CN217766906U - Ultra-wideband flat all-fiber circular polarizer - Google Patents
Ultra-wideband flat all-fiber circular polarizer Download PDFInfo
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Abstract
The utility model relates to an all-fiber circular polarizer with flat ultra-wideband, which is a double-helix chirp type chiral long-period fiber grating, wherein a plurality of double-helix chiral long-period fiber gratings are connected in sequence to form the all-fiber circular polarizer; 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 axis. The all-fiber circular polarizer works near a dispersion turning point, can realize ultra-wideband circular polarization filtering characteristics, and simultaneously introduces a chirp effect to the fiber grating, so that the grating period changes along with the position of the fiber axis, and the flat filtering characteristics can be realized, thereby realizing the all-fiber circular polarizer with flat working bandwidth, wider bandwidth, small volume and low cost.
Description
Technical Field
The utility model belongs to the technical field of the passive device of optic fibre type, concretely relates to flat all-fiber type circular polarizer of ultra wide band.
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 to 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, circularly polarized light is generally generated using the following techniques: 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.
SUMMERY OF THE UTILITY MODEL
In order to solve the above-mentioned problem that exists among the prior art, the utility model provides a flat all-fiber type circular polarizer of ultra wide band. The to-be-solved technical problem of the utility model is realized through following technical scheme:
the embodiment of the utility model provides a flat full optical fiber type circular polarizer of ultra wide band, full optical fiber type circular polarizer is double helix chirp type chirality long period fiber grating, double helix chirp type chirality long period fiber grating is formed by connecting a plurality of double helix chirality long period fiber gratings in proper order; 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 the 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 axis.
In an embodiment of the present invention, the double-spiral chirped chiral long-period fiber grating is a right-handed double-spiral chirped chiral long-period fiber grating.
In an embodiment of the present invention, the double-spiral chirped chiral long-period fiber grating is a left-handed double-spiral chirped chiral long-period fiber grating.
In an embodiment of the present invention, the grating period of the double-helix chirped chiral long-period fiber grating gradually increases with an increase in the position of the optical fiber axis.
In an embodiment of the present invention, the grating period of the double-helix chirped chiral long-period fiber grating gradually decreases as the position of the optical fiber axis increases.
In one embodiment of the present invention, each section of the double-spiral chiral long-period fiber grating decreases as the coupling strength of the section of the grating increases.
In one embodiment of the present invention, the double-helix chiral long period fiber grating comprises a fiber core, a stress region, and a cladding, wherein,
the fiber core is located in the center of the cladding, the stress region is located inside the cladding, and the stress region forms a double-spiral path.
In an embodiment of the present invention, the cross-sectional type of the double-helix chirped chiral long-period fiber grating includes a panda type, a bow tie type, an elliptical cladding type, and an elliptical core structure type.
Compared with the prior art, the beneficial effects of the utility model are that:
the utility model discloses an all-fiber circular polarizer work near dispersion turning point can realize ultra wide band circular polarization filtering characteristic, introduces the chirp effect to fiber grating simultaneously for the grating period can realize flat filtering characteristic along with the change of optic fibre axle position, thereby realizes that the working bandwidth is flat, bandwidth broad, small, with low costs all-fiber type circular polarizer.
Drawings
Fig. 1 is a schematic structural diagram of 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 double-helix chiral long-period fiber grating according to an embodiment of the present invention;
fig. 3 a-3 b are schematic diagrams of phase matching curves of a right-handed double-helix 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;
fig. 5 a-5 b are schematic structural diagrams of two double-spiral chirped chiral long-period fiber gratings according to an embodiment of the present invention;
fig. 6 is a schematic view illustrating modulation of an axial refractive index of a double-spiral chirped chiral long-period fiber grating according to an embodiment of the present invention;
fig. 7 is a transmission spectrum of a double-spiral 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 present invention is not limited thereto.
Example one
Referring to fig. 1, fig. 1 is a schematic structural diagram of an ultra-wideband flat all-fiber circular polarizer according to an embodiment of the present invention. The full-optical-fiber 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.
It should be noted that, the double-helix chirped chiral long-period fiber grating may be formed integrally, for example, a molten polarization maintaining fiber is twisted at a high speed and a variable speed to form the double-helix chiral long-period fiber grating, and the twisted grating may be divided into a plurality of double-helix chiral long-period fiber gratings; the double-helix chirp type chiral long-period fiber grating can also be formed by sequentially connecting multiple sections of double-helix chiral long-period fiber gratings, wherein the ends of the multiple sections of double-helix chiral long-period fiber gratings are in contact.
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, and thus the double-spiral chirped long-period fiber grating may be a right-handed double-spiral chirped long-period fiber grating or a left-handed double-spiral chirped long-period fiber grating.
In one embodiment, 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 operates at the dispersion inflection point.
Referring to fig. 2, fig. 2 is a structural diagram of a right-handed double-spiral chiral long-period fiber grating according to an embodiment of the present invention. In fig. 2, a polarization maintaining fiber in a molten state (for example, a panda-type polarization maintaining fiber) 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. The double-helix chiral long-period fiber grating comprises a fiber core, a stress area and a cladding, wherein the fiber core is positioned in the center of the cladding, the stress area is positioned in the cladding, the stress area forms a double-helix path, and the surface of the cladding forms an uneven shape.
Different from the traditional long-period fiber grating, the double-helix chiral long-period fiber grating couples the fiber core fundamental mode LP01 to the high-order cladding mode LP0n, and the first phase matching condition is met:
λ res ＝(n eff,01 -n eff,0n )Λ (1)
wherein λ is res Denotes the resonance wavelength, Λ denotes the grating period, n eff,01 Represents LP 01 Effective refractive index of fundamental core mode, n eff,0n Represents LP 0n The 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 first 14-order (i.e., m = 2-15) resonant cladding mode of the double-helix chiral long-period fiber grating can be obtained according to the first phase matching condition of the formula (1), and for convenience, the resonant cladding mode is given by a phase matching curve, please refer to fig. 3 a-3 b, and fig. 3 a-3 b are schematic diagrams of phase matching curves of a right-hand double-helix chiral long-period fiber grating according to an embodiment of the present invention, where fig. 3a is a phase matching curve of a cladding mode with a cladding order of m = 2-8, fig. 3b is a phase matching curve of a cladding mode with a cladding order of m = 9-15, and a black dot in the diagram represents 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 the cladding modes of m =2 to 8. In fig. 3b, the slope of the curve changes from positive to negative for the cladding modes with m =9 to 15, and there is a point where the slope is infinite, as shown by the black dot in fig. 3b, 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 resonance peak bandwidth of the fiber grating can be expressed as:
in the formula (2), Δ λ 3dB 3-dB bandwidth representing the resonant peak of the fiber grating, L grating length, Δ n g Representing 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 between the core and the claddingΔn g The smaller and thus the wider the bandwidth of the resonance peak of the fiber grating.
Therefore, the grating parameters of the double-helix chiral long-period fiber grating are designed to enable the double-helix chiral long-period fiber grating to work at the dispersion turning point of a high radial order cladding mode, and an ultra-wideband circular polarization mode can be effectively generated. 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.
According to the first phase matching condition of the formula (1), taking a right-handed double-helix chiral long period fiber grating as an example, the fiber core model (LP) thereof 01 ) 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 in a right-handed dual-spiral 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 a right-handed dual-spiral chiral long-period fiber grating operating at a dispersion turning point according to an embodiment of the present invention 01 ) Respectively associated with cladding modes (LP) 02 ，LP 0,11 ) Coupling is performed, wherein, cladding mode LP 02 Not operating at the point of dispersion transition, cladding mode LP 0,11 Operating 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.
The double-helix chiral long-period fiber grating realizes the conversion of an ultra-wideband circular polarization mode with a 3-dB bandwidth of about 190nm, however, under such a bandwidth, the extinction ratio of a spectral resonance peak is not flat, namely, the extinction ratios at different wavelength positions in the broadband working wavelength have a large difference, as shown in FIG. 4, the extinction ratio can reach 60dB at the center of the resonance peak, and is only 3dB at the edge. The extinction ratio of the resonance peak has a great influence on the mode purity of the converted circular polarization mode, so that in order to obtain ultra-wideband and flat circularly polarized light beams, chirp is introduced to the double-helix chiral long-period fiber grating working at the dispersion turning point to form the double-helix chirp type chiral long-period fiber grating.
In one embodiment, the grating period of a double helix chirped chiral long period fiber grating varies with fiber axis position. With the increase of the optical fiber axis position, the grating period of the double-helix chirped chiral long-period optical fiber grating can be gradually increased or gradually decreased.
In this embodiment, the example that the grating period of the double-helix chirped chiral long-period fiber grating gradually increases with the increase of the position of the fiber axis is described.
Referring to fig. 5 a-5 b, fig. 5 a-5 b are schematic structural diagrams of two dual-spiral chirped chiral long-period fiber gratings according to an embodiment of the present invention, where fig. 5a is a right-handed dual-spiral chirped chiral long-period fiber grating, and fig. 5b is a left-handed dual-spiral chirped chiral long-period fiber grating. In fig. 5a and 5b, the grating period of the double-helix chirped chiral long-period fiber grating is gradually increased with the increase of the position of the optical fiber axis.
Specifically, after introducing chirp to the period of the double-helix chiral long-period fiber grating, the grating period is no longer constant, but varies with the axial direction of the fiber, as shown in fig. 5a and 5 b. For a double-spiral 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-spiral 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 grating parameters (including grating period and 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 ultra-wideband circular polarization mode coupling can be effectively generated.
Specifically, the period of each grating segment is:
Λ＝Λ 0 +cz (3)
wherein, Λ 0 The 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 = delta lambda/L, c represents the chirp coefficient, and delta lambda represents the total period variation of the chirped grating.
It can be understood that the period of each grating section is located in the grating period range corresponding to the dispersion turning point of the double-helix chiral long-period fiber grating, the grating period of the grating section located in 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 section located at the starting end is close to the smaller period in the grating period range, and the grating period of the grating section located at the tail end is close to the larger period in the grating period range.
In one embodiment, each segment of the double-helix chiral long period fiber grating decreases as the coupling strength of the segment of the grating increases. That is, the length of each double-helix chiral long-period fiber grating is determined by the coupling strength of the section, the stronger the coupling strength, the shorter the length, and the weaker the coupling strength, the longer the strength.
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 LP 01 Coupled to cladding mode LP 0n At the dispersion turning point (n is more than or equal to 2), the second phase matching condition is met:
λ res ＝(n eff,01 -n eff,0n )Λ(z) (4)
wherein λ is res Representing the resonance wavelength, Λ representing the grating period, z representing the fiber axis position, n eff,01 Represents LP 01 Effective refractive index of fundamental core mode, n eff,0n Represents LP 0n The effective refractive index of the cladding modes.
The core layer and the cladding layer of the right-handed direction chiral long period fiber grating formed by the twisted commercial panda fiber (similar to the left-handed direction) have radius distributions of a =4.15 μm and b =62.5 μm, and the refractive indexes of the core and the cladding are respectively n 1 =1.452 and n 2 =1.444, panda polarization maintaining optical fiber beat length L b Is 2.5mm, so that the anisotropy perturbation difference Delta epsilon of the dielectric constant x -Δε y This can be obtained from the following relationship:
where B is the birefringence of the panda fiber.
Through calculation, the anisotropic disturbance difference Delta epsilon of the dielectric constant x -Δε y Is 1.8X 10 -3 。
Based on this, taking a panda fiber with a right-handed helical structure and high twist rate as an example, the coupling equations of the circular polarization modes of the core mode and the cladding mode of the polarization fiber in the first coordinate system x and the second coordinate system y in the local coordinate system are as follows:
where κ and κ' both represent coupling coefficients, τ (τ =2 π/P) represents the twist rate, P represents the 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 the core and cladding layers of a right-handed circularly polarized fiber co Representing the core mode HE in an ideal isotropic fiber 11 The 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, epsilon 0 Denotes the isotropic part of the dielectric constant distribution,. DELTA.. Di-elect cons x Showing the anisotropically perturbed part of the dielectric constant distribution in the x-polarization mode, deltasima y Denotes 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.
For all coupling modes, β co Greater than beta cl For 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 relationship is met:
β co -τ(z)＝β cl +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 that formula (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 amplitude 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 co Representing the core mode HE in an ideal isotropic fiber 11 The 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, epsilon 0 Denotes the isotropic part of the dielectric constant distribution,. DELTA.. Di-elect cons x Showing the anisotropically perturbed part of the dielectric constant distribution in the x-polarization mode, deltasima y Denotes 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.
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 is co (z M+1 ) Representing the amplitude of the mode of the core at the output, A cl (z M+1 ) Representing the amplitude of cladding mode at the output end, F i The element of the matrix representing the ith raster segment, A co (z 1 ) =1 denotes fiber core model initial condition, A cl (z 1 ) =0 represents the cladding mode initial condition, a ij Two mode coupling coefficients in the matrix element representing the ith grating segment, k represents the coupling strength, a represents the imaginary part of the coupling strength, z i+1 Denotes the i +1 th segment grating length, z i Represents the ith segment grating length, i represents the grating number.
And (3) substituting the amplitude relation into a target coupling equation, namely equation (9), to obtain the final chirped grating amplitude, thereby calculating the chirped grating transmission spectrum (namely the transmission 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.
In a specific embodiment, a polarization maintaining fiber in a molten state is subjected to high-speed variable-rate torsion (for example, panda-type polarization maintaining fiber, and the like in other types of polarization maintaining fibers) under the condition of known grating period and length of each grating segment, and the fiber forms a chirp-type spiral path with gradually-changed period after torsion, so that a double-helix chirp-type chiral long-period fiber grating is formed, wherein 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.
Specifically, the type of polarization maintaining fiber includes, but is not limited to, a panda type polarization maintaining fiber, a bow tie type polarization maintaining fiber, an elliptic cladding type polarization maintaining fiber, and an elliptic core structure type polarization maintaining fiber, and thus, the cross-sectional type of the double-helix chirped chiral long period fiber grating formed by twisting includes, but is not limited to, a panda type, a bow tie type, an elliptic cladding type, and an elliptic core structure type.
Referring again to fig. 1, by twisting the polarization maintaining fiber, a double-helix chirped chiral long period fiber grating having a period Λ that gradually changes with the length of the fiber as shown in fig. 1 is formed. The fiber grating is provided with a plurality of grating sections, the grating period of each grating section is uniform, 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 the 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, the grating period of each grating section is uniform, 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. 6, fig. 6 is a schematic view illustrating an axial refractive index modulation of a double-spiral chirped chiral long-period fiber grating according to an embodiment of the present invention. The refractive index modulation in fig. 6 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. 7, fig. 7 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. 7 is for operation at LP 0,11 The 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. 7 compared to the spectrum of fig. 4.
The all-fiber circular polarizer of the embodiment works near a dispersion turning point, can realize ultra-wideband circular polarization filtering characteristics, and simultaneously introduces a chirp effect to the fiber grating, so that the grating period changes along with the position of the fiber axis, and the flat filtering characteristics can be realized, thereby realizing the all-fiber circular polarizer with flat working bandwidth, wider bandwidth, small volume and low cost.
The foregoing is a more detailed description of the present invention, taken in conjunction with the specific preferred embodiments thereof, and it is not intended that the invention be limited to the specific embodiments shown and described. To the utility model belongs to the technical field of ordinary technical personnel, do not deviate from the utility model discloses under the prerequisite of design, can also make a plurality of simple deductions or replacement, all should regard as belonging to the utility model discloses a protection scope.
Claims (8)
1. The full-fiber circular polarizer with the flat ultra-wide band is characterized in that the full-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 the 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 axis.
2. The ultra-wideband flat all-fiber circular polarizer according to claim 1, wherein said double-helix chirped chiral long-period fiber grating is a right-handed double-helix chirped chiral long-period fiber grating.
3. The ultra-wideband flat all-fiber circular polarizer according to claim 1, wherein the double-helix chirped chiral long-period fiber grating is a left-handed double-helix chirped chiral long-period fiber grating.
4. The ultra-wideband flat all-fiber circular polarizer according to claim 1, wherein the grating period of the double-helix chirped chiral long period fiber grating gradually increases with increasing fiber axis position.
5. The ultra-wideband flat all-fiber circular polarizer according to claim 1, wherein the grating period of said double-helix chirped chiral long period fiber grating gradually decreases with increasing position of the fiber axis.
6. The ultra-wideband flat all-fiber circular polarizer according to claim 1, wherein each segment of the double-helix chiral long period fiber grating decreases as the coupling strength of the segment increases.
7. The ultra-wideband planar all-fiber circular polarizer of claim 1, wherein said double helix chiral long period fiber grating comprises a core, a stress region, and a cladding, wherein,
the core is located in the center of the cladding, the stress region is located inside the cladding, and the stress region forms a double-helix path.
8. The ultra-wideband flat all-fiber circular polarizer according to claim 1, wherein the cross-sectional types of the double-helix chirped chiral long period fiber grating include panda type, bow-tie type, elliptical cladding type, and elliptical core structure type.
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| CN114563840B (en) * | 2022-01-27 | 2024-01-23 | 西安邮电大学 | Ultra-wideband flat all-fiber circular polarizer and manufacturing method thereof |
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