Optoelectronic oscillators with hollow-core fibres

Abstract

Optoelectronic oscillators (OEOs) are capable of generating microwave signals with high spectral purity (i.e. with high short-term stability) using optical energy sources and optical energy storage elements. Optical fibre delay-lines are commonly used in OEOs as the optical energy storage element and the spectral purity of the generated microwave signals can be increased by increasing the length of the incorporated optical fibre. However since the delay induced by an optical fibre is susceptible to temperature, the ambient temperature induced delay changes in the optical fibre can vary the OEO output signal frequency (by varying loop delay), degrading the longterm stability of the OEO. This thesis investigates the temperature stability improvement that can be obtained by replacing standard SMF delay-lines in OEOs with long-length hollow-core photonic bandgap fibres (HCPBGFs). Using 860 m long HC-PBGF with 19-cell core structure, it is shown that when the temperature sensitivity of the HC-PBGF is limited by temperature induced length changes, a temperature stability improvement of more than 15 times can be achieved in an OEO by using a HC-PBGF delay-line instead of an SMF delay-line. Furthermore by using a 1.09 km long 19-cell HC-PBGF it is shown that by operating the laser source in the OEO at the long-wavelength-edge of the bandgap of the HC-PBGF (where the group velocity gets increased when the temperature is increased), the effect of temperature induced fibre length changes in the HC-PBGF can be further reduced to reach zero and even negative temperature sensitive characteristics. Furthermore it is shown that within a 3 nm wide wavelength window, the temperature sensitivity of this particular fibre is at least 100 times lower than that of an SMF. Phase noise of the long-length HC-PBGF delay line incorporated OEOs are measured and it is shown that phase noise degradation due to multipath interference in HCPBGFs can be supressed by modulating the laser source. In order to supress theundesired polarization properties of HC-PBGFs such as polarization dependent loss (PDL) and polarization mode coupling that could potentially degrade the performance of HC-PBGF delay-line incorporated OEOs, two passive fibre-optic depolarizers capable of depolarizing input signals with 20 MHz and 1 MHz linewidths were developed. The characterization results of these depolarizers are also shown in this thesis. These depolarizers are now being commercialized by Phoenix Photonics, UK.

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Identifiers

ORCID for Radan Slavik: orcid.org/0000-0002-9336-4262

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