In-situ measurement device and method for fiber nonlinear coefficient and nonlinear refractive index coefficient

Abstract

This invention belongs to the field of optical parameter measurement technology, and specifically discloses an in-situ measurement device and method for the nonlinear coefficient and nonlinear refractive index coefficient of optical fibers. The device includes a tunable laser module for outputting laser with adjustable power, amplitude modulation, and frequency modulation depth; a beam splitter for splitting the laser into two paths, one leading to a power meter and the other to a spectrometer; and a processor for receiving measurement results and acquiring the measured values ​​of the nonlinear coefficient and nonlinear refractive index coefficient of the optical fiber when the tunable laser module compresses the laser spectrum to its narrowest point. The method includes: instrument calibration to obtain the frequency modulation depth when the spectrum is compressed to its narrowest point; measurement: connecting the optical fiber under test, adjusting the tunable laser module until the spectrum is compressed to its narrowest point, and calculating the nonlinear coefficient and nonlinear refractive index coefficient based on the measured parameters. This invention enables in-situ measurement of the nonlinear coefficient and nonlinear refractive index coefficient of optical fibers.

Description

Technical Field

[0001] This invention belongs to the field of optical parameter measurement technology, and more specifically, relates to an in-situ measurement device and method for fiber nonlinear coefficient and nonlinear refractive index coefficient. Background Technology

[0002] When high-intensity light pulses propagate through a medium, the refractive index of the medium changes with the light intensity due to nonlinear effects. With the development of high-power pulsed fiber lasers, the nonlinear refractive index effect in optical fibers has attracted increasing attention and become the basis for many fiber optic sensors. Currently, the most widely used method for measuring the nonlinear refractive index coefficient of a medium is the Z-scan method. However, the Z-scan method is only suitable for thin media; for thicker media, data analysis is often very difficult, and the results contain significant errors. Therefore, the Z-scan method cannot perform in-situ measurements of the nonlinear refractive index and nonlinear coefficient of optical fibers in a slender state. Especially for new optical fibers such as photonic crystal fibers, their nonlinear refractive index and nonlinear coefficient are largely affected by the internal structure of the fiber and cannot be accurately described using the nonlinear refractive index coefficient of the medium.

[0003] Given the aforementioned shortcomings and deficiencies, in-situ measurement of the nonlinear coefficient and nonlinear refractive index of optical fibers is of paramount importance. Existing measurement devices based on Mach-Zehnder interferometers are generally difficult to manufacture, structurally complex, and challenging to measure the nonlinear coefficient and nonlinear refractive index at high power. Summary of the Invention

[0004] To address the aforementioned deficiencies or improvement needs of existing technologies, this invention provides an in-situ measurement device and method for the nonlinear coefficient and nonlinear refractive index coefficient of optical fibers. By splitting the laser beam and sending it separately to a power meter and a spectrometer, and adjusting the frequency modulation depth of the tunable laser module to compress the spectrum to its narrowest point, the nonlinear coefficient and nonlinear refractive index coefficient of the optical fiber are calculated by a processor. This method features a simple structure, convenient operation, and high-precision measurement of the nonlinear coefficient and nonlinear refractive index under high power conditions. It can achieve in-situ measurement of the nonlinear coefficient and nonlinear refractive index coefficient of optical fibers under different laser powers, wavelengths, polarizations, etc., and is applicable to special optical fibers such as photonic crystal fibers.

[0005] To achieve the above objectives, according to one aspect of the present invention, an in-situ measurement device for fiber optic nonlinear coefficient and nonlinear refractive index coefficient is provided, comprising:

[0006] Tunable laser module for lasers with adjustable output power, amplitude modulation, and frequency modulation depth;

[0007] A beam splitter is used to split the laser into two paths, one of which goes to a power meter and the other to a spectrometer.

[0008] The processor is used to receive the measurement results from the power meter and the spectrometer, and to acquire the measured values ​​of the nonlinear coefficient and nonlinear refractive index coefficient of the optical fiber when the tunable laser module compresses the laser spectrum to its narrowest point.

[0009] As a further preferred embodiment, the tunable laser module includes a laser, an electro-optic crystal, a radio frequency source, and an attenuation amplifier connected in sequence. The radio frequency source performs synchronous amplitude modulation and frequency modulation on the laser output by the laser through the electro-optic crystal, and then achieves independent adjustment of the amplitude modulation and frequency modulation depth through the attenuation amplifier. The output power, intensity, and frequency modulation depth data of the laser are transmitted to the processor in real time and displayed on the display.

[0010] As a further preferred embodiment, the power meter generates a power signal by measuring the reflected light from the beam splitter and transmits the power signal to the processor, which calculates the power value at the output end of the optical fiber under test based on the power signal.

[0011] As a further preferred embodiment, the spectrometer measures the spectrum of light transmitted through the beam splitter and transmits the spectrum to the processor for display.

[0012] As a further preferred embodiment, the processor calculates the nonlinear coefficient of the optical fiber under test according to the following formula:

[0013]

[0014] Wherein, P1 is the measured power of the power meter when the spectrum is compressed to its narrowest point without the fiber under test connected, and β1 is the corresponding frequency modulation depth; P2 is the measured power of the power meter when the spectrum is compressed to its narrowest point after the fiber under test is connected, and β2 is the corresponding frequency modulation depth; m is the amplitude modulation depth of the tunable laser module, and L is the length of the fiber under test.

[0015] As a further preferred embodiment, the processor calculates the nonlinear refractive index coefficient of the optical fiber according to the following formula:

[0016]

[0017] Among them, A eff λ represents the effective mode field area of ​​the optical fiber under test, and λ is the laser wavelength.

[0018] According to another aspect of the present invention, an in-situ measurement method for fiber nonlinearity coefficient and nonlinear refractive index coefficient is also provided, comprising the following steps:

[0019] The laser output from the tunable laser module is split into two paths, one of which goes to the power meter and the other goes to the spectrometer.

[0020] With the amplitude modulation depth of the tunable laser module fixed at m, and without connecting the optical fiber to be tested, the frequency modulation depth of the tunable laser module is gradually adjusted so that the spectrum observed by the display is compressed to the narrowest point. The frequency modulation depth β1 and the power meter power measurement value P1 are recorded at this time.

[0021] After connecting the fiber under test, the frequency modulation depth of the tunable laser module is gradually adjusted again to compress the spectrum observed by the display to the narrowest point. The frequency modulation depth β2 and the power meter power measurement value P2 are recorded at this time.

[0022] The processor calculates the nonlinear coefficient and nonlinear refractive index coefficient of the optical fiber under test.

[0023] As a further preferred embodiment, the processor calculates the nonlinear coefficient of the optical fiber under test according to the following formula:

[0024]

[0025] Wherein, P1 is the measured power of the power meter when the spectrum is compressed to its narrowest point without the fiber under test connected, and β1 is the corresponding frequency modulation depth; P2 is the measured power of the power meter when the spectrum is compressed to its narrowest point after the fiber under test is connected, and β2 is the corresponding frequency modulation depth; m is the amplitude modulation depth of the tunable laser module, and L is the length of the fiber under test.

[0026] As a further preferred embodiment, the processor calculates the nonlinear refractive index coefficient of the optical fiber according to the following formula:

[0027]

[0028] Among them, A eff λ represents the effective mode field area of ​​the optical fiber under test, and λ is the laser wavelength.

[0029] As a further preferred embodiment, the power meter generates a power signal by measuring the reflected light from the beam splitter and transmits the power signal to the processor, which calculates the power value at the output end of the optical fiber under test based on the power signal.

[0030] The spectrometer measures the spectrum of light transmitted through the beam splitter and transmits the spectrum to the processor, where it is displayed.

[0031] In summary, compared with the prior art, the above-described technical solutions conceived by this invention mainly possess the following technical advantages:

[0032] 1. This invention enables in-situ measurement of the nonlinear coefficient and nonlinear refractive index coefficient of optical fibers under different laser power, laser wavelength, polarization, and other conditions, and is applicable to special optical fibers such as photonic crystal fibers.

[0033] 2. This invention splits the laser beam and sends it to a power meter and a spectrometer separately. By adjusting the frequency modulation depth of the tunable laser module, the spectrum is compressed to the narrowest point. The nonlinear coefficient and nonlinear refractive index coefficient of the optical fiber are calculated by the processor. It features a simple structure, convenient operation, and high-precision measurement of the nonlinear coefficient and nonlinear refractive index under high power.

[0034] 3. This invention has a simple structure and is easy to operate. Compared with Z-scan and other methods for measuring nonlinear refractive index, it can achieve in-situ measurement of the nonlinear coefficient and nonlinear refractive index coefficient of optical fibers under different laser powers, wavelengths, and polarization conditions, and is applicable to special optical fibers such as photonic crystal fibers. The device and method of this invention differ from measurement methods based on Mach-Zehnder interferometers, enabling accurate measurement of the nonlinear coefficient and nonlinear refractive index coefficient of optical fibers at extremely high power. Therefore, this device and method have broader application prospects in the field of high-power lasers.

[0035] 4. This invention provides two methods for determining the narrowest spectrum. The first method directly observes the change in spectrum with frequency modulation depth using a spectrometer in real time, suitable for roughly determining the frequency modulation depth range corresponding to the narrowest spectrum, and is efficient and fast. The second method determines the narrowest spectrum by finding the maximum value of the normalized center power, which avoids errors caused by human factors and has higher measurement accuracy. Attached Figure Description

[0036] Figure 1 This is a schematic diagram of the structure of an in-situ measurement device for fiber nonlinear coefficient and nonlinear refractive index coefficient according to a preferred embodiment of the present invention;

[0037] Figure 2 These are schematic diagrams of three spectra involved in an in-situ measurement method for the nonlinear coefficient and nonlinear refractive index coefficient of an optical fiber according to a preferred embodiment of the present invention.

[0038] Figure 3 This is a schematic diagram illustrating the determination of the narrowest spectral β by the normalized center power in an in-situ measurement method for the nonlinear coefficient and nonlinear refractive index coefficient of an optical fiber, according to a preferred embodiment of the present invention.

[0039] In all the accompanying drawings, the same reference numerals denote the same technical features, specifically: 1-tunable laser module, 2-fiber under test, 3-beam splitter, 4-power meter, 5-spectrometer, 6-processor, 7-display. Detailed Implementation

[0040] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the invention. Furthermore, the technical features involved in the various embodiments of this invention described below can be combined with each other as long as they do not conflict with each other.

[0041] like Figure 1 As shown in the illustration, this invention provides an in-situ measurement device for the nonlinear coefficient and nonlinear refractive index coefficient of an optical fiber. The device includes a tunable laser module, an optical fiber under test, a beam splitter, a power meter, a spectrometer, a processor, and a display. The tunable laser module includes a laser, an electro-optic crystal, a radio frequency (RF) source, and an attenuation amplifier. The RF source synchronously modulates the amplitude and frequency of the output laser light through the electro-optic crystal, and the attenuation amplifier enables independent adjustment of the amplitude modulation and frequency modulation depths. The output power, intensity, and frequency modulation depth data of the laser module are transmitted to the processor in real time and displayed on the display. More specifically, in the above embodiment, the device requires initial calibration. Specifically, the optical fiber under test is not connected between the tunable laser module and the beam splitter. The amplitude modulation depth of the tunable laser module is fixed at m, and the frequency modulation depth is gradually adjusted to compress the spectrum observed on the display to its narrowest point. The frequency modulation depth β1 and the power meter power measurement value P1 are recorded at this point. After obtaining the calibration values, the fiber under test is connected between the tunable laser module and the beam splitter. The frequency modulation depth of the tunable laser module is then gradually adjusted to compress the spectrum observed on the display to its narrowest point. The frequency modulation depth β2 and the power meter power measurement value P2 are recorded at this point. The processor calculates the nonlinear coefficient and nonlinear refractive index coefficient of the fiber under test based on the calibration and measurement data.

[0042] In the above embodiments, the tunable laser module is used to provide laser output with adjustable power, amplitude modulation, and frequency modulation depth.

[0043] In the above embodiments, the laser output of the tunable laser module is transmitted through the optical fiber under test and then split by a beam splitter to reach the power meter and the spectrometer, respectively.

[0044] In the above embodiments, the power meter generates a power signal by measuring the reflected light from the beam splitter and transmits the power signal to the processor. The processor calculates the power value at the output end of the optical fiber under test based on the power signal. The spectrometer measures the spectrum of the transmitted light from the beam splitter and transmits the spectrum to the processor, where it is displayed on the display.

[0045] In one embodiment of the present invention, by adjusting the frequency modulation depth of the tunable laser module, the change in the spectrum of the display is observed until the spectrum is compressed to its narrowest point. At this point, the result displayed on the display is the measured value of the nonlinear coefficient and nonlinear refractive index coefficient of the optical fiber.

[0046] Of course, in another embodiment of the present invention, it can also be determined by judging the normalized center power corresponding to the spectrum, that is, the spectrum is narrowest when the normalized center power reaches its maximum value, wherein the normalized center power is defined as:

[0047]

[0048] In the formula, A(ω) is the spectral amplitude, and w is the half-width of the spectral center power window.

[0049] In the above embodiments, the processor calculates the nonlinear coefficient of the optical fiber under test according to the following formula:

[0050]

[0051] Wherein, P1 is the measured power of the power meter when the spectrum is compressed to its narrowest point without the fiber under test connected, and β1 is the corresponding frequency modulation depth; P2 is the measured power of the power meter when the spectrum is compressed to its narrowest point after the fiber under test is connected, and β2 is the corresponding frequency modulation depth; m is the amplitude modulation depth of the tunable laser module, and L is the length of the fiber under test.

[0052] In the above embodiments, the processor calculates the nonlinear refractive index coefficient of the optical fiber according to the following formula:

[0053]

[0054] Among them, A eff λ represents the effective mode field area of ​​the optical fiber under test, and λ is the laser wavelength.

[0055] Of course, the present invention also provides an in-situ measurement method for the nonlinear coefficient and nonlinear refractive index coefficient of optical fiber, comprising the following steps:

[0056] Step 1, Instrument Connection: Split the laser output from the adjustable laser module into two paths, one of which goes to the power meter and the other to the spectrometer.

[0057] Step 2, Instrument Calibration: Fix the amplitude modulation depth of the tunable laser module 1 to m. Without connecting the fiber under test, gradually adjust the frequency modulation depth of the tunable laser module 1 so that the spectrum observed by the display 7 is compressed to the narrowest. Record the frequency modulation depth β1 at this time, as well as the power measurement value P1 of the power meter.

[0058] Step 3, Measurement: Connect the fiber under test 2 to the output end of the tunable laser module 1. Gradually adjust the frequency modulation depth of the tunable laser module 1 again until the spectrum observed on the display 7 is compressed to its narrowest point. Record the frequency modulation depth β2 at this point, and the power measurement value P2 from the power meter. Calculate the nonlinear coefficient of the fiber under test using the following formula:

[0059]

[0060] Where L is the length of the fiber under test. The above calculation formula takes into account the fiber loss. By adjusting the power of the tunable laser module, the nonlinear coefficient of the fiber under test 2 can be measured at different powers. The nonlinear refractive index coefficient of the fiber is calculated by the following formula:

[0061]

[0062] Among them, A eff λ represents the effective mode field area of ​​the optical fiber under test, and λ is the laser wavelength.

[0063] This invention achieves the narrowest possible spectral compression by adjusting the frequency modulation depth of the tunable laser module, allowing the processor to calculate the nonlinear coefficient and nonlinear refractive index coefficient of the optical fiber. The device is simple in structure and easy to operate. This measurement method can achieve in-situ measurement of the nonlinear coefficient and nonlinear refractive index coefficient of optical fibers under different laser powers, wavelengths, and polarization conditions, and is applicable to special optical fibers such as photonic crystal fibers.

[0064] Example 1

[0065] This invention provides an in-situ measurement device for the nonlinear coefficient and nonlinear refractive index of optical fibers. Figure 1 A schematic diagram of an in-situ measurement device for the nonlinear coefficient and nonlinear refractive index of an optical fiber is shown. It includes: an adjustable laser module 1, an optical fiber under test 2, a beam splitter 3, a power meter 4, a spectrometer 5, a processor 6, and a display 7.

[0066] The tunable laser module 1 consists of a laser, an electro-optic crystal, an RF source, and an attenuator amplifier. The RF source synchronously modulates the amplitude and frequency of the output laser through the electro-optic crystal, and the attenuator amplifier enables independent adjustment of the amplitude modulation and frequency modulation depth. The output power, intensity, and frequency modulation depth data of the laser module are transmitted to the processor 6 in real time and displayed on the display 7.

[0067] The tunable laser module 1 provides laser output with adjustable power, amplitude modulation, and frequency modulation depth. The laser output passes through the fiber under test 2 and then through the beam splitter 3 to the power meter 4 and the spectrometer 5, respectively.

[0068] The power meter 4 generates a power signal by measuring the reflected light from the beam splitter 3 and transmits the power signal to the processor 6. The processor 6 calculates the power value at the output end of the optical fiber under test based on the power signal.

[0069] The spectrometer 5 measures the spectrum of the light transmitted through the beam splitter 3 and transmits the spectrum to the processor 6, which is then displayed on the monitor 7.

[0070] Processor 6 calculates the nonlinear coefficient and nonlinear refractive index coefficient of the optical fiber based on the obtained data, and transmits the results to display 7 for display.

[0071] This invention provides an in-situ measurement method for the nonlinear coefficient and nonlinear refractive index of optical fibers, the process of which consists of two steps:

[0072] Step 1, Instrument Calibration: Fix the amplitude modulation depth of the tunable laser module 1 to m. Without connecting the fiber under test, gradually adjust the frequency modulation depth of the tunable laser module 1 so that the spectrum observed on the display 7 is compressed to the narrowest. Record the frequency modulation depth β1 at this time, as well as the power measurement value P1 of the power meter.

[0073] Step 2, Measurement: Connect the fiber under test 2 to the output of the tunable laser module 1. Gradually adjust the frequency modulation depth of the tunable laser module 1 again until the spectrum observed on the display 7 is compressed to its narrowest point. Record the frequency modulation depth β2 at this point, and the power measurement value P2 from the power meter. Calculate the nonlinear coefficient of the fiber under test using the following formula:

[0074]

[0075] Where L is the length of the fiber under test. The above calculation formula takes into account the fiber loss. By adjusting the power of the tunable laser module, the nonlinear coefficient of the fiber under test 2 can be measured at different powers. The nonlinear refractive index coefficient of the fiber is calculated by the following formula:

[0076]

[0077] Among them, A eff λ represents the effective mode field area of ​​the optical fiber under test, and λ is the laser wavelength.

[0078] This invention also provides two methods for determining the narrowest spectrum. The first method is as follows: Figure 2 As shown, the narrowest spectrum is determined directly by the spectral shape, corresponding to β = 1.25. The second method is as follows... Figure 3 As shown, the spectrum is narrowest when the normalized center power reaches its maximum value. The normalized center power is defined as:

[0079]

[0080] A(ω) is the spectral amplitude, and w is the half-width of the spectral center power window.

[0081] Those skilled in the art will readily understand that the above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. An apparatus for in-situ measurement of fiber nonlinear coefficient and nonlinear refractive index coefficient, characterized in that, include: Tunable laser module for lasers with adjustable output power, amplitude modulation, and frequency modulation depth; A beam splitter is used to split the laser into two paths, one of which goes to a power meter and the other to a spectrometer. The optical fiber under test can be selectively connected in the optical path between the tunable laser module and the beam splitter; The amplitude modulation depth of the tunable laser module is fixed at m The frequency modulation depth of the tunable laser module is adjusted stepwise while the fiber under test is not connected, so that the spectrum observed by the display is compressed to the narrowest, and the frequency modulation depth at this time is recorded And the power measurement value of the power meter P 1; After connecting the fiber under test, the frequency modulation depth of the tunable laser module is gradually adjusted again to compress the spectrum observed by the display to its narrowest point, and the frequency modulation depth at this point is recorded. and power meter power measurement value P 2; The processor is used to receive the measurement results from the power meter and the spectrometer, and to acquire the measured values ​​of the nonlinear coefficient and nonlinear refractive index coefficient of the optical fiber under test when the tunable laser module compresses the laser spectrum to its narrowest point.

2. The in-situ measurement device for fiber optic nonlinear coefficient and nonlinear refractive index coefficient according to claim 1, characterized in that, The tunable laser module includes a laser, an electro-optic crystal, a radio frequency source, and an attenuation amplifier connected in sequence. The radio frequency source synchronously modulates the amplitude and frequency of the laser output by the laser through the electro-optic crystal. Then, the attenuation amplifier enables independent adjustment of the amplitude modulation and frequency modulation depth. The output power, intensity, and frequency modulation depth data of the laser are transmitted to the processor in real time and displayed on the display.

3. The in-situ measurement device for fiber optic nonlinear coefficient and nonlinear refractive index coefficient according to claim 1, characterized in that, The power meter generates a power signal by measuring the reflected light from the beam splitter and transmits the power signal to the processor, which calculates the power value at the output end of the optical fiber under test based on the power signal.

4. The in-situ measurement device for fiber optic nonlinear coefficient and nonlinear refractive index coefficient according to claim 2, characterized in that, The spectrometer measures the spectrum of light transmitted through the beam splitter and transmits the spectrum to the processor, where it is displayed.

5. The in-situ measurement device for fiber optic nonlinear coefficient and nonlinear refractive index coefficient according to any one of claims 1-4, characterized in that, The processor calculates the nonlinear coefficient of the optical fiber under test according to the following formula: in, P 1 represents the measured power of the power meter when the spectrum is compressed to its narrowest point without the fiber under test being connected. This corresponds to the frequency modulation depth; P 2 represents the measured power of the power meter when the spectrum is compressed to its narrowest point after the fiber under test is connected. This corresponds to the frequency modulation depth; m The amplitude modulation depth of the tunable laser module. L The length of the optical fiber to be measured is denoted as .

6. The in-situ measurement device for fiber optic nonlinear coefficient and nonlinear refractive index coefficient according to any one of claims 1-4, characterized in that, The processor calculates the nonlinear refractive index coefficient of the optical fiber according to the following formula: in, The effective mode area of ​​the optical fiber under test. is the laser wavelength.

7. A method for in-situ measurement of the nonlinear coefficient and nonlinear refractive index coefficient of an optical fiber, implemented using the in-situ measurement device for the nonlinear coefficient and nonlinear refractive index coefficient of an optical fiber as described in any one of claims 1-6, characterized in that, Includes the following steps: The laser output from the tunable laser module is split into two paths, one of which goes to the power meter and the other goes to the spectrometer. The amplitude modulation depth of the tunable laser module is fixed at 1. m Without connecting the fiber under test, the frequency modulation depth of the tunable laser module is gradually adjusted until the spectrum observed on the display is compressed to its narrowest point, and the frequency modulation depth at this point is recorded. and power meter power measurement value P 1; After connecting the fiber under test, the frequency modulation depth of the tunable laser module is gradually adjusted again to compress the spectrum observed by the display to its narrowest point, and the frequency modulation depth at this point is recorded. and power meter power measurement value P 2; The processor calculates the nonlinear coefficient and nonlinear refractive index coefficient of the optical fiber under test.

8. The in-situ measurement method for the nonlinear coefficient and nonlinear refractive index coefficient of optical fiber according to claim 7, characterized in that, The processor calculates the nonlinear coefficient of the optical fiber under test according to the following formula: in, P 1 represents the measured power of the power meter when the spectrum is compressed to its narrowest point without the fiber under test being connected. This corresponds to the frequency modulation depth; P 2 represents the measured power of the power meter when the spectrum is compressed to its narrowest point after the fiber under test is connected. This corresponds to the frequency modulation depth; m The amplitude modulation depth of the tunable laser module. L The length of the optical fiber to be measured is denoted as .

9. The in-situ measurement method for the nonlinear coefficient and nonlinear refractive index coefficient of optical fiber according to claim 7, characterized in that, The processor calculates the nonlinear refractive index coefficient of the optical fiber according to the following formula: in, The effective mode area of ​​the optical fiber under test. is the laser wavelength.

10. The in-situ measurement method for the nonlinear coefficient and nonlinear refractive index coefficient of an optical fiber according to claim 7, characterized in that, The power meter generates a power signal by measuring the reflected light from the beam splitter and transmits the power signal to the processor. The processor calculates the power value at the output end of the optical fiber under test based on the power signal. The spectrometer measures the spectrum of light transmitted through the beam splitter and transmits the spectrum to the processor, where it is displayed.

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