Nowadays, Microwave photonics has been extensively studied. Microwave photonic filter(MPF) is a new method in recent years that is widely used in optical sensing, which is a new method developed in recent years. In microwave photonic sensing technology, microwave photonic interference is the key method. In this experiment, the microwave photonic interference technology is used to complete the identification of different types of fiber connector, which can be identify optical fiber connectorr or certain types of optical devices in the optical fiber sensing link. MPF is formed by Mach-Zehnder interferometer(MZI) composed of coupler and optical fiber. People usually use the optical path differences between the upper and lower arms to form microwave photon interferometer for optical sensing. We form the optical path difference by adding optical fibers of different lengths. The formation of MPF is due to the interference of light modulated by the microwave signal interferometer. We has found that the different fiber connector of the optical fiber also have a greater impact on the performance of the microwave photonic filter. This paper selects the optical fiber of 3 meter with different fiber connectors, and measures optical power and radio frequency intensity at different temperature. This paper mainly discusses the differences of different connectors of the fiber and the microwave photonic sensing at different temperatures.
The silicon Mach–Zehnder modulator is a significant type of optical modulator, which is widely utilized in optical communication and microwave photonics. The performance of the second harmonic microwave generation by a silicon Mach–Zehnder modulator is simulated and analyzed. The power and the purity of the second harmonic microwave mainly depend on two significant parameters: 1) the electrical nonlinearity of the diodes on arms of the modulator, 2) the bias voltage on the diode. Simulations show following results:1) when the built-in voltage Vb decreases from 8V to 3.5V, the incremental amounts of second harmonics and the fourth harmonics are 7.9dB and 15.6dB under the condition that the RF power is 15dBm and the bias voltage is 0V; 2) when the bias voltage increases from -0.5V to 4V, the ratio between second harmonics and the fourth harmonics varies from 20.0dB to 11.3dB under the condition that the RF power is 15dBm and the built-in voltage Vb is 3.5V. 3) when the RF power increases from 5dBm to 20dBm, the ratio between second harmonics and the fourth harmonics varies from 27.6dB to 11.6dB under the condition that is the bias voltage is 2V and the built-in voltage Vb is 3.5V. To conclude, the powers of second harmonics will increase with the reduction of the ratios between the second harmonics and the fourth harmonics.
When we use FBG to filter the optical signal, we pay more attention to its optical power spectrum and less attention to its dispersion fluctuation. However, if the dispersion fluctuation of FBG is too large, it may lead to signal distortion, so it is very important to detect the dispersion fluctuation of FBG. In this paper, we choose the FBG with wavelength of 1550nm and reflectivity of 95%, and use the method of phase to amplitude conversion to detect the second-order dispersion fluctuation of FBG. In the temperature range of 23 ° - 33.6 °, we have measured the optical power spectrum and the second-order dispersion fluctuation curve of the fiber grating, and analyzed the experimental results. Then we find that when the FBG with flat optical power spectrum but large dispersion fluctuation is used for filtering, the output signal will be distorted.
For a VLC system, it transmits information by modulating LED with electrical signal. To ensure the sufficient optical power at the receiver, the electrical signal with large amplitude is hope to be applied on the LED , however, the LED has the turn-on voltage and the saturated voltage, so large electrical signal in transmitter will lead to the clipping distortion on the LED in theory. In this paper, the modulating efficiencies of white light LED with the rated power 1W, 0.5W and 0.1W are measured in experiment with the biased voltage 3.4V, and their modulating property are compared when the peak-peak AC voltage of the frequency 500KHz are in the range 0.5V-2.1Vpp and 2.2-5Vpp. In the experiment, an Arbitrary waveform generator (AWG) with the voltage bias function is utilized to generate a biased sine waveform Experimental results show that the slopes of Pe-Vt curve(modulating efficiency) increase with the drop of the rated power of LED at the frequency 500KHz in region I. In region II, the slopes of 1W LED and 0.1W LED are larger than that of 0.5W LED. The modulating efficiency is higher in region II than that in region I. When LED operate at the clipping distortion region, it has large modulating efficiency. Data fitting analysis shows that the 1W LED has the poor linearity in both region because its R-squares in both regions are smaller than other LEDs. The nonlinearity of the Pe-Vt curve will affect the modulating efficiency.
Microwave photonic filter is one of the key technologies of microwave photonics. Its main purpose is to replace the traditional method to process radio frequency signal, modulate optical carrier by radio frequency signal, and process it directly in the optical domain. The advantage of FIR microwave photonic filter is that it has no system poles and is more stable. It guarantees linear phase, which is very important in signal processing. In this paper, the incoherent microwave photon sensor cascaded by FBG is verified experimentally. In the experiment, three FBG wavelengths are 1530 nm, 1550.12nm and 1539.5nm respectively to form a high-order FIR microwave photon filter, two 3dB couplers and three km fibers to form an unbalanced M-Z interferometer. The incoherent light generated by EDFA is modulated by a filter electro-optic modulator through an optical fiber Bragg grating, demodulated by an M-Z interferometer, and eventually received by a photodetector. In the experiment, 60 MHz near notch is chosen as the modulation frequency, the amplitude of modulation signal is 4 Vpp, the temperature range is 30-40°C with the temperature interval of 1°C. The sensitivities of 0.2219dBm/°Care obtained by processing the average values of 3 points, 5 points, 7 points, 10 points and 15 points.
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.
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