Optical coupler and/or multiplexer

Details Optical coupler and/or multiplexer Optical coupler and/or multiplexer

2000-02-14

2001-08-21

Spyrou, Cassandra (Department: 2872)

Optical waveguides

With optical coupler

Input/output coupler

C385S024000, C385S042000, C385S043000, C359S199200, C359S199200, C359S199200

Reexamination Certificate

active

06278818

ABSTRACT:

This invention relates to optical couplers and/or multiplexers.
Fused optical fibre couplers, in which two optical fibres are fused together at a coupling region, are known.
JP-A-62 017 709 and U.S. Pat. No. 5,170,450 propose and optical fibre coupler having at least an m-core optical fibre optically coupled to an n-core optical fibre, where m and n are positive integers and m is greater than 1.
In such a coupler, at least one of the fibres involved in the coupler has two or more light-transmitting cores.
In wavelength division multiplexed (WDM) optical transmission systems, multiple information channels are transmitted at different respective wavelengths so that the channels can all be carried on a single waveguide (e.g. an optical fibre).
There is a need in such systems for so-called channel-add multiplexers and so-called channel-drop demultiplexers (these two functions can be combined in a single channel add/drop multiplexer).
A channel add multiplexer is an optical device capable of receiving two optical signals—one generally being a multi-channel WDM signal and the other generally being a new wavelength channel to be added to the WDM signal. These two signals are received at respective input ports of the device, and a composite WDM signal comprising the original WDM signal and the new wavelength channel is supplied at an output port of the device.
Similarly, in a channel drop demultiplexer, a multi-channel WDM signal is received at an input port of the device. One or more of the wavelength channels of the WDM signal is separated from the others and is supplied at a first output port, while the remainder of the WDM signal is supplied at another output port.
FIG. 1
of the accompanying drawings schematically illustrates a previously prosed optical fibre channel add/drop multiplexer having two input ports A
1
, A
2
and two output ports A
3
, A
4
. The device comprises two optical circulators
10
,
20
and a fibre Bragg grating
30
.
In
FIG. 1
, a multi-channel WDM signal &lgr;
IN
is launched into port A
1
. A channel to be added, &lgr;
ADD
, is launched into port A
4
and a channel &lgr;
DROP
to be separated from the WDM signal &lgr;
IN
is output at port A
2
. The WDM signal with the channel &lgr;
DROP
dropped and the new channel &lgr;
ADD
is output at port A
3
.
The device works in a straightforward way. The Bragg grating is arranged. to reflect light at the wavelength &lgr;
DROP
of the channel to be dropped and the channel to be added. So, the channel to be dropped passes from the circulator towards the grating, is reflected by the grating and is output by port A
2
of the circulator. Similarly, the channel to be added enters at port A
4
, passes from the circulator to the Bragg grating where it is reflected, and is output at port A
3
of the circulator. The remaining channels of the WDM signal, &lgr;
OTHERS
, are unaffected by the Bragg grating and so emerge at port A
3
of the circulator.
The device of
FIG. 1
makes good use of the wavelength-selective properties of a fibre Bragg grating, but because the grating
30
is a two-port device the multiplexer needs the two circulators
10
,
20
. Circulators are expensive bulk optical devices, so it is undesirable to use them in an all-fibre system. Also, there are inevitable losses caused by the need to connect fibres to the bulk optical circulators. A simpler device could use two 50:50 fused fibre couplers, but at the expense of an increased insertion loss of 6 dB (decibels) for the add/drop multiplexer.
This invention provides a channel drop demultiplexer comprising an optical fibre coupler having at least an m-core optical fibre optically coupled to an n-core optical fibre, where m and n are positive integers and m is greater than 1 in which:
the fibres are coupled at a coupling region;
a grating is disposed on at least one core of the m-core fibre away from the coupling region;
a core of the m-core optical fibre to a first side of the coupling region provides an input port for a WDM signal;
that core of the m-core fibre, to a second side of the coupling region, provides an output port for a WDM signal; and
the grating promotes coupling of light of a channel to be dropped between cores of the m-core fibre.
This invention also provides a channel add multiplexer comprising an optical fibre coupler having at least an m-core optical fibre optically coupled to an n-core optical fibre, where m and n are positive integers and m is greater than 1 in which:
the fibres are coupled at a coupling region;
a grating is disposed on at least one core of the m-core fibre away from the coupling region;
a core of the m-core optical fibre to a first side of the coupling region provides an input port for a WDM signal;
that core of the m-core fibre, to a second side of the coupling region, provides an output port for WDM signal; and
the grating promotes coupling of light of a channel to be added between cores of the m-core fibre.
In the invention a channel add multiplexer and a channel drop demultiplexer are provided which do not require expensive and lossy bulk optical devices such as optical circulators but which do not introduce the insertion losses of conventional fused taper couplers.
Embodiment of the invention can provide a way of selectively coupling light into or out of one core of the m-core fibre. Embodiments of the invention can use established simple fused coupler techniques, and can introduce very low (e.g. less than 1 dB) insertion losses for the coupled light.
A exemplary embodiment of the invention comprises a dual core fibre coupled to a single core fibre. If one core of the dual core fibre is substantially identical (in terms of its optical propagation constants) to the core of the single core fibre, light can be made to couple between that core of the dual core fibre and the single core fibre, whereas light in the other core of the dual core fibre is essentially unaffected.
The insertion loss can be very low (e.g. <1 dB or even <0.5 dB), which is much lower than the simple coupler arrangement described in a paper by Bilodeau et al: IEEE Photonics Technology Letters, 7, 1995, pp388-390 and a paper by Bakhti et al: Electronics Letters, 33, 1997, pp803-804. Compared to these prior art devices, embodiments of the invention are not interferometric devices and so there is no need to balance optical path lengths within the devices to a few wavelengths of the light. This makes embodiments of the invention much easier to manufacture.
Again, compared to the paper by Bakhti et al and also WO97/08574, the grating can be written over a length of fibre instead of onto the waist of the coupler. this means that coupler manufacture and grating manufacture are different processes which can reliably be done separately without affecting the other process. In any event, it is difficult to produce a coupler waist of good uniformity over a long length, so any irregularities will affect the grating performance and restricts the length and number of gratings which can be used in the prior art devices. In contrast, in the present embodiments, multiple gratings can be used, of good quality because they are impressed onto fibre rather that onto a coupler waist. Tuning of the gratings e.g. by compression or stretching is also possible, helped by the non-interferometric nature of the embodiments and the high physical strength of fibre away from the couplerwaist.


REFERENCES:
patent: 5170450 (1992-12-01), Dahlgren
patent: 5574807 (1996-11-01), Snitzer
patent: 5778119 (1998-07-01), Farries
patent: 5805751 (1998-09-01), Kewitsch
patent: 5940556 (1999-08-01), Moslehi
patent: 0 435 194 (1991-07-01), None
patent: 62-017709 (1987-01-01), None
patent: 62-127807 (1987-06-01), None
patent: 97/08574 (1997-03-01), None
L. Dong, “Novel add/drop filters for WDM optical fiber systems using a Bragg grating assisted mismatched coupler”, IEEE Photonics technology Letters, 8(12), pp. 1656-1658, Dec. 1996.*
Patent Abstracts of Japan, vol. 011, No. 194 (P-588), Jun. 23, 1987 —& JP 62 017709 A (Nippon Telegr & Teleph Corp), Jan. 26, 1987, see abstract.
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