JP2008076830A - Optical fiber module - Google Patents

Abstract

Propagation loss is reduced in an optical fiber module including a first multimode optical fiber having a tapered portion and a second multimode optical fiber joined thereto.
A first multimode optical fiber 11 having a tapered portion 11f partially following an output end 11c, and a second multimode optical fiber 12 joined to the output end 11c of the first multimode optical fiber 11. In the optical fiber module, the numerical apertures and core diameters of the first multimode optical fiber 11 other than the taper portion are NA ₁ and d ₁ , respectively, and the core diameter ratio (minimum core diameter / maximum core diameter) of the taper portion is α. and then, the numerical aperture of the second multi-mode optical fiber 12, respectively and NA _2, d ₂ the core diameter, when the diameter on the first multimode optical fiber entrance end 11d of the incident light 16 was d _in, d ₂ ≧ α × d ₁ and the divergence angle of incident light 16 is θ _in , and the conditions of d _in × tan θ _in ≦ d ₁ × NA ₁ , tanθ _in ≦ α × NA ₁ , tanθ _in ≦ α × NA ₂ Satisfy.
[Selection] Figure 1

Description

  The present invention relates to an optical fiber module in which another multimode optical fiber is joined to a multimode optical fiber having a tapered portion.

  Conventionally, as shown in, for example, Patent Document 1, the light emitting ends of a plurality of first optical fibers are coupled to the light incident ends of one second optical fiber, and light respectively incident on the plurality of first optical fibers is first 2. Description of the Related Art An optical fiber module that multiplexes two optical fibers is known. According to such an optical fiber module, combined high-output light can be obtained.

  Note that multimode optical fibers are generally used in optical fiber modules for the purpose of multiplexing. The first multimode optical fiber and the second multimode optical fiber are usually bonded to each other with the cores fused together.

Further, in this type of optical fiber module, as shown in Patent Document 1, it is considered that a part following the emission end of the first multimode optical fiber is a tapered portion. By doing so, it becomes possible to join a larger number of first multimode optical fibers to one second multimode optical fiber.
JP 2004-325957 A

  However, in the optical fiber module formed by joining the first multimode optical fiber having the tapered portion as described above to the second multimode optical fiber, light leaks from the optical fiber after the tapered portion and the joined portion thereof. There is a problem that the light propagation efficiency is reduced.

  The present invention has been made in view of the above circumstances, and in an optical fiber module formed by joining a first multimode optical fiber having a tapered portion to a second multimode optical fiber, light leakage is prevented. The purpose is to obtain high propagation efficiency.

An optical fiber module according to the present invention includes:
A first multimode optical fiber in which a part following the emission end is a tapered portion;
The first multimode optical fiber and the core are coupled to each other, and the incident end includes a second multimode optical fiber joined to the output end of the first multimode optical fiber.
In an optical fiber module used by coupling incident light collected to a diameter smaller than the core diameter of the optical fiber to the incident end of the first multimode optical fiber,
The numerical aperture and core diameter other than the tapered portion of the first multimode optical fiber are NA ₁ and d ₁ , respectively, and the core diameter ratio (minimum core diameter / maximum core diameter) of the tapered portion is α, and the second multimode optical fiber. When the numerical aperture and the core diameter are NA ₂ and d ₂ respectively, and the diameter of the incident light on the first multimode optical fiber entrance end is d _in ,
d ₂ ≧ α × d ₁ and
The incident light divergence angle is θ _in , and d _in × tan θ _in ≦ d ₁ × NA ₁ , tan θ _in ≦ α × NA ₁ , tanθ _in ≦ α × NA ₂ are satisfied. Is.

  The above-mentioned “divergence angle” should be called “convergence angle” from the viewpoint of entering the first multimode optical fiber in a state where the incident light is collected, but the light propagating in the optical fiber. In consideration of the correspondence with the divergence angle, it is referred to as an “expansion angle” for easy understanding.

  The optical fiber module according to the present invention is particularly preferably configured in a form in which a plurality of first multimode optical fibers are provided and bonded to one second multimode optical fiber.

  Further, in the optical fiber module of the present invention, when the divergence angles of incident light are different in two directions orthogonal to each other, for example, when a semiconductor laser is used as an incident light source, before entering the first multimode optical fiber. It is desirable to provide means for reducing the NA of the incident light only in the direction of a large divergence angle.

  On the other hand, the optical fiber module according to the present invention is preferably configured on the assumption that the wavelength of incident light is in the range of 160 nm to 500 nm.

  Furthermore, in the optical fiber module according to the present invention, the first multimode optical fiber and / or the second multimode optical fiber is preferably a quartz optical fiber having a cladding doped with F.

  As a result of examining the cause of the light leakage described above, the present inventor has found that the light divergence angle increases when light propagates through the tapered portion of the first multimode optical fiber, which is larger than the NA of the optical fiber. From the fact that it was getting larger, we found out that light leaked. In other words, the NA of the optical fiber having a tapered portion is effectively lowered, which causes the generation of leakage light.

The above will be described in more detail below. When light having a divergence angle θ _in is incident on a multimode optical fiber with a tapered portion whose core diameter at the incident end is d ₁ and whose core diameter at the output end is α × d ₁ (α is the above-mentioned core diameter ratio), The tangent of the divergence angle of the light emitted from the optical fiber is tanθ _in / a. In addition, when the core diameter at the output end is reduced by taper, the numerical aperture NA _tf of the optical fiber is lower than the numerical aperture NA ₁ when the taper is not tapered because the NA of the light is enlarged at the tapered portion. ,
NA _tf = a × NA ₁
It becomes. For this reason, if the divergence angle of the incident light is determined in accordance with the numerical aperture NA ₁ when the taper is not tapered, a loss occurs in the tapered portion. In order to prevent loss, it is conceivable to reduce the taper ratio, that is, to increase the core diameter ratio α. However, by doing so, the core diameter of the second multimode optical fiber inevitably increases. This causes a problem that high brightness light cannot be obtained.

Furthermore, the present inventor has found that when light is incident at a divergence angle θ _in (tan θ _in ≦ NA) smaller than the numerical aperture NA of the optical fiber, the light propagates through the multimode optical fiber immediately. not spread up to the numerical aperture, divergence angle theta _in the time of incidence has been found to be retained. Accordingly, the tangent tan θ _in of the divergence angle of the light incident on the first multimode optical fiber is made sufficiently smaller than the numerical aperture of the optical fiber, specifically, the numerical aperture NA _tf or less of the optical fiber which is lowered by the taper. Thus, it can be seen that it is possible to prevent the occurrence of a large loss by making light incident. Based on this knowledge, in the optical fiber module of the present invention, by setting tan θ _in ≦ NA _tf = α × NA ₂ , it is possible to prevent light from leaking out by the tapered portion and realize high propagation efficiency. Yes.

The condition of d ₂ ≧ α × d ₁ in the optical fiber module of the present invention is that the diameter of the light incident on the second multimode optical fiber is made smaller than the core diameter of the optical fiber. This is a condition that prevents light from leaking out of the core when incident.

The condition of d _in × tan θ _in ≦ d ₁ × NA ₁ is a condition for coupling incident light with the core of the first multimode optical fiber.

The condition of tan θ _in ≦ α × NA ₂ is a condition for preventing light from leaking out in the coupling between the first multimode optical fiber and the second multimode optical fiber. That is, the incident light becomes larger at the exit end of the first multimode optical fiber, and the divergence angle becomes larger as tanθ _in / α (converted to the divergence angle in the air). The divergence angle is equal to the divergence angle of the light incident on the incident end of the second multimode optical fiber. In order to prevent light from leaking through the second multimode optical fiber, tanθ _in / α ≦ NA _2, that is, tanθ _in ≦ α × NA ₂

  As a light source constituting the light input system, a semiconductor laser is useful because it can be obtained at low cost. However, when a semiconductor laser is used, since the divergence angle of incident light is different in two directions orthogonal to each other, the optical system constituting the optical input system has an unnecessarily large magnification according to the direction in which the divergence angle is large. A lens must be used. If so, the minimum value of the core diameter of the first multimode optical fiber is determined according to the lens magnification, and it is difficult to increase the luminance.

  Therefore, in such a case, if a means for reducing the NA of incident light before entering the first multimode optical fiber only in the direction with a large divergence angle is provided, the beam cross-sectional shape of the incident light becomes a circle. By making them closer, the NA can be reduced even with the same beam diameter, and the above-described problems can be dealt with.

  On the other hand, in order to prevent contamination of the incident end of the first multimode optical fiber, it is conceivable to dispose a dielectric block so as to be in optical contact with the incident end. In that case, if the wavelength of the incident light is in the range of about 160 nm to 500 nm, particularly when the beam diameter of the incident light is small, the optical fiber incident end and the dielectric block may be adhered by fusion or the like. In this case, the positions of the two members move due to the attachment and detachment of the optical fiber, vibration, and the like, causing a problem that the attached portion is peeled off and the coupling efficiency to the optical fiber is significantly reduced.

  In such a case, the above-mentioned sticking can be suppressed by lowering the luminance of the incident light at the optical fiber incident end face. In such a case, generally, the luminance of the output light tends to be low. However, in the optical fiber module of the present invention, since leakage of light can be prevented as described above, output light having relatively high luminance can be obtained even when the luminance of incident light is relatively low. Even in the range of about 500 nm to 500 nm, the brightness of the output light can be increased while the brightness of the incident light is lowered to prevent the sticking problem.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.

[First Embodiment]
FIG. 1 shows a schematic side shape of an optical fiber module according to a first embodiment of the present invention. This optical fiber module includes a first multimode optical fiber 11 comprising a core 11a and a clad 11b formed around the core 11a with a lower refractive index, a core 12a, and a lower refractive index. The second multimode optical fiber 12 made of the clad 12b formed around the core 12a and the light input system 13 for coupling the incident light 16 to the first multimode optical fiber 11 are configured.

  The light input system 13 includes a semiconductor laser 14 as an incident light source and a condenser lens 15 that condenses incident light 16 emitted from the semiconductor laser 14 in a divergent light state.

  The first multimode optical fiber 11 has a tapered portion 11f that is partially tapered after the emission end 11c, and is arranged so that the incident light 11 is irradiated to the incident end 11d. ing. On the other hand, the second multimode optical fiber 12 has a constant core diameter, and the first multimode optical fiber 11 and the core are coupled to each other, and the incident end 12d thereof is the emission end of the first multimode optical fiber 11. It is joined to 11c by fusion or the like.

The numerical aperture NA ₁ and the core diameter d ₁ of the first multimode optical fiber 11 other than the tapered portion 11f are NA ₁ = 0.18 and d ₁ = 33 μm, respectively. The core diameter at the emission end 11c is 25 μm, and therefore the core diameter ratio (minimum core diameter / maximum core diameter) α = 0.76 of the tapered portion 11f. On the other hand, the numerical aperture NA ₂ and the core diameter d ₂ of the second multimode optical fiber 12 are NA ₂ = 0.22 and d ₂ = 60 μm, respectively.

The operation of this optical fiber module will be described below. Incident light 16 emitted from the semiconductor laser 14 is condensed by the condenser lens 15 into a state having a predetermined diameter d _in on the incident end 11d of the first multimode optical fiber 11 (specifically, the incident end of the core 11a). From there, it enters the core 11 and propagates. The propagated incident light 16 exits from the exit end 11c of the first multimode optical fiber 11 (specifically, the exit end of the core 11a), and enters the core 12a from the entrance end 12d of the second multimode optical fiber 12. The incident light 16 propagates through the core 12a and exits from the exit end 12c, and is used for a predetermined application.

Here, since d ₁ = 33 μm, d ₂ = 60 μm, and α = 0.76 as described above, the above-described relationship of d ₂ ≧ α × d ₁ is satisfied.

  Further, as shown in FIG. 2, the optical axis of the optical input system 13 is tilted at an angle θ with respect to the core axis of the first multimode optical fiber 11, and the first multimode light is changed stepwise. The divergence angle of the light propagating through the fiber 11 was increased effectively, and the output of the light 16 emitted from the second multimode optical fiber 12 at that time was measured by the photodiode 20.

At this time, obtains the ratio of the outgoing light output to the optical output of the incident light 16 is incident on the first multimode optical fiber 11 as a propagation efficiency, also obtains a tangent tan .theta _in the effective spread angle theta _in the incident light 16 FIG. 3 shows a plot of the relationship between these two points. As shown here, _in the region where the tangent of incident light divergence angle tanθ _in is 0.13 or less, almost no loss is observed, but when it exceeds this, the loss increases, and tanθ _in = 0.18 and the propagation efficiency is up to 66% descend. This, NA of the incident light is increased by the tapered portion 11f, is for more than NA ₁ = 0.18 of the first multimode optical fiber 11. The tangent tan θ _in of the incident light divergence angle at which the loss occurs substantially coincides with a value (0.18 × 0.76 = 0.137) obtained by multiplying NA ₁ of the first multimode optical fiber 11 by the core diameter ratio α.

Using other first multimode optical fibers having core diameter ratio α values of 0.6, 0.44, and 0.42, respectively, the tangent tan θ _in of the incident light divergence angle at which the propagation efficiency rapidly decreases is examined. Thus, it was found that the core diameter ratio α substantially coincides with α × NA ₁ determined by NA ₁ of the optical fiber. From this, it can be seen that high propagation efficiency can be obtained if the above-described condition of tan θ _in ≦ α × NA ₁ is satisfied.

In the present embodiment, it is assumed that the optical fiber module is used under the condition that tan θ _in is 0.13 or less from the result when the angle θ is changed stepwise as described above. In this case, since α = 0.76 and NA ₂ = 0.22, α × NA ₂ = 0.17, so that tan θ _in ≦ 0.13 <0.17 = α × NA ₂ , and the above-described condition of tan θ _in ≦ α × NA ₂ is satisfied, Propagation loss is hardly observed.

In this embodiment, d _in = 28 μm. Since tanθ _in ≦ 0.13, d ₁ = 33 μm, and NA ₁ = 0.18 as described above, d _in × tan θ _in ≦ 3.64 <5.94 = d ₁ × NA ₁ , and d _in × tan θ _in ≦ d ₁ described above. The relationship of × NA ₁ is satisfied.

  In the embodiment described above, since the second multimode optical fiber 12 having a sufficiently large NA is applied, there is no loss in the second multimode optical fiber 12, but it is more than the tapered portion 11f of the first multimode optical fiber 11. When the NA is small, the loss in the second multimode optical fiber 12 is similarly observed.

  In the described embodiment, one first multimode optical fiber 11 is joined to one second multimode optical fiber 12, but one plurality of first multimode tapered optical fibers is provided. The conditions for suppressing the propagation loss are the same in the multiplexer fused and joined to the second multimode optical fiber.

[Second Embodiment]
FIG. 5 and FIG. 6 show the schematic side surface shape and schematic plane shape of the optical fiber module according to the second embodiment of the present invention, respectively. The optical fiber module according to the second embodiment basically has a configuration in which a cylindrical lens 30 is additionally installed in the embodiment shown in FIG. That is, in the present embodiment, a semiconductor laser 14 having a light emission width of 7 μm, a full width of FFP (far field pattern) of 20 ° in the horizontal direction, and 45 ° in the vertical direction is used. Only through a cylindrical lens 30 having a reduction ratio of 1/2. As a result, the outgoing light 16 is beam-shaped so as to have an divergence angle of about 45 ° with respect to the horizontal direction, and then condensed by the condensing lens 15 having a magnification of 4 times, and the first diameter having a core diameter d ₁ = 33 μm. Coupled to the multimode optical fiber 11.

In the case where the above-described cylindrical lens 30 is not disposed, the beam diameter in the horizontal direction at the incident end 11d of the first multimode optical fiber 11 is 28 μm, and the light enters the first multimode optical fiber 11 having a core diameter d ₁ = 33 μm. When this is done, the coupling efficiency is reduced due to misalignment due to assembly variations during module manufacture. On the other hand, by arranging the cylindrical lens 30 described above, the beam diameter in the horizontal direction at the optical fiber incident end becomes 14 μm, and a light source module capable of stably obtaining high coupling efficiency is obtained.

[Third Embodiment]
FIG. 7 shows a schematic side shape of an optical fiber module according to the third embodiment of the present invention. This third embodiment basically has a form in which a dielectric block 40 is additionally provided in the second embodiment shown in FIGS. That is, in the present embodiment, a short wavelength semiconductor laser 14 having an emission width of 7 μm, an FFP (far field pattern) full width of 20 ° in the horizontal direction, 45 ° in the vertical direction, and an oscillation wavelength of 405 nm is used. The emitted incident light 16 is condensed by the cylindrical lens 30 and the condenser lens 15 and coupled to the first multimode optical fiber 11 having a core diameter d ₁ = 33 μm. A dielectric block 40 is attached and fixed to the incident end 11d of the first multimode optical fiber 11 by an optical contact in order to prevent contamination by an organic substance or the like.

  When propagating the incident light 16 having a short wavelength as described above, it has been found that there is a phenomenon in which the dielectric block and the optical fiber incident end 11d are adhered by fusion or the like, particularly when the luminance is high. This phenomenon becomes prominent when the luminance is high, and may not occur when the luminance is low. Therefore, it is effective to lower the luminance at the optical fiber incident end 11d. For this reason, for example, if the magnification of the condenser lens 15 is increased, the luminance at the end surface of the optical fiber incident end 11d is lowered and the sticking phenomenon is reliably suppressed. can do.

  In particular, when using a semiconductor laser, the beam shape of the emitted light from the semiconductor laser is an ellipse. By rounding with a cylindrical lens or the like, the beam diameter is effectively expanded within the core diameter of the optical fiber, and the brightness is increased. Can be lowered.

1 is a schematic side view showing an optical fiber module according to a first embodiment of the present invention. Schematic side view showing another state of the optical fiber module A graph showing the relationship between propagation efficiency and tangent tanθ _in of the divergence angle of light incident on an optical fiber Graph showing the relationship between the product of the taper core diameter ratio α and the numerical aperture NA ₁ other than the taper portion with respect to the tangent tan θ _{in the} incident light divergence angle at which the propagation efficiency drops sharply in a tapered optical fiber The schematic side view which shows the optical fiber module by the 2nd Embodiment of this invention Schematic top view which shows the optical fiber module by the 2nd Embodiment of this invention The schematic side view which shows the optical fiber module by the 3rd Embodiment of this invention

Explanation of symbols

DESCRIPTION OF SYMBOLS 11 1st multimode optical fiber 11a Core of 1st multimode optical fiber 11b Cladding of 1st multimode optical fiber 11c Outgoing end of 1st multimode optical fiber 11d Incidence end of 1st multimode optical fiber 11f 1st multimode Tapered portion of optical fiber 12 Second multimode optical fiber 12a Core of second multimode optical fiber 12b Cladding of second multimode optical fiber 12c Output end of second multimode optical fiber 12d Incident end of second multimode optical fiber 13 Optical Input System 14 Semiconductor Laser 15 Condensing Lens 16 Incident Light 30 Cylindrical Lens 40 Dielectric Block

Claims (4)

A first multimode optical fiber in which a part following the emission end is a tapered portion;
The first multimode optical fiber and the core are coupled to each other, and the incident end includes a second multimode optical fiber joined to the output end of the first multimode optical fiber.
In an optical fiber module used by coupling incident light collected to a diameter smaller than the core diameter of the optical fiber to the incident end of the first multimode optical fiber,
The numerical aperture and core diameter other than the tapered portion of the first multimode optical fiber are NA ₁ and d ₁ , respectively, and the core diameter ratio (minimum core diameter / maximum core diameter) of the tapered portion is α, and the second multimode optical fiber. When the numerical aperture and the core diameter are NA ₂ and d ₂ respectively, and the diameter of the incident light on the first multimode optical fiber entrance end is d _in ,
d ₂ ≧ α × d ₁ and
The incident light divergence angle is θ _in , and d _in × tan θ _in ≦ d ₁ × NA ₁ , tan θ _in ≦ α × NA ₁ , tanθ _in ≦ α × NA ₂ are satisfied. Fiber optic module.   2. The optical fiber module according to claim 1, wherein a plurality of the first multimode optical fibers are provided and are joined to one second multimode optical fiber. The divergence angle of the incident light is different in two directions orthogonal to each other,
3. The optical fiber module according to claim 1, further comprising means for reducing the NA of incident light before entering the first multimode optical fiber only in a direction with a large divergence angle.   The optical fiber module according to any one of claims 1 to 3, wherein the wavelength of the incident light is in a range of 160 nm to 500 nm.

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