Local Oscillator Sideband Research Articles

All-optical, filter-free, image-reject, and fundamental/sub-harmonic downconverted RoF link

An all-optical, image-reject, and fundamental/sub-harmonic downconverted radio-over-fiber (RoF) link using a polarization-division multiplexing Mach–Zehnder modulator (PDM-MZM) and fiber-dispersion effect is proposed. At the remote antenna unit, the receiving radio frequency (RF) signal and the local oscillator (LO) signal are used to drive two sub-MZMs of the PDM-MZM. After a length of optical fiber transmission, at the central office, by using the fiber-dispersion effect and controlling the polarization controller to introduce an appropriate phase shift between the RF and LO sidebands, image rejection is achieved and power fading is avoided simultaneously. Because the scheme is a simple and all-optical system without electrical frequency-dependent devices or filters, it has a good image-reject capability over a large working frequency range. Besides, the downconverter in sub-harmonic mode can decrease frequency requirement of the LO signal. Experimental results indicate that in the fundamental and sub-harmonic downconverters, for a single-tone signal, IRRs are over 47 dB. Image rejection for the 100-Mbaud 16-QAM signal and demodulated desired IF signal with a good EVM of less than 7% are realized. The proposed structure can be applied in RoF systems and microwave and millimeter-wave receivers.

Reconfigurable microwave photonic frequency upconverter with local oscillator doubling or local oscillator quadrupling

A reconfigurable microwave photonic frequency upconverter with local oscillator (LO) doubling or LO quadrupling is proposed based on two cascaded dual-parallel Mach–Zehnder modulators (DPMZMs) and a fiber Bragg grating (FBG) with a wide bandwidth (0.4 nm). The first DPMZM is used to generate the ±1st-order or ±2nd-order sidebands of the LO signal. The FBG is inserted to separate the positive and negative sidebands. Only the transmitted LO sideband is sent to the second DPMZM, and the carrier-suppressed single-sideband (CS-SSB) modulation of the intermediate frequency (IF) signal is realized by using an electrical 90 deg hybrid coupler. The reflected LO sideband via FBG and the −1st-order IF sideband are combined. Then, the upconverted signal with the frequency of 2ωLO + ωIF or 4ωLO + ωIF can be generated by a photodetector. The experimental results show that the spur suppression ratios of the upconverter with LO doubling and quadrupling are 21.3 and 11.3 dB, respectively. The proposed approach greatly reduces the frequency requirement of the LO signal. The purity of electrical spectrum is largely improved in both the LO doubling and LO quadrupling upconversion benefiting from the CS-SSB modulation. Furthermore, these two upconversion modes can be easily switched by tuning the DC biases of the first DPMZM.

A Microwave Photonic Mixer Using a Frequency-Doubled Local Oscillator

A microwave photonic mixer based on a frequency-doubled local oscillator (LO) is proposed. The carrier-suppressed single-sideband (CS-SSB) modulation of the –first-order radio frequency (RF) and +second-order LO sidebands can be generated by using a dual-polarization dual-parallel Mach–Zehnder modulator (DP-DPMZM), an electrical 90° hybrid coupler, and an optical bandpass filter. The upconverted intermediate frequency (IF) signal with the frequency of ${\omega _{\boldsymbol{RF}}} + 2{\omega _{\boldsymbol{LO}}}$ is output by frequency beat at a photodetector. Meanwhile, the –first-order RF sideband can be easily changed to +first-order RF sideband by adjusting one dc bias voltage of the DP-DPMZM. The downconverted IF signal with the frequency of ${\omega _{\boldsymbol{RF}}} - 2{\omega _{\boldsymbol{LO}}}$ is obtained correspondingly. The experimental results show that the spur suppression ratios of upconverted and downconverted IF signal are 19.0 dB and 25.5 dB, respectively. The spurious-free dynamic range of the proposed mixer is 96.5 dB·Hz2/3. The output electrical spectrums are pure owing to the optimal design of the CS-SSB modulation. The proposed mixer reduces the frequency requirement of LO signal since the second-order LO single sideband is used, and the upconversion and downconversion can be switched to each other by a simple electrical operation.

Broadband Photonic Microwave Signal Processor With Frequency Up/Down Conversion and Phase Shifting Capability

A new dual-function photonic microwave signal processing structure that has the ability to realize both frequency up/down conversion and RF/IF phase shifting, is presented. In the proposed signal processor, a dual-polarization dual-parallel Mach Zehnder modulator (DP-DPMZM) is used to generate an orthogonally polarized single RF/IF signal and local oscillator (LO) modulation sideband without an optical carrier. The optical phase difference between the two sidebands can be controlled by controlling a DC voltage applied to a LiNbO3 electro-optic phase modulator that is connected after the DP-DPMZM. Beating between the two sidebands at a photodetector generates an RF/IF signal with a phase equal to the optical phase difference between the IF/RF signal and LO sidebands. The dual-function photonic microwave signal processor has a wide bandwidth and does not require a precise control of the laser wavelength. Experimental results demonstrate that a flat >−3 dB down/up conversion efficiency is achieved for an RF signal from 3.5 to 26.5 GHz and for an IF signal from 0.5 to 3 GHz, and a full 360° continuous phase shift of the output IF/RF signal.

All-Optical Microwave Photonic Downconverter With Tunable Phase Shift

An all-optical microwave photonic frequency downconverter with tunable full range continuous phase shift is presented. In the proposed system, the radio frequency (RF) and local oscillator (LO) signals modulate two sub Mach–Zehnder modulators (MZMs) in an integrated dual-parallel MZM (DPMZM), respectively. The two sub-MZMs are both biased at the minimum transmission point to implement the carrier-suppressed double-sideband modulation. An optical bandpass filter is then used to retain the +1st order RF and LO sidebands. When the two sidebands are send to a photodetector, an intermediate frequency (IF) signal is produced, and the phase of the converted IF signal is linearly changed by adjusting the bias voltage of the parent MZM in the DPMZM. An experiment is carried out. An RF signal can be downconverted to a phase-tunable IF signal, and the measured spurious-free dynamic range reaches 100.2 dB·Hz2/3. The phase deviation and power ripple of the IF signal are less than 2° and 0.26 dB, respectively. The all-optical design makes the system bandwidth unlimited by electrical components. Meanwhile, since the proposed microwave photonic link can simultaneously implement frequency downconversion and full range phase shift of the converted IF signal, it provides a compact alternative for the applications including radio-over-fiber system and phased-array beamforming.

Dual-parallel Mach-Zehnder modulator based microwave photonic down-conversion link with high dynamic range

With the rapid development of the microwave photonic communication technology, the frequency of the microwave signal is expanded to the Ka waveband, since most of low frequency bands are occupied. However, the current commercial detectors and signal processing modules are limited by bandwidth. Therefore, the traditional method of directly detecting the microwave signals cannot meet the actual demands. It is essential to achieve the microwave photonic down-conversion from the high frequency microwave signal (~10 GHz) to the lower frequency signal (~100 MHz). Meanwhile, the down-conversion low frequency signal can be processed by the existing mature technology and low cost devices. The microwave down-conversion link can effectively avoid leaking the local oscillator, and it possesses many advantages such as high bandwidth and spurious free dynamic range, low loss and low noise. In this paper, a microwave photonic down-conversion system is presented based on the integrated dual-parallel Mach-Zehnder modulator (DPMZM) to increase the spurious-free dynamic range as well as conversion efficient of microwave photonic link. The integrated DPMZM is mainly comprised of two intensity modulators (MZM-a and MZM-b), and a phase shifter. The radio frequency (RF) signal is loaded into DPMZM to modulate the optical signal. The local oscillator is loaded into the MZM-a to produce the 1st local oscillator sideband, and two RF signals are fed to the MZM-b to form the 1st RF signal sideband. The direct current bias of the DPMZM is adjusted to output a high carrier suppressed double sideband (DSB) signal. The erbium-doped fiber amplifier is used to increase the power of light to match the power range of the detector. The RF signal sideband and local oscillator sideband are mixed to produce the beat frequency, and the frequency down-conversion can be achieved. The principle of frequency down-conversion is elaborated by theoretical analysis. The conversion efficiency and spurious free dynamic range are analyzed and simulated. On this basis, the microwave photonic link of frequency down-conversion is built. The performance of the system is tested. The ratio of optical carrier power to sideband power of the DSB signal is 26 dB. The experimental result shows that the conversion efficiency is 7.43 dB and spurious-free dynamic range is 110.85 dB/Hz2/3. The down-conversion method based on the DPMZM can optimize the output spectrum of the sideband. The structure of system is simple and easy to implement, so it is a good option for improving the conversion efficiency and spurious-free dynamic range.

Microwave photonic mixer based on a single bidirectional Mach–Zehnder modulator

A microwave photonic mixer based on a single electro-optic Mach-Zehnder intensity modulator operating in both directions is presented. In this mixer structure, the light from the optical source travels in opposite directions inside the modulator and is modulated by both the RF signal and the local oscillator (LO). The output optical spectrum comprises the RF signal and LO sidebands without the optical carrier. This enables a high conversion efficiency mixing operation to be obtained. The mixer has a simple structure, and its performance is insensitive to the modulator bias voltage; hence no DC bias voltage and no modulator bias controller are required to obtain robust high conversion efficiency mixing operation. Experimental results are presented showing large conversion efficiency improvement of 25.7dB compared to the conventional dual Mach-Zehnder modulator-based microwave photonic mixer and a modulator bias insensitive mixing performance.

Bandwidth enhancement of electro-optic field sensing using photonic down-mixing with harmonic sidebands

We demonstrate that harmonic sidebands of an electro-optic modulator's driving frequency can be used as the local oscillator in a photonic down-mixing process in order to significantly enhance the bandwidth of near-field, electro-optic, microwave measurements. The creation of second- and third-order-harmonic modulation sidebands on a laser-diode output are described, with heterodyne down-conversion of microwave signals taking place within an electro-optic sensor crystal. The measurement bandwidth of an electro-optic microwave probe can thus be enhanced by as much as a factor of three with respect to the use of conventional, fundamental-harmonic sidebands. Carrier-sideband analysis from the measured optical spectrum indicates that millimeter-wave-frequency local-oscillator sidebands can be created using a Ku-band electro-optic modulator and that the electro-optic-signal-modulation depth can be enhanced by suppressing the light-beam carrier component. Transverse near-field distributions from high frequency patch antennas are extracted using both second- and third-order-harmonic sidebands.

On the effect of IF power nulls in Schottky diode harmonic mixers

Experimental results reveal the existence of a null in the IF output power for certain bias voltage levels in single Schottky diode harmonic mixers at terahertz frequencies. This dramatic loss of IF power is due to a drastic increase of conversion loss and is governed by intrinsic diode parameters, as well as external parameters. For a second harmonic mixer, this is due to a competition of two mixing paths; one is the mixing of the RF with the doubled local oscillator (LO) frequency and the other a mixing of the RF with the LO to an LO sideband followed by a second mixing with the LO, both leading to the IF. In this paper, we present a systematic investigation of and some basic relations between these parameters and the depth and voltage levels where the conversion loss increases. Good agreement is obtained between a simple analysis and the experimental data on second and fourth harmonic mixers.