Optical waveguides – Integrated optical circuit
Reexamination Certificate
2003-01-30
2004-09-28
Lee, Michael G. (Department: 2876)
Optical waveguides
Integrated optical circuit
C385S088000, C385S129000
Reexamination Certificate
active
06798932
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a parallel LD/PD module which transmits and receives a plurality of optical signals via a plurality of channels. A parallel communication system transmits optical signals of four, eight, sixteen or, in general, 2
m
(m=integer) channels via a tape fiber including parallel individual M element fibers (M=2
m
). A single-mode fiber has a 125 &mgr;m cladding diameter. A standard tape fiber arrays individual element fibers at a 250 &mgr;m pitch.
This invention claims the priority of Japanese Patent Application No. 2002-35177 filed on Feb. 13, 2002, which is incorporated herein by reference.
A laser diode chip has at least a 300 &mgr;m length and a 300 &mgr;m width. A parallel LD module should have M laser diodes (LDs). If lightpaths were formed in a parallel LD module at a 250 &mgr;m pitch, the virtual module could not allot laser diodes having sides longer than 300 &mgr;m at ends of the 250 &mgr;m pitch lightpaths.
2. Description of Related Art
{circle around (1)} M. Shishikura, K. Nagatsuma, T. Ido, M. Tokuda, K. Nakahara, E. Nomoto, T. Sudoh and H. Sano, “10 Gbps×4-channel parallel LD module”, Proceeding of the 2001 Communications Society Conference of IEICE, C-3-50, p160 (2001)
This pointed out problems of multichannel LD modules which contain a plurality of laser diodes. The problems cited were interchannel interference, crosstalk, heating, and fluctuation of properties induced to a multichannel LD module which arranged laser diodes without enough margins. For overcoming the problems, {circle around (1)} proposed a four channel laser diode module with four laterally extending lightwaveguides formed on a silicon bench and four laser diodes placed at ends of the lightwaveguides on the silicon bench. The laser diodes are arranged side by side at a wide pitch. The wide separation reduces crosstalk between an LD and a neighboring LD.
FIG. 16
is a perspective view of the LD module proposed by {circle around (1)}. {circle around (1)} has a (100) orientation single crystal silicon bench
222
(or base), lightwaveguides of an SiO
2
type formed upon the silicon bench
222
, and laser diodes LDa, LDb, LDc and LDd. The lightwaveguides consist of a lower cladding, a core and an upper cladding. The cores are made of Ge-doped SiO
2
. The lower and upper claddings are made of SiO
2
. The four channel module has four lightwaveguides A, B, C and D. Front ends of the lightwaveguides have a spatial period (pitch d) of 250 &mgr;m which is equal to a pitch of element fibers in standardized ribbon fibers and MT connectors. The lightwaveguides have width enlarging region. Rear ends of the lightwaveguides have a wide spatial period (pitch D) of 1000 &mgr;m (1 mm). The four laser diodes LDa, LDb, LDc and LDd are mounted on a rear end
226
of the silicon bench
222
behind the final ends of the lightwaveguides at a 1000 &mgr;m pitch.
The known LD module {circle around (1)} expands the pitch of channels from 250 &mgr;m, which is equal to the pitch of ribbon fibers, to 1000 &mgr;m which is suitable for mounting the four laser diodes. {circle around (1)} multiplies the channel pitch by four. {circle around (1)} reported that crosstalks between the neighboring laser diodes (LDa-LDb, LDb-LDc and LDc-LDd) were −40 dB at 10 GHz. {circle around (1)} gave wide separation between the neighboring laser diodes for suppressing LD—LD mutual crosstalks. Enlargement of the lightwaveguides requires a long enlarging region on the silicon bench. A full length of the silicon bench should be 15 mm to 20 mm. The long silicon bench results in a large, bulky four-channel LD module.
An arrangement of separated, individual laser diode chips at final ends of plural lightpaths in parallel is unavoidable for high speed parallel signal transmission. Separation of the laser diodes enables the module to freely choose p-type substrate LDs or n-type substrate LDs and to select oscillation frequencies of laser diodes independently. Installment of independent laser diodes enhances freedom of designing.
The known reference {circle around (1)} has only a signal transmitting device but lacks a signal receiving device. {circle around (1)} will require a signal receiving device and another ribbon fiber containing another set of four-channel element fibers for establishing a bi-directional simultaneous optical communications system. {circle around (1)} will allocate a common wavelength (e.g., 1.3 &mgr;m) in both upward signals and downward signals. An extension of {circle around (1)}, which contains a separated signal transmitting device and a separated signal receiving device, will not be annoyed by crosstalks between the transmitting portion and the receiving portion. The known reference {circle around (1)} requires two (binary) fibers for a single channel. The extension of {circle around (1)} is a binary fiber type system which needs 2M fibers for M channels. Modules for the binary fiber type {circle around (1)} will be expensive, large-sized modules due to an independent LD device and an independent PD device.
The binary fiber system like {circle around (1)} has, in addition, a drawback of laying 2M fibers between a subscriber (ONU) and a central station. In the case of a four channel binary system, eight (4×2=8) element fibers should be built between the ONU and the station, which raises the cost of constructing the system.
A preferable one is a multichannel system which can exchange four channel signals by four fibers by allotting two different wavelengths (e.g., 1.3 &mgr;m and 1.55 &mgr;m) to an upward signal stream (from ONU to station) and a downward signal stream (from station to ONU). One purpose of the present invention is to provide a bi-directional simultaneous multichannel LD/PD module which can carry signals up and down by a plurality of fibers whose number is equal to the number of channels. Another purpose of the present invention is to provide a bi-directional simultaneous multichannel LD/PD module of low-cost, small-size and high reliability.
There is a known single-fiber LD/PD module, which aims at simultaneous, bi-directional signal transmission of a single channel, positioning a laser diode, a photodiode and a wavelength selective filter at a point of a y-branch formed upon a lightwaveguide layer on a silicon bench in two dimensions. For example,
{circle around (2)} Japanese Patent Laying Open No.11-68705, “Two-way WDM optical transmission reception module” proposed a single-channel LD/PD module which has a silicon bench, a y-branched SiO
2
lightwaveguide formed on the silicon bench, a laser diode (transmitting 1.3 &mgr;m light) deposited at an upper left end of “y”, a photodiode (receiving 1.55 &mgr;m light) deposited at a bottom end of “y”, an end of a fiber fitted at an upper right end of “y”, and a WDM (wavelength division multiplexer) at the branch for allowing 1.55 &mgr;m light to pass and reflecting 1.3 &mgr;m light. On the silicon bench, the 1.3 &mgr;m LD beam depicts a V-shaped locus and the 1.55 &mgr;m PD beam a /-shaped locus. The known reference {circle around (2)} contrives to reduce electrical crosstalk by positioning the LD and the PD in reverse directions regarding the WDM. Since {circle around (2)} is a module on an ONU (optical network unit; subscriber), a single-channel is sufficient.
An ONU is satisfied with a module having a single LD (1.3 &mgr;m) and a single PD (1.55 &mgr;m). The relation of the wavelengths is reversed for an ONU and a station. The central station should be equipped with a station module having an LD which emits 1.55 &mgr;m and a PD which senses 1.3 &mgr;m. The central station may utilize single-channel modules similar to the module of an ONU. The central station should have N single-channel modules for exchange signals with N ONUs. N, which is a number of ONUs, is a very large number. Installment of N single-channel modules would occupy a vast volume in the central station.
Instead of the single-channel modules, multi-channel modules are favorable for the cent
Kuhara Yoshiki
Yamabayashi Naoyuki
Fish & Richardson P.C.
Lee Michael G.
Lee Seung H
Sumitomo Electric Industries Ltd.
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