JP2016139716A - Light receiving module - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a light receiving module capable of achieving both a sufficiently low ORL and sufficiently high receiving light efficiency without precisely performing the positional adjustment in each optical system.SOLUTION: A light receiving module includes: a collimating optical system 20 including one or more lenses, which receives incident light incident from an input optical system 10 and converts the incident light into collimating light; a focusing optical system 42 including one or more lenses, which receives the collimating light and converts the collimating light into focused light having astigmatism; and a light receiving part 50 for receiving the focused light.SELECTED DRAWING: Figure 3

Description

  The present invention relates to a light receiving module.

  In optical communication, for example, 100GBASE-LR4 and 100GBASE-ER4 are known as international standards for realizing communication at a transmission rate of 100 Gbps by a four-wave multiplexing method (IEEE 802.3ba). In the light receiving module conforming to such a standard, a plurality of collimator optical systems that convert light input from an input optical system such as an optical fiber into parallel light and a condensing optical system that condenses the parallel light on the light receiving unit are provided. May be.

Further, for example, by ITU-T (International Telecommunication Union Telecommunication Standardization Sector), the light reflection attenuation amount of the light receiving module (hereinafter referred to as ORL (Optical Return Loss)) is required to be −27 dB or less. Here, ORL means that when the incident light intensity to the light receiving module is P _in [W / cm ² ] and the reflected light intensity from the light receiving module is P _back [W / cm ² ], ORL [dB] = It is an amount defined by −10 × log ₁₀ (P _in / P _back ).

  Patent Document 1 below describes an optical assembly having a tilting mechanism that tilts a light beam incident on a bounce cavity.

  Patent Document 2 describes an optical module in which the optical axis of a first lens is arranged to be inclined with respect to the optical axis of outgoing light.

  Patent Document 3 describes an optical transceiver module in which the center of the opening is decentered so that the light reflected by the end face of the optical fiber cut obliquely does not reach the light receiving element.

  Patent Document 4 describes an optical functional module that makes the size and position of the beam waist substantially coincide with each other by making the characteristic variable parameters of the collimator different from the characteristic variable parameters of other collimators.

Special table 2009-503567 JP 2009-260118 A Japanese Patent Laid-Open No. 11-064687 JP 2002-107566 A

  In a light receiving module that receives a plurality of light beams, tolerances on the positions of optical systems (including collimating optical systems and condensing optical systems) used for the respective light beams are strictly limited.

  FIG. 12 is a schematic diagram showing the relationship between light coupling efficiency and defocus (defocus: deviation from the focal length of the optical system). Here, the curve A represents the relationship between the defocusing efficiency (the ratio between the light intensity of the light received by the light receiving unit and the light intensity of the light input from the input optical system) in the light receiving unit and the defocus. Curve B shows the coupling efficiency of light in the input optical system (ratio of the light intensity reflected from the light receiving unit and returning to the input optical system and the light intensity input from the input optical system) and the defocus. Represents a relationship. Since the light returning to the input optical system passes through the optical system twice in the forward path and the return path, the light coupling efficiency in the input optical system is smaller than the light coupling efficiency in the light receiving unit when the defocus is the same. .

  A point P in FIG. 12 represents a defocus at which the light coupling efficiency in the input optical system is 0.5. When the defocus is larger than the point P, the ORL of the light receiving module can be made sufficiently small. Further, a point Q in FIG. 12 represents a defocus at which the light coupling efficiency in the light receiving unit is 0.75. When the defocus is larger than the point Q, the light coupling efficiency in the light receiving unit becomes insufficient and the light receiving efficiency is deteriorated. For this reason, in the light receiving module, in order to achieve both a sufficiently small ORL and a sufficiently large light receiving efficiency, it is desirable to set the defocus between the point P and the point Q.

  However, if such a condition is realized by adjusting the position of each optical system, the tolerance for the position of each optical system is severely limited, and the production accuracy of each optical system is sufficient. However, the ORL of the light receiving module may not be sufficiently small.

  Accordingly, an object of the present invention is to provide a light receiving module that can achieve both a sufficiently small ORL and a sufficiently large light receiving efficiency without precisely adjusting the position of each optical system.

  (1) In order to solve the above-described problem, a light receiving module according to the present invention includes a collimator including one or more lenses that receives input light input from an input optical system and converts the input light into collimated light. An optical system; a condensing optical system including one or a plurality of lenses that receives the collimated light and converts the collimated light into convergent light with astigmatism; and a light receiving unit that receives the convergent light; It is characterized by providing.

  (2) The light receiving module according to (1), wherein the condensing optical system includes an asymmetric lens that generates astigmatism.

  (3) In the light receiving module according to (1), the condensing optical system includes a symmetric lens for converging the collimated light and light converged by the symmetric lens with astigmatism. And a convergent asymmetrical single convex lens, wherein the asymmetrical single convex lens is affixed to the daylighting surface of the light receiving section.

  (4) In the light receiving module according to (1), the condensing optical system includes a cylindrical lens.

  (5) In the light receiving module according to (1), the condensing optical system includes a transparent flat plate arranged so as to be oblique to a traveling direction of the collimated light.

(6) In the light receiving module according to (2), when the input light is light approximated by a Gaussian beam having a wavelength λ and a beam waist radius w, the input optical system of the collimating optical system a back focal length f _1, and the average focal length f _2AV of the light receiving portion side focal length of the light receiving unit side focal distance and the sagittal plane at the meridional plane of the asymmetric lens, the in the meridional plane of the asymmetric lens The difference Δf ₂ between the light receiving unit side focal length and the light receiving unit side focal length in the _{spherical surface} has a relationship satisfying | Δf ₂ |> 6πw ² (f _2av ) ² / λ (f ₁ ) ^2. It is characterized by.

(7) In the light receiving module according to (3), the input light is light approximated by a Gaussian beam having a wavelength λ and a beam waist radius w, and the light receiving unit extends from a main surface of the asymmetrical single convex lens. When the distance to the light receiving surface is d, the input optical system side focal length f _{1 of the} collimating optical system, the light receiving unit side focal length f _{3 of the} symmetric lens, and the meridional surface of the asymmetrical single convex lens The focal length f _4av of the light receiving portion side focal length in the light receiving portion side and the average focal length f _4av of the light receiving portion side focal length in the _spherical surface, and the light receiving portion side focal length in the meridional surface of the asymmetrical single convex lens The light-receiving-part-side focal length difference Δf ₄ satisfies the relationship of | Δf ₄ |> 6πw ² (f ₃ ) ² (f _4av −d) ² / λ (f ₁ ) ² d ^2. To do.

  The present invention provides a light receiving module that can achieve both a sufficiently small ORL and a sufficiently large light receiving efficiency without precisely adjusting the position of each optical system.

It is a functional block diagram of the light reception module which concerns on embodiment of this invention. It is a figure which shows conversion of the light ray by an optical system. It is a figure which shows the optical system contained in the light reception module which concerns on the 1st Embodiment of this invention. It is a perspective view of the asymmetric lens used in the 1st Embodiment of this invention. It is a figure which shows the focal distance in the meridian surface of an asymmetrical lens. It is a figure which shows the focal distance in the spherical surface of an asymmetrical lens. It is a figure which shows the optical system contained in the light reception module which concerns on the 2nd Embodiment of this invention. It is a perspective view of the cylindrical lens which is the 1st specific example of an asymmetrical optical system. It is a perspective view of the transparent flat plate which is the 2nd specific example of an asymmetrical optical system. It is a figure which shows the optical system contained in the light reception module which concerns on the 3rd Embodiment of this invention. It is a figure which shows the optical system contained in the light reception module which concerns on the 4th Embodiment of this invention. It is a schematic diagram showing the relationship between the coupling efficiency of light and a focus shift.

  Hereinafter, embodiments of the present invention will be described specifically and in detail based on the drawings. Note that components having the same function are denoted by the same reference symbols throughout the drawings for describing the embodiments, and the repetitive description thereof will be omitted. In addition, the figure shown below demonstrates the Example of embodiment to the last, Comprising: The magnitude | size of a figure and the reduced scale as described in a present Example do not necessarily correspond.

  FIG. 1 is a functional block diagram of a light receiving module 1 according to an embodiment of the present invention. A light receiving module 1 according to an embodiment of the present invention is a light receiving module that receives an optical signal from an optical fiber 5, and includes an input optical system 10 (Input OS), a collimating optical system 20 (Collimating OS), and a duplexer 30. (DMP: Demultiplexer), a condensing optical system 40 (Collecting OS), and a light receiver 50 (OR: Optical Receiver).

  The input optical system 10 is, for example, an optical fiber, and guides an optical signal transmitted through the optical fiber 5 to the inside of the light receiving module 1 to give input light. As for the connection between the input optical system 10 and the optical fiber 5, it is desirable that the fiber end face is axially aligned and angled so that reflection of light does not occur at the connection location.

In the embodiment described below, in order to simplify the description, the light emitted from the input optical system 10 is approximated by a Gaussian beam having a wavelength λ and a beam waist radius w. Here, the beam waist radius is the smallest radius among the radii (1 / e ² radius) at which the intensity of the beam is 1 / e ² (e is the base of the natural logarithm) as compared with the intensity at the center of the beam. . In the case of light emitted from the input optical system 10, the 1 / e ² radius at the emission side end face of the optical fiber constituting the input optical system 10 is the beam waist radius.

In this specification, geometrical optical approximation is used in analyzing the conversion of light by the optical system. In that case, the Gaussian beam emitted from the input optical system 10 is regarded as a bundle of a plurality of light beams, and the light beam at the 1 / e ² radius is defined as the outer edge of the light. In addition, when performing ray tracing, a thin lens approximation that ignores the thickness of the lenses constituting each optical system is used, and a paraxial approximation is used as necessary. Of course, these approximations are used only for the sake of simplifying the explanation, and it goes without saying that the present invention can be applied even when the approximations do not hold. In addition, if the light emitted from the input optical system 10 cannot be accurately approximated by a Gaussian beam, for example, when it is appropriate to represent it as a Hermitian Gaussian beam or a Laguerre Gaussian beam, the intensity of the beam is compared with the maximum intensity of the beam. Then, the position that becomes 1 / e ² is defined as the outer edge of the light, and the geometrical optical approximation may be applied by regarding the beam as a bundle of a plurality of light beams.

The collimating optical system 20 is, for example, a biconvex lens, and is a lens having a focal length f ₁ on the input optical system 10 side. The collimating optical system 20 is a collimator lens that converts input light passing through the focal point on the input optical system 10 side into collimated light (parallel light). The collimating optical system 20 may be configured by a plano-convex lens having a flat surface on the input optical system 10, or may be configured by using a combination of a concave lens and a convex lens, or a combination of a convex lens and a convex lens.

  The duplexer 30 decomposes the optical signal for each wavelength and includes a diffraction grating and the like. In the case of a transmission method that transmits light signals by superimposing light of a plurality of wavelengths, such as a four-wave multiplexing method, a demultiplexer 30 is necessary in the receiving module 1 because the superimposed light must be decomposed for each wavelength. It becomes. The demultiplexer 30 demultiplexes one input optical signal into four output optical signals, for example, when the receiving module 1 receives a four-wave multiplexing optical signal. In FIG. 1, two output optical signals are shown and the others are omitted in order to avoid complications due to repeated description.

  The condensing optical system 40 is an optical system that receives collimated light having a wavelength demultiplexed by the demultiplexer 30 among collimated light emitted from the collimating optical system 20 and converts the collimated light into convergent light with astigmatism. Yes, it is an optical system including one or more lenses. Here, astigmatism refers to an aberration in which the focal length differs between the meridian plane (cross section including the optical axis, meridional surface) and the spherical surface (cross section orthogonal to the meridian plane, sagittal surface). Since an actual lens is usually not completely symmetric with respect to the optical axis, it can be considered that any lens is accompanied by some astigmatism. In this specification, an optical system with astigmatism (asymmetric optical system) means that the absolute value of the difference between the focal length at the meridian plane and the focal length at the spherical surface of the optical system is the wavelength of light to be transmitted. Let's say that the wavelength is longer than the longest. Further, in this specification, an optical system without astigmatism (a symmetric optical system) means that the absolute value of the difference between the focal length on the meridian plane and the focal length on the spherical surface of the optical system is that of the transmitted light. The wavelength that is equal to or shorter than the shortest wavelength is used. Here, for an optical system with astigmatism, it is desirable that the absolute value of the difference between the focal length on the meridian plane and the focal length on the spherical surface is larger than the tolerance for the dimensions of the optical system. . Similarly, in an optical system without astigmatism, the absolute value of the difference between the focal length at the meridian plane and the focal length at the spherical surface of the optical system is smaller than the tolerance for the dimensions of the optical system. desirable. Details of the condensing optical system 40 will be described later.

  The light receiver 50 includes, for example, a photodiode, and outputs an electrical signal corresponding to the intensity of convergent light from the condensing optical system 40. The electrical signal output from each light receiver 50 is sent to the outside of the light receiving module 1, and the content of information encoded as an optical signal is read.

FIG. 2 is a diagram illustrating light beam conversion by the optical system 100 (OS). In FIG. 2, it proceeds at the position of x _{1 in} the x-axis direction (optical axis direction) and at an angle α ₁ (> 0, unit is radians) from the position of h ₁ in the y-axis direction (direction orthogonal to the optical axis). The light is converted by the optical system 100 and becomes light traveling in the direction of angle α ₂ (<0) at the position of x _{2 in} the x-axis direction and at the position of h ₂ (<h ₁ ) in the y-axis direction. It is shown. Here, the angle is an angle formed between the x-axis direction and the light traveling direction, and is measured with the counterclockwise direction being positive and the clockwise direction being negative in the xy plane with respect to the x-axis direction. Such a ray conversion is expressed as (Equation 1) below, assuming that the position h in the y-axis direction and the ray traveling angle α are collectively expressed as (h, α) ^t as a two-component vector. be able to.

Here, the 2 × 2 matrix M having matrix components of M ₁₁ , M ₁₂ , M ₂₁ , and M ₂₂ is a parameter that characterizes the optical system 100. Each component of the matrix M may be represented by A, B, C, and D, respectively, and is often called an ABCD matrix. For example, the optical system 100 is a free space having a length L (a space without a lens or the like, which is air in the embodiment of the present invention, but is not limited thereto, and is a space filled with another gas. In other words, h ₂ = h ₁ + Lα ₁ and α ₂ = α ₁ , so that M ₁₁ = 1, M ₁₂ = L, M ₂₁ = 0, M ₂₂ = It turns out that it becomes 1. The refractive index of free space (air) is 1 and constant.

Further, in a thin lens having a focal length f (a lens with negligible thickness), the position of the light beam in the y-axis direction does not change before and after the main surface (h ₂ = h ₁ ), and the angle is α ₂ = α _1. Since −h ₁ / f, it can be seen that M ₁₁ = 1, M ₁₂ = 0, M ₂₁ = −1 / f, and M ₂₂ = 1. Even if the lens thickness cannot be ignored, each matrix element can be expressed using the refractive index and thickness of the lens, and the same analysis as described below can be performed.

[First Embodiment]
FIG. 3 is a diagram showing an optical system included in the light receiving module 1 according to the first embodiment of the present invention. Although the light receiving module 1 according to the present embodiment includes the duplexer 30, in FIG. 3, the duplexer 30 is not illustrated in order to avoid complexity due to repeated description of the light receiver 50 and the like, and the input optical system 10 Only one unit of optical system reaching the light receiver 50 is shown. FIG. 3 shows an input optical system 10, a collimating optical system 20, an asymmetric lens 42 as a condensing optical system 40, and a light receiver 50. The solid arrow in the figure indicates the traveling direction of the light emitted from the input optical system 10, and the dotted arrow indicates the traveling direction of the return light from the light reflected by the light receiver 50 returning to the input optical system 10.

In the present embodiment, the distance from the end face of the optical fibers constituting the input optics 10 to the main surface of the biconvex lens constituting the collimating optical system 20 and d _1, from the main surface of the biconvex lens constituting the collimator optical system 20 the distance to the main surface of the asymmetric lens 42 is L, the distance from the main surface of the asymmetric lens 42 to the light receiving surface of the photodetector 50 and d _2.

The focal length of the biconvex lens constituting the collimating optical system 20 is f ₁ , the focal length on the light receiver 50 side in the meridian plane of the asymmetric lens 42 is f 2 _mer , and light reception in the spherical surface of the asymmetric lens 42 is received. The focal length on the side of the device 50 is assumed to be f _2sag . Here, the focal length on the light receiver 50 side in the meridian plane of the asymmetric lens 42 and the average focal length of the focal length on the light receiver 50 side in the _spherical surface are _expressed as f _2av = (f _2mer + f _2sag ) / 2, The difference between the focal length on the light receiver 50 side in the meridian plane of the asymmetric lens 42 and the focal length on the light receiver 50 side in the _{spherical surface} is expressed as Δf ₂ = f _2mer −f _2sag .

In the present embodiment, a biconvex elliptical lens is used as the asymmetric lens 42. In addition, for example, an anamorphic lens or a biconic lens may be used. In any case, _lenses having different focal lengths (f _2mer ≠ f _2sag ) between the meridional surface and the _spherical surface, and the difference is not less than the wavelength of the transmitted light (Δf ₂ > λ). It is desirable to use it. The asymmetric lens 42 is a lens that is asymmetric with respect to the optical axis. That is, the asymmetric lens 42 is a lens that does not have rotational symmetry with respect to the optical axis.

In order to analyze the return light of the light emitted from the input optical system 10, ray tracing is performed considering the conversion of light rays by each optical system. _First , considering light rays converted by a collimating optical system 20 including a free space having a length d ₁ and a thin lens having a focal length f ₁ , the conversion matrix M _{20 ← 10} is expressed as (Equation 2) below. Is done. However, it is assumed that the collimating optical system 20 can produce ideal collimated light, and d ₁ = f ₁ .

Subsequently, since there is a free space of length L between the collimating optical system 20 and the asymmetric lens 42, the ray conversion matrix _{M42 ← 20} is expressed as (Equation 3) below.

Finally, the focal length f _2mer in the meridian _plane, and considering the focal length light conversion by the free space of the asymmetric lens 42 and the length d ₂ with f _2Sag in sagittal plane, each of the meridional plane and Tamaketsumen The ray conversion matrix M _{50 ← 42} ^{mer (sag)} is expressed as (Equation 4) below. However, the _{f 2mer (sag)} during equation, if when considering the ray in the meridian plane using the _{f 2mer,} consider a ray in Tamaketsumen indicating the use of _{f 2sag.}

  Approximately 1% of the light reaching the light receiver 50 is reflected by the light receiving surface or the like to become return light. Similarly, when considering the conversion matrix of light rays, the return light is expressed as (Equation 5-7) below in order from the light receiver 50 side.

By multiplying the above six transformation matrices, how the rays with the position and angle of (h _in , α _in ) ^t emitted from the input optical system 10 are reflected on the meridional surface 200 and the spherical surface 300, respectively. (H _back ^{mer (sag)} , α _back ^{mer (sag)} ) ^t representing the position to return to the input optical system 10 at a certain position and angle can be obtained as in the following (Equation 8).

  Before describing the details of the ORL, it will be described how the return light can be reduced in this embodiment. FIG. 4 is a perspective view of the asymmetric lens 42 used in the first embodiment of the present invention. Here, the meridional surface 200 (the xy plane in the drawing) that is a surface including the optical axis of the asymmetric lens 42 and the spherical surface 300 (the xz plane in the drawing) that is a surface orthogonal to the meridional surface 200. Is virtually illustrated.

FIG. 5 is a diagram showing a focal length _f2mer on the meridian plane 200 (xy plane in the drawing) of the asymmetric lens 42, and FIG. 6 shows a spherical surface 300 ( _xz in the drawing) of the asymmetric lens 42. It is a figure which shows the focal distance _f2sag in a plane. In the present embodiment, f _2sag > f _2mer and the two do not match. Therefore, no matter how the distance d ₂ from the main surface of the asymmetric lens 42 to the light receiving surface of the light receiver 50 is selected, the light converged by the asymmetric lens 42 is at least one of the meridional surface 200 or the spherical surface 300. No image is formed on the light receiving surface of the light receiver 50.

According to the present embodiment, for example, when positioning to d ₂ = f _2av , regardless of which direction the position of the asymmetric lens 42 (or the position of the light receiver 50) is shifted in any direction along the optical axis direction, The image on the light receiving surface of the light receiver 50 remains blurring to the same extent. Therefore, even if a deviation occurs in any direction, the ORL does not increase so much and the light receiving efficiency of the light receiver 50 does not decrease so much, so even if the position of each optical system is not adjusted precisely, it is sufficiently small. It is possible to achieve both ORL and sufficiently high light receiving efficiency. On the other hand, when a normal lens is used as the condensing optical system 40, if the positions of the condensing optical system 40 and the light receiver 50 are shifted along the optical axis direction, the image becomes clearer and the ORL increases, or the image becomes more There is a possibility that the light receiving efficiency of the light receiver 50 may be reduced or the light receiving efficiency of the light receiver 50 may be reduced.

Hereinafter, ORL is analyzed with h _in = 0. In the analysis, d ₂ is set as d ₂ = f _2av + Δd ₂ and the first order of Δd ₂ is left. Here, Δd ₂ represents a deviation from the ideal positional relationship between the asymmetric lens 42 and the light receiver 50, d ₂ = f _2av . Further, the focal length _{f 2mer = f 2av + Δf 2} /2 in the meridional plane 200, and the focal length _{f 2sag = f 2av -Δf 2/} 2 in Tamaketsumen 300, and leaving until the primary Delta] f _2, respectively .

The coupling efficiency η of the return light to the input optical system 10 is obtained by η = (η _mer η _sag ) ^1/2 . Here, η _mer is the light coupling efficiency at the meridian surface 200, and η _sag is the light coupling efficiency at the spherical surface 300. η _mer is obtained by using the magnification m ^mer = −α _in / α _back ^mer of the return light on the meridian plane 200 and the focal position shift Δz ^mer = −h _back ^mer / α _back ^mer of the return light on the meridian plane 200, It is calculated as (Equation 9) below. Here, the shift Δz ^mer of the focal position of the return light is 0 when the distance from the main surface of the collimating optical system 20 to the end surface of the optical fiber constituting the input optical system 10 coincides with the focal position on the meridian plane 200. Suppose that

Also for η _sag , the return light magnification m ^sag = −α _in / α _back ^sag on the spherical surface 300 and the focal position shift Δz ^sag = −h _back ^sag / α _back ^sag on the spherical surface 300. And can be similarly determined. The result is obtained by replacing m ^mer and Δz ^mer in Equation 9 with m ^sag and Δz ^sag , respectively.

Hereinafter, a case where | Δd ₂ / f _{2mer (sag)} | << 1, | Δf ₂ / f _{2mer (sag)} | << 1 and f _2av / L˜1 is considered. This is because the position shift of the asymmetric lens 42 is sufficiently smaller than the focal length on the meridional surface 200 (or the spherical notch surface 300) of the asymmetric lens 42, and the focal length on the meridional surface 200 of the asymmetric lens 42 and the focal point on the spherical notch surface 300. The difference in distance is sufficiently smaller than the focal length of the meridional surface 200 (or the spherical surface 300) of the asymmetric lens 42, and the average focal length of the asymmetric lens 42 is larger than that of the main surface of the asymmetric lens 42. It means the case where it is almost the same as the distance to the surface. In this case, the coupling efficiency η of the return light to the input optical system 10 is obtained as follows (Formula 10).

As seen from Equation 10, the light receiving module 1 according to the present embodiment, the position of the asymmetric lens 42 (or the position of the optical receiver 50) is also displaced in either direction of the optical axis, i.e. [Delta] d ₂ is a positive or negative value Even so, the coupling efficiency η of the return light to the input optical system 10 changes in the same way. In other words, the coupling efficiency η of the return light to the input optical system 10 is an even function with respect to Δd ₂ . Further, the coupling efficiency η of the return light to the input optical system 10 takes a maximum value when Δd ₂ = 0. Therefore, in the light receiving module 1 according to the present embodiment, the coupling efficiency η of the return light to the input optical system 10 is Δd ₂ = 0 even if the asymmetric lens 42 and the light receiver 50 are not precisely aligned. (When the alignment of the asymmetric lens 42 and the light receiver 50 is completely performed), the input optical system regardless of which direction the positions of the asymmetric lens 42 and the light receiver 50 are shifted along the optical axis. The coupling efficiency η of the return light to 10 is similarly reduced.

In order to sufficiently reduce the coupling efficiency η of the return light to the input optical system 10, the difference Δf between the focal length f ₁ of the collimating optical system 20, the average focal length f _{2av of} the asymmetric lens 42, and the focal length of the asymmetric lens 42. ₂ may satisfy the following conditions. Since the coupling efficiency η of the return light to the input optics 10 becomes maximum [Delta] d ₂ = 0, imposes an upper limit to η in the case of [Delta] d ₂ = _0, to _{f 1,} f _2av, Δf ₂ satisfies Finding the conditions is sufficient.

In this embodiment, in order to achieve −30 dB as the ORL, an upper limit value of η _| Δd2 _{= 0} <0.1 is imposed. In this case, the coupling efficiency η of the return light to the input optical system 10 is smaller than 0.1 no matter what value Δd ₂ takes. In the present embodiment, since the reflection of light is about 1% at a light receiving surface of the photodetector 50, if eta <0.1, the light intensity P _in the light emitted from the input optical system 10, the input optical The ratio of the light intensity P _back of the light returning to the system 10 is P _in / P _back = 10 ³ at the maximum. Therefore, the ORL is at most ORL = −10 × log ₁₀ (P _in / P _back ) = − 30 [dB], and a sufficiently small ORL of −27 dB or less can be achieved.

When the condition of η _{| Δd2 = 0} <0.1 is modified and the conditions to be satisfied by f ₁ , f _2av , and Δf ₂ are obtained, the following (Formula 11) is obtained.

Here, for example, _assuming that the wavelength λ = 1.5 μm and the beam waist radius w = 5 μm, | Δf ₂ |> 300 (f _2av ) ² / (f ₁ ) ² [μm]. Therefore, for example, if f ₁ = f _2av , it can be seen that ORL can be suppressed to −30 dB or less if | Δf ₂ |> 0.3 [mm]. Further, if a lens satisfying f _2av / f ₁ <1 is selected, the lower limit value of | Δf ₂ | is determined by the square of f _2av / f ₁ , so even an asymmetric lens 42 with smaller astigmatism can be obtained. An ORL of −30 dB or less can be achieved.

As a more desirable case, let _{us consider a} case where | Δd ₂ / f _{2mer (sag)} | << 1, | Δf ₂ / f _{2mer (sag)} | << 1 and f _2av / L << 1. This is because the position shift of the asymmetric lens 42 is sufficiently smaller than the focal length on the meridional surface 200 (or the spherical notch surface 300) of the asymmetric lens 42, and the focal length on the meridional surface 200 of the asymmetric lens 42 and the focal point on the spherical nose surface 300 The difference in distance is sufficiently smaller than the focal length of the meridional surface 200 (or the spherical surface 300) of the asymmetric lens 42, and the average focal length of the asymmetric lens 42 is larger than that of the main surface of the asymmetric lens 42. It means a case sufficiently smaller than the distance to the surface. In this case, the term (m ^mer + (m ^mer ) ⁻¹ ) ² in η _mer (or η _sag ) is larger than 1, and the coupling efficiency η of the return light to the input optical system 10 is further reduced.

Specifically, if L> f ₁ + 6f _2av , the contribution of magnification m is added, and the coupling efficiency η of the return light to the input optical system 10 is further reduced. However, even if | Δd ₂ / f _{2mer (sag)} | << 1, | Δf ₂ / f _{2mer (sag)} | << 1, and f _2av / L˜1, f ₁ , f _2av , Δf ₂ satisfies the relationship of Equation 11, the ORL can be suppressed to −30 dB or less. _If the condition of L> f ₁ + 6f _2av (the condition of f _2av / L << 1) is satisfied, the ORL can be further reduced. If this condition is not satisfied, the ORL is reduced to −30 dB or less. It doesn't mean you can't let it happen.

  Conventionally, in a light receiving module that receives a plurality of light beams, the tolerance of alignment of each optical system for each light beam has been severely limited. For example, in the case of a light receiving module used in four-wave multiplex optical communication, alignment is required about four times as many parts as in the case of handling one light beam. For this reason, the tolerances on the positions of the optical systems are more strictly limited due to the accumulation of tolerances. Therefore, as the number of optical signals handled by the multiplexing system increases, it has been difficult to achieve both a sufficiently small ORL and a sufficiently large light receiving efficiency by adjusting the position of each optical system. However, according to the light receiving module 1 according to the present embodiment, a sufficiently small ORL can be achieved if the condition of Expression 11 is satisfied. Therefore, it can be said that the light receiving module 1 according to the present embodiment is suitable for receiving a multiplexed optical signal. The light receiving module according to the present invention is not limited to the light receiving module that handles the optical signal of the multiplex system like the light receiving module 1 according to the present embodiment, and one unit of optical system (input optical system 10, collimating optics). It may be a light receiving module that includes the system 20, the condensing optical system 40, and the light receiver 50) and receives one light beam (one light signal). However, since the tolerance for the position of each optical system is more strictly limited when dealing with a multiplex optical signal, the present invention is more effective in the case of a light receiving module that receives the multiplex optical signal. .

  In the above description, the beam is considered to be a single mode. However, if it is a multi-mode, there are as many beam waist radii w as the number of modes, and the coupling efficiency value for each mode is determined by their contribution. The sum total over time is the coupling efficiency of the entire multimode beam. Therefore, the above analysis can also be applied to the multimode.

  In order to further reduce the reflected return light, a circular so-called aperture may be arranged so as to surround the end face of the optical fiber constituting the input optical system 10. When such a configuration is adopted, a part of the return light can be blocked by the aperture, so that the ORL can be further reduced.

  Further, the end face of the optical fiber constituting the input optical system 10 may be cut obliquely so as to be inclined by 6 to 8 degrees as measured from a plane orthogonal to the central axis of the optical fiber. In that case, the optical axis of the light emitted from the optical fiber is inclined by 2.7 to 3.8 °, and the optical fiber is shifted in the direction along the main surface of the collimating optical system 20, so that the optical fiber is arranged. The reflected return light by itself can be suppressed, and the ORL can be further reduced.

  In addition, as a lens used in this embodiment, in addition to a spherical lens and an aspherical lens, a condensing function as a lens, such as a diffraction lens, a refractive index distribution lens, a hologram lens, a lens using a resonance phenomenon such as plasmon or polariton, etc. Anything you have can be used. A combination of these lenses may be used.

  As the light receiving element constituting the light receiver 50, a photodiode is used in the present embodiment, but an optical element including a waveguide or a light guide may be used. In this case, the light receiving module 1 outputs an optical signal to the outside of the housing, and the reflection surface that generates the reflected return light is the end surface of the optical element that includes the waveguide and the light guide.

As for the astigmatism magnitude Δf ₂ , the ORL can be reduced as the magnitude of the astigmatism increases. However, if the magnitude is too large, the light coupling efficiency with respect to the light receiver 50 decreases. Therefore, Δf ₂ is approximately equal to or less than the Rayleigh length of the convergent light that converges toward the light receiver 50 (distance where the radius of the converged Gaussian beam is 2 ^−1/2 times, z _R = πw ² / λ). It is desirable to do.

[Second Embodiment]
FIG. 7 is a diagram showing an optical system included in the light receiving module 1 according to the second embodiment of the present invention. Although the light receiving module 1 according to the present embodiment also includes the duplexer 30, as in the first embodiment, the duplexer 30 is not illustrated in FIG. 7, and one unit from the input optical system 10 to the light receiver 50 is not illustrated. Only the optical system is shown. FIG. 7 shows the input optical system 10, the collimating optical system 20, the symmetric lens 44 a and the asymmetric optical system 46 as the condensing optical system 40, and the light receiver 50.

  The difference between the first embodiment and the second embodiment is the configuration of the condensing optical system 40. In the second embodiment, the condensing optical system 40 includes a symmetric lens 44 a and an asymmetric optical system 46. The symmetric lens 46a is, for example, a spherical biconvex lens, and is a normal lens without astigmatism. The symmetric lens 46a is a lens that is symmetric with respect to the optical axis. As the asymmetric optical system 46, any one of the following specific examples is adopted.

  FIG. 8 is a perspective view of a cylindrical lens 46 a which is a first specific example of the asymmetric optical system 46. In FIG. 8, the meridian surface 200 (xy plane in the drawing) and the spherical notch surface 300 (xz plane in the drawing) of the cylindrical 46a are virtually illustrated. When the light condensed by the symmetric lens 44a is incident on the cylindrical 46a, the light is further converged on the meridian surface 200, and is emitted as it is without being converged or diffused on the spherical surface 300. As a result, the light passing through the condensing optical system 40 including the symmetric lens 44a and the cylindrical 46a has a focal length on the meridian plane 200 that is smaller than a focal length on the spherical surface 300, and the focal lengths do not match. It becomes. Therefore, for the same reason as described in the first embodiment, the coupling efficiency η of the return light to the input optical system 10 can be reduced, and the ORL can be suppressed to −30 dB or less.

  FIG. 9 is a perspective view of a transparent flat plate 46 b as a second specific example of the asymmetric optical system 46. Also in FIG. 9, the meridian surface 200 (the xy plane in the drawing) and the spherical notch surface 300 (the xz plane in the drawing) of the transparent flat plate 46 b are virtually illustrated. The transparent flat plate 46b is disposed so as to be oblique to the traveling direction of the collimated light emitted from the collimating optical system 20. In other words, the transparent flat plate 46b is arranged to be inclined by θ (0 <θ <π / 2) with respect to the spherical surface 300. Therefore, when the light condensed by the symmetric lens 44a is incident, the light is further converged on the meridian plane 200, and is emitted as it is without being converged or diffused on the spherical surface 300. The light that has passed through the condensing optical system 40 constituted by the transparent flat plate 46b has a focal length on the meridian plane 200 that is smaller than a focal length on the spherical surface 300, and for the same reason as described in the first embodiment. The coupling efficiency η of the return light to the input optical system 10 can be reduced, and the ORL can be suppressed to −30 dB or less.

[Third Embodiment]
FIG. 10 is a diagram showing an optical system included in the light receiving module 1 according to the third embodiment of the present invention. Although the light receiving module 1 according to the present embodiment also includes the duplexer 30, as in the first embodiment, the duplexer 30 is not illustrated in FIG. 10, and one unit from the input optical system 10 to the light receiver 50 is not illustrated. Only the optical system is shown. FIG. 10 shows an input optical system 10, a collimating optical system 20, a symmetric lens 44 a and an asymmetric half-convex lens 48 as a condensing optical system 40, and a light receiver 50. The asymmetric single convex lens 48 is affixed to the daylighting surface of the light receiver 50.

Here, the distance from the end face of the optical fiber constituting the input optical system 10 to the main surface of the collimating optical system 20 is d ₁ , the distance from the main surface of the collimating optical system 20 to the main surface of the symmetric lens 44a is L, and symmetric. distance from the main surface of the lens 44a to the main surface of the asymmetric piece convex lens 48 is a distance d _3, from the main surface of the asymmetric piece convex lens 48 to the light receiving surface of the photodetector 50 is is d. Further, the focal length of the input optical system 10 side of the biconvex lens constituting the collimating optical system 20 is f _1, the photodetector 50 side focal length of the symmetric lens 44a is f _3. Further, it is _assumed that the focal length on the light receiver 50 side in the meridian plane of the asymmetrical single convex lens 48 is _f4mer , and the focal length on the light receiver 50 side in the spherical surface of the asymmetrical single convex lens 48 is _f4sag . Here, the focal length on the light receiver 50 side in the meridional plane of the asymmetrical convex lens 48 and the average focal length of the focal length on the light receiver 50 side in the _spherical surface are _expressed as f _4av = (f _4mer + f _4sag ) / 2. The difference between the focal length on the light receiver 50 side in the meridian plane of the asymmetrical convex lens 48 and the focal distance on the light receiver 50 side in the _{spherical surface} is expressed as Δf ₄ = f _4mer −f _{4 sag} .

In order to obtain the coupling efficiency η of the return light to the input optical system 10 in this embodiment, the light conversion matrix M _{20 ← 10} , M _{44a ← 20} , M _{50 ← 48 ← 44a} ^{mer (sag)} in each optical system, It is necessary to know ^M44a _{← 48 ← 50} ^{mer (sag)} , ^M20 _{← 44a} , and ^M10 _{← 20} . Here, M _{20 ← 10} and M _{10 ← 20} are the same as Equation 2 and Equation 7, respectively. M _{44a ← 20} = M _{42 ← 20} (Formula 3), and M _{20 ← 44a} = M _{20 ← 42} (Formula 6). About the remainder, it calculates | requires by calculating the following (Formula 12 and 13).

By multiplying the above six transformation matrices, the position and angle of the light (h _in , α _in ) ^t emitted from the input optical system 10 on the meridional plane 200 and the spherical notch plane 300, respectively. (H _back ^{mer (sag)} , α _back ^{mer (sag)} ) ^t can be obtained as shown below (Formula 14).

The coupling efficiency η of the return light to the input optical system 10 is obtained by η = (η _mer η _sag ) ^1/2 as in the case of the first embodiment. Here, η _mer can be obtained by Equation 9, and the same applies to η _sag . _{Assuming that} h _in = 0, d ₃ = (f ₃ −f _4av d / (f _4av −d)) + Δd _{3 is set} and the first order of Δd ₃ is left, and the focal point of the asymmetrical single convex lens 48 on the meridional plane 200 distance _{f 4mer = f 4av + Δf 4} /2, and the focal length _{f 4sag = f 4av -Δf 4/} 2 in Tamaketsumen 300, respectively When leaving until primary Delta] f _4, in the first embodiment The same analysis as the case will be performed. If d ₃ = (f ₃ −f _4av d / (f _4av −d)), the intermediate position between the focal position of the convergent light on the meridian plane 200 and the focal position of the spherical surface 300 is This coincides with the position of the light receiving surface. However, since the combined optical system of the symmetric lens 44a and the asymmetrical single convex lens 48 has a positive effective focal length, it is necessary that f _4av > d.

Here, | Δd ₃ / f _{4mer (sag)} | << 1, | Δd ₃ / f ₃ | << 1, | Δf ₄ / f _{4mer (sag)} | << 1, | Δf ₄ / f ₃ | < Consider the case of <1 and f ₃ / L˜1. This is because the displacement of the symmetric lens 44a is sufficiently smaller than the focal length of the meridional surface 200 (or the spherical surface 300) of the asymmetrical convex lens 48 and the focal length of the symmetric lens 44a. The difference between the focal length at the spherical surface 300 and the focal length at the spherical surface 300 is sufficiently smaller than the focal length at the meridional surface 200 (or the spherical surface 300) of the asymmetrical single convex lens 48 and the focal length of the symmetric lens 44a. This means that the focal length of 44a is about the same as the distance from the main surface of the symmetric lens 44a to the main surface of the collimating optical system 20. In such a case, an upper limit value is imposed on η _{| Δd3 = 0} to obtain a condition that f ₁ , f ₃ , f _4av , Δf ₄ , and d should satisfy. Also in this embodiment, since about 1% of light is reflected by the light receiving surface of the light receiver 50, η _{| Δd3 = 0} <0.1 is imposed as a condition for the ORL to be −30 dB or less. In this case, the conditions to be satisfied by f ₁ , f _4av , Δf ₄ , and d are as follows (Formula 15).

Here, if the asymmetrical single-convex lens 48 is selected so that 0 <(f _4av −d) / d <1, the term of (f _4av −d) ² / d ² reduces the lower limit value of | Δf ₄ |. Even if the asymmetrical single-convex lens 48 plays a role and has smaller astigmatism, the ORL can be set to -30 dB or less. Further, even if a lens satisfying f ₃ / f ₁ <1 is selected, or the asymmetrical single convex lens 48 with smaller astigmatism, the ORL can be set to −30 dB or less.

More desirably, | Δd ₃ / f _{4mer (sag)} | << 1, | Δd ₃ / f ₃ | << 1, | Δf ₄ / f _{4mer (sag)} | << 1, | Δf ₄ / f ₃ Consider a case where | << 1 and f ₃ / L << 1 are satisfied. This is because the displacement of the symmetric lens 44a is sufficiently smaller than the focal length of the meridional surface 200 (or the spherical surface 300) of the asymmetrical convex lens 48 and the focal length of the symmetric lens 44a. The difference between the focal length at the spherical surface 300 and the focal length at the spherical surface 300 is sufficiently smaller than the focal length at the meridional surface 200 (or the spherical surface 300) of the asymmetrical single convex lens 48 and the focal length of the symmetric lens 44a. This means that the focal length of 44a is sufficiently smaller than the distance from the main surface of the symmetric lens 44a to the main surface of the collimating optical system 20. In this case, as in the case of the first embodiment, the contribution of the magnification m is also added, and the coupling efficiency η of the return light to the input optical system 10 is further reduced.

Specifically, if L> f ₁ + f ₃ +5 (f ₃ ) ² (f _4av −d) ² / f _4av d ² , the return light to the input optical system 10 is added with the contribution of the magnification m. The coupling efficiency η is further reduced. However, | Δd ₃ / f _{4mer (sag)} | << 1, | Δd ₃ / f ₃ | << 1, | Δf ₄ / f _{4mer (sag)} | << 1, | Δf ₄ / f ₃ | << 1 and f ₃ / L˜1, the ORL can be suppressed to −30 dB or less if f ₁ , f _4av , Δf ₄ , and d satisfy the condition of Expression 15. If the condition of L> f ₁ + f ₃ +5 (f ₃ ) ² (f _4av −d) ² / f _4av d ² (f ₃ / L << 1) is satisfied, the ORL can be further reduced. However, this does not mean that the ORL cannot be reduced to -30 dB or less unless this condition is satisfied.

[Fourth Embodiment]
FIG. 11 is a diagram showing an optical system included in the light receiving module 1 according to the fourth embodiment of the present invention. Unlike FIG. 3, FIG. 7, and FIG. 10, which show the cases of the first to third embodiments, FIG. 11 shows the duplexer 30 and shows the entire optical system included in the light receiving module 1. . FIG. 11 shows the input optical system 10, the collimating optical system 20, the branching filter 30, the integral symmetric lens 44 b and the asymmetrical single convex lens 48 as the condensing optical system 40, and the light receiver 50. In the fourth embodiment, an integral symmetric lens 44 b and an asymmetrical single convex lens 48 are used as the condensing optical system 40. Here, the integrated symmetric lens 44b is disposed so as to straddle a plurality of light beams demultiplexed by the duplexer 30, and a convex lens is formed at a portion where each light beam is incident. It is desirable to form a biconvex lens in such a portion. The integral symmetric lens 44b can be produced by injection molding a synthetic resin such as polycarbonate. Therefore, there is an advantage that the components of the light receiving module 1 can be created at a low cost and in a short time compared to the case where the lenses are individually created. In addition, the four light receivers 50 each having the four asymmetrical single convex lenses 48 attached thereto are provided on a common base.

  In this embodiment, the position of the condensing optical system 40 is set for each lens by adopting the integral symmetric lens 44b and the asymmetrical single-convex lens 48 attached to the daylighting surface of the light receiver 50 provided on the common pedestal. It is no longer necessary to match. By adopting such a configuration, if one unit of the optical system is adjusted so as to satisfy the condition of Expression 15, the ORL can be reduced to −30 dB or less for all four units of the optical system.

  In addition, instead of using the asymmetrical single convex lens 48, an integral symmetric lens 44b and a cylindrical lens 46a arranged so as to straddle the four light receivers 50 or a transparent flat plate 46b may be used. The four light receivers 50 do not necessarily have to be provided on a common pedestal, and may be individually positioned. Naturally, a combination of the above-described configurations may be used, an aperture may be arranged so as to surround the end face of the optical fiber that constitutes the input optical system 10, and the input optical system 10 is configured. It is good also as employ | adopting the structure which cuts the end surface of an optical fiber diagonally.

  According to the light receiving module 1 described above, it is possible to provide the light receiving module 1 that can achieve both a sufficiently small ORL and a sufficiently large light receiving efficiency without precisely adjusting the position of each optical system. .

  DESCRIPTION OF SYMBOLS 1 Light receiving module, 5 Optical fiber, 10 Input optical system, 20 Collimating optical system, 30 Demultiplexer, 40 Condensing optical system, 42 Asymmetric lens, 44a Symmetric lens, 44b Integrated symmetrical lens, 46 Asymmetric optical system, 46a Cylindrical Lens, 46b Transparent flat plate, 48 Asymmetrical single-convex lens, 50 Light receiver, 100 Optical system, 200 Meridian surface, 300 Sphere missing surface

Claims (7)

A collimating optical system including one or a plurality of lenses that receives input light from the input optical system and converts the input light into collimated light;
A condensing optical system including one or a plurality of lenses that receives the collimated light and converts the collimated light into convergent light with astigmatism;
A light receiving unit for receiving the convergent light;
A light receiving module comprising: The light receiving module according to claim 1,
The condensing optical system is
Including asymmetric lenses that produce astigmatism,
A light receiving module characterized by that. The light receiving module according to claim 1,
The condensing optical system is
A symmetric lens for converging the collimated light;
An asymmetrical single convex lens that further converges the light converged by the symmetric lens with astigmatism, and
The asymmetrical single convex lens is affixed to the daylighting surface of the light receiving unit,
A light receiving module characterized by that. The light receiving module according to claim 1,
The condensing optical system is
Including cylindrical lenses,
A light receiving module characterized by that. The light receiving module according to claim 1,
The condensing optical system is
Including a transparent flat plate arranged obliquely with the traveling direction of the collimated light,
A light receiving module characterized by that. The light receiving module according to claim 2,
When the input light is light approximated by a Gaussian beam having a wavelength λ and a beam waist radius w,
The input optical system side focal length f _{1 of the} collimating optical system;
An average focal length f _2av of the light receiving unit side focal length in the meridional surface of the asymmetric lens and the light receiving unit side focal length in the _{spherical surface} ,
A difference Δf ₂ between the light receiving unit side focal length in the meridional plane of the asymmetric lens and the light receiving unit side focal length in the spherical surface is,
| Δf ₂ |> 6πw ² (f _2av ) ² / λ (f ₁ ) ²
A light receiving module characterized by that. The light receiving module according to claim 3,
The input light is light approximated by a Gaussian beam having a wavelength λ and a beam waist radius w;
When the distance from the main surface of the asymmetrical single convex lens to the light receiving surface of the light receiving unit is d,
The input optical system side focal length f _{1 of the} collimating optical system;
And the light receiving unit side focal length f ₃ of the symmetrical lens,
An average focal length f _4av of the light receiving unit side focal length in the meridional surface of the asymmetrical convex lens and the light receiving unit side focal length in the _{spherical surface} ,
A difference Δf ₄ between the light receiving unit side focal length in the meridional plane of the asymmetrical single convex lens and the light receiving unit side focal length in the spherical surface is,
| Δf ₄ |> 6πw ² (f ₃ ) ² (f _4av −d) ² / λ (f ₁ ) ² d ²
A light receiving module characterized by that.

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