Abstract
Broadband image-reject mixing using a dual-channel polarization-modulated photonic microwave phase shifter is proposed and experimentally demonstrated. The polarization-modulated photonic microwave phase shifter is realized by an orthogonal circularly-polarized wavelength generator, an optical coupler, two polarizers, and two photodetectors (PDs). When a local oscillator (LO) signal, a radio-frequency (RF) signal, and an image interference are supplied to the polarization-modulated phase shifter, opposite phase shifts would be introduced to the RF- and image-induced intermediate frequency (IF) signals. By carefully adjusting the polarizers to make the two channels of the phase shifter quadrature, a phase difference of −90 degree is introduced to the RF-induced IF signals in the two channels, while 90-degree phase difference between the image-induced IF signals is produced. With an electrical 90 degree hybrid to combine the two channels, the two RF-induced IF signals will be in phase while those from the image are out of phase. As a result, the image in the downconverted signal is cancelled. An experiment is carried out. An image-reject mixer (IRM) with an image rejection ratio of ∼60 dB for a single-frequency signal and 23 dB for a 4-GHz linear frequency-modulated signal is obtained.
Highlights
Photonic microwave mixers are considered as an essential part in RF transceivers for radio over fiber systems, radar frontends and satellite payloads to convert the received high-frequency signals to a specific frequency band thanks to the advantages brought by the photonic technologies with respect to high frequency, large operational bandwidth, low transmission loss, and immunity to electromagnetic interference [1]–[5]
When a local oscillator (LO) signal, a radio-frequency (RF) signal, and an image interference are supplied to the polarization-modulated phase shifter, opposite phase shifts would be introduced to the RF- and image-induced intermediate frequency (IF) signals
When a lightwave and two pairs of quadrature RF signals with expressions of exp(jωct), VLOcos(ωLOt + φLO) and VLOcos(ωLOt + φLO + π /2), and VRFcos(ωRFt + φRF) and VRFcos(ωRFt + φRF + π /2) are applied to the PDM-dual-parallel MZM (DPMZM), where VRF and VLO are the amplitudes of the RF and LO signals, and φRF and φLO are the initial phases of the RF and LO signal, orthogonally-polarized carrier-suppressed single sideband (CS-SSB) modulations can be realized by each sub-DPMZM via adjusting the DC biases [28], Ex = −2J±1 exp [ j]
Summary
Photonic microwave mixers are considered as an essential part in RF transceivers for radio over fiber systems, radar frontends and satellite payloads to convert the received high-frequency signals to a specific frequency band thanks to the advantages brought by the photonic technologies with respect to high frequency, large operational bandwidth, low transmission loss, and immunity to electromagnetic interference [1]–[5]. Based on this scheme, an optical 90 degree hybrid is employed to introduce the quadrature LO signals in an IRM structure [16]–[19]. The unideal 90 degree phase shift of the optical 90 degree hybrid would lead to a relatively small image rejection ratio To solve this problem, broadband tunable photonic microwave phase shifters are used to introduce the precise phase shift [22]–[26]. Thanks to the good scalability and phase tunability of the polarization-based photonic microwave phase shifter, the IRM can be scaled into multiple channels to realize multichannel IRMs with independent phase shifts by sharing the same laser source and the same modulator, and the phase variations introduced by the electrical 90-degree hybrids can be properly compensated
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