JP2005266217A - Fiber collimator - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a fiber collimator which is allowed to attain high optical coupling efficiency by using a spherical lens. <P>SOLUTION: One end surface 102a of a coreless fiber 102 made of a material which has nearly the same uniform refractive index as that of a core 101a at a center part is joined with an end surface of an optical fiber 101 having the core 101a and a clad 101b at its outer periphery, and the spherical lens 50 is arranged on the optical axis of the optical fiber on the side of the other end surface 102b of the coreless fiber. The spherical lens has its refractive index and diameter set so that an aberration circle diameter of the spherical lens is less than a mode field diameter of the optical fiber and also has its refractive index and diameter set so that the back focus BF of the spherical lens is larger than the minimum length of the coreless fiber prescribed to secure a necessary reflection loss quantity of the coreless fiber. A correction value which varies with the coreless fiber length is used as the value of the back focus. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to an optical component used for optical communication and the like, and in particular, a fiber collimator in which one end face of a coreless fiber is bonded to an end face of an optical fiber having a core and a cladding, and a spherical lens is disposed on the other end face side of the coreless fiber. It is about.

  With the development of optical communication, miniaturization of optical devices and optical components to be used is desired. Of the methods of optical coupling between optical fibers, a lens is installed near the light beam exit end of the fiber as a method of optical coupling that can be installed with various optical elements between them with little loss. In general, a method is used in which the emitted light beam is a parallel light beam, and a similar optical system is assembled on the light receiving side to couple the light to the fiber again. An element having a function of converting a light beam emitted from a fiber into a parallel light beam is generally called a fiber collimator, and the above-described combination of a fiber and a lens is one type of fiber collimator.

  In general, an optical coupling loss and a reflection loss are used as indexes representing the performance of this type of fiber collimator. The optical coupling loss reflects the light flux quality of the parallel light flux, and is affected by the numerical aperture (hereinafter abbreviated as NA) of the fiber used, the exit end face shape, the aberration at the exit end face, the lens performance, and the like.

  On the other hand, the reflected light expressed using the reflection loss as an index has an effect of losing the emission energy at the fiber emission end or impairing the stable operation of the light source. Of these influences of reflected light, particularly the influence on the stable operation of the light source is one of the serious problems in the optical communication field.

  In recent years, in the optical communication field, a distributed feedback laser is generally used as a light source. This type of laser light source is characterized in that laser oscillation tends to become unstable due to so-called return light that travels backward in the fiber and reaches the light source, and as a result, fluctuations in output power tend to occur. That is, when the reflected light increases, in other words, when the reflection loss is small, it means that the return light is large, and the fluctuation of the output power is increased.

Generally, in a fiber collimator, in order to suppress the output fluctuation of the laser light source described above to a level that can be ignored, 50 dB or more is required as an end face reflection loss shown in the following equation (1).
End face reflection loss = −10 × log (IR / IO) (1)
In equation (1), IR represents the amount of reflected light and IO represents the amount of incident light.

  As a method for obtaining a reflection loss at present, a method in which the fiber end face is inclined with respect to the optical axis is used. This type of optical fiber terminal can be obtained by inserting a fiber into a glass capillary and polishing the end face of each capillary with an angle of about 4 ° to 8 °. As a result, the reflected light at the end face is attenuated in a clad mode, so that a large reflection loss can be obtained, and a reflection loss of 50 dB or more can be obtained together with the AR coating on the surface. Since this method is extremely simple, it has been the mainstream method so far.

  By the way, when using a collimator using an optical fiber terminal having such an oblique end surface in an optical component, there is a particular point to be noted. This point will be described with reference to FIG.

  A refractive index distribution lens (GRIN lens) often used as a collimator lens of a fiber collimator has an image formation state that depends on the lens length (Pitch). Therefore, in the case of 0.25 pitch, a refractive index distribution lens as shown in (a). When a light source is placed on the end face of (GRIN Lens) 1101, collimated light is emitted from the other end face. Actually, as shown in (b), the refractive index distribution lens (GRIN Lens) 1102 has a lens length of about 0.23 Pitch, so that the position adjustment of the light source has a degree of freedom.

  An optical path when collimated light with this configuration is combined is shown in FIG. Due to the influence of the oblique end surface described above, the light emitted from the optical fiber terminal 1103 is inclined by about 3.8 ° and enters the lens 1104, so that it is deviated from the optical axis of the optical fiber terminal 1103 by δ1. Since the end surface of the gradient index lens (GRIN Lens) 1104 is inclined in the same manner as the end surface of the optical fiber terminal 1103, the incident beam is incident on the gradient index lens (GRIN Lens) 1104 at an angle. It has an angle (θ) with respect to the optical axis of the light emitted from the optical fiber terminal 1103. Therefore, in this combination of collimators, in order to make the optical axes coincide, optical coupling cannot be performed unless it is shifted by δ2 with respect to the original optical axis. This is the reason why position adjustment is difficult when optical coupling is performed with a conventional collimator.

  As the collimator lens, in addition to the above-mentioned gradient index lens (GRIN lens), a spherical lens, an aspherical lens, a spherical lens (global lens or drum lens) can be used, but the optical axis of the optical fiber terminal And the optical axis of the lens are arranged on a straight line, the same beam inclination and beam eccentricity occur.

  In general, in order to simplify the assembly of components, it is preferable to employ a so-called passive alignment method as much as possible so that the target performance can be obtained by simply arranging the components at a predetermined position. In the case of optical components, for example, if optical coupling can be performed simply by placing the fiber and collimator lens facing each other on the same straight line such as a V-groove, assembly is greatly simplified and stable at a low price. It is possible to produce a product of the same quality.

  However, in the current mainstream method, the above-described beam inclination and beam eccentricity occur, and thus it is impossible in principle to adopt such a passive alignment method.

  In order to eliminate the optical path deviation as described above, the optical fiber terminal and the lens end face may be all perpendicular to the optical axis. However, in this case, all end face reflections are reflected as return light. The reflection loss caused by the difference in refractive index between the glass end face and air is 14.7 dB. Even if a good AR coating (R <0.2%: 27 dB) is applied to this, the reflection loss at the end face is about 42 dB. Therefore, the required specification of 50 dB or more cannot be achieved.

  As a means for solving such a problem, there has conventionally been a structure in which a coreless fiber is fused to an end of a single mode fiber by an appropriate length, and a necessary return loss is obtained by a diffusion action of a light beam in the coreless fiber. It is known (see Patent Document 1).

  The basic structure of the optical fiber terminal in this case is shown in FIG. According to this, it is possible to easily produce an optical fiber terminal that is free from optical axis deviation and has a reflection loss of 50 dB or more. Therefore, a fiber collimator capable of passive alignment can be realized by combining this optical fiber terminal and a collimator lens such as an aspheric lens, a spherical lens, and a spherical lens (including a drum lens).

  In general, aspherical lenses with corrected spherical aberration have few aberrations and excellent imaging performance, and are used as collimator lenses mainly for high-end products that require low loss. An aspherical lens is a lens in which the curvature of the lens surface is made location-dependent, so that it is difficult to produce by polishing, and is produced by press molding using a mold at the mass production site. Therefore, the manufacturing cost is expensive, and due to the structure, eccentricity occurs between the lens outer diameter and the lens surface at a level of several μm or more, and it is used in combination with an optical fiber terminal as shown in FIG. However, there were problems such as tilting the beam.

Therefore, in practice, it is conceivable to use a spherical lens, particularly a ball lens (spherical lens) as the collimator lens. A lens having a spherical surface as a lens surface can be mass-produced by polishing and has the advantage of being manufactured at low cost. In particular, since all surfaces are isotropic in a spherical lens, no eccentricity occurs in the optical axis of the incident / exit surface. In addition, the focal length f, which is the most important optical parameter, can be easily defined as shown in the following equation (2) using only the refractive index n and the spherical radius r (diameter R = 2r) of the material. There are benefits.

  Accordingly, when a spherical lens is used as a collimator lens, if the spherical lens 50 has the same diameter as the fiber or the fiber holding jig 80 as shown in FIG. 14A, it is placed in a V-shaped guide groove (not shown). It is possible to easily obtain a collimated beam having excellent linearity. However, when the value of the focal length f has to be increased, the sphere radius r increases and the size does not match the fiber holding jig 50. In such a case, as shown in FIG. 14 (b), the outer periphery of the spherical lens is sharpened to the shape of the drum lens 52, so that the spherical lens having the lens radius r in the optical axis direction can be obtained. Since the same performance can be obtained, such a drum lens can be substituted. Rather, this is more stable and advantageous in fixing the lens in the guide groove. In FIGS. 14A and 14B, 101 is a single mode optical fiber (SMF) having a core and a clad, and 102 is a coreless fiber (CLF).

Japanese Patent Laid-Open No. 7-281054

従って、光結合効率は、上式（３）で示される収差円径δの大きさと、受光する側の光ファイバのモードフィールド径との相対的な大小関係で決定されることになる。 By the way, when a lens having such a spherical surface is used, there is a spherical aberration as shown in FIG. 15, and the imaging performance is clearly inferior to that of an aspherical lens. However, considering only the optical energy transmission means, such as an optical coupler, the presence or absence of aberration is irrelevant if all of the spread area of the image at the focal position is captured. This spread of the image is called an aberration circle. If the diameter is δ (aberration circle diameter), it can be obtained by the following equation (3) using the numerical aperture NA of the fiber.

Therefore, the optical coupling efficiency is determined by the relative magnitude relationship between the magnitude of the aberration circle diameter δ expressed by the above formula (3) and the mode field diameter of the optical fiber on the light receiving side.

  In consideration of the above circumstances, an object of the present invention is to provide a fiber collimator that can achieve high optical coupling efficiency using a spherical lens.

  According to the first aspect of the present invention, one end face of a coreless fiber made of a material having a uniform refractive index is substantially the same as the core is bonded to the end face of an optical fiber having a core at the center and a clad at the outer periphery. A fiber collimator in which a spherical lens is disposed on the other end surface side of the coreless fiber on the optical axis of the optical fiber, and the focal point of the spherical lens passing through the coreless fiber is positioned on the end surface of the core of the optical fiber. The refractive index and the diameter of the spherical lens are set so that the aberration circle diameter of the spherical lens is equal to or smaller than the mode field diameter of the optical fiber, and specified in advance in order to secure the necessary reflection loss amount of the coreless fiber. The refractive index and diameter of the spherical lens are set so that the back focus of the spherical lens is larger than the minimum length of the coreless fiber to be And butterflies.

  Thus, by setting the refractive index and diameter of the spherical lens so that the aberration circle diameter of the spherical lens is less than or equal to the mode field diameter of the optical fiber, the optical coupling loss between the optical fiber and the spherical lens is reduced. Therefore, a fiber collimator with high optical coupling efficiency can be realized. In addition to the so-called ball lens (global lens), the concept of “spherical lens” here means that the spherical surface of the original spherical lens remains as both lens surfaces intersecting the optical axis, but other optically unnecessary. Also included is a partially spherical lens whose outer periphery has been removed, for example, a drum lens (D lens).

  In optical communication components, a single mode fiber is generally used, and its mode field diameter is about 10 μm. Therefore, by design, if the refractive index n and radius r of the lens are determined so that the value δ of the above equation (3) is less than 10 μm, the collimator lens can realize high coupling efficiency.

  When NA = 0.14 and the refractive index n and sphere diameter R (R = 2r) at which δ = 10 μm are calculated for the formulas (2) and (3), the curve shown by the solid line in FIG. 3 is obtained. Therefore, it is the region above this curve that satisfies δ <10 μm.

  By the way, as shown in FIGS. 14A and 14B, the inventors tried to produce a collimator by combining an optical fiber terminal with a coreless fiber and a spherical lens 50 or a drum lens 52. I realized that there was another important limitation to achieve this.

  In order to obtain collimated light, as shown in FIG. 2A, it is necessary to place a fiber core end face (end face of the core 101a of the optical fiber 101) to which light is coupled at the position of the focal length f of the spherical lens 50. is there. Here, when the back focus length (BF) of the spherical lens 50 is shorter than the length L of the coreless fiber 102, as shown in FIG. 2B, the fiber core end face cannot be installed at the focal position. Therefore, there arises a problem that it becomes impossible to generate collimated light.

  As one measure for avoiding this, as shown in FIG. 2C, there is a method of making the coreless fiber length L shorter than the back focus length BF, but the length L of the coreless fiber 102 is the same as that of the optical fiber terminal. In order to increase the reflection loss amount to 50 dB or more, it is necessary to set the reflection loss amount to a certain threshold value or more, so there is a limit in avoiding the problem with this method.

  In the field of optical components, the collimator configured as shown in FIGS. 14 (a) and 14 (b) is not common, so the problem of the technical limitation caused by the back focus length required for the collimator lens is now being investigated. Not done at all.

  Therefore, as a result of technical studies on the material refractive index n and the sphere diameter R of the spherical lens, the present inventor has selected both parameters so as to satisfy certain conditions, and thereby has a low loss and excellent linearity. Has been found to be possible, thereby obtaining another feature of the invention of claim 1.

  That is, another feature of the invention of claim 1 is that, as described above, the back focus of the spherical lens is larger than the minimum length of the coreless fiber that is defined in advance in order to ensure the necessary reflection loss amount of the coreless fiber. In this way, the refractive index and diameter of the spherical lens are set.

  By setting the refractive index and diameter of the spherical lens under such conditions, a collimator with low loss and excellent linearity can be obtained.

  The invention according to claim 2 is the fiber collimator according to claim 1, wherein the refractive index and the diameter of the spherical lens when the aberration circle diameter of the spherical lens is equal to the mode field diameter of the optical fiber. Using the relationship curve as the lower limit curve, the length of the coreless fiber set to be equal to or greater than the minimum length of the coreless fiber specified in advance to ensure the required amount of reflection loss of the coreless fiber, and the length of the coreless fiber are corrected. The relationship between the refractive index and the diameter of the spherical lens when the back focus of the spherical lens becomes equal is the upper limit curve, and the refraction of the spherical lens is within a range between the upper limit curve and the lower limit curve. The rate and diameter are set.

  The actual back focus length of the lens varies depending on the light traveling medium. Here, there is a coreless fiber between the spherical lens and the end face of the optical fiber, and light travels through the coreless fiber, so it is desirable to correct the back focus value calculated in the air, and the correction value Varies depending on the length of the coreless fiber. Therefore, in the invention of claim 2, the refractive index and the diameter of the spherical lens are set by using a correction value that changes according to the length of the coreless fiber as the back focus value. In addition, in order to obtain the optimum refractive index and diameter of the spherical lens, various conditions must be taken into consideration, but as described above, the refractive index of the spherical lens is within the range between the upper limit curve and the lower limit curve. The diameter can be set. By doing so, it is possible to easily find the optimum conditions and to improve the efficiency of the lens design.

  The invention of claim 3 is the fiber collimator according to claim 2, characterized in that the length of the coreless fiber is set to 200 μm or more and 800 μm or less.

  According to the present invention, since the optical fiber and the spherical lens are set in an optimum relationship, a fiber collimator with high optical coupling efficiency can be provided at low cost. In particular, according to the second and third aspects of the invention, a fiber collimator that can sufficiently satisfy the specifications of reflection loss and coupling efficiency can be easily manufactured.

Embodiments of the present invention will be described below.
1A and 1B are schematic configuration diagrams of the fiber collimator of the embodiment. These fiber collimators are formed on the end face of a single-mode optical fiber (SMF) 101 having a standard outer diameter of 125 μm having a core 101a at the center and a clad 101b at the outer periphery of the core 101a. One end face 102a of a coreless fiber (CLF) 102 made of a material having a refractive index is fusion bonded, and the length of the coreless fiber 102 is set to be less than 1 mm, and the other end face 102b of the coreless fiber 102 is connected to an optical fiber. A spherical lens (ball lens) 50 or a drum lens 52 is ground and polished at 0 ° with respect to a surface perpendicular to the optical axis of 101 and positioned on the optical axis of the optical fiber 101 on the other end surface 102b side of the coreless fiber. Is arranged. The drum lens 52 is a partially spherical lens in which the outer circumference of the original spherical lens is left as both lens surfaces intersecting with the optical axis and the other optically unnecessary outer periphery is removed.

  The reflection loss amount of the optical fiber terminal depends on the length of the coreless fiber 102 and has the characteristics as shown in FIG. The reflection loss that can be guaranteed with the current antireflection film (AR coating) is 0.2% (27 dB). Therefore, in order to obtain a reflection loss amount of 50 dB or more as a fiber terminal, the reflection of 23 dB or more without the AR coating is required. It turns out that the amount of loss is necessary. Therefore, it can be seen from FIG. 4 that the minimum required length of the coreless fiber 102 fused to the optical fiber terminal is 200 μm or more.

  Since the refractive index of the coreless fiber 102 is substantially the same as that of the core 101a of the optical fiber 101, the outline of the optical path when the coreless fiber 102 is inserted between the optical fiber 101 and the spherical lens 50 is precisely shown in FIG. However, here, as a first approximation, the optical path when assuming that it has the same refractive index as air as shown in FIG. 5A will be considered first.

The back focus length (BF) of the spherical lens 50 of radius r is given by the following formula (4)
BF = fr (4)
Therefore, it can be understood that the relational expression of n, r, and BF for constituting the target collimator is expressed by the following inequality (5) from the expressions (2) and (4).

  BF in the equation (5) indicates a limit length (maximum value) of the coreless fiber 102 to which this design can be applied. Here, when 200 μm, which is the minimum value of the coreless fiber length L, is entered as BF, the region satisfying the above equation (5) is the lower range of the curve B indicated by the one-dot chain line in FIG.

  From the above, the refractive index n and the diameter R (R = 2r) of the spherical lens 50 are set so that the aberration circle diameter δ of the spherical lens 50 is equal to or smaller than the mode field diameter of the core 101a of the optical fiber 101. In addition, the refractive index n and the diameter R of the spherical lens 50 are set so that the back focus BF of the spherical lens 50 becomes larger than the minimum length 200 μm of the coreless fiber defined in advance in order to ensure the necessary reflection loss amount of the coreless fiber 102. It can be seen that it should be set.

  That is, if the spherical diameter R is the same, the region defined by the aberration circle diameter δ has the meaning of determining the lower limit of the refractive index n, and the region defined by the back focus length BF has the meaning of determining the upper limit of the refractive index n. I can say that. In other words, the refractive index n and the spherical diameter R (curvature) of the spherical collimator lens (spherical lens) satisfying both the conditions of the aberration circular diameter δ and the back focus length BF are the solid line A (aberration circular diameter δ = 10 μm) in FIG. It can be said that this is a region sandwiched between a dotted line B (a curve satisfying BF = 200 μm).

Next, the coreless fiber 102 is examined considering that it is a glass medium. At this time, as shown in FIG. 5C, since the optical path length becomes long, the fiber end face becomes farther by δf than the focal position considered in the first approximation. That is, it means that the length of the coreless fiber 102 can be increased by this length. From geometric considerations, if the angle formed with the optical axis in the glass medium is θ1, and the incident angle from the air medium is θ2, δf is obtained as the following equation (6).

Since it has been obtained by optical simulation that θ1 and θ2 have the coreless fiber length dependence as shown in FIG. 7, when δf at each angle is plotted based on this, the correlation shown in FIG. Was found to be obtained. The relationship between the two has a linear relationship. From the inclination of this straight line, it has been found that the coreless fiber 102 can be made longer by a distance corresponding to 32% of the length of the coreless fiber 102 used in the optical fiber terminal. Therefore, the required back focus length BF of the collimator lens (sphere lens 50) is obtained as the following equation (7) by transforming equation (4) as an effective value corresponding to the coreless fiber length L.
BF = fr−L × 0.32 (7)
A curve obtained from this relationship is a curve C indicated by a dotted line in FIG.

  That is, as compared with the case of the first approximation, a material having a higher refractive index can be obtained with the same R, and a sphere having a smaller R can be obtained with the same refractive index. The region is sandwiched between the solid line A and the dotted line C. If the spherical diameter (curvature) r and the material refractive index n of the spherical lens 50 are selected so that the result falls within the range shown here, a collimator lens that can realize a fiber collimator having the desired performance can be obtained. For the first time in principle.

  To summarize the above, from the above examination, the relationship curve between the refractive index n and the diameter R of the spherical lens 50 when the aberration circle diameter δ of the spherical lens 50 and the diameter of the core 101a of the optical fiber 101 become equal is the lower limit. A length L of the coreless fiber 102 set to a minimum length of 200 μm or more and a coreless length of the coreless fiber 102 defined in advance in order to secure a necessary reflection loss amount (50 dB) of the coreless fiber 102 as a curved line (solid line A) When the relationship curve between the refractive index n and the diameter R of the spherical lens 50 when the back focus BF of the spherical lens 50 corrected according to the length L of the fiber 102 becomes equal is the upper limit curve (dotted line C), It can be seen that the refractive index n and the diameter R of the spherical lens 50 may be set within a range between the upper limit curve and the lower limit curve.

  Incidentally, when the coreless fiber length is 800 μm, the intersection with the lower limit curve A of the aberration circle diameter does not exist as shown by the thin line D in FIG. This indicates that, from the viewpoint of the collimator lens, even if the coreless fiber length is 800 μm or more, there is no meaning in principle.

  When practical problems are taken into consideration, the aberration circle diameter and the back focus length, that is, the coreless fiber length are more strongly limited, and the compatible region tends to be narrowed.

  When considering the diameter of the aberration, not only spherical aberration but also aberrations represented by Seidel's five aberrations occur, and the influence of wavefront aberration that combines them must be considered. Actually, the blurring is asymmetrical and larger than the diameter of the aberration caused by spherical aberration. Even when an optimum design is performed with a single wavelength, when a wavelength multiplexed signal is handled, the δ value becomes large due to chromatic aberration depending on the refractive index dispersion of the material. Furthermore, for the same lens, δ tends to increase as the wavelength increases. Therefore, if the δ value is filled to the full mode field diameter at a single design wavelength, coupling loss may occur, so it should be set somewhat smaller.

  As for the coreless fiber length, the reflectance of the AR coating on the fiber end face is about 23 dB, so it is necessary to take about 350 μm at the shortest, and when it is 500 μm or more, loss occurs in the light emitted from the fiber end. There is a case. Accordingly, the preferred length of the coreless fiber is about 350 μm to 500 μm.

  In view of the above, FIG. 9 shows the compatible range of the lens diameter R and the refractive index n when the upper limit of the aberration circle diameter is 8 μm and the coreless fiber lengths are 350 μm and 500 μm. In this way, when considering the parameters in reality, the design of the compatible lens becomes very narrow.

  By the way, the most important lens parameter in the normal optical design is f. Since f is a value determined by two parameters n and r as shown in the equation (2), the designed n and r are the collimating lenses. The contents of the present invention are extremely useful when it is determined whether or not it is appropriate.

  As a specific example, a procedure for selecting n and r when the design value of the focal length f is 2.0 mm will be described. When f = 2.0 mm, n and R (= 2r) are realized by the combination indicated by the solid line P in FIG. However, for the purpose of producing a collimator having high coupling efficiency, as described above, the limitations of aberration circle diameter δ <8 μm or less and coreless fiber length of 350 μm or more are added, and the range of n and R is strongly limited. receive. When there is no guideline, in order to confirm which combination is appropriate, it is necessary to perform calculation or to make a trial product, but if the present invention is used, the region can be clearly defined.

  That is, from FIG. 10, if a sphere diameter and a glass material are selected in a range of 3.17 mm <R <3.52 mm and 1.65 <n <1.79, a collimator lens (sphere Lens).

  This limitation becomes more severe as the length of the coreless fiber becomes longer, and the region is subjected to stronger limitations, so selection of the glass material is usually extremely difficult. However, even in such a case, the spherical diameter and the glass material can be selected very simply by using the n-R curve expressed in FIG. 10 when the coreless fiber length is 500 μm. In this case, selection may be made within the range of 3.17 mm <R <3.32 mm and 1.65 <n <1.71. There are only a dozen or more glass glass types that meet this range, and it is very easy to select a glass glass type having dispersion and chemical / mechanical properties suitable for the intended use.

  As described above, the method of defining the usable material region shown in the present invention can be said to be a very useful tool because the lens design can be easily and reliably performed.

  A collimator pair was actually fabricated by combining an optical fiber terminal with a coreless fiber and a collimator lens, and the coupling loss was measured. The optical fiber terminal was manufactured according to the method described above, and an optical fiber terminal with a coreless fiber having a capillary outer diameter of φ1.80 and a coreless fiber length of 300 μm to 400 μm was prepared. As a collimator lens, a spherical lens having the outer diameter shown in the table was prepared for the glass material (HOYA glass material) shown in Table 1 below. The prepared lenses are designed in accordance with the contents of the present invention so that the outer diameter of the spherical lens and the refractive index of the glass material are all designed so that the aberration circle diameter δ is 8 μm or less. All values are for a wavelength of 1550 nm.

  Next, as shown in FIG. 11, a spherical lens 50 is fixed to the tip of a glass tube 60 having an inner diameter φ of 1.85 and a length of 7 mm with an adhesive (UV adhesive) 65, and a capillary 67 with a fiber is inserted from the opposite side. . The position of the capillary 67 with the fiber was adjusted and fixed with an adhesive so as to optimize the relative position between the optical fiber and the spherical lens 50 with the maximum propagation distance obtained from the focal length of the lens 50.

  A plurality of fiber collimators were manufactured by such a method, and the distance dependence of the coupling loss of the collimator pair was measured using an LD light source of λ = 1550 nm, and the minimum coupling loss value was obtained. As a result, as shown in Table 1, it was proved that a collimator with a sufficiently low loss can be obtained with any lens.

It is explanatory drawing of embodiment of this invention, (a) is a case where a ball lens (full ball lens) is used as a spherical lens, (b) is a figure which shows the case where a drum lens (partial ball lens) is used as a spherical lens. It is. It is explanatory drawing which shows the relationship between the back focus length of a spherical lens, and a coreless fiber length. It is a figure which shows the relationship between n (refractive index) and R (spherical diameter) which satisfy | fill the aberration circular diameter (delta) <10micrometer of a spherical lens. It is a figure which shows the relationship between coreless fiber length and reflection loss. It is explanatory drawing about the change of the back focus value by a coreless fiber. It is a figure which shows the relationship between the diameter of a spherical lens, and refractive index for every condition. It is explanatory drawing about the fiber length dependence of the incident angle in a coreless fiber end surface. It is a figure which shows the coreless fiber length dependence of the value related to the correction value of the back focus of a spherical lens. It is a figure for demonstrating how to grasp the diameter and refractive index of a spherical lens when a specific coreless fiber length is given. It is a figure for demonstrating how to grasp the diameter and refractive index of a spherical lens when a specific coreless fiber length is given. It is an external view which shows the specific structural example of this invention. It is explanatory drawing of the problem in the conventional optical coupling. It is a block diagram of the conventional optical fiber terminal with a coreless fiber. It is explanatory drawing of the fiber collimator in the case of using a spherical lens. It is explanatory drawing of the aberrational circle diameter of a spherical lens.

Explanation of symbols

101 optical fiber 101a core 101b clad 102 coreless fiber 50 ball lens 52 drum lens

Claims (3)

One end face of a coreless fiber made of a material having a uniform refractive index substantially the same as the core is bonded to the end face of the optical fiber having a core at the center and a clad at the outer periphery thereof, on the optical axis of the optical fiber. A fiber collimator in which a spherical lens is disposed on the other end surface side of the coreless fiber, and the focal point of the spherical lens passing through the coreless fiber is positioned on the end surface of the core of the optical fiber,
The refractive index and the diameter of the spherical lens are set so that the aberration circle diameter of the spherical lens is equal to or smaller than the mode field diameter of the optical fiber, and is specified in advance to ensure the necessary reflection loss amount of the coreless fiber. A fiber collimator, wherein a refractive index and a diameter of a spherical lens are set so that a back focus of the spherical lens is larger than a minimum length of a coreless fiber. The fiber collimator according to claim 1, wherein
The relationship curve between the refractive index and the diameter of the spherical lens when the aberration circle diameter of the spherical lens is equal to the mode field diameter of the optical fiber is a lower limit curve,
The length of the coreless fiber set to be equal to or larger than the minimum length of the coreless fiber defined in advance in order to secure the necessary reflection loss amount of the coreless fiber, and the back of the spherical lens corrected according to the length of the coreless fiber The relationship curve between the refractive index and the diameter of the spherical lens when the focus becomes equal is the upper limit curve,
A fiber collimator characterized in that the refractive index and diameter of the spherical lens are set within a range between the upper limit curve and the lower limit curve. The fiber collimator according to claim 2, wherein
The length of the said coreless fiber is set to 200 micrometers or more and 800 micrometers or less, The fiber collimator characterized by the above-mentioned.

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