Polarization resolved measurement of Rayleigh backscatter in fiber-optic components

Transcription

1 1. Introduction Polarization reolved meaurement of Rayleigh backcatter in fiber-otic comonent B. J. Soller, M. Wolfe, M. E. Froggatt Luna Technologie Inc., 2020 Kraft Dr. Suite 2000, Blackburg, VA Fiber-otic comonent, module and aemblie for modern networking alication are becoming more comlex. At the comonent and module level, failure uch a bad lice, bend, crim, and other nonreflective event can be extremely difficult to locate and troublehoot. The available meaurement technique deigned to identify thee tye of failure fall into two baic categorie: otical time domain reflectometry (OTDR), and frequency domain reflectometry. Tyically, OTDR lack ufficient atial reolution to be ueful at the comonent and module level where one might be intereted in, ay, locating a bad lice in a concatenation of a dozen WDM comonent. Otical frequency domain reflectometry (OFDR) i a tunable laer-baed frequency domain technique that ha everal ditinct advantage over time domain and low coherence technique when the otical ytem under conideration are everal ten of meter in length [1,2]. Secifically, OFDR ha advantage in that it i caable of ub-millimeter reolution meaurement over multile ten of meter of otical length with high enitivity and dynamic range. In thi aer, we introduce a method for fiber-otic teting and troublehooting at the aembly level that i baed on uing OFDR to meaure the ditributed Rayleigh backcatter along the length of the fiberotic network. Rayleigh cattering in otical fiber i caued by material denity and radiu fluctuation along the length of the core. Meauring the ditribution of the light cattered in the backward direction a a function of length down a fiber-otic aembly can be ueful in identifying break, bad ice and nonreflective event. Rayleigh catter can alo be ued to meaure ditributed lo and gain [3], induced tre and train [4], temerature [5], and local birefringence [6]. We reort olarization reolved meaurement of the ditributed reflection in an otical ytem with -130 db enitivity and 60 db dynamic range with 40 µm reolution over 30 meter of otical length. The alication reented how that the ytem can be ued to quantitatively meaure both inertion and return lo with accuracy and reolution on the order of 0.1 db. We alo how that the ytem can be ued to track change in the tate of olarization (SOP) of the robe light a it roagate through the network. Thi method ha ignificant advantage in range, reolution, eed and uability when comared to conventional reflectometer. 2. Exeriment The otical network ued to meaure olarization reolved Rayleigh backcatter i hown in Fig. 1. Light from a tunable laer ource i lit into meaurement and reference otical ath. In the meaurement ath, the light i further lit by a 50/50 couler. A third couler i ued to recombine the light from the meaurement ath with the light from the reference ath. After recombination, the light i lit by a olarization beam litter. Interference i detected at two PIN hotodiode that are connected via amlification circuitry to a 12-bit, 5 mega-amle er econd National Intrument data acquiition card (DAQ). The laer ource i a commercially available continuouly tunable external cavity laer diode. The tyical tuning range i from 2.5 nm to 40 nm centered at 1550 nm with a tuning rate of 20 nm/. The laer i controlled via analog voltage ram from the DAQ card. The line-width of the laer i 200 khz. Not hown in the figure i an auxiliary interferometer ued to monitor hae error during laer tuning. Thi technique i called triggered acquiition and i common in OFDR ytem to remove laer tuning error from the data [1]. Alo not hown i a ortion of the network wherein a HCN ga-cell i ued to monitor the intantaneou wavelength of the canning laer. Hence, for a given can data i being taken acro the DAQ on four channel imultaneouly.

2 Fig. 1. Otical network ued to erform olarization divere meaurement of Rayleigh backcatter. The couler are all 50/50 and the ADC rereent a high-eed digital data acquiition card (12 bit, 5 mega-amle er econd). The maximum meaurable length for thi intrument i determined by the amling reolution in the frequency domain which i in turn determined by the hyical delay difference of the auxiliary interferometer ued for data triggering. In thi cae the delay in the auxiliary interferometer wa 800 n. Dividing thi number by 2 twice, once for Nyquit and once becaue of the double-a nature of the meaurement interferometer, give u a maximum meaurable delay difference of 200 n, or a maximum length differential of 40 m in fiber. Of coure a longer differential delay in the auxiliary interferometer would reult in acce to more length in the meaurement arm. Thi raie a central difficulty. The bandwidth in the acquiition and amlification electronic mut be carefully choen o a to eliminate a much noie from the meaurement a oible. Meaurement of longer length require greater bandwidth which raie the noie level in the ytem beyond the Rayleigh level. 3. Interference and olarization effect Conider a reflection within the device under tet (DUT) with a time-of-flight delay differential with the reference ath of τ. For a laer wee decribed by the intantaneou frequency ω() t = ω + γt + σ() t where γ i the tuning rate and σ(t) i the (mall) tuning error, interference ignal at o the detector labeled and in Fig. 1 for thi reflection will be of the form r r E () 2 ( ) ( ˆ ) ( ˆ t = rτ g τ TE mea TEref) co ( ωτ o + γτt+ σ( t τ) τ + φτ ), (1) r r E () 2 ( ) ( ˆ ) ( ˆ t = rτ g τ TEmea TEref) co ( ωτ o + γτt+ σ( t τ) τ + φτ ), (2) where r τ and φτ are the amlitude and hae of the comlex reflectivity of the event, E r mea i the vector electric field amlitude from the meaurement ath at the beam litter, E r ref i the vector amlitude from the field in the reference arm, T ˆ and T ˆ are rojection oerator that rereent the orthogonal tate of the beam litter [7] and g ( τ ) and g ( ) τ are the frequency deendent gain of the two data acquiition channel. Equation (1) and (2) clearly illutrate the olarization enitivity of the OFDR technique. In the abence of the beam litter and two-channel acquiition, the interference amlitude i a function of the relative alignment of the reference and meaurement field. The olarization controller in the reference arm of the network combined with the olarization beam litter allow the reference field to be aligned uch that it i lit evenly between the two tate and of the beam litter. Thi enure that an interference ignal will be reent on at leat one of the detector irreective of the olarization tate of the field reflected from the DUT; a condition not necearily met in the abence of the beam litter. Thi i of articular imortance for the reult decribed here, becaue for a given launch tate in the meaurement arm, mall amount of birefringence along the length of the DUT caue the olarization tate of the reflected ignal to be comletely undetermined.

3 Fig. 2. The reflectivity of a 25 meter ool (2 in radiu) of tandard ingle-mode fiber. The olid curve i the vector um of the two dahed curve. The dahed curve are the amlitude of the Fourier tranform of the interference ignal meaured on the two detector hown in Fig. 1 labeled and. The ocillation in the dahed curve are caued by birefringence (or PMD) induced in the fiber due to the wind. The equation above that decribe the interference are written a a function of time, t. Thi i the lab time and reflect the fact that the tunable laer i canned a a function of time, ω = ω() t. So, Eq. (1) and (2) are a frequency domain decrition of the DUT. By tranforming thee equation to the time domain, we can build a rereentation of the reflection in a device or aembly under tet a a function of length. Thi i accomlihed by a Fourier tranform of the frequency domain reult into the time domain. Figure 2 how time domain reult for a 25 meter ool of fiber. The laer can wa from 1535 nm to 1575 nm which correond to a bandwidth of 5 THz. The atial reolution of in the data i dictated by the can bandwidth according to z=c/2n g f where n g i the grou delay of material under tet, c i the eed of light and f i the bandwidth of the meaurement can. For thi can thi come to 20 µm. In reality, mall amount of reidual dierion, laer tuning error and other environmental effect combine to reult in a two-oint reolution of cloer to 40 µm. Scanning at 20 nm/, the total meaurement time including roceing for thi data et i aroximately 3.5 econd. No averaging wa erformed to achieve thi data, however the data dilayed in Fig. 2 wa filtered with an effective bandwidth of aroximately 10 mm. The two dahed curve rereent the amlitude of the Fourier tranform of the ignal on the - and - detector. If the Fourier tranform of the frequency domain ignal E ( t) and E () t (remembering that thee are function of frequency, ω=ω(t), and t i in the lab frame) then the tranform can be written: E% ( τ ) and E% ( τ ). The total reflectivity in the time domain i given by the vector um 2 2 r( τ ) = E% ( τ) + E% ( τ). (3) The olid curve in Fig. 2 i the vector um of the two Fourier tranformed ignal E% ( τ ) and E% ( τ ). Note that erforming thi oeration in the time domain a ooed to the frequency domain i advantageou in that one avoid the non-linear oeration of umming and quaring inuoidally varying ignal. The abolute referencing of the reflectivity axi i accomlihed by meauring the reone of a nearly erfectly reflecting gold lated olihed fiber ti. In doing thi, one mut confirm that the amlifier reone i linear over the entire range of the ytem (130 db). 4. Reult and dicuion The ditributed reflectivity of the fiber hown in Fig. 2 at -120 db i due to the Rayleigh backcatter. The figure alo how the noie floor of the ytem (beyond 26.5 m) at about -130 db. Excluding the lo in the firt connector, thi give a 13 db overhead from the catter level to the noie floor. With a carefully calibrated ytem, thi dynamic range allow one to monitor the level of Rayleigh catter to determine certain metric of the DUT. Secifically, inertion lo and return lo event can be located atially with high reolution, and their value can be meaured quantitatively.

4 Fig. 3. Rayleigh backcatter meaured through three known bad lice in SMF-28. Becaue the backcatter exerience the lo twice, a 0.1 db lo i recorded a a 0.2 db dro in the backcatter level. The ame length of fiber wa ued leading u to each lice. Note the reeatability in the catter ignature from the fiber leading u to the lice. Figure 3 dilay the reult of a meaurement of the backcatter through three known bad lice. The backcatter level can be integrated on either ide of the lice and comared in order to get a meaure of the lice inertion lo. In thi cae, the inertion lo reolution i on the order of 0.05 db, a all three bad lice are clearly identifiable. Becaue the reflected wave exerience the lo of an event twice, once for forward roagation and once backward, the backcatter level from the aembly through a lo event dro by two time the amount of lo. That i, for a 0.1 db lo, the backcatter level dro 0.2 db. The ame length of fiber wa ued leading u to each lice hown in Fig. 4. Note the reeatability in the catter ignature from the fiber leading u to the lice. Thi i tyical of the Rayleigh catter ignature in otical fiber. Though we have not conducted extenive tudie, it aear that thi ignature i ermanent. That i, for a given length of fiber, the Rayleigh catter ignature i tatic over a very long eriod of time. Thi lead one to believe that thi ignature can actually be ued to ermanently identify certain imortant fiber length and locate them in a comlex otical network. An early demontration of thi caability i given in reference 5. We teted the inertion lo accuracy by meauring a nine ole otical witch uing both the method decribed here and the more tandard cut-back and lice method. The reult are hown in Fig. 4. The to anel dilay the meaurement of the catter level through the witch including connector on both the inut and outut ide and the different length of fiber through the aembly. Figure 4 (b) how a blow u of the witching ection of the network. The individual interface within the witching mechanim are clearly reolved. The dro in the catter internal to the witch i mot likely due to ection of free ace and other material that don t catter a trongly a fiber. Uing otical backcatter reflectometry, the inertion lo on either ide of the witch i etimated by averaging the cattering ignature over a certain length. For thi examle, the integration width wa et to 0.25 m. The meaured lo value are hown in Fig. 4 (c). The reult uing the backcatter reflectometer decribed here and the tandard licing method how agreement to within 0.1 db. It i worth noting that for the examle of lo meaurement through a witch, there i a ubtantial time aving when uing the reflectometric technique. The reflectometric technique require only one otical connection to meaure the inertion lo of every channel of the witch; it require only that the witch tate i toggled to meaure the lo through each ort. Thi reulted in a total time of tet of 4.5 minute uing backcatter reflectometry a comared to an average time of tet of 20 minute uing the tandard technique. The reflectometric technique ha the further advantage of being able to meaure the lo of the witch, in a ingle can, both with and without the connector. So, when accuracie of 0.1 db are required, the reflectometric technique dilay clear advantage over conventional technique. The lat examle i hown in Figure 5. Thi figure dilay the reflectivity through a 1 db ta couler. One of the fiber on the outut ide of the couler i inched off very cloe to the couler. The other fiber i about 8 meter long and a bad lice i clearly viible jut after nine meter. In the inet of thi figure i dilayed an exanion of the couling region. Thi couler how a ignature that i tyical of fued taered couler. In the taer region, the catter level dro. We believe that thi i caued by the mixing of the mode field of the two couled fiber and the tranfer of ower denity from the core of the fiber to the cladding.

5 (a) (b) (c) Fig. 4. (a) Reflection through one ole of a nine ole otical witch howing both inut and outut connector and fiber, and the internal witching mechanim, (b) a high reolution of the witching mechanim, and (c) inertion lo data uing otical backcatter reflectometry (OBR, black dot) and a ower meter and licing (white dot). Fig. 5. Backcatter reflectivity through a 1 db ta couler with one outut howing a bad lice.

6 5. Concluion We have demontrated a method for olarization divere meaurement of Rayleigh backcatter in otical fiber and fiber-couled comonent. With thi method, we achieved 40 µm reolution meaurement over 30 m of otical aembly length in a ingle can of a tunable laer, the entire roce taking only econd. Senitivitie of greater than -125 db were achieved by uing a tunable laer with favorable hae noie characteritic, uing quite amlification electronic and carefully chooing the receiver bandwidth. We oberved a total dynamic range of jut over 60 db. That i, reflection in a tet ytem tronger than -70 db began to affect the level of the noie floor. Though 60 db dynamic range i quite good, it i likely that thi could be extended by the ue of higher reolution DAQ electronic. Careful calibration of the effect of the amlifier on the backcatter ignal a a function of length allow the ytem to be ued a both a recie meaurement tool for both return lo and inertion lo. Otical backcatter reflectometry not only allow one to locate reflective event in an aembly, but to alo locate non-reflective inertion lo event uch a bend and bad lice. Comarion with conventional technique how that the backcatter technique i caable of reolving inertion lo event of 0.05 db and etimating the lo to within 0.1 db. The backcatter technique ha ignificant advantage in eed of tet when comared to traditional technique. Backcatter reflectometry alo allow one to meaure loe in a ytem indeendent of connector. While the ytem i olarization enitive, the meaurement reult rereent the reone of the DUT to a ingle inut olarization tate. Polarization deendent loe can be meaured by averaging data et taken over a diverity of inut olarization tate. Invetigation into the efficacy of thi technique are underway. The two channel olarization diverity detection give thi technique the ability to track change in the olarization tate of the light that roagate through the device or ytem under tet. Thi unique and owerful feature allow one to identify location within a device or aembly that have the great effect on the olarization tate of the light. Thi i evident in the beating of the two dahed curve in Fig. 2. It hould be noted that the olarization tracking i not accomlihed by meauring the actual olarization tate of the light a it roagate down an aembly. Polarization tracking i accomlihed by monitoring the amlitude of the interference between the light reflected from the DUT and the two reference field determined by the olarization beam litter. However, the information rovided by thi method i quite ueful nonethele when faced with diagnoing olarization related iue in an aembly of comonent. The technique reented here ha ditinct advantage in range, reolution, eed, accuracy, enitivity and dynamic range for meaurement alication from the comonent to the aembly level. Becaue it i baed on the ue of widely available tunable laer technology and relatively imle hardware, we believe that equiment baed on thi technique will rove to be indienable to the roce of building, teting and troublehooting otical equiment. Reference [1] U. Glombitza and E. Brinkmeyer, Otical frequency domain reflectometry for characterization of ingle-mode integrated otical waveguide, J. Lightwave Tech. 11, (1993). [2] J. P. von der Weid, R. Pay, G. Mui, and N. Giin, On the characterization of otical fiber network comonent with otical frequency domain reflectometry, J. Lightwave Tech. 15, (1997). [3] J. P. von der Weid, R. Pay, and N. Giin, Coherent reflectometry of Otical Fiber Amlifier, IEEE Photon. Tech. Lett. 9, (1997). [4] M. Froggatt and J. Moore, High reolution train meaurement in otical fiber with Rayleigh catter, Al. Ot. 37, (1998). [5] M. Froggatt, B. Soller, D. Gifford, and M. Wolfe, Correlation and keying of Rayleigh catter for lo and temerature ening in arallel otical network, OFC Technical Diget, Lo Angele, March, 2004, aer PDP 17. [6] B. Huttner, J. Reecht, N. Giin, R. Pay, and J. P. von der Weid, Local birefringence meaurement in ingle-mode fiber with coherent otical frequency-domain reflectometry, IEEE Photon. Tech. Lett. 10, (1998). [7] Thee oerator denote the litting of the inut light by the beam litter into two orthogonal olarization denoted and.