JP2009158515A - Bidirectional optical module - Google Patents

Abstract

A bidirectional optical module free from optical crosstalk is provided by arranging the wavelength signal light emitted from a laser diode (LD) to be difficult for a photodiode (PD) to receive.
A package in which a cap having a light transmission window is sealed on a light emitting element that emits transmission light, a light receiving element that receives reception light, and a wavelength multiplexing / demultiplexing filter. 2 is a bi-directional optical module. The light receiving element 5 is disposed in a recess 12 that is recessed from the upper surface of the stem, and is mounted so that the light receiving surface of the light receiving element 5 does not protrude from the upper surface of the stem 3. Further, stray light generated in the package is blocked from entering the light receiving surface of the light receiving element by the side wall of the recess 12.
[Selection] Figure 1

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 advancement of optical communication technology, PON (Passive), which enables higher-speed and large-capacity immediate communication at an appropriate cost, in the subscriber network (access system) represented by FTTH (Fiber To The Home) The introduction of the Optical Network system is progressing. The PON system adopts a single-core bidirectional communication system, reduces the number of fibers in the optical fiber transmission line between the station side and the user side, and realizes a high-speed service at the same low price as a metal cable. . Specifically, it is a wavelength division multiplexing (WDM: Wavelength Division Multiplexing) in which transmission and reception are performed with two wavelengths of a 1.31 μm band and a 1.55 μm band (or 1.49 μm band) using one optical fiber.

  In a single-core bidirectional optical module (Bi-D: Bi-Directional Module) for use in such a PON system, transmission light emitted from a light emitting element such as a laser diode (hereinafter referred to as LD) is coupled to an optical fiber. On the other hand, the received light transmitted through the optical fiber is coupled to a light receiving element (hereinafter referred to as PD) such as a photodiode. Many proposals have been made for a method and an apparatus for coupling an optical fiber and transmitted / received light.

  Conventionally, development has proceeded because an optical module having a two-package structure using a CAN package containing a transmission LD and a CAN package containing a reception PD can be assembled by a conventional method. However, as described at the beginning, it is necessary to further reduce the cost by single-core bidirectional communication, which has a strong demand for low prices, and both LD and PD are built in one CAN package. Development is also underway for single-core bidirectional optical modules with a single package structure.

  FIG. 4 is an example of a conventional single-core bidirectional optical module having a single package structure. The package 2 is formed of a coaxial package, and the LD 4 that transmits signal light to the stem 3 has a heat sink member for cooling. The PD 5 that receives the received light is mounted via a submount or the like. A cap 8 provided with a lens 7 for condensing transmission light (LD light) and reception light is attached so as to seal the LD 4 and PD 5 mounted on the stem 3 and other electronic components. A holder 9 for positioning and holding a sleeve member (not shown) that forms a connection with the optical fiber is attached and fixed to the cap 8.

  A WDM filter 6 that reflects the transmission light and transmits the reception light is arranged on the optical path of the lens 7 and the LD 4 and PD 5. The transmission light emitted from the LD 4 is reflected by the WDM filter 6, collected by the lens 7, passes through the opening 9a of the holder 9, and enters the fiber portion 10a of the fiber stub 10 fixed to the sleeve member. It is sent to an external optical fiber transmission line. The received light transmitted from the external optical fiber transmission line is emitted from the fiber portion 10a of the fiber stub 10, is condensed by the lens 7 through the opening 9a of the holder 9, passes through the WDM filter 6, and passes to the PD 5. Incident.

  As shown in FIG. 5, a part of the signal light emitted from the LD 4 is reflected by the WDM filter 6 and incident on the fiber stub and passes through or reflects the WDM filter 6 in addition to the transmission. It becomes stray light that repeats reflection while being attenuated in the cap 8 without being emitted. When the light that has become stray light in the cap 8 is received by the PD 5 for reception, it causes optical crosstalk and deteriorates reception characteristics.

As a method for improving this, a cut filter 11 (also referred to as a band-pass filter that transmits light of a specific wavelength and blocks other wavelengths) is disposed between the PD 5 and the WDM filter 6 to transmit light. Leakage light or reflected light is prevented from being received by the PD 5 as stray light in the cap.
Patent Document 1 discloses that a black film such as Zn-Ni plating is provided on the cap surface to suppress stray light due to reflection of LD light. Patent Document 2 discloses matte palladium plating on the cap surface. In addition, it is disclosed that stray light due to reflection of LD light is similarly suppressed.
JP 2003-204006 A JP 2006-120864 A

By mounting a transmission LD and a reception PD in one package, space and cost are reduced compared to mounting a transmission LD and a reception LD in separate packages (two packages). There is a great merit, but there is a problem of occurrence of optical crosstalk as described above.
When the cut filter 11 is provided between the PD 5 and the WDM filter 6 as shown in FIG. 5 described above, the angle tolerance is narrower than the angle at which the received light is incident on the PD 5 due to the characteristics of the cut filter 11. It is weak against large stray light incidence, and is insufficient to prevent optical crosstalk.

Further, as disclosed in Patent Document 1, a black film is formed on the surface of the package cap, or by applying matte palladium plating on the surface of the package cap as disclosed in Patent Document 2, the LD Although the effect of reducing the reflection of light is recognized, it has not been improved sufficiently.
The present invention has been made in view of the above-described circumstances, and is configured such that the wavelength signal light emitted from the laser diode (LD) is not easily received by the photodiode (PD), thereby preventing both optical crosstalk. The purpose is to provide a directional module.

  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. Is a bi-directional optical module. The light receiving element is disposed in a recess provided to be recessed from the upper surface of the stem, and is mounted so that the light receiving surface of the light receiving element does not protrude from the upper surface of the stem. Further, stray light generated in the package is blocked from entering the light receiving surface of the light receiving element by the side wall of the recess.

  According to the present invention, stray light caused by part of the signal light emitted from the light emitting element being reflected in the package can be efficiently reflected and extinguished so as not to be received by the light receiving surface of the light receiving element. It is possible 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, FIG. 2 is a schematic diagram of the package of FIG. 1, and FIG. 3 is a diagram showing a crosstalk penalty. In the figure, 2 is a package, 3 is a stem, 3a is a lead pin, 4 is a light emitting element (LD), 5 is a light receiving element (PD), 6 is a wavelength multiplexing / demultiplexing filter (WDM filter), 7 is a lens, and 8 is a cap. , 9 is a holder, 9a is an opening, 10 is a fiber stub, 10a is an optical fiber portion, and 12 is a recess.

  The bidirectional optical module is formed of a coaxial package 2 as shown in FIG. In this package 2, for example, an LD 4 that transmits signal light is mounted on a stem 3 to which a plurality of lead pins 3 a are sealed via a heat sink member for cooling and the PD 5 that receives received light. Are mounted via a submount or the like. In the present invention, as will be described later, in particular, the PD 5 is disposed in a recess 12 provided to be recessed from the upper surface of the stem 3 and mounted so that its light receiving surface does not protrude above the upper surface of the stem. Yes.

On the stem 3, a wavelength multiplexing / demultiplexing filter 6 (hereinafter referred to as a WDM filter) is mounted on an optical path where transmission light and reception light having different wavelengths intersect with each other, so that transmission light and reception light are transmitted. Separated by selective reflection or transmission.
The LD 4 and PD 5 and the WDM filter 6 mounted on the stem 3 are protected and sealed from the outside by attaching the cap 8 together with other attached electronic components. The cap 8 also functions as a housing of the package 2, and is usually formed of metal, and a lens 7 is attached to an opening that transmits transmission light (LD light) and reception light.

  A cap-shaped holder 9 is fitted to the cap 8 so as to be aligned in the optical axis direction (to adjust the focal position of the LD light condensed by the lens 7) and fixed by adhesion or welding. An opening 9a through which signal light passes is provided in the upper wall portion of the holder 9, and a sleeve member (not shown) for forming a connection with the optical fiber is positioned and fixed. The fiber stub 10 in which the optical fiber 10a through which the signal light is emitted and incident is embedded is positioned above the holder 9 by the sleeve member.

  In the bidirectional optical module configured as described above, the transmission light emitted from the LD 4 is reflected by the WDM filter 6, collected by the lens 7, passes through the aperture 9 a of the holder 9, and the light from the fiber stub 10. The light enters the fiber 10a. On the other hand, the signal light from the external optical fiber transmission line is emitted from the optical fiber 10a of the fiber stub 10, enters the package 2 through the opening 9a of the holder 9, is condensed by the lens 7, and is collected by the WDM filter. 6 is received by PD5.

  Here, the signal light emitted from the LD 4 is designed to be incident on the optical fiber 10a of the fiber stub 10 and transmitted to an external optical fiber transmission line. However, as shown in FIG. 2A, a part of the output signal light passes through the WDM filter 6 and is reflected and reflected while being attenuated by hitting the inner wall of the cap 8 without being emitted to the outside. Repeated stray light. When the stray light enters the PD 5, there is a risk of causing optical crosstalk. In the present invention, the PD5 is arranged so that stray light generated in the package does not enter the light receiving surface of the PD5.

  FIG. 2 (B) shows an example of the arrangement form of the PD according to the present invention, and the recess 12 for accommodating and arranging the PD 4 is formed so as to be recessed from the upper surface of the stem. The recess 12 is formed at a position where the received light that has passed through the WDM filter can be received by the light receiving surface of the PD, and has a size and depth that can be easily formed by a mechanical press or the like. Further, it is desirable that the light receiving surface of the PD 5 is formed so as not to protrude from the upper surface of the stem 3 when the PD 5 is mounted in the recess 12 via a submount.

  By arranging the PD 5 so as not to protrude from the upper surface of the stem 3 as described above, stray light that has passed through the WDM filter 6 or reflected and hits the inner wall of the cap 8 without being emitted to the outside is generated by the PD 5. It becomes difficult to enter the light receiving surface. In particular, when the light receiving surface of the PD 5 is made lower than the upper surface of the stem 3, the PD 5 becomes a shadow portion by the side wall 12 a of the recess 12, and the stray light is blocked from entering the light receiving surface of the PD 5. As a result, it is possible to suppress the stray light generated in the package from entering the light receiving surface of the PD 5.

  FIG. 3 is a diagram showing the results of verifying the above-described stray light suppression effect. The vertical axis shows the code error rate and the horizontal axis shows the reception sensitivity. FIG. 4B) and FIG. 3B are the products of the present invention (the configuration of FIG. 1 in which the PD is disposed in the recess of the stem).

In the product of the present invention that is not implemented in FIG. 3A, the LD on (within the same package) is compared with the PD receiving sensitivity when the LD is off (the signal light is not emitted from the LD in the same package). In the case of signal light being emitted from the LD), the receiving sensitivity is reduced by 1.3 dB on average (crosstalk penalty).
On the other hand, in the product according to the present invention shown in FIG. 3B, the reception sensitivity in the case of LD on with respect to the reception sensitivity in the case of LD off is a crosstalk penalty of about 0.8 dB on average, which is normally acceptable. It can be suppressed to 1.0 dB or less.

It is a figure explaining the outline of the bidirectional | two-way optical module made into the object of this invention. It is a figure explaining the detail of the package by this invention. It is a figure which shows the crosstalk penalty of the conventional product and this invention product. It is a figure explaining the prior art. It is a figure explaining the detail of the conventional package.

Explanation of symbols

2 ... package, 3 ... stem, 3a ... lead pin, 4 ... light emitting element (LD), 5 ... light receiving element (PD), 6 ... wavelength multiplexing / demultiplexing filter (WDM filter), 7 ... lens, 8 ... cap, 9 ... Holder, 9a ... opening, 10 ... fiber stub, 10a ... optical fiber, 11 ... cut filter, 12 ... recess.

Claims (2)

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
The bidirectional optical module is characterized in that the light receiving element is disposed in a recess provided to be recessed from the upper surface of the stem, and is mounted so that the light receiving surface does not protrude from the upper surface of the stem.   2. The bidirectional optical module according to claim 1, wherein stray light generated in the package is blocked from entering a light receiving surface of the light receiving element by a side wall of the concave portion.

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