Photonics-assisted microwave instantaneous frequency measurement based on frequency-power mapping

A photonic-assisted approach for instantaneous frequency measurement (IFM) based on frequency-power mapping is proposed and demonstrated. The principle of the technique is based on frequency-to-optical power mapping. In this scheme, in order to obtain the amplitude comparison function (ACF), two counter-propagation optical channels are constructed by four intensity modulators (IM) and two RF time delay lines. The RF signal-under-test (SUT) is divided evenly into four parts: two of them are time-delayed compared to other two. For the clockwise channel, the optical carrier is firstly intensity modulated by SUT in a manner of double sideband signal with suppression central carrier (CS-DSB). When traveling to the next IM, the CS-DSB optical light is further CS-DSB modulated by a time-delayed replica of SUT. After output from the two cascaded IMs, the RF modulated optical signal is launched into the last two IMs used in a reverse direction and filtered by an optical band-pass filter (OBPF) before received by an optical power meter. Here, it should note that the IM worked in a reverse direction has weak intensity modulation on the transmitted optical wave and can be regarded as a transmission waveguide. Similarly, the optical power traveled in anticlockwise channel can also be detected. For the anticlockwise direction, the optical carrier is CS-DSB modulated by another SUT and time-delayed SUT in sequence. Hence, the ACF can be established to convert the frequency into optical power ratio between two optical signals propagated in opposite directions. Experimental results show that RF signals varying from 0 to 14 GHz can be measured with an acceptable error.