JP2005148267A - Optical system and module for optical communication - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an optical system which can roughly linearly map changes in wavelengths of luminous fluxes on an imaging face and a module for optical communication which can roughly linearly map luminous fluxes of optical signals which are transmitted through an optical communication system by a wavelength division multiplexing system and whose wavelengths has changed on the imaging face of a light receiving element array or the like and can highly accurately detect the luminous fluxes whose wavelengths are different. <P>SOLUTION: The optical system includes a grating element 3 having a light separating function and an fθ lens on which each of rays b1 to b3 from the grating element 3 are made incident and which is arranged so that diffracted rays are converged respectively on position coordinates P1, P2, P3 which are substantially proportional to their angles of incidence. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to an optical communication module and an optical system for detecting an optical signal for each wavelength in an optical communication system using a wavelength division multiplexing system.

  2. Description of the Related Art Conventionally, in an optical communication system using WDM (wavelength division multiplexing), a light receiving element or the like is arranged at an optical signal receiving terminal, and received optical signals having different wavelengths are separated and monitored. For example, as shown in FIG. 4, the optical demultiplexer described in Patent Document 1 below includes a single-core input fiber 100, a collimator lens 120, a diffraction grating 140, and a light receiving element array 160 that is a photodetector. . The divergent light beam introduced from the input fiber 100 into the cylindrical body 180 through the connecting portion 220 is converted into a parallel light beam by the collimator lens 120 and then reaches the diffraction grating 140. The light beam separated for each wavelength in accordance with the wavelength dispersion characteristic of the diffraction grating 140 is converted into a convergent light beam for each demultiplexed wavelength by the collimator lens 120, and is received by the light receiving element array 160 at each light receiving element arranged in a line at equal intervals. By entering, light detection for each wavelength is performed.

  As shown in FIG. 5, when each diffracted light from the diffraction grating 140 is focused on the imaging surface 161 of the light receiving element array 160 at the diffraction angles θ1 and θ2 for two different wavelengths λ1 and λ2, the diffraction angle changes. Δθ is proportional to the wavelength change Δλ of the diffracted light, but the position change amount Δx on the image plane 161 is related to the diffraction angles θ1 and θ2 and takes into account sin θ = n · λ / d (n = 1) become that way.

Δx = f (tan θ1−tan θ2)
= F {tan (sin ⁻¹ (λ1 / d)) − tan (sin ⁻¹ (λ2 / d))}
Where d: pitch of the diffraction grating f: distance between the diffraction grating and the imaging plane

In the above equation, as λ1 / d and λ2 / d increase, the linearity of the tan function is lost, and the position change amount Δx is not proportional to the wavelength change. For this reason, as shown in FIGS. 4 and 5, in the light receiving element array 160 in which the respective light receiving elements are arranged at equal intervals on the imaging surface 161, an error in position change becomes large on both sides of the wavelength range. In a module for a WDM (wavelength division multiplexing) optical communication system using a conventional optical demultiplexer such as 4, there arises a problem that detection errors increase.
JP 2000-349305 A

  In view of the above-described problems of the prior art, the present invention provides an optical system capable of accurately and linearly mapping a change in the wavelength of a light beam, and a wavelength transmitted by an optical communication system using a wavelength division multiplexing system. It is an object of the present invention to provide an optical communication module capable of mapping a light beam of an optical signal having changed substantially linearly onto an imaging surface such as a light receiving element array and accurately detecting light beams having different wavelengths.

  In order to achieve the above object, an optical system according to the present invention includes a grating element having a spectroscopic function, and each diffracted light from the grating element, and each of the diffracted lights at a position coordinate that is at least substantially proportional to the input angle. And a lens disposed so that the diffracted light is focused.

  According to this optical system, diffracted light having different wavelengths diffracted by the grating element is focused on the position coordinates that are at least substantially proportional to the input angle by the lens, so that each diffracted light having different wavelengths corresponds to each position coordinate. It can be mapped at least approximately linearly to the image plane.

  The optical system can be configured such that a light beam enters the grating element from the end face of the optical fiber. Thereby, the optical system of the present invention can be applied to an optical communication module that separates and detects light beams of optical signals having different wavelengths transmitted in an optical communication system using a wavelength division multiplexing system.

  Further, since the lens is composed of an fθ lens or an orthographic lens, it is possible to focus on position coordinates substantially proportional to the input angle. The fθ lens can be provided at low cost, and the optical system of the present invention can be easily realized. The fθ lens is a lens having a distortion characteristic such that the image height is represented by fθ, and is realized by a spherical lens + toroidal lens, and is widely used in a scanning optical system such as a laser printer. The orthographic lens has an image height represented by 2 f sin (θ / 2) and is often used as a fisheye lens together with the f θ lens.

  An optical communication module according to the present invention includes an optical fiber disposed in an input unit, a grating element having a spectral function and receiving a light beam from an end surface of the optical fiber, and each diffracted light from the grating element being input. A lens disposed so that each diffracted light is focused at a position coordinate that is at least substantially proportional to the input angle; and a light receiving means having a plurality of light receiving portions disposed at equal intervals on the focal plane of the lens; It is characterized by providing.

  According to this optical communication module, light beams of optical signals having different wavelengths transmitted in an optical communication system using the wavelength division multiplexing system are diffracted by the grating element, and each diffracted light having different wavelengths is at least substantially at the input angle by the lens. Since the focal point is focused on the position coordinates proportional to each other, each diffracted light can be mapped at least almost linearly to the focal plane corresponding to each position coordinate. Since a plurality of light receiving portions are arranged at equal intervals on this focal plane, each diffracted light having different wavelengths can be linearly mapped onto an imaging surface such as a light receiving element array, and light beams having different wavelengths can be detected accurately. Can do.

  In the optical communication module, it is preferable that the plurality of light receiving portions are arranged on the focal plane of the lens at intervals corresponding to the resolution defined by the grating.

  Further, the light receiving means can be constituted by a light receiving element array in which a plurality of light receiving elements are arranged at equal intervals or an optical fiber array in which a plurality of optical fiber terminals are arranged at equal intervals.

  Further, it is preferable that the grating element and the lens are integrated by a cylindrical body or the like. This eliminates the need for axial alignment between the grating element and the lens, and also allows the optical fiber terminal and grating element to be aligned with the outer dimensions of the cylindrical body, eliminating the need for alignment of the optical system or It can be done easily.

  In addition, since the lens is composed of an fθ lens, the optical communication module of the present invention can be realized inexpensively and easily. The lens may be an orthographic lens.

  According to the optical system of the present invention, the change in the wavelength of the light beam can be mapped onto the image plane at least almost linearly. Further, according to the optical communication module of the present invention, the light flux of the optical signal having a changed wavelength transmitted in the optical communication system using the wavelength division multiplexing method is formed at least almost linearly on the light receiving element array or the optical fiber array. It is possible to map onto a surface and detect light beams having different wavelengths over a wide wavelength range with high accuracy.

  The best mode for carrying out the present invention will be described below with reference to the drawings.

  <First Embodiment>

  FIG. 1 is a diagram schematically showing an optical system according to the first embodiment. As shown in FIG. 1, in the optical system of the present embodiment, the grating element 3 on which the light beam emitted from the end face 2 of the optical fiber 1 enters and the diffracted lights b1, b2, and b3 from the grating element 3 are input. The fθ lens 4 is arranged so that the diffracted lights b1 to b3 are focused on the position coordinates on the image plane 5 proportional to the input angle. The grating element 3 has a diffractive surface 3a on which a diffraction grating is formed on the image surface side, and has a spectral function by the diffractive surface 3a.

  As shown in FIG. 1, when the light beam emitted from the end face 2 of the optical fiber 1 includes, for example, a light beam b1 having a wavelength λ1, a light beam b2 having a wavelength λ2, and a light beam b3 having a wavelength λ3 (λ1 <λ2 <λ3). ), The light beam incident on the grating element 3 is diffracted by the grating element 3 and separated into diffracted lights b1, b2, and b3. However, the input angle when the diffracted lights b1 to b3 are input to the fθ lens 4 is substantially reduced. Focus is made on the position coordinates P1, P2 and P3 on the proportional imaging plane 5.

  The distance Δxa between the position coordinates P1 and P2 is substantially proportional to the wavelength difference Δλa (= λ2-λ1), and the distance Δxb between the position coordinates P2 and P3 is substantially proportional to the wavelength difference Δλb (= λ3-λ2). The diffracted beams b1 to b3 having different wavelengths can be mapped almost linearly on the imaging plane 5 corresponding to each position coordinate.

  In addition, the use of a grating for the spectroscopic element facilitates circuit design of the optical system, and an optical system having a new diffracted beam focusing function can be realized. Further, the fθ lens is widely used in a scanning optical system such as a laser printer in order to make the scanning speed constant, and the optical system in FIG. 1 can be configured inexpensively and easily.

  Next, the effect of the fθ lens 4 in the optical system of FIG. 1 will be described with reference to FIG. 6 in comparison with a conventional lens. When the image height of the fθ lens is represented by fθ, a lens having the image height represented by ftanθ is a conventional lens. An orthographic lens whose image height is expressed by 2 f sin (θ / 2) will be described together.

fθ lens image height variable θ (= arcsin (λ / d)), orthographic lens image height variable 2sin (θ / 2) (= sin (arcsin (λ / 2d))), and Table 1 below shows the results obtained by calculating the value of the variable tan θ (= tan (arcsin (λ / d))) of the image height of the conventional lens. Table 1 also shows the distortion obtained by the following equation from each shift amount and the ideal value (λ / d) for the fθ lens and the conventional lens.
Distortion = (displacement-ideal value) / ideal value

  FIG. 6 shows the relationship between the normalized wavelength (λ / d) and each shift amount in Table 1. Where λ is the wavelength and d is the pitch of the diffraction grating.

  From Table 1 and FIG. 6, when the normalized wavelength (λ / d) is small, both the fθ lens and the conventional lens have a small distortion rate and little difference between them, but the normalized wavelength (λ / d) is about 0. It can be seen that when the value exceeds 3, the distortion rate of the conventional lens is considerably larger than that of the fθ lens. For this reason, the error in position change is large on the imaging plane in the conventional lens, whereas the error in position change is small in the fθ lens. Therefore, the diffracted lights b1 to b3 having different wavelengths by the fθ lens 4 in FIG. The image plane 5 can be mapped almost linearly. In addition, since the error in the position change of the orthographic lens is slightly smaller than that in the fθ lens, the error in the position change can be further reduced by using the orthographic lens instead of the fθ lens.

  <Second Embodiment>

  Next, a second embodiment in which the above-described optical system is applied to an optical communication module that separates and detects light beams of optical signals having different wavelengths transmitted in an optical communication system using a wavelength division multiplexing system will be described with reference to FIG. Will be described with reference to FIG.

  FIG. 2A is a diagram schematically showing an optical communication module according to the second embodiment, and FIG. 2B is a diagram schematically showing an optical fiber array that can be arranged in place of the CCD array.

  As shown in FIG. 2, the module for optical communication according to the present embodiment includes the optical system of FIG. 1, and is arranged such that a CCD array 6 as a light receiving means is positioned on the imaging surface 5 of FIG. .

  That is, the optical communication module 10 receives the grating element 3 on which the light beam emitted from the end face 2 of the optical fiber 1 enters, and the diffracted lights b1, b2, and b3 from the grating element 3, and is proportional to the input angle. An fθ lens 4 disposed so that each diffracted light b1 to b3 is focused on a position coordinate on the imaging surface 5, a CCD array 6 disposed such that a light receiving surface 6a is positioned on the imaging surface 5, including. The grating element 3 has a diffractive surface 3a on which a diffraction grating is formed on the image surface side, and has a spectral function by the diffractive surface 3a.

  On the light receiving surface 6a of the CCD array 6, a plurality of light receiving elements are arranged in a straight line in the x direction in FIG.

  As shown in FIG. 2A, when optical signals having different wavelengths are transmitted from an optical communication system using a wavelength division multiplexing system and emitted from the end face 2 of the optical fiber 1, the light flux is, for example, a light flux b1 having a wavelength λ1. When the light beam b2 having the wavelength λ2 and the light beam b3 having the wavelength λ3 are multiplexed (λ1 <λ2 <λ3), the light beam incident on the grating element 3 is diffracted by the grating element 3 and is diffracted into the diffracted lights b1, b2, and b3. The diffracted lights b1 to b3 are separated and focused on the position coordinates P1, P2, and P3 on the image plane 5 proportional to the input angle when the diffracted lights b1 to b3 are input to the fθ lens 4.

  The position change amount Δxa, which is the distance between the position coordinates P1 and P2, is proportional to the wavelength difference Δλa (= λ2-λ1), and the position change amount Δxb between the position coordinates P2 and P3 is proportional to the wavelength difference Δλb (= λ3-λ2). Therefore, the diffracted beams b1 to b3 having different wavelengths can be mapped almost linearly on the imaging plane 5 corresponding to each position coordinate. Accordingly, the diffracted beams b1 to b3 are incident on the light emitting elements linearly arranged at equal intervals on the light receiving surface 6a of the CCD array 6 positioned on the imaging surface 5, thereby accurately fluxing the light beams b1 to b3 having different wavelengths. Can be detected well. Therefore, according to the module 10 for optical communication, it is possible to accurately detect the optical signal from the optical communication system using the wavelength division multiplexing method for each wavelength.

  Conventionally, especially when the diffraction angle is increased, the position change amount Δx is not proportional to the wavelength change, and the error of the position change is increased on both sides of the wavelength range. In the present embodiment, each diffracted light is input. Since the fθ lens 4 is arranged so that each diffracted light is focused on a position coordinate on the light receiving surface 6a proportional to the angle, each diffracted light can be mapped almost linearly on the light receiving surface 6a, and detection errors can be reduced. .

  The plurality of light receiving elements arranged on the light receiving surface 6a of the CCD array 6 are arranged at intervals corresponding to the resolution of the grating element 3. For example, in order to correspond to this resolution, the shift amount of the fθ lens in FIG. When it is necessary to further approximate the ideal value, an additional lens may be disposed, or the fθ lens may be replaced with an orthographic lens. Further, the fθ lens may be replaced with a lens designed to have an ideal value.

  In addition, as shown in FIG. 2B, instead of the CCD array 6 shown in FIG. 2A, an optical fiber array 7 may be arranged so that each end face 7a is positioned on the image plane 5. The optical fiber array 7 is configured by arranging a plurality of optical fibers at equal intervals. Each diffracted light can be detected by disposing a CCD array or the like on each end face 7 b side opposite to the optical fiber array 7.

  Next, an example in which the grating element 3 and the fθ lens 4 are integrated in the optical communication module 10 of FIG. 2A will be described with reference to FIG. FIG. 3 is a view showing an example in which the grating element 3 and the fθ lens 4 of FIG.

  As shown in FIG. 3, the grating element 3 is attached to and held by one end 8 a of the cylindrical body 8 via the holding member 9. Further, the fθ lens 4 is attached to and held by the other end 8b of the cylindrical body 8.

  Since the rating element 3 and the fθ lens 4 are attached to the central axis a of the cylindrical body 8 so as to be an axial object, the axis alignment of the rating element 3 and the fθ lens 4 is not necessary, and the optical communication module 10 For example, the center axis a only needs to be aligned with the end face 2 of the optical fiber 1 indicated by a broken line in FIG. 3, so that assembly and adjustment are facilitated.

  As described above, the best mode for carrying out the present invention has been described. However, the present invention is not limited to these, and various modifications are possible within the scope of the technical idea of the present invention. For example, in the optical system in FIG. 1 and the optical communication module in FIG. 2A, the grating element and the fθ lens are separately configured. However, when the fθ lens and the grating element are integrated, for example, fθ You may make it integrate by forming a grating in the lens surface of a lens.

It is a figure which shows schematically the optical system by 1st Embodiment. . FIG. 6A is a diagram schematically illustrating an optical communication module according to a second embodiment, and FIG. 6B is a diagram schematically illustrating an optical fiber array that can be arranged instead of a CCD array. It is a figure which shows the example which integrated the rating element 3 and the f (theta) lens 4 of Fig.2 (a) with the cylindrical body. It is a figure which shows the optical demultiplexer for a monitor of the conventional optical communication system. FIG. 5 is a diagram for explaining a relationship between a diffraction angle and an imaging position in the optical demultiplexer of FIG. 4. It is a figure which shows the effect of an f (theta) lens and an orthographic lens in the optical system of FIG. 1 compared with the conventional lens.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Optical fiber 2 End surface 3 Grating element 3a Diffraction surface 4 f (theta) lens 5 Imaging surface (focal plane)
6 CCD array 6a Light receiving surface 7 Optical fiber array 8 Cylindrical body 10 Optical communication module Δλa Wavelength difference Δλb Wavelength difference Δxa Interval, position change Δxb Interval, position change λ1, λ2, λ3 Wavelength P1, P2, P3 Position coordinate a Central axes b1, b2, b3 Diffracted light

Claims (8)

A grating element having a spectroscopic function, and a lens disposed so that each diffracted light from the grating element is input and the diffracted light is focused on a position coordinate that is at least substantially proportional to the input angle. An optical system including the optical system. The optical system according to claim 1, wherein a light beam is incident on the grating element from an end face of an optical fiber. The optical system according to claim 1, wherein the lens includes an fθ lens or an orthographic lens. An optical fiber arranged in the input section;
A grating element having a spectroscopic function and into which a light beam from the end face of the optical fiber is incident;
Each diffracted light from the grating element is input, and a lens disposed so that each diffracted light is focused on a position coordinate that is at least substantially proportional to the input angle;
And a light receiving means having a plurality of light receiving portions arranged at equal intervals on a focal plane of the lens. 5. The optical communication module according to claim 4, wherein the plurality of light receiving units are arranged on the focal plane of the lens at intervals corresponding to the resolution defined by the grating. The light receiving means is a light receiving element array in which a plurality of light receiving elements are arranged at equal intervals or an optical fiber array in which a plurality of optical fiber terminals are arranged at equal intervals. Module for optical communication. The optical communication module according to claim 4, wherein the grating element and the lens are integrated. 8. The optical communication module according to claim 4, 5, 6 or 7, wherein the lens is composed of an fθ lens or an orthographic lens.

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