Wangzhe Li

A Narrow-Passband and Frequency-Tunable Microwave Photonic Filter Based on Phase-Modulation to Intensity-Modulation Conversion Using a Phase-Shifted Fiber Bragg Grating

A novel approach to implementing a narrow-passband and frequency-tunable microwave photonic filter (MPF) based on phase-modulation to intensity-modulation conversion in a phase-shifted fiber Bragg grating (PS-FBG) is proposed and experimentally demonstrated. In the proposed MPF, a phase-modulated signal is sent to a PS-FBG. If one of the sidebands falls in the notch of the PS-FBG, the phase-modulated signal is converted to an intensity-modulated signal. Due to the ultra-narrow notch of the PS-FBG, a microwave filter with an ultra-narrow passband is realized. The tunability of the microwave filter is achieved by tuning the wavelength of the optical carrier. A theoretical analysis is performed in which the value of the phase shift and the location of the phase shift in the PS-FBG on the frequency response of the MPF are studied. Two PS-FBGs with different reflection bandwidths and different phase-shift values introduced at the center of the gratings are fabricated and incorporated into the proposed MPF. For the two PS-FBGs, the 3-dB bandwidths are 120 and 60 MHz and the tunable ranges are 5.5 and 15 GHz.

A Wideband Frequency Tunable Optoelectronic Oscillator Incorporating a Tunable Microwave Photonic Filter Based on Phase-Modulation to Intensity-Modulation Conversion Using a Phase-Shifted Fiber Bragg Grating

An optically tunable optoelectronic oscillator (OEO) with a wide frequency tunable range incorporating a tunable microwave photonic filter implemented based on phase-modulation to intensity-modulation conversion using a phase-shifted fiber Bragg grating (PS-FBG) is proposed and experimentally demonstrated. The PS-FBG in conjunction with two optical phase modulators in the OEO loop form a high-Q, wideband and frequency-tunable microwave photonic bandpass filter, to achieve simultaneously single-frequency selection and frequency tuning. Since the tuning of the microwave filter is achieved by tuning the wavelength of the incident light wave, the tunability can be easily realized at a high speed. A theoretical analysis is performed, which is verified by an experiment. A microwave signal with a frequency tunable from 3 GHz to 28 GHz is generated. To the best of our knowledge, this is the widest frequency tunable range ever achieved by an OEO. The phase noise performance of the OEO is also investigated.

Investigation of Photonically Assisted Microwave Frequency Multiplication Based on External Modulation

Microwave frequency multiplication based on external modulation using two cascaded Mach-Zehnder modulators (MZMs) has been considered an effective solution for high-frequency and frequency-tunable microwave signal generation. Different techniques have been demonstrated recently, but no generalized approach has been developed. In this paper, a generalized approach to achieving microwave frequency multiplication using two cascaded MZMs is presented. A theoretical analysis leading to the operating conditions to achieve frequency quadrupling, sextupling, or octupling is developed. The system performance in terms of phase noise, tunability, and stability is also investigated.

Microwave Generation Based on Optical Domain Microwave Frequency Octupling

A novel approach to achieving microwave frequency octupling in the optical domain is proposed and demonstrated. The proposed system consists of two cascaded Mach-Zehnder modulators (MZMs) that are both biased at the maximum transmission point, with a tunable optical phase shifter connected in between to introduce a phase shift. An input microwave signal is applied to the MZMs with its power adjusted to ensure the two MZMs having an identical phase modulation index. A theoretical analysis that leads to the conditions for achieving frequency octupling is provided. The approach is verified by experiments. The phase noise performance and the frequency tunability are also experimentally investigated.

An Optically Tunable Optoelectronic Oscillator

An optically tunable optoelectronic oscillator (OEO) implemented by employing a two-port optical phase modulator without using any electronic microwave filters is proposed and experimentally demonstrated. The key device in the system is the two-port phase modulator, which functions, in conjunction with a dispersive element in the loop, to form a high-Q microwave filter to perform microwave frequency selection. The central frequency of the microwave filter is a function of the optical wavelength and the chromatic dispersion of the dispersive element, therefore, the oscillation frequency can be simply tuned by tuning the wavelength of the laser source or the chromatic dispersion of the dispersive element. A theoretical analysis is provided, which is verified by experiments. The phase noise performance and the frequency tunability are both experimentally investigated.

Photonic Generation of Phase-Coded Microwave Signal With Large Frequency Tunability

A photonic approach to realizing phase-coded microwave signal generation with large frequency tunability is proposed and demonstrated. Two coherent optical wavelengths are generated based on external modulation by biasing a Mach-Zehnder modulator (MZM) at the minimum transmission point to generate ±1 -order sidebands while suppressing the optical carrier. The two ±1-order sidebands are then sent to a fiber Sagnac interferometer (SI) incorporating an optical phase modulator (PM) and a broadband flat-top fiber Bragg grating (FBG), with one of the sidebands being phase modulated at the PM. A frequency tunable phase-coded microwave signal is generated by beating the two sidebands at a photodetector (PD). The proposed technique is experimentally investigated. The generation of a frequency tunable phase-coded microwave signal at 22 and 27 GHz is demonstrated.

Microwave and Terahertz Generation Based on Photonically Assisted Microwave Frequency Twelvetupling With Large Tunability

A novel approach to achieving photonically assisted microwave frequency twelvetupling with large frequency tunability is proposed and demonstrated. The frequency twelvetupling is realized through a joint operation of polarization modulation, four-wave mixing, and stimulated-Brillouin-scattering-assisted filtering. The key significance of the technique is that the system is capable of realizing simple frequency tuning over a wide frequency range. An experiment is performed. An electrical signal with a frequency that is tunable from 48 to 132 GHz is generated.

Transverse load sensing based on a dual-frequency optoelectronic oscillator

We propose and experimentally demonstrate a fiber-optic sensor implemented based on a dual-frequency optoelectronic oscillator (OEO) for transverse load sensing. In the OEO loop, a phase-shifted fiber Bragg grating (PS-FBG) is employed to which a transverse load is applied to introduce a birefringence to create two orthogonally polarized notches, which leads to the generation of two oscillating frequencies. The beat frequency between the two oscillating frequencies is a function of the load force applied to the PS-FBG. The proposed sensor is experimentally demonstrated. The sensitivity and the minimal detectable load are measured to be as high as ~9.73 GHz/(N/mm) and 2.06×10(-4) N/mm, respectively. The high-frequency purity and stability of the generated microwave signal by the OEO permit extremely reliable and high-accuracy measurement. The frequency interrogation allows the system to operate at an ultra-high speed. In addition, the sensing signal is insensitive to the variations of both the environmental temperature and the optical carrier wavelength.

Dynamic range improvement of a microwave photonic link based on bi-directional use of a polarization modulator in a Sagnac loop.

A novel microwave photonic link (MPL) with an improved spurious-free dynamic range (SFDR) based on a bidirectional use of a polarization modulator (PolM) in a Sagnac loop is proposed and demonstrated. The PolM in the loop functions, in conjunction with a polarization controller and a polarization beam combiner, as a Mach Zehnder modulator (MZM), which only modulates the incident light wave along the clockwise direction, leaving the counter-clockwise light wave unmodulated due to the velocity mismatch. Two clockwise intensity-modulated signals along two paths (Path 1 and Path 2) are generated, with one (Path 2) combined with the non-modulated light wave from the counter-clockwise direction to suppress part of the optical carrier. By controlling the power relationship between the two paths, the third-order intermodulation distortion (IMD3) can be fully suppressed, and thus an MPL with improved dynamic range is achieved. A theoretical analysis is presented, which is validated by an experiment. The IMD3 can be suppressed by 50 dB, giving an improvement in SFDR of 16 dB.

Photonic-Assisted Microwave Channelizer With Improved Channel Characteristics Based on Spectrum-Controlled Stimulated Brillouin Scattering

A photonic-assisted microwave channelizer with improved channel characteristics based on spectrum-controlled stimulated Brillouin scattering (SBS) is proposed and experimentally demonstrated. In the proposed system, N lightwaves from a laser array are multiplexed and then split into two paths. In the upper path, the lightwaves are modulated by a microwave signal with its frequency to be measured. In the lower path, for each lightwave, the wavelength is shifted to a specific shorter wavelength via carrier-suppressed single-sideband modulation and the spectrum is then shaped. The wavelength-shifted and spectrum-shaped lightwaves are used to pump a single-mode fiber to trigger SBS. Thanks to the SBS effect, multiple gain channels at the N wavelengths are generated. The channel profile of each channel, determined by the designed spectral shape of the pump source, is improved with a flat top and a reduced shape factor. The characteristics including the bandwidth, channel spacing, and channel profile can be controlled by adjusting the spectral shape of the pump source. A proof-of-concept experiment is performed. A microwave channelizer with a shape factor less than 2, a tunable channel bandwidth of 40, 60, or 90 MHz, and a tunable channel spacing of 50, 70, or 80 MHz, is demonstrated.