A method for stabilizing the operating point of a lithium niobate electric field sensor

Abstract

The application discloses a working point stabilizing method of a lithium niobate electric field sensor, and is applied to the fields of electric field sensing and optoelectronic technology, and aims at the problem that the working point of a lithium niobate MZI type electric field sensor will drift due to changes of external conditions such as temperature and humidity and the like after the sensing system works for a period of time according to the self characteristics of the lithium niobate crystal; no external electric field is applied when the calibration is performed, at this time, the wavelength of the laser output light has not changed, and the output light power of the electric field sensor changes due to the working point drift caused by the change of the external conditions; since the electric field is not applied and the output light wavelength has not changed during the calibration, the change amount of the output light power represents the drift amount of the working point of the electric field sensor, and thus the working point of the electric field sensor can be quickly stabilized by analyzing the value of the output light power. The method is simple in processing process and fast in working point stabilizing speed.

Description

Technical Field

[0001] This invention belongs to the fields of electric field sensing and optoelectronics, and specifically relates to a sensor operating point stabilization technology. Background Technology

[0002] Lithium niobate electric field sensors primarily rely on the electro-optic effect of lithium niobate material to sense electric fields. When an external electric field acts on the sensor's probe, the refractive index of the lithium niobate crystal within the probe changes with the external electric field. However, due to the pyroelectric, thermo-optic, piezoelectric, and elasto-optic effects of lithium niobate crystals—meaning changes in factors such as temperature, humidity, and stress—the refractive index of the lithium niobate crystal will ultimately change. This causes the operating point of the electric field sensor to drift. The stability of the electric field sensor's operating point directly affects whether the extracted electrical signal can reflect the true nature of the measured electric field with the least distortion.

[0003] Therefore, how to obtain a stable operating point method for a lithium niobate electric field sensor is a technical problem that needs to be solved in this field. Summary of the Invention

[0004] To solve the above-mentioned technical problems, this invention proposes a method for stabilizing the operating point of a lithium niobate electric field sensor, which only requires the addition of a single-chip microcomputer control module to achieve the stabilization of the operating point.

[0005] The technical solution adopted in this invention is: a method for stabilizing the operating point of a lithium niobate electric field sensor, based on an electric field sensing system including: a tunable laser, a polarization-maintaining fiber, an asymmetric MZI type electric field sensor, a single-mode fiber, a coupler, a photodiode PIN, an operational amplifier circuit, a microcontroller control module, a photodetector PD, a low-noise amplifier, and an oscilloscope.

[0006] The linearly polarized light output from the tunable laser travels through a polarization-maintaining fiber to an asymmetric MZI-type electric field sensor. After passing through the asymmetric MZI-type electric field sensor, it is output through a single-mode fiber to a coupler. The coupler outputs a portion of the light to a photodetector (PD) for electrical signal extraction, while the other portion is input to a photodiode (PIN). The operational amplifier circuit then converts the current signal output from the photodiode (PIN) into a voltage signal, filters and amplifies it. Finally, the microcontroller control module provides a feedback signal to the tunable laser based on the extracted, filtered, and amplified voltage signal for operating point stabilization.

[0007] The process of using a microcontroller control module to provide a feedback signal to the tunable laser based on the extracted, filtered, and amplified voltage signal for operating point stabilization is as follows:

[0008] During the first calibration, record the current external condition as a. The microcontroller control module commands the tunable laser to perform a complete wavelength scan. A complete wavelength scan specifically means that the tunable laser can tunably output several wavelength channels, and these several wavelength channels are output at equal time intervals. During the scan, for each output wavelength channel, record the current channel number and the sampling voltage collected by the microcontroller control module. Based on all the wavelength channel numbers and their corresponding sampling voltages, obtain the fitting curve corresponding to a.

[0009] Find the maximum voltage value and the minimum voltage value of the fitting curve corresponding to a, and take the average of the maximum voltage value and the minimum voltage value to obtain the best linear working point on the fitting curve corresponding to a. Take the channel number λ1 and the sampling voltage u1 corresponding to this best linear working point as the best working wavelength and the best linear voltage of the tunable laser.

[0010] When the external condition changes, perform re - calibration. Record the changed external condition as b. The fitting curve corresponding to b is translated along the wavelength direction based on the fitting curve corresponding to a, that is, the best linear voltage corresponding to b is u1. Record the best working wavelength corresponding to b as λ2.

[0011] Calculate λ2 by finding the number of wavelength channel numbers between λ1 and λ2.

[0012] The process of finding the number of wavelength channel numbers between λ1 and λ2 is as follows:

[0013] When the external condition is b and the input wavelength of the tunable laser is λ1, record the sampled voltage value as u2. According to the slope of the position of u2 on the fitting curve corresponding to a and the magnitude relationship between u1 and u2, determine the search direction. Record the current search count as j, and initially j = 1. When the search condition is not met, j is incremented by 1. The finally obtained j that meets the search condition is the number of channel numbers between λ1 and λ2. The search condition is one of the following four search conditions:

[0014] When the slope is positive and u2 > u1, determine whether u2 is less than the voltage value corresponding to λ1 + Δλ * j on the fitting curve corresponding to a.

[0015] When the slope is positive and u2 < u1, determine whether u2 is greater than the voltage value corresponding to λ1 - Δλ * j on the fitting curve corresponding to a.

[0016] When the slope is negative and u2 > u1, determine whether u2 is less than the voltage value corresponding to λ1 - Δλ * j on the fitting curve corresponding to a.

[0017] When the slope is negative and u2 < u1, determine whether u2 is greater than the voltage value corresponding to λ1 + Δλ * j on the fitting curve corresponding to a.

[0018] Beneficial effects of the present invention: The present invention has the following advantages compared with the prior art:

[0019] (1) The entire operating point stabilization system is small in size and contains few internal components, which reduces the complexity of the system and makes it easy to use;

[0020] (2) The calibration time required by this operating point stabilization method is short, which is more conducive to electric field measurement by electric field sensing system;

[0021] (3) The present invention does not limit the source of working point drift and can be applied to the influence of changes in external conditions such as temperature and humidity on the working point;

[0022] (4) The components included in the stable operating point system are affordable, which reduces the cost of the system and achieves low price, easy operation, and all domestic components can be selected. Attached Figure Description

[0023] Figure 1 Block diagram of a lithium niobate electric field sensor system;

[0024] Figure 2 The fitting curves of sampling voltage versus operating wavelength under different external conditions;

[0025] Figure 3 This is a flowchart of the operating point stabilization system of the present invention. Detailed Implementation

[0026] To facilitate understanding of the technical content of this invention by those skilled in the art, the following description, in conjunction with the accompanying drawings, further illustrates the invention.

[0027] To address the aforementioned technical difficulties, this invention proposes a method for stabilizing the operating point of a lithium niobate electric field sensor. First, this method requires few components; by adding a single-chip microcomputer control module to the electric field sensing system, operating point stabilization can be achieved, resulting in a low-cost operating point stabilization system. Second, this method offers short calibration times and ease of use.

[0028] This invention achieves rapid stabilization of the operating point of an electric field sensor by analyzing the final optical power output from the sensor without applying an external electric field. The key to the entire solution lies in photoelectric conversion and filtering amplification, electrical signal sampling, and microcontroller control of the tunable laser.

[0029] Based on the above principles, see Figure 1The electric field sensing system mainly includes ten parts: a tunable laser 01, a polarization-maintaining fiber 02, an asymmetric MZI type electric field sensor 03, a single-mode fiber 04, a coupler 05, a photodiode PIN 06, a microcontroller control module 07, a photodetector PD 08, a low-noise amplifier 09, and an oscilloscope 10.

[0030] The linearly polarized light output from the tunable laser reaches the asymmetric MZI electric field sensor through the polarization-maintaining fiber. After passing through the sensor, the light intensity has been modulated. Therefore, it only needs to pass through a single-mode fiber to the coupler. The coupler outputs most of the light to the photodetector for electrical signal extraction, while the other part of the light is input to the PIN photodiode to complete the conversion between optical and current signals. Then, the current signal is converted into a voltage signal and filtered and amplified. Finally, it is extracted by the microcontroller as a feedback signal for operating point stabilization.

[0031] See Figure 2 , Figure 3 The steps of the method for stabilizing the operating point of a lithium niobate electric field sensor are as follows:

[0032] 1. The sampling voltage of the microcontroller control module is R is the photoelectric responsivity, k is the splitting ratio of the coupler, α is the light attenuation coefficient in the sensing system, and P in λ is the optical power output by the laser, λ is the wavelength of the tunable laser output, and n is the wavelength of the laser output. eff Let be the effective refractive index of the optical waveguide, and ΔL be the difference in arm length between the two waveguide arms in the MZI structure. This refers to the phase difference change caused by the combined effects of various external factors. Assuming the laser's output wavelength λ ranges from 1530 nm to 1560 nm, RkαP in Substituting 1, the arm length difference ΔL is 40μm, and the effective refractive index n of the single-mode optical waveguide is taken. eff The value is 2.138, which gives us the following: Figure 2 The figures show the fitting curves of sampling voltage versus operating wavelength under different external conditions. When the external condition is a... When the value is set to π / 2, and the external condition is b... Let it be 5π / 8.

[0033] 2. Assuming external condition 'a', during the first operating point calibration, the microcontroller control module will command the tunable laser to perform a complete wavelength scan. For example, the laser can tunably output 40 wavelength channels, with a wavelength interval of 0.8 nm for each channel. The program will record the channel number transmitted and its corresponding sampling voltage V(i) during the scan, thus obtaining... Figure 2Curve a is formed by connecting 40 points. After completing the wavelength scan, the minimum and maximum voltage values ​​are found and averaged to obtain the optimal linear operating point A of curve a. The x and y coordinates of point A are the optimal operating wavelength λ1 and the optimal linear voltage u1 of curve a, respectively.

[0034] 3. After a period of time following the first calibration, the external conditions change from external condition a to external condition b. At this point, the wavelength channel remains unchanged, and the laser still outputs λ1, but the curve has changed to b. Therefore, the operating point becomes point C, and the sampling voltage of the microcontroller module changes to u2. Clearly, λ1 is not the optimal linear operating point for curve b. Since the curve shifts in the wavelength direction when external conditions change, the ordinate of the optimal linear operating point, i.e., the optimal linear voltage, remains u1 for curve b. Therefore, point B is the optimal linear operating point for curve b. Thus, the goal of the second and subsequent calibrations is to find the optimal operating wavelength λ2 at the abscissa of point B. Those skilled in the art should understand that u2 ≠ u1 in this invention.

[0035] 4. Since the output wavelengths of a tunable laser each have a corresponding channel number, and the wavelength interval between adjacent channel numbers is 0.4 nm, for the optimal linear wavelength λ2 of curve b, it is only necessary to find the difference Δi between the channel numbers corresponding to λ1 and λ2. In other words, it is also necessary to find how many output operating points exist between points B and C in curve b. Since curves a and b are translated, the number of sampling points between points A and D in curve a represents the number of wavelength channels between λ2 and λ1.

[0036] 5. Since the wavelength scanning method was used during the first calibration, the microcontroller control module obtained the sampling points corresponding to the 40 wavelength channels on curve a. By comparison, it first determined whether the slope of the curve where point A is located was positive or negative, and then compared whether u2 was greater than u1. If it was greater, then u2 was compared with the voltage value corresponding to the wavelength channel adjacent to point A.

[0037] like Figure 2 As shown, the voltage value u1 at point A is 0.5V, and the voltage value u2 at points D and C is 0.691V. In curve a, the voltage value corresponding to the adjacent wavelength channel 1540.6nm at point A is 0.534V. The voltage value of 0.534V corresponding to channel 1540.6nm is greater than the voltage value of 0.5V corresponding to channel 1541nm at point A. Figure 3 As shown, the slope of curve a is negative at this time; and u2 is greater than u1, so the channel with the interval is found by reducing the wavelength starting from the current channel 1541nm. Specifically: determine whether u2 is less than the channel (1540.6nm-0.4*jnm), where j is the current comparison number. If the current comparison result is not, then j is incremented by 1 and the comparison is repeated.

[0038] like Figure 2 As shown, when j=1, u2 is compared with the voltage value of 0.534V corresponding to the wavelength channel 1541nm-0.4nm=1540.6nm, u2=0.691V. Obviously, 0.691V>0.534V, so j=2. Next, u2 is compared with the voltage value of 0.579V corresponding to the wavelength channel 1541nm-0.4*2nm=1540.2nm, u2=0.691V. Obviously, 0.691V>0.579V, so j=3. Then, u2 is compared with the voltage value of 0.623V corresponding to the wavelength channel 1541nm-0.4*3nm=1539.8nm. Comparing the values, it is clear that 0.691V > 0.623V, therefore j = 4. Next, u2 is compared with the voltage value of 0.667V corresponding to the wavelength channel 1541nm - 0.4 * 4nm = 1539.4nm. Obviously, 0.691V > 0.667V, therefore j = 5. Next, u2 is compared with the voltage value of 0.708V corresponding to the wavelength channel 1541nm - 0.4 * 5nm = 1539nm. Obviously, 0.691V < 0.708V. Let the laser output channel wavelength λ2 be 1541nm + 0.4 * (5 - 1)nm = 1542.6nm, which means that the stabilization of the operating point of the electric field sensor has been completed.

[0039] Those skilled in the art should know that if the slope of the curve containing point A is negative and u2 is less than u1, then wavelength should be added for searching; if the slope of the curve containing point A is positive and u2 is greater than u1, then wavelength should be added for searching; if the slope of the curve containing point A is positive and u2 is less than u1, then wavelength should be reduced for searching.

[0040] 6. After outputting light with a wavelength of λ2, if calibration is successful, the feedback voltage should also be near the optimal linear voltage, i.e., between the voltage value of 0.534V corresponding to channel 1540.6nm and the voltage value of 0.466V corresponding to channel 1541.4nm on curve a. If the feedback voltage does not meet this condition, the wavelength scan should be repeated to obtain curve a and the optimal operating wavelength again. If the condition is met, it means that the operating point calibration was successful.

[0041] Specifically, during the comparison process of adding or subtracting wavelength, if the number of comparisons j exceeds 10, the wavelength is rescanned to obtain a new curve. Furthermore, the number of channels between adjacent peaks and troughs of any fitted curve is greater than 10.

[0042] In this way, by combining the above specific implementation steps, the function of stabilizing the operating point of the lithium niobate electric field sensor can be achieved.

[0043] Those skilled in the art will recognize that the embodiments described herein are intended to help the reader understand the principles of the invention, and should be understood that the scope of protection of the invention is not limited to such specific statements and embodiments. Various modifications and variations can be made to the invention by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the invention should be included within the scope of the claims of the invention.

Claims

1. A method for stabilizing the operating point of a lithium niobate electric field sensor, characterized in that, The electric field sensing system includes: tunable laser, polarization-maintaining fiber, asymmetric MZI electric field sensor, single-mode fiber, coupler, photodiode PIN, operational amplifier circuit, microcontroller control module, photodetector PD, low-noise amplifier and oscilloscope. The linearly polarized light output from the tunable laser travels through a polarization-maintaining fiber to an asymmetric MZI-type electric field sensor. After passing through the asymmetric MZI-type electric field sensor, it is output through a single-mode fiber to a coupler. The coupler outputs a portion of the light to a photodetector PD for electrical signal extraction, while the other portion is input to a photodiode PIN. The operational amplifier circuit then converts the current signal output from the photodiode PIN into a voltage signal, filters and amplifies it. Finally, the microcontroller control module provides a feedback signal to the tunable laser based on the extracted, filtered and amplified voltage signal for operating point stabilization. The process of using a microcontroller control module to provide a feedback signal to the tunable laser based on the extracted, filtered, and amplified voltage signal for operating point stabilization is as follows: During the first calibration, the current external condition is denoted as 'a'. The microcontroller control module commands the tunable laser to perform a complete wavelength scan. A complete wavelength scan is specifically as follows: the tunable laser can output several wavelength channels at equal time intervals. During the scan, each time a wavelength channel is output, the current channel number and the sampling voltage collected by the microcontroller control module are recorded. Based on all wavelength channel numbers and their corresponding sampling voltages, the fitting curve corresponding to 'a' is obtained. Find the maximum and minimum voltage values ​​of the fitted curve corresponding to 'a', and take the average of the maximum and minimum voltage values ​​to obtain the optimal linear operating point on the fitted curve corresponding to 'a'. Use the channel number λ1 and sampling voltage u1 corresponding to the optimal linear operating point as the optimal operating wavelength and optimal linear voltage of the tunable laser. When external conditions change, recalibrate. Let the changed external conditions be b. The fitting curve corresponding to b is the curve after shifting along the wavelength direction based on the fitting curve corresponding to a. That is, the optimal linear voltage corresponding to b is u1; let the optimal operating wavelength corresponding to b be λ2. λ2 is calculated by finding the number of wavelength channel numbers between λ1 and λ2.

2. The method for stabilizing the operating point of a lithium niobate electric field sensor according to claim 1, characterized in that, Find the number of wavelength channel numbers between λ1 and λ2, specifically: When the external condition is b and the input wavelength of the tunable laser is λ1, the sampled voltage value is denoted as u2. Based on the slope of u2 on the fitted curve corresponding to a, and the relationship between u1 and u2, the search direction is determined, and the current search count is denoted as j, initially j=1. When the search condition is not met, j is incremented by 1. The final j that satisfies the search condition is the number of channel numbers between λ1 and λ2. The search condition is one of the following four search conditions: When the slope is positive and u2 > u1, determine whether u2 is less than the voltage value corresponding to λ1 + ∆λ*j on the fitted curve corresponding to a; When the slope is positive and u2 < u1, determine whether u2 is greater than the voltage value corresponding to λ1-∆λ*j on the fitted curve corresponding to a; When the slope is negative and u2 > u1, determine whether u2 is less than the voltage value corresponding to λ1-∆λ*j on the fitted curve corresponding to a; When the slope is negative and u2 < u1, determine whether u2 is greater than the voltage value corresponding to λ1 + ∆λ*j on the fitted curve corresponding to a.

3. The method for stabilizing the operating point of a lithium niobate electric field sensor according to claim 2, characterized in that, When the slope is positive and u2 > u1, or when the slope is negative and u2 < u1, λ2 = λ1 - ∆λ*(j-1).

4. The method for stabilizing the operating point of a lithium niobate electric field sensor according to claim 3, characterized in that, When the slope is positive and u2 < u1, or when the slope is negative and u2 > u1, λ2 = λ1 + ∆λ*(j-1).

5. A method for stabilizing the operating point of a lithium niobate electric field sensor according to claim 3 or 4, characterized in that, λ2 is the input wavelength of the tunable laser when the external condition is b.

6. The method for stabilizing the operating point of a lithium niobate electric field sensor according to claim 5, characterized in that, When the external condition is b and the input wavelength of the tunable laser is λ2, the process also includes determining whether the sampling voltage collected by the microcontroller control module is between the two voltage values ​​corresponding to λ1-∆λ and λ1+∆λ on the fitting curve corresponding to a; if so, the calibration is successful; otherwise, a complete wavelength scan is performed again.

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