Coupled optical system and optical device using the same

Description: TECHNICAL FIELD [Detailed description of the invention]

【0001】

[Technical field to which the invention belongs]

The present invention relates to an optical communication system, particularly to a collimator vs. optical element used in combination with an optical fiber.

【0002】

[Conventional technology]

In recent years, with the rapid spread of the Internet, there is a strong demand for an increase in the capacity of optical fiber communication networks, and the development of WDM (wavelength division multiplexing) communication is rapidly being promoted as a means for this. In WDM communication, since light with a slight wavelength difference transmits individual information, optical functional elements such as an optical demultiplexer, an optical filter, an optical isolator, and an optical circulator with good wavelength selectivity are required. Needless to say, mass productivity, miniaturization, integration, stability, etc. are strongly required for the above functional elements.

【0003】

In the above optical functional element, the light emitted from the end face of the emitting side optical fiber is converted into a parallel light beam by a collimator, transmitted through a flat plate-shaped optical functional element having a function of a filter or an isolator, and then again by a condenser lens. In many cases, the light is collected and sent to the end face of the incident side optical fiber. As the collimator and the condenser lens, a rod lens having a refractive index distribution in the radial direction, a glass ball lens, an aspherical molded lens, or the like is used. In terms of shape and aberration correction, the refractive index distribution type rod lens is the easiest lens to use.

【0004】

FIG. 1 is a schematic view of a collimator parallel pair using a first plano-convex lens 3 and a second plano-convex lens 4 made of a homogeneous material as an example. In a normal parallel pair, two identical lenses (lens thickness: Z) are placed on both sides with a distance of 2 L. Further, in the case of a lens having an asymmetric lens surface shape as shown in FIG. 1, both lenses 3 and 4 are oriented in opposite directions. That is, in the case of FIG. 1, in the first lens 3, the plane 30 side is the entrance surface and the curved surface 130 side is the exit surface. On the contrary, in the second lens 4, the curved surface 140 side is the entrance surface and the plane 40 side is the exit surface. The curved surfaces 130 and 140 may be spherical or aspherical. Further, for the incident side optical fiber 1 (hereinafter referred to as a light source fiber) and the outgoing side optical fiber 2 (hereinafter referred to as a light receiving fiber), optical fibers having the same mode field diameter and the same characteristics are used. The distance WD between the end surface 10 of the light source fiber 1 and the incident surface 30 of the lens 3, the exit surface 40 of the lens 4 and the end surface 20 of the light receiving fiber 2 is made equal to obtain a completely symmetrical optical system.

【0005】

FIG. 1 is a geometrical optics schematic diagram showing the light beam 5. However, the luminous flux actually emitted from the single-mode optical fiber can be regarded as a Gaussian beam as shown in FIG. In this case, in order to obtain better coupling efficiency with the collimator parallel pair, it is necessary to arrange so that the beam waist (BW) 26 of the Gaussian beam 7 is formed at the midpoint of both lenses 3 and 4. That is, the first beam waist 16 (radius w1) corresponding to the emitted light 17 from the light source fiber 1 once forms the second beam waist 26 (radius w2) at the midpoint of the optical system, and further, the second beam waist 26 (radius w2) is formed. It is coupled to the light receiving fiber 2 by the lens 4 at the position of the third beam waist 36 (equal to the radius w3 and w1).

【0006】

Here, if the wavelength used, the NA (numerical aperture) of the optical fiber, the focal point and the principal point position of the lens are known, WD and L in the configuration of FIG. 2 are calculated by the so-called ABCD law using the elements of the ray matrix. Can be designed. Theoretically, for example, Kenji Kono, "Basics and Applications of Optical Coupling Systems for Optical Devices," Hyundai Engineering Co., Ltd. (1991) shows detailed mathematical formulas, and ABCD is included in commercially available optical design software. Some have a calculation function. However, since the maximum value Lmax exists in the distance between the lenses, that is, the distance L between the lenses 3 or 4 and the second BW26, the value cannot be made larger than that. Fig. 3 schematically shows the relationship between WD and L for a lens with a focal length f.

【0007】

In such a coupled optical system, the ratio of the optical power emitted from the light source fiber to the optical power incident on the light receiving fiber, that is, the coupling efficiency or the coupling loss is an important characteristic parameter. If L is Lmax or less, 100% coupling efficiency (coupling loss 0dB) can be theoretically obtained by selecting an appropriate WD, but if L exceeds Lmax, the coupling loss increases rapidly (coupling loss increases rapidly). Figure 4). The value of Lmax increases in proportion to the square of the approximate lens focal length.

【0008】

Although a completely symmetrical optical system has been described above as an example, there are also coupled optical systems in which the light source is not an optical fiber but a light emitting element such as a semiconductor laser, and the light receiving side is also a photodetector such as a photodiode. Yes, even in such a case, a design can be made by applying the Gaussian beam as described above.

【0009】

[Problems to be Solved by the Invention]

However, the result of the ABCD calculation is based only on the paraxial data, and it is premised that each lens has no aberration and is not affected by eclipse due to insufficient effective diameter. It is inevitable that a lens used in an actual optical system will cause a loss due to various residual aberrations. Therefore, the relationship between the lens-to-lens distance of 2 L and the coupling loss is not always simple as shown in FIG. Further, if the focal length of the lens and the aberration condition change due to changes in temperature and humidity, it is possible that the coupling loss also fluctuates. In addition, fluctuations in the volume and length of the parts that hold the lens and optical fiber also cause coupling loss.

【0010】

The present invention has been made to solve the above-mentioned problems, and provides a configuration condition that minimizes the coupling loss for a coupled optical system in which some aberrations, defects, changes due to the environment, etc. exist. It is a thing.

【0011】

[Means for solving problems]

In the first aspect of the coupled optical system in the present invention, a Gaussian beam-like light beam emitted from a light source is converted into a substantially parallel light beam by a first lens having a positive refractive force, and the substantially parallel light beam is converted into a first substantially parallel light beam. In a coupled optical system that is the same lens as the lens and is incident on a second lens in which the light inlet / output surfaces are arranged in opposite directions, and the light beam converted into convergent light by the second lens is incident on the light receiving means. It is characterized in that the distance between the lenses is substantially matched with the maximum distance at which a beam waist can be formed at equal distances from both lenses.

【0012】

That is, when the distance between the two lenses is 2L and the maximum distance at which the beam waist can be formed equidistant from both lenses is 2Lmax, the distance 2L is in the range of 1.8Lmax 2L 2Lmax. Is desirable. Further, the coupling loss in the coupling optical system of the first aspect is equal to or smaller than the coupling loss that occurs when the distance 2L between the two lenses is in the range of 0 2L <1.8L max. The coupling loss value is preferably 0.05 dB or less.

【0013】

In the second aspect of the coupled optical system in the present invention, a Gaussian beam-like light beam emitted from a light source is converted into a substantially parallel light beam by a lens having a positive refractive force, and the substantially parallel light beam is reflected behind the lens. In a coupled optical system in which a light beam that is reflected by a surface and returned to the lens and converted into convergent light by the lens is incident on a light receiving means arranged at or near the position of a light source, the distance between the lens and the reflecting surface is determined by the lens. It is characterized by roughly matching the maximum distance that a waist can be formed.

【0014】

That is, when the distance between the lens and the reflecting surface is L and the maximum distance at which the beam waist can be formed by the lens is Lmax, the distance L is 0.9Lmax L Lmax It is desirable that it is in the range of. Further, the coupling loss in the coupling optical system of the second aspect is equal to or smaller than the coupling loss that occurs when the distance L between the lens and the reflecting surface is in the range of 0 L <0.9 Lmax. The coupling loss value is preferably 0.05 dB or less.

【0015】

For the light source and the light receiving means, end faces of optical fibers having the same mode field diameter can be used. In the case of the coupled optical system of the second aspect, the end face of one optical fiber can be used as both a light source and a light receiving means.

【0016】

Examples of the lens having a positive refractive power include a rod lens having a refractive index distribution in the radial direction, a plano-convex lens having a refractive index distribution in the optical axis direction, a plano-convex lens made of a homogeneous material, and a spherical lens made of a homogeneous material. , A diffractive lens surface or the like can be used.

【0017】

In the coupled optical system of the first aspect, the optical element can be configured by installing an optical functional element between the two lenses. Further, in the coupled optical system of the second aspect, the optical element can be configured by installing the optical functional element between the lens and the reflecting surface. Further, in these optical elements, the same coupled optical system can be arranged in an array of one row or a plurality of rows.

【0018】

The coupled optics of the present invention, in which the distance between the lenses or between the lenses and the reflective surfaces is set to correspond to the maximum distance at which the lens can form a beam waist, provide some degree of contrast to the ideal optics. The fluctuation of the coupling loss is small even when the aberration increases or a defect occurs, or when the performance of the optical system fluctuates due to an environmental change or the like. Further, the optical element to which the coupled optical system of the present invention is applied has little fluctuation in performance due to deviation from ideal design conditions and environmental fluctuation.

【0019】

BEST MODE FOR CARRYING OUT THE INVENTION

Fig. 5 shows the relationship between the graph of Fig. 3 and the lens whose focal length is slightly longer than f (f + Δf). For example, when WD = WD1, the difference in L value (ΔL) between the two lenses represents the difference in the second BW position due to the difference in focal length. By the way, it can be seen that when WD is set so that the L value is close to Lmax in FIG. 5, the value of ΔL becomes a very small value. This means that when the L value approaches Lmax, the fluctuation of the second and third BW positions is small even if the focal length changes.

【0020】

According to the result of calculating the relationship between the BW position and the coupling loss (design example described later) for the actual parallel pair optical system, Set the distance between the lenses of the parallel pair to near 2Lmax. The fluctuations in the focal lengths of both lenses for some reason are equal to each other. It can be seen that when both of the above conditions are satisfied, the increase in coupling loss due to the fluctuation of the focal length becomes very small.

【0021】

The following points need to be considered as the causes of the fluctuation of the BW position that causes an increase in the bond loss. (1) Due to the axial chromatic aberration of the lens, the BW position fluctuates due to the wavelength difference of the wavelength used. (2) The BW position fluctuates due to changes in the focal length of the lens at the wavelength used due to fluctuations in the environment such as temperature and humidity expected in the usage state. (3) The BW position fluctuates due to changes in the volume of the structure holding the optical system and changes in the refractive index of the medium installed in the middle of the optical system due to fluctuations in the environment such as temperature and humidity expected in the usage state. To do. (4) If the lens has optical axis asymmetry and astigmatism occurs on the optical axis, there will be multiple BW positions. (5) Due to the birefringence of the lens, there are multiple BW positions.

【0022】

In response to these factors, the "configuration in which the distance between lenses matches Lmax" according to the present invention is effective for the specific cases listed below.

【0023】

[1] When using a lens with large axial chromatic aberration In the case of a lens pair used for WDM (wavelength division multiplexing), if the axial chromatic aberration is large, the coupling loss increases at a wavelength far from the design reference wavelength. However, according to the configuration of the present invention, the influence of axial chromatic aberration can be suppressed to a small extent. Examples of the lens having a large chromatic aberration include a kinoform-shaped diffractive lens, a glass single lens having a long focal length, and a plastic lens. In particular, since the diffractive lens has a remarkably large axial chromatic aberration (several tens of times that of the glass convex lens), the configuration of the present invention is particularly effective.

【0024】

[2] For lenses that change significantly depending on the environment (temperature, humidity, etc.) Plastic aspherical lenses are inexpensive because they can be mass-produced by pressing, but there is a problem that fluctuations in focal length and aberration due to temperature and humidity are much larger than those of glass lenses and the like. However, according to the configuration of the present invention, the fluctuation of the bond loss due to the above-mentioned causes can be reduced.

【0025】

[3] When keeping fluctuations due to the environment particularly small In an optical system for communication, it is often strictly required to suppress fluctuations in characteristics due to temperature (athermal property) not only in the lens but also in the entire device. In order to improve the athermal property, methods such as keeping the entire device at a constant temperature, using a material with little temperature change, and canceling the temperature change by a combination of materials are taken. If the configuration of the present invention is adopted in addition to these methods, there is an effect of suppressing the fluctuation of the coupling loss due to the change in the optical path length to a small value.

【0026】

[4] In the case of a rod lens with distortion For rod lenses that have a refractive index distribution in the optical axis direction, the coefficient of thermal expansion of the material also changes depending on the distance from the optical axis, so radial distortion often remains, and the amount of distortion is particularly large for lenses with a long focal length. growing. In a distorted lens, the focal length differs depending on the polarization direction of the incident light due to the birefringence phenomenon, so that the focal point is blurred and the coupling loss becomes large. However, according to the configuration of the present invention, even if there is a difference in the focal length, the positions of the BWs are aligned, so that the focal point becomes sharp and the coupling loss can be reduced.

【0027】

As shown in FIG. 6, the reflector 8 is installed at a position corresponding to the position of the second beam waist 26 in the middle of the lens of FIG. 2, and the light is sent back to the optical fiber 11 which also serves as the light source fiber and the light receiving fiber. However, the effect of the present invention is similarly exhibited. In the case of FIG. 6, the above conditions, Set the lens spacing of parallel pairs to near 2Lmax. The fluctuations in the focal lengths of both lenses for some reason are equal to each other. Is always filled. Therefore, if the distance between the lens 13 and the reflector 8 is set to a value close to Lmax, the lens to be used will be Aberration due to optical axis asymmetry (such as astigmatism that occurs on the optical axis) Variation of focal length from the standard value Variation of lens thickness from the standard value Even if there are some drawbacks such as these, there is an effect of suppressing an increase in bond loss due to these.

【0028】

Even if the light source fiber 21 and the light receiving fiber 22 are close to each other as shown in FIG. 7, the same effect as that of the configuration of FIG. 6 can be obtained. The results of calculating the relationship between the amount of astigmatism and the coupling loss for a specific lens system will be described below. The calculation was performed using the lens design software "OSLO Six" manufactured by Sinclair Optics of the United States.

【0029】

[Design example 1] A collimator parallel pair with a diffractive lens was designed, and the coupling loss was calculated. (Design value of diffractive lens) Fig. 8 shows the configuration of a single lens. A diffractive lens (quinoform shape) 25 with a focal length of 1 mm and NA = 0.2 was set on the surface of quartz glass 9 having a thickness of Zg: 1 mm. The design wavelength λ is 1550 nm and uses the focus of the primary light. The wave plane due to the diffraction plane was parabolic with respect to the distance r from the optical axis 50. Wave surface function: φ (r) = (2π / λ) d_f1 R²Constant: d_f1=-0.500 Zone depth: 0.00351 mm Aberration on the axis: RMS-OPD = 0.0062λ And said. Spherical aberration can be ignored, and there is no need for correction by higher-order terms.

【0030】

(Chromatic aberration on the axis) Fig. 9 shows the longitudinal aberration of an infinite single lens at three wavelengths (1520, 1550, 1580 nm). It can be seen that the chromatic aberration on the axis is extremely large, and the focal position moves about 20 μm with respect to Δλ = 30 nm. This is a problem with diffractive lenses.

【0031】

(Collimator parallel pair coupling efficiency) As shown in FIG. 10, the light source fiber 1 and the light receiving fiber 2 having the same mode field diameter have the diffractive lenses 23 and 24 formed on the surfaces of the quartz glasses 19 and 29 as parallel pairs. The coupling loss was calculated by the following procedure for the case of symmetrical arrangement between them. In the calculation, the diameter and position of BW (according to ABCD calculation) were taken into consideration, but the loss due to surface reflection, internal absorption, diffraction efficiency, and spherical aberration was ignored. Both the light source fiber 1 and the light receiving fiber 2 have NA = 0.1 (1 / e) at a wavelength of λ = 1550 nm.²Strength).

【0032】

(1) First, set the distance between lenses to 2L and optimize the WD so that the beam waist is in the middle. (2) Calculate the bond loss at λ = 1550 nm. (3) With the same configuration, set the conditions of both optical fibers to NA = 0.1 at λ = 1520 nm, and calculate the coupling loss at λ = 1520 nm. (4) Calculate the coupling loss in the same way with the light source λ = 1580 nm.

【0033】

(a) When L is short Table 1 shows the calculation results when L = 0.0836 mm and WD = 0.3053 mm. The bond loss at λ = 1550 nm is small. However, when λ changes, the BW position on the receiving side moves significantly, so a large loss of 0.5 to 0.8 dB occurs in the range of Δλ = ± 30 nm.

【0034】

[table 1]

<img file="JP2002243991A_D0001.tif" />【0035】

(b) When L = Lmax In this optical system, when WD = 0.3561 mm, Lmax = 11.237 mm. The calculation results are shown in Table 2. The bond loss at λ = 1550 nm is small and hardly changes as compared with the case where L is short. However, even if λ is changed, the fluctuation of the BW position on the light receiving side is small, so the coupling loss hardly changes and remains less than 0.05 dB.

【0036】

[Table 2]

<img file="JP2002243991A_D0002.tif" />【0037】

From the above results, in the vicinity of the optimum design wavelength of λ = 1550 nm, the coupling loss is small and hardly depends on the interlens distance L, but when the wavelength changes from the optimum design wavelength, the coupling loss strongly depends on L. It turns out that However, in the configuration of the present invention in which L = Lmax, the coupling loss due to chromatic aberration is significantly smaller than when L is sufficiently smaller than Lmax (typically when L to 0 are brought close to each other). That is, according to the coupling optical system of the present invention, the influence of coupling loss due to chromatic aberration of an actual diffractive lens can be suppressed to a small value.

【0038】

Since the problem of chromatic aberration exists more or less in lenses other than diffractive lenses, the coupled optical system in the configuration of the present invention is effective for lenses with chromatic aberration in general.

【0039】

[Design Example 2] A collimator parallel pair using an "aspheric plano-convex lens" was designed, and the coupling loss when the R of the convex surface changed (the aspherical coefficient was constant) was calculated. (Design value of plano-convex lens) A lens with a glass refractive index of 1.520 and a lens thickness of 1.00 mm was set with a design wavelength of λ of 1550 nm. Convex surface R: 1.716mm, aspherical coefficient (4th order): -0.0152mm^-4The spherical aberration was corrected as. Aberration on the axis: RMS-OPD = 0.0067λ Focal length: 3.30mm NA: 0.20 (effective diameter φ1.32mm) And said.

【0040】

(Collimator parallel pair coupling efficiency) As shown in Fig. 11, the aspherical plano-convex lenses 43 and 44 of the above specifications are designed to be symmetrically arranged between the light source fiber 1 and the light receiving fiber 2 having the same mode field diameter as parallel pairs. The coupling loss was calculated in the same way as in Example 1. As variables, R of convex surfaces 150 and 160 was changed at the same time for both lenses. The wavelength λ, the aspherical coefficient, L, WD, the lens thickness Z, and the glass refractive index were kept constant. When L is short (L = 5.00mm WD = 2.6425mm) When L = Lmax (L = 114.8mm WD = 2.6900mm) The calculation results for are shown in Table 3 and Fig. 12.

【0041】

[Table 3]

<img file="JP2002243991A_D0003.tif" />【0042】

Even in this optical system, if the convex surface R is close to the design value, the coupling loss does not depend much on L in the range where L is smaller than Lmax. However, as shown in Fig. 12, when L is short, the range of 1 / R where the coupling loss is 0.05 dB or less is ± 0.004 mm with respect to the design value.^-1Very narrow. On the other hand, in the coupled optical system of the present invention in which L = Lmax, the range of 1 / R at which the coupling loss is 0.05 dB or less is ± 0.02 mm with respect to the design value.^-1It expands to a large extent. Also, the value is always smaller than when L is short. From the above results, it is clarified that, by adopting the configuration of the present invention, even if the R of the plano-convex lens changes due to the influence of the environment and the focal length fluctuates, the fluctuation of the coupling loss can be suppressed to be extremely small. It was. Similar effects can be obtained for factors other than R that cause changes in the focal length (fluctuations in refractive index, fluctuations in lens thickness, etc.). Therefore, the configuration of the present invention is effective for lenses in general having a positive refractive power other than the aspherical plano-convex lens.

【0043】

[Design Example 3] An optical system using a "refractive index distribution type rod lens" and a reflector was designed, and the coupling loss when the rod lens was asymmetric in the optical axis was calculated. (Design value of refractive index distribution type rod lens) The refractive index distribution in the radial direction of the rod lens follows the following equation. n (r)² = n₀²{1-(g r)²+ h₄(g r)⁴} However, n₀ Is the axial index of refraction, r is the distance from the optical axis, g and h₄Is the refractive index distribution coefficient. At the design wavelength of 1550 nm n₀1.600 Lens length Z: 4.430mm g = 0.326 (1 / mm) h₄= 0.67 Lens radius: 0.90mm Aberration on the axis: RMS-OPD = 0.0050λ Focal length: 1.933mm NA: 0.20 (effective diameter φ0.774mm) And said.

【0044】

(Coupling efficiency when there is optical axis asymmetry) As shown in FIG. 13, the lens 53 of the above specifications and the optical fiber 11 for both input and output are arranged, and the reflecting surface 8 by the reflecting mirror is installed at the second BW position 26. The coupling loss was calculated for this case. First, the optimum WD was calculated for the L values shown in Table 4 below.

[Table 4]

<img file="JP2002243991A_D0004.tif" />【0045】

Next, astigmatism on the axis was generated with the lens surface facing the reflecting surface 8 as a cylindrical surface having a radius of curvature Rc, and the change in coupling efficiency was calculated. Wavelength λ, L, WD, lens length Z_REtc. were fixed. The calculation results are shown in Table 5 and Fig. 14.

【0046】

[Table 5]

<img file="JP2002243991A_D0005.tif" />【0047】

In the vicinity of Rc = 0, the axial astigmatism is small and the dependence on L is also small. However, when the L / Lmax value is less than 0.9, the 1 / Rc range where the coupling loss is 0.05 dB or less is ± 0.01 mm.^-1Degree and narrow. On the other hand, when L / Lmax = 1, which is the configuration of the present invention, the range of Rc at which the coupling loss is 0.05 dB or less is ± 0.02 mm.^-1Expand to more. If L / Lmax exceeds 1, the beam waist cannot be formed on the reflecting surface even if there is no astigmatism, so that coupling loss occurs, which is not suitable as a coupling optical system.

【0048】

From the above results, it was clarified that by adopting the configuration of the present invention, the fluctuation of the coupling loss can be suppressed to a very small value even when the lens has an optical axis asymmetry. Similar effects can be obtained for factors other than the factors on the outer shape of the lens that cause optical axis asymmetry (optical axis asymmetry of refractive index distribution, poor centering, pulse, etc.).

【0049】

In the above design examples 1 to 3, a diffractive lens surface, a plano-convex aspherical lens, and a radial refractive index distribution type rod lens have been described, but the same effect can be obtained if the lens has a positive refractive power and can form a coupled optical system. Is obtained. In addition to the above, spherical lenses, spherical lenses, plano-convex lenses with an optical axis refractive index distribution, and the like can be used.

【0050】

Although the case where both the light source and the light receiving means are optical fibers has been described above, the light source may be a semiconductor laser or the like as long as it can be regarded as emitting a Gaussian beam. Further, the light receiving means may also be a light receiving element or the like.

【0051】

The coupled optical system of the present invention is applied as the following optical elements. As shown in FIG. 15, for example, the optical functional element 100 is inserted between the two lenses of the coupled optical system of FIG. As the optical functional element, an optical filter, an optical isolator, an optical modulation element, an optical switch element, or the like can be used. Any element that operates by incident light beams that are almost parallel can be widely applied. It is also possible to provide the same coupled optical system in a plurality of arrays and insert optical functional elements having the same function or different functions into each of them.

【0052】

For example, if wavelength division multiplexing light in which multiple wavelengths are combined is emitted from a plurality of light source fibers and band filters having different pass bands are inserted into each coupled optical system, light in different wavelength ranges is coupled to the light receiving fibers. , Demultiplexing function can be obtained. Since the coupling optical system having the configuration of the present invention has an extremely small coupling loss, an optical element having good characteristics can be obtained.

【0053】

Further, as shown in FIG. 16, for example, an optical functional element may be inserted into the coupled optical system of FIG. In this case, the light passes through the optical functional element twice in a round trip. FIG. 16 shows an example in which a plurality of lenses 53-1, 53-2, ..., 53-n are arranged to form an array-shaped coupled optical system. Similar to the above, the optical functional elements 100-1, 100-2, ..., 100-n may be the same or different depending on the purpose. Further, it is not always necessary to arrange a plurality of reflecting surfaces 8, and one reflecting surface 8 may be used in common for all coupled optical systems as shown in the figure.

【0054】

[Effect of the invention]

As described above, if the coupled optical system composed of two lenses according to the present invention is used, it is due to the essential drawbacks of the lenses (chromatic aberration, birefringence due to distortion, etc.) and changes in the environment (temperature, humidity, etc.). Both fluctuations in coupling loss can be kept small. Further, by using the coupled optical system composed of one lens and a reflecting mirror according to the present invention, it is possible to suppress fluctuations in coupling loss due to lens defects (variation in focal length, astigmatism due to optical axis asymmetry). it can. Therefore, even if the lens has some essential defects, the influence on the performance of the system is small, so that the manufacturing tolerance is widened and the product yield is improved. In addition, performance fluctuations due to environmental fluctuations are reduced, and system reliability is improved.

[Simple explanation of drawings]

[Figure 1]

It is explanatory drawing which showed the coupling of the optical fiber by the collimator parallel pair geometrically and optically.

[Figure 2]

It is explanatory drawing which showed the coupling of the optical fiber by the collimator parallel pair by the schematic Gaussian beam.

[Fig. 3]

It is the figure which showed typically the relationship between the distance WD of an optical fiber and a lens, and the distance (half value) L between lenses.

[Fig. 4]

It is the figure which showed typically the relationship between the interlens distance (half value) L of an ideal lens, and the coupling loss.

[Fig. 5]

It is the figure which showed typically the relationship between WD and L when the focal length f of a lens changes.

[Fig. 6]

It is a figure explaining the structure using one lens, one optical fiber and a reflective surface.

[Fig. 7]

It is a figure explaining the structure using one lens, two optical fibers and a reflective surface.

[Fig. 8]

It is an optical path diagram of a diffractive lens.

[Fig. 9]

It is a longitudinal spherical aberration diagram of a diffractive lens.

[Fig. 10]

It is a figure which shows the structure of the coupling optical system of the 1st design example.

[Fig. 11]

It is a figure which shows the structure of the coupling optical system of the 2nd design example.

[Fig. 12]

It is a figure which shows the calculation result of the coupling loss in the 2nd design example.

[Fig. 13]

It is a figure which shows the structure of the coupling optical system of the 3rd design example.

[Fig. 14]

It is a figure which shows the calculation result of the coupling loss in the 3rd design example.

[Fig. 15]

It is a figure which shows the structure of the optical element using the coupling optical system of the 2nd design example.

[Fig. 16]

It is a figure which shows the structure of the optical element which used a plurality of coupled optical systems of 3rd design example.

[Explanation of symbols]

1, 21 Light source fiber 2, 22 light receiving fiber 3, 4, 13 Plano-convex lens 5 rays 7, 37,47 Gaussian beam 8 Reflective surface 9,19,29 Quartz glass 11 Optical fiber 16,26,36 Beam waist 23,24,25 Diffractive lens 43,44 Aspheric plano-convex lens 53 Refractive index distribution type rod lens