 2. Terahertz chip layer; 3. Optical fiber fixing layer; 4. Polarization-maintaining fiber.

[0063] Figure 2The diagram shows a schematic of the terahertz chip layer in an on-chip terahertz electro-optic sampling device based on thin-film lithium niobate. From bottom to top, this layer comprises three parts: a substrate layer, a buried insulator layer, and a lithium niobate waveguide thin film layer. The substrate layer has a thickness of 200 μm to 500 μm and can be made of lithium niobate or silicon, primarily used to support the device structure. The buried insulator layer is located on top of the substrate layer and is preferably made of materials with low dielectric constants and low absorption loss for terahertz waves, such as SiO2 or Si3N4, with a thickness of approximately 1 μm to 4 μm. It is mainly used to fix and connect the waveguide thin film layer and the ground metal layer, achieving bonding of the composite epitaxial wafer. The lithium niobate waveguide thin film layer is located on top of the buried insulator layer and has a thickness of 300 nm to 900 nm. It is preferably made of x-cut y-transmission lithium niobate crystal and is mainly used to fabricate a single-mode transmission ridge on-chip optical waveguide structure. The terahertz electro-optic sampling chip fabricated on a terahertz chip layer mainly consists of two parts: an on-chip optical waveguide and an on-chip traveling wave electrode. Furthermore, to achieve phase velocity matching, an insulating layer is placed between the on-chip optical waveguide and the on-chip traveling wave electrode. Figure 3 As shown. The on-chip optical waveguide of the terahertz electro-optic sampling chip is a ridge waveguide structure, comprising, from left to right, an input grating, a straight transmission waveguide, a 3dB beam splitter, vertically parallel waveguide interferometer arms, and an output grating. The 3dB beam splitter can be a Y-shaped branch with rising cosine bending, a 1×2 multimode interference coupler, or a directional coupler structure. The on-chip traveling wave electrode of the terahertz electro-optic sampling chip is a coplanar waveguide, and its structure can be rectangular or T-shaped, such as... Figure 4 As shown, the on-chip traveling wave electrode is placed on both sides of the waveguide interference arm that is parallel to the top and bottom of the on-chip optical waveguide.

[0064] Figure 2 The meanings of the numbers are as follows: 5. Substrate layer; 6. Buried insulator layer; 7. Lithium niobate waveguide thin film layer; 8. Input grating; 9 / 10. Output grating; 11. On-chip optical waveguide; 12. On-chip traveling wave electrode. Figure 3 13 in the text is the insulating layer. Figure 4 This is a top view of the on-chip coplanar waveguide traveling wave electrode structure of the terahertz electro-optic sampling chip in an on-chip terahertz electro-optic sampling device based on thin-film lithium niobate according to the present invention. (a) is a rectangular metal electrode, and (b) is a T-shaped metal electrode.

[0065] Figure 5The diagram shows a schematic of the fiber fixing layer structure in an on-chip terahertz electro-optic sampling device based on thin-film lithium niobate. Its horizontal dimension is the same as that of the thin-film lithium niobate layer, and its thickness is 1.5 mm to 3 mm. As can be seen from the cross-sectional view, there is a reserved optical input / output fiber coupling hole with a diameter of about 1.5 mm at an angle of 8° to the vertical direction. As can be seen from the top view, the position of the optical input fiber coupling hole on the lower surface of the fiber fixing layer is consistent with the position of the optical input grating on the thin-film lithium niobate sheet, and the position of the optical output fiber coupling hole on the lower surface of the fiber fixing layer is consistent with the position of the optical output grating on the thin-film lithium niobate sheet.

[0066] Figure 6 The diagram shows a schematic of a terahertz signal measurement system based on an on-chip terahertz electro-optic sampling device using thin-film lithium niobate. According to the different propagation media, it can be divided into two parts: an optical link and a terahertz link. The optical link includes a frequency-locked ultrafast femtosecond transducer with a center wavelength of 1550 nm, a repetition rate of 50 MHz, a pulse width of 100 fs, and an output power of 100 mW, sequentially connected; an on-chip optical waveguide structure based on thin-film lithium niobate; a photodetector; and a signal acquisition and analysis module. The terahertz link includes a terahertz signal generator consisting of a microwave signal source, a spread spectrum module, and a gain horn antenna; an on-chip traveling wave electrode based on thin-film lithium niobate; and a trigger signal source. A femtosecond laser is incident on an input polarization-maintaining fiber to an input coupling grating. After transmission through an on-chip optical waveguide, it is received by the output polarization-maintaining fiber with high coupling efficiency through an output coupling grating and connected to a balanced photodetector. The photodetector converts the optical signal into an electrical signal, which is then connected to a data acquisition and analysis module to be converted into waveform data. Simultaneously, a radio frequency signal radiated by a low-frequency signal source is spread spectrum to generate a terahertz wave signal with a frequency range of 110 GHz to 500 GHz. This signal is radiated directionally to an on-chip traveling-wave electrode through a gain horn antenna. The traveling-wave electrode receives the terahertz wave in the far-field region (the distance from the horn antenna satisfies 2D² / λRF) and propagates in the same direction as the optical wave in the electro-optic crystal. Due to the electro-optic effect, the terahertz wave signal is loaded onto the optical signal. Subsequent demodulation and other processes are then performed to extract the terahertz wave signal. During system testing, a small rubidium clock with a short-term time base stability better than 10⁻¹¹ is used to provide a reference clock to each instrument via a frequency beam splitter. The test trigger signal is a square wave signal with a repetition frequency of 50Hz to 1kHz and an amplitude of approximately 500mV to 1V.

[0067] Figure 6The meanings of the numbers are as follows: 14. On-chip terahertz electro-optic sampling device based on lithium niobate thin film; 15. Miniature rubidium clock; 16. Clock distributor; 17. Microwave signal source 1; 18. Spread spectrum module; 19. Gain horn antenna; 20. Microwave signal source 2; 21. Femtosecond laser with frequency repetition rate locking; 22. Trigger signal source; 23. Balanced photodetector; 24. Signal acquisition and analysis module.

[0068] Figure 7 A schematic diagram illustrating a method for fabricating an on-chip terahertz electro-optic sampling device based on thin-film lithium niobate, provided by the present invention, the method comprising the following steps:

[0069] S1. Clean and dry the thin film lithium niobate substrate, fabricate the on-chip optical waveguide structure of the electro-optic sampling chip using electron beam lithography and dry etching processes, and deposit a SiO2 insulating isolation layer on the top of the optical waveguide.

[0070] S2. The traveling wave electrode structure on the electro-optic sampling chip is completed using an electroplating process.

[0071] S3. Use UV adhesive to complete the curing and encapsulation of the device support layer and terahertz chip layer;

[0072] S4. Using a dual six-axis coupling platform, pre-fix the polarization-maintaining fiber and the fiber fixing layer to achieve high-efficiency coupling between the polarization-maintaining fiber and the grating coupler of the electro-optic sampling chip. Then, use an ultraviolet lamp to sequentially solidify the relative positions of the polarization-maintaining fiber and the fiber fixing layer, and the relative positions of the fiber fixing layer and the terahertz chip layer.

[0073] Although preferred embodiments of the invention have been described, those skilled in the art, upon learning the basic inventive concept, can make other changes and modifications to these embodiments. Therefore, the appended claims are intended to be interpreted as including both the preferred embodiments and all changes and modifications falling within the scope of the invention.

[0074] Obviously, those skilled in the art can make various modifications and variations to this invention without departing from its spirit and scope. Therefore, if these modifications and variations fall within the scope of the claims of this invention and their equivalents, this invention also intends to include these modifications and variations.

Claims

1. An on-chip terahertz electro-optic sampling device based on thin-film lithium niobate, characterized in that, From bottom to top, it consists of four parts: a device support layer, a terahertz chip layer, an optical fiber fixing layer, and a single-mode polarization-maintaining fiber. Each layer is encapsulated by UV adhesive. The device support layer has a thickness of 500μm to 5mm and is made of polyethylene or glass. It is used to support the device. The terahertz chip layer is located on top of the device support layer and is made of thin-film lithium niobate. It is used to prepare the terahertz electro-optic sampling chip. The optical fiber fixing layer is located on top of the terahertz chip layer and is made of polyethylene or glass. It is used to encapsulate the polarization-maintaining optical fiber.

2. The device according to claim 1, characterized in that, The terahertz chip layer, from bottom to top, includes a substrate layer, a buried insulator layer, and a lithium niobate waveguide thin film layer; The substrate layer has a thickness of 200μm to 500μm and is made of lithium niobate or silicon. It is used to support the device structure. The buried insulator layer is located on top of the substrate layer and has a thickness of 1μm to 4μm. The lithium niobate waveguide thin film layer is located on top of the buried insulator layer and has a thickness of 300nm to 900nm. It is used to fabricate a terahertz electro-optic sampling chip.

3. The device according to claim 2, characterized in that, The buried insulating layer is SiO2 or Si3N4, used to fix and connect the lithium niobate waveguide thin film layer and the substrate layer.

4. The device according to claim 2, characterized in that, The terahertz electro-optic sampling chip includes two parts: an on-chip optical waveguide and an on-chip traveling wave electrode. An insulating layer covers the surface between the on-chip optical waveguide and the on-chip traveling wave electrode.

5. The device according to claim 4, characterized in that, The on-chip optical waveguide is a ridge waveguide structure, which includes, in sequence, an optical input grating, a transmission straight waveguide, a 3dB beam splitter, vertically parallel waveguide interferometer arms, and an optical output grating.

6. The device according to claim 4, characterized in that, The transmission line type of the on-chip traveling wave electrode is a coplanar waveguide, with a rectangular or T-shaped structure, and is placed on both sides of the waveguide interference arms that are parallel to each other on the upper and lower parts of the on-chip optical waveguide.

7. The device according to claim 4, characterized in that, The insulating layer is SiO2 or Si3N4.

8. The device according to claim 1, characterized in that, The transverse dimension of the optical fiber fixing layer is the same as that of the thin-film lithium niobate layer, and the thickness is 1.5mm-3mm.

9. A method for fabricating an on-chip terahertz electro-optic sampling device of thin-film lithium niobate according to any one of claims 1-8, comprising the following steps: The thin-film lithium niobate substrate is cleaned and dried for later use. A chromium layer of a predetermined thickness is deposited on the surface of the thin-film lithium niobate substrate using an electron beam evaporator as a mask. The desired on-chip optical waveguide pattern structure is fabricated on the mask using electron beam lithography. The thin-film lithium niobate on-chip optical waveguide is obtained using SF6 and Cr gas. The remaining chromium mask is removed. Finally, a SiO2 insulating layer of a certain thickness is deposited on top of the optical waveguide layer to complete the fabrication of the on-chip optical waveguide of the electro-optic sampling chip. A Ti / Au seed layer with a preset thickness is deposited on the surface of the thin-film lithium niobate substrate; The pattern of the coplanar waveguide traveling wave electrode is exposed and developed on the seed layer using contact photolithography. The thin film lithium niobate substrate, after being softened and hardened, is placed in the electroplating solution. The seed layer of the thin film lithium niobate substrate to be plated is connected to the negative terminal of the power supply, and the anode plate placed in the electroplating bath is connected to the positive terminal of the power supply. By setting the electroplating current and the electrolysis current, the on-chip traveling wave electrode of the electro-optic sampling chip is fabricated. UV adhesive was applied to the upper surface of the device support layer and attached to the lower surface of the thin-film lithium niobate. After the position was fixed, UV lamp was used for curing and encapsulation. Then, UV adhesive was applied to the four edges of the upper surface of the thin-film lithium niobate for later use. The optical input / output polarization-maintaining fiber is pre-placed in the reserved coupling hole of the fiber fixing layer. A dual six-axis coupling platform is used to couple the fiber fixing layer and the terahertz chip layer with the pre-fixed polarization-maintaining fiber. When the laser light incident from the optical input polarization-maintaining fiber enters the optical waveguide on the electro-optic sampling chip through the optical input coupling grating, it is output through the optical output coupling grating and received by the optical output polarization-maintaining fiber. The relative position of the polarization-maintaining fiber and the fiber fixing layer is first solidified by an ultraviolet lamp, and then the fiber fixing layer and the terahertz chip layer are solidified.

10. A terahertz signal measurement system based on an on-chip terahertz electro-optic sampling device of thin-film lithium niobate according to any one of claims 1-8, characterized in that, It includes an optical link and a terahertz link, wherein the optical link includes a repetition rate locked femtosecond laser, an on-chip terahertz electro-optic sampling device, a photodetector and a signal acquisition module connected in sequence; the terahertz link includes a terahertz signal generator composed of a microwave signal source, a spread spectrum module and a gain horn antenna, an on-chip terahertz electro-optic sampling device and a trigger signal source.

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