We show a parity-time (PT) symmetric microwave photonic system in the optical wavelength space within a single spatial resonator, in which the gain and loss modes can perfectly overlay spatially but are distinguishable in the designated parameter space. To prove the concept, a PT-symmetric optoelectronic oscillator (OEO) in the optical wavelength space is implemented. The OEO has a single-loop architecture, with the microwave gain and loss modes carried by two optical wavelengths to form two mutually coupled wavelength-space resonators. The operation of PT symmetry in the OEO is verified by the generation of a 10-GHz microwave signal with a low phase noise of −129.3 dBc/Hz at 10-kHz offset frequency and small sidemodes of less than −66.22 dBc/Hz. Compared with a conventional spatial PT-symmetric system, a PT-symmetric system in the wavelength space features a much simpler configuration, better stability and greater resilience to environmental interferences.

A photonic true-time delay beamforming network to provide tunable progressive time delays for phased array beamforming is proposed and demonstrated. In the proposed system, a microwave signal is modulated on multiwavelength optical carriers and sent to an optical dispersive recirculating loop that contains a linearly chirped fiber Bragg grating (LCFBG) to provide a wavelength-dependent round trip time. As the microwave-modulated optical signals recirculate in the loop, the microwave signals carried by different wavelengths will experience different time delays at the output of the loop. By controlling the number of round trips, tunable progressive true-time delays are achieved, which are required for true-time delay phased array beamforming. The proposed true time delay beamforming network is experimentally evaluated. Two dispersive loops that can introduce a single-round-trip true-time delay difference of 2.5 ns and 160 ps are designed, which are implemented by using two LCFBGs with two different dispersion coefficients of 2500 and 200 ps/nm. The progressive time delays for beam steering achieved by the two dispersive loops can cover a beam scanning range from -60° to 60°.

An optoelectronic oscillator (OEO) for high sensitivity temperature sensing is proposed and experimentally demonstrated. The oscillation frequency of the OEO is determined by a single passband microwave photonic filter (MPF) in the OEO loop, which is implemented using a broadband light source, a Mach-Zehnder interferometer (MZI), a dispersion compensating fiber, and a photodetector. One arm of the MZI is used as a sensing arm, which is exposed to temperature variations and the other arm is used as a reference arm. When the temperature is changed, the length difference between the two arms is changed, which leads to the change of the free spectral range of the MZI. Since the central frequency of the MPF is a function of the FSR, the oscillation frequency of the OEO is affected by the temperature variations. By measuring the frequency change, the temperature change to the sensing arm is estimated. The proposed approach is experimentally evaluated. High sensitivity temperature sensing with a sensitivity of 3.7 MHz/°C is experimentally demonstrated.

A wideband and continuously tunable optoelectronic oscillator (OEO) incorporating a single passband microwave photonic filter (MPF) is proposed and experimentally demonstrated. The single passband MPF is implemented using a broadband light source, a Mach-Zehnder interferometer (MZI) in which a phase modulator is incorporated in one arm, a dispersion compensating fiber (DCF), and a photodetector (PD). The light wave from the broadband light source is sent to the MZI, and the output signal from the MZI is sent to the DCF and then detected at the PD. The overall operation is equivalent to a single passband MPF with the central frequency of the passband tunable by tuning the length difference between the two arms of the MZI. By incorporating the MPF into an OEO, a microwave signal with a tunable frequency is generated. The proposed OEO is experimentally evaluated. A microwave signal with a tunable frequency from 1 to 12 GHz and a tuning step of 0.5 GHz is generated. The phase noise performance of the generated microwave signal is evaluated. At 6.02 GHz, the phase noise is measured to be -107 dBc/Hz at an offset of 10 kHz.

A dual-passband microwave photonic filter (MPF) implemented based on phase-modulation to intensity-modulation (PM-IM) conversion using a phase modulator and an equivalent phase-shifted fiber Bragg grating (EPS-FBG) is proposed and experimentally demonstrated. The key component in the system is the EPS-FBG, which is designed and fabricated based on the equivalent phase-shift technique. The unique feature of the EPS-FBG is that equivalent phase shifts are introduced to both of the ±1^st channels, leading to a notch in each of the two channels. Thus, by implementing PM-IM conversion in the two channels, two passbands are produced. The central frequency of each passband is determined by the wavelength different between the notch and the optical carrier. In the design and fabrication, two phase shifts are introduced to the EPS-FBG to decrease the shape factor, which is defined as the ratio between the 20- and 3-dB bandwidths. In addition, a stimulated Brillouin scattering (SBS) assisted filter is incorporated in the system for carrier suppression to increase the spurious-free dynamic range (SFDR) and decrease the noise figure (NF) of the MPF. An experiment is performed. A dual passband filter with a 3-dB bandwidth and a shape factor of 167.3 MHz and 3.8, and 143.3 MHz and 3.3 for the 1^st and 2^nd passband is achieved. The frequency tunable ranges of the 1^st and 2^nd passbands are 5.4 and 7.4 GHz, respectively, with the magnitude variations of about ±0.5 dB during the turning. Due to the SBS-assisted filter, the SFDRs are increased by 7 dB and the NFs are decreased by 10 dB.

A novel fiber-optic twist sensor based on a dual-polarization distributed Bragg reflector (DBR) fiber grating laser is proposed and experimentally demonstrated. By beating the signal between the two polarizations of the laser which operates at 1543.154nm, a signal of 30.78MHz in frequency domain is observed. The twist will change the fiber birefringence, and resulting in the beat frequency variation between the two polarization modes from the fiber laser. The result shows the beat frequency shifts as a Sinc function curve with the twist angle and both the measuring curve period and twist sensitivity depend on the twist length of the laser cavity. A high twist sensitivity of 6.68MHz/rad has been obtained at the twist length of 17.5cm. Moreover, the sensor is insensitive to the environmental temperature, as well as strain along the fiber axis with ultralow beat frequency coefficients, making temperature and axial strain compensation unnecessary.