♦ The Wavelength Add/Drop Multiplexer for
`Lightwave Communication Networks
`C. Randy Giles and Magaly Spector
`
`Lightwave systems are progressing toward optical networks capable of manipulating
`data paths by optical means rather than by traditional electronic switching. This is
`facilitated by wavelength multiplexed transmission, in which narrow bandwidth
`optical filters can be used to remove specific channels and reinsert new ones
`anywhere in the optical link. Wavelength add/drop multiplexers performing this
`optical channel processing can range in capability from providing dedicated
`add/drop of a single channel to having fully reconfigurable add/drop of many, if not
`all, of the wavelength division multiplexed (WDM) channels. Careful placement of
`wavelength add/drop multiplexers can dramatically improve a network’s flexibility
`and robustness while providing significant cost advantages. This paper summarizes
`the rationale for incorporating wavelength add/drop multiplexers in modern optical
`networks, outlines their logical and optical characteristics, and introduces the
`predominant technology choices.
`
`Introduction
`The astonishing demand for lightwave communi-
`cation networks has spawned aggressive efforts to
`invent desperately needed optical components and
`subsystems. In this paper, we will concentrate on
`wavelength add/drop multiplexers (WADMs)—
`versatile optical subsystems that facilitate the evolution
`of lightwave systems from single-wavelength point-to-
`point transmission lines to wavelength division multi-
`plexed (WDM) optical networks. The need for greater
`flexibility in wavelength management is apparent con-
`sidering the enormous transmission capacity of optical
`fibers that can carry hundreds of WDM channels. The
`resultant fiber capacity, now in the terabit/second
`range, can exceed that required to simply connect two
`network nodes; more economical fiber utilization is
`needed. WADMs facilitate management of fiber capac-
`ity by enabling the selective removal and reinsertion
`of WDM channels at intermediate points in the line
`system. There are also many new advantages for pro-
`visioning and protecting a network by manipulating
`the optical granularity created by the wavelength mul-
`tiplexing of channels.
`
`Lucent Technologies’ WaveStarTM 400G optical
`line system (OLS)—an 80-channel, 400-Gb/s aggre-
`gate capacity system—exemplifies the introduction of
`WADM technology. Initial deployment of WaveStar
`400G will have a fixed 4-channel add/drop capability
`dispersed along the optical link, and later releases will
`include a 16-WDM-channel rearrangable add/drop
`multiplexer. Similar capabilities are expected from
`other lightwave system manufacturers as optical net-
`works evolve to capitalize on the advantages of wave-
`length multiplexed signals.
`Metropolitan WDM lightwave services constitute
`another area of intense activity where interoffice and
`business premises wavelength add/drop plays an
`important role. Proposals range from rearrangable
`add/drop management of 1 to 8 channels in a small
`business access ring to complete add/drop manage-
`ment of 40 or more channels in an interoffice ring.
`Furthermore, each WDM channel may carry different
`data rates and formats as expected in a shared media
`serving diverse business needs. This breadth of appli-
`cations and the urgency to deploy WADMs demand a
`
`Copyright 1999. Lucent Technologies Inc. All rights reserved.
`
`Bell Labs Technical Journal N January–March 1999 207
`
`Petitioner Ciena Corp. et al.
`Exhibit 1035-1
`
`

`

`Time
`
`Single span point-to-point
`transmission
`
`Amplified multiple span
`WDM transmission
`
`Amplified WDM
`transmission
`with WADM nodes
`
`WDM network with
`multiple WADM nodes
`and WDM optical
`cross-connect capability
`
`Through
`Drop Add
`
`WADM
`
`WADM
`
`WADM
`
`WDM
`OXC
`
`WDM
`OXC
`
`WADM
`
`WADM
`
`WADM
`
`OXC – Optical cross connect
`WADM – Wavelength add/drop multiplexer
`WDM – Wavelength division multiplexer
`
`Figure 1.
`Evolution of fiber-optic transmission from single-span transmission to optical networking.
`
`methodical evaluation of technology options similar to
`that which we will develop in this paper.
`To illustrate where we have come from and where
`we are headed in lightwave communications, Figure 1
`depicts the evolutionary course of fiber-optic systems
`and networks, beginning with single-channel point-to-
`point transmission systems and leading to optical net-
`working. Ten years ago, a long distance fiber-optic
`transmission system consisted of a series of optical
`transmitters and receivers linked through short fiber
`spans. The individual span lengths rarely exceeded
`40km since laser transmitter power was limited to
`1mW, and practical optical amplifier repeaters were
`unavailable. Consequently, at that time the main ben-
`efit to incorporating multiple WDM channels in a
`single fiber was to increase the overall optical
`
`208 Bell LabsTechnical Journal u January–March 1999
`
`Panel 1. Abbreviations, Acronyms, and Terms
`MONET—Multiwavelength Optical Networking
`NRZ—nonreturn to zero
`OC-192—optical carrier digital signal rate of
`9.953 Gb/s in a SONET system
`OC-48—optical carrier digital signal rate of
`2.488 Gb/s in a SONET system
`OLS—optical line system
`RF—radio frequency
`SONET—synchronous optical network
`SPM—self-phase modulation
`WADM—wavelength add/drop multiplexer
`WDM—wavelength division multiplexed/
`multiplexing
`WIS—wavelength-independent switch
`WSS—wavelength-selective switch
`
`Petitioner Ciena Corp. et al.
`Exhibit 1035-2
`
`

`

`Panel 2. Nomenclature of WADMs
`Add-channel: WDM channel inserted locally,
`appearing at the out-port WDM stream.
`Add-port: WADM input port carrying channels to be
`added to the optical stream appearing at the out-port.
`Branching function: The capability of selecting
`one or more dropped WDM channels to exit
`from a single drop-port.
`Drop-and-continue function: The capability to
`simultaneously drop and continue (pass on to
`the out-port) a particular WDM channel through
`the WADM.
`Drop-channel: WDM channel removed from the
`in-port WDM stream.
`Drop-port: WADM output port carrying channels
`removed from the input optical stream.
`East-west separability: A design specification
`requiring that the reliability, maintenance, and
`upgrade of the in- and drop-ports be autonomous
`from that of the out- and add-ports. East-west
`separability prevents unprotected failures and
`maintenance procedures that could otherwise
`occur in some optical networks.
`Fixed WADM: A WADM permanently configured
`to drop, add, and express preassigned WDM
`channels.
`Flexible WADM: A WADM that can be scaled
`with minimum intervention to accommodate
`varying numbers of add/drop channels. An
`example is a set of serially connected single-
`channel WADM modules. Both fixed and
`reconfigurable WADM may be flexible.
`In-port: WADM input optical port.
`Noninterrupting reconfigurable WADM: A
`reconfigurable WADM that interrupts service
`
`during reconfiguration only on the WDM chan-
`nels being reconfigured.
`Optical through: WDM channels propagate
`through the WADM only as optical signals.
`Optoelectronic through: WDM channels propa-
`gate through the WADM with optical-to-
`electrical-to-optical conversion.
`Out-port: WADM output port carrying the out-
`put optical stream altered by the add/drop
`function.
`Reconfigurable WADM: A WADM that can be
`reconfigured—manually or automatically—to
`change the drop, add, and express conditions for
`various WDM channels.
`Remotely reconfigurable WADM: A WADM that
`can be programmatically reconfigured through
`the network software to change the drop-, add-,
`and through-states for various WDM channels.
`Through- (continue-, express-) channel: WDM
`channel carrying the same information payload
`from in-port to out-port of the WADM.
`WADM input state: The state defined by the
`channels present at the in- and add-ports of the
`WADM.
`WADM node operational state: The state of the
`WADM node defined by the input and output
`states and the connection matrix C.
`WADM output state: The state defined by the
`channels present at the out- and drop-ports of
`the WADM.
`Wavelength-reuse WADM: A WADM that
`accommodates drop- and add-channels at the
`same wavelengths.
`
`transmission capacity. The introduction of WDM
`transmission was challenging as it required new opti-
`cal components (multiplexers, demultiplexers, and
`improved laser sources), and there was strong compet-
`itive pressure from increasing single-channel (time
`division multiplexing and optical time division multi-
`plexing) bit rates. Transmitting farther than 40 km
`raised unavoidable costs as signals had to be processed
`
`through expensive optoelectronics on a per-wavelength
`channel basis.
`The discovery of erbium-doped fiber amplifiers
`provided compelling reasons to employ WDM signal-
`ing. With optical amplifiers, optical transmission reach
`was extended to thousands of kilometers, allowing
`widely separated regions to exchange large quantities
`of voice and data at a reasonable expense. Technology
`
`Bell Labs Technical Journal u January–March 1999
`
`209
`
`Petitioner Ciena Corp. et al.
`Exhibit 1035-3
`
`

`

`In[1]
`
`Out[2]
`
`WADM
`
`Channels 1, 2, 4
`
`In[1]
`
`3
`
`3
`
`Out[2]
`
`Add[3]
`
`Drop[4]
`
`Add[3]
`
`Drop[4]
`
`(a) Channel connections from input ports (In[1]
`and Add[3]) to output ports (Out[2] and Drop[4])
`
`(b) Example of 4-channel ADM
`with channel 3 drop and add
`
`ADM – Add/drop multiplexer
`WADM – Wavelength add/drop multiplexer
`
`Figure 2.
`Four-port model of the WADM.
`
`has also responded to the demands for higher capacity,
`and terabit/second transmission on a single optical fiber
`is now feasible.1 While petabit/second data exchange in
`dense metropolitan areas has been considered,2 near-
`term expectations are for data exchange rates between
`major centers to be around a terabit/second, commen-
`surate with the capacity of a single optical fiber. The
`WADM enables greater bandwidth efficiency by allow-
`ing this capacity to interconnect geographically diverse
`centers along the fiber transmission link. Inefficient
`loopback of data streams to smaller nodes between
`major nodes can also be avoided.
`The WDM channels used to communicate
`between nodes in a network can be permanently pro-
`visioned or adapted to changing network conditions. A
`fixed WADM is appropriate in the first case, facilitating
`the removal and reinsertion of data streams on dedi-
`cated WDM channels. This capability—fixed wave-
`length add/drop of selected channels—is the state of
`the art for commercial systems
`in 1999.
`Reconfigurable wavelength add/drop—the ability to
`manually or programmatically alter the wavelength
`connections through the WADM—has been widely
`demonstrated and sought for imminent deployment.
`Flexible optical provisioning—the ability to set up and
`tear down wavelength connections to follow traffic
`demands in a network for efficient capacity
`utilization—is one advantage of reconfigurable
`
`add/drop multiplexers. Reconfigurable WADMs can
`also be used for optical restoration, providing the abil-
`ity to reroute traffic around failed lines or nodes.
`Details concerning provisioning and restoration using
`WADMs are vague, as they must consider the full net-
`work, not just the behavior of the WADM. For exam-
`ple, synchronous optical network (SONET) rings
`already incorporate protection mechanisms through
`spare channels and fibers, and the interaction with
`optical protection using a WADM must be understood.
`We will not attempt to resolve these network issues in
`this paper; instead, we will focus on describing the
`WADM as an optical component and introducing a
`few of the predominant technology choices.
`
`Functional and Logical Descriptions of the WADM
`The complexity inherent in lightwave optical net-
`works and subsystems is captured in the nomenclature
`describing their function and operation. For this dis-
`cussion, we adopt a vocabulary to distinguish key
`attributes of WADMs in order to associate WADM
`characteristics with their functions in the network. The
`definitions, listed in Panel2,also help to classify the
`technology options as viewed from the perspective of
`system architecture.
`A wavelength add/drop multiplexer is character-
`ized in terms of total numbers of input-, through-,
`drop-, and add-channels (WDM data streams). The
`
`210 Bell LabsTechnical Journal u January–March 1999
`
`Petitioner Ciena Corp. et al.
`Exhibit 1035-4
`
`

`

`C1
`
`C2
`
`C1
`
`C2
`
`In[1]
`
`Out[2]
`
`WADM
`
`Add[3]
`
`Drop[4]
`
`Channel
`
`10
`
`11
`
`12
`
`13
`
`14
`
`15
`
`16
`
`0 1 0 0
`
`1 0 0 0
`
`0 1 0 0
`
`1 0 0 0
`
`0 1 0 0
`
`1 0 0 0
`
`0 1 0 0
`
`1 0 0 0
`
`1 0 0 0
`
`1 0 0 0
`
`1 0 0 0
`
`0 1 1 0
`
`1 0 0 0
`
`1 0 0 0
`
`9 1 0 0 0
`
`1 0 0 0
`
`8 1 0 0 0
`
`1 0 0 0
`
`7 1 0 0 0
`
`1 0 0 0
`
`6 1 0 0 0
`
`1 0 0 0
`
`5 1 0 0 0
`
`1 0 0 0
`
`4 0 0 1 0
`
`1 0 0 0
`
`3 0 0 1 0
`
`1 0 0 0
`
`2 0 0 1 0
`
`0 1 1 0
`
`1 0 0 1 0
`
`1 0 0 0
`
`C1
`
`Port
`connection
`
`C2
`
`Port
`connection
`
`1-2
`
`1–4
`
`3–2
`
`3–4
`
`1-2
`
`1–4
`
`3–2
`
`3–4
`
`WADM – Wavelength add/drop multiplexer
`
`Figure 3.
`WADM connection diagram.
`
`function of the WADM is defined in terms of the
`WDM connections among its optical ports (physical
`input and output optical paths), including the ability to
`rearrange the connections. These considerations lead
`to a formal matrix description of the connectivity from
`input to output ports, which assists in the unambigu-
`ous classification of the logical characteristics of a
`WADM.Figure 2 shows a WADM having in-, out-,
`add-, and drop-ports numerically designated from
`1 to 4. It is implicit that there are madd- and drop-
`ports available (m≤N ) to implement full add/drop
`capability with ports available for each channel. Later,
`we will distinguish between the “wavelength-
`selective-switch-centric” WADM in Figure 2 and the
`“space-switch-centric” WADM that inherently multi-
`plexes the add-channels and fully demultiplexes the
`
`through- and drop-channels. Channel pathways are
`indicated from the in- and add-ports to the out- and
`drop-ports—the logical traffic directions normally asso-
`ciated with the WADM. A connection matrix is con-
`structed with rows corresponding to optical paths
`through the WADM and columns of 0s and 1s indicat-
`ing the state of connections for each WDM channel.
`A fixed WADM is represented by a single connection
`matrix while a reconfigurable WADM is represented by
`a set of matrices. A fully flexible WADM that can
`add/drop any and all of Nchannels has at least 2 N pos-
`sible connection states (more than 2N are possible if the
`in- and add-ports can independently connect to the
`out- and drop-ports). Figure 3shows two representa-
`tive connection matrices—C1 and C2—for 16 WDM
`channels in a WADM. In C1, channels 5 to 12 are
`
`Bell LabsTechnical Journal u January–March 1999 211
`
`Petitioner Ciena Corp. et al.
`Exhibit 1035-5
`
`

`

`4 1 1 1 1
`
`4 1 0 1 0
`
`4 1 0 0 1
`
`4 1 0 0 0
`
`3 1 1 1 1
`
`3 1 1 1 1
`
`3 0 1 1 0
`
`3 0 1 1 0
`
`C =
`
`2 1 1 1 1
`
`2 1 0 1 0
`
`2 1 0 0 1
`
`2 1 0 0 0
`
`1 1 1 1 1
`
`1 1 0 1 0
`
`1 1 0 0 1
`
`1 1 0 0 0
`
`1-2
`
`1–4
`
`3–2
`
`3–4
`
`1-2
`
`1–4
`
`3–2
`
`3–4
`
`1-2
`
`1–4
`
`3–2
`
`3–4
`
`1-2
`
`1–4
`
`3–2
`
`3–4
`
`Coupler
`
`Coupler
`and filter
`
`In[1]
`
`Out[2]
`
`Add[3]
`
`Drop[4]
`
`In[1]
`
`Out[2]
`
`Add[3]
`
`Drop[4]
`
`Bragg grating
`WADM
`
`In[1]
`
`Out[2]
`
`Drop[4]
`
`Add[3]
`
`MUX/DEMUX
`WADM
`
`In[1]
`
`Out[2]
`
`Drop[4]
`
`Add[3]
`
`DEMUX – Demultiplexer
`MUX – Multiplexer
`WADM – Wavelength add/drop multiplexer
`
`Figure 4.
`Four examples of connection diagrams for a 4-channel WDM system dropping channel 3.
`
`through-channels, channels 1 to 4 are added, and
`channels 13 to 16 are dropped. In C2, channels 2 and 11
`are both dropped and added, while all others are
`through-channels.
`Connection matrices also assist in differentiating
`among WADM options that, on cursory inspection,
`appear to have the same function. For example,
`Figure 4shows four versions of a fixed, single-filter
`WADM and their connection matrices for the
`add/drop of channel 3 of a 4-channel WDM system.
`The 3-dB coupler, though designated a fixed
`
`WADM in Figure 4a, performs no wavelength filter
`function, and the connection matrix shows that the
`simple coupler is a very restrictive WADM. Placing a
`bandpass filter at the drop-port of the coupler
`changes its connectivity, and the resultant WADM
`has “drop-and-continue without add” capability for
`channel 3. The well-established fiber Bragg grating
`and multiplexed/demultiplexed WADM (Figures 4c
`and 4d, respectively), often claimed to have the
`same logical behavior, in fact have different connec-
`tion matrices. The main difference is the
`
`212 Bell LabsTechnical Journal u January–March 1999
`
`Petitioner Ciena Corp. et al.
`Exhibit 1035-6
`
`

`

`Add
`
`In
`
`Wavelength-
`independent
`switch array
`
`In
`
`Out
`
`Out
`
`Add
`
`Drop
`
`Wavelength-
`selective
`switch
`
`Drop
`
`(b) WADM using a wavelength-
`selective switch fabric
`
`(a) WADM using a wavelength-independent switch array
`
`Figure 5.
`Reconfigurable WADMs.
`
`transparency from add- to drop-port in the Bragg
`grating WADM, which is a benign circumstance
`under normal operation.
`The operational state of a WADM node is com-
`pletely defined from the channels entering the WADM
`and its connection matrix. In a particular operational
`state, channels leaving the WADM through the out-
`and drop-ports are calculated from simple algebra.
`Output states of the out- and drop-ports for the i th of
`N channels become
`O (OUT, i) = I (IN, i) 3 C (IN ! OUT, i)
`+ I (ADD, i) 3 C (ADD ! OUT, i)
`O (DROP, i) =I (IN, i) 3 C (IN ! DROP, i)
`+ I (ADD, i) 3 C (ADD ! DROP, i)
`where I (IN, i) and I (ADD, i), the input states to the
`WADM, assume values of 0 or 1, and C (xx, i) designates
`elements of the connection matrix. The total number of
`operational states of a WADM node may be only one in
`the case of a fixed add/drop, while a conventional, fully
`flexible WADM may have 23N states. Normal conditions
`
`require that for all operational states, O (OUT, i) and
`O (DROP, i) evaluate to either 0 or 1. However, it is con-
`ceivable that some WADM fabrics could support opera-
`tional states where a channel is present at both the in-
`and add-ports, and both ports are simultaneously con-
`nected to the out-port. Then O (OUT, i) evaluates to 2
`and channel interference results. Conditions during
`state changes, as in changing a channel from a drop- to
`a through-state or the reverse, are also captured as tran-
`sient operational states. For instance, a common
`requirement is for the WADM to be noninterrupting;
`reconfiguration should only affect the channels being
`rearranged. It is important to know the operational
`states of a WADM node in both normal and failed con-
`ditions in order to design a robust lightwave system
`that, for example, avoids O = 2. While the WADM fabric
`itself may fail, requiring alarm and protection features,
`other failures need to be considered as well. Two com-
`mon failures are an incorrect wavelength from a laser
`source or a failed upstream WADM node sending unex-
`pected downstream signals.
`
`Bell Labs Technical Journal u January–March 1999
`
`213
`
`Petitioner Ciena Corp. et al.
`Exhibit 1035-7
`
`

`

`Table I. Common scattering parameter matrix terms and their association with WADM add-, drop-, and
`through-channels.
`
`Specification (50, 100, 200…GHz channel spacing,
`OC-12, 0C-48, OC-192…transmission factors…)
`
`Through-port channel loss (1, 2, 5, 10…dB)
`
`Through-port channel frequency accuracy (1, 2, 5…GHz)
`Through-port channel bandwidth (BWthrough @ -3, -10, -20…dB)
`Through-port channel uniformity (0.1, 0.2, 0.5…dB)
`
`Through-port channel PDL (0.1, 0.2, 0.5…dB)
`
`Drop-port channel loss (5, 10…dB)
`
`Drop-port channel frequency accuracy (1, 2, 5…GHz)
`Drop-port channel bandwidth (BWdrop @ -3, -10, -20…dB)
`Drop-port channel uniformity (0.1, 0.2, 0.5…dB)
`
`Drop-port channel PDL (0.1, 0.2, 0.5…dB)
`
`Add-port channel loss (5, 10…dB)
`
`Add-port channel frequency accuracy (1, 2, 5…GHz)
`Add-port channel bandwidth (BWadd @ -3, -10, -20….dB)
`Add-port channel uniformity (0.1, 0.2, 0.5…dB)
`
`Add-port channel PDL (0.1, 0.2, 0.5…dB)
`
`Through-port isolation of drop-channel (20, 30, 40…dB)
`
`Add
`
`Drop-port isolation of through-channel (20, 30, 40…dB)
`
`Drop-port isolation of add-channels (20, 30, 40…dB)
`
`Through-port channel filter shape (power and dispersion)
`
`Through
`
`Drop-port channel filter shape (power and dispersion)
`
`Add-port channel filter shape (power and dispersion)
`
`S21
`
`S41
`
`S23
`
`S43
`
`Through
`
`Through
`
`Through
`
`Through
`
`Through
`
`Drop
`
`Drop
`
`Drop
`
`Drop
`
`Drop
`
`Drop
`
`Drop
`
`Add
`
`Add
`
`Add
`
`Add
`
`Add
`
`Add
`
`Drop
`
`1
`
`2
`
`3
`
`4
`
`5
`
`6
`
`7
`
`8
`
`9
`
`10
`
`11
`
`12
`
`13
`
`14
`
`15
`
`16
`
`17
`
`18
`
`19
`
`20
`
`21
`
`BW – Bandwidth
`OC – Optical carrier
`
`PDL – Polarization-dependent loss
`S – Scattering parameter
`
`The reconfigurable WADM supports several con-
`nection states that are typically configured through
`optical switches or tunable optical filters. These
`WADMs are differentiated by the functionality of the
`switches (tunable filters) and their placement relative
`to other optical filter elements.Figure 5shows fully
`reconfigurable N-channel WADMs having either an
`array of 2N1 32 wavelength-independent switches
`(WISs) or a single 232 N-wavelength-selective
`switch (WSS). The use of 2N132 optical switches as
`the WIS core rather than N232 switches is moti-
`vated by east-west separability concerns. The
`N-wavelength-selective switch (sometimes called a
`wavelength branching unit) can set up cross or bar switch
`
`states on a per-wavelength basis, and it achieves the full
`functionality of an N-channel reconfigurable WADM
`upon the addition of an add-multiplexer and drop-
`demultiplexer. Even without identifying specific hard-
`ware, differences in performance and function of the
`two WADMs are expected. For example, the WSS-core
`WADM subjects through-channels to 1 optical bandpass
`filter and drop- and add-channels to 2 bandpass filters.
`Conversely, the WIS-core WADM subjects through-
`channels to 2 optical filters and the drop- and
`add-channels to 1 filter. These considerations lead to
`important details of WADM insertion loss, channel
`filtering, dispersion, and crosstalk. In the next section,
`we will develop a general description of the physical
`
`214 Bell LabsTechnical Journal u January–March 1999
`
`Petitioner Ciena Corp. et al.
`Exhibit 1035-8
`
`

`

`Channel
`distortion
`
`Ripple
`
`Crosstalk
`impairment
`
`b-dB (filter bandwidth)
`
`b-dB (filter bandwidth)
`
`X-dB (crosstalk level)
`
`X-dB (crosstalk level)
`
`fi – 1
`
`fi
`
`fi + 1
`
`fi – 1
`
`fi
`
`fi + 1
`
`(a) WADM filter template determined from
`bandwidth, ripple, and crosstalk specifications
`
`(b) WADM filter template determined from
`known filter shape specification
`
`Figure 6.
`The extension of filter specifications to graphical templates and filter-shape families.
`
`optical characteristics that must be considered in ana-
`lyzing either WADM and understanding its behavior
`and effect on the WDM channels.
`
`Scattering Parameters of the WADM
`The logical attributes of a WADM are established
`from connection matrices; operational states relate this
`interconnectivity within the WADM and the input
`states. The physical attributes also need to be catego-
`rized in order to facilitate hardware engineering.
`Specifications relating to insertion loss, channel spac-
`ing, channel filtering, and crosstalk are commonly
`cited and determined after considering system and line
`engineering requirements. System requirements
`include the size of a WADM and the number and type
`of WADM nodes, while line engineering requirements
`include span length, system reach, bit rate, and chan-
`nel capacity. Scattering parameters, analogous to those
`used in radio frequency (RF) circuit analysis, provide a
`convenient method of capturing a complete descrip-
`tion of the linear optical properties of a WADM.
`Designating the in-, out-, add-, and drop-ports as “1,”
`“2,” “3,” and “4,” respectively, the optical scattering
`parameter, Sij (C|fk), is the optical frequency response
`from port j to port i while the WADM is in either static
`or transient connection state C. The additional
`s-parameter k subscript is needed to distinguish the
`
`individual add- and drop-ports of multi-channel
`WADMs. The most important s-parameters are:
`• S21(C|f), the forward scattering term indicating
`the filter response from the in-port to the out-
`port;
`• S41(C|fk), the scattering term describing the filter
`response from the in-port to a drop-port; and
`• S23(C|fk), the term describing the filter response
`from an add-port to the out-port.
`The crosstalk of add-channels to the drop-ports,
`S43 (requiring a second subscript on f to identify indi-
`vidual add- and drop-ports), and the reverse scattering
`parameters (reflectivity) of the in- and add-ports, S11
`and S33, are less often cited. Generally, s-parameters
`have complex values that incorporate both amplitude
`and phase responses. In optical measurements, these
`parameters are measured as power response and
`group delay or dispersion response. Furthermore, the
`s-parameters may be expressed as Jones matrices that
`describe polarization properties to account for effects
`such as polarization-dependent loss and birefringence.
`A significant refinement to the s-parameters is the
`inclusion of undesired optical pathways that lead to
`the multi-path interference of channels. For example,
`a WIS-core WADM constructed using a demultiplexer
`and multiplexer pair (as shown in Figure 5) has
`
`Bell Labs Technical Journal u January–March 1999
`
`215
`
`Petitioner Ciena Corp. et al.
`Exhibit 1035-9
`
`

`

`10
`
`0
`
`–10
`
`–20
`
`–30
`
`–40
`
`–50
`
`–60
`
`Loss (dB)
`
`–70
`1555.0
`
`1555.5
`
`1556.0
`
`1556.5
`
`1557.0
`
`Wavelength (nm)
`
`Reflection (drop)
`Transmission (through)
`
`(a) Fiber Bragg grating
`
`–10
`
`–20
`
`–30
`
`–40
`
`–50
`
`–60
`
`–70
`
`Loss (dB)
`
`1537.0
`
`1537.5
`
`1538.0
`
`1538.5
`
`1547.0
`
`1547.5
`
`1548.0
`
`1548.5
`
`Wavelength (nm)
`
`Reflection (through)
`Transmission (drop)
`
`Wavelength (nm)
`
`Transmission
`
`(b) Thin film filter
`
`(c) Arrayed waveguide grating
`
`10
`
`0
`
`–10
`
`–20
`
`–30
`
`–40
`
`–50
`
`–60
`
`Loss (dB)
`
`–70
`1536.5
`
`Figure 7.
`Examples of reflection and transmission spectra.
`
`parallel optical pathways that can carry weak crosstalk
`components that can ultimately interfere with the
`intended channel. These multi-paths-parameters can
`be designated as Smpij (C|f); the most notable ones are
`Smp21 (C|f) and Smp 41 (C|f), which can corrupt
`through- and drop-channels, respectively. As these
`multi-path s-parameters result in in-band optical inter-
`ference, it is important to keep their values small—
`typically less than –40 dB below the intended signal.
`
`Table Isummarizes the relationship between
`s-parameters and optical filter specifications associated
`with the WADM. The intent is to capture a minimum
`set of numbers that adequately bounds the optical
`design of the WADM to ensure satisfactory perfor-
`mance of the lightwave network under all plausible
`operational, maintenance, and failure conditions.
`However, filter response is vague and is established for
`a particular filter technology (for example, thin film or
`
`216 Bell LabsTechnical Journal u January–March 1999
`
`Petitioner Ciena Corp. et al.
`Exhibit 1035-10
`
`

`

`Receiver eye
`
`(a) No self-phase modulation with
`negligible eye penalty and 0.1 dB filtering loss
`
`1
`
`0
`
`1
`
`0
`
`050
`
`050
`
`(b) 50-GHz self-phase modulation with
`0.9 dB penalty and 1.2 dB filtering loss
`
`Figure 8.
`Simulated OC-192 eye diagrams with 40-GHz bandwidth supergaussian-shaped receiver optical filter.
`
`fiber Bragg grating) after the line system is defined and
`the optical signal characteristics at the WDM node are
`known. Figure 6illustrates the extension of filter
`specifications to graphical templates and filter-shape
`families to more precisely define requirements.
`Typically, minimum insertion loss at center frequency
`(Lmin), minimum bandwidth at b-dB down from cen-
`ter frequency (BWb-dB), ripple (R), and maximum
`adjacent-channel crosstalk (Xmax) are specified. In
`Figure 6, WDM channels are indicated at fi-1, fi, and
`fi+1, and white dots correspond to nominal specifica-
`tions for center frequency, filter bandwidth, and
`adjacent-channel crosstalk. Constructing piece-wise
`linear approximations to possible filter shapes and
`assuming monotonicity on either side of center fre-
`quency, Figure 6a shows that these specifications can
`result in peculiar filter templates with the inner bound
`likely overfiltering realistic signals and the outer bound
`
`resulting in excess crosstalk. The situation improves if
`the characteristic filter shape is known; waveguide
`grating routers (Gaussian and “flattened” responses),
`fiber Bragg gratings, and thin film optical filters all can
`have distinguishing filter responses. A waveguide grat-
`ing router with a Gaussian passband response
`
`(
`)
`T f L
`
`exp
`
`(cid:236)(cid:237)(cid:238)
`
`)[]
`()(
`ff BW=--
`ln
`0
`
`/2 2
`3
`
`2
`-min
`
`(cid:252)(cid:253)(cid:254)
`
`would have an idealized template similar to that in
`Figure6b. A family of Gaussian shapes can fit the area
`with an inner bound determined from the minimum
`bandwidth requirement and an outer bound set by the
`maximum adjacent-channel crosstalk. For fixed Lmin
`and center frequency f0, the Gaussian filter bandwidth
`and the crosstalk are uniquely determined by one
`parameter. Ill-posed specifications are possible with a
`minimum filter bandwidth requirement resulting in
`
`Bell LabsTechnical Journal u January–March 1999 217
`
`Petitioner Ciena Corp. et al.
`Exhibit 1035-11
`
`

`

`0
`
`–5
`
`–10
`
`–15
`
`–20
`
`–15
`
`–30
`
`–35
`
`–40
`
`Transmission loss (dB)
`
`–45
`154615481550155215541556155815601562
`
`Wavelength (nm)
`(a) 50-GHz channel spacing, flatband spectra
`
`0
`–5
`–10
`–15
`–20
`–15
`–30
`–35
`–40
`–45
`–50
`–55
`153015341538154215461550155415581562
`
`Transmission loss (dB)
`
`Wavelength (nm)
`
`(b) 100-GHz channel spacing, Gaussian spectra
`
`Figure 9.
`Transmission spectra of 40-channel arrayed waveguide grating router.
`
`excessive crosstalk or a maximum crosstalk value
`forcing excessively narrow filter bandwidths.
`Of course, filters are not ideal, and families of real
`filter curves must be used. The in-band fine structure
`of the filter response and crosstalk limitations are usu-
`ally worse than predicted from idealized calculations,
`and the filter template must account for them.
`Figure 7shows spectra of a fiber Bragg grating filter, a
`thin film filter, and a waveguide grating filter for com-
`parison. The obvious differences among the spectral
`shapes necessitate careful engineering to obtain the
`intended optical performance required in the network;
`
`filter loss, signal spectrum clipping, dispersion, and
`crosstalk all factor into the design.
`Scattering parameters and filter responses are
`reduced to specifications, with actual numbers relating
`to the performanceof the optical network. Performance
`parameters quantify channel integrity through the
`system—that is, degradation from line propagation
`coupled with filtering and crosstalk originating from
`the WADM. Performance analysis follows the propa-
`gation of WDM signals through nodes in the system,
`beginning at the first transmitter location. The trans-
`mission penalty on a signal originating at the Lth node
`
`218 Bell LabsTechnical Journal u January–March 1999
`
`Petitioner Ciena Corp. et al.
`Exhibit 1035-12
`
`

`

`(a) Packaged array of twenty 2 X 2 lithium niobate switches
`
`Lever arm
`Spring-suspended
`capacitor plate
`
`Gold-coated
`shutter
`
`Optical
`fiber
`
`(b) SEM photomicrograph views of Lucent’s micromechanical 1 X 2 optical switch and schematic of switch element
`
`Figure 10.
`Optical switches for wavelength-independent switch WADM.
`
`and terminating at the Mth node is PML, where PML, the
`receiver power penalty at a specified bit error rate,
`includes all impairments—those originating from the
`line system, optical filtering in the WADM, crosstalk,
`and multi-path interference. Requirements on scatter-
`ing parameters or specifications derived from them are
`established from the acceptable worst-case PML. Worst-
`
`case performance would likely occur at the last
`WADM site in the line system where signals have
`been exposed to cumulative noise, interference, and
`nonlinear distortion. Ideally, limitations in signal trans-
`mission would only be associated with line impair-
`ments such as optical signal-to-noise ratio and fiber
`nonlinearities, but, in reality, the number of WADM
`
`Bell Labs Technical Journal u January–March 1999
`
`219
`
`Petitioner Ciena Corp. et al.
`Exhibit 1035-13
`
`

`

`(a) Waveguide grating routers
`and 2 3 2 switch fabric
`
`(b) Fiber Bragg gratings and optical circulators
`
`Grating
`
`Mirror array
`
`(c) Free-space grating and
`microelectromechanical mirrors
`
`f
`
`(d) Planar lightwave circuit integration
`with thermo-optic phase shifters
`
`(e) Acousto-optic tunable filter
`
`Figure 11.
`2 3 2 wavelength-selective switches.
`
`nodes is restricted by a blend of signal-to-noise ratio
`and signal distortion effects.
`The effect of WADM filtering on optical signals
`leading to signal distortion may be illustrated using a
`source modified with a self-phase modulation (SPM)
`term to imitate nonlinear transmission in an optical
`line system. This very simple approach is useful pri-
`marily for discussion, not practical system design. In
`the analysis, the signal is filt