Abstract
A simple and all-optical Doppler frequency shift (DFS) measurement system is presented. It is based on a dual-drive Mach Zehnder modulator (DDMZM) driven by a transmitted signal and an echo signal received by an antenna. A low-frequency sawtooth wave is applied to the DDMZM DC port to frequency shift the carrier and sidebands generated by the transmitted signal. Beating of a frequency-shifted transmitted signal sideband and an echo signal sideband at the photodetector produces a low-frequency electrical signal. The DFS, and consequently the speed and moving direction of a target, can be obtained from the frequency of this low-frequency electrical signal. The DDMZM used in the proposed DFS measurement system does not need to be operated at a specific point in the transfer function and hence it has no bias drift problem. The proposed DFS measurement technique can be used in both CW and pulsed radar systems. Experimental results demonstrate the proposed DFS measurement system has a wide operating frequency range of 10 GHz to 19.95 GHz, small DFS measurement error of less than ±0.6 Hz and long-term stable performance. Results also demonstrate, for the first time, DFS measurement of a pulsed signal using a microwave photonic technique.
Highlights
There is a growing interest in using microwave photonic techniques for radar signal measurement
A sawtooth wave is shown at the input of the dual-drive Mach Zehnder modulator (DDMZM) DC port in [14], it is not used for frequency translation
A new microwave photonic based Doppler frequency shift (DFS) measurement system has been presented. It is based on using Serrodyne frequency translation to frequency shift a transmitted signal sideband, which beats with an echo signal sideband at a PD to generate a low-frequency electrical signal
Summary
There is a growing interest in using microwave photonic techniques for radar signal measurement. Many reported techniques rely on measuring the phase difference of two output signals via an oscilloscope to obtain the sign of a DFS in order to determine a target moving direction [3], [6]-[9], [11], [14]. The problem of these techniques is that they can only be used in continuous wave (CW) radar systems. We present a simple single-laser, single-modulator and single-PD based DFS measurement system It does not involve electrical components and does not require a high-frequency reference signal. Long-term stable performance, and DFS measurement of a pulsed signal and multiple echo signals, are demonstrated
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