Optical communications module

Details Optical communications module Optical communications module

2002-12-11

2004-09-21

Nguyen, Chau N. (Department: 2831)

Optical waveguides

With disengagable mechanical connector

Optical fiber to a nonfiber optical device connector

C385S024000, C385S088000

Reexamination Certificate

active

06793410

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to an optical communications module having M laser diodes (LDs), M photodiodes (PDs) and M lightwaveguides (M≧1) suitable for bi-directional simultaneous optical communications which allocate a single fiber to both upward and downward signal streams. This invention also relates to an optical communications module having M laser diodes (LDs), M monitoring photodiodes and M lightwaveguides (M≧1) suitable for optical communications which allocate a fiber to an upward signal stream.
This application claims the priority of Japanese Patent Applications No. 2001-391143 filed on Dec. 25, 2001 and No. 2002-320705 filed on Nov. 5, 2002, which are incorporated herein by reference.
Bidirectional communications gives an optical fiber a role of carrying optical signals in both directions between a central station and a plurality of subscribers (ONU: optical network units). LD/PD modules are equipped at the ONUs and the station. An LD/PD module should separate light paths into a PD part and an LD part at ends of optical nets. Various separation elements have been proposed. Requirements for the LD/PD path separation elements are low-division loss, low optical crosstalk, low electrical crosstalk, and low electromagnetic crosstalk.
Optical crosstalk means that strong light emitted from an LD goes into a highly sensitive PD and induces noise in receiving signals. Simultaneous bidirectional communications system uses two different wavelengths &lgr;1 and &lgr;2. &lgr;1(1.3 &mgr;m) is a upward signal wavelength which is sent from ONUs to a central station. &lgr;2(1.55 &mgr;m) is a downward signal wavelength which is sent from the central station to the ONUs. Photodiodes (PDs) generally used in optical communications have an InGaAs light receiving layer (sensing layer or active layer) which has sensitivity between 1.0 &mgr;m and 1.6 &mgr;m. The InGaAs-PDs sense both &lgr;1 and &lgr;2. Thus, optical crosstalk from LDs to PDs should be eliminated. “Optical” means that the medium of transmitting noise is light.
Another problem is electric crosstalk from LDs to PDs. Electric crosstalk means that strong driving currents for LDs mixe with weak photocurrents of PDs via a package or wirings. “Electric” means that the medium of transmitting noise is electric currents. A further problem is electromagnetic crosstalk. LDs generate electromagnetic waves which fly in space to the PDs and induce noise in the receiving signals. “Electromagnetic” means that the medium of carrying noise is electromagnetic waves. Reduction of optical, electrical and electromagnetic crosstalk is ardently required for LD/PD modules in the optical communications.
2. Description of Related Art
There are some types of modules for allocating an LD and a PD.
FIG. 12
shows a discrete type of an LD/PD module. The discrete type LD/PD module aligns an optical fiber
85
and an LD
86
along a straight beam line, positions a Wavelength Division Multiplexer (WDM)
87
slanting by 45 degrees to the beam line at a middle point in the straight beam line, and disposes a PD
88
in a vertical direction which crosses the beam line at right angles. In the module, transmitting light signals emitted from the LD
86
simply pass the WDM
87
and go into the optical fiber
85
. Receiving light signals propagating in the optical fiber
85
are reflected at the WDM
87
and are guided into the PD
88
for generating photocurrents which represent receiving signals.
The WDM
87
is employed for selecting wavelengths in the module. The WDM, which is an optical device made by piling a plurality of sets of more than two kinds of transparent dielectric thin films having different refractive indices, allows a first wavelength (&lgr;1) to pass at a ratio of nearly 100% but reflects a second wavelength (&lgr;2) at a rate of nearly 100%. The WDM has an intermediate rate of transparency and another intermediate rate of reflection for other wavelengths.
{circle around (1)} Masahiro Ogusu, Tazuko Tomioka, Shigeru Ohshima, “Receptacle Type Bi-directional WDM Module I”, Proceeding of the 1996 Electronics Society Conference of IEICE, C-208, p208 (1996).
The module proposed by {circle around (1)} has light paths in a free space. A PD and an LD form an independent PD module and an independent LD module which are separated in the free space. Spatial separation decreases crosstalk between the PD and the LD, which is an advantage. Since separated modules are integrated in the free space, the module is bulky, large, heavy and expensive.
Another known module divides an optical path by a Y branch coupler. The y-branch module is made by fabricating an inverse y-shaped branch lightwaveguide on a silicon bench, positioning a photodiode at an end of the lightwaveguide, providing a wavelength division multiplexer (WDM) at a branch point, putting a laser diode (LD) at an end of a right branched waveguide, and putting an end of an optical fiber at another end of a left branched lightwaveguide. The optical fiber emits 1.55 &mgr;m receiving signals to the left branched lightwaveguide. The 1.55 &mgr;m receiving signals make a straight way via the WDM in the lightwaveguide into the PD. The LD emits 1.3 &mgr;m transmitting signals. The 1.3 &mgr;m signals run in the lightwaveguide. The transmitting 1.3 &mgr;m signals are reflected by the WDM and go into the optical fiber.
{circle around (2)} Japanese Patent Laying Open No.11-68705, “Two-way WDM Optical Transmission Reception Module”,
proposed such a Y-branched WDM based planar type module, in which receiving light going out from an optical fiber propagates in a lightwaveguide, passes the WDM and reaches a PD and transmitting light yielded by an LD is reflected by the WDM and enters the optical fiber. The LD and the PD are allocated at opposite ends of an silicon bench with a wide distance. {circle around (2)} asserted that such a structure decreases electric crosstalk and optical crosstalk.
The PD and the LD are mounted upon the common silicon bench. Silicon, which is a semiconductor with high conductivity, conducts electricity. Thus, electric crosstalk would be large. Silicon is transparent for 1.3 &mgr;m light. Thus, optical crosstalk would be large in the silicon bench based module. The planar Y-branch type module has another drawback that the LD and the PD mounted on the surface require wide areas on a silicon bench. It is difficult to modify the planar single LD/PD module into a multichannel module having a plurality of pairs of LDs and PDs.
A third type known module is an upward branch type which is prepared by making a linear lightwaveguide/linear optical fiber on a planar bench, disposing a laser diode (LD) at an end of the lightwaveguide/fiber, providing a upward slanting WDM in the lightwaveguide/fiber and positioning a photodiode on a submount put at a point slanting to the WDM on the bench. Receiving light propagating in an external fiber and going into the lightwaveguide is reflected by the WDM upward to the PD.
{circle around (3)} T. Uno, T. Nishikawa, M. Mitsuda, G. Tohmon, Y. Matsui, “Hybridly Integrated LD/PD Module with Passive-alignment Technology”, Proceeding of the 1997 Electronics Society Conference of IEICE, C-3-89, p 198 (1997).
The above proposed such an upward branch type which was made by preparing a silicon bench having a lower front step and a higher rear step with a V-groove, gluing a glass substrate having a V-groove on the front step of the silicon bench with the V-groove aligning the bench V-groove, mounting an optical fiber on the V-grooves, installing a WDM in a slit at an intermediate spot of the fiber, mounting a photodiode on a submount at a point slantingly in front of the WDM, and fitting a laser diode at the end of the fiber. Receiving light running in the fiber is reflected upward by the WDM and is guided into the PD. Light passage is vertically divided for the LD and the PD.
Since the PD should be mounted slightly higher than the LD, the PD is mounted on the submount giving an additional height. An interval between the

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