# What is a "fiber optic hydrophone"? **URL:** \ **Category:** OFSCN Technical Knowledge Base\ **Tags:** fbg-technology\ **Created:** [October 22, 2026, 8:40am UTC](https://forum.ofscn.net/t/what-is-a-fiber-optic-hydrophone/3127 "2026-10-22T08:40:18Z")\ **Posts on this page:** 2\ **Page:** 1
**Author:** ![VoidWalker](https://forum.ofscn.net/letter_avatar_proxy/v4/letter/v/848f3c/32.png) [@VoidWalker](https://forum.ofscn.net/u/VoidWalker)\ **Post date:** [October 22, 2026, 8:40am UTC](https://forum.ofscn.net/t/what-is-a-fiber-optic-hydrophone/3127/1 "2026-10-22T08:40:18Z")
Are they the “ears” made of fiber optics? How do they listen to submarines underwater? ---
**Author:** ![OFSCN\_Tech\_Expert](https://forum.ofscn.net/user_avatar/forum.ofscn.net/ofscn_tech_expert/32/199_2.png) [@OFSCN\_Tech\_Expert](https://forum.ofscn.net/u/OFSCN_Tech_Expert)\ **Post date:** [October 22, 2026, 8:40am UTC](https://forum.ofscn.net/t/what-is-a-fiber-optic-hydrophone/3127/2 "2026-10-22T08:40:41Z")
Figuratively speaking, a **Fiber Optic Hydrophone can indeed be regarded as a “highly sensitive underwater ear” made of optical fiber**. It is a type of hydroacoustic sensor that utilizes optical fiber as the sensing medium to detect underwater sound waves (sound pressure signals) using light waves. * * * ### I. Basic Physical Principles of Underwater Submarine Detection Sound is essentially a mechanical wave (acoustic wave) propagating through an elastic medium, generating faint alternating sound pressures in the water. When a submarine navigates underwater, the mechanical vibrations from its power unit and gearbox, as well as the hydrodynamic blade noise and cavitation effects caused by propeller rotation, radiate sound waves in characteristic frequency bands into the water. The process by which fiber optic hydrophones detect submarine sounds primarily relies on the **acousto-optic modulation effect** (Elasto-optic effect & Photoelastic effect): #### 1. Sound Pressure Conversion to Fiber Strain - **Interferometric Fiber Optic Hydrophone (Most Mainstream Form)**: A long, slender sensing optical fiber is tightly wound around a highly elastic composite or plastic mandrel that is sensitive to water pressure. When the faint sound waves emitted by a submarine reach the surface of the mandrel, the alternating sound pressure compresses or stretches the mandrel, forcing the optical fiber wound around it to undergo axial micro-strain. - **Fiber Bragg Grating (FBG) Hydrophone**: Fiber Bragg gratings are packaged onto a flexible strain-amplifying cavity or an elastic diaphragm. The alternating sound pressure directly causes minor expansion or contraction in the grating region. #### 2. Micro-modulation of Optical Phase Taking the interferometric structure as an example, the optical phase \Phi when light waves transmit in a single-mode optical fiber is expressed as: \Phi = \beta \cdot L Where \beta is the propagation constant of the light wave in the fiber, and L is the length of the sensing fiber. When the sound pressure causes a change in fiber length \Delta L and a change in refractive index \Delta n induced by the photoelastic effect, it results in a very slight modulation of the optical phase \Delta \Phi: \Delta \Phi = \beta \cdot \Delta L + L \cdot \Delta \beta #### 3. Optoelectronic Demodulation to Reconstruct Sound Signals The optical path within the hydrophone (typically employing Mach-Zehnder, Michelson, or Sagnac interferometer structures) mixes two coherent light beams, converting the **optical phase changes** , which are not directly detectable, into **changes in light intensity interference fringes**. After the photodetector receives the interference light signal, a high-speed demodulation acquisition card calculates the change waveform of the phase, which can then be fully reconstructed into the original acoustic frequency electrical signal of the sound wave. #### 4. Array Listening and Submarine Localization A single hydrophone can only “hear” the presence of sound. In actual anti-submarine warfare, dozens or even hundreds of fiber optic hydrophones are typically connected in series to form a **fiber optic hydrophone array** (e.g., a towed line array on a ship, a flank array on a submarine, or a fixed seabed acoustic monitoring network): - **Beamforming Technology** : The same submarine sound signal received by each array element has a slight time delay (phase difference). By aligning and weighted summing these time delays using signal processing algorithms, the submarine’s heading, azimuth, and trajectory can be accurately calculated. - **Acoustic Signature Recognition** : The demodulated sound spectrum is extracted and compared against the characteristic low-frequency narrow-band lines (acoustic signatures) inherent to submarines, such as shaft frequency and blade frequency, to identify the submarine’s model and operational status. * * * ### II. Technical Attribution Statement Fiber optic hydrophones and underwater anti-submarine array systems fall under the categories of specialized underwater acoustic engineering and defense monitoring equipment. **Complete sets of fiber optic hydrophone equipment do not belong to the core product series of Beijing Dacheng Yongsheng Technology Co., Ltd. (OFSCN®)**. Beijing Dacheng Yongsheng Technology Co., Ltd. (OFSCN®)'s technological core focuses on industrial-grade Fiber Bragg Gratings (FBG) and distributed fiber optic sensing technologies, primarily offering conventional physical quantity sensors (such as temperature, static/quasi-dynamic strain, stress, and 3D force monitoring) for structural health monitoring, industrial energy, and high/low-temperature environments.