Distributed fiber-optic hydrophone is based on heterodyne coherent detection

Underwater acoustic monitoring using hydrophones (specialized microphones) has obvious military applications such as submarine location, and civilian uses as well, such as monitoring sea animals or exploring for marine mineral sources. Hydrophones based on optical fiber have advantages over other types of hydrophones such as thinness and ruggedness, no underwater electrical devices, and immunity to electromagnetic interference. These devices are typically interferometer-based and include an array of sensing elements along the fiber. However, these arrays are limited to about a hundred or less sensors due to technology restrictions; in addition, the sensor array-element spacing is typically fixed and lacks the flexibility to satisfy different frequency detection needs in the field of marine acoustic detection.

Recently, researchers from the Shanghai Institute of Optics and Fine Mechanics (SIOM) of the Chinese Academy of Sciences (CAS), in cooperation with the 23rd Institute of China Electronics Technology Group Corporation, have demonstrated a distributed optical-fiber hydrophone (DOFH) based on heterodyne coherent detection (het Φ-OTDR) and have field-tested a 104-m-long version of the apparatus, showing a sensitivity up to -146 dB rad/μPa/m (much higher than traditional optical cable).

In the experimental version, light from a <3 kHz-linewidth laser diode is split into probe and reference light using a fiber-optic coupler (OC); an acousto-optic modulator (AOM) then chops the probe light into pulses of 100 ns duration and also shifts the frequency by 160 MHz. After amplification by an erbium-doped optical fiber amplifier (EDFA) and passage through a circulator (CIR) that mixes reference and probe light, the light is sent to the fiber under test (FUT). Return signals are detected by a balanced photodetector (BPD) and sent to a data-acquisition board (DAQ) and a signal processor.

Fiber-wound mandrel within a cable

The 12.5-mm-diameter optical cable itself consists of a supporting mandrel, special sensitized optical fiber, and cable sheath. The bend-insensitive fiber is wound in a spiral path (7.5 m of fiber for every 1 m of cable) on a sound-sensitive mandrel at the central axis of the cable (which is sheathed for protection). Acoustic signals disturb the mandrel and the fiber, causing phase changes in Rayleigh-scattered light, which are the desired signal.

An array signal-processing model for DOFH was constructed to analyze the equivalence and specificity of the acoustic wave response using het Φ-OTDR as a distributed array of acoustic sensors (although the fiber senses continuously along its length, the model can select a spacing); the model shows, for example, that signals at 375 Hz are detected with high angular specificity throughout 360°, which at 675 Hz there are some angular blind spots.

The field test was conducted in a reservoir, with the Φ-OTDR system placed on a platform by the water and a 104 m length of cable extended into the water, with an inert section of fiber tens of meters long connecting the Φ-OTDR system with the sensing portion of the cable. The acoustic wavelength for the 375 and 625 Hz test signals were 4 and 2.4 m, respectively, with corresponding array element spacings of 1.34 and 0.67 m. The acoustic source was placed underwater at various positions ranging from 189 to 367 m away from the cable.

Through array signal processing, underwater acoustic signal source signal orientation and motion trajectory tracking can be realized accurately. Angular location of the source was precise at both acoustic wavelengths (see figure).

The researchers note that fabrication of the cable can be easily automated, and say that, while the current system noise floor is still much higher than that of traditional interferometric hydrophones, multiplexing or other methods could be used to further optimize the detection bandwidth of the system.

REFERENCE

1. B. Lu et al., Opt. Express (2021); https://doi.org/10.1364/oe.414598.