RELATED APPLICATIONS

The present application is a continuation-in-part of the Applicants' U.S. patent application Ser. No. 09/240,550, entitled “Optical Wavelength Route”, now U.S. Pat. No. 5,978,116 filed on Jan. 29, 1999, which is a continuation of U.S. patent application Ser. No. 08/739,424, entitled “Programmable Wavelength Router”, filed on Oct. 29, 1996, now U.S. Pat. No. 5,867,291, issued on Feb. 2, 1999. The present application is also a continuation-in-part of the Applicant's U.S. patent application Ser. No. 09/036,202, entitled “Optical Add/Drop Wavelength Switch”, filed on Mar. 6, 1998, now U.S. Pat. No. 6,166,838, issued on Dec. 26, 2000, which is based on the Applicants' U.S. Provisional Patent Application 60/042,373, filed on Mar. 24, 1997. The present application is also a continuation-in-part of the Applicants' U.S. patent application Ser. No. 09/156,211 entitled “Programmable Optical Add/Drop Multiplexer,” filed on Sep. 17, 1998 now U.S. Pat. No. 6,285,478, and U.S. patent application Ser. No. 09/048,557, entitled “Programmable Optical Multiplexer,” filed on Mar. 26, 1998 now U.S. Pat. No. 6,208,442.

GOVERNMENT INTERESTS

The invention was made with government support under Contract DARPA II: DAAH01-97-C-R308 awarded by U.S. Army Missile Command, AMSMI-AC-CRAY, Redstone Arsenal, AL 35898. The government has certain rights in the invention.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to the field of optical communications. More specifically, the present invention discloses an optical wavelength add/drop multiplexer for use in wavelength division multiplex (WDM) optical communications.

2. Statement of the Problem

Optical wavelength division multiplexing has gradually become the standard backbone network for fiber optic communication systems. WDM systems employ signals consisting of a number of different wavelength optical signals, known as carrier signals or channels, to transmit information on optical fibers. Each carrier signal is modulated by one or more information signals. As a result, a significant number of information signals may be transmitted over a single optical fiber using WDM technology.

Despite the substantially higher fiber bandwidth utilization provided by WDM technology, a number of serious problems must be overcome, for example, multiplexing, demultiplexing, and routing optical signals, if these systems are to become commercially viable. The addition of the wavelength domain increases the complexity for network management because processing now involves both filtering and routing. Multiplexing involves the process of combining multiple channels (each defined by its own frequency spectrum) into a single WDM signal. Demultiplexing is the opposite process in which a single WDM signal is decomposed into individual channels. The individual channels are spatially separated and coupled to specific output ports. Routing differs from demultiplexing in that a router spatially separates the input optical channels to output ports and permutes these channels according to control signals to a desired coupling between an input channel and an output port.

The Applicants' U.S. Pat. No. 5,724,165 and U.S. patent application Ser. No. 08/739,424 (Kuang-Yi Wu et al.) teach two independent methods for high performance signal routing (U.S. Pat. No. 5,724,165) and wavelength de-multiplexing (Ser. No. 08/739,424). In U.S. Pat. No. 5,724,165, new structures for realizing optical switches (routers) were disclosed that achieve very high extinction ratios. However, these switches are wavelength independent. In Ser. No. 08/739,424, a system is disclosed to provide the functions of wavelength de-multiplexing and routing. However, this single stage design relies primarily on the filter design. The transmission function of the filter has to be close to an ideal square flat-top to realize the desired low crosstalk operation.

Other prior art in the field includes the following:

Inventor

U.S. Pat. No.

Issue Date

Glance

5,488,500

Jan. 20, 1996

Patel et al.

5,414,541

May 9, 1995

Meadows

5,381,250

Jan. 10, 1995

DeJule et al.

5,363,228

Nov. 9, 1994

Nelson

4,919,522

Apr. 24, 1990

Ammann, “Synthesis of Electro-Optic Shutters Having A Prescribed Transmission vs. Voltage Characteristic,”

Journal of the Optical Society of America

, vol. 56, no. 8, pp. 1081-1088 (August 1966)

Harris et al., “Optical Network Synthesis Using Birefringent Crystals* I. Synthesis of Lossless Networks of Equal-Length Crystals,”

Journal of the Optical Society of America

, vol. 54, no. 10, pp. 1267-1279 (October 1964)

Patel et al. disclose an optical switch using a series of birefringent layers and ferroelectric cells to route an input beam to any of a plurality of output positions.

Glance discloses a tunable add/drop filter using a 1×N optical switch, a wavelength grating router (WGR), and a multiplexer. The WGR outputs include a set of retain outputs that are coupled directly to the multiplexer and a drop output. The particular WDM frequency component that is routed to the drop output is determined by the WGR input port at which the WDM signal is received. The 1×N switch provides the WDM signal to the proper WGR input so that a selected frequency is provided to the drop output. The retained signals and any added signals are multiplexed by the multiplexer.

DeJule et al. disclose an optical switching device using a plurality of polarization-independent switching cells arranged in matrix form. Each switching cell consists of a spatial light modulator and a number of polarized beamsplitters that can be used to selectively direct an input optical beam along either of two axes.

Nelson discloses an optical switch employing an electro-optical crystal that exhibits birefringence in each of two different light paths when the crystal is disposed in orthogonally-oriented electric fields. Each light path is sensitive to a different one of the two electric fields and has its own set of fast and slow axes.

Meadows discloses a 2×2 electro-optical switch that employs dielectric film polarizing beamsplitters and a switchable electro-optic retarder.

Ammann and Harris et al. provide general background in the field of optical filter design.

3. Solution to the Problem

None of the prior art references discussed above show an optical wavelength add/drop multiplexer that uses a network of wavelength slicers to separate the input WDM channels from a first optical link into a plurality of sets of channels. At least one set of channels is then separated into individual channels by an interference filter to interface with an add/drop switch array. The output channels from the add/drop switch array and a selected set of channels from the wavelength slicer network can be combined and transmitted over a second optical link.

SUMMARY OF THE INVENTION

This invention provides an optical wavelength add/drop multiplexer for communications between two optical links supporting wavelength division multiplexing (WDM). A wavelength slicer spatially separates the input signal into two sets of channels. An optical filter, such as an interference filter, spatially separates the a subset of the input channels into an array of separated channels. A programmable optical add/drop switch array selectively routes channels from an array of input ports to an array of drop ports, substitutes channels from an array of add ports in place of the dropped channels, and routes the remaining input channels and added channels to an array of output ports. The channels from the output ports of the said add/drop switch array are then combined and transmitted into the second optical link. A network of wavelength slicers can be used to spatially separate the input signal into a larger number of sets of channels that can either be accessed by a number of add/drop switch arrays, or pass unchanged as “express lanes” to the second optical link. In an alternative embodiment, a circulated drop filter consisting of an optical circulator and a series of fiber Bragg gratings is used to select a predetermined series of input channels to be processed by the add/drop switch array, with the remaining channels being passed by the circulated drop filter as express lanes.

A primary object of the present invention is to provide an optical wavelength add/drop multiplexer that can separate multiple channels from an input WDM signal and selectively substitute channels from series of add ports in place of the input channels.

Another object of the present invention is to provide an optical wavelength add/drop multiplexer that can be use to augment the channel capacity of an existing central office equipment for optical communications.

These and other advantages, features, and objects of the present invention will be more readily understood in view of the following detailed description and the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention can be more readily understood in conjunction with the accompanying drawings, in which:

FIG. 1

is a schematic block diagram of the overall optical wavelength add/drop multiplexer.

FIG. 2

is a simplified block diagram illustrating the functionality of the wavelength slicer

101

.

FIG. 3

is a schematic block diagram of the wavelength slicer

101

.

FIG. 4

is a spectral diagram of the transmission function of a wavelength slicer for separating adjacent 50 GHz input channels into two sets of output channels.

FIG. 5

is a schematic block diagram of the optical add/drop switch array

500

.

FIGS.

6

(

a

) and

6

(

b

) are simplified block diagrams illustrating the functionality of an individual optical add/drop switch

510

in the bridge state and add/drop state, respectively.

FIGS.

7

(

a

) and

7

(

b

) are schematic block diagrams of an optical add/drop switch

510

in the bridge state and add/drop state, respectively.

FIG. 8

is a schematic block diagram of one implementation of the present invention to expand the communications capability over an optical link between two central offices.

FIG. 9

is a schematic block diagram of an alternative embodiment of the present invention using circulated drop filters.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1

provides an overall schematic diagram of an embodiment of the present invention using two networks

100

and

110

of wavelength slicers. Input WDM signals

10

from an optical link are coupled to the input port of a first wavelength slicer

101

. The input WDM signal consists of multiple channels with each channel having its own range of wavelengths or frequencies. As used herein, the terms “channel” or “spectral band” refer to a particular range of frequencies or wavelengths that define a unique information signal. Each channel is usually evenly spaced from adjacent channels, although this is not necessary. Uneven spacing may result in some complexity in design, but, as will be seen, the present invention can be adapted to such a channel system. This flexibility is important in that the channel placement is driven largely by the technical capabilities of transmitters (i.e., laser diodes) and detectors and so flexibility is of significant importance.

Operation of the wavelength slicer

101

is describe in detail below. However, to summarize, each wavelength slicer

101

-

105

in the wavelength slicer network

100

spatially separates a set of input WDM channels into two complementary sets of output channels. In the preferred embodiment, each wavelength slicer separates alternating adjacent input channels into first and second sets of output channels as shown in FIG.

2

. Thus, returning to

FIG. 1

, the first wavelength slicer

101

separates the network input signal

10

into a first set of channels

11

that are routed to wavelength slicer

102

, and a second set of channels

12

that are routed to wavelength slicer

103

. For example, the initial wavelength slicer

101

can separate channels based on a 50 GHz spacing between adjacent channels, as depicted in FIG.

4

.

As illustrated in

FIG. 1

, the first set of channels output by the initial wavelength slicer

101

is routed along a first optical path

11

to second and third stage wavelength slicers

102

,

104

, and

105

. The second set of channels output by the initial wavelength slicer

101

is routed along a second optical path

12

to wavelength slicer

103

. The second stage of wavelength slicers

102

,

103

further separate the input channels into four sets of channels. For example, in the embodiment shown in

FIG. 1

, the second stage of wavelength slicers

102

,

103

separates channels based on a 100 GHz spacing between adjacent channels. This process can be continued by cascading additional stages of wavelength slicers to achieve up to 2

N

sets of channels, where N is the number of stages. For example, the wavelength slicer network

100

shown in

FIG. 1

has a partial third stage consisting of wavelength slicers

104

and

105

.

In contrast, the output channels from wavelength slicer

103

exit the first wavelength slicer network

100

along optical path

13

without further processing. These output channels are referred to as “express lanes” and pass directly to the second wavelength slicer network

110

used to recombine the optical signals that are to be returned to the optical network, as will be discussed below. Optionally, wavelength slicers

103

and

113

could be eliminated so that the second set of optical signals from the initial wavelength slicer

101

would serve as the express lanes and pass directly to the final wavelength slicer

111

in the second wavelength slicer network

110

.

Returning to the third stage of wavelength slicers

104

and

105

in

FIG. 1

, the first set of channels are further subdivided into four sets of channels that are received as inputs by an array of optical filters

201

,

202

,

203

, and

204

. These optical filters

201

-

204

separate the input sets of channels into an array of separated channels. The implementation shown in

FIG. 1

is based on commercially-available grating interference filter arrays that can separate up to eight channels. However, other types of optical filters can be employed for spatially separating the channels. The type of optical filter used and the number of filters in the array are purely matters of convenience of design.

The array of separated channels are connected to the input ports of a series of programmable optical add/drop switch arrays

500

. Here again, any number of add/drop switch arrays can be employed to handle any desired number of channels based on design requirement.

Each add/drop switch array

500

also has a corresponding arrays of output ports, add ports, and drop ports. The add/drop switch array

500

selectively routes channels from the input ports to its drop ports; substitutes channels from the add ports in place of the dropped channels; and routes the remaining input channels and the added channels to the output ports of the add/drop switch array

500

.

FIG. 5

is a schematic block diagram of the optical add/drop switch array

500

showing a diagonal array of eight individual add/drop switches

510

-

517

than can be individually controlled to selectively replace one of the input channels with one of the add channels.

The details of the structure and operation of an individual add/drop switch are discussed at length below with regard to

FIGS. 3

,

7

(

a

) and

7

(

b

). FIGS.

6

(

a

) and

6

(

b

) provide a simplified overview illustrating the bridge state and add/drop state, respectively, for an individual add/drop switch

510

. In the bridge state depicted in FIG.

6

(

a

), the channels at the input port

711

are routed unchanged to the output port

712

, and the add channel

713

is routed unchanged to the drop port

714

of the add/drop switch

510

. In contrast, FIG.

6

(

b

) shows the add/drop state in which the add/drop switch

510

selectively routes one of the input channels

711

to its drop port

714

, substitutes the add channel

713

in place of the dropped channel, and routes the remaining input channels and the added channel to the output port

712

of the add/drop switch

510

.

Returning to

FIG. 1

, it should be noted that this combination of a network slicer network

100

, optical filters

201

-

204

, and add/drop switch arrays

500

, allow any combination of input channels to be replaced with any combination of add channels. In contrast, many prior art systems permit only an incomplete set of the possible combinations of input channels and add channels due to blocking problems.

The array of output channels from the add/drop switch array

500

passes through a regulator

250

which adjustably regulates the optical power level of each channel. The output channels are then combined so that they can be transmitted through a second optical link

15

in the optical network. In the embodiment of the present invention illustrated in

FIG. 1

, the means for combining the output channels consists of a second array of interference filters

211

-

214

and a second wavelength slicer network

110

. These devices are inherently bidirectional, and therefore can be used to multiplex as well as demultiplex the WDM signal for the optical network. Each of the interference filters

211

-

214

in the second array combine eight channels as an inverse operation of that performed by the first array of interference filters

201

-

204

. The second array of interference filters

211

-

214

also serve to purify the spectral characteristics of the output channels and reduce cross-talk. Wavelength slicers

114

,

115

, and

112

in the second wavelength slicer network

110

multiplex the sets of channels from the second array of interference filters

211

-

214

as an inverse operation to the demultiplexing provided by wavelength slicers

102

,

104

, and

105

in the first wavelength slicer network

100

. Similarly, wavelength slicers

113

and

111

combine the express lanes

13

with the multiplexed channels from the wavelength slicer

112

to reconstitute the entire WDM signal for the optical network.

It should be understood that other means could be readily substituted to combine output channels from the add/drop switch arrays

500

and the express lanes

13

since each channel has a unique wavelength. For example, a network of polarized beamsplitters could be employed to combine the channels.

Wavelength Slicer.

FIG. 2

is a block diagram illustrating the general functionality of an individual wavelength slicer

101

as a component in the overall system shown in FIG.

1

. The input WDM signal is coupled using conventional optical signal coupling techniques to the input port of the wavelength slicer

101

. The wavelength slicer

101

separates the input signal into two sets of channels, which are routed to the output ports as depicted in FIG.

2

. In the preferred embodiment, the wavelength slicer

101

separates alternating adjacent input channels into the first and second sets of output channels as shown in FIG.

2

.

FIG. 4

illustrates the transmission characteristics of a wavelength slicer with a channel spacing of 50 GHz. If multiple stage of wavelength slicers are employed as shown in

FIG. 1

, it should be noted that the channel spacing for each successive stage will be twice that of the previous stage due to the interdigital slicing of adjacent channels.

FIG. 3

is a schematic diagram of an individual wavelength slicer

101

. Each of the optical paths is labeled with either a horizontal double-headed line indicating horizontal polarization, or a vertical double-headed line indicating vertical polarization, or both horizontal and vertical double-headed lines indicating mixed horizontal and vertical polarizations in the optical signal at that point.

The input signal

311

enters the first birefringent element

330

that spatially separates horizontal and vertically polarized components of the input signal. The first birefringent element

330

consists of a material that allows the vertically polarized portion of the optical signal to pass through without changing course because they are ordinary waves in the birefringent element

330

. In contrast, horizontally-polarized waves are redirected at an angle because of the birefringent walk-off effect. The angle of redirection is a well-known function of the particular materials chosen. Examples of materials suitable for construction of the birefringent element include calcite, rutile, lithium niobate, YVO

4

-based crystals, and the like. The horizontally-polarized component travels along a path

401

as an extraordinary signal in the first birefringent element

330

while the vertically polarized component

402

travels as an ordinary signal and passes through without spatial reorientation. The resulting signals

401

and

402

both carry the full frequency spectrum of the input signal

311

.

At least one of the beam components

401

and

402

are coupled to a polarization rotator

340

which selectively rotates the polarization state of either beam component

401

or

402

by a predefined amount. In the preferred embodiment, the rotator

340

rotates the signals by either 0° (i.e., no rotation) or 90°. In

FIG. 3

, the vertically-polarized component

402

is rotated by 90° so that both signals

403

,

404

exiting the polarization rotator

340

have a horizontal polarization. Again, at this stage, both the horizontal and vertical components

402

,

403

contain the entire frequency spectrum of channels in the input WDM signal

311

.

The stacked waveplates element

361

is a stacked plurality of birefringent waveplates at selected orientations that generate two eigen states. The first eigen state carries a first set of channels with the same polarization as the input, and the second eigen state carries a complementary set of channels at the orthogonal polarization. The polarization of the incoming beam and the two output polarizations form a pair of spectral responses, where (H, H) and (V, V) carry the first set of channels from the input spectrum and (H, V) and (V, H) carry the complementary (second) set of channels of the input spectrum, where V and H are vertical and horizontal polarization, respectively. With horizontal polarizations

403

,

404

input to the stacked waveplates element

361

as illustrated in

FIG. 3

, orthogonal vertical and horizontal polarizations are generated with the first set of channels residing in horizontal polarization and the second set of channels residing in vertical polarization.

Returning to

FIG. 3

, the pairs of optical responses

405

,

406

output by the stacked waveplates element

361

are coupled to a second birefringent element

350

. This birefringent element

350

has a similar construction to the first birefringent element

330

and spatially separates the horizontally and vertically polarized components of the input optical signals

405

and

406

. As shown in

FIG. 3

, the optical signals

405

,

406

are broken into vertically-polarized components

407

,

408

containing the second set of channels and horizontally-polarized components

409

,

410

containing the first set of channels. Due to the birefringent walk-off effect, the two orthogonal polarizations that carry first set of channels

409

,

410

in horizontal polarization and second set of channels

407

,

408

in vertical polarization are separated by the second birefringent element

350

.

Following the second birefringent element

350

, the optical elements on the input side of the second birefringent element

350

can be repeated in opposite order, as illustrated in FIG.

3

. The second stacked waveplates element

362

has substantially the same composition as the first stacked waveplates element

361

. The horizontally-polarized beams

409

,

410

input to the second stacked waveplates element

362

, are further purified and maintain their polarization when they exit the second stacked waveplates element

362

. On the other hand, the vertically-polarized beams

407

,

408

experience a 90° polarization rotation and are also purified when they exit the second stacked waveplates element

362

. The 90° polarization rotation is due to the fact that the vertically-polarized beams

407

,

408

carry the second set of channels and are in the complementary state of stacked waveplates element

362

. At the output of the stacked waveplates element

362

, all four beams

411

,

412

and

413

,

414

have horizontal polarization. However, the spectral bands defined by the filter characteristics of the stacked waveplates elements

361

,

362

are separated with the second set of channels on top and the first set of channels below.

To recombine the spectra of the two sets of beams

411

,

412

and

413

,

414

, a second polarization rotator

341

and a third birefringent element

370

are used. The second rotator

341

intercepts at least two of the four parallel beams

411

-

414

and rotates the polarization of the beams to produce an orthogonally-polarized pair of beams

415

,

416

and

417

,

418

for each spectral band at the output of the second polarization rotator

341

. In the case of

FIG. 3

, the polarization of beams

411

and

413

is rotated by 90°, and beams

412

and

414

are passed without change of polarization. Finally, a third birefringent element

370

recombines the two orthogonally-polarized beam pairs

415

,

416

and

417

,

418

using the walk-off effect to produce two sets of channels that exit at the output ports

314

and

313

, respectively.

Optical Add/Drop Switch Array. As mentioned above,

FIG. 5

is a schematic diagram of an array

500

of optical add/drop switches

510

-

517

. FIGS.

6

(

a

) and

6

(

b

) illustrate in block diagram form the general functionality of an individual add/drop switch

510

in the array. The input WDM signal

711

is fed into an input port

701

using conventional optical signal coupling techniques to the add/drop wavelength switch

510

. In the bridge state shown in FIG.

6

(

a

), the input signal

711

passes through the add/drop switch

510

uninterrupted and exits at port

703

. The add port

702

and drop port

704

are connected to form a “bridge” in which no add/drop operation occurs. The add signal

713

that is input through the add port

702

passes through the add/drop switch

510

and exits at the drop port

704

as the drop signal

714

.

In contrast, when the add/drop switch

510

is switched to the add/drop state shown in FIG.

6

(

b

), a pre-defined optical channel is extracted from the input signal

711

and exits as the drop signal

714

at the drop port

704

. The add signal

713

, on the other hand, is combined with the pass-through portion of the input signal to exit at the output port

703

as shown in

FIG. 1

b.

FIGS.

7

(

a

) and

7

(

b

) further illustrate an individual add/drop wavelength switch

510

in schematic form in the bridge and add/drop control states, respectively. In accordance with the preferred embodiment, the add/drop switch

510

is under binary control from a control bit. In FIGS.

7

(

a

) and

7

(

b

), bold solid lines indicate the optical paths for the full spectrum of channels in the WDM input signal

701

. Solid thin lines indicate the optical paths of signals comprising a first subset of channels from the WDM signals that are to pass through the add/drop switch

510

undisturbed (i.e., the pass-through channels). Thin-intermittent dashed lines indicate the optical paths for the drop channels that comprise a second subset of the input channels. Thin dotted lines indicate the optical path for the add signal. Finally, thick dotted lines are the optical paths for the pass-through subset of input channels combined with the add signal. It is important to understand that each of these subsets may comprise more than one channel and may itself be a set of WDM signals. Each of the lines representing optical paths are further labeled with either a short perpendicular line indicating horizontal polarization, or a large dot indicating vertical polarization, or both a perpendicular line and a large dot indicating mixed horizontal and vertical polarizations in the optical signal at that point.

In FIG.

7

(

a

), the input signals

701

and the add signal

702

enter a first birefringent element

600

that spatially separates the horizontal and vertically polarized components of these signals

701

,

702

. The first birefringent element

600

is made of a material that allows the vertically polarized portion of the optical signal to pass through without changing course because they are ordinary waves in the birefringent element

600

. In contrast, horizontally polarized waves are redirected at an angle because of the birefringent walk-off effect. The angle of redirection is a well-known function of the particular materials chosen. Examples of materials suitable for construction of the birefringent elements used in the preferred embodiments include calcite, rutile, lithium niobate, YVO

4

-based crystals, and the like. A polarization beam splitter can also be used to perform a similar function for polarization separation.

The vertically polarized components from the input WDM signal

701

are coupled into a fixed polarization rotator

601

such that the state of polarization (SOP) becomes horizontal. The add signal

702

is coupled to a switchable polarization rotator

602

under control of a control bit. Switchable polarization rotator

602

consists of two sub-element rotators that form a complementary state, i.e. when one turns on the other turns off. Rotator

602

serves to selectively rotate the polarization of the add signal

702

by a predefined amount. In the preferred embodiment, rotator

602

rotates the polarization of the signals by either 0° (i.e., no rotation) or 90°. In FIGS.

7

(

a

) and

7

(

b

), gray-shaded areas indicate polarization rotation and white (transparent) areas indicate no polarization rotation. The switchable polarization rotator

602

can be made of one or more types of known elements including parallel aligned liquid crystal rotators, twisted nematic liquid crystal rotators, ferroelectric liquid crystal rotators, pi-cell liquid crystal rotators, magneto-optic based Faraday rotators, acousto-optic and electro-optic polarization rotators. Commercially available rotators using liquid crystal based technology are preferred, although other rotator technologies may be applied to meet the needs of a particular application. The switching speeds of these elements range from a few milliseconds to nanoseconds, and therefore can be applied to a wide variety of systems to meet the needs of a particular application. These and similar basic elements are considered equivalents and may be substituted and interchanged without departing from the spirit of the present invention.

FIG.

7

(

a

) illustrates the bridge state in which the signals exiting the polarization rotators

601

and

602

have horizontal polarization. A first stacked waveplates element

603

is made of a stacked plurality of birefringent, composite waveplates at selected orientations that generate two eigen states. The first eigen state carries a first sub-spectrum with the same polarization as the input, and the second eigen state carries a complementary sub-spectrum at the orthogonal polarization. The polarization of the incoming beam and the two output polarizations form a pair of spectral responses, where (H, H) and (V, V) carry the first part of the input spectrum and (H, V) and (V, H) carry the complementary (second) part of the input spectrum, where V and H are vertical and horizontal polarizations, respectively. Further details of the design and the filtering mechanism of the stacked waveplates element are disclosed in the Applicants' U.S. patent application Ser. No. 08/739,424 and U.S. Pat. No. 5,694,233. In the case of this add/drop wavelength switch, the first eigen state carries the first sub-spectrum (i.e., the pass-through channels) with the same polarization as the input, and the second eigen state carries a complementary sub-spectrum (i.e., the add/drop channels) at the orthogonal polarization.

The input channels

701

are decomposed into two components having orthogonal polarizations as they pass through the first stacked waveplates element

603

. The pass-through spectrum

705

is coded in the horizontal polarization and the drop spectrum

706

is coded in the vertical polarization. The add signal

702

, has a horizontal polarization before entering the stacked waveplates element

603

. It is rotated by 90° as it passes through the first stacked waveplates element

603

, because it has the same spectrum as the drop channel. At the plane after the first stacked waveplates element

603

as shown in FIG.

7

(

a

), the add/drop channels are vertically polarized, while the pass-through spectrum is horizontally polarized.

Optical signals

705

,

706

, and

707

represent the pass-through, drop, and add signals that are coupled to the second birefringent element

604

. The second birefringent element

604

has a similar construction to the first birefringent element

600

and serves to spatially separate the horizontally and vertically polarized components of the optical signals

705

,

706

, and

707

. The two orthogonal polarizations that carry the pass-through spectrum

705

in horizontal polarization and the add/drop spectrum

707

,

706

in vertical polarization are separated by the second birefringent element

604

because of the birefringent walk-off effect.

A second set of polarization rotators

605

and

606

follow the second birefringent element

604

. The pass-through signal

705

passes through a fixed-type rotator

605

that rotates the polarization by 90°. The add/drop signals

707

,

706

pass through a switchable polarization rotator

606

that is also set to rotate the polarization by 90° in the bridge state (see FIG.

7

(

a

)). At the exit plane of the polarization rotators

605

and

606

, the pass-through spectrum has vertical polarization and the add/drop spectra have horizontal polarization, as indicated in FIG.

7

(

a

).

Following the second set of polarization rotators

605

and

606

, the preceding components can be repeated, but arranged in opposite order. As shown for the bridge state in FIG.

7

(

a

), a third birefringent element

607

recombines the pass-through spectrum

705

and the drop signal

706

because of the walk-off effect. Thus, in the bridge state, no add/drop operation occurs. The add signal

707

propagates upward in the third birefringent element

607

and keeps its horizontal polarization.

The second stacked waveplates element

608

has the same structure and composition as to the first stacked waveplates element

603

. With the horizontally-polarized beams

706

,

707

input to the second stacked waveplates element

608

, the add/drop spectrum is further purified and rotates its polarization by 90°. On the other hand, the vertically-polarized beam

705

(carrying the pass-through signals) input to the second stacked waveplates element

608

maintains its polarization but is also purified when it exits the second stacked waveplates element

608

. The 90° polarization rotation of the horizontally polarized beams

706

,

707

is due to the fact that the add/drop spectrum is the complementary state of the second stacked waveplates element

608

. At the output of second stacked waveplates element

608

, all four beams have vertical polarization. In the bridge state depicted in FIG.

7

(

a

), the upper two beams carry the full input spectrum and the lower two beams carry the add signal's spectrum.

To recombine the two sets of beams, a third set of polarization rotators

609

and

610

and a fourth birefringent element

611

are used. Again, the third set of polarization rotators consists of a fixed-type polarization rotator

609

and a switchable polarization rotator

610

. Both polarization rotators

609

and

610

have two sub-elements that intercept the two sets of beams. The pass-through signals (i.e., the complete input signals in the bridge state) carried by the upper two beams (indicated by the heavy solid lines after the third birefringent element

607

in FIG.

7

(

a

)) pass through the fixed polarization rotator

609

such that one of the upper beams has its polarization is rotated by 90°. The two orthogonal polarizations are then recombined by the fourth birefringent element

611

to exit at output port

703

.

The two lower beams carrying the add signal pass through the switchable polarization rotator

610

so that the polarization of one of the lower beams is rotated by 90°. They are then recombined by the fourth birefringent element

611

. In this design, the sub-elements of the third set of polarization rotators

609

and

610

are set at complementary states to the corresponding sub-elements in the first set of polarization rotators

601

and

602

. This complementary design assures high contrast operation for the polarization rotators and further assures high isolation for spectral filtering. This completes the bridge state of operation for the add/drop wavelength switch

510

.

In the add/drop state, the optical paths are shown in FIG.

7

(

b

). The three switchable polarization rotators

602

,

606

and

610

have switched to their complimentary states (i.e., from on to off or off to on, depending on their original states). In this state of operation, the light paths for the pass-through spectrum

705

remain unchanged as in the bridge state. This design ensures that the pass-through signals are not affected by the add/drop operation and flow through the optical node without being interrupted. This can be seen from the optical paths for the pass-through spectrum

705

through the fixed polarization rotators

601

,

605

, and

609

shown in FIGS.

7

(

a

) and

7

(

b

). The pass-through spectrum

705

passes undisturbed through the entire add/drop switch

510

along an optical path that remains unchanged between the bridge state (FIG.

7

(

a

)) and the add/drop state (FIG.

7

(

b

)).

In contrast, the paths of the add signal and the drop signal are interchanged between the add/drop state and the bridge state, i.e., the drop signal

706

now exits at drop port

704

and the add signal

707

is combined with the pass-through signal

705

that exits through output port

703

. In FIG.

7

(

b

), the add signal

702

is again decomposed into two orthogonal polarizations. Because the first polarization rotator

602

is now set to have the output polarizations all vertical, they pass through the first stacked waveplates element

603

which rotates the polarization by 90° so that both components of the add signal

702

become horizontally polarized. These horizontally-polarized beams propagate upward in the second birefringent element

604

due to its extraordinary wave characteristic. The add signal

707

meets the drop signal

706

at the exit plane of the second birefringent element

604

. These two signals containing the add/drop spectra then pass through the second polarization rotator

606

, which is set for no polarization rotation. The add signal

707

continues to propagate upward through the third birefringent element

607

and meets the pass-through signal

705

at the exit plane of the third birefringent element

607

. The drop signal

706

, however, propagates straight through the third birefringent element

607

because it is an ordinary wave in this birefringent element

607

. It is clear up to this point that the add signal and the drop signal have exchanged their paths in comparison to the bridge state shown in FIG.

7

(

a

).

These four beams pass through the second stacked waveplates element

608

. The pass-through signal

705

keeps its polarization and the add/drop signals

707

,

706

rotate their polarizations by 90°. They pass through the fourth set of polarization rotators

609

and

610

such that orthogonal polarizations result. These two sets of beams are recombined by the fourth birefringent element

611

and exit to ports

703

and

704

, respectively. This completes the add/drop operation of the add/drop wavelength switch

510

.

Central Office Embodiment.

FIG. 8

provides a schematic diagram of an implementation of the present invention to supplement and enhance the capabilities of existing central offices in an optical network. Existing optical communications systems typically provide only point-to-point communications between two central offices

801

,

802

over an optical link

841

,

842

. As WDM technology has progressed, the number of channels carried over the optical link has increased. However, many early WDM systems use central office equipment

805

,

806

that is capable of handling only a relatively small number of widely spaced channels. In order to upgrade the channel capacity of such systems, it is generally necessary to completely replace the existing central office equipment, which involves considerable expense. In contrast, the present invention allows existing central office equipment to be retrofitted to handle an increased number of channels, as shown in FIG.

8

.

At the receiving central office

802

, the initial wavelength slicer

804

separates the WDM signal into two sets of adjacent channels, as previously discussed. One set consists of the existing channels used by the existing central office equipment

806

. The other set consists of new channels that are interdigitally spaced between the existing channels. The existing channels are routed by the initial wavelength slicer

804

to the existing central office equipment

806

to be processed in accordance with the communications protocol for the existing central office equipment

806

.

In contrast, the new interdigital channels are routed by the initial wavelength slicer

804

to additional wavelength slicer stages, and thence to optical filters

808

that separate the set of new channels into an array of separated channels for an add/drop switch array

810

, as previously discussed. Express lanes

812

can also be provided. For example, the add ports and drop ports of the add/drop switch array

810

can be connected to a digital cross switch (DCS)

816

to interface with an external communications network. The output ports of the add/drop switch array

810

can be routed to a second wavelength slicer network

814

that combines the array of output channels and the express lanes

812

into a WDM signal that can be transmitted over a second optical link

818

.

At the transmitting central office

801

, these components provide the inverse operations to those described above due to their inherently bi-directional characteristics. The wavelength slicer network

807

,

803

combines the existing output channels from the old central office equipment

805

with the new interdigital channels output by the add/drop switch array

809

and the express lanes

811

. Here again, the add/drop switch array can be interfaced with a digital cross switch (DCS)

815

and a third optical link

817

. The present invention can also be used to add or drop channels at an intermediate station

850

in the optical link

841

,

842

, as shown in FIG.

8