Improved device for measuring transmission mode of hollow-core anti-resonance optical fiber

Abstract

The invention relates to the technical field of laser photoelectrons, in particular to an improved device for measuring a transmission mode of a hollow-core anti-resonance optical fiber. Comprising a tunable laser, an output tail fiber, an optical fiber to be measured, a lens group, a CCD camera and a computer, the output tail fiber is used for coupling laser; the optical fiber to be measured is used for receiving the coupled laser; the lens group is used for amplifying the coupled laser; the CCD camera is used for imaging the amplified and coupled laser; the tunable laser is used for tuning at a set frequency interval, and the CCD camera records a light spot mode pattern under each frequency; and the computer is used for collecting and processing the light spot mode pattern under each frequency, separating various modes existing in the optical fiber to be detected, and quantitatively representing the relative content of each mode. The method is rapid in measurement, simple in operation and accurate in measurement, and the number and types of modes and the relative power level of each mode and the fundamental mode are determined by obtaining high-quality images of different frequencies in a certain bandwidth.

Description

Technical Field

[0001] The invention relates to the field of laser optoelectronic technology, in particular to an improved device for measuring hollow-core anti-resonance optical fiber transmission modes. Background Art

[0002] For the accurate measurement of the modes contained in the optical fiber, the method in the prior art combines a broadband light source and a spectrometer. The light emitted from the broadband light source is coupled into the optical fiber to be measured. The light coming out of the optical fiber to be measured is amplified by a lens group and then incident on the detection surface. There is a probe optical fiber on the detection surface. The probe optical fiber is fixed on a two-dimensional translation stage. The translation stage is controlled by a computer to scan the light spot in the detection surface point by point, measure the spectral information of the light spot point by point, and then perform Fourier transformation on the spectrum and superimpose it, so as to measure the mode dispersion, mode components and relative content of each mode in the optical fiber.

[0003] The measurement method in the prior art is complicated to operate, and requires accurate positioning of the relative position between the light spot output by the optical fiber to be measured and the probe optical fiber, and requires point-by-point acquisition, which takes a long time to measure. In addition, this method requires the use of the probe optical fiber for two-dimensional scanning, which will introduce new errors and affect the accuracy of the measurement results. Therefore, a device with fast measurement, simple operation and accurate measurement is needed to replace this measurement method. Summary of the invention

[0004] In view of the above-mentioned shortcomings of the prior art, an object of the present invention is to provide an improved device for measuring the transmission mode of hollow-core anti-resonance optical fiber, which has the advantages of fast measurement, simple operation and accurate measurement.

[0005] To achieve the above-mentioned and other related purposes, the present invention provides an improved device for measuring the transmission mode of hollow-core anti-resonance optical fiber, comprising:

[0006] A tunable laser for emitting laser light;

[0007] An output fiber pigtail, used for coupling the laser;

[0008] The optical fiber to be tested is used to receive the coupled laser;

[0009] A lens group, used for amplifying the coupled laser;

[0010] A CCD camera is used to image the laser after amplification and coupling;

[0011] The tunable laser is used to tune at a set frequency interval, and the CCD camera records the light spot pattern at each frequency;

[0012] The computer is used to collect and process the light spot pattern diagram at each frequency to separate various modes existing in the optical fiber to be tested so as to quantitatively characterize the relative content of each mode.

[0013] In one embodiment of the present invention, the tunable laser, the output pigtail, the optical fiber to be tested, the lens group, and the CCD camera are placed on the same optical axis in sequence.

[0014] In one embodiment of the present invention, the tunable laser and the CCD camera are both connected to a computer, and the computer is also used to adjust the output wavelength of the tunable laser and to perform data processing on images of different wavelengths collected by the CCD camera.

[0015] In one embodiment of the present invention, the tunable range of the wavelength of the tunable laser output is 1550nm-1554nm.

[0016] In one embodiment of the present invention, the output pigtail is a single-mode 28 optical fiber.

[0017] In one embodiment of the present invention, the optical fiber to be tested is placed on a three-dimensional translation platform for three-dimensionally adjusting the optical fiber to be tested.

[0018] In one embodiment of the present invention, the CCD camera is used to collect near-field magnified images in the 1550nm-1554nm band.

[0019] The present invention also provides a method for measuring an improved device for measuring a hollow-core anti-resonance optical fiber transmission mode, comprising the above-mentioned improved device for measuring a hollow-core anti-resonance optical fiber transmission mode, wherein the method for measuring the improved device for measuring a hollow-core anti-resonance optical fiber transmission mode comprises:

[0020] The tunable laser is combined with the CCD camera, and the mode interference generated by the high-order mode and the fundamental mode in the optical fiber during the transmission process is utilized. The spectrum interference is solved spatially to measure the optical fiber mode. The basic principle is as follows:

[0021] The electric field distribution at the optical fiber incident end face z = 0 is:

[0022]

[0023] Among them, A 01 and A 11 are the amplitudes of the two modes, and are the initial phases of the two modes respectively;

[0024] After the mode in the optical fiber propagates a distance L, the electric field distribution at the output end face (Z = L) is:

[0025]

[0026] According to the electric field intensity at the exit end face, the light intensity at the exit end face is calculated as:

[0027]

[0028] From formula (3), we can see that the light spot at the optical fiber output end face is decomposed into countless points. Each point (x, y) contains the interference information of two modes. The interference is related to the frequency of the light wave. The Taylor expansion of the propagation constant β with respect to the frequency ω is:

[0029]

[0030] According to the definition of mode group delay in formula (1), substituting formula (4) into it, we can obtain:

[0031]

[0032] In formula (5), the first term of the expression on the far right of the equation is related to the mode dispersion, which is the mode group delay at the angular frequency ω0. The second term is related to the material dispersion and waveguide dispersion. When material dispersion and waveguide dispersion are ignored, substitute formula (5) into formula (4) and then substitute it into formula (3) to obtain:

[0033]

[0034] In formula (6), Δω=ω-ω0, τ 11 is the modal group delay of the LP11 mode, τ 01 is the mode group delay of LP01, and equation (6) is further simplified into the form of cosine function:

[0035]

[0036] In formula (7), is a constant (fixed phase difference), ΔT 11 is the delay caused by the two modes transmitting along the optical fiber for a distance of L, ΔT 11 =(τ 11 -τ 01 )L;

[0037] From formula (7), we can see that the light intensity at any point (x, y) on the optical fiber output end face is related to the frequency;

[0038] The expression of interference light intensity supporting multiple high-order mode transmission is:

[0039]

[0040] Where m and n represent the mode numbers of the higher-order modes. By Fourier transforming formula (7) with respect to the angular frequency ω, we can obtain the group delay difference between the higher-order mode and the fundamental mode, which is ΔT after passing through the optical fiber with a length of L. mn ;

[0041] The spatial correlation between different modes enables multiple modes with different group delay differences propagating in the optical fiber to interfere with each other and form images simultaneously. The interference pattern of two modes depends on the MPI value between the two modes, which reflects the relative power level between the two modes.

[0042]

[0043] The MPI value between two interference modes is the relative power level between the two modes. The theoretical derivation process of the calculation assumes that the amplitude of light of different modes in the detection plane is related to the spatial position and frequency. The intensity between the two coherent modes is related by a constant α that is only related to the spatial position. α is independent of the wavelength. The intensity relationship between the two modes is expressed by formula (10):

[0044] I2(x,y,ω)=α 2 (x,y)I1(x,y,ω) (10)

[0045] Substituting formula (10) into formula (9), we can obtain the interference light intensity of the two modes:

[0046] I(x,y,ω)=I1(x,y,ω)[1+α 2 (x,y)+2α(x,y)cos(τ b ω)] (11)

[0047] In formula (11), τ b Refers to the period of the beat formed by the group delay difference between the two modes, τ b The product of the circular frequency ω represents the phase difference between the two modes. In formula (11), the interference light intensity of a certain wavelength at different detection points is mainly determined by the constant α which is only related to the spatial position. The spectral intensity in formula (11) is transformed by Fourier transform to obtain the form shown in the following formula:

[0048] B(x,y,τ)=[1+α 2 (x,y)]B1(x,y,τ)+α(x,y)[B1(x,y,τ-τ b )+B1(x,y,τ+τ b )] (12)

[0049] Wherein, B1(x, y, τ) = F{I1(x, y, ω)}, which is the Fourier transform of the light intensity of a single mode with intensity I1;

[0050] The Fourier transform of the spatially correlated spectrum is used to calculate α(x, y). At any point (x, y) in the detection plane, f(x, y) is defined as the ratio of the intensity of the peak corresponding to the peak of the mode interference to the intensity at zero group delay. Considering that τ in B1(x, y, τ) should be much smaller than τ, b , so the definition expression of f(x,y) is as shown in formula (13):

[0051]

[0052] According to formula (13), the value of α(x, y) is calculated as follows:

[0053]

[0054] Within the measurement bandwidth, including multiple beat frequency cycles, the total light intensity IT(x,y) obtained by integrating the measurement bandwidth at any point (x,y) is equal to the incoherent superposition of the light intensities of the two modes, that is, IT(x,y)=I1(x,y)+I2(x,y). Then the light intensity of each mode can be expressed by the following two formulas:

[0055]

[0056]

[0057] The relative power MPI value of the two modes is the relative power of the higher-order mode to the fundamental mode:

[0058]

[0059] As described above, the improved device for measuring the transmission mode of hollow-core anti-resonant optical fiber of the present invention has the following beneficial effects:

[0060] (1) The improved device for measuring the transmission mode of hollow-core antiresonant optical fiber of the present invention comprises a tunable laser, an output pigtail, an optical fiber to be measured, a lens group, a CCD camera, and a computer. The present invention has fast measurement, simple operation, and accurate measurement.

[0061] (2) The improved device for measuring the transmission mode of hollow-core antiresonant optical fiber of the present invention can determine the number and type of modes and the relative power level of each mode and the fundamental mode by acquiring high-quality images of different frequencies within a certain bandwidth.

[0062] (3) The improved device for measuring the transmission mode of hollow-core antiresonant optical fiber of the present invention only performs one-dimensional scanning of wavelength instead of two-dimensional scanning of space, which saves measurement time and improves measurement accuracy.

[0063] (4) The improved device for measuring the transmission mode of hollow-core antiresonant optical fiber of the present invention is based on interference and is also sensitive to high-order modes with very low power and can detect them.

[0064] (5) The measurement method of the improved device for measuring the transmission mode of hollow-core antiresonant optical fiber of the present invention does not require other complex theoretical knowledge to support, thus reducing the difficulty of the experiment. BRIEF DESCRIPTION OF THE DRAWINGS

[0065] Figure 1 A structural diagram of an improved device for measuring the transmission mode of a hollow-core antiresonant optical fiber provided in an embodiment of the present application.

[0066] Figure 2 An improved device for measuring the transmission mode of a hollow-core antiresonant optical fiber provided in an embodiment of the present application is provided, and a light spot pattern diagram of 1552 nm is measured using a CCD camera.

[0067] Figure 3 An improved device for measuring the transmission mode of a hollow-core antiresonant optical fiber provided in an embodiment of the present application is provided, and a light spot pattern diagram of 1552 nm is measured using a CCD camera.

[0068] Figure 4 An improved device for measuring the transmission mode of a hollow-core antiresonant optical fiber provided in an embodiment of the present application is provided with a group delay curve of a 5 m hollow-core antiresonant optical fiber tested under the conditions of a bending radius of R=48 cm and optimal coupling between the output pigtail and the hollow-core antiresonant optical fiber to be tested.

[0069] Figure 5 An improved device for measuring the transmission mode of a hollow-core antiresonant optical fiber provided in an embodiment of the present application provides a group delay curve of a 10.5 m hollow-core antiresonant optical fiber under the conditions of a bending radius of R=48 cm, optimal coupling between the output fiber pigtail and the hollow-core antiresonant optical fiber to be measured, 5 micron offset coupling, and 15 micron offset coupling.

[0070] Component number description

[0071] 1 Tunable laser

[0072] 2 Output pigtail

[0073] 3 Fiber to be tested

[0074] 4 lens groups

[0075] 5 CCD camera

[0076] 6 Computer DETAILED DESCRIPTION

[0077] The following describes the embodiments of the present invention by specific examples, and those skilled in the art can easily understand other advantages and effects of the present invention from the contents disclosed in this specification. The present invention can also be implemented or applied through other different specific embodiments, and the details in this specification can also be modified or changed in various ways based on different viewpoints and applications without departing from the spirit of the present invention. It should be noted that the following embodiments and features in the embodiments can be combined with each other without conflict.

[0078] It should be noted that the illustrations provided in the following embodiments are only used to illustrate the basic concept of the present invention in a schematic manner, and thus the illustrations only show components related to the present invention rather than being drawn according to the number, shape and size of components in actual implementation. In actual implementation, the type, quantity and proportion of each component may be changed arbitrarily, and the component layout may also be more complicated.

[0079] Terms such as first or second can be used to describe various components, but these components are not limited by the above terms. The above terms are used to distinguish one component from another component, for example, without departing from the scope of the concept according to the present disclosure, a first component can be referred to as a second component, and similarly, a second component can be referred to as a first component.

[0080] In addition, "connected / coupled" means that one component is directly electrically coupled to another component or is indirectly electrically coupled through another component. The singular form may include the plural form unless explicitly stated in the sentence. In addition, "include / comprise" or "includes / comprises" used in this specification indicates that one or more components, steps, operations, and elements exist or have been added. The specific structural or functional descriptions of the examples of the implementation of the concepts disclosed in this specification are only illustrated to describe the examples of the implementation of the concepts, and the examples of the implementation of the concepts can be implemented in various forms, but these descriptions are not limited to the examples of the implementation described in this specification.

[0081] According to the concept, various modifications and changes can be applied to the examples of the embodiments, so that the examples of the embodiments will be illustrated in the drawings and described in the specification. However, the examples of the embodiments according to the concept are not limited to the specific embodiments, but include all changes, equivalents or replacements included in the spirit and technical scope of the present disclosure.

[0082] It should be understood that when an element is described as being "coupled" or "connected" to another element, the element may be directly coupled or directly connected to the other element, or may be coupled or connected to the other element through a third element. Conversely, it should be understood that when an element is referred to as being "directly coupled to" or "directly coupled to" another element, no other element is placed between them. Other expressions describing the relationship between components (i.e., "between" and "directly between" or "adjacent to" and "directly adjacent to") need to be interpreted in the same manner.

[0083] The terms used in this specification are only used to describe specific examples of the embodiments and are not intended to limit the present disclosure. If there is no clear opposite meaning in the context, the singular form may include the plural form. In this specification, it should be understood that the term "including" or "having" indicates the presence of the features, quantities, steps, operations, components, parts or combinations thereof described in the specification, but the possibility of the presence or addition of one or more other features, quantities, steps, operations, components, parts or combinations thereof cannot be precluded.

[0084] If there is no contrary definition, all terms (including technical terms or scientific terms) used herein have the same meaning as those generally understood by ordinary technicians in the field. If the terms defined in the commonly used dictionary are not clearly defined in this specification, they should be interpreted as having the same meaning as in the context of the relevant technology, and not interpreted as ideal or overly formal meanings.

[0085] Descriptions of well-known components and processing techniques may be omitted so as not to unnecessarily obscure the embodiments of the disclosure.

[0086] Throughout the specification, the same reference numerals refer to the same elements. Thus, even if a reference numeral is not mentioned or described with reference to one figure, it may be mentioned or described with reference to another figure. Furthermore, even if a reference numeral is not shown in one figure, it may be mentioned or described with reference to another figure.

[0087] In addition, the logic level of the signal may be different or opposite to the described logic level. For example, a signal described as having a logic "high" level may alternatively have a logic "low" level, and a signal described as having a logic "low" level may alternatively have a logic "high" level.

[0088] The following will describe the various embodiments of the present disclosure in detail with reference to the accompanying drawings. However, it will be appreciated by those skilled in the art that in the various embodiments of the present disclosure, many technical details are provided in order to enable the reader to better understand the present disclosure. However, even without these technical details and various changes and modifications based on the following embodiments, the technical solutions claimed in the present disclosure can be implemented.

[0089] The present invention provides an improved device for measuring the transmission mode of hollow-core antiresonant optical fiber, which adopts a combination of a tunable laser light source and a CCD camera to replace a combination of a broadband light source and a spectrometer to achieve the purpose of measuring the transmission mode components of the hollow-core antiresonant optical fiber.

[0090] See also Figure 1 , Figure 1 A structural diagram of an improved device for measuring the transmission mode of a hollow-core antiresonant optical fiber provided in an embodiment of the present application. The present invention provides an improved device for measuring the transmission mode of a hollow-core antiresonant optical fiber, comprising a tunable laser 1, an output pigtail 2, an optical fiber to be tested 3, a lens group 4, a CCD camera 5, and a computer 6. The tunable laser 1 is used to emit laser light; the output pigtail 2 is used to couple the laser light; the optical fiber to be tested 3 is used to receive the coupled laser light; the lens group 4 is used to amplify the coupled laser light; the CCD camera 5 is used to image the amplified coupled laser light; the tunable laser 1 is used to tune at a set frequency interval, and the CCD camera 5 records the light spot pattern diagram at each frequency; the computer 6 is used to collect and process the light spot pattern diagram at each frequency to separate the various modes existing in the optical fiber to be tested 3 to quantitatively characterize the relative content of each mode.

[0091] In one embodiment of the present invention, the light emitted from the tunable laser 1 is coupled to the hollow core anti-resonant optical fiber to be tested 3 through the output pigtail 2, and after being amplified by the lens group 4, it is directly imaged on the CCD camera 5. The tunable laser 1 is tuned at a certain frequency interval, and then the CCD camera 5 records the light spot pattern at each frequency. After data acquisition and processing by the computer 6, the various modes existing in the optical fiber can be separated, and the relative content of each mode can be quantified and characterized.

[0092] Specifically, the tunable laser 1, the output pigtail 2, the optical fiber to be tested 3, the lens group 4, and the CCD camera 5 are placed on the same optical axis in sequence.

[0093] Specifically, the tunable laser 1 and the CCD camera 5 are both connected to a computer 6 . The computer 6 is also used to adjust the output wavelength of the tunable laser 1 . The computer 6 is also used to perform data processing on images of different wavelengths collected by the CCD camera 5 .

[0094] Specifically, the tunable range of the wavelength of the tunable laser 1 is 1550nm-1554nm. The output pigtail 2 is a single-mode 28 optical fiber. The optical fiber 3 to be tested is placed on a three-dimensional displacement platform for three-dimensional adjustment of the optical fiber 3 to be tested. The CCD camera 5 is used to collect near-field magnified images in the 1550nm-1554nm band.

[0095] The improved device for measuring the transmission mode of hollow-core antiresonant optical fiber of the present invention combines a tunable laser and a CCD camera, utilizes the mode interference generated by the high-order mode and the fundamental mode in the optical fiber during the transmission process, and solves the spectrum interference spatially to achieve the purpose of measuring the optical fiber mode.

[0096] The basic principle of fiber mode interferometry used in this measurement method is as follows:

[0097] The electric field distribution at the optical fiber incident end face z = 0 is:

[0098]

[0099] Among them A 01 and A 11 are the amplitudes of the two modes, and are the initial phases of the two modes respectively;

[0100] After the mode in the optical fiber propagates a distance L, the electric field distribution at the output end face (Z = L) is:

[0101]

[0102] According to the electric field intensity at the exit end face, the light intensity at the exit end face is calculated as:

[0103]

[0104] From the above formula, we can see that the light spot at the optical fiber output end face is decomposed into countless points. Each point (x, y) contains the interference information of two modes. The interference is related to the frequency of the light wave. Taylor expansion of the propagation constant β with respect to the frequency ω is:

[0105]

[0106] According to the definition of mode group delay in formula (1), substituting formula (4) into it, we can obtain:

[0107]

[0108] In equation (5), the first term of the expression on the far right of the equation is related to the mode dispersion, which is the mode group delay at the angular frequency ω0, and the second term is related to the material dispersion and waveguide dispersion. When material dispersion and waveguide dispersion are ignored, substituting equation (5) into equation (4) and then into equation (3) yields:

[0109]

[0110] In the formula, Δω=ω-ω0, τ 11 is the modal group delay of the LP11 mode, τ 01 is the mode group delay of LP01. Equation (6) is further simplified into the form of cosine function:

[0111]

[0112] In formula (7), is a constant (fixed phase difference), ΔT 11 is the delay caused by the two modes transmitting along the optical fiber for a distance of L, ΔT 11 =(τ 11 -τ 01 )L.

[0113] It can be seen from the above formula that the light intensity at any point (x, y) on the optical fiber output end face is related to the frequency and changes periodically with the frequency. Knowing the frequency period of the light intensity change, we can solve the group delay time difference between the two modes.

[0114] Then the expression of the interference light intensity supporting multiple high-order mode transmission is:

[0115]

[0116] In the above formula, m and n represent the modulus numbers of the high-order modes. By performing Fourier transform on the above formula with respect to the angular frequency ω, the high order can be obtained;

[0117] The group delay difference between the mode and the fundamental mode (through an optical fiber with a length of L) ΔT mn .

[0118] The spatial correlation between different modes enables multiple modes with different group delay differences propagating in the optical fiber to interfere with each other and form images simultaneously. The interference pattern of two modes depends on the MPI value between the two modes, which reflects the relative power level between the two modes.

[0119]

[0120] Next, we introduce the theoretical derivation process of calculating the MPI value between two interference modes (i.e., the relative power level between the two modes). We assume that the amplitude of light of different modes in the detection plane is related to the spatial position and frequency, and the intensity between the two coherent modes can be related by a constant α that is only related to the spatial position, and α is independent of the wavelength. Then the intensity relationship between the two modes can be expressed by formula (10):

[0121] I2(x,y,ω)=α 2 (x,y)I1(x,y,ω) (10)

[0122] Substituting equation (10) into equation (9), the interference light intensity of the two modes can be obtained as follows:

[0123] I(x,y,ω)=I1(x,y,ω)[1+α 2 (x,y)+2α(x,y)cos(τ b ω)] (11)

[0124] In formula (11), τ b Refers to the period of the beat formed by the group delay difference between the two modes, τ b The product of the circular frequency ω represents the phase difference between the two modes. It can be seen from formula (11) that the interference light intensity of a certain wavelength at different detection points is mainly determined by the constant α that is only related to the spatial position. The spectral intensity in formula (11) is transformed by Fourier to obtain the form shown in the following formula:

[0125] B(x,y,τ)=[1+α 2 (x,y)]B1(x,y,τ)+α(x,y)[B1(x,y,τ-τ b )+B1(x,y,τ+τ b )] (12)

[0126] Among them, B1(x, y, τ) = F{I1(x, y, ω)}, which is the Fourier transform of the light intensity of a single mode with intensity I1. Although the spectrum in the measurement process is plotted relative to the wavelength, the Fourier transform is transformed relative to the frequency, so in the process of Fourier transform, the spectrum plotted by wavelength must be changed to a spectrum plotted by frequency.

[0127] The Fourier transform of the spatially correlated spectrum can be used to calculate α(x, y). At any point (x, y) in the detection plane, f(x, y) is defined as the ratio of the intensity of the peak corresponding to the peak of the mode interference to the intensity at zero group delay. Considering that τ in B1(x, y, τ) should be much smaller than τ, b, so the definition expression of f(x,y) is as shown in formula (13):

[0128]

[0129] From the above formula, the value of α(x, y) can be calculated as follows:

[0130]

[0131] Because the measurement bandwidth contains multiple beat frequency cycles, the total light intensity IT(x,y) obtained by integrating the measurement bandwidth at any point (x,y) is equal to the incoherent superposition of the light intensities of the two modes, that is, IT(x,y)=I1(x,y)+I2(x,y). The light intensity of each mode can be expressed by the following two formulas:

[0132]

[0133] Then the relative power MPI value of the two modes (higher-order mode relative to the fundamental mode) is:

[0134]

[0135] In one embodiment of the present invention,

[0136] See also Figure 2 , Figure 2 The improved device for measuring the transmission mode of hollow core anti-resonance optical fiber provided in the embodiment of the present application uses a CCD camera to measure the light spot mode of 1552nm. The mode of the hollow core anti-resonance optical fiber is measured. The cross section of the hollow core anti-resonance optical fiber is as follows: Figure 2 The improved measuring device is shown in Figure 1 The tunable wavelength range of the tunable laser 1 used includes 1550-1554nm, the output pigtail 2 is a single-mode 28 optical fiber, the optical fiber to be tested 3 is a hollow-core anti-resonant optical fiber, and the lens group 4 amplifies the light spot output by the optical fiber to be tested. Figure 1 As shown in the figure, the positions of various components are arranged and connected. The measurement steps are as follows:

[0137] The light emitted from the tunable laser 1 is coupled to the hollow-core antiresonant optical fiber 3 to be tested through the output pigtail 2, and after being amplified by the lens group 4, it is directly imaged on the CCD camera 5. The CCD camera 5 transmits the collected data to the computer 6. The tunable laser 1 is tuned at a frequency interval of 0.05nm, and then the CCD camera 5 records the light spot pattern at each frequency. The Fourier transform is performed on the light spots at multiple frequencies collected, and the frequency domain information is converted to the time domain, so that different modes and their relative intensities can be distinguished.

[0138] Figure 2The following are some basic parameters of the hollow-core antiresonant optical fiber to be tested, where (a) is a scanning electron microscope image of the optical fiber; (b) is the transmission spectrum of 330m and 5m optical fibers; (c) is a comparison of the simulated and measured loss of the optical fiber. It can be seen that the optical fiber has extremely low loss in the communication band, which is comparable to single-mode optical fiber, but its transmission capacity is better than that of single-mode optical fiber.

[0139] See also Figure 3 , Figure 3 The 1552nm spot pattern diagram measured by a CCD camera is a 1552nm spot pattern diagram measured by an improved device for measuring the transmission mode of a hollow core anti-resonance optical fiber provided in an embodiment of the present application. Figure 3 shown.

[0140] See also Figure 4 , Figure 5 , Figure 4 An improved device for measuring the transmission mode of a hollow-core antiresonant optical fiber provided in an embodiment of the present application is provided with a group delay curve of a 5 m hollow-core antiresonant optical fiber tested under the conditions of a bending radius of R=48 cm and optimal coupling between the output pigtail and the hollow-core antiresonant optical fiber to be tested. Figure 5 An improved device for measuring the transmission mode of a hollow-core antiresonant optical fiber provided in an embodiment of the present application shows a group delay curve of a 10.5 m hollow-core antiresonant optical fiber under the conditions of a bending radius of R=48 cm, optimal coupling between the output pigtail and the hollow-core antiresonant optical fiber to be measured, 5 micron coupling offset, and 15 micron coupling offset. The transmission mode of a 5 m and 10.5 m hollow-core antiresonant optical fiber was measured using the improved measurement device. The group delay curve of a 5 m hollow-core antiresonant optical fiber under the conditions of a bending radius of R=48 cm, optimal coupling between the output pigtail 2 and the hollow-core antiresonant optical fiber 3 to be measured is shown as follows: Figure 4 As shown in the figure, it can be seen that in addition to the fundamental mode (LP01 mode), the 5m optical fiber also includes higher-order modes such as LP11 mode, LP21 mode and LP02 mode. The group delay curve of the 10.5m hollow-core antiresonant optical fiber is tested under the conditions of optimal coupling, 5 micron offset coupling and 15 micron offset coupling with the output pigtail 2 and the hollow-core antiresonant optical fiber 3 to be tested, as shown in the figure. Figure 5 As shown in the figure, it can be seen that after passing through the 10.5m long hollow-core antiresonant optical fiber, only the fundamental mode (LP01 mode) is transmitted.

[0141] By using the improved device for measuring the transmission mode of hollow-core antiresonant optical fiber of the present invention, the mode transmission performance of hollow-core antiresonant optical fibers of 5 m and 10.5 m was compared, which not only proved the good single-mode transmission performance of the hollow-core antiresonant optical fiber, but also verified the accuracy and practicality of the measurement of the improved optical fiber mode measurement device.

[0142] In summary, the improved device for measuring the transmission mode of hollow-core antiresonant optical fiber of the present invention comprises a tunable laser, an output pigtail, an optical fiber to be measured, a lens group, a CCD camera, and a computer. The present invention has fast measurement, simple operation, and accurate measurement. The present invention can determine the number and type of modes and the relative power level of each mode and the fundamental mode by acquiring high-quality images of different frequencies within a certain bandwidth. The present invention only performs one-dimensional scanning of wavelengths instead of two-dimensional scanning of space, which saves measurement time and improves measurement accuracy. The present invention is based on interference and is also very sensitive to high-order modes with very low power, and can also detect them.

[0143] The above embodiments are merely illustrative of the principles and effects of the present invention, and are not intended to limit the present invention. Anyone familiar with the art may modify or alter the above embodiments without departing from the spirit and scope of the present invention. Therefore, all equivalent modifications or alterations made by a person of ordinary skill in the art without departing from the spirit and technical concept disclosed by the present invention shall still be covered by the claims of the present invention.

Claims

1. An improved device for measuring the transmission mode of hollow core anti-resonant optical fiber, characterized in that: include: A tunable laser (1) for emitting laser light; An output pigtail (2) for coupling the laser; The optical fiber to be tested (3) is used to receive the coupled laser light; A lens group (4) for amplifying the coupled laser light; A CCD camera (5) is used to image the laser light after amplification and coupling; The tunable laser (1) is used to tune at a set frequency interval, and the CCD camera (5) records the light spot pattern at each frequency; The computer (6) is used to collect and process the light spot pattern diagram at each frequency to separate various modes existing in the optical fiber (3) to be tested, so as to quantitatively characterize the relative content of each mode.

2. The improved device for measuring hollow-core anti-resonance optical fiber transmission mode according to claim 1, characterized in that: The tunable laser (1), the output pigtail (2), the optical fiber to be tested (3), the lens group (4), and the CCD camera (5) are sequentially placed on the same optical axis.

3. The improved device for measuring hollow core anti-resonance optical fiber transmission mode according to claim 2, characterized in that: The tunable laser (1) and the CCD camera (5) are both connected to a computer (6); the computer (6) is also used to adjust the output wavelength of the tunable laser (1); and the computer (6) is also used to perform data processing on images of different wavelengths collected by the CCD camera (5).

4. The improved device for measuring hollow core anti-resonance optical fiber transmission mode according to claim 1, characterized in that: The tunable range of the wavelength of the laser emitted by the tunable laser (1) is in the 1550nm-1554nm band.

5. The improved device for measuring hollow core anti-resonance optical fiber transmission mode according to claim 1, characterized in that: The output pigtail (2) is a single-mode 28 optical fiber.

6. The improved device for measuring hollow core anti-resonance optical fiber transmission mode according to claim 1, characterized in that: The optical fiber (3) to be tested is placed on a three-dimensional displacement platform for three-dimensionally adjusting the optical fiber (3) to be tested.

7. The improved device for measuring hollow core anti-resonance optical fiber transmission mode according to claim 3, characterized in that: The CCD camera (5) is used to collect near-field magnified images within the 1550nm-1554nm wavelength band.

8. A method for measuring an improved device for measuring hollow core anti-resonance optical fiber transmission mode, characterized in that: The improved device for measuring hollow-core anti-resonance optical fiber transmission mode according to any one of claims 1 to 7, wherein the measuring method of the improved device for measuring hollow-core anti-resonance optical fiber transmission mode comprises: The tunable laser (1) and the CCD camera (5) are combined to utilize the mode interference generated by the high-order mode and the fundamental mode in the optical fiber during the transmission process, and the spectrum interference is solved spatially to measure the optical fiber mode. The basic principle is as follows: The electric field distribution at the fiber incident end face z = 0 is: Among them, A 01 and A 11 are the amplitudes of the two modes, and are the initial phases of the two modes respectively; After the mode in the optical fiber propagates a distance L, the electric field distribution at the output end face (Z = L) is: According to the electric field intensity at the exit end face, the light intensity at the exit end face is calculated as: From formula (3), we can see that the light spot at the optical fiber output end face is decomposed into countless points. Each point (x, y) contains the interference information of two modes. The interference is related to the frequency of the light wave. The Taylor expansion of the propagation constant β with respect to the frequency ω is: According to the definition of mode group delay in formula (1), substituting formula (4) into it, we can obtain: In formula (5), the first term of the expression on the far right of the equation is related to the mode dispersion and is the angular frequency ω The second term is related to the material dispersion and waveguide dispersion. When material dispersion and waveguide dispersion are ignored, (5) is substituted into (4) and then into (3) to obtain: In formula (6), Δω=ω-ω0, τ 11 is the modal group delay of the LP11 mode, τ 01 is the mode group delay of LP01, and equation (6) is further simplified into the form of cosine function: In formula (7), is a constant (fixed phase difference), ΔT 11 is the delay caused by the two modes transmitting along the optical fiber for a distance of L, ΔT 11 =(τ 11 -τ 01 )L; From formula (7), it can be seen that the light intensity at any point (x, y) on the optical fiber output end face is related to the frequency; The expression of interference light intensity supporting multiple high-order mode transmission is: Where m and n represent the mode numbers of the higher-order modes. By Fourier transforming formula (7) with respect to the angular frequency ω, we can obtain the group delay difference between the higher-order mode and the fundamental mode, which is ΔT after passing through the optical fiber with a length of L. mn ; The spatial correlation between different modes enables multiple modes with different group delay differences propagating in the optical fiber to interfere with each other and form images simultaneously. The interference pattern of two modes depends on the MPI value between the two modes, which reflects the relative power level between the two modes. The MPI value between two interference modes is the relative power level between the two modes. The theoretical derivation process of the calculation assumes that the amplitude of light of different modes in the detection plane is related to the spatial position and frequency. The intensity between the two coherent modes is related by a constant α that is only related to the spatial position. α is independent of the wavelength. The intensity relationship between the two modes is expressed by formula (10): I2(x,y,ω)=α 2 (x,y)I1(x,y,ω) (10) Substituting formula (10) into formula (9), we can obtain the interference intensity of the two modes: I(x,y,ω)=I1(x,y,ω)[1+a 2 (x,y)+2α(x,y)cos(τ b oh)] (11) In formula (11), τ b Refers to the period of the beat formed by the group delay difference between the two modes, τ b The product of the circular frequency ω represents the phase difference between the two modes. In formula (11), the interference light intensity of a certain wavelength at different detection points is mainly determined by the constant α which is only related to the spatial position. The spectral intensity in formula (11) is transformed by Fourier transform to obtain the form shown in the following formula: B(x,y,τ)=[1+α 2 (x, y)]B1(x, y, τ)+α(x, y)[B1(x, y, τ-τ b )+B1(x,y,τ+τ b )] (12) Wherein, B1(x, y, τ) = F{I1(x, y, ω)}, which is the Fourier transform of the light intensity of a single mode with intensity I1; The Fourier transform of the spatially correlated spectrum is used to calculate α(x, y). At any point (x, y) in the detection plane, f(x, y) is defined as the ratio of the intensity of the peak corresponding to the peak of the mode interference to the intensity at zero group delay. Considering that τ in B1(x, y, τ) should be much smaller than τ, b , so the definition expression of f(x, y) is as shown in formula (13): According to formula (13), the value of α(x, y) is calculated as follows: Within the measurement bandwidth, including multiple beat frequency cycles, the total light intensity IT(x, y) obtained by integrating the measurement bandwidth at any point (x, y) is equal to the incoherent superposition of the light intensities of the two modes, that is, IT(x, y) = I1(x, y) + I2(x, y). Then the light intensity of each mode can be expressed by the following two formulas: The relative power MPI value of the two modes is the relative power of the higher-order mode to the fundamental mode:

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