© 2009 OSA/OFC/NFOEC 2009 a1062_1.pdf NTuC5.pdf NTuC5.pdf
Testing MPI Threshold in Bend Insensitive Fiber Using Coherent Peak-To-Peak Power Method David Z. Chen, Vijay X. Jain, Robert C. Ditmore, George N. Bell Verizon, 2400 N. Glenville Dr., Richardson, Texas 75082, (972) 729-5624,
[email protected]
David Boivin, Louis-Anne de Montmorillon, Lionel Provost, Pierre Sillard Draka Communications, Route de Nozay, 91460 Marcoussis, France, Tel: 33-1-30-77-67-36, email:
[email protected]
Abstract: We present a low frequency coherent peak-to-peak detection method to evaluate Draka’s (BendBright-Elite™) new bend insensitive fiber MPI and show that a -30dB maximum threshold is satisfied in simulating all harsh field deployment scenarios. ©2009 Optical Society of America
OCIS codes: (060.2430) Fibers, single-mode; (060.2400) Fiber properties
1. Introduction Technology for improving optical fiber macro/micro bend losses has progressed, directly enhancing optical power delivery for FTTH applications. This enables cable installation for situations such as tight 90o corners, enforced cable stapling, and 5mm bend radii. However, there are many fiber parameters that must be optimized such as mode field diameter, fusion splice loss, optical connector loss, cable cut-off wavelength, chromatic dispersion and PMD [1]. By following the ITU-T standards of G.652D, G.657A and G.657B, backwards compliance is ensured with minimal sacrifice. There is one essential parameter, however, that has previously not been addressed; optical multipath-interference (MPI). Verizon initiated a bend insensitive fiber (BIF) request for information (RFI) specifically addressing MPI performance in April, 2008. MPI is associated with cable cut-off wavelength, bend loss, fusion and connector loss. The MPI specification however is not fully characterized in ITU-T standards. This is an absolutely new area due to the nature of the physics and its applications in the FTTH installations. The traditional test methods used for MPI characterization in ultra long haul terrestrial and submarine cables [2, 3] may not be valid for BIF cables. Indeed, these are generally incoherent methods that characterize interactions occurring on longer scales than the laser coherent length. With the new generation BIFs [4-6], the application and physics have changed, especially in FTTH applications. In particular, splices, connectors, staples and bends are located very close to each other and the separation distances are on the order of laser coherent regime. A proper test to characterize and evaluate the MPI in such conditions becomes very important. Its final purpose is to ensure MPI below -30dB in order to guarantee successful deployments in Verizon’s FiOS and MDU environment. In this paper, we introduce a coherent peak-to-peak power test method that incorporates polarization dependent contributions to estimate MPI in FTTH environments. Care is taken to ensure that maximum MPI in the BIF is under control and that it does not unnecessarily increase fiber manufacturing cost. In the following, we present in more detail a proper lab test method to validate the MPI values for BIF. 2. Measurement method and validation The most appropriate technique to characterize coherent MPI must be able to capture its low frequency essence [7], any methods acting as a high pass filter will underestimate the short length true coherent MPI level. The specific behavior of coherent MPI therefore requires particular attention when both choosing its measurement method and specifying the transmitter/receiver performances. If relative intensity noise and swept wavelength scanning techniques are traditionally used for measuring incoherent MPI, they are less adequate in the FTTH context. This is why we focus on measuring transmission fluctuations of the received optical power. The experimental setup used to conduct all the tests reported in this present paper is represented on Figure 1. The transmitter consists of a highly coherent and tunable laser source that is connected to a polarization controller. Because polarization modifies the visibility of interferences [3], changing it at the device-under-test (DUT) input ensures that all states of the interferometer are observed within a measurement time frame. The MPI level estimation is therefore more accurate and obtained in a time shorter than the one required when just letting the system evolve for itself. Nevertheless, one can argue that this particular method also includes polarization dependent loss (PDL) contributions in the total amount of fluctuations observed with no means to distinguish it from MPI-induced ones. In the system application, this measurement is still of interest because it leads to an MPI overestimation (worst case scenario). If greater accuracy is needed, PDL can be determined by other techniques (Jones Matrix) and then subtracted maximum value to give the exact MPI level.
© 2009 OSA/OFC/NFOEC 2009 a1062_1.pdf NTuC5.pdf NTuC5.pdf
The source output power stability is critical since it ultimately defines the dynamic range of the bench. The guaranteed 0.01 dB stability over 1 hour and a lower typical value over the measurement time window give a backto-back MPI less than –65 dB. The receiver consists of a power meter with a zero low frequency cut-off operating in Min-Max mode (10 ms averaging) and displaying peak-to-peak (ptp) fluctuations.
Fig.1: Experimental Setup (Pol. C: polarization controller, DUT: device under test)
Provided that the measurement is long enough for the system to explore various states and interference conditions, the MPI is linked to the Peak-To-Peak (ptp) value expressed in dB through
⎛ 10 ptpdB / 20 − 1 ⎞ MPI = 20 log⎜⎜ ptpdB / 20 ⎟⎟ . + 1⎠ ⎝ 10 A fiber-based interferometer has been jointly designed to validate this MPI measurement method. It consists of two arms optically isolated, one including a variable optical attenuator while the other one being a 2 m SMF length for the optical path delay. The MPI level, controlled by the variable attenuator present in one arm of the interferometer, is determined by measuring the optical power in both arms with a power meter. So after a full and accurate calibration that takes all losses into account, one is able to compute the true MPI level and to compare it with the direct measurement. For MPI levels down to -30 dB, the difference between the true and measured values is less than 0.1 dB, making us confident on our ability to capture a good MPI estimation. 3. Test results interpretation and discussions The following experiments Fig. 2 and Fig. 3 have been made to mimic the extreme field installation conditions through Verizon’s BIF RFI, using 3 and 5 mm cables made with the BendBright-Elite™ fiber. This fiber is an optimized single-trench-assisted bend-insensitive fiber fully compliant with G.652.D and G.657.B ITU-T recommendations and with maximum bend losses
