JP2012027410A - Optical module - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an low cost optical module for high-speed optical communication free from reflection return light.SOLUTION: An optical module 1 reflects transmitted light which is output from an LD 11 and whose polarization direction is a predetermined direction by an optical filter 14 and makes it transmit through a rotor 40a that rotates a polarization direction to the same direction regardless of an incident direction and a polarizer 40b which transmits only light of a particular polarization direction to output it into an optical fiber 21a. The optical filter 14 is a polarization dependency filter that reflects light whose polarization direction is the predetermined direction and blocks light whose polarization direction is different from the predetermined direction regarding the wavelength of the transmission light. Reflection return light from the optical fiber 21a can be prevented from reaching the LD 11 by making the reflection return light transmit through the polarizer 40b and rotor 40a in sequence to turn it into polarization light whose polarization direction is different from the predetermined direction and making it transmit through the optical filter 14.

Description

  The present invention relates to an optical module used in optical communication, which includes a semiconductor laser element.

  In an optical module equipped with a semiconductor laser (LD: Laser Diode) used for optical communication, reflection occurs on the optical path from the LD to the optical fiber that is the transmission path of the optical signal, and the reflected light is returned to the LD. When returning, the laser oscillation of the LD becomes unstable. In order to prevent this, it is necessary to take measures against reflected light from the LD in the optical module.

As a countermeasure against reflected return light, an isolator having a function of transmitting light from one direction (transmitted light) without loss, but blocking and not transmitting light from the reverse direction (reflected return light), that is, a one-way transmission function of light. Can be considered between the LD and the optical fiber.
6 and 7 are diagrams illustrating an isolator used in the optical module disclosed in Patent Document 1. FIG.

  As shown in FIG. 6, the optical isolator 100 includes a Faraday rotator 100a that rotates the polarization direction of transmitted light by 45 degrees, and polarizers 100b and 100c that transmit only light having a predetermined polarization direction. Yes. In this specification, the direction in which the transmitted light from the LD 101 travels is referred to as the forward direction, and the direction in which the reflected return light of the transmitted light travels is referred to as the reverse direction. Is referred to as the A direction. The Faraday rotator 100a rotates the polarization direction in the same direction as the forward direction and the same direction by rotating 45 degrees with respect to the A direction, and the polarizer 100b is a light whose polarization direction is 0 degrees with respect to the A direction. The polarizer 100c is disposed so as to transmit only light whose polarization direction is 45 degrees with respect to the A direction.

  The transmission light emitted from the LD 101 is polarized light that oscillates in a specific direction, that is, the A direction, as shown in FIG. 7A, and the polarization direction does not change after being transmitted through the polarizer 100b. It remains in the A direction as shown in FIG. Further, as shown in FIG. 7C, the transmitted light that has been transmitted through the polarizer 100b and transmitted through the Faraday rotator 100a has a polarization direction of A + 45 degrees and is transmitted through the polarizer 102b. As shown in FIG. 4, the optical fiber 102 is coupled while the polarization direction is in the A + 45 degree direction.

  On the other hand, the return light reflected by the near-end reflection and the far-end reflection has an undefined polarization direction as shown in FIG. 7E, but the polarization direction is A + 45 as shown in FIG. 7F. Only through the polarizer 100c is transmitted through the polarizer 100c. The light transmitted through the polarizer 100c is rotated by 45 degrees in the polarization direction by the Faraday rotator 100a, and becomes polarized light having a polarization direction of A + 90 degrees as shown in FIG. Since the light in the polarization direction of A + 90 degrees cannot pass through the polarizer 100b, the reflected return light can be blocked.

JP 2007-226182 A

  However, an isolator composed of two polarizers and a rotor as described above is expensive. A configuration using an isolator composed of one polarizer and a rotator is also conceivable. However, in this configuration, polarized light whose polarization direction is the A + 90 degree direction shown in FIG. 7 is returned to the LD, so 25 Gbps or 10 Gbps or the like. When a distributed feedback type DFB-LD for high-speed optical communication is used, the oscillation of the LD is affected and the transmission characteristics are deteriorated.

  The present invention has been made in view of the above circumstances, and an object of the present invention is to provide an inexpensive optical module for high-speed optical communication without reflected return light.

  The optical module of the present invention reflects a transmission light whose polarization direction is a predetermined direction output from a laser diode by an optical filter, rotates a polarization direction in the same direction regardless of the incident direction, and a specific polarization. A polarizer that transmits only light in the wave direction is sequentially transmitted and output to an optical fiber, and the optical filter reflects light whose polarization direction is a predetermined direction with respect to the wavelength of the transmitted light, and the polarization direction. Is a polarization-dependent optical filter that blocks light different from a predetermined direction. The reflected return light from the optical fiber side is transmitted through a polarizer and a rotor in order, and the polarization direction is changed to a polarized light different from the predetermined direction. By passing through the filter, the reflected return light is prevented from reaching the laser diode.

  The optical module includes a photodiode and a wavelength multiplexing / demultiplexing filter that transmits or reflects light according to the wavelength. The wavelength multiplexing / demultiplexing filter transmits transmission light and receives light from an optical fiber. The bidirectional optical module may be provided in the optical path between the polarizer and the optical fiber so that the light is reflected and received by the photodiode.

  According to the optical module of the present invention, since an expensive polarizer is used as one piece and a polarization-dependent optical filter is used instead, and there is no reflected return light, the optical module can be manufactured at low cost.

It is sectional drawing of an example of the optical transmission module which concerns on one Embodiment of this invention. It is a figure explaining the mode of the transmission light and reflected return light of LD in the optical transmission module of FIG. It is a figure which shows the characteristic of an example of the polarization dependence optical filter used for the optical transmission module of FIG. It is a figure explaining the outline of an example of the bidirectional module which concerns on one Embodiment of this invention. It is a figure explaining the mode of the transmitted light and reflected return light of LD in the bidirectional module of FIG. It is a figure explaining the isolator used with the conventional optical transmission module. It is a figure explaining the isolator used with the conventional optical transmission module.

  FIG. 1 is a diagram for explaining an outline of an example of an optical transmission module according to an embodiment of the present invention. The optical transmission module of the present invention includes an optical transmission device 10 in which a semiconductor light emitting element (LD) 11 is incorporated in a CAN package, an LD 11 in the optical reception device 10 and an external optical transmission line, as illustrated by reference numeral 1 in the figure. And a joint sleeve (J sleeve) 30 for connecting the optical transmission device 10 and the sleeve portion 20 to each other. In the optical transmission module 1, a sleeve portion 20 is coupled to the optical transmission device 10 via a J sleeve 30. When an optical connector is inserted into the sleeve portion 20, an external transmission path is connected via the optical connector. So that optical signals can be transmitted and received.

  The optical transmission device 10 includes, for example, an LD 11, a stem 12, a lens cap 13, and an optical filter 14. The LD 11 is a distributed feedback type that emits light having a polarization direction in a direction parallel to the active layer, and outputs transmission light (for example, wavelength 1310 nm) to an external transmission path. The LD 11 is mounted on the upper surface 12a, which is the mounting surface of the stem 12, via the submount 11a so that the outgoing optical axis is parallel to the mounting surface, and the oscillation polarization direction is a predetermined direction, In the example of FIG. 1, the transmission light is output toward the optical filter 14 in the A direction that is perpendicular to the paper surface.

The stem 12 forms a CAN package having an element mounting space in cooperation with the lens cap 13, and the lead pin 12b for mounting the LD 11 and electrically connecting to an external circuit is glass-sealed. .
The lens cap 13 functions as a cap that hermetically seals the LD 11 in cooperation with the stem 12. A lens 13 a that collects transmission light from the LD 11 is glass-sealed in the upper central opening 13 b. Yes. The opening 13b (that is, the lens 13a) serves as a transmission light emitting port, and an inexpensive ball lens can be used as the lens 13a.

  The optical filter 14 relates to the wavelength of the transmitted light, for example, reflects light (S wave: vertical polarization) whose polarization direction is the A direction, and the polarization direction is other light, for example, the polarization direction is It is a polarization-dependent optical filter that blocks light in a direction perpendicular to the A direction (P wave: parallel polarization). The optical filter 14 is provided at an angle of 45 ° with respect to the upper surface 12a of the stem 12, and transmits light from the LD 11 having an optical axis parallel to the upper surface 12a to light having an optical axis perpendicular to the upper surface 12a. Is reflected toward the lens 13a.

Next, the sleeve portion 20 will be described. The sleeve portion 20 receives an optical connector (not shown) for optical connection with an external optical transmission line, and includes a stub 21, a sleeve 22, a sleeve shell 23, and a bush 24.
The columnar stub 21 is for optically coupling the LD 11 and the optical connector in the optical transmission device 10 with high optical coupling efficiency, and a single optical fiber 21a is inserted in the center thereof. The sleeve 22 is configured to allow the optical fiber in the ferrule and the optical fiber 21a in the stub 21 to abut each other when the ferrule of the optical connector is inserted. Hold together.

  The sleeve shell 23 receives the optical connector and houses and protects the sleeve 22. As will be described later, the bush 24 is used for alignment in the direction perpendicular to the axial direction, and a sleeve shell 23 for housing the sleeve 22 fitted with the stub 21 is mounted thereon. The end face of the stub 21 on the optical transmission device 10 side is polished obliquely together with the optical fiber 21a so as to suppress the reflection return of the signal light at the end face.

  Next, the J sleeve 30 will be described. The above-described optical transmission device 10 and the sleeve portion 20 are coupled via the following J sleeve 30. The J sleeve 30 has a flat upper end surface 30a and an opening 30b at the center of the upper portion. The signal light condensed by the lens 13a passes through the opening 30b.

  The J sleeve 30 includes an optical member 40 including a Faraday rotator 40a and a polarizer 40b between the opening 30b and the optical transmission device 10 (that is, between the stub 21 and the lens 13a). The Faraday rotator 40a rotates the polarization direction of the transmitted light by 45 degrees in the same direction regardless of whether it is incident from the LD 11 side or the opposite side, and the polarizer 40b has a predetermined polarization direction. It transmits only light having Around the optical member 40, a magnet 30c for the Faraday rotator 40a is provided.

  In the optical transmission module 1, axial alignment and radial alignment are performed via the J sleeve 30. The axial position (condensing distance of incident / exited light) between the optical transmission device 10 and the sleeve portion 20 is adjusted by the axial fitting length of the J sleeve 30 and the lens cap 13. On the other hand, the radial position between the optical transmission device 10 and the sleeve portion 20 (the position of incident / exit light) is adjusted by changing the relative position of the J sleeve 30 and the bush 24. After these adjustments, the J sleeve 30 is fixed to and integrated with the lens cap 13 and the bush 24 by bonding or welding.

  When the optical transmission module 1 is aligned as described above, the transmission light output from the LD 11 is reflected by the filter 14 because its polarization direction is the A direction and enters the optical filter 14 as a P wave. The light is condensed by the lens 13 a, passes through the Faraday rotator 40 a and the polarizer 40 b, and enters the optical fiber 21 a of the stub 21 through the opening 30 b of the J sleeve 30.

  Most of the signal light emitted and output from the LD 11 is incident on the optical fiber 21a of the stub 21 and transmitted to the external transmission path, but a part of the output signal light is reflected by, for example, the end face of the stub 21. Then, it returns to the optical transmission device 10 side as reflected return light. However, most of the reflected return light is blocked by the polarizer 40b, and the light transmitted through the polarizer 40b is also changed to the direction perpendicular to the A direction by the Faraday rotator 40a. Since it is incident, it passes through the filter 14 and does not reach the LD 11.

  A state in which the transmission light of the LD 11 is coupled to the optical fiber 21a and a state of reflected return light of the transmission light in the optical transmission module 1 will be described with reference to FIG. The polarizer 40b is arranged so that only the light whose polarization direction is A + 45 degrees is transmitted.

  The transmission light emitted from the LD 11 is polarized light that oscillates in the A direction as shown in FIG. 2A, and therefore enters the optical filter 14 as an S wave and is reflected by the filter 14. The reflected light is incident on the Faraday rotator 40a with the polarization direction being the A direction as before reflection as shown in FIG. 2B, and as shown in FIG. 2C by the rotor 40a. The polarization direction is A + 45 degrees and the light is transmitted through the polarizer 40b. As shown in FIG. 2D, the polarization direction is coupled to the optical fiber 21a as polarized light having the A + 45 degrees direction.

  On the other hand, the return light reflected by the near-end reflection and the far-end reflection has an undefined polarization direction as shown in FIG. 2E, but the polarization direction is A + 45 as shown in FIG. 2F. The light passes through the polarizer 40b only in the direction of degrees. The light transmitted through the polarizer 40b is rotated by 45 degrees in the polarization direction by the Faraday rotator 40a, and becomes polarized light having a polarization direction of A + 90 degrees as shown in FIG. Since the light in the polarization direction of A + 90 degrees is transmitted through the optical filter 14, it is reflected by the filter 14 and does not reach the LD 11, so that the reflected return light can be blocked.

  As described above, since the optical transmission module 1 blocks the reflected return light by one polarizer, one Faraday rotator, and one optical filter, the two polarizers, one Faraday are blocked by the above-described blocking. Compared to a configuration using a rotator, an optical filter can be obtained / manufactured at a lower cost than a polarizer, so that the cost is low.

  In addition, since the emitted light from LD11 is a diverging light, it injects with the angle of 45 degree +/- alpha degree with respect to the optical filter arrange | positioned 45 degrees. α depends on the characteristics of the FFP (Far Filed Pattern) of the LD, but it is 25 degrees in consideration of the coupling loss of light having FFP of 20 degrees or more, the structural space, and the like. That is, it is preferable that the optical filter reflects transmission light incident at an angle of 45 degrees ± 25 degrees (20 degrees to 70 degrees). Similarly, it is preferable that the reflected return light whose incident angle to the optical filter is not 45 degrees and the incident angle return of 45 degrees ± 25 degrees (20 degrees to 70 degrees) are also transmitted.

  Therefore, an optical filter that exhibits characteristics as shown in FIG. 3 is preferable. That is, in the transmission light wavelength band (1310 ± 10 nm), the S wave having an incident angle of 20 ° to 75 ° has a high reflectance, a transmittance of −30 dB or less, and a P wave having an incident angle of 20 ° to 70 °. For waves, a polarization-dependent optical filter having a high transmittance of −0.3 dB or higher is preferable.

The present invention can be configured not only as an optical transmission module having an optical transmission function but also as a bidirectional optical module having an optical transmission / reception function.
FIG. 4 is a diagram illustrating an outline of an example of a bidirectional module according to an embodiment of the present invention. In the figure, only the main part of the bidirectional optical module is shown, and the same components as those of the optical transmission module of FIG.

  The bi-directional optical module of FIG. 4 includes the optical transmission device 10 having the LD 11, the optical filter 14, and the optical member 40 made of a Faraday rotator and a polarizer, like the optical transmission module 1 of FIG. 1. In addition, a light receiving device 50 having a photodiode (PD) 51 and a condenser lens 52, and a wavelength multiplexing / demultiplexing filter 60 that transmits or reflects light according to the wavelength are provided.

  The wavelength multiplexing / demultiplexing filter 60 is in the optical path between the polarizer and the optical fiber so that the transmission light is transmitted and the reception light from the optical fiber 21a is reflected and received by the PD 51 through the condenser lens 52. Provided. The wavelength multiplexing / demultiplexing filter 60 is an optical filter having a property different from that of the optical filter 14 that is a polarization-dependent optical filter. For example, the transmittance of the light in all polarization directions is −0 in the transmission optical wavelength band. .3 dB or more, and an optical filter having a high reflectance and a transmittance of −30 dB in the received light wavelength band (1490 ± 10 nm).

A state in which the transmission light of the LD 11 is coupled to the optical fiber 21a and a state of reflected return light of the transmission light in the bidirectional optical module of FIG. 4 will be described with reference to FIG.
The state until the transmission light emitted from the LD 11 passes through the polarizer 40b is the same as that in FIG. Then, the light transmitted through the polarizer 40b is incident on the wavelength multiplexing / demultiplexing filter 60 with the polarization direction being the A + 45 degree direction as before transmission, as shown in FIG. 5D. As shown in E), the polarization direction is coupled to the optical fiber 21a with the polarization in the A + 45 degree direction.

  On the other hand, the polarization direction of the reflected return light is not determined as shown in FIG. 5 (F), and is not determined after passing through the wavelength multiplexing / demultiplexing filter 60 as shown in FIG. 5 (G). As shown in FIG. 2H, the polarization direction is transmitted through the polarizer 40b only in the direction of A + 45 degrees. Thereafter, the process proceeds in the same manner as that in FIG. 2, and as shown in FIG. 5I, the polarized light having a polarization direction of A + 90 degrees is incident on the optical filter 14. Since the light in the polarization direction of A + 90 degrees is transmitted through the optical filter 14, it is reflected by the filter 14 and does not reach the LD 11, so that the reflected return light can be blocked even in the bidirectional optical module. .

DESCRIPTION OF SYMBOLS 1 ... Optical transmission module, 10 ... Optical transmission device, 11 ... Semiconductor laser (LD), 11a ... Submount, 12 ... Stem, 13 ... Lens cap, 14 ... Optical filter, 20 ... Sleeve part, 21 ... Stub, 22 ... Sleeve, 23 ... Sleeve shell, 24 ... Bush, 30 ... J sleeve, 30c ... Magnet, 40 ... Optical member, 40a ... Faraday rotator, 40b ... Polarizer, 50 ... Optical receiving device, 51 ... Photodiode (PD), 60: Wavelength multiplexing / demultiplexing filter.

Claims (2)

The transmitted light whose polarization direction is output from the laser diode is reflected by the optical filter, and only the light having a specific polarization direction and a rotor that rotates the polarization direction in the same direction regardless of the incident direction are transmitted. An optical module that sequentially transmits a polarizer and outputs it to an optical fiber,
The optical filter is a polarization-dependent optical filter that reflects light whose polarization direction is the predetermined direction with respect to the wavelength of the transmission light, and blocks light whose polarization direction is different from the predetermined direction;
The reflected return light from the optical fiber side is sequentially transmitted through the polarizer and the rotor, the polarization direction is different from the predetermined direction, and the reflected return light is transmitted through the optical filter. An optical module characterized by preventing reaching to a laser diode. A wavelength multiplexing / demultiplexing filter that transmits or reflects light according to a wavelength, and the wavelength multiplexing / demultiplexing filter transmits the transmission light and reflects the reception light from the optical fiber. An optical module comprising a bidirectional optical module provided in an optical path between the polarizer and the optical fiber so that the photodiode receives light.

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