CN114895399A - Echo wall micro-cavity coupling system based on long-period fiber grating and manufacturing method thereof - Google Patents
Echo wall micro-cavity coupling system based on long-period fiber grating and manufacturing method thereof Download PDFInfo
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- CN114895399A CN114895399A CN202210553655.6A CN202210553655A CN114895399A CN 114895399 A CN114895399 A CN 114895399A CN 202210553655 A CN202210553655 A CN 202210553655A CN 114895399 A CN114895399 A CN 114895399A
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- G—PHYSICS
- G02—OPTICS
- G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
- G02B6/00—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings
- G02B6/02—Optical fibres with cladding with or without a coating
- G02B6/02057—Optical fibres with cladding with or without a coating comprising gratings
- G02B6/02076—Refractive index modulation gratings, e.g. Bragg gratings
- G02B6/0208—Refractive index modulation gratings, e.g. Bragg gratings characterised by their structure, wavelength response
- G02B6/02085—Refractive index modulation gratings, e.g. Bragg gratings characterised by their structure, wavelength response characterised by the grating profile, e.g. chirped, apodised, tilted, helical
- G02B6/02095—Long period gratings, i.e. transmission gratings coupling light between core and cladding modes
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- G—PHYSICS
- G02—OPTICS
- G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
- G02B6/00—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings
- G02B6/02—Optical fibres with cladding with or without a coating
- G02B6/02057—Optical fibres with cladding with or without a coating comprising gratings
- G02B6/02076—Refractive index modulation gratings, e.g. Bragg gratings
- G02B6/02123—Refractive index modulation gratings, e.g. Bragg gratings characterised by the method of manufacture of the grating
Abstract
The invention provides a long-period fiber grating-based echo wall microcavity coupling system and a manufacturing method thereof, wherein the long-period fiber grating-based echo wall microcavity coupling system comprises a long-period fiber grating and an echo wall microcavity, the long-period fiber grating-based echo wall microcavity comprises an optical fiber, a plane parallel to a fiber core of the optical fiber is formed on the optical fiber after a part of a cladding on the optical fiber is removed, a grating is formed on the plane, and each refractive index modulation unit in the grating is vertical to the fiber core of the optical fiber; the echo wall micro-cavity is fixed in the grating area on the plane, and the equatorial plane of the echo wall micro-cavity is parallel to the fiber core of the optical fiber; after the optical signal is transmitted to the long-period fiber grating along the fiber core, the optical signal is coupled to the cladding mode from the fiber core module and then coupled to the echo wall micro-cavity from the cladding mode, and the echo wall mode is excited. The invention can improve the coupling efficiency and physical strength of the optical fiber echo wall microcavity coupling system.
Description
Technical Field
The invention belongs to the field of echo wall microcavity optical fiber coupling, and particularly relates to an echo wall microcavity coupling system based on a long-period optical fiber grating and a manufacturing method thereof.
Background
The echo wall micro-cavity limits an optical field by adopting continuous total reflection, has ultrahigh quality factor and ultra-small mode volume, can rapidly enhance the interaction of light and substances, and is applied to the fields of low-threshold nonlinear optics, high-sensitivity sensing, quantum optics and the like. The tapered fiber with the beam waist diameter of about 2 microns can efficiently excite the resonance mode in the passive echo wall microcavity, but the tapered fiber has low physical strength and is easy to be interfered by the outside world, so that the long-term stability of the echo wall microcavity coupling system is low. The tapered fiber with the beam waist diameter of 18 microns is manufactured after the long-period fiber grating to excite the whispering gallery mode, so that the physical strength of the tapered fiber coupler can be improved to a certain extent, but the coupling system can only work in a single direction, namely, incident light can be transmitted along the direction from the long-period fiber grating to the tapered fiber to excite the whispering gallery mode. The D-type optical fiber has the characteristics of small volume and high physical strength, but the mode field distribution characteristics of the D-type optical fiber cause the excitation efficiency of the whispering gallery mode to be lower and are only suitable for large-size whispering gallery micro-cavities. The microstructure fiber coupling echo wall micro-cavity has higher integration level and stability, but is only suitable for the spherical echo wall micro-cavity, so that the quality factor and the material selection of the echo wall micro-cavity are limited.
Disclosure of Invention
The invention provides a echo wall micro-cavity coupling system based on a long-period fiber grating and a manufacturing method thereof, and aims to solve the problem that the existing echo wall micro-cavity fiber coupling system cannot give consideration to both coupling efficiency and physical strength.
According to a first aspect of the embodiments of the present invention, a long-period fiber grating-based echo wall microcavity coupling system is provided, including a long-period fiber grating and an echo wall microcavity, where the long-period fiber grating includes an optical fiber, a plane parallel to a fiber core of the optical fiber is formed on the optical fiber after a part of a cladding on the optical fiber is removed, a grating is formed on the plane, and each refractive index modulation unit in the grating is perpendicular to the fiber core of the optical fiber; the echo wall micro-cavity is fixed in the grating area on the plane, and the equatorial plane of the echo wall micro-cavity is perpendicular to the plane and is parallel to the fiber core of the optical fiber; after the optical signal is transmitted to the plane along the fiber core, the optical signal is coupled to the cladding mode from the fiber core module and then coupled to the echo wall micro-cavity from the cladding mode, and the echo wall mode is excited.
In an optional implementation manner, the refractive index modulation amount of the grating changes periodically along the axial direction of the optical fiber, and the fixed position of the echo wall microcavity in the grating region is changed by moving the echo wall microcavity along the axial direction of the optical fiber, so that the coupling coefficient k2 between the cladding mode and the echo wall mode is adjusted.
In another alternative implementation manner, when the distance L between the whispering gallery microcavity fixing position and the head end or the tail end of the grating in the axial direction of the optical fiber satisfies the following formula, the coupling efficiency between the core mode and the cladding mode is the highest:
in another optional implementation manner, the resonance wavelength of the echo wall microcavity is located within the resonance bandwidth of the long-period fiber grating, and the resonance wavelength λ 1 of the long-period fiber grating needs to satisfy phase matching: λ 1 ═ nco-ncl, nco denotes the core effective refractive index, ncl denotes the cladding effective refractive index, and Λ denotes the grating period.
In another optional implementation manner, the resonant wavelength of the long-period fiber grating is determined by the width and the interval of each refractive index modulation unit in the grating, and by adjusting at least one parameter of the width, the interval, and the L of the refractive index modulation units, the resonant wavelength of the echo wall microcavity can be matched with the resonant wavelength of the long-period fiber grating, and the coupling efficiency between the fiber core mold and the echo wall microcavity can be maximized.
According to a second aspect of the present invention, there is provided a method for manufacturing the above system, comprising:
step S100, removing a part of coating in the optical fiber to form a plane parallel to a fiber core on the optical fiber;
step S200, manufacturing refractive index modulation units in the grating on the plane according to the set refractive index modulation unit interval, and executing step S300, wherein a corresponding grating period exists for the width of each refractive index modulation unit;
step S300, determining the refractive index modulation amount of each grating period one by one from the head end of the grating along the axial direction of the optical fiber, determining the effective refractive index of the fiber core and the coupling coefficient between the fiber core mold and the cladding mold according to the refractive index modulation amount of the ith grating period, wherein i is an integer and the initial value is 1, judging whether a point which enables the coupling efficiency between the fiber core mold and the cladding mold to be the highest exists in the ith grating period or not according to the coupling coefficient, if so, executing step S500, otherwise, executing step S400;
step S400, judging whether i is the maximum grating period number, if so, widening the width of the refractive index modulation unit, returning to the step S200, otherwise, adding 1 to i, and returning to the step S300;
step S500, judging whether the long-period fiber grating is matched with a corresponding echo wall micro-cavity or not according to the fiber core effective refractive index, the cladding effective refractive index and the grating period, if so, judging that the width of the refractive index modulation unit and the distance between the set refractive index modulation units are qualified, and executing step S600, otherwise, returning to execute step S400;
and S600, fixing the echo wall micro-cavity at the point with the highest coupling efficiency between the fiber core mold and the cladding mold.
In an optional implementation manner, in step S400, before widening the width of the refractive index modulation unit, the method further includes:
step S700, judging whether the width of the refractive index modulation unit is equal to a set width, if so, replacing a new optical fiber, adjusting the distance between the set refractive index modulation units, setting the width of the refractive index modulation unit as an initial value, returning to execute step S100, otherwise, widening the width of the refractive index modulation unit, and returning to execute step S200.
In another alternative implementation manner, the determining whether there is a point in the ith grating period at which the coupling efficiency between the core mode and the cladding mode is the highest in step S300 includes:
multiplying the distances between each point in the grating period and the head end of the grating by the coupling coefficient k1 respectively, and judging whether the multiplication result is equal to
If yes, the grating period exists to enable the coupling efficiency between the fiber core mode and the cladding mode to be the maximumA high point, otherwise, indicates that there is no point in the grating period at which the coupling efficiency between the core and cladding modes is highest.
In another optional implementation manner, in the step S500, determining whether the long-period fiber grating is matched with the corresponding echo wall microcavity according to the fiber core effective refractive index, the cladding effective refractive index, and the grating period includes:
judging whether the following formula is satisfied, wherein λ 1 is the resonance wavelength of the long-period fiber grating, λ 2 is the resonance wavelength of the echo wall microcavity, λ 1 is (nco-ncl), nco represents the effective refractive index of the fiber core, ncl represents the effective refractive index of the cladding, and Λ represents the grating period; λ 2 ═ 2 π nr/m, n denotes the whispering gallery mode effective index, r denotes the whispering gallery microcavity radius, and m denotes an integer;
if yes, the long-period fiber grating is matched with the corresponding echo wall micro-cavity, otherwise, the long-period fiber grating is not matched with the corresponding echo wall micro-cavity.
In another optional implementation manner, in step S300, if it is determined that there is a point in the ith grating period at which the coupling efficiency between the core mode and the cladding mode is the highest and the point is located at the middle position of the ith grating period, the coupling efficiency between the core mode and the cladding mode can be the highest whether the optical signal is transmitted from the head end to the tail end of the grating or from the tail end to the head end of the grating.
The invention has the beneficial effects that:
1. the invention removes the partial cladding on the fiber, forms the level parallel to fiber core on the fiber, the invention sets up the grating on the level, the refractive index modulation amount of the grating presents the periodic variation, through moving the echo wall microcavity along the axial of the fiber, change the fixed position of the echo wall microcavity in the grating area, can make the coupling efficiency between fiber mandrel and cladding mode reach the highest, and realize the coupling coefficient is adjusted between cladding mode and echo wall mode, it can be seen that the invention has improved the coupling efficiency between fiber grating of long period and echo wall microcavity; the invention fixes the echo wall micro-cavity on the plane, canThe physical strength of the echo wall micro-cavity coupling system is improved, and the structure of the echo wall micro-cavity coupling system is more compact; because the fixed position of the echo wall micro-cavity satisfies the formula by moving the echo wall micro-cavity along the axial direction of the optical fiber
The coupling efficiency of the long-time period fiber grating and the echo wall micro-cavity reaches the highest, and the formula shows that the coupling efficiency can reach the highest at a plurality of positions in the grating region, so that a plurality of echo wall micro-cavities can be simultaneously integrated in a single optical fiber; in addition, the long-period fiber grating has the performance of wavelength selection, which provides a wavelength primary screen for the excitation of the whispering gallery mode, so that the excitation of a high-order mode can be avoided, and a pure resonance spectrum can be obtained;
2. the invention determines the width and the interval of the refractive index modulation units of the grating by adopting the method, so that the manufactured long-period fiber grating has high coupling efficiency and can be matched with the echo wall microcavity; in addition, under the condition that the length of the grating along the axial direction of the optical fiber is fixed, the grating with the set width and the set interval of the refractive index modulation unit is determined, whether a point exists in each grating period is determined one by one, so that the long-period optical fiber grating can be matched with the corresponding echo wall microcavity and the coupling efficiency of the long-period optical fiber grating and the echo wall microcavity is the highest, when the point exists, the width and the interval of the refractive index modulation unit at the moment are directly taken as the processing parameters of the grating, when the point does not exist, the width of the refractive index modulation unit is widened, and whether a point exists in each grating period of the grating with the width and the interval of the refractive index modulation unit is further determined, so that the long-period optical fiber grating can be matched with the corresponding echo wall microcavity and the coupling efficiency of the echo wall is the highest, and the whole method is very simple and can quickly determine the grating processing parameters required by tuning the optical signal with the set wavelength;
3. the invention selects a point which has the highest coupling efficiency and is positioned at the middle position of the corresponding grating period as the fixed point of the echo wall micro-cavity, so that the optical signal transmission in the system has no directional dependence and the bidirectional work is realized.
Drawings
FIG. 1 is a schematic structural diagram of an embodiment of a echo wall microcavity coupling system of the present invention;
fig. 2 is a flow chart of optical signal transmission of the echo wall microcavity coupling system of the invention.
Detailed Description
In order to make the technical solutions in the embodiments of the present invention better understood and make the above objects, features and advantages of the embodiments of the present invention more comprehensible, the technical solutions in the embodiments of the present invention are described in further detail below with reference to the accompanying drawings.
In the description of the present invention, unless otherwise specified and limited, it is to be noted that the term "connected" is to be interpreted broadly, and may be, for example, a mechanical connection or an electrical connection, or a communication between two elements, or may be a direct connection or an indirect connection through an intermediate medium, and a specific meaning of the term may be understood by those skilled in the art according to specific situations.
Fig. 1 is a schematic structural diagram of an echo wall microcavity coupling system according to an embodiment of the present invention. The echo wall microcavity coupling system can comprise a long-period fiber grating and an echo wall microcavity 2, wherein the long-period fiber grating comprises an optical fiber 1, a plane 13 parallel to a fiber core 12 of the optical fiber 1 is formed on the optical fiber 1 after a part of a cladding 11 on the optical fiber 1 is removed, a grating 3 is formed on the plane 13, and each refractive index modulation unit in the grating 3 is vertical to the fiber core of the optical fiber 1; the echo wall micro-cavity 2 is fixed in the grating area on the plane 13, and the equatorial plane of the echo wall micro-cavity 2 is perpendicular to the plane 13 and is parallel to the fiber core of the optical fiber 1; after the optical signal is transmitted to the grating 3 along the fiber core 12, the optical signal is coupled to the cladding mode from the fiber core mode, and is coupled to the echo wall micro-cavity from the cladding mode, so that the echo wall mode is excited. After the optical signal is tuned into resonant light by the echo wall micro-cavity, the resonant light is coupled to the cladding mode from the echo wall mode and is coupled to the fiber core mode from the cladding mode.
As shown in connection with FIG. 2, the optical signal is along the fiberThe optical field of the core 12, transmitted without being transmitted to the plane 13, is E 1 When the optical signal is transmitted to the grating 3, a part (or all) of the optical signal is coupled from the core 12 to the cladding 11 (i.e. coupled from the core mode to the cladding mode), the coupling coefficient between the core mode and the cladding mode is k1, and the optical field after the optical signal is transmitted to the cladding 11 is E 2 (ii) a After receiving the optical signal, the cladding 11 couples and transmits the optical signal to the echo wall microcavity 2 (i.e. couples and excites the echo wall mode from the cladding mode to the echo wall microcavity), the coupling coefficient between the cladding mode and the echo wall mode is k2, and the optical field after the optical signal is transmitted to the echo wall microcavity is E 3 (ii) a After the optical signal is received by the echo wall microcavity, the optical signal circulates along the echo wall microcavity, a loss alpha is generated in the annular process, when the optical signal circulates for a circle and reaches the coupling point again, the optical signal is tuned into resonant light, and the optical field of the resonant light is E 4 (ii) a The whispering gallery microcavity 2 couples the resonant light to the cladding 11 (i.e. couples the resonant light from the whispering gallery mode to the cladding mode), the coupling coefficient between the whispering gallery mode and the cladding mode is also k2, and the optical field of the resonant light after being transmitted to the cladding 11 is E 5 (ii) a The cladding 11, upon receiving the resonance light, couples the resonance light to the core 12 (i.e., couples from the cladding mode to the core mode), the coupling coefficient between the cladding mode and the core mode is also k1, and the optical field after the resonance light is transmitted to the core 12 is E 6 。
In this embodiment, the optical fiber may be a common single-mode optical fiber, and after a portion of the cladding on the optical fiber is removed, a plane parallel to the fiber core is formed on the optical fiber, thereby manufacturing a D-type optical fiber with a plane. The grating may be composed of a plurality of refractive index modulation units, the intervals between the adjacent refractive index modulation units may be equal, the widths of the refractive index modulation units may be equal and may be formed on a plane by arc discharge or the like, and each refractive index modulation unit may be a long strip. Part of the cladding 11 on the optical fiber can be removed by grinding, and the key parameters in the grinding process include grinding time, axial stress, the mesh number of the grinding paper and the like. Fig. 1 establishes three-dimensional coordinate systems for the optical fiber and the whispering gallery microcavity, respectively, with the fiber core in the z-axis direction and the cross-sectional plane in the x-y plane for the optical fiber, and the whispering gallery microcavity with the equatorial plane in the x '-z' plane and the y-axis perpendicular to the equatorial plane, the z-axis and the z-axis being in the same direction.
Because the optical signal is coupled from the fiber core mold to the cladding mold, the optical field is attenuated in the radial transmission process of the optical signal along the optical fiber, the invention removes the upper part of the cladding of the optical fiber, and forms a plane parallel to the fiber core on the optical fiber, so that the echo wall micro-cavity positioned on the plane can receive the optical signal with stronger optical field, thereby improving the coupling efficiency between the fiber core mold and the cladding mold. In general, the larger the height difference between the plane 13 and the cladding 11, the closer the plane 13 is to the fiber core 12, and the stronger the optical field of the optical signal received by the whispering gallery microcavity on the plane, but when the plane 13 intersects with the fiber core 12, the extra loss is introduced by the abrasion of the fiber core 12, so in the present invention, the plane 13 and the fiber core 12 are arranged close to each other but spaced apart from each other.
In addition, after a plane parallel to the fiber core is formed on the optical fiber, if the whispering gallery micro-cavity is directly arranged on the plane, although the whispering gallery mode can also be excited, just like the existing D-type optical fiber, the whispering gallery mode has low excitation efficiency and is only suitable for a large-size whispering gallery micro-cavity. Therefore, the grating is arranged on the plane, the refractive index modulation quantity of the grating is periodically changed along the axial direction of the optical fiber, and the fixed position of the echo wall microcavity in the grating region is changed by moving the echo wall microcavity along the axial direction of the optical fiber, so that the coupling coefficient k2 between the cladding mode and the echo wall mode can be adjusted; by moving the echo wall micro-cavity along the axial direction of the optical fiber and changing the fixed position of the echo wall micro-cavity in the grating region, the coupling efficiency between the fiber core mold and the cladding mold can reach the highest, thus the invention improves the coupling efficiency between the long-period optical fiber grating and the echo wall micro-cavity.
When the distance L between the fixed position of the echo wall microcavity and the head end or the tail end of the grating along the axial direction of the optical fiber meets the following formula, the coupling efficiency between the fiber core mode and the cladding mode is the highest: coefficient of coupling
The resonance wavelength of the echo wall microcavity is located in the resonance bandwidth of the long-period fiber grating, and the resonance wavelength λ 1 of the long-period fiber grating needs to satisfy phase matching: λ 1 ═ nco-ncl, nco denotes the core effective refractive index, ncl denotes the cladding effective refractive index, and Λ denotes the grating period. λ 1 ═ λ 2, λ 2 is the echo wall microcavity resonance wavelength, λ 2 ═ 2 pi nr/m, n denotes the echo wall mode effective refractive index, r denotes the echo wall microcavity radius, and m denotes an integer.
The invention fixes the echo wall micro-cavity on the plane, can improve the physical strength of the echo wall micro-cavity coupling system and make the structure of the echo wall micro-cavity coupling system more compact, and because the invention has higher coupling efficiency, the invention can be suitable for the long-period fiber grating to match with the echo wall micro-cavities of various types, the applicable scope is wider, for example the echo wall micro-cavity can be spherical, annular and cylindrical, etc.; because the fixed position of the echo wall micro-cavity satisfies the formula by moving the echo wall micro-cavity along the axial direction of the optical fiber
The coupling efficiency of the long-time period fiber grating and the echo wall micro-cavity reaches the highest, and the formula shows that the coupling efficiency can reach the highest at a plurality of positions in the grating region, so that a plurality of echo wall micro-cavities can be simultaneously integrated in a single optical fiber; in addition, the long-period fiber grating has the property of wavelength selection, which provides a wavelength primary screen for the excitation of the whispering gallery mode, so that the excitation of a high-order mode can be avoided, and a pure resonance spectrum can be obtained.
The refractive index modulation amount of the grating is determined by the width and the distance of each refractive index modulation unit in the grating. By adjusting at least one parameter of the width, the distance and the L of the refractive index modulation unit, the echo wall microcavity resonance wavelength can be matched with the long-period fiber grating resonance wavelength, and the coupling efficiency between the fiber core model and the echo wall model can reach the highest.
In order to realize the matching of the resonant wavelength of the existing echo wall microcavity and the long-period fiber grating, the invention also provides a preparation method of the echo wall microcavity coupling system based on the long-period fiber grating, which comprises the following steps:
step S100, removing a part of a coating in the optical fiber to form a plane parallel to a fiber core on the optical fiber.
Step S200, manufacturing the refractive index modulation units in the grating on the plane according to the set refractive index modulation unit interval, wherein the width of each refractive index modulation unit has a corresponding grating period, and executing step S300. In this step, when the refractive index modulation unit in the grating is fabricated on the plane, the refractive index modulation unit can be fabricated by using methods such as carbon dioxide laser, femtosecond pulse laser, and arc discharge, and when the width of each refractive index modulation unit in the grating is synchronously widened, taking the arc discharge fabrication method as an example, the method can be implemented by increasing the single discharge amount or increasing the number of discharges.
Step S300, determining the refractive index modulation amount of each grating period one by one from the head end of the grating along the axial direction of the optical fiber, determining the effective refractive index of the fiber core and the coupling coefficient between the fiber core mold and the cladding mold according to the refractive index modulation amount of the ith grating period, wherein i is an integer and the initial value is 1, judging whether a point which enables the coupling efficiency between the fiber core mold and the cladding mold to be the highest exists in the ith grating period or not according to the coupling coefficient, if so, executing step S500, otherwise, executing step S400;
step S400, judging whether i is the maximum grating period number, if so, widening the width of the refractive index modulation unit, returning to the step S200, otherwise, adding 1 to i, and returning to the step S300;
step S500, judging whether the long-period fiber grating is matched with the corresponding echo wall micro-cavity or not according to the fiber core effective refractive index, the cladding effective refractive index and the grating period, if so, judging that the width of the refractive index modulation unit and the distance between the set refractive index modulation units are qualified, and executing step S600, otherwise, returning to execute step S400. The invention determines the width and the distance of the refractive index modulation units of the grating by adopting the method, so that the manufactured long-period fiber grating has high coupling efficiency and can be slightly matched with a corresponding echo wall to meet the requirement of tuning a wavelength light signal with set wavelength; in addition, under the condition that the length of the grating along the axial direction of the optical fiber is fixed, the invention can aim at the grating with the width and the interval of the refractive index modulation unit, judge whether a point exists in each grating period one by one, can enable the long-period optical fiber grating to be matched with the corresponding echo wall microcavity and enable the coupling efficiency of the long-period optical fiber grating and the echo wall microcavity to be the highest, when the point exists, the width and the interval of the refractive index modulation unit at the moment are directly taken as the processing parameters of the grating, when the point does not exist, the width of the refractive index modulation unit is widened, and then further judge whether a point exists in each grating period of the grating with the width and the interval of the refractive index modulation unit, can enable the long-period optical fiber grating to be matched with the corresponding echo wall microcavity and enable the coupling efficiency of the echo wall to be the highest, and the whole method is very simple and can rapidly determine the grating processing parameters needed by tuning the wavelength optical signal with the set wavelength.
In step S400, before widening the width of the refractive index modulation unit, the method may further include: step S700, determining whether the width of the refractive index modulation unit is equal to a set width (which is an allowable maximum width), if so, replacing a new optical fiber, adjusting the distance between the set refractive index modulation units, setting the processing width of the refractive index modulation unit as an initial value, returning to perform step S100, otherwise, widening the width of the refractive index modulation unit, and returning to perform step S200. According to the invention, after the width of the refractive index modulation unit is widened to the limit, the distance between the refractive index modulation units is adjusted, and the grating processing parameters required by tuning the wavelength optical signal with the set wavelength can be further ensured to be determined.
The step S300 of determining whether or not a point exists in the ith grating period at which the coupling efficiency between the core mode and the cladding mode is the highest includes: the distance between each point in the grating period and the head end of the grating,respectively multiplied by the coupling coefficients k1, and whether the multiplication result is equal to
If so, it indicates that there is a point in the grating period at which the coupling efficiency between the core mode and the cladding mode is the highest, otherwise, it indicates that there is no point in the grating period at which the coupling efficiency between the core mode and the cladding mode is the highest.
In step S500, determining whether the long-period fiber grating matches the corresponding echo wall microcavity according to the fiber core effective refractive index, the cladding effective refractive index, and the grating period includes:
judging whether the following formula is satisfied, wherein λ 1 is the grating resonance wavelength, λ 2 is the echo wall microcavity resonance wavelength, λ 1 is (nco-ncl), nco represents the effective refractive index of the fiber core, ncl represents the effective refractive index of the cladding, and Λ represents the grating period; λ 2 ═ 2 π nr/m where n denotes the whispering gallery mode effective index, r denotes the whispering gallery microcavity radius, and m denotes an integer; if yes, the long-period fiber grating is matched with the corresponding echo wall micro-cavity, otherwise, the long-period fiber grating is not matched with the corresponding echo wall micro-cavity.
In step S300, if it is determined that there is a point in the ith grating period at which the coupling efficiency between the core mode and the cladding mode is the highest and the point is located at the middle position of the ith grating period, the coupling efficiency between the core mode and the cladding mode can be maximized whether the optical signal is transmitted from the head end to the tail end of the grating or from the tail end to the head end of the grating. Therefore, the invention can ensure that the optical signal transmission in the system has no directional dependence by selecting a point which has the highest coupling efficiency and is positioned in the middle of the grating period as the fixed point of the echo wall micro-cavity, thereby realizing bidirectional work.
Other embodiments of the invention will be apparent to those skilled in the art from consideration of the specification and practice of the invention disclosed herein. This application is intended to cover any variations, uses, or adaptations of the invention following, in general, the principles of the invention and including such departures from the present disclosure as come within known or customary practice within the art to which the invention pertains. It is intended that the specification and examples be considered as exemplary only, with a true scope and spirit of the invention being indicated by the following claims.
It will be understood that the invention is not limited to the precise arrangements described above and shown in the drawings and that various modifications and changes may be made without departing from the scope thereof. The scope of the invention is to be controlled solely by the appended claims.
Claims (10)
1. A echo wall microcavity coupling system based on a long-period fiber grating is characterized by comprising the long-period fiber grating and an echo wall microcavity, wherein the long-period fiber grating comprises an optical fiber, a plane parallel to a fiber core of the optical fiber is formed on the optical fiber after a part of a cladding on the optical fiber is removed, a grating is formed on the plane, and each refractive index modulation unit in the grating is vertical to the fiber core of the optical fiber; the echo wall micro-cavity is fixed in the grating area on the plane, and the equatorial plane of the echo wall micro-cavity is perpendicular to the plane and is parallel to the fiber core of the optical fiber; after the optical signal is transmitted to the plane along the fiber core, the optical signal is coupled to the cladding mode from the fiber core module and then coupled to the echo wall micro-cavity from the cladding mode, and the echo wall mode is excited.
2. The system of claim 1, wherein the grating has a periodically varying refractive index modulation along the axial direction of the fiber, and the fixed position of the whispering gallery microcavity in the grating region is changed by moving the whispering gallery microcavity along the axial direction of the fiber, so as to adjust the coupling coefficient k2 between the cladding mode and the whispering gallery mode.
3. The whispering gallery microcavity coupling system based on the long-period fiber grating as claimed in claim 1 or 2, wherein along the axial direction of the optical fiber, when a distance L between the whispering gallery microcavity fixing position and the head end or the tail end of the grating satisfies the following formula, the coupling efficiency between the core mode and the cladding mode is the highest:
coefficient of coupling
An odd number.
4. The echo wall microcavity coupling system according to claim 1, wherein the resonant wavelength of the echo wall microcavity is within the resonant bandwidth of the long-period fiber grating, and the resonant wavelength λ 1 of the long-period fiber grating needs to satisfy phase matching: λ 1 ═ nco-nc Λ where nco represents the core effective index, nc represents the cladding effective index, and Λ represents the grating period.
5. The echo wall microcavity coupling system according to claim 4, wherein the resonant wavelength of the long-period fiber grating is determined by the width and the spacing of each refractive index modulation unit in the grating, and by adjusting at least one of the width, the spacing, and the L of the refractive index modulation units, the resonant wavelength of the echo wall microcavity can be matched with the resonant wavelength of the long-period fiber grating, and the coupling efficiency between the fiber core mold and the echo wall microcavity can be maximized.
6. A method of making the system of any one of claims 1 to 5, comprising:
step S100, removing a part of coating in the optical fiber to form a plane parallel to a fiber core on the optical fiber;
step S200, manufacturing refractive index modulation units in the grating on the plane according to the set refractive index modulation unit interval, and executing step S300, wherein a corresponding grating period exists for the width of each refractive index modulation unit;
step S300, determining the refractive index modulation amount of each grating period one by one from the head end of the grating along the axial direction of the optical fiber, determining the effective refractive index of the fiber core and the coupling coefficient between the fiber core mold and the cladding mold according to the refractive index modulation amount of the ith grating period, wherein i is an integer and the initial value is 1, judging whether a point which enables the coupling efficiency between the fiber core mold and the cladding mold to be the highest exists in the ith grating period or not according to the coupling coefficient, if so, executing step S500, otherwise, executing step S400;
step S400, judging whether i is the maximum grating period number, if so, widening the width of the refractive index modulation unit, returning to the step S200, otherwise, adding 1 to i, and returning to the step S300;
step S500, judging whether the long-period fiber grating is matched with a corresponding echo wall micro-cavity or not according to the fiber core effective refractive index, the cladding effective refractive index and the grating period, if so, judging that the width of the refractive index modulation unit and the distance between the set refractive index modulation units are qualified, and executing step S600, otherwise, returning to execute step S400;
step S600, fixing the echo wall micro-cavity at the point where the coupling efficiency between the fiber core mold and the cladding mold is highest.
7. The method of manufacturing according to claim 6, wherein in step S400, before widening the width of the refractive index modulation unit, the method further comprises:
step S700, judging whether the width of the refractive index modulation unit is equal to a set width, if so, replacing a new optical fiber, adjusting the distance between the set refractive index modulation units, setting the width of the refractive index modulation unit as an initial value, returning to execute step S100, otherwise, widening the width of the refractive index modulation unit, and returning to execute step S200.
8. The method according to claim 6, wherein the step S300 of determining whether a point exists in the ith grating period at which the coupling efficiency between the core mode and the cladding mode is the highest comprises:
multiplying the distances between each point in the grating period and the head end of the grating by the coupling coefficient k1 respectively, and judging whether the multiplication result is equal to
An odd number indicates that there is a point in the grating period at which the coupling efficiency between the core mode and the cladding mode is the highest, and otherwise indicates that there is no point in the grating period at which the coupling efficiency between the core mode and the cladding mode is the highest.
9. The method according to claim 6, wherein in step S500, determining whether the long-period fiber grating matches the corresponding whispering gallery microcavity according to the effective refractive index of the fiber core, the effective refractive index of the cladding, and the grating period includes:
judging whether the following formula is satisfied, wherein λ 1 is the resonance wavelength of the long-period fiber grating, λ 2 is the resonance wavelength of the echo wall microcavity, λ 1 is (nco-ncl), nco represents the effective refractive index of the fiber core, ncl represents the effective refractive index of the cladding, and Λ represents the grating period; λ 2 ═ 2 π nr/m, n denotes the whispering gallery mode effective index, r denotes the whispering gallery microcavity radius, and m denotes an integer;
if yes, the long-period fiber grating is matched with the corresponding echo wall micro-cavity, otherwise, the long-period fiber grating is not matched with the corresponding echo wall micro-cavity.
10. The method of claim 6, wherein in step S300, if it is determined that a point at which the coupling efficiency between the core mode and the cladding mode is highest exists in the i-th grating period and the point is located at an intermediate position of the i-th grating period, the coupling efficiency between the core mode and the cladding mode can be maximized whether the optical signal is transmitted from the head end to the tail end of the grating or from the tail end to the head end of the grating.
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