Optical: systems and elements – Deflection using a moving element – Using a periodically moving element
Reexamination Certificate
1998-07-29
2001-12-11
Tweel, Jr., John (Department: 2632)
Optical: systems and elements
Deflection using a moving element
Using a periodically moving element
C359S199200, C359S199200
Reexamination Certificate
active
06330090
ABSTRACT:
This invention relates to optical fiber wavelength multiplexers and demultiplexers.
These devices become more and more important with the development of optical fiber telecommunications. Indeed, wavelength multiplexing and demultiplexing technologies enable transmission of an increased volume of information in the same optical fiber. Direct optical amplification is now reliable and allows one to amplify a set of channels, at different wavelengths, with a single optical amplifier. It does not require any more to demultiplex the channel wavelengths for amplifying them separately, as it would be the case with electronic amplifiers. Such dense wavelength division multiplexing (D-WDM) is particularly efficient in the 1530 nm-1565 nm window of erbiumi-doped-fiber amplifier (EDFA).
The operation of a device according to the previous art is illustrated on
FIGS. 1 and 2
.
FIG. 1
represents a multiplexer. Input single-mode fibers
1
to
5
have their ends located on a plane
6
constituting the input plane of the multiplexer. This multiplexer comprises a dispersing element or grating
7
, a collimation optical element
8
, a reflector system
9
and produces an output beam
10
collected by an output single-mode fiber
61
. The optical elements of the multiplexer, the grating
7
and the collimation optical elements
8
as well as the reflector optical system
9
are laid out in such a way that the input beams, spatially separate in the input plane
6
, are superimposed at the output point
62
and coupled in the output fiber
61
. This arrangement with a grating and a reflector is usually called the Littman-Metcalf configuration.
On
FIG. 2
, each of the input single-mode fibers
1
to
5
ends has been represented, together with their optical cores
11
,
21
,
31
,
41
,
51
, their claddings
12
,
22
,
32
,
42
,
52
and their coatings
13
,
23
,
33
,
43
,
53
. In such a system, the input plane
6
defines, in its geometrical dimension x, the input function F(&lgr;) of the multiplexer, represented approximately on
FIG. 2
, each of the fibers cutting through an associated elementary passband
14
,
24
,
34
,
44
and
54
.
The widths &Dgr;&lgr;
1
, . . . ,&Dgr;&lgr;
5
of each of these elementary bands depend on the diameters of the cores
11
,
21
,
31
,
41
,
51
of each single-mode optical fiber
1
to
5
and are generally small in relation to the distance d(&lgr;
1
, &lgr;
2
), . . . , d(&lgr;
4
, &lgr;
5
) separating the central wavelengths &lgr;
1
, . . . , &lgr;
5
of the elementary bands, consecutive to the beams provided by each input fiber
1
to
5
and superimposed on the output fiber
61
.
We shall designate later on by &Dgr;&lgr; the width of the elementary bands &Dgr;&lgr;i, . . . , &Dgr;&lgr;n and by d(&lgr;i, &lgr;i+1) the distance between the central wavelengths of two consecutive elementary bands.
Various propositions have already been made in order to increase the &Dgr;&lgr;/d(&lgr;i, &lgr;i+1) ratio. We know that this &Dgr;&lgr;/d(&lgr;i, &lgr;i+1) ratio =&agr;/&dgr;, where &agr; corresponds to the diameter of the fundamental transmitted mode, which is substantially equal to the core diameter of the single-mode fiber and where &dgr; is the distance between two cores of consecutive fibers. In practice, when the coating is removed, this &dgr; distance is at least equal to the diameter of the cladding.
It has been suggested to reduce the thickness of the claddings
12
,
22
,
32
,
42
,
52
, which enables reducing
8
and hence the distance d(&lgr;i, &lgr;i+1) without reducing the widths of the bands &Dgr;&lgr;. However, this lay-out is difficult to control and to implement.
Several other attempts have been made for improving the &Dgr;&lgr;/d(&lgr;i, &lgr;i+1) ratio. UK patent application N° 2.219.869 proposes to provide the multiplexer with waveguide having a tapered optical field spot size achieved, either by physically tapering the fibers, or by changing the core index at their ends by diffusion. This also is difficult to control and to implement.
D. R. Wisely in an article published in Electronics Letters (14
th
March 1991, vol. 27, N° 6) proposed to place a microlens array at the end of the fibers. Such an array enhances the relative bandwidth ratio and directs the light beams directly to the diffraction grating. To have these beams illuminating a common area on the grating and then to reduce loss, each of the microlenses has to be offset relatively to the core of the associated fiber; the amplitude of this offset depends on the position of the fiber which is possible using a microlens array having a pitch smaller than the pitch of the fiber ends and carefully controlled. In practice, this requires the microlens array to be manufactured to this particular use and the accurate relation between the pitch of the fiber ends and the pitch of the microlens array is difficult to obtain. Furthermore, the beams exciting the microlenses are slightly diverging because of diffraction since their diameter is limited by the small diameter of the microlenses. This yields astigmatism when these beams experience dispersion on the grating, which increases the loss of the device.
The purpose of the invention is to suggest an optical fiber wavelength optical multiplexer-demultiplexer which exhibits a significant improvement of the &Dgr;&lgr;/d(&lgr;i, &lgr;i+1) ratio, is easy to manufacture, can be realized with standard components easy to obtain and has a low loss.
It is another purpose of the invention to construct such multiplexing-demultiplexing device in which the elementary passband associated to each fiber is widened and shows front edges towards low frequencies and towards high frequencies which are as steep as possible and in which each transmitted wavelength undergoes the same attenuation. Such an elementary transfer function, ideally rectangular in shape, enables to obtain accurate delimitation of the passband and uniform transmission within this band.
To this end, the invention relates to an optical fiber wavelength multiplexing device comprising:
an array of input single-mode fibers designed for carrying light beams at different wavelengths &lgr;
1
, &lgr;
2
, . . . , &lgr;n,
an output single-mode fiber designed for carrying the whole set of such light beams,
a dispersing system receiving light beams form the input fibers in an input plane and generating superimposed light beams designed for the output fiber in an output plane,
an array of converging microlenses being located in the input plane, whereas a microlens corresponds to each input fiber, wherein the microlens array has the same pitch as the input fiber an-ay and produces diverging beams whose respective central axes are parallel and which are directed to a collimating lens which produces collimated beams whose respective central axes are converging on the dispersing system.
According to the invention, it is also possible to construct a demultiplexing device. The device according to the previous alt described above with reference to
FIGS. 1 and 2
can also operate in reverse direction, as a demultiplexer. The single-mode fiber
61
is then an input fiber carrying a light beam at various wavelengths and the fibers
1
to
5
become thus output fibers, each receiving a beam at a given wavelength, separated spatially from the beams coming out at the other wavelengths. Thus, although it will be mainly described embodied as a multiplexer, the invention can also be applied to such a demultiplexer.
The device according to the invention is then a fiber wavelength demultiplexer comprising an output fiber array designed for carrying light beams at different wavelengths &lgr;
1
, . . . , &lgr;n, an input fiber designed for carrying the whole set of such light beams, a dispersing system receiving the light beam from the input fiber in an end plane and generating spatially separate light beams designed for the output fibers in an output plane,
a converging microlens array is located in the output plane, whereas a microlens corresponds to each output fiber.
Accordin
LaLoux Bernard
Lefevre Herve
Martin Philippe
Merigot Bertrand
Arent Fox Kintner & Plotkin & Kahn, PLLC
Photonetics
Tweel , Jr. John
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