JP2005128397A - Optical transmission and reception module - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an optical transmission and reception module that needs no demultiplexing filter. <P>SOLUTION: The module is equipped with a light emitting element 21 for transmitting transmission light and a light receiving element 22 for reception for receiving reception light, an optical fiber 25 for performing two-way communication between the light emitting element and the light receiving element for reception, and a spherical lens 24 installed between the light emitting element and the optical fiber, and the light receiving element for reception and the optical fiber. The light emitting element is mounted at a position where no light axis passes through the center of curvature of the spherical lens. The optical fiber is such that a line passing through the core center is parallel to the optical axis of the light emitting element and that the end face is inclined so as to be higher along the optical axis direction of the light emitting element relative to a plane orthogonally crossing the optical axis of the optical fiber. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention is an optical transmission / reception for optical communication that has both a light emitting element and a light receiving element and performs conversion from electricity to light and conversion from light to electricity in bidirectional optical communication using a single-core optical fiber. Regarding modules.

  FIG. 7 is a block diagram showing a conventional known optical transceiver module (see, for example, Patent Document 1 below). The conventional optical transmission / reception module shown in FIG. 7 allows light of the first wavelength λ1 to pass in the optical axis direction on the distal end surface optical axis of the ferrule 13 incorporating the optical fiber 12, and light of the second wavelength λ2. The prism-type wavelength multiplexing / demultiplexing coupler 15 sandwiching the demultiplexing filter 14 that reflects the light in the direction perpendicular to the optical axis is fixed, and the transmitted light is transmitted through the lens 16 in the optical axis direction and the direction perpendicular to the optical axis. The optical transmitting element 17 and the optical receiving element 19 that receives the received light through the lens 18 are arranged, and these members are fixedly supported by a single case member 11.

Transmission light λ1 (eg, 1.3 μm) from the optical transmission element 17 passes through the wavelength multiplexing / demultiplexing coupler 15 as it is in the optical axis direction (goes straight) and is transmitted to the optical fiber 12. On the other hand, the received light λ 2 (for example, 1.55 μm) from the optical fiber 12 is reflected by the wavelength multiplexing / demultiplexing coupler 15 in the direction perpendicular to the optical axis and received by the optical receiving element 19. With this configuration, it is possible to obtain a precise optical coupling with a small number of parts compared to the previous configuration, and to provide a small, robust and highly reliable optical transceiver module at low cost.
JP 2000-180671 A (FIG. 1)

  However, the conventional optical transceiver module requires the demultiplexing filter 14 to pass the light of the first wavelength λ1 in the optical axis direction and reflect the light of the second wavelength λ2 in the direction perpendicular to the optical axis. In addition, since the demultiplexing filter 14 is required to have high accuracy in the mounting angle, there are problems that the number of parts is increased and the manufacturing cost is increased.

  The present invention has been made to solve the conventional problems, and an object thereof is to provide an optical transceiver module that does not require a demultiplexing filter.

  In order to solve the above problems, an optical transceiver module according to the present invention includes a light emitting element for transmitting transmission light, a light receiving element for receiving reception light, and an external light using the light emitting element and the light receiving element for reception. An optical fiber for performing bidirectional communication between communication devices, and a spherical lens provided between the light emitting element, the receiving light receiving element, and the optical fiber, the light emitting element having an optical axis Is mounted at a position that does not pass through the center of curvature of the spherical lens, and the optical fiber has a straight line that passes through the center of the core parallel to the optical axis of the light emitting element, and an end surface that is parallel to the optical axis of the optical fiber. It is configured to be inclined with respect to an orthogonal plane so as to increase along the optical axis direction of the light emitting element. With this configuration, it is possible to increase the misalignment between the position of the light emitting point and the condensing point of the received light without reducing the fiber coupling efficiency of the transmitted light, and the transmission / reception light can be separated without using a demultiplexing filter. Communication can be performed without transmitting / receiving light having different wavelengths.

  Also, a monitor end face incident type light receiving element is disposed on the back surface of the light emitting element, and the reception light receiving element is disposed behind the monitor end face incident type light receiving element. With this configuration, the light emitting element, the receiving light receiving element, and the monitoring light receiving element can be mounted in a small area, and the module can be downsized.

  In addition, a mirror or a reflective multilayer film is formed on the back surface of the light emitting element, and the light receiving element for reception is disposed behind the light emitting element. With this configuration, the light emitted from the back surface of the light emitting element is prevented from being incident on the light receiving surface of the light receiving element for reception, and crosstalk degradation can be prevented when there is no light shielding means behind the light emitting element.

  Further, the light emitting element is mounted on a housing or a heat sink by a junction up method. With this configuration, the distance between the light emitting element surface opposite to the mounting surface and the active layer is shortened, so that even when the positional deviation between the light emitting point and the condensing point of the received light is small, the received light is transmitted at the end face of the light emitting element. It becomes unobstructed.

  The light emitting element is mounted on a housing or a heat sink by a junction down method. With this configuration, the distance between the reception light reflecting slope provided on the housing or the heat sink on which the light emitting element is mounted and the active layer is shortened. Even if it is small, the received light is not blocked by the end face of the light emitting element, and the heat radiation effect of the light emitting element to the heat sink is enhanced.

  The casing or the heat sink is a slope whose surface closest to the light emitting point of the light emitting element among one or more surfaces facing the spherical lens is not parallel to a straight line passing through the core center of the optical fiber. The output light from the optical fiber is configured to be incident on the light receiving element for reception after being reflected by the slope of the casing or the heat sink. With this configuration, even when the cavity length of the light emitting element and the thickness below the light receiving portion of the monitor light receiving element are thick, the received light is blocked by the light emitting element or the monitor light receiving element, or the light collection characteristic of the received light is deteriorated. Can be prevented.

  The casing or the heat sink is made of metal. With this configuration, the reflectance of the received light can be ensured without performing processing such as mirror vapor deposition or metallization on the inclined surface for reflecting the received light provided on the housing or the heat sink.

  Further, the reception light reflecting slope of the casing or the heat sink is mirror-deposited or metallized. With this configuration, even when the housing or the heat sink is made of a material with low reflectance, the reflectance of received light can be ensured.

  According to the present invention, the light emitting element is disposed at a position where the optical axis does not pass through the center of curvature of the spherical lens, the straight line passing through the core center of the optical fiber is shifted from the optical axis, and the end face of the optical fiber is projected. By tilting the incident light so as to bend toward the optical axis, it is possible to increase the positional deviation between the position of the light emitting point and the condensing point of the received light without reducing the fiber coupling efficiency of the transmitted light. As a result, it is possible to separate transmitted / received light without using a demultiplexing filter, and it is possible to provide an optical transceiver module with a reduced number of parts and man-hours. Further, communication can be performed without using transmission / reception light of different wavelengths.

Hereinafter, embodiments of the present invention will be described with reference to the drawings.
<First Embodiment>
FIG. 1 is a configuration diagram showing an optical transceiver module according to the first embodiment of the present invention. 1 includes a light emitting element 21 such as an edge emitting semiconductor laser that transmits transmission light, a light receiving element 22 such as a photodiode that receives received light, and an edge incident photo that monitors the power of the light emitting element 21. A single mode optical fiber whose end face is obliquely polished (hereinafter simply referred to as “bidirectional communication” with an external optical communication device using the light receiving element 23 for monitoring such as a diode, the light emitting element 21 and the light receiving element 22). (Referred to as an optical fiber) 25, a light emitting element 21, a ball lens 24 as a spherical lens provided between the light receiving element 22 and the optical fiber 25, and a ferrule 26 incorporating the optical fiber 25.

  Here, the light emitting element 21 is mounted on a heat sink (or simply a housing) 27 by a junction down method at a position where the optical axis does not pass through the center of curvature of the ball lens 24, and the optical fiber 25 is A straight line passing through the center of the core is parallel to the optical axis of the light emitting element 21, and the end face is inclined with respect to a plane perpendicular to the optical axis of the optical fiber 25 along the optical axis direction of the light emitting element 21. doing. That is, the end face of the optical fiber 25 is inclined so that the light emitted from the optical fiber 25 bends to the optical axis side of the light emitting element 21.

The configuration and operation of this optical transceiver module will be described below by taking specific element sizes, materials, and arrangements as examples. An angle θ _f formed by the end faces of the optical fiber 25 and the ferrule 26 and a plane perpendicular to the optical axis of the optical fiber 25 is 8 degrees. The light-emitting element 21 has a wavelength of 1310 nm and a divergence angle of 25 degrees with an intensity of 1 / e ² with respect to the peak. The ball lens 24 has a base material BK7 and a diameter of 1.5 mm.

  The light emitting element 21 is mounted at a position where the optical axis does not pass through the center of curvature of the ball lens 24, and the light emitting point of the light emitting element 21 is located at the point C. Point A is a point corresponding to the intersection of a straight line passing through the center of curvature of the ball lens 24 and parallel to the optical axis of the light emitting element 21 and a straight line passing through the point C and perpendicular to the optical axis. The distance between A and C is 140 μm. The distance from the point A to the surface of the ball lens 24 is 1300 μm. The ball lens 24 has an infinite number of optical axes. For convenience, a straight line passing through the point A and the center of curvature of the ball lens 24 is hereinafter referred to as a lens optical axis.

  First, an optical system for transmission light will be described. The light emitted from the light emitting element 21 passes through the ball lens 24 and enters the optical fiber 25. The point D where the end face of the core of the optical fiber 25 is placed is the point where the coupling efficiency between the light emitting element 21 and the optical fiber 25 is the highest, and the positive direction of the Y axis of the illustrated XYZ coordinate system with respect to the lens optical axis. About 127 μm.

The point where the light beam along the optical axis of the light emitting element 21 crosses the surface S _f having the Z coordinate equal to the point D is a point shifted about 158 μm in the positive direction of the Y axis with respect to the lens optical axis. The incident angle incident on the surface S _f is about 7.3 degrees, and the incident angle on the end face of the optical fiber is about 15.3 degrees. The incident angle θ _{t with} respect to the surface S _f of the light beam that cuts the point D is slightly smaller than this angle.

Further, the simulation result of the intensity distribution on the surface S _f is as shown in FIG. 3 as compared to FIG. 2 showing the simulation result when the optical axis of the light emitting element 21 is matched with the lens optical axis of the ball lens 24. Compared to the simulation result shown in FIG. 2, the result is slightly expanded in the Y-axis direction. Further, the convergence angle of light incident on the optical fiber 25 is substantially the same as the case where the optical axis of the light emitting element 21 is aligned with the lens optical axis of the ball lens 24. The horizontal axis of FIG. 2 and FIG. 3 is a ray passing through the optical axis of the light emitting element is obtained by the zero position to cut the surface S _f, it does not have the point B 0. The vertical axis shows the relative energy intensity distribution when the energy at position 0 is 1.

The coupling efficiency to the optical fiber 25 depends on the spread of the light intensity at the end face of the optical fiber, the convergence angle θ _A of the incident beam, and the angle θ _B between the traveling direction of the light immediately after entering the optical fiber and the optical fiber optical axis. In each case, comparing the first embodiment with the case where light along the optical axis is incident on a standard oblique polishing fiber having an inclination angle of 8 degrees, the spread of light intensity is greater in the first embodiment. Slightly wider, θ _A is equivalent, and θ _B is slightly smaller in the first embodiment. For this reason, in the first embodiment, the coupling efficiency deterioration depending on the spread of the light intensity is slightly larger, the coupling efficiency deterioration depending on the convergence angle of the incident beam is equal, and the traveling direction of the light immediately after the incidence The coupling efficiency degradation that depends on is slightly reduced.

  Regarding the coupling efficiency deterioration depending on the traveling direction of light immediately after the incidence, some supplementary explanation will be given below. The coupling efficiency to the optical fiber 25 increases as the traveling direction of the light beam passing through the center of the incident light beam immediately after entering the optical fiber becomes smaller with respect to the optical axis of the optical fiber 25. When light enters an oblique polishing fiber having an inclination angle of 8 degrees as shown in FIG. 1 from the direction of the extension line of the optical axis of the optical fiber 25, it is 2.6 degrees with respect to the extension line due to refraction at the end face of the optical fiber 25. It becomes a ray with an angle of. On the other hand, when light having an angle of 7.6 degrees with respect to the extension line of the optical axis of the optical fiber 25 and an incident angle of 15.6 degrees with respect to the end face of the optical fiber is incident, the light is 2.6 in the opposite direction with respect to the extension line. It becomes a ray with a degree angle. In the optical transceiver module, since the light beam having an incident angle with respect to the end face of the optical fiber smaller than 15.3 degrees is the center of the light beam, the coupling efficiency deterioration is estimated as described above.

Next, the received light optical system will be described. The received light of the optical transceiver module of the present invention is emitted from the vicinity of the point D on the end face of the optical fiber 25. The angle θ _r of the light beam passing through the center of the light beam with respect to the extension line of the optical axis of the optical fiber 25 is about 3.8 degrees due to refraction at the end face of the optical fiber. This light passes through the ball lens 24 and is collected at the point E. Point E deviates from point C by about 28 μm in the positive direction of the Y axis due to lens aberration. This deviation is a value larger than about 7 μm, which is the deviation when the light emitting point of the light emitting element 21 is arranged at the point A and the core end face of the optical fiber 25 is arranged at the point B. Such a difference occurs because in an optical system using a spherical lens, the aberration becomes larger as it deviates from the paraxial ray. This difference can be further increased by increasing the amount of deviation of the light emitting point in the Y-axis direction and the distance between the light emitting point and the lens.

The simulation result of the intensity distribution on the surface S _L including the light emitting point and the point A is as shown in FIG. 4. When the difference between the light emitting point C and the received light condensing point E is 7 μm, the spread of the received light is increased. The light emitting point C is included therein, but if the difference is about 28 μm, the light emitting point C is located outside the spread of the received light. Thereby, transmission / reception light can be spatially separated. Note that the horizontal axis in FIG. 4 indicates that the position at which the light beam forming the center of the light beam of the optical fiber output light cuts the surface S _L is 0, and the point A is not 0.

  As shown in FIG. 1, the light emitting element 21 is mounted on a heat sink 27, where the heat sink 27 is located at a light emitting point C of the light emitting element 21 among one or a plurality of surfaces facing the ball lens 24. The closest surface is formed by a slope that is not parallel to a straight line passing through the core center of the optical fiber 25, and the output light from the optical fiber 25 is reflected by the slope of the heat sink 27 and then enters the light receiving element 22.

  The reflectance on the slope may be ensured by configuring the heat sink 27 with a material that hardly absorbs light, such as a metal. If such a material cannot be used, the heat sink 27 is provided on the heat sink 27. Alternatively, the received light reflecting slope may be subjected to a process such as mirror deposition or metallization.

  The received light reflected on the inclined surface of the heat sink 27 is incident on the light receiving element 22 having a light receiving diameter of about 100 μm. The light receiving element 22 is mounted on a heat sink 28 in contact with the inclined surface of the heat sink 27. The position of the light receiving element 22 on the YZ plane is adjusted by the relative position of the heat sink 27 and the heat sink 28. Since the light receiving surface is shifted from several tens to several hundreds μm with respect to the point E, the intensity distribution of the received light on the light receiving surface is slightly wider than the distribution at the point E, but the light receiving diameter is about 100 μm. If there is, even if there is a deviation of about several hundred μm, sufficient light enters the light receiving diameter.

  The configuration and operation of the optical transceiver module according to the first embodiment have been described above with reference to numerical examples. However, the points of the present embodiment are limited to the following points. Other than this point, there is no problem even if it is changed.

  The first point is that a ball lens 24 (spherical lens) is used and the optical axis of the light emitting element 21 is shifted in parallel to the lens optical axis of the ball lens 24. If the ball lens 24 has no aberration, the light emitted from the end face of the optical fiber 25 placed at a position where the image of the light emitting point of the light emitting element 21 is the clearest is condensed at the light emitting point C. However, when the ball lens 24 is used, the aberrations of the transmitted and received light overlap, and the position of the light emitting point C and the position of the condensing point E of the received light shift. This deviation increases as the light passing through the ball lens 24 deviates from the paraxial ray.

  As a method of shifting the transmission / reception light from the paraxial ray, the light emitting point C and the end face of the optical fiber 25 are placed on the lens optical axis of the ball lens 24, and the optical axis of the light emitting point C and the optical axis of the optical fiber 25 are placed on the ball lens 24. And the optical axis of the light emitting point C and the optical axis of the optical fiber 25 are parallel to the lens optical axis of the ball lens 24, and the light emitting point C and the optical fiber. Although the method of shifting the end face of 25 from the lens optical axis of the ball lens 24 is conceivable, the coupling to the optical fiber 25 is increased as the amount of deviation between the position of the light emitting point C and the position of the condensing point E of the received light increases. The latter configuration is adopted in the present invention in consideration of the small deterioration in efficiency and the ease of manufacturing the fiber holder and other members.

  The second point is that the end face of the optical fiber 25 is obliquely polished, and this inclination direction is set to a direction in which the direction in which the light emitting element 21 is shifted is increased. That is, the end face of the optical fiber 25 is inclined so as to be higher along the optical axis direction of the light emitting element 21 with respect to a plane perpendicular to the optical axis of the optical fiber 25. Increasing the positional deviation of the light emitting element 21 with respect to the lens optical axis of the ball lens 24 shifts the incident angle of the transmitted light to the optical fiber 25, but the optical axis in the traveling direction immediately after the optical fiber is incident due to refraction at the end face of the optical fiber 25. Therefore, the coupling efficiency is easily secured.

  As described above, according to the first embodiment, it is possible to increase the positional shift between the position of the light emitting point and the condensing point of the received light without reducing the fiber coupling efficiency of the transmitted light. Transmission / reception light can be separated without using a demultiplexing filter. Further, communication can be performed without using transmission / reception light of different wavelengths.

  The heat sink 27 is formed of a slope that is not parallel to a straight line passing through the center of the core of the optical fiber 25 with the surface closest to the light emitting point of the light emitting element 21 among one or more surfaces facing the ball lens 24. The output light from the fiber 25 is reflected by the slope of the heat sink 27 and then enters the light receiving element 22, so that even when the cavity length of the light emitting element 21 and the thickness below the light receiving portion of the monitor light receiving element 23 are thick, It is possible to prevent the received light from being blocked by the light emitting element 21 or the monitor light receiving element 23 or to deteriorate the condensing characteristic of the received light.

  Further, since the light emitting element 21 is mounted on the heat sink 27 by the junction down method, the distance between the reception light reflecting slope provided on the heat sink 27 on which the light emitting element 21 is mounted and the active layer is shortened. Even when the positional deviation between the position of the light emitting point and the condensing point of the received light is small, the received light is not blocked by the end face of the light emitting element 21 and the heat dissipation effect of the light emitting element 21 to the heat sink 27 is enhanced. .

  Further, if the heat sink 27 is made of metal, the received light reflecting slope provided on the heat sink 27 does not need to be subjected to a process such as mirror deposition or metallization, and the reflectance of the received light can be ensured.

  Further, the received light reflecting slope of the heat sink 27 is mirror-deposited or metallized, so that the reflectivity of the received light can be ensured even when the heat sink 27 is made of a material with low reflectivity.

  The cavity length of the light emitting element 21 and the thickness below the light receiving portion of the monitor light receiving element 23 are thin, so that the received light is blocked by the light emitting element 21 and the monitor light receiving element 23 or the light receiving element 22 falls backward. If there is no danger that the light collection characteristic of the received light will deteriorate, the configuration as in the second and third embodiments described below may be adopted.

<Second Embodiment>
FIG. 5 is a layout diagram of the light emitting element 21 and the light receiving elements 22 and 23 according to the second embodiment of the present invention. The structure of other parts not shown in FIG. 5 is the same as that of the first embodiment. In the optical transceiver module according to the second embodiment, the light emitting element 21 is mounted on the heat sink (or may be simply a casing) 27 by a junction-up method. The monitoring light receiving element 23 may be an end face incident type light receiving element integrated with the light emitting element 21, or may be an element placed behind the light emitting element 21 and independent of the light emitting element 21. The light receiving element 22 is placed behind the light emitting element 21.

  Therefore, according to the second embodiment, since the light emitting element 21 is mounted on the heat sink 27 by the junction-up method, the distance between the light emitting element surface opposite to the mounting surface and the active layer is shortened. Even when the positional deviation between the position of the light emitting point and the condensing point of the received light is small, the received light is not blocked by the end face of the light emitting element 21.

  In addition, an end-incident monitor light-receiving element 23 is disposed on the back surface of the light-emitting element 21, and the reception light-receiving element 22 is disposed behind the monitor light-receiving element 23, whereby the light-emitting element 21 and the light-receiving element 22. The monitor light receiving element 23 can be mounted in a small area, and the module is downsized.

<Third Embodiment>
FIG. 6 is a layout diagram of the light emitting element 21 and the light receiving element 22 in the third embodiment of the present invention. The configuration of other parts not shown in FIG. 6 is the same as that of the first embodiment. In the optical transceiver module according to the third embodiment, the light emitting element 21 has a mirror or a reflective multilayer film formed on the back surface, and is mounted on the heat sink 27 (or may be simply a case) by the junction-up method. Has been. The light receiving element 22 is placed behind the light emitting element 21.

  Therefore, according to the third embodiment, a mirror or a reflective multilayer film is formed on the back surface of the light emitting element 21, and the light receiving element 22 is arranged behind the light emitting element 21. The incident light is not incident on the light receiving surface of the light receiving element 22, and it is possible to prevent the deterioration of the crosstalk when there is no light shielding means behind the light emitting element 21.

  Similarly to the second embodiment, since the light emitting element 21 is mounted on the heat sink 27 by the junction-up method, the distance between the light emitting element surface opposite to the mounting surface and the active layer is shortened. Even when the positional deviation between the position of the light emitting point and the condensing point of the received light is small, the received light is not blocked by the end face of the light emitting element.

  Since the optical transmission / reception module of the present invention does not require a demultiplexing filter, it can be realized at low cost, and thus is useful in the field of optical communication.

1 is a configuration diagram of an optical transceiver module according to a first embodiment of the present invention. FIG. 10 is a diagram for explaining the first embodiment of the present invention, and shows a simulation result of intensity distribution on the surface S _f when the optical axis of the light emitting element is made coincident with the lens optical axis. FIG. 9 is a diagram for explaining the first embodiment of the present invention, and showing the intensity distribution simulation result on the surface S _f of FIG. 1. FIG. 5 is a diagram for explaining the first embodiment of the present invention, and showing the intensity distribution simulation result on the surface _SL in FIG. 1. Arrangement diagram of light emitting element and light receiving element in the second embodiment of the present invention Arrangement of light-emitting elements and light-receiving elements in the third embodiment of the present invention Configuration diagram of conventional optical transceiver module

Explanation of symbols

21 Light Emitting Element 22 Light Receiving Element (Receiving Light Receiving Element)
23 Light-receiving element for monitor 24 Ball lens (spherical lens)
25 Optical fiber (single mode optical fiber)
26 Ferrule 27, 28 Heat sink

Claims (8)

A light emitting element that transmits transmission light and a light receiving element for reception that receives reception light; and
An optical fiber for performing bidirectional communication between an external optical communication device using the light emitting element and the receiving light receiving element;
A spherical lens provided between the light emitting element and the receiving light receiving element and the optical fiber;
The light emitting element is mounted at a position where the optical axis does not pass through the center of curvature of the spherical lens,
The optical fiber has a straight line passing through the center of the core parallel to the optical axis of the light emitting element, and an end surface along the optical axis direction of the light emitting element with respect to a plane orthogonal to the optical axis of the optical fiber. Optical transmission / reception module configured to be inclined so as to be higher.   2. The optical transmission / reception module according to claim 1, wherein an end face incident light receiving element for monitoring is disposed on a back surface of the light emitting element, and the receiving light receiving element is disposed behind the end face incident light receiving element for monitoring.   2. The optical transceiver module according to claim 1, wherein a mirror or a reflective multilayer film is formed on a back surface of the light emitting element, and the light receiving element for reception is disposed behind the light emitting element.   4. The optical transceiver module according to claim 1, wherein the light emitting element is mounted on a housing or a heat sink by a junction-up method. 5.   4. The optical transceiver module according to claim 1, wherein the light emitting element is mounted on a housing or a heat sink by a junction down method. 5.   The housing or the heat sink is configured by an inclined surface that is not parallel to a straight line passing through the core center of the optical fiber, the surface closest to the light emitting point of the light emitting element among one or more surfaces facing the spherical lens. The optical transmission / reception module according to claim 4, wherein output light from the optical fiber is incident on the light receiving element for reception after being reflected by an inclined surface of the housing or the heat sink.   The optical transceiver module according to claim 6, wherein the housing or the heat sink is made of metal. The optical transmission / reception module according to claim 6, wherein the reception light reflecting slope of the housing or the heat sink is subjected to mirror vapor deposition or metallization.

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