Optical add-drop module with low loss and high isolation

Details Optical add-drop module with low loss and high isolation Optical add-drop module with low loss and high isolation

1999-06-15

2003-06-10

Pascal, Leslie (Department: 2633)

Optical: systems and elements

Deflection using a moving element

Using a periodically moving element

C359S199200, C359S199200, C359S199200

Reexamination Certificate

active

06577415

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates generally to passive optical devices for channel selection, and more particularly, to devices in which dielectric thin-film filters are used for channel selection.
BACKGROUND OF THE INVENTION
There is currently a market need, experienced by, for example, telephone carriers and cable operators, to provide short-haul transmission of internet protocol (IP) packets among interconnected nodes of a network that are typically spaced apart by 20 km or less. Proposed systems send IP packets, via a SONET interface, over a fiber-optic transmission medium using wavelength division multiplexing (WDM). A particular wavelength channel is assigned to each node. At each node, optical demultiplexing is required to extract the received data from the medium in the channel of interest, and optical multiplexing is required to inject the transmitted data into the medium in the channel of interest.
The optical components that effectuate optical multiplexing and demultiplexing add cost to the network installation, and in operation, they also add optical losses that accumulate over the network. For greater profitability, it is desirable both to reduce installation cost, and to reduce the accumulated losses that would otherwise limit transmission distances. For both purposes, it would be advantageous to reduce the number of optical components at each node that perform optical multiplex-demultiplex functions.
One known device for providing the multiplex-demultiplex function described above is an optical add-drop module (OADM) using in-line dielectric thin-film filters (TFFs). Such a device is represented in FIG.
1
. Very briefly, incoming traffic on optical fiber
10
is incident on drop filter
15
. Signals in undesired wavelength channels, i.e. all but the channel assigned to the node of interest, are reflected into bypass optical fiber
20
. From fiber
20
, these signals are incident on add filter
25
and reflected into optical fiber
30
, from which they propagate downstream and away from the node of interest. Signals in the channel of interest are transmitted through filter
15
to receiver
35
. Signals in the channel of interest from transmitter
40
are transmitted through filter
25
into optical fiber
30
.
Some incoming optical power in the channel of interest may leak through the OADM by reflection from filters
15
and
25
and transmission into fiber
30
. Such leakage is undesirable because it leads to interference in that channel. The attenuation of the undesired power, relative to the desirably transmitted power in the channel, is referred to herein as channel isolation. Those skilled in the art will appreciate that the term adjacent channel isolation is often used to denote a measure of the suppression of channel crosstalk at receiver
35
. What we mean by channel isolation, however, is the isolation provided by virtue of the reflections from filters
15
and
25
. Such isolation is also sometimes referred to as reflection isolation.
In some cases, the add and drop filters fail to provide sufficient channel isolation. In those cases, it is customary to add a third filter
45
, which supplements filters
15
and
25
by continuing the bypass path from fiber
20
to fiber
20
′. The additional reflection from this third filter further reduces the power in the local add/drop channel that is transmitted into fiber
30
, and thus further increases the channel isolation.
There are certain drawbacks to the use of a third filter, such as filter
45
, in the bypass fiber. Such a filter adds some loss to all of the traffic being routed through the bypass fiber, and it adds component cost to the fiber-optic network.
SUMMARY OF THE INVENTION
Represented in
FIG. 2
is a pair of characteristic curves for an illustrative TFF. Curve
50
is a transmission characteristic, and curve
55
is a reflection characteristic. Evident in curve
55
are ripple features
60
having peaks
61
and valleys
62
. A ripple peak corresponds to a wavelength, within the transmission passband of the TFF, which is also partially reflected. Thus, the TFF affords somewhat less isolation near ripple peaks than it does near ripple valleys. Herein, we use the term across-the-band isolation to refer to the smallest isolation achieved at any point within the passband of a given TFF.
We have found that the combined isolation afforded in an OADM by an add filter and a drop filter can be improved by selecting a pair of filters having nominally similar characteristics, but having relatively displaced ripple features, such that at least one ripple peak of the add filter falls at approximately the same wavelength as a ripple valley of the drop filter and vice versa. The resulting improvement is sufficent to obviate the need for a third filter in the bypass fiber, even for some highly demanding applications. Thus, appropriate matching of the add and drop filters can lead to reduced loss in the network, and reduced network cost.
Accordingly, the invention in one embodiment is an OADM having a passband and a through-path, and in the throughpath, exactly two broadband TFFs having nominally similar transmission and reflection characteristics. In this context, the throughpath is the path taken by traffic that is not added or dropped. With reference to
FIG. 1
, it is the path from fiber
10
to fiber
30
via fibers
20
and
20
′. The respective TFFs are selected to have at least partially complementary reflection characteristic curves, such that at least some ripple peaks in the characteristic curve of one TFF overlap ripple valleys in that of the other TFF. The selection of TFFs is further characterized in that the OADM achieves a level of channel isolation over the entire said passband that is greater than that achievable by pairing either of the individual TFFs with an identical counterpart.


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