CN112781840B - A method for measuring the absorption coefficient of few-mode erbium-doped fibers - Google Patents
A method for measuring the absorption coefficient of few-mode erbium-doped fibers Download PDFInfo
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- 239000000835 fiber Substances 0.000 title claims abstract description 126
- 238000010521 absorption reaction Methods 0.000 title claims abstract description 41
- 238000000034 method Methods 0.000 title claims abstract description 23
- 230000003287 optical effect Effects 0.000 claims abstract description 34
- 238000009826 distribution Methods 0.000 claims abstract description 13
- 229910052691 Erbium Inorganic materials 0.000 claims abstract description 10
- 230000008878 coupling Effects 0.000 claims description 13
- 238000010168 coupling process Methods 0.000 claims description 13
- 238000005859 coupling reaction Methods 0.000 claims description 13
- 230000005284 excitation Effects 0.000 claims description 7
- 239000002245 particle Substances 0.000 claims description 5
- 238000010586 diagram Methods 0.000 claims description 4
- 238000003780 insertion Methods 0.000 claims description 3
- 230000037431 insertion Effects 0.000 claims description 3
- 238000012360 testing method Methods 0.000 claims description 2
- 238000005086 pumping Methods 0.000 abstract description 5
- 238000005259 measurement Methods 0.000 description 10
- 239000013307 optical fiber Substances 0.000 description 9
- 230000005540 biological transmission Effects 0.000 description 6
- -1 erbium ions Chemical class 0.000 description 5
- 238000013461 design Methods 0.000 description 3
- UYAHIZSMUZPPFV-UHFFFAOYSA-N erbium Chemical compound [Er] UYAHIZSMUZPPFV-UHFFFAOYSA-N 0.000 description 3
- 238000004891 communication Methods 0.000 description 2
- 238000005516 engineering process Methods 0.000 description 2
- 238000012966 insertion method Methods 0.000 description 2
- 239000004576 sand Substances 0.000 description 2
- 238000010008 shearing Methods 0.000 description 2
- 238000003466 welding Methods 0.000 description 2
- 238000013459 approach Methods 0.000 description 1
- 230000009286 beneficial effect Effects 0.000 description 1
- 238000007796 conventional method Methods 0.000 description 1
- 230000007547 defect Effects 0.000 description 1
- 230000002950 deficient Effects 0.000 description 1
- 238000011161 development Methods 0.000 description 1
- 230000000694 effects Effects 0.000 description 1
- 238000002474 experimental method Methods 0.000 description 1
- 239000000463 material Substances 0.000 description 1
- 238000010606 normalization Methods 0.000 description 1
- 230000010287 polarization Effects 0.000 description 1
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Abstract
The invention discloses a method for measuring the absorption coefficient of a few-mode erbium-doped fiber, which comprises the steps of measuring the power of signal light and pump light which are input and output to the few-mode erbium-doped fiber under the pumping condition of a fundamental mode fiber to obtain the gain of the fundamental mode signal light and calculate the population inversion number; then changing the power of the pump light, measuring a change curve of gain along with population inversion, and fitting rho according to the curvelGain G when equal to 00Thereby calculating the absorption coefficient of the fundamental mode optical signal; and finally, calculating the mode field distribution of each mode according to the erbium ion concentration and the refractive index distribution of the few-mode erbium-doped fiber, and further calculating the absorption coefficient of the high-order mode signal light of the few-mode erbium-doped fiber according to the measured fundamental mode absorption coefficient.
Description
Technical Field
The invention belongs to the technical field of optical communication, and particularly relates to a method for measuring the absorption coefficient of a few-mode erbium-doped optical fiber.
Background
At present, based on technologies such as wavelength division multiplexing, polarization multiplexing, multi-level modulation and the like, the transmission capacity of a single optical fiber can reach 100Tbit/s, and the transmission capacity of a Single Mode Fiber (SMF) approaches the Shannon limit. To further increase the transmission capacity of the fiber, it is necessary to utilize a new degree of fiber-spatial mode. The Mode Division Multiplexing (MDM) technology based on few-mode optical fiber has great potential in improving transmission capacity, and is continuously concerned at home and abroad.
The wavelength division multiplexing long-distance transmission system based on the single-mode fiber can not be separated from the traditional erbium-doped fiber amplifier (EDFA); naturally, the long-distance transmission of the Mode Division Multiplexing (MDM) fiber communication also necessarily requires the few-mode EDFA, wherein many parameter information such as the refractive index distribution, doping concentration, absorption coefficient and the like of the adopted few-mode erbium-doped fiber are essential to the design and development of the few-mode EDFA. For example, the absorption loss coefficients of erbium-doped fibers in different modes directly affect the differential-mode gain characteristics of few-mode EDFA systems.
It is known that some parameters are determined during the drawing of erbium-doped fiber, but parameters such as absorption coefficient (related to absorption cross section) of each mode of few-mode erbium-doped fiber are needed to be further measured. Conventionally, the absorption coefficient of an erbium-doped fiber is experimentally measured by an insertion method or a shearing method, and for a few-mode fiber, when a mode optical signal to be tested is injected into the few-mode erbium-doped fiber, other crosstalk modes are usually excited, so that the measurement result of the insertion method is inaccurate. The shearing method causes loss to the optical fiber, which increases the experimental cost, and especially in the present stage, the price of the few-mode erbium-doped optical fiber is very expensive, which is about 30-50 times of that of the single-mode erbium-doped optical fiber. More importantly, the measurement results of these conventional methods further introduce errors due to the absorption effect of erbium ions in the erbium deficient erbium fiber. Therefore, how to accurately measure the absorption coefficient of the used few-mode erbium-doped fiber in the working bandwidth range of the few-mode EDFA becomes a problem to be solved in the design of the few-mode EDFA.
Disclosure of Invention
The invention aims to overcome the defects of the prior art and provides a method for measuring the absorption coefficient of a few-mode erbium-doped fiber.
In order to achieve the above object, the present invention provides a method for measuring the absorption coefficient of a few-mode erbium-doped fiber, which is characterized by comprising the following steps:
(1) completing the power test of the sample fiber;
(1.1) selecting a section of bare fiber with L length and less mode erbium doping as a sample fiber, and welding two ends of the sample fiber with a single-mode tail fiber; then, calculating a mode excitation relation graph between the single-mode tail fiber and the sample fiber according to the refractive index distribution of the single-mode tail fiber and the sample fiber, thereby obtaining the mode coupling efficiency and the crosstalk magnitude;
(1.2) carrying out wavelength division multiplexing on the signal light and the pump light through a single-mode wavelength division multiplexer;
(1.3) injecting single-mode multiplexed light into the 1: in the 99 optical splitter, the optical power of 1% output port is measured by an optical power meter, and the optical power of the signal output from the 99% port is calculated according to the splitting ratio of the optical splitter
And pump light power
(1.4) injecting the multiplexed light output by the 99% port into the sample fiber through the single-mode tail fiber, and calculating the signal light power injected into the sample fiber according to the mode coupling efficiency and the crosstalk between the single-mode tail fiber and the sample fiber
And pump light power
(1.5) separating the signal light and the pump by the single-mode wavelength division multiplexer, and measuring the power of the signal light respectively
And pump light power
Then, calculating the signal light power output by the sample fiber according to the mode coupling efficiency and crosstalk from the sample fiber to the single-mode tail fiber and the insertion loss of the single-mode wavelength division multiplexer
And pump light power
(2) Calculating the absorption coefficient of a basic mode in the sample fiber;
(2.1) calculating the signal light gain G;
(2.2) calculating the inverse particle number ρl；
Wherein v issAnd vpFrequency of the signal light and the pump light, h is Planck constant, T1Is the relaxation time;
(2.3) changing the pump power input into the sample fiber without changing the optical power of the input signal
Then according to the method of the steps (2.1) to (2.2), respectively calculating different pumping powers
G and p atlThen drawing G-rholA curve;
(2.4) fitting to obtain G-rholCurve and rholIntercept G of 00Then calculating the base mode LP in the sample fiber01Absorption coefficient of signal light
(3) And repeating the step (2) to calculate the absorption coefficient of the sample fiber to the pump light
(4) Calculating the absorption coefficient of a high-order mode in the sample fiber;
wherein N is0Is the concentration of erbium ions in the sample fiber,
the normalized mode field distribution of each mode signal light corresponds to a fundamental mode when i is 0, and (x, y) is the abscissa and ordinate of the cross section of the optical fiber.
The invention aims to realize the following steps:
the invention relates to a method for measuring the absorption coefficient of a few-mode erbium-doped fiber, which comprises the steps of measuring the power of signal light and pump light which are input and output to the few-mode erbium-doped fiber under the pumping condition of a fundamental mode fiber to obtain the gain of the fundamental mode signal light and calculate the population inversion number; then changing the power of the pump light, measuring a change curve of gain along with population inversion, and fitting rho according to the curvelGain G when equal to 00Thereby calculating the absorption coefficient of the fundamental mode optical signal; and finally, calculating the mode field distribution of each mode according to the erbium ion concentration and the refractive index distribution of the few-mode erbium-doped fiber, and further calculating the absorption coefficient of the high-order mode signal light of the few-mode erbium-doped fiber according to the measured fundamental mode absorption coefficient.
Meanwhile, the method for measuring the absorption coefficient of the few-mode erbium-doped fiber has the following beneficial effects:
(1) the invention implements measurement when the few-mode erbium-doped fiber is in an optical pumping state, is consistent with the actual working state of the few-mode EDFA, and has more accurate measurement result compared with the traditional method in a passive non-working state.
(2) The measuring system of the invention is composed of single-mode optical devices with lower cost, complex spatial coupling devices or expensive spatial light modulators and the like are not needed, and the method is easy to implement.
(3) The method can complete the measurement of the absorption loss of each mode of the few-mode erbium-doped fiber only by measuring the optical power of a small section of erbium-doped fiber, does not need to cut off the erbium-doped fiber for multiple times for measurement, saves expensive experimental materials for experiments, and has lower experimental cost.
Drawings
FIG. 1 is a flow chart of a method for measuring the absorption coefficient of a few-mode erbium-doped fiber according to the present invention;
FIG. 2 is a diagram of a measurement apparatus for mode loss coefficients of a few-mode erbium-doped fiber;
FIG. 3 is a graph showing the refractive index distributions and mode excitations of G.652 single-mode and erbium-doped fibers to be tested;
FIG. 4 is a diagram of the fundamental mode LP of the fiber01Gain G with inverse particle number ρlMeasurement profile of change.
Detailed Description
The following description of the embodiments of the present invention is provided in order to better understand the present invention for those skilled in the art with reference to the accompanying drawings. It is to be expressly noted that in the following description, a detailed description of known functions and designs will be omitted when it may obscure the subject matter of the present invention.
Examples
FIG. 1 is a flow chart of a method for measuring the absorption coefficient of a few-mode erbium-doped fiber according to the present invention.
In this embodiment, as shown in fig. 1, a method for measuring an absorption coefficient of an erbium-doped fiber with few modes according to the present invention includes the following steps:
s1, welding sample fibers;
in this embodiment, fig. 2 shows a diagram of an apparatus for measuring the absorption coefficient of an erbium-doped fiber with few modes; selecting a section of bare erbium-doped fiber with a length L of 1.9m as a sample fiber, and fusing two ends of the sample fiber with a G.652 single-mode tail fiber together, as shown in fig. 3 (a); the refractive indexes of the single-mode tail fiber and the sample fiber are shown in fig. 3(b), and a mode excitation relation graph between the single-mode tail fiber and the sample fiber is calculated according to the refractive index distribution of the single-mode tail fiber and the sample fiber, so that the mode coupling efficiency and the crosstalk magnitude are obtained;
in this embodiment, the mode excitation relationship between the g.652 single mode and the erbium-doped fiber to be tested is shown in fig. 3(c) and fig. 3(d), wherein fig. 3(c) shows the excitation process of 1550nm wavelength light, and fig. 3(d) shows the 980nm wavelength lightThe erbium-doped fiber adopts fiber core uniform doping, and the doping concentration of erbium ions is N0＝9.8184×1024m-3。
As can be seen from FIG. 3(a), LP01The coupling loss of the signal light from the single-mode tail fiber to the few-mode erbium-doped fiber or from the few-mode erbium-doped fiber to the single-mode tail fiber is about 1.6 dB; the coupling loss of the input pump light from the single-mode tail fiber to the few-mode erbium-doped fiber is 0.29dB, and the loss of the output pump light from the few-mode erbium-doped fiber to the single-mode tail fiber is 2.8 dB; the additional loss introduced by the other devices is about 0.4 dB.
In addition, the few-mode erbium-doped fiber can be fused with the few-mode tail fiber, and then the information of the mode coupling efficiency and the crosstalk magnitude can be obtained according to a mode excitation relation graph between the few-mode tail fiber and the few-mode tail fiber.
S2, measuring the power of the sample fiber;
s2.1, firstly, according to the measurement requirement, the wavelength of the signal light is set to 1550nm, the wavelength of the pump light is set to 980nm, and the modes are the fundamental mode LP01(ii) a Then, as shown in fig. 2, 1550nm signal light and 980nm pump light are multiplexed and combined by a 1550nm/980nm single-mode wavelength division multiplexer;
s2.2, after the two optical signals are combined, injecting single-mode multiplexing light into the optical fiber 1: in the 99 optical splitter, the optical power of 1% output port is measured by an optical power meter, and the optical power of the signal output from the 99% port is calculated according to the splitting ratio of the optical splitter
And pump light power
S2.3, injecting the multiplexed light output by the 99% port into the sample fiber through the single-mode tail fiber, and calculating the signal light power injected into the sample fiber according to the mode coupling efficiency and the crosstalk between the single-mode tail fiber and the sample fiber
And pump light power
S2.4, separating the signal light and the pump by the 1550nm/980nm wavelength division multiplexer, and measuring the power of the signal light respectively
And pump light power
Then, calculating the signal light power output by the sample fiber according to the mode coupling efficiency and crosstalk from the sample fiber to the single-mode tail fiber and the insertion loss of the single-mode wavelength division multiplexer
And pump light power
S3, calculating the absorption coefficient of the fundamental mode in the sample fiber;
s3.1, calculating signal light gain G;
s3.2, calculating the number rho of inversion particlesl；
Wherein v issAnd vpThe frequencies of the signal light and the pump light are respectively, and v is takens193.4THz, vp306.1THz, h is Planck's constant, h is 6.62607015 × 10-34，T1For relaxation time, take T1＝10ms；
S3.3, changing the pump power input into the sample fiber under the condition of not changing the optical power of the input signal
Then, according to the method of the steps S3.1-S3.2, different pumping powers are respectively calculated
G and p atlThen, as shown in FIG. 4, G to ρ are plottedlA curve;
s3.4, fitting out G to rholCurve and rholIntercept G of 00The fundamental mode LP in the sample fiber is calculated after-19.71 dB01Absorption coefficient of signal light
In the present embodiment, it is preferred that,
s4, calculating the absorption coefficient of the pump light in the sample fiber according to the method in the step S3
S5, calculating the absorption coefficient of the high-order mode in the sample fiber;
wherein N is0Is the concentration of erbium ions in the sample fiber,
the normalized mode field distribution of each mode signal light corresponds to a fundamental mode when i is 0, and (x, y) is the abscissa and ordinate of the cross section of the optical fiber.
In the present embodiment, according to the measurement to be performedErbium particle concentration N of the sample fiber0And LP11Mode normalization mode field distribution to calculate LP of few-mode erbium-doped fiber pair11The absorption loss coefficient of the mode signal light is
Although illustrative embodiments of the present invention have been described above to facilitate the understanding of the present invention by those skilled in the art, it should be understood that the present invention is not limited to the scope of the embodiments, and various changes may be made apparent to those skilled in the art as long as they are within the spirit and scope of the present invention as defined and defined by the appended claims, and all matters of the invention which utilize the inventive concepts are protected.
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