JPH08146250A - Condenser lens and its production - Google Patents

Abstract

PURPOSE: To provide a condenser lens capable of converting a flat straight ray into an axisymmetrical circular gaussian beam with constitution simpler than the constitution of a system to combine special discrete lenses and efficiently taking the flat rays of large output LD as well and which is applicable to applications to improve the optical coupling efficiency between the LD and an optical fiber. CONSTITUTION: The end of a coreless silica rod 2 having a uniform refractive index and the diameter larger than the diameter of the optical fiber 1 is joined as a condense lens chip formed with the cylindrical curved surfaces intersecting orthogonally with each other and varying in the radii of curvature on the same curved surface to the optical fiber 1. The front end is formed to a curved surface consisting of desired curvature (radius R1 of curvature) by polishing, melting, etc., and thereafter, the front end is finished to a wedge shape and edge tip part in such a manner that the radius R2 of curvature on the front end face in a direction orthogonal with the curved surface of the radius R1 attains R1>R2.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to light ray conversion and optical coupling, which are expected to have various uses in optical communication, optical image processing, optical measuring instruments and the like.

[0002]

2. Description of the Related Art In various optical equipment fields and optical communication systems using a laser beam, it has been a problem in the prior art that a light intensity generated from a laser or other optical device is a Gaussian distribution. Among the Gaussian beams that are approximated by, there was how to convert a ray whose beam profile is flattened in the vertical and horizontal directions into a ray that is circularly symmetric and easy to handle with a lens.

Generally, a flat ray is converted into a circularly symmetric ray by combining two individual cylindrical lenses at a right angle. In this case, the combination of the individual parts naturally increases the size of the entire structure and is applied to a semiconductor laser or the like, and there is a limit in assembling the size of the whole.

These problems are not limited to the collection of semiconductor laser (hereinafter referred to as LD) light beams, but also in the light coupling between various flat plate-shaped optical waveguide elements and optical fibers, or the light beams emitted from the series array LD at once. This is an important issue in various devices that require a small configuration such as an image conversion lens for performing image processing and the like by converting the light profile into a circularly symmetric light profile.

In particular, various attempts have been made to convert an LD light beam into a circularly symmetric light beam. For example, an attempt has been made to arrange a cylindrical lens in the immediate vicinity of the LD to focus the light on the optical fiber core, and a coupling loss of 4 to 6 dB has been obtained. Furthermore, the aspect ratio of the light emitting window (aperture) of the LD is made as close as possible to each other, and coupling with the optical fiber is also performed through a spherical lens or aspherical lens, and high optical coupling of 1 to 3 dB is also obtained. Came to be.

However, the aspect ratio of the light emitting portion is usually 0.
If the method is about 5 to 1 μm and the width is about 2 to 5 μm, the above method is sufficiently small and can be achieved in an optical communication system, etc., but if the aspect ratio (hereinafter referred to as “aspect ratio”) is larger, it becomes a problem.

For example, in order to increase the injection current in an LD to form a high-power laser, the lateral width of the active layer is increased so that the LD active layer is not damaged by an overcurrent. In this case, the vertical width is 0.5 to 1 μm, and the horizontal width is several tens to
It may be expanded to several hundreds of μm, and the aspect ratio becomes several hundreds.

These high aspect ratio light sources have a core diameter of several μm.
When coupled into a single mode fiber (hereinafter referred to as SMF) having a size of m to 10 μm, most of the light rays are scattered and only a small portion can be captured in the optical fiber.

For example, when an LD having a length of 1 μm and a width of 200 μm is to be coupled to an SMF having a core diameter of 5 μm, when a conventional single lens is considered and a condition of capturing a 1 μm light beam is set, the expected coupling efficiency η Becomes like Equation 1.

[Equation 1]

Where ω ₀ is the core radius of the optical fiber, and ω
_X is the radius of the ray when it propagates to the optical fiber core position of the ray in the lateral direction (X-axis direction), and for a ray with an LD aperture diameter of 200 μm, even if it propagates in a space of several hundred μm, ω _X = 100 μm And the coupling efficiency η expected from Equation 1
Becomes η = 13 dB.

Of course, the longer the propagation distance, the further the deterioration. This relationship is a problem that occurs not only in the LD emitted light but also when converting a horizontally long light emitting source into an axisymmetric Gaussian beam in general, and a small and appropriate beam conversion lens has been desired.

In particular, the high-power LD as shown in this example cannot transmit its capability sufficiently through the optical fiber, so that the problem is not limited to the optical communication field, but various fields using an axially symmetric laser beam, for example, There has been a problem in forming a small and highly sensitive device by mounting a powerful LD light source in a system for various applications such as holograms, various optical interference systems, and measurement equipment using optical space propagation.

SUMMARY OF THE INVENTION In view of the above problems, the present invention provides a condensing lens for converting a flat light beam into an axially symmetric light beam with a simple structure, in contrast to a method of combining individual cylindrical lenses. Therefore, it is an object of the present invention to disclose a method of using a condenser lens and a method of manufacturing the same.

[0014]

SUMMARY OF THE INVENTION The present invention has been made to solve the above-mentioned problems of the prior art. A condensing lens according to the present invention has a compound curvature which is orthogonal to one lens and has different radii of curvature. The main purpose is to facilitate conversion from a light source with a high aspect ratio to an axisymmetric Gaussian beam by forming a surface or forming a cylindrical curved surface having a different radius of curvature on a normal convex lens surface.

In general, when the light source has a beam waste of 2ωp in the vertical direction and 2ωh in the horizontal direction, when this ray is converted into an axially symmetric ray through a lens, the distance between lenses at the position where the symmetrical ray is formed is L, At this time, the beam diameter is 2ω ₀ , the distance from the flat light source to the lens is Z, and the condensing lens is provided with cylindrical curved surfaces having curvatures R1 and R2 orthogonal to each other. The former captures a wide 2ωh and the latter captures 2ωp. Assuming that the lens is for the beam conversion, the beam conversion becomes the ray matrix of the equation (2).

[Equation 2]

The matrix elements A, B, C and D are expressed by
When h, ωp, and 2ω ₀ are known numbers, Z, L
It is possible to calculate R1 and R2 that give a desired beam conversion for a ray having a beam waste at the position of from the following calculation. That is, from the Gaussian beam ray matrix, there is a relation of Equation 3, and a relation of Equations 4 and 5 can be derived.

(Equation 3)

[Equation 4]

(Equation 5)

Therefore, from the equations 4 and 5, the beam curvature position L can be estimated from the optimum curvature R1 and the lens as a function of Z. Next, when considering the beam conversion that spreads in the vertical direction, it is required that the light beams have the same diameter at the same position L. At this time, the basic ray matrix is expressed by Equation 6.

(Equation 6)

In the equation (6), the equation (5) is introduced as L so that the vertical direction and the horizontal direction can be matched. At this time, R2 can be determined from the relationship of Equation 7.

(Equation 7)

R1, R2, L obtained as a function of Z
And ωh and ωp are determined, the beam waste diameter imaged at the position of L is calculated back by applying Equation 3 and the like,
All the conditions can be set by deriving h0 and ωp0 and finding Z that maximizes the coupling efficiency η of Equation 8.

(Equation 8)

The matters described above are based on the present invention.
The outline of the image formation conversion using the compound curvature condensing lens having different radii of curvature in the perpendicular direction is disclosed, but the effect of the present invention is impaired even when the portion R1 is not a cylindrical shape but a convex shape (convex shape). Not a thing.

Various methods can be considered for the basic manufacturing method of the compound curvature surface according to the present invention depending on the size of the lens, the property of the beam to be handled, the use, etc., but the basic method is to use the first cylindrical surface or the convex surface. It can be manufactured by forming and then rotating 90 degrees to form a cylindrical curved surface having a desired radius of curvature in the orthogonal direction. Of course, forming by the reverse procedure is also included as long as the gist of the present invention can be achieved. The details will be described below with reference to examples.

[0022]

Embodiment 1 FIG. 5 shows an example of a compound curvature condenser lens of the present invention. The tip of a silica rod with a diameter of 500 μm is fixed, and a curved surface with a curvature of 305 μm is polished. Next, it was rotated 90 degrees and ground to have a curvature of 133 μm. In the case of a lens that collects LD light having an aspect ratio of 1 μm in the vertical direction and 200 μm in the horizontal direction and forms an image of 5 μm on the other end surface without curvature, cut the silica rod to about 970 μm from the lens tip and polish the end surface. And then processed flat.

When the converted beam shape was measured by disposing a complex curvature condenser lens formed at the emission center of the LD, an axially symmetric beam spot having a diameter of approximately 5 μm was observed. The beam diameter calculated from the above estimation is 4.
It was confirmed that the actual beam shape agrees with 98 μm / 5.00 μm in the lateral direction. In this case, focusing the beam waste position farther than the lens can be easily achieved by adjusting the curvature. From this result, it can be seen that the present invention can achieve the lens coupling, in which two cylindrical lenses are made orthogonal to each other and are converted into axial symmetry by using one minute compound curvature converging lens, with one lens.

It is self-evident that this chip-shaped lens introduces an axially symmetric ray from the opposite direction and couples the flat beam to the exit side, and the aperture of the LD for amplification having a large aspect ratio for semiconductor optical amplification or the like. If an image is formed on a portion, a light beam can be projected with high efficiency, and extremely effective beam conversion can be achieved.

[0025]

[Embodiment 2] FIGS. 1 and 2 show a complex curvature condenser lens chip 2 made of a silica rod in order to propagate a silica rod (diameter 250 μm) similar to that in Embodiment 1 to an SMF 1 having a core diameter of 5 μm. SM in the center part based on the outer diameter
F1 was discharge fused. In this case, when considering the coupling with the fiber core, it is necessary to design the lens according to the refractive index NA of the fiber as a condition different from the first embodiment.

That is, the SMF is guided through the silica lens portion 2.
Light rays that are coupled to the core are excluded from light with an incident angle that is greater than the angle specified by the refractive index NA, so the light rays that are converted by the lens are those that have a convergence angle that includes the refractive index NA of the fiber. Must match the beam spread when is emitted.

When there is a cylindrical lens waveguide length L having the same refractive index as that of an optical fiber core having a refractive index n, the maximum allowable angle θ given by the refractive index NA is given by Equation 9, and the beam waste in the vicinity of the core is substantially It moves to the SMF side from the fusion spliced part.

[Equation 9]

Assuming that the substantial beam waste diameter is ω and the distance from the junction end face to be ω is d, it is estimated from the known amount of ωf, and is given by Equation 10.

[Equation 10]

Where tan θ = λd / (π · n · ω _f ² ), N
If A = 0.14 and refractive index n = 1.459, then from Equations 9 and 10, ω = 2.477 μm and d = 4.8 μm at a wavelength of 0.8 μm.

That is, in FIG. 2, the ray propagating from the fiber core to the curved surface of the lens is expanded as a Gaussian beam when traveling through the silica lens rod, and on the lens surface, the same beam as the ray propagating on the light source side through the lens interface. The condition is to form a spread. That is, the beam matching conditions of L and Z must be reflected also in the ray matrix of Equations 2 and 6.

That is, as the coupling conditions, the vertical beam diameter ωps from the light source side on the lens surface and the horizontal beam diameter ωhs from the light source side.
Also, the axially symmetric beam diameter ωfs that propagates from the SMF1 through the silica waveguide portion and reaches the lens interface must be considered. These are estimated from the following formula 11.

[Equation 11]

Therefore, the excess loss ηe when coupled to an optical fiber and coupled to a light emitting source or the like is given by Equation 12, and the total coupling loss is given by η · ηe.

[Equation 12]

In Equation 12, if ω _fs = ω _hs and ω _fs = ω _ps , the ηe term is the optimum condition, but after all, considering that Equation 13 is a precondition, L, R1, and R2 are The corresponding Z can be determined.

[Equation 13]

By substituting these conditions into the above-mentioned calculation formula to determine numerical values, when applied to the same LD as in Example 1,
Table 1 shows the relationship.

[0035]

[Table 1]

From Table 1, the position where the optimum combination can be expected is Z
= 270 μm, L = 1414 μm, and the radii of curvature of the orthogonal cylindrical lenses are R1 = 446 μm and
R2 = 97 μm. The beam spot collected from the LD at the fiber core position is 1.9 μm in the vertical direction and in the horizontal direction.
It becomes 3.6 μm, and a slight mismatch of the beam spots is recognized, but the coupling efficiency is expected to be 0.7 dB, and it is clear that the coupling efficiency is dramatically improved from the conventional coupling efficiency of 13 dB.

[0037]

Embodiment 3 FIGS. 3 and 4 show a method of manufacturing an optical fiber with a condenser lens. As shown in the figure, in the method of manufacturing an optical fiber with a condenser lens according to the present invention, the end surface of a coreless silica rod 2 having a uniform refractive index and a diameter larger than that of SMF1 is joined to SMF1, and the tip is polished and melted. Etc. to form a curved surface having a desired curvature (curvature radius R1), and form a curved surface with a different curvature (curvature radius R2) at the tip in a direction orthogonal to the curved surface so that R1> R2 Is.

1 and 2 are a plan view and a front view of the shape. In the figure, 1 is SMF, 2 is a diameter larger than SMF1, and two types of curvature radii R1 where the tip is processed into a wedge shape from a curved surface>
Coreless silica rod with R2, 4 LD chip, 5
Is the LD active layer, Z is the distance from the lens end surface to the exit end surface, L is the distance from the lens surface to the SMF fusion point, 2ω
h and 2ωp are the beam wastes in the horizontal and vertical directions of the LD end face, respectively.

The design conditions for the compound curvature condensing lenses orthogonal to each other are derived from the above-mentioned principle. 3 and 4, the coreless silica rod 2 with a diameter of 250 μm is attached to the SMF.
Fused to 1. The diameter of the lens forming part to be fused depends on the lateral width of the light beam to be captured. Width 200 as described above
A typical 125 μm SMF1 for a μm emission source
In the present invention, the diameter of the coreless silica rod forming the lens is not limited because the entire light emitting portion cannot be collected with a fiber whose outer diameter is matched with. Of course, if the width of the light emitting part is about 50 μm or less, 125
Fusing a coreless silica rod of μm is preferable for the reason of increasing the coaxiality between the fibers.

The fused coreless silica rod is cut to a length that allows L to be obtained after processing, and inserted and fixed in a protective tube such as a ferrule or a capillary. As the material of the protective tube, any material can be applied as long as it does not interfere with the subsequent process. Next, the cylindrical lens surface having a radius of curvature of R1 including the protective tube is polished and formed. Here, a convex lens surface having the same curvature may be formed. In that case, it is possible not only to perform polishing but also to melt the tip to secure a desired curvature by using the principle of surface tension.

Next, cylindrical lens surfaces having different radii of curvature are formed in a direction perpendicular to the first processed curved surface. When the required width of light collection is narrow, grind the cylindrical surface created in advance at right angles to the wedge shape from both sides,
It is preferable to form a curved cutting edge at R1. Next, the cutting edge is polished to produce a desired curvature R2. Whether to grind and polish the wedge or to grind the curved surface in the perpendicular direction immediately depends on the condition of the light source to be handled. The lens surface after polishing can be used as it is, but it is preferable to form an antireflection film.

Even when the diameter of the cylindrical lens is the same as that of the SMF to be fusion-spliced, the fiber with a condenser lens according to the present invention can be produced from the same steps.

[0043]

Fourth Embodiment FIG. 6 is an application example of a configuration in which a collimator lens is arranged at the other end of the fiber SMF1 with a condenser lens manufactured from the third embodiment. Since the light beam that has already been taken into the SMF has been converted into a typical axially symmetric Gaussian beam, it is possible to form a collimated light beam with an expanded beam diameter of 2ω _ex with a simple lens configuration as shown in FIG.

The collimated light beam of ω _ex is formed when the correlation between the beam expanding lens (focal length f) and the equation 14 is obtained. That is, the focal length of the lens can be designed from the beam diameter to be expanded.

[Numerical equation 14]

In particular, when a high power LD is used as a light emitting source and a compact and high power axisymmetric light beam is required, it can be formed from the structure of this embodiment.

[0046]

[Embodiment 5] In FIG. 7, several LDs formed in an array are arranged two-dimensionally (for example, M × N), and linear LD emission rays emitted from the individual LDs are replaced with an axisymmetric ray matrix. When the condenser lens (diameter
(250 μm) are similarly arranged in a plane and can be converted into M × N axisymmetric Gaussian beams shown in FIG. At this time, the curvature radii of the cylindrical lenses orthogonal to each other are naturally limited by the aperture size and aspect ratio of the LD.

As described above, since the fiber with a lens of the present invention is manufactured as described above, the fiber and the lens have an integral structure, and the lens has a minute shape close to the fiber diameter, so that the coupling with the LD is easy. is there.

Further, even if the lateral length of the LD chip is large, since the diameter of the waveguide portion is larger than the diameter of the SMF, the waveguide portion length L can be adjusted to the optimum length independently of the curvature of R1. The mechanical part can be significantly downsized. Further, it is possible to transmit flat rays having a large aspect ratio generated from the high-power LD, which is the original purpose, to the optical fiber with high coupling efficiency.

Further, since the tip is formed in a wedge shape and the radius of curvature R2 of the tip end surface in the direction orthogonal to the curved surface of radius R1 is gradually increased toward both sides, the short axis length is formed. The radius of curvature of the axis is similar to that of a spheroid cut into slices from the rotating end, and higher optical coupling is possible not only in the vertical and horizontal directions of the LD chip, but also in the 360 ° direction, including the diagonal direction.

[0050]

As described above, the condenser lens of the present invention provides a simple structure for converting a flat ray having a high aspect ratio into an axially symmetric Gaussian beam, and discloses a method of applying the same. The main idea is to convert a linear ray into a circular ray. In particular, when coupling a high-power LD to an optical fiber, high coupling efficiency can be realized and connection with the optical fiber can be easily formed. It can be mass-produced on a market scale, has long-term reliability, and can be used in a wide range of fields as a low-cost condenser lens.

[Brief description of drawings]

FIG. 1 is a plan view illustrating coupling between an optical fiber with a condenser lens according to the present invention and an LD.

FIG. 2 is a front view illustrating coupling between an optical fiber with a condenser lens according to the present invention and an LD.

FIG. 3 is a diagram illustrating a method of manufacturing a fiber with a condenser lens according to an embodiment of the present invention.

FIG. 4 is a diagram illustrating a method of manufacturing a fiber with a condenser lens according to an embodiment of the present invention.

FIG. 5 is a diagram illustrating the principle of light ray conversion of a condenser lens according to an embodiment of the present invention.

FIG. 6 is a diagram illustrating the principle of Embodiment 4 of the present invention.

FIG. 7 is a diagram illustrating the principle of a fifth embodiment of the present invention.

[Explanation of symbols]

 1 SMF 2 Coreless silica fiber 3 Fiber protection tube 4 LD chip 5 Active layer

 ─────────────────────────────────────────────────── ─── Continuation of front page (72) Inventor Takako Hayakawa 3-8-22 Nitta, Adachi-ku, Tokyo Inside Namiki Seimitsu Takara Stone Co., Ltd. (72) Inventor Tetsuya Miyazaki Oita, Seiraku-cho, Kyoto Pref. No. 5 In the AR Optical Communications Research Institute (72) Inventor Yoshio Karasawa, Mihaya, Seika-cho, Seika-cho, Soraku-gun, Kyoto No. 5 In the Optical Optical Communications Research Institute, Ltd.

Claims (6)

[Claims] 1. At least two curved surfaces orthogonal to each other,
A condenser lens characterized by different curvatures. 2. A condenser lens characterized in that a tip of a coreless silica rod having a uniform refractive index and a diameter equal to or larger than an optical fiber diameter has at least two curved surfaces orthogonal to each other and the curvatures thereof are different. 3. A coreless silica rod having a uniform refractive index and a diameter equal to or larger than that of an optical fiber has at least two curved surfaces having mutually different curvatures and having the other end joined to an optical fiber. A condensing lens characterized in that 4. A coreless silica rod having a uniform refractive index and a diameter equal to or larger than that of an optical fiber has at least two curved surfaces which are orthogonal to each other and have different curvatures, and the other end is joined to an optical fiber. A condensing lens, characterized in that a desired lens is provided at the other end of the optical fiber to make a light beam emitted from the optical fiber core a collimator light beam. 5. A step of polishing one end of a glass or silica rod into a cylindrical lens shape or a convex lens shape having a desired curvature, and forming a cylindrical lens surface having a different curvature in a direction perpendicular to a previously curved surface or in an arbitrary direction. A method of manufacturing a condensing lens mainly including the step of: 6. A method of manufacturing an optical fiber terminal with a condenser lens, the step of fusing a fiber to a coreless silica rod, and the step of cutting the fused coreless silica rod to a required length for a waveguide portion. The process of inserting and fixing the fiber protection tube, the process of polishing the tip of the coreless silica rod previously cut into a cylindrical lens shape or convex lens shape with a desired curvature, and the direction perpendicular or arbitrary direction of the curved surface that was previously polished And a step of forming cylindrical lens surfaces having different curvatures.