Model analysis method for interference signal superposition effect in light interference displacement measurement

Abstract

The invention discloses a model analysis method for an interference signal superposition effect in light interference displacement measurement, and the method comprises the steps: building an interference superposition model which is used for simulating superposition effects after primary reflected light and secondary reflected light interfere with reference light; the reference light is a light beam reflected by the end face of the optical fiber; obtaining model key parameters, and based on the model key parameters, calculating an interference signal of the primary reflected light and the reference light, an interference signal of the secondary reflected light and the reference light, and a superposed signal of the primary reflected light and the reference light through a model; analyzing a nonlinear effect of an interference signal by using a simulation result of the superposed signal so as to optimize the displacement measurement precision of the optical fiber Fabry-Perot interferometer; according to the method, the reflected light interference superposition model is established, the nonlinear effect of multiple reflection interferences is constructed, and the signal processing method is optimized, so that the precision, the stability and the applicability of displacement measurement are improved.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of optical fiber sensing and interferometric measurement, and particularly relates to a model analysis method for interference signal superposition effect in optical interference displacement measurement. BACKGROUND

[0002] Optical interference displacement measurement technology has become a core supporting technology in the fields of precision manufacturing, micro-nano processing, aerospace precision calibration, optical element detection, etc. due to its ultra-high measurement precision of nanometer level or even sub-nanometer level. The principle is based on the quantitative mapping relationship between the interference fringe change caused by the superposition of coherent light and the displacement amount. Typical technologies such as Michelson interferometer, Fabry-Perot interferometer and laser interferometer realize accurate measurement of micro displacement by detecting the phase, intensity or frequency change of the interference signal.

[0003] However, in actual measurement scenarios, the superposition effect of the interference signal often leads to deviation of the measurement precision from the theoretical value, which becomes a key bottleneck restricting the performance improvement. This superposition effect is mainly caused by the following factors: multiple reflections and scattering of optical elements (such as mirrors and beam splitters) in the optical system produce stray light, which forms a non-ideal superposition with the main interference light beam, resulting in a decrease in fringe contrast and a decrease in signal-to-noise ratio (SNR), and even introduces false phase jumps; environmental factors such as thermal expansion and contraction of optical elements caused by temperature fluctuations, micro changes in optical path length caused by mechanical vibration, and non-uniformity of air refractive index, will cause additional dynamic shifts in the interference optical path difference, which is superimposed with the ideal optical path difference caused by displacement, resulting in signal phase drift; and the system itself characteristics such as insufficient coherence of the light source, nonlinear response of the detector, and polarization state change of the optical path, will cause nonlinear superposition of the interference signal intensity and phase, which destroys the linear mapping relationship between displacement and signal change.

[0004] At present, the analysis of the superposition effect of the interference signal in the prior art is mainly focused on the qualitative description of a single factor (such as stray light or temperature interference), and lacks quantitative modeling of the coupling and superposition of multiple factors. Therefore, it is urgent to establish a model analysis method for the superposition effect of the interference signal that can consider the coupling of multiple factors, to quantitatively describe the correlation mechanism between the superposition effect and the displacement measurement error, to provide theoretical support and optimization direction for high-precision optical interference displacement measurement, and to meet the demand for sub-nanometer level measurement precision in high-end manufacturing and micro-nano technology fields. Therefore, a model analysis method for the superposition effect of the interference signal in optical interference displacement measurement is needed. SUMMARY

[0005] The purpose of the present application is to provide a model analysis method for the superposition effect of the interference signal in optical interference displacement measurement.

[0006] To achieve the above purpose, the present application is implemented according to the following technical solutions:

[0007] The present application comprises the following steps:

[0008] (1) an interference superposition model is established, which is used to simulate the superposition effect of the first reflected light and the second reflected light after interference with the reference light; wherein the first reflected light is the measurement beam reflected back to the fiber end face for the first time by the target reflector, the second reflected light is the measurement beam reflected back to the fiber core for the second time by the target reflector after being reflected by the fiber cladding or coating layer due to the deviation of the first reflected light from the fiber core caused by the inclination of the target reflector, and the reference light is the light beam reflected by the fiber end face;

[0009] (2) key parameters of the model are obtained, including the core diameter of the fiber, the cladding diameter, the fiber end face reflectivity R1, the target reflector reflectivity R2, the cladding reflectivity R3, the coating reflectivity R4, and the offset distance d of the first reflected light at the fiber end face;

[0010] (3) based on the key parameters of the model, the interference signals of the first reflected light and the reference light, the interference signals of the second reflected light and the reference light, and the superposition signals of the two are calculated by the model;

[0011] (4) using the simulation results of the superposition signals, the non-linear effect of the interference signals is analyzed, which is used to optimize the displacement measurement accuracy of the fiber Fabry-Perot interferometer;

[0012] In the simulation process, the light intensities incident back to the fiber core and hitting the cladding when reflected back to the fiber end face for the first time are calculated by integration, and the simulation of the first and second reflected simulation interference signals is performed, as shown in the following formula:

[0013] Signal1=A1*sin(2πf1t)+B1

[0014] Signal2=A2*sin(2πf2t)+B2

[0015] Combined_Signal=Signal1+Signal2

[0016] Wherein A1, A2, B1, B2 are determined according to the light power incident back to the fiber core and hitting the cladding when reflected back to the fiber end face for the first time, and f2=2f1.

[0017] f1=f0+m*sin(2πΩt)

[0018] f2=2f1

[0019] Wherein f0 is the center frequency, m is the modulation depth, and Ω is the modulation frequency.

[0020] Further, the fiber of the interference superposition model is a single-mode fiber, the fiber core diameter of the FC / PC joint is 8.2 μm, and the cladding diameter is 125 μm.

[0021] Further, the fiber end face reflectivity R1 of the interference superposition model is 3.6%, the target reflector reflectivity R2 is greater than 96%, the cladding reflectivity R3 is 3.5%, and the coating layer reflectivity R4 is 13%.

[0022] Further, in step (3), the interference signal period of the reference light of the secondary reflected light is 1 / 2 of the interference signal period of the primary reflected light and the reference light.

[0023] Further, in step (4), the simulation result of the superposition signal is used to analyze the influence of different target reflector reflectivities R2 on the interference signal, so as to optimize the sensor head design.

[0024] Further, a 1550 nm tunable distributed feedback laser is used, and the interference signal is detected and recorded by an InGaAs detector and a data acquisition card.

[0025] The beneficial effects of the present application are:

[0026] The present application is a model analysis method for interference signal superposition effect in optical interference displacement measurement, which has the following technical effects compared with the prior art:

[0027] The present application establishes a reflected light interference superposition model, deeply studies the nonlinear effect of multiple reflection interference, optimizes the signal processing method, and thus improves the precision, stability and applicability of displacement measurement. BRIEF DESCRIPTION OF DRAWINGS

[0028] Figure 1 The present application is a model analysis method for interference signal superposition effect in optical interference displacement measurement, which has the following technical effects compared with the prior art:

[0029] Figure 2 The present application is a model analysis method for interference signal superposition effect in optical interference displacement measurement, which has the following technical effects compared with the prior art:

[0030] Figure 3 The present application is a model analysis method for interference signal superposition effect in optical interference displacement measurement, which has the following technical effects compared with the prior art:

[0031] Figure 4 The present application is a model analysis method for interference signal superposition effect in optical interference displacement measurement, which has the following technical effects compared with the prior art:

[0032] Figure 5The schematic diagram of simulated interference fringes in the superposition state and the Lissajous figure of signal 1 and signal 2 of the model analysis method of the superposition effect of interference signals in the optical interference displacement measurement of the present application; DETAILED DESCRIPTION

[0033] The present application will be further described below through specific embodiments, and the illustrative embodiments of the present application and the description are used to explain the present application but do not limit the present application.

[0034] The model analysis method of the superposition effect of interference signals in the optical interference displacement measurement of the present application comprises the following steps:

[0035] The present application establishes a model for a reflection light interference superposition phenomenon in the displacement measurement process of a fiber Fabry-Perot interferometer, simulates and reproduces the interference superposition phenomenon of the once-reflected light and the twice-reflected light in the interference process through the model, and greatly meets the needs of high-precision measurement and scientific research experiments.

[0036] The experimental device diagram of the method is shown in Figure 1 The experimental system adopts a 1550nm tunable distributed feedback laser, the laser output light is input through a port 1 of a fiber circulator, output through a port 2 of the fiber circulator, then connected through a fiber flange and a single-mode fiber jumper, the other output end of the single-mode fiber is an FC / PC joint, and the fiber end face constitutes a first reflection surface of a Fabry-Perot cavity with a reflectivity of about 4%. When the light beam passes through the fiber end face, about 4% of the light is reflected back to the single-mode fiber as a reference light beam, and 96% of the outgoing light is coupled into the cavity and exits to the object surface. At this time, the light beam returns along the original path after being reflected, re-couples into the single-mode fiber to form a measurement light beam, and interference occurs between the measurement light beam and the reference light beam to generate an interference signal. The interference signal is detected at a port 3 of the fiber circulator through an InGaAs detector, and the interference signal information is recorded by a data acquisition card.

[0037] The twice-reflected light path in the sensing cavity is shown in Figure 2 When the target reflecting object of the sensing cavity is slightly tilted at an inclination angle a, the incident light is not vertically incident to the reflecting object surface, and then the reflected light returns to the fiber end face at a certain angle. At this time, part of the light directly enters the fiber core, and the other part of the light is reflected by the cladding, passes through the cavity, is twice-reflected on the target reflecting object, and then enters the fiber core again.

[0038] Due to the tilt of the target reflecting object, the reflected light beam does not directly couple back to the fiber core, but forms a small radial offset at the fiber end face. The distance d between the first reflected light spot at the fiber end face and the center of the fiber core is used to represent the radial offset. Figure 3When d changes, the position of the light spot reflected once on the end face of the optical fiber also changes, and the position of the light spot can be determined according to the value of d.

[0039] As shown in Figure 3 , the FC / PC head of the single-mode optical fiber is composed of a fiber core, a cladding layer and a coating layer, the diameter of the fiber core is 8.2 μm, the reflectivity R1 = 3.6%, the diameter of the cladding layer is 125 μm, the reflectivity R3 = 3.5%, and the reflectivity R4 = 13% of the coating layer.

[0040] As shown in Figure 4 , the distribution of the once-reflected light and the twice-reflected light interferes with the reference light, and the superposition of the two kinds of interference signals causes the shape of the interference signal detected by the photodetector to change. The once-reflected interference signal, the twice-reflected interference signal and the superposition signal can be simulated by establishing a model, as shown in Figure 5 . Among them, Signal 1 and Signal 2 are the simulated interference signals of the once-reflected and twice-reflected light respectively, and the superposition signal (Combined Signal) is the superposition of the two, that is, the simulation of the actual signal (Interference Signal). When the light beam passes through the sensing cavity twice (double pass), the interference signal period is 1 / 2 of the single pass of the light beam, which is due to the interference characteristics of the Fabry-Perot cavity. In the simulation process, the light intensity incident into the fiber core and hitting the cladding layer when the light is reflected back to the end face of the fiber for the first time needs to be calculated by integration respectively. And the simulated interference signals of the once-reflected and twice-reflected light are simulated respectively, as shown in the following formula:

[0041] Signal1 = A1*sin(2πf1t) + B1

[0042] Signal2 = A2*sin(2πf2t) + B2

[0043] Combined_Signal = Signal1 + Signal2

[0044] Among them, A1, A2, B1, B2 are determined according to the light power incident into the fiber core and hitting the cladding layer when the light is reflected back to the end face of the fiber for the first time, f1 and f2 are related to the modulation depth and the modulation frequency and satisfy f2 = 2f1. From Figure 4 , it can be seen that the superposition causes the interference signal of the fiber optic interferometer system to appear nonlinear effect, which reduces the accuracy of displacement measurement and reduces the sinusoidal degree and usability of the interference signal.

[0045] f1 = f0 + m*sin(2πΩt)

[0046] f2 = 2f1

[0047] where f0 is the center frequency, m is the modulation depth, and Ω is the modulation frequency.

[0048] The present application establishes a mathematical model to simulate the superposition effect. The model can be used to analyze the nonlinear effect of the interference signal, improve the accuracy of displacement measurement and system optimization ability, when the target reflector is slightly tilted, the reflected light will deviate from the fiber core, causing a part of the light to be reflected twice, through numerical simulation and experimental data comparison, the relationship between the first reflected light, the second reflected light and the combined signal is verified. The double pass effect makes the period of the interference signal become 1 / 2 of the original, which is of great significance to the error compensation and optimization of the measurement system. The influence of the reflectivity of different target reflecting surfaces on the interference signal can be analyzed, and the design of the sensor head can be optimized.

[0049] The above only describes the preferred embodiments of the present application and is not intended to limit the present application. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present application shall be included in the protection scope of the present application.

Claims

1. A model analysis method for the superposition effect of interference signals in optical interferometric displacement measurement, characterized in that, Includes the following steps: (1) Establish an interference superposition model, which is used to simulate the superposition effect of the primary reflected light and the secondary reflected light after interfering with the reference light respectively; wherein, the primary reflected light is the measurement beam that is first reflected back to the fiber end face after being reflected by the target reflector, the secondary reflected light is the measurement beam that is reflected back to the fiber core after being deflected by the fiber core due to the tilt of the target reflector, the primary reflected light is reflected by the fiber cladding or coating layer, and then reflected back to the fiber core by the target reflector, and the reference light is the beam reflected by the fiber end face; (2) Obtain key parameters of the model, including the core diameter of the optical fiber, the cladding diameter, the reflectivity of the optical fiber end face R1, the reflectivity of the target reflector R2, the cladding reflectivity R3, the reflectivity of the coating layer R4, and the offset distance d of the first reflected light at the end face of the optical fiber. (3) Based on the key parameters of the model, the interference signal between the primary reflected light and the reference light, the interference signal between the secondary reflected light and the reference light, and the superposition signal of the two are calculated by the model. (4) Using the simulation results of the superimposed signal, the nonlinear effect of the interference signal is analyzed to optimize the displacement measurement accuracy of the fiber Fabry-Perot interferometer; During the simulation, the light intensity incident on the fiber core and striking the cladding during the first reflection back to the fiber end face is calculated by integration. The simulated interference signals of the first and second reflections are then simulated, as shown in the following equations: Signal1 = A1 * sin(2πf1t) + B1 Signal2 = A2 * sin(2πf2t) + B2 Combined_Signal=Signal1+Signal2 A1, A2, B1, and B2 are determined based on the optical power incident on the fiber core and the optical power hitting the cladding during the first reflection back to the fiber end face, respectively, and f2 = 2f1.

2. The method according to claim 1, characterized in that, In step (2), the optical fiber of the interference superposition model is a single-mode optical fiber with a core diameter of 8.2 μm and a cladding diameter of 125 μm for its FC / PC connector.

3. The method according to claim 1, characterized in that, In step (2), the optical fiber end face reflectivity R1 = 3.6%, the target reflectivity R2 > 96%, the cladding reflectivity R3 = 3.5%, and the coating reflectivity R4 = 13% in the interference superposition model.

4. The method according to claim 1, characterized in that, In step (2), the distance d between the light spot reflected back to the fiber end face and the fiber core center is calculated by the effective focal length f of the collimating lens and the tilt angle α of the target reflector. The value of distance d is used to determine the position of the light spot on the fiber end face of the first reflected light.

5. The method according to claim 1, characterized in that, In step (3), the interference signal period of the reference light of the secondary reflected light is 1 / 2 of the interference signal period of the primary reflected light and the reference light.

6. The method according to claim 1, characterized in that, In step (4), the simulation results of the superimposed signal are used to analyze the influence of the reflectivity R2 of different target reflectors on the interference signal, so as to optimize the sensor head design.

7. The method according to claim 1, characterized in that, In step (4), f1 and f2 exhibit a sinusoidal relationship with time, as shown in the following equation: f1 = f0 + m*sin(2πΩt) f2 = 2f1 Where f0 is the center frequency, m is the modulation depth, and Ω is the modulation frequency.

8. The method according to claim 1, characterized in that, A 1550nm tunable distributed feedback laser was used, and the interference signal was detected and recorded by an InGaAs detector and a data acquisition card.

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