What is Spatial Multiplexing or Space-Division Multiplexing (SDM)?

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- GoPhotonics

Jan 20, 2025

Spatial Multiplexing, also known as Space-Division Multiplexing (SDM), is a technique used to increase the capacity of a communication or sensing system by exploiting multiple spatial channels within the same medium. Unlike other multiplexing methods such as Wavelength Division Multiplexing (WDM) or Time Division Multiplexing (TDM), which use distinct optical properties or time slots to separate signals, SDM utilizes spatial separation of channels. In the context of Fiber Bragg Grating (FBG) systems, SDM involves the use of multiple optical fibers, each containing FBG sensors, to increase the total number of sensors in the system.

Working

The core idea behind Spatial Multiplexing (SDM) is to use spatially distinct optical fibers, each functioning as an independent channel, to carry multiple signals or sensor outputs simultaneously. This approach leverages physical separation within the medium to increase sensing or communication capacity without relying on additional wavelength or time-division mechanisms within a single fiber. Here’s a detailed explanation of how SDM operates in Fiber Bragg Grating (FBG) systems:

1. Deployment of Multiple Fibers

In SDM-based FBG systems, multiple optical fibers are laid out, typically in parallel, with each fiber carrying its own set of FBG sensors. Each fiber is spatially separated, ensuring that signals from one fiber do not interfere with those in another. For example, if an application requires monitoring strain at 100 points, SDM allows the system to divide these points across several fibers, with each fiber hosting a subset of the FBG sensors.

2. Independent Sensing in Each Fiber

Each optical fiber operates independently, hosting its unique FBG sensors. The sensors within a fiber reflect specific wavelengths of light when exposed to a broadband optical source. These reflections correspond to physical parameters such as strain, temperature, or pressure at each sensor location.

  • In Wavelength Division Multiplexing (WDM) within SDM: Each FBG sensor in a single fiber reflects a unique wavelength. Different fibers can use the same set of wavelengths, as they are spatially separated and do not interfere.
  • In Time Division Multiplexing (TDM) within SDM: Sensors are interrogated sequentially based on time intervals, with each fiber independently timed.

3. Optical Signal Transmission

The reflected signals from the FBG sensors in each fiber are transmitted to a receiver or interrogation system. Each fiber’s reflected signal is processed separately, as the spatial separation ensures no overlap or crosstalk between fibers. The system can use identical signal processing techniques for all fibers, streamlining data acquisition.

4. Data Collection and Processing

At the interrogation unit, signals from all fibers are collected and analyzed. The interrogation unit typically includes:

  • Optical Demultiplexers: For isolating individual sensor signals in each fiber.
  • Photo-detectors and Spectrometers: For converting optical signals into electrical data and determining changes in reflected wavelengths.
  • Signal Processing Algorithms: For analyzing the data to determine physical parameters being measured by the sensors.

In an SDM setup, the interrogation system handles data from multiple fibers in parallel, either through separate optical ports or by switching between fibers using a multiplexer.

5. Enhanced Sensing Capacity Through Spatial Multiplexing

The use of multiple fibers allows the system to multiply its sensing capacity. For instance:

  • If one fiber supports 50 sensors using WDM, deploying 10 fibers via SDM increases the total sensing capacity to 500 sensors.
  • SDM also supports hybrid configurations, combining WDM and TDM in each fiber, significantly enhancing the overall scalability of the system.

Advantages of SDM in FBG Systems:

  1. Enhanced Scalability: SDM significantly increases the number of sensors that can be deployed, as each fiber can host a complete set of sensors. This makes SDM an attractive option for large-scale sensing applications.
  2. Reduced Signal Interference: By separating sensing channels spatially, SDM avoids cross-talk and signal interference, which can occur in dense multiplexing systems like WDM.
  3. Redundancy for Reliability: Multiple fibers provide inherent redundancy. If one fiber fails, others can continue to operate, enhancing system reliability.
  4. Ease of Deployment in Distributed Systems: SDM can be used in geographically dispersed systems, such as monitoring pipelines or power grids, where multiple fibers can run in parallel to monitor different sections of the infrastructure.

Applications of SDM in FBG Systems:

  • Structural Health Monitoring: In large infrastructures like bridges or tunnels, multiple fibers with SDM can monitor strain, displacement, and temperature across different parts of the structure, allowing comprehensive and distributed sensing.
  • Energy Sector: In power grids and pipelines, SDM enables monitoring of multiple lines or segments simultaneously, ensuring that each line is independently assessed for strain, temperature, and pressure variations.
  • Railway Systems: SDM allows for parallel monitoring of multiple rail lines, where each fiber can cover a specific track section, providing distributed sensing over long distances.
  • Aerospace and Marine Systems: In aircraft, spacecraft, and submarines, SDM supports localized monitoring of various components or sections, ensuring detailed data acquisition for safety and performance analysis.

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