Abstract
We demonstrate experimentally an optoelectronic oscillator (OEO) in which high side-mode suppression is achieved by cascading a phase modulator-based single passband tunable microwave photonic (MWP) filter with an optoelectronic feedback loop-based infinite impulse response (IIR) MWP filter. The OEO provides an RF oscillation that can be tuned from 6.5 GHz to 17.8 GHz with a phase noise lower than -103 dBc/Hz. Experimental results show that inclusion of the IIR section leads to a 20 dB reduction of phase noise close to the carrier and an increase of 10 dB in side mode suppression, compared to the equivalent OEO without an IIR section. The OEO was used to drive an optical frequency comb generator to generate a THz signal at 242.6 GHz by optical heterodyning; inclusion of the IIR section increases suppression of the side modes neighboring the THz carrier. A radio over fiber link was then implemented at a 242.6 GHz carrier frequency, with transmission of a 24 Gbps signal over 40 km of fiber and a 30 cm wireless path at a bit error rate below the forward error correction limit. The proposed system may be applied to frequency reconfigurable THz links and radars.
Highlights
Optoelectronic oscillators (OEO) [1], [2] have many applications due to their low phase noise, including microwave signal generation, instrumentation [3], sensing [4], radar systems and frequency down conversion [5]
The RF output signal of the OEO can be tuned from 6.5 GHz to 17.8 GHz by tuning the optical bandwidth of the tunable optical bandpass filter (TOBF) of the single passband microwave photonic (MWP) filter section; the tuning range is limited by the bandwidth of the RF components of the OEO loop, and the second amplifier (EA-2) in the OEO loop which operates between 6–18 GHz
The infinite impulse response (IIR) section in the OEO suppresses the side modes by an additional 10 dB as compared to the OEO without the IIR section that was reported in [13]. This improvement is due to the enhancement of the Q factor of the OEO loop that results from cascading the tunable single passband section with the IIR section
Summary
Optoelectronic oscillators (OEO) [1], [2] have many applications due to their low phase noise, including microwave signal generation, instrumentation [3], sensing [4], radar systems and frequency down conversion [5]. In [7], a side mode suppressed tunable OEO was implemented using an EBPF and injection locking (via an external electrical synthesizer), but the tuning range (from 8.5 to 9.5 GHz) was limited to the passband of the electrical bandpass filter. By cascading a single passband filter with an optoelectronic IIR section, the single passband section determines the center frequency of oscillation and the IIR section provides the high Q filtering for side mode suppression. We modify the tunable dual-loop OEO architecture [13] by cascading the single passband MWP filter of that topology with an additional optoelectronic IIR MWP filter section (Fig. 1), in order to increase the SMSR both in the microwave and THz band.
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