JP2005078027A - Optical multiplex transmission system and manufacturing method thereof - Google Patents

Abstract

PROBLEM TO BE SOLVED: To suppress significant reception deterioration due to mode conversion even when a bent portion is generated in a multimode optical fiber, and to easily change according to use without using a precise device such as a condensing lens or a beam expander, and is inexpensive. An optical multiplex transmission system and a manufacturing method thereof are provided.
An optical multiplex transmission system according to the present invention includes a refractive index distribution type optical fiber 1 in which optical signals La to Ld are arranged one-dimensionally in the Y-axis direction and is perpendicular to the Y-axis without bending in the Y-axis direction. It is arranged on the XZ plane. As a result, the optical axis of the optical fiber 1 is always perpendicular to the amplitude direction of each propagation mode, and the influence of mode conversion due to the bending of the optical fiber 1 can be minimized. In the optical multiplex transmission system, the four optical signals La to Ld emitted from the VCSEL array 10 are accurately received by the photodetector array 20 through the single gradient index optical fiber 1 having a bent portion. can do.
[Selection] Figure 1

Description

  The present invention relates to an optical multiplex transmission system and a manufacturing method thereof, and more particularly to an optical multiplex transmission system using a gradient index multimode optical fiber and a manufacturing method thereof.

  Conventionally, optical fibers are excellent in broadband and low-loss characteristics, and their introduction into backbone systems for the purpose of high-speed and large-capacity transmission typified by the Internet is progressing. In the future, optical fibers are expected to be applied to access systems from trunk lines to homes and to home networks.

Optical fibers are roughly classified into single mode optical fibers (SMF) and multimode optical fibers (MMF) due to their characteristics. SMF is generally characterized in that both core and clad layers are made of silica (SiO ₂ ), the core diameter is as small as about 10 μm, and only a specific mode is transmitted, so the transmission band is wide. is there. Therefore, SMF has been developed and widely spread for long distance and large capacity transmission such as trunk line systems.

  On the other hand, MMF has a core diameter of 50 μm to 1 mm and is larger than SMF. The MMF is further classified according to the core or clad base material. For example, an MMF whose core and clad layer material is made of silica is called GOF (Glass Optical Fiber), and whose core layer is made of silica and whose clad layer is made of polymer is called PCF (Polymer Clad Fiber). A material whose core and cladding layers are all made of plastic is called POF (Plastic Optical Fiber). And since MMF has many propagation modes which are the course of light, the transmission form turns into multimode transmission (multimode transmission).

  FIG. 11 is a diagram schematically showing the difference in propagation modes in the MMF described above. In the MMF 100, a fiber core 102 and a clad 101 are formed. In general, in each light propagation mode in the MMF 100, light travels through the fiber core 102 while being repeatedly reflected at the interface between the core 102 and the clad 101. Therefore, the closer the light is parallel to the interface (that is, parallel to the optical axis of the MMF 100), the wider the interval of reflection at the interface, and the longer the distance that can be reached on the optical axis from one reflection to the next. A mode in which such reflection is repeatedly transmitted is called a low-order mode. On the other hand, when the light has a large angle with respect to the interface, the interval of reflection at the interface is narrow, and the distance that can be reached on the optical axis from one reflection to the next is short. That is, since the light has a large angle with respect to the optical axis, when the same fiber length is considered, the number of reflections at the interface is larger than that in the low-order mode, and an optical path difference occurs. A mode in which such reflection is repeatedly transmitted is called a higher-order mode.

  By the way, in each propagation mode mentioned above, since an optical path difference arises, the optical signal transmitted as a pulse train differs in the time which reaches | attains with respect to the same fiber length, respectively. Therefore, each propagation mode in the MMF propagates through the fiber at a unique transmission rate, so the pulse train included in the low-order mode with a high transmission rate and the pulse train included in the high-order mode with a low transmission rate are the same. In spite of being an optical signal, a time lag occurs and reception may be impossible. This is called mode dispersion and is a factor that greatly restricts the transmission band of the MMF compared to the SMF. In general, the transmission band of these optical fibers is represented by the product of the transmission speed and the transmission distance (for example, [Mbps × km]), and the higher the signal rate, the shorter the transmittable distance. In order to lengthen the signal rate, the signal rate becomes slow. In addition, the influence of the mode dispersion is higher as the signal rate is higher and the transmission distance is longer. Therefore, in a transmission system using MMF, it is often possible to shorten the transmission distance in order to transmit a desired signal rate.

  On the other hand, the merit of MMF is that the cost of the optical fiber and the optical transmission system is low. This is because the core diameter of the MMF is 50 μm to 1 mm in GOF, which is larger than that of the SMF, making it easy to align the optical axis when connecting the optical fiber, and reducing the mounting accuracy of the connector, thus reducing the cost of the optical transmission system. Because it contributes greatly. Therefore, MMF is often used in optical transmission at a transmission distance where the influence of mode dispersion does not matter.

  For example, when the transmission distance is a short distance of about several centimeters to several meters, in the optical transmission using MMF, the angular propagation mode is kept almost constant and is hardly affected by mode dispersion. In this case, since each propagation mode has an individual transmission path, optical multiplexing transmission can be performed within one MMF if each propagation mode carries individual information in an optical signal. As a result, an optical transmission system that optically transmits high-speed digital signals can be realized in a space-saving size. There is an optical multiplex transmission system using one POF using such characteristics (see, for example, Patent Document 1). The optical multiplex transmission system will be described below with reference to FIG. FIG. 12 is a diagram showing a schematic configuration of the optical multiplex transmission system.

  In FIG. 12, the optical multiplexing transmission system includes a large-diameter POF 100, a VCSEL array 110 including VCSELs (Vertical Cavity Surface Emitting Lasers) 110a to 110c, a condensing lens 111, a beam expander 121, and a light receiving unit. It includes a light receiver array 120 including light receivers 120a to 120c. The large-diameter POF 100 has a fiber core 102 and a clad 101 formed therein.

The VCSELs 110a to 110c are semiconductor lasers that oscillate light in a direction perpendicular to the substrates, and emit optical signals La to Lc, respectively. The condensing lens 111 gives individual incident angles to the optical signals La to Lc emitted from the VCSELs 110 a to 110 c and makes them incident on the incident side end face of the core 102 formed in the large diameter POF 100. In the large-diameter POF 100, the respective optical signals La to Lc are transmitted in individual propagation modes (angles θa to θc, respectively, with respect to the optical axis of the POF 100), and the propagation modes are maintained during short distance transmission. Then, the optical signals La to Lc propagated through the large-diameter POF 100 are emitted from the exit side end face of the core 102 at individual exit angles, and the exit angle is further emphasized by the beam expander 121 to the light receivers 120a to 120c. Reach. In each of the light receivers 120a to 120c, each outgoing light is separated at a sufficient interval by the beam expander 121, so that the respective optical signals La to Lc can be received without error.
Japanese Patent Laid-Open No. 2000-22643

  Here, the precondition for establishing the optical multiplex transmission system is that the large-diameter POF 100 is required to be short and ideally linear, and in this case, each propagation mode (angles θa to θc) is maintained. The And when each propagation mode is hold | maintained, the exact angle and position of each propagation mode which arrives at the beam expander 121 from the output side end surface of large diameter POF100 can be grasped | ascertained. The optical multiplex transmission system can strictly design the condensing lens 111 and the beam expander 121 based on these information, and can guide the optical signals La to Lc of each propagation mode to the light receivers 120a to 120c, respectively.

  However, in the optical multiplex transmission system, when the large-diameter POF 100 has a bent portion, it is not possible to guide each propagation mode to the light receivers 120a to 120c as shown in FIG. In FIG. 13, the optical signal Lx incident on the incident side end face of the large-diameter POF 100 propagates through the straight line portion while maintaining the angle θ1. Since the straight portion of the large diameter POF 100 is a sufficiently short distance, the optical signal Lx is transmitted while maintaining the propagation mode (angle θ1). When the optical signal Lx enters the bent portion of the large-diameter POF 100, the interface between the core 102 and the clad 101 is curved, so that the incident angle to the interface changes, and the reflected angle changes accordingly. The angle θ1 is not maintained. When the optical signal Lx comes out from the bent portion to the straight line portion, it has an angle θ2 and undergoes mode conversion so that θ1 ≠ θ2. As described above, mode conversion occurs in the bent portion of the large diameter POF 100, and the angle and position of each propagation mode arriving at the beam expander 121 are shifted.

  MMF (POF) having such a bent portion is beyond the scope of the optical multiplex transmission system, and when the bent portion is generated in the MMF, this mode conversion becomes a fatal phenomenon. The mode conversion is caused not only by the intentionally provided bent portion but also by non-flatness of the core / cladding interface, nonuniformity of the core refractive index, or impurities in the core. Although these factors are not assumed in an ideal MMF, they are phenomena that frequently occur in an actual optical multiplex transmission system. When the number of propagation modes in the MMF increases, the optical multiplex transmission system is significantly affected.

  The optical multiplex transmission system capable of transmitting four propagation modes will be described with reference to FIG. FIG. 14 is a diagram showing a schematic configuration of an optical multiplex transmission system capable of transmitting four propagation modes.

  14, the difference from the optical multiplex transmission system that can transmit the three propagation modes described with reference to FIG. 12 is that the number of propagation modes is three (angles θa to θc) because the optical signal Ld is increased. To four (angles θa to θd), and accordingly, the VCSEL 110d and the light receiver 120d are added. Of the four propagation modes in the large-diameter POF 100, the propagation modes θa and θd that are components emitted from the VCSELs 110a and 110d are higher-order than the propagation modes θb and θc that are components emitted from the VCSELs 110b and 110c. Propagation mode (angle θ is large). Here, in the example of FIG. 14, the arrangement of the optical signals La to Ld reaching the beam expander 121 is Ld, Lb, Lc, and La in order from the top of the drawing. However, when mode conversion is caused in the large-diameter POF 100 due to a bend or an unexpected factor described above, the arrangement of the optical signals La to Ld on the exit-side end face after propagating the fiber length of the large-diameter POF and Those emission angles will fluctuate. As a result, the arrangement of the optical signals La to Ld reaching the beam expander 121 may be La, Lb, Lc, and Ld in order from the top.

  Here, the condensing lens 111 and the beam expander 121 are rigorously designed on the assumption that each propagation mode is maintained and transmitted without error to the optical receiver array 120. The mode conversion that occurs in is not allowed. Furthermore, considering that the condensing lens 111 and the beam expander 121 require a precise design and are expensive, the number of propagation modes transmitted through the large-diameter POF 100 may be increased or decreased, and the length of the fiber may be adjusted. Changes cannot be made easily. As a result, the optical multiplex transmission system lacks system flexibility and becomes an expensive optical transmission system.

  Therefore, the object of the present invention is to suppress significant reception deterioration due to mode conversion even when a bent portion is generated in a multimode optical fiber, and further, without using a precise device such as a condenser lens or a beam expander. It is an object to provide an optical multiplex transmission system that can be easily changed according to the cost and a manufacturing method thereof.

  In order to achieve the above object, the present invention employs the following configuration. Note that reference numerals and the like in parentheses indicate correspondence with embodiments described later to help understanding of the present invention, and do not limit the scope of the present invention.

  The optical multiplex transmission system of the present invention includes a multimode optical fiber (optical fiber 1), a plurality of light emitting units (VCSELs 10a to 10d), and a plurality of light receiving units (light receivers 20a to 20d). The plurality of light emitting units are arranged in series in at least the first direction (Y-axis direction), and each of the emitted light signals (La to Ld) is incident on the incident side end surface (A) of the multimode optical fiber. The plurality of light receiving units are arranged in series in at least the first direction, and respectively receive the plurality of optical signals that propagate through the multimode optical fiber and are emitted from the emission side end face (D) of the multimode optical fiber. The multimode optical fiber is disposed on a plane (XZ plane) based on the first direction.

  The first direction may be perpendicular to the plane on which the multimode optical fiber is disposed. As an example, the plurality of light emitting units are one-dimensionally arranged on a straight line in a first direction intersecting the optical axis on the incident side end face of the multimode optical fiber. In this case, the plurality of light receiving portions are one-dimensionally arranged on a straight line in the first direction intersecting the optical axis at the emission side end face of the multimode optical fiber. As another example, the plurality of light emitting units are two-dimensionally arranged by a first group (VCSELs 10a to 10d) and a second group (VCSELs 10e and 10f). The first set is arranged on a straight line in the first direction intersecting the optical axis at the incident side end face of the multimode optical fiber. The second set is arranged on a straight line in a second direction (X-axis direction) that is perpendicular to the first direction and intersects the optical axis parallel to the incident-side end surface. In this case, the plurality of light receiving units are two-dimensionally arranged by the third group (light receivers 20a to 20d) and the fourth group (light receivers 20e and 20f). The third set is arranged on a straight line in the first direction intersecting the optical axis at the output side end face of the multimode optical fiber, and the optical signals (La to Ld) emitted from the light emitting units belonging to the first set. Are respectively received. The fourth group is arranged on a straight line in the third direction perpendicular to the first direction and intersecting the optical axis parallel to the emission side end face, and emitted from the light emitting unit belonging to the second group. The optical signals (Le, Lf) are received. And the light emission part which belongs to the said 2nd group may be arrange | positioned in the position relatively near with respect to the optical axis in the incident side end surface of a multimode optical fiber.

  The multimode optical fiber may emit a plurality of optical signals incident from the incident-side end face from the emission-side end face while maintaining a unique propagation mode. Specifically, as a first example, the multimode optical fiber is a gradient index optical fiber. In this case, the gradient index optical fiber transmits an optical signal having a specific amplitude (Ymax) in a propagation mode having the same period (Λ). The exit-side end face of the gradient index optical fiber is formed at a position where the amplitude of the propagation mode of the optical signal is maximized (“antinode”). As a second example, the multimode optical fiber is a graded-index polymer optical fiber (GI-POF) in which the core (3) and the cladding (2) are all made of plastic. . As a third example, the multi-mode optical fiber is a multi-layer type plastic optical fiber (ML-POF: Multi-layer Polymer Optical Fiber).

  Specifically, the plurality of light emitting units are composed of a plurality of vertical cavity surface emitting lasers (VCSEL) integrated on the same wafer parallel to the incident side end face of the multimode optical fiber. . The plurality of vertical cavity surface emitting lasers may each be provided with a condensing lens. Further, specifically, the plurality of light receiving units are configured by a plurality of photodiodes integrated on the same wafer parallel to the incident side end face of the multimode optical fiber. Each of the plurality of photodiodes may be provided with a condensing lens.

  In the manufacturing method of the present invention, a plurality of light signals emitted from a plurality of light emitting portions are incident on an incident side end face of a multimode optical fiber, propagated through the multimode optical fiber, and emitted from the emission side end face. This is a method of manufacturing an optical multiplex transmission system in which an optical signal is received by a plurality of light receiving units. As a manufacturing method of the optical multiplex transmission system, two methods can be considered as described below. In the first manufacturing method, first, a plurality of light emitting units are arranged in series in at least a first direction, and a plurality of light receiving units are arranged in series in at least a first direction. Thereafter, the multimode optical fiber is disposed on a plane perpendicular to the first direction, and the plurality of light emitting units and the plurality of light receiving units are optically connected. In the second manufacturing method, first, a multimode optical fiber is disposed on a predetermined plane. Thereafter, the plurality of light emitting units are arranged in series in at least a first direction perpendicular to the plane, and the plurality of light receiving units are arranged in series in at least the first direction.

  Specifically, the multimode optical fiber is a gradient index optical fiber. In this case, the gradient index optical fiber transmits an optical signal having a specific amplitude in a propagation mode having the same period. Before arranging the gradient index optical fiber, the output side end face of the gradient index optical fiber is set to the amplitude of the propagation mode of the optical signal with respect to the incident end face of the gradient index optical fiber. Is formed at a position where is the maximum.

  According to the optical multiplex transmission system of the present invention, it is possible to suppress significant reception deterioration due to mode conversion even if a bent portion is generated in the multimode optical fiber. In addition, when the first direction is perpendicular to the plane on which the multimode optical fiber is disposed, significant reception deterioration due to the mode conversion can be greatly reduced.

  The optical multiplex transmission system can perform optical multiplex transmission using a plurality of light emitting units and light receiving units arranged one-dimensionally in series in the same direction. Further, even in a two-dimensional arrangement in which a plurality of light emitting units and light receiving units are further added in a direction perpendicular to the above direction with respect to the plurality of light emitting units and light receiving units arranged one-dimensionally, optical multiplexing is similarly performed. Transmission can be performed. In this case, by arranging the added light emitting unit in the vicinity of the optical axis, the propagation mode of those optical signals becomes a low-order mode, so that higher-order propagation modes due to bending of the optical fiber can be allowed. .

  When the multimode optical fiber is composed of a refractive index distribution type optical fiber, a change according to the application is facilitated without using a precise device such as a condenser lens or a beam expander. Further, since the optical signal transmitted through the gradient index optical fiber is not easily affected by the core / cladding interface, mode conversion due to non-flatness of the core / cladding interface can be prevented. Furthermore, by forming the exit-side end face of the gradient index optical fiber at a position where the amplitude of the propagation mode of the optical signal is maximized, each propagation mode interval is maximized on the exit-side end face, and the optical signal is received at the light receiving section. Each transmission error is minimized and transmission performance is optimized.

  In addition, the optical multiplex transmission system can be composed of a refractive index distribution type or multi-layer type plastic optical fiber as a multimode optical fiber, a vertical cavity surface emitting laser as a light emitting part, and a photodiode as a light receiving part. It can be easily configured by combining the respective components.

  According to the method of manufacturing an optical multiplex transmission system of the present invention, an operation of incorporating an optical multiplex transmission system inside a device can be easily realized, for example, when incorporating an optical signal into various devices that multiplex-transmit an optical signal. Further, the built-in optical multiplex transmission system can similarly obtain the above-described effects.

  An optical multiplex transmission system according to an embodiment of the present invention will be described with reference to FIGS. 1 is a top view showing the overall configuration of the optical multiplex transmission system, FIG. 2 is a side view of section AB in FIG. 1 viewed from the m direction, and FIG. 3 is a CD in FIG. 4 is a side view of the section viewed from the n direction, FIG. 4 is a view of the incident side end face A of FIG. 1 viewed from the optical signal traveling direction, and FIG. 5 is a sectional view of FIG. FIG. 6 is a cross-sectional view, and FIG. 6 is a view of the emission side end face D of FIG. 1 to 3, the inside of the optical fiber is seen through in order to show the state of the optical signal transmitted through the optical multiplex transmission system.

  1 to 3, the optical multiplex transmission system includes an optical fiber 1, a VCSEL array 10 including VCSELs (Vertical Cavity Surface Emitting Lasers) 10a to 10d, and light receivers 20a to 20d. A light receiver array 20 is provided. The optical fiber 1 is a refractive index distribution type large-diameter multimode optical fiber (MMF) in which a fiber core 3 and a cladding 2 are formed. For example, the core 3 and the cladding 2 are all made of plastic. A refractive index distribution type plastic optical fiber (GI-POF: Graded-index Polymer Optical Fiber).

  Specifically, each of the light receivers 20a to 20d is a photodiode. Each of these photodiodes may be accompanied by a condenser lens, and the photodetector array 20 can be configured by integrating the photodiodes on the same wafer. The wafer on which the photodiodes are integrated is arranged in parallel with the emission side end face D of the optical fiber 1. Further, the VCSELs 10a to 10d can also be integrated on the same wafer to constitute the VCSEL array 10. The wafer on which the VCSELs 10 a to 10 d are integrated is arranged in parallel with the incident side end face A of the optical fiber 1. Further, each of the VCSELs 10a to 10d may be accompanied by a condensing lens.

  In order to make the following description concrete, X, Y, and Z axis directions perpendicular to each other are defined (see the drawings). The VCSELs 10a to 10d included in the VCSEL array 10 are juxtaposed on a straight line in the Y-axis direction, and the straight line intersects the central axis of the core 3 (the optical axis of the optical fiber 1) on the incident side end face A of the optical fiber 1. To be arranged. The light receivers 20 a to 20 d included in the light receiver array 20 are also arranged in parallel on a straight line in the Y-axis direction, and the straight line is the central axis of the core 3 (on the optical fiber 1 of the optical fiber 1) on the output-side end face D of the optical fiber 1. It is arranged so as to intersect the optical axis. And the optical fiber 1 is arrange | positioned on the XZ plane perpendicular | vertical with respect to the Y-axis direction in which VCSEL10a-10d and light receiver 20a-20d are arranged in parallel.

  The shape of the optical fiber 1 in the XZ plane will be described in detail. The optical fiber 1 includes a linear part AB section from the incident side end face A to the point B, a bent part BC section from the point B to the point C, and a linear part from the point C to the exit side end face D. Each of which is disposed on the XZ plane. The bending direction of the optical fiber 1 is such that the central axis of the core 3 (the optical axis of the optical fiber 1) at the incident side end face A is the Z axis and the central axis of the core 3 at the outgoing side end face D is the X axis. That is, description will be made using an example in which the optical fiber 1 is bent by 90 ° parallel to the XZ plane.

  In FIG. 2, VCSELs 10 a to 10 d enter optical signals La to Ld carrying individual information from the incident side end face A of the optical fiber 1 to the core 3. As shown in FIG. 4, the points at which the respective optical signals La to Ld are incident on the incident side end surface A are the same straight line passing through the optical axis of the optical fiber 1 (that is, parallel to the Y-axis direction and the optical fiber 1. The position on the straight line passing through the optical axis. In this case, a complicated optical system for strict angling as described in the background art is unnecessary between the VCSEL array 10 and the optical fiber 1. For example, the optical signals La to Ld may be directly incident on the incident side end surface A from the VCSELs 10a to 10d, respectively, or may be incident via an optical system device of a simple ball lens.

  As shown in FIG. 2, the optical signals La to Ld incident on the optical fiber 1 propagate through the cores 3 in respective propagation modes in the linear portion AB. Here, since the optical multiplex transmission system of the background art described above (see FIG. 14) simply uses a large-diameter POF (SI-POF), each propagation mode has a unique propagation angle θa to θd. The angles θa to θd are always maintained with respect to the optical axis (the central axis of the core 102 in the optical fiber 100). However, since the optical multiplex transmission system of the present invention uses the refractive index distribution type optical fiber 1, each propagation mode does not have a specific propagation angle, and as they propagate in the Z-axis direction in the figure, The angle of changes periodically. Further, each propagation mode of the present invention has the same propagation period Λ, and each propagation mode can be identified by each maximum amplitude Ymax. In the example of FIG. 2, since the optical signals La and Ld are incident on the outer side of the optical axis of the optical fiber 1 than the optical signals Lb and Lc, the optical signals La and Ld are propagated relatively outside the optical axis and have a maximum amplitude. Ymax increases. That is, the propagation modes of the optical signals La and Ld are higher order modes than the propagation modes of the optical signals Lb and Lc.

  Here, as shown in FIG. 7, the optical signals La to Ld incident on the gradient index optical fiber 1 are not maintained in the propagation mode in the bent portion B-C section, and are converted into different maximum amplitudes Ymax. May end up. FIG. 7 shows a state where the propagation mode of the gradient index optical fiber 1 is converted by the bent portion.

  In FIG. 7, the bending direction of the optical fiber 1 shown in FIG. 1 (that is, bending on the XZ plane perpendicular to the Y-axis direction in which the VCSELs 10a to 10d and the light receivers 20a to 20d are arranged side by side) Differently, a bent portion BE section where the optical fiber 1 is bent in the Y-axis direction and a linear portion EF section continuous therewith are formed. In this case, as shown in FIG. 7, an optical signal Ly (for example, an optical signal Lb having a small maximum amplitude Ymax) that is incident in a relatively low-order mode is mode-converted in the bent portion BE and the optical signal Ly The maximum amplitude Ymax is expanded and a higher mode is set. Therefore, in an optical multiplex transmission system in which a plurality of propagation modes are propagated by a single gradient index POF, the optical fiber is bent regardless of whether it is SI-POF or gradient index POF. Is considered extremely difficult.

  However, in the optical multiplex transmission system of the present invention, the four VCSELs 10a to 10d of the VCSEL array 10 are arranged in series (one-dimensional) in the Y-axis direction (see FIGS. 2 and 4). If the propagation distance is short in the linear portion A-B section of the gradient index optical fiber 1, the optical signal La is maintained substantially spatially (in series in the Y-axis direction). ~ Ld can be transmitted (see FIGS. 1 and 2). FIG. 5 shows an example of the propagation mode states of the optical signals La to Ld held in series in the Y-axis direction at the point B. As described above, when a bent portion is formed in the gradient index optical fiber 1, reception becomes difficult if mode conversion occurs. Therefore, in the present invention, the optical fiber 1 in which the optical signals La to Ld are one-dimensionally arranged in the Y-axis direction is not bent in the Y-axis direction and only in the X-axis direction in the cross-sectional view at point B shown in FIG. It is bent and arranged on the XZ plane perpendicular to the Y axis (see FIG. 1). As a result, the optical axis of the optical fiber 1 is always perpendicular to the amplitude direction (Y-axis direction) of each propagation mode, and the influence of mode conversion caused by bending the optical fiber 1 can be minimized.

  Next, as shown in FIG. 1, the optical signals La to Ld propagated through the bent portion B-C section of the optical fiber 1 reach the linear portion CD section without mode conversion. As shown in FIG. 3, if the propagation distance is short in the linear portion CD section of the gradient index optical fiber 1, the propagation modes of the optical signals La to Ld are held almost spatially. The light is transmitted as it is (in series in the Y-axis direction) and emitted from the emission-side end face D. FIG. 6 shows an example of a state in which the optical signals La to Ld held in series in the Y-axis direction are emitted from the emission side end face D, respectively.

  As shown in FIGS. 1 and 3, the optical signals La to Ld emitted from the emission side end face D of the optical fiber 1 are received by the light receiver array 20. As shown in FIG. 6, if each propagation mode array of the optical signals La to Ld is one-dimensionally arrayed in the Y axis direction, the four light receivers 20a to 20d included in the light receiver array 20 are also in the Y axis direction. Are arranged one-dimensionally. In this case, a complicated optical system for strict angling such as a beam expander as described in the background art is not required between the optical receiver array 20 and the optical fiber 1. For example, the optical signals La to Ld from the emission side end face D may be directly incident on the light receivers 20a to 20d, respectively, or may be incident via an optical system device such as a simple ball lens.

  Next, the fiber length L of the optical fiber 1 for realizing the optical multiplex transmission system of the present invention will be described. As described above, the propagation period Λ of each propagation mode propagating through the optical fiber 1 is the same (see, for example, FIG. 2). That is, a position indicating the maximum amplitude Ymax (hereinafter referred to as “antinode portion”) and a position indicating the minimum amplitude Ymin (hereinafter referred to as “node portion”) in all of the propagation periods Λ are all. Are equal in the propagation mode. Therefore, in order to detect the optical signals La to Ld accurately and without error in the light receiver array 20, it is preferable that the state of each propagation mode on the emission-side end face D of the optical fiber 1 is “antinode”. In addition, in order to make each propagation mode arrangement | sequence in the output side end surface D of the optical fiber 1 match with a "belly part", the state of the optical signals La-Ld directly emitted from the output side end surface D of the optical fiber 1 is confirmed. However, it is also possible to design the fiber length L of the optical fiber 1 by calculation.

  Hereinafter, with reference to FIG. 8, in the gradient index optical fiber 1, the state of each propagation mode with respect to the fiber length L is calculated. FIG. 8 is a graph showing the state of each propagation mode with the horizontal axis representing the fiber length L (au: arbitrary unit) and the vertical axis representing the distance Y (μm) from the optical axis of the optical fiber 1. is there.

  In FIG. 8, each series in the graph represents each propagation mode of the optical fiber 1 and is classified by the maximum amplitude Ymax. For example, the propagation mode showing the maximum amplitude Ymax = 100 μm is indicated by a solid line, and the propagation mode is separated from the optical axis of the optical fiber 1 by Y = 100 μm at the fiber length L = 0.0 (that is, the incident side end face A). The locus of the optical signal when the light is incident in parallel with the optical axis. In each propagation mode, an “antinode” showing the maximum amplitude appears with the fiber length L = 1.2, 2.4, 3.6, 4.8. Then, with respect to the value of the distance Y at the fiber length L = 0.0, the sign is opposite at L = 1.2 and 3.6. That is, when the distance Y coincides with the Y-axis direction described above, the propagation mode arrangement is Y for the fiber lengths L = 1.2 and 3.6 with respect to the propagation mode arrangement (order) on the incident side end face A. Invert on the axis. Considering these things, if the fiber length L of the optical fiber 1 is made to coincide with the “antinode portion”, each propagation mode interval is maximized at the emission side end face D, and the optical signal La˜ The error in receiving Ld is minimized and the transmission performance is optimized.

  As described above, according to the optical multiplex transmission system of the present invention, the four optical signals La to Ld emitted from the VCSEL array 10 are accurately transmitted through the single gradient index optical fiber 1 having the bent portion. The light receiver array 20 can receive the light. In addition, the optical multiplex transmission system can further increase the number of optical signals multiplexed on the optical fiber 1. Since the optical multiplex transmission system of the background art uses a large-diameter POF, if the number of optical signals to be multiplexed is increased, the propagation mode arrangement on the fiber exit side end surface changes, and the design of the beam expander has to be changed. However, in the optical multiplex transmission system of the present invention, even if the number of multiplexed optical signals is increased, the “belly part” with respect to the fiber length L is equal, so the number of VCSELs included in the VCSEL array 10 and the receiver array are included. Changes other than the number of receivers are not required. Therefore, the optical multiplex transmission system of the present invention can realize a flexible system according to the application.

  The optical multiplex transmission system of the present invention may be incorporated into various devices (for example, digital cameras) that multiplex-transmit optical signals therein. The operation of incorporating the optical multiplex transmission system in such a device can be easily realized. For example, a VCSEL array in which a plurality of VCSELs are provided in series (one-dimensional) in a predetermined direction, and a light receiver array in which a plurality of light receivers are provided in series in the same predetermined direction, It is assumed that each is fixed at a location and at the same height. In this case, the refractive index distribution type optical fiber having the fiber length L calculated as described above is used to optically connect the plurality of VCSELs and the optical receiver, and the optical fiber is connected to the predetermined direction. If the optical multiplex transmission system is arranged on a vertical plane, the optical multiplexing transmission system can be incorporated. Further, it is assumed that the optical fiber is disposed on a predetermined plane inside the device. In this case, a VCSEL array including a plurality of VCSELs provided in series in a direction perpendicular to the plane and a receiver array including a plurality of light receivers provided in series in the same direction are arranged on both end surfaces of the optical fiber. In the same manner, the optical multiplex transmission system can be incorporated.

  In the above description, the bending direction of the optical fiber 1 is the center axis of the core 3 on the incident side end face A (the optical axis of the optical fiber 1) as the Z axis, and the central axis of the core 3 on the output side end face D is the X axis. In other words, the optical fiber 1 is bent by 90 ° parallel to the XZ plane. However, the angle at which the optical fiber 1 is bent may not be 90 °. When the propagation mode arrays on the incident side end face A of the optical fiber 1 are arranged in series (one-dimensional) in the Y-axis direction, the bending direction of the optical fiber 1 is maintained on a plane perpendicular to the Y-axis direction. For example, regardless of the angle at which the optical fiber 1 is bent, the optical signal La to Ld can be received by the optical receiver array 20 similarly.

  In the above description, the case where the propagation mode arrays at the point B are arranged in series (one-dimensional) in the Y-axis direction (see FIG. 5) is used. However, even if each propagation mode is slightly shifted from the Y-axis direction, if the bending direction of the optical fiber 1 is maintained on a plane perpendicular to the Y-axis direction, the effect of mode conversion can be reduced. Can do. In the system in which the path of the optical fiber 1 is determined according to the request of the application, it is possible to arrange the VCSEL array so as to suppress the influence of mode conversion from the bending direction.

  In the above description, the case where the propagation mode arrays at the point B are arranged in series (one-dimensional) in the Y-axis direction is used. However, the light of the present invention can be used even if each propagation mode array is two-dimensional. The optical fiber 1 of the multiplex transmission system can have a bent portion. An example of each propagation mode array in the two-dimensional case will be described with reference to FIGS. 9 and 10. 9 shows an example of the propagation mode states of the optical signals La to Lf incident on the incident side end surface A, and FIG. 10 shows an example of the propagation mode states of the optical signals La to Lf on the emission side end surface D. Yes.

  In FIG. 9, six optical signals La to Lf are incident on the incident side end face A of the optical fiber 1. The optical signals La to Ld are incident on the incident side end face A of the optical fiber 1 from the VCSELs 10a to 10d (see FIGS. 1 and 2) arranged in the same manner as described above. That is, the point at which each of the optical signals La to Ld enters the incident side end surface A is on a straight line that is parallel to the Y-axis direction and passes through the optical axis of the optical fiber 1. On the other hand, the optical signals Le and Lf are incident on the incident side end face A of the optical fiber 1 from the VCSELs 10e and 10f (not shown). The VCSELs 10e and 10f are arranged perpendicularly to the Y-axis direction (X-axis direction) in which the VCSELs 10a to 10d are arranged in series and at a predetermined interval on a straight line passing through the optical axis of the optical fiber 1. -10f are two-dimensionally arranged. Therefore, the points at which the respective optical signals Le and Lf are incident on the incident-side end face A are positions parallel to the X-axis direction and spaced by a predetermined distance on a straight line passing through the optical axis of the optical fiber 1. The interval between the points where the optical signals Le and Lf are incident on the incident side end face A is set so that the order of each propagation mode (maximum amplitude Xmax) becomes relatively small (that is, the optical fiber 1 of the optical fiber 1). It is arranged at a position relatively close to the optical axis).

  Then, the optical signals La to Lf incident on the incident side end face A are multiplexed and transmitted through the linear portion AB section (see FIG. 1) from the incident side end face A to the point B of the optical fiber 1 and converted into modes. Without reaching B point. Here, like the optical signals La to Ld, the propagation modes of the optical signals Le and Lf also do not have a specific propagation angle, and their angles change periodically as they propagate in the Z-axis direction. . The propagation modes of the optical signals Le and Lf have the same propagation period as the above-described propagation period Λ, and each propagation mode can be identified by the maximum amplitude Xmax. Further, the point at which the optical signals Le and Lf are incident on the incident side end face A is a position where the order (maximum amplitude Xmax) of each propagation mode is relatively small, and therefore the propagation modes of the optical signals Le and Lf. Is a lower mode than the propagation modes of the optical signals La and Ld.

  Next, the optical signals La to Lf are multiplexed and transmitted through the bent section B-C section (see FIG. 1) of the optical fiber 1 to the point C. Here, in the bent section B-C section of the optical fiber 1, the optical fiber 1 in which the optical signals La to Ld are one-dimensionally arranged in the Y-axis direction is bent only in the X-axis direction without bending in the Y-axis direction at all. Are arranged on an XZ plane perpendicular to the Y axis. Therefore, as described above, the optical signals La to Ld reach the point C without being subjected to mode conversion. On the other hand, the optical signals Le and Lf are affected by mode conversion because the optical fiber 1 is bent in the amplitude direction (X-axis direction) of each propagation mode. However, since the propagation modes of the optical signals Le and Lf are low-order modes, it is possible to allow higher-order propagation modes due to bending of the optical fiber 1. On the contrary, when the propagation modes of the optical signals Le and Lf are already higher order modes, they become higher order at the bent portion of the optical fiber 1 and are repelled out of the optical fiber 1 so that the optical signals Le and Lf cannot be transmitted. It is possible.

  And the optical signals La-Lf which propagated the bending part BC section of the optical fiber 1 arrive at the linear part CD section (refer FIG. 1). If the propagation distance is short in the linear portion CD section of the gradient index optical fiber 1, the optical signals La to Lf are transmitted while each propagation mode at the point C is held substantially spatially. The light is emitted from the emission side end face D. FIG. 10 shows an example of a state in which the optical signals La to Lf are emitted from the emission side end face D, respectively. The state in which the optical signals La to Ld are respectively emitted from the emission side end face D is the same as the state described with reference to FIG. 6, and four light receivers 20a to 20d arranged one-dimensionally in the Y-axis direction (FIG. 1 and FIG. (See FIG. 3). On the other hand, since the optical signals Le and Lf are converted into a higher-order propagation mode by bending of the optical fiber 1, the higher-order propagation mode is maintained even in transmission in the linear portion CD section. The light is emitted from the emission side end face D. Accordingly, the light receivers 20e and 20f (not shown) that receive the optical signals Le and Lf are provided in the light receiver array 20 in accordance with the optical signals Le and Lf emitted from the emission side end face D, respectively. Thus, even if the mode array is two-dimensional, according to the optical multiplex transmission system of the present invention, the six optical signals La to Lf emitted from the VCSEL array 10 are converted into one refractive index having a bent portion. Light can be accurately received by the light receiver array 20 via the distributed optical fiber 1.

  As described above, according to the optical multiplex transmission system of the present invention, even if a bent portion is generated in the multimode optical fiber, it is possible to suppress a significant reception deterioration due to mode conversion, and to perform a precise operation such as a condensing lens or a beam expander. An inexpensive optical multiplex transmission system that can be easily changed according to the application without using an optical device can be realized. In addition, since the optical signal transmitted through the optical fiber is hardly affected by the core / cladding interface of the optical fiber, mode conversion due to non-flatness of the core / cladding interface can be prevented.

  In the above description, the refractive index distribution type optical fiber is used in the optical multiplex transmission system of the present invention, but other optical fibers may be used. For example, a multilayer optical fiber may be used in the optical multiplex transmission system of the present invention. The present invention can also be realized in the same manner by using a multi-layer polymer optical fiber (ML-POF).

  An optical multiplex transmission system and a manufacturing method thereof according to the present invention suppress significant reception deterioration due to mode conversion even when a bent portion occurs in a multi-mode optical fiber, and perform optical multiplex transmission by optically connecting the transmission and reception units with an optical fiber. Useful for.

1 is a top view showing an overall configuration of an optical multiplex transmission system according to an embodiment of the present invention. Side view of section AB in FIG. 1 viewed from the m direction The side view which looked at the CD section of FIG. 1 from the n direction FIG. 1 is a view of the incident side end surface A of FIG. Sectional view of section B of FIG. The figure which looked at the output side end surface D of FIG. 1 from the optical signal advancing direction The figure showing a mode that the propagation mode of the refractive index distribution type optical fiber 1 is converted by the bending part. Graph showing the state of each propagation mode with the horizontal axis representing the fiber length L and the vertical axis representing the distance Y from the optical axis of the optical fiber 1 The figure which shows an example of the propagation mode state of each optical signal La-Lf which injects into the incident side end surface A of FIG. The figure which shows an example of each propagation mode state of optical signal La-Lf in the output side end surface D of FIG. The figure which showed schematically the difference in the propagation mode in the conventional MMF The figure which shows schematic structure of the conventional optical multiplex transmission system FIG. 12 is a diagram for explaining the propagation mode when the large-diameter POF 100 of the optical multiplex transmission system has a bent portion. The figure which shows schematic structure of the conventional optical multiplexing transmission system which can transmit four propagation modes

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 ... Optical fiber 2 ... Cladding 3 ... Core 10 ... VCSEL array 10a-10f ... VCSEL
20 ... Photoreceiver arrays 20a to 20f ... Photoreceiver

Claims (18)

A multimode optical fiber;
A plurality of light emitting units arranged in series in at least the first direction, each of which emits an outgoing optical signal to the incident side end face of the multimode optical fiber;
A plurality of light receiving units arranged in series in at least the first direction, each receiving a plurality of optical signals propagating through the multimode optical fiber and emitted from the output side end surface of the multimode optical fiber,
The optical multiplex transmission system, wherein the multimode optical fiber is disposed on a plane based on the first direction.   2. The optical multiplex transmission system according to claim 1, wherein the first direction is perpendicular to a plane on which the multimode optical fiber is disposed. The plurality of light emitting units are one-dimensionally arranged on a straight line in the first direction intersecting an optical axis at an incident side end face of the multimode optical fiber,
3. The optical multiplexing according to claim 2, wherein the plurality of light receiving units are one-dimensionally arranged on a straight line in the first direction intersecting an optical axis at an output side end face of the multimode optical fiber. Transmission system. The plurality of light emitting units are:
A first set arranged on a straight line in the first direction intersecting the optical axis at the incident side end face of the multimode optical fiber;
Two-dimensionally arranged by a second set arranged on a straight line in a second direction intersecting the optical axis perpendicular to the first direction and parallel to the incident side end face;
The plurality of light receiving units are
A third set that is arranged on a straight line in the first direction intersecting the optical axis at the output-side end face of the multimode optical fiber and that receives the optical signals emitted from the light emitting units belonging to the first set, respectively. When,
An optical signal emitted from a light emitting unit belonging to the second set, arranged on a straight line in a third direction intersecting the optical axis perpendicular to the first direction and parallel to the emission side end face. The optical multiplex transmission system according to claim 2, wherein the optical multiplex transmission system is two-dimensionally arranged by a fourth set that receives light.   5. The optical multiplex transmission according to claim 4, wherein the light emitting units belonging to the second set are arranged at a position relatively close to an optical axis on an incident side end face of the multimode optical fiber. system.   2. The optical multiplexing according to claim 1, wherein the multi-mode optical fiber emits a plurality of optical signals incident from the incident side end face from the emission side end face while maintaining a specific propagation mode. Transmission system.   2. The optical multiplex transmission system according to claim 1, wherein the multi-mode optical fiber is a gradient index optical fiber. The refractive index distribution type optical fibers each transmit an optical signal having a specific amplitude in a propagation mode of the same period,
8. The optical multiplex transmission system according to claim 7, wherein an output-side end face of the gradient index optical fiber is formed at a position where the amplitude of the propagation mode of the optical signal is maximized.   The multimode optical fiber is a graded-index polymer optical fiber (GI-POF: Graded-index Polymer Optical Fiber) whose core and cladding are all made of plastic. Optical multiplex transmission system.   2. The optical multiplex transmission system according to claim 1, wherein the multi-mode optical fiber is a multi-layer polymer optical fiber (ML-POF). 3.   The plurality of light emitting units are configured by a plurality of vertical cavity surface emitting lasers (VCSELs) integrated on the same wafer parallel to the incident side end face of the multimode optical fiber. The optical multiplex transmission system according to claim 1.   12. The optical multiplex transmission system according to claim 11, wherein each of the plurality of vertical cavity surface emitting lasers is provided with a condensing lens.   2. The optical multiplex transmission system according to claim 1, wherein the plurality of light receiving units include a plurality of photodiodes integrated on the same wafer parallel to an incident side end face of the multimode optical fiber. .   The optical multiplex transmission system according to claim 13, wherein each of the plurality of photodiodes is provided with a condensing lens. Each optical signal emitted from a plurality of light emitting units is incident on the incident side end face of the multimode optical fiber, and propagates through the multimode optical fiber and receives a plurality of optical signals emitted from the emission side end face. A method of manufacturing an optical multiplex transmission system that receives light at a portion,
The plurality of light emitting units are arranged in series in at least the first direction,
After arranging the plurality of light receiving units in series in at least the first direction,
A method of manufacturing an optical multiplex transmission system, wherein the multimode optical fiber is disposed on a plane perpendicular to the first direction, and the plurality of light emitting units and the plurality of light receiving units are optically connected. A plurality of optical signals emitted from a plurality of light emitting sections are incident on an incident side end face of the multimode optical fiber, and a plurality of optical signals propagated through the multimode optical fiber and emitted from the emission side end face are respectively received. A method of manufacturing an optical multiplex transmission system including a light receiving unit of
After arranging the multimode optical fiber on a predetermined plane,
The plurality of light emitting units are arranged in series in a first direction perpendicular to at least the plane,
A method of manufacturing an optical multiplex transmission system, wherein the plurality of light receiving units are arranged in series in at least the first direction.   The method of manufacturing an optical multiplex transmission system according to claim 15 or 16, wherein the multimode optical fiber is a gradient index optical fiber. The refractive index distribution type optical fibers each transmit an optical signal having a specific amplitude in a propagation mode of the same period,
Before the distribution of the refractive index distribution type optical fiber, the output side end face of the refractive index distribution type optical fiber is set to the propagation mode of the optical signal with respect to the incident side end face of the refractive index distribution type optical fiber. The method of manufacturing an optical multiplex transmission system according to claim 17, wherein the optical multiplex transmission system is formed at a position where the amplitude is maximized.

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