Beat length measurement method for few-mode polarization-maintaining optical fiber

Abstract

A method for measuring the beat length of a few-mode polarization-maintaining optical fiber, the method comprising measuring the interference signal between different polarization modes of the few-mode polarization-maintaining optical fiber, converting it into a timing diagram through inverse Fourier transform, separating different polarization modes, and distinguishing them according to the polarization sensitivity of different modes, thereby obtaining the group delay difference between different polarization modes, and calculating the beat length of the few-mode polarization-maintaining optical fiber accordingly. The current mainstream polarization-maintaining optical fiber beat length measurement method and device are only for single-mode polarization-maintaining optical fiber. The present invention has the characteristics of easy construction, simple operation, fast measurement speed and high measurement accuracy, and has a certain guiding role in the development and application of few-mode polarization-maintaining optical fiber.

Description

Technical Field

[0001] The invention relates to a polarization-maintaining optical fiber, in particular to a beat length measurement method for a few-mode polarization-maintaining optical fiber. Background Art

[0002] Polarization-maintaining fiber can transmit linear polarization states over a long fiber length and has been widely used in fiber-optic communications, interferometric fiber sensors, polarization-sensitive optical devices, and other fields. Polarization-maintaining fiber achieves polarization maintenance by introducing (geometric or stress) asymmetry in the fiber structure and suppressing the cross-coupling of optical power between two perpendicular polarization modes. Modal birefringence indicates the polarization-maintaining ability of an optical fiber. Beat length is an important parameter for measuring the birefringence of an optical fiber. Therefore, the accurate measurement of the beat length of a polarization-maintaining fiber is of great significance for the research, development, production, and application of polarization-maintaining fiber.

[0003] Few-mode polarization-maintaining fiber has good application prospects because of its characteristics of few-mode and polarization-maintaining. However, the current mainstream beat length measurement method is for single-mode polarization-maintaining fiber, and there are few methods for measuring the beat length of few-mode polarization-maintaining fiber.

[0004] At present, there are several methods for measuring beat length: stress (strain), torsion, truncation, Rayleigh scattering, magneto-optical modulation, polarized light time domain reflectometry and interferometry. Interferometry includes Sagnac interferometry, white light interferometry, broadband interferometry and S 2 method.

[0005] The stress (strain) method requires external stress to be applied to the optical fiber under test. This method is complex to operate, sensitive to external conditions, and has low accuracy. The torsion method calculates the beat length by measuring the polarization state of the optical fiber under various torsion conditions, mainly for single-mode polarization-maintaining optical fiber. The truncation method is a destructive measurement method that calculates the beat length by measuring the polarization state of the output at different optical fiber lengths. This method is complex to operate, slow to measure, and difficult to verify the consistency of the method. The Rayleigh scattering method uses the polarization dependence effect of Rayleigh scattering to measure the beat length. This method is non-destructive, but multiple scattering will reduce the influence of polarization-related modulation of the scattered light intensity. The measurement method is complex and has low accuracy. The strong magnetic field required for magneto-optical modulation measurement is difficult to control, which increases the difficulty and complexity of the measurement.

[0006] The above methods as well as polarization time domain reflectometry, Sagnac interferometry, and broadband interferometry are all for single-mode polarization-maintaining fibers. They cannot eliminate the influence of high-order modes in few-mode polarization-maintaining fibers and are not suitable for measuring the beat length of few-mode polarization-maintaining fibers.

[0007] White light interferometry is based on the Michelson interferometer, which compensates for the optical path difference between modes with different polarization directions, analyzes the interference pattern to calculate the beat length between different modes, and can be used to measure the beat length of few-mode polarization-maintaining optical fibers. However, this method is relatively complicated to operate, and the measurement accuracy is limited by the accuracy of the interferometer arm translation stage, making it difficult to achieve real-time measurement. 2 This method has very high requirements for light source and incident conditions, is expensive, and has complex data processing methods. Summary of the invention

[0008] The purpose of the present invention is to overcome the deficiencies of the above-mentioned prior art and provide a method for measuring the beat length of a few-mode polarization-maintaining optical fiber. The method belongs to non-contact measurement and has the characteristics of simple equipment construction, simple operation, fast measurement speed, high measurement accuracy and wide application range.

[0009] In order to achieve the above invention purpose, the present invention adopts the following technical solution:

[0010] A method for measuring the beat length of a few-mode polarization-maintaining optical fiber, characterized in that it comprises the following steps:

[0011] a. Build a beat length measurement device for a few-mode polarization-maintaining fiber, which includes a light source, an interference system, a receiving system, and a control and computing terminal. The interference system, the receiving system, the control and computing terminal, and the interference system are sequentially connected to form a feedback path. The few-mode polarization-maintaining fiber to be measured is placed in the interference system 2, and the light source is turned on. The control and computing terminal adjusts the interference system through the feedback path according to the image signal obtained from the receiving system, so as to couple the light output by the light source into the core of the few-mode polarization-maintaining fiber to be measured;

[0012] b. The control computing terminal adjusts the polarization direction of the polarization analyzer of the interference system through a feedback path so as to accurately identify the group delay difference between different polarization modes, so that the receiving system can obtain the required interference signal;

[0013] c. The control computing terminal uses a built-in few-mode polarization-maintaining fiber beat length measurement program to separate different polarization modes and convert the interference signal obtained by the receiving system into a timing diagram;

[0014] d. Repeat steps b and c to obtain multiple sets of timing diagrams. The built-in few-mode polarization-maintaining fiber beat length measurement program of the control and computing terminal performs peak search and comparative analysis on the obtained timing diagrams to identify the interference peaks of different polarization modes and obtain the group delay difference Δτ between different polarization modes;

[0015] e. The built-in few-mode polarization-maintaining fiber beat length measurement program of the control computing terminal calculates the fiber beat length according to the group delay difference between different polarization modes and the geometric length of the fiber to be measured in the interference system. The beat length calculation formula is: L B=λL / cΔτ,

[0016] Wherein, λ is the central wavelength of the interference signal received by the receiving system, L is the geometric length of the optical fiber to be measured in the interference system, c is the speed of light in vacuum, and Δτ is the group delay difference between different polarization modes.

[0017] Compared with the prior art, the present invention has the following characteristics and advantages:

[0018] 1. The present invention adopts interference technology to measure the interference signal between different polarization modes of few-mode polarization-maintaining optical fiber, and converts it into a timing diagram through inverse Fourier transform, so as to separate different polarization modes. Based on the polarization sensitivity of different modes, the peak value changes in the timing diagram are compared and analyzed by adjusting the polarization direction of the polarization analyzer in the interference system, and the fundamental mode and the high-order mode are distinguished to obtain the group delay difference between different polarization modes, and the beat length of the optical fiber to be measured in the interference system is calculated accordingly.

[0019] 2. The present invention can distinguish the fundamental mode and the higher-order mode in the few-mode polarization-maintaining optical fiber, thereby measuring the beat length of the few-mode polarization-maintaining optical fiber.

[0020] 3. The present invention is a non-contact measurement and will not cause damage to the optical fiber.

[0021] 4. The present invention does not use complicated devices and equipment, and is simple to build and easy to implement.

[0022] 5. During the measurement process of the present invention, the group delay difference between different polarization modes can be accurately identified by adjusting the polarization direction of the polarization analyzer in the interference system, thereby calculating the beat length. The operation is simple and the measurement speed is fast.

[0023] 6. The measurement accuracy of the present invention is related to the geometric length of the optical fiber to be measured, the bandwidth of the light source, and the resolution of the spectrum signal receiving system. The geometric length of the optical fiber to be measured and the bandwidth of the light source can be adjusted accordingly according to the required measurement accuracy, and the measurement accuracy is flexible.

[0024] 7. The present invention is applicable to the measurement of the beat length of various polarization-maintaining optical fibers and has a wide range of applications. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Figure 1 It is a schematic diagram of the beat length measurement device of the few-mode polarization-maintaining optical fiber of the present invention.

[0026] Figure 2 Schematic diagram of interference signal of few-mode polarization-maintaining fiber according to an embodiment of the present invention.

[0027] Figure 3 Schematic diagram of a timing diagram obtained during the measurement of a few-mode polarization-maintaining optical fiber according to an embodiment of the present invention.

[0028] Figure 4It is a schematic diagram of the change of the fundamental mode interference peak value with the polarization direction of the polarization analyzer in the timing diagram obtained during the measurement of the few-mode polarization-maintaining optical fiber according to an embodiment of the present invention.

[0029] In the figure: 1-light source, 2-interference system, 3-receiving system, 4-control and computing terminal. DETAILED DESCRIPTION

[0030] In order to facilitate the understanding of the present invention, the above scheme is further described below in conjunction with the accompanying drawings and specific embodiments. The embodiments of the present invention are described in detail as follows:

[0031] Example:

[0032] See also Figure 1 The present invention provides a device for measuring the beat length of a few-mode polarization-maintaining optical fiber, comprising a light source 1, an interference system 2, a receiving system 3, and a control and computing terminal 4. The interference system 2, the receiving system 3, the control and computing terminal 4, and the interference system 2 are sequentially connected to form a feedback path. The few-mode polarization-maintaining optical fiber to be measured is placed in the interference system 2. The few-mode polarization-maintaining optical fiber beat length measurement program built into the control and computing terminal 4 controls the interference system 2 through the feedback path according to the signal received by the receiving system 3. The few-mode polarization-maintaining optical fiber beat length measurement program built into the control and computing terminal 4 analyzes and processes the signal received by the receiving system 3 and calculates the beat length.

[0033] The light source 1 is a broadband light source. The interference system 2 includes a polarizing device, a few-mode polarization-maintaining fiber to be measured, a coupling system, and a polarization analyzer. The receiving system 3 includes a beam splitting system, an imaging system, and a spectrum signal receiving system. The control and computing terminal 4 includes an image display system, a control system, a feedback system, and a few-mode polarization-maintaining fiber beat length measurement program.

[0034] The method for measuring the beat length of a few-mode polarization-maintaining optical fiber in this embodiment includes the following steps:

[0035] a. Build a beat length measurement device for a few-mode polarization-maintaining fiber, which includes a light source 1, an interference system 2, a receiving system 3, and a control and computing terminal 4. The interference system 2, the receiving system 3, the control and computing terminal 4, and the interference system 2 are sequentially connected to form a feedback path. The few-mode polarization-maintaining fiber to be measured is placed in the interference system 2, and the light source 1 is turned on. The control and computing terminal 4 adjusts the interference system 2 through the feedback path according to the image signal obtained by the receiving system 3, so as to couple the light output by the light source 1 into the core of the few-mode polarization-maintaining fiber to be measured;

[0036] b. The control computing terminal 4 adjusts the polarization direction of the polarization analyzer of the interference system 2 through the feedback path so as to accurately identify the group delay difference between different polarization modes. The control computing terminal 4 obtains the required interference signal from the receiving system 3, see Figure 2 ;

[0037] c. The built-in few-mode polarization-maintaining fiber beat length measurement program of the control computing terminal 4 converts the interference signal obtained by the receiving system 3 into a timing diagram, see Figure 3 ;

[0038] d. Repeating process b and c, the built-in few-mode polarization-maintaining fiber beat length measurement program of the control computing terminal 4 performs peak search and comparative analysis on the acquired timing diagram, identifies the interference peaks of different polarization modes, and obtains the group delay difference between different polarization modes, see Figure 3 ;

[0039] e. The control and calculation terminal 4 has a built-in program for measuring the beat length of a few-mode polarization-maintaining optical fiber. The control and calculation terminal 4 calculates the beat length of the optical fiber according to the group delay difference between different polarization modes and the geometric length of the optical fiber to be measured in the interference system 2. The beat length calculation formula is: L B =λL / cΔτ,

[0040] Wherein, λ is the central wavelength of the interference signal of the interference system 2 received by the receiving system 3, L is the geometric length of the optical fiber to be measured in the interference system 2, c is the speed of light in vacuum, and Δτ is the group delay difference between different polarization modes.

[0041] In the timing diagram obtained during the measurement process of this embodiment, the peak value of the fundamental mode interference changes with the polarization direction of the polarization analyzer. Figure 4 .

[0042] Experiments show that the present invention has the characteristics of easy construction, simple operation, fast measurement speed and high measurement accuracy, and has a certain guiding role in the development and application of few-mode polarization-maintaining optical fibers.

[0043] The above embodiments of the present invention are described in conjunction with the accompanying drawings. The above embodiments are only exemplary embodiments of the present application and are not intended to limit the present application. The protection scope of the present application is defined by the claims. Those skilled in the art may make various modifications or equivalent substitutions to the present application within the essence and protection scope of the present application, and such modifications or equivalent substitutions shall also be deemed to fall within the protection scope of the present application.

Claims

1. A method for measuring the beat length of a few-mode polarization-maintaining optical fiber, characterized in that: The following steps are involved: a. constructing a beat length measurement device for a few-mode polarization-maintaining optical fiber, the device comprising a light source (1), an interference system (2), a receiving system (3), and a control and computing terminal (4), wherein the interference system (2), the receiving system (3), the control and computing terminal (4), and the interference system (2) are sequentially connected to form a feedback path, the few-mode polarization-maintaining optical fiber to be measured is placed in the interference system (2), the light source (1) is turned on, and the control and computing terminal (4) adjusts the interference system (2) through the feedback path according to the image signal obtained from the receiving system (3), so as to couple the light output by the light source (1) into the core of the few-mode polarization-maintaining optical fiber to be measured; b. The control computing terminal (4) adjusts the polarization direction of the polarization analyzer of the interference system (2) through a feedback path so as to accurately identify the group delay difference between different polarization modes, so that the receiving system (3) can obtain the required interference signal; c. The control computing terminal (4) uses a built-in few-mode polarization-maintaining fiber beat length measurement program to separate different polarization modes and convert the interference signal obtained by the receiving system (3) into a timing diagram; d. Repeat steps b and c to obtain multiple sets of timing diagrams. The built-in few-mode polarization-maintaining fiber beat length measurement program of the control and computing terminal (4) performs peak search and comparative analysis on the obtained timing diagrams to identify the interference peaks of different polarization modes and obtain the group delay difference Δτ between different polarization modes; e. The built-in few-mode polarization-maintaining fiber beat length measurement program of the control computing terminal (4) calculates the fiber beat length according to the group delay difference between different polarization modes and the geometric length of the few-mode polarization-maintaining fiber to be measured in the interference system (2). The beat length calculation formula is: L B =λL / cΔτ, Wherein, λ is the central wavelength of the interference signal received by the receiving system (3), L is the geometric length of the few-mode polarization-maintaining optical fiber to be measured in the interference system (2), c is the speed of light in vacuum, and Δτ is the group delay difference between different polarization modes.

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