Abstract
We present a free-space, continuous-wave laser interferometric system capable of multi-target dynamic phase measurement at acoustic frequencies up to a Nyquist bandwidth of 10.2 kHz. The system uses Digitally-enhanced Heterodyne Interferometry to range gate acoustic signals simultaneously from multiple in-line reflections while isolating coherent cross-talk between them. We demonstrate sub-nanometer displacement sensitivity across the audio band for each individual reflection surface and 1.2 m resolution between successive surfaces. Signals outside the 1.2 m range-gate of the system were suppressed by greater than 30 dB in amplitude, enabling high fidelity independent acoustic measurements.
Highlights
A well-known technical challenge in high precision interferometric measurement has been the management of spurious noise due to unwanted coherent reflections
Multiplexing architectures have been developed to meet the fast data transfer requirements of modern communication systems. These techniques exploit one or more properties of an optical system to allow for simultaneous transmission of multiple channels of information, and can be broadly separated into two categories
We have demonstrated a multiplexed interferometric readout of four measurement channels, showing isolation of three audio band signals using Digitally enhanced Heterodyne interferometry
Summary
A well-known technical challenge in high precision interferometric measurement has been the management of spurious noise due to unwanted coherent reflections. This enables gating of optical signals, allowing for both multiplexed readout and rejection of spurious interference The bandwidth of these systems is limited by the repetition rate, and averaging time required to extract phase information, reducing the measurement duty cycle [4, 5]. While similar to RM-CW LIDAR techniques [13, 14], DI builds on the fundamental principle of heterodyne interferometry to retain the high displacement sensitivity seen in interferometric sensors [15] This allows the optical complexity and multiplexing to be transferred into the digital signal processing realm [16]. We show the separation of audio-band displacement signals from four in-line reflection surfaces, with sufficient fidelity and isolation for applications in perimeter monitoring and surveillance
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