JP4270454B2 - Optical coupling method and connector - Google Patents

Description

  The present invention relates to an optical coupling method for plastic optical fibers used in optical communications and the like, and a connector used for this optical coupling.

  One of the characteristics of plastic optical fibers is that they have a larger diameter than silica-based optical fibers. Light emitting diodes (LEDs, light emitting diodes), semiconductor lasers (LDs, laser diodes), or surface emitting semiconductor lasers (VCSELs, The advantage is that optical coupling with a light source such as Vertical Cavity Surface Emitting Laser is easy. In recent years, it has become possible to produce a gradient index plastic optical fiber, and the use of the above-described plastic optical fiber has begun to be studied as a transmission line for a broadband optical signal transmission link of 1 Gbps (100 m) or more. Even in a broadband optical signal transmission link of 1 Gbps (100 m) or more, the superiority of the optical coupling between the light source and the large-diameter plastic optical fiber is the same as that of the silica-based optical fiber. In optical coupling with a receiving optical device (for example, a photodiode), efficient optical coupling cannot be achieved unless a condensing device such as an optical lens is disposed in the gap between the plastic optical fiber and the photodiode 41. This is because the light emitted from the large-diameter plastic optical fiber spreads beyond the opening diameter of the light receiving optical device by diffraction.

  A similar problem exists in a dedicated connector for connecting between plastic optical fibers in an optical signal transmission link. The connector for the plastic optical fiber is designed so that a space of about several hundred μm is opened between the optical fibers to be connected in consideration of expansion of the plastic optical fiber due to temperature change. For this reason, when light emitted from the plastic optical fiber enters the next connected plastic optical fiber, the beam diameter becomes greater than the fiber diameter (opening diameter) due to diffraction, and optical coupling loss (diffraction loss) occurs. Therefore, in order to obtain efficient optical coupling even in a connector between plastic optical fibers, it is necessary to arrange a condensing device such as a relay lens in the connector. This is a problem that is not present in the connection between the quartz fibers that physically contact the fiber end faces.

  Furthermore, in an optical transmission link using a plastic optical fiber, a wavelength division multiplexing optical transmission system that multiplexes and transmits several different types of wavelengths has been attracting attention in order to further increase the bandwidth. In such a transmission link, it is necessary to insert an optical functional element such as a wavelength filter for individually extracting multiplexed wavelength signals into the transmission path. However, if a space for inserting an optical element that does not have a condensing function is created between the optical transmission paths, diffraction loss occurs and optical coupling efficiency decreases. Usually, in order not to reduce the optical coupling efficiency, an optical collimator composed of lenses is arranged between the optical transmission paths.

  As described above, using a condensing optical device for optical coupling is a general method for reducing diffraction loss, that is, improving the efficiency of optical coupling, in an optical signal transmission link using a plastic optical fiber. Condensing optical devices are more important when using a plastic optical fiber with a larger outgoing beam diameter as the optical transmission path than when using a silica-based fiber as the transmission path of the optical signal transmission link. In order to efficiently couple power to the light receiving device, the configuration and design of the condensing optical device are very important compared to the silica-based optical fiber.

  In the conventional technology, as a specific condensing optical device, a lens is interposed between an optical fiber and a light receiving optical device (see, for example, Patent Document 1), a gradient-index lens (GRIN-lens, GRIN lens). ) (For example, refer to Patent Document 2), and fiber end surfaces processed into a curved surface such as a spherical surface or a taper shape (for example, refer to Patent Document 3 and Patent Document 4). It has been known that the emitted light from the optical fiber is collected by the above, and the emitted light is combined with other optical devices to improve the efficiency of optical coupling.

JP 2000-147294 A Japanese Patent Laid-Open No. 2001-264592 JP-A-8-21929 Japanese Patent Laid-Open No. 8-75935

  However, the methods described in Patent Documents 1 to 4 require precise positioning because there is an optimal position (for example, the focal length of the lens) of the optical member that collects light in the direction of the optical fiber output axis. . Therefore, it is essential to increase the number of components for optical coupling and alignment between the optical fiber / optical lens / light receiving device, resulting in an increase in the cost of the optical transmission link system.

  In addition, optical elements such as lenses are interposed between the plastic optical fiber and the light receiving element, which increases costs or reduces the optical power of transmitted light due to Fresnel loss. Therefore, optical coupling with fewer optical elements is required. It was done. In addition, it is possible to use a form in which the emitting end face and the light receiving medium are in direct contact with each other as described above. However, if this is performed with a simple structure, the light receiving element is broken or the end face is broken. Therefore, a precise design and firm fixing means are required between the light emitting side and the light receiving side, and it has been difficult to use as simple optical coupling.

  An object of this invention is to provide the optical coupling method which improves coupling efficiency, and the connector used for this optical coupling, without using a condensing optical device.

  As a result of diligent studies to solve the above-described problems, the optical transmission core substantially covers the first core part having a refractive index distribution and the outer periphery of the first core part in the core part where light propagates. By using a refractive index distribution type plastic optical fiber (hereinafter also referred to as a GI type plastic optical fiber) having two regions with a second core portion that does not have a refractive index distribution in the optical fiber, from the output end face of the optical fiber. By obtaining the knowledge that has a region where the outer diameter of the emitted light of the light is maintained for a certain distance (hereinafter referred to as a parallel region) and designing the first core portion and the second core portion, The present invention that can be made unnecessary has been reached. Aspects of the present invention are as follows.

The optical coupling method of the present invention is an optical coupling method of a GI type plastic optical fiber that optically couples between a GI type plastic optical fiber or between the GI type plastic optical fiber and a light receiving optical device, wherein the GI type plastic optical fiber includes: A first core part having a refractive index distribution in which the refractive index gradually decreases from the center toward the outer periphery in the radial direction as a core through which light propagates, and covers the outer periphery of the first core part, and the first core A second core portion having a constant refractive index that is substantially the same as the refractive index of the outer periphery of the portion, the outer diameter of the second core portion being not less than 250 μm and not more than 1000 μm, The outer diameter ratio between the core part and the second core part is 0.67 or more and 0.87 or less, and the exit end face of the GI type plastic optical fiber and the entrance end of the GI type plastic optical fiber A gap with a distance of 150 μm or more and 500 μm or less between the surface and the light receiving optical device is provided for optical coupling, and the outer diameter of the emitted light from the emission end face of the GI plastic optical fiber is maintained at a certain distance in the gap. It is characterized by being in the area.

  It is preferable to arrange an optical functional element that does not have a light convergence function in the gap.

  It is preferable that the refractive index distribution coefficient g of the first core portion is in a range of 1.5 ≦ g ≦ 4.0.

  Further, the output NA of the GI type plastic optical fiber is preferably in the range of 0.12 ≦ (output NA) ≦ 0.22. Further, it is preferable to use a light emitting device having an emission NA of 0.05 ≦ (output NA) ≦ 0.22 as a light source for making light incident on the GI type plastic optical fiber. The light emitting device is preferably a surface emitting semiconductor laser.

The connector of the present invention is a connector for optically coupling between a GI type plastic optical fiber or between the GI type plastic optical fiber and a light receiving optical device, and a first holding member for holding the GI type plastic optical fiber; A second holding member for holding the plastic optical fiber or the light receiving optical device; one or both of the holding members; an emission end face of the GI type plastic optical fiber; an incident end face of the GI type plastic optical fiber; Holding a gap where the distance to the light receiving optical device is 150 μm or more and 500 μm or less, and connecting means for connecting the first holding member and the second holding member, the refractive index distribution type plastic optical fiber, As a core for light propagation, gradually from the center to the outer periphery in the radial direction A first core portion having a refractive index distribution in which the refractive index decreases, and a second core that covers the outer periphery of the first core portion and has a constant refractive index that is substantially the same as the refractive index of the outer periphery of the first core portion. The outer diameter of the second core portion is not less than 250 μm and not more than 1000 μm, and the outer diameter ratio of the first core portion to the second core portion is 0.67. The gap is 0.87 or less, and the gap is in a region where the outer diameter of the outgoing light from the outgoing end face of the gradient index plastic optical fiber is maintained at a constant distance . Further, when the first holding member and the second holding member are connected by the connecting means, a light is applied to at least one of the first holding member and the second holding member so as to be disposed in the gap. It is preferable to provide an optical functional element that does not have a convergence function.

  According to the optical coupling method and connector of the present invention, there is no need to collect or collimate the emitted light because the emitted light emitted from the exit end face of the gradient index plastic optical fiber is radiated in parallel. It is. For this reason, the number of components in the optical coupling unit is reduced, and the adjustment process for positioning the components of the optical coupling unit can be simplified, so that the cost of the optical transmission link system can be reduced. Further, even if an active optical element such as a wavelength filter is inserted between the optical fiber and the light receiving optical device, the diffraction loss can be reduced.

  FIG. 1 is a cross-sectional view showing the configuration of the connector of the present invention. The connector 2 includes a holding member (first holding member) 3 that holds a GI plastic optical fiber (hereinafter referred to as POF) 10 and a holding member (second holding member) 5 that holds the light receiving optical device 4. ing. The holding member 3 exposes the emission end face 10a of the POF 10 forward (on the holding member 5 side). Further, the holding member 3 has a connecting portion 3a having a convex shape, and the holding member 5 has a concave shape and has a connecting portion 5a that engages with the connecting portion 3a. The connecting portions 3a and 5a are formed of an elastic resin or the like. By inserting the holding member 3 into the holding member 5 from the emission end face 10a side, the connecting portion 3a is engaged with the connecting portion 5a. The holding member 3 and the holding member 5 are connected. At this time, the aforementioned emission end face 10a and the light receiving optical device 4 face each other, and the distance X that is the gap is set to about 300 μm. This distance X can be used in the range of 150 μm or more and 500 μm or less, and more preferably 200 μm or more and 400 μm or less. This distance is when the gap is air, and the gap may be filled with a substance that does not have a light convergence function, and the distance X at that time is shorter than the refractive index of the filled substance. Become. The light receiving optical device 4 is a light receiving optical element such as a PIN photodiode, for example.

  In addition, when the holding member 3 and the holding member 5 are connected, an optical functional element that does not have a light convergence function is provided in at least one of the holding member 3 and the holding member 5 so as to be disposed in the gap. May be.

  The opening diameter of the photodiode (PD) as the light receiving element is preferably selected in consideration of the balance between the light receiving area and the operation performance, and the diameter is about 300 to 400 μm for 1 Gbps transmission and about 100 μm for 3 Gbps transmission. Can do. If it is necessary to use a photodiode with a small area due to factors such as high-speed communication or a reduction in fiber diameter, use avalanche photodiodes with improved sensitivity to improve the disadvantages of narrow areas. Can do. Further, since the optical coupling of the present invention does not require strict setting of the optical axis, sufficient coupling can be obtained even by passive alignment, and the mass productivity is excellent. In particular, when the diameter of the emitted light is small with respect to the area of the light receiving element, the amount of light can be secured with respect to the alignment shift, and therefore, it works more effectively.

  Next, the GI type plastic optical fiber used in the optical coupling method and connector of the present invention will be described. FIG. 2 is a cross-sectional view of the POF 10. The POF 10 includes a core portion 11 disposed in the center and a clad portion 12 that covers the outer periphery of the core portion 11. The core part 11 includes two regions, a central inner core (first core part) 13 and an outer core (second core part) 14 that covers the outer periphery of the inner core 13. The core radius of the inner core 13 is a, and the core radius of the outer core 14 is b. The optical fiber of this mode is known to have high transmission performance because the first core portion (inner core 13) has a refractive index distribution, and the refractive index distribution at this time is a square distribution mode. It is known that it has the highest transmission performance.

  FIG. 3 is an explanatory diagram showing the refractive index distribution of the POF 10. As shown in FIG. 3, the refractive index n1 of the inner core 13 gradually decreases from the central axis CL toward the outer peripheral side (radial direction), and the g-th power distribution of a so-called GI (graded index) type plastic optical fiber. It has a refractive index profile.

  As a manufacturing method for providing the refractive index of the inner core 13, a refractive index adjusting substance for changing the refractive index as described in the pamphlet of International Publication WO 93/08488 is added to the monomer, and the excluded volume effect is utilized in the polymerization of the monomer. Thus, a method of giving a high refractive index toward the central part by concentrating the refractive index adjusting substance in the central part of the core part, or a monomer having a different refractive index as described in JP-A-2001-215345 A method of changing the refractive index of a copolymer by adjusting the copolymerization ratio from the outer peripheral portion toward the central portion, and diffusion of a refractive index adjusting substance as described in JP-A-08-334635 by heat And a method of changing the refractive index by distributing the abundance ratio of the refractive index adjusting substance according to the degree of diffusion at that time.

  The refractive index n2 of the outer core 14 is substantially the same value as the refractive index of the outer periphery of the inner core 13, and the refractive index is made uniform. The refractive index n2 of the outer core 14 is preferably uniform, but may have a refractive index distribution as long as higher-order modes propagate. Further, the refractive index nc of the cladding portion 12 is smaller than the refractive index n2 of the outer core 14.

  Moreover, the manufacturing method in particular of the outer core 14 is not prescribed | regulated, You may produce by any well-known methods, such as the method by throwing a polymer in the hollow tube which rotates and shape | molds by extrusion of the melted polymer, and rotates. At this time, if the transmission performance has irregularities such as fine unevenness and distortion at the interface between the first core portion (inner core 13) and the second core portion (outer core 14), loss due to scattering or the like occurs. Therefore, care should be taken so that there is no interface irregularity. There is no order in manufacturing the first core part and the second core part. For example, after the first core part is created, the second core part may be provided around the first core part. The first core portion that is formed by superposing or separately forming the first core portion in the hollow tube may be fitted.

  As described above, the refractive index n1 of the inner core 13 gradually decreases from the center CL of the inner core 13 toward the outer periphery (radius) direction, and has a g-th power distribution with respect to the distance.

This g value is called a refractive index distribution coefficient, and is an index that determines the refractive index distribution shape when the refractive index distribution shape of the optical fiber is approximated by a power law as shown in the following equation. a is the radius of the first core part, the refractive index at the center of the first core part where the refractive index is maximum, Mn1, and the refractive index of the outer periphery of the core part where the refractive index distribution starts to change (in the present invention, the inner core 13 The refractive index of the outer peripheral surface of the outer core 14, that is, the refractive index of the outer core 14) n2, Δ is a radius from the center of the first core portion when the difference between Mn1 and n2 is divided by Mn1 (specific refractive index difference). It is defined by the following formula (1) that is a core portion where the refractive index at an arbitrary position separated by r is n1 (r).

n1 (r) = Mn1 (1-2Δ (r / a) ^g ) ^1/2 ... (1)
(However, Δ = (Mn1-n2) / Mn1)

  Furthermore, regarding the diameter ratio of the first core part and the second core part, the first core part outer diameter / second core part outer diameter is 0.67 or more and 0.87 or less, preferably 0.67. It is preferable to arrange so as to be 0.80 or less. When the diameter ratio is too large, the parallel beam diameter is undesirably increased, and when the diameter ratio is too small, the exit light is not converged. The area of the light guide portion, that is, the outer diameter of the second core portion is preferably 250 μm or more and 1000 μm or less, and preferably 300 μm or more and 750 μm or less. If the outer diameter, that is, the fiber diameter is too large, the collimated beam diameter is undesirably large, and if it is too small, it is difficult to make light incident.

  The inner core 13 is configured such that the refractive index distribution coefficient g falls within the range of 1.5 ≦ g ≦ 4.0. The refractive index distribution coefficient of the inner core 13 that is the first core portion having the refractive index distribution is preferably arranged so as to be 2.0 or more and 2.5 or less. If this value is too large, a mode delay occurs and transmission characteristics deteriorate, which is not preferable.

  The embodiment of the present invention will be described as an example. The outer diameter 2b of the outer core 14 is in the range of 250 μm ≦ 2b ≦ 1000 μm, and the ratio of the outer diameter 2a of the inner core 13 to the outer diameter 2b of the outer core 14 is 0.67 ≦ a / b ≦. It is formed so as to be within the range of 0.87. Further, in the POF 10, the output NA is formed so as to be within the range of 0.12 ≦ output NA ≦ 0.22.

  Next, the emitted light characteristics of the POF 10 having the above configuration will be described. FIG. 4 is an explanatory diagram illustrating the configuration of the measurement system 20 that measures the width of the laser beam emitted from the emission end face of the POF 10. This measurement system 20 uses a VCSEL 21 which is a surface emitting laser as a light source. The maximum number of emission spots of the VCSEL 21 used for this measurement is 4, and its emission NA is 0.05 or more and 0.22 or less. Further, regarding the incident beam diameter to the POF 10 of the present invention, it is preferable that the diameter of the incident light is smaller than the outer diameter 2b of the outer core 14 so as not to cause a coupling loss when entering the core portion 11. Is preferably equal to or smaller than the outer diameter 2 a of the inner core 13. Accordingly, the distance between the VCSEL 21 and the incident end face of the POF 10 can be varied so as to be the optimum beam outer diameter. Further, a CCD camera type detector 23 for detecting the characteristics of the outgoing beam, that is, the outer diameter of the beam, is disposed on the outgoing end face side of the POF 10 at a position away from the distance Z. Further, an ND filter 22 is disposed between the VCSEL 21 and the POF 10. This is arranged for the purpose of preventing the power detection capability of the CCD camera of the detector 23 from being saturated when the distance between the VCSEL 21 and the incident end face of the POF 10 is small and the output light power from the POF 10 is too strong. .

  The beam emitted from the VCSEL 21 enters from the incident end face of the POF 10 via the ND filter 22. The POF 10 guides the incident beam and emits it from the emission end face to the space. The detector 23 measures the outer diameter of the beam emitted into the space at the position of the distance Z.

  A measurement result obtained by measuring the outer diameter of the beam emitted from the emission end face of the POF 10 to the space by the measurement system 20 will be described below. The result of measurement using a POF 10 having an outer diameter of the cladding portion 12 of 475 μm is shown in the graph of FIG. The POF 10 has a refractive index nc made of PVDF of 1.42, an outer core of 475 μm in outer diameter made of PMMA inside a cladding of 500 μm in outer diameter, and further refracted by adding diphenyl sulfide to PMMA inside. An inner core 13 having a rate distribution was provided. The ratio (a / b) of the outer diameter 2b of the outer core 14 to the outer diameter 2a of the inner core 13 was set to 0.78. Further, the respective refractive indexes were 1.505, the maximum refractive index n1 of the central portion of the inner core 13 was 1.505, the refractive index n2 of the outer core 14 was 1.492, and the refractive index distribution coefficient g was 2.5.

The VCSEL 21 has a wavelength of 780 nm and the number of light emission spots is four. Further, the emitted light intensity of the VCSEL 21 has a Gaussian beam distribution, and the divergence angle is 18 degrees. The divergence angle here uses a general definition method. That is, it was defined by an angle that is 1 / e ² of the power at the center where the light intensity is highest in the Gaussian beam distribution. The length of the POF 10 is 30 m.

  As can be seen from the graph of FIG. 5, the distance Z ranges from 0 to 400 μm, and the outer diameter of the beam does not increase, and the beam diameter is about 400 μm, and when the distance Z exceeds 400 μm, A divergence angle of 14 °, that is, an output NA of a plastic optical fiber of 0.12 spreads in space. This measurement was performed for 3 lots, and almost the same result was obtained. Moreover, even when the same measurement was performed with the length of the POF 10 set to 10 m, 5 m, and 2 m, the same result as in FIG. 5 was obtained. The present invention is not particularly limited to a light receiving optical device. In addition to coupling with a semiconductor device such as a light receiving element, the present invention may be used for connection between plastic optical fibers and optical members such as an optical coupler. Can do.

  The same measurement was performed for POF having an outer diameter of the outer core portion 14 of 300 μm. This POF also has a refractive index nc made of PVDF of 1.42, an outer core of 300 μm in outer diameter made of PMMA inside the cladding of 316 μm in outer diameter, and diphenyl sulfide added to PMMA inside to add a refractive index distribution. The inner core 13 is provided. The ratio (a / b) of the outer diameter 2b of the outer core 14 to the outer diameter 2a of the inner core 13 was set to 0.78. Each refractive index is created so that the maximum refractive index n1 at the center of the inner core 13 is 1.505, the refractive index n2 of the outer core 14 is 1.492, and the refractive index distribution coefficient g is 2.5. did.

  The result is shown in the graph of FIG. Other conditions are the same as the measurement in FIG. As can be seen from the graph in FIG. 6, the distance Z is from 0 to 320 μm, and the outer diameter of the beam does not widen, but the beam diameter is a parallel beam of about 300 μm, and when the distance Z exceeds 320 μm, A divergence angle of 24 degrees, that is, an output NA 0.21 of a plastic optical fiber spreads in space.

  FIG. 7 shows the result of the same measurement as in FIG. 5 or FIG. 6 using a 30-m step index type POF (hereinafter referred to as SI type POF). The outer diameter of the POF clad is 750 μm. Other conditions are the same as those in FIG. 5 or FIG. As can be seen from FIG. 7, it can be seen that the emitted light of the SI-type POF is emitted from the exit end face of the POF and is spread by diffraction.

  FIG. 8 shows the result of the same measurement as in FIG. 5 or FIG. 6 for a GI optical fiber having a structure that does not have an outer core different from the GI optical fiber used in the present invention. The outer diameter of the POF clad is 125 μm, the length is 30 m, and other conditions are the same as in FIG. 5 or FIG. As can be seen from FIG. 8, in the GI optical fiber having another structure different from the GI optical fiber (POF10) used in the present invention, the emitted light has a clear parallel region like the POF10 used in the present invention. It is not seen, and the emitted light begins to spread about 50-100 μm after emission. At these distances, it is not an appropriate distance to arrange an optical element having no light collecting function between the fibers without interposing an optical system.

  From the above results, the phenomenon that the outer diameter of the outgoing beam from the POF 10 propagates without expanding to the interval where the optical functional elements can be arranged is not found in the conventional SI-type POF or GI-type POF, and is used in the present invention. It shows that it is clear that it is expressed only by the structure of POF10.

  FIG. 9 is a result of schematically measuring the coupling efficiency between the light emitted from the emission end face of the POF 10 and the light receiver (PD). The aperture diameter of the optical receiver was 400 μm, and the POF having an outer core outer diameter of 475 μm described as the first embodiment was used. It can be seen that the light emitted from the exit end face of the POF has an optical coupling efficiency of approximately 100% when the distance between the exit end face and the light receiver (PD) is within 300 μm.

  In FIG. 10, when the exit end face and the entrance end face of the two POFs 10 face each other at a distance Y, and the light emitted from one POF (A) is received by the entrance end face of the other POF (B), It is a plot of how the coupling efficiency changes with distance Y. The outer diameter of the cladding portion of the POF used in the experiment is 475 μm, and the length of the POF is 30 m for both POF (A) and POF (B). The VCSEL 21 used for the measurement in FIG. 4 was used as the light source. As can be seen from FIG. 10, the light emitted from the exit end face of POF (A) is optically coupled within the range where the distance Y between the exit end face of POF (A) and the entrance end face of POF (B) is within 300 μm. It can be seen that the efficiency is approximately 100%. This is nothing but the fact that the outer diameter of the outgoing beam from the POF does not spread by diffraction when the distance Y is within 300 μm.

  The experimental results in FIGS. 9 and 10 indicate that no diffraction loss occurs even if a space of 300 μm or less exists in the optical coupling between the POF and the light receiver (PD) and the optical coupling between the POFs. That is, no means for condensing or collimating is required. In addition, an optical device such as a wavelength filter can be inserted into this space without loss in principle.

  Such a characteristic is a completely new function not found in conventional silica-based optical fibers and plastic optical fibers. Further, when the plastic optical fibers of the present invention are connected by a connector, low-loss optical coupling can be realized without the need to physically contact the fiber end faces (physical contact) as seen in a conventional quartz fiber connector. Further, as seen in conventional plastic fiber connectors, low-loss optical coupling can be realized without the need for strict variation in the spatial distance for absorbing the expansion and contraction of the fiber. Therefore, it is possible to design a new connector housing structure with low optical coupling and low cost.

  As described above, the POF used in the present invention is emitted in a parallel beam over a certain spatial range while maintaining the large aperture and the high light receiving angle that are the characteristics of the conventional plastic optical fiber. It also has the characteristics of a conventional small-diameter fiber that can achieve high-efficiency optical coupling with a high-bandwidth small-aperture light receiver. For this reason, in the optical coupling of the present invention, high accuracy is not required in the component positioning process, so that the positioning process can be simplified and the cost of the optical transmission link system can be reduced.

  Further, the light emitted from the POF 10 having the above-described structure propagates through the space with a parallel beam for a certain distance (about 300 μm), and then is emitted into the space with a certain spread angle. Accordingly, in the parallel beam propagation region as described above, the optical coupling efficiency with a light receiver having an aperture diameter larger than the parallel beam diameter is approximately 100% without using a condensing optical device such as an optical lens. In addition, since no condensing optical device is used, the number of components in optical coupling is reduced, and the adjustment process for positioning optical coupling components can be simplified, so that the cost of the optical transmission link system can be reduced. Furthermore, it is possible to have a space for inserting an optical functional element.

It is sectional drawing which shows the structure of the connector of this invention. It is sectional drawing which shows the structure of POF. It is explanatory drawing which shows the characteristic of the refractive index of POF. It is explanatory drawing which shows the structure of a measurement system. It is a graph which shows the characteristic of the beam diameter with respect to distance from the POF output end surface of an outer core outer diameter of 475 micrometers. It is a graph which shows the characteristic of the beam diameter with respect to distance from the POF output end surface of an outer core outer diameter of 300 micrometers. It is a graph which shows the characteristic of the beam diameter with respect to distance from SI type POF output end surface of an outer core outer diameter of 750 micrometers. It is a graph which shows the characteristic of the beam diameter with respect to the distance from the GI type | mold POF output end surface with an outer core outer diameter of 125 micrometers used other than this invention. It is a graph which shows the characteristic of the optical coupling efficiency with respect to the distance of an outer core outer diameter 475 micrometer POF and a light receiver. It is a graph which shows the characteristic of the optical coupling efficiency with respect to the distance between outer core outer diameters 475 micrometers POF.

Explanation of symbols

2 Connector 3, 5 Holding member 4 Light receiving optical device 10 POF
10a Output end face 11 Core part 12 Clad part 13 Inner core 14 Outer core 21 VCSEL
22 ND filter 23 Detector a Inner core radius b Outer core radius n1, n2, nc Refractive index

Claims (8)

In an optical coupling method of a graded index plastic optical fiber that optically couples between a graded index plastic optical fiber or between the graded index plastic optical fiber and a light receiving optical device,
The refractive index distribution type plastic optical fiber includes a first core portion having a refractive index distribution in which a refractive index gradually decreases from a center to an outer periphery in a radial direction as a core through which light propagates, and the first core portion And has two regions of the second core part having a constant refractive index that is substantially the same as the refractive index of the outer periphery of the first core part,
The outer diameter of the second core part is 250 μm or more and 1000 μm or less, and the outer diameter ratio between the first core part and the second core part is 0.67 or more and 0.87 or less,
A gap between the exit end face of the gradient index plastic optical fiber and the entrance end face of the gradient index plastic optical fiber or the light receiving optical device is provided with a gap of 150 μm or more and 500 μm or less, and optically coupled,
The optical coupling method , wherein the gap is in a region where the outer diameter of the outgoing light from the outgoing end face of the gradient index plastic optical fiber is maintained at a constant distance .   The optical coupling method according to claim 1, wherein an optical functional element having no light convergence function is disposed in the gap. Claim 1 or claim 2 the optical coupling method wherein a refractive index distribution coefficient of the first core portion is in the range of 1.5 to 4.0. The exit NA graded index plastic optical fiber, according to claim 1 to claim 3 optical coupling method wherein a is in the range of 0.12 or more 0.22 or less. As a light source of light incidence onto the refractive index distribution type plastic optical fiber, the exit NA is claimed in any one claims 1 to 4, characterized by using a light-emitting device is 0.05 or more 0.22 or less Optical coupling method. 6. The optical coupling method according to claim 5 , wherein the light emitting device is a surface emitting semiconductor laser. In the connector for optically coupling between the gradient index plastic optical fiber or between the gradient index plastic optical fiber and the light receiving optical device,
A first holding member for holding the gradient index plastic optical fiber; a second holding member for holding the gradient index plastic optical fiber or the light receiving optical device;
Provided in one or both of these holding members, the distance between the exit end face of the gradient index plastic optical fiber and the entrance end face of the gradient index plastic optical fiber or the light receiving optical device is 150 μm or more and 500 μm or less. Connecting means for holding the gap and connecting the first holding member and the second holding member;
The refractive index distribution type plastic optical fiber includes a first core portion having a refractive index distribution in which a refractive index gradually decreases from a center to an outer periphery in a radial direction as a core through which light propagates, and the first core portion There are two regions of the second core portion that is substantially the same as the refractive index of the outer periphery of the first core portion and has a constant refractive index,
The outer diameter of the second core part is 250 μm or more and 1000 μm or less, and the outer diameter ratio between the first core part and the second core part is 0.67 or more and 0.87 or less,
The said gap is in the area | region where the outer diameter of the emitted light from the output end surface of the said gradient index plastic optical fiber is maintained a fixed distance . When at least one of the first holding member and the second holding member is disposed in the gap when the first holding member and the second holding member are connected by the connecting means, a light convergence function is provided. 8. The connector according to claim 7, further comprising an optical functional element that does not have an optical element.

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