A fiber-optic interferometry-based refractive index measurement system and method

Abstract

This invention specifically relates to a refractive index measurement system and method based on fiber optic interferometry, used for real-time monitoring of the refractive index of transparent gases and liquids. It addresses the technical problems of existing air refractive index measurement methods, which either struggle to accurately obtain the absolute value of the refractive index, or involve complex procedures, cumbersome optical path adjustments, susceptibility to vibration interference, and low dynamic response speed. The fiber optic interferometry-based refractive index measurement system includes a tunable fiber laser, a first fiber coupler, a transmitting probe, a receiving probe, an attenuator, a second fiber coupler, a photoelectric converter, and a recording device. The first fiber coupler splits the optical signal emitted from the tunable fiber laser into two paths, one input to the transmitting probe and the other to the attenuator. The input of the second fiber coupler is connected to the output of the attenuator and the output of the receiving probe. This invention offers high reliability and a fast response time of 0.2 ms; it can perform refractive index monitoring in various scenarios.

Description

Technical Field

[0001] This invention specifically relates to a refractive index measurement system based on fiber optic interferometry and a measurement method using the system, for real-time monitoring of the refractive index of transparent gases and liquids. Background Technology

[0002] The refractive index of a medium is related to parameters such as density, temperature, and composition. In some experiments, it is often necessary to dynamically monitor the refractive index to facilitate the analysis and calculation of physical processes. For example, in detonation and shock wave experiments, a schlieren observation system is used to obtain refractive index information images of the flow field; further analysis of the refractive index information images can reveal information such as the position and propagation velocity of the shock wave front.

[0003] Commonly used methods for measuring the refractive index of air include schlieren imaging and vacuum cavity interferometry. Schlieren imaging is generally used to visualize differences in refractive index distribution within an imaging region, but it is difficult to accurately obtain the absolute value of the refractive index. Vacuum cavity interferometry is more complex; when conducting field experiments, the optical path adjustment process is cumbersome, it is easily affected by vibration interference, and its dynamic response speed is relatively low.

[0004] In summary, current traditional methods for measuring the refractive index of air are either difficult to obtain an accurate absolute value of the refractive index, or involve complex procedures, cumbersome optical path adjustments, susceptibility to vibration interference, and low dynamic response speed. Summary of the Invention

[0005] The purpose of this invention is to address the technical problems of existing air refractive index measurement methods, which are either difficult to accurately obtain the absolute measurement value of the refractive index, or have complicated steps, cumbersome optical path adjustment process, are easily affected by vibration interference, and have low dynamic response speed. The invention provides a refractive index measurement system and method based on fiber optic interferometry.

[0006] To solve the above-mentioned technical problems, the technical solution adopted by the present invention is as follows:

[0007] A refractive index measurement system based on fiber optic interferometry is characterized by comprising: a tunable fiber laser, a first fiber coupler connected to the tunable fiber laser, a transmitting probe and an attenuator connected to the first fiber coupler, a receiving probe disposed opposite to the transmitting probe, a second fiber coupler, a photoelectric converter connected to the second fiber coupler, and a recording device connected to the photoelectric converter.

[0008] The tunable fiber laser is used to provide optical signals with periodic frequency variations and interference capabilities.

[0009] The first fiber coupler is used to split the optical signal emitted by the tunable fiber laser into two paths, one of which is input to the transmitting probe and the other is input to the attenuator.

[0010] The input end of the second fiber optic coupler is connected to the output end of the receiving probe and the output end of the attenuator, respectively.

[0011] The receiving probe is used to receive the interference signal containing refractive index information emitted by the transmitting probe and input it into the second fiber coupler;

[0012] The attenuator is used to attenuate the intensity of the optical signal;

[0013] The second fiber coupler is used to couple the interference signal and the attenuated optical signal to output a sensitive signal;

[0014] The photoelectric converter is used to convert the sensitive signal into an electrical signal; the recording device is used to record and store the electrical signal.

[0015] Furthermore, it also includes a bracket for setting up the transmitting and receiving probes.

[0016] Furthermore, the center wavelength of the tunable fiber laser is 1550nm, the sweep period is 5kHz, and the sweep width is 10GHz.

[0017] The distance L between the transmitting probe and the receiving probe is 200mm; the response frequency of the photoelectric converter is DC~200MHz;

[0018] The recording device is a signal recorder.

[0019] Meanwhile, this invention also provides a refractive index measurement method based on fiber optic interferometry, which is characterized by including the following steps:

[0020] 1) Construct the aforementioned refractive index measurement system based on fiber optic interferometry;

[0021] 2) Set the distance between the transmitting probe and the receiving probe, using the transmitting probe and the receiving probe as the measuring arm and the attenuator as the reference arm;

[0022] 3) Turn on the tunable fiber laser and set its scanning cycle;

[0023] 4) Obtain the phase difference change between the measuring arm and the reference arm within the scanning period under initial conditions. and refractive index n 1 ;

[0024] 5) Measure the phase difference change of the interference signal within the test area during the scanning period. Based on the direct proportionality between the phase difference change and the refractive index, the refractive index of the region to be measured is calculated. n 2 .

[0025] Further, step 4) specifically involves:

[0026] 4.1 Record the environmental parameters under the initial conditions, including temperature and pressure, and calculate the refractive index under the initial conditions using the Edlén formula. n 1 ;

[0027] 4.2 Measurement of the phase difference change between the measuring arm and the reference arm under initial conditions during the scanning cycle. :

[0028]

[0029] In the formula: L The distance between the transmitting probe and the receiving probe. λ 1 To measure the phase difference between the measuring arm and the reference arm The corresponding operating wavelength, λ 2 To measure the phase difference between the measuring arm and the reference arm The corresponding operating wavelength.

[0030] Further, step 5) specifically involves:

[0031] 5.1 Measurement of the phase difference change between the measuring arm and the reference arm in the area to be measured during the scanning cycle. ;

[0032] 5.2 From step 4.2 It can be seen that the change in phase difference is directly proportional to the refractive index:

[0033] ;

[0034] 5.3. Based on the proportional relationship between the phase difference change obtained in step 5.2 and the refractive index, determine the refractive index of the interference signal in the region to be measured. n 2 .

[0035] Furthermore, in step 3), the scanning period of the tunable fiber laser is 5 kHz.

[0036] Furthermore, in step 4.1, the initial conditions are normal temperature and pressure.

[0037] Compared with the prior art, the beneficial effects of the technical solution of the present invention are:

[0038] (1) The refractive index measurement system based on fiber optic interferometry of the present invention has a simple structure, low cost, high reliability, and fast response time, which can reach 0.2ms.

[0039] (2) The refractive index measurement method based on fiber optic interference of the present invention forms a double-beam fiber optic interference by establishing a measuring arm and a reference arm. By obtaining the phase difference change in each scanning cycle, the refractive index is calculated. The absolute measurement value of the refractive index of the area to be measured can be accurately obtained and is not easily affected by vibration interference.

[0040] (3) The refractive index measurement method based on optical fiber interference of the present invention can realize static and dynamic measurement of refractive index, and can also complete refractive index monitoring in various scenarios.

[0041] (4) The refractive index measurement method based on optical fiber interference of the present invention has simple measurement steps and convenient optical path adjustment process, which can significantly improve work efficiency. Attached Figure Description

[0042] Figure 1 This is a schematic diagram of the structural principle of the refractive index measurement system based on fiber optic interferometry of the present invention (recording equipment omitted).

[0043] Figure 2 This is a schematic diagram of the output electrical signal of the photoelectric converter under normal temperature and pressure conditions in an embodiment of the refractive index measurement method based on fiber optic interferometry of the present invention.

[0044] Figure 3 This is a schematic diagram of the output electrical signal of the photoelectric converter at 264 kPa in an embodiment of the refractive index measurement method based on fiber optic interferometry of the present invention.

[0045] The attached figures are labeled as follows:

[0046] 1-Tunable fiber laser, 2-First fiber coupler, 3-Transmitting probe, 4-Receiving probe, 5-Attenuator, 6-Second fiber coupler, 7-Photoelectric converter. Detailed Implementation

[0047] like Figure 1 As shown, a refractive index measurement system based on fiber optic interferometry includes a tunable fiber laser 1, a first fiber coupler 2, a transmitting probe 3, a receiving probe 4, an attenuator 5, a second fiber coupler 6, a photoelectric converter 7, and a recording device.

[0048] The input end of the first fiber coupler 2 is connected to the tunable fiber laser 1, and the output end of the first fiber coupler 2 is connected to the transmitting probe 3 and the attenuator 5 respectively; the input end of the receiving probe 4 is set opposite to the output end of the transmitting probe 3; the input end of the second fiber coupler 6 is connected to the output end of the receiving probe 4 and the output end of the attenuator 5 respectively, and the output end of the second fiber coupler 6 is connected to the photoelectric converter 7, which is connected to the recording device.

[0049] A tunable fiber laser 1 is used to provide a periodically varying and interferometric optical signal; a first fiber coupler 2 is used to split the optical signal emitted from the tunable fiber laser 1 into two paths, one of which is input to the transmitting probe 3 and the other is input to the attenuator 5; a receiving probe 4 is used to receive the interference signal containing refractive index information emitted from the transmitting probe 3 and input it to the second fiber coupler 6; the attenuator 5 is used to attenuate the intensity of the optical signal; the second fiber coupler 6 is used to couple the interference signal and the attenuated optical signal and output a sensitive signal; a photoelectric converter 7 is used to convert the sensitive signal into an electrical signal; and a recording device is used to record and store the electrical signal.

[0050] In this embodiment, the optical signal emitted by the tunable fiber laser 1 is input to the measurement arm (i.e., the optical path where the transmitting probe 3 and the receiving probe 4 are located) and the reference arm (i.e., the optical path where the attenuator 5 is located) respectively through the first fiber coupler 2, and finally enters the photoelectric converter 7 through the second fiber coupler 6.

[0051] In this system, the transmitting probe 3 and the receiving probe 4 are used in pairs with a fixed spacing. The area between the transmitting probe 3 and the receiving probe 4 is the sensitive region for refractive index measurement. The tunable fiber laser 1 is a key component of the measurement system. Its scanning frequency period is comparable to the refractive index measurement period, and the width of the scanning frequency is related to the density of the interference fringes output by the system. In this embodiment, the center wavelength of the tunable fiber laser 1 is 1550 nm, the scanning period is 5 kHz, and the scanning width is 10 GHz. The spacing L between the transmitting probe 3 and the receiving probe 4 is 200 mm. The larger the spacing, the greater the phase difference change, corresponding to a denser interference fringes output by the measurement system of this invention. To ensure stable light intensity during the measurement process, a sturdy bracket is used to lock the transmitting probe 3 and the receiving probe 4, and the bracket is adjusted to ensure that the transmitting probe 3 and the receiving probe 4 are basically aligned. To ensure the stability of the measurement system, the response frequency of the photoelectric converter 7 includes the DC band, and the response frequency ranges from DC to 200 MHz. In addition, a signal recorder needs to be configured to record the output signal of the photoelectric converter 7, store the test data, and use external peak data processing software to analyze the data in order to quickly obtain the refractive index to be measured.

[0052] Meanwhile, the present invention also provides a refractive index measurement method based on optical fiber interferometry, comprising the following steps:

[0053] 1) Construct the aforementioned refractive index measurement system based on fiber optic interferometry;

[0054] 2) Set the distance between the transmitting probe 3 and the receiving probe 4, and use the optical path where the transmitting probe 3 and the receiving probe 4 are located as the measuring arm, and the optical path where the attenuator 5 is located as the reference arm.

[0055] 3) Turn on the tunable fiber laser 1 and set the scanning period of the tunable fiber laser 1.

[0056] 4) Obtain the phase difference change between the measuring arm and the reference arm within the scanning period under initial conditions. and refractive index n 1 In this embodiment, the scanning period is set to 1ms; the initial conditions are room temperature and pressure.

[0057] 4.1 Record the environmental parameters under the initial conditions, including temperature and pressure, and calculate the refractive index under the initial conditions using the Edlén formula. n 1 ;

[0058] 4.2 Measurement of the phase difference change between the measuring arm and the reference arm under initial conditions during the scanning cycle. :

[0059] (1)

[0060] (2)

[0061] (3)

[0062] From equations (1) and (2), we can obtain equation (3), where: L The distance between the transmitting probe and the receiving probe. λ 1 To measure the phase difference between the measuring arm and the reference arm The corresponding operating wavelength, λ 2 To measure the phase difference between the measuring arm and the reference arm The corresponding operating wavelength.

[0063] 5) Measure the phase difference change of the interference signal within the test area during the scanning period. Based on the direct proportionality between the phase difference change and the refractive index, the refractive index of the region to be measured is calculated. n 2 .

[0064] 5.1 Measure the change in phase difference between the measuring arm and the reference arm within the area to be measured during the scanning period. From equation (3), we can obtain

[0065] (4)

[0066] 5.2 According to formulas (3) and (4), it can be seen that within the same period, the change in phase difference is directly proportional to the refractive index:

[0067] (5)

[0068] 5.3. Use equation (5) to determine the refractive index of the interference signal in the region to be measured. n 2 ;

[0069] Since the phase difference of each scanning cycle can be obtained from the interference waveform of the output optical signal of the tunable fiber laser 1 in the measurement system of the present invention, the relative change value of refractive index in any scanning cycle can be obtained according to the direct proportional relationship of formula (5), and the refractive index measurement is completed.

[0070] In this embodiment, the scanning period of the output frequency of the tunable fiber laser 1 is set to vary in a sinusoidal manner, with a sweep period of 5 kHz. Under normal temperature and pressure (100 kPa) conditions, the output electrical signal of the photoelectric converter 7 is obtained, see [link to documentation]. Figure 2 Under a pressure of 264 kPa, the electrical signal output by photoelectric converter 7 is obtained, see [reference]. Figure 3 In comparison, Figure 3 The stripes of the electrical signal shown are denser, corresponding to a larger refractive index value.

Claims

1. A refractive index measurement method based on fiber optic interferometry, characterized in that, Includes the following steps: 1) Construct a refractive index measurement system based on fiber interferometry; the system includes a tunable fiber laser (1), a first fiber coupler (2) connected to the tunable fiber laser (1), a transmitting probe (3) and an attenuator (5) connected to the first fiber coupler (2), a receiving probe (4) arranged opposite to the transmitting probe (3), a second fiber coupler (6), a photoelectric converter (7) connected to the second fiber coupler (6), and a recording device connected to the photoelectric converter (7); The tunable fiber laser (1) is used to provide a frequency-period-varying and interferometric optical signal; The first fiber coupler (2) is used to split the optical signal emitted by the tunable fiber laser (1) into two paths, one of which is input to the transmitting probe (3) and the other is input to the attenuator (5). The input end of the second fiber optic coupler (6) is connected to the output end of the receiving probe (4) and the output end of the attenuator (5), respectively; The receiving probe (4) is used to receive the interference signal containing refractive index information emitted by the transmitting probe (3) and input it into the second fiber coupler (6). The attenuator (5) is used to attenuate the light intensity of the optical signal; The second fiber coupler (6) is used to couple the interference signal and the attenuated optical signal to output a sensitive signal; The photoelectric converter (7) is used to convert the sensitive signal into an electrical signal; the recording device is used to record and store the electrical signal; 2) Set the distance between the transmitting probe (3) and the receiving probe (4), and use the transmitting probe (3) and the receiving probe (4) as the measuring arm, and the attenuator (5) as the reference arm; 3) Turn on the tunable fiber laser (1) and set the scanning period of the tunable fiber laser (1); 4) Obtain the phase difference change between the measuring arm and the reference arm within the scanning period under initial conditions. and refractive index n 1 ; 5) Measure the phase difference change of the interference signal within the test area during the scanning period. Based on the direct proportionality between the phase difference change and the refractive index, the refractive index of the region to be measured is calculated. n 2 .

2. The refractive index measurement method based on fiber optic interferometry according to claim 1, characterized in that: In step 1, the refractive index measurement system based on fiber optic interferometry also includes a bracket for setting up the transmitting probe (3) and the receiving probe (4).

3. The refractive index measurement method based on fiber optic interferometry according to claim 2, characterized in that: In step 1, the center wavelength of the tunable fiber laser (1) is 1550nm, the sweep period is 5kHz, and the sweep width is 10GHz. The distance L between the transmitting probe (3) and the receiving probe (4) is 200mm; the response frequency of the photoelectric converter (7) is DC~200MHz; The recording device is a signal recorder.

4. The refractive index measurement method based on fiber optic interferometry according to claim 1, characterized in that, Step 4) specifically involves: 4.1 Record the environmental parameters under the initial conditions, including temperature and pressure, and calculate the refractive index under the initial conditions using the Edlén formula. n 1 ; 4.2 Measurement of the phase difference change between the measuring arm and the reference arm under initial conditions during the scanning cycle. : In the formula: L The distance between the transmitting probe and the receiving probe. λ 1 To measure the phase difference between the measuring arm and the reference arm The corresponding operating wavelength, λ 2 To measure the phase difference between the measuring arm and the reference arm The corresponding operating wavelength.

5. The refractive index measurement method based on fiber optic interferometry according to claim 1, characterized in that, Step 5) specifically involves: 5.1 Measurement of the phase difference change between the measuring arm and the reference arm in the area to be measured during the scanning cycle. ; 5.2 From step 4.2 It can be seen that the change in phase difference is directly proportional to the refractive index: ； 5.

3. Based on the proportional relationship between the phase difference change obtained in step 5.2 and the refractive index, determine the refractive index of the interference signal in the region to be measured. n 2 .

6. The refractive index measurement method based on fiber optic interferometry according to claim 5, characterized in that: In step 3), the scanning period of the tunable fiber laser (1) is 5 kHz.

7. The refractive index measurement method based on fiber optic interferometry according to claim 6, characterized in that: In step 4.1, the initial conditions are normal temperature and pressure.

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