Report on the Sensitivity of a Fiber Optic Hydrophone-type Sensor

Constantin Fenton David Randin Harbin Engineering University, China. Supervisor: Dr. Zhang Jian Zhong Spring 2011

Abstract

A fiber optic hydrophone mandrel sensor has been designed, fabricated and tested. The elastomer is rela-

tively small in size and is made from a clear silicone gel that was cast from a mold. Three cylinders were

first tested with Fiber Bragg Grating sensors to determine which was the most sensitive. Then, a fiber

length of 2.24m was wrapped around the most sensitive elastomer and then tested for sensitivity using a

Mach-Zehnder interferometer.

1. Introduction

In recent years, fiber optic sensors have developed to practical applications such as

medicine, defense, aerospace applications, and telecommunications. They can be used to

measure temperature, pressure, humidity, acceleration, and strain.[9](p.xi) Intrinsic fiber optic

sensors rely on the light beam propagating through the optical fiber being modulated by the

environmental effect either directly or through environmentally induced path length changes in

the fiber itself.[10](p.3)

An optical fiber hydrophone is an acoustic sensor that uses optical fiber as the sensing element.

Many of its features make it a good alternative to the conventional piezoelectric ceramic sensor.

FO hydrophones include features such as high sensitivity, large dynamic range, and freedom

from electromagnetic interference.[11](p.368)

There are a vast number of fiber optic hydrophone designs that employ different variations of

sensor head design and similar interferometric methods.[1]-[8] A novel design is proposed in

which a coil of optical fiber is wound on a compliant mandrel. The mandrel is housed in a

cylindrical aluminum casing and is designed to carry a flexible diaphragm that seals the unit

from water. The sensitivity of the mandrel depends mostly on the compressibility of the silicone

elastomer and the length of the optical fiber coiled around the elastomer.

The purpose of this study was to evaluate the sensitivity of an interferometric optical fiber

coil hydrophone-type sensor wound on a compliant mandrel. The goal was to design a small

hydrophone-type optical fiber sensor and to test its response capabilities and its sensitivity.

The cylindrical sensor was placed inside its aluminum packaging to see if the packaging would

hinder its response, also to keep the sensor in place. The sensor was tested with a piezoelectric

actuator that allowed for frequency and voltage amplitude variations. A Mach-Zehnder

interferometer was used to interrogate the signal coming from the fiber. The interferometer,

along with the software required to view the response of the sensor, were built by students of

Harbin Engineering University’s School of Science.

The sensor was exposed to direct vibrations at different frequencies and amplitudes and its

response was recorded and evaluated.

2. Sensor Design

Figure 1 shows the design of the

sensor. The design for the cylinder where

the fiber was wrapped was to be less than

10 mm high and ~ 50 mm in diameter.

These physical limitations made it difficult

to wrap longer lengths of fiber on the

cylindrical elastomer that would raise the

Fig. 1. Exploded view of the optical fiber sensor and its components, the fiber is not shown (to scale).

sensitivity of the sensor. The longer the length of fiber wrapped on a compliant mandrel, the

more sensitive the sensor will be. This is due to the fact that the change in fiber length is

proportional to the phase shift created by differences in length between the sensor arm and the

reference arm of the interferometer.[1], [3], [7] The cylindrical elastomer should be made of a

material that has a low Young’s Modulus and Poisson’s ratio [1], this will make the elastomer

react more efficiently to the stimulus; the cylinder will warp with greater ease and the fiber

wrapped around it will lengthen and shorten more efficiently in the presence of sound or

vibration.

The elastomer was made out of silicone gel and was cast from a mold. The elastomer is a

cylinder made out of clear silicone gel and is 7.66 mm high by 54.8 mm in diameter. The coil is

illustrated in Figure 2. The diaphragm is intended to

transfer the vibratory energy to the elastomer as efficiently

as possible. This requires a thin and flexible membrane

that also seals the elastomer inside the aluminum housing.

Fig. 2. Optical fiber and elastomer configuration.

The fiber was wrapped 13 times in a single layer around the cylinder. The length of the fiber

wrapped around the mandrel is equal to 2 R*N , N is the number of fiber windings round the

mandrel and R is the radius of the mandrel. This yields, 2 27.4 mm * 13 = 2.24 meters. In

accordance with interferometry, a length of optical fiber of 2.24 m was wrapped on the sensor

cylinder and another fiber of the same length was wrapped on a reference cylinder that was also

made out of silicone gel. The lengths of fiber that led into and out of the cylinders had to be kept

the same as well. The reference cylinder was made the same shape and size as the sensor

cylinder in order to keep the bending losses the same on both the reference and sensor arms.[16]

The fiber was fixed to the elastomer at the beginning and end of the coil with cyanoacrylate

adhesive (super glue) and then the elastomer along with the fiber coil, were coated with a less

than 1 mm layer of the same silicone gel used to make the elastomer. The application of a

coating layer to the mandrel and fiber coil is a feature found in previous hydrophone

designs[1],[2],[5],[14],[15] and variations in this layer may lead to different sensitivities.

The design for this sensor differs from the designs of some previously developed

hydrophones[1]-[8], [14]-[15] in the sense that this sensor is designed to pick up vibrations from

the direction perpendicular to the curved surface of the mandrel. This design coupled with the

housing should allow for a directional sensor.

3. Experimental Procedure

Before performing tests on the optical fiber coil sensor, Fiber Bragg Grating Sensors were

attached to three different types of cylinders. This was done to find the type of cylinder material

and size that would yield the highest transfer of energy onto the coil. Three silicone gel cylinders

were made; one clear and large, another clear and small, and the third one white and large. The

large cylinders were the same size and shape but the silicone gels from which they were made

were different. The three cylinders with FBGs were submitted to the same pressure variations by

adding weight and the change in wavelength was observed. The cylinder which displayed the

highest change in wavelength would be determined to be the most sensitive and chosen to be

tested in the hydrophone-type sensor. Figure 3 shows the sensitivities of the three types of

Fig. 3. Compared sensitivities of three types of elastomer cylinders, tested with FBGs attached.

cylinders. These test revealed that the most sensitive type of cylinder is the clear

(transparent)/large elastomer. This elastomer was chosen to be tested in the hydrophone housing

with the PZT actuator.

The sensor was tested in the housing unit with a PZT actuator using Mach-Zehnder

Interferometry, illustrated in Figure 4. The actuator was placed directly on the bare mandrel with

no diaphragm to serve as a control test and to raise the sensitivity and obtain the maximum

Fig. 4. Diagram of Mach-Zehnder Interferometer used for this study.

output from the sensor. The actuator was fed different voltages at different frequencies from

1kHz to 15kHz. The response at each frequency/voltage combination was displayed on the

computer in energy-time domain with an accompanying FFT in real-time, and the results were

recorded for 5 second periods. Frequencies below 1kHz were not tested since the system noise

below 1khz was too high to obtain any observable response. The process was repeated with the

copper diaphragm fastened in place and the PZT actuator placed on top, which was fed 20 volts

to obtain the highest response possible.

4. Data Analysis

The sensor with the diaphragm removed was excited with a PZT actuator being fed different

voltages and different frequencies. The frequency response graphs are shown in Figure 5.

Fig. 5. Frequency response plots of the sensor across various voltages with no diaphragm on the sensor’s housing.

Figure 5 reveals that that the frequency responses are not flat, this could be due to the unstable

output signal inherent in Mach-Zehnder Interferometers and 2x2 coupling [11], [12]. Another

possible reason for such en erratic response is the resonances in the elastomer itself, the housing,

or even resonances of the PZT actuator as well.

A frequency response of the sensor with the copper diaphragm was also obtained with 20

volts being delivered to the actuator for maximum response. For comparison reasons, Figure 6

shows the frequency response graphed alongside the response of the sensor without a diaphragm.

Fig. 6. Frequency response graphs of the sensor with and without a diaphragm.

Figure 6 shows that the copper diaphragm reduces the output of the sensor when compared the

output of the sensor with no diaphragm. This is most significant at low frequencies between

1kHz and 2kHz. Although the output is reduced, the frequency response seems to be flatter from

3kHz and above with the copper diaphragm. Perhaps the diaphragm fastened into the housing,

pressing down slightly on the elastomer has the characteristic of stabilizing the sensor during

operation.

The “change in dB” values for Figures 5 and 6 were obtained by taking the level of signal

above the noise floor in the FFT graphs obtained from the raw data as illustrated in Figure 7(a),

which is the response from 20 volts at 8kHz. Figure 7(b) is the energy/time curve with its

corresponding FFT and the filtered FFT and the inverse FFT corresponding to the energy/time on

the bottom-left corner.

Fig. 7(a). FFT of energy/time curve of sensor responding to 8kHz with 20v being fed to the actuator

Fig. 7(b). Full view of energy/time curve with its corresponding FFT and the filtered FFT and the inverse FFT corresponding to the energy/time on the bottom-left corner (20v at 8kHz).

From the sinusoidal signals in the inverse FFT images such as the one in Figure 7(b), the

peak-to-peak voltage outputs of the photodetector are displayed in Figures 8(a) and 8(b).

Fig. 8(a). Output of photodetector at voltages 4, 8 and 12; 2kHz-15kHz

Fig. 8(b). Output of photodetector at voltages 12, 16 and 20; 2kHz-15kHz

5. Conclusion

An optical fiber hydrophone-type sensor was designed and developed for the purposes of

evaluating its sensitivity and response. The sensor was tested with a PZT actuator in a Mach-

Zehnder interferometer. Ideally, the sensor should be tested with acoustic energy in an under-

water environment but due to the size requirements on the sensor, such tests would be unlikely to

yield a significant response. The sensor does display a roughly flat frequency response at 12

volts and it creates an output change of up to 25dB with 4 volts of electromechanical energy

being radiated on the top of the sensor cylinder.

There are various improvements that can lead to better results in the future experimentation

of this mandrel. Jong-in Im and Yong-rae Roh’s research on fiber optic hydrophones explains

how sensor’s sensitivity would increase if the elastomer were larger in height since this would al-

low for a longer length of fiber wrapped in the coil. This is the compensation one must deal with

when selecting the elastomer size.1 Acquiring an elastomer with an even lower Young’s

Modulus and Poisson’s ratio would also help increase the sensitivity.[1] The frequency response

as well as the omni-directional directivity increases when the mandrel length is shortened.[1] A

thinner diaphragm would also increase the sensor’s sensitivity as long as the diaphragm’s

damping characteristics were minimized. Changing the fiber might also yield better sensitivity

results. In an experiment done at Drexel University, results show that a down-tapered gold

coated fiber can provide up to15dB improvement in sensitivity when compared to a down-

tapered uncoated fiber or a straight cleaved fiber.[13] Lastly, switching the Mach-Zehnder

Interferometer for a Michelson Interferometer would theoretically double the sensitivity since the

signal passes through the sensor head twice.[11] With the dimensions of mandrel being very

short other components of the system should be improved to make up for the sensitivity loss of

the shortened mandrel height as well as the system noise should be reduced as much as possible

to achieve better results.

Acknowledgements

The work was supported by Harbin Engineering University with the help of Dr. Zhang

Jian Zhong and his students who first introduced us to fiber optic hydrophone systems. We

would also like to thank Khalil Later and Amine Touati for their help and contribution.

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