Device and method for measuring rare earth ions doped in photon chip at low temperature

Abstract

The invention provides a device and a method for measuring rare earth ions doped in a photon chip at low temperature, and belongs to the field of photon chip low-temperature optical detection. The problem that fluorescence measurement of an existing integrated optical chip is inconvenient at low temperature is solved. Comprising a to-be-tested sample, wherein the to-be-tested sample is composed of a photon chip and an optical fiber array which are optically coupled; wherein the photon chip adopts a thin-film lithium niobate photon chip doped with rare earth ions; the low-temperature sample table is used for placing a sample to be detected and enabling the sample to be in a low-temperature environment; the semiconductor laser source is used for emitting a laser source; the polarization controller is connected to an optical fiber output port of the to-be-detected sample and used for controlling the polarization state of the laser source, irradiating light to the to-be-detected sample, introducing laser through an optical fiber array on the to-be-detected sample and collecting a fluorescence signal; the light detector is used for separating exciting light and fluorescence signals to realize fluorescence detection of rare earth ions; the method is applied to fluorescence measurement of rare earth ions in the photon chip at the liquid helium temperature.

Description

Technical Field

[0001] This application relates to the field of low-temperature optical detection technology for photonic chips, and in particular to a device and method for measuring rare earth ions doped in photonic chips at low temperatures. Background Technology

[0002] In fields such as quantum information technology and integrated optics, thin-film lithium niobate has become an important material for next-generation integrated photonic devices due to its excellent physical properties, including a transparent window of 400nm-5000nm, a large electro-optic modulation coefficient, a large nonlinear coefficient, and low optical loss. With the development of integrated photonics, the application prospects of thin-film lithium niobate photonic chips are becoming increasingly broad, especially in applications requiring high precision and high reliability, such as quantum computing and high-speed optical communication, where they have demonstrated great potential. However, their packaging technology still faces many challenges, particularly in low-temperature adaptability packaging, where the following key issues exist.

[0003] 1. In low-temperature environments, traditional packaging materials (such as UV adhesives, epoxy resins, etc.) are prone to brittleness or debonding, which affects the performance and long-term stability of photonic chips.

[0004] 2. Due to the significant difference in the coefficients of thermal expansion between lithium niobate and traditional substrate materials such as silicon and silicon dioxide, for example, at 60K, the coefficients of thermal expansion are 4.3×10⁻⁶ / K (lithium niobate), 0.077×10⁻⁶ / K (silicon), and 0.4×10⁻⁶ / K (silicon dioxide). This typical difference between lithium niobate films and silicon dioxide leads to mechanical stress at low temperatures, causing fiber-to-chip coupling misalignment and increasing coupling loss.

[0005] 3. Existing room temperature packaging technologies are often unsuitable for low temperature environments, especially in terms of heat conduction and material stability. Therefore, it is necessary to design a packaging structure with both excellent thermal and mechanical properties for low temperature environments.

[0006] Therefore, measuring the optical properties of rare earth ions in thin-film lithium niobate photonic chips at liquid helium temperature is a significant challenge that urgently requires new solutions to overcome the bottlenecks of existing processes and measurement technologies in extreme environments and to promote the widespread application of rare earth ion-doped lithium niobate photonic chips in quantum storage, quantum repeaters, and other fields. Summary of the Invention

[0007] To address the inconvenience of low-temperature fluorescence measurement in existing integrated optical chips, this application proposes a device and method for measuring rare-earth ions doped in photonic chips at low temperatures.

[0008] The technical solution adopted in this application is: a device for measuring rare earth ions doped in a photonic chip at low temperature, comprising: The sample to be tested consists of an optically coupled photonic chip and an optical fiber array; the photonic chip is a thin-film lithium niobate photonic chip doped with rare earth ions. Low-temperature sample stage: used to place the sample to be tested and keep it in a low-temperature environment; Semiconductor laser source: Used to emit laser light; Polarization controller: Connected to the fiber optic output port of the sample under test, it is used to control the polarization state of the laser source, and then the light is irradiated onto the sample under test. The laser is guided through the fiber optic array on the sample under test and the fluorescence signal is collected. Photodetector: Used to separate excitation light and fluorescence signal to achieve fluorescence detection of rare earth ions.

[0009] Furthermore, a grating coupler array is disposed on the photonic chip, wherein the grating coupler array and the fiber array are optically coupled and fixed by UV adhesive and two-component optical adhesive.

[0010] Furthermore, the input port of each grating coupler in the grating coupler array is connected to a first straight waveguide, the output port of each grating coupler is connected to a second straight waveguide, and a curved waveguide connects the first and second straight waveguides.

[0011] Furthermore, the fiber array includes at least two single-mode fibers with an end-face tilt angle of 8°.

[0012] Furthermore, the photodetector employs a time-gated detector.

[0013] Furthermore, the semiconductor laser employs a tunable semiconductor laser.

[0014] Furthermore, the two-component optical adhesive uses the two-component high-performance optical adhesive EPO-TEK301-2.

[0015] A method for measuring rare-earth ions doped in a photonic chip at low temperature, using the aforementioned measuring device, includes the following steps: S1: Sample preparation; S2: Load the sample to be tested onto the low-temperature sample stage and cool it to the target temperature; S3: Connect the polarization controller to the fiber optic output port, turn on the semiconductor laser source, and guide the excitation light through the fiber optic array to collect the fluorescence signal; S4: The photodetector uses time-gated technology to separate the excitation light and fluorescence signal; the fluorescence spectral characteristics and time properties are analyzed.

[0016] Further, step S1 includes: S11: Prepare thin-film lithium niobate photonic chips and fiber arrays; S12: Collimate the fiber array with the grating coupler array on the photonic chip; S13: Pre-encapsulation is performed. After collimation is completed, the fiber array is pre-encapsulated by dropping UV adhesive onto one side of the fiber array and irradiating it with UV lamps from multiple angles until the output power is stable. S14: After pre-encapsulation, use a glass dropper to add a drop of two-component optical adhesive to the same side of the fiber array, and cure at room temperature.

[0017] Furthermore, it also includes S15: After the fiber array and photonic chip are coupled, the sample to be tested is transferred to a low-temperature sample stage, and the fiber output port is fixed on a bracket using two sets of optical adhesive to release stress.

[0018] The advantages of this application compared to existing technologies are as follows: This application uses a fiber optic array combined with a fan-shaped grating coupler for packaging, which results in a higher packaging success rate, smaller efficiency changes before and after packaging, less efficiency fluctuation with temperature changes, and a stable coupling curve throughout the temperature drop process. By adding a pre-packaging step, the loss of coupling efficiency during packaging and the probability of packaging failure due to operational errors are overcome. A low-temperature-compatible two-component optical adhesive is used to ensure the stability of optical signal input and output of the packaged chip at temperatures as low as liquid helium (1.5K). Due to the optical input and output of the fiber optic array, the fluorescence measurement of rare earth ions in the thin film at low temperatures can be directly completed through the fiber optic array. The photonic chip packaging technology of this application is simple to operate, has a high success rate, and exhibits small efficiency loss before and after packaging, with efficiency remaining stable with temperature changes, which is beneficial for the fluorescence measurement of rare earth ions at low temperatures. Attached Figure Description

[0019] The following description, in conjunction with the accompanying drawings, further illustrates this application: Figure 1 This is a schematic diagram of the structure of the photonic chip provided in an embodiment of this application.

[0020] Figure 2 This is a schematic diagram of the photonic chip packaging technology provided in the embodiments of this application.

[0021] Figure 3 This is a schematic diagram of the packaging structure of the photonic chip packaging technology provided in the embodiments of this application.

[0022] Figure 4 A schematic diagram illustrating the specific steps of the photonic chip packaging technology provided in the embodiments of this application.

[0023] Figure 5 This is a schematic diagram of the measuring device provided in an embodiment of this application.

[0024] Figure 6 The measurement of fiber-photonic chip coupling efficiency as a function of temperature is provided in the embodiments of this application.

[0025] Figure 7 A graph showing the fiber-photonic chip coupling efficiency measured by multiple heating and cooling cycles, as provided in an embodiment of this application.

[0026] Figure 8 Fluorescence lifetime and spectral width of rare earth ions in a lithium niobate photonic chip measured at 1.5K temperature, provided for embodiments of this application; In the figure: 1 is a grating coupler, 2 is a straight waveguide, 3 is a curved waveguide, 10 is a clamping base plate, 20 is a photonic chip, 30 is a fiber array, 40 is a fiber output port, 50 is UV adhesive, 60 is a two-component adhesive, 70 is a UV lamp, and 80 is a mounting bracket. Detailed Implementation

[0027] like Figures 1 to 8 As shown, this application provides a device for measuring rare-earth ions doped in a photonic chip at low temperatures, such as... Figure 5 As shown, it includes: The sample to be tested consists of an optically coupled photonic chip 20 and an optical fiber array 30. Low-temperature sample stage: used to place the sample to be tested and keep it in a low-temperature environment; Semiconductor laser source: used to emit laser light; wherein the semiconductor laser source uses a tunable semiconductor laser; Polarization controller: Connected to the fiber optic output port 40 of the sample under test, it is used to control the polarization state of the laser source and then irradiate the sample under test with light; the laser is introduced through the fiber optic array 30 on the sample under test and the fluorescence signal is collected. Photodetector: Used to separate excitation light and fluorescence signal to realize fluorescence detection of rare earth ions; after the light is received by the optical fiber, it is connected to the optical fiber interface of the sample to be tested through the polarization controller, and then measured by the photodetector after passing through the photonic chip 20.

[0028] The photonic chip 20 is a thin-film lithium niobate photonic chip doped with rare earth ions, including a grating coupler array for pump light input and fluorescence output, and various optical waveguides (including straight waveguide 2 and curved waveguide 3).

[0029] Fiber array 30, which contains two or more single-mode fibers with an end-face tilt angle of 8°.

[0030] The grating coupler array on the photonic chip 20 is coupled to the fiber array 30 through UV adhesive 50 and two-component optical adhesive 60. The photonic chip 20 and the fiber array 30 after optical coupling are used as a whole for the measurement of rare earth ions.

[0031] Semiconductor lasers can generate light sources in the wavelength range of 1470 nanometers to 1570 nanometers. By adjusting the wavelength of the emitted laser, the fluorescence signal values ​​at different wavelengths can be recorded.

[0032] The photodetector specifically employs a time-gated detector, using time-gated technology to separate the excitation light and fluorescence signals, and analyze the fluorescence spectral characteristics and temporal properties.

[0033] like Figure 1 The diagram shows a schematic of the pattern on a photonic chip. Grating coupler 1 serves as the input, output, and transmission device for optical signals. It has an opening angle of 60°, a radius of 31.5 micrometers, a fill factor of 0.5, a grating period of 1 micrometer, and is etched with 17 grooves at a depth of 170 nanometers. The distance between two grating couplers 1 is 200 micrometers. Straight waveguide 2 is 2 micrometers wide and 1500 micrometers long. The curved waveguide 3 has a radius of 100 micrometers.

[0034] like Figure 2 As shown, when encapsulating and coupling the photonic chip 20 with the fiber array 30, the photonic chip 20 can be placed on the clamping base plate 10. In this embodiment, the clamping base plate 10 is made of resin material using 3D printing, and the photonic chip 20 is fixed to the clamping base plate 10 using low-temperature varnish. The grating coupler array on the photonic chip 20 and the fiber array 30 need to be collimated. The fiber array 30 is fixed to the grating coupler 1 using UV adhesive 50 and two-component adhesive 60.

[0035] In this embodiment, the photonic chip platform selected is a lithium niobate thin film, which is doped with rare earth ions. The lithium niobate thin film with rare earth ions is a 300nm-600nm thick film grown on silicon dioxide. It has a high refractive index contrast and can be used in quantum devices such as quantum storage at low temperatures.

[0036] In this embodiment, the grating coupler 1 used on the photonic chip 20 for optical signal input and output and optical signal transmission between the optical fiber and the single-mode waveguide has many advantages, including a large coupling error (3dB error reaching more than 3 micrometers), a large coupling bandwidth (3dB bandwidth greater than 80nm), and the ability to be fabricated together with waveguides and other devices. The large coupling error can help overcome the thermal expansion between materials at low temperatures; therefore, the grating coupler 1 is suitable for the optical packaging of the low-temperature photonic chip 20.

[0037] To reduce efficiency losses caused by second-order reflections on the fiber surface and back reflections from the grating coupler 1, the selected fiber array 30 is a multi-channel fiber array with an 8° tilt angle. This fiber array 30 is manufactured using a special adhesive (two-component optical adhesive), which does not crack at low temperatures and exhibits minimal deformation, demonstrating good stability.

[0038] This application also proposes a method for measuring rare earth ions doped in a photonic chip at low temperatures, which includes the following steps: S1: Preparation of the sample to be tested, such as... Figure 4 As shown, it includes: S11: Prepare the thin-film lithium niobate photonic chip 20 and fiber array 30 used in the above device.

[0039] S12: Collimate the fiber array 30 with the grating coupler array on the photonic chip 20.

[0040] In the process of aligning the fiber array 30 with the grating coupler array, in order to protect the photonic chip 20 and facilitate clamping, this embodiment uses 3D printing to make a resin clamping base plate 10, and uses low-temperature varnish to fix the photonic chip 20 to the clamping base plate 10.

[0041] After securing the photonic chip 20, the clamping base plate 10 is pressed and fixed onto the clamping device and placed on a six-axis displacement stage. After roughly adjusting the relative positions of the fiber array 30 and the photonic chip 20, a microscope is used to locate the specific position of the fiber array 30, and the displacement stage is used to align the fiber array 30 with the pre-made collimation auxiliary marks on the photonic chip 20. Once the signal is found, the power meter reading is increased by rotating the x, y, and z axis knobs of the displacement stage. Then, the other degrees of freedom of the displacement stage and the polarization controller are used to bring the power meter reading to its maximum value, close to the simulated value, and ensure power stability. Care must be taken in the z-direction when moving the displacement stage to avoid damaging the photonic chip 20 or the fiber array 30.

[0042] S13: Pre-encapsulation is performed. After collimation, pre-encapsulation of the fiber array 30 begins. During pre-encapsulation, an optical fiber is used as a dispensing device to apply a drop of UV adhesive 50 to one side of the fiber array 30, and a UV lamp 70 is used to irradiate it from multiple angles until the output power stabilizes. If there is any loss in efficiency, it can be recovered by moving the displacement stage.

[0043] S14: Use a glass dropper to drop a drop of two-component optical adhesive 60 on the same side of the fiber array 30. The adhesive will spread around the bottom of the fiber array 30 due to capillary action and completely encapsulate it. Then, expose it to room temperature for 36 hours to allow it to cure.

[0044] S15: Transfer the photonic chip 20. To prevent the stress of the optical fiber from affecting the packaging effect, use double-sided optical adhesive 60 to fix the optical fiber output port 40 at a distance of 2 cm from the optical fiber array 30. Fix the optical fiber output port 40 on the fixing frame 80 of the moving displacement stage and release the stress.

[0045] S2: Load the sample to be tested onto the low-temperature sample stage and cool it to the target temperature; S3: Connect the polarization controller to the fiber optic output port 40, turn on the semiconductor laser source, introduce the excitation light through the fiber optic array 30 and collect the fluorescence signal; S4: The photodetector uses time-gated technology to separate the excitation light and fluorescence signal; the fluorescence spectral characteristics and time properties are analyzed.

[0046] In this embodiment, the adhesive used for pre-encapsulation is UV adhesive 50. UV adhesive 50 with suitable parameters such as viscosity, optical refractive index, and optical transmittance was selected for pre-encapsulation, taking advantage of its rapid curing. The UV adhesive 50 selected in this embodiment is NTT6001, with a viscosity of 470 pcs, a refractive index of 1.489 matching the refractive index of the optical fiber, and a transmittance greater than 91%.

[0047] During pre-encapsulation, 50 drops of UV adhesive are applied to one side of the fiber array 30, and then UV lamp 70 is used to cure the pre-encapsulation. The fiber is irradiated from multiple angles for 3 minutes until the output power of the fiber stabilizes, thus completing the pre-encapsulation at room temperature. This pre-encapsulation is very important. Without pre-encapsulation, the coupling efficiency of the encapsulation needs to be continuously monitored during the curing process of the two-component optical adhesive 60, and the displacement stage needs to be adjusted synchronously. The pre-encapsulation process makes the subsequent encapsulation process efficient and the steps simple and convenient.

[0048] The two-component optical adhesive 60 used in the encapsulation process needs to operate stably at liquid helium temperatures and possess suitable refractive index, transmittance, modulus, and hardness. The selected two-component optical adhesive 60 is the high-performance two-component optical adhesive EPO-TEK301-2. The two-component designation means it consists of two components, A and B. Before mixing, both components are colorless and transparent liquids, with a mixing ratio of 3:1. The adhesives only cure after being mixed in the specified ratio. After curing, it is suitable for low-temperature applications with a refractive index of approximately 1.5, a transmittance greater than 98%, and a modulus of 432279 psi.

[0049] After the two-component optical adhesive 60 completely encapsulates the fiber array 30, the coupling efficiency remains stable during the curing process due to the advantages of pre-encapsulation and does not require further optimization and adjustment. The two-component optical adhesive 60 is used to bond the fiber array 30 to the photonic chip 20, and the thermal expansion coefficients of the two-component optical adhesive 60 and the photonic chip 20 are matched, so that the coupling efficiency of the optical fiber remains stable during the cooling process and at low temperatures, and remains stable in multiple thermal cycles.

[0050] The measurement data shows that the efficiency decreased by only 1.7% before and after encapsulation. The loss was mainly due to minor errors caused by the amount of UV adhesive during pre-encapsulation and uneven exposure during curing. The encapsulation efficiency loss can be further reduced by operating more precisely.

[0051] When measuring at low temperatures, a slight blue shift occurs at the wavelength center as the temperature decreases, but the overall trend of the wavelength-efficiency curve remains basically unchanged. Measurement reference is as follows: Figure 6 As shown, the temperature range is 1.5K-300K. Furthermore, the peak efficiency of the package remains stable across all temperature zones, with efficiency fluctuations of approximately ±0.5% of the average peak efficiency. In addition, multiple heating and cooling tests were performed on the packaged test samples, such as... Figure 7 As shown, the curve trend did not change significantly after multiple temperature increases and decreases, and the efficiency remained stable.

[0052] Finally, low-temperature fluorescence testing was performed on the rare-earth ion-doped photonic chip packaged with a fiber array, such as... Figure 8 As shown. Pumping and fluorescence collection were performed using an optical fiber array 30, and the measurement results showed that the fluorescence lifetime was approximately 2.25 ms and the spectral width was 1.98 nm.

[0053] Based on the above experimental data, after the fiber array 30 and the fiber coupler array in this application are packaged, the fluorescence measurement at low temperature can be directly completed by the fiber array 30 for input and output, and the fluorescence of rare earth ions can be directly measured by the optical fiber connected to the photodetector.

[0054] This application reduces the probability of encapsulation failure by incorporating a pre-encapsulation process, while ensuring that encapsulation efficiency is not lost during the long curing time required for the two-component high-performance optical adhesive. Furthermore, this encapsulation technology is simple, easy to implement, and has a high success rate. The efficiency reduction before and after encapsulation is minimal, and the efficiency remains stable with temperature changes. Even after multiple temperature reduction processes, the encapsulation effect maintains the original coupling level, demonstrating good robustness. Moreover, this encapsulation technology has good scalability and integrability. The fiber array 30 used in the example provides the possibility of performing multi-channel operation on a single chip. The fiber array 30 can be used to complete the input of pump light and the output of fluorescence signals, conveniently enabling on-chip ion property measurements at low temperatures.

[0055] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of this application, and are not intended to limit them. Although this application has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features therein. Such modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of this application.

Claims

1. A device for measuring rare-earth ions doped in a photonic chip at low temperatures, characterized in that: include: Test sample: The test sample consists of an optically coupled photonic chip and an optical fiber array; The photonic chip uses a thin-film lithium niobate photonic chip doped with rare earth ions. Low-temperature sample stage: used to place the sample to be tested and keep it in a low-temperature environment; Semiconductor laser source: Used to emit laser light; Polarization controller: Connected to the fiber optic output port of the sample under test, it is used to control the polarization state of the laser source, and then the light is irradiated onto the sample under test. The laser is guided through the fiber optic array on the sample under test and the fluorescence signal is collected. Photodetector: Used to separate excitation light and fluorescence signal to achieve fluorescence detection of rare earth ions.

2. The measuring device for rare earth ions doped in a photonic chip at low temperature according to claim 1, characterized in that: The photonic chip is equipped with a grating coupler array, which is optically coupled and fixed to the fiber array by UV adhesive and two-component optical adhesive.

3. The measuring device for rare earth ions doped in a photonic chip at low temperature according to claim 2, characterized in that: The input port of each grating coupler in the grating coupler array is connected to a first straight waveguide, the output port of each grating coupler is connected to a second straight waveguide, and a curved waveguide connects the first and second straight waveguides.

4. The measuring device for rare earth ions doped in a photonic chip at low temperature according to claim 1, characterized in that: The fiber array includes at least two single-mode fibers with an end-face tilt angle of 8°.

5. The measuring device for rare earth ions doped in a photonic chip at low temperature according to claim 1, characterized in that: The photodetector is a time-gated detector.

6. The measuring device for rare earth ions doped in a photonic chip at low temperature according to claim 1, characterized in that: The semiconductor laser uses a tunable semiconductor laser.

7. The measuring device for rare earth ions doped in a photonic chip at low temperature according to claim 1, characterized in that: The two-component optical adhesive uses the two-component high-performance optical adhesive EPO-TEK301-2.

8. A method for measuring rare-earth ions doped in a photonic chip at low temperature, characterized in that: The measuring device as described in any one of claims 1-7 includes the following steps: S1: Sample preparation; S2: Load the sample to be tested onto the low-temperature sample stage and cool it to the target temperature; S3: Connect the polarization controller to the fiber optic output port, turn on the semiconductor laser source, introduce the excitation light through the fiber optic array and collect the fluorescence signal; S4: The photodetector uses time-gated technology to separate the excitation light and fluorescence signal; the fluorescence spectral characteristics and time properties are analyzed.

9. The method for measuring rare earth ions doped in a photonic chip at low temperature according to claim 8, characterized in that: Step S1 includes: S11: Prepare thin-film lithium niobate photonic chips and fiber arrays; S12: Collimate the fiber array with the grating coupler array on the photonic chip; S13: Pre-encapsulation is performed. After collimation is completed, the fiber array is pre-encapsulated by dropping UV adhesive onto one side of the fiber array and irradiating it with UV lamps from multiple angles until the output power is stable. S14: After pre-encapsulation, use a glass dropper to drop a drop of two-component optical adhesive on the same side of the fiber array, and cure at room temperature.

10. The method for measuring rare earth ions doped in a photonic chip at low temperature according to claim 9, characterized in that: It also includes S15: After the fiber array and photonic chip are coupled, the sample to be tested is transferred to a low-temperature sample stage, and the fiber output port is fixed on a bracket using two sets of optical adhesive to release stress.

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