Evaluation of transfer functions for punctured turbo codes - Electronics Letters

Fig. 2 shows a plot of the measured temporal waveform of the returned THz signal. The first signal (left) is the THz reflection from the metallic chopper blade once the chopper blade was set to be perpendicular to the propagation direction of the THz beam, and the second peak is the THz signal transmitted through the chopper but reflected back from the metallic mirror. Because there is a n phase difference between the phases of the transmitted and reflected THz beam from the chopper blade (compared to the chopper reference and measured with the lock-in amplifier), these two signals reflected from the chopper and the mirror show opposite polarities. The time delay between two THz signals is the round trip time of a THz pulse which has travelled between the chopper and the metallic mirror. The reflection from the chopper blade automatically serves as a reference marker for the system calibration. Because the photocurrent generated by the optical beams (both pump and probe) and the bias voltage contributes to the noise, the measured THz waveform has a signal-to-noise ratio (SNR) of 200. Nevertheless, with better selection of the filtering circuit and operational conditions (such as a modulation frequency of several kilohertz), we expect a substantial improvement in the SNR. A noise characterisation and analysis are currently being carried out.