Background of the Invention This invention relates to wavelength division multiplexed (WDM) optical communication systems. The invention is more particularly concerned with a novel wavelength selective switching scheme for such systems that is based on a simple serial architecture, and with optical switching apparatus designed to implement the architecture.

In a preferred mode, the apparatus is constituted as a wavelength-selective add/drop switch. The invention is also concerned with optical communications apparatus and with methodology that take advantage of the architecture.

In WDM optical communication systems, the optical transmission spectrum is divided into a plurality of wavelength bands or channels for communication. Multiple optical signals can be transmitted simultaneously over a common path (usually an optical fiber) with each signal being on a different wavelength channel. This allows

different groups of end users or devices to communicate at the same time on different channels.

A typical WDM optical communication system is constructed as a network of nodes interconnected by fiber- optic links. End users and devices connect to the network at corresponding nodes. To optimize utilization of the network, node designs commonly incorporate signal add/drop functionality, whereby signals on one or any combination of wavelength channels can be dropped and/or added at the node.

For this purpose, a node may be constructed as, or to include, a wavelength add/drop multiplexer (WADM). The components forming the node should add as little loss as possible to the system, should be highly reliable, and should provide active switchability of the WADM so that signals on individual channels can be passed, dropped, and added at the node as communication requirements dictate.

Most node designs with WADM capability have relied on parallel architectures for providing signal add/drop functionality. For example, one proposed design uses an arrangement of switches connected in parallel between a multiplexer and a demultiplexer to enable switching of the individual channels. Another proposed design uses a pair of star couplers interconnected by a wavelength-selective parallel switching arrangement.

Fig. la shows a WADM 10 of the first-mentioned design.

The WADM 10 includes a demultiplexer (DEMUX) 12 connected to an input line 14 (e. g., an optical fiber or planar optical

path) to receive multiplexed optical signals at wavelengths The The DEMUX 12 demultiplexes the optical signals, and it outputs them individually to corresponding 2x2 optical switches Su-son connected to its output side. As shown in Fig. la, the switches Su-son are connected to the input side of a multiplexer (MUX) 16, which combines the signals from the switches for transmission on an output line 20.

The switches Sl-Sn, under electronic control, can each assume either a"bar"state or a"cross"state. In the bar state, a signal entering a switch from the DEMUX 12 passes through to the MUX 16, so that it is retained for transmission on the output line 20. The channel carrying such a signal is said to be in a"through"state. The switches for wavelength channels A1 and An are shown in the bar state. In the cross state, shown by the switch for channel Ai, the signal entering the switch is directed to a corresponding drop line 18, such as for transmission to an end user, and does not pass to the output line 20.

Optionally, another signal at the same wavelength Xi can be input to the system, via a corresponding add line 19, for transmission on the output line 20. The channel for wavelength Xi is thus said to be in an"add/drop"state.

The WADM 10 shown in Fig. la is complex, expensive, and based on an inflexible design. The design, more specifically, is not readily expandable to accommodate the addition of new wavelength channels to the communication network. This means that the initial node design must

include excess capacity to allow for possible future wavelength channels, or that specialty components and an additional WADM structure must be added to accommodate new channels at a later time. The former option is not cost- effective since capital must be allocated for equipment to handle more channels than initially required. The latter option may require substantial future expense and may be problematical due to additional system losses.

Parallel architectures based on star couples are also problematical. For example, the star coupler approach is inherently lossy, and the loss increases with the number of channels (loss increases as n2 where n is the number of add/drops required at a node). Further, like the design in Fig. la, star-coupler based designs are complex, expensive, and not readily expandable to accommodate additional wavelength channels beyond the initial design capacity.

Fig. lb illustrates a known signal add/drop component 30 based on serial architecture. This component can be fabricated by arranging a Bragg grating device 33 tuned to a desired wavelength Xi between two optical circulators 32,36 as shown. Bragg grating devices can be implemented in various forms including fiber and planar devices. Each of the circulators 32,36 includes respective ports 1,2, and 3.

Component 30 receives a composite group of signals at different wavelengths xi-un on an input line 14 (e. g., an optical fiber or planar optical path) at port 1 of optical

circulator 32. The signals propagate via port 2 of the circulator 32 to the Bragg grating 33. The Bragg grating passes all of the signals except for the signal at wavelength Xi to an output line 20 via ports 2 and 3 of circulator 36. The signal at wavelength Xi, which is the signal to be dropped, is reflected by the Bragg grating and propagates to a drop line 38 via ports 2 and 3 of circulator 32. A signal to be added at wavelength Xi can be input at port 1 of circulator 36, via an add line 39, and is combined with the remaining signals for transmission on the output line 20.

The component 30 has the advantage of a relatively simple design, but it is not switchable. Thus, the signal on the channel reflected by the Bragg grating 33 must be dropped. The component can be designed to drop/add signals on multiple wavelengths by including additional gratings between the circulators. But signals at all wavelengths that are reflected by the gratings must still be dropped.

The component 30 therefore cannot provide the discretionary add/drop functionality required for efficient WDM network utilization.

Fig. lc shows a proposed WADM design 30'based on a switchable serial architecture. This design includes a plurality of series-connected 2x2 optical switches Sl-Sn+l (n is the number of channels) arranged between a pair of optical circulators 32,36, each having three ports as previously described in connection with Fig. lb. The

optical circulators 32,36 are connected, respectively, to an input line 14 and an output line 20. Adjacent switches of the series are coupled to each other by wavelength- selective Bragg gratings 33i (I = 1 to n) tuned to corresponding wavelengths of the system and by complementary bypass lines 35. A drop line 38 and an add line 39 are connected, respectively, to the optical circulators 32,36.

In operation of the WADM 30', the switches Sl-Sn+l are configured (using the bar and cross states) to route the input WDM signal to the grating (s) corresponding to the signal (s) to be dropped. The gratings reflect the corresponding signals back to the optical circulator 32 to be dropped via the drop line 38. The remaining signals (through channels) pass through the gratings to circulator 36 and to the output line 20. Dropped signals can be replaced by new signals inputted to circulator 36 via the add line 39.

When necessary to accommodate additional wavelengths, the WADM 30'can be expanded. This is accomplished by adding new switches and suitably tuned Bragg gratings into the existing series arrangement. The WADM 30'can thus be tailored to meet a network's initial channel capacity, without providing excess capacity, and thereafter expanded as needed.

Although its switchable serial architecture affords good flexibility for expansion, the WADM 30'poses a significant risk of data loss. This is because the signals

on all wavelength channels, including the through channels, are subjected to switching. For example, when the signal at wavelength A1 is to be dropped, the corresponding switch S is switched to the cross state so that all of the channels are routed to grating 331. As a result, signal data on the through channels can be lost during the switching interval.

Summary of the Invention The present invention provides an improved switchable serial architecture for WDM network applications. As will be seen hereinafter, the invention offers the simplicity and easy expandability associated with serial architecture, while avoiding the potential for data loss associated with designs that require switching of through channels (see the earlier discussion of Fig. lc).

According to one of its principal aspects, the invention provides switch apparatus for WDM optical communications, comprising a wavelength-selective optical switching assembly including an input port constructed to receive multiple optical wavelength channels, an output port, a wavelength-selective optical filter, and an optical switching device. The wavelength-selective filter is constructed and disposed to direct signals on a plurality of the received wavelength channels for propagation to the output port and to direct a signal on another of the received wavelength channels to the optical switching device. The optical switching device is disposed and

operative to switch the other wavelength channel between a through state and a drop or add/drop state without switching the plurality of wavelength channels.

In a preferred mode, the optical switching assembly comprises first and second optical circulators, each having at least first, second, and third ports. The first port of the first circulator constitutes the input port, while the third port of the second circulator constitutes the output port. The wavelength-selective filter includes a reflective grating connected between the second ports of the first and second circulators. The optical switching device is connected between the third port of the first circulator and the first port of the second circulator.

In another preferred mode, the optical switching assembly comprises first and second optical couplers, each having at least first, second, and third ports. The first port of the first coupler constitutes the input port, while the second port of the second coupler constitutes the output port. The wavelength-selective filter includes a reflective grating connected between the second port of the first coupler and the first port of the second coupler. The optical switching device is connected between the third ports of the first and second couplers.

In yet another preferred mode, the wavelength-selective filter comprises a four-port filter device having a thin- film notch filter coupled to first through fourth ports.

The first port and fourth port constitute the input port and

output port, respectively. The signals on the plurality of channels received at the first port reflect from the thin- film filter to the fourth port, and the signal on the other received channel passes through the filter to the second port. The optical switching device is connected between the second and third ports.

Still another preferred mode, employs a wavelength- selective Mach-Zehnder filter device. The Mach-Zehnder device may include first and second 2x2 optical couplers, each having first, second, third, and fourth ports. The first port of the first coupler constitutes the input port.

The third and fourth ports of the first coupler are connected by first and second phase-shift optical paths to the first and second ports, respectively, of the second coupler. A reflective grating portion is disposed in the first and second phase-shift optical paths. The optical switching device is connected between the second port of the first coupler and the third port of the second coupler, and the fourth port of the second coupler constitutes the output port.

Still another preferred mode employs a wavelength- selective thin-film filter which is reflective to the plurality of received wavelength channels and transmissive to the other received wavelength channel, and disposed in a path of signals propagating from the input port. The switching device has a member switchable between a first position and a second position. In the first position, the

switchable member intercepts the signal transmitted through the thin-film filter to cause that signal to propagate to the output port. In the second position, the switchable member allows the signal transmitted through the filter to be dropped.

In yet another preferred mode, all of the optical components of the wavelength-selective filter and the optical switching device are free-optics components (non- waveguide components). Structures based on free optics can be advantageous from the point of view of minimizing the number of components, and thus reducing the overall cost of the apparatus.

According to another principal aspect of the invention, a switching apparatus for WDM optical communications may comprise an input port constructed to receive a plurality of multiplexed optical signals each on a different wavelength channel, an output port, a first optical path from the input port to the output port, and a second optical path from the input port to the output port. The second optical path includes an optical switching device, and the first optical path includes a wavelength-selective filter which is constructed to cause at least a selected one of the signals to propagate to the switching device and to cause the remainder of the signals to propagate to the output port by way of a path including the first optical path. The switching device has a first state to cause the at least one selected signal propagated from the wavelength-selective

filter to propagate to the output port by way of the second optical path, and a second state in which the at least one selected signal propagated from the wavelength-selective optical filter is dropped so as not to propagate to the output port.

Again, preferred modes include implementations based on the use of optical circulators, optical couplers, a notch filter device, or free-optics components.

According to yet another principal aspect, the present invention provides signal add/drop apparatus for a WDM optical communication system, the apparatus comprising a plurality of wavelength-selective add/drop switches coupled in series, with each switch being constructed to switch a corresponding wavelength channel between a through state and an add/drop state without switching another wavelength channel present at that switch.

As will be seen hereinafter, still other principal aspects of the invention relate to the design of a wavelength-selective add/drop switching device having redundant add/drop switching capability, and to apparatus and methods that take advantage of the design.

The aforementioned and other aspects of the invention, as well as its features and advantages, will be more fully appreciated from the following detailed description taken in conjunction with the accompanying drawings.

Brief Description of the Drawinqs Fig. la is a schematic illustration of a WADM design based on parallel architecture.

Fig. lb is a schematic illustration of a signal add/drop component based on serial architecture.

Fig. lc is a schematic illustration of a WADM design based on a switchable serial architecture.

Fig. 2 schematically illustrates the serial connection of a plurality of switch modules in accordance with the invention.

Fig. 3 is a schematic illustration of a wavelength selective add/drop switch according to the invention.

Figs. 4a and 4b illustrate the"through"state and the "add/drop"state, respectively, of a 4-port optical switch.

Fig. 5 schematically illustrates the serial connection of two fiber Bragg gratings.

Figs. 6 and 7 illustrate 4-port optical fiber switches that can be used in wavelength-selective add/drop switches of the invention.

Fig. 8 is a schematic illustration of another embodiment of a wavelength-selective add/drop switch employing fiber couplers.

Fig. 9 schematically illustrates another embodiment of a wavelength-selective add/drop switch employing a Mach-Zehnder filter.

Fig. 10 schematically illustrates another embodiment of a wavelength-selective add/drop switch employing a 4-port thin-film notch filter.

Fig. 11 shows the output spectra in both the add/drop and the through states for a wavelength selective add/drop switch.

Fig. 12 shows the drop port transmission spectra in both the add/drop and the through states for a wavelength selective add/drop switch.

Fig. 13 schematically illustrates another embodiment of the invention based on a free-optics design.

Fig. 14 shows a modification of the embodiment in Fig.

13.

Fig. 15 schematically illustrates an additional embodiment based on a free-optics design.

Fig. 16 shows a modification of the embodiment in Fig.

15.

Fig. 17 schematically illustrates an application of the invention for bi-directional communication.

Fig. 18 schematically illustrates a wavelength- selective add/drop device used in the embodiment of Fig. 17.

Figs. 19 and 20 show variations of the device of Fig.

18.

Description of the Preferred Embodiments Fig. 2 illustrates a WADM system based on switchable serial architecture in accordance with the present

invention. The basic building block of the system is a wavelength-selective add/drop switch (WSA/D switch). In the form shown, the system includes a series connection of WSA/D switch modules, with one switch module being provided for each wavelength channel Au-an present in the system.

Depending on system requirements, switch modules constructed to switch more than one wavelength channel may be used. The modular nature of the system allows for easy reconfiguration and expansion to accommodate new channel plans and the addition of new channels. Reconfiguration can be accomplished by simply rearranging individual modules within the system. Expansion simply involves adding new switch modules constructed to switch the new channels being added to the system.

A preferred WSA/D switch design for the system shown in Fig. 2 will be substantially transparent to all wavelength channels except the channel or channels of interest. It will also allow active switching of the channel (s) of interest between a through state and an add/drop state (or drop state if no signal is to be added) without switching the other channels. Switching the channel of interest therefore does not disrupt the transmission of the other channels.

Fig. 3 is a schematic diagram of one such WSA/D switch design. The WSA/D switch 40 shown in Fig. 3 is composed of an optical switching device-here, a 4-port 2 x 2 switch Si-connected to a wavelength-selective filter assembly 45

which includes two optical circulators 42 and 46 and a single fiber Bragg grating 43 tuned to a selected wavelength Ai. Port 1 of circulator 42 and port 3 of circulator 46 respectively constitute the input port and output port of the WSA/D switch. The switch Si, which preferably is wavelength insensitive (not wavelength selective) is connected to the assembly 45 at port 3 of circulator 42 and port 1 of circulator 46 via lines 44. The WSA/D switch can take either of two states-a through state or an add/drop state-with respect to the channel at wavelength Xi, depending on the state of the 4-port switch Si. The corresponding states of the switch Si are shown diagrammatically in Figs. 4a and 4b.

Incoming signals on channel wavelengths Xl-Xn are supplied to the WSA/D switch input port through an input line 14. All of the incoming signals propagate through circulator 42 to grating 43, which is tuned to the selected wavelength Xi. The through channel signals (those at all wavelengths except Xi) propagate through grating 43 and circulator 46 to output line 20.

The signal on channel Xi that is selected by grating 43 is reflected back through circulator 42 and propagates from port 3 of thereof to switch Si. When switch Si is in the through state (Figs. 3 and 4a), the selected signal propagates via port 4 of the switch to port 1 of circulator 46, and from port 3 of the circulator to the output line 20.

When switch Si is in the add/drop state (Fig. 4b), the

selected signal propagates via port 3 of the switch to a drop line 48. In this state, a signal at wavelength Xi can be added at port 2 of switch Si via an add line 49. Ports 2 and 3 of switch Si thus constitute the add and drop ports, respectively, of the WSA/D switch 40.

As will be appreciated from the preceding description, the structure of Fig. 3 provides two paths from the input port to the output port of the WSA/D switch 40. More particularly, signals on the through channels propagate to the output port via a first path including circulator 42, grating 43, and circulator 46. The signal on the selected channel at wavelength Xi propagates to the output port via a second path including circulator 42, the assembly of lines 44 and switch Si, and circulator 46. Since the through channel signals propagate only through circulators 42,46 and grating 43, there is no disruption of their propagation during the switching interval of switch Si.

The arrangement of switch Si in the second path thus allows for switching of the selected channel between a through state and an add/drop state without subjecting the through channels to switching. This avoids the risk of data loss that would accompany switching of the through channels.

Other attributes of the WSA/D switch 40 include its low-loss, and its modularity and reduced complexity-the switch can be constructed with just two discrete components, the filter assembly 45 and the 4-port switch Si.

WSA/D switch 40 can be modified to add or drop a plurality of signals by providing a grating for each signal wavelength. For example, signals at wavelengths A1 and X could be dropped and added from the WSA/D switch by replacing grating 43 with the series connection of gratings 66 and 67 shown in Fig. 5.

Various types of optical switches can be used as the 4- port switch Si. Fig. 6 shows an overclad fiber-optic switch 50, the principles of operation of which are disclosed in U. S. patents 4,763,977 and 5,353,363 (both incorporated herein by reference). Switch 50 includes a WDM fiber-optic coupler 51 having two optical fibers. Ends 52a and 52b of one fiber protrude from opposite ends of coupler 51, and ends 53a and 53b of the other fiber protrude from opposite ends of the coupler. Coupler 51 is fixed at one end by suitable fixing means 54, and the other end of the coupler is switchably bent by a bending device 56. Electromagnetic, piezoelectric, bimetallic and other types of devices can provide the small, controlled movement that is required for bending the coupler. Switch 50 functions such that an optical signal applied to fiber end 52a couples to the other fiber and appears at fiber end 53b when the coupler is unbent. Similarly, a signal applied to fiber end 53a couples to the other fiber and appears at fiber end 52b when the coupler is unbent. When the coupler is bent, an optical signal applied to fiber end 52a remains uncoupled and appears at fiber end 52b. Coupler 51 could be deflected to

the bent condition by means such as those disclosed in U. S.

Patents Re. 31,579; 4,204,744; 4,303,302; 4,318,587 and 4,337,995 (all incorporated herein by reference). The rotary action of the switch disclosed in U. S. Patent 5,146,519 (incorporated herein by reference) is also well suited for switching switch 50; the linear motion of the switch actuating device would simply be converted to a twisting motion.

4-port switch Si could also be constructed in accordance with Fig. 7. In the structure of Fig. 7, a switchable optical fiber 59, which is connected to an input port 1, can be switched between a drop port 3 and an output port 4 as indicated by the double-headed arrow a.

Similarly, a switchable optical fiber 58, which is connected to an add port 2, can be switched to and away from output port 4 as indicated by double-headed arrow b.

In the unswitched state, switchable fibers 58 and 59 are in the position represented by solid lines, and the signal channel is connected from port 1 to port 4. In the switched state, switchable fibers 58 and 59 are in the positions represented by dashed lines 58'and 59'.

Therefore, the signal channel is connected from port 1 to port 3, and the port 2 is connected to port 4. Switchable fibers 58 and 59 can be switched between the two illustrated states by means such as those disclosed in U. S. Patents Re.

31,579; 4,204,744; 4,303,302; 4,318,587 and 4,337,995.

Fig. 8 illustrates a WSA/D switch 70 in which the circulators of Fig. 3 have been replaced by optical couplers 72,76. As shown in Fig. 8, a filter assembly 75 includes two 3dB couplers 72 and 76, each with respective ports 1-3, and a fiber Bragg grating 43. Channel signals at wavelengths Au-an are received on input line 14 of switch 70 and coupled by through ports 1 and 2 of coupler 72 to grating 43. Except for the signal at wavelength Xi, the received signals propagate to port 1 of coupler 76 and from port 2 thereof to output line 20. The signal at wavelength xi is reflected back to port 2 of coupler 72 and from port 3 thereof to 4-port switch Si, where it may or may not be dropped depending upon the state of switch Si. When the switch Si is in the through state, the signal at wavelength Xi propagates to port 3 of coupler 76, where it is coupled to port 2 and placed on output line 20. When the switch Si is in the add/drop state, the signal propagates to drop line 48. Depending upon the system in which WSA/D switch 70 is used, it may be desirable to place an isolator in line 14.

It is apparent that the WSA/D switch 70 provides two optical paths from its input port to its output port similarly to the arrangement of Fig. 3, with attendant advantages as previously described.

Fig. 9 shows a further WSA/D switch 80 according to the invention, in which the two optical circulators and fiber Bragg grating of Fig. 3 have been replaced by a Mach-Zehnder (MZ) wavelength-selective filter assembly 85, the operation

of which is described in U. S. Patent 4,900,119 (incorporated herein by reference).

Briefly, the MZ filter assembly 85 operates as follows.

The incoming channel signals at multiple wavelengths are supplied by input line 14 and enter filter assembly 85 through port 1 of a first coupler 82. Upon passing through coupler 82, the wavelengths are split, and the signals in each arm are phase shifted by w/2. The wavelengths which are not resonant with Bragg gratings 83 in the phase shift paths are transmitted to a coupler 86 where again, due to an additional tir/2 phase shift, all the light is interferometrically coupled to port 4 of coupler 86 and exits the MZ-WSA/D switch to output line 20. The wavelength Xi is reflected by the Bragg gratings and experiences a second s/2 phase shift upon propagation back to the coupler 82 and thus exits the filter assembly at port 2 of the coupler and propagates to 4-port switch Si. Any signal at Xi propagated from 4-port switch Si to coupler 86 will be reflected by the Bragg gratings and routed from port 3 to port 4 of coupler 86 in the same manner as the input signal at Xi is routed from port 1 to port 2 of coupler 82.

The 4-port switch Si operates in the manner described in connection with Fig. 3. Thus, when the 4-port switch is in the through state, the signal selected by the MZ filter assembly 85 is reflected around the path containing the switch Si and routed out of the MZ-WSA/D switch 80 by way of ports 3 and 4 of coupler 86. When switch Si is in the

add/drop state, the signal selected by MZ filter assembly 85 is dropped at port 3 of the switch Si, and a new signal at the same wavelength can be added at port 4 of the switch (see Fig. 4b).

Fig. 10 shows yet another WSA/D switch 90 according to the invention. This switch employs a 4-port thin-film notch filter assembly 95 and does not use a Bragg grating. Notch filter assembly 95 may be constructed in accordance with teachings found in, for example, Macleod, H. A., Thin-film Optical Filters, American Elsevier, 1969 (incorporated herein by reference).

The switch 90 operates as follows. The incoming channel signals at different wavelengths Au-an are supplied via input line 14 and enter WSA/D switch 90 through input port 1 of the filter assembly 95. All of the incoming channel signals except the Xi channel signal reflect from surface 92a of a thin-film notch filter 92 and exit at output port 4 of the filter assembly to output line 20. The Xi channel signal propagates through notch filter 92 and from surface 92b thereof to port 2 of the filter assembly, where it is coupled via one of two lines 44 to 4-port switch Si. When the 4-port switch is in the through state, theX channel signal propagates around the path containing lines 44 and switch Si and is routed to port 3 of the filter assembly, from which it propagates back through notch filter 92 to be output from port 4. The ports 1-4, incidentally, are optically coupled to the thin-film filter 92, as by

respective GRIN lenses (gradient refractive index lenses-not shown) connected to those ports. When switch Si is in the add/drop state, the Xi channel signal from port 2 is routed to drop line 48, and a new signal can be added on the same channel via add line 49. The added signal propagates from switch Si to port 3, from which it propagates through notch filter 92 to port 4 and output line 20.

The operation of a two channel wavelength-selective add/drop switch of the type disclosed in connection with Figs. 3 and 5 has been demonstrated. The switch was built from two commercially available optical circulators and a multi-clad bending coupler switch of the type shown in Fig.

6. Two serially connected fiber Bragg gratings were fabricated to operate at wavelengths of 1554.8 nm and 1555.8 nm. Additionally, a single channel switch has been demonstrated which operates at a wavelength of 1557 nm. The performance of the single channel switch is similar to the two channel switch.

Fig. 11 shows the output port transmission spectrum of the two-channel device when the 4-port switch is in both the add/drop and through states. In the through state, the insertion loss was 3.7 dB and 1.9 dB for the selected and adjacent channels, respectively (curves 98 and 99). The directivity was 36 dB (curves 100 and 101) and was limited by the bending coupler switch.

The transmission spectrum at the drop port is shown in Fig. 12. The insertion loss with the switch in the add/drop state was 1.8 dB (curves 102 and 103) and the directivity was 34 dB (curves 104 and 105). The adjacent channel rejection was limited by the side-bands of the fiber Bragg gratings used. The insertion loss of the switch was limited by the loss of the circulators and the fusion-splices between the high-delta fiber used for the Bragg gratings and the standard single-mode optical fiber used for the circulators (the insertion loss of the bending switch is only 0.15 dB).

By reducing the splice losses to negligible levels, the insertion loss could be reduced to 1.75 and 0.8 dB for selected and adjacent channels, respectively. Assuming these low losses, it is estimated that 32 single channel switches could be concatenated before accumulating 30 dB of total insertion loss. In fact, by using a Mach-Zehnder filter type switch as described in connection with Fig. 9, the total insertion loss for 32 switches could be as low as 18 dB.

The experimental results indicate that WSA/D switches according to the invention can be fabricated with low insertion loss and high directivity.

Figs. 13-16 illustrate additional WSA/D switch designs according to the invention. The designs in Figs. 13-16 use wavelength-selective thin-film filters, but unlike the embodiment of Fig. 10, are based on the use of free-optics

components (non-waveguide components) to accomplish both the wavelength selection and channel switching functions. This allows the thin-film filter and the channel switching portion to be integrated into a single device. The part count and the number of fiber splices in the overall switch design, and consequently the production costs, can thus be reduced.

Fig. 13 shows a WSA/D switch 100. The switch has four optical ports, including an input port 1 connected to input line 14, an output port 4 connected to output line 20, an add port 2 connected to add line 109, and a drop port 3 connected to drop line 108. The input port is coupled to the other ports via respective GRIN lenses 1021-1024 and a wavelength-selective switching assembly 105, including a thin-film filter 103 and a switchable member constituted here by a movable mirror member Mi. Of course, if signal- add capability is not desired, the add port and associated GRIN lens may be omitted.

The thin-film filter 103 is transmissive to light of a selected channel wavelength Xi and is reflective to light of the remaining channel wavelengths. The filter is appropriately disposed to reflect light of the remaining channel wavelengths for propagation to the output port via GRIN lens 1024. The light of the selected wavelength is transmitted by the filter toward the drop port GRIN lens 1023, which is substantially optically aligned with the input port GRIN lens 1021 across the thin-film filter.

The switchable member Mi has first and second mirror surfaces 104,106 mounted on a common support member 107 and is movable between a position corresponding to the through state of the channel at wavelength Ai (position shown in solid lines in Fig. 13) and a position corresponding to the add/drop state of the channel (position shown in phantom).

In the through position, the first mirror surface 104 is disposed to intercept the light of wavelength Xi transmitted by the thin filter 103. This light is then reflected to the second mirror surface 106 which, in turn, reflects the light back through the thin-film filter to output port GRIN lens 1024 for placement on output line 20. In the add/drop position of the switchable member Mi, the first and second mirror surfaces 104,106 are removed from the respective optical paths between the input and drop GRIN lenses 1021, 1023 and the output and add GRIN lenses 1024,1022. Thus, the light of wavelength Xi transmitted by the thin-film filter propagates to the drop port 3 and line 108 via the GRIN lens 1023. Optionally, the dropped signal may be replaced by a signal of the same wavelength introduced at the add port 2 via line 109. The new signal propagates from the add port, through GRIN lens 1022, the thin-film filter 103, and the output port GRIN lens 1024, to the output port 4.

The motive power for the movable member Mi can readily be provided by a variety of mechanisms. For example, a permanent magnet can be attached to the mirror support and

two electro-magnets can be disposed at respective movement stops corresponding to the through-state and add/drop-state positions of the movable member.

It will be appreciated that, as in the earlier described embodiments, the light of the through channels (channels not to be switched by the optical switching device) and the light of the selected channel wavelengthX follow different paths from the input port to the output port of the WSA/D switch 100, with only the light path of the selected channel wavelength being subjected to switching. More particularly, the through channels follow a path including the input port GRIN lens 1021, the incidence surface of the thin-film filter 103, and the output port GRIN lens 1024. Light of the selected wavelength Xi follows a path including the input port GRIN lens 1021, a first pass through the thin-film filter 103, the first and second reflective surfaces 104,106 of the movable member Mi, a second pass through the thin-film filter 103, and the output port GRIN lens 1024. Thus, as in the previous embodiments, the selected channel at wavelength Xi can be switched between the through and add/drop states without switching the through channels.

Although not necessary in practice, the illustrative arrangement of the first and second mirror surfaces 104,106 on a common movable support member is advantageous because it facilitates accurate and stable alignment of the mirror surfaces. As an alternative, the switchable member may be

constituted by a prism, with the mirror surfaces being constituted by reflective end surfaces on the prism.

Alignment of the GRIN lenses 1021-1024 during construction can be accomplished by simply"following the light path" starting with the input port and proceeding to the output port, the add port, and the drop port (with the mirrors moved out of the way in the latter two cases). The output port and add port GRIN lenses 1024,1022 are substantially optically aligned with one another across the thin-film filter 103, as are the input port and drop port GRIN lenses 1021, 1023.

Another advantage of the free-optics design is that it allows for permanent attachment of all optical fibers associated with the WSA/D switch 100 such that they are immobile. By contrast, opto-mechanical switches, such as those discussed in connection with earlier embodiments, allow for movement of fibers within the switch.

The optical performance of the WSA/D switch is optimized for the through channels, for which the insertion loss is expected to be only about 0.5 dB. The switchable channel will see the most loss in the through state, but even in this case, the insertion loss is expected to be less than 1.5 dB. Since switching occurs on the back side of the filter 103, the through channels are not affected during the switching interval. Cross-talk is also limited only to out- of-band cross-talk that can be obtained by the filter element. Additionally, since the thin-film filter will only

allow light of the selected wavelength Xi to pass through it, the optical signal bundle is protected from unauthorized wavelengths. Any wavelength out of the band of the device passed into the add port will be reflected by the thin-film filter into the drop port and away from the output port.

This is advantageous for security purposes. Further, because the second reflective surface 106 is positioned in the path from the add port to the output port in the through position of the switchable member, even light at the selected wavelength cannot be introduced via the add port except in the add/drop state of the switch.

Fig. 14 illustrates an embodiment which provides additional channel add/drop capability in a modification of the Fig. 13 design. In Fig. 14, the WSA/D switch 100' includes two wavelength-selective optical switching assemblies 105,105'coupled in series, each having a thin- film filter tuned to a different wavelength, disposed (optically) between the input port 1 and the output port 4.

Additional add and drop ports 2', 3'and associated GRIN lenses 1022'and 1023'are provided to accommodate the add/drop functionality for the additional wavelength.

In the embodiment of Fig. 14, the through channel signals follow a first optical path including the input port GRIN lens 1021, the first thin-film filter 103, the second thin-film filter 103', and the output port GRIN lens 1024.

The channel signal switchable by the first wavelength- selective switching assembly 105 follows (in a through

state) a path from the input port to the output port that includes the through channel path just described, plus a portion including switchable member Mi'. More particularly, this light follows a path including the input port GRIN lens 1021, a first pass through thin-film filter 103, the first and second mirror surfaces of member Mi, a second pass through thin-film filter 103, and the incidence surface of the second thin-film filter 103', from which it is reflected to the output port GRIN lens 1024. The channel signal switched by the second optical switching assembly 105' follows (in a through state) an analogous path except that the light thereof is reflected by the first thin-film filter 103 and transmitted by the second thin-film filter 103'and redirected by the second switchable member Mi'to the output port GRIN lens 1024.

As will be appreciated from Fig. 14, the basic free- optics design of Fig. 13 advantageously can be expanded simply by inserting additional wavelength-selective switching devices and corresponding add and drop GRIN lenses, without having to go back to fiber.

The arrangements shown in Figs. 13 and 14, incidentally, may not provide adequate optical performance for some applications, because of polarization dependent losses due to the large angles of incidence onto the filter.

However, the free-optics approach can readily be implemented using smaller angles of reflection. Fig. 15 illustrates such an embodiment.

In the embodiment of Fig. 15, a WSA/D switch 300 includes a plurality of wavelength-selective thin-film filters 303a-303c tuned to selected wavelength Xi and mounted on a set of parallel mounting rails 350 to define a zig-zag portion of an optical path coupling the input port 1 and the output port 4. Through channel signals received at port 1 via input line 14 propagate from GRIN lens 3021 to the first filter 303a. Filter 303a, which is reflective to the through channel wavelengths, reflects the signals to filter 303b, from which the signals reflect to filter 303c and then to GRIN lens 3024 for propagation on output line 20.

A switchable member Mi'includes a pair of mirrors 304, 306 mounted to a common movable support platform 307.

Platform 307 is movable, as indicated by a double-headed arrow, between a first position (solid line) corresponding to the through state for the wavelength Xi and a second position (in phantom) corresponding to the add/drop state.

The alternate positions of mirrors 304,306 are not shown in Fig. 15 to simplify the drawing.

In the first position of platform 307, the mirror 304 is disposed between the drop port GRIN lens 3023 and the first thin-film filter 303a, and the mirror 306 is disposed between add port GRIN lens 3022 and the third thin-film filter 303c. Light entering the switch at the selected channel wavelength Xi is initially transmitted by the first filter 303a to propagate toward drop port GRIN lens 3023.

The transmitted light is intercepted, however, by the mirror 304, which reflects the light to mirror 306. Mirror 306 reflects the light back into the through channel path, via filter 303c. The light thus propagates to GRIN lens 3024 and output port 4. Also in this state, the positioning of the mirror 306 will prevent an extraneous signal from being introduced by way of the add port 2.

In the add/drop position, the movable support member 307 is disposed such that mirrors 304,306 do not obstruct the respective optical paths between the drop GRIN lens 3023 and the first thin-film filter 303a and between the add port GRIN lens 3022 and the third thin-film filter 303c.

Accordingly, light entering the switch on selected channel wavelength Xi is transmitted through the first thin-film filter 303a and then propagates to the drop port 3 via the GRIN lens 3023. An additional signal on channel wavelength Xi can be added via the add port 2, from which the added signal will propagate through GRIN lens 3022, the third thin- film filter 303c, and GRIN lens 3024 to the output port 4.

In one variation of the construction shown in Fig. 15, the second thin-film filter 303b can be replaced by a mirror. However, use of the filter as shown may be preferred. In particular, the filter will transmit, and thereby remove, residual light at wavelength Xi not removed at filter 303a, thus permitting use of filters with somewhat reduced transmissivity for wavelength Xi.

The WSA/D switch shown in Fig. 15, like the previous embodiments, provides two optical paths from the input port to the output port. For the through channels, the path includes the input port GRIN lens 3021, the first to third filter elements 303a-303c, and the output port GRIN lens 3024. The switchable channel, on the other hand, follows a second path including the input port GRIN lens 3021, a pass through filter 303a, mirror 304, mirror 306, and a pass through filter 303c to output port GRIN lens 3024. In this design as well, switching of the add/drop channel occurs beyond a thin-film filter transmissive to that channel and not to the through channels, whereby propagation of the through channels is not disturbed by the switching operation.

The arrangement of Fig. 15 is readily expandable by providing additional appropriately tuned thin-film filters on the mounting rails 350 to extend the zig-zag path, and by providing additional add and drop ports, movable mirrors, and GRIN lenses arranged analogously to the corresponding structures in Fig. 15. The output port would, of course, be relocated in correspondence with the end of the extended zig-zag path.

Fig. 16 illustrates an embodiment in which the arrangement of Fig. 15 has been expanded to provide selective add/drop functionality for a second channel at wavelength Aj. Added components corresponding to components controlling the first channel at wavelength Xi are denoted

with corresponding primed reference numbers in Fig. 16. In this embodiment, the signal light at wavelength Xi passing through filter 303c from mirror 306 (through state) or from GRIN lens 3022 (add/drop state) propagates in the zig-zag optical path portion from filter 303c to output port GRIN lens 3024, because the thin-film filters 303a'-303c'are tuned to Aj. The channel at wavelength Xi is switchable between a through state and an add/drop state as described in connection with Fig. 15. The channel at wavelength Xi is switchable, by the corresponding additional components, in the same manner.

Fig. 17 illustrates how the basic serial architecture of the invention can be utilized to provide a node construction that supports redundant communication, as in a bi-directional ring network, for example. Simply speaking, a bi-directional ring network utilizes a plurality of nodes interconnected by one or more pairs of fiber-optic transmission lines to form a ring. The two fiber lines may be installed along different routes and carry information in opposite directions from each other about the ring. This enhances the survivability of the network in the event of multiple faults such as fiber cuts and/or node component failures. For a more comprehensive description of bi- directional networks, including ring networks and network fault protection, see Ramaswami, R. et al., Optical Networks, A Practical Perspective, Morgan Kaufmann Publishers, Inc., 1998 (incorporated herein by reference).

In the arrangement of Fig. 17, a network node N includes a plurality of bi-directional wavelength-selective add/drop devices ADl-ADn. These devices include respective signal processing devices SPD,-SPD,, each constructed to receive and transmit signals on a corresponding one of wavelengths Al-An. The signal processing devices may, for example, be synchronous optical network (SONET) add/drop multiplexing terminals, SONET line terminal equipment, or Internet Protocol (IP) routers. Of course, different types of signal processing devices may be used for the different wavelength channels depending upon the design requirements of the node. The signal processing devices electronically process data received and data to be sent as optical signals over the WDM optical communications network.

Each signal processing device is connected to the respective add and drop ports of a pair of wavelength- selective add/drop (WSA/D) switches for the corresponding optical wavelength channel. Each WSA/D switch belongs to one of two series arrangements provided to switch eastbound and westbound signals, respectively.

The WSA/D switches are each of a construction according to the principles of the invention as previously described.

For example, the construction in any one of Figs. 3,8-10, and 13 may be used, or a plurality thereof may be used in combination. Arrangements such as those shown in Figs. 14 and 16 may, of course, be used to provide add/drop switching

for plural signal processing devices unless discrete switch modules for the individual wavelengths are preferred.

It will be appreciated that the arrangement shown in Fig. 17 can readily be modified and/or expanded to meet changing system requirements. To modify the arrangement, the add/drop devices AD-AD may be arranged in a different series order. Alternatively, one (or more) of the devices may be replaced with a like device operating on a new wavelength (or respective new wavelengths). Still another modification could involve replacing or exchanging the WSA/D switch pair of one or more devices and setting the associated signal processing devices to operate on the respective wavelengths to which their new WSA/D switch pairs are tuned. Expansion is accomplished by adding one or more add/drop devices, each operating at a respective new wavelength, either at the end or at an intermediate point of the series arrangement.

Fig. 18 is a more detailed diagram showing an exemplary construction of a bi-directional wavelength-selective add/drop device ADi of Fig. 17. The device includes a signal processing device SPDi, an eastbound (upper as shown) WSA/D switch, and a westbound (lower as shown) WSA/D switch.

In the illustrative construction, each WSA/D switch includes a wavelength-selective filter assembly tuned to wavelength Xi and an optical switching device Si constituted by a 2x2 fiber-optic switch. Thus, the specific construction may be as explained in connection with any of Figs. 3 and 8-10, for

example. The signal processing device SPDI is connected by add lines 49 and drop lines 48 to the respective 2x2 switches Si of the eastbound and westbound WSA/D switches.

In the through state of the eastbound 2x2 switch, the signal received on the eastbound input fiber 14 at channel wavelength Xi will propagate to the eastbound output fiber 20 for transmission with eastbound through channel signals.

In the add/drop state of the eastbound 2x2 switch, the signal received on channel wavelength Xi is dropped via the west drop line to the signal processing device SPDi. The signal processing device may also introduce a new signal on the same channel via the east add line for propagation with through channels on eastbound output fiber 20. Switching of the westbound 2x2 switch provides the same add/drop functionality for westbound transmissions at channel wavelength X.

The signal processing device SPDiand the optical switches are controlled by a common network management and control system (now shown). The specific control operations of the network management and control system will depend on the type of network involved and its fault protection procedures. For example, in a so-called unidirectional path switched ring (UPSR) network, signal traffic is transmitted concurrently in both the eastbound and westbound directions.

In this case, the signal processing device will process the received signal from one of the drop lines 48 and output any new signal on the selected wavelength channel in both

directions via the add lines 49. In the event of a fault, such as a fiber cut or a 2x2 switch failure on the side of the selected drop line, the signal processing device will switch to a"protect"mode to receive data via the other drop line and continue to transmit new signals via one or both add lines depending upon the failure mode. For a more comprehensive discussion of UPSR and other ring networks, see the aforementioned text by Ramaswami et al.

Figs. 19 and 20 illustrate modified embodiments utilizing optical switching devices Si'that incorporate plural interconnected switches (preferably wavelength- insensitive) to collectively provide the add/drop functionality of the 2x2 switches previously described.

Referring to Fig. 19, each switching device includes two interconnected 1x2 optical switches Sli, S2i. The four lx2 switches are preferably independently powered so that a power failure to one switch will not render any other switch inoperative.

Each switch Sli has an input connected to fiber 44 leading from the corresponding wavelength/selective optical filter assembly to receive the selected signal on wavelength channel Ai, a first output port connected to the corresponding drop line 48, and a second output port connected to an input port of the corresponding switch S2i.

The input port of each switch Sli is switchable between the two output ports thereof so that the signal on the input

port may propagate either to the corresponding switch S2ior to the signal processing device SPDi.

Each switch S2i has a second input port connected to the corresponding add line 49 and an output port connected to the fiber 44 leading back to the corresponding filter assembly. The output port of each switch S2i is switchable between the two input ports so that the signal on either of the input ports may propagate to the corresponding filter assembly for transmission on the corresponding output fiber.

In Fig. 19 the add/drop state of each optical switching device Si'is represented as the solid line state of the respective switch pair Sli, S2i. The through state is represented as the dashed line state.

The construction shown in Fig. 19 increases the ability of the node to tolerate second faults as compared with the construction in Fig. 18. For example, if the eastbound 2x2 switch in Fig. 18 fails (mechanically or due to loss of power to the switch), the add/drop device ADi can still transmit and receive via the westbound 2x2 switch.

But, in the event of a second fault that occurs on the westbound side, such as a westbound fiber cut or a failure of the westbound 2x2 switch, the add/drop device ADi is isolated from the network (cannot transmit and/or receive).

In the add/drop device ADi'of Fig. 19, a single switch failure on the eastbound (or westbound) side will only prevent eastbound (or westbound) reception or transmission, but not both. The remaining switch on that side can still

be used. For example, if the west drop switch fails, the east add switch can still be used for eastbound transmission. Then, the only additional faults on the westbound side that could isolate the add/drop device from the network would be those that prevent westbound reception, such as a cut on input fiber 14'or a failure of the east drop switch. An additional fault that disrupts westbound transmission, such as a cut on output fiber 20'or a failure of the west add switch, would not isolate the add/drop device because the device can still transmit eastbound on output fiber 20 via the east add switch.

Fig. 20 shows an add/drop device ADi"having the same switching arrangement as in Fig. 19. This device differs from that of Fig. 19 in that the west add and drop switches share a power supply and the east add and drop switches share another power supply. The fault tolerance of the Fig.

20 device for mechanical switch failures is similar to that of the Fig. 19 device. However, the tolerance for switch power failures is reduced relative to the Fig. 19 device because of the shared power supply arrangement. Still, the overall reliability is greater than for the device in Fig.

18.

It will be appreciated by those skilled in the art that the preferred embodiments shown and described herein are merely illustrative and that various changes and modifications are possible in keeping with the basic principles and scope of the present invention.