JP2007232834A - Optical communication module - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a single core two-way optical communication module that makes long-distance communication possible by improving coupling efficiency between signal light and an optical fiber. <P>SOLUTION: The optical communication module includes one optical fiber, an LD light source for outputting signal light, a light converging means for converging the signal light on the core center of the optical fiber, and parallel plates arranged with an inclination to the optical axis in the optical path of the signal light. The module is characterized in that aberration generated by the parallel plates is corrected by arranging the light converging means with an inclination to the optical axis. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a single-core bidirectional optical communication module that transmits and receives information signal light using a single optical fiber.

  Conventionally, a single-core bidirectional transmission / reception optical communication module that performs transmission / reception of signal light using a single optical fiber is known. For example, in Patent Document 1, a laser light source that emits signal light, a light deflecting unit (condensing lens) for guiding the signal light from the laser light source to the core center of the optical fiber, and the light deflecting unit deflect the light. First control means for performing negative feedback control on the light deflecting means so that the position of the signal light on the optical fiber incident surface faces the center of the core, and the laser so that the light quantity of the signal light becomes a predetermined light quantity. An optical communication device including a second control unit that performs APC control on a light source is described. The first control means drives the condenser lens by negative feedback control so that the incident position of the signal light is directed toward the core center of the optical fiber.

  Further, in Patent Document 2, a predetermined amount of step is formed between the core and the clad at a predetermined end face of the optical fiber, and the intensity of the first-order diffracted light of the reflected light generated by the step is disclosed. An optical communication module capable of detecting the incident position of the light from the light source on the predetermined end surface based on the distribution is described.

  In addition, by using the optical fiber described in Patent Document 2 for the optical communication device of Patent Document 1, the first control means generates a signal based on the intensity distribution of the first-order diffracted light of the reflected light generated from the end face of the optical fiber. An optical communication module that can detect the incident position of light and perform negative feedback control has also been proposed.

  In general, in a single-core bidirectional optical communication module, signal light is transmitted and received by a single optical fiber. Therefore, WDM (Wavelength Division) for separating light of different wavelengths is included in the optical communication module. Multiplex) filters and elements for receiving the received light are provided. As the light source, a laser diode (LD) capable of monitoring the amount of light with a built-in photodiode is used (for example, a Fabry-Perot LD).

JP 2004-96299 A JP 2004-157360 A

  However, as described above, a single-core bidirectional optical communication module requires a WDM filter (its shape is a plane-parallel plate). However, such a plane-parallel plate is not included in the focused light of signal light. It is known that aberrations (astigmatism and coma) occur in the signal light when arranged. When aberration occurs, the spot diameter formed on the end face of the optical fiber increases, and the coupling efficiency between the signal light and the optical fiber decreases. The lower the coupling efficiency between the signal light and the optical fiber, the lower the amount of light taken into the optical fiber, making it difficult to transmit signal light over long distances.

  In view of the above circumstances, an object of the present invention is to provide a single-core bidirectional optical communication module that enables long-distance communication by improving the coupling efficiency between signal light and an optical fiber.

  In order to solve the above problems, the present invention provides a single optical fiber, an LD light source that outputs signal light, a condensing means for condensing the signal light at the core center of the optical fiber, A plane parallel plate disposed at an inclination with respect to the optical axis in the optical path, and the aberration generated by the plane parallel plate is corrected by arranging the light collecting means at an angle with respect to the optical axis. An optical communication module is provided.

  By setting it as such a structure, the optical communication module which concerns on this invention can correct | amend the aberration in signal light by inclining a condensing means. Therefore, the coupling efficiency between the signal light and the optical fiber is improved by correcting the aberration in the signal light. In addition, since the condensing means is merely tilted, it is not necessary to add a new part to correct the aberration.

  In the optical communication module according to the present invention, the light collecting means is inclined in the same direction as the inclination direction of the plane parallel plate. Alternatively, the condensing means is characterized in that it is inclined in a direction rotated by 90 degrees around the optical axis with respect to the inclination direction of the plane parallel plate.

  More specifically, the light condensing means is inclined from 1.9 degrees to 2.5 degrees with respect to the optical axis.

  Further, the light condensing means is movably arranged to guide the signal light to the core center on the incident end face of the optical fiber.

  Therefore, according to the present invention, it is possible to provide a single-core bidirectional optical communication module that enables long-distance communication by improving the coupling efficiency between the signal light and the optical fiber.

  Hereinafter, an optical communication module according to the present invention will be described with reference to the drawings.

  FIG. 1 is a diagram showing a configuration of an optical communication module 100 of the present invention. The optical communication module 100 is incorporated, for example, in an ONU for connecting an optical fiber drawn into a subscriber's house and a PC or the like. The optical communication module 100 uses, for example, one optical fiber to bidirectionally transmit an upstream signal for transmission with a wavelength of 1.3 μm and a downstream signal for reception with a wavelength of 1.5 μm by wavelength division multiplexing (WDM). It is comprised so that it can transmit to.

  The optical communication module 100 includes a laser diode (LD) 1, a condensing lens 3, a WDM filter 7, an optical fiber 9, a position detector 11, a condensing lens 13, a reception light detector 15, a negative A feedback control circuit 19 and an actuator drive circuit 21 are included.

  The LD 1 is a light source configured to output signal light (laser light) having a wavelength of 1.3 μm modulated by transmission information. The LD 1 is, for example, a Fabry-Perot type LD that uses a cleaved surface of a crystal, and a light amount monitoring element (photodiode or the like) is provided inside the hermetic seal. In LD1, APC (Automatic Power Control) is performed based on the amount of light monitored by a built-in photodiode or the like. The LD 1 has a cover glass, and the emitted light is emitted through the cover glass. The LD 1 has a function of outputting a light amount monitor signal to the negative feedback control circuit 19.

  Here, in FIG. 1, XYZ coordinate axes are defined. The direction in which the LD 1 emits laser, that is, the direction perpendicular to the semiconductor substrate in the LD 1 (left and right direction in the drawing) is defined as the Z axis direction, and the direction in which the laser emitted from the LD 1 travels is defined as the positive direction of the Z axis. Also, the direction perpendicular to the paper surface is defined as the X-axis direction, and the upward direction (front side) perpendicular to the paper surface is defined as the positive direction of the X-axis. In addition, a direction perpendicular to the Z axis and the X axis (up and down direction on the paper surface) is a Y axis direction, and a direction toward the upper side of the paper surface is a positive direction of the Y axis.

  Transmitted light (signal light having a wavelength of 1.3 μm) emitted from the LD 1 enters the condenser lens 3, exits the condenser lens 3, and then gradually converges, passes through the WDM filter 7, and reaches the end face of the optical fiber 9. A spot is formed at 9a. Note that the LD 1, the WDM filter 7, and the optical fiber 9 are respectively fixed to a housing (not shown) of the optical communication module 100. On the other hand, the condenser lens 3 is arranged so as to be finely movable in a direction perpendicular to the Z axis, and is driven by an actuator (not shown) controlled by the actuator drive circuit 21. When the optical communication module 100 is assembled, the light is roughly collected on a straight line extending in the positive direction of the Z-axis from the center of the laser radiation position on the LD 1 (this straight line is defined as “optical axis” for convenience). The lens 3, the WDM filter 7, and the end face 9a of the optical fiber 9 are aligned so that the center positions thereof are arranged.

  The condensing lens 3 is preferably an aspherical lens that can reduce aberrations. The condensing lens 3 has an XY plane including the semiconductor substrate surface (light emitting surface) of the LD 1 and a core on the end surface 9 a of the optical fiber 9 when the main plane is arranged perpendicular to the optical axis. The XY plane including a position slightly on the Z-axis positive direction side from the surface is arranged so as to be conjugate (to form a spot having a predetermined size on the end face 9a). Since the WDM filter 7 is disposed between the condenser lens 3 and the optical fiber 9 and nothing is disposed between the condenser lens 3 and the LD 1, the condenser lens 3 is closer to the LD 1 than the fiber 9. It is preferable to arrange them.

  In the embodiment of the present invention, the condenser lens 3 is disposed so as to have a predetermined angle with respect to the optical axis. For example, in FIG. 1, the condenser lens 3 is arranged in a state rotated (tilted) by a predetermined amount around a rotation axis 3 a that is parallel to the X axis and passes through the center of the condenser lens 3. The condenser lens 3 is moved in a direction perpendicular to the optical axis (direction parallel to the XY plane) by an actuator (not shown) driven by the actuator drive circuit 21 while maintaining the tilted state.

  The WDM filter 7 passes the transmission light toward the optical fiber 9 (when the transmission light passes through the WDM filter 7, an aberration occurs in the transmission light). On the other hand, the WDM filter 7 reflects all the 1.5 μm received light emitted from the optical fiber 9 at the reflecting surface 7 a on the optical fiber 9 side and deflects it in the direction of the condenser lens 13. The condensing lens 13 condenses the received light onto the light receiving surface of the received light detector 15. The received light detector 15 includes a PD on the light receiving surface, converts the received light received by the PD into an electrical signal, and demodulates the electrical signal as information.

  The end face 9a of the optical fiber 9 is composed of a clad surface and a core surface protruding a predetermined amount from the clad surface. The clad surface is formed to be parallel to the X axis and have a predetermined angle (for example, 45 degrees) with respect to the Z axis. The end face 9a is configured to reflect a part of the incident transmission light. The position detector 11 is disposed at a position where the transmission light (diffracted light) reflected by the end face 9a can be received. For example, if the end surface 9a is formed so as to be inclined by 45 degrees with respect to the Z axis, the position detector 11 is disposed at a position in the positive direction of the Y axis with respect to the end surface 9a. When the position detector 11 is arranged, the center of the light receiving surface of the position detector 11 is arranged at a position facing the center of the end face 9a (the center of the core surface).

  The position detector 11 has a four-divided PD that is divided into four areas by two boundary lines whose light receiving surfaces extend perpendicularly to each other at the center thereof. The position detector 11 detects a change in the amount of transmitted light received for each area. The negative feedback control circuit 19 monitors the change in the amount of light detected by the position detector 11.

  The negative feedback control circuit 19 is a signal for controlling the position of the condenser lens 3 so that the transmission light is incident on the center (core surface) of the end face 9a of the optical fiber 9 based on the detection result by the position detector 11. Is transmitted to the actuator drive circuit 21.

  The actuator drive circuit 21 is a circuit for driving an actuator (not shown) for finely adjusting the position of the condenser lens 3 (controlled by negative feedback control).

  In the optical communication module 100, before negative feedback control is performed on the condenser lens 3 by the actuator drive circuit 21, first, an electric current is applied to the LD 1 so that the amount of transmission light emitted from the LD 1 becomes a predetermined value. To do. Thereafter, when the amount of light detected by the light amount monitoring element in the LD 1 exceeds a predetermined value, the negative feedback control circuit 19 transmits a signal to the actuator drive circuit 21 and starts control of the condenser lens 3. Further, after negative feedback control is performed, the output of LD1 is adjusted by APC.

  Further, in the optical communication module 100, astigmatism generated by the WDM filter 7 can be corrected by tilting the condenser lens 3. That is, in the optical communication module 100, astigmatism is generated when the condenser lens 3 is tilted, and an amount of astigmatism approximately opposite to the astigmatism generated by the WDM filter 7 in the condenser lens 3 is obtained. As a result, the astigmatism of the entire optical system is corrected by generating the aberration. In general, the smaller the amount of astigmatism in the transmission light incident on the optical fiber 9, the higher the coupling efficiency between the optical fiber 9 and the transmission light. The coupling efficiency is a ratio of light taken into the optical fiber out of the light collected on the end face of the optical fiber.

  Hereinafter, the relationship between the astigmatism amount and the coupling efficiency with respect to the tilt amount of the condensing lens 3, the thickness of the WDM filter 7 (parallel plane plate), and the tilt amount in the optical communication module 100 will be described using an optical system model. . Hereinafter, Comparative Examples 1 to 3 and Examples 1 and 2 will be described. The optical system models corresponding to the optical communication module 500 are Examples 1 and 2, respectively. Description will be made by comparing Comparative Examples 1 and 2 with Example 1 and Comparative Examples 1 and 3 with Example 2.

<Comparative Example 1>
FIG. 2 shows an optical system model when the WDM filter 7 is not included. That is, the system does not cause astigmatism due to the WDM filter 7. In the optical system, a cover glass 1a (thickness: 0.25 mm) and a condenser lens 3 (no tilt) are arranged. The wavelength used for the transmitted light is 1310 nm. The cover glass 1a is a plane parallel plate, and its glass material is equivalent to BK7, and has a d-line refractive index nd = 1.51633 and an Abbe number ν = 64.1. Further, the imaging magnification by the condenser lens 3 is −1.144, and the one having the d-line refractive index nd = 1.58913 and the Abbe number ν = 61.2 is used. The entrance pupil diameter is 7 mm and the pupil apodization is 0.402. Comparative Example 1 is for confirming the coupling efficiency when no astigmatism occurs due to the plane parallel plate.

The pupil apodization is a value indicating where in the normalized pupil coordinates a light amount that is ½ of the peak is located when the light amount distribution is expressed by a Gaussian distribution. Assuming that the apodization value is a with respect to the normalized pupil coordinates, the light quantity distribution I (x) can be expressed as follows.

Table 1 shows lens data of the optical system of Comparative Example 1. r is a radius of curvature, d is a lens thickness or a lens interval, and n is a refractive index at a working wavelength of 1310 nm. The 0th surface is a light source surface. In addition, a surface in which the r column is blank represents a “plane (r = infinity)”.

<Comparative example 2>
FIG. 3 shows an optical system model including a cover glass 1 a, a condenser lens 3 (no tilt), and a WDM filter 7. The cover glass 1a and the WDM filter 7 are parallel flat plates, and the glass material is equivalent to BK7, and has a d-line refractive index nd = 1.51633 and an Abbe number ν = 64.1. The thickness of the WDM filter 7 is 0.5 mm. The wavelength of the transmission light, the magnification of the condenser lens, the glass material of the condenser lens, the entrance pupil diameter, and the pupil apodization are the same as those in the optical system of Comparative Example 1. The WDM filter 7 is tilted 30 degrees (that is, tilted 30 degrees with respect to the optical axis of the transmission light). Comparative Example 2 is for confirming how much the coupling efficiency is reduced by astigmatism generated by the WDM filter 7.

Table 2 shows lens data of Comparative Example 2.

  4 is a lateral aberration diagram of the optical system of Comparative Example 2, and FIG. 5 is a longitudinal aberration diagram of the optical system of Comparative Example 2. From the lateral aberration diagram of FIG. 4, it can be seen that there is aberration in the meridional plane (Y-FAN) and the sagittal plane (X-FAN).

<Example 1>
FIG. 6 shows an optical system model of Example 1, in which the condenser lens 3 of the optical system of Comparative Example 2 is tilted. The tilt amount (tilt angle with respect to the optical axis) is set to a value that provides the highest coupling efficiency in this optical system (in this case, 1.9 degrees in the same direction as the tilt direction of the WDM filter 7). ).

Table 3 shows lens data of Example 1.

  FIG. 7 is a lateral aberration diagram of the optical system of Example 1, and FIG. 8 is a longitudinal aberration diagram of the optical system of Example 1. As can be seen from the lateral aberration diagram of FIG. 7, the aberration is corrected in the meridional plane (Y-FAN) and the sagittal plane (X-FAN) as compared with Comparative Example 2 (FIG. 4).

  Table 4 shows the calculation results of aberration and coupling efficiency of Comparative Example 1, Comparative Example 2, and Example 1. In Comparative Example 1, RMS wavefront aberration and PV wavefront aberration do not occur, and the coupling efficiency is 0.972.

  In Comparative Example 2, the RMS wavefront aberration was 0.082λ, the PV wavefront aberration was 0.449λ, and the coupling efficiency was 0.910. It can be seen that the coupling efficiency is 0.062 lower than that of the optical system of Comparative Example 1 due to the influence of the WDM filter 7.

In Example 1, the RMS wavefront aberration was 0.011λ, the PV wavefront aberration was 0.054λ, and the coupling efficiency was 0.970. The amount of aberration is reduced as compared with the optical system of Comparative Example 2 (both RMS and PV are reduced to about 13%). Further, the coupling efficiency is 0.060 higher than that of the optical system of Comparative Example 2 where the condenser lens 3 is not tilted, and is only 0.002 lower than the coupling efficiency of Comparative Example 1 where there is no WDM filter 7 (no aberration). there were. Therefore, in the optical system of Example 1, astigmatism is corrected by the tilt of the condensing lens 3 in spite of the occurrence of astigmatism in the WDM filter 7, and as a result, a decrease in coupling efficiency is suppressed. You can see that

<Comparative Example 3>
FIG. 9 shows an optical system model of Comparative Example 3, in which the tilt of the WDM filter 7 of the optical system of Comparative Example 2 is 45 degrees (that is, 45 degrees with respect to the optical axis).

Table 5 shows lens data of Comparative Example 3.

  10 is a lateral aberration diagram of the optical system of Comparative Example 3, and FIG. 11 is a longitudinal aberration diagram of the optical system of Comparative Example 3. From the lateral aberration diagram of FIG. 10, it can be seen that aberration exists in the meridional plane (Y-FAN) and the sagittal plane (X-FAN).

<Example 2>
FIG. 12 is an optical system model of Example 2, in which the condenser lens 3 of the optical system of Comparative Example 3 (FIG. 9) is tilted. The tilt amount (tilt angle with respect to the optical axis) is set to a value that provides the highest coupling efficiency in this optical system (in this case, 2.5 degrees in the same direction as the tilt direction of the WDM filter 7). ).

Table 6 shows lens data of Example 2.

  FIG. 13 is a lateral aberration diagram of the optical system of Example 2, and FIG. 14 is a longitudinal aberration diagram of the optical system of Example 2. From the lateral aberration diagram of FIG. 13, it can be seen that the aberration is corrected in the meridional plane (Y-FAN) and the sagittal plane (X-FAN) as compared with Comparative Example 3 (FIG. 10).

  Table 7 shows the calculation results of aberration and coupling efficiency of Comparative Example 1, Comparative Example 3, and Example 2. In Comparative Example 3, the RMS wavefront aberration was 0.323λ, and the PV wavefront aberration was 1.735λ. It can be seen that the amount of aberration generated is larger than that of the optical system of Comparative Example 2 in which the tilt of the WDM filter 7 is 30 degrees (see Table 4). In addition, since the coupling efficiency is 0.748, it can be seen that the coupling efficiency decreases as the aberration increases. Compared with the optical system of Comparative Example 1 without the WDM filter 7, the coupling efficiency is reduced by 0.224, and compared with Comparative Example 2 in which the tilt amount of the WDM filter 7 is different, the coupling efficiency is reduced by 0.162.

  In Example 2, the RMS wavefront aberration was 0.022λ, the PV wavefront aberration was 0.124λ, and the coupling efficiency was 0.950. The amount of aberration is reduced as compared with the optical system of Comparative Example 3 (both RMS and PV are reduced to about 7%). Further, the coupling efficiency is 0.202 higher than that of the optical system of Comparative Example 2 in which the condenser lens 3 is not tilted, and only 0.022 lower than the coupling efficiency of Comparative Example 1 in which the WDM filter 7 is not provided (no aberration). there were. Accordingly, in the optical system of Example 2, astigmatism is corrected by the tilt of the condenser lens 3 even though the astigmatism is greatly generated in the WDM filter 7, and as a result, the coupling efficiency is reduced. It turns out that it is suppressing. The coupling efficiency after correction of astigmatism is higher in the optical system of Example 1, but the correction rate is higher in the optical system of Example 2.

That is, when the tilt amount of the WDM filter 7 is about 30 degrees, the coupling efficiency is approximately the maximum value. However, there may be a case where it is desired to further tilt the WDM filter 7 due to the arrangement in the apparatus. In this case, astigmatism due to the WDM filter 7 increases, but it can be sufficiently corrected by the tilt of the condenser lens 3.

  In the first and second embodiments, the condenser lens 3 is tilted in the same direction as the tilt of the WDM filter 7. However, the tilting direction of the condensing lens 3 is also a direction obtained by rotating the tilting direction of the WDM filter 7 by 90 degrees around the optical axis (that is, the condensing lens 3 in FIG. It is possible to reduce aberrations in the entire optical system by rotating (tilting) a predetermined amount around the rotation axis passing through the center of the optical lens 3. Therefore, the direction of tilt of the condenser lens 3 is not limited to the direction of the first and second embodiments.

  Therefore, in the optical communication module 100 of the present invention, the astigmatism caused by the WDM filter can be canceled and the astigmatism of the entire optical system can be corrected by tilting the condenser lens to generate astigmatism. . By correcting the astigmatism, the coupling efficiency between the optical fiber and the transmission light is improved, and as a result, the optical communication module 100 enables the long-distance communication of the transmission light. Since the tilt of the lens is used as a means for generating astigmatism, the aberration can be corrected at a lower cost than when astigmatism is corrected using another member. In addition, a module can be easily created in that an ordinary rotationally symmetric lens can be used rather than a method of correcting astigmatism by placing a cylindrical component on the lens to make a rotationally asymmetric shape.

It is a figure which shows the structure of the optical communication module of this invention. 6 is a diagram illustrating an optical system model of Comparative Example 1. FIG. 10 is a diagram illustrating an optical system model of Comparative Example 2. FIG. 6 is a lateral aberration diagram of Comparative Example 2. FIG. 6 is a longitudinal aberration diagram of Comparative Example 2. FIG. 2 is a diagram illustrating an optical system model of Example 1. FIG. FIG. 3 is a lateral aberration diagram of Example 1. 2 is a longitudinal aberration diagram of Example 1. FIG. 10 is a diagram illustrating an optical system model of Comparative Example 3. FIG. 10 is a lateral aberration diagram of Comparative Example 3. FIG. 10 is a longitudinal aberration diagram of Comparative Example 3. FIG. 6 is a diagram illustrating an optical system model of Example 2. FIG. FIG. 6 is a lateral aberration diagram of Example 2. FIG. 6 is a longitudinal aberration diagram of Example 2.

Explanation of symbols

1 LD
DESCRIPTION OF SYMBOLS 3 Condensing lens 7 WDM filter 9 Optical fiber 11 Position detector 13 Condensing lens 15 Reception light detector 17 Light quantity detector 19 Negative feedback control circuit 21 Actuator drive circuit

Claims (5)

A single optical fiber,
An LD light source that outputs signal light;
Condensing means for condensing the signal light at the core center of the optical fiber;
A plane parallel plate disposed in the optical path of the signal light and inclined with respect to the optical axis thereof,
An optical communication module, wherein the aberration generated by the plane-parallel plate is corrected by arranging the condensing means to be inclined with respect to the optical axis.   The optical communication module according to claim 1, wherein the condensing unit is inclined in the same direction as an inclination direction of the parallel flat plate.   2. The optical communication module according to claim 1, wherein the condensing unit is inclined in a direction rotated 90 degrees around an optical axis with respect to an inclination direction of the parallel flat plate.   The optical communication module according to any one of claims 1 to 3, wherein the condensing means is inclined at 1.9 to 2.5 degrees with respect to the optical axis.   5. The optical communication module according to claim 1, wherein the condensing unit is arranged to be movable so as to guide the signal light to a core center on an incident end face of the optical fiber. .

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