JP2009026869A - Bidirectional optical module - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a bidirectional optical module which is free of optical crosstalk by suppressing generation of stray light by efficiently absorbing wavelength light that an LD element emits to eliminate it. <P>SOLUTION: The bidirectional optical module has a package with a sealed cap 7 having a light transmission window on a stem 3 on which a light emitting element 4 emitting transmission light, a light receiving element 5 receiving reception light, and a wavelength multiplexing demultiplexing filter 6 are mounted. In the package, a light absorbing member 20 made of a semiconductor material having a light absorption end longer than the light emission wavelength of the light emitting element 4 is disposed to absorb stray light in the package. As the semiconductor material, one of Ge, InGaAs, and InGaAsP is preferably used. Further, low-fusion-point glass may be used which contains powder of the semiconductor material. The light absorbing member may be provided on an internal wall surface of the cap 8. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a coaxial bidirectional optical module in which a light emitting element and a light receiving element are mounted in one package and used for transmission / reception of an optical signal.

  With the progress of optical communication technology, introduction of a GE-PON (Gigabit Ethernet (registered trademark) -Passive Optical Network) system of a subscriber communication network represented by FTTH (Fiber To The Home) is progressing. In this GE-PON system, the center terminal (OLT: Optical Line Terminal) on the station side is reduced with one optical fiber to reduce the number of optical fibers in order to realize a higher speed service at the same price as a metal cable. A single-fiber bidirectional communication method has been proposed in which upstream and downstream bidirectional communication is performed between a user terminal (ONU: Optical Network Unit) and a user-side subscriber terminal. Specifically, it is a wavelength division multiplexing (WDM) that performs transmission / reception using two wavelengths of a 1.31 μm band and a 1.55 μm band (or 1.49 μm band).

  As a single-core bidirectional optical module used for this, transmission light emitted from a semiconductor laser (LD element), which is a light emitting element, is reflected by a wavelength filter and coupled to an optical fiber while being transmitted through the optical fiber. The received light transmitted through the wavelength filter is positioned so as to be optically coupled to a photodiode (PD element) that is a light receiving element, and is fixed to the casing by YAG welding or the like. However, with the recent expansion of the use of optical access systems, there is an increasing demand for downsizing and cost reduction of single-fiber bidirectional optical modules. LD elements and PD elements are housed in one package, and wavelength filters (WDM filters) are used. A form of demultiplexing has been studied (see, for example, Patent Document 1).

  When a single package is used, the emitted light has a half-value width of 25 to 35 degrees in the horizontal direction and 30 to 40 degrees in the vertical direction, depending on the structure of the LD element, and is coupled to the optical fiber. There is no light. The light not coupled to the optical fiber is attenuated while being reflected in the package. In the edge-emitting LD element, light is emitted also from an end face that emits light coupled to the optical fiber and an end face on the opposite side. The emitted light from the opposite side is used to monitor the intensity of the emitted light from the LD element, but a part of it is repeatedly reflected in the package. Some of the light emitted from the LD element becomes stray light that repeats reflection while being attenuated in the package. When this stray light is incident on the receiving PD element, optical crosstalk occurs, and it may cause other noise or malfunction.

In order to avoid the influence of stray light as described above, in Patent Document 1, the PD element for reception is surrounded by a cap different from the cap of the package so that the reflected light and the stray light are not received. Patent Document 2 discloses that a black film such as Zn-Ni plating is provided on the cap surface to suppress stray light due to reflection of laser light. Patent Document 3 discloses matte palladium plating on the cap surface. Further, it is disclosed that stray light due to reflection of laser light is similarly suppressed. Patent Document 4 discloses that an absorbent such as carbon is provided around a window portion of a package that transmits signal light to suppress stray light due to reflection.
JP 2004-133463 A JP 2003-204006 A JP 2006-120864 A JP 2003-209315 A

  By mounting a transmitting LD element and a receiving PD element in one package, the space is smaller than that in which a transmitting LD element and a receiving LD element are mounted in separate packages (two packages). However, there is a problem of the occurrence of optical crosstalk as described above. As disclosed in Patent Document 1 described above, enclosing the LD element with the internal cap requires a space for arranging the internal cap, and light entering through the WDM filter cannot be prevented.

Further, as disclosed in Patent Document 2, a black film is formed on the surface of the package cap, or matte palladium plating is applied to the surface of the package cap as disclosed in Patent Document 3, As disclosed in FIG. 4, although an effect of reducing the reflection of the laser beam by providing a light absorption film around the window portion of the package is recognized, it has not been improved sufficiently.
The present invention has been made in view of the above circumstances, and provides a bidirectional optical module that efficiently absorbs and extinguishes the wavelength light emitted by the LD element, suppresses the generation of stray light, and has no optical crosstalk. With the goal.

A bidirectional optical module according to the present invention includes a light-emitting element that emits transmission light, a light-receiving element that receives reception light, and a package on which a cap having a light transmission window is sealed on a stem on which a wavelength multiplexing / demultiplexing filter is mounted. The light absorption member made of a semiconductor material having a light absorption end longer than the light emission wavelength of the light emitting element is disposed in the package, and stray light in the package is absorbed.
For example, Ge, InGaAs, or InGaAsP is preferably used as the semiconductor material. Alternatively, the semiconductor material may be powdered and contained in low-melting glass. The light absorbing member can be provided on the inner wall surface of the cap.

  According to the present invention, stray light resulting from reflection of a part of signal light emitted from a light emitting element within a package can be efficiently absorbed and extinguished according to the wavelength emitted by the light emitting element. Therefore, it is possible to prevent light reception by the light receiving element and to suppress the occurrence of optical crosstalk to the extent that there is no problem.

  Embodiments of the present invention will be described with reference to the drawings. FIG. 1 is a diagram for explaining an example of a bidirectional optical module as an object of the present invention, and FIG. 2 is an enlarged schematic diagram of a package portion. In the figure, 1 is a bidirectional optical module, 2 is a package unit, 3 is a stem, 4 is a light emitting element (LD), 5 is a light receiving element (PD), 6 is a wavelength multiplexing / demultiplexing filter (WDM filter), and 7 is a lens. , 8 is a cap, 9 is a holder, 10 is a partition wall, 11 is an opening, 12 is a sleeve portion, 13 is a stub, 13a is a short optical fiber, 14 is a sleeve shell, 15 is a guide sleeve, and 16 is a bush. .

  As shown in FIG. 1, the bidirectional optical module 1 includes a sleeve portion 12 coupled to a package portion 2 via a holder 9, and an optical connector (not shown) that is detachably inserted into the sleeve portion 12. The optical signal is transmitted / received via an external optical transmission line. The package unit 2 is formed of, for example, a CAN type (coaxial type), and a light emitting element 4 (hereinafter referred to as an LD 4) such as a laser diode that transmits signal light to the stem 3, and a photo that receives signal light from an external circuit. It is configured as a one package type optical module device on which a light receiving element 5 (hereinafter referred to as PD5) such as a diode is mounted.

  As shown in an enlarged view in FIG. 2, a wavelength multiplexing / demultiplexing filter 6 (hereinafter referred to as a WDM filter) is disposed on the stem 3 of the package unit 2 on an optical path where transmission light and reception light having different wavelengths intersect. ) Are inclined and are separated by selectively reflecting or transmitting transmitted light and received light. A cap 8 that also functions as a housing of the package unit 2 is provided with a lens 7 that collects signal light and reception light at the center of the upper part, and serves as an output port for signal light and an incident port for reception light. The cap 8 is sealed to the stem 3 with a sealing structure, and protects internal mounting components.

  The sleeve portion 12 is a portion where an optical connector is inserted to form an optical connection with the external optical transmission line. For example, the sleeve portion 12 houses a guide sleeve 15 in which a stub 13 is closely fitted to a cylindrical sleeve shell 14. Is configured to fit into the bush 16. The stub 13 is formed in a ferrule shape in which a short optical fiber 13a is inserted in the center, and is abutted with a ferrule end of an optical connector inserted into the guide sleeve 15, so that the optical fiber 13a and an external optical cable ( An optical connection is formed with an optical fiber (not shown).

  The package part 2 and the sleeve part 12 are coupled via a holder 9 and are aligned in the radial direction. The holder 9 has a partition wall 10 at the top, and an opening 11 through which the signal light condensed by the lens 7 passes is provided at the center of the partition wall 10, and is attached to the stem 3 of the package unit 2. It is fixed by welding. The radial position (position of incident / exited light) between the package part 2 and the sleeve part 12 is adjusted by changing the relative position of the holder 9 and the bush 16. After these adjustments, the bush 16 is fixed to and integrated with the holder 9 by adhesion or welding.

  When the bidirectional optical module 1 is aligned as described above, the transmission light output from the LD 4 is reflected by the WDM filter 6 and collected by the lens 7, and passes through the aperture 11 of the holder 9 to stub. It is incident on 13 optical fibers 13a. On the other hand, the signal light from the external transmission path is emitted from the optical fiber 13 a of the stub 13, enters the package portion 2 through the opening 11 of the holder 9, is condensed by the lens 7, and passes through the WDM filter 6. The light is received by PD5.

  Here, the signal light emitted and output by the LD 4 is designed to be incident on the optical fiber 13a of the stub 13 and transmitted to the external transmission path, but a part of the output signal light is packaged 2 It hits the inner wall of the cap 8 and is reflected. The reflected signal light becomes stray light and is repeatedly reflected and incident on the PD 5 to cause crosstalk. In the present invention, stray light generated in the package portion 2 is absorbed and extinguished.

  FIG. 3 shows an example according to the present invention, and FIG. 3 shows a light absorbing member 20 that absorbs stray light in a cap 8 of the package part 2. The basic structure of the package unit 2 is almost the same as that shown in FIG. 2 (for example, a CAN structure), and a light emitting element 4 such as an LD that transmits signal light on the stem 3 and a PD that receives signal light from an external circuit. The light receiving element 5 is mounted as a one-package type optical module device. Then, the WDM filter 6 is mounted on an optical path where transmission light and reception light having different wavelengths cross each other, and the transmission light and the reception light are selectively reflected or transmitted to be separated. The cap 8 that also functions as the housing of the package unit 2 is provided with a lens 7 that collects signal light and received light at the center of the upper part, and is sealed to the stem 3 with a sealing structure so that the internal mounted components. Protect.

  On the stem 3 of the package part 2, in addition to the LD 4, PD 5, and WDM filter 6, a light absorbing member 20 according to the present invention is fixed by a resin material and absorbs stray light generated by reflection in the cap 8. Like to do. The light absorbing members 20 are dispersed and arranged in the space portion on the stem 3, but are desirably arranged in a form surrounding the periphery of the PD 5 so that stray light other than the received light does not reach the PD 5.

  The light absorbing member 20 is formed of a material having an absorption coefficient capable of absorbing the light emission wavelength (for example, 1.31 μm) of the LD 4 that is a light emitting element, that is, a material having a light absorption edge equal to or greater than the light emission wavelength of the light emitting element. . For example, a Ge (light absorption edge is 1.55 μm) substrate, or InGaAs (light absorption edge is 1.65 μm), InGaAsP (light absorption edge is 1.30 to 1.55 μm) or the like on an InP substrate with a predetermined thickness. A semiconductor substrate grown in this manner can be used. As a result, a part of the signal light from the LD 4 hits the inner wall of the cap 8 and is reflected, hits the light absorbing member 20 before reaching the PD 5 and disappears.

  Further, instead of disposing the light absorbing member on the stem 3, a light absorbing member 21 may be provided on the inner wall surface of the cap 8 of the package 2 as shown in FIG. 4. However, the inner wall surface of the cap 8 is usually formed with a curved surface in many cases, and it is not easy to install the substrate on which the above-described semiconductor is grown. Therefore, for example, the semiconductor substrate formed as described above may be powdered and contained in a low-melting glass or the like and attached to the inner wall surface of the cap 8.

When the above-described InGaAs is used as a stray light absorbing member, assuming that the emission wavelength of the LD 4 is 1.31 μm, the absorption coefficient of InGaAs for this wavelength light is about 1.3 × 10 ⁴ cm ⁻¹ . 99.85% of light can be absorbed with a thickness of 5 μm. If the emission wavelength is 1.55 μm, the absorption coefficient of InGaAs with respect to this wavelength light is about 1.0 × 10 ⁴ cm ⁻¹ , and it absorbs 99.3% light at a thickness of 5 μm. Can do.

  Since the reflectance of the black Ni plating is about several to 10%, it can be said that the stray light absorbing effect in the package by the light absorbing members 20 and 21 is large. Further, the thickness of about 5 μm effective as a light absorbing member is not different from the normal plating thickness, does not occupy a limited space in the cap of the package, and does not need to be enlarged.

  When a semiconductor crystal substrate or the like is used as the light absorbing member, reflection may occur when the refractive index of the semiconductor material itself is, for example, n = 3.56 for InGaAs and n = 4.0 for Ge. Therefore, reflection on the surface of the absorbing member may be suppressed by further applying an AR coat (antireflection film) to the surface of the light absorbing members 20 and 21. As the AR coating material, for example, a Si oxide film (SiNx, SiON) can be used.

  The above-described light absorbing members 20 and 21 are manufactured by a normal semiconductor manufacturing technique and can be obtained at low cost, and can be easily obtained by a simple method such as adhesion or adhesion in a package of a bidirectional optical module having a general structure. Can be placed. The signal light from the light emitting element of the bidirectional optical module effectively absorbs and extinguishes the stray light reflected on the wall surface in the package and prevents the stray light from being received on the light receiving element side, thereby suppressing the occurrence of crosstalk. be able to.

It is a figure explaining the outline of the bidirectional | two-way optical module made into the object of this invention. It is an enlarged view of the package part of FIG. It is a figure explaining embodiment of this invention. It is a figure explaining other embodiment of this invention.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 ... Bidirectional optical module, 2 ... Package part, 3 ... Stem, 4 ... Light emitting element (LD), 5 ... Light receiving element (PD), 6 ... Wavelength multiplexing / demultiplexing filter (WDM filter), 7 ... Lens, 8 ... Cap, 9 ... Holder, 10 ... Partition wall, 11 ... Opening, 12 ... Sleeve, 13 ... Stub, 13a ... Short optical fiber, 14 ... Sleeve shell, 15 ... Guide sleeve, 16 ... Bush, 20, 21 ... Light absorbing member.

Claims (4)

A bidirectional optical module comprising a light emitting element that emits transmitted light, a light receiving element that receives received light, and a package in which a cap having a light transmission window is sealed on a stem on which a wavelength multiplexing / demultiplexing filter is mounted. And
A bidirectional optical module, wherein a light absorbing member made of a semiconductor material having a light absorption end longer than the light emitting wavelength of the light emitting element is disposed in the package to absorb stray light in the package.   The bidirectional optical module according to claim 1, wherein the semiconductor material is any one of Ge, InGaAs, and InGaAsP.   The bidirectional optical module according to claim 2, wherein the semiconductor material is powdered and contained in a low-melting glass.   The bidirectional light module according to claim 1, wherein the light absorbing member is provided on an inner wall surface of the cap.

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