Measuring acoustic waves propagation in solid or fluid media is an important task in applications such as Structural Health Monitoring (SHM), seismology, oceanography, underwater acoustic communications and more. While there are quite a few acoustic sensors that are considered to be highly sensitive and broadband, such as geophones for seismic applications or hydrophones for underwater applications, they are all point sensors. Point sensors are limited since they cannot provide spatiotemporal measurement of propagating acoustic waves. In addition, their coverage volume is limited due to the attenuation of the acoustic waves in the medium. These limitations can be alleviated by using an array of acoustic sensors which can provide the required spatiotemporal measurement capability in addition to extended detection volume. This work describes the implementation of an underwater fiber-optic sensor array for ultrasonic (US) waves. To overcome the well-known trade-off between update rate and sensing fiber length a Coded Array Matched Interrogation (C-AMI) method was implemented. The method enabled an enhancement of the theoretical sampling rate by a factor of 54. The system successfully measured the propagation of an ultrasonic pulse with a carrier of 95kHz along a 20m long test pool.
Laser ranging measurements using incoherent pulse compression of complementary code pairs is reported. The two bipolar codes are converted to unipolar representations using a pulse position modulation algorithm, and used in succession in intensity modulation of a laser ranging source. Reflected echoes from a wall target are directly and incoherently detected. The cross-correlation between each of the two collected echoes and its respective, reference bipolar sequence, that is digitally stored at the receiver, is calculated. The two correlation functions are then added together. The off-peak aperiodic correlation functions of two codes sum up to zero, hence they are particularly suitable for low-sidelobe radar and laser ranging and detection systems. The scheme does not require the preservation of phase information in transmission or reception and provides superior sidelobe suppression compared with that of longer single codes. The code pairs are scalable to arbitrary lengths through simple procedures. Simulated and experimental ranging measurements in the presence of additive noise are discussed. The distance to the target could be recovered based on weak collected echoes, with an average optical power as low as 2 nW, without averaging over repeating measurements.
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