Objective Microwave instantaneous frequency measurement (IFM) is part of electronic measurement technology. The measured signal is a series of periodic and narrow duration microwave pulses, and the carrier frequency of a single pulse may change rapidly. In this case, the measured signal cannot be measured multiple times. Rapid frequency measurement in a short period of pulse duration enables the acquisition of frequency- related information when the RF signal is intercepted. IFM technology has applications in electronic countermeasures, radar early warning, and modern communications. However, traditional electronic IFM systems are restricted by limited bandwidth, vulnerability to electromagnetic interference (EMI), and high power consumption. In recent years, photonic-assisted frequency measurement systems have been proposed and proved because of their large bandwidth, low loss, light weight, and anti- electromagnetic interference. At present, the common implementation methods of microwave photon IFM systems are roughly divided into frequent- to- time mapping (FTTM), frequent- to- space mapping (FTSM), and frequent- to- power mapping (FTPM). As one of the most commonly employed methods in IFM, FTPM maps the RF to be measured into the power ratio between two optical/ electrical power channels and constructs the amplitude comparison function (ACF) to identify the RF frequency instantaneously. The FTPM method usually adopts dispersive media, optical filters, or polarization control to achieve optical/electrical power mapping. The dispersion class scheme utilizes the power fading caused by fiber dispersion to construct electrical power ACF, which is limited by the medium material and usually does not have the continuous tunability of measuring range and accuracy. Optical filtering schemes leverage spectral complementarity to construct optical power ACF. However, filters with specific spectral responses are greatly affected by the wavelength drift of the light source, thus affecting the measurement error. The polarization control scheme employs polarization interference characteristics to construct wavelength- independent optical power ACF, which can avoid the utilization of optical filters and thus reduce the measurement error caused by the wavelength drift of the light source. However, the polarization state is unstable and greatly affected by environmental factors. Methods We propose a transient frequency measurement scheme for AC/DC power detection based on a double- parallel Mach-Zehnder modulator (DP-MZM). The proposed scheme is modulated by DP-MZM, introduces an adjustable time delay, maps the RF frequency information to the phase of the optical field, and constructs an electrical power ACF based on the AC and DC power values of photocurrent after photoelectric detection. The DC and AC power values will be determined by detecting the photodetector output and the divider output respectively to employ a single detection branch for frequency to power mapping. Finally, the corresponding instantaneous frequency is obtained by the inverse solution of the constructed ACF. The scheme design will help to reduce the utilization of high- frequency devices and decrease the implementation cost and complexity. Results and Discussions The system employs DP-MZM to modulate the signal while utilizing only one photodetector (Fig. 1). The system has an important characteristic of wavelength independence, and the maximum frequency range is determined by the time delay. A larger frequency measurement range can be obtained by reducing the time delay (Fig. 4). Simulation results show that the error tolerance is 200 MHz in the whole frequency range. Meanwhile, the system deterioration is analyzed with higher harmonics considered (Fig. 9). We also analyze the factors affecting the frequency range and accuracy of the system, such as modulation coefficient, bias voltage drift, and extinction ratio. The respective error tolerances are obtained. Conclusions We propose a transient frequency measurement scheme for AC/DC power detection based on DP-MZM modulation. The RF frequency information is mapped to the optical field phase by an adjustable delay, and after being converted to the electrical domain by PIN, the electric power ACF is constructed by employing the AC and DC power values of the current. The DC and AC power values are determined by detecting the PIN output and the output of the divider respectively. Meanwhile, only a single detection branch is needed to realize frequency power mapping. While verifying the feasibility of the theory, we discuss the error sources that affect the measurement frequency accuracy of the system, such as modulation coefficient, MZM bias voltage drift, and extinction ratio. Finally, it is found that if sufficient accuracy needs to be guaranteed, which means the error range is within +/- 200 MHz and sufficient frequency measurement range is required, the system tolerance should meet the small signal modulation. This indicates that the modulation coefficient m < 0.5, the bias voltage drift degree Delta V-bias12 < 2.0 degrees o, Delta V-bias3 < 2.0 degrees o, and the system tolerance should meet the small signal modulation. The extinction ratio should meet E-r > 30 dB.
