CN117761847A - Lateral optical coupling enhancement method based on 45-degree inclined optical well straight Fresnel lens - Google Patents
Lateral optical coupling enhancement method based on 45-degree inclined optical well straight Fresnel lens Download PDFInfo
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Abstract
The invention belongs to the technical field of lateral optical coupling of plastic optical fibers, and solves the problem of low lateral coupling efficiency of an LED-plastic optical fiber system. The lateral optical coupling enhancement method of the straight Fresnel lens of the 45-degree inclined optical well comprises the following steps: s1: processing a side coupling optical well on the side surface of the plastic optical fiber; s2: the design and parameter selection of a primary focusing Fresnel lens are carried out, the primary focusing Fresnel lens converges LED radiation light, the light spot area of the primary focusing Fresnel lens is reduced, and the primary focusing Fresnel lens is focused above a light well; s3: and designing and selecting parameters of a secondary subarea collimating Fresnel lens, wherein the secondary subarea collimating Fresnel lens is calculated and subarea according to the position and angle of incident light, and comprises a central hollowed-out area and subarea collimating areas, so that the LEDs are optically coupled into the light well to form an effective conduction mode to the maximum extent. The light well is combined with the Fresnel lens group, so that the lateral coupling efficiency of the LED-plastic optical fiber coupling system is more than 6%, and is far higher than the current level below 1%.
Description
Technical Field
The invention belongs to the technical field of plastic optical fiber lateral light coupling, and particularly relates to a lateral light coupling enhancement method based on a 45-degree inclined optical well straight Fresnel lens.
Background
The multi-source scanning is a novel quasi-distributed optical fiber sensing positioning technology which is proposed in recent years, and low-cost quasi-distributed water leakage measurement, angle measurement, motion analysis, flexible wearable devices, shape reconstruction and the like can be realized by utilizing the multi-source scanning. The multi-source scanning adopts a low-cost LED lamp strip and a lateral light coupling technology to realize space positioning, and the lateral coupling efficiency is an important parameter of the optical fiber lateral coupling technology, and is characterized in that the total power of the light source forming an effective conduction mode in the optical fiber accounts for the percentage of the total radiation power of the light source after the lateral coupling of the total radiation power of the light source.
Light that can be coupled into the fiber and form a conduction mode without a lens group or other optical path control system is weak, typically less than 1% of the total radiated power of the light source. Because the optical fiber system can cause attenuation, the higher the lateral coupling efficiency is, the longer the measurement distance is and the lower the system power consumption is; the low lateral coupling efficiency limits the sensitivity and measurement range of sensors based on the multi-source scanning technique described above and increases system power consumption.
The high-efficiency lateral coupling technology and the related technology of the LED-plastic optical fiber coupling system are a technical problem which is still to be solved at present. The light emitting area of the LED is larger than the size of the plastic optical fiber side surface coupling structure, the LED is equivalent to a surface light source, the relation between the divergence angle and the normalized radiation power is shown in figure 1, the LED radiation light is hemispherical and diverged, and the radiation angle coverage range is 0-90 degrees and is far larger than the laser beam divergence angle. The current optical fiber lateral coupling technology mainly comprises: fusion cone coupling, macrobend coupling, cladding defect coupling and the like.
Fusion cone coupling: because of the need to heat and fuse two fibers and taper, the taper is extremely fragile and requires packaging protection to be completely isolated from the outside environment. The device manufactured by the method is limited to the application of a beam splitter, a coupler, a beam splitter and the like, and is difficult to be practically applied to sensing measurement.
Macrobending coupling: the macroscopic bending of the flexible plastic optical fiber is utilized to generate a large number of cladding radiation modes, and the radiation mode light is laterally coupled into the cladding and the fiber core of the optical fiber through the other bent optical fiber adjacent to the cladding radiation mode light, so that a conduction mode is formed partially. However, it should be ignored that macrobend coupling process causes a lot of optical energy loss, and the use as a single-point sensor is better, while the use as a long-distance, distributed sensor is limited to a large extent by the rapid attenuation of optical power.
Cladding defect coupling: the original complete cladding of the optical fiber is damaged to a certain extent, and the defect forms of the optical fiber can be various, including a cylinder, a cone, a ball, a V-shaped groove and the like. The processing mode is mainly grinding, drilling and cutting. Taking V-shaped groove coupling as an example, specifically cutting a smooth and flat V-shaped structure on the side surface of an optical fiber, focusing laser on the reflecting surface of the V-shaped groove by utilizing a lens system, and realizing external light coupling to the fiber core of the optical fiber through proper angle control. At present, equipment capable of cutting V-shaped grooves with specific angles on an optical fiber is hardly found, and the robustness of the cut optical fiber is hardly ensured. Therefore, the process has high difficulty, the processing parameters are not easy to control, and the popularization and the use are difficult. In addition, the traditional lens system is developed for a laser-optical fiber coupling system, cannot adapt to the high divergence characteristic of an LED light source, and cannot be directly transplanted to the LED-plastic optical fiber coupling system.
Disclosure of Invention
The invention provides a lateral optical coupling enhancement method based on a 45-degree inclined optical well straight Fresnel lens, which aims to solve at least one technical problem in the prior art.
The invention is realized by adopting the following technical scheme: a lateral optical coupling enhancement method based on a 45-degree inclined optical well straight Fresnel lens comprises the following steps:
s1: processing a side coupling optical well on the side surface of the plastic optical fiber;
s2: the design and parameter selection of a primary focusing Fresnel lens are carried out, the primary focusing Fresnel lens converges LED radiation light, the light spot area of the primary focusing Fresnel lens is reduced, and the primary focusing Fresnel lens is focused above a light well;
s3: designing and selecting parameters of a secondary subarea collimating Fresnel lens, calculating and subareas according to the position and angle of incident light, wherein the secondary subarea collimating Fresnel lens comprises a central hollowed-out area and subarea collimating areas, so that LEDs are optically coupled into a light well to form an effective conduction mode to the greatest extent;
s4: and the primary focusing Fresnel lens and the secondary zoned collimating Fresnel lens are arranged between the LED lamp and the plastic optical fiber with the inclined optical well, so that the lateral optical coupling enhancement of the plastic optical fiber is realized.
Preferably, the inclination angle of the side coupling light well is 45 degrees, and the specific processing steps comprise:
s101: cutting plastic optical fiber with required length and grinding end face;
s102: drawing by using a laser etching software system, and determining the minimum processing size and the graph as primitives;
s103: placing the plastic optical fiber by using an inclination angle instrument to enable the plastic optical fiber to generate a required inclination angle;
s104: vertically and unidirectionally punching the plastic optical fiber, and cleaning the punched optical well;
s105: and measuring the processed optical fiber single-gate straight-through optical power and side coupling optical power, and calculating the insertion loss and the side coupling efficiency of the optical fiber single-gate straight-through optical power and the side coupling optical power to determine whether a processed finished product is qualified or not.
Preferably, one surface of the primary focusing Fresnel lens is planar, and the other surface of the primary focusing Fresnel lens is saw-toothed with equal groove depth; the angles among the saw teeth are different, and each saw tooth ring can collect all projected light rays at one point;
the relationship between the angles in the primary focusing fresnel lens and the expression of the conventional fresnel lens formula are:
wherein mu is n 、μ n 'represents the angle between the incident light and the refracted light and the optical axis FF'; r is R n And R'. n The distances from the point A and the point B to the optical axis of the lens; f and F 'are the distances from points F and F' to point O, respectively; k' n The distance between the center point B on the small sawtooth inclined surface and the straight surface of the collecting lens can be expressed as: k'. n ＝tanα n ×ΔR/2，a n Representing the included angle between each small triangle inclined plane and the bottom side;
when the LED light is incident on one side of the plane of the lens, the f and f' sizes required by the experiment are set according to the size of the LED structure, and the lens with equal saw tooth width is adopted, namely the width of each small saw tooth is delta R, and the height k of the saw tooth n Far less than the focal length f', the formula tan alpha n Can be expressed as:
wherein N represents the ratio of the refractive index of the lens to the refractive index of air; and all sawtooth parameters of the primary focusing Fresnel lens can be calculated according to the parameters of the required lens.
Preferably, the secondary zone collimating fresnel lens design method comprises the steps of:
s301: calculating the light angle distribution according to the minimum focal length f' of the primary focusing Fresnel lens;
s302: calculating the total internal reflection incoming light angle range of the light well;
s303: calculating the angle range of guided wave incoming light;
s304: obtaining the incoming light angle range which simultaneously meets the steps S301-S303;
s305: determining the size and the position of a secondary partition collimating Fresnel lens;
s306: performing eccentric hollowed-out treatment on the secondary partition collimating Fresnel lens;
s307: and carrying out collimation design on the secondary subarea collimation Fresnel lens except the eccentric hollowed-out area according to the incoming light angle.
Preferably, the angle μ ' between the refracted light and the optical axis FF ' is ' n May be defined as the secondary collimating fresnel lens acceptance angle; calculating the angle of the incoming light mu' n The value range is as follows: the +/-45 degrees is distributed in a cone shape.
Preferably, the total internal reflection light angle of the light well refers to the light angle range of light which is within the light range of +/-45 degrees and is distributed in a cone and reflected by the light well, and meets the light angle range of light which is subjected to total internal reflection at the light well-air interface; the calculation method comprises the following steps: θ c -45°＜μ' n < 45 DEG, wherein theta c Is the total internal reflection angle of the optical well interface,the total internal reflection light angle range caused by the 45 ° inclined light well is calculated as follows: -2.76 °<μ' n <+45°。
Preferably, the guided wave light incoming angle range means that the optical fiber with a specific numerical aperture receives light incoming within a certain angle range, and a conduction mode can be formed in the waveguide; the conditions for forming forward guided waves in the plastic optical fiber through 45 DEG optical well coupling are as follows: theta is 0 to or less in ＝μ' n < 180 DEG arcsin (NA), wherein θ in The NA is the numerical aperture of the optical fiber, which is the included angle between the light reflected by the optical well and the optical fiber axis; it is found from the calculation that the angle of arrival μ 'is obtained when the numerical aperture NA of the optical fiber is 0.5' n Forward bias: 0 to +30°; the angle range of the incoming light obtained in step S304 is: forward bias 0 deg. +30 deg..
Preferably, the secondary zoned collimating fresnel lens is placed against the fiber surface; the secondary zone collimating fresnel lens has a size of: the projection surface of the cone with the cone angle of 90 degrees on the upper surface position of the optical fiber takes the focus of the primary focusing Fresnel lens as the vertex.
Preferably, the hollowed-out range is as follows: taking the focus of the primary focusing Fresnel lens as a vertex, forward biasing a half cone formed by 30 degrees to form a projection area of the half cone formed on the secondary subarea collimating Fresnel lens; the collimation is carried out to enable the light incoming angle of the light coming from the projection area of the secondary subarea collimation Fresnel lens, which is used for removing the hollowed-out part, to be deflected to 0 degrees and to irradiate the light well, so that the condition of forming guided waves is met.
Compared with the prior art, the invention has the beneficial effects that:
the invention discloses a process method and process parameters for processing a high-efficiency side coupling optical well on the side surface of a plastic optical fiber, wherein the side coupling optical well with an inclination angle of 45 degrees and controllable depth is etched on the side surface of the plastic optical fiber by utilizing a laser processing technology. Compared with polishing, drilling, cutting and other processes, the processing method disclosed by the patent can ensure processing consistency more easily, reduce the damage area of the optical fiber, ensure the robustness of the optical fiber to the maximum extent and reduce the insertion loss and the crosstalk between coupling structures.
The patent also discloses a lateral coupling enhancement method of the LED-optical fiber coupling system based on the two-stage Fresnel lens group. The lens group includes a primary focusing fresnel lens and a secondary zoned collimating fresnel lens. The primary focusing lens can make the LED radiation light be converged as much as possible, reduce the light spot area and focus the LED radiation light above the light well. The secondary partition collimating lens calculates and partitions according to the position and the angle of the incident light, and comprises an eccentric hollow area and a collimating area, so that the lateral coupling efficiency is effectively improved.
The light well is combined with the Fresnel lens group, so that the lateral coupling efficiency of the LED-plastic optical fiber coupling system is more than 6%, and is far higher than the current level below 1 mill.
Drawings
In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings that are needed in the embodiments will be briefly described below, and it is obvious that the drawings in the following description are only some embodiments of the present invention, and other drawings can be obtained according to these drawings without inventive effort for a person skilled in the art.
FIG. 1 is a graph of normalized radiant power versus divergence angle for a typical LED;
FIG. 2 is a schematic illustration of a 45℃inclined optical well etched on the surface of a plastic optical fiber using a laser;
FIG. 3 is a schematic diagram of Fresnel lens condensation;
fig. 4 is a mounting position diagram of the fresnel lens group.
In the figure: 1-an LED lamp strip; 2-LED lamp beads; 3-plastic optical fiber; 4-45 degrees inclined light well; 5-primary focusing fresnel lens; 6-secondary zoned collimating fresnel lenses; 7-collimation zone; 8-hollow areas; 9-hollow area range angle: 0 to +30°; 10-optical well tilt angle: 45 deg..
Detailed Description
Technical solutions in the embodiments of the present invention will be clearly and completely described with reference to the drawings in the embodiments of the present invention, and it is apparent that the described embodiments are only some embodiments of the present invention, but not all embodiments. All other embodiments, which can be made by those skilled in the art based on the examples of this invention without making any inventive effort, are intended to fall within the scope of this invention.
It should be understood that the structures, proportions, sizes, etc. shown in the drawings are merely for the purpose of understanding and reading the disclosure, and are not intended to limit the scope of the invention, which is defined by the appended claims, and any structural modifications, proportional changes, or dimensional adjustments, which may be made by those skilled in the art, should fall within the scope of the present disclosure without affecting the efficacy or the achievement of the present invention, and it should be noted that, in the present disclosure, relational terms such as first and second are used solely to distinguish one entity from another entity without necessarily requiring or implying any actual relationship or order between such entities.
The present invention provides an embodiment:
as shown in fig. 1 to 4, a lateral optical coupling enhancement method based on a 45-degree inclined optical well straight fresnel lens includes the following steps:
s1: processing a side coupling optical well on the side surface of the plastic optical fiber;
s2: the design and parameter selection of a primary focusing Fresnel lens are carried out, the primary focusing Fresnel lens converges LED radiation light, the light spot area of the primary focusing Fresnel lens is reduced, and the primary focusing Fresnel lens is focused above a light well;
s3: designing and selecting parameters of a secondary subarea collimating Fresnel lens, calculating and subareas according to the position and angle of incident light, wherein the secondary subarea collimating Fresnel lens comprises a central hollowed-out area and subarea collimating areas, so that LEDs are optically coupled into a light well to form an effective conduction mode to the greatest extent;
s4: and the primary focusing Fresnel lens and the secondary zoned collimating Fresnel lens are arranged between the LED lamp and the plastic optical fiber with the inclined optical well, so that the lateral optical coupling enhancement of the plastic optical fiber is realized.
In this embodiment, the inclination angle of the side-coupled light well is 45 degrees, and the specific processing steps include:
s101: cutting plastic optical fibers with required lengths, grinding the end surfaces of the plastic optical fibers to enable the end surfaces to be flat, and reducing insertion loss;
s102: drawing by using a laser etching software system, and determining the minimum processing size and the graph as primitives; taking a 2mm diameter ck-80 optical fiber as an example, the optimal graphic element diameter size is 110 mu m, and the graphic element shape is circular; the diameter of the graphic primitive is smaller than 110 mu m, the etched air gap is too narrow, and the lateral coupling effect is reduced; the diameter of the graphic primitive is larger than 110 mu m, so that the insertion loss can be increased too quickly, and the damage area of the surface of the optical fiber is increased too greatly to increase the crosstalk between the coupling structures; thus, a comprehensive consideration of 110 μm can give consideration to both the lateral coupling efficiency and the insertion loss.
S103: placing the plastic optical fiber by using an inclination angle instrument to enable the plastic optical fiber to generate a required inclination angle; then, punching operation is carried out, as shown in fig. 2, wherein an arrow is the laser direction, and an angle alpha is the inclination angle; the repeated test proves that the 45-degree inclined angle is the optimal inclined angle under the condition of no lens system; the 45-degree tilt angle is selected to be compatible with systems with or without lens combinations, and is the most compatible choice for products.
S104: vertically and unidirectionally punching the plastic optical fiber, cleaning the punched optical well, and washing away residues after laser burning; taking a laser engraving machine with power of 40W and a 2 mm-diameter ck-80 optical fiber as an example, the optimal output power is 16.5 percent in a vertical unidirectional mode. The excessive laser power causes the increase of insertion loss, influences robustness, and has insufficient etching depth due to the excessively low power and excessively low side coupling efficiency.
S105: and measuring the processed optical fiber single-gate straight-through optical power and side coupling optical power, and calculating the insertion loss and the side coupling efficiency of the optical fiber single-gate straight-through optical power and the side coupling optical power to determine whether a processed finished product is qualified or not.
The Fresnel lens is formed by a plurality of fine wedge-shaped laminas, is formed by injection molding polyolefin materials, is lighter in weight than a common lens, and can effectively reduce the manufacturing time and cost. The Fresnel lens can focus a wide and large light path to a point, and the light condensing capacity is higher than that of a common lens. The patent adopts 45 degrees inclined light well to combine fresnel lens group to realize the reinforcing of LED-plastics optical fiber system optical coupling efficiency.
One surface of the primary focusing Fresnel lens is in a plane shape, and the other surface of the primary focusing Fresnel lens is in a sawtooth shape with equal groove depth; and the angles among the saw teeth are different from each other, and each saw tooth ring can collect all the projected light rays at one point.
In the design of a primary focusing fresnel lens, the lens condensing performance is analyzed, and the refraction law can be used to obtain:
wherein N represents the ratio of the refractive index of the lens to the refractive index of air; beta n And beta' n Respectively represent the light rays P at A n An angle of incidence and angle of refraction at the lens working surface; θn and θn represent the incident angle and refraction angle, respectively, of the lens base plane at B. As can be seen from fig. 3
θ′ n ＝μ′ n +a n ,a n ＝θ n +β′ n ,β n ＝μ n (1-2)
Wherein a is n Represents the included angle between the inclined plane and the base of each small triangle, mu n 、μ n 'represents the angle between the incident light and the refracted light and the optical axis FF'.
sinθ′ n ＝sin(μ′ n +a n ),sinθ′ n ＝Nsinθ＝Nsin(a n -β′ n ) (1-3)
Nsin(a n -β′ n )＝sin(μ′ n +a n ) (1-4)
The two sides of the upper part are unfolded to obtain
sina n (Ncosβ′ n -cosμ′ n )＝cosa n (sinμ′ n +Nsinβ′ n ) (1-5)
Namely:
obtained from (1-1)
Substituting the formula (1-7) into the formula (1-6) to obtain the following formula:
the relationship between the angles can be obtained from the geometric relationship and the traditional fresnel lens formula:
wherein F and F 'are the distances from points F and F' to point O, respectively; r is R n And R'. n The distances from the lens optical axis are point a and point B, respectively.
k' n The distance between the center point B on the small sawtooth inclined surface and the straight surface of the collecting lens can be expressed as:
K′ n ＝tanα n ×ΔR/2 (1-10)
when the LED light is incident on one side of the plane of the lens as shown in FIG. 3, the sizes of f and f' required by the experiment are set according to the size of the LED structure, and further, the lens required by the design of equal saw tooth width is adopted, namely the width of each small saw tooth is delta R, and the height k of the saw tooth is also used n Much smaller than the focal length f', then equation (1-8) can be expressed as:
and calculating all sawtooth parameters of the Fresnel lens according to the parameters of the required lens.
In particular in this patent, according to fig. 1: LED typical normalized radiation power vs. divergence angle, mu n The maximum value cannot be 90 degrees (the incident light is parallel to the lens plane), and mu can be taken only by taking the light loss of the divergence angle as small as possible as the design basis nmax Is 80 deg.. Considering the nested installation relation of the primary Fresnel lens and the LEDs, f is required to be larger than the heights of the LED lamp beads, and f is selected to be just larger than the heights of the LED lamp beads in order to achieve the minimization of the primary Fresnel lens size because the f and the Fresnel lens radius are in a direct proportion relation. The LED sizes in this patent are specifically: 5mm 1.6mm, the corresponding primary fresnel lens design is shaped as a square base, with dimensions 20.43 x 20.43mm, inscribed circle radius r=10.21 mm, internal height h=3.4 mm, and object distance f=1.8 mm.
From a performance standpoint, the smaller the sawtooth width should be, the smoother the lens surface becomes when the sawtooth width is wider, and the less obvious the refractive angle change per small area, the less accurate the focusing will result, affecting the optical performance of the lens. But if the saw tooth width is too small, it causes manufacturing difficulty and cost increase. Thus, considering both cost and performance factors, the saw tooth width is selected to be 0.3mm.
The choice of focal length f 'is related to the angle of the incoming light at the focal point, which determines whether the light reaching the optical well can form guided waves, and therefore the choice of f' is critical. The larger f ' is better from a light-converging performance perspective, whereas on the other hand f ' affects the overall structure thickness, the smaller f ' is better from a packaging perspective. Therefore, single stage fresnel lenses cannot achieve optimal design of lateral coupling optical performance of LED-plastic fiber systems subject to structural size limitations. For this purpose, this patent proposes a design scheme of primary focusing fresnel lens superimposed with secondary zoned collimating fresnel lens.
The secondary zone collimating fresnel lens design method comprises the steps of:
s301: calculating the light angle distribution according to the minimum focal length f' of the primary focusing Fresnel lens; the minimum focal length f' of the primary focusing fresnel lens that is acceptable is determined from the standpoint of the structural dimensions, this patent taking a value of 10.21mm as an example.
S302: calculating the total internal reflection incoming light angle range of the light well; the angle between the refracted light and the optical axis FF 'is mu' n May be defined as the secondary collimating fresnel lens acceptance angle; calculating the angle of the incoming light mu' n The value range is as follows: the +/-45 degrees is distributed in a cone shape. (shown in FIG. 3, μ' n Anticlockwise as forward bias and clockwise as reverse bias)
S303: calculating the angle range of guided wave incoming light; the total internal reflection light incoming angle of the light well means that the light incoming angle range of + -45 degrees is in cone distribution in the light incoming range obtained in the step S301, the light incoming angle range of the light which is reflected by the total internal reflection at the light well-air interface is met; the calculation method comprises the following steps: θ c -45°＜μ' n < 45 DEG, wherein theta c Is the total internal reflection angle of the optical well interface,the total internal reflection light angle range caused by the 45 ° inclined light well is calculated as follows: -2.76 °<μ' n <+45°。
S304: obtaining the incoming light angle range which simultaneously meets the steps S301-S303; the guided wave light incoming angle range means that an optical fiber with a specific numerical aperture receives incoming light within a certain angle range, and a conduction mode can be formed in the waveguide; the conditions for forming forward guided waves in the plastic optical fiber through 45 DEG optical well coupling are as follows: theta is 0 to or less in ＝μ' n < 180 DEG arcsin (NA), wherein θ in The NA is the numerical aperture of the optical fiber, which is the included angle between the light reflected by the optical well and the optical fiber axis; it is found from the calculation that the angle of arrival μ 'is obtained when the numerical aperture NA of the optical fiber is 0.5' n Forward bias: 0 to +30°; the angle range of the incoming light obtained in step S304 is: forward bias 0 deg. +30 deg.. (shown in FIG. 3, μ' n Counterclockwise is forward biased, and clockwise is reverse biased).
S305: determining the size and the position of a secondary partition collimating Fresnel lens; the secondary subarea collimating Fresnel lens is placed against the surface of the optical fiber; the secondary zone collimating fresnel lens has a size of: the projection surface of the cone with the cone angle of 90 degrees on the upper surface position of the optical fiber takes the focus of the primary focusing Fresnel lens as the vertex. Taking a 2mm plastic optical fiber as an example, the primary focusing Fresnel lens focus is arranged at the axial center of the optical fiber, and the primary focusing Fresnel lens focus can be obtained through calculation: the radius of the secondary Fresnel lens is 1mm (the radius of the actual working area), and the actual machining radius of the secondary zoned collimating Fresnel lens is as follows in consideration of the installation fault tolerance performance: 2mm, and the saw tooth width is 0.1mm.
S306: since S304 determines that the light angle range satisfying steps S301, S302 and S303 is forward biased by 0 ° to +30°, the portion of light can naturally form a conduction mode in the optical fiber, and cannot be collimated, otherwise, most of the light cannot be irradiated to the optical well, and the coupling efficiency is greatly reduced. Therefore, the secondary subarea collimation Fresnel lens is subjected to eccentric hollowed-out treatment; the hollowed-out range is as follows: taking the focus of the primary focusing Fresnel lens as a vertex, forward biasing a half cone formed by 30 degrees to form a projection area of the half cone formed on the secondary subarea collimating Fresnel lens;
s307: carrying out collimation design on the secondary subarea collimation Fresnel lens except the eccentric hollowed-out area according to the incoming light angle; the collimation is carried out to enable the light incoming angle of the light coming from the projection area of the secondary subarea collimation Fresnel lens, which is used for removing the hollowed-out part, to be deflected to 0 degrees and to irradiate the light well, so that the condition of forming guided waves is met.
In this example, the effect of 45 ° inclined optical wells on lateral coupling efficiency was etched on the surface of CK80 plastic optical fibers.
Experimental test excitationThe percentage of photoetching power is 15.5%, 16% and 16.5%, and six combinations with graphic element diameter of 0.11mm and 0.12mm are adopted, and 40W CO of the large family is adopted 2 The laser cutting machine etches the optical well on the surface of the CK80 plastic optical fiber. Lateral coupling tests are carried out by adopting a cable Lei Bogao stability 660nmLED light source, and under the condition that the light source condition is unchanged, the lateral coupling efficiency of a 0-degree vertical etching light well and a 45-degree inclined etching light well is compared, and the results are shown in table 1. Under the six combined conditions, the actual measurement lateral coupling efficiency of the 45-degree inclined etching optical well is improved by more than 300 percent.
TABLE 1 side coupling efficiency of coupling structures with different tilt angles
Effect of fresnel lens group on lateral coupling efficiency
And on the basis of the 45-degree inclined etching light well, a double-stage Fresnel lens is adopted to focus and partition and collimate the LED light source. The design parameters of the two lenses are shown in table 2:
TABLE 2 Fresnel lens group design parameters
Taking the above parameters as an example, the theoretical side coupling efficiency of the single-stage fresnel lens is known (neglecting the light distribution non-uniformity) by estimation using the projection method as follows: 5.305, after the secondary partition collimating Fresnel lens is added, the theoretical side coupling efficiency of the two-stage Fresnel lens is as follows: 6.785% the theoretical side coupling efficiency of a two-stage fresnel lens assembly is nearly 28% improved over that of a single-stage fresnel lens assembly.
By adopting the 45-degree inclined optical well combined with the two-stage Fresnel lens group, the theoretical side coupling efficiency can reach more than 6 percent, and the overall theoretical improvement of the side coupling efficiency can reach more than 150 times relative to the side coupling efficiency (0.04 percent according to an actual measurement value) of the 0-degree vertical etching optical well.
The foregoing is only a preferred embodiment of the present invention, but the scope of the present invention is not limited thereto, and any changes or substitutions easily contemplated by those skilled in the art within the technical scope of the present invention should be included in the scope of the present invention. Therefore, the protection scope of the present invention shall be subject to the protection scope of the claims.
Claims (9)
1. The lateral optical coupling enhancement method based on the 45-degree inclined optical well straight Fresnel lens is characterized by comprising the following steps of:
s1: processing a side coupling optical well on the side surface of the plastic optical fiber;
s2: the design and parameter selection of a primary focusing Fresnel lens are carried out, the primary focusing Fresnel lens converges LED radiation light, the light spot area of the primary focusing Fresnel lens is reduced, and the primary focusing Fresnel lens is focused above a light well;
s3: designing and selecting parameters of a secondary subarea collimating Fresnel lens, calculating and subareas according to the position and angle of incident light, wherein the secondary subarea collimating Fresnel lens comprises a central hollowed-out area and subarea collimating areas, so that LEDs are optically coupled into a light well to form an effective conduction mode to the greatest extent;
s4: and the primary focusing Fresnel lens and the secondary zoned collimating Fresnel lens are arranged between the LED lamp and the plastic optical fiber with the inclined optical well, so that the lateral optical coupling enhancement of the plastic optical fiber is realized.
2. The lateral optical coupling enhancement method based on the 45-degree inclined optical well straight fresnel lens according to claim 1, wherein the method comprises the following steps: the inclination angle of the side coupling light well is 45 degrees, and the specific processing steps comprise:
s101: cutting plastic optical fiber with required length and grinding end face;
s102: drawing by using a laser etching software system, and determining the minimum processing size and the graph as primitives;
s103: placing the plastic optical fiber by using an inclination angle instrument to enable the plastic optical fiber to generate a required inclination angle;
s104: vertically and unidirectionally punching the plastic optical fiber, and cleaning the punched optical well;
s105: and measuring the processed optical fiber single-gate straight-through optical power and side coupling optical power, and calculating the insertion loss and the side coupling efficiency of the optical fiber single-gate straight-through optical power and the side coupling optical power to determine whether a processed finished product is qualified or not.
3. The lateral optical coupling enhancement method based on the 45-degree inclined optical well straight fresnel lens according to claim 2, wherein the method comprises the following steps: one surface of the primary focusing Fresnel lens is in a plane shape, and the other surface of the primary focusing Fresnel lens is in a sawtooth shape with equal groove depth; the angles among the saw teeth are different, and each saw tooth ring can collect all projected light rays at one point;
the relationship between the angles in the primary focusing fresnel lens and the expression of the conventional fresnel lens formula are:
wherein mu is n 、μ n 'represents the angle between the incident light and the refracted light and the optical axis FF'; r is R n And R'. n The distances from the point A and the point B to the optical axis of the lens; f and F 'are the distances from points F and F' to point O, respectively; k' n The distance between the center point B on the small sawtooth inclined surface and the straight surface of the collecting lens can be expressed as: k'. n ＝tanα n ×ΔR/2，a n Representing the included angle between each small triangle inclined plane and the bottom side;
when the LED light is incident on one side of the plane of the lens, the f and f' sizes required by the experiment are set according to the size of the LED structure, and the lens with equal saw tooth width is adopted, namely the width of each small saw tooth is delta R, and the height k of the saw tooth n Far less than the focal length f', the formula tan alpha n Can be expressed as:
wherein N represents the ratio of the refractive index of the lens to the refractive index of air; and all sawtooth parameters of the primary focusing Fresnel lens can be calculated according to the parameters of the required lens.
4. A method for enhancing lateral optical coupling based on 45-degree inclined optical straight fresnel lenses according to claim 3, wherein: the secondary zone collimating fresnel lens design method comprises the steps of:
s301: calculating the light angle distribution according to the minimum focal length f' of the primary focusing Fresnel lens;
s302: calculating the total internal reflection incoming light angle range of the light well;
s303: calculating the angle range of guided wave incoming light;
s304: obtaining the incoming light angle range which simultaneously meets the steps S301-S303;
s305: determining the size and the position of a secondary partition collimating Fresnel lens;
s306: performing eccentric hollowed-out treatment on the secondary partition collimating Fresnel lens;
s307: and carrying out collimation design on the secondary subarea collimation Fresnel lens except the eccentric hollowed-out area according to the incoming light angle.
5. The lateral optical coupling enhancement method based on the 45-degree inclined optical well straight fresnel lens according to claim 4, wherein the method comprises the following steps: the angle between the refracted light and the optical axis FF 'is mu' n May be defined as the secondary collimating fresnel lens acceptance angle; calculating the angle of the incoming light mu' n The value range is as follows: the +/-45 degrees is distributed in a cone shape.
6. The lateral optical coupling enhancement method based on the 45-degree inclined optical well straight fresnel lens according to claim 5, wherein the method comprises the following steps: the total internal reflection light incoming angle of the light well means that the light incoming angle range of + -45 degrees is in cone distribution in the light incoming range obtained in the step S301, the light incoming angle range of the light which is reflected by the total internal reflection at the light well-air interface is met; the calculation method comprises the following steps: θ c -45°＜μ' n < 45 DEG, wherein theta c Is the total internal reflection angle of the optical well interface,the total internal reflection light angle range caused by the 45 ° inclined light well is calculated as follows: -2.76 °<μ' n <+45°。
7. The lateral optical coupling enhancement method based on the 45-degree inclined optical well straight fresnel lens according to claim 6, wherein the method comprises the following steps: the guided wave light incoming angle range means that an optical fiber with a specific numerical aperture receives incoming light within a certain angle range, and a conduction mode can be formed in the waveguide; the conditions for forming forward guided waves in the plastic optical fiber through 45 DEG optical well coupling are as follows: theta is 0 to or less in ＝μ' n < 180 DEG arcsin (NA), wherein θ in The NA is the numerical aperture of the optical fiber, which is the included angle between the light reflected by the optical well and the optical fiber axis; it is found from the calculation that the angle of arrival μ 'is obtained when the numerical aperture NA of the optical fiber is 0.5' n Forward bias: 0 to +30°; the angle range of the incoming light obtained in step S304 is: forward bias 0 deg. +30 deg..
8. The lateral optical coupling enhancement method based on the 45-degree inclined optical well straight fresnel lens according to claim 7, wherein: the secondary subarea collimating Fresnel lens is placed against the surface of the optical fiber; the secondary zone collimating fresnel lens has a size of: the projection surface of the cone with the cone angle of 90 degrees on the upper surface position of the optical fiber takes the focus of the primary focusing Fresnel lens as the vertex.
9. The lateral optical coupling enhancement method based on the 45-degree inclined optical well straight fresnel lens according to claim 8, wherein the method comprises the following steps: the hollowed-out range is as follows: taking the focus of the primary focusing Fresnel lens as a vertex, forward biasing a half cone formed by 30 degrees to form a projection area of the half cone formed on the secondary subarea collimating Fresnel lens; the collimation is carried out to enable the light incoming angle of the light coming from the projection area of the secondary subarea collimation Fresnel lens, which is used for removing the hollowed-out part, to be deflected to 0 degrees and to irradiate the light well, so that the condition of forming guided waves is met.
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