Balanced Heterodyne Architecture for Electro-Optic Probing

Technology Electro-optic (EO) probing is generally used for contactless measurement of electric fields. It enables measurements up to the terahertz range without disruption to the signals present and 3D mapping of electromagnetic field leakage from electronic devices such as mobile phones. EO probing relies on the polarisation changes of an optical beam from which voltage waveforms can be derived. One of the advantages of EO probing used for spectrum analysis is its capability to acquire voltage waveforms in the extremely high frequency THz range (0.3THz to 10THz). THz radiation has significant properties that enable many materials and living tissues to be imaged and identified. Unlike many other forms of radiation, the non-ionizing property of THz radiation allows screening to be conducted safely. Also, many materials have unique spectral characteristics in the THz range. The need and use of EO probing will become significant as more electronic devices shift to operate within the THz range, electronic components are further miniaturised, and techniques are refined for imaging or characterising biological tissue. A conventional EO probe scheme operating within the THz range uses heterodyne detection. The main constraint with heterodyne detection is its sensitivity for monitoring or detecting signals with very low amplitude, thus reducing the usefulness and applicability of EO probing. Enhancing the sensitivity of EO probing have generally focused on refining individual components: • EO probe material and structure; and • Optimisation of photonic components. The relative complexity and subsequent price considerations for these improvements however do not provide significant value or benefits. The technique proposed modifies a conventional heterodyne EO probe scheme and improves the sensitivity by integrating a balanced detection arrangement. The balanced heterodyne architecture takes advantage of two image frequency signal in phase op-position generated from the output signal of the probe. The modification is easy to implement and adaptable to conventional heterodyne EO probe schemes. The new architecture only requires an additional optical coupler and Mach-Zehnder modulator to be integrated with an heterodyne EO probe scheme. The technique does not require any polarisation manipulation or modification to the EO probe. As such, the architecture can easily be implemented with a conventional heterodyne EO probing scheme, and immediately benefit from the performance improvements. The noise performance of the balanced heterodyne architecture has been evaluated and compared to the conventional scheme. Performance improvements and benefits, particularly at low input powers, over the conventional scheme have been shown in key specifications: Increased sensitivity • Input power for a minimum detectable signal is approximately 7dB lower (~400%). Improved signal amplification • Average of 5.5dB improvement in the linear region of operation (~250%). Increased stability • Standard deviation of output signal is consistently lower.