**Who we are ?**

Télécom Paris, part of the IMT (Institut Mines-Télécom) and a founding member of the Institut Polytechnique de Paris, is one of France's top 5 general engineering schools.

The**mainspring** of Télécom Paris is to train, imagine and undertake to design digital models, technologies and solutions for a society and economy that respect people and their environment.

We are looking for our next postdoctoral researcher to work on assessing the vulnerability of a PON (Passive Optical Network) telecoms access network to potential eavesdropping via DAS (Distributed Acoustic Sensors), to join the Communications and Electronics Department (COMELEC).

This study is conducted within the framework of Project \<\< EAT \>\> RAPID (Régime d'Appui Pour l'Innovation Duale), resulting from a partnership between the company Febus Optics and Telecom Paris.

The innovative nature of this work lies in defining the network conditions allowing for a cybersecurity audit on already deployed infrastructures.

Distributed Acoustic Sensing (DAS) via optical fiber is a recent technology with a constantly diversifying range of applications. These include perimeter intrusion detection, seismic measurements, and borehole diagnostics (oil \& gas, geothermal), among others. The measurement bandwidth, ranging from sub-Hertz to hundreds of kilohertz, also enables the capture of audio signals, thereby allowing for acoustic measurements over optical fibers. It is thus possible to exploit a building's telecom optical network for eavesdropping by using a modified DAS system. This involves integrating specific hardware artifacts (fiber coils, antennas, connectors) near the listening zone and employing dedicated signal processing to achieve sufficient performance.

**Postdoctoral Objectives**

The scope of this postdoctoral research is to evaluate the vulnerability of telecom networks to potential DAS-based eavesdropping. Connecting to an active local optical telecom network requires adapting to its specific architecture, particularly regarding the wavelength plan.

The audit of telecommunication systems to assess their vulnerability is a major challenge for telecom networks. An example of an access network configuration is described in the figure below. From the Backbone network and starting from the Central Office (CO), telecom signals are routed to subscribers via a Passive Optical Network (PON). Inside buildings, an FTTX (Fiber To The Home, Room, Office, etc.) or an optical Local Area Network (LAN) is used to deliver the optical signal to the final terminal or to enable communication between local terminals via optical links.

The PON consists of a feeder fiber followed by at least one 1×N splitter. Information encoding relies on a combination of Coarse Wavelength Division Multiplexing (CWDM) and Time Division Multiplexing (TDM). Each output port of the 1×N splitter is sent to groups of 1 to *n* subscribers through at least one filter that selects their allocated wavelength. After this filter, the signal becomes monochromatic and TDM-encoded. A final 1×n splitter distributes the signal among the *n* subscribers. Consequently, this PON includes passive splitters, connectors, splices, and other components.

**Goals and Work Packages :**

The objective of this postdoctoral fellowship is to conduct an assessment of the different types of deployed networks and evaluate their impact on DAS measurement capabilities:

* Description of various existing local area network types.

* Assessment of different transmission schemes: CWDM, TDM, etc.

* Identification of the various access points for DAS measurements.

* Discrimination of different fibers interrogated after a splitter and the influence of reflective points.

* Study of power margins required for DAS measurements.

* Analysis of potential add-drop nodes and the limits of optical transparency.

* Sensitivity analysis of DAS measurements based on fiber types and their environment.

* Impact of the final reflector on eavesdropping capabilities.

The innovative nature of this work lies in defining the network conditions, DAS configurations, and performance metrics required to conduct a cyber audit on already deployed infrastructures.

Beyond a synthesis of field-deployed configurations, the project will involve conducting tests either on an authorized live network or on a model network implemented specifically for this study. This work will lead to the identification and specification of the required interfacing modules, followed by their design and fabrication.

**Your main responsabilities** **:**

* To carry out research missions in the field of optical fiber sensor.

* To ensure supervision and tutoring missions.

* To contribute to the reputation of the School, the Institut Mines-Télécom and the Institut Polytechnique de Paris.