CN107806981A - A Measuring Device for Beat Length of Polarization Maintaining Optical Fiber - Google Patents

Abstract

The invention discloses a measuring device for beat length of a polarization maintaining optical fiber, which is characterized by comprising a wide-spectrum light source, an optical fiber polarizer, a frequency spectrograph and a computer, wherein the wide-spectrum light source enters a high-birefringence polarization maintaining optical fiber to be measured through the optical fiber polarizer; the high birefringence polarization maintaining optical fiber to be tested is connected with the other end of the optical fiber polarizer in a 45-degree welding manner, so that the connection part is very tight and firm, no gap is formed, and the stability is very high; the device has the characteristics of large treatment capacity, convenience, practicability and the like.

Description

A Measuring Device for Beat Length of Polarization Maintaining Optical Fiber

technical field

The invention relates to the field of measuring the beat length of polarization-maintaining optical fibers, in particular to a measuring device for the beat length of polarization-maintaining optical fibers.

Background technique

Polarization-maintaining optical fiber transmits linearly polarized light, and is widely used in various fields of national economy such as aerospace, aviation, navigation, industrial manufacturing technology and communication. In the interferometric optical fiber sensor based on optical coherent detection, the use of polarization-maintaining optical fiber can ensure that the linear polarization direction remains unchanged, improve the coherent signal-to-noise ratio, and achieve high-precision measurement of physical quantities. As a special optical fiber, polarization maintaining optical fiber is mainly used in optical fiber gyroscopes, optical fiber hydrophones and other sensors and optical fiber communication systems such as DWDM and EDFA.

High birefringence polarization-maintaining fibers are widely used in fields such as fiber optic gyroscopes, fiber optic current transformers, and polarization-maintaining optical devices. In these applications, the polarization-maintaining performance of the polarization-maintaining fiber is the key to determine its application. Generally, the beat length is used to measure the polarization-maintaining performance of the polarization-maintaining fiber. It reflects the difference in refractive index between the two polarization eigen axes of the polarization maintaining fiber, that is, the birefringence of the polarization maintaining fiber.

The measurement of polarized fiber beat length mainly includes torsion method, pressure method, electro-optical or magneto-optical modulation method, optical polarization method, shearing method, polarization mode dispersion method, prism coupling method, Rayleigh scattering method, wavelength scanning method, optical frequency domain reflection counting and other methods. During the drawing process of the polarization-maintaining optical fiber, the structural defects generated inside the optical fiber will cause the degradation of the polarization-maintaining performance. The invention provides a measuring device for the beat length of the polarization-maintaining optical fiber.

Contents of the invention

The technical problem to be solved by the present invention is to overcome the defect that the polarization maintaining performance of the existing polarization maintaining optical fiber is difficult to detect during the working process, and provide a measurement device for the beat length of the polarization maintaining optical fiber, so as to solve the above problems.

In order to solve the problems of the technologies described above, the present invention provides the following technical solutions:

The present invention is a kind of measurement device of polarization-maintaining optical fiber beat length, which comprises wide-spectrum light source, optical fiber polarizer, spectrum analyzer and computer. The end of the birefringent polarization-maintaining fiber is connected with a polarizer, and the signal output of the polarizer has an electro-optic modulator. The optical-carrying microwave signal on the electro-optic modulator is incident on the high-speed photodetector after passing through the dispersion fiber, and the microwave on the high-speed photodetector After the signal is amplified by a low-noise amplifier and passes through a microwave power divider, a part of the microwave signal is injected into the electro-optical modulator, and a part of the microwave signal reaches the computer through the spectrum analyzer.

As a preferred technical solution of the present invention, the high birefringence polarization-maintaining optical fiber to be tested is connected to the optical fiber polarizer by welding, and the welding angle is 45°, so that the connection is very tight and firm without gaps.

As a preferred technical solution of the present invention, the electro-optic modulator, dispersion fiber, high-speed photodetector, low-noise amplifier and microwave power divider form a photoelectric oscillator, which converts optical signals into microwave signals, making it digital, which is beneficial to Analytical calculations and recycling.

As a preferred technical solution of the present invention, the wide-spectrum light source can use a Gaussian or rectangular light source as the emitting light source, so that the selectivity of the emitting light source is higher.

The beneficial effects achieved by the present invention are: the present invention is a measurement device for the beat length of a polarization-maintaining optical fiber. A Mach-Zehnder interferometer is jointly formed by a highly birefringent polarization-maintaining optical fiber to be measured and a polarization analyzer, so that the wide-spectrum light source After passing through the interferometer, when the optical path difference between the two arms of the interferometer is within the coherent range of the light source, interference fringes will be generated at the output end of the interferometer, and the interference fringes are a sinusoidal comb spectrum in the frequency domain; The high birefringence polarization-maintaining fiber to be tested is tightly connected to the fiber polarizer, so that there will be no gaps, and it is also very stable; there will be no problems of cumbersome assembly, easy falling off, unstable connection, and poor aesthetics. The electro-optical modulator, dispersive fiber, high-speed photodetector, low-noise amplifier and microwave power divider constitute the photoelectric oscillator loop, convert the optical signal into a microwave signal and measure the center frequency of the output microwave signal, so that it can be big data It greatly saves manpower, material resources and time, and improves the accuracy.

Description of drawings

The accompanying drawings are used to provide a further understanding of the present invention, and constitute a part of the description, and are used together with the embodiments of the present invention to explain the present invention, and do not constitute a limitation to the present invention.

In the attached picture:

Fig. 1 is a schematic diagram of the framework structure of the system of the present invention.

Reference numerals in the figure: 101, wide-spectrum light source; 102, optical fiber polarizer; 103, high birefringence polarization-maintaining optical fiber to be tested; 104, polarizer; 105, electro-optic modulator; 106, dispersion optical fiber; 107, high-speed photoelectric detection 108. Low noise amplifier; 109. Microwave power splitter; 201. Spectrum analyzer; 202. Computer.

Detailed ways

The preferred embodiments of the present invention will be described below in conjunction with the accompanying drawings. It should be understood that the preferred embodiments described here are only used to illustrate and explain the present invention, and are not intended to limit the present invention.

In the description of the present invention, it should be noted that the orientation or positional relationship indicated by the terms "vertical", "upper", "lower", "horizontal" etc. is based on the orientation or positional relationship shown in the drawings, and is only In order to facilitate the description of the present invention and simplify the description, it does not indicate or imply that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation, and thus should not be construed as limiting the present invention.

In the description of the present invention, it should also be noted that, unless otherwise clearly specified and limited, the terms "installation", "installation", "connection" and "connection" should be understood in a broad sense, for example, it may be a fixed connection, It can also be a detachable connection or an integral connection; it can be a mechanical connection or an electrical connection; it can be a direct connection or an indirect connection through an intermediary, and it can be the internal communication of two components. Those of ordinary skill in the art can understand the specific meanings of the above terms in the present invention according to specific situations.

Embodiment: as shown in Figure 1, the present invention provides a kind of measurement device of polarization maintaining fiber beat length, it is characterized in that, comprises broadband light source 101, optical fiber polarizer 102, spectrometer 201 and computer 202; Broad spectrum light source 101 Enter the high birefringence polarization maintaining fiber 103 to be tested through the optical fiber polarizer 102, the signal output end of the high birefringence polarization maintaining fiber 103 to be tested is connected with a polarization analyzer 104, the high birefringence polarization maintaining fiber 103 to be tested and the polarization analyzer 104 Together constitute a Mach-Zehnder interferometer, the signal output end of the Mach-Zehnder interferometer has an electro-optic modulator 105, and the light-carrying microwave signal on the electro-optic modulator 105 is incident on the high-speed photodetector 107 after passing through the dispersion fiber 106, and the high-speed photoelectric The detector 107 converts the optical signal into a microwave signal and amplifies it through the low-noise amplifier 108. The output of the low-noise amplifier 108 is connected to the microwave power divider 109, and the microwave power divider 109 injects a part of the microwave signal into the electro-optic modulator 105, and at the same time Another part of the microwave signal is input to the spectrum analyzer 201 , and the end of the spectrum analyzer 201 is connected to the computer 202 .

The high birefringence polarization-maintaining optical fiber 103 to be tested is connected to the optical fiber polarizer 102 by fusion splicing, and the fusion splicing angle is 45°, so that the connection is very tight and firm without gaps; the electro-optic modulator 105, the dispersion optical fiber 106, A high-speed photodetector 107, a low-noise amplifier 108 and a microwave power divider 109 form a photoelectric oscillator, and the input end of the photoelectric oscillator loop is connected with the output end of the Mach-Zehnder interferometer, and the optical signal is changed into a microwave signal to make it It can be digitized, which is beneficial for analysis, calculation and recycling; the wide-spectrum light source 101 can use a Gaussian or rectangular light source as the emission light source, so that the selectivity of the emission light source is higher.

The principle of this measurement method is that the mode birefringence of the high birefringence polarization-maintaining fiber 103 to be tested makes the optical path difference of the two paths of light propagating in the high-birefringence polarization-maintaining fiber 103 to be tested different, and the output of the photoelectric oscillator The center frequency of the microwave signal is related to the optical path difference, and the beat length of the highly birefringent polarization-maintaining fiber 103 to be tested can be obtained according to the center frequency of the microwave signal. After the wide-spectrum light source passes through the high birefringence polarization-maintaining fiber 103 to be tested with a beat length of _Lp , select a suitable length of the high-birefringence polarization-maintaining fiber 103 to be tested so that the two polarization local oscillator axes of the high-birefringence polarization-maintaining fiber 103 to be tested are The two paths of light transmitted above meet the interference conditions at the end of the highly birefringent polarization-maintaining fiber 103 to be tested and interfere, then the output of the interference fringes can be expressed in the frequency domain as:

Where A is the visibility of the interference fringes output by the interferometer, Δω is the frequency interval of the output interference fringes when the optical path difference of the interferometer is different, is the phase shift of the interferometer, and ω ₀ is the central circular frequency of the laser. Δω can be expressed as:

Δω＝2πcL _p /λ ₀ l (2)

Where c is the speed of light, λ ₀ is the central wavelength of the broadband light source 101, and l is the length of the highly birefringent polarization-maintaining fiber 103 to be tested. Then the free spectral range of the interferometer can be expressed as:

The output light of interference is wavelength-dependent, and its electric field can be characterized as:

E(t)＝∫E(ω)e ^jωt dω (4)

Then the optical power spectral density of the broadband light source 101 can be expressed as:

T(ω)＝|E(ω)| ² (5)

After the interference fringes output by the interferometer pass through the electro-optic modulator 105, each frequency component E(ω) of the spectrum is modulated, and a microwave signal with a frequency of ξ is generated by the photoelectric oscillator loop, and the light output by the electro-optic modulator 105 field can be expressed as:

E(ω)＝e ^jωt (1+e ^jξt +e ^-jξt ) (6)

Dispersion fiber 106 is used as a delay line in the optoelectronic oscillator, and the electric field transfer function of the time delay line can be expressed as:

H(ω)＝|H(ω)|e ^-jφ(ω) (7)

φ(ω) is the phase introduced by the delay of the dispersion fiber 106. According to the Taylor series expansion, the phase can be expressed as:

In the formula, τ(ω ₀ ) is the group delay when the center frequency is ω ₀ , β is the dispersion of the dispersion fiber 106, and its unit is ps ² /km, and β can be expressed as:

In the formula, D (ps/km/nm) is the dispersion coefficient of the dispersion fiber 106, and λ ₀ is the wavelength of the broadband light source 101.

According to formulas (5)-(9), the response function of the photoelectric oscillator can be obtained as:

in

It can be seen that the center frequency of the microwave signal output by the photoelectric oscillator can be expressed as:

Thus, the beat length of the highly birefringent polarization-maintaining fiber 103 to be tested can be obtained as:

As can be seen from the above formula, according to the center frequency of the photoelectric oscillator output microwave signal, the center wavelength of the broadband light source 101, the dispersion coefficient and the length of the dispersion fiber 106 and the length of the high birefringence polarization-maintaining fiber 103 to be measured, the height to be measured can be obtained. The beat length of the birefringent polarization-maintaining fiber 103. From formula (12), if the beat length of the high birefringence polarization-maintaining fiber 103 to be measured is 1m, the dispersion coefficient of the dispersion fiber 106 is -150ps/km/nm, the length of the dispersion fiber 106 is 1km, and the central wavelength of the light source is 1550nm, then The beat length measurement resolution can reach 0.4mm. Since the dispersion coefficient of the dispersion fiber 106 is generally several fixed values, the resolution of beat length measurement can be improved by increasing the length of the dispersion fiber 106 .

The workflow of the high birefringence polarization-maintaining fiber beat length measurement device is as follows:

After being powered on, the driving board of the electro-optic modulator 105 automatically controls the intensity-type optical modulator to work at the linear operating point through a program. After the working point of the electro-optic modulator 105 is determined, the center frequency of the microwave signal output by the photoelectric oscillator. According to formula (12), the beat length of the high birefringence polarization-maintaining fiber 103 to be tested can be obtained.

Finally, it should be noted that the above are only preferred embodiments of the present invention, and are not intended to limit the present invention. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art can still understand The technical solutions recorded in the foregoing embodiments are modified, or some of the technical features are equivalently replaced. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principles of the present invention shall be included within the protection scope of the present invention.

Claims (4)

1. A measurement device for polarization-maintaining optical fiber beat length, characterized in that it comprises a wide-spectrum light source (101), an optical fiber polarizer (102), a spectrum analyzer (201) and a computer (202), and a wide-spectrum light source (101) Enter the high birefringence polarization-maintaining fiber (103) to be tested through the optical fiber polarizer (102), the signal output end of the high birefringence polarization-maintaining fiber to be tested (103) is connected with a polarization analyzer (104), and the high birefringence polarization-maintaining fiber to be tested The polarizing fiber (103) and the polarizer (104) together form a Mach-Zehnder interferometer, the signal output end of the Mach-Zehnder interferometer has an electro-optic modulator (105), and the light-borne microwave signal on the electro-optic modulator (105) After passing through the dispersion fiber (106), it is incident on the high-speed photodetector (107), and the high-speed photodetector (107) converts the optical signal into a microwave signal and amplifies it through the low-noise amplifier (108), and the low-noise amplifier (108) outputs The microwave power divider (109) is connected to the microwave power divider (109), and the microwave power divider (109) injects a part of the microwave signal into the electro-optical modulator (105), and simultaneously inputs another part of the microwave signal into the spectrum analyzer (201), and the spectrum analyzer (201) terminal A computer is connected (202). 2. The measuring device of a kind of polarization-maintaining optical fiber beat length according to claim 1, characterized in that, the high birefringence polarization-maintaining optical fiber (103) to be measured is connected with the optical fiber polarizer (102) by fusion splicing, and the fusion splicing The angle is 45°. 3. The measuring device of a kind of polarization-maintaining fiber beat length according to claim 3, characterized in that, electro-optic modulator (105), dispersion fiber (106), high-speed photodetector (107), low noise amplifier (108 ) and microwave power divider (109) to form a photoelectric oscillator, and the input end of the photoelectric oscillator loop is connected to the output end of the Mach-Zehnder interferometer. 4. A measuring device for the beat length of polarization maintaining optical fiber according to claim 3, characterized in that the wide-spectrum light source (101) can use a Gaussian or rectangular light source as the emission light source.