Unit-4 new

1 Md.Ismail,EED,MJCET

UNIT-4 FIBRE OPTIC INSTRUMENTATION

Measurement of Displacement: Here measurement of distance is done digitally by using a morine fringe modulator. In this case the opaque lined gratings produce dark morine fringes. Transverse movement of one grating with respect to the other causes the fringe to move up or down. Thus a count of the fringes as the gratings are moved provides a measurement of distance.

Advantages:

1. The fringe counting is independent of instabilities with in the s/m components. 2. Optical intensity is not affected.

Disadvantages: 1. Mechanical vibrations severely affect the measurement. 2. Accuracy is limited because of vibrations.

Measurement of Pressure by Photo elastic Pressure Sensor:


Here the device called photo elastic sensor is induced by a mechanical stress. The photo elastic material (eg:- Piezo optic glass, epoxy resin) is utilized to rotate the optical polarization between a pair of crossed polarizer’s. The phenomenon known as binefringence occurs with the application of mechanical stress to the transparent isotropic material , whereby it becomes optically anisotropic ( exhibits variations in physical properties with the direction of measurement) giving a variation in transmitted light through the sensor. Advantages: Stress may be induced directly without the need for an intermediate mechanism (eg: pressure to displacement) . Temperature Measurement:


This instrument relies on the temperature variation of the fluorescence in europium doped lanthanum oxy sulfide. A small amount of this material is placed on the object whose temp is to be measured and the fluorescence is excited by illuminating it with UV. Light transmitted down a fairly large diameter plastic coated silica fiber The source of radiation is a quartz halogen lamp where output has been filtered to remove any unwanted higher wavelengths. Another similar fiber picks up some of the emitted fluorescence and carries it back to the detector system. In fact the phosphor emits at more than one wavelength and it is the intensity ratio of two of the lines which is measured because a ratio is measured, any fluctuations in the irradiance of the source are not important. At the detector end of the fiber, the radiation is split into two using a beam splitter. Each beam falls onto a Si photo-diode having an appropriate filter in front to isolate the particular wavelength required. The ratio of the two signals then provides the temp true information. It has a range of -50 to 250 degree Celsius. Current Measurement: This device consists of a single polarization maintaining fiber which passes up from earth to loop ground. The current carrying conductor before passing back to earth. A He-Ne laser beam is linearly polarizer and launched into the fiber which is then stripped of any cladding modes.

The directions of polarization of the light are the fiber core is rotated by the longitudinal magnetic field around the loop, via the outers of the faraday magnetic optic effect. A Wollaston prism is used to sense the resulting rotation and resolves the emerging light into two orthogonal components. These components are separately detected with a photo diode prior to generator of a sum and difference signal of the two intensities (I1 & I2). The difference signal normalized to the sum give a parameter which is proportional to the polarization rotation Kp. Where Kp = (I1-I2)/(I1+I2)


Hence a measurement (ac or dc) may be obtained which is independent of the received light power. Voltage Measurement:

The electric fields which are produced by the voltage differences, affect the linear, rather than the rotary motion of the atomic electrons. For a material like glass, without any natural directionality , we can only affect a transverse wave motion via a transverse electric field. An electron vibrating in the direction of light propagations has no effects at the polarized light. The field travels more slowly than the polarized light in perpendicular direction of polarized light. As a result if light waves enter fiber in each of these two directions a phase difference between them is emerged .This phase difference will depend on the integrated effect of the transverse electric field along the length of the fiber. This effect is used to measure the vg which cause the fields to reflect back light from high voltage end to the fiber. Liquid Level Measurement: When the fluid, which has a refractive index greater than the glass forming the optical dipstick reaches the chambered end, total internal reflection ceases and the light is transmitted into the fluid. Hence an indication of the fluid level is obtained at the optical detector. Although this system is somewhat crude and will not provide continuous measurement of a fluid level, it is simple and safe for use with flammable liquids


. Fiber Optic Sensors: There are optical transducers used for monitoring & telemetry in industrial applications. It requires an electrically passive optical fiber sensor for modulating a light beam either directly or indirectly by the measurand (eg: pressure, temp,flow) without any electrical interfacing. Advantages of fiber optic sensor over electrical sensors:

1. Non conducting and immune to electromagnetic and radio-frequency interference. 2. Safe, even in explosive environments. 3. Not sensitive to temperature changes. 4. Highly reliable and easily available to suit any design.

Classification: Fiber optic sensor are classified into 3 general categories 1. Intensity- modulated sensors 2. Phase sensors 3. Diffraction sensors 1.Intensity- modulated sensors: These works on the principle of intensity variations, generally these use multimode fiber and they convert a physical parameter into a change in amount of light transmitted . e.g.: Displacement measurement, level measurement. Displacement measurement by intensity modulated sensor:

As the displacement delta x increases, the intensity of the detected light decreases.


2.Phase Sensors: It utilizes the principle of phase variation with respect to temp. When the sensor element is heated, the fiber elongates and thereby increases the length of the light path. As a result, the phase of the light that reaches the detector is changed. The reference fiber provides a phase reference for the phase comparator.e.g. Temp measurement. 3.Diffraction grating sensor: In this sensor two diffraction gratings are butted together to form a single grating.

As one of the gratings is displaced, the effective grating distance and the diffraction angle are changed as shown in the fig below by dotted lines.

The detector will sense it as a change in the intensity. As the grating move, we can count digitally the high and low intensity cycles. Thus we can get the direct digital measurement of displacement. Interferometric Sensors: Here the principle is to detect the influence of physical parameters on the phase of light propagating in a single mode fiber.


Comparison between Active(Intrinsic) fiber sensor and Passive (Extrinsic) fiber sensor: Intrinsic or Intensity Sensor Extrinsic or Interferometric sensor 1. In a fiber optic sensor s/m if the light signal received by the detector changes as a result of some change in fiber, than sensor is called Intrinsic. 2. Here physical parameter to be sensed modulates the transmission properties of the sensing fiber. 3. Physical properties of light e.g. Intensity, Phase, wavelength etc are modulated by the measurand. 4. The detectors circuitry used is analog or digital.

1. Here detector changes as a result of some transducer type of device. 2. Here modulation takes place outside the fiber. 3. Here light is just transmitted without in modulations. 4. Detectors circuitry is fringe counting.

Fiber Optic Sensors: Optical fibers provide an innovative approach to the design of an optical transducer/sensor. The following are some of the characteristics useful in the sensor design: 1. Optical fibers are (i) non-conducting, (ii) Immune to electromagnetic and radio frequency interference, and (iii) Safe, even in explosive environments. 2. Their low attention coefficient allows the monitoring station to be located at a safe place, away from the hazardous environments. 3. Fiber optic sensors are not so sensitive to temperature changes. 4. Optical fibers are easily available to suit any design. These sensors find very wide application. Temperature or pressure variations produce change in the effective length of the optical fiber, which in turn, creates a minute phase change. For a long fiber, even this minute phase change, gets largely magnified over the entire length, and thereby produces a measurable phase change. So it automatically measures the variation in temperature or pressure. Classification of fiber optic sensors is as follows:

(i) Extrinsic sensors (ii) Intrinsic sensors

In extrinsic sensors, sensing takes place in the region external to the fiber( outside of the fiber) and the fiber essentially serves as a conduit for the to and fro transmission of light to the sensing region.

On the other hand, in intrinsic sensors one or more of the physical characteristics of the fiber undergo a change causing modulation of intensity, phase or frequency etc.

a) Displacement Sensor: (Intensity modulation type of sensor):


This type of sensor converts a physical parameter into a change in the amount of light which is transmitted as shown in figure. Intensity modulated sensors generally use multimode fibers and they work on the principle of intensity variation.

The intensity of the light , which is arriving at the detector , is related to the displacement of an object. As the displacement delta x increases, the intensity of the detected light decreases. In this type of sensor, spurious light incidence at the detector is avoided by

(i) Illuminating the object with the help of a comparatively narrow line light source and (ii) Placing a filter matching the source wavelength in the detector.

Also the illumination of fiber by spurious light is reduced by extending the sleeves beyond the end of the fibers. b) Displacement Sensor ( using Y guide probe): The most common Y-guide probe is a bifurcated bundle fiber. The principle of operation of the displacement sensor using Y-guide probes is shown in the figure.

The light emitted from one bundle is back- reflected by the object to be measured and collected by another bundle (receiving fiber). As a result, the returned light at the detector is intensity-modulated to a degree dependent on the distance between the end of the bundle fiber and the object. The sensitivity and the dynamic range are determined by the geometrical arrangement of the array of bundle fibers, as well as the number and type of fibers.

The displacement sensor using the Y-guide probe provides resolution of 0.1micro meter and dynamic range of 100 micro meter displacement-guide probe displacement sensor are well suited for robotics applications as position sensors and for gauging the surface assessment since they have high sensitivity to small distances. Strain Sensor: (Intensity Modulation type) A portion of bare fiber, without plastic jacket, is put between two corrugated blocks as shown in figure. It is known as micro bend strain sensor.


Micro bending losses are produced in the fiber when the top block presses the fiber by the applied external force applied on the top block. The intensity changes produced by the applied force are measured with reference to a direct unmodulated signal from the source of light, eliminating source variations as an error source. Hybrid Sensors: These sensors allow a broad range of modulation schemes and treat the fiber as a light pipe, transmitting the light to a remote sensor. It is the simplest sensor which acts as an on-off switch for detecting the pressure or absence of a stimulus at the sensor site. Liquid Level Sensor: (Hybrid Sensor) A number of optical fiber liquid level sensors based on the direct interaction between light and liquid have been developed. The most common method employs a prism as a sensing element as shown in figure.

The prism is attached to the ends of two single optical fibers. The input light from the LED is totally internally reflected and returns into the output fiber when the prism is immersed in liquid, the light refracts into the fluid with low reflection, resulting in negligible returned light. Thus this device works as a liquid- level switch. The sensitivity of this sensor is determined by the contrast ratio, which depends on the refractive index of the liquid. The signal output of a well- designed sensor can be switched for a change in liquid level of only 0.1mm. Advantages: Optical fiber liquid- sensors have advantages of low cost and electrical isolation.


Applications:

(i) Their application is wide spread in petroleum and chemical plants, where conventional sensors have problems in these hazardous environments.

(ii) Liquid level monitoring of storage tanks in a petroleum plant. Limitations:

(i) The influence of dirt contamination on the prism surface. Dirt problem can be solved by enclosing the sensing element with a fine filter.

(ii) The possible influence of bubbles which could be generated in the liquid at high temperatures.

Current Sensor: Electrical current sensors are based on magneto-optic devices. Electrical current can be gauged by measuring the magnetic field around the conductor, if the contour integral of the magnetic field is determined. This can be done conveniently in a fiber optic sensor by employing the Faraday Effect. The rotation of the plane of vibration of linearly polarized light by a longitudinal magnetic field. Extrinsic fiber optic current sensor : The design of extrinsic current sensors has the advantage of mechanical stability, freedom of choice of magneto-optic material.

When a beam of plane polarized light passes through a substance subjected to a magnetic field, its plane of polarization is observed to rotate by an amount proportional to the magnetic field component parallel to the direction of propagation.

The rotation of the plane of polarization is given by ө=VBL Where V is the Verdet constant, ө is the magnetic flux parallel to the direction of propagation and L is the path length in the material. Since the magneto-optic material is placed in the magnetic field of a current carrying conductor the rotation of polarization ө is proportional to the current carried in the conductor.


Precautions are necessary to prevent sensitivity disturbance due to other magnetic fields, mainly from the two other phase currents. Magnetic shielding of the conductor with the sensor located within the shielding can be used. The sensitivity and dynamic range of the current sensor depends on the magneto-optic material and the optical wavelength of the system.The Verdant constant (magneto-optic constant) is inversely proportional to the square of the wavelength of light. The faraday effect in a current sensor is typically a few tenths of a degree per kilo ampere. The most commonly used materials for the sensor element are flint glass or Bismuth Silicon Oxide(BSO) Voltage sensors: Passive fiber optic voltage sensor are based on electro-optic effects, i.e, the change in linear birefringence by an applied electric field. The linear electro-optic effect is restricted to non Centro symmetric crystals.

When an electric field is applied across an optical medium the distribution of electrons within it is distorted so that the polarisability and hence the refractive index of the medium changes ansiotropically.


The transmittance is a function of applied voltage. Fiber optic voltage sensors are in research and early development stage. Most experimental work has been done on extrinsic Pockles effect (electro-optic effect) sensors. Most of these sensors resemble the linear faraday effect sensor as shown in figure. The main difference is the addition of a quarter wave plate to introduce a 90 phase bias. By suitable choice of the sensing crystal and the sensor shape, undesirable temperature sensitivity or acoustic resonance due to piezoelectric behavior can be reduced. Fiber optic voltage sensors are currently in use for experimental measurements in power stations. B) Photo-elastic effect sensor: The second acceleration sensor is based on photo elastic effect in a material, i.e., it relies on the measurement of a stress-dependent birefringence in the material.

When the incident light, linearly polarized at an angle of 45degree relative to the two optical principal axes passes through photo elastic material, a phase difference is induced between the two orthogonal polarization components. This phase difference is converted into the intensity by an optical analyzer. The figure shows a vibration acceleration sensor which has a weight on the photo elastic material to increase the response. The pressure applied to the photo elastic material is proportional to the product of the mass of the weight and the amplitude of the vibrational acceleration. As shown in the figure, in order to obtain high reliability the sensing element is constructed in one piece. When BK-7 glass was used as he photo elastic material, the acceleration range of 2.9/squre -14 Om/square was obtained with a good linearity in the frequency region of 10Hz-3KHz.

OTDR (Optical Domain Reflectometer)

An OTDR measures the fraction of light that is reflected back because of Rayleigh scattering & Fresnel reflection. This technique is often called the back-scatter measure method. It provides measurement of the attenuation on an optical link down its entire length. It gives information of


the length dependent of the link loss. It is non-destructive attenuation measuring instrument which requires access to one end of the optical link only.

1 Type of Fiber SM 2 Central Wavelength 1310/1550 nm ± 20 nm

1550/1625 nm ± 20 nm 3 Dynamic Wavelength (dB) 44/4 4 Dead Zone

Event Attenuation

2 m 8 m

5 Pulsewidth 10 ns to 20µs 6 Distance Range (Km) 0 to 320 km 7 Distance Accuracy 0 to 320 km 8 IOR Setting 1.400000 to 1.699999 9 Readout Resolution 0.01 dB 10 Connection Check On/Off Switchable

Figure A.1 illustrates a schematic block diagram of optical time domain reflectometry (OTDR) or back-scattering technique. In this technique a pulsed laser is used to transmit light pulses into the fiber in forward direction. A directional coupler or a beam splitter with a system of lenses can be used to launch light into the fiber. The light beam transmitted through the fiber is scattered back from either joint or faulty region of the fiber or fiber end. This back scattered light is detected by an avalanche photodiode (APD) receiver. The output of APD drives an integrator as shown in the figure. The integrator provides arithmetic average of number measurement for successive points within the fiber. Thus OTDR technique provides attenuation values depending on location of back scattering points. Therefore, reflection from splices, joints, faults and Fresnel reflection loss from the end of the fiber is determined using OTDR.

Documents

Unit-4 new