Phoonics and Opoelecronics (P&O) Oc. 1, Vol. 1 Iss. 3, PP. 6-64 A Novel Phase-Noise Cancelled Opical Frequency Domain Reflecomery Using Modulaion Sidebands Shua Hiramasu, Kasushi Iwashia Kochi Universiy of Technology 185 Miyanokuchi, Tosayamada-machi, Kami-shi, 78-85, Japan hiramasu.h134@gmail.com; iwashia.kasushi@kochi-ech.ac.jp Absrac-We propose a novel OFDR mehod which improves spaial resoluion by cancelling laser phase noise. Laser ligh is modulaed wih linearly swep. Boh he backscaered ligh and he reference ligh are combined and divided ino upper- and lower-sideband componens by an filer. These wo componens are convered wo elecrical signals and muliplied each oher. The laser phase noise which is conained hese signals can be cancelled compleely by his muliplicaion. The proposed mehod is confirmed experimenally. The laser phase noise having 1.5-MHz linewidh is cancelled by his mehod. These resuls confirm ha his mehod is feasible o improve spaial resoluion for OFDR. Keywords- Phase Noise; OFDR; Inensiy-modulaion; Modulaion Sidebands I. INTRODUCTION Opical fibers have low loss and broadband properies. They are used for long-haul and huge capaciy ransmission sysems. Moreover, fiber characerisics can be changed by he environmen emperaure and he pressure which applied o hem. Therefore, fibers can be used for sensors. Opical domain reflecomery (OFDR) and ime-domain reflecomery (OTDR) [1][] are mehods for measuring reflecion poins over fibers. OTDR has been widely used o diagnose he break poin of fibers. The spaial resoluion of OTDR is limied by he pulse widh used, and is abou 1 o 1 m. OTDR is required o increase he power for long disance measuremens. On he oher hand, OFDR observes bea signals which are produced by he reference ligh and he backscaered ligh a reflecion poins. The bea frequencies are proporional o he disances from he reflecion poins because he source is linearly swep. OFDR has beer spaial resoluion and excellen sensiiviy han OTDR. However, he OFDR measuremen range is limied by he laser coherence lengh because laser phase noise causes serious degradaion o he signal-o-noise raio (SNR) as he measuremen disance approaches he laser coherence lengh. The coherence lengh is abou several ens of meer, because ypical linewidh of DFB-LD is a few MHz. Therefo re, i is impossible o apply his echnique o ransmission fibers. Recenly, he phase noise compensaion OFDR (PNC- OFDR) on he concaenaed reference mehod has been proposed [3]. By he PNC-OFDR, km have been repored o achieve sub-meer range resoluion measuremens[4]. However, his mehod need addiional reference arm, complicae signal processing and narrow linewidh source. In his paper, we propose a novel phase noise cancelled OFDR mehod. The proposed mehod is modulaed by linearly swep wih double sideband-suppressed carrier modulaion. The backscaered ligh and he reference ligh are combined and divided ino upper- and lowersideband. The divided lighs are deeced and each signals are muliplied each oher. Then he phase noise cancelled signals are obained. We will demonsrae he proposed mehod o find break poin. Nex secion, we will show he principle of he proposed phase noise cancelled OFDR mehod. Then we will show experimenal resuls o confirm our proposed mehod. II. PRINCIPLE OFDR mehod has meris of high spaial resoluion and high sensiiviy. The spaial resoluion L is deermined by L = c/ ( nfiber f) where c is he ligh velociy in vacuum, n fiber is he fiber refracive index and f is he sweep range. For insance, if f =5 GHz, he spaial resoluion L = cm. Bu, is measuring range is limied by he coherence lengh. The coherence lengh l c is calculaed as lc = c/ δ f where δf is laser linewidh. For example, if he linewidh is 1 MHz, he coherence lengh is 3 m. I corresponds o he measuring lengh of 15 m. Therefore, i is impossible o measure long disance fiber. Then, he OFDR mehod is used for only defecing poins of componens. We propose a novel phase noise cancelled OFDR mehod. Figure 1 shows he proposed configuraion, schemaically. Figure shows he principle of phase noise cancellaion. Alhough he ligh source of he ordinary OFDR is swep by saw eeh, ha of he proposed mehod is no swep. The ligh source is modulaed wih double sidebandsuppressed carrier. The field E() can be described as ( ω m πβ ) ωc θ ( ) { { ω+ πβ θ ( ) { ω πβ θ ( ) E ( ) = E cos + cos + E = cos + + + cos + where ω + =ω C +ω m, ω + =ω C -ω m, ω C is he carrier angular, ω m is he modulaion angular, β is he linear sweep rae, θ() represens he random phase resuled from phase noise. (1) - 6 -
Phoonics and Opoelecronics (P&O) Oc. 1, Vol. 1 Iss. 3, PP. 6-64 The angular of he firs erm is φ θ 1 = ωc + ( ωm + πβ ) + ( ) The angular of he second erm is φ c m ( ) θ = ω ( ω + πβ ) + The firs erm increases wih modulaion while he second erm decreases. However, he phase noise erm is added o boh. () (3) ω Opical is swep linearly. Reference ligh Probe ligh (c) Ligh source DSB-SC Fiber Under Tes ω Ch LPF DEMUX LPF Ch1 Fig. 1 Configuraion of he proposed OFDR. DSB-SC: double sideband modulaor, demux : demuliplexer When he ligh source is modulaed by he double sideband-suppressed carrier mehod, wo sidebands appear as shown in Fig.. The modulaed ligh is divided ino wo arms. One arm's ligh is for he reference ligh and acs as local oscillaor ligh for heerodyne deecion. The oher arm is fed o fiber under es () o find he reflecion poins along he fiber. Rayleigh backscaered ligh from is coupled wih he reference ligh as shown in Fig.. The refleced ligh wih he delay ime τ is expressed as α { ω+ ( τ ) πβ τ ) θ( τ ) cos { ω ( τ ) πβ ( τ ) θ( τ ) (4) E E ( ) = cos + ( + + + where α represens he fiber loss and he refleciv iy of he reurned signal, he delay ime τ =L/v, L is he fiber lengh and v is he ligh velociy in he fiber. The combined lighs are divided ino wo componens using he demul iplexer. The divided wo componens are deeced wih phoo deecors and convered o elecrical signals as shown in Fig.(c). The elecrical componens as signals are described wihou dc I1() cos{ πβτ + θ () θ ( τ ) + ϕ1 I () cos[ πβτ { θ () θ ( τ ) + ϕ ] (5) (d) Fig. Principle of he phase noise cancellaion. oupu specrum from he modulaor, specra reference ligh and signal, (c) deeced elecrical signal specra and (d) muliplied specrum. Where ϕ1 = ω+ τ πβτ, ϕ = ω τ πβτ. These frequencies are proporional o he delay ime τ. Therefore, he refleced poin can be known, bu, hey conain phase noise. The phase noise componens are he opposie sign wih he angular πβτ. By mul iply ing he wo signals, ake higher componen, hen he signal is ( ) ( ) cos( 4πβτ ϕ ϕ ) I I + + (6) 1 1 We can ignore he consan phase erm ϕ 1 +ϕ for his measuremen. The phase noise is compleely cancelled by his process and he componens βτ will appear. If he ligh source is swep linearly like OFDR, he modulaed and phase noise have he same sign and hey canno be cancelled. On he oher hand, by our mehod, he sidebands frequencies are swep by he applied sweep signal. Then he boh sidebands are spread and he sign has he opposie each oher bu he sign of he phase noise is he same. Then, he phase noise is cancelled by he mu lip lica ion. The meris of he proposed mehod are simple configuraion and his can cancel phase noise compleely. Moreover, since wo channel signals ravel he same pass, sable deecion can be realized because he wo signals experience he same environmens such as hermal disurbance, vibraion and so on. We confirm he proposed mehod by simulaion. We assume ha he ligh source has he linewidh of 1.5 MHz and modulaed by he 16 GHz/s linear sweep rae (β=1.6x1 11 Hz/s). The signal ligh is passed hrough a delay fiber o apply consan delay wih 16 µs (τ =1.6x1-4 s) which corresponds o MHz bea. The - 61 -
Phoonics and Opoelecronics (P&O) Oc. 1, Vol. 1 Iss. 3, PP. 6-64 ransmied ligh is combined wih he reference ligh. The combined lighs are divided ino wo componens, ha is, higher/lower componens compared wih carrier. The divided lighs are deeced and muliplied each oher. 8 6 6 8 6 6 7 6 5 3 1 1 3 5 6 Fig. 3 Specra for received a ch1 and ch and muliplied of he wo signal a simula ion The simulaed specra are shown in Fig.3. Bea signals appears around MHz which are broadened by phase noise as shown in Fig. 3 for each channel. The muliplied specrum as shown in Fig. 3 shows sharp peak a around MHz. This resul shows he phase noise is compleely cancelled by his process. A linewidh a db down from he peak in Fig.3 is khz which is resriced by he sampling period. The meris of his mehod are simple configuraion and his can cancel phase noise compleely and we can use normal DFB-LD. III. EXPERIMENTAL SET -UP The purpose of his experimen is o confirm he feasibiliy of he proposed mehod. Firs, we ry o check a consan reflecion poin, nex we measure he characerisics wih OFDR configuraion. The experimenal seup is shown in Fig.4. A Disribued Feed-Back (DFB)-laser diode wih he linewidh of 1.5 MHz a 155 nm is used as ligh source. The oupu is inensiymodulaed by LiNbO 3 dual drive MZM inensiy modulaor. The double sideband-suppressed carrier modulaion is performed by applying opposie phase signals o wo arms which is described as n C m n C m E( ) = J ( m)cos( ω + nω ) J ( m)cos{( ω + nω ) + nπ n = {1 ( 1) J ( m)cos( ω + nω ) n C m = + J 3( m)cos( ωc 3 ωm) + J 1( m)cos( ωc ωm) + J ( m)cos( ω + ω ) + J ( m)cos( ω + 3 ω ) + 1 C m 1 C m where m is he modulaion index. The swep signal is divided ino wo signals using 18 degree hybrid. The bias volage of he modulaor is adjused so as o suppress he carrier componen. The modulaed specrum wih 1 GHz consan modulaion is shown in Fig.5. The carrier componen is suppressed more han 18 db down fro m he signal. The modulaed ligh is amplified and divided ino wo arms. One is used for he local oscillaor as he reference ligh. The oher arm s ligh is used for he follo wing wo experimens. A. Confirmaion of phase noise cancellaion The oher arm's ligh is fed o he fiber wih he lengh of 5. km o clarify our proposed mehod in he firs experimen. The modulaion is swep from 1 GHz o 15 GHz wih 16 GHz/s average sweep rae. The reference ligh and he signal are combined. The combined lighs are divided using arrayed waveguide graing(awg) filer wih 5 GHz channel spacing and 18 GHz bandwidh. The carrier is se o he sop band(beween wo pass-band). Then, he firs lower/upper sidebands will pass hrough AWG and oupu a adjacen pors. The oupus of he wo pors are deeced by phoo deecors and sampled A/D converer( MS/s). The number of sampling poins is 1,, hen he sep afer FFT is khz. The deeced signals are mulip lied each oher. The specrum of he muliplied signal is measured. B. Phase noise cancellaion of Rayleigh backscaer signals In he second experimen, 5. km single-mode fiber is conneced o he circulaor as. Moreover, km single-mode fiber is insered beween he circulaor and he coupler. This is because he fold back of he received specrum decreases he performance in shor disance. The modulaion is swep from 1 GHz o 15 GHz. Sweep rae is 9.6 GHz/s. The following seup is he same as firs experimen. Laser 1~15GHz sweep AM 18 HYB Trigger OAMP AWG(5Hz) : Polarizaion Conroller AM : Ampliude Modulaor : Fiber Under Tes AWG : Arrayed Waveguide Graing Ch1 A/D converer BPF AWG(5Hz) BPF Fig. 4 Experimen al se-up Ch Delay 5.km 5.km Delay km - 6 -
Phoonics and Opoelecronics (P&O) Oc. 1, Vol. 1 Iss. 3, PP. 6-64 - -5-3 -35 18dB - -45-5 -55-65 1549.35 1549.6 1549.85 155.1 155.35 Wave lengh[nm] Fig. 5 Opical specrum wih double sideband-suppressed carrier modulaion IV. EXPERIMENTAL RESULT The firs experimenal resuls are shown in Fig.6. Figure 6 is he specra which are he individual arm and Fig. 6 is he mulip lied specrum wih wo arm signals. The sweep rae is 16 GHz/s and he fiber delay corresponds o 16 µs, hen he bea is appeared around MHz including he phase noise. The specra are broadened by phase noise. Figure 6 shows a sharp peak a around MHz and noise level depressed by his muliplicaion. A linewidh a db down from he peak as shown in he inse of Fig.6 is 1 khz. The measured linewidh is broader han he simulaed one. This is considered ha he propagaion ime fro m he coupler o phoo deecors is no same. -8-1 -1 6-1 -1-15 -16-8 -1-1 6 1dB -15-17 37 37.5 38 38.5 39-17 1 3 5 6 Fig. 6 Specra for received a ch1 and ch and muliplied of he wo signal Second experimenal resuls are shown in Fig. 7. The divided phase-modulaed ligh is fed o which is 5. km single-mode fiber. The backscaered ligh is passed hrough km single-mode fiber o add he delay which is for -7-8 -9-1 -1 5 1 15-1 -1-15 -16 5 1 15 Fig. 7 Spec ra for received a ch1 and muliplied of he wo signal he heerodyne deecion. This ligh is deeced as he previous experimen does. Figure 7 shows he deeced specrum a channel 1. There is a dull peak around 7.5 MHz. I is broadened by laser phase noise. The muliplied specrum shown in Fig.7 has a Fresnel sharp peak a around 15 MHz. This means ha he phase noise is compleely cancelled by his mehod. Therefore, his spaial resoluion can be improved. V. CONCLUSION We have proposed a novel OFDR mehod which improves spaial resoluion by cancelling laser phase noise. Opical source wih fin ie linewidh is phase modulaed wih modulaion sweep insead of sweeping he source. The modulaed signal is divided ino he reference ligh and he es signal. The backscaered ligh and he reference ligh are combined and filered o divide uppersideband and lower-sideband. These wo signals are deeced and convered o elecrical signals and muliplied each oher. The oupu signal is phase cancelled. The proposed mehod is confirmed by he preliminary experimen. The laser phase noise having 1.5 MHz linewidh is cancelled by his mehod. This resul confirming ha his mehod is feasible o improve spaial resoluion for OFDR. We are going o invesigae he characerisics of mu liple reflecion poins. REFERENCES [1] J. Rogers, Polarizaion- ime domain reflecomeey : a echnique for he measuremen of field disribuions, Opical Sociey of America, Appl. Op. 1981, 16-174. [] W. Eickhoff and R. Ulrich, Opical domain reflecomery in single mode fiber, Appl 1981, Phys. Le., vol. 39, pp. 693 695. [3] X. Fan, Y. Koshikiya, and F. Io, Phase-Noise-Compensaed Opical Frequency-Domain Reflecomery, IEEE J. Quanum Elecron., vol. 45, no. 6, june, pp.594, 9. - 63 -
Phoonics and Opoelecronics (P&O) Oc. 1, Vol. 1 Iss. 3, PP. 6-64 [4] Y. Koshikiya, X. Fan and F. Io, -km Range, 1-m Resoluion Measuremen Based on Phase-noisecompensaed Coheren Opical Frequency Domain Reflecomery, ECOC 8, Sepember 8, Brussels, p.1.11, 8. Shua Hiramasu received degr ees in elecrical engineering from Kochi Naional College of Technilogy, Kochi, in 9. and in elecrical and phoniics engineering from Kochi Universiy og Technology, Kochi, in 11.He is maser cource suden a he graduae school of elecrical and phoonic engineering cource, a Kochi Universiy of Technology. His research ineres includes fiber sensors. Kasushi Iwashia received B.S., M.S., and Ph.D. degrees in elecrical engineering from Kyusyu Universiy, Fukaoka, in 1977,1979 and 1987, respecively. In 1979, he joined he Yokosuka Elecrical Communicaion Laboraory, NTT, where he has communicaion and adapive newark conrol and manageman echnologies. From 4, he has served as a professor of Kochi Universiy of Technology. He has received he IECE Young Engineers Award in 1985 and direcor of Science and Technology Agency Award. He is an IEICE Fellow. - 64 -