CN104792502B - A kind of method for measuring reflectivity of weak fiber bragg grating - Google Patents

Abstract

The invention relates to a method for measuring the reflectivity of weak optical fiber gratings, belonging to the technical field of spectrum measurement. The method specifically includes the following steps: placing the fiber grating to be tested in a low-reflectivity fiber grating reflectivity test system; then cleaning the UPC type fiber optic connector end of the fiber grating to be tested, and placing it in the medium 1 to obtain The reflection spectrum of the fiber Bragg grating under test; then keep the system fixed, quickly clean the UPC fiber optic connector and place it in the medium 2, and obtain the reflection spectrum of the fiber Bragg grating under test again; finally use the peak value of the two reflection spectra to calculate the measured Reflectivity of weak fiber gratings. A method for measuring the reflectivity of weak fiber gratings provided by the present invention calculates and measures the reflectivity of weak fiber gratings by obtaining double reflection spectra, which eliminates the influence of fiber end faces and fiber connector connections on the measurement, and greatly reduces the power of the light source The problem of jitter can meet the requirements of accurately measuring the reflectivity of weak fiber gratings.

Description

A Method of Measuring the Reflectivity of Weak Fiber Bragg Grating

technical field

The invention belongs to the technical field of spectrum measurement technology and relates to a method for measuring the reflectivity of weak optical fiber gratings.

Background technique

Fiber Bragg Grating is a kind of fiber optic device widely used in the fields of fiber optic communication, fiber laser, fiber optic sensing, etc. It has the function of reflecting specific wavelengths. When applying fiber gratings, the reflectivity is an index to characterize the performance of fiber gratings. According to the reflectivity, fiber gratings are divided into strong and weak points. In fiber optic communication and fiber laser applications, weak fiber gratings often play an important role in filtering and wavelength locking; when applied to fiber optic sensing, it helps to avoid the limitation of multiplexing capacity and realize high-density quasi-distributed sensing. When using weak fiber gratings for design or application, it is very important to be able to know the size of its reflectivity, but it is a difficult problem to measure its reflectivity accurately.

For the measurement of the reflectivity of the fiber Bragg grating, it can be calculated by obtaining the transmission spectrum or reflection spectrum of the fiber Bragg grating.

It is a commonly used method to measure the reflectivity of FBG by its transmission spectrum. When obtaining the transmission spectrum of the fiber grating, a broadband light source or a tunable laser can be used as an input light source, and a spectrometer or an optical power meter can be used to obtain the transmission spectrum. Finally, use the formula R=1-10 ^{-△T(dBm)/10} to calculate the reflectivity of the fiber grating, where △T(dBm) is the depth of the valley in the transmission spectrum, as shown in Figure 1. However, the value of ΔT (dBm) decreases with the decrease of the reflectivity of the fiber grating. When the reflectivity is less than 20%, the value of ΔT (dBm) will be less than 1dB. The unevenness of the input light source, the jitter of the output power, and the inaccuracy of the power measurement of the spectrometer and optical power meter will greatly affect the determination of the actual △T (dBm), resulting in a large measurement error of the reflectivity. Therefore, for weak fiber gratings (R<20%), it is not suitable to measure the reflectance by obtaining the transmission spectrum.

Contents of the invention

In view of this, the object of the present invention is to provide a method for measuring the reflectivity of weak fiber gratings, which greatly reduces the problem of power jitter of the light source and eliminates the influence of the unevenness of the light source on the test.

To achieve the above object, the present invention provides the following technical solutions:

A method for measuring the reflectivity of a weak fiber grating, the method comprising the following steps:

S1: Place the fiber Bragg grating to be tested in a low reflectivity fiber Bragg grating reflectivity test system;

S2: Clean the UPC fiber optic connector end of the fiber grating under test, and place it in medium 1 to obtain the reflection spectrum of the fiber grating under test; the refractive index of the medium 1 is a known parameter;

S3: keep the system fixed, quickly clean the UPC fiber optic connector and place it in the medium 2, and obtain the reflection spectrum of the measured fiber grating again; the refractive index of the medium 2 is a known parameter;

S4: Calculate the reflectance of the measured weak fiber grating by using the peak values of the two reflection spectra.

Further, in S4, the reflectivity of the measured weak fiber grating is calculated by the following formula,

Among them, R _s1 is the interface reflectance of the fiber end face of the UPC fiber connector placed in the medium 1, R _s2 is the interface reflectance of the fiber end face of the UPC fiber connector placed in the medium 2; _RF is the measured fiber grating reflection ΔP (dBm) is the peak difference between the two reflection spectra; the reflectance of the measured fiber grating and the reflectance of the interface where the UPC fiber connector is placed in medium 1 and medium 2 are all greater than 10 ^-4 .

Further, the R _s1 and R _s2 are calculated by the following formula,

Among them, n ₁ is the refractive index of the fiber core, and n ₂ is the refractive index of medium 1 or medium 2.

Further, the low-reflectivity fiber grating reflectivity test system in the S1 includes an APC-type fiber optic connector, a UPC-type fiber optic connector, a 1X2 splitter, a broadband light source and a spectrometer, and a fiber Bragg grating to be tested;

One end of the fiber grating to be tested is fused with an APC-type optical fiber connector, and the other end is fused with a UPC-type optical fiber connector; the APC-type optical fiber connector is connected to one end of the 1X2 splitter; the other two ends of the 1X2 splitter are respectively connected to a broadband light source connected to the spectrometer.

The beneficial effects of the present invention are: a method for measuring the reflectivity of weak fiber Bragg gratings provided by the present invention is to calculate and measure the reflectivity of weak fiber Bragg gratings by obtaining double reflection spectra. The optical fiber connector and the reasonable selection of two reflection interfaces eliminate the influence of the optical fiber end face and the optical fiber connector connection on the measurement; in addition, the optical connection remains fixed during the measurement, and the interval between two spectral measurements is short, which can greatly reduce the elimination of light sources The problem of power jitter; from the principle of double-spectrum calculation, it is known that it uses the ratio of power at the same wavelength, and there is no need to determine the valley depth or peak height mentioned earlier, thus eliminating the unevenness of the light source in principle impact on testing. This method can meet the requirement of accurately measuring the reflectivity of weak fiber gratings.

Description of drawings

In order to make the purpose, technical solutions and advantages of the present invention clearer, the present invention will be described in further detail below in conjunction with the accompanying drawings, wherein:

Figure 1 shows the principle of measuring the reflectivity of fiber gratings by transmission spectrum;

Fig. 2 is a flowchart of the method of the present invention;

Figure 3 is a low reflectivity fiber grating reflectivity test system;

Fig. 4 is a principle diagram of accurately calculating the reflectivity of weak fiber gratings by double reflection spectrum;

Figure 5 shows the actual measurement results of the low-reflectivity fiber grating by the dual-spectrum method.

detailed description

The preferred embodiments of the present invention will be described in detail below with reference to the accompanying drawings.

The present invention provides a method for measuring the reflectivity of weak fiber Bragg gratings. The method calculates and measures the reflectivity of weak fiber Bragg gratings by acquiring double reflection spectra, which can reduce and eliminate the unevenness of the input light source and output power jitter, fiber end faces and fiber optics. Influence of splice connection etc. on weak FBG reflectivity measurement. The method flow chart is shown in Figure 2, and specifically includes the following steps;

S1: Place the fiber Bragg grating to be tested in a low reflectivity fiber Bragg grating reflectivity test system;

The low-reflectivity fiber grating reflectivity test system is shown in Figure 3. The system includes APC-type fiber connectors, UPC-type fiber connectors, 1X2 splitters, broadband light sources and spectrometers, and fiber gratings under test;

One end of the fiber grating under test is fused with an oblique spherical (APC) optical fiber connector, and the other end is fused with a super physical end face (UPC) optical fiber connector. Connect the APC end to one end of the 1X2 splitter (APC optical fiber connector), and then connect the 1X2 The other two ends of the router are respectively connected to the broadband light source and the spectrometer;

S2: Clean the UPC type fiber optic connector end of the fiber grating under test, and place it in medium 1 to obtain the reflection spectrum of the fiber grating under test; the refractive index of the medium 1 is a known parameter; the medium 1 can be Air or a liquid with known refractive index.

S3: Keep the system fixed, quickly clean the UPC fiber optic connector and place it in the medium 2, and obtain the reflection spectrum of the fiber grating under test again; the refractive index of the medium 2 is a known parameter; the medium 2 can be a A liquid with a known refractive index.

S4: Calculate the reflectance of the measured weak fiber grating by using the peak values of the two reflection spectra.

When selecting the reflective medium in S2 and S3, try to select the reflectance difference between the two mediums is relatively large, so as to ensure that the two reflective spectra have obvious peak differences, which is convenient for calculation.

The principle of calculating the reflectivity of weak fiber gratings by using double reflection spectrum:

Assuming that the two reflection spectra obtained by the above test method are shown in Figure 4, since the return loss of the APC fiber optic connector can reach 60dB, which is equivalent to the reflectance of its end face is 10 ^-6 , as long as the reflectance of the fiber grating and the UPC end The reflectivity is above 10 ^-4 , and the influence of reflection at the fiber connection can be basically ignored. Therefore, the light only has multiple reflections between the interface between the weak fiber grating and the UPC fiber connector, and its equivalent reflectivity R _eff can be expressed as

Among them, R _F is the reflectivity of the fiber grating, and R _s is the reflectivity of the UPC end interface.

Calculate the reflectivity of the fiber grating under test, and find the peak power at the center wavelength of the two reflection spectra respectively, and the ratio of the peak power is equal to the ratio of the equivalent reflectivity under the two reflection interfaces, from which it can be concluded that

Among them, R _s1 is the interface reflectance of the fiber end face of the UPC fiber connector placed in the medium 1, R _s2 is the interface reflectance of the fiber end face of the UPC fiber connector placed in the medium 2; _RF is the measured fiber grating reflection ΔP (dBm) is the peak difference between the two reflection spectra; the reflectance of the measured fiber grating and the reflectance of the interface where the UPC fiber connector is placed in medium 1 and medium 2 are all greater than 10 ^-4 .

Through formula (2), the following quadratic equation can be obtained

By solving this equation, the reflectivity of the fiber grating under test can be calculated.

Since the fiber end face of the UPC fiber optic connector is flat, the reflectivity of the interface placed in the medium can be calculated according to formula (4)

In the formula, n ₁ is the refractive index of the fiber core, and n ₂ is the refractive index of medium 1 or medium 2.

In order to satisfy R _s >10 ^-4 , generally deionized water or alcohol can be used as the liquid.

From the above analysis, it can be seen that as long as R _s1 and R _s2 are calculated, and according to the peak power ratio of the two measurements, the reflectance _RF of the fiber grating can be calculated.

Using the following test conditions, the test results shown in Figure 5 can be obtained.

Test conditions: broadband light source (Shanghai Hanyu desktop ASE light source), spectrometer (Ando-AQ6317B), 1X2 fiber splitter, fiber grating under test (the fiber is SMF-28e single-mode fiber, the reflectivity is 3.39%), two kinds of reflection Medium (air, deionized water).

Due to the use of high return loss optical fiber connectors and the reasonable selection of two reflection interfaces when measuring the reflection spectrum twice, the influence of the fiber end face and the connection of the fiber connector on the measurement is eliminated; The interval between sub-spectral measurements is short, which can greatly reduce the problem of eliminating light source power jitter; from the principle of double-spectrum calculation, it is known that it uses the power ratio at the same wavelength, and there is no need to determine the depth of the valley mentioned above Or the peak height, thus eliminating the influence of the unevenness of the light source on the test in principle. Therefore, the method of the present invention can meet the requirement of accurately measuring the reflectivity of weak fiber gratings.

Finally, it should be noted that the above preferred embodiments are only used to illustrate the technical solutions of the present invention and not to limit them. Although the present invention has been described in detail through the above preferred embodiments, those skilled in the art should understand that it can be described in terms of form and Various changes may be made in the details without departing from the scope of the invention defined by the claims.

Claims (2)

1. A kind of 1. method for measuring reflectivity of weak fiber bragg grating, it is characterised in that：This method comprises the following steps：

   S1：Tested optical fiber grating is placed in antiradar reflectivity fiber grating reflectance test system；

   S2：The UPC type fibre-optical splices end cleaning of tested optical fiber grating is clean, and place it into medium 1, obtain tested optical fiber The reflectance spectrum of grating；The refractive index of the medium 1 is known parameters；

   S3：Holding system is fixed, and will be placed in medium 2 rapidly, is obtained again by light-metering after the cleaning totally of UPC types fibre-optical splice again The reflectance spectrum of fine grating；The refractive index of the medium 2 is known parameters；

   S4：The reflectivity of weak optical fiber Bragg grating is tested using the peak computational of two reflectance spectrums；Calculated in S4 by below equation The reflectivity of tested weak optical fiber Bragg grating,

   Wherein, R_s1The interface reflectivity being placed in for the fiber end face of UPC type fibre-optical splices in medium 1, R_s2For UPC type fibre-optical splices The interface reflectivity that is placed in medium 2 of fiber end face；R_FFor tested optical fiber optical grating reflection rate；Δ P (dBm) is two reflected lights The peak difference of spectrum；Reflectivity, the UPC type fibre-optical splices of the tested optical fiber grating are placed in the reflection of the interface in medium 1 and medium 2 Rate is all higher than 10^-4；Antiradar reflectivity fiber grating reflectance test system in the S1 includes APC types fibre-optical splice, UPC type light Fine joint, 1X2 shunts, wideband light source and spectrometer and tested optical fiber grating；

   One end welding APC type fibre-optical splices of the tested optical fiber grating, other end welding UPC type fibre-optical splices；The APC types One end of fibre-optical splice and 1X2 shunts；The other both ends of the 1X2 shunts are connected with wideband light source with spectrometer respectively.
2. A kind of 2. method for measuring reflectivity of weak fiber bragg grating according to claim 1, it is characterised in that：The R_s1And R_s2 Calculated by below equation,

   Wherein, n₁For fiber core refractive index, n₂For the refractive index of medium 1 or medium 2.