The microwave photonic sensing system is widely investigated recently. The transmission character between two types of optical sensing schemes interrogated by microwave approach is investigated. The first case is that the incoherent light is modulated by the electro-optic modulator firstly and then filtered, and the second case is that the incoherent light is filtered firstly and then modulated. Experimental results show that the optical loss and microwave loss of the links between two cases are different. By utilizing a fiber with the length of 2km between and optical filter, the optical loss of the links in case I is 1.8dB lower than that in case II, and the microwave loss of the links in case I is 3.8dB lower than that in case II. To conclude, the non-reciprocal character appears in this incoherent microwave photonics links.
In this paper, a method of temperature sensing based on microwave signal generation with fiber Bragg grating (FBG) as a frequency discriminator is proposed and experimentally demonstrated. According to the proposed approach, the optical carrier frequency is placed at the slope of the phase response of the FBG. The measure and to the phase of microwave signal generated by the frequency discriminator is mapped through an electrical spectrum analyzer. The proposal can describe the change in the flat area of the amplitude response, which improves the sensing resolution of the temperature near the center wavelength. Higher precision can be distinguished from the slope of the phase response. A result that the sensor responds to temperature with a sensitivity of 5.2dB/°C is achieved.
A structure in photonic integrated circuit with a microring serially connected by an optical attenuator is proposed to generate the non-reciprocity by the nonlinear optical effect in microring. We find that when the information of the transmission spectra of both forward and backward directions is utilized for sensing, the precision of sensing can be enhanced. We calculate the non-reciprocity ratio of our proposed structure, and utilize this spectrum for sensing. Simulation shows the non-reciprocal spectrum varies with the attenuation value of the attenuator. The slope of 4.34dB/GHz can be achieved by utilizing 10dB attenuator, and the sensing frequency range is 7.35GHz. These results benefit its application in sensing.