Photonically wired spacecraft panels have been demonstrated within a recent ESA ARTES project by integrating mechanically packaged fiber Bragg grating (FBG) based optical temperature sensors into a honeycomb satellite test panel. Replacing electrical sensors with optical fiber sensors for testing satellites should have the advantage of reducing the harness mass and AIT. Fiber optic sensing also comes with clear benefits including immunity to electromagnetic interference and the capability of supporting arrays of sensors on a single fiber. However, standard FBG based temperature sensors are sensitive to both strain and temperature and in order to measure both strain and temperature simultaneously, two FBG sensors are required. An alternative solution using birefringent FBGs inscribed in Polarization Maintaining (PM) fiber (PM-FBG) in combination with high precision optical interrogators offers the same capabilities of standard FBG based optical sensors with high accuracy measurements, and can simultaneously measure both strain and temperature using only one sensor. PM-FBG sensors can also be multiplexed on a single fiber and therefore offer a simplified installation option by mounting them on the surface of the structure without the requirement for complex transducer packaging designs. With the support from an ESA GSTP project, we have developed an optical interrogator that measures PM-FBGs with high accuracy. The aim of the project is to demonstrate an optical strain independent temperature measurement system using PM-FBGs installed on a satellite test panel in atmospheric pressure and thermal vacuum environments with an operating temperature range from -20°C to + 80°C.
Fiber Bragg Gratings (FBGs) have been used and deployed in several applications and industries over the past years. Standard FBG based temperature sensors are sensitive to both strain and temperature and in order to measure temperature, the strain influence needs to be isolated from the FBG by careful transducer design, packaging and calibration of the sensor. Birefringent FBGs such as polarization maintaining FBGs (PM-FBG) that can simultaneously measure strain and temperature have been demonstrated in recent publications. Such sensors exhibit a double FBG response which is polarization dependent and the wavelength peak separation is an important parameter to enable measurements beyond standard FBGs. To achieve the best performance of a birefringent FBG, an optimized interrogation technique that can measure both FBG orthogonal polarization responses with high precision is required. In addition to the need for an optimized interrogator, the selection of sensor inscription method, coating type, mounting technique, and calibration are very important parameters to deliver the best overall system performance. PM-FBG sensors can be multiplexed on a single fiber and offer a simplified installation option without the requirement for complex transducer packaging designs. Here we have developed as part of an ESA GSTP project, a fiber optic sensing system for birefringent FBGs based on a high precision tunable laser interrogator system. We have also evaluated different types of PM-FBG sensors with different coatings and mounting techniques and demonstrated an optical temperature measurement system with an operating temperature range from -20°C to + 80°C using PM-FBGs with improved calibration techniques.
Recently optical sensing solutions based on fiber Bragg grating (FBG) technology have been proposed for temperature monitoring in telecommunication satellite platforms with an operational life time beyond 15 years in geo-stationary orbit. Developing radiation hardened optical interrogators designed to be used with FBG sensors inscribed in radiation tolerant fibers offer the capabilities of multiplexing multiple sensors on the same fiber and reducing the overall weight by removing the copper wiring harnesses associated with electrical sensors.
Here we propose the use of a tunable laser based optical interrogator that uses a semiconductor MG-Y type laser that has no moving parts and sweeps across the C-band wavelength range providing optical power to FBG sensors and optical wavelength references such as athermal Etalons and Gas Cells to guarantee stable operation of the interrogator over its targeted life time in radiation exposed environments. The MG-Y laser was calibrated so it remains in a stable operation mode which ensures that no mode hops occur due to aging of the laser, and/or thermal or radiation effects.
The key optical components including tunable laser, references and FBGs were tested for radiation tolerances by emulating the conditions on a geo-stationary satellite including a Total Ionizing Dose (TID) radiation level of up to 100 krad for interrogator components and 25 Mrad for FBGs.
Different tunable laser control, and signal processing algorithms have been designed and developed to fit within specific available radiation hardened FPGAs to guarantee operation of a single interrogator module providing at least 1 sample per second measurement capability across <20 sensors connected to two separate optical channels.
In order to achieve the required temperature specifications of ±0.5°C across a temperature range of -20°C to +65°C using femtosecond inscribed FBGs (fs-FBG), a polarization switch is used to mitigate for the polarization dependent frequency shift (PDFS) induced from fs-FBG which could be in the order of < 20 pm causing < 2°C error in the measurement. Also special transducers were designed to isolate the strain from the FBGs to reduce any strain influence on the FBG temperature measurements while ensuring high thermal conductivity.
In this paper we demonstrate the operation of an optical FBG interrogator as part of a hybrid sensor bus (HSB) engineering model system developed in the frame of an ESA-ARTES program and is planned to be deployed as a flight demonstrator on-board the German Heinrich Hertz geo-stationary satellite.
The sensing of hydrocarbons, such as the BTEX compounds in water, is described. These hydrocarbons, which are constituents of petroleum can find their way into groundwater due to leaks in underground tanks and in associated piping, are known to be carcinogenic and threaten flora and fauna. An infrared fiber optic sensor based on the evanescent wave generated around the bare core fiber is utilised to perform qualitative and quantitative measurements on these analytes. A silver halide fiber is used for its low spectral attenuation between 4-16mm, for within this wavelength range the analytes have characteristic absorption peaks, which allow their concentrations in water to be determined using the Beer Lambert Law. Using narrow bandpass filters centred on a characteristic peak, the sensor can be selectively tuned to a single analyte. Coating the bare core with a hydrophobic plasticised PVC film increases analyte concentration within the active region of the sensor and minimizes water interference, which is considerable at these wavelengths.