Real time FPGA-based crosstalk elimination for multichannel interferometry systems in fusion diagnostics

Infrared (IR) interferometry is a well-known method for measuring the Line-Integrated electron Density (LID) of fusion plasmas. In the TJ-II stellarator, an FPGA-based IR-interferometer has been recently installed to accurately measure the LID of the plasma in real time. To guarantee the correct functionality of the interferometer and achieve high precision rates, it is essential to maximize the Signal-to-Noise Ratio (SNR) of the output density signal. In the measurement process, one of the most important distortion sources is the crosstalk or interchannel interference. Thus, in order to increase the SNR of the system, a crosstalk reduction stage has been designed and implemented in an FPGA. This paper presents a novel crosstalk elimination algorithm that provides very high spectral selectivity while preserves the phase linearity. Experimental results have shown that the proposed algorithm successfully separates the desired channels, improving the SNR over 30 dB, and keeping track of the phase of the original signal. In addition, the proposed solution has been optimized for its high-performance hardware implementation in the TJ-II. Resource consumption in the FPGA is now 22% smaller and 71% faster than with the traditional implementation based on linear-phase digital filtering.