High resolution measurement of telecommunication component polarization mode dispersion by means of quantum interferometry
Egorov, Roman V.
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As optical transport networks migrate towards supporting optical channel (or superchannel) bitrates of 100Gbps, 400Gbps, or even 1 Tbps, it becomes more important to conduct careful analysis, precise characterization, and optimized mitigation of polarization mode dispersion (PMD) in order to deliver optimal performance in fiber-optic communication systems. Telecommunication service providers need their system vendors to match the performance of their platforms to the overall system PMD accumulation, including both fiber and component PMD. The fiber PMD is a characteristic of the optical fiber plant and is very heterogeneous in both its design and time of installation. Today all installed fiber plants require mitigation solutions for PMD-induced impairments when transporting optical channels with bitrates of 40Gbps and higher. The component PMD is due to discrete components present in the optical path of a lightwave channel, such as Wavelength Selective Switches (WSS), amplifiers, Dispersion Compensating Modules (DCM), and multiplexer/demultiplexer structures. The proliferation of components, especially WSSs, has rapidly accelerated recently with the introduction of Reconfigurable Optical Add Drop Multiplexers (ROADM). Each component contributes a relatively small value to the overall system PMD. Until recently, these components were not considered to be important contributors to the total. The significance of component PMD suddenly became very apparent because ROADM introduction into the fiber-optic network fabric increased the number of components at least ten-fold. Depending on the Link Design Value (LDV) of network fabric, the component PMD can now contribute an amount similar to or even greater than the PMD penalty incurred in the fiber. Today's state of the art tedmiques in PMD measurement focus solely on characterizing fiber PMD. In our research we have developed a new method to measure very small values of PMD (or Differential Group Delay (DOD)) in discrete components of optical fiber communication systems based on the utilization of quantum interferometry. We designed a polarization counterpart of the Hong-Ou-Mandel (HOM) interferometer with entangled photons operating in the telecommunication region of the optical spectrum. Using this setup we were able to demonstrate close to 1 fs resolution while measuring DGD values of a 1x9 port Wavelength Selective Switch (WSS), based on Micro Electro Mechanical System (MEMS). We used a specially designed source of broadband polarization entangled photon pairs generated in the process of collinear type-II Spontaneous Parametric Down Conversion (SPDC). Our result paves the way for building future test and measurement devices that will be capable of resolving even smaller PMD values in discrete components of future optical communication systems, ultimately with atto-second resolution.
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