Low return loss short dimension fiber collimator

Abstract

This invention discloses a low-return-loss, short-size fiber collimator, comprising: a first freeform surface and a second freeform surface, wherein the shapes of both the first and second freeform surfaces conform to Chebyshev polynomials; the first and second freeform surfaces are respectively the surface near the fiber end and the surface away from the fiber end in the fiber collimator; both the first and second freeform surfaces are x-axis symmetric and y-axis asymmetric; the optical power of the first freeform surface is negative, and all surfaces except those within the effective light transmission area are planes perpendicular to the optical axis. Through the above technical solution, this invention provides a single-mode fiber collimator with ultra-short element length, long working distance, high point accuracy, and low return loss.

Description

Technical Field

[0001] This invention relates to the field of optical technology, and in particular to a low-return-loss, short-size fiber collimator. Background Technology

[0002] A fiber optic collimator is a collimating coupling device in a fiber optic communication system. Its function is to expand the light from the fiber into a collimated beam, and then perform operations such as splitting, combining, and coupling the collimated beam to achieve functions such as optical information connection, energy demultiplexing / combining, wavelength division multiplexing / demultiplexing, optical path conversion, energy attenuation, and reverse isolation.

[0003] Optical communication is currently developing towards higher speeds, higher bandwidths, and longer distances. To meet these demands, the loss and cost of fiber optic collimators need to be reduced.

[0004] Currently, there are two main types of fiber optic collimators: self-focusing lenses (GRIN Lens) and constant refractive index lenses known as CLens.

[0005] A GRIN lens is an optical collimator that uses a graduated refractive index lens (the exit end is flattened, and the incident end is tilted vertically at an 8° angle). The refractive index of the GRIN lens material gradually decreases radially, allowing for continuous refraction of light propagating along the axial direction, thus achieving smooth and continuous convergence of the outgoing light to a single point. Its advantages include the ease of manufacturing due to its flat end face, and the ability to change the lens's length to alter its focal length and characteristics. A very short focal length can be achieved simply by adjusting the gradient depth of the refractive index distribution and the lens length. Disadvantages include the inability to achieve absolute standardization during the manufacturing process of self-focusing lenses, leading to errors in parameters such as the focusing constant and period length, resulting in a mismatch in the mode field diameters of the incoming and outgoing Gaussian beams. Furthermore, it is relatively expensive in terms of material cost.

[0006] Due to cost considerations, C-Lens are currently more widely used, and they have advantages over G-Lens in long-distance applications. C-Lens uses a fixed refractive index, with the incident light end face angled at 8° and the exiting end shaped like a convex lens. The fiber optic end is placed at the focal length of the C-Lens, ensuring that the Gaussian beam emitted from the fiber is collimated and parallel after passing through the collimating lens. The main parameters of a C-Lens include insertion loss, return loss, and point accuracy. Point accuracy refers to the angle between the emitted beam and the collimating lens axis, which is caused by the tilt of the pigtail and the lens end face. Figure 3 As shown.

[0007] Normal point accuracy Expressed as follows in The refractive index represents the environment. Optical fibers are typically used in air and have a refractive index of 1. is the refractive index of the collimating lens; The refractive index of the optical fiber is typically fused silica, around 1.48; L is the length of the C-LENS collimating lens; R is the end face radius of the C-LENS collimating lens; d is the distance between the fiber exit end face and the C-LENS end face. The tilt angle between the optical end face and the end face of the C-LENS collimator is typically 8°. The above formula represents the factors affecting the point accuracy of the fiber optic collimator, including R, L, and [other factors]. , d.

[0008] Point precision ensures that the angle of the emitted light is at a certain angle to the optical axis. When the ends of the two optical fibers are swapped over a long working distance, the impact of point precision on coupling efficiency is mainly reflected in the difference in the radial distance between the collimating lenses of the two optical fibers and the difference in the incident angle. The change in coupling efficiency caused by the difference in radial distance between the fiber collimating lenses is given by the following formula: in This represents the radial drift distance between the two interleaved fiber collimating lenses. Indicates the waist radius as follows Figure 5 As shown, the greater the difference in radial drift distance between the two interleaved fiber collimating mirrors, the lower the coupling efficiency. The change in coupling efficiency caused by the difference in the incident angle of the fiber collimating lens is given by the following formula: in This indicates the angle between the coupled ray and the optical axis of the collimating lens. Indicates the wavelength of the coupled light rays. The refractive index of a collimating lens is represented by... This represents the mode field radius of the collimating lens. The larger the angle between the coupled ray and the optical axis of the collimating lens, the lower the coupling efficiency.

[0009] When the optical axes of the two collimating lenses are the same, the point accuracy... ;Point accuracy It is positively correlated with the radial drift distance of the two interleaved fiber collimating mirrors.

[0010] In practice, when dealing with fiber optic coupling efficiency, the optical axes of the two fibers are usually not parallel. Workers need to continuously adjust the spatial position and rotate the fiber collimating lens, guided by coupling efficiency, to obtain the optimal result. This process is quite tedious.

[0011] In summary, the point accuracy of traditional fiber optic collimators affects the improvement of coupling efficiency, and next-generation products need to consider higher point accuracy and coupling efficiency. Summary of the Invention

[0012] To address the point accuracy issues in existing C-LENS structures and the fact that existing structures do not consider shortening the length of the collimating lens, thus failing to effectively improve coupling efficiency, this invention provides a low-return-loss, short-size fiber collimating lens. This single-mode fiber collimating lens offers ultra-short element length, long working distance, high point accuracy, and low return loss.

[0013] To achieve the above technical objectives, the present invention provides the following technical solution: a low-return-loss, short-size fiber collimating lens, comprising: The first free-form surface and the second free-form surface, wherein the shapes of the first free-form surface and the second free-form surface both conform to Chebyshev polynomials; the first free-form surface and the second free-form surface are respectively the surface near the end of the optical fiber and the surface away from the end of the optical fiber in the optical fiber collimating lens. Both the first and second freeform surfaces are symmetric along the x-axis and asymmetric along the y-axis. The first freeform surface has a negative optical power and all surfaces except those within the effective light transmission area are planes perpendicular to the optical axis.

[0014] Optionally, the fiber collimating lens is used in conjunction with an 8D oblique-cut fiber.

[0015] Optionally, the Chebyshev polynomial is: ; Where c is the vertex radius of curvature on the surface, x0 and y0 are the normalized lengths, and c(i,j) are the normalized polynomial coefficients. , These are the fundamental terms of the Chebyshev polynomial; The Chebyshev polynomials corresponding to the shapes of the first and second free-form surfaces are even functions in the x-direction and odd functions in the y-direction.

[0016] Optionally, the optical fiber collimator has an aperture of 1.26 mm and an applicable wavelength of 1550 nm.

[0017] Optionally, the fiber optic collimating lens is made of Chengdu Guangming's D-K9 material and is manufactured using a molding process.

[0018] The present invention has the following technical effects: 1. High point accuracy: The Gaussian beam emitted from the 8° oblique fiber passes through two asymmetrical surfaces in the y-direction, forming a Gaussian beam with the emitted light parallel to the optical axis of the collimating lens.

[0019] 2. Low return loss: This collimating lens is used in conjunction with an 8° angled optical fiber. The return loss is given by the following formula: in The Fresnel reflection coefficient of the fiber end face; The refractive index of the fiber cladding; The mode field radius of the optical fiber; The angle of cut of the optical fiber; λ is the incident wavelength.

[0020] Fiber optic cut angle The larger the angle, the lower the return loss. Considering the impact of large-angle polarization separation, the fiber's tangent angle... Setting it to 8° is a relatively ideal specification. This invention is designed for use with 8° oblique-cut optical fibers.

[0021] 3. Ultra-long working distance: The aperture of this fiber optic collimator is 1.26mm, the wavelength of the fiber is 1550nm, and its Rayleigh distance is [not specified]. .

[0022] 4. High coupling efficiency: The receiving fiber mode field diameter is 9.6um, the system efficiency is 0.9959; the receiving efficiency is 0.968695; and the coupling efficiency is 0.964753.

[0023] 5. Ultra-short optical element length: The negative optical power of both sides of the collimating lens reduces the length of the optical element. Attached Figure Description

[0024] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0025] Figure 1 This is a schematic diagram of a single-mode fiber collimator in the XZ direction provided in an embodiment of the present invention; Figure 2 This is a schematic diagram of a YZ-direction planar single-mode fiber collimating lens provided in an embodiment of the present invention; Figure 3 A schematic diagram illustrating the principle of point accuracy provided in an embodiment of the present invention; Figure 4 This is a schematic diagram of the coupling efficiency of a single-mode fiber collimating lens provided in an embodiment of the present invention; Figure 5 This is a schematic diagram of the radial drift distance of the collimating lens for the interlocking fiber provided in an embodiment of the present invention; Figure 6 A schematic diagram of a mirror image of a Chebyshev polynomial provided in an embodiment of the present invention, where all coefficients of the exponential terms are 0. Figure 7 A schematic diagram of a mirror image of a Chebyshev polynomial with odd-degree polynomials whose coefficients are all 0, provided in an embodiment of the present invention. Figure 8 A schematic diagram of a mirror image of a Chebyshev polynomial provided in an embodiment of the present invention, where the coefficients of the multiple terms are not zero when the X-axis and Y-axis coefficients of the polynomial have odd-degree terms. Figure 9 This is a schematic diagram of a mirror image that is symmetrical about the X-axis and asymmetrical about the Y-axis, provided for an embodiment of the present invention. Detailed Implementation

[0026] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0027] To address the problems existing in the prior art, the present invention provides the following solution: This invention discloses a low-return-loss, short-size fiber optic collimator, which provides a single-mode fiber optic collimator with ultra-short element length, long working distance, high point accuracy, and low return loss. It is applicable in optical and data communication fields. The fiber optic collimator is designed with a double freeform surface; it is manufactured using a molding process; it is used in conjunction with an 8D oblique-cut fiber; the first freeform surface of the fiber optic collimator, the surface closest to the fiber, has a structure perpendicular to the optical axis; the freeform surfaces of the two end faces of the fiber optic collimator have a structure that is symmetrical along the X-axis and asymmetrically tilted along the Y-axis. These coordinate axes correspond to the XY axes in the shape space of a Chebyshev polynomial function. The XYZ axes in the attached figures correspond one-to-one with the XYZ axes of the Chebyshev polynomial. Through the above technical solution, this invention provides an ultra-short collimator manufactured using molding, which has an absolute advantage in point accuracy.

[0028] like Figure 1-2 The present invention provides a low return loss, short-size fiber collimator design, wherein the single-mode fiber collimator structure adopts two Chebyshev polynomial freeform surfaces. The two freeform surfaces of the single-mode fiber collimating lens have negative optical power values, which serve to diffuse the Gaussian beam emitted from the fiber. Both freeform surfaces of the single-mode fiber collimating lens are symmetric along the X-axis and asymmetric along the Y-axis. The single-mode fiber collimating lens material used is Chengdu Guangming's D-K9, which reduces costs and supports molding. The end face of the single-mode fiber collimating lens near a section of the fiber is perpendicular to the optical axis, as shown in the attached figure. Figure 1 , 2 As shown; The optical power of the two freeform surfaces of the single-mode fiber collimator is negative, which serves to diffuse the Gaussian beam coming out of the fiber; the optical power of the surface near the fiber end face is also negative, which serves to diffuse the light. The setting of this surface shortens the element length of the fiber collimator. The single-mode fiber collimating lens structure employs two Chebyshev polynomial freeform surfaces, both symmetric along the X-axis and asymmetric along the Y-axis. An angled Gaussian beam emitted from an 8° obliquely cut single-mode fiber undergoes its exit angle transformation via these two asymmetric Y-axis freeform surfaces, achieving high point accuracy. The single-mode fiber collimating lens uses Chengdu Guangming's D-K9 material. The end face of the single-mode fiber collimating lens near the fiber section is perpendicular to the optical axis; its function is to reduce the difficulty of molding and ultimately reduce costs.

[0029] To address the point accuracy issue in existing C-LENS structures and to shorten the collimator structure length while improving coupling efficiency, this invention designs an ultra-long working distance single-mode fiber collimator with zero point accuracy and a coupling efficiency >95%.

[0030] To achieve the above technical objectives, the present invention provides the following technical solution: a low-return-loss, short-size fiber collimator design; The collimating lens is made of Chengdu Guangming's D-K9 material, which facilitates molding and reduces costs.

[0031] The collimating lens has a negative optical power at the end closest to the fiber end face, which serves to diffuse the light beam. The introduction of this face makes the element length required to achieve the same working distance shorter compared to a collimating lens with a conventional C-LENS structure.

[0032] Both surface types of the collimating mirror are Chebyshev polynomials. Where c is the radius of curvature at the vertex of the freeform surface, x is the length of the freeform surface in the X direction, and y is the length of the freeform surface in the Y direction. (The plane defined by the off-axis object point and the optical axis is called the YZ plane, and the plane passing through the principal ray and perpendicular to the YZ plane is called the XZ image plane.) The XY directions of the polynomial here are related to the attached... Figure 1-2 The X and Y directions correspond, z is the distance along the Z-axis from each surface point to the position x=0, y=0, where the value of z when x and y are both equal to 0 represents the surface coordinate vertex, i and j represent the coefficients of the X and Y axes respectively, which are constants. The larger i and j are, the greater the degree of surface refinement. 0、y0 is the standardized length. To normalize the polynomial coefficients, ,in, Let represent the coefficients of the Chebyshev polynomials corresponding to coefficients i and j. These are the fundamental terms of the Chebyshev polynomial.

[0033] The Chebyshev polynomials of the two surfaces of the collimating mirror are even functions in the X direction and odd functions in the Y direction. This content is related to the appendix. Figure 1 , Figure 2 Correspondingly, both surfaces are X-axis symmetric but Y-axis asymmetric. This structure can address the issue of increased point accuracy caused by optical fiber exiting at an 8° angle. To facilitate understanding, numerical and graphical examples of the Chebyshev polynomial function graph are provided, as shown in Table 1.

[0034] Table 1 In this context, 'i' represents the coefficient of the X-axis, and 'j' represents the coefficient of the Y-axis; when the coefficient i of the X-axis corresponds to... When there are odd-numbered terms, the corresponding odd-numbered terms If all parameters are equal to 0, then the Chebyshev polynomial graph is said to be symmetric about the X-axis; when the coefficients j corresponding to the Y-axis are all equal to 0, the graph is said to be symmetric about the X-axis. When there are odd-numbered terms, the corresponding odd-numbered terms If all parameters are equal to 0, then the graph of the Chebyshev polynomial is said to be symmetric about the Y-axis. Figure 6 This represents the coefficients of the multiple terms in the Chebyshev polynomial. When both values ​​are 0, the image is symmetrical about both the X and Y axes; Figure 7 This represents the case where the coefficients of odd-degree polynomials are all 0, and the graph is symmetrical about the X and Y axes. Figure 8 This represents the coefficients of the higher-order terms when the X-axis and Y-axis coefficients of a Chebyshev polynomial contain terms of odd degree. If the value is not 0, the image is asymmetrical about the X-axis and Y-axis; Figure 9 This indicates that the coefficients of odd-degree polynomials in the Chebyshev polynomial are all 0, meaning that the coefficients of terms corresponding to i are all 0. When the number of terms is odd, the corresponding All are 0, the Y-axis coefficient has an odd number of terms and the coefficient of the polynomial term is not 0, that is, j corresponds to When the number of terms is odd, the corresponding If the value is not zero, the image is symmetrical about the X-axis and asymmetrical about the Y-axis. This example merely illustrates the effect of Chebyshev polynomial coefficients on whether the surface shape is symmetrical about the X and Y axes.

[0035] The collimating lens, near the fiber end face, has a plane perpendicular to the optical axis on all surfaces except for the effective light transmission area. Figure 1 , Figure 2 As shown, this setting reduces the difficulty of molding and lowers costs. The parameters of the collimating lens are shown in Table 1 below. Table 2 The present invention has the following technical effects: 1. High point accuracy: The Gaussian beam emitted from the 8° oblique fiber passes through two asymmetrical surfaces in the Y direction, forming a Gaussian beam with the emitted light parallel to the optical axis of the collimating lens.

[0036] 2. Low return loss: This collimating lens is used in conjunction with an 8° angled optical fiber. The return loss is given by the following formula: in The Fresnel reflection coefficient of the fiber end face; The refractive index of the fiber cladding; The mode field radius of the optical fiber; The angle of cut of the optical fiber; incident wavelength Fiber optic cut angle The larger the angle, the lower the return loss. Considering the impact of large-angle polarization separation, the fiber's tangent angle... Setting it to 8° is a relatively ideal specification. This invention is designed for use with 8° oblique-cut optical fibers.

[0037] 3. Ultra-long working distance: The aperture of this fiber optic collimator is 1.26mm, the wavelength of the fiber is 1550nm, and its Rayleigh distance is [not specified]. .

[0038] 4. High coupling efficiency: The receiving fiber mode field diameter is 9.6µm, the system efficiency is 0.9959; the receiving efficiency is 0.968695; and the coupling efficiency is 0.964753. (See details...) Figure 4 . 5. Ultra-short optical element length: The negative optical power of both sides of the collimating lens reduces the length of the optical element.

[0039] This design can be directly manufactured by molding. The invention achieves zero point accuracy, low return loss, high coupling efficiency, ultra-short optical element length, and ultra-long working distance by simultaneously acting on the first and second curved surfaces.

[0040] The design concept described in this invention is applicable to any collimating lens that requires ultra-long working distance, high point accuracy, and high coupling efficiency.

[0041] The above are merely preferred embodiments of this application, but the scope of protection of this application is not limited thereto. Any variations or substitutions that can be easily conceived by those skilled in the art within the scope of the technology disclosed in this application should be included within the scope of protection of this application. Therefore, the scope of protection of this application should be determined by the scope of the claims.

Claims

1. A low-return-loss, short-size fiber collimating lens, characterized in that, include: The first free-form surface and the second free-form surface, wherein the shapes of the first free-form surface and the second free-form surface both conform to Chebyshev polynomials; the first free-form surface and the second free-form surface are respectively the surface near the end of the optical fiber and the surface away from the end of the optical fiber in the optical fiber collimating lens. Both the first and second freeform surfaces are symmetric along the x-axis and asymmetric along the y-axis. The first freeform surface has a negative optical power and all surfaces except those within the effective light transmission area are planes perpendicular to the optical axis.

2. The low return loss short-size fiber collimator according to claim 1, characterized in that: The fiber collimating lens is used in conjunction with an 8D oblique-cut fiber.

3. The low return loss short-size fiber collimator according to claim 1, characterized in that: The Chebyshev polynomial mentioned therein is: ； Where c is the vertex radius of curvature on the surface, x0 and y0 are the normalized lengths, and c(i,j) are the normalized polynomial coefficients. , These are the fundamental terms of the Chebyshev polynomial; The Chebyshev polynomials corresponding to the shapes of the first and second free-form surfaces are even functions in the x-direction and odd functions in the y-direction.

4. The low return loss short-size fiber collimator according to claim 1, characterized in that: The optical fiber collimator has an aperture of 1.26 mm and is suitable for a wavelength of 1550 nm.

5. The low return loss short-size fiber collimator according to claim 1, characterized in that: The fiber optic collimating lens is made of D-K9 material from Chengdu Guangming and is manufactured using a molding process.

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