An optically tunable radio frequency (RF) downconversion scheme is proposed based on an optoelectronic oscillator (OEO) incorporating a tunable microwave photonic filter. The local oscillation (LO) is generated in the OEO, whose frequency is varied through simply tuning the frequency difference between the optical carrier and the reflection notch of a phase-shifted fiber Bragg grating (PS-FBG). The LO and the input RF signal are combined and added to the OEO loop by a single phase modulator. The RF modulation sidebands and one of the LO modulation sidebands are extracted out of the OEO loop by the PS-FBG and sent to a photo-detector to achieve RF downconversion. In the experiment, optically tunable LOs in the frequency range of 6 GHz to 15 GHz are generated, and RF signals in the frequency range of 7 GHz to 16 GHz are successfully down-converted to intermediate frequency band around 1 GHz. The proposed scheme has the potential to cover a frequency range beyond 40 GHz.
We propose and demonstrate a self-calibrating approach to measure the magnitude response of a broadband electro-optic intensity modulator based on photonic downconversion sampling. A low-repetition-rate mode-locked laser is employed to sample a swept-frequency microwave signal via the modulator under test, where the magnitude response of the modulator is calculated from the relative intensity of the Fourier frequency components in the first Nyquist frequency range and the direct current. In the experiment, magnitude response and half-wave voltage versus frequency of a commercial Mach-Zehnder electro-optic modulator within a frequency range of 0-40 GHz is successfully measured using the proposed method.
An approach to retrieving intrinsic phase response of optical filters from magnitude response only measurement is proposed and demonstrated. Phase retrieval is based on the fact that the intrinsic phase response of an optical filter with a minimum phase response has a unique relationship with its magnitude response. A phase retrieval algorithm based on fast Fourier transform is proposed. In the experiment, the intrinsic phase response of notch in the reflection spectrum of a phase-shifted fiber Bragg grating is successfully retrieved from the measured magnitude response, which is free of the influence of fiber pigtail in the measurement setup.
KEYWORDS: Microwave radiation, Microwave photonics, Digital signal processing, Signal processing, Photonic microstructures, Modulation, Distortion, Analog electronics, Electronic components, Quantization
A new microwave photonic approach to microwave frequency measurement with a high resolution and a large
bandwidth is proposed. In this method, three photonic sampling analog-to-digital converters (ADCs) with co-prime
sampling rates are employed. Three Fourier frequencies acquiring through down-converted analog-to-digital conversion
of the unknown microwave signal are utilized to recovery the frequency of the unknown signal. The simulation results
show that a microwave frequency measurement system which is feasible for multi-frequency microwave signal achieves
a large measurement range of 0-50GHz and an accuracy of±1MHz. In addition, the spur-free dynamic range of
101.1dB-Hz2/3@50GHz is also numerically demonstrated.
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