SAGNAC INTERFEROMETER BASED STRAIN SENSOR

INTRODUCTION

Afiber optic sensoris asensorthat usesoptical fibereither as the sensing element ("intrinsic sensors"), or as a means of relaying signals from a remote sensor to the electronics that process the signals ("extrinsic sensors"). Fibers have many uses in remote sensing. Depending on the application, fiber may be used because of its small size, or because noelectrical poweris needed at the remote location, or because many sensors can bemultiplexedalong the length of a fiber by using light wavelength shift for each sensor, or by sensing the time delay as light passes along the fiber through each sensor. Time delay can be determined using a device such as anoptical time-domain reflectometerand wavelength shift can be calculated using aninstrumentimplementing optical frequency domain reflectometry.Fiber optic sensors are also immune toelectromagnetic interference, and do not conduct electricity so they can be used in places where there ishigh voltageelectricity or inflammable material such asjet fuel. Fiber optic sensors can be designed to withstand high temperatures as well.INTRINSIC SENSORSOptical fibers can be used as sensors to measurestrain, temperature,pressureand other quantities by modifying a fiber so that the quantity to be measured the intensity,phase,polarization,wavelengthor transit time of light in the fiber. Sensors that vary the intensity of light are the simplest, since only a simple source and detector are required. A particularly useful feature of intrinsic fiber optic sensors is that they can, if required, provide distributed sensing over very large distances. Temperature can be measured by using a fiber that hasevanescentloss that varies with temperature, or by analyzing theRaman scatteringof the optical fiber. Electrical voltage can be sensed bynonlinear opticaleffects in specially-doped fiber, which alter the polarization of light as a function of voltage or electric field. Angle measurement sensors can be based on theSagnac effect.Special fibers likelong-period fiber grating(LPG) optical fibers can be used for direction recognition. Photonics Research Group ofAston Universityin UK has some publications on vectorial bend sensor applications. Optical fibers are used ashydrophonesfor seismic andsonarapplications. Hydrophone systems with more than one hundred sensors per fiber cable have been developed. Hydrophone sensor systems are used by the oil industry as well as a few countries' navies. Both bottom-mounted hydrophone arrays and towed streamer systems are in use. The German companySennheiserdeveloped alaser microphonefor use with optical fibers. Afiber optic microphoneand fiber-optic based headphone are useful in areas with strong electrical or magnetic fields, such as communication amongst the team of people working on a patient inside a magnetic resonance imaging (MRI) machine during MRI-guided surgery. Optical fiber sensors for temperature and pressure have been developed for down hole measurement in oil wells.The fiber optic sensor is well suited for this environment as it functions at temperatures too high for semiconductor sensors (distributed temperature sensing).Optical fibers can be made intointerferometricsensors such asfiber optic gyroscopes, which are used in theBoeing 767and in some car models (for navigation purposes). They are also used to makehydrogen sensors.Fiber-optic sensors have been developed to measure co-located temperature and strain simultaneously with very high accuracy usingfiber Bragg gratings.This is particularly useful when acquiring information from small complex structures.Brillouin scatteringeffects can be used to detect strain and temperature over larger distances (2030kilometres)

EXTRINSIC SENSORSExtrinsic fiber optic sensors use anoptical fiber cable, normally amultimodeone, to transmitmodulatedlight from either a non-fiber optical sensor, or an electronic sensor connected to an optical transmitter. A major benefit of extrinsic sensors is their ability to reach places which are otherwise inaccessible. An example is the measurement of temperature insideaircraftjet enginesby using a fiber to transmitradiationinto a radiationpyrometerlocated outside the engine. Extrinsic sensors can also be used in the same way to measure the internal temperature ofelectrical transformers, where the extremeelectromagnetic fieldspresent make other measurement techniques impossible.Extrinsic fiber optic sensors provide excellent protection of measurement signals against noise corruption. Unfortunately, many conventional sensors produce electrical output which must be converted into an optical signal for use with fiber. For example, in the case of aplatinum resistance thermometer, the temperature changes are translated into resistance changes. The PRT must therefore have an electrical power supply. The modulated voltage level at the output of the PRT can then be injected into the optical fiber via the usual type of transmitter. This complicates the measurement process and means that low-voltage power cables must be routed to the transducer.Extrinsic sensors are used to measure vibration, rotation, displacement, velocity, acceleration, torque, and twisting.

SAGNAC INTERFEROMETERThe interference of two waves occurs because the waves superimpose, forming a single wave with an amplitude thats either greater or lower than the initial waves. For a beam of light, many photons are interfering with each other and as the resulting waves are detected, a pattern emerges that illustrates the various properties of the light. The pattern changes with the frequency, phase and amplitude of the light. An interferometer measures differences in the interference pattern to determine certain properties of the light. This can reveal information about the light source or the effects on the interferometer that caused the shifting of the interference fringes.In a Sagnac interferometer, a laser is used whose light is monochromatic and coherent. Because the beam is of a single frequency, interfering light behaves more predictably and because it is coherent, a phase shift affects the interference in way that can be measured. The laser is split into two beams of approximately equal power using a beam splitter and each beam follows a path single path directed by mirrors but in opposite directions. Once the beams reach the point where they were split, therefore enclosing an area, they recombine and resulting beam is detected. The setup is shown in figure1. Figure 1. The half-silvered mirror acts as a beam splitter, resulting in one beam travelling in one direction and the other beam travelling in the other direction. The two beams are combined and detected using interferometry where an interference pattern can be analyzed

A Sagnac interferometer is usually arranged so that the beams have a triangular or rectangular trajectory using mirrors, or a circular trajectory guided by the fiber optics. Rotation of the interferometer changes the paths of the beams because the position of the point where the beams combine has rotationally shifted relative to the position where the initial beam split. This causes one of the split beams to have a longer travel time than the other because they are travelling in a constant medium and therefore have the same speed. The distance travelled is shifted, as shown in fig 2

Fig 2: This illustrates the effect that the rotation of the system has on the counter propagating beams. a beam travelling in the direction of rotation ultimately has to travel more distance than the beam travelling in the direction opposite the rotation

Change in travel time of the beam is given by the following equation.Time difference, t = 4A./v2 This time difference causes a shift in the interference fringes.The phase shift = 2vt/ =8A./ v Where A=4R2 R is the radius of circular ring is the angular velocity v is the speed of light & is the wavelength of light source used.

PROPOSED SYSTEM

FIBER OPTIC STRAIN SENSOR USING SAGNAC INTERFEROMETER

Optical fiber sensors have been developed in different sensing applications due to their significant advantages, like accuracy, compactness, low cost, and immunity to electromagnetic waves. Sagnac fiber loop is a kind of optical fiber sensor, which just consists of a fiber coupler and a section of optical fiber, and it can be used to measure many parameters, such as strain, temperature, liquid level, and curvature. By adopting different kinds of fibers, the sensing characteristics of the Sagnac fiber loop are varied. In recent years, the polarization-maintaining photonic crystal fiber(PM-PCF)-based Sagnac loop has attracted much interest. A temperature-independent strain sensor by a highly birefringent PCF-based Sagnac interferometer has also been presented and a pressure sensor with PM-PCF-based Sagnac interferometer has been proposed. An elliptical hollow-core photonic bandgap fiber based on Sagnac configuration with a strain sensitivity of -0.81pm/ has been presented, and the birefringence of the fiber was measured to be 3x10-3. However, all of them used polarization-maintaining fiber (PMF) or high birefringence (Hi-Bi) fiber inserted into Sagnac fiber loop. In our project, a low-birebirefringence PCF based Sagnac loop employed as a strain sensor is proposed. Due to the low birefringence, just one dip in the wavelength range of 15001600 nm appears. The experimental setup and principle are described in Section I. The results and discussion are presented in Section II.

I. EXPERIMENTAL SETUP AND WORKING PRINCIPLE

Fig 3 :Experimental setup strain measurement by using low birefringence photonic crystal based Sagnac loop

The experimental setup of the strain sensor by the use of the low-birefringence PCF-based Sagnac loop is shown in Fig 3. It includes a 3-dB single-mode-fiber (SMF) coupler and a 40-cmlong PCF (NL-1550-NEG-1, Crystal Fiber A/S). The mode field diameter of the PCF is about 2.8 m, with seven rings of air holes in the cladding, and an attenuation coefficient in the wavelength range of 15101620 nm is less than 9 dB/km. The Sagnac loop was formed by splicing the two ends of the PCF to the arms of the 3-dB SMF coupler. The combined loss of these two splicing points was measured to be about 6 7dB, high due to the mode-field mismatch. A broadband light which was source was connected to the input of the Sagnac loop, and the output spectrum was observed with an optical spectrum analyser.The input light was split by the 3-dB SMF coupler and two counter propagating light beams were propagated inside the Sagnac loop. When they passed through the PCF and encountered at the same coupler, the counter propagating light beams introduced the relative phase difference due to the birefringence property of the PCF. So it led to the minima (dips) and maxima (peaks) in the output spectrum. The transmission spectrum of the Sagnac loop is approximately a periodic function of the wavelength.

T = [1- Cos () ]/2(1)Where = 2L0B/is the phase difference; is the operating wavelength;L0 is the length of the PCF; and B is the birefringence of the PCF. The wavelength spacing (S) between the adjacent transmission dips or peaks is given by

S = 2/(B.L0)(2)

A section of PCF is fixed straightly on translation stages with 140-mm separation, employing as sensing element, and the length of the sensing PCF is denoted as .L When the sensing PCF was stretched by moving one of the translation stages, the strain applied on the sensing PCF was varied, which introduced an elongation L(a strain = L/L ) and led to the change of phase difference ( ) = 2(LB+LB)/ (3)

where B is the variation of birefringence of the PCF caused by photo elastic effect. Then the wavelength of the dip or peak in the Sagnac output spectrum is changed by= S /2. (4)So the change of the strain can be obtained by measuring the wavelength shift of the dip or peak in the output spectrum.

II. EXPERIMENTAL RESULTSThe transmission spectrum of the low-birefringence PCF-based Sagnac loop at room temperature of about 25 0C is shown in Fig. 4

Fig 4: Spectra of input super continuum light (solid line), Sagnac output (dashed line), and the normalized transmission (dotted line)

Only one dip appears in the spectral range of 15001600 nm. The extinction ratio is about 21 dB, and the wavelength of this dip can be tunable in the range of 15001600 nm by tuning a polarization controller which is inserted inside the Sagnac loop. Therefore, this characteristic could be used as a band stop filter for the C- and L-band. It can be seen from equation (2) that S will become large when the product of B and L is very small. Thus only one dip appears in the wavelength range of 15001600 nm. A broadband, super continuum light source in the experiment. In Fig. 4, two dips appear in the spectral range of 14001700 nm, and the wavelength spacing between the two adjacent dips is about 110 nm. The birefringence value of 5.8x10-5was estimated by (2), which is about one or two orders less than that of PM-PCF or high-birefringence fiber (the order of 10-4 10-3 ).When the applied strain was varied from 0 to 2520 by increasing the separation distance between the two stages, the Sagnac output spectra under different strain levels are shown Fig.5.

Fig 5 : Transmission spectrum of Sagnac loop under different strain

The wavelength of the dip in the Sagnac transmission spectrum was changed from 1581.76 to 1580.6 nm, corresponding to a total wavelength shift of about 1.16 nm. The wavelength shift of the transmission dip as a function of strain change is shown in Fig. 6.

Fig6: Wavelength shift of the transmission dip versus strain

As can be seen, the wavelength shift of the dip has a linear relationship with the strain change, and a sensitivity of about -0.457 pm/ was achieved. This strain sensitivity is two times higher than that of the reported PM-PCF-based Sagnac interferometer. But the resolution of strain measurement is limited by the 40-pm wavelength error due to the flat-profile dip, which is calculated to about 87 . The Hi-Bi fiber-based Sagnac sensor had a small fringe separation, which could lead to the overlap of the fringes when the wavelength shift is larger than the fringe separation. By comparison, our proposed sensor has a potential ability to acquire larger measurement range due to the wider fringe spacing.The influence of temperature on the Sagnac loop was also investigated. The 40-cm PCF was placed on the temperature-controlled oven which was set to increase from 250 C to 750 C with a step of 10 C. The wavelength shift versus temperature is shown in Fig. 7

Fig 7: Wavelength shift of the transmission dip versus temperature

It shows that the wavelength shift has a linear relationship with the temperature. A temperature sensitivity of about -80 pm/0 C was achieved. So the cross sensitivity of theTemperature on the strain is about 175/0 C. But the strain experiment was performed in a temperature-controlled environment, and the temperature variation was less than 0.10 C, so the error of strain measurement induced by temperature is just about 17.5 . Moreover, the temperature effect on the proposed strain sensor could be compensated by placing a fiber Bragg grating or a long-period grating outside the Sagnac loop, which can be realized easily.

EXISTING SYSTEM I

Fig 8: Sagnac interferometer using SM fiber

Instead of photonic crystal fiber fixed on translational stage here we used single mode step index fiber and we fixed optical fiber cable on steel scale by using araldite. The setup is shown above in fig 8.The experimental setup of the strain sensor by the use of the single mode silica fiber based Sagnac loop is shown in Fig 8. It includes a 3-dB single-mode-fiber (SMF) coupler and a 5m long single mode silica fiber. The Sagnac loop was formed by splicing the two ends of the single mode silica fiber to the arms of the 3-dB SMF coupler. The combined loss of these two splicing points was measured to be about .3dB. A broadband light which was source was connected to the input of the Sagnac loop, and the output spectrum was observed with an optical spectrum analyser.The input light was split by the 3-dB SMF coupler and two counter propagating light beams were propagated inside the Sagnac loop. When they passed through the single mode silica fiber and encountered at the same coupler. Then it will undergo interference, So it led to the minima (dips) and maxima (peaks) in the output spectrum. The transmission spectrum of the Sagnac loop is approximately a periodic function of the wavelength.

Fig 9: experimental setup

Fig 10 : Broadband source and spectrum analyser

EXISTING SYSTEM II

Fig 9: strain measurement using laser

We replaced broadband light source with a laser and optical spectrum analyser with a photo detector. Then measured detector voltage corresponding to different weights. It is observed that the output voltage decreases as the weight increases. Using the measurement plotted a graph of weight versus output voltage. The observations and graph are given below.

Weight (gm)Output voltage (mv)

1262

2261

5260

10254

20231

5070

Table 1: weight out put voltage

Fig 10 : weight versus output voltage graph

Fig 11: Strain measurement using laser

CONCLUSION

For the strain measurement PCF fiber was required. But in our experimental set up we used single mode step index silica fiber. For silica fiber there is no birefringence property, it is present in photonic crystal fiber. Broadband source used here has low coherence. To produce interference by using a low coherent source, fiber must have birefringent property. So we cannot produce interference effectively in the single mode silica fiber. Also the 3dB coupler provided was not effectively coupling 50% to the two arms. So we are unable to produce interference pattern.So we used laser instead of broadband source and detector for output voltage measurement. Strain is applied to the fiber by varying the mass. Output voltage is measured for different values of weights. Output voltage versus weight graph is plotted. From this graph we can measure strain.

REFERENCES

1.Strain Sensor Realized by Using Low-Birefringence Photonic-Crystal-Fiber-Based SagnacLoop By Huaping Gong, Member, IEEE, Chi Chiu Chan, Member, IEEE, Lihan Chen, and Xinyong Dong2.G. Sun, D. S. Moon, and Y. Chung, Simultaneous temperature andstrain measurement using two types of high-birefringence fibers in Sagnac loop mirror, IEEE Photon. Technol. Lett.,vol. 19, no. 24, pp.20272029, Dec. 15, 2007.3. D. S. Moon, B. H. Kim, A. Lin, G. Sun, Y. G. Han, W. T. Han, and Y. Chung, The temperature sensitivity of Sagnac loop interferometer based on polarization maintaining sidehole fiber, Opt. Express, vol.15, no. 13, pp. 79627967, Jun. 2007.4. www.sciencemag.org/content/330/6007/10815. http://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=057059266. Pressure sensor realized with polarization-maintaining photonic crystal fiber-based Sagnac interferometer H. Y. Fu,1,* H. Y. Tam,1 Li-Yang Shao,1 Xinyong Dong,1 P. K. A. Wai,2 C. Lu,2 and Sunil K. Khijwania3

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