JP4112126B2 - Optical system - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a collimating / condensing optical system that converts light emitted from an optical fiber cord into parallel light and collects the parallel light onto the optical fiber cord.
[0002]
[Prior art]
Conventionally, when limiting the power and wavelength band of light propagating through the core of an optical fiber cord, an optical system having a configuration as shown in FIG. 7 is generally employed.
[0003]
In the optical system 21 shown in FIG. 7, the light propagating in the core of the optical fiber cord 22 is converted into parallel light in the air using the lens 23, and then passed through a desired attenuation element 24 or filter element 25. Then, the light is condensed again on the optical fiber cord 27 by the lens 26.
[0004]
Incidentally, in recent years, as the ferrule 28 provided at the tip of the optical fiber cords 22 and 27, as shown in FIG. 8, an oblique polishing ferrule 28A having an end face 28a cut obliquely is being used. As a result, the light reflected by the end face 28a is incident on the optical fiber cords 22 and 27 again and does not affect the stability of the light source as return light.
[0005]
However, when the oblique polishing ferrule 28A is used as the ferrule 28 of the optical fiber cords 22 and 27, as shown in FIG. 8, the optical axis L10 of the emitted light does not coincide with the central axis L11 of the oblique polishing ferrule 28A. The optical axis of the commonly used ferrule 28 coincides with the center of the core.
[0006]
When assembling a pair of collimating / condensing optical systems, after assembling the optical system on the exit side, optical components are arranged, and finally, the optical system on the condensing side that is the receiving side is aligned.
[0007]
[Problems to be solved by the invention]
However, skill is required for the above-described axis alignment operation. In order to solve this problem, as shown in FIGS. 9A and 9B, the oblique polishing ferrule 28A and the lens 23 (or 26) are cylindrical so that the cylindrical holder 31 and the optical axis of the emitted light coincide with each other. A collimating / condensing optical system fixed to the holder 31 has been proposed.
[0008]
In this collimating / condensing optical system, as shown in FIGS. 9A and 9B, the center of the inner diameter of the cylindrical holder 31 is set so that the center axis of the outer shape of the cylindrical holder 31 coincides with the optical axis. Cut from the center of the outer shape.
[0009]
Then, a collimator / condensing optical system in which the oblique polishing ferrule 28A and the lens 23 (or 26) are incorporated in the off-axis cylindrical holder 31 as described above is a U-shaped holding base as shown in FIG. When used as a pair in two holes 33 formed by penetrating a drill through 32, the center of the outer shape of the cylindrical holder 31 can coincide with the optical axis. Thereby, an optical system with a small loss can be manufactured without adjusting the optical axis.
[0010]
However, in the optical system using the conventional off-axis cylindrical holder 31 as described above, there is a drawback in that it requires precise processing that slightly shifts the center of the outer shape and the inner diameter.
[0011]
Therefore, the present invention has been made in view of the above-described problems, and simplifies the production without requiring precise processing so that the optical axes of outgoing light or incident light shifted by the oblique polishing ferrule are matched. An object of the present invention is to provide an optical system capable of performing the above.
[0012]
[Means for Solving the Problems]
To achieve the above object, the invention of claim 1 is directed to an optical system 1 that converts the light output from the optical fiber cord 2 into parallel light, emits it, or collects the parallel light onto the optical fiber cord.
An obliquely polished ferrule 3 provided at an end of the optical fiber cord and having an end surface 3a polished obliquely;
A collimator lens 4,
An oblique polishing optical element 5 in which both end faces 5a and 5b are obliquely polished in parallel;
Said oblique polishing ferrule exit position 3a is a focal position of the collimator lens, and so that the optical axis of the swash central axis L1 and the collimating lens polishing the ferrule coincides, the swash polishing ferrule, the collimating lens, and a holder 6 for fixed these parts in the order of the swash polishing optical element,
The shape and material refractive index of the oblique polishing optical element are defined so that the optical axis of the emitted or incident light coincides with the central axis of the holder.
[0013]
The invention of claim 2 is the optical system of claim 1,
The oblique polishing ferrule, the collimating lens and the oblique polishing optical element each have a cylindrical shape,
The holder 6 is composed of a split sleeve.
[0014]
DETAILED DESCRIPTION OF THE INVENTION
FIG. 1 is a diagram showing an embodiment of an optical system according to the present invention.
[0015]
As shown in FIG. 1, the optical system 1 in this example includes a slant polishing ferrule 3, a lens 4, a slant polishing optical element 5, and a holder 6 provided at the tip of an optical fiber cord 2. In the single holder 6, the oblique polishing ferrule 3, the lens 4, and the oblique polishing optical element 5 are fixed in this order at a predetermined interval. The oblique polishing optical element 5 has a shape and a refractive index of the material so that the optical axis of the emitted or incident light coincides with the central axis of the holder 6.
[0016]
In the optical system 1 in this example, when light is incident from the oblique polishing ferrule 3 side, the light is converted into parallel light through the lens 4 and emitted from the oblique polishing optical element 5. On the other hand, when parallel light is incident from the oblique polishing optical element 5 side, the parallel light is condensed through the lens 4 onto the core central axis (L1 indicated by a one-dot chain line in the figure) of the oblique polishing ferrule 3. ing.
[0017]
The holder 6 is a cylindrical split sleeve for the purpose of absorbing the component machining accuracy. The split sleeve 6 forming the holder is formed with a slit-like cut 6a which is parallel to the central axis of the cylinder and penetrates along the central axis.
[0018]
The oblique polishing ferrule 3 having a cylindrical shape is cut obliquely with an angle of, for example, about 4 ° to 8 ° with respect to a surface whose tip surface 3a is orthogonal to the core central axis L1. As a result, the light reflected by the distal end surface 3a is incident on the optical fiber cord 2 again and does not affect the stability of the light source as return light. The oblique polishing ferrule 3 is installed and fixed in the split sleeve 6 so that the front end surface 3a faces the lens 4 side.
[0019]
The lens 4 converts the light incident from the oblique polishing ferrule 3 side into parallel light and outputs it, and condenses the parallel light incident from the oblique polishing optical element 5 on the core central axis L1 of the oblique polishing ferrule 3. ing. The lens 4 can be constituted by a collimator lens such as a Selfoc lens, in addition to a cylindrical drum lens whose both end surfaces are convex spherical surfaces as shown in FIG. The lens 4 is installed between the oblique polishing ferrule 3 and the oblique polishing optical element 5 at a substantially central position in the split sleeve 6 and at a position where the distance from the tip surface 3a to the principal point of the lens is the focal length. It is fixed.
[0020]
The oblique polishing optical element 5 is composed of a cylindrical optical element whose both end faces are obliquely polished in parallel. The oblique polishing optical element 5 is installed and fixed in the split sleeve 6 so that one obliquely polished surface faces the lens 4 side. In the oblique polishing optical element 5, two surfaces (both end surfaces) 5 a and 5 b where light enters and exits are parallel to each other with high accuracy, and the optical axis of the emitted or incident light coincides with the central axis of the split sleeve 6. Thus, the shape, the refractive index of the material, the oblique polishing angle, the length, the distance from the oblique polishing ferrule, the angle with the optical axis, the installation angle around the axis, etc. are defined.
[0021]
Here, the parallelism of the both end faces 5a and 5b of the oblique polishing optical element 5 needs to be finished with high accuracy, and is particularly important when propagating parallel light over a long distance. For example, when both end faces of a cylindrical optical element are obliquely polished independently, the parallelism is very poor and cannot be used practically.
[0022]
For this reason, the columnar oblique polishing optical element 5 is manufactured by the procedure of the processing method described below.
[0023]
First, as shown in FIG. 2A, the flat plate 11 finished with high accuracy is cut out into a quadrangular prism shape. Subsequently, as shown in FIG. 2 (b), what is cut out into a quadrangular prism shape 12 is cut into a quadrangular prism shape inclined at an angle of 8 ° with general processing accuracy. At this time, it is not necessary to cut into a cylindrical shape. Thereafter, as shown in FIG. 2 (c), a piece 13 cut into a square column inclined at an angle of 8 ° is inserted into the cylindrical holder 14, and the periphery is adhered and fixed. Thereby, the cylindrical oblique polishing optical element 5 is completed.
[0024]
Next, a specific example of the oblique polishing optical element 5 will be described with reference to FIGS.
[0025]
As shown in FIG. 3A, the refractive index of the core portion of the oblique polishing ferrule 3 is n0 (= 1.5), the oblique polishing angle of the tip surface 3a is θ0 (= 8 °), and the refractive index of air is n1. If (= 1), the diffraction angle θ1 of the emitted light is n0 · sin θ0 = n1 · sin θ1 and θ1 = sin ⁻¹ (n0 / n1 · sin θ0) ≈12 ° according to Snell's law. Therefore, the deflection angle ψ (the angle formed by the core central axis L1 of the oblique polishing ferrule 3 and the outgoing optical axis L2) is ψ = θ1−θ0 = 4 °.
[0026]
As shown in FIG. 3B, assuming that the focal length of the lens 4 with respect to the tip surface 3a of the oblique polishing ferrule 3 is f (= 2 mm), the deviation x of the parallel optical axis (the core central axis L1 of the inclined polishing ferrule 3). -Distance from L1 to the optical axis L3-L3 of parallel light) is x = f · tan ψ≈0.14 mm.
[0027]
As the oblique polishing optical element 5, the length I of the element in the case of using BK7 (borosilicate crown glass) having a refractive index n3 (= 1.5) at an oblique polishing angle θ2 (= 8 °) is used. The following procedure is used.
[0028]
The intersection of the optical axis L4 and the incident surface 5a is a point A, the intersection of the optical axis L4 and the exit surface 5b is a point B, and the intersection of the perpendicular from the point A to the core central axis L1 of the oblique polishing ferrule 3 is a point C. A point D is an intersection of the incident surface 5a and the core central axis L1 of the oblique polishing ferrule 3.
[0029]
The refraction angle θ3 is n1 · sin θ2 = n3 · sin θ3 and θ3 = sin ⁻¹ (n1 / n3 · sin θ2) ≈5.3 ° according to Snell's law.
[0030]
Therefore, ∠CAB becomes ∠CAB = 90 ° −θ2 + θ3≈87.3 °. The length of the side CB is CB = x · tan ・ CAB≈2.97 mm. Further, the length of the side DC is DC = x · tan θ2≈0.02 mm.
[0031]
Therefore, the length I of the oblique polishing optical element 5 is I = CB + DC≈3 mm.
[0032]
By the way, in the optical system as shown in FIG. 10, when using a slant polishing ferrule 28A having a core diameter of 10 μm and a slant polishing angle of the tip surface of 8 °, and a lens 23 (or 26) having a focal length of 2 mm. When the relationship between the amount of misalignment of parallel light and the coupling efficiency is calculated, the characteristics shown in FIG. 4 are obtained.
[0033]
And, as in the above optical system, the worst value of the amount of misalignment when the oblique polishing optical element 5 is not used is 140 μm × 2 = 280 μm. Therefore, in an optical system that does not use the oblique polishing optical element 5, a loss of about 10 dB at worst is generated.
[0034]
On the other hand, in this example, the oblique polishing optical element 5 is fixed to the same split sleeve (holder) 6 in addition to the oblique polishing ferrule 3 and the lens 4, and the optical system 1 is configured. As described below, It is designed to withstand practical use with minimal loss.
[0035]
First, the effect of the processing accuracy of the length I of the oblique polishing optical element 5 on the coupling efficiency will be described. The length I of the oblique polishing optical element 5 and the axis deviation amount x are in a proportional relationship, and the amount of change in the length I A change amount Δx of the axis deviation amount x with respect to ΔI is expressed as Δx / x = ΔI / I. Substituting the above-mentioned value x = 140 μm, I = 3 mm, and ΔI = ± 0.1 mm as general processing accuracy into this equation, Δx≈ ± 4.7 μm.
[0036]
Accordingly, the amount of axial misalignment between the two collimates is 10 μm or less, and the worst value of loss is 0.02 dB or less, which can be used practically.
[0037]
Next, the effect of the processing accuracy of the oblique polishing angle θ2 of the oblique polishing optical element 5 on the coupling efficiency will be described. The processing accuracy of the oblique polishing optical element 5 at the oblique polishing angle θ2 is about ± 0.5 °. The amount of change Δθ3 of the refraction angle θ3 at this time is obtained by differentiating the equation for obtaining θ3 by θ2, and θ3≈ ± 0.33 °. At this time, the amount of change Δx in the amount of axial deviation x is Δx≈BC / cos θ3 · tan Δθ3≈ ± 17 μm.
[0038]
Accordingly, the amount of axial misalignment between both collimates is 35 μm or less, and the worst value of loss is 0.2 dB or less, which is larger than that in the case of the parallel axis misalignment. However, even if it is used practically, it is a sufficiently acceptable range.
[0039]
Next, the rotation tolerance around the axis will be described. When the oblique polishing ferrule 3 and the oblique polishing optical element 5 rotate relative to the central axis L5 of the split sleeve 6, the outgoing optical axis L6 does not coincide with the central axis L5. As shown by the broken line in FIG. 5, when the optical system 1 is viewed from the emission side, one end of the outgoing optical axis L6 passes through the central axis L5 and is located on the circumference having a radius of 140 μm. If the relative rotation angle is a small angle α, the change amount Δx of the axis deviation amount x is Δx≈x · α, where α is rad. Therefore, if the processing (adjustment) accuracy is ± 2 °, Δx≈5 μm.
[0040]
Therefore, as in the case of the parallel axis deviation, the worst value of the loss is as small as 0.02 dB or less, and can be used practically enough.
[0041]
By the way, when designing the optical system 1, the influence of multiple reflection must be taken into consideration.
[0042]
Specifically, as shown in FIG. 6, multiple reflection between the end face 3a of the oblique polishing ferrule 3 and the end face 5a of the oblique polishing optical element 5, multiple reflection between both ends 5a and 5b of the oblique polishing optical element 5, It is necessary to consider complex multiple reflection, multiple reflection on surfaces other than the above, and the like.
[0043]
The influence of this multiple reflection appears as a deterioration of the wavelength dependency of the coupling efficiency, or as a deterioration of the stability of the light source when returning to the light source as light.
[0044]
Therefore, the coupling efficiency to the light receiving system (and the ratio of the return light) from the beam diameter of the main light beam and the multiple reflected light, the amount of axial deviation of the multiple reflected light, the amount of angular deviation of the multiple reflected light, the reflectance on the reflecting surface, the transmittance, etc ) Is calculated, and the material (refractive index) of the oblique polishing optical element 5, the oblique polishing angle, the length, the anti-reflection coating, the distance from the oblique polishing ferrule 3 and the like are determined so that the values do not affect the system. To do.
[0045]
As described above, in the optical system 1 according to the present embodiment, in addition to the configuration of the oblique polishing ferrule 3 and the lens 4, the oblique polishing optical element 5 in which both ends 5a and 5b are obliquely polished in parallel is provided with the same holder (split sleeve). ) 6 is housed and fixed. The oblique polishing optical element 5 has a shape and a refractive index that are defined so that the optical axis of the emitted or incident light coincides with the central axis L5 of the holder 6. As a result, it is possible to simplify the production of the optical system without requiring precise processing as in the prior art, and to match the optical axes of the outgoing light or the incident light shifted by the oblique polishing ferrule 3.
[0046]
When the power or wavelength band of light propagating through the optical fiber cord is limited, each component (the oblique polishing ferrule 3, the lens 4, and the oblique polishing optical element 5) is arranged symmetrically. If the optical system 1 of this example is attached to a portion surrounded by a broken line (two holes 33, 33 of the holding base 32) as a pair, a parallel optical system with small loss can be configured without performing special optical axis adjustment. can do.
[0047]
【The invention's effect】
As is clear from the above description, according to the present invention, the optical axis of the emitted light or incident light shifted by the oblique polishing ferrule can be matched without using a conventional high-precision processing technique, There is an effect that the production can be simplified.
[0048]
In particular, according to the optical system of claim 2, in a state in which the processing accuracy of each component (oblique polishing ferrule, collimating lens, oblique polishing optical element) is absorbed, without using a conventional high-precision processing technique, The optical axis of the outgoing light or incident light shifted by the oblique polishing ferrule can be matched.
[Brief description of the drawings]
FIG. 1 is a side sectional view showing an embodiment of an optical system according to the present invention. FIGS. 2 (a) to (c) are diagrams showing a processing procedure of an oblique polishing optical element. FIG. 3 (a) to (c). FIG. 4 is a diagram for explaining a specific example of a slant polishing optical element. FIG. 4 is a characteristic diagram of loss with respect to an axis deviation amount when a slant polishing optical element is not used. FIG. 6A is a diagram showing multiple reflections between an end surface of an oblique polishing ferrule and an end surface of an oblique polishing optical element. FIG. 6B is a diagram showing multiple reflections between both end surfaces of an oblique polishing optical element. FIG. FIG. 8 is a side view of an optical fiber cord having a slant polishing ferrule. FIG. 9A is a diagram illustrating the alignment of the optical axis of outgoing light. Cross-sectional side view of a conventional optical system that uses a cylindrical holder for the cylindrical holder (b) Sectional view of a state of attaching the saw cross-sectional view [FIG. 10] an optical system that employs a cylindrical holder of Figure 9 in holder EXPLANATION OF REFERENCE NUMERALS
DESCRIPTION OF SYMBOLS 1 ... Optical system, 2 ... Optical fiber cord, 3 ... Slant-polishing ferrule, 3a ... Tip surface, 4 ... Lens, 5 ... Slant-polishing optical element, 5a, 5b ... Input / output surface, 6 ... Holder (split sleeve), 6a ... cuts.

Claims (2)

In the optical system (1) that converts the light output from the optical fiber cord (2) into parallel light and emits it, or collects the parallel light on the optical fiber cord.
A slant polishing ferrule (3) provided at an end of the optical fiber cord and having an end surface (3a) polished obliquely;
A collimating lens (4),
An oblique polishing optical element (5) whose both end faces (5a, 5b) are obliquely polished in parallel;
The oblique polishing ferrule, so that the exit position (3a) of the oblique polishing ferrule is a focal position of the collimating lens and the central axis (L1) of the oblique polishing ferrule and the optical axis of the collimating lens coincide with each other . the collimating lens, and a holder (6) to secure the these parts in the order of the swash polishing optical element,
An optical system characterized in that the shape and material refractive index of the oblique polishing optical element are defined so that the optical axis of emitted or incident light coincides with the central axis of the holder. The oblique polishing ferrule, the collimating lens and the oblique polishing optical element each have a cylindrical shape,
The optical system according to claim 1, wherein the holder comprises a split sleeve.

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