Reconfigurable, all optical add/drop nodes using...

Details Reconfigurable, all optical add/drop nodes using... Reconfigurable, all optical add/drop nodes using...

2001-04-27

2003-07-01

Sanghavi, Hemang (Department: 2874)

Optical waveguides

With optical coupler

Switch

C385S018000, C385S037000, C359S199200, C359S199200

Reexamination Certificate

active

06587608

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to reconfigurable optical add/drop nodes which use non-interrupting switching apparatus and methods.
2. Description of the Prior Art
In Wavelength Division Multiplexed (WDM) fiber optics communications, one fiber carries many data streams, each on a separate wavelength signal. In networks using WDM, ideally each node should be able to separate out (drop) any wavelength in use on the fiber and redirect it to a detector or sub-network. At the same time, it is desirable that each node be able to add data to the fiber on any wavelength channel that is currently unused at the node, either because such wavelength is not present at the node, or because it was just dropped at said node.
In addition, if network nodes are able to switch between the state where a given wavelength channel is dropped and the state where it is passed (not dropped) fast enough (in a way that does not interrupt other network traffic while switching), then the network controller can Time-Division Multiplex (TDM) a wavelength to several subscribers. This is highly desirable, since many customers do not want or need the full data rate possible on a single wavelength. A fast enough switching time for this application is on the order of 2 milliseconds.
In today's optical WDM optical networks, nodes are actually implemented in two different ways, neither of which is ideal:
1. Optical→Electronic→Optical (OEO) Conversion: This is the most common (and expensive) method of constructing nodes. All wavelengths coming into the node along the input fiber are demultiplexed into separate channels and detected (i.e., converted to electronic signals). The signals which are not being dropped at the node are used to modulate lasers and the resulting wavelengths are multiplexed back onto the output fiber. The multiplexing/demultiplexing is typically done with either arrays of filters or with diffraction grating techniques.
The advantage of this method is that the node is completely flexible
any wavelength can be dropped or added at the node. In addition, signals may be transferred from one wavelength to another.
The disadvantages of this method are:
a) Expensive hardware components (the detector, electronics, laser, and modulator) are needed for each wavelength on the fiber. This rapidly becomes very expensive as numbers of wavelengths grow.
b) Much of the hardware (detectors, electronics, and laser modulators) are data-rate dependent: If the network is upgraded from 2.5 Gigabits/sec to 10 Gigabits/sec per wavelength, for example, all electronics at all nodes must also be expensively upgraded.
2. Fixed Optical Add/Drop Filters: There are, at most, two nodes in a WDM network (the terminal nodes) that need to drop all wavelengths on the fiber—all other nodes (intermediate nodes) usually need to drop or add only a few wavelengths. This can be done inexpensively by passing the fiber through several fixed-wavelength optical drop/add filters. Only the wavelengths these filters are designed for are dropped or added—all other wavelengths simply continue on with no change. These filters are usually constructed using thin-film interference filters or fiber Bragg gratings.
Advantages: This node style is considerable less expensive than an OEO node—filters, electronics, and lasers are only required for the number of wavelengths actually to be dropped at the node. If the wavelengths are being sent on to a sub-network, only the inexpensive filters are needed, and the node is data-rate independent.
Disadvantages: Fixed-wavelength nodes don't allow the network to adjust to varying loads, and make network expansion more difficult. When the network grows complicated enough, “wavelength blocking” occurs: even though the network may be far from it's theoretical carrying capacity, certain pathways are blocked from use as no single wavelength can connect them. The network could be manually re-configured to remove any given block, but this would create other blocked paths during different load conditions. This problem grows rapidly with network complexity. In addition, current fixed-drop technologies cannot be switched on and off without interrupting the rest of the network traffic.
Neither of the above methods of constructing optical add/drop network nodes adequately address the need for networks to be both inexpensive and easily and quickly reconfigurable—the OEO nodes achieve wavelength flexibility at the cost of a very high price and data-rate sensitivity; the fixed wavelength add/drop filter nodes are data-rate insensitive and inexpensive, but are completely inflexible as to the dropped wavelengths. The ideal network node would, therefore, have the following characteristics:
I. The node would be all optical—there would be no optical to electronic conversions. Thus the node would be completely insensitive to data-rate upgrades.
II. The node would have the flexibility to drop (and add) any wavelength on the fiber, and the wavelengths to drop could be changed remotely at any time without data interruption to the rest of the network.
III. The node could be constructed relatively inexpensively, using proven components.
IV. The node would have low loss, at least for the passed (undropped) wavelengths, so as to minimize the requirement for expensive optical amplifiers.
Two methods of addressing the need for flexibility in choosing which wavelengths to drop (or add) at an intermediate network node that are being developed are tunable add/drop filters and re-configurable Optical Add/Drop Multiplexers (OADMs).
Tunable Add/Drop Filters: This method uses a tunable optical filter with a relatively broad tuning range, capable of tuning across several WDM channels.
FIG. 1
(Prior Art) shows a possible configuration for using a Fiber Bragg Grating (FBG) filter
108
as a tunable OADM. The FBG (which can be tuned either by stretching or heating) is placed between two optical circulators
104
,
110
. Inputs
102
are &lgr;
1
, &lgr;
2
, &lgr;
3
in this example. The wavelength
106
that the FBG is currently tuned to (shown as &lgr;
2
in
FIG. 1
) is reflected back toward the input, whence it is diverted by the input-side circulator
104
to the drop fiber. The rest of the channels (&lgr;
1
, &lgr;
3
) pass the FBG and go to the Pass-Through output
112
back to the network. To add back to the network, the dropped wavelength
114
(but probably carrying different information—designated as &lgr;
2
′ in FIG.
1
), is input to the channel of the output circulator
110
that sends it back to the FBG, whence it is reflected to the Pass-Through output
112
along with the other passed wavelengths. The problem with this technique is that the filter momentarily drops all wavelengths that it tunes through.
For example, if the filter is currently dropping, say &lgr;
2
, and is commanded to switch to &lgr;
7
; then all of the intermediate channels, &lgr;
3
. . . &lgr;
6
are momentarily interrupted as the FBG tunes through them. This is unacceptable behavior for a network component.
Reconfigurable OADMs: A second method of building flexibility in wavelength use at a network node, without incurring the cost of a complete Mux/Demux (OEO) node, is to configure a number of fixed add/drop filters with optical switches such that they can be switched into or out of the data fiber at will.
FIG. 2
(Prior Art) shows a typical arrangement of switches
202
-
205
and OADM filters
206
,
208
that can switch any or all of the wavelengths addressed by the filters off of the network fiber onto a drop fiber. Mux
210
provides the Drop output. Demux
212
inserts the Add input.
The OADMs can be any suitable device; e.g., based on FBGs or thin film (TF) filters. The switches themselves can be of two basic kinds:
1. A “make and break” switch which can be as simple as a fiber patch cord moved between different jacks on a panel, or as sophisticated as a micro-mirror switch with active alignment. In any case, the prime characteristic of the switch is that the connection betwe

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