WO2002011332A2

WO2002011332A2 - Wavelength selectable optical add-drop multiplexer

Info

Publication number: WO2002011332A2
Application number: PCT/US2001/023911
Authority: WO
Inventors: Gayle R. Link; Calvin J. Martin; Zelda Gills; Derek Robertson; Marcus W. Shute
Original assignee: Luxcore Networks, Inc.
Priority date: 2000-08-01
Filing date: 2001-07-30
Publication date: 2002-02-07
Also published as: WO2002011332A9; AU2001283039A1; WO2002011332A3

Abstract

In a optical add-drop multiplexer, a tunable wavelength selection module selectively separates a channel or several channels from wavelength division multiplexed (WDM) channels received by the multiplexer. While the separated channel or channels are dropped by the multiplexer, the remaining channels are passed through the multiplexer and coupled to a fiber. Before the pass-through channels are coupled to the fiber, they are transmitted to a wavelength conversion module where they are multiplexed with a locally added channel that has been converted to a wavelength not present among the pass-through channels. For added flexibility, the specific wavelength of the converted channel may be varied by tuning the wavelength conversion module.

Description

WAVELENGTH SELECTABLE OPTICAL ADD-DROP MULTIPLEXER

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to multiplexers for optical communication networks and, more particularly, to multiplexers that can remove wavelength channels from, and add wavelength channels to, wavelength division multiplexed optical signals.

2. Background

The explosive growth of telecommunications is, to a large degree, both a cause and an effect of the proliferation of fiber optic communication systems. Because of its many advantages, silica-based optical fiber has now been used in telecommunications for approximately three decades. The advantages include low signal attenuation, immunity to electromagnetic interference (EMI), low crosstalk, fast propagation speed, physical flexibility, small size, and low weight — all at a reasonable cost.

In a typical optical network, light modulated with a data signal is coupled to a single mode fiber at a source node, transmitted to a destination node, possibly through several intermediate nodes, received at the destination node, demodulated and converted into an electrical data signal. "Light" in the present context includes infrared light; in fact, two of the more commonly used bands are centered around 1550 nanometers and 1310 nanometers, both lying in the near infrared region of the electromagnetic spectrum. The continuing growth of telecommunication services impels service providers to accommodate ever-higher bandwidths requirements. At this time, bandwidth available on a single wavelength channel (i.e., on a single transmission frequency) is increasing from 10 Gbits/s (OC-192/STM-64) to 40 Gbits/s (OC- 768/STM-256). These rates, however, are just small fractions of the total bandwidth potentially available from an optical fiber, which is of the order of 20 Terahertz. As the need for more bandwidth exerts its relentless pressure, wavelength division multiplexed (WDM) systems have evolved to wring more carrying capacity from a single fiber. In WDM systems, separate data channels are transmitted through the same fiber on different wavelengths. As more and more distinct channels are squeezed into a single fiber, narrowband wavelength division multiplexed (NWDM) systems are replaced by dense wavelength division multiplexed (DWDM) systems having as many as 160 wavelength channels at the present time.

The expansion of capacity of existing fiber networks through the use of WDM systems may reduce the need to install more fiber. Moreover, the use of WDM overcomes bandwidth limitations of the existing electronic end-point equipment, because each of the bandwidth channels can be processed separately. These, however, are not the only reasons for using WDM systems. Another reason is that such systems provide much needed flexibility in selecting protocol and network topology. Both topology and protocol selection are severely restricted in telecommunication systems where data of multiple channels are embedded in the same stream. An example of such transmission scheme is the synchronous optical network/synchronous digital hierarchy (SONET/SDH), a three-layer transport network architecture. In a SONET/SDH network, individual data flows, e.g., tributaries, are mapped into payloads and transported across the network's spans in envelopes, in a synchronous time division multiplexed (TDM) manner. The data flows of a SONET/SDH network must therefore be extracted from the envelopes before they can be switched individually.

In contrast, the data format and bit rate of each multiplexed wavelength channel can be independent from formats and rates of other channels propagating in the same fiber, because each multiplexed wavelength channel is independent from other channels. For example, one fiber can carry κ_l5 κ , and κ₃ wavelength channels, where κι is a 2.5 Gbit/s SONET OC-48 channel, κ₂ is a 10 Gbit/s SONET OC-192 channel, and κ₃ is a proprietary format channel. Unlike multiplexed data flows carried by the same wavelength channel, each of the three wavelength channels can be optically routed or switched. In other words, each wavelength channel is not transported as a payload of another communication layer, and therefore can be switched independent of other channels.

Independent switching avoids the need for opto-electronic (O-E) conversion of the aggregate data carried by the fiber, electronic processing of the data, and electro- optic (E-O) conversion for further transmission. The conversions and electronic processing typically require arrays of photodetectors and transponders. Photodetectors optically detect signals, and translate them into electronic signals that can be de-multiplexed and switched electronically. Transponders can then be employed to receive the detected and separated wavelength channels and translate them to different wavelengths for subsequent multiplexing and transmission through appropriate fibers. The use of photodetector and transponder arrays is expensive. Even more important is that photodetectors and transponders are usually wavelength-specific components, requiring a priori knowledge of the wavelengths. Switching flexibility is therefore lost. And redundancy, often needed for reliability expected from modern providers of telecommunication services, becomes a rather costly one-to-one redundancy, instead of the more affordable N-to-M redundancy with N < M.

To benefit from the above-described advantages offered by WDM, many optical networks implement all-optical wavelength-based routing (or wavelength routing) architectures. Such networks can separately route distinct wavelength channels from node to node, across spans, as directed by the routing algorithms used. This means that a wavelength channel can be removed — i.e., "dropped" — from a bundle of WDM channels propagating in one fiber, and multiplexed with — i.e., "added" to — another bundle of WDM channels in a second fiber. Similarly, a channel may be dropped or added at a terminal node, e.g., the channel's origination node, destination node, or an edge device node connecting the WDM network to a legacy network. Optical add-drop multiplexers perform the functions of adding and dropping selected wavelength channels, while allowing other wavelength channels to pass through the node.

A prior art add-drop multiplexer 100 is illustrated in Figure 1. Fiber 105 carries inbound traffic that includes wavelength channels λ_ls λ₂, and λ into a de- multiplexer 110, which separates the three channels and outputs them on three outputs. Each channel is fed into one of 1 x 2 optical switches 130a, 130b, or 130c. Each of the switches 130 can be configured to couple its respective signal either into one of the receivers 150 or into one of the 2 x 1 switches 140. Each of the switches 140 selects either the signal from its corresponding switch 130 or a signal from one of transponders 160, and couples the selected signal into one of the input ports of a multiplexer 120.

To drop a channel, for example, λ_l5 the switch 130a is configured to route λ₁ to the receiver 150a. To add a channel at the same wavelength as the dropped channel, the switch 140a is configured to couple output of transponder 160a to the multiplexer 120. In this way, any combination of the inbound channels can be dropped, and any combination of the outbound channels can be added.

In the basic arrangement of the prior art add-drop multiplexer 100, the pass- through signals are attenuated by the de-multiplexer 110, the multiplexer 120, and two the switches 130 and 140. Practically, the signal losses in the de-multiplexer and in the multiplexer are of the order of 7 dB each, and the loss in each of the switches is approximately 1 dB, for a combined attenuation of about 16 dB. This is a significant number, especially because the pass-through signals have likely already traversed at least one span of the network, and will be traversing at least one more span. It is desirable to provide an optical add-drop multiplexer with low signal attenuation.

Further, the add-drop multiplexer 100 imposes additional constraints on the routing algorithms of the optical network because the receivers and transponders are associated with specific ports of the de-multiplexer and the multiplexer. Thus, ingress and egress port-wavelength associations are rather inflexible. It is desirable to provide an optical add-drop multiplexer with a less rigid port-wavelength association scheme.

Another basic wavelength add-drop multiplexer architecture is illustrated in Figure 2. The multiplexer 200 comprises circulators 210 and 220, and filter 240. The filter 240 transmits all wavelength except λj.

Multiplexed wavelength channels λι...λj...λN are input into a port 212 of the circulator 210. The multiplexed channels propagate through the circulator 210 and couple into a waveguide 230 through a port 214. The channels then encounter the filter 240, which is tuned to reflect wavelength λ; corresponding to one of the channels, and to pass through the remaining channels. The reflected channel λ; returns to the port 214 of the circulator 210, and exits out of port a 216 into a waveguide 250. The channel λj has thus been dropped.

The remaining channels propagate through the waveguide 230, enter the circulator 220 through a port 224, and exit through a port 226. In addition, a new channel λ^ , having the same wavelength as the dropped channel λj enters port 222 of the circulator 220, and exits through the port 224. When the new channel λ_π- encounters the filter 240, it is reflected back towards the port 224 of the circulator 220. The new channel λ^ then exits the port 226 with the other wavelength channels. In this way, one channel has been dropped and another channel at the same wavelength has been added.

The filter 240 may be a combination of filters such that multiple wavelengths can be dropped and added, but the dropped channels will be commingled together at the drop output 216. It is therefore desirable to provide an optical add-drop multiplexer that separates dropped channels.

Yet another add-drop scheme uses a series combination of two multi-layered dielectric filters. Each filter reflects all wavelengths except one, and both filters are tuned to reflect the same wavelengths. When the multiplexed channels strike the first of the two filters, one channel passes through the filter and is dropped. The remaining channels (i.e., the pass-through channels) are reflected towards a first surface of the second filter, and then reflected again towards the output. A new channel is directed at the second surface of the second filter, passing through it to combine with the pass- through channels. One problem with this combination is maintaining physical and wavelength alignment of the two filters. Another problem is that only a single wavelength can generally be dropped and added by the combination.

There are a number of variations removed to various degrees from the basic schemes discussed above. See, for example, U.S. Patent No. 6,154,585 to Copner et al.; U.S. Patent No. 6,091,869 to Sundelin; U.S. Patent No. 6,069,719 to Mizrahi; U.S. Patent No. 5,974,207 to Aksyuk et al.; U.S. Patent No. 5,926,300 to Miyakawa et al.; U.S. Patent No. 5,822,095 to Taga et al.; U.S. Patent No. 5,812,291 to Bendelli et al; U.S. Patent No. 5,726,785 to Chawki et al.; U.S. Patent No. 5,712,717 to Hamel et al.; and U.S. Patent No. 5,612,805 to Fevrier et al. But it appears that all of these schemes suffer from at least one of the disadvantages discussed above. Furthermore, these devices do not provide for wavelength conversion of the added channels, or for primary path fault protection.

SUMMARY OF THE INVENTION The present invention is directed to a wavelength add-drop multiplexer having a wavelength selection module and a wavelength conversion module. The wavelength selection module includes an input port for receiving multiplexed wavelength channels on different wavelengths, a wavelength filter for selecting one or more wavelength channels to be dropped by the add-drop multiplexer through a drop output of the add-drop multiplexer, and a pass-through port for outputting the wavelength channels that have not been dropped.

The wavelength conversion module includes an input port optically coupled to the pass-through port of the wavelength selection module for receiving the pass- through wavelength channels that have not been dropped, and an add port for receiving an add wavelength channel to be multiplexed with the pass-through wavelength channels. The wavelength conversion module further includes a wavelength converter for transforming the add channel to a different, transformed wavelength, and a channel multiplexer for combining the transformed add channel and the pass-through wavelength channels received by the input port. The combined channels are outputted through an output port of the add-drop multiplexer.

BRIEF DESCRIPTION OF THE DRAWINGS The present invention will now be explained, by way of examples only, with reference to the following description, appended claims, and accompanying figures where:

Figure 1, described above, illustrates an optical add-drop multiplexer using a wavelength channel multiplexer, a wavelength channel de-multiplexer, and a plurality of switches;

Figure 2, described above, illustrates an optical add-drop multiplexer using a pair of circulators and a filter;

Figure 3 illustrates a schematic diagram of an embodiment of an add-drop multiplexer in accordance with the present invention; Figure 4 illustrates a schematic diagram of an embodiment of a wavelength selection module for use in the add-drop multiplexer;

Figure 5A illustrates a schematic diagram of another embodiment of a wavelength selection module for use in the add-drop multiplexer;

Figure 5B illustrates a schematic diagram of a third embodiment of a wavelength selection module for use in the add-drop multiplexer;

Figure 6 illustrates a schematic diagram of a fourth embodiment of a wavelength selection module for use in the add-drop multiplexer;

Figure 7 illustrates a schematic diagram of an embodiment of a wavelength conversion module for use in the add-drop multiplexer; Figure 8 illustrates a schematic diagram of a second embodiment of a wavelength conversion module for use in the add-drop multiplexer; and

Figure 9 illustrates a schematic diagram of a network node with two principal and one redundant add-drop multiplexers providing primary path fault protection.

DETAILED DESCRIPTION An add-drop multiplexer 300 in accordance with the present invention is shown in Figure 3. A wavelength selection module 310 receives multiplexed wavelength channels λi . . . λ_N at an input 312. One or more of the multiplexed channels, including λj, are filtered out and dropped via an output 314 of the wavelength selection module 310. The remaining, i.e., pass-through, channels are transmitted to an output 316. The output 316 is coupled to input 332 of a wavelength conversion module 330. In addition to receiving the pass-through channels coupled to its input 332, the wavelength conversion module 330 receives an "add" signal having a wavelength λ_a at input 336. The wavelength conversion module 330 spectrally transforms the add signal from the wavelength λ_a to a wavelength λ_j that is not present among the wavelengths of the pass-through channels. The transformed signal is then multiplexed with the pass-through channels, and the multiplexed channels are outputted from port 334 of the device. If the add-drop multiplexer 300 operates in a blocking manner, i.e., filters out the dropped wavelength channel from the pass-through channels, the wavelength λj can be the same as the dropped wavelength λj.

The wavelength selection module is essentially a filtering element, and may be tunable across a range of wavelengths. Several optical filters are known in the art. One example of an optical filter is a Bragg grating. A Bragg grating will reflect a specific wavelength, while allowing a broad band of surrounding wavelengths to pass through it. Thus, the wavelength selection module 310 can be realized as a combination of a Bragg grating and a circulator for collecting the reflected wavelength channels. A circulator is a multi-port device, with signals propagating in one direction.

In a three-port optical circulator having a first port, a second port, and a third port, in this order, signals input at the first port are transmitted to the second port; and signals input at the second port are transmitted to the third port. But the signals are not transmitted in the reverse direction. For example, a signal input at the third port will not be transmitted to the second port.

Figure 4 illustrates an exemplary embodiment of a wavelength selection module 400 built with a circulator 410 and a Bragg grating 420. Port 412 of the circulator 410 serves as the input to the wavelength selection module 400, while port

416 of the circulator 410 is the "drop" output of the module. Output 424 of the Bragg grating serves as the pass-through output.

The filtering element in the wavelength selection modules 310 and 400 may be a Fabry-Perot resonator (an etalon), i.e., an optical resonator formed by mirrors. Fabry-Perot resonators can be tuned, for example, with low voltage piezoelectric actuators varying the gap between a resonator's mirrors by positioning one or more of the mirrors.

A Fabry-Perot filter can also be tuned by inserting a liquid crystal layer between the opposed mirrors of the filter, and applying an electric field across the layer. The electric field changes the refractive index of the liquid crystal material, thereby changing the resonant frequency of the cavity. Tunable Fabry-Perot liquid crystal filters are described in, for example, U.S. Patents with numbers 5,068,749 and 5,111,321, both to Patel, and U.S. Patent No. 6,154,591 to Kershaw.

Another type of optical filter is a tunable acousto-optical filter. Acousto- optical filters operate based on elasto-optical effect, which is the phenomenon of physical stresses in a material causing changes in the material's refractive index. To take advantage of the elasto-optical effect, radio frequency waves are often used to generate surface acoustic waves in appropriate electro-optic medium, such as LiNbO₃ crystal. The compressions and rarefications of the surface acoustic waves create a temporary grating within the crystal. The temporary grating is tuned by controlling the radio frequency emitter.

United States Patent No. 6,157,025 to Katagiri teaches an optical filter layer deposited on a disc-shaped transparent substrate. The filter layer is such that the center wavelength of the band-pass region varies with the angular dimension of the filter. Rotating the disc in relation to a light beam incident upon it exposes different angular portions of the disc to the beam, thereby changing the center wavelength of the filter. Different wavelengths can thus be selected by rotating the disc.

More generally, a tunable filter can be realized in an arrangement that allows physical movement of a filter element in some dimension in relation to an optical path of a beam of light being filtered. If the center wavelength of the band-pass region of the filter element varies with the dimension, the filter can be tuned by controlling an actuator that moves the filter element in the dimension of interest. The actuator may include a servomechanism, a position encoder, and a controller. The servomechanism moves the filter element, whose position the encoder senses and transmits to the controller. The controller receives the position data from the encoder and directs the servomechanism to place the filter element in accordance with an input signal. See U.S. Patent No. 6,111,997 to Jeong for examples of such tunable filters.

Yet another example of a tunable optical filter is found in U.S. Patent No. 6,058,226 to Starodubov. Starodubov teaches an optical fiber including a core covered by a cladding. A grating within the core couples light either into the cladding or into a coating surrounding the fiber adjacent to the grating, depending on the resonant wavelength of the structure. The resonant wavelength is a function of the refractive index of the coating, which is made of a material whose refractive index varies with an externally controllable stimulus, such as an electric or a magnetic field. A tunable optical filter somewhat similar to that taught by Starodubov is disclosed in U.S. Re-Examined Patent No. RE. 36,710 to Baets et al. Baets's filter is also based on a tunable optical grating embedded in a multi-waveguide structure.

Another type of a tunable optical filter uses an optical splitter to divide a beam into several components. The several components are transmitted through different phase shifters, and then combined. The combined components interfere constructively or destructively, depending on their relative phases, which depend on the phase shifters and on the wavelength of the beam. Controlling the phase shifters tunes such interferometric filter to reject different wavelengths. Still another type of optical filter uses a dielectric multi-layered filter element.

Varying the optical lengths of the layers varies the passband of the filter. A simple method of varying the optical lengths of the layers is to change the angle of incidence of a beam upon the filter element. This can be done by, for example, rotating the filter element. See U.S. Patent No. 5,481,402 to Cheng et al. for a polarization- independent tunable filter based on this principle.

Other tunable optical filters exist, including those based on polarization interference effects. But the precise type of filter or filters is not critical to the operation of the present invention. The wavelength selection module may use a fused fiber optical power splitter/coupler in combination with one or more filters to perform the function of dropping one or more channels. This scheme is illustrated in Figure 5A, where a wavelength selection module 500 includes a power splitter 510 and a filter 520. The aggregate multiplexed signal is fed into an input 512 of the splitter 510, which divides the power between a pass-through output 514 and a "drop" output 516. The-pass through output 514 is filtered by the filter 520 to remove the dropped wavelength λ , providing blocking operation. The output 516 may be filtered by filter 530 to isolate the dropped wavelength λ_<j. The power splitter 510 may have a plurality of drop outputs for dropping a plurality of channels. This is illustrated in Figure 5B. In such case, the filter 520 may have several band-reject areas for filtering out multiple wavelength channels.

Because the power splitter inherently attenuates both the pass-through and the dropped channels, active fiber filler may be provided within the power splitter to amplify all the multiplexed channels, only the pass-through channels, or only the dropped channel. Figure 6 illustrates the case with active fiber filler 640 located within a drop output 616 to amplify only the dropped wavelength channel. This arrangement allows the power splitter to be designed with a relatively small portion of the total power, e.g., less than 10%, to be diverted into the drop output 616, thereby mimmizing the power losses incurred by the pass-through channels. At the same • time, the effect on the signal-to-noise ration of the dropped channel is also minimized, because the amplified spontaneous emissions (ASE) generated in the active fiber 640 are suppressed by a bandpass filter 630. The filter 630 may be made relatively narrow-band, with a passband just wide enough to transmit only the dropped channel or channels.

Typical active fiber is fiber doped with rare earth element ions. The doped fiber becomes fluorescent, meaning that it can absorb excitation energy at one wavelength and emit the absorbed energy at a different wavelength. For optical amplification, active fiber is excited or "pumped" by a source of light ("optical pump"), e.g., a diode laser, at a wavelength other than the wavelengths of the multiplexed channels, elevating the energy states of the fiber's constituent particles. The particles then emit light triggered by the propagating channels at the channels' wavelengths, thus amplifying the channels. Fluorescent dopants often used in active fiber of non-coherent optical systems operating in the 1310 nm and 1550 nm bands are erbium and praseodymium.

We turn now to a discussion of the wavelength conversion module. In Figure

7, an embodiment of the module 700 comprises a combining unit 710 and a wavelength converting unit 720. The combining unit 710 is depicted as a circulator, but may be any kind of an optical power combining mechanism, including, for example, a fused fiber optical power coupler.

The wavelength converting unit 720 converts an "add" channel at a wavelength λ_a input at a port 722 into a channel at a wavelength not present among the pass-through channels input into the module. Several methods of optical wavelength conversion are known to those of ordinary skill in the art, including the following: difference frequency mixing, cross-gain modulation, cross-phase modulation, and four- wave mixing.

Difference frequency mixing manipulates second-order nonlinearities in a quasi-phasematching structure to mix a modulated information-carrying signal at a free-space wavelength λ_s (corresponding to angular frequency ω_s) with a locally- generated continuous wave pump signal at a wavelength κ_p (corresponding to ξ_p) to obtain a difference product at an angular frequency of ξ_p-ξ_s and a wavelength κ_p-s.

Note that in this document, we designate the wavelength corresponding to the angular frequency of ξ_p-ξ_s as κ_p-s, despite the fact that in the wavelength domain the frequency relationships are inverted.

The technique of optical difference frequency mixing is described in the commonly-assigned patent application entitled Optical Wavelength-Converting Apparatus of the present inventors. The disclosure of that patent application is hereby incorporated by reference as if fully set forth herein. Additional information is available in Martin M. Fejer et al., Quasi-Phase-Matched Second Harmonic Generation: Tuning and Tolerances, 28 J. QUANTUM ELEC. 2631-54 (LEEE 1992); and in U.S. Patent No. 5,815,307 to Arbore et al., which patent is hereby incorporated by reference as if fully set forth herein. Cross-gain modulation and cross-phase modulation are two related techniques of wavelength conversion (or, more accurately, translation) that use nonlinear effects of semiconductor optical amplifiers (SOAs). In an SOA, light is amplified by stimulated emissions when the light propagates in an active region of a forward- biased p-n semiconductor junction. Using the nomenclature of the above example, when the modulated signal and the pump signal at a different wavelength enter an SOA, the presence of one wavelength will deplete the minority carrier concentration by the stimulated emission process, so that the population inversion experienced by the other signal will be reduced. The carrier populations are restored by spontaneous emissions from a high-energy state to a low-energy state, which process in many instances has a lifetime of the order of a nanosecond. As the input power of the first one of the two signals increases, carriers in the gain region of the SOA get depleted, resulting in gain-saturation with a concomitant reduction in the amplitude of the second signal. Conversely, a reduction in the power level of the first signal results in an increase in the power level of the second signal. Because carrier fluctuations happen quickly, typically in a picosecond timeframe, the gain experienced by the pump signal will respond to fluctuations in the information-carrying signal on a bit- by-bit basis. Thus, the amplified pump signal will be modulated with the logically- inverted pattern of the modulation of the information-carrying signal. This effect is known as wavelength conversion through cross-gain modulation.

In a typical cross-phase modulation wavelength conversion scheme, two SOAs are built into two arms of a Mach-Zehnder interferometer. The interferometer is adjusted so that the signals at the pump wavelength add destructively at its output, canceling each other. The modulated signal is injected into one of the arms of the Mach-Zehnder interferometer, modulating the refractive index experienced by the pump signal in the SOA of that arm. The interferometer is now unbalanced, and its pump wavelength output becomes modulated by the data of the information-carrying signal. For more information on cross-gain and cross-phase wavelength conversion techniques, the reader is referred to B. Mikkelson et al, Polarisation insensitive wavelength conversion of lOGbit/s signal with SOAs in a Michelson interferometer, 30 ELEC LETTERS, 260-61 (Feb. 1994); and to T. Durhus et al, All Optical Wavelength Conversion by SOA's in a Mach-Zehnder Configuration, 6 PHOTONICS TECH. LETTER, 53-55, (JEEE Jan. 1994). Both articles are hereby incorporated by reference as if fully set forth herein.

The fourth wavelength conversion technique is four-wave mixing. In short, the field intensity pattern of two interfering pump signal waves (with free-space wavelength of κ_p) form a grating in an SOA or in a nonlinear medium. The grating can be a population density grating or a refractive index grating. The modulated information-carrying signal with a wavelength λ_s and an angular frequency ω_s is scattered by the grating, resulting in a scattered wave with an angular frequency equal to of 2ξ_p-ξ_s. The modulation of the scattered wave corresponds to a spectral content that is a phase conjugate of the spectral content of the information-carrying signal. For a more detailed treatment of the four-wave mixing technique, see Govind P. Agrawal, Population pulsations and non degenerate four-wave mixing in semiconductor laser and amplifiers, 5 OPT. Soc'Y AM. B, 147-59 (Jan. 1988); and Jianhui Zhou et al., Four-Wave Mixing Wavelength Conversion Efficiency in Semiconductor Traveling-Wave Amplifiers Measured to 65 nm of Wavelength Shift, 6 PHOTONICS TECH. LETTERS, 984-87 (LEEE Aug. 1994). These two articles are hereby incorporated by reference as if fully set forth herein.

To increase the flexibility of the add-drop multiplexer, the pump signals used in any of the conversion schemes can be made tunable, so that the added signal can be converted to one of a plurality of wavelength channels.

Returning to the discussion of the wavelength conversion module, an equalizer may be employed to bring the power levels of the added channel λj and of the pass- through channels into relative parity. The equalizer may include an adjustable attenuator or an adjustable amplifier, and an optical power sensor. Figure 8 illustrates a wavelength conversion module 800 having an equalizer 815 interposed between a power combining unit 810 and a wavelength converting unit 820.

Several add-drop multiplexers in accordance with the present invention may be combined in the same network node. First, two or more add-drop multiplexers may process WDM channels of a fiber in series, one after another, each multiplexer dropping and/or adding different channels. Second, the add-drop multiplexers may process channels from different fibers connected to the node. This is illustrated in Figure 9, where add-drop multiplexers 910 and 930 receive WDM channels from fibers 901 and 905 via switches 940 and 950, respectively. The multiplexers 910 and 930 then output the processed WDM channels through fibers 902 and 904. A third add-drop multiplexer 920 acts in conjunction with a combiner 960 and the switches 940 and 950 to provide path fault protection through redundancy. For example, if the multiplexer 910 or the fiber 902 fails, the WDM channels inbound on the fiber 901 may be switched to the multiplexer 920 and fiber 903. In this configuration, the combiner 960 may be, for example, a coupler or a switch.

We have described the inventive add-drop multiplexer and some of its features in considerable detail for illustration purposes only. Neither the specific embodiments of the invention as a whole nor those of its features limit the general principles underlying the invention. In particular, the invention is not limited to specific regions of the light spectrum mentioned in this document, or to use in WDM optical transmission systems. The specific wavelength-converting techniques, filters, power splitters, and couplers described may be used in some embodiments, but not in others, without departure from the spirit and scope of the invention as set forth. Different geometries of the optical splitters also fall within the intended scope of the invention, and components such as the filters and the wavelength-conversion modules may, but need not, be tunable. Many additional modifications are intended in the foregoing disclosure, and it will be appreciated by those of ordinary skill in the art that in some instances some features of the invention will be employed in the absence of a corresponding use of other features. The illustrative examples therefore do not define the metes and bounds of the invention, which function has been reserved for the following claims and their equivalents.

Claims

We claim:

1. An optical add-drop multiplexer comprising: a wavelength selection module comprising an input port for receiving a plurality of wavelength channels, each channel being on a different wavelength, the plurality of wavelength channels comprising a drop subset of wavelength channels, the drop subset comprising at least a first wavelength channel on a first wavelength, a wavelength filter for selecting the drop subset of wavelength channels from the plurality of wavelength channels, a drop port for outputting the drop subset of wavelength channels, and a pass-through port for outputting at least one wavelength channel of the plurality of wavelength channels; and a wavelength conversion module comprising an input port optically coupled to the pass-through port of the wavelength selection module for receiving the at least one wavelength channel outputted by the pass-through port, an add port for receiving an add wavelength channel on an add wavelength, a wavelength converter for transforming the add wavelength channel to a transformed wavelength, a channel multiplexer for combining the transformed add wavelength channel and the at least one wavelength channel received by the input port, and an output port for outputting the combined wavelength channels.

2. An add-drop multiplexer according to claim 1, wherein the wavelengths of the at least one wavelength channel outputted from the pass-through port do not include the transformed wavelength.

3. An add-drop multiplexer according to claim 2, wherein wavelength associated with the first wavelength channel is substantially identical to the transformed wavelength.

4. An add-drop multiplexer according to claim 1, wherein the wavelength filter comprises a tunable wavelength filter capable of varying wavelengths in the drop subset.

5. An add-drop multiplexer according to claim 4, wherein the tunable wavelength filter comprises a tunable Bragg grating for reflecting the first wavelength channel and a circulator for receiving the reflected first wavelength channel and guiding the reflected first wavelength channel to the drop port.

6. An add-drop multiplexer according to claim 4, wherein the tunable wavelength filter comprises a tunable Fabry-Perot resonator.

7. An add-drop multiplexer according to claim 4, wherein the tunable wavelength filter comprises a tunable acousto-optical filter.

8. An add-drop multiplexer according to claim 4, wherein the tunable wavelength filter comprises a band-pass filter element positioned in the optical path of the plurality of wavelength channels, the filter element having a dimension-dependent filter function, the filter element being movable to vary the filter function experienced by the plurality of wavelength channels, thereby varying wavelengths of the drop subset.

9. An add-drop multiplexer according to claim 4, wherein the tunable wavelength filter comprises a multi-waveguide structure.

10. An add-drop multiplexer according to claim 9, wherein the tunable wavelength filter further comprises a grating embedded in the multi-waveguide structure.

11. An add-drop multiplexer according to claim 4, wherein the tunable wavelength filter comprises a first multi-layered dielectric filtering element positioned in the optical path of the plurality of wavelength channels.

12. An add-drop multiplexer according to claim 11, wherein the tunable wavelength filter further comprises a second multi-layered dielectric filtering element positioned in the optical path of wavelength channels reflected by the first multi- layered dielectric element.

13. An add-drop multiplexer according to claim 4, wherein the tunable wavelength filter comprises an interferometric filter comprising a splitter for dividing power of the plurality of wavelength channels between a first filter path and a second filter path, a first adjustable phase shifter in the first filter path, and a combiner for combining the divided power of the plurality of wavelength channels after the wavelength channels of the plurality of wavelength channels exit the first and the second filter paths.

14. An add-drop multiplexer according to claim 1, wherein the wavelength selection module further comprises: a fused fiber optical power splitter comprising a first input for receiving the plurality of the wavelength channels, a first output path for outputting the at least one wavelength channel, and a second output path for outputting the first wavelength channel; and a first drop channel band-pass filtering element with a first pass-band capable of transmitting the first wavelength channel, the first drop channel band-pass filtering element being positioned in the second output path of the fused fiber optical power splitter.

15. An add-drop multiplexer according to claim 14, wherein: the drop subset further comprises a second wavelength channel; the fused fiber optical power splitter further comprises a third output path for outputting the second wavelength channel; the wavelength selection module further comprises a second drop channel band-pass filtering element with a second pass-band capable of transmitting the second wavelength channel, the second drop channel band-pass filtering element being positioned in the third output path of the fused fiber optical power splitter.

16. An add-drop multiplexer according to claim 14, wherein the wavelength selection module further comprises active fiber filler for amplifying the first wavelength channel.

17. An add-drop multiplexer according to claim 14, wherein the first drop channel band-pass filtering element is a tunable filtering element with a variable center wavelength.

18. An add-drop multiplexer according to claim 1, wherein the channel multiplexer of the wavelength conversion module comprises a fused fiber optical combiner.

19. An add-drop multiplexer according to claim 1, wherein the channel multiplexer of the wavelength conversion module comprises a circulator.

20. An add-drop multiplexer according to claim 1, wherein the wavelength converter comprises an optical pump source and a quasi-phasematching structure for performing difference frequency mixing of light outputted by the optical pump source and the add wavelength channel to obtain the transformed add wavelength channel.

21. An add-drop multiplexer according to claim 20, wherein the optical pump source comprises a tunable optical pump source capable of generating light in a range of wavelengths.

22. An add-drop multiplexer according to claim 1, wherein the wavelength converter comprises an optical pump source and a semiconductor optical amplifier for performing wavelength translation through cross-gain modulation.

23. An add-drop multiplexer according to claim 22, wherein the optical pump source comprises a tunable optical pump source capable of generating light in a range of wavelengths.

24. An add-drop multiplexer according to claim 1, wherein the wavelength converter comprises: an optical pump source generating light at the third wavelength; and a Mach-Zehnder interferometer comprising: a first waveguiding arm, a second waveguiding arm, an optical power divider for receiving light generated by the optical pump source and dividing the received light between the first arm and the second arm, a first semiconductor optical amplifier in the first arm, a second semiconductor optical amplifier in the second arm, an input for receiving the add wavelength channel and directing the add wavelength channel into the first arm, and a power combiner for combining light output by the first and the second arms so that in the absence of the add wavelength channel the light output by the first arm at the third wavelength substantially cancels the light output by the second arm at the third wavelength, the Mach-Zehnder interferometer thereby transforming the add wavelength channel to the third wavelength through cross-phase modulation.

25. An add-drop multiplexer according to claim 24, wherein the optical pump source comprises a tunable optical pump source capable of generating light in a range of wavelengths.

26. An add-drop multiplexer according to claim 1, wherein the wavelength converter comprises an optical pump source generating light at a pump frequency, a nonlinear propagation medium for four- wave mixing the add wavelength channel and the light generated by the optical pump source to transform the add wavelength channel to the third wavelength, the third wavelength corresponding to a third frequency, the third frequency being equal to twice the pump frequency less a frequency corresponding to the second wavelength.

27. An add-drop multiplexer according to claim 26, wherein the optical pump source comprises a tunable optical pump source capable of generating light in a range of wavelengths.

28. An add-drop multiplexer according to claim 1, wherein the wavelength converter comprises an optical pump source generating light at a pump frequency, an optical amplifier for four-wave mixing the add wavelength channel and the light generated by the optical pump source to transform the add wavelength channel to the third wavelength, the third wavelength corresponding to a third frequency, the third frequency being equal to twice the pump frequency less a frequency corresponding to the second wavelength.

29. An add-drop multiplexer according to claim 28, wherein the optical pump source comprises a tunable optical pump source capable of generating light in a range of wavelengths .

30. An add-drop multiplexer according to claim 1, wherein the wavelength conversion module further comprises an equalizer for adjusting power level of the transformed add wavelength channel into substantial parity with power of the at least one wavelength channel combined with the transformed add wavelength channel in the channel multiplexer of the wavelength conversion module.

31. An optical add-drop multiplexer comprising: a wavelength selection module means comprising a main output and a drop output, the wavelength selection module means being for receiving a plurality of wavelength channels, each channel on a different wavelength, for separating a drop subset of the plurality of wavelength channels, the drop subset comprising at least a first wavelength channel, for outputting the drop subset through the drop output, and for outputting at least one wavelength channel of the plurality of wavelength channels through the main output, the at least one wavelength channel not belonging to the drop subset; and a wavelength conversion module comprising an input port coupled to the main output of the wavelength selection module means to receive the at least one wavelength channel output from the main output, an add port for receiving an add wavelength channel having a second wavelength, a wavelength converter means for transforming the add wavelength channel to a third wavelength, a channel multiplexer means for combining the transformed add wavelength channel and the at least one wavelength channels received by the input port, and an output port for outputting the combined wavelength channels.

32. An optical add-drop multiplexer according to claim 31, wherein the wavelength converter means comprises a tunable wavelength converter means capable of transforming the add wavelength channel to the third wavelength in a range of wavelengths.

33. An optical add-drop multiplexer according to claim 32, wherein the wavelength selection module means further comprises a tunable filter, the wavelength selection module means being capable of varying wavelengths associated with the drop subset.

34. An optical add-drop multiplexer means according to claim 33, wherein the wavelength conversion module further comprises an equalizer means for adjusting power level of the transformed add wavelength channel into substantial parity with average power of the wavelength channels combined with the transformed add wavelength channel in the multiplexer of the wavelength conversion module.

35. An optical node comprising: a first optical switch comprising a first switch input, a first switch upper output, and a first switch lower output, the first optical switch being for selectively switching a first plurality of wavelength channels from a first inbound fiber to the first switch upper output and the first switch lower output, each channel of the first plurality being on a different wavelength; a first wavelength selection module means coupled to the first switch upper output, the first wavelength selection module comprising a first main output, and a first drop output, the first wavelength selection module means being for receiving the first plurality of wavelength channels, for separating a first drop subset of the first plurality of wavelength channels, the first drop subset comprising at least a first wavelength channel, for outputting the first drop subset through the first drop output, and for outputting at least one wavelength channel of the first plurality of wavelength channels through the first main output, the at least one wavelength channel of the first plurality of wavelength channels not belonging to the first drop subset; a first wavelength conversion module comprising a first input port coupled to the first main output of the first wavelength selection module means to receive the at least one wavelength channel of the first plurality output from the first main output, a first add port for receiving a first add wavelength channel having a first add wavelength, a first wavelength converter means for transforming the first add wavelength channel to a first transformed wavelength, a first channel multiplexer means for combining the first transformed add wavelength channel and the at least one wavelength channel received by the first input port, and a first output port for outputting the combined transformed first add wavelength channel and the at least one wavelength channel received by the first input port; a second optical switch comprising a second switch input, a second switch upper output, and a second switch lower output, the second optical switch being for selectively switching a second plurality of wavelength channels from a second inbound fiber to the second switch upper output and the second switch lower output, each channel of the second plurality being on a different wavelength; a second wavelength selection module means coupled to the second switch lower output, the second wavelength selection module comprising a second main output and a second drop output, the second wavelength selection module means being for receiving the second plurality of wavelength channels, for separating a second drop subset of the second plurality of wavelength channels, the second drop subset comprising at least a second wavelength channel, for outputting the second drop subset through the second drop output, and for outputting at least one wavelength channel of the second plurality of wavelength channels through the second main output, the at least one wavelength channel of the second plurality of wavelength channels not belonging to the second drop subset; a second wavelength conversion module comprising a second input port coupled to the second main output of the second wavelength selection module means to receive the at least one wavelength channel of the second plurality output from the second main output, a second add port for receiving a second add wavelength channel having a second add wavelength, a second wavelength converter means for transforming the second add wavelength channel to a second transformed wavelength, a second channel multiplexer means for combining the second transformed add wavelength channel and the at least one wavelength channel received by the second input port, and a second output port for outputting the combined transformed second add wavelength channel and the at least one wavelength channel received by the second input port; a combining means comprising a first combining means input coupled to the first switch lower output, a second combimng means input coupled to the second switch upper output, and a combimng means output, the combining means being for coupling pluralities of wavelength channels from the first combining means input and the second combining means input into the combimng means output; a third wavelength selection module means coupled to the combining means output, the third wavelength selection module comprising a third main output and a third drop output, the third wavelength selection module means being for receiving a plurality of wavelength channels from the combimng means output, for separating a third drop subset of the plurality of wavelength channels from the combining means output, the third drop subset comprising at least a third wavelength channel, for outputting the third drop subset through the third drop output, and for outputting at least one wavelength channel of the plurality of wavelength channels from the combining means output through the third main output, the at least one wavelength channel of the plurality of wavelength channels from the combining means output not belonging to the third drop subset; and a third wavelength conversion module comprising a third input port coupled to the third main output of the third wavelength selection module means to receive the at least one wavelength channel of the plurality of wavelength channels from the combining means output, a third add port for receiving a third add wavelength channel having a third add wavelength, a third wavelength converter means for transforming the third add wavelength channel to a third transformed wavelength, a third channel multiplexer means for combining the third transformed add wavelength channel and the at least one wavelength channel received by the third input port, and a third output port for outputting the combined transformed third add wavelength channel and the at least one wavelength channel received by the third input port.