Based Fiber Optic Sensors

Sensing with Light Intensity-Based Intensity Based Fiber Optic Sensors

Sensing with Fibers Fiber optic sensors are based on monitoring the change of optical signals propagating along the sensitive area. In general, a monochromatic optic field, E r, t, can be written as: E r, t A r, t e i ( t ( r, t )) Where: A r, t is the amplitude of the complex light field, is the angular frequ- ency of the monochromatic light field, and is the phase of the complex field. The key parameters used to describe the light fields, may be subject to change due to external perturbations. By detecting changes in these parameters the external perturbations can be detected or sensed. ( r, t) 1. The polarization - direction of the electric field A r 2. The amplitude (i.e., t ) or intensity (i.e., I r, t A r, t 2, ) 3. The frequency (i.e., f / 2 ) or wavelength of the light field (i.e., ) 4. The phase ( r, t)

Fiber Optic Sensor Capabilities Temperature and pressure Rotation, ti acceleration Electric and magnetic fields Acoustics and vibration i Strain, humidity, and viscosity i

Fiber Optic Sensor Advantages Lightweight i h / nonobtrusive Passive / low power EMI resistant t High sensitivity and bandwidth Environmental ruggedness Complementary to telecom / optoelectronics

Extrinsic Fiber Optic Sensors Input fiber Light modulator Output fiber Environmental signal Extrinsic fibre optic sensors consist of optical fibres that lead in to and out of a "black box" that modulates the light beam passing through it in response to an environmental effect.

Intrinsic Fiber Optic Sensors Optical lfiber Environmental signal 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 optical path length changes in the fiber itself.

Types Comparison Extrinsic sensors are: Less sensitive, more easily multiplexed, and easier to use. They also exhibit connection problems (into and out of the light modulator). Intrinsic sensors are: more sensitive and more difficult to shield from unwanted external perturbations. the all-fiber design reduces or eliminates the connection problems experienced with extrinsic sensors, but they usually require more elaborate signal demodulation. From a commercial standpoint, intrinsic sensors tend to be significantly more expensive than extrinsic sensors but are more versatile for installation and are more robust.

Intensity-Based Fiber Optic Sensors The first one developed Most commonly used nowadays Working in hazard conditions: High/Low tempertures Strong electromagnetic fields High/Low preasure Vibration Chemicely agresive inveroments Easy to coupled with existing communication fiber optics networks Relatively l cheap LED, multimode fiber and polymer connectors could be used

Closure and Vibration Sensors Based on Numerical laperture d Fiber optic sensors based on numerical aperture can be used as door closure indicators and measure levels of vibration in machinery.

NA Based Displacement Sensors NA FOS measuring small vibrations and displacements. These sensors were used to measure thermal distortions of critical components at low temperatures for the Hubble replacement telescope imager. http://www.philtec.com

Samba Sensors technology Ultra-miniature i t fiber optic pressure sensor http://www.sambasensors.com

Measuring hydrostatic pressure Space filled with GAS or LIQUID Hydrostatic pressure Sensor Low pressure 0.42mm High pressure 0.36mm Optical fiber 0.25mm See TSD170 Series http://www.sambasensors.com/

Inserting transducer into a mouse heart http://www.sambasensors.com/ 49

Translation Sensor Based on Numerical laperture Detectors Input light Collection fibers Fiber optic translation sensor based on numerical aperture uses the ratio Fiber optic translation sensor based on numerical aperture uses the ratio of the output on the detectors to determine the position of the input fibre.

Rotary Position Based on Reflectance Variable reflectance shaft Input/output fibers Fibre optic rotary position sensor based on reflectance used to measure rotational position of the shaft via the amount of light reflected from dark and light patches.

Critical Angle Pressure/Index of Refraction Measurement Fiber core n o outside medium index of refraction Light input, output Fiber cladding Mirror Fibre sensor using critical angle properties of a fibre for pressure/index of refraction measurement via measurements of the light reflected back into the fiber.

Liquid Level Sensor Based on TtlIt Total Internal lrfl Reflection Liquid Liquid level detected by total internal reflection. The presence of a liquid causes an absence of a return light signal. The biggest problem is that the effective shape of the sensing element changes The biggest problem is that the effective shape of the sensing element changes when a drop or a thin film of liquid adheres to the tip of the sensor.

Liquid Level Sensor Based on Total Internal Reflection Total Internal Reflection Liquid level sensor based on total internal reflection detects the presence or Liquid level sensor based on total internal reflection detects the presence or absence of liquid by the presence or absence of a return light signal.

Liquid Level Sensor Based on Total Internal Reflection Total Internal Reflection Task: Using TracePro optimise sensor shape 0-7803-8879-8/05/ 2005 IEEE T. Eftimov, W., J. Bock Liquid level sensor based on total internal reflection detects the presence or Liquid level sensor based on total internal reflection detects the presence or absence of liquid by the presence or absence of a return light signal.

Evanescence Based Sensors Active core sensor J.P. Singh - Institute for Clean Energy Technology (ICET) - Mississippi St. University

Evanescence Based Sensors U-bend fibre sensor: J.P. Singh - Institute for Clean Energy Technology (ICET) - Mississippi St. University

Bending losses Macro-bending losses Micro-bending Losses Example bending loss 1 turn at 32 mm diameter causes 0.5 db Property of fiber, under loss control of fabricator, now very Index profile can be adjusted to reduce loss small, usually included in the but this degrades the fibers other total t attenuation ti numbers characteristics This loss is mode dependent

Macro-Bending losses Detecting opening and closing of clamshell devices Plastic optical fiber-based displacement sensor.

Micro-Bending losses Light source Microbend transducer Detector Microbend fiber sensors are configured so that an environmental effect results in an increase or decrease in loss through the transducer due to light loss resulting from small bends in the fiber.

Evanescence Based Sensors L Interaction length Light in d Fiber cores Light outputs Evanescence based fiber optic sensors rely on the cross coupling of light between two closely spaced fiber optic cores. Variations in this distance due to temperature, pressure or strain offer environmental sensing capabilities.

Limitations of Intensity Based Fiber Optic Sensors Variable loss due to connectors Microbending losses Macrobending losses Mechanical losses due to creep and misalignment

Blue Road Research Fly by Light System