A Comprehensive Study on Highly Sensitive Photonics Based Acoustic Sensors for Under Water Applications

By referring few researcher works which is related to the work of optical sensor and able to detect the acoustic signal, this review paper provides the detailed study of acoustic signal in under water. Researchers has worked with the optical sensor and emerged as great promising sensing device. From various research papers it is observed that the detection of acoustic signal is done with the help of various optical sensors. The various sensors are fiber bragg grating (FBG) sensor, fiber optic interferometric sensor and Mach zehnder interferometer (MZI) sensor. So this review paper covers all these sensors and their application with a particular focus on the different structure of optical sensor. The structure of the various sensors and their application is discussed from the different research paper. The overall work is reported and corresponding result is demonstrated. From the demonstrated work it has been observed that with good quality and proper sensitivity in all optical sensors are able to detect the acoustic signal for under water due to their compact size and able to provide accurate results.


Introduction
The OFAS (optical fiber acoustic sensing) system is popular to monitor the acoustic wave which is generated by the external sources. Acoustic wave is kind of mechanical wave which is very important to carrying the information. With different frequency band the detected acoustic wave can be applicable for various fields such as ultrasonic medicine, underwater acoustic monitoring and aerospace.
Since few decades the development in acoustic sensing domain, this optical sensing devices has been observed and provides the detailed information. The advantage of this optical sensor is anti-electromagnetic interference and less loss for long distance transmission. Due to their high sensitivity, compact size, these sensors are applied in different field of applications.
Another most important and popular sensor is Mach Zehnder Interferometer. It was demonstrated as temperature sensor and pressure sensor. The mechanism of this mach Zehnder interferometer is that it uses laser source as source. This light beam will be decoupled by the two arms and recoupled at the receiver end. Because of its high sensitivity MZI can be used as acoustic sensor. When sensing arm of MZI is conveying the acoustic signal and include a change in optical path and then introducing phase shift at receiver end.
Ian F. Akyildiz et.al has worked with sensors which has potential to unexplored application and to enhance ability to observe and predict the ocean. So authors worked with sensors, which are related to network and gathered information about under water environment. In this paper they worked with underwater acoustic sensors which constitute a basis from Shengye HUANG discussion of the challenges associated with the underwater environment. Tao Fu et.al, they worked with different sensor such as two dimensional sensor networks, three dimensional sensor network. [1]. Problem can be observed from this paper in approaching of different network layer under water is difficult.
Geoffrey A et. al. focused on work of detection of acoustic signal under water using Fiber-Optic Interferometric Sensors. This sensor is electro ceramic transducers. Xiaohong Bai et.al, the authors discussed about this sensor array. Work has been done by a hydrophones installed named as the fiber-optic bottom mounted array, which is large and time division multiplexed architecture.
In military sonar and seismic survey system this sensor can be used [2]. The designed hydrophone is used to get consistent with the good acoustic resolution but it should be specified depth. There should be proper frequency response with the operating frequency range of interest.
The mechanical properties of the materials and the geometry of the design factors which can influence the above design matter. The design of hydrophone is very important and design should be such that mechanical properties of the material frequency are higher than operating frequency. So choosing of material is crucial part to demonstrate this kind sensor. So another approach can be proposed to detect the acoustic signal in under water are Photonic crystal sensor, fiber bragg grating sensor or mach Zehnder interferometer can be proposed for the experiment.
Shengye HUANG. et. al [3] worked with optical fiber hydrophone which responds with temperature compensation package and able to provide improved sensitivity with a frequency from 2.5 kHz to 12 kHz. This acoustic sensor is one kind of transducer which is able to convert pressure into elastic vibration as shown in resultant output.
Graham Wild et. al. has done experiment with Fiber Bragg Grating (FBG) sensors to detect the acoustic signal. There are numerous advantages of FBG sensor so it can be useful for detecting of acoustic signal in under water.
Authors observed the advantages of FBG and have done experiment on FBG sensor.
The merits of FBG as sensing elements, which can be either, stress or temperature. In this work we also showcase recent outputs.
Optical fiber grating have been immerged in optical platform to detect chemical and bio chemical elements as sensor. By inducing this interaction in the transmission spectrum along the fibers so there will be change in the measurement of refractive index.
There will be combination of optical fibers, Based on this approach the optical fiber gratings are considered because of its advantages and limitations [5]. Another way to approach the FBG sensor is to detect the DNA sequence. FBG is inscribed and the inner surface of a microstructure fiber has been functionalized by covalent linking of a peptide nuclei acid probe targeting a DNA sequence bearing a single point mutation implication in crystal fibrosis disease. The essential part of new, fast and lowest price technology for healthcare, medical platform and detection of any chemical and organic, inorganic substances.
Sensor which is used to detect of bimolecular in an aqueous solution. Many researchers have put effort to implement a sensor for bio-applications. Fluorophorelabeled DNA is inserted in the air holes of the microstructure part of a PCF. In the presence of the target bimolecular, the transmission peak located in the fluorophore revealed [5].
For a many applications in various areas optical fiber gratings have developed including physical sensing for temperature, strain, acoustic waves and pressure.
The fiber bragg grating is etched as the sensing element by a narrow bandwidth [6].Optical refract metric type biosensor (ORTB) based on the FBG to determine the octane number of gasoline.
For a number of sensing applications, Fiber grating sensors are of significant. But this fiber bragg grating (FBG) sensor can used to detect acoustic signal in under water. Hill et al. in 1978 demonstrated the optical fiber bragg grating (FBG) [9]. Meltz et al developed this method which is called as transverse holographic fabrication method. After that, FBG has become very much popular.
There is a very important advantage of optical Fiber Sensors compare to other sensors that is small in size, good quality sensitivity and immunity to EMI. The advantage of this FBG is sensitive to a number of measured while being multiplexed. By discussing about application of FBG acoustic sensor, many researchers worked on the FBG and represent the detection.
Kavya V. Ullal, et al. In this paper authors explores the movement of the micro optical elements, which manipulates the light passes through all the dimensional spaces for micro opted-electro mechanical systems (MOEMS). These are used to detect stress, strain and other mechanical parameters based on the displacement using photonic crystal sensors for detection of acoustic signals [8].
T. Zouache et al, In this work the two-dimensional photonic crystal waveguide coupled to a point-defect resonant micro cavity. The resonant wavelength will shift when pressure variation induces change in the refractive indexes of the structure and these sensors have compactness, high sensitivity, and various choices of materials [28].

Fundamental Theory Of Fiber Bragg Grating
A FBG which is a spectrally reflective element and consist of core of an optical fiber. The FBG is consisting of various refractive indices with alternating regions. The difference in refractive indices and as results in Fresnel reflection at each interface which is named as the Bragg wavelength, λB. The wavelength of Bragg is , Where the effective refractive index of the grating in the fiber core is and is the grating period.
Strain is applied for measuring the acoustic and ultrasonic signal. If there is change in grating period which means strain is applied and change in refractive index then resultant effect is on strain optic sensor.

Figure 2. Basic principle operation of Fiber Bragg Grating
The alternative method is to detect the acoustic signal using fiber optic sensor, because this kind of sensors is extremely sensitive. Their size is small and it as oil-well monitoring where space is constraint [9].
For measuring sound in water a hydrophone is simply a device [10][11]. A fiber hydrophone is an acoustic sensor used for sensing the element.

Fiber Bragg Grating Acoustic Emission
This Sensor is used in various fields. The sensing principle is co-ordination between the AE wave and the sensor is introduced based on an FBG AE sensor. For FBG sensor, on the surface of the sensor, a thin polymer bonded explosive material is used. In order to improve the accuracy of the sensor the time coefficient Location method is proposed.
There are different applications which come underwater such as oil exploration, underwater pollution examination, and so on. In order to work on the application such as under water wireless sensor grids are involved in such case [12]. Sub aquatic sensor involved to provide sensitization. But there will be problem with wireless sensor network application for sensing. So FBG can diploid for this research work. Much other work is carried out for sensing research work [13]. Researcher's works on another approach to detect the acoustic signal and the method is three-dimensional photo acoustic imaging method. This method uses a MZI sensor for calculation of acoustic waves generated in an object. With short laser pulses. Fig 3 shows that basic working principle of interferometric [27]. Researcher worked on that Interferometer and explained the working principle in detail.

Figure 4. Fiber optical interferometer setup to detect the acoustic signal
Considering conventional solid core fiber and hollow core photonic crystal fiber (HCPCF), higher sensitivity was reported.
Another way to utilize the fiber Bragg Grating sensor is to detect the octane number of gasoline and the presence of organic compounds in biological fuels. Fiber Bragg Grating sensor is using π shifted etched. The π phase shifted FBGs which as narrow resonance bandwidth dimensions with the size of a grating and a good sensitivity to changes in the refractive index [14]. So this FBG with π shifted is used as fuel detection sensor.

Interferometric Ultrasound Detector
The above Fig. 3. Shows the experimental setup of the MZI The continuous laser beam is separated into signal beam and reference beam is incident on an arm of a MZI sensor. Object is placed near to the signal beam and both beams Travers a water tank. To improve the temporal resolution of the sensor, the diameter of the beam near the object is reduced.
The output of the differential amplifier as a function of path length difference is a sine wave [15]. In order to monitor low frequency underwater acoustic signal, the PCF based acoustic sensor was reported by Dnyandeo et al. [16].
A Comprehensive Study on Highly Sensitive Photonics Based Acoustic Sensors for Under Water Applications The construction of FBGs which is able to reflects required wavelengths of light and transmits remaining all others. It depends on Bragg condition which means The Bragg reflected wavelength changes depends on pressure, temperature, strain etc. This physical parameter changes refractive index which in turn affect the reflected wavelength [17][18][19].

PCF-FBG Sensor:
It is constructed on to the core of the photonic crystal fiber. This sensor has a fiber optic sensor system which is capable of distinguish the effects of physical Parameters [22]. The improved sensor system as power, energy scaling and discrimination of cross-sensitivities with good SNR [20,21].

Figure 8. PCF FBG Sensor
There are many advantages for FBG sensor and this sensor is applicable in numerous fields. Based on its advantages and good characteristic FBG sensor can be applicable for detection of acoustic signal.

Temperature Sensing
Fiber bragg grating can be used as temperature sensor. By changing the temperature of the fiber which can produce a shift in the Bragg wavelength due to thermal expansion and that can change the grating spacing. If there is change in temperature resultant change in index of refraction. For temperature change the fractional Bragg wavelength will be changed [23,24].
The detection of acoustic signal is difficult and very crucial for marine fields. By demonstrating MZI hydrophone using polarization maintaining PCF, operated at 1550 nm source with these data will be compared.  The market of optical sensor is growing very fast and essential. The number of advantages is required for fiber optic technology which is compared with conventional piezoelectric hydrophone techniques [25,26].

Merits
• Fiber optic sensor does not have any disturbance from EMI (Electromagnetic Interference) and RFI (Radio Frequency Interference). • It is safe and suitable to be used in extreme vibration and harse environments. • It is tolerant against high temperature (i.e. >1450oC) and corrosive environments. • It offers good sensitivity • It is less weight and small in size.
• It offers wide dynamic range and large bandwidth.
• It offers multiplexing and remote sensing capabilities.
• It can be used in multifunctional sensing capabilities for mechanical measurement, electric measurements, Magnetic measurements, chemical & biological sensing. • It can measure nearly all of the physical measurands.

Demerits
• It is very expensive.
• Detection system is difficult.
• It is complex to develop usable measurement systems using fiber optic sensors

Conclusion
The review paper demonstrated the reported work and corresponding results. Observation is done with characteristic of fiber bragg grating sensor with good sensitivity. FBG sensor is applied in different field. The structure and properties of this sensor is described in detail. The knowledge about structure and working principle of FBG sensor and broaden their idea about this sensor and can provide new solution for further exploit the potential of FBG sensor. The technology of FBG and Mach Zehnder based sensor will be controllability and integration and the exploration on new mechanism and new method.