Localization and quantification of reflective events along an optical fiber using a bidirectional TRA technique Min Cen, * Véronique Moeyaert, Patrice Mégret, and Marc Wuilpart Electromagnetism and Telecommunication Department, Faculté Polytechnique, Université de Mons, Boulevard Dolez 31, 7000 Mons, Belgium * [email protected]
Abstract: We report on the theory and the implementation of a novel approach for the detection and localization of a reflective event along a fiber link. By launching a continuous-wave signal into both fiber ends and by analyzing the transmitted and reflected/backscattered optical powers, it is possible to localize an optical event and to quantify the induced insertion and return losses simultaneously. The novel idea of utilizing bi-directional measurement allows the localization of both reflective and non-reflective events. Theoretical and experimental studies show that for a 10 km-long single mode fiber, the localization accuracy can be in the range of 5.0 m. ©2014 Optical Society of America OCIS codes: (060.2270) Fiber characterization; (060.2370) Fiber optics sensors; (280.1350) Backscattering; (290.5870) Scattering, Rayleigh.
References and links 1. 2.
D. Derickson, Fiber Optic Test and Measurement (Prentice Hall PTR, 1997), chap. 11. K. Yüksel, M. Wuilpart, and P. Mégret, “Analysis and suppression of nonlinear frequency modulation in an OFDR,” Opt. Express 17, 5845 (2009). 3. J. W. Berthold, “Historical review of microbend fiber-optic sensors,” J. Lightwave Technol. 13(7), 1193–1199 (1995). 4. E. Udd and W. B. Spillman, Fiber Optic Sensors: An Introduction for Engineers & Scientists, 2nd Edition (Wiley, 2011), chap. 6. 5. V. V. Spirin, F. J. Mendieta, S. V. Miridonov, M. G. Shlyagin, A. A. Chtcherbakov, and P. L. Swart, “Localization of a Loss-Inducing Perturbation With Variable Accuracy Along a Test Fiber Using TransmissionReflection Analysis,” IEEE. Photonic Tech. L. 16(2), 569–571 (2004). 6. V. V. Spirin, M. G. Shlyagin, S. V. Miridonov, and P. L. Swart, “Transmission/reflection analysis for distributed optical fiber loss sensor interrogation,” Electron. Lett. 38(3), 117–118 (2002). 7. A. Girard, FTTx PON technology and testing (EXFO Electro-Optical Engineering Inc., 2005), chap. 3. 8. A. D. Kersey and A. Dandridge, “Applications of fiber-optic sensors,” IEEE Trans. Compon., Hybrids, Manuf. Technol. 13, 137–143 (1990). 9. L. Thevenaz, Advanced Fiber Optics: Concepts and Technology (CRC, 2011), chap. 8. 10. W. Lee, S. I. Myong, J. C. Lee, and S. Lee, “Identification method of non-reflective faults based on index distribution of optical fibers,” Opt. Express 22(1), 325–337 (2014). 11. G. Lietaert, JDSU White Paper, “Fiber Water Peak Characterization” (JDSU, 2009). http://www.jdsu.com/ProductLiterature/fiber-water-peak-characterization_fwpc_wp_fop_tm_ae.pdf.
1. Introduction Nowadays, Distributed Monitoring Systems (DMS) are widely used for fiber monitoring in both telecommunication and sensing applications. Among them, Optical Time Domain Reflectometry (OTDR) and Optical Frequency Domain Reflectometry (OFDR) are commonly used. However, OTDR and OFDR require either time- or frequency-modulated light sources, which makes them not cost-effective. Moreover, improvements and breakthroughs are still required to address certain limitations, e.g. long measurement time (several minutes are normally needed for monitoring a fiber with a length of tens of kilometers by OTDR [1]) and short measurement range (only a few hundred meters for OFDR [2]). In addition to these two methods, other DMS can be found in the literature, such as bending-based and Fabry-Perot interferometer sensors [3,4]. However, for those solutions, the measurand should be
#207359 - $15.00 USD (C) 2014 OSA
Received 28 Feb 2014; revised 3 Apr 2014; accepted 7 Apr 2014; published 16 Apr 2014 21 April 2014 | Vol. 22, No. 8 | DOI:10.1364/OE.22.009839 | OPTICS EXPRESS 9839
associated with complex manufacturing techniques of the sensing fiber. In [5,6], a Transmission-Reflection Analysis (TRA) method is reported, which is based on the measurement of transmitted and backscattered powers of an un-modulated light source. This method was proposed for the detection and localization of a non-reflective optical event, such as a fiber bending. However, most optical events in a fiber are reflective such as discontinuities, misalignments, and breaks that can notably be present in telecommunication networks [7]. In sensing applications, some transducing processes also result in the presence of a reflective event along the sensing fiber [8]. In order to address the aforementioned problems, a novel solution is proposed to localize both non-reflective and reflective optical events by using a Bi-Directional TransmissionReflection Analysis (BD-TRA) technique. In addition, the proposed interrogation scheme is able to quantify the return loss of the reflective event. 2. Operating principle The TRA technique is based on the unique relationship between the transmitted (PT) and the backscattered (PB) powers for a given loss (i.e. non-reflective event) location [5]. The definition of transmitted (PT) and backscattered (PB) powers can be found in Fig. 1. PT is the power transmitted through the fiber link when a Continuous optical Wave (CW) of power P0 is launched. PB is described as the power reflected/backscattered all along the fiber link. PB is mainly due to Rayleigh backscattering, which results from the presence of inhomogeneities in the fiber material [9]. In the presence of loss for a given fiber, the relationship between PT and PB only depends on the loss location. The measurement of PB and PT therefore allows localizing the event [5]. The TRA approach already presented in the literature is only suitable for non-reflective events.
Fig. 1. Definition of transmitted (PT) and backscattered (PB) powers.
In order to make the TRA approach compatible with reflective events, a novel BiDirectional TRA (BD-TRA) technique is presented in this paper. The schematic diagram of the proposed BD-TRA is shown in Fig. 2. In Fig. 2(a), a continuous-wave light emitted by a Super Luminescent Diode (SLD) is launched into a single mode fiber (Corning® SMF-28e + ® optical fiber) through an optical circulator (from point 1 to point 2, forward measurement). An optical isolator is implemented to minimize the back reflections from the fiber end. The transmitted power (PT1) is measured by the powermeter located after the isolator. The integrated reflected/Rayleigh backscattered power (PB1) is measured by the second powermeter connected to the circulator. The same measurement is repeated a second time by emitting the light from the other end of the same fiber in the opposite direction (i.e. see Fig. 2(b), from point 2 to point 1, backward measurement) in order to get the corresponding transmitted and backscattered powers (PT2 and PB2 respectively). Note that for future applications, a 2 × 2 switch can be applied to realize an automatic interchange process. As developed hereafter, the localization process of a reflective event can be based on the unique relationship between the backscattered (PB1 and PB2) and transmitted (PT1 and PT2) powers for a given event location zp and return loss (RL).
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Received 28 Feb 2014; revised 3 Apr 2014; accepted 7 Apr 2014; published 16 Apr 2014 21 April 2014 | Vol. 22, No. 8 | DOI:10.1364/OE.22.009839 | OPTICS EXPRESS 9840
Fig. 2. Schematic diagram of BD-TRA technique. (a) Forward measurement for getting PT1 and PB1. (b) Backward measurement for getting PT2 and PB2.
Let us consider a fiber with a total length L and an initial transmitted power (PT0) of the undisturbed reference system (here we assume that PT0 are not changed with the measurement direction): PT 0 = P0 ⋅ 10
(−
ILcir 1 ) 10
⋅ T ( L ) ⋅ 10
(−
ILiso ) 10
(1)
.
where P0 is the input power, ILcir1 [dB] and ILiso [dB] are insertion losses of the circulator (from port a to port b) and isolator respectively, T(L) is the fiber transmission coefficient associated with the fiber attenuation coefficient α, which can be expressed as: T ( Δx ) = e −αΔx . Taking into account the directivity of the circulator DIR [dB] (i.e. power transmitted directly from port a to port c) and the return loss of the isolator RLiso [dB], the total backscattered/reflected power of the initial undisturbed system (PB0) can be expressed as: PB 0 = P0 ⋅ [10
(−
DIR 10
)
+ RAY ( L ) + T ( L ) ⋅ 10 2
(−
RLiso ) 10
] ⋅ 10
(−
ILcir 2 ) 10
.
(2)
where ILcir2 [dB] is the insertion losses of the circulator (from port b to port c), RAY(L) is the Rayleigh backscattered power coefficient that can be expressed as [9]: RAY ( Δx ) = S ⋅
αS 2α
⋅ (1 − e
−2 αΔx
).
(3)
αs is the scattering coefficient due to the Rayleigh scattering and is proportional to 1/λ4 (0.34 dB/km at 1310 nm [9]), Δx is the length of the fiber segment and S is the capture coefficient which is 10−3 at the wavelength of 1310 nm [9]. Let us now consider that a reflective event with a return loss RL [dB] (practically speaking, the RL for a non-reflective event is usually higher than 65 dB, e.g. bending) and an insertion loss IL [dB] is introduced into the fiber at the event location zp. RL is defined as: 10 ⋅ log10 (ϕ ) , where φ is the ratio between the incident and reflected powers of the event. Here we suppose that RL and IL are not changed with the measurement direction. The transmitted (PT1) and the backscattered (PB1) powers for the forward measurement (as shown in Fig. 2(a)) can be written as: PT 1 = PT 0 ⋅ 10
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(−
IL
10
)
.
(4)
Received 28 Feb 2014; revised 3 Apr 2014; accepted 7 Apr 2014; published 16 Apr 2014 21 April 2014 | Vol. 22, No. 8 | DOI:10.1364/OE.22.009839 | OPTICS EXPRESS 9841
PB1 = P0 ⋅ [10
(−
DIR ) 10
T ( L ) ⋅ 10 2
+ RAY ( z p ) + T ( z p ) ⋅ 10 2
IL (− ) 5
⋅ 10
RL ( − iso ) 10
] ⋅ 10
IL ( − cir 2 ) 10
(−
RL ) 10
+ [ RAY ( L) − RAY ( z p )] ⋅ 10
(−
IL ) 5
+
(5)
.
For the backward measurement (the optical signal is launched from point 2 as shown in Fig. 2(b)), PT2 and PB2 can be expressed as: PT 2 = PT 0 ⋅ 10
PB 2 = P0 ⋅ [10
(−
DIR ) 10
(−
IL 10
)
(6)
.
+ RAY ( L − z p ) + T 2 ( L − z p ) ⋅ 10
[ RAY ( L ) − RAY ( L − z p )] ⋅ 10
IL (− ) 5
(−
+ T ( L ) ⋅ 10 2
RL ) 10
IL (− ) 5
+ ⋅ 10
(−
RLiso 10
)
] ⋅ 10
(−
ILcir 2 10
(7) )
.
With Eqs. (2), (5) and (7), the normalized power reflection coefficient R1 and R2 can be written as: R1 =
PB1 PB 0
=
10
DIR RL IL ) (− ) (− ) 10 + RAY ( z ) + T 2 ( z ) ⋅ 10 10 + [ RAY ( L ) − RAY ( z )] ⋅ 10 5 p p p
(−
10
DIR ) 10
(−
+ RAY ( L ) + T ( L ) ⋅ 10 2
RLiso IL (− ) (− ) T 2 ( L ) ⋅ 10 5 ⋅ 10 10
10
(−
DIR ) 10
+ RAY ( L ) + T ( L) ⋅ 10
10 P R2 = B 2 = PB 0
2
( − DIR ) 10
10
(−
RLiso ) 10
(−
RLiso 10
2
10
+ RAY ( L) + T ( L) ⋅ 10 2
[ RAY ( L ) − RAY ( L − z p )] ⋅ 10
10
(−
DIR ) 10
(−
IL ) 5
(8)
.
+ RAY ( L − z p ) + T ( L − z p ) ⋅ 10
( − DIR )
+
)
(−
10
RLiso 10
+ T ( L) ⋅ 10 2
+ RAY ( L) + T ( L) ⋅ 10 2
( − RL )
(−
(−
+
) IL ) 5
⋅ 10
RLiso ) 10
(−
(9)
RLiso ) 10
.
Since DIR, α, αs, RLiso and L are known parameters, and since PB0, PB1, PB2 and PT0, PT1, PT2 can be obtained by measurements, the problem defining zp and RL finally turns in solving a set of two equations with only two unknown variables (i.e. zp and RL). Based on the above calculation, analytical studies have been undertaken. The parameters used in the calculation model are listed in Table 1. Here we consider a reflective event with a 10 dB insertion loss and an event location (zp) varied from 0 to L. The event return loss (RL) in our simulation is varied from 20 to 80 dB to cover different applications (e.g. for a fiber break RL is usually less than 30 dB, while referring a fiber bending, a 70 dB return loss is normally employed). Table 1. Parameters related to the calculation model [9]
Interrogating wavelength λ
S
α
αs
DIR
RLiso
1310nm
0.001
0.34 dB/km
0.33 dB/km
60 dB
65 dB
L 6 km
Figures 3(a) and 3(b) show the relationship between the normalized power reflection coefficients (i.e. R1 and R2), the return loss (RL) and the event location (zp) when a single reflective event is introduced. We can clearly see the variation of R1 and R2 with the change of the event return loss and location. In Figs. 3(a) and 3(b), one can see for an introduced event
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Received 28 Feb 2014; revised 3 Apr 2014; accepted 7 Apr 2014; published 16 Apr 2014 21 April 2014 | Vol. 22, No. 8 | DOI:10.1364/OE.22.009839 | OPTICS EXPRESS 9842
with a fixed RL and zp, there is only a single corresponding (R1, R2) point. Therefore by substituting the measured R1 and R2 into Eqs. (8) and (9) respectively, only one real solution for both RL and zp can be calculated.
Fig. 3. Calculated normalized power reflection coefficients (R1 and R2) for different event return loss (RL) and event location (zp) when a reflective event is introduced. For an introduced event with a fixed RL and zp, there is only a single corresponding (R1, R2) point. (a) Forward measurement results (R1 vs zp and RL). (b) Backward measurement results (R2 vs zp and RL).
3. Experimental results for different optical events
The experimental set up is shown in Fig. 2. The SLD used in our experiment is operated at 1307.5 nm (central wavelength) with an 80.4 nm optical bandwidth. A CW light emitted by the SLD was launched into a 4.7 km-long standard single-mode fiber. The input power (P0) was 10.73 mW and the fiber attenuation coefficient α, which was measured by an OTDR at 1310 nm, is equal to 0.34 dB/km. In the experiments, different types of reflective events that can be found in real applications, e.g. longitudinal connector-mismatch and fiber bending have been introduced along the fiber under test. As a comparison, the localizations and return losses were also measured by a conventional OTDR, which has a localization and return loss measurement accuracy of 1 m and 2 dB (with pulse duration of 10 ns) at 1310 nm respectively. 3.1 Longitudinal connector-mismatch In the case of the longitudinal connector-mismatch, Fresnel reflection occurs at the interfaces. In our experiment, two types of FC connectors were applied to testify the BD-TRA’s capability of measuring reflective events with different RL levels. (1) FC/PC connector In this experiment, an FC/PC connector is used as a reflective event and five different locations were tested (i.e. 0 km, 0.7 km, 1.7 km, 4.2 km and 4.7 km). At each location, the measurements were repeated 10 times in order to evaluate the repeatability of the measurement. The FC/PC connector is characterized by a 55 dB-return loss and a 0.4 dBinsertion loss (both measured by the OTDR). In Figs. 4(a) and 4(b), the normalized power reflection coefficients (R1 and R2) are plotted as a function of the event location zp along the test fiber. The pink curves refer to the theoretical expected R1(2) obtained with Eqs. (8) and (9). The blue points represent the mean experimental results from the ten measurements set. The maximum standard deviation (STD) values of R1 and R2 are 0.005 and 0.004 respectively.
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Received 28 Feb 2014; revised 3 Apr 2014; accepted 7 Apr 2014; published 16 Apr 2014 21 April 2014 | Vol. 22, No. 8 | DOI:10.1364/OE.22.009839 | OPTICS EXPRESS 9843
Fig. 4. Theoretical (pink solid line) and experimental results (blue dots) of the normalized power reflection coefficients (R1 and R2) for different event locations (zp) when a FC/PC connector is introduced as an event. (a) R1 vs zp, R1 increases with zp. (b) R2 vs zp, R2 decreases with zp.
For the forward measurements (Fig. 4(a)), R1 increases with zp. Regarding the backward measurement (Fig. 4(b)), R2 decreases. As predicted by the theory described above, reflective powers from both directions are dependent on zp. All the experimental data agree reasonably well with the theoretical calculations. A maximum R1(2) error of 0.0047 (0.010) has been observed. To verify the proposed Bi-Directional TRA technique, all the measured data, i.e. PB0, PT0, PB1, PT1, PB2, and PT2, were implemented in the calculation model to derive the RL and zp. (See Eqs. (8) and (9)). The calculated results are shown in Fig. 5. The BD-TRA experimental results agree well with the OTDR measurement results i.e. with a maximum localization difference of 10m and a maximum RL difference of 0.7 dB. From the partial enlargement drawing of Fig. 5, one can see a good concentration of different measurement points, which shows a high repeatability of the measurement (the standard deviation (STD) values of RL and zp are 0.35 dB and 3.5 m respectively). The measurement accuracy of the proposed BDTRA technique is mainly affected by the accuracy of the powermeters.
Fig. 5. Comparison between the experimentally measured RL and zp by OTDR (pink dots) and BD-TRA (blue dots) when a FC/PC connector is introduced as an event, showing a good accuracy and a high repeatability of both RL and zp measurement using the BD-TRA method.
(2) FC/APC connector In order to introduce another level of RL, we apply a FC/APC connector at one position i.e. zp = 1.7 km along the test fiber. The FC/APC connector has been characterized by a 66.5 dBreturn loss and a 0.25 dB-insertion loss through an OTDR measurement.
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Received 28 Feb 2014; revised 3 Apr 2014; accepted 7 Apr 2014; published 16 Apr 2014 21 April 2014 | Vol. 22, No. 8 | DOI:10.1364/OE.22.009839 | OPTICS EXPRESS 9844
The corresponding experimental results are presented in Fig. 6. A good agreement between OTDR and BD-TRA experimental results has also been achieved (i.e. with a maximum localization difference of 15 m and maximum RL difference of 1.2 dB). Compared with the previous case of FC/PC connector, the FC/APC connector shows a slight increase of the measurement inaccuracy. It is mainly due to its larger RL value (i.e. a small reflectivity) and the inaccuracy of the powermeters (see section 4.1), when RL is larger than 60 dB, the estimated RL values will decrease). Good repeatability and resolution of the APC connector measurement can still be expected according to the partial enlargement drawing in Fig. 6 (the STD values of RL and zp are 0.75 dB and 7.4 m respectively).
Fig. 6. Comparison between the experimentally measured RL and zp by OTDR (pink dot) and BD-TRA (blue dots) when a FC/APC connector is introduced as an event, showing a good accuracy and a high repeatability of both RL and zp measurement using the BD-TRA method.
The aforementioned two FC connectors’ cases (i.e. FC/PC connector and FC/APC connector) confirm the capability of localizing and quantifying a reflective event with typical RLs. To prove the applicability of the proposed BD-TRA technique in a wide range of practical applications, the study of fiber bending (usually with a large RL and small IL) will be introduced in the following paragraph. 3.2 Experimental results of macro bending In this experiment, a fiber bending with a 2-cm bending diameter (one-turn bending) was applied along the fiber (zp is equal to 1.7 km). The RL of the bending has been measured by a photon-counting OTDR (Photon Kinetics Model 6500 OTDR, the accuracies of zp and RL are 0.5 m and 0.05 dB respectively), which is 66.5 dB. The BD-TRA measurement results are depicted in Fig. 7, with a maximum localization difference of 17 m and a maximum RL difference of 12.4 dB, which shows a good accuracy for the localization and a large inaccuracy for the RL measurement. Regarding the measurement repeatability, the STD values of RL and zp are 0.98 dB and 3.5 m respectively, indicating a relatively large fluctuation of the RL measurement.
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Received 28 Feb 2014; revised 3 Apr 2014; accepted 7 Apr 2014; published 16 Apr 2014 21 April 2014 | Vol. 22, No. 8 | DOI:10.1364/OE.22.009839 | OPTICS EXPRESS 9845
Fig. 7. Comparison between the experimentally measured RL and zp by OTDR (pink dot) and BD-TRA (blue dots) when a fiber bending is introduced as an event, showing a good accuracy and a high repeatability of localization and a large inaccuracy and fluctuation of RL measurement using the BD-TRA method.
For bending loss measurement, since RLs of the bending are identical for both forward and backward measurement, we can expect from Eqs. (8) and (9) that good localization accuracy can be derived. On the other hand, due to the large RL value (bending is usually treated as a non-reflective event), low measurement accuracy of RL can be expected. This can be explained if we plot the RL error expected values and the corresponding STDs by taking the inaccuracy of the powermeters into consideration. To do so, we introduce two measurement uncertainty coefficients ζ1 and ζ2, which correspond to the maximum measurement errors of the two powermeters, respectively. According to our previous experiments, both ζ1 and ζ2 are equal to 0.001 (0.1%, repeatability test). The measurements can be therefore positioned inside the ranges PB ± PB ·ζ1 and PT ± PT ·ζ2 (here PT and PB are the exact power values). Simulations taking into account a uniform distribution of the measured powers within the above-mentioned ranges were undertaken (10000 samples). Also, RL is varied from 10 to 80 dB to cover different applications. The considered test fiber has a 4.7 km fiber length and the event location zp equals to 1.7 km. The simulation results are presented in Fig. 8. We can observe that, when RL is larger than 60 dB, the estimated RL values decreases instead of approaching the real value. The growing STD value indicates that the measurement fluctuations become much larger.
Fig. 8. Calculated STDs and expected errors of event return loss (RL) for different RL values when an event with a zp of 1.7 km is introduced. When RL is larger than 60 dB, low accuracy and large fluctuation of the measurement can be expected.
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Received 28 Feb 2014; revised 3 Apr 2014; accepted 7 Apr 2014; published 16 Apr 2014 21 April 2014 | Vol. 22, No. 8 | DOI:10.1364/OE.22.009839 | OPTICS EXPRESS 9846
Moreover, since the effective refractive index of the optical mode in the bent fiber is sensitive to the interrogating wavelength [10], the bending loss caused by Rayleigh scattering is wavelength dependent. Considering that OTDR uses a narrow-band laser source (0.5 nmoptical bandwidth) while TRA uses a very wide-band super luminescent diode light source (over 80nm optical bandwidth), it is reasonable to find discrepancy between the measurement results of the two approaches. Besides, the bending event of our experiment is not a point even but distributes itself along a certain length of fiber (i.e. 2 cm-bending diameter utilized in our experiment), the proposed calculation model is inaccurate in this case and an induced measurement error can be expected. Note that although the proposed solution may underestimate the exact value of the return loss, the relatively precise localization function makes this solution practically applicable. 4. Theoretical analysis of the localization accuracy with BD-TRA method
In this section, the influence of the critical parameters (e.g. return loss, insertion loss and the total length of the fiber) on the measurement accuracy will be discussed in detail. Besides, methods to improve the localization accuracy are proposed. As for the experiments, the analytical studies are conducted under the interrogating wavelength of 1310 nm. 4.1 Effect of the event return loss (RL) Utilizing the similar methodology for drawing Fig. 8, we plotted in Fig. 9 the expected value of the localization error and the corresponding STDs versus the return loss along a 5 km-long fiber (L = 5 km). An event location zp equals to L/4 is considered. The insertion loss (IL) of the event is set to 30 dB and the return loss (RL) is varied from 10 to 80 dB. As depicted in Fig. 9, the localization error and the STD values (here localization error means the difference between BD-TRA estimated event locations and the theoretical values) are found to be strongly dependent on the value of RL. In particular, when the return loss is around 33 dB, the localization standard deviation shows a peak. By analyzing the relationship between R1 and R2 under different RLs, one may trace the origin of the phenomenon found in Fig. 9. Note that due to the inaccuracy of the powermeter, the measurements of R1 and R2 are distributed inside the ranges: R1 ± 3σ1 and R2 ± 3σ2, where σ1 and σ2 are the experimentally evaluated standard deviations of R1 and R2, respectively, both of them are determined to be around 0.01. In the (R1, R2) domain, a rectangle with sides equal to 3σ1·3σ2 can be used to represent the space of measurement variations. In Fig. 10, each curve represents the dependency between R1 and R2 when an event is introduced at various locations along the 5 km-long fiber for a given RL, for example, the blue curve which located in the button left corner depicts the relationship between R1 and R2 when RL equals to 65 dB. By taking into account the measurement variation space defined by 3σ1·3σ2 (the rectangles in Fig. 10 are symbolic, the size the rectangles are magnified for better visibility), the interaction length of each curve with the measurement variation space leads to a zp variation range. As depicted in Fig. 10, when RL is getting close to 33 dB, the corresponding curve for different event locations is very much squeezed and has a large overlap with the measurement variation space. Therefore, a large localization error can be expected.
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Received 28 Feb 2014; revised 3 Apr 2014; accepted 7 Apr 2014; published 16 Apr 2014 21 April 2014 | Vol. 22, No. 8 | DOI:10.1364/OE.22.009839 | OPTICS EXPRESS 9847
Fig. 9. Calculated localization STDs and expected localization errors for different RL values when an event with a zp of L/4 is introduced. When the RL is around 33 dB, the localization STD shows a peak.
Fig. 10. Calculated normalized power reflection coefficients (R1 and R2) for different RL values when an event is introduced at various locations along the fiber. When RL is getting close to 33 dB, a large localization error can be expected.
By conducting analytical and simulation studies, it has been demonstrated that the localization STD mainly depends on the ratio between the attenuation coefficient α and the scattering coefficient αs. In order to solve this problematic, we proposed to exploit the waterpeak absorption phenomenon in fibers [11]. Generally speaking, Rayleigh scattering is the main reason for fiber attenuation, therefore the ratio between α and αs always remains similar under different wavelengths, e.g. α/αs = 1.1@1310 nm, α/αs = 1.16@1550 nm. However, when the interrogating wavelength is set at certain values i.e. 950, 1244, and 1383 nm, the respective attenuation coefficients are increased compared to other wavelength values due to the absorption of hydroxide ions present inside the fiber. The α/αs value will then have a rapid increase (e.g. α/αs = 1.76@1383 nm, with an S of 0.001, α of 0.48 dB/km and αs of 0.273 dB/km [11]). As shown in Fig. 11, in the case of a light source at 1383 nm, the localization accuracies under 33 dB return loss can be greatly improved (i.e. from 0.125 km to 0.03 km).
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Received 28 Feb 2014; revised 3 Apr 2014; accepted 7 Apr 2014; published 16 Apr 2014 21 April 2014 | Vol. 22, No. 8 | DOI:10.1364/OE.22.009839 | OPTICS EXPRESS 9848
Fig. 11. Calculated localization STDs and expected localization errors for different RL values under two wavelengths (1310 nm and 1383 nm) when an event with a zp of L/4 is introduced. The localization STD peak can be greatly decreased if a 1383 nm-light source is applied.
Note that low cost broadband light sources with a 1383 nm central wavelength are commercially available. The possibility of utilizing 1383 nm as the interrogating wavelength will be discussed later. 4.2 Effect of the event insertion loss Figure 12 shows the localization error as well as the corresponding STDs as a function of the event insertion loss (with a fixed 20 dB RL) along a 5 km-long fiber with a 1.25 km event location. As shown in Fig. 12, over a wide range of insertion loss (0 to 60 dB in this case), the localization error is kept sufficiently low (i.e. less than 3 m). Therefore, with the BD-TRA technique, one can expect that events with both weak and strong insertion losses can accurately be localized. Taking into account the previous mentioned 33 dB-return loss case as depicted in Fig. 13, one can find that in the worst case of 33 dB-RL, the BD-TRA solution still enjoys high measurement accuracy over a wide range of insertion loss at 1383 nm interrogating wavelength, e.g. less than 0.1 m localization error and 7 m STD.
Fig. 12. Calculated localization STDs and expected localization errors for different IL values when an event with a zp of L/4 and a RL of 20 dB is introduced, showing a good accuracy and a high repeatability of localization over a wide range of insertion loss.
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Received 28 Feb 2014; revised 3 Apr 2014; accepted 7 Apr 2014; published 16 Apr 2014 21 April 2014 | Vol. 22, No. 8 | DOI:10.1364/OE.22.009839 | OPTICS EXPRESS 9849
Fig. 13. Calculated localization STDs and expected localization errors for different IL values under two wavelengths (1310 nm and 1383 nm) when an event with a zp of L/4 and a RL of 33 dB is introduced. The localization accuracies can be greatly improved if a 1383 nm-light source is applied.
4.3 Effect of the fiber length In Fig. 14, the relationship between the localization error and the length of the fiber link is presented. An event location zp of L/4, a RL of 20 dB, and an IL of 60 dB were considered. When the fiber length approaches 50km, the curve of the localization error starts to show a rapid increase (i.e. up to 1.8 km for a 60 km fiber length), which indicates the limitation of applying this approach in a long-distance application. Similar to the previous return loss analysis, in Fig. 15, we plotted the dependency between R1 and R2 when an event is introduced along the fiber for different L values. Each curve refers to a certain fiber length (L). The introduced points on the curves correspond to different zp (the distance between two adjacent points along one curve is 1km). As shown in Fig. 15(a), when the fiber length increases from 5 km to 60 km, the related points become much more squeezed, which means that for a larger L, the interaction length of the curve with the measurement variation space represents a larger possible zp variation range (e.g. when L equals to 5 km, the interaction length represents a 5 m zp variation range; when L equals to 60 km, the interaction length represents a 2 km zp variation range). Consequently, for a large L value, a large localization error can be expected.
Fig. 14. Calculated localization STDs and expected localization errors for different L values when an event with a zp of L/4 and a RL of 20 dB is introduced. When L is larger than 30 km, a large localization error can be expected.
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Received 28 Feb 2014; revised 3 Apr 2014; accepted 7 Apr 2014; published 16 Apr 2014 21 April 2014 | Vol. 22, No. 8 | DOI:10.1364/OE.22.009839 | OPTICS EXPRESS 9850
Besides the above case, other two cases with 33 dB and 60 dB RLs are also presented. In the case of RL equals to 33 dB as shown in Fig. 15(b), all the related points are squeezed together despite any L values, therefore large location errors can be expected (at 1310 nm). Figure 15(c) depicts the R1 and R2 dependency with a 60 dB RL under different L values. When L is increased from 5 to 60 km, the localization inaccuracy can be relatively large due to the more squeezed related point, which is similar with the 20 dB RL case.
Fig. 15. Calculated normalized power reflection coefficients (R1 and R2) for different fiber lengths (L) when an event with a zp of L/4 is introduced at three different RL values. (a) RL equals to 20 dB, localization inaccuracy increases with L. (b) RL equals to 33 dB, large location errors can be expected despite any L values. (c) RL equals to 60 dB, localization inaccuracy increases with L.
By conducting analytical and simulation studies, we found that the localization accuracy can be greatly improved by changing the interrogating wavelength. By analyzing Eqs. (8) and (9), one could find that the calculation results are strongly dependent on the critical parameters α and αs, while α and αs are highly wavelength dependent. Therefore, optimization of the interrogating wavelength could improve the measurement accuracy. Figures 16(a) and 16(b) illustrate the measurement accuracy under four wavelengths, i.e. 1310 nm, 1383 nm, 1550 nm (with an S of 0.0012, α of 0.23 dB/km and αs of 0.135 dB/km [9]) and 1625 nm (1625 nm is widely used as the monitoring wavelength for OTDR, with an S of 0.0012, α of 0.23 dB/km and αs of 0.135 dB/km [9]). It shows that if the interrogating wavelength is changed from 1310 nm to 1550 nm, localization accuracy can be improved more than ten times e.g. 1.87 km@1310 nm, 0.13 km@1550 nm. Let us note that when the fiber length is less than 15 km, all of the four wavelengths give very good localization accuracies (around 5 m). However, as the fiber length approaches over 20 km, the localization STD values of two wavelengths i.e. 1383 nm and 1310 nm, show a rapid increase. In comparison, acceptable localization accuracy can be obtained from the other two wavelengths i.e. 1550 nm and 1625 nm, in long reach applications. From the discussion above, one can expect that by selecting different interrogating wavelengths, high measurement accuracy can be achieved for both #207359 - $15.00 USD (C) 2014 OSA
Received 28 Feb 2014; revised 3 Apr 2014; accepted 7 Apr 2014; published 16 Apr 2014 21 April 2014 | Vol. 22, No. 8 | DOI:10.1364/OE.22.009839 | OPTICS EXPRESS 9851
short range and long reach applications (i.e. for short range applications, one can choose 1383 nm thanks to its high measurement accuracy in the 33 dB RL case; while for the long reach cases, 1550 nm can be utilized).
Fig. 16. Calculated localization STDs and expected localization errors for different L values when an event with a zp of L/4 and a RL of 20 dB is introduced at four different wavelengths. The localization accuracy and repeatability for large L values can be greatly improved if a 1550 nm or 1650 nm-light source is applied. (a) Localization STDs. (b) Expected localization errors.
5. Discussion
The experimental and analytical results presented in this paper demonstrate the capability of the proposed BD-TRA technique to measure the location and value of the return loss for different types of reflective event with a sufficient accuracy. Table 2 summarizes the measurement accuracies for various optical events employed in our experiments. Table 2. BD-TRA measurement accuracies
Event
Expected/ Maximum localization error
Expected/ Maximum RL measurement error
Localization STDs
RL STDs
FC/PC connector (§ 3.1(1))
4 m/10 m
0.25 dB/0.7 dB
3.5 m
0.35 dB
FC/APC connector (§ 3.1(2))
8 m/15 m
0.8 dB/1.2 dB
7.4 m
0.75 dB
11 m/17 m
10 dB/12.4 dB
3.5 m
0.98 dB
Fiber bending (§ 3.2)
The measurement accuracy of the proposed BD-TRA approach is highly dependent on the return loss (RL) and the fiber length (L). As discussed in section 4, the interrogating wavelength can be optimized to overcome the limitation of low measurement accuracy in different applications. For example, in access network monitoring systems (L is usually from 5 to 20 km) and sensing applications (L is less than a few kilometers), a 1383 nm central wavelength light source can be utilized to deal with the inaccurate fault localization when the return loss is around 33 dB. For long range system applications, e.g. long-reach PON monitoring (L is usually larger than 40 km), the interrogating wavelength can be set to 1550 or 1625 nm to reduce the localization STDs.
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Received 28 Feb 2014; revised 3 Apr 2014; accepted 7 Apr 2014; published 16 Apr 2014 21 April 2014 | Vol. 22, No. 8 | DOI:10.1364/OE.22.009839 | OPTICS EXPRESS 9852
In addition to its good measurement accuracy, BD-TRA outstands other solutions by its superior detection speed thanks to its simple approach that requires only the measurement of power variations (PT and PB). Compared with OTDR, which requires a long measurement time for the averaging process, the proposed BD-TRA dramatically reduced the measurement time, e.g. 2~3 seconds for BD-TRA compared to 2 minutes for OTDR. Besides, the BD-TRA technique also provides a simple and low cost system construction (utilizing un-modulated light source) that makes it a very competitive practical solution in both telecommunication area and sensing applications. 6. Conclusion
A Bi-Directional TRA based approach has been proposed in this paper. The capability of localization and quantification of single reflective event along an optical fiber is theoretically analyzed and experimentally realized. The whole system is characterized by its simple configuration and low cost. Its superior detection speed and the well-functioned localization process make it a very competitive practical solution for both telecommunication and sensing applications. We have also demonstrated that the proposed BD-TRA technique exhibits different measurement accuracy for different types of optical events. However, the measurement accuracy can be greatly improved if an appropriate interrogating wavelength is utilized. Acknowledgments
The authors would like to thank the financial support of the F.R.S.-FNRS (F.R.I.A). This research was supported by the Interuniversity Attraction Poles program of the Belgian Science Policy Office, under grant IAP P7-35 photonics@be.
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Received 28 Feb 2014; revised 3 Apr 2014; accepted 7 Apr 2014; published 16 Apr 2014 21 April 2014 | Vol. 22, No. 8 | DOI:10.1364/OE.22.009839 | OPTICS EXPRESS 9853
Abstract: We report on the theory and the implementation of a novel approach for the detection and localization of a reflective event along a fiber link. By launching a continuous-wave signal into both fiber ends and by analyzing the transmitted and reflected/backscattered optical powers, it is possible to localize an optical event and to quantify the induced insertion and return losses simultaneously. The novel idea of utilizing bi-directional measurement allows the localization of both reflective and non-reflective events. Theoretical and experimental studies show that for a 10 km-long single mode fiber, the localization accuracy can be in the range of 5.0 m. ©2014 Optical Society of America OCIS codes: (060.2270) Fiber characterization; (060.2370) Fiber optics sensors; (280.1350) Backscattering; (290.5870) Scattering, Rayleigh.
References and links 1. 2.
D. Derickson, Fiber Optic Test and Measurement (Prentice Hall PTR, 1997), chap. 11. K. Yüksel, M. Wuilpart, and P. Mégret, “Analysis and suppression of nonlinear frequency modulation in an OFDR,” Opt. Express 17, 5845 (2009). 3. J. W. Berthold, “Historical review of microbend fiber-optic sensors,” J. Lightwave Technol. 13(7), 1193–1199 (1995). 4. E. Udd and W. B. Spillman, Fiber Optic Sensors: An Introduction for Engineers & Scientists, 2nd Edition (Wiley, 2011), chap. 6. 5. V. V. Spirin, F. J. Mendieta, S. V. Miridonov, M. G. Shlyagin, A. A. Chtcherbakov, and P. L. Swart, “Localization of a Loss-Inducing Perturbation With Variable Accuracy Along a Test Fiber Using TransmissionReflection Analysis,” IEEE. Photonic Tech. L. 16(2), 569–571 (2004). 6. V. V. Spirin, M. G. Shlyagin, S. V. Miridonov, and P. L. Swart, “Transmission/reflection analysis for distributed optical fiber loss sensor interrogation,” Electron. Lett. 38(3), 117–118 (2002). 7. A. Girard, FTTx PON technology and testing (EXFO Electro-Optical Engineering Inc., 2005), chap. 3. 8. A. D. Kersey and A. Dandridge, “Applications of fiber-optic sensors,” IEEE Trans. Compon., Hybrids, Manuf. Technol. 13, 137–143 (1990). 9. L. Thevenaz, Advanced Fiber Optics: Concepts and Technology (CRC, 2011), chap. 8. 10. W. Lee, S. I. Myong, J. C. Lee, and S. Lee, “Identification method of non-reflective faults based on index distribution of optical fibers,” Opt. Express 22(1), 325–337 (2014). 11. G. Lietaert, JDSU White Paper, “Fiber Water Peak Characterization” (JDSU, 2009). http://www.jdsu.com/ProductLiterature/fiber-water-peak-characterization_fwpc_wp_fop_tm_ae.pdf.
1. Introduction Nowadays, Distributed Monitoring Systems (DMS) are widely used for fiber monitoring in both telecommunication and sensing applications. Among them, Optical Time Domain Reflectometry (OTDR) and Optical Frequency Domain Reflectometry (OFDR) are commonly used. However, OTDR and OFDR require either time- or frequency-modulated light sources, which makes them not cost-effective. Moreover, improvements and breakthroughs are still required to address certain limitations, e.g. long measurement time (several minutes are normally needed for monitoring a fiber with a length of tens of kilometers by OTDR [1]) and short measurement range (only a few hundred meters for OFDR [2]). In addition to these two methods, other DMS can be found in the literature, such as bending-based and Fabry-Perot interferometer sensors [3,4]. However, for those solutions, the measurand should be
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Received 28 Feb 2014; revised 3 Apr 2014; accepted 7 Apr 2014; published 16 Apr 2014 21 April 2014 | Vol. 22, No. 8 | DOI:10.1364/OE.22.009839 | OPTICS EXPRESS 9839
associated with complex manufacturing techniques of the sensing fiber. In [5,6], a Transmission-Reflection Analysis (TRA) method is reported, which is based on the measurement of transmitted and backscattered powers of an un-modulated light source. This method was proposed for the detection and localization of a non-reflective optical event, such as a fiber bending. However, most optical events in a fiber are reflective such as discontinuities, misalignments, and breaks that can notably be present in telecommunication networks [7]. In sensing applications, some transducing processes also result in the presence of a reflective event along the sensing fiber [8]. In order to address the aforementioned problems, a novel solution is proposed to localize both non-reflective and reflective optical events by using a Bi-Directional TransmissionReflection Analysis (BD-TRA) technique. In addition, the proposed interrogation scheme is able to quantify the return loss of the reflective event. 2. Operating principle The TRA technique is based on the unique relationship between the transmitted (PT) and the backscattered (PB) powers for a given loss (i.e. non-reflective event) location [5]. The definition of transmitted (PT) and backscattered (PB) powers can be found in Fig. 1. PT is the power transmitted through the fiber link when a Continuous optical Wave (CW) of power P0 is launched. PB is described as the power reflected/backscattered all along the fiber link. PB is mainly due to Rayleigh backscattering, which results from the presence of inhomogeneities in the fiber material [9]. In the presence of loss for a given fiber, the relationship between PT and PB only depends on the loss location. The measurement of PB and PT therefore allows localizing the event [5]. The TRA approach already presented in the literature is only suitable for non-reflective events.
Fig. 1. Definition of transmitted (PT) and backscattered (PB) powers.
In order to make the TRA approach compatible with reflective events, a novel BiDirectional TRA (BD-TRA) technique is presented in this paper. The schematic diagram of the proposed BD-TRA is shown in Fig. 2. In Fig. 2(a), a continuous-wave light emitted by a Super Luminescent Diode (SLD) is launched into a single mode fiber (Corning® SMF-28e + ® optical fiber) through an optical circulator (from point 1 to point 2, forward measurement). An optical isolator is implemented to minimize the back reflections from the fiber end. The transmitted power (PT1) is measured by the powermeter located after the isolator. The integrated reflected/Rayleigh backscattered power (PB1) is measured by the second powermeter connected to the circulator. The same measurement is repeated a second time by emitting the light from the other end of the same fiber in the opposite direction (i.e. see Fig. 2(b), from point 2 to point 1, backward measurement) in order to get the corresponding transmitted and backscattered powers (PT2 and PB2 respectively). Note that for future applications, a 2 × 2 switch can be applied to realize an automatic interchange process. As developed hereafter, the localization process of a reflective event can be based on the unique relationship between the backscattered (PB1 and PB2) and transmitted (PT1 and PT2) powers for a given event location zp and return loss (RL).
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Received 28 Feb 2014; revised 3 Apr 2014; accepted 7 Apr 2014; published 16 Apr 2014 21 April 2014 | Vol. 22, No. 8 | DOI:10.1364/OE.22.009839 | OPTICS EXPRESS 9840
Fig. 2. Schematic diagram of BD-TRA technique. (a) Forward measurement for getting PT1 and PB1. (b) Backward measurement for getting PT2 and PB2.
Let us consider a fiber with a total length L and an initial transmitted power (PT0) of the undisturbed reference system (here we assume that PT0 are not changed with the measurement direction): PT 0 = P0 ⋅ 10
(−
ILcir 1 ) 10
⋅ T ( L ) ⋅ 10
(−
ILiso ) 10
(1)
.
where P0 is the input power, ILcir1 [dB] and ILiso [dB] are insertion losses of the circulator (from port a to port b) and isolator respectively, T(L) is the fiber transmission coefficient associated with the fiber attenuation coefficient α, which can be expressed as: T ( Δx ) = e −αΔx . Taking into account the directivity of the circulator DIR [dB] (i.e. power transmitted directly from port a to port c) and the return loss of the isolator RLiso [dB], the total backscattered/reflected power of the initial undisturbed system (PB0) can be expressed as: PB 0 = P0 ⋅ [10
(−
DIR 10
)
+ RAY ( L ) + T ( L ) ⋅ 10 2
(−
RLiso ) 10
] ⋅ 10
(−
ILcir 2 ) 10
.
(2)
where ILcir2 [dB] is the insertion losses of the circulator (from port b to port c), RAY(L) is the Rayleigh backscattered power coefficient that can be expressed as [9]: RAY ( Δx ) = S ⋅
αS 2α
⋅ (1 − e
−2 αΔx
).
(3)
αs is the scattering coefficient due to the Rayleigh scattering and is proportional to 1/λ4 (0.34 dB/km at 1310 nm [9]), Δx is the length of the fiber segment and S is the capture coefficient which is 10−3 at the wavelength of 1310 nm [9]. Let us now consider that a reflective event with a return loss RL [dB] (practically speaking, the RL for a non-reflective event is usually higher than 65 dB, e.g. bending) and an insertion loss IL [dB] is introduced into the fiber at the event location zp. RL is defined as: 10 ⋅ log10 (ϕ ) , where φ is the ratio between the incident and reflected powers of the event. Here we suppose that RL and IL are not changed with the measurement direction. The transmitted (PT1) and the backscattered (PB1) powers for the forward measurement (as shown in Fig. 2(a)) can be written as: PT 1 = PT 0 ⋅ 10
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(−
IL
10
)
.
(4)
Received 28 Feb 2014; revised 3 Apr 2014; accepted 7 Apr 2014; published 16 Apr 2014 21 April 2014 | Vol. 22, No. 8 | DOI:10.1364/OE.22.009839 | OPTICS EXPRESS 9841
PB1 = P0 ⋅ [10
(−
DIR ) 10
T ( L ) ⋅ 10 2
+ RAY ( z p ) + T ( z p ) ⋅ 10 2
IL (− ) 5
⋅ 10
RL ( − iso ) 10
] ⋅ 10
IL ( − cir 2 ) 10
(−
RL ) 10
+ [ RAY ( L) − RAY ( z p )] ⋅ 10
(−
IL ) 5
+
(5)
.
For the backward measurement (the optical signal is launched from point 2 as shown in Fig. 2(b)), PT2 and PB2 can be expressed as: PT 2 = PT 0 ⋅ 10
PB 2 = P0 ⋅ [10
(−
DIR ) 10
(−
IL 10
)
(6)
.
+ RAY ( L − z p ) + T 2 ( L − z p ) ⋅ 10
[ RAY ( L ) − RAY ( L − z p )] ⋅ 10
IL (− ) 5
(−
+ T ( L ) ⋅ 10 2
RL ) 10
IL (− ) 5
+ ⋅ 10
(−
RLiso 10
)
] ⋅ 10
(−
ILcir 2 10
(7) )
.
With Eqs. (2), (5) and (7), the normalized power reflection coefficient R1 and R2 can be written as: R1 =
PB1 PB 0
=
10
DIR RL IL ) (− ) (− ) 10 + RAY ( z ) + T 2 ( z ) ⋅ 10 10 + [ RAY ( L ) − RAY ( z )] ⋅ 10 5 p p p
(−
10
DIR ) 10
(−
+ RAY ( L ) + T ( L ) ⋅ 10 2
RLiso IL (− ) (− ) T 2 ( L ) ⋅ 10 5 ⋅ 10 10
10
(−
DIR ) 10
+ RAY ( L ) + T ( L) ⋅ 10
10 P R2 = B 2 = PB 0
2
( − DIR ) 10
10
(−
RLiso ) 10
(−
RLiso 10
2
10
+ RAY ( L) + T ( L) ⋅ 10 2
[ RAY ( L ) − RAY ( L − z p )] ⋅ 10
10
(−
DIR ) 10
(−
IL ) 5
(8)
.
+ RAY ( L − z p ) + T ( L − z p ) ⋅ 10
( − DIR )
+
)
(−
10
RLiso 10
+ T ( L) ⋅ 10 2
+ RAY ( L) + T ( L) ⋅ 10 2
( − RL )
(−
(−
+
) IL ) 5
⋅ 10
RLiso ) 10
(−
(9)
RLiso ) 10
.
Since DIR, α, αs, RLiso and L are known parameters, and since PB0, PB1, PB2 and PT0, PT1, PT2 can be obtained by measurements, the problem defining zp and RL finally turns in solving a set of two equations with only two unknown variables (i.e. zp and RL). Based on the above calculation, analytical studies have been undertaken. The parameters used in the calculation model are listed in Table 1. Here we consider a reflective event with a 10 dB insertion loss and an event location (zp) varied from 0 to L. The event return loss (RL) in our simulation is varied from 20 to 80 dB to cover different applications (e.g. for a fiber break RL is usually less than 30 dB, while referring a fiber bending, a 70 dB return loss is normally employed). Table 1. Parameters related to the calculation model [9]
Interrogating wavelength λ
S
α
αs
DIR
RLiso
1310nm
0.001
0.34 dB/km
0.33 dB/km
60 dB
65 dB
L 6 km
Figures 3(a) and 3(b) show the relationship between the normalized power reflection coefficients (i.e. R1 and R2), the return loss (RL) and the event location (zp) when a single reflective event is introduced. We can clearly see the variation of R1 and R2 with the change of the event return loss and location. In Figs. 3(a) and 3(b), one can see for an introduced event
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Received 28 Feb 2014; revised 3 Apr 2014; accepted 7 Apr 2014; published 16 Apr 2014 21 April 2014 | Vol. 22, No. 8 | DOI:10.1364/OE.22.009839 | OPTICS EXPRESS 9842
with a fixed RL and zp, there is only a single corresponding (R1, R2) point. Therefore by substituting the measured R1 and R2 into Eqs. (8) and (9) respectively, only one real solution for both RL and zp can be calculated.
Fig. 3. Calculated normalized power reflection coefficients (R1 and R2) for different event return loss (RL) and event location (zp) when a reflective event is introduced. For an introduced event with a fixed RL and zp, there is only a single corresponding (R1, R2) point. (a) Forward measurement results (R1 vs zp and RL). (b) Backward measurement results (R2 vs zp and RL).
3. Experimental results for different optical events
The experimental set up is shown in Fig. 2. The SLD used in our experiment is operated at 1307.5 nm (central wavelength) with an 80.4 nm optical bandwidth. A CW light emitted by the SLD was launched into a 4.7 km-long standard single-mode fiber. The input power (P0) was 10.73 mW and the fiber attenuation coefficient α, which was measured by an OTDR at 1310 nm, is equal to 0.34 dB/km. In the experiments, different types of reflective events that can be found in real applications, e.g. longitudinal connector-mismatch and fiber bending have been introduced along the fiber under test. As a comparison, the localizations and return losses were also measured by a conventional OTDR, which has a localization and return loss measurement accuracy of 1 m and 2 dB (with pulse duration of 10 ns) at 1310 nm respectively. 3.1 Longitudinal connector-mismatch In the case of the longitudinal connector-mismatch, Fresnel reflection occurs at the interfaces. In our experiment, two types of FC connectors were applied to testify the BD-TRA’s capability of measuring reflective events with different RL levels. (1) FC/PC connector In this experiment, an FC/PC connector is used as a reflective event and five different locations were tested (i.e. 0 km, 0.7 km, 1.7 km, 4.2 km and 4.7 km). At each location, the measurements were repeated 10 times in order to evaluate the repeatability of the measurement. The FC/PC connector is characterized by a 55 dB-return loss and a 0.4 dBinsertion loss (both measured by the OTDR). In Figs. 4(a) and 4(b), the normalized power reflection coefficients (R1 and R2) are plotted as a function of the event location zp along the test fiber. The pink curves refer to the theoretical expected R1(2) obtained with Eqs. (8) and (9). The blue points represent the mean experimental results from the ten measurements set. The maximum standard deviation (STD) values of R1 and R2 are 0.005 and 0.004 respectively.
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Received 28 Feb 2014; revised 3 Apr 2014; accepted 7 Apr 2014; published 16 Apr 2014 21 April 2014 | Vol. 22, No. 8 | DOI:10.1364/OE.22.009839 | OPTICS EXPRESS 9843
Fig. 4. Theoretical (pink solid line) and experimental results (blue dots) of the normalized power reflection coefficients (R1 and R2) for different event locations (zp) when a FC/PC connector is introduced as an event. (a) R1 vs zp, R1 increases with zp. (b) R2 vs zp, R2 decreases with zp.
For the forward measurements (Fig. 4(a)), R1 increases with zp. Regarding the backward measurement (Fig. 4(b)), R2 decreases. As predicted by the theory described above, reflective powers from both directions are dependent on zp. All the experimental data agree reasonably well with the theoretical calculations. A maximum R1(2) error of 0.0047 (0.010) has been observed. To verify the proposed Bi-Directional TRA technique, all the measured data, i.e. PB0, PT0, PB1, PT1, PB2, and PT2, were implemented in the calculation model to derive the RL and zp. (See Eqs. (8) and (9)). The calculated results are shown in Fig. 5. The BD-TRA experimental results agree well with the OTDR measurement results i.e. with a maximum localization difference of 10m and a maximum RL difference of 0.7 dB. From the partial enlargement drawing of Fig. 5, one can see a good concentration of different measurement points, which shows a high repeatability of the measurement (the standard deviation (STD) values of RL and zp are 0.35 dB and 3.5 m respectively). The measurement accuracy of the proposed BDTRA technique is mainly affected by the accuracy of the powermeters.
Fig. 5. Comparison between the experimentally measured RL and zp by OTDR (pink dots) and BD-TRA (blue dots) when a FC/PC connector is introduced as an event, showing a good accuracy and a high repeatability of both RL and zp measurement using the BD-TRA method.
(2) FC/APC connector In order to introduce another level of RL, we apply a FC/APC connector at one position i.e. zp = 1.7 km along the test fiber. The FC/APC connector has been characterized by a 66.5 dBreturn loss and a 0.25 dB-insertion loss through an OTDR measurement.
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Received 28 Feb 2014; revised 3 Apr 2014; accepted 7 Apr 2014; published 16 Apr 2014 21 April 2014 | Vol. 22, No. 8 | DOI:10.1364/OE.22.009839 | OPTICS EXPRESS 9844
The corresponding experimental results are presented in Fig. 6. A good agreement between OTDR and BD-TRA experimental results has also been achieved (i.e. with a maximum localization difference of 15 m and maximum RL difference of 1.2 dB). Compared with the previous case of FC/PC connector, the FC/APC connector shows a slight increase of the measurement inaccuracy. It is mainly due to its larger RL value (i.e. a small reflectivity) and the inaccuracy of the powermeters (see section 4.1), when RL is larger than 60 dB, the estimated RL values will decrease). Good repeatability and resolution of the APC connector measurement can still be expected according to the partial enlargement drawing in Fig. 6 (the STD values of RL and zp are 0.75 dB and 7.4 m respectively).
Fig. 6. Comparison between the experimentally measured RL and zp by OTDR (pink dot) and BD-TRA (blue dots) when a FC/APC connector is introduced as an event, showing a good accuracy and a high repeatability of both RL and zp measurement using the BD-TRA method.
The aforementioned two FC connectors’ cases (i.e. FC/PC connector and FC/APC connector) confirm the capability of localizing and quantifying a reflective event with typical RLs. To prove the applicability of the proposed BD-TRA technique in a wide range of practical applications, the study of fiber bending (usually with a large RL and small IL) will be introduced in the following paragraph. 3.2 Experimental results of macro bending In this experiment, a fiber bending with a 2-cm bending diameter (one-turn bending) was applied along the fiber (zp is equal to 1.7 km). The RL of the bending has been measured by a photon-counting OTDR (Photon Kinetics Model 6500 OTDR, the accuracies of zp and RL are 0.5 m and 0.05 dB respectively), which is 66.5 dB. The BD-TRA measurement results are depicted in Fig. 7, with a maximum localization difference of 17 m and a maximum RL difference of 12.4 dB, which shows a good accuracy for the localization and a large inaccuracy for the RL measurement. Regarding the measurement repeatability, the STD values of RL and zp are 0.98 dB and 3.5 m respectively, indicating a relatively large fluctuation of the RL measurement.
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Received 28 Feb 2014; revised 3 Apr 2014; accepted 7 Apr 2014; published 16 Apr 2014 21 April 2014 | Vol. 22, No. 8 | DOI:10.1364/OE.22.009839 | OPTICS EXPRESS 9845
Fig. 7. Comparison between the experimentally measured RL and zp by OTDR (pink dot) and BD-TRA (blue dots) when a fiber bending is introduced as an event, showing a good accuracy and a high repeatability of localization and a large inaccuracy and fluctuation of RL measurement using the BD-TRA method.
For bending loss measurement, since RLs of the bending are identical for both forward and backward measurement, we can expect from Eqs. (8) and (9) that good localization accuracy can be derived. On the other hand, due to the large RL value (bending is usually treated as a non-reflective event), low measurement accuracy of RL can be expected. This can be explained if we plot the RL error expected values and the corresponding STDs by taking the inaccuracy of the powermeters into consideration. To do so, we introduce two measurement uncertainty coefficients ζ1 and ζ2, which correspond to the maximum measurement errors of the two powermeters, respectively. According to our previous experiments, both ζ1 and ζ2 are equal to 0.001 (0.1%, repeatability test). The measurements can be therefore positioned inside the ranges PB ± PB ·ζ1 and PT ± PT ·ζ2 (here PT and PB are the exact power values). Simulations taking into account a uniform distribution of the measured powers within the above-mentioned ranges were undertaken (10000 samples). Also, RL is varied from 10 to 80 dB to cover different applications. The considered test fiber has a 4.7 km fiber length and the event location zp equals to 1.7 km. The simulation results are presented in Fig. 8. We can observe that, when RL is larger than 60 dB, the estimated RL values decreases instead of approaching the real value. The growing STD value indicates that the measurement fluctuations become much larger.
Fig. 8. Calculated STDs and expected errors of event return loss (RL) for different RL values when an event with a zp of 1.7 km is introduced. When RL is larger than 60 dB, low accuracy and large fluctuation of the measurement can be expected.
#207359 - $15.00 USD (C) 2014 OSA
Received 28 Feb 2014; revised 3 Apr 2014; accepted 7 Apr 2014; published 16 Apr 2014 21 April 2014 | Vol. 22, No. 8 | DOI:10.1364/OE.22.009839 | OPTICS EXPRESS 9846
Moreover, since the effective refractive index of the optical mode in the bent fiber is sensitive to the interrogating wavelength [10], the bending loss caused by Rayleigh scattering is wavelength dependent. Considering that OTDR uses a narrow-band laser source (0.5 nmoptical bandwidth) while TRA uses a very wide-band super luminescent diode light source (over 80nm optical bandwidth), it is reasonable to find discrepancy between the measurement results of the two approaches. Besides, the bending event of our experiment is not a point even but distributes itself along a certain length of fiber (i.e. 2 cm-bending diameter utilized in our experiment), the proposed calculation model is inaccurate in this case and an induced measurement error can be expected. Note that although the proposed solution may underestimate the exact value of the return loss, the relatively precise localization function makes this solution practically applicable. 4. Theoretical analysis of the localization accuracy with BD-TRA method
In this section, the influence of the critical parameters (e.g. return loss, insertion loss and the total length of the fiber) on the measurement accuracy will be discussed in detail. Besides, methods to improve the localization accuracy are proposed. As for the experiments, the analytical studies are conducted under the interrogating wavelength of 1310 nm. 4.1 Effect of the event return loss (RL) Utilizing the similar methodology for drawing Fig. 8, we plotted in Fig. 9 the expected value of the localization error and the corresponding STDs versus the return loss along a 5 km-long fiber (L = 5 km). An event location zp equals to L/4 is considered. The insertion loss (IL) of the event is set to 30 dB and the return loss (RL) is varied from 10 to 80 dB. As depicted in Fig. 9, the localization error and the STD values (here localization error means the difference between BD-TRA estimated event locations and the theoretical values) are found to be strongly dependent on the value of RL. In particular, when the return loss is around 33 dB, the localization standard deviation shows a peak. By analyzing the relationship between R1 and R2 under different RLs, one may trace the origin of the phenomenon found in Fig. 9. Note that due to the inaccuracy of the powermeter, the measurements of R1 and R2 are distributed inside the ranges: R1 ± 3σ1 and R2 ± 3σ2, where σ1 and σ2 are the experimentally evaluated standard deviations of R1 and R2, respectively, both of them are determined to be around 0.01. In the (R1, R2) domain, a rectangle with sides equal to 3σ1·3σ2 can be used to represent the space of measurement variations. In Fig. 10, each curve represents the dependency between R1 and R2 when an event is introduced at various locations along the 5 km-long fiber for a given RL, for example, the blue curve which located in the button left corner depicts the relationship between R1 and R2 when RL equals to 65 dB. By taking into account the measurement variation space defined by 3σ1·3σ2 (the rectangles in Fig. 10 are symbolic, the size the rectangles are magnified for better visibility), the interaction length of each curve with the measurement variation space leads to a zp variation range. As depicted in Fig. 10, when RL is getting close to 33 dB, the corresponding curve for different event locations is very much squeezed and has a large overlap with the measurement variation space. Therefore, a large localization error can be expected.
#207359 - $15.00 USD (C) 2014 OSA
Received 28 Feb 2014; revised 3 Apr 2014; accepted 7 Apr 2014; published 16 Apr 2014 21 April 2014 | Vol. 22, No. 8 | DOI:10.1364/OE.22.009839 | OPTICS EXPRESS 9847
Fig. 9. Calculated localization STDs and expected localization errors for different RL values when an event with a zp of L/4 is introduced. When the RL is around 33 dB, the localization STD shows a peak.
Fig. 10. Calculated normalized power reflection coefficients (R1 and R2) for different RL values when an event is introduced at various locations along the fiber. When RL is getting close to 33 dB, a large localization error can be expected.
By conducting analytical and simulation studies, it has been demonstrated that the localization STD mainly depends on the ratio between the attenuation coefficient α and the scattering coefficient αs. In order to solve this problematic, we proposed to exploit the waterpeak absorption phenomenon in fibers [11]. Generally speaking, Rayleigh scattering is the main reason for fiber attenuation, therefore the ratio between α and αs always remains similar under different wavelengths, e.g. α/αs = 1.1@1310 nm, α/αs = 1.16@1550 nm. However, when the interrogating wavelength is set at certain values i.e. 950, 1244, and 1383 nm, the respective attenuation coefficients are increased compared to other wavelength values due to the absorption of hydroxide ions present inside the fiber. The α/αs value will then have a rapid increase (e.g. α/αs = 1.76@1383 nm, with an S of 0.001, α of 0.48 dB/km and αs of 0.273 dB/km [11]). As shown in Fig. 11, in the case of a light source at 1383 nm, the localization accuracies under 33 dB return loss can be greatly improved (i.e. from 0.125 km to 0.03 km).
#207359 - $15.00 USD (C) 2014 OSA
Received 28 Feb 2014; revised 3 Apr 2014; accepted 7 Apr 2014; published 16 Apr 2014 21 April 2014 | Vol. 22, No. 8 | DOI:10.1364/OE.22.009839 | OPTICS EXPRESS 9848
Fig. 11. Calculated localization STDs and expected localization errors for different RL values under two wavelengths (1310 nm and 1383 nm) when an event with a zp of L/4 is introduced. The localization STD peak can be greatly decreased if a 1383 nm-light source is applied.
Note that low cost broadband light sources with a 1383 nm central wavelength are commercially available. The possibility of utilizing 1383 nm as the interrogating wavelength will be discussed later. 4.2 Effect of the event insertion loss Figure 12 shows the localization error as well as the corresponding STDs as a function of the event insertion loss (with a fixed 20 dB RL) along a 5 km-long fiber with a 1.25 km event location. As shown in Fig. 12, over a wide range of insertion loss (0 to 60 dB in this case), the localization error is kept sufficiently low (i.e. less than 3 m). Therefore, with the BD-TRA technique, one can expect that events with both weak and strong insertion losses can accurately be localized. Taking into account the previous mentioned 33 dB-return loss case as depicted in Fig. 13, one can find that in the worst case of 33 dB-RL, the BD-TRA solution still enjoys high measurement accuracy over a wide range of insertion loss at 1383 nm interrogating wavelength, e.g. less than 0.1 m localization error and 7 m STD.
Fig. 12. Calculated localization STDs and expected localization errors for different IL values when an event with a zp of L/4 and a RL of 20 dB is introduced, showing a good accuracy and a high repeatability of localization over a wide range of insertion loss.
#207359 - $15.00 USD (C) 2014 OSA
Received 28 Feb 2014; revised 3 Apr 2014; accepted 7 Apr 2014; published 16 Apr 2014 21 April 2014 | Vol. 22, No. 8 | DOI:10.1364/OE.22.009839 | OPTICS EXPRESS 9849
Fig. 13. Calculated localization STDs and expected localization errors for different IL values under two wavelengths (1310 nm and 1383 nm) when an event with a zp of L/4 and a RL of 33 dB is introduced. The localization accuracies can be greatly improved if a 1383 nm-light source is applied.
4.3 Effect of the fiber length In Fig. 14, the relationship between the localization error and the length of the fiber link is presented. An event location zp of L/4, a RL of 20 dB, and an IL of 60 dB were considered. When the fiber length approaches 50km, the curve of the localization error starts to show a rapid increase (i.e. up to 1.8 km for a 60 km fiber length), which indicates the limitation of applying this approach in a long-distance application. Similar to the previous return loss analysis, in Fig. 15, we plotted the dependency between R1 and R2 when an event is introduced along the fiber for different L values. Each curve refers to a certain fiber length (L). The introduced points on the curves correspond to different zp (the distance between two adjacent points along one curve is 1km). As shown in Fig. 15(a), when the fiber length increases from 5 km to 60 km, the related points become much more squeezed, which means that for a larger L, the interaction length of the curve with the measurement variation space represents a larger possible zp variation range (e.g. when L equals to 5 km, the interaction length represents a 5 m zp variation range; when L equals to 60 km, the interaction length represents a 2 km zp variation range). Consequently, for a large L value, a large localization error can be expected.
Fig. 14. Calculated localization STDs and expected localization errors for different L values when an event with a zp of L/4 and a RL of 20 dB is introduced. When L is larger than 30 km, a large localization error can be expected.
#207359 - $15.00 USD (C) 2014 OSA
Received 28 Feb 2014; revised 3 Apr 2014; accepted 7 Apr 2014; published 16 Apr 2014 21 April 2014 | Vol. 22, No. 8 | DOI:10.1364/OE.22.009839 | OPTICS EXPRESS 9850
Besides the above case, other two cases with 33 dB and 60 dB RLs are also presented. In the case of RL equals to 33 dB as shown in Fig. 15(b), all the related points are squeezed together despite any L values, therefore large location errors can be expected (at 1310 nm). Figure 15(c) depicts the R1 and R2 dependency with a 60 dB RL under different L values. When L is increased from 5 to 60 km, the localization inaccuracy can be relatively large due to the more squeezed related point, which is similar with the 20 dB RL case.
Fig. 15. Calculated normalized power reflection coefficients (R1 and R2) for different fiber lengths (L) when an event with a zp of L/4 is introduced at three different RL values. (a) RL equals to 20 dB, localization inaccuracy increases with L. (b) RL equals to 33 dB, large location errors can be expected despite any L values. (c) RL equals to 60 dB, localization inaccuracy increases with L.
By conducting analytical and simulation studies, we found that the localization accuracy can be greatly improved by changing the interrogating wavelength. By analyzing Eqs. (8) and (9), one could find that the calculation results are strongly dependent on the critical parameters α and αs, while α and αs are highly wavelength dependent. Therefore, optimization of the interrogating wavelength could improve the measurement accuracy. Figures 16(a) and 16(b) illustrate the measurement accuracy under four wavelengths, i.e. 1310 nm, 1383 nm, 1550 nm (with an S of 0.0012, α of 0.23 dB/km and αs of 0.135 dB/km [9]) and 1625 nm (1625 nm is widely used as the monitoring wavelength for OTDR, with an S of 0.0012, α of 0.23 dB/km and αs of 0.135 dB/km [9]). It shows that if the interrogating wavelength is changed from 1310 nm to 1550 nm, localization accuracy can be improved more than ten times e.g. 1.87 km@1310 nm, 0.13 km@1550 nm. Let us note that when the fiber length is less than 15 km, all of the four wavelengths give very good localization accuracies (around 5 m). However, as the fiber length approaches over 20 km, the localization STD values of two wavelengths i.e. 1383 nm and 1310 nm, show a rapid increase. In comparison, acceptable localization accuracy can be obtained from the other two wavelengths i.e. 1550 nm and 1625 nm, in long reach applications. From the discussion above, one can expect that by selecting different interrogating wavelengths, high measurement accuracy can be achieved for both #207359 - $15.00 USD (C) 2014 OSA
Received 28 Feb 2014; revised 3 Apr 2014; accepted 7 Apr 2014; published 16 Apr 2014 21 April 2014 | Vol. 22, No. 8 | DOI:10.1364/OE.22.009839 | OPTICS EXPRESS 9851
short range and long reach applications (i.e. for short range applications, one can choose 1383 nm thanks to its high measurement accuracy in the 33 dB RL case; while for the long reach cases, 1550 nm can be utilized).
Fig. 16. Calculated localization STDs and expected localization errors for different L values when an event with a zp of L/4 and a RL of 20 dB is introduced at four different wavelengths. The localization accuracy and repeatability for large L values can be greatly improved if a 1550 nm or 1650 nm-light source is applied. (a) Localization STDs. (b) Expected localization errors.
5. Discussion
The experimental and analytical results presented in this paper demonstrate the capability of the proposed BD-TRA technique to measure the location and value of the return loss for different types of reflective event with a sufficient accuracy. Table 2 summarizes the measurement accuracies for various optical events employed in our experiments. Table 2. BD-TRA measurement accuracies
Event
Expected/ Maximum localization error
Expected/ Maximum RL measurement error
Localization STDs
RL STDs
FC/PC connector (§ 3.1(1))
4 m/10 m
0.25 dB/0.7 dB
3.5 m
0.35 dB
FC/APC connector (§ 3.1(2))
8 m/15 m
0.8 dB/1.2 dB
7.4 m
0.75 dB
11 m/17 m
10 dB/12.4 dB
3.5 m
0.98 dB
Fiber bending (§ 3.2)
The measurement accuracy of the proposed BD-TRA approach is highly dependent on the return loss (RL) and the fiber length (L). As discussed in section 4, the interrogating wavelength can be optimized to overcome the limitation of low measurement accuracy in different applications. For example, in access network monitoring systems (L is usually from 5 to 20 km) and sensing applications (L is less than a few kilometers), a 1383 nm central wavelength light source can be utilized to deal with the inaccurate fault localization when the return loss is around 33 dB. For long range system applications, e.g. long-reach PON monitoring (L is usually larger than 40 km), the interrogating wavelength can be set to 1550 or 1625 nm to reduce the localization STDs.
#207359 - $15.00 USD (C) 2014 OSA
Received 28 Feb 2014; revised 3 Apr 2014; accepted 7 Apr 2014; published 16 Apr 2014 21 April 2014 | Vol. 22, No. 8 | DOI:10.1364/OE.22.009839 | OPTICS EXPRESS 9852
In addition to its good measurement accuracy, BD-TRA outstands other solutions by its superior detection speed thanks to its simple approach that requires only the measurement of power variations (PT and PB). Compared with OTDR, which requires a long measurement time for the averaging process, the proposed BD-TRA dramatically reduced the measurement time, e.g. 2~3 seconds for BD-TRA compared to 2 minutes for OTDR. Besides, the BD-TRA technique also provides a simple and low cost system construction (utilizing un-modulated light source) that makes it a very competitive practical solution in both telecommunication area and sensing applications. 6. Conclusion
A Bi-Directional TRA based approach has been proposed in this paper. The capability of localization and quantification of single reflective event along an optical fiber is theoretically analyzed and experimentally realized. The whole system is characterized by its simple configuration and low cost. Its superior detection speed and the well-functioned localization process make it a very competitive practical solution for both telecommunication and sensing applications. We have also demonstrated that the proposed BD-TRA technique exhibits different measurement accuracy for different types of optical events. However, the measurement accuracy can be greatly improved if an appropriate interrogating wavelength is utilized. Acknowledgments
The authors would like to thank the financial support of the F.R.S.-FNRS (F.R.I.A). This research was supported by the Interuniversity Attraction Poles program of the Belgian Science Policy Office, under grant IAP P7-35 photonics@be.
#207359 - $15.00 USD (C) 2014 OSA
Received 28 Feb 2014; revised 3 Apr 2014; accepted 7 Apr 2014; published 16 Apr 2014 21 April 2014 | Vol. 22, No. 8 | DOI:10.1364/OE.22.009839 | OPTICS EXPRESS 9853
