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`CERTIFICATE OF MAILING BY “EXPRESS MAIL” (37 CFR 1.10)
`
`Docket No.
`2102393-991101
`
`Applicant: Capella Photonics, Inc.
`
`Serial No.
`Not Yet Assigned
`
`Filing Date
`Herewith
`
`Examiner
`NIA
`
`Group Art Unit
`NIA
`
`Invention:
`
`RECONFIGURABLE OPTICAL ADD-DROP MULTIPLEXERS W’ITH SERVO
`CONTROL AND DYNAMIC SPECTRAL POWER MANAGEMENT CAPABILITIES
`
`E3
`£0
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`50
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`
`UTILITY PATENT APPLICATION
`(Identify gape afcarrespondence)
`is being deposited with the United States Postal Service “Express Mail Post Ofiice to Addressee”
`service under 37 CFR 1.10 in an envelope addressedt
`: The Commissioner of Patents and
`
`97.3 2&[
`
`(Date)
`
`I hereby certify that this
`
`Trademarks, Washington, D.C., 20231-0001 on
`
`&;a... c. W,.,a%
`
`d or Pri‘r:£ea'Name zy'Per
`
`M'n‘l'I.‘ng Corresparldsnce)
`
`(SigrIaru.re ofP
`
`:1 Mailing Correspondence)
`
`EL 904925981 US
`("Express Mail" Malling Lube! Number)
`
`Note: Each paper must have its own certificate of mailing.
`
`Gray CaI'y\El'UIl70ll2446.l
`2102393 -990000
`
`9
`
`

`
`Docket No. 2102393-991101
`
`ABSTRACT OF THE DISCLOSURE
`
`This invention provides a novel wavelength-separating-routing (WSR) apparatus that uses a
`
`diffraction grating to separate a rnulti-wavelength optical signal by wavelength into multiple
`
`spectral channels, which are then focused onto an array of corresponding channel
`
`micromirrors. The channel micromirrors are individually controllable and continuously
`pivotable to reflect the spectral channeis into selected output ports: As such, the inventive
`
`WSR apparatus is capable of routing the spectral channels on a channel-by-channel basis and
`coupling any spectral channel into any one of the output ports. The WSR apparatus of the
`
`further equipped with servo-control and spectral power--
`invention may be
`present
`rnanagcmcnt capabilities,
`thereby maintaining the coupling efficiencies of the spectral
`
`channels into the output ports at desired values. The WSR apparatus of the present invention
`
`can be used to construct a novel class of dynamically reconfigurable optical add—drop
`
`multiplexers (OADMS) for WDM optical networking applications.
`
`10
`
`

`
`Docket No. 2102393-991101
`
`PATENT APPLICATION
`
`RECONFIGURABLE OPTICAL ADD-DROP MULTIPLEXERS WITH SERVO
`
`CONTROL AND DYNAMIC SPECTRAL POWER MANAGEMENT CAPABILITIES
`
`INVENTOR
`
`Jeffrey P. Wilde
`
`53
`ED
`is
`Lu
`Eli.
`gs
`EU
`El‘!
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`
`CROSS-REFERENCE TO RELATED APPLICATIONS
`
`I
`
`This application claims priority of US. Provisional Patent Application No. 60!2'?”;‘,21?, filed
`
`19 March 2001, which is incorporated herein by reference. ‘>
`
`FIELD OF THE INVENTION
`
`This invention relates generally to optical communication systems. More specifically,
`
`it
`
`relates to a novel class of dynamically reconfigurable optical add-drop multiplexers
`
`(OADMS) for wavelength division multiplexed optical networking applications.
`
`BACKGROUND
`
`As fiber-optic communication networks rapidly spread into every walk of modern life, there
`
`is a growing demand for optical components and subsystems that enable the fiber-optic
`
`communications networks to be increasingly scalable, versatile, robust, and cost-effective.
`
`Contemporary fiber-optic communications networks commonly employ wavelength division
`
`multiplexing (WDM),
`
`for
`
`it allows multiple information (or data) channels
`
`to be
`
`simultaneously transmitted on a single optical fiber by using different wavelengths and
`
`thereby significantly enhances the infonnation bandwidth of the fiber. The prevalence of
`
`- WDM technology has made optical add-drop multiplexers indispensable building blocks of
`
`modern fiber-optic communication networks. An optical add-drop multiplexer (OADM)
`
`11
`
`

`
`Docket No. 2102393-991101
`
`serves to selectively remove (or drop) one or more wavelengths from a multiplicity of
`
`wavelengths on an optical fiber, hence taking away one or more data channels from the tratfic
`
`stream on the fiber.
`
`It further adds one or more wavelengths back onto the fiber, thereby
`
`inserting new data channels in the same stream of traffic. As such, an OADM makes it
`possible to launch and retrieve multiple data channels (each characterized by a distinct
`
`wavelength) onto and from an optical fiber respectively, without disrupting the overall trafiic
`
`flow along the fiber.
`
`Indeed, careful placement of the OADMs can dramatically improve an -
`
`optical communication networlc’s flexibility and robustness, while providing significant cost
`
`advantages.
`
`Conventional 0ADMs
`
`in the art
`
`typically _ employ multiplexersfdemultiplexers
`
`(e.g,
`
`waveguide grating routers or arrayed-waveguide gratings), tunable filters, optical switches,
`
`and optical circulators in a parallel or serial architecture to accomplish the add and drop
`
`functions.
`
`In the parallel architecture, as exemplified in U.S. Patent 5,974,203’, a
`
`demultiplexer (e.g., a waveguide grating router) first separates a multi-wavelength signal into
`
`its constituent spectral components.
`
`A wavelength switching/routing means (e.g., a
`
`combination of optical switches and optical circulators) then serves to drop selective
`
`wavelengths and add others. Finally, a multiplexer combines the remaining (i.e., the pass-
`
`through) wavelengths into an output multi-wavelength optical signal.
`In the serial
`architecture. as exemplified in U.S. Patent 6,205,269,
`tunable filters (e.g., Bragg fiber
`gratings) in combination with optical circulators are used to separate the drop wavelengths
`
`from the pass-through wavelengths and subsequently launch the add channels into the pass-
`
`through path. And if multiple wavelengths are to be added and dropped, additional
`
`multiplexers and demultiplexers are required to dernultiplex the drop wavelengths and
`
`multiplex the add wavelengths, respectively.
`
`Irrespective of the underlying architecture, the
`
`OADMS currently in the art are characteristically high in cost, and prone to significant optical
`
`loss accumulation. Moreover, the designs of tl'1ese.0ADMs are such that it is inherently
`
`difficult to reconfigure them in a dynamic fashion.
`
`12
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`
`Docket No. 2102393-991101
`
`U.S. Patent 6,204,946 to Askyuk et al. discloses an OADM that makes use of free-space
`
`optics in a parallel construction.
`
`In this case, a rnulti-wavelengtli optical signal emerging
`
`from an input port is incident onto a ruled diffraction grating. The constituent spectral
`channels thus separated are then focused by a focusing lens onto a linear array of binary
`micromachined mirrors. Each rnicromirror is configured to operate between two discrete
`
`states, such that it either retroreflects its corresponding spectral channel back into the input
`port as a pass-tiuough channel, or directs its spectral channel to an output port as a drop
`channel. As such, the pass-through signal (i.e., the combined pass-through channels) shares
`
`the same input port as the input signal. An optical circulator is therefore coupled to the input
`
`port, to provide necessary routing of these two signals. Likewise, the drop channels share the
`
`output port with the add channels. An additional optical circulator is thereby coupled to the _
`
`output port, from which the drop channels exit and the add channels are introduced into the
`
`output port. The add channels are subsequently combined with the pass-through signal by
`
`way of the diffraction grating and the binary micromirrors.
`
`Although the aforementioned OADM disclosed by Askyuk et a]. has the advantage of
`performing wavelength separating and routing in free space and thereby incurring less optical
`
`loss, it suffers a number of limitations. First, it requires that the pass-through signal share the
`
`same portffiber as the input signal. An optical circulator therefore has to be implemented, to
`
`provide necessary routing of these two signals. Likewise, all the add and drop channels enter
`and leave the OADM through the same output port, hence the need for another optical
`
`circulator. Moreover, additional means must be provided to multiplex the add channels
`
`before entering the system and to demultiplex the drop channels after exiting the system.
`
`This additional multiplexing/demultiplexing requirement adds more cost and complexity that
`
`can restrict the versatility of the OADM thus-constructed. Second, the optical circulators
`
`implemented in this OADM for various routing purposes introduce additional optical losses,
`which can accumulate to a substantial amount. Third, the constituent optical components
`
`must be in a precise alignment, in order for the system to achieve its intended purpose. There
`
`are, however, no provisions provided for maintaining the requisite -alignment; and no
`
`mechanisms
`
`implemented for overcoming degradation in the alignment owing to
`
`3
`
`13
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`

`
`Docket No. 2102393-991 101
`
`environmental effects such as thermal and mechanical disturbances over the course of
`
`operation.
`
`U.S. Patent 5,906,133 to Tomlinson discloses an OADM that makes use of a design similar to
`
`that of Aksyuk et at. There are input, output, drop and add ports implemented in this case.
`
`By positioning the four ports in a specific arrangement, each micromirror, notwithstanding
`
`switchable between two discrete positions, either reflects its corresponding channel (coming
`
`from the input port) to the output port, or concomitantly reflects its channel to the drop port
`
`and an incident add channel to the output port. As such, this OADM is able to perforrn both
`
`‘the add and drop functions without involving additional optical components_(such as optical
`
`circulators used in the system of Al-zsyuk et al.). However, because a single drop port is
`
`designated for all the drop channeis and a single add port is designated for all the add I
`
`channels, the add channels would have to be multiplexed before entering the add port and the
`
`drop channels likewise need to be demutiplxed upon exiting from the drop port. Moreover,
`
`as in the case of Askyuk et 31., there are no provisions provided for maintaining requisite
`
`optical alignment in the system, and no mechanisms implemented for combating degradation
`
`in the alignment due to environmental effects over the course of operation.
`
`the prevailing drawbacks suffered by the OADMS currently in the art are
`As such,
`summarized as follows:
`
`1)
`
`2)
`I
`
`The wavelength routing is intrinsically static, rendering it difiicult to dynarnicaliy
`
`reconfigure these OADMS.
`
`Add andfor drop channels often need to be_ multiplexed andfor demultiplexed, thereby
`imposing additional complexity and cost.
`'
`Stringent
`fabrication tolerance and painstaking optical alignment are required.
`
`Moreover, the optical alignment is not actively maintained, rendering it susceptible to
`
`environmental effects such as thermal and mechanical disturbances over the course of
`
`operation.
`
`In an optical communication network, OADMs are typically in a ring or cascaded
`
`configuration.
`
`In order to mitigate the interference amongst OADMs, which often
`
`14
`
`

`
`Docket No. 2I02393~99110I
`
`adversely affects the overall performance of the network, it is essential that the power
`
`levels of spectral channels entering and exiting each OADM be managed in a
`
`systematic way, for instance, by introducing power (or gain) equalization at each
`
`stage. Such a power equalization capability is also needed for compensating for non-
`
`uniform gain caused by optical amplifiers (e.g., erbium doped fiber amplifiers) in the
`
`network. There lacks, however, a systematic and dynamic management of the power
`
`levels of various spectral channels in these OADMs.
`
`The inherent high cost and heavy optical loss further impede the wide application of
`these OADMS.
`
`In view of the foregoing, there is an urgent need in the art for optical add-drop multiplexers
`
`that overcome the aforementioned shortcomings in a simple, effective, and economical-
`construction.
`
`SUMMARY
`
`The present
`
`invention provides a wavelength-separating-routing (WSR) apparatus and
`
`method which employ an array of fiber collirnators serving as an input port and a plurality of
`
`output ports; a wavelength-separator; a beam-focuser; and an array of channel micrornirrors.
`
`In operation, a multi—waveleng’r.h optical signal emerges From the input port. The wavelength-
`
`separator separates the rnulti-wavelength optical signal into multiple spectral channels, each
`
`characterized by a distinct center wavelength and associated bandwidth. The beam-focuser
`
`focuses the spectral channels into corresponding spectral spots. The channel micromirrors
`are positioned such that each channel micrornirror receives one of the spectral channels. The
`channel micromirrors are individually controllable and movable, e.g., continuously pivotable
`
`(or rotatable), so as to reflect the spectral channels into selected ones of the output ports. As
`
`such, each channel micrornin-or is assigned to a specific spectral channel, hence the name
`
`“channel micromirror”. And each output port may receive any number of the reflected
`
`spectral charmels.
`
`15
`
`

`
`Docket No. 210239339110]
`
`A distinct feature of the channel micromirrors in the present invention, in contrast to those
`
`used in the- prior art,
`
`is that the motion, e.g., pivoting (or rotation), of each channel
`
`mic-romirror is under analog control such that its pivoting angle can be continuously adjusted.
`
`This enables each channel micromirror to scan its corresponding spectral channel across all
`
`possible output ports and thereby direct the spectra] channel to any desired output port.
`
`In the WSR apparatus of the present invention, the wavelength-separator may be provided by
`
`a ruled diffraction grating, a holographic diffraction grating, an echelle grating, a curved
`
`diffraction grating, a dispersing prism, or other wavelength-separating means known in the
`
`art. The beam-focuser may be a single lens, an assembly of lenses, or other beam-focusing
`
`means known in the art.
`The channel micromirrors may be provided by silicon
`micrornachined mirrors, reflective ribbons (or membranes), or other types of beam-deflecting
`
`means known in the art. And each channel micrornirror may be pivotable about one or two
`
`axes. The fiber coliimators serving as the input and output ports may be arranged in a one-
`
`dimensional or two-dimensional array. In the latter case, the channel microntirrors must be
`
`pivotable biaxially.
`
`The WSR apparatus of the present invention may fiirther comprise an array of collimator-
`
`aiignment mirrors, in optical communication with the wavelengthseparator and the fiber
`
`collimators, for adjusting the alignment of the input multi-wavelength signal and directing the
`spectral channels into the selected output ports by way of angular control of the coilimatcd
`
`beams. Each collimatonalignment mirror may be rotatable about one or two axes. The
`
`collimator-alignment mirrors may be arranged in a one-dimensional or two-dimensional
`
`array. First and second arrays of imaging lenses may additionally be optically interposed
`
`between the coltimator-aligitrnent mirrors and the fiber collimators
`
`in a telecentric
`
`arrangement, thereby ‘‘imaging‘‘ the collimator-alignment mirrors onto the corresponding
`
`fiber collimators to ensure an optimal alignment.
`
`The WSR apparatus of the present invention may further include a servo-control assembly. in
`
`communication with the channel micromirrors and the output ports. The servo-control
`
`16
`
`

`
`Docket No. 2102393-991101
`
`assembly serves to monitor the power levels of the spectral channels coupled into the output
`ports and further provide control of the channel micromirrors on an individual basis, so as to
`maintain a predetermined coupling efficiency of each spectral channel in one of the output
`ports. As such, the servo-control assembly provides dynamic control of the coupling of the
`spectral channels into the respective output ports and actively manages the power levels of
`the spectral channels coupled into the output ports.
`(If the WSR apparatus includes an array
`of collimator-alignment mirrors as described above,
`the servo-control assembly may
`additionally provide dynamic control of the coI1i_mator—alignment mirrors.) Moreover, the
`utilization of such a servo-control assembly effectively relaxes the requisite fabrication
`tolerances and the precision of optical alignment during assembly of a WSR apparatus of the
`present invention, and further enables the system to correct for shift in optical alignment over
`the course of operation. A WSR apparatus incorporating a servo-control assembly thus
`described is termed a WSR-S apparatus, thereinafter in the present invention.
`
`Accordingly, the WSR-S (or WSR) apparatus of the present invention may be used to
`construct a variety of optical devices, including a novel class of dynamically reconfigurable
`optical add-drop multiplexers (0ADMs), as exemplified in the following embodiments.
`
`One embodiment of an OADM of the present invention comprises an aforementioned WSR-S
`(or WSR) apparatus and an optical combiner. The output ports of the WSR-S apparatus
`include a pass-through port and one or more drop ports, each carrying any number of the
`spectral channels. The optical combiner is coupled to the pass-through port, serving to
`combine the pass-through channels with one or more add spectral channels. The combined
`optical signal constitutes an output signal of the system. The optical combiner may be an
`Nxl (N 2 2) broadband fiber~optic coupler, for instance, which also serves the purpose of
`multiplexing a multiplicity of add spectral channels to be coupled into the system.
`
`In another embodiment of an OADM of the present invention, a first WSR-S (or WSR)
`apparatus is cascaded with a second WSR-S (or WSR) apparatus. The output ports of the
`first WSR-S (or WSR) apparatus include a pass-through port and one or more drop ports.
`
`7
`
`17
`
`

`
`Docket No. 2102393-991101
`
`The second WSR-S (or WSR) apparatus includes a plurality of input ports and an exiting
`
`port. The configuration is such that the pass-through channels from the first WSR-S
`
`apparatus and one or more add channels are directed into the input ports of the second WSR-
`
`S apparatus, and consequently multiplexed into an output multi—wavelengtl1 optical signal
`
`directed into the exiting port of the second WSR-S apparatus. That is to say that in this
`
`embodiment, one WSR~S apparatus (e.g., the first one) effectiveiy performs a dynamic drop
`
`function, whereas the other WSR-S apparatus (e.g., the second one) carries out a dynamic add
`
`function. And there are essentially no fundamental restrictions on the wavelengths that can
`
`be added or dropped, other than those imposed by the overall communication system.
`
`Moreover, the underlying OADM architecture thus presented is intrinsically scalable and can
`be readily extended to any number of the WSR-S (or WSR) systems, if‘so desired for
`
`performing intricate add and drop functions in a network environment.
`
`Those skilled in the art will recognize that the aforementioned embodiments provide only two
`
`of many embodiments of a dynamically reconfigurable OADM according to the present
`
`invention. Various changes, substitutions, and alternations can be made herein, without
`
`departing from the principles and the scope of the invention. Accordingly, a skilled artisan
`
`can design an OADM in accordance with the present
`
`invention,
`
`to best suit a given
`
`application.
`
`' All in all, the 0ADMs of the present invention provide many advantages over the prior
`devices. notably:
`
`1)
`
`By advantageously employing an array of channel micromirrors that are individually
`
`and continuously controllable, an OADM of the present invention is capable of
`
`routing the spectral channels on a channel—by«channel basis and directing any spectral
`channel
`into any one of the output ports. As such,
`its underlying operation is
`
`I
`
`dynamically reconfigurable, and its underlying architecture is intrinsically scalable to
`
`a large number of channel counts.
`
`18
`
`

`
`Docket No. 2102393-991101
`
`2)
`
`The add and drop spectral channels need not be multiplexed and demultiplexed before
`
`entering and after leaving the OADM respectively. And there are not fundamental
`
`restrictions on the wavelengths to be added or dropped.
`
`The coupling of the spectral channels into the output ports is dynamically controlled
`
`by a servo-control assembly, rendering the OADM less susceptible to environmental
`
`effects (such as thennal and mechanical disturbances) and therefore more robust in
`
`performance. By maintaining an optimal optical alignment, the optical losses incurred
`
`by the spectral channels are also significantly reduced.
`
`The power levels of the Spectral channels coupled into the output ports can be,
`
`dynamically managed according to demand, or maintained at desired values (e.g.,
`
`equalized at a predetermined value) by way of the servo-control assembly. This
`
`spectral power'-management capability as an integral part of the OADM will be
`
`particularly desirable in WDM optical networking applications.
`
`The use of free-space optics provides a simple,
`
`low loss, and cost-effective
`
`construction. Moreover,
`
`the utilization of the servo-control assembly effectively
`
`relaxes the requisite fabrication tolerances and the precision of optical alignment
`‘during initial assembly, enabling the OADM to be simpler and more adaptable in
`
`structure, lower in cost and optical loss.
`
`The underlying OADM architecture allows a multiplicity of the OADMs according to
`
`the present invention to be readily assembled (e.g., cascaded) for WDM optical
`networking applications.
`‘
`
`The novel features of this invention, as well as the invention itself, will be best understood
`
`from the foilowing drawings and detailed description.
`
`BRIEF DESCRIPTION OF THE FIGURES
`
`FIGS. IA-1D show a first embodiment of a wavelengtb-separatingrouting (WSR) apparatus
`according to the present
`invention, and the modeling results demonstrating the
`performance of the WSR apparatus;
`
`FIGS. 2A—2C depict second and third embodiments of a WSR apparatus according to the
`present invention;
`
`19
`
`

`
`Docket No. 2102393-9_91l01
`
`FIG. 3 shows a fourth embodiment of a WSR apparatus according to the present invention;
`FIGS. 4A-4B show schematic illustrations of two embodiments of a WSR-S apparatus
`
`comprising a WSR apparatus and a servo-control assembly, according to the present
`
`invention;
`
`FIG. 5 depicts an exemplary embodiment of an optical add-drop multiplexer (OADM)
`
`according to the present invention; and
`
`FIG. 6 shows an alternative embodiment of an OADM according to the present invention.
`
`DETAILED DESCRIPTION
`
`In this specification and appending claims, a “spectral channel” is characterized by a distinct
`
`center wavelength and associated bandwidth. Each spectral channel may carry a unique
`
`information signal, as in WDM optical networking applications.
`
`FIG. 1A depicts a firstembodiment of a wavelength-separating-routing (WSR) apparatus
`
`according to the present invention. By way of example to illustrate the general principles and
`
`the topological structure of a wavelength-separatingwouting [WSR) apparatus of the present
`
`invention, the WSR apparat