A high-speed and high-resolution temperature sensor based on two cascaded fiber-optic Sagnac loops (FSLs) interrogated using a microwave photonic (MWP) method is proposed and experimentally demonstrated. In the proposed system, an FSL is used as a sensor element. When it is experiencing temperate change, its spectrum is shifted, which leads to the shift in the spectrum of the cascaded FSLs. Instead of measuring the wavelength shift of the spectrum by using an optical spectrum analyzer (OSA), which has a low wavelength resolution and a slow scanning speed, we convert the optical spectrum to the time domain based on spectral shaping and wavelength-to-time (SS-WTT) mapping, and use a digital signal processor (DSP) to extract the sensing information. In our experiment, two cascaded FSLs with two different free spectrum ranges (FSRs) are employed. Due to the Vernier effect, multiple dips in the envelope of the optical spectrum are introduced. By passing a broadband frequency-chirped optical pulse generated by a frequency-swept laser source to the two cascaded FSLs, the sensing information is encoded in the optical spectrum and it is converted to the time domain due to SS-WTT mapping. By detecting the optical waveform at a photodetector (PD), a temporal microwave waveform with its shape identical to the optical spectrum is generated. By using a DSP, the time shift of a dip is measured and the sensing information is precisely demodulated. Experimental result shows that the proposed fiber-optic temperature sensor can provide a temperature resolution of 1.33 × 10 ^-5 °C at a sensing speed of 23.497 kHz.

Microwave photonic interrogation of a high-speed and high-resolution multipoint refractive index (RI) sensor based on an Au-coated tilted fiber grating (Au-TFBG) array is proposed and experimentally demonstrated. Due to the surface plasmon resonance (SPR), the optical spectrum of an Au-TFBG has a dip in the spectral envelope. When an Au-TFBG is immersed in a solution and the RI of the solution changes, the location of the dip will shift, reflecting the change of the RI. For multipoint sensing, an Au-TFBG array with the array elements located at different locations is employed. However, due to the overlap of the spectra of the Au-TFBGs in the array, the dips cannot be precisely located by optical spectrum measurement. A solution is to convert the spectra to the time domain based on spectral shaping and wavelength-to-time (SS-WTT) mapping with the mapped temporal waveforms separated by different time delays. By using a digital signal processor (DSP), the sensing information can be extracted at a high speed and high resolution. The proposed approach is evaluated experimentally. Experimental results show that the sensor has high RI accuracies of 3.1×10⁻⁵, 2.5×10⁻⁵ and 2.6×10⁻⁵ refractive index unit (RIU) and a fast sensing speed of 11.75 kHz.

Microwave photonic interrogation of a high-speed and high-resolution refractive index (RI) sensor based on a tilted fiber Bragg grating with surface plasmon resonance (TFBG-SPR) is proposed and experimentally demonstrated. Instead of demodulating the wavelength shift or intensity change of a TFBG-SPR spectrum in the optical domain, we convert the TFBG-SPR spectrum to the time domain based on spectral shaping (SS) and wavelength-to-time (WTT) mapping and use a digital signal processor (DSP) to extract the RI information at a high speed and high resolution. In the experiment, when an Au-coated TFBG is immersed in a solution, the TFBG-SPR spectrum will produce a dip. When the RI changes, the location of the dip in the TFBG-SPR spectrum will shift, which is a function of the RI. By passing a broadband frequency-chirped optical pulse generated by a frequency swept laser source to the TFBG-SPR and detecting the optical pulse at the output of the TFBG-SPR at a photodetector (PD), due to SS-WTT mapping, a temporal waveform with its shape identical to the optical spectrum is produced. A DSP is then used to extract the SPR envelope information from the temporal waveform. By monitoring the changes of the SPR envelope, the RI information is interrogated with a high resolution of 1.123 × 10 ^-6 RIU at a high speed over 20 kHz.

An actively and harmonically mode-locked optoelectronic oscillator (OEO) is demonstrated to generate a broadband microwave frequency comb (MFC) with a low near-carrier phase noise and a suppressed supermode noise. To achieve a low near-carrier phase noise, the OEO is designed to have a long feedback loop, to increase its Q factor. However, an OEO with a long loop has a large number of densely spaced longitudinal modes, making the OEO, if harmonically mode locked, have a high supermode noise. In this work, a novel approach to suppressing supermode noise by incorporating a microwave photonic filter (MPF) is proposed and experimentally demonstrated. The MPF deployed in the OEO loop has a free spectral range (FSR) equal to the harmonic mode locking frequency. This ensures that the desired longitudinal modes are selected, while the unwanted supermodes are effectively suppressed. The proposed approach is experimentally evaluated. For an OEO with 2nd-order and 3rd-order harmonic mode locking, by using a two-tap and three-tap MPF, the supermode suppression ratio is increased from 8 dB to 59 dB for 2nd-order harmonic mode locking and from 5 dB to 55 dB for 3rd-order harmonic mode locking, indicating the suppression ratio is increased by 51 dB and 50 dB as compared with a harmonically mode-locked OEO without an MPF. The phase noise of the proposed mode-locked OEO at an offset frequency of 10 kHz are measured to be about −94 dBc/Hz and −104 dBc/Hz at a central frequency of 10 GHz and 4 GHz, respectively. The proposed technique can also be utilized for higher order harmonically mode-locked OEOs, showing its capability for high-repetition-rate MFC generation with low near-carrier phase noise and low supermode noise.

A high-speed fiber-optic real-time organic gas sensor based on a microfiber Mach-Zehnder interferometer (MZI) interrogated based on microwave photonic pulse compression is proposed and experimentally demonstrated. A taper-based microfiber MZI (mMZI) with a nonuniform free spectral range (FSR) is used as a crucial sensor element. The mMZI is designed to be sensitive to the surrounding refractive index (RI) in the air. When the mMZI is exposed to an organic gas ambiance with varying concentrations, the optical spectrum of the mMZI will shift with the gas concentration change. Instead of measuring the wavelength shift of the mMZI in the optical domain, which has a low interrogation resolution and a slow interrogation speed, we convert the mMZI spectrum to the time domain based on spectral shaping and wavelength-to-time (SS-WTT) mapping by using a frequency swept laser (FSL) source. The time-domain waveform is then processed by a digital signal processor (DSP), where the time-domain waveform is compared with a reference waveform through cross-correlation. A narrow correlation peak indicating the time shift of the waveform is obtained. Since the time shift is linearly proportional to the wavelength shift of the mMZI, the wavelength shift of the mMZI is measured. In the experiment, the surrounding RI of ethanol gas is measured at a scanning speed as high as 1.6 MHz. The results show that the sensor has a sensitivity and resolution of -0.3663 ps/ppm and 54.6 ppm, respectively, without any surface modification. The proposed sensing system used to monitor the diffusion of ethanol vapor in real time and to mark the gas concentration after the diffusion has also been studied.

Microwave photonic frequency conversion, with flexible tunability and a simple structure based on a wavelength swept laser (WSL), is proposed and experimentally demonstrated. In the proposed frequency conversion system, a broadband WSL was used to generate a frequency-chirped optical carrier, which was sent to a Mach Zehnder modulator (MZM). A microwave signal, with its frequency to be converted, was applied to the MZM. The modulated signal was then sent to a dispersive device, where the waveform was compressed or expanded depending on the dispersion of the dispersive device. After photodetection, a frequency up-converted or down-converted microwave signal was generated. The tuning of the conversion frequency was achieved by adjusting the chirp rate of the optical carrier from the WSL or the dispersion of the dispersive device. The proposed approach was experimentally demonstrated. A microwave signal, with its frequency at 5 GHz, was up-converted to 5.577 GHz and down-converted to 4.936 GHz. The quality of the frequency converted microwave signal was also evaluated. The conversion performance could be further improved by introducing a WSL with a high duty cycle.