In this paper we present the basic working principle of a fiber based optical frequency comb, the advantages for using this technology for space applications and the limitations and technical challenges arising from environmental conditions in space.
The Hybrid Sensor Bus is a space-borne temperature monitoring system for telecommunication satellites com bining electrical and fiber-optical Fiber Bragg Grating (FBG) sensors. Currently, there is no method available for testing the functionality and robustness of the system without setting up an actual sensor-network implying numerous FBG sensors in which each has to be heated/cooled individually.
As a verification method of the mentioned system, FBG reflection based scanning laser interrogator, an FBG emulator is implemented to emulate the necessary FBG sensors. It is capable of immediate emulation of any given FBG spectrum, thus, any temperature. The concept provides advantages like emulating different kinds of FBGs
with any peak shape, variable Bragg-wavelength λB, maximal-reflectivity τmax, spectral-width and degradation
characteristics. Further, it facilitates an efficient evaluation of different interrogator peak-finding algorithms and the capability of emulating up to 10000 sample points per second is achieved.
In the present paper, different concepts will be discussed and evaluated yielding to the implementation of a Variable Optical Attenuator (VOA) as the main actuator of the emulator. The actuator choice is further restricted since the emulator has to work with light in unknown polarization state. In order to implement a fast opto-ceramic VOA, issues like temperature dependencies, up to 200 V driving input and capacitive load have to be overcome. Furthermore, a self-calibration procedure mitigates problems like attenuation losses and long-term drift.
In this paper measurement results of the fiber-optic interrogator module for telecommunication satellite applications are presented. The sensor interrogator features from fiber Bragg grating (FBG) based sensing. Benefits are intrinsic sensor distribution capability and the possibility to embed optical fibers in composite structures like tanks and satellite panels.
The fiber-optic interrogator module is based on a narrow-band monolithic laser diode where the output wavelength is spectrally tuned by electric control signals. By evaluating the intensities of the sensor response, the peak of the FBG can be monitored. The correct evaluation of the sensor response is a challenging task, therefore different computational methods are presented, namely centroid, finite impulse response filter and curve fitting algorithms. The algorithms shall met the performance requirements in terms of measurement accuracy, robustness against laser degradation and measurement rate. Furthermore the algorithms shall be implemented in an FPGA, which means a detailed point of view to fixed-point arithmetic and necessary amount of hardware resources at constant performance.
Measurement results based on the different FBG evaluation algorithms are presented and traded regarding accuracy robustness and their possible implementation in an FPGA.