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`IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 13, NO. 10, OCTOBER 2001
`
`Reconfigurable Multichannel Optical Add–Drop
`Multiplexers Incorporating Eight-Port Optical
`Circulators and Fiber Bragg Gratings
`
`An Vu Tran, Student Member, IEEE, Wen De Zhong, Member, IEEE, Rodney S. Tucker, Fellow, IEEE, and
`Kai Song, Member, IEEE
`
`Abstract—We propose and demonstrate two new strictly
`nonblocking reconfigurable multichannel optical add–drop
`multiplexers (RM-OADMs) using optical circulators and fiber
`Bragg gratings. By effectively using eight-port optical circulators,
`the new structures significantly reduce component count and
`insertion loss, and achieve good crosstalk performance. One of
`the new RM-OADMs potentially achieves the lowest insertion loss
`among existing RM-OADMs.
`Index Terms—Circulator, crosstalk, fiber grating, multichannel,
`nonblocking, optical add–drop multiplexer, reconfigurable, wave-
`length-division multiplexing.
`
`(a)
`
`I. INTRODUCTION
`
`I N WAVELENGTH-DIVISION-MULTIPLEXING (WDM)
`
`networks, reconfigurable multichannel optical add–drop
`multiplexers (RM-OADMs) are required to flexibly configure
`and reconfigure optical paths. Critical issues in the design
`of RM-OADMs are insertion loss, crosstalk, and component
`count. Many types of RM-OADMs, based on different op-
`tical devices, have been proposed and demonstrated [1]–[5].
`RM-OADMs can also be constructed by cascading multiple
`conventional single-channel OADMs [6]. Among these, the
`optical circulator
`(OC)-fiber Bragg grating (FBG)-based
`RM-OADMs [3]–[6] are very promising because of their low
`crosstalk, and temperature and polarization insensitivity. In
`addition, OC-FBG-based RM-OADMs do not cause band-
`width-narrowing when WDM signals pass through many
`OADM nodes. However, these OC-FBG-based RM-OADMs
`still suffer from high component count and high insertion
`loss due to the use of many circulators [3], [4], [6], and a
`mux–demux pair [5]. Moreover, the use of the mux–demux
`pair in the RM-OADM structure proposed in [5] prevents the
`flexible add–drop of channels in WDM systems.
`In this letter, we present
`two new strictly nonblocking
`RM-OADMs incorporating OCs and FBGs [7]. The devices
`have separate add and drop ports for each channel and can
`accommodate any arbitrary wavelength add–drop schemes.
`
`Manuscript received May 8, 2001; revised July 11, 2001.
`A. V. Tran and R. S. Tucker are with the Australian Photonics Coopera-
`tive Research Centre, Photonics Research Laboratory, Department of Electrical
`and Electronic Engineering, The University of Melbourne, Vic. 3010, Australia
`(e-mail: a.tran@ee.mu.oz.au).
`W. D. Zhong is with the School of Electrical and Electronic Engineering,
`Nanyang Technological University, 639 798 Singapore.
`K. Song is with Sycamore Networks, Chelmsford, MA 01824 USA.
`Publisher Item Identifier S 1041-1135(01)08197-6.
`
`1041–1135/01$10.00 © 2001 IEEE
`0001
`
`(b)
`Fig. 1. New RM-OADM structures. (a) Structure I. (b) Structure II.
`
`The devices significantly reduce the required number of OCs
`and insertion loss by effectively using eight-port OCs. One of
`the new RM-OADMs potentially achieves the lowest insertion
`loss for through channels among existing RM-OADMs [1]–[6].
`
`II. NEW RM-OADM STRUCTURES
`A. Structure I
`The first RM-OADM structure is shown schematically in
`Fig. 1(a). The device consists of
`eight-port OCs,
`FBGs
`and
`2
`2 optical switches (OSWs) to accommodate
`add–drop channels. The Bragg wavelength
`. An
`of the
`is designed to match the WDM channel
`OSW is connected between ports four and five of each OC.
`If the OSW is in the bar state, the channel corresponding to
`the FBG connected between ports three and six of each OC
`Capella 2011
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`TRAN et al.: RM-OADMs INCORPORATING EIGHT-PORT OPTICAL CIRCULATORS AND FBGs
`
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`goes from port four to port five and through the device together
`with other through channels. If the OSW is in the cross state,
`the channel is dropped through port four and the OSW, and
`another channel at the same wavelength can be added through
`the OSW to port five, similarly to that described in [6]. An
`OSW is also connected between port eight of each OC and
`port one of the next OC. If the OSW is in the bar state, the
`channel corresponding to the FBG connected between port
`seven of each OC and port two of the next OC is not dropped
`and goes through the device. If the OSW is in the cross state,
`the channel is dropped through port eight and the OSW, and the
`add channel at the same wavelength can enter the device via
`port one of the next OC. Port one of the first OC and port eight
`of the last OC are left for use as additional add–drop ports, if
`required. The RM-OADM Structure I is strictly nonblocking.
`Only the channels to be drop/added are affected during the
`switching operation.
`The number of eight-port OCs required is
`represents the smallest integer greater than or equal to ).
`(
`In other words, an increase of one additional eight-port OC
`can provide two additional add–drop channels. In comparison,
`if we use four-port OCs [4] or six-port OCs [6] to build a
`RM-OADM with
`add–drop channels, the number of OCs
`required is
`or
`, respectively, which is about twice
`that for eight-port OCs. Note that the cost for a commercial
`eight-port OC is not much higher than that for six-port and
`four-port OCs and their sizes are the same. Therefore, the new
`RM-OADM is more compact and cost-effective.
`If
`is the insertion loss between two adjacent ports of an
`OC, and
`is the out-of-band transmission insertion loss
`of a FBG,
`then the insertion loss for through channels is
`. Assuming
`dB and
`dB, for a RM-OADM with five add–drop chan-
`nels, the insertion loss is 6.5 dB, which is smaller than most of
`other existing OC-FBG-based RM-OADMs [3], [4], [6].
`
`B. Structure II
`Although Structure I has reduced insertion loss for through
`channels, the insertion loss is still proportional to the number of
`OCs used. The second structure aims to substantially reduce the
`insertion loss for through channels. It is shown schematically in
`Fig. 1(b). Structure II consists of the same number of eight-port
`OCs and 2
`2 OSWs, and twice the number of FBGs to accom-
`modate the same number of add–drop channels as Structure I.
`and
`have the same Bragg wavelength
`corre-
`sponding to the WDM channel
`. A number of FBGs corre-
`sponding to the channels to be reconfigured are connected be-
`tween ports two and seven of the first OC. An OSW is connected
`between port four of the first OC and port one of the second OC.
`If the OSW is in the bar state, the channel at the corresponding
`Bragg wavelength of the FBG connected between port three of
`the first OC and port two of the second OC is not dropped and
`goes through the device and exits the OUT port. If the OSW
`is in the cross state, the channel is dropped through port four
`of the first OC and the OSW, and the add channel at the same
`wavelength can enter port one of the second OC via the OSW.
`An OSW is also connected between port five of the first OC and
`port eight of the last OC. The OSW is used to drop/add a channel
`
`Fig. 2. WDM test setup using new RM-OADM structures.
`
`corresponding to the FBG connected between port six of the first
`OC and port seven of the last OC. The arrangement of FBGs and
`OSWs, and the add–drop operations from the second OC to the
`last OC, are the same as that of Structure I. The RM-OADM
`Structure II is also strictly nonblocking.
`The function of the first OC is to separate the through chan-
`nels from the add–drop channels, which makes the insertion loss
`independent of the number of OCs used. This is very important
`in networks with multiple OADM nodes and few or no in-line
`optical amplifiers. The insertion loss for through channels is
`, which is independent of the number
`of OCs used. Assuming
`dB and
`dB as
`before, for a RM-OADM with five add–drop channels, the in-
`sertion loss is 2.5 dB, which is the smallest among all existing
`RM-OADMs [1]–[6].
`For both RM-OADM structures, the in-band crosstalk be-
`tween the add–drop channels depends on the reflectivity of the
`FBG and the leakage through the OSW, whereas the out-of-band
`crosstalk from the adjacent to the drop channels depends on the
`FBG reflection of adjacent channels [6].
`
`III. EXPERIMENTAL SETUP AND RESULTS
`The performance of the new RM-OADMs is experimentally
`investigated in the eight-channel WDM test setup, shown in
`Fig. 2. The RM-OADMs consist of two eight-port OCs, and
`three FBGs of wavelengths 1549.32, 1550.92, and 1551.72 nm
`corresponding to channels one, three, and four of the input
`WDM signal, respectively. For Structure II, three pairs of
`gratings at the same Bragg wavelengths are used. The average
`interport insertion loss and isolation of each OC are 1.2 and
`45 dB, respectively. The 3-dB bandwidth, adjacent channel
`reflection and reflectivity of the FBGs are 0.3 nm,
`30 dB,
`and 99.99%, respectively, except for the second grating at
`1550.92 used in Structure II, which only has 99.7% reflectivity.
`We operate the RM-OADMs in such a way that all channels
`corresponding to the FBGs are add/dropped. No OSWs are
`used in the experiment. When OSWs are employed in the
`RM-OADM structures, the insertion loss for the reconfigured
`channels is increased and depending on the OSW leakage, the
`in-band crosstalk between the add and drop channels may be
`higher. Eight laser sources from 1549.32 to 1554.92 nm with
`0.8-nm spacing are multiplexed by an 8
`8 arrayed-wave-
`guide grating (AWG) and externally modulated with a
`nonreturn-to-zero pseudorandom bit sequence (PRBS) at
`2.5 Gb/s to represent eight WDM channels. The modulated
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`IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 13, NO. 10, OCTOBER 2001
`
`Fig. 3. Optical spectra at different RM-OADM ports. (a) IN port of both
`Structure I and II. (b) OUT port of Structure I without any add channels present.
`(c) OUT port of Structure II without any add channels present. (d) DROP1 port
`of Structure I with all add channels present. (e) DROP2 port of Structure I with
`all add channels present. (f) DROP3 port of Structure I with all add channels
`present.
`
`sources are split into two paths. Path one goes through 36.3 km
`of standard single-mode fiber (SMF) for bit decorrelation, and
`amplified by an erbium-doped fiber amplifier (EDFA) before
`entering the IN port of the RM-OADM. Path two is amplified
`by an EDFA, and demultiplexed by another 8
`8 AWG to
`represent three add channels. Optical attenuators are used after
`the AWG to equalize the add and through channels powers at
`the OUT port of the RM-OADM. The bit-error rates (BERs)
`and spectra of the three drop, and one through channels are
`measured at the DROP1, DROP2, DROP3, and OUT ports of
`the RM-OADM, respectively.
`The measured optical spectra at different RM-OADM ports
`are shown in Fig. 3. The spectrum at the OUT port of Struc-
`ture I without any add channels [see Fig. 3(b)] shows small
`in-band crosstalk of less than
`40 dB from the three drop chan-
`nels one, three, and four. The insertion loss for through channels
`for Structure I is 8.5 dB. Fig. 3(c) shows the spectrum obtained
`at the OUT port of Structure II. The insertion loss for through
`channel for Structure II is only 3.5 dB. Due to the poor reflec-
`tivity of the particular FBG used in the experiment for Struc-
`ture II, an in-band crosstalk of
`25 dB is observed at 1550.92
`nm. The spectra at the DROP1, DROP2, and DROP3 ports of
`Structure I with all add channels present [see Fig. 3(d)–(f), re-
`spectively] show very small out-of-band crosstalk of less than
`30 dB from the through channels and adjacent add channels.
`The spectra at different drop ports of Structure II are similar to
`those of Structure I (not shown).
`Fig. 4(a) and (b) show the BERs measured at the DROP1
`and DROP3 ports of Structure I with and without the add
`channels present, along with a back-to-back measurement for
`the drop channel one (1549.32 nm) and drop channel four
`(1551.72 nm), respectively. The result shows no power penalty
`between the back-to-back with and without add channels
`measurements, which indicates negligible crosstalk from the
`add and through to the drop channels. We also show BERs for
`
`Fig. 4. Bit-error rate curves for Structure I. (a) Drop channel one. (b) Drop
`channel four. (c) Through channel five.
`
`the through channel five (1552.52 nm) with and without the
`three add channels, together with a back-to-back measurement
`in Fig. 4(c). The result shows negligible penalty of 0.2 dB be-
`tween the back-to-back and the with and without add channels
`cases. This is due to imperfect filtering before the receiver. The
`BERs obtained for Structure II are similar to those of Structure
`I (not shown), which also indicates negligible in-band and
`out-of-band crosstalk from the add and through to the drop
`channels.
`
`IV. CONCLUSION
`We have proposed and demonstrated two new strictly non-
`blocking RM-OADMs using OCs and FBGs. By effectively
`using eight-port OCs, the new structures significantly reduce
`the component count and insertion loss and achieve good
`crosstalk performance. Structure II potentially achieves the
`lowest
`insertion loss for through channels among existing
`RM-OADMs.
`
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