Design challenges of a tunable laser interrogator for geo-stationary communication satellites

Abstract

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.

Fiber Optic Sensing in Spacecraft Engineering: An Historical Perspective From the European Space Agency

“…This leads to increased Assembly, Integration and Testing (AIT) effort, weight and overall cost. Replacing electrical sensors with optical fiber sensors for testing satellites should have the advantage of reducing the harness mass and AIT [30]. In addition to strain and temperature sensing, approximately 250 accelerometers are employed to measure the structure and qualify it for the launch loads during the mechanical testing (shock and vibration) of the satellite.…”

“…An important element of developing technology for space is the effort dedicated to evaluating and qualifying components and systems for the space environment. This section details some of the results achieved at component and system level aimed at improving the Technology Level Readiness of FBG sensor technology for future flight opportunities [30].…”

“…Several projects along the years have been devoted to investigating these effects and develop radiation tolerant optical sensors and interrogation equipment for space applications. A radiation hardened tunable laser based optical interrogator was developed by OHB and Optics11 FAZ (FAZ Technology) as part of an ARTES project to develop a hybrid sensor bus, designed to be used with FBG sensors inscribed in radiation tolerant fibers as shown in Figure 10A [30]. 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.…”

Laser Technology in Photonic Applications for Space

“…ESA has also been investigating the application of fiber sensors for other applications, such as the ISS furnace temperature measurements, strain measurement in solar cells, or temperature monitoring during the re-entry of a spacecraft in the atmosphere [117], for instance, in the frame of ARTES 5.2 and Future Launchers Preparatory Programme (FLPP) phase 3 programs, in which a fiber ring tunable laser emission is swept between 1500 nm and 1600 nm. Several types of lasers have been considered, for instance, a semiconductor optical amplifier has been used as an active medium [118] and a tunable laser-based optical interrogator has been designed with semiconductor modulated grating Y-branch laser diode (MG-Y) type laser [119].…”

Demonstration of strain independent temperature measurements using optical PM-FBG sensors for ground testing of satellite panels

“…Today telecommunication satellite sensing systems require a complex wiring harness which leads to increased Assembly, Integration and Testing (AIT) effort, weight and overall cost 1,2 . Traditional point-to-point wiring of electrical temperature sensors result in the vast size and weight of the harness.…”