华中科技大学研究生先进光纤传感课程optical fiber sensor lecture notes_D2-2015new

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Differential reflective fiber-optic angular displacement sensor
Suppress the common mode noise caused by the disturbance in intensity and detector‫׳‬s electrical drift, and achieve higher anti-interference capacity;
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S2: Intensity modulated OFS
• • • • Basic structures Other types of intensity based OFS Property, pros & cons Special structures
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S2-1: Basic structures
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• Displacement underground induce microbending of fibers; • Microbending loss is measured by OTDR for distributed displacement sensing application. 2015/9/10
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S2-1: Basic structures
Transmission type
• Similar to a movable reflector • Used as distance or strain sensors
Limitation: no reference signal so it suffers from light source intensity fluctuation and fiber loss varies
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S2-2: Other intensity based types:
Microbending fiber sensor
No bending, fiber is straight
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Heavy pressure applied cause microbending loss
ቤተ መጻሕፍቲ ባይዱ
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S2-3: Property of intensity based types: • Versatile • Simple design and easy signal interpretation • Usually suffer from intensity fluctuations and low sensitivity • Variation of light source, fiber loss, modal power distribution, long-term drift
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S2-2: Other intensity based types:
Intensity based force sensor
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S2-2: Other intensity based types:
Liquid level fiber sensor
Liquid
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Recall what we learned last lecture:
• • • • • Overview Sensor capabilities Advantages Sensor types Review of fiber optics
– Absorption, scattering, NA…
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Sensors and Actuators B 208 (2015) 315– 319
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A novel and compact fiber MI-based sensor was demonstratedby means of splicing a section of TCF to a standard SMF with acore offset. The core offset and the TCF length were improved to8 m and 3 mm, respectively, in order to achieve a large fringe con-trast of up to 18 dB. Such a MI-based RI sensor based on intensitymodulation exhibits a high sensitivity of at the −202.46 dB/RIU atthe refractive index of 1.42, which is two or three times higherthan that of other intensity-modulated RI sensors reported previ-ously. Moreover, the RI sensor is insensitive to temperature, thusovercoming the cross sensitivity problem between surrounding RIand temperature. Our MIbased RI sensor also exhibits the advantages of compact size (8 mm), simple structure, easy fabrication,and good repeatability.
Optics & Laser Technology 2015/9/10 Volume 68, 2015, 124–128
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Intensity modulated refractive index sensor based on optical fiberMichelson interferometer
光纤传感系统与网络 Optical fiber sensor system and networks
鲁平 Ping Lu
华中科技大学光学与电子信息学院 光通信与光网络系 Email: pluriver@
Lecture note 2
Session 1 – Fundamental concept and components
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S2-4: Special structures
Differential intensity type
• Mitigating the power fluctuation issue by using differential intensity signal from sensing arm and reference arm • Pout1-Pout2 ∞ L1-L2
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Split-spectrum intensity-based optical fiber sensors for measurement of microdisplacement,strain, and pressure
The power ratio of the sensing signal to the reference signal is thus a function of the air-gap separation. However, because the two signals travel along the same optical path in the fiber and are also radiated from the same source, they carry the same information concerning the fiber losses and sourcepower fluctuations. The power ratio should therefore be immune to variations in both the fiber losses and the source power. Thus a low drift and a high long-term accuracy may be expected for this system. The output of a broadband source is coupled into a multimode fiber and propagates through a fiber coupler and mode scrambler to the sensor.The short-wave-pass interference edge filter, which is attached to or directly fabricated on the output end face of the lead-in–lead-out optical fiber, reflects the portion of the spectrum above its cutoff wavelength but allows the portion of the spectrum below its edge wavelength to transmit. The reflected signal, referred to as the reference signal, returns directly to the fiber. The light transmitted through the filter,referred to as the sensing signal, propagates across the air gap to the mirror. The light is reflected by the mirror and is then partially recoupled into the fiber. The two reflections travel along the same fiber to the detector end and are separated spectrally by the wavelength division multiplexer WDM or demultiplexer. The function of the mode scrambler is to ensure similar MPD’s for the two signals. The intensity of the reference signal is independent of the air gap in the sensor,but the intensity of the sensingsignal is a function of the air-gap separation. 17