Switchable instantaneous frequency measurement by optical power monitoring based on DP-QPSK modulator

We propose and analyze an instantaneous frequency measurement system by using optical power monitoring technique with improved resolution. The primary component adopted in the proposal is a dual-polarization quadrature phase shift keying (DP-QPSK) modulator which is used to modulate the microwave signal that has a designed time delay and phase shifting. The generated optical signal is sent to polarization beam splitter (PBS) in DP-QPSK modulator. Owing to the complementary transmission nature of polarization interference introduced by PBS, the frequency information is converted into the optical power and the relationship between the amplitude comparison function (ACF) and microwave frequency to be measured is established. Thus, the frequency of the microwave signal can be easily measured through monitoring the optical powers of the two output ports of the PBS. Furthermore, by adjusting the direct current (DC) biases of the DP-QPSK modulator instead of changing the electrical delay, the measurement range and resolution can be switched. In this paper, the basic principle of the instantaneous frequency measurement system is derived in detail, and simulation has been performed to investigate the resolution, the measurement range, and the influence of imperfection devices. The proposed scheme is wavelength-independent and its measurement range is switchable, which can avoid the laser wavelength drifting problem and thus greatly increasing the system flexibility.