A waveguide loss measurement method based on double-channel detection

Abstract

The application relates to a waveguide loss measurement method based on double-channel detection, which is used for accurately obtaining the linear loss coefficient of a nonlinear waveguide to be measured. Pump light, probe light and auxiliary laser are combined and injected into the waveguide to be measured to trigger a four-wave mixing effect to generate idler light; then, the waveguide output light is split into two paths through a light splitting element, and the idler light signal and the auxiliary laser signal are extracted as a measurement channel and a reference channel; the waveguide temperature is adjusted to stabilize the idler light power, and a linear interval of the probe light and the idler light power is obtained through fitting; the probe light power is fixed in the interval, a noise compensation factor is generated by using the average power and the instantaneous power of the auxiliary laser, the idler light power is corrected in real time, and finally the waveguide loss is calculated based on the corrected data. The method realizes real-time noise cancellation through double channels, combines parameter locking and linear interval screening, greatly improves the measurement stability and precision, and adapts to the loss detection requirements of various nonlinear waveguides.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of waveguide loss measurement, and in particular to a waveguide loss measurement method based on double-channel detection. BACKGROUND

[0002] With the rapid development of optical communication, integrated optics and optical quantum computing, the loss characteristics of waveguides, as the core devices for optical signal transmission and processing, have become a key indicator for measuring device performance, directly affecting optical transmission efficiency, signal fidelity and system integration. Currently, waveguide loss measurement techniques mainly include the truncation method, backscattering method, interference method and four-wave mixing (FWM) method based on nonlinear optical effects. Among them, the truncation method is simple to operate, but it requires the preparation of waveguides of different lengths from the same batch, which is destructive and difficult to adapt to miniaturized integrated devices; the backscattering method is a non-destructive measurement, but its spatial resolution is low, and the measurement accuracy of low-loss waveguides is limited; the interference method has high sensitivity, but it is easily disturbed by environmental vibration, temperature drift and other factors, and has poor stability.

[0003] In recent years, the measurement method based on four-wave mixing effect has gradually become a research hotspot for low-loss waveguide detection due to its advantages of high sensitivity, non-destructive and integrability. However, the existing four-wave mixing measurement technology still has significant deficiencies: first, environmental disturbances such as mechanical vibration, air flow change and light source power fluctuation will simultaneously affect the transmission stability of signal light and pump light, leading to random errors in the measurement of idler light power, and the existing single-channel detection scheme lacks effective real-time noise compensation mechanism, making it difficult to eliminate such disturbances; second, the four-wave mixing conversion efficiency is significantly affected by waveguide temperature, pump light wavelength and other parameters, and if the key parameters are not accurately locked, the conversion efficiency will fluctuate, thereby introducing system errors; third, the linear relationship between probe light power and idler light power is the premise to ensure the accuracy of measurement, but the existing technology does not explicitly select the linear interval, which is easy to measure in the nonlinear region, resulting in deviation of the loss coefficient calculation.

[0004] The above problems restrict the application of the four-wave mixing method in high-precision waveguide loss measurement, and there is an urgent need for a measurement scheme that can realize real-time noise compensation, parameter stability control and accurate selection of linear interval. SUMMARY

[0005] In view of the deficiencies of the prior art, the purpose of the present application is to provide a waveguide loss measurement method based on double-channel detection, which can improve the measurement accuracy and stability of the linear loss coefficient of the waveguide.

[0006] The above purpose of the present application is realized by the following technical scheme:

[0007] A waveguide loss measurement method based on double-channel detection, comprising the following steps: step S1: merging pump light, probe light and auxiliary laser light and then injecting into a nonlinear waveguide to be measured to trigger four-wave mixing effect to generate idler light; step S2: splitting the waveguide output light into two paths by a light splitting element, extracting the signal of the idler light as a measurement channel, and extracting the signal of the auxiliary laser light as a reference channel; step S3: adjusting the temperature of the waveguide, and locking the temperature of the waveguide and the wavelength of the pump light when the idler light power of the measurement channel reaches a stable state; step S4: keeping the temperature of the waveguide and the wavelength of the pump light locked in step S3, recording the idler light power under different probe light powers and fitting the relationship between them to obtain an interval of the probe light power showing a linear relationship; step S5: keeping the temperature of the waveguide and the wavelength of the pump light locked in step S3, and fixing the probe light power in the interval obtained in step S4, generating a noise compensation factor by measuring the average power and instantaneous power of the auxiliary laser light, and using the noise compensation factor to correct the measured idler light power in real time; and step S6: calculating the waveguide loss based on step S5. ; wherein, is the four-wave mixing conversion efficiency; is the pump light power; is the probe light working power; is the corrected idler light power; is the waveguide length; is the linear loss coefficient.

[0008] As a preferred embodiment of the present application, the following conditions need to be met in step S1: rad / m; wherein, is the wave vector mismatch; is the wave vector of the pump light in the waveguide; is the wave vector of the probe light in the waveguide, is the wave vector of the idler light in the waveguide, and the wave vector mismatch is adjusted by adjusting the temperature of the waveguide to meet the requirements.

[0009] As a preferred embodiment of the present application, in step S1, the polarization states of the pump light, the probe light and the auxiliary laser light are consistent, and all match the principal polarization direction of the waveguide to be measured, so as to maximize the four-wave mixing conversion efficiency.

[0010] As the preferred of the present application, the step S4 comprises: step S4.1: keeping the temperature of the waveguide locked in the step S3 and the wavelength of the pump light; step S4.2: setting the detection light adjustment range, adjusting the power of the detection light by 1mW~2mW step by step from the lower limit of the adjustment range, and staying for 3s~5s after each power point adjustment; step S4.2: collecting 100 groups of idler light power data at the power point with a sampling frequency of 10Hz, taking the arithmetic mean value as the effective value of the idler light power corresponding to the detection light power after removing 3 maximum values and 3 minimum values; step S4.3: importing all the detection light power and idler light power effective value data into data processing software, and performing linear regression fitting by using the least square method to calculate the goodness of fit R²; screening out the continuous detection light power sub-interval with R²≥0.998, which is the detection light power interval in a linear relationship in the step S4.

[0011] As the preferred of the present application, the generation of the noise compensation factor in the step S5 also comprises outlier rejection, when the deviation absolute value of the instantaneous power of a certain auxiliary laser from the average power is ≥10%, it is determined as abnormal data, and the noise compensation factor is calculated by using the sliding average value of the previous 3 normal data to replace the instantaneous power of the auxiliary laser.

[0012] As the preferred of the present application, the noise compensation factor is calculated in the following manner: ; wherein, is the noise compensation factor; is the average power of the auxiliary laser; is the instantaneous power of the auxiliary laser; and the corrected idler light power is calculated in the following manner: ; wherein, is the instantaneous power of the idler light; is the system error of the measurement channel.

[0013] As the preferred of the present application, the four-wave mixing conversion efficiency in the step S6 needs to be corrected according to the actual working temperature of the waveguide; ; wherein, is the conversion efficiency at the reference temperature; is the reference temperature; is the temperature coefficient; is the actual working temperature of the waveguide.

[0014] As the preferred of the present application, when the four-wave mixing conversion efficiency is unknown in the step S6, the iterative method is used to calculate the linear loss coefficient.

[0015] As the preferred of the present application, the iterative formula is: ; ; wherein, is the four-wave mixing conversion efficiency of the n th iteration; is the linear loss coefficient of the n th iteration; is the linear loss coefficient of the n+1 th iteration; the initial value of the linear loss coefficient is set according to the waveguide material characteristics; the iteration termination condition is that dB / cm.

[0016] As a preferred embodiment of the present application, the linear loss coefficient a calculated in step S6 needs to be verified by multiple measurements. The measurement is repeated 3-5 times under the same experimental conditions. If the relative standard deviation of multiple measurement results is ≤2%, the average value is taken as the final result; otherwise, steps S3-S6 are re-executed to exclude system errors.

[0017] In summary, the beneficial technical effects of the present application are:

[0018] 1. The present application introduces auxiliary laser in step S1, and uses a light splitting element to split the waveguide output light into an idler light measurement channel and an auxiliary laser reference channel in step S2. In step S5, the average power and instantaneous power of the auxiliary laser are measured to generate a noise compensation factor, which is used to correct the idler light power in real time. This design solves the problem that the existing single-channel detection scheme cannot eliminate environmental disturbances. The design takes advantage of the fact that the auxiliary laser and the idler light share the same transmission path and are subject to the same environmental interference. By using the compensation factor to offset the noise in real time, the random error of the idler light power measurement is effectively reduced, and the reliability of the original data is improved.

[0019] 2. In step S3, the waveguide temperature is adjusted to be stable after the idler light power is stable, and the waveguide temperature and the pump light wavelength are locked. This design solves the problem that the four-wave mixing conversion efficiency fluctuates due to inaccurate control of temperature and pump wavelength in the prior art. The locking of temperature and pump wavelength ensures the stability of the waveguide phase matching state, avoids the system deviation introduced by the change of conversion efficiency, provides a stable four-wave mixing conversion efficiency basis for subsequent calculation of loss coefficient based on the four-wave mixing formula, and reduces the system level measurement error.

[0020] 3. In step S4, the idler light power under different probe light powers is recorded, and the relationship is fitted to determine the linear interval. In step S5, the probe light power is fixed in the linear interval. This design solves the problem that the prior art is prone to measurement in the nonlinear region, resulting in distortion of the relationship between the idler light power and the probe light power. The selection of the linear interval and the fixing of the power ensure that the premise of the linear relationship between the idler light power and the probe light power in the four-wave mixing effect is met, avoiding the interference of nonlinear region data on the loss calculation, and significantly improving the accuracy of the formula-based back calculation of the linear loss coefficient. BRIEF DESCRIPTION OF DRAWINGS

[0021] Figure 1 Flow chart of the waveguide loss measurement method based on dual-channel detection.

[0022] Figure 2 Flow chart of the method for obtaining the interval of the probe light power in linear relationship. DETAILED DESCRIPTION

[0023] The application will be further described in detail below with reference to the accompanying drawings.

[0024] As shown in Figure 1 , a waveguide loss measurement method based on dual-channel detection is shown. First, the merging and incidence of multiple laser beams need to be completed to trigger the target nonlinear effect. The pump light, probe light and auxiliary laser involved here are preferably selected from the 1550 nm band, the 1560 nm band and the 1310 nm band, respectively. To achieve effective merging of the three beams, a fiber coupler or a spatial light beam combiner in the prior art can be used. By adjusting the light path alignment accuracy of the beam combining element, the collinear incidence of the three beams to the input end of the waveguide under test is ensured.

[0025] The waveguide under test needs to have nonlinear optical characteristics. The waveguide is selected from the commonly used nonlinear waveguides in the prior art, including lithium niobate waveguides, chalcogenide glass waveguides and quantum well waveguides based on semiconductor materials. When the three beams are injected into the nonlinear waveguide under test at a set power, the four-wave mixing process will be triggered due to the third-order nonlinear polarization effect in the waveguide, and then generate idler light of a new frequency. The pump light power is higher than that of the probe light and the auxiliary laser to ensure the degree of the four-wave mixing effect. The frequency of the idler light is determined by the frequencies of the pump light and the probe light, and satisfies the energy conservation relationship, i.e. f4=2f1-f2, where f4 is the idler light frequency, f1 is the pump light frequency, and f2 is the probe light frequency.

[0026] Further, the mixed light output from the waveguide is split and the signal is extracted to construct a double-channel detection structure. The splitting element used for splitting is preferably a half-transmission half-reflection mirror or a fiber splitter in the prior art, and the splitting ratio can be set according to the subsequent detection requirements. The two paths of light after splitting respectively undertake the functions of measurement and reference. For the path as the measurement channel, the idler light signal needs to be extracted, which can be achieved by setting a band-pass filter matching the idler light wavelength in the optical path. The center wavelength of the band-pass filter needs to be accurately aligned with the idler light wavelength. If the idler light wavelength is 1540 nm, the filter center wavelength is set to 1540 nm, and the bandwidth is controlled within 1 nm-2 nm to filter out the stray light interference of the pump light, probe light and auxiliary laser, so that the measurement channel finally outputs only the idler light signal for subsequent power detection. For the other path as the reference channel, the auxiliary laser signal needs to be extracted, which is also achieved by setting a band-pass filter matching the auxiliary laser wavelength. The purpose is to use the auxiliary laser as a reference benchmark for subsequent compensation of environmental noise in the measurement process, such as waveguide vibration, power fluctuation, and detector dark current change. Since the auxiliary laser and the idler light are transmitted in the same waveguide and split by the same splitting element, they are consistent in the influence of environmental noise, so the power fluctuation of the auxiliary laser can reflect the noise level.

[0027] Further, the waveguide temperature and the pump light wavelength are locked to ensure the stability of the measurement conditions. The adjustment of the waveguide temperature can be realized by a semiconductor thermoelectric cooler (TEC) temperature control station in the prior art. The waveguide to be measured is fixed on the stage of the temperature control station, and the temperature of the waveguide is monitored in real time by the feedback control module of the temperature control station. The temperature detection accuracy can reach ±0.01℃, and the refrigeration / heating power is adjusted to change the temperature of the waveguide. During the temperature adjustment process, the change of the idler light power needs to be monitored in real time by the measurement channel. Preferably, the optical power can be converted into an electrical signal by an optoelectronic detector, and then recorded by a data acquisition card. When the fluctuation amplitude of the idler light power is ≤±0.1% within a continuous monitoring time of, for example, 30 seconds, it is determined that the idler light power reaches a stable state, and the temperature control station needs to be controlled to maintain the current temperature to realize the locking of the waveguide temperature. At the same time, the pump light wavelength needs to be locked, because the slight change of the pump light wavelength will directly affect the phase matching condition of the four-wave mixing, and then cause the fluctuation of the idler light power. The wavelength locking can be realized by a wavelength locking module based on a Fabry-Perot interferometer or a wavelength feedback control device based on a fiber grating. The pump light wavelength is monitored in real time, and the output wavelength of the pump laser is adjusted, for example, by controlling the temperature or injection current of the laser, so that the pump light wavelength is stabilized at the wavelength value when the idler light power is maximum, to realize the locking of the pump light wavelength.

[0028] After the temperature and wavelength are locked, the linear response interval of the probe light power needs to be determined to lay the foundation for subsequent accurate measurement. In this process, the locked waveguide temperature and pump light wavelength remain unchanged, and the output power of the probe light is adjusted. This can be achieved by using a variable optical attenuator (VOA) or directly controlling the injection current of the probe laser. The adjustment step can be set to 1 mW~2 mW. The specific step size needs to be determined according to the probe light power range and measurement accuracy requirements. Within the set probe light power range, for example, 0 mW~20 mW, different power points are selected in turn. For each power point, after adjustment, it needs to stay for 3s~5s, and after the probe light power is stable, the idle frequency light power data is collected through the measurement channel at a sampling frequency of 10Hz. Preferably, 100 sets of data are collected to reduce the influence of random noise. Then, the 100 sets of collected data are preprocessed to remove 3 maximum values and 3 minimum values to eliminate burst noise interference. Then, the arithmetic mean of the remaining 94 sets of data is calculated as the effective value of the idle frequency light power at this probe light power. All the probe light power values and the corresponding idle frequency light power effective values are arranged as data pairs and imported into data processing software such as Origin, Matlab, etc. Linear regression fitting is performed using the least squares method to calculate the goodness of fit R². The closer R² is to 1, the stronger the linear relationship. Select the continuous probe light power sub-interval with R²≥0.998. This interval is the linear response interval of the probe light power. Within this interval, the idle frequency light power and the probe light power have a strict linear relationship, which meets the theoretical assumptions of the subsequent waveguide loss calculation formula, and can avoid calculation errors caused by probe light power exceeding the linear interval.

[0029] Further, based on the reference channel, real-time noise compensation and correction of the idle frequency light power are realized. In this process, the locked waveguide temperature and pump light wavelength remain unchanged, and the probe light power is fixed within the above linear response interval, for example, the power value at the midpoint of the linear interval is selected to balance the measurement sensitivity and stability. The average power and instantaneous power of the auxiliary laser are determined through the reference channel. The power data of the auxiliary laser is collected through the photodetector and data acquisition card of the reference channel at a sampling frequency of 10Hz~100Hz. The arithmetic mean of all instantaneous power data in this time period is the average power of the auxiliary laser ; at the same time, the instantaneous power of the auxiliary laser is collected in real time , and the sampling frequency is consistent with the idle frequency light power collection frequency to ensure data synchronization. During the instantaneous power collection process, outlier rejection needs to be performed. When the absolute value of the deviation between a certain instantaneous power and the average power is ≥10%, the data is considered as an outlier and is removed , the data is determined as an abnormal value, at this time, the sliding average of the first three normal instantaneous power data is used to replace the abnormal value to ensure the reliability of the reference signal. Based on the processed auxiliary laser average power and instantaneous power, a noise compensation factor is generated, and the calculation formula is The physical meaning of the factor is that the noise level of the current measurement environment is inverted through the instantaneous power fluctuation of the auxiliary laser, and then the idler light power is compensated. At the same time, the instantaneous power of the idler light is collected in real time through the measurement channel , and the noise compensation factor is used for real-time correction. The calculation formula of the corrected idler light power is , wherein is the system error of the measurement channel, which is mainly derived from the dark current of the photodetector in the measurement channel, the insertion loss of the band-pass filter, the quantization error of the data acquisition card, etc. The specific value of is obtained by pre-calibration, for example, measuring the power value corresponding to the dark current of the detector under the condition of no light, and measuring the power loss of the filtering and acquisition link under the illumination of the standard light source with known power.

[0030] Finally, the linear loss coefficient of the waveguide is calculated based on the corrected idler light power. The core calculation formula used here is , wherein the meanings and acquisition methods of the parameters are as follows: is the four-wave mixing conversion efficiency, which is related to the nonlinear coefficient of the waveguide, the wavelength of the pump light, the temperature of the waveguide and the mode matching degree. It can be calibrated by pre-experiment, for example, measuring , , under different conditions in the standard waveguide with known loss, and substituting the formula to invert ; is the actual incident power of the pump light, which can be directly measured by connecting a power meter in the pump light path; is the fixed detection light working power, which is also measured by a power meter; is the corrected idler light power obtained in the above steps; is the physical length of the waveguide to be measured, which can be directly measured by using a vernier caliper or optical coherence tomography (OCT) technology; is the waveguide linear loss coefficient to be solved, and the unit is usually dB / cm or cm⁻¹.

[0031] Mathematical transformation is performed on the above formula to derive the calculation formula of : The actual measurement values of the parameters are substituted into the formula to calculate the linear loss coefficient of the waveguide to be measured, thereby completing the measurement of the waveguide loss.

[0032] Furthermore, to ensure that the four-wave mixing effect triggered by the pump and probe beams within the waveguide under test has sufficient efficiency to generate stable and accurately detectable idler light, a specific wave vector mismatch condition must be strictly satisfied when the three beams are combined and injected into the nonlinear waveguide under test. Specifically, this condition is expressed as follows: The calculation is in rad / m, and the implementation methods of each physical quantity and condition need further explanation. The physical meaning and essence of each parameter in the formula should be clarified. The wave vector mismatch, at its core, describes the degree of phase matching between the three participating beams—pump beam, probe beam, and generated idler beam—as they propagate within the waveguide during four-wave mixing. Phase matching is a crucial prerequisite for the efficient occurrence of nonlinear optical effects like four-wave mixing, and the wave vector mismatch is the key indicator for quantifying this degree of matching. The formula... , , These correspond to the wave vectors of the pump light, probe light, and idler light in the waveguide under test, respectively. The wave vector, as a physical quantity characterizing the propagation properties of light in a medium, is directly related to the wavelength of the light and the refractive index of the waveguide material, specifically satisfying… ,in, For wave vectors; Let be the refractive index of the waveguide material under test for the corresponding wavelength of light. Let be the wavelength of the light in a vacuum. This means that when the same beam of light propagates in media with different refractive indices, the wave vector will change accordingly. Since the refractive index of the same medium differs for different wavelengths of light, the wave vectors of pump light, probe light, and idler light will vary due to their different wavelengths within the same waveguide under test. , , Different values ​​will be displayed.

[0033] Limit the wave vector mismatch to rad / m. From a physical perspective, during four-wave mixing, the energy and momentum of photons must be conserved simultaneously, and the wave vector mismatch... Essentially, it reflects the degree of deviation from the conservation of momentum, that is The smaller the value, the closer the momentum conservation is to the ideal state, the more efficiently the photons participating in the mixing can convert energy, the higher the efficiency of idler light generation, and the stronger the final output idler light power; if If the magnitude is too large, the momentum conservation deviation will cause most of the photon energy to fail to be converted into idler light, instead being dissipated as heat. This will not only cause a sharp drop in idler light power, but may even result in a signal that is too weak to be accurately captured by subsequent measurement channels, directly affecting the accuracy and reliability of waveguide loss measurement. Therefore, Controlled to ≤10⁻ 4Within the range of rad / m, this is a necessary condition to ensure the efficient occurrence of the four-wave mixing effect and to provide a stable signal source for subsequent measurements.

[0034] The refractive index of waveguide materials is temperature-dependent. Almost all materials used to fabricate nonlinear waveguides, such as lithium niobate and chalcogenide glasses, exhibit a slight but tunable change in refractive index with temperature. For example, the temperature coefficient of refractive index of lithium niobate is approximately 1 × 10⁻⁻⁻⁴. 5 / ℃, meaning that for every 1℃ change in temperature, the refractive index will increase by approximately 1×10⁻⁻. 5 The change in the wave vector. The relationship shows that when the waveguide temperature changes, the refractive index of the material... The changes will directly lead to , , The value changes accordingly, thereby altering... The result.

[0035] The specific adjustment process can be achieved using existing high-precision temperature control equipment, such as a thermoelectric cooler (TEC) temperature control station. The waveguide under test is tightly fixed on the stage of the temperature control station, and the actual temperature of the waveguide is monitored in real time through the temperature feedback module of the temperature control station. The monitoring accuracy can reach ±0.01℃. At the same time, the power signal of the idler light is collected in real time through the measurement channel. The idler light power is related to... Negative correlation The smaller the value, the larger the idler power; therefore, the idler power can be used as... Indirect monitoring indicators for whether standards are met. During adjustment, the set temperature of the temperature control station is slowly changed in small increments, such as 0.1℃ / min. After each temperature adjustment, it is held for 3 to 5 seconds to allow the waveguide temperature to stabilize. Then, the trend of the idler optical power is observed. As the temperature is adjusted, the idler optical power will gradually increase and reach a maximum value. At this point, the corresponding... Typically at its minimum; when the idler power stabilizes near its maximum value, and through theoretical calculations, combined with the refractive index data of the waveguide material at that temperature and the wavelengths of each beam, it is substituted into... calculate , , Then obtain ,confirm ≤10⁻ 4 When the temperature reaches rad / m, temperature adjustment is stopped, at which point the wave vector mismatch meets the requirements.

[0036] To further improve the conversion efficiency of the four-wave mixing effect and ensure that the generated idler light has sufficient intensity to support subsequent accurate measurement, the polarization states of the three laser beams and their adaptation relationship with the waveguide need to be clearly specified. Specifically, the polarization states of the pump light, probe light, and auxiliary laser need to be consistent, and the polarization states of these three beams need to be accurately matched with the principal polarization direction of the waveguide to be measured. Light is a transverse wave, and the direction of its electric field vibration is perpendicular to the direction of propagation. The polarization state in the above is a physical quantity used to describe the distribution characteristics of the electric field vibration direction. Common polarization states include linear polarization, circular polarization, and elliptical polarization. In nonlinear optical effects, linearly polarized light is the most commonly used choice because its vibration direction is single and easy to control, and it can be more stably adapted to the transmission characteristics of the waveguide. The requirement for consistent polarization states of the three beams is essentially to ensure that the electric fields of the pump light and probe light vibrate in the same direction within the waveguide. The essence of the four-wave mixing effect is a third-order nonlinear polarization process, and the interaction strength of the optical field in this process is directly related to the consistency of the electric field vibration direction. If there is a difference in the polarization states of the three beams, the electric fields of the two beams cannot form effective superposition within the waveguide, and the third-order nonlinear polarization intensity will be greatly weakened, resulting in a significant reduction in the generation efficiency of the idler light. In some cases, the signal may be too weak to be captured by the subsequent measurement channel. Conversely, when the polarization states of the three beams are completely consistent, the electric fields can be superimposed in the same direction to form a stronger optical field interaction, providing a prerequisite for efficient four-wave mixing.

[0037] The waveguide to be measured will exhibit polarization dependence when transmitting optical signals due to its structural characteristics. That is, when light with different polarization directions is transmitted within the waveguide, it will face different transmission losses, refractive indices, and mode stabilities. Among them, there is one or two specific polarization directions in which the light transmission loss is minimal, the refractive index is most stable, and polarization mode dispersion, which is the signal broadening caused by the difference in transmission speed of different polarization modes, is unlikely to occur. This specific direction is referred to as the principal polarization direction of the waveguide. For example, the principal polarization directions of a single-mode optical fiber are typically horizontal polarization and vertical polarization, corresponding to the two orthogonal polarization modes of the LP01 mode of the optical fiber. However, a lithium niobate planar waveguide may have a principal polarization direction related to the optical axis direction of the crystal due to the optical anisotropy of the crystal.

[0038] It is necessary to maintain the polarization state of the three beams matching the main polarization direction of the waveguide to avoid polarization loss and polarization distortion during light transmission in the waveguide. If the polarization state of the laser does not match the main polarization direction of the waveguide, for example, if the laser is 45° polarized while the main polarization direction of the waveguide is horizontal, the optical power of the laser after injection into the waveguide will be decomposed into components along the main polarization direction and components along non-main polarization directions. The components along non-main polarization directions will attenuate rapidly during transmission due to high loss. At the same time, the polarization mode coupling effect of the waveguide may also cause random distortion of the polarization state of the light during transmission. This will not only cause power loss of the three beams, especially the loss of pump light power, which will directly weaken the driving capability of four-wave mixing, but also cause the polarization state of the light field reaching the waveguide output to deviate from the initial setting, destroying the consistency of the polarization state of the three beams, and further reducing the conversion efficiency of four-wave mixing. When the polarization states of the three beams are precisely matched with the main polarization direction of the waveguide, the light propagates only along the main polarization direction when it is transmitted in the waveguide. This can minimize power loss and ensure that the polarization state remains stable during transmission. This ensures that the three beams maintain a consistent polarization state throughout the entire waveguide, providing a stable transmission environment for the efficient generation of idler light.

[0039] In practice, the above-mentioned polarization state control and matching can be achieved using existing mature equipment. For the polarization state consistency adjustment of the three laser beams, a polarization controller can be connected in series in the optical path of each laser beam. By adjusting the parameters of the polarization controllers, such as the position of the squeezed fiber and the rotation angle of the waveplate, the polarization state of the three beams is monitored in real time using a polarization analyzer until the analyzer shows that the polarization state parameters of the three beams are consistent, such as the vibration direction angle and degree of polarization of linear polarization. For matching with the main polarization direction of the waveguide, the main polarization direction of the waveguide to be tested needs to be determined first by polarization scanning method. Specifically, a laser beam with a known polarization state, such as a linearly polarized laser, is injected into the waveguide. By rotating the polarization direction of the laser, the laser power at the output end of the waveguide is monitored with a power meter. When the power meter shows the maximum power and the fluctuation is minimal, the polarization direction of the laser at this time is the main polarization direction of the waveguide. After determining the main polarization direction, the polarization states of the three beams are adjusted so that the polarization states of the three beams completely coincide with the main polarization direction, which satisfies the polarization state requirements and ultimately maximizes the four-wave mixing conversion efficiency.

[0040] like Figure 2 As shown, this illustrates the method for determining the range in which the probe optical power and the idler optical power have a linear relationship. This range is a key prerequisite for subsequently fixing the probe optical power and accurately calculating the waveguide loss. The specific operation process must strictly follow the logic below.

[0041] First, the stability of the measurement conditions must be maintained, that is, the locked waveguide temperature and the pump light wavelength remain unchanged. The idle light power is stabilized by adjusting the temperature. The essence is to keep the waveguide in a high-efficiency and stable four-wave mixing state. The locking of the pump light wavelength ensures the constancy of the phase matching condition. If the waveguide temperature or the pump light wavelength deviates at this time, it will directly change the conversion efficiency of four-wave mixing, causing the relationship between the idle light power and the probe light power to deviate from the true law, and the subsequent fitted linear interval will lose reference significance. In actual operation, the semiconductor cooling plate (TEC) temperature control console needs to be used to continuously monitor the waveguide temperature. Once the temperature deviates from the locked value, the temperature control console needs to immediately start feedback adjustment. At the same time, the wavelength locking module of the pump light also needs to monitor the wavelength change in real time. By adjusting the laser injection current or temperature, the pump light wavelength is always stabilized at the locked value, providing a constant experimental background for subsequent probe light power adjustment and data acquisition.

[0042] Subsequently, the adjustment of the probe light power and the collection of the idle light power data are entered. The first step is to set a reasonable probe light power adjustment range. This range needs to be determined in combination with the nonlinear response capability of the waveguide to be tested and the upper limit of the output power of the probe light laser. If the adjustment range is too narrow, it may not cover the complete linear interval. If it is too wide, such as exceeding the maximum output power of the laser or the nonlinear saturation threshold of the waveguide, the idle light power will no longer change linearly with the probe light power. After setting the adjustment range, start adjusting the probe light power from the lower limit of the range with a step size of 1 mW~2 mW. The reason for choosing this step size is that a small step size will result in too many data points and prolong the experiment time, while a large step size may miss the key changes in the linear interval. After adjusting to a new probe light power point, stop for 3s~5s before collecting data. This is because after adjusting the probe light power, whether it is through a tunable optical attenuator to change the light intensity or through the control of the laser injection current to change the output, the light power needs a short time to stabilize. If data is collected immediately, the idle light power data will deviate due to power fluctuations. A 3s~5s stop time can ensure that the probe light power is stable at the set value, thereby ensuring the authenticity of the idle light power data.

[0043] The data acquisition link needs to use high-frequency sampling combined with abnormal value elimination and finally mean value calculation to process the idle frequency optical power signal to reduce the influence of random noise. Specifically, the idle frequency optical power data at the current probe optical power point is collected at a sampling frequency of 10 Hz, a total of 100 groups. The 10 Hz sampling frequency can avoid data redundancy caused by too fast sampling, and can obtain enough data points in a short time to reflect the statistical characteristics of the signal; the size of 100 groups of data provides a basis for subsequent abnormal value elimination, which can effectively reduce the influence of single burst interference such as instantaneous electromagnetic noise and probe dark current fluctuation on the result. After the collection is completed, the three maximum values and the three minimum values in the 100 groups of data need to be removed, because such extreme values are usually caused by non-systematic burst interference, such as temporary vibration in the laboratory environment and instantaneous voltage fluctuation of the power supply. If they are retained, they will raise or lower the overall data level, resulting in distorted results. After removing the extreme values, the arithmetic mean of the remaining 94 groups of data is calculated. The average value is the effective value of the idle frequency optical power corresponding to the current probe optical power. Compared with a single data point, the average value can more accurately reflect the true level of the idle frequency optical power, further weakening the interference of random noise.

[0044] Finally, the fitting of linear relationship and the screening of linear interval. All the probe optical power points and their corresponding idle frequency optical power effective values are arranged as data pairs, imported into commonly used data processing software such as Origin and Matlab, and linear regression fitting is performed using the least squares method. The core advantage of the least squares method is that it can minimize the sum of squares of errors between data points and the fitted straight line to obtain the fitting result closest to the true linear relationship, which is the standard method for handling linear fitting problems in scientific experiments. After fitting, the goodness of fit R² needs to be calculated. This parameter is used to quantify the degree of fit between data points and the fitted straight line: the value of R² ranges from 0 to 1, and the closer R² is to 1, the stronger the linear relationship between the idle frequency optical power and the probe optical power. If R² is much less than 1, it means that there may be a nonlinear relationship between the two or there is a large disturbance in the data. To ensure that the relationship between the idle frequency optical power and the probe optical power in subsequent measurements strictly meets the linear assumption, a continuous probe optical power sub-interval with R²≥0.998 needs to be selected. R²≥0.998 means that the data points and the fitted straight line have a very high degree of fit, and the linear relationship can be considered to be established. At the same time, it is required to be continuous because during the adjustment of the probe optical power, the linear relationship may only be established in a certain middle section. For example, the low power section or the high power section may have nonlinearity due to waveguide nonlinear saturation. Only a continuous sub-interval can ensure that when a fixed probe optical power is selected, it can meet the requirements of the linear relationship. The final selected continuous sub-interval is the required probe optical power interval that shows a linear relationship.

[0045] Further, the setting of the auxiliary laser facilitates the generation of the noise compensation factor, which is the core step of achieving accurate correction of the idle frequency light power, and the abnormal value elimination is the key prerequisite for ensuring the reliability of the noise compensation factor. If the instantaneous power of the auxiliary laser abnormally fluctuates due to sudden interference, the unprocessed abnormal data will directly cause the noise compensation factor to deviate from the true noise level, and then cause the idle frequency light power correction result to deviate. Therefore, in the process of generating the noise compensation factor, an abnormal value elimination step needs to be additionally added, and the specific determination criteria and processing method are as follows. First, the determination basis of the abnormal value is determined, which is based on the average power of the auxiliary laser and the fluctuation amplitude of the instantaneous power. In the previous step, a plurality of power data of the auxiliary laser is collected through the reference channel and the average power is calculated , which reflects the reference power level of the auxiliary laser in a stable environment and can be regarded as the true power without sudden interference. The subsequently collected instantaneous power of the auxiliary laser needs to be compared with the average power . When the absolute value of the deviation of a certain instantaneous power from the average power satisfies , it is determined that the instantaneous power data is an abnormal value. The selection of 10% as the deviation threshold is based on the environmental interference characteristics commonly seen in waveguide loss measurement experiments. In a normal experimental environment, such as a constant temperature, shockproof, and electromagnetic shielded optical laboratory, the instantaneous power fluctuation of the auxiliary laser is usually controlled within ±5%. The threshold of 10% can effectively cover the acceptable small fluctuations caused by short-term air flow disturbance, slight electromagnetic radiation, and accurately identify the unacceptable abnormal fluctuations caused by sudden strong electromagnetic interference, such as the start and stop of high-power equipment near the laboratory, instantaneous failure of the detector, such as sudden change of the dark current of the photodetector, etc. This avoids misjudging normal fluctuations as abnormal, and also does not miss the real abnormal data.

[0046] For the data determined to be abnormal, the sliding average of the previous 3 normal data is used to replace the abnormal instantaneous power , which is then used to calculate the noise compensation factor. The core reason for choosing this processing method is to ensure the time continuity and authenticity of the data. The sliding average value can reflect the change trend of the auxiliary laser power based on the recent continuous normal data, avoiding data breakage caused by directly discarding abnormal values or replacing them with fixed values. If the abnormal values are directly discarded, there will be no corresponding compensation factor at that moment, and real-time correction of the idle frequency light power cannot be achieved. If the average power Direct replacement, ignoring the auxiliary laser power in the normal range of slow drift, such as the slight attenuation of the laser output power in the long-term experiment, resulting in the compensation factor cannot track the real-time noise level. On the one hand, the volume of 3 data can smooth the small error caused by normal fluctuations, and can maximize the power state before the abnormal data occurs, avoiding the lag caused by introducing too much historical data; On the other hand, the characteristics of the moving average ensure that each abnormal value processing is based on the latest normal data, which can dynamically adapt to the slow change of the auxiliary laser power, so that the replaced power value is closer to the real instantaneous power without abnormal interference.

[0047] Specifically, the moving average in the above is also called moving average, which is a data processing term. The core is to calculate the average value of continuous, fixed number of recent data points to smooth the short-term fluctuations in the original data, and more clearly reflect the overall trend or true level of the data. With the generation of new data, the data window for calculating the average value will move forward, always based on the latest fixed number of data points for calculation, rather than using all the data in the entire data set.

[0048] In the waveguide loss measurement method, the application scenario of the moving average, that is, the moving average of the previous 3 normal data is used to replace the abnormal auxiliary laser instantaneous power, can be specifically understood as follows: when the instantaneous power of the auxiliary laser at a certain time is determined to be abnormal, the continuous 3 normal instantaneous power data before the abnormal value appears are selected, for example, if the 10th data is abnormal, the 7th, 8th and 9th normal data are taken, and the arithmetic mean of the 3 data is calculated. This average is the moving average.

[0049] The moving average is used to maintain data continuity, avoid data breakage caused by discarding abnormal values, ensure that there is a corresponding power value at each time for calculating the noise compensation factor, meet the real-time correction requirements, and close to the real trend. The previous 3 normal data can reflect the power change trend before the abnormality occurs, which is more reflective of the real-time state than using a fixed average power to replace it, which not only eliminates the interference of sudden abnormalities, but also continues the reasonable trend of the data, providing a reliable basis for accurate calculation of the noise compensation factor.

[0050] The specific operation process needs to be seamlessly connected with the process of instantaneous power acquisition and compensation factor calculation. After the auxiliary laser instantaneous power is collected through the reference channel at a certain frequency , first calculate the deviation percentage of the current and the average power ; If the deviation percentage < 10%, it is determined that the data is normal, and it is directly retained and used to calculate the noise compensation factor at the current time ; if the deviation percentage is greater than or equal to 10%, the data is marked as abnormal, and then the previous three normal instantaneous power data collected continuously are called, the sliding average of the three data is calculated, and the average is taken as the instantaneous power record; finally, the formula is substituted In the formula, the calculation of the current time noise compensation factor is completed.

[0051] Through the above steps of abnormal value elimination and data replacement, the sudden interference in the auxiliary laser instantaneous power can be effectively filtered, it is ensured that each noise compensation factor can accurately reflect the real noise level of the current experimental environment, and then the subsequent idle frequency light power correction based on the factor is more accurate, and a reliable light power data basis is provided for the accurate calculation of the waveguide loss coefficient.

[0052] Noise compensation factor The calculation formula is , which involves two key parameters: one is the average power of the auxiliary laser , and the other is the instantaneous power of the auxiliary laser . The reference power value of the auxiliary laser in a stable environment is obtained by the photoelectric detector of the reference channel and the data acquisition card, and the power data of the auxiliary laser is continuously collected at a set frequency, and the collection time usually needs to cover more than 1 minute to ensure that the data quantity is sufficient to reflect the stable state, and then the arithmetic average of all collected effective data is calculated. The average is . The essence of is the real power level of the auxiliary laser without sudden noise interference, which can be used as a reference to judge whether the subsequent instantaneous power is affected by noise.

[0053] And is the real-time collected auxiliary laser power data, and the collection frequency needs to be completely consistent with the collection frequency of the measurement channel idle frequency light instantaneous power, so that the idle frequency light power at each moment can correspond to the auxiliary laser power at the same moment, and synchronous compensation is realized. will fluctuate due to environmental noise, such as slight vibration of the waveguide, fluctuation of the laboratory power supply voltage, instantaneous change of the detector dark current, etc. When the noise causes the auxiliary laser power to decrease, will be less than ; when the noise causes the auxiliary laser power to rise, will be greater than .

[0054] The physical meaning of the noise compensation factor is to quantify the influence degree of the current noise on the light power. When decreases due to noise, the calculation result will be greater than 1, which means that the idle frequency light power of the measurement channel is also likely to be underestimated due to the same noise, and the idle frequency light power of the measurement channel is also likely to be underestimated due to the same noise. Forward compensation is performed; when the noise increases, it will be less than 1, and the idle frequency optical power may be overestimated, which needs to be corrected by backward correction; if and are basically the same, then is close to 1, indicating that no additional compensation is needed. This auxiliary laser-based measurement method essentially uses the consistency of the auxiliary laser and the idle frequency light in the same waveguide transmission and the same beam splitting element to make the auxiliary laser a noise probe, thereby indirectly correcting the noise of the idle frequency light power.

[0055] Based on the above noise compensation factor , the idle frequency light power can be corrected, and the corrected idle frequency light power is calculated by the formula , wherein two key parameters, the instantaneous power of the idle frequency light and the system error of the measurement channel, are involved.

[0056] is the original power data of the idle frequency light collected by the measurement channel in real time, and its collection process is synchronized with . After the idle frequency light signal is converted into an electrical signal by the bandpass filter and photodetector of the measurement channel, the instantaneous power value is recorded by the data acquisition card. However, it will be disturbed by two aspects: one is the environmental noise described above, which is compensated by , and the other is the inherent system error of the measurement channel, which needs to be corrected by , so it needs to be multiplied by to offset the environmental noise and then introduce to correct the system error.

[0057] Here, is the system error of the measurement channel, which is different from the randomness of the environmental noise, is a stable and pre-calibrated inherent error determined by the hardware characteristics of the measurement channel, and its main sources include three types: one is the dark current error of the photodetector, which will produce a weak current due to its own semiconductor characteristics even without light, and this current will be misjudged as the power signal of the idle frequency light, forming a fixed deviation; the second is the insertion loss error of the bandpass filter, which will produce a certain power attenuation of the idle frequency light while filtering the stray light, and the attenuation is fixed; the third is the quantization error of the data acquisition card, which will produce a small error due to the limitation of the minimum quantization unit when the analog electrical signal is converted into a digital signal, and the error shows a stable distribution in multiple measurements.

[0058] ​ The specific values ​​need to be obtained through calibration experiments before the experiment. For example, for dark current error, the power value corresponding to the detector output current can be recorded when there is no idler light incident; this is the error component caused by dark current. For filter insertion loss, a standard idler light source with known power can be used to measure the power of the light source directly incident on the detector and the power incident on the detector after passing through the filter; the difference between the two is the insertion loss error component. After superimposing the various error components, the result can be obtained. The specific value needs to be determined. The symbol.

[0059] If the error causes the measured value to be too small, It should be a positive value; if it causes the measured value to be too large, It is a negative value. Therefore, in the formula... It needs to be determined based on the calibration results to ultimately ensure It can accurately reflect the true power level of idler light, which not only cancels out the random interference of environmental noise, but also corrects the inherent deviation of the hardware system, providing reliable idler light power data for the accurate calculation of waveguide loss coefficient.

[0060] Furthermore, in calculating the linear loss coefficient of waveguides... At that time, the four-wave mixing conversion efficiency It is one of the core parameters, and its value is not constant but fluctuates significantly with the actual operating temperature of the waveguide. If a fixed value is directly used... Values, such as initial measurements at room temperature, when substituted into the loss calculation formula, will be affected by... Deviations from actual operating conditions lead to significant errors in the final loss result. Therefore, it is necessary to consider the already determined actual operating temperature of the waveguide when calculating the loss. Targeted corrections are made, and the specific correction method is achieved through the following mathematical relationship. The physical meaning, acquisition method, and correction principle of each parameter can be explained in detail below.

[0061] This represents the four-wave mixing conversion efficiency at the reference temperature and is the baseline value for the correction calculation; For the corresponding reference temperature, a common stable temperature in the experimental environment is usually selected, such as standard room temperature of 25℃, i.e., 298.15K. This can also be adjusted according to the application scenario of the waveguide under test. For example, if the actual operating environment of the waveguide is 50℃, then... Set the temperature to 50℃ to reduce the correction range; The temperature coefficient of four-wave mixing conversion efficiency reflects the efficiency of a 1°C change in temperature. The key parameter for the change ratio is determined by the inherent properties of the waveguide material. This refers to the locked actual operating temperature of the waveguide. This temperature is the optimal temperature for triggering the efficient four-wave mixing effect, and it is also the true operating temperature of the waveguide throughout the entire measurement process. It must be compared with the temperature in the correction formula. Strict consistency is necessary to ensure the accuracy of the revised version. It conforms to actual working conditions.

[0062] Next, we need to explain the specific methods for obtaining each parameter, which is the practical basis for implementing the correction. Regarding the reference temperature... Corresponding conversion efficiency The temperature needs to be obtained through a pre-calibration experiment: Before the experiment begins, place the waveguide under test on a temperature control platform and adjust the platform to stabilize the waveguide temperature. Keeping the pump light wavelength, probe light power, and other conditions consistent with the subsequent formal measurements, the pump light power at this point is then measured. Detection of optical power Idle frequency optical power Combined with the known waveguide length Substitute into the basic formula of four-wave mixing At this point, the typical loss coefficient of the waveguide material can be used for approximate calculation, or the loss coefficient of a standard waveguide from the same batch with known loss can be used for calibration to deduce the loss coefficient. Below This process needs to be repeated 3 to 5 times, and the average value is taken as the final result. This avoids random errors in a single calibration.

[0063] temperature coefficient The acquisition of this data relies on the characteristic data of the waveguide material or specific measurements: different nonlinear waveguide materials Significant differences exist, for example, in lithium niobate (LiNbO3) waveguides. Approximately 1×10⁻³~5×10⁻³ / ℃, chalcogenide glass waveguides It is even smaller, about 1×10⁻ 4 ~5×10⁻ 4 / ℃, if the waveguide supplier provides a temperature coefficient handbook for the material, you can directly refer to the handbook. Value; for higher precision, experimental measurement can also be performed. Select multiple temperature points, such as 20℃, 25℃, 30℃, and 35℃, and label each temperature point accordingly. Then, with temperature as the x-axis, Plot a curve on the ordinate and calculate the slope of the curve using linear fitting. Slope = And then we can deduce =Slope / It should be noted that, The sign can be positive or negative; for most materials, the sign increases with temperature. is in a weak upward trend, so is positive, but no matter the sign, it must be kept complete to ensure the accuracy of the correction formula.

[0064] Regarding the core principle of temperature correction, we need to return to the nature of the four-wave mixing effect: the four-wave mixing conversion efficiency is directly related to the third-order nonlinear susceptibility of the waveguide and the phase matching condition, and both of these factors are regulated by the temperature of the waveguide. When the temperature of the waveguide changes, the refractive index n of the material will also change. The change in refractive index not only affects the phase matching condition, thereby changing the degree of matching of photon conservation, but also indirectly affects the third-order nonlinear susceptibility. The third-order nonlinear susceptibility is related to the electron cloud distribution characteristics of the material. Temperature changes will change the lattice vibration state and affect the electron cloud response. The superposition of these two factors will cause to change with temperature, for example, when the temperature rises, if the change in refractive index makes closer to 0, and the third-order nonlinear susceptibility increases slightly, then will rise accordingly; on the contrary, if the temperature change destroys the phase matching, then it will decrease. Therefore, the actual working temperature locked in step S3 is different from the reference temperature , if there is a difference, will inevitably deviate from , if not corrected, it will directly lead to calculation deviation of after being substituted into the loss formula, for example, if the actual is higher than , and is positive, the uncorrected will be less than the true , and the calculated will be larger, because in the formula is positively correlated with , if it is smaller, a larger is needed to satisfy the equation.

[0065] In actual correction calculations, the actual working temperature of the waveguide needs to be read and recorded through the temperature control console sensor first, and then the pre-calibrated , and values are retrieved, and the corrected is calculated by substituting them into the formula . For example, if = 25℃, = 0.08 (i.e. 8%), and = 3×10⁻³ / ℃, the locked = 27.5℃, then the corrected = 0.08 x [1 + 3 x 10-3 x (27.5 - 25)] = 0.08 x (1 + 0.0075) = 0.0806, i.e. 8.06%. This revised Substitute the waveguide loss calculation formula , to ensure that the calculation results are consistent with the actual loss state of the waveguide, avoid introducing additional measurement errors due to temperature-induced deviation, and ultimately improve the overall accuracy of waveguide loss measurement.

[0066] In a preferred embodiment, in the case where the value of cannot be determined in advance, the linear loss coefficient is calculated using an iterative method, which relies on a clear iterative formula and convergence criterion to ensure the normativity of the calculation process and the reliability of the results. Specifically, the iterative process updates the parameters through two core formulas, and determines the iteration endpoint based on the preset termination condition, while the reasonable setting of the initial value provides a basis for the convergence of the iteration. The details of each link are as follows.

[0067] The core formulas of iterative calculation include the iterative formula of four-wave mixing conversion efficiency and the update formula of linear loss coefficient. Among them, the four-wave mixing conversion efficiency of the nth iteration is calculated by formula , which is derived from the power relationship formula of four-wave mixing. After moving the core formula , the relationship between and other parameters can be obtained, where the loss coefficient of the nth iteration is substituted to obtain the conversion efficiency of the corresponding round. The source of each parameter in the formula needs to be clear, is the effective value of the idler light power after noise compensation and system error correction; is the actual incident power of the pump light, which is directly measured by a power meter; is the working power of the probe light in the determined linear interval, which is also measured by a power meter; is the physical length of the waveguide to be measured, which is obtained by high-precision measurement tools; is the linear loss coefficient assumed in the nth iteration; is the exponential term reflecting the influence of waveguide loss on light power attenuation.

[0068] Based on the conversion efficiency of the nth round , the linear loss coefficient of the nth+1 round can be calculated by formula . The derivation of this formula is based on the logarithmic transformation of the core formula. Taking the natural logarithm of both sides , after rearrangement, the expression of can be separated, which is substituted into the After that, the updated The essence of this process is to use the conversion efficiency obtained in the previous iteration to back-calculate the loss coefficient closer to the true value, thereby gradually optimizing the parameters.

[0069] Linear loss coefficient assumption value of initial round It needs to be reasonably set according to the material properties of the waveguide to be measured, which is the key to ensure the rapid convergence of the iteration. Different types of waveguide materials have typical ranges of linear loss coefficients due to differences in preparation process and structural characteristics. For example, the linear loss of silicon-based photonic crystal waveguide is usually 0.1 dB / cm to 2 dB / cm, the loss of lithium niobate (LiNbO3) waveguide is usually 0.5 dB / cm to 5 dB / cm, and the loss of chalcogenide glass waveguide is usually about 1 dB / cm to 10 dB / cm. The initial value The middle value of the material loss range can be selected, such as 1 dB / cm for silicon-based waveguide and 2 dB / cm for lithium niobate waveguide. If there are historical measurement data of waveguides of the same batch and structure, the historical data can also be used to set the initial value to further shorten the iteration time. The initial value does not need to be absolutely accurate, as long as it is within the typical loss range of the material, which can ensure the convergence of the iteration process to the true value.

[0070] The termination of the iteration process needs to meet the preset condition. When the absolute value of the difference between the linear loss coefficients obtained in the adjacent two iterations is less than 10⁻³ dB / cm dB / cm, the iteration is stopped, and the is the final linear loss coefficient. Selecting 10⁻ 5 dB / cm as the termination threshold is the result of considering the measurement accuracy requirements and computational efficiency. The conventional accuracy requirement for waveguide loss measurement is 10⁻³ dB / cm, and the threshold of 10⁻ 5 dB / cm is much higher than the actual requirement, which can ensure the reliability of the results. At the same time, this threshold can avoid the waste of computational resources caused by too many iterations. In actual operation, for most waveguides, 5 to 10 iterations can meet the condition, and the entire process can be automatically completed through programming tools such as Python and Matlab without human intervention.

[0071] Through the synergistic effect of the above iteration formula, initial value setting principle and termination condition, even in the case of unknown four-wave mixing conversion efficiency, the linear loss coefficient can be calculated systematically and efficiently, and the result accuracy can meet the stringent requirements of waveguide loss measurement, further improving the calculation logic.

[0072] In the above waveguide loss measurement method based on double-channel detection, the linear loss coefficient Multiple measurements are required to verify the results and eliminate any random or potential systematic errors that may exist in a single measurement, ensuring the reliability and accuracy of the final result. This verification process must strictly follow the logic of repeated measurements under the same conditions and statistical error analysis to determine the validity of the final result. The specific operation and judgment criteria are as follows.

[0073] First, the linear loss coefficient measurement procedure must be repeated under identical experimental conditions, with the number of repetitions set to 3 to 5. These identical experimental conditions are the core prerequisite for ensuring the comparability of multiple measurement data. Specifically, this includes: maintaining a constant waveguide temperature and pump light wavelength, with temperature fluctuations ≤ ±0.01℃ and wavelength fluctuations ≤ ±0.01nm; ensuring consistency between the determined linear range of the probe light power and the fixed probe light power value; keeping the acquisition parameters of the auxiliary laser and idler light, such as sampling frequency and outlier rejection rules, unchanged; and maintaining the initial state of the waveguide placement and optical path alignment, confirmed by observing the stability of the output light power. Choosing 3 to 5 repetitions is a result of comprehensively considering error statistics requirements and experimental efficiency. Too few repetitions cannot effectively reflect the dispersion of the data and make it difficult to distinguish between random and systematic errors; too many repetitions will significantly prolong the experimental time and have limited gain for error analysis. 3 to 5 repetitions satisfy the statistical significance requirements while also considering experimental efficiency.

[0074] After completing multiple measurements, the relative standard deviation (RSD) of these results needs to be calculated to quantify the stability and consistency of the data. The formula for calculating the relative standard deviation is RSD = (standard deviation / arithmetic mean) × 100%, where the standard deviation reflects the dispersion of each measurement from the average, and the arithmetic mean represents the central tendency of multiple measurements. The physical meaning of the relative standard deviation is that it expresses the relative fluctuation of the measurement results as a percentage, avoiding error evaluation bias caused by differences in absolute values. For example, if three measurements yield… The values ​​were 0.8 dB / cm, 0.82 dB / cm, and 0.78 dB / cm, respectively, with an average value of 0.8 dB / cm, a standard deviation of approximately 0.02 dB / cm, and a relative standard deviation of (0.02 / 0.8) × 100% = 2.5%.

[0075] Based on the measurement accuracy requirements, when the relative standard deviation of multiple measurement results is ≤2%, the measurement data is considered to have good repeatability and stability. In this case, the arithmetic mean of these 3 to 5 measurement results is taken as the final linear loss coefficient. This is because the relative standard deviation ≤ 2% means that the fluctuation of each measurement value around the average value is small, indicating that the experimental system is less affected by random errors, and there is no significant systematic error, such as slow shift of the optical path, drift of the detector sensitivity, etc., and the average value can be closer to the true value of the waveguide loss; at the same time, the threshold of 2% is the conventional accuracy requirement in the field of waveguide loss measurement, which can ensure the reliability of the results, and will not cause reasonable data to be misjudged due to excessive strictness.

[0076] If the relative standard deviation of multiple measurement results is > 2%, it indicates that the measurement system may have uneliminated systematic errors, at which time the current measurement process needs to be terminated, and all operations after the temperature determination of the waveguide are re-executed, and the error sources are eliminated by systematic investigation. Possible systematic errors include, the stability of temperature or wavelength locking decreases, the determined linear interval has deviation, the calculation of noise compensation factor is affected by sudden disturbance, the optical path alignment state changes, etc. When re-executing the process, the above-mentioned links need to be checked, for example, by extending the temperature monitoring time to confirm the locking stability, re-verifying the fitting goodness of the linear interval, checking the long-term power stability of the auxiliary laser, etc., until the relative standard deviation of multiple measurements is ≤ 2%, and then taking the average value as the final result.

[0077] Through the above multiple measurement verification and error analysis steps, random errors and systematic errors can be effectively distinguished, and the linear loss coefficient The true reflection of the inherent characteristics of the waveguide to be measured can further improve the reliability and accuracy of the entire measurement method.

[0078] The embodiments of the specific implementation mode are the preferred embodiments of the present application, and do not limit the protection scope of the present application, so: any equivalent changes made according to the structure, shape, principle of the present application should be covered within the protection scope of the present application.

Claims

1. A waveguide loss measurement method based on dual-channel detection, characterized in that, Includes the following steps: Step S1: The pump light, probe light and auxiliary laser are combined and injected into the nonlinear waveguide under test to trigger the four-wave mixing effect to generate idler light; Step S2: The output light of the waveguide is split into two paths by a beam splitter. The signal of the idler light is extracted from one path as a measurement channel; the signal of the auxiliary laser is extracted from the other path as a reference channel. Step S3: Adjust the temperature of the waveguide until the idler power of the measurement channel reaches a stable state, then lock the temperature of the waveguide and the wavelength of the pump light at this time; Step S4: Maintain the waveguide temperature and pump light wavelength locked in step S3, record the idler light power under different probe light power and fit the relationship between the two, use the least squares method to perform linear regression fitting, calculate the goodness of fit R², screen out the continuous probe light power sub-intervals with R²≥0.998, and obtain the probe light power interval with a linear relationship. Step S5: Maintain the waveguide temperature and pump light wavelength locked in step S3, and fix the probe light power within the range obtained in step S4. Generate a noise compensation factor by measuring the average power and instantaneous power of the auxiliary laser. The noise compensation factor is calculated as follows: The noise compensation factor is used to correct the measured power of the idler light in real time. The corrected idler light power is calculated as follows: ; in, The noise compensation factor is mentioned above. The average power of the auxiliary laser; The instantaneous power of the auxiliary laser; The instantaneous power of the idler frequency light; To measure the systematic error of the channel; Step S6: Calculate the waveguide loss based on step S5; ; in, For four-wave mixing conversion efficiency; The pump light power; The operating power of the probe light; The corrected idler frequency optical power; The length of the waveguide; This is the linear loss coefficient.

2. The waveguide loss measurement method based on dual-channel detection according to claim 1, characterized in that, The following conditions must be met in step S1; rad / m; in, This is the wave vector mismatch. The wave vector of the pump light in the waveguide; The wave vector of the probe light in the waveguide, The wave vector of the idler frequency light in the waveguide is adjusted by regulating the temperature of the waveguide to ensure that the wave vector mismatch meets the requirements.

3. The waveguide loss measurement method based on dual-channel detection according to claim 1, characterized in that, In step S1, the pump light, the probe light, and the auxiliary laser have the same polarization state and are all matched with the main polarization direction of the waveguide to be tested, so as to maximize the four-wave mixing conversion efficiency.

4. The waveguide loss measurement method based on dual-channel detection according to claim 1, characterized in that, Step S4 includes, Step S4.1: Maintain the temperature of the waveguide and the wavelength of the pump light locked in step S3; Step S4.2: Set the detection light adjustment range. Starting from the lower limit of the adjustment range, adjust the power of the detection light upward in steps of 1mW to 2mW, and hold for 3s to 5s after each power adjustment. Step S4.2: Collect 100 sets of idler optical power data at the power point using a sampling frequency of 10Hz, remove the 3 maximum values ​​and 3 minimum values, and take the arithmetic mean as the effective value of the idler optical power corresponding to the probe optical power; Step S4.3: Import the effective values ​​of all probe light power and idler light power into the data processing software, perform linear regression fitting using the least squares method, and calculate the goodness of fit R²; select the continuous probe light power sub-interval with R² ≥ 0.998, which is the probe light power interval with a linear relationship mentioned in step S4.

5. The waveguide loss measurement method based on dual-channel detection according to claim 1, characterized in that, The generation of the noise compensation factor in step S5 also includes outlier removal. When the absolute value of the deviation between the instantaneous power and the average power of a certain auxiliary laser is ≥10%, it is determined to be abnormal data. The moving average of the first 3 normal data is used to replace the instantaneous power of the auxiliary laser to calculate the noise compensation factor.

6. The waveguide loss measurement method based on dual-channel detection according to claim 1, characterized in that, The four-wave mixing conversion efficiency in step S6 needs to be corrected according to the actual operating temperature of the waveguide. ; in, The conversion efficiency is at the reference temperature; For reference temperature; Temperature coefficient; This represents the actual operating temperature of the waveguide.

7. The waveguide loss measurement method based on dual-channel detection according to claim 1, characterized in that, In step S6, when the four-wave mixing conversion efficiency is unknown, the linear loss coefficient is calculated using an iterative method.

8. The waveguide loss measurement method based on dual-channel detection according to claim 7, characterized in that, The iterative formula is: ; ; in, The four-wave mixing conversion efficiency in the nth iteration; The linear loss coefficient for the nth iteration; The linear loss coefficient for the (n+1)th iteration; The assumed value of the linear loss coefficient in the initial round is set based on the waveguide material properties; The iteration termination condition is: dB / cm。 9. The waveguide loss measurement method based on dual-channel detection according to claim 1, characterized in that, The linear loss coefficient α calculated in step S6 needs to be verified by multiple measurements. The measurements should be repeated 3 to 5 times under the same experimental conditions. If the relative standard deviation of the multiple measurement results is ≤2%, the average value should be taken as the final result. Otherwise, repeat steps S3 to S6 to eliminate system errors.

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