CN114563840A - Ultra-wideband flat all-fiber circular polarizer and manufacturing method thereof - Google Patents
Ultra-wideband flat all-fiber circular polarizer and manufacturing method thereof Download PDFInfo
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Abstract
The invention relates to an ultra-wideband flat all-fiber circular polarizer and a manufacturing method thereof, wherein the manufacturing method comprises the following steps: calculating the grating period range when the fiber core fundamental mode is operated at a dispersion turning point according to a first phase matching condition met by coupling the fiber core fundamental mode to a high-order cladding mode by the double-helix chiral long-period fiber grating; introducing chirp into the double-helix chiral long-period fiber grating to obtain the double-helix chirp type chiral long-period fiber grating; acquiring the grating period of each grating section in the double-helix chirped chiral long-period fiber grating according to the grating period range; establishing a target coupling equation of the double-helix chirp type chiral long-period fiber grating in a local coordinate system; calculating the length of each grating section according to the amplitude relation between the output end and the input end of each grating section and by combining a target coupling equation; and manufacturing the all-fiber circular polarizer according to the grating period and the grating length. The full-optical-fiber circular polarizer can be manufactured by the manufacturing method, and has the advantages of flat working bandwidth, wide bandwidth, small volume and low cost.
Description
Technical Field
The invention belongs to the technical field of optical fiber type passive devices, and particularly relates to an ultra wide band flat all-fiber type circular polarizer and a manufacturing method thereof.
Background
The circular polarization technology has important application in the fields of optical fiber sensing, optical fiber communication and the like. The polarization characteristic of the circularly polarized light has important research significance and application value in the aspects of underwater optical communication, optical fiber gyroscopes, optical fiber current sensors, polarization imaging and the like. Generally, the circular polarization polarizer is composed of discrete components, so that the whole optical network has the defects of large volume, difficulty in integration, large insertion loss, unstable work and the like. With the research and application requirements, higher requirements are put forward on the functions and performances of the circularly polarized light modulation device. In recent years, the optical fiber type circular polarizer becomes a research hotspot, and related performance parameters of the optical fiber type circular polarizer are rapidly optimized. Since the optical fiber type circular polarizer has: the optical fiber type circular polarizer has the advantages of simple structure, easy manufacture and many excellent and unique circular polarization and sensing characteristics, and more research groups are dedicated to the manufacture and application research of the optical fiber type circular polarizer.
However, the current circular polarizer has narrow bandwidth (several nanometers) and uneven power, which not only weakens the spectrum efficiency, communication capacity, detection accuracy and applicability of a communication system, but also has the problem that the working index/performance of the device is sharply reduced due to the working waveband shift generated along with the environmental change. These factors limit the popularity and application of circular polarizers in all-optical communication, sensing, and modulation systems. Meanwhile, the emergence of the optical fiber type circular polarizer brings hope for the design of a high-performance circular polarizer, and more importantly, provides possibility for realizing the transmission of circularly polarized light in an all-fiber system. Therefore, research on optical fiber type circular polarizers has attracted a great deal of attention from many well-known colleges and research institutions at home and abroad. However, the related research is gradually developed, and the research on the optical fiber type circular polarizer with flat broadband which is urgently needed has not been reported. In summary, designing and implementing an all-fiber circular polarizer with ultra-wideband flatness is a topic worthy of further study.
In the prior art, the following techniques are generally used to generate circularly polarized light: circular polarization glass slide, polarization beam splitter prism, spatial light modulator, spiral polarization maintaining fiber and the like. However, the circular polarizer realized by the above technology is large in volume, relatively expensive, narrow in working bandwidth and uneven in power, and most circular polarizers require a complex space structure, have serious dispersion effect of device performance, and cannot realize full optical fiber in an all-optical communication system.
Disclosure of Invention
In order to solve the problems in the prior art, the invention provides an ultra-wideband flat all-fiber circular polarizer and a manufacturing method thereof. The technical problem to be solved by the invention is realized by the following technical scheme:
the embodiment of the invention provides a method for manufacturing an ultra-wideband flat all-fiber circular polarizer, which comprises the following steps:
calculating a grating period range of the double-helix chiral long-period fiber grating when the double-helix chiral long-period fiber grating works at a dispersion turning point according to a first phase matching condition which is met by the double-helix chiral long-period fiber grating when a fiber core fundamental mode is coupled to a high-order cladding mode;
introducing chirp into the double-helix chiral long-period fiber grating to obtain a double-helix chirp type chiral long-period fiber grating;
Acquiring the grating period of each grating section in the double-helix chirped chiral long-period fiber grating according to the grating period range, wherein the grating period changes along with the position of an optical fiber shaft;
establishing a target coupling equation under the interaction between a right-hand circularly polarized fiber core mode and a left-hand circularly polarized cladding mode in the double-helix chirped chiral long-period fiber grating in a local coordinate system;
calculating the length of each grating segment according to the amplitude relation between the output end and the input end of the grating segment and by combining the target coupling equation;
and manufacturing the all-fiber type circular polarizer according to the grating period and the length.
In one embodiment of the present invention, the first phase matching condition is:
λres＝(neff,01-neff,0n)Λ
wherein λ isresDenotes the resonance wavelength, Λ denotes the grating period, neff,01Representation LP01Effective refractive index of fundamental core mode, neff,0nRepresents LP0nThe effective refractive index of the cladding modes.
In one embodiment of the present invention, the grating period of each of the grating segments is:
Λ＝Λ0+cz
wherein, Λ0The initial period size of the chirp period is shown, z is the axial position of the optical fiber, the value of z ranges from 0 to L, c is delta lambda/L, c represents the chirp coefficient, and delta lambda represents the total period variation of the chirp grating.
In one embodiment of the present invention, establishing a target coupling equation between a right-handed circularly polarized core mode and a left-handed circularly polarized cladding mode in the double-helix chirped chiral long-period fiber grating in a local coordinate system includes:
establishing a circular polarization mode coupling equation of a polarization fiber core mold and a cladding mold in a first coordinate system and a second coordinate system in a local coordinate system;
based on the mutual coupling effect between the fiber core mode of the right-handed circular polarization and the cladding mode of the left-handed circular polarization, and the fiber core fundamental mode LP of the double-helix chirp type chiral long-period fiber grating01Coupled to higher-order cladding modes LP0nAnd simplifying the circular polarization mode coupling equation to obtain the target coupling equation under the second phase matching condition.
In one embodiment of the present invention, the circular polarization mode coupling equation is:
where κ and κ' both represent coupling coefficients, τ (τ ═ 2 π/P) represents twist rate, P represents fiber pitch,
showing the amplitude of the right-handed circularly polarizing core layer,
showing the amplitude of the right-handed circularly polarizing core layer,
showing the amplitude of the left-handed circularly polarized core and cladding,
amplitude, beta, of right-handed circularly polarizing core and cladding coRepresenting the core mode HE in an ideal isotropic fiber11The propagation constant of (a) is set,
indicating the normalized electric field distribution of the different modes, respectively, the superscript x or y indicating the polarization direction of the principal transverse component of the mode, epsilon0Denotes the isotropic part of the dielectric constant distribution,. DELTA.. di-elect consxAnisotropic perturbation fraction representing the dielectric constant distribution in the x-polarization mode，ΔεyDenotes the anisotropically perturbed part of the dielectric constant distribution in the y-polarization mode, j denotes the complex number, ω denotes the optical wave angular frequency, and s denotes the integral area.
In one embodiment of the present invention, the second phase matching condition is:
λres＝(neff,01-neff,0n)Λ(z)
wherein λ isresRepresenting the resonance wavelength, Λ representing the grating period, z representing the fiber axis position, neff,01Representation LP01Effective refractive index of fundamental core mode, neff,0nRepresentation LP0nThe effective refractive index of the cladding modes.
In one embodiment of the present invention, the target coupling equation is:
where κ denotes a coupling coefficient, τ (τ ═ 2 π/P) denotes a twist rate, P denotes a fiber pitch,
showing the amplitudes of the right-handed circularly polarized core and cladding layers,
showing the amplitude of the left-handed circularly polarized core and cladding layers; beta is acoRepresenting the core mode HE in an ideal isotropic fiber11The propagation constant of (a) is determined,
indicating the normalized electric field distribution of the different modes, respectively, the superscript x or y indicating the dominance of the mode Polarization direction of transverse component, epsilon0Denotes the isotropic part of the dielectric constant distribution,. DELTA.. di-elect consxShowing the anisotropically perturbed part of the dielectric constant distribution in the x-polarization mode, DeltasimayDenotes the anisotropically perturbed part of the dielectric constant distribution in the y-polarization mode, j denotes the complex number, ω denotes the optical wave angular frequency, and s denotes the integral area.
In one embodiment of the present invention, the amplitude relationship is:
wherein A isco(zM+1) Representing the amplitude of the mode of the core at the output, Acl(zM+1) Representing the amplitude of cladding mode at the output end, FiThe element of the matrix representing the ith raster segment, Aco(z1) 1 denotes the initial condition of the core mold, Acl(z1) 0 denotes the cladding mode initiation condition, aijTwo mode coupling coefficients in matrix elements representing the ith grating segment, k represents the coupling strength, a represents the imaginary part of the coupling strength, zi+1Denotes the i +1 th segment grating length, ziRepresents the ith segment grating length, i represents the grating number.
Another embodiment of the present invention provides an ultra-wideband flat all-fiber circular polarizer, which is a double-spiral chirped chiral long-period fiber grating formed by sequentially connecting a plurality of double-spiral chiral long-period fiber gratings, wherein,
each section of the double-helix chiral long-period fiber grating works in a preset range on two sides of a dispersion turning point, and the grating period of the double-helix chirped chiral long-period fiber grating changes along with the position of an optical fiber shaft.
In one embodiment of the invention, the grating period of each section of the double-helix chiral long-period fiber grating gradually increases or decreases with the increase of the position of the optical fiber axis.
Compared with the prior art, the invention has the following beneficial effects:
in the manufacturing method, the grating period range is obtained when the double-helix chiral long-period fiber grating works at the dispersion turning point, the full-fiber type circular polarizer with the ultra-wideband circular polarization filtering characteristic can be realized, and the chirp is introduced into the double-helix chiral long-period fiber grating, the full-fiber type circular polarizer with the flat filtering characteristic can be realized, so that the full-fiber type circular polarizer with flat working bandwidth, wide bandwidth, small volume and low cost is manufactured.
Drawings
FIG. 1 is a schematic flow chart of a method for manufacturing an ultra-wideband flat all-fiber circular polarizer according to an embodiment of the present invention;
FIG. 2 is a structural diagram of a right-handed dual-spiral chiral long-period fiber grating according to an embodiment of the present invention;
FIGS. 3 a-3 b are schematic diagrams of phase matching curves of a right-handed dual-spiral chiral long-period fiber grating according to an embodiment of the present invention;
fig. 4 is a schematic diagram of a right-handed circularly polarized light transmission spectrum in a right-handed double-helix chiral long-period fiber grating operating at a dispersion turning point according to an embodiment of the present invention;
FIGS. 5 a-5 b are schematic structural diagrams of two types of double-helix chirped chiral long-period fiber gratings according to an embodiment of the present invention;
FIG. 6 is a schematic structural diagram of a double-helix chirped chiral long-period fiber grating obtained by fusion twisting of a polarization maintaining fiber according to an embodiment of the present invention;
FIG. 7 is a schematic diagram illustrating axial refractive index modulation of a double-helix chirped chiral long-period fiber grating according to an embodiment of the present invention;
fig. 8 is a transmission spectrum of a double-helix chirped chiral long-period fiber grating according to an embodiment of the present invention.
Detailed Description
The present invention will be described in further detail with reference to specific examples, but the embodiments of the present invention are not limited thereto.
Example one
Referring to fig. 1, fig. 1 is a schematic flow chart of a method for manufacturing an ultra-wideband flat all-fiber circular polarizer according to an embodiment of the present invention. The manufacturing method of the ultra-wideband flat all-fiber circular polarizer comprises the following steps:
s1, according to the double helix chiral long period fiber grating, the core fundamental mode LP01Coupled to higher-order cladding modes LP0nAnd calculating the grating period range of the double-helix chiral long-period fiber grating working at the dispersion turning point under the satisfied first phase matching condition.
Referring to fig. 2, fig. 2 is a structural diagram of a right-handed double-helix chiral long-period fiber grating according to an embodiment of the present invention. In fig. 2, a polarization maintaining fiber (for example, a panda-type polarization maintaining fiber) in a molten state is twisted at a high speed, and a core forms a spiral path after the twisting, thereby forming a double-helix chiral long-period fiber grating.
It should be noted that the optical fiber shown in fig. 2 is only a schematic diagram, the double-helix chiral long-period fiber grating includes a fiber core, a stress region and a cladding, the fiber core is located in the center of the cladding, the stress region is located inside the cladding, the stress region forms a double-helix path, and the surface of the cladding forms an uneven shape.
In this embodiment, the double-spiral chiral long-period fiber grating may be a right-handed double-spiral chiral long-period fiber grating or a left-handed double-spiral chiral long-period fiber grating.
Unlike conventional long-period fiber gratings, the double-helix chiral long-period fiber grating couples the core fundamental mode LP01Coupled to higher-order cladding modes LP0nAbove, it satisfies the first phase matching condition:
λres＝(neff,01-neff,0n)Λ (1)
wherein λ isresDenotes the resonance wavelength, Λ denotes the grating period, neff,01Represents LP01Effective refractive index of fundamental core mode, neff,0nRepresents LP0nThe effective refractive index of the cladding modes.
Further, the resonant cladding mode of each order of the double helix chiral long period fiber grating can be calculated according to the first phase matching condition of the formula (1). In this embodiment, the resonant cladding mode of the first 14 orders (i.e., m is 2 to 15) of the double-helix chiral long-period fiber grating is calculated according to the first phase matching condition of the formula (1), and for convenience, the resonant cladding mode is given as a phase matching curve, please refer to fig. 3 a-3 b, and fig. 3 a-3 b are schematic diagrams of the phase matching curve of the right-handed double-helix chiral long-period fiber grating according to the embodiment of the present invention, where fig. 3a is a phase matching curve of a cladding mode with a cladding mode order of m is 2 to 8, fig. 3b is a phase matching curve of a cladding mode with a cladding mode order of m is 9 to 15, and a black dot in the diagram indicates a dispersion turning point of the phase matching curve.
As can be seen from fig. 3a, the slope of each curve is always positive for cladding modes with m-2 to 8. In fig. 3b, m is the cladding mode from 9 to 15, the slope of the curve changes from positive to negative, and there is a point where the slope is infinite, as shown by the black point in fig. 2b, which is called the dispersion turning point of the fiber grating. It is apparent that each dispersion inflection point corresponds to the maximum grating period of each resonant cladding mode. For the cladding mode with m being more than or equal to 9, when the grating period is less than the grating period at the dispersion turning point, the slope of the curve is changed from positive to negative, and one grating period corresponds to the resonant wavelengths at two different positions, namely, the double resonance phenomenon is generated. Then, along with the increase of the grating period, the two different resonant wavelength positions are simultaneously close to the resonant wavelength position at the dispersion turning point, and are superposed to one point when the grating period is increased to the grating period at the dispersion turning point, namely the resonant wavelength position corresponding to the dispersion turning point. In this case, the resonance peak at the dispersion inflection point has a wider bandwidth than that at the single resonance due to the mutual superposition of the dual resonance peaks.
Further, the bandwidth of the resonance peak of the fiber grating can be expressed as:
in the formula (2), Δ λ3dB3-dB bandwidth representing the resonant peak of the fiber grating, L grating length, Δ ngRepresenting the group index difference between the core and the cladding.
As can be seen from the formula (2), the closer the resonance wavelength is to the dispersion inflection point of the fiber grating, the group refractive index difference Δ n between the core and the claddinggThe smaller and thus the wider the resonance peak bandwidth of the fiber grating.
Therefore, by designing the grating parameters of the double-helix chiral long-period fiber grating, the double-helix chiral long-period fiber grating works at the dispersion turning point of the high-radial-order cladding mode, and can effectively generate an ultra-wideband circular polarization mode. In the embodiment, the grating period range of the double-helix chiral long-period fiber grating is set, so that the double-helix chiral long-period fiber grating can work near a dispersion turning point; in other words, when the double-helix chiral long-period fiber grating works near the dispersion turning point, a wider resonance peak bandwidth can be realized, and therefore, a period in a certain range at two sides of a grating period corresponding to the dispersion turning point is taken as a grating period range. Specifically, the grating period range can be set according to actual requirements, the larger the period range is, the wider the working bandwidth is, but the farther the period is from the dispersion turning point, the weaker the coupling strength is, for example, the period at the dispersion turning point of the double-helix chiral long-period fiber grating is 160 μm, 159 to 161 μm can be selected as the grating period range, and 3dB bandwidth to 200nm can be obtained. And the grating period range can determine the twisting speed of the melting and twisting of the polarization maintaining fiber.
Further, according to the first phase matching condition of the equation (1), taking a right-handed double-helix chiral long period fiber grating as an example, the core mode (LP) thereof is selected01) And cladding mode (LP)0,11) Please refer to fig. 4, fig. 4 is a schematic diagram of a right-handed circularly polarized light transmission spectrum of a right-handed double-helix chiral long-period fiber grating operating at a dispersion turning point according to an embodiment of the present invention, and fig. 4 is a schematic diagram of a fiber core model (LP) of the right-handed double-helix chiral long-period fiber grating01) Respectively associated with cladding modes (LP)02，LP0,11) Coupling is performed, wherein, cladding mode LP02Not operating at the point of dispersion transition, cladding mode LP0,11Operating at the dispersion break-over point. As can be seen in fig. 4, the modal coupling bandwidth is significantly increased for operation at the dispersion break-over point compared to the modal coupling not operating at the dispersion break-over point. Therefore, the broadband mode coupling can be realized based on the dispersion turning characteristic of the double-helix chiral long-period fiber grating.
And S2, introducing chirp into the grating period range to obtain the double-helix chirp type chiral long-period fiber grating.
Generally, to achieve broadband transmission, chirped long period fiber gratings need to have a substantial chirp period variation to achieve a wide range of wavelength resonances. In this case, the grating length needs to be increased to increase the effective coupling length at different resonance wavelength positions in the spectrum; for example, a chirp period variation of 41 μm is required to achieve a wideband transmission of 3-dB bandwidth 100nm, and a chirp grating length of up to 41.5cm is required. The grating length is too large, so that inconvenience is caused in practical application, packaging becomes a great challenge due to the too long grating length, and in addition, the spectrum quality is reduced due to the fact that the longer grating is more easily affected by external factors, and the application is very unfavorable. In addition, for the chiral long period fiber grating working at the non-dispersion turning point, the mode coupling of the core fundamental mode and a plurality of cladding modes occurs in a wide resonance wavelength range, which greatly affects the conversion efficiency and the mode purity of the circular polarization mode, so that the spectral performance of the fiber grating is limited.
Specifically, the double-helix chiral long-period fiber grating in step S1 realizes ultra-wideband circular polarization mode conversion with a 3-dB bandwidth of about 190nm, however, at such a bandwidth, the extinction ratio of the spectral resonance peak is not flat, i.e., there is a large difference between the extinction ratios at different wavelength positions within the broadband operating wavelength, as shown in fig. 4, the extinction ratio can reach 60dB at the center of the resonance peak and only 3dB at the edge. Since the extinction ratio of the resonance peak has a great influence on the mode purity of the converted circular polarization mode, chirp is introduced to the double-helix chiral long-period fiber grating operating at the dispersion turning point in step S1 in order to obtain an ultra-wideband and flat circularly polarized light beam.
Referring to fig. 5 a-5 b, fig. 5 a-5 b are schematic structural diagrams of two types of double-spiral chirped chiral long-period fiber gratings according to an embodiment of the present invention, where fig. 5a is a right-handed double-spiral chirped chiral long-period fiber grating, and fig. 5b is a left-handed double-spiral chirped chiral long-period fiber grating.
Specifically, after chirp is introduced to the period of the double-helix chiral long-period fiber grating, the grating period is no longer constant, but changes along the axial direction of the fiber, as shown in fig. 5a and 5 b. For the double-helix chirp type chiral long-period fiber grating with the length of L, the grating can be divided into M grating sections, and each grating section can be regarded as a double-helix chiral long-period fiber grating with uniform period; the length of each grating section is determined by the coupling strength of the grating section, the stronger the coupling strength is, the shorter the length of the grating section is, and the weaker the coupling strength is, the longer the length of the grating section is; each grating section works at the dispersion turning point, and the grating period of each grating section is located near the dispersion turning point of the double-helix chiral long-period fiber grating. The number of grating segments depends on different system accuracy requirements, for example, M may be taken to be 100.
Furthermore, by designing the grating parameters (including the grating period and the length of each grating section) of each grating section, namely the double-helix chiral long-period fiber grating, the fiber grating works at the dispersion turning point of a high radial order cladding mode, and the ultra-wideband circular polarization mode coupling can be effectively generated.
S3, obtaining the grating period of each grating section in the double-helix chirped chiral long-period fiber grating according to the grating period range, wherein the grating period changes along with the position of the optical fiber axis.
Specifically, the period of each grating segment is:
Λ＝Λ0+cz (3)
wherein Λ is0The initial period size of the chirp period is represented, z is the axial position of the optical fiber, the value of z ranges from 0 to L, c is delta lambda/L, c represents the chirp coefficient, and delta lambda represents the total period variation of the chirp grating.
It can be understood that the period of each grating segment is located near the dispersion turning point of the double-helix chiral long-period fiber grating, the grating period of the grating segment located at the center is equal to the period corresponding to the dispersion turning point of the double-helix chiral long-period fiber grating, the grating period of the grating segment located at the starting end is close to the smaller period in the range of the grating period, and the grating period of the grating segment located at the tail end is close to the larger period in the range of the grating period.
S4, establishing a target coupling equation under the interaction between a right-handed circularly polarized fiber core mode and a left-handed circularly polarized cladding mode in the double-helix chirped chiral long-period fiber grating in a local coordinate system. The method specifically comprises the following steps:
and S41, establishing circular polarization mode coupling equations of the polarization fiber core mode and the cladding mode in the first coordinate system and the second coordinate system in the local coordinate system.
Specifically, similar to the double-helix chiral long-period fiber grating, the double-helix chirped chiral long-period fiber grating couples the core fundamental mode LP01Coupled to cladding mode LP0nAt the dispersion turning point (n is more than or equal to 2), the second phase matching condition is met:
λres＝(neff,01-neff,0n)Λ(z) (4)
wherein λ isresRepresenting the resonance wavelength, Λ representing the grating period, z representing the fiber axis position, neff,01Represents LP01Effective refractive index of fundamental core mode, neff,0nRepresents LP0nThe effective refractive index of the cladding modes.
The core layer and the cladding layer of the right-handed chiral long-period fiber grating formed by the twisted commercial panda fiber have radius distribution of a being 4.15 mu m and b being 62.5 mu m, and the refractive indexes of the core and the cladding are n respectively11.452 and n21.444, panda polarization maintaining fiber beat length LbIs 2.5mm, so that the anisotropy perturbation difference Delta epsilon of the dielectric constant x-ΔεyThis can be obtained from the following relationship:
where B is the birefringence of the panda fiber.
Through calculation, the anisotropy fluctuation difference Delta epsilon of the dielectric constantx-ΔεyIs 1.8X 10-3。
Based on this, taking a panda fiber with a right-handed helical structure and high twist rate as an example, a linear coupling mode equation of a first coordinate system x, a second coordinate system y polarization fiber core mode and a cladding mode is directly established in a local coordinate system, and then a mode conversion is used to convert a linear polarization mode into a circular polarization mode, so that the coupling equation of the circular polarization mode is obtained as follows:
where κ and κ' both represent coupling coefficients, τ (τ ═ 2 π/P) represents twist rate, P represents fiber pitch,
showing the amplitude of the right-handed circularly polarizing core layer,
showing the amplitude of the right-handed circularly polarizing core layer,
showing the amplitude of the left-handed circularly polarized core and cladding,
amplitude, beta, of right-handed circularly polarizing core and claddingcoRepresenting the core mode HE in an ideal isotropic fiber11The propagation constant of (a) is determined,
indicating the normalized electric field distribution of the different modes, respectively, the superscript x or y indicating the polarization direction of the principal transverse component of the mode, epsilon0Denotes the isotropic part of the dielectric constant distribution,. DELTA.. di-elect consxShowing the anisotropically perturbed part of the dielectric constant distribution in the x-polarization mode, Deltasima yDenotes the anisotropically perturbed part of the dielectric constant distribution in the y-polarization mode, j denotes the complex number, ω denotes the angular frequency of the light wave, and s denotes the integral area.
S42, simplifying the circular polarization mode coupling equation based on the mutual coupling effect between the right-hand circular polarization fiber core mode and the left-hand circular polarization cladding mode and the second phase matching condition which is met by the double-helix chirped chiral long-period fiber grating by coupling the fiber core fundamental mode LP01 to the high-order cladding mode LP0n, and obtaining the target coupling equation.
In particular, for all coupling modes, βcoGreater than betaclFor right-handed configurations, τ (z) is a positive value (corresponding to the situation shown in fig. 5 a), and therefore the second phase matching condition can only be met when the following relation is met:
βco-τ(z)＝βcl+τ(z) (8)
in theoretical research, only the right-handed circularly polarized (RCP) core mode and the left-handed circularly polarized (LCP) cladding mode are considered to be strongly coupled, and other modes with low coupling strength are not considered, so equation (6) is simplified in combination with the coupling effect between the right-handed circularly polarized (RCP) core mode and the left-handed circularly polarized (LCP) cladding mode and the second phase matching condition, and a target coupling equation representing the coupling between the RCP core mode and the LCP cladding mode is obtained:
Wherein,
where κ denotes a coupling coefficient, τ (τ ═ 2 π/P) denotes a twist rate, P denotes a fiber pitch,
showing the amplitudes of the right-handed circularly polarized core and cladding layers,
showing the amplitude of the left-handed circularly polarized core and cladding layers; beta is a betacoRepresenting the core mode HE in an ideal isotropic fiber11The propagation constant of (a) is determined,
indicating the normalized electric field distribution of the different modes, respectively, the superscript x or y indicating the polarization direction of the principal transverse component of the mode, epsilon0Denotes the isotropic part of the dielectric constant distribution,. DELTA.. di-elect consxShowing the anisotropically perturbed part of the dielectric constant distribution in the x-polarization mode, DeltasimayThe anisotropic perturbation part of the dielectric constant distribution in the y-polarization mode is shown, j represents a complex number, omega represents the angular frequency of the light wave, and s represents the integral area。
And S5, calculating the length of each grating segment according to the amplitude relation between the output end and the input end of the grating segment and by combining the target coupling equation.
Specifically, by dividing the periodic grating, the transmission characteristics of the whole chirped grating can be simulated and calculated by a transmission matrix method. For a double-helix chirped chiral long period fiber grating, each grating segment can be described by a 2 × 2 matrix, and the amplitude relationship between the output end and the input end can be expressed as:
Wherein,
wherein A isco(zM+1) Representing the amplitude of the mode of the core at the output, Acl(zM+1) Representing the amplitude of cladding mode at the output end, FiThe element of the matrix representing the ith raster segment, Aco(z1) 1 denotes the initial conditions of the core mold, Acl(z1) 0 denotes the cladding mode initiation condition, aijTwo mode coupling coefficients in matrix elements representing the ith grating segment, k represents the coupling strength, a represents the imaginary part of the coupling strength, zi+1Denotes the i +1 th segment grating length, ziRepresents the ith segment grating length, i represents the grating number.
And substituting the amplitude relation into a target coupling equation, namely equation (9), to obtain the final chirped grating amplitude, so as to calculate the chirped grating transmission spectrum (namely the coupling spectrogram). Furthermore, the coupling strength of each grating segment can be obtained according to the chirped grating transmission spectrum, and the length of the grating segment can be further obtained.
And S6, manufacturing the all-fiber circular polarizer according to the grating period and the length.
Specifically, after obtaining the grating parameters, the polarization maintaining fiber in the molten state is twisted at a high speed and a variable speed (for example, panda-type polarization maintaining fiber, and other types of polarization maintaining fibers are similar), and the twisted fiber forms a chirp-type spiral path with gradually-changed period, so that a double-helix chirp-type chiral long-period fiber grating is formed, and the double-helix chirp-type chiral long-period fiber grating is an all-fiber circular polarizer. The twisting speed of the melting twisting of the polarization maintaining fiber is determined by the grating period of each grating section, and the moving speed of the polarization maintaining fiber is determined by the length of each grating section.
Referring to fig. 6, fig. 6 is a schematic structural diagram of a double-spiral chirped chiral long-period fiber grating obtained by melting and twisting a polarization maintaining fiber according to an embodiment of the present invention. In fig. 6, a double-helix chirped chiral long-period fiber grating with a period Λ gradually changing with the fiber length is formed by twisting the polarization-maintaining fiber. The fiber grating is provided with a plurality of grating sections, each grating section works near a dispersion turning point, the grating section positioned at the center works at the dispersion turning point, and the grating period of each grating section is gradually increased along with the increase of the length. The length of each grating segment decreases with the increase of the coupling strength in the grating segment, the greater the coupling strength, the shorter the length of the grating segment, and the smaller the coupling strength, the longer the length of the grating segment.
In another embodiment, a double helix chirped chiral long period fiber grating with a period Λ that varies with fiber length is formed by twisting the polarization maintaining fiber. The fiber grating is provided with a plurality of grating sections, each grating section works near a dispersion turning point, the grating section positioned at the center works at the dispersion turning point, and the grating period of each grating section is gradually reduced along with the increase of the length.
Further, according to the second phase matching condition, in the double-helix chirped chiral long-period fiber grating, each grating segment corresponds to a different resonant wavelength, and the resonant peak of the final transmission spectrum of the chirped chiral long-period fiber grating can be regarded as being obtained by mutually overlapping the resonant peaks at different wavelength positions of each grating segment, so that broadband transmission can be realized.
Referring to fig. 7, fig. 7 is a schematic diagram illustrating axial refractive index modulation of a double-helix chirped chiral long-period fiber grating according to an embodiment of the present invention. The refractive index modulation in fig. 7 is a projection of the refractive index modulation of a double-helix chirped chiral long-period fiber grating on a plane, and it can be seen that the grating refractive index changes according to the periodicity, and due to the chirped structure design of the grating period, the size of each period changes according to the formula (3) along the length direction of the fiber.
Referring to fig. 8, fig. 8 is a transmission spectrum of a double-spiral chirped chiral long-period fiber grating according to an embodiment of the present invention. The transmission spectrum in FIG. 8 is for operation at LP0,11The transmission spectrum of the chirped chiral long period fiber grating near the mode dispersion inflection point is better flat for the fiber grating of fig. 8 compared to the spectrum of fig. 4.
In the manufacturing method of the embodiment, the grating period range is obtained when the double-helix chiral long-period fiber grating works at the dispersion turning point, the all-fiber type circular polarizer with the ultra-wideband circular polarization filtering characteristic can be realized, and the chirp is introduced into the double-helix chiral long-period fiber grating, the all-fiber type circular polarizer with the flat filtering characteristic can be realized, so that the all-fiber type circular polarizer with the flat working bandwidth, the wide bandwidth, the small volume, the low cost and the easy manufacture can be manufactured. Therefore, the method realizes the manufacture of the ultra-wideband flat all-fiber circular polarizer, the bandwidth and the flat shape of the circular polarizer are simultaneously and obviously improved, the use of the circular polarizer for light field regulation can realize all-fiber, the performance of all-optical communication is improved to a great extent, and the manufacture cost is saved.
Example two
On the basis of the first embodiment, the present embodiment provides an ultra-wideband flat all-fiber circular polarizer, which is manufactured by the preparation method described in the first embodiment, and the structure of the all-fiber circular polarizer is shown in fig. 6.
Specifically, the all-fiber circular polarizer is a double-helix chirped chiral long-period fiber grating, and the double-helix chirped chiral long-period fiber grating is formed by sequentially connecting a plurality of double-helix chiral long-period fiber gratings; wherein, the grating period of the double-helix chiral long-period fiber grating is within the corresponding grating period range when the double-helix chiral long-period fiber grating works at a dispersion turning point; the grating period of the double-helix chirp type chiral long-period fiber grating changes along with the position of the optical fiber shaft.
In a specific embodiment, the grating period of each section of the double-helix chiral long-period fiber grating gradually increases with the increase of the position of the optical fiber axis, or the grating period of each section of the double-helix chiral long-period fiber grating gradually decreases with the increase of the position of the optical fiber axis. The length of each section of the double-helix chiral long-period fiber grating is reduced along with the increase of the coupling strength of the section of the grating.
Further, the double-spiral chirped chiral long-period fiber grating may be a right-handed double-spiral chirped chiral long-period fiber grating or a left-handed double-spiral chirped chiral long-period fiber grating.
Specifically, please refer to embodiment one for the second phase matching condition, the circular polarization coupling equation, and the amplitude relationship between the output end and the input end of each section of the double-spiral chiral long-period fiber grating that are satisfied by the all-fiber circular polarizer, which is not described in detail in this embodiment.
The all-fiber circular polarizer of the embodiment realizes the ultra-wideband circular polarization filtering characteristic by using the dispersion turning characteristic and realizes the flat filtering characteristic by using the chirp effect, and the all-fiber circular polarizer has flat and wide working bandwidth and small volume, thereby greatly improving the performance of all-optical communication.
The foregoing is a more detailed description of the invention in connection with specific preferred embodiments and it is not intended that the invention be limited to these specific details. For those skilled in the art to which the invention pertains, several simple deductions or substitutions can be made without departing from the spirit of the invention, and all shall be considered as belonging to the protection scope of the invention.
Claims (10)
1. A method for manufacturing an ultra-wideband flat all-fiber circular polarizer is characterized by comprising the following steps:
calculating a grating period range of the double-helix chiral long-period fiber grating when the double-helix chiral long-period fiber grating works at a dispersion turning point according to a first phase matching condition which is met by the double-helix chiral long-period fiber grating when a fiber core fundamental mode is coupled to a high-order cladding mode;
introducing chirp into the double-helix chiral long-period fiber grating to obtain a double-helix chirp type chiral long-period fiber grating;
acquiring the grating period of each grating section in the double-helix chirped chiral long-period fiber grating according to the grating period range, wherein the grating period changes along with the position of an optical fiber shaft;
establishing a target coupling equation under the interaction between a right-handed circularly polarized fiber core mode and a left-handed circularly polarized cladding mode in the double-helix chirped chiral long-period fiber grating in a local coordinate system;
calculating the length of each grating section by combining the target coupling equation according to the amplitude relation between the output end and the input end of the grating section;
and manufacturing the all-fiber type circular polarizer according to the grating period and the length.
2. The method of claim 1, wherein the first phase matching condition is:
λres＝(neff,01-neff,0n)Λ
Wherein λ isresDenotes the resonance wavelength, Λ denotes the grating period, neff,01Represents LP01Effective refractive index of fundamental core mode, neff,0nRepresents LP0nThe effective refractive index of the cladding modes.
3. The method of claim 1, wherein the grating period of each grating segment is:
Λ＝Λ0+cz
wherein, Λ0The initial period size of the chirp period is shown, z is the axial position of the optical fiber, the value of z ranges from 0 to L, c is delta lambda/L, c represents the chirp coefficient, and delta lambda represents the total period variation of the chirp grating.
4. The method of claim 1, wherein establishing a target coupling equation between the core mode of the right-handed circular polarization and the cladding mode of the left-handed circular polarization in the double-helix chirped chiral long-period fiber grating in a local coordinate system comprises:
establishing a circular polarization mode coupling equation of a polarization fiber core mode and a cladding mode of a first coordinate system and a second coordinate system in a local coordinate system;
based on the mutual coupling effect between the fiber core mode of the right-handed circular polarization and the cladding mode of the left-handed circular polarization, and the fiber core fundamental mode LP of the double-helix chirp type chiral long-period fiber grating 01Coupled to high-order cladding modes LP0nAnd simplifying the circular polarization mode coupling equation according to the second phase matching condition to obtain the target coupling equation.
5. The method according to claim 4, wherein said circular polarization mode coupling equation is:
where κ and κ' both represent coupling coefficients, τ (τ ═ 2 π/P) represents twist rate, P represents fiber pitch,
representing the amplitude of the right-handed circularly polarized core layer,
showing the amplitude of the right-handed circularly polarizing core layer,
showing the amplitude of the left-handed circularly polarized core and cladding,
amplitude, beta, of right-handed circularly polarizing core and claddingcoRepresenting the core mode HE in an ideal isotropic fiber11The propagation constant of (a) is determined,
indicating the normalized electric field distribution of the different modes, respectively, the superscript x or y indicating the polarization direction of the principal transverse component of the mode, epsilon0Denotes the isotropic part of the dielectric constant distribution,. DELTA.. di-elect consxShowing the anisotropically perturbed part of the dielectric constant distribution in the x-polarization mode, DeltasimayRepresenting the distribution of the dielectric constant in the y-polarization modeThe anisotropic perturbation portion, j represents a complex number, ω represents the optical angular frequency, and s represents the integrated area.
6. The method of claim 4, wherein the second phase matching condition is:
λres＝(neff,01-neff,0n)Λ(z)
wherein λ isresRepresenting the resonance wavelength, Λ representing the grating period, z representing the fiber axis position, neff,01Represents LP01Effective refractive index of fundamental core mode, neff,0nRepresents LP0nThe effective refractive index of the cladding modes.
7. The method of claim 1 or 4, wherein the target coupling equation is:
where κ denotes a coupling coefficient, τ (τ ═ 2 π/P) denotes a twist rate, P denotes a fiber pitch,
showing the amplitudes of the right-handed circularly polarized core and cladding layers,
showing the amplitude of the left-handed circularly polarized core and cladding layers; beta is acoRepresenting the core mode HE in an ideal isotropic fiber11The propagation constant of (a) is determined,
indicating the normalized electric field distribution of the different modes, respectively, the superscript x or y indicating the polarization direction of the principal transverse component of the mode, epsilon0Denotes the isotropic part of the dielectric constant distribution,. DELTA.. di-elect consxShowing the anisotropically perturbed part of the dielectric constant distribution in the x-polarization mode, DeltasimayDenotes the anisotropically perturbed part of the dielectric constant distribution in the y-polarization mode, j denotes the complex number, ω denotes the optical wave angular frequency, and s denotes the integral area.
8. The method of claim 1, wherein the amplitude relationship is:
wherein A isco(zM+1) Representing the amplitude of the mode of the core at the output, Acl(zM+1) Representing the amplitude of cladding mode at the output end, FiRepresents the ithElement of the grating segment, Aco(z1) 1 denotes the initial conditions of the core mold, Acl(z1) 0 denotes the cladding mode initiation condition, aijTwo mode coupling coefficients in matrix elements representing the ith grating segment, k represents the coupling strength, a represents the imaginary part of the coupling strength, zi+1Denotes the i +1 th segment grating length, ziRepresents the ith segment grating length, and i represents the grating ordinal number.
9. An ultra-wideband flat all-fiber circular polarizer is characterized in that the all-fiber circular polarizer is a double-helix chirp type chiral long-period fiber grating formed by sequentially connecting a plurality of double-helix chiral long-period fiber gratings, wherein,
each section of the double-helix chiral long-period fiber grating works in a preset range on two sides of a dispersion turning point, and the grating period of the double-helix chirped chiral long-period fiber grating changes along with the position of an optical fiber shaft.
10. The ultra-wideband flat all-fiber circular polarizer according to claim 9, wherein the grating period of each segment of said double-helix chiral long period fiber grating gradually increases or decreases as the position of said fiber axis increases.
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