1、Fibre Optic Sensors光纖傳感器光纖傳感器 第1頁/共58頁第一頁，編輯于星期六：二十點 五十二分。2Smart Material? An optical fibre is an optically pure glass (光學(xué)純玻璃) or plastic fibre as thin as a human hair that carries information in the form of light over long distances. Is it a smart material? A smart material is a material that conve

2、rts energy from one form to another.Changes in many different types of energy in the surrounding environment can cause a change in the energy/properties of the light transmitted along the optical fibre. Normally a smart material is used in a situation where the energy conversion produces a response

3、which improves the situation. Piezoelectric - lighterSMA change the shape of an aircraft wing MR fluids reduce vibrations Chromism make your clothes look good第2頁/共58頁第二頁，編輯于星期六：二十點 五十二分。3Smart Material? Optical fibres, however, do not do anything to improve the situation, they are pure sensors. They

4、 produce data which then needs to be analysed before anything useful can be done by another device to improve the situation. However, optical fibres are very important in smart structures for one important reason they can provide a distributed sensing system (分布式傳感系統(tǒng)), in some ways similar to our ow

5、n nervous system (中樞神經(jīng)系統(tǒng)). The information they have gathered can then be used by other systems (sometime smart sometimes not smart) to provide an appropriate response to improve the situation. Therefore optical fibres are very important in the design and operation of smart systems/structures.第3頁/共5

6、8頁第三頁，編輯于星期六：二十點 五十二分。4Lecture 10 Teaching Aims Optical Fibres Introduction What is an Optical Fibre Principle of Operation Types of Fibres Dispersion losses (色散損失) Fibre Optic Sensors Introduction Types of Sensors Light Properties that can be modified Fibre Optic Sensing Systems Applications第4頁/共58

7、頁第四頁，編輯于星期六：二十點 五十二分。Optical Fibres第5頁/共58頁第五頁，編輯于星期六：二十點 五十二分。6Introduction Traditionally coaxial cables (同軸電纜) are used for data transfer. However, over the past 20 years optical fibres have revolutionized the telecommunications industry (電信業(yè)革命) by providing higher performance, more reliable telec

8、ommunications links with ever decreasing bandwidth (帶寬) cost. As a result the cost of optical fibres has fallen and the quality has increased. 第6頁/共58頁第六頁，編輯于星期六：二十點 五十二分。7Advantages Faster - Coaxial cables (同軸電纜) transmit radio signals, which travel at two-thirds the speed of light. In comparison,

9、optical fibres transmit light which travels 30% faster than radio signals. Less expensive - Several kilometres of optical fibres can be manufactured at a cheaper cost than equivalent lengths of copper wire (銅線) or coaxial cable. This reduces the costs for cable providers, which leads to cheaper pric

10、es for customers. Can carry more information Optical fibre cables can transmit more information relative to the size of the cable than copper or coaxial cables. This is because light is faster and more efficient than electricity for transmitting information. Transmission distance Optical fibres allo

11、w longer distances to be travelled before the signal has to be amplified by expensive amplifiers.第7頁/共58頁第七頁，編輯于星期六：二十點 五十二分。8Advantages Less signal loss - Signal loss, or degradation (退化) results in an unclear signal being received. Optical fibre cables suffer less signal degradation than coaxial c

12、ables, particularly over long distances. No interference (干擾) (e.g. electromagnetic)- Unlike the electrical signals used in copper and coaxial cables, light signals from one optical fibre do not interfere with other fibres nearby. This means the fibres can be gathered closer together to save space.

13、It also means there is even less signal loss. Fibre is thinner - Each optical fibre can be smaller in diameter than individual copper wires, meaning more can fit in a smaller space and be gathered together into an optical cable (光纜). 第8頁/共58頁第八頁，編輯于星期六：二十點 五十二分。9Advantages Flexible (柔性) - Optical fi

14、bre cables can be bent (彎曲) without a big risk (風(fēng)險) of breaking the cable. Copper and coaxial cables are less flexible. Use less energy - Because there is less signal loss, lower-power transmitters can be used to send information through optical fibres than through coaxial cables. This reduces cost

15、and maintenance, saving money for customers. Less risk of injury - Because no electricity is sent through optical fibres, there is no risk of electrocution (觸電) or burns when handling or accidentally cutting through optical fibre cables (光纜). There is also less risk of the cables causing fires. 第9頁/

16、共58頁第九頁，編輯于星期六：二十點 五十二分。10What is an Optical Fibre? A flexible, optically transparent fibre, usually made of glass or plastic, through which light can be transmitted by internal reflections (內(nèi)部反射). 0.5mmAn average human hair is 18-80mmlConstructionlCore (玻璃芯) - Thin glass centre of the fibre where t

17、he light travels lCladding (玻璃封套) - Outer optical material (can also be glass) surrounding the core that reflects the light back into the core (以使光纖保持在芯內(nèi)).lBuffer coating (塑料外套) - Plastic coating that protects (保護) the fibre from damage and moisture第10頁/共58頁第十頁，編輯于星期六：二十點 五十二分。11Principle of Operati

18、on The light in an optical fibre travels through (傳播) the core by reflecting (反射) off the cladding.Because the cladding does not absorb any light from the core, the light wave can travel great distances. However, some of the light signal degrades (退化) within the fibre, mostly due to impurities in th

19、e glass (雜質(zhì)在玻璃). The amount that the signal degrades depends on the purity of the glass (玻璃的純度) and the wavelength (波長) of the transmitted light The reflection of the light within the core is based on a principle called total internal reflection (TIR) (全內(nèi)反射). TIR is based on refraction (a change in

20、the velocity of light as it changes media). Wavelength (nm)Signal degradation (%/km)85060-75 (50) 1,31050-60 (20) 1,55050 (0.5) 第11頁/共58頁第十一頁，編輯于星期六：二十點 五十二分。12Refraction Refraction (折射) occurs when light passes from a material with one index of refraction (折射率) to a material of a different index of

21、 refraction, n. When light passes from one material with a high n to another material with a lower n, it refracts away from the normal line - a line perpendicular to the interface (界面) between the two materials. As i becomes greater, r also becomes greater. Glass fibren = 1.52n 1.52Normal line第12頁/共

22、58頁第十二頁，編輯于星期六：二十點 五十二分。13Total Internal Reflection (全內(nèi)反射) When i reaches a particular angle called the critical angle, c, (臨界角) the refracted light will not enter the second material, but instead will travel along the interface between the two material If the beam travels through the glass at an an

23、gle greater than the critical angle, then the refracted beam will be reflected entirely back into the glass. In an optical fibre, the light travels through the core by constantly (不斷) reflecting from the cladding because i is always greater than the c. The critical angle is normally described with r

24、espect to (關(guān)于) the normal line. However, in optical fibres the critical angle is described with respect to the axis (軸) running down the middle of the fibre. n2)(n1 12sinnncGlass fibren = 1.52n 1.52Normal line第13頁/共58頁第十三頁，編輯于星期六：二十點 五十二分。14How Light Enters the Fibres The most common devices used as

25、 the light source (光源) in optical fibres are: light emitting diode (LED) (發(fā)光二極管) laser diode (LD) (激光二極管) In an optical fibre system, the diode is placed as close as possible to the optical fibre so that as much light as possible enters the fibre. LEDs emit (發(fā)光) incoherent light (非相干光) in a broad pa

26、ttern which cannot be focused (定焦點) into the core, therefore only a small percentage of the light will enter the core. LDs emit a narrow light pattern and therefore much more light can enter the core.第14頁/共58頁第十四頁，編輯于星期六：二十點 五十二分。15Types of Fibre There are two primary types of fibre. Multi-mode fibr

27、e (多模光纖) - very common for short distances (2km). A core diameter of 50 or 62.5 mm is used. Multi-mode fibres are divided in to 2 sub-categories: step index (跳變式光纖) & graded index (漸變式光纖). Single-mode fibre (單模光纖) - common for long distances and high speed. A core diameter of about 9 mm is used.

28、 第15頁/共58頁第十五頁，編輯于星期六：二十點 五十二分。16Multi-mode: Step Index Fibre (多模光纖: 跳變式光纖) Step index fibre have of a glass core (玻璃芯) with a low index of refraction (低折射率) surrounded by a glass cladding of even lower index of refraction. This difference in refractive index between the core and cladding causes lig

29、ht to continually be reflected by the core/cladding interface. Can simultaneously transmit light of many different modes (能以多個模式同時傳輸?shù)墓饫w). The number of modes of light and thus the amount of light that can be transmitted by the optical fibre, is determined by the core size and the difference in the c

30、ores and the claddings refractive index. 第16頁/共58頁第十六頁，編輯于星期六：二十點 五十二分。17Multi-mode: Step Index Fibre (多模光纖: 跳變式光纖) Because high-angle modes (高角度模式) of light have a longer distance to travel relative to (相對于) the low-angle modes of light, an initially sharp pulse (最初急劇脈沖) made up of many modes (多模式)

31、 broadens (拓寬) as it travels along the fibre because the modes of light do not arrive at the end at the same time. This is called pulse dispersion or pulse broadening (離散脈沖或脈沖擴大). This limits the frequency at which light can be sent down the fibre and the distance the light can travel because it det

32、ermines how closely the input pulses can be spaced without overlapping at the output end. This in turn limits the bandwidth (帶寬).第17頁/共58頁第十七頁，編輯于星期六：二十點 五十二分。18Multi-mode: Graded Index Fibre (多模光纖: 漸變式光纖) In graded index fibres the core and cladding are made of a single type of glass however it is

33、treated so that the index of refraction gradually decreases as the distance from the core centreline increases. (折射率隨著半徑的增加按一定規(guī)律減小) The result of this design is that light continuously bends toward the centre of the fibre much like a continuous lens.第18頁/共58頁第十八頁，編輯于星期六：二十點 五十二分。19Multi-mode: Graded

34、 Index Fibre (多模光纖: 漸變式光纖) Since light travels slower in the high-index region of the fibre relative to the low-index region, significant equalization of the transit time for the various modes can be achieved to reduce pulse broadening. High-angle modes (高角度模式) of light have to travel further, but,

35、since they pass through the low-index regions, they can travel faster than light in the higher-index regions. Thus the low and high-angle modes of light arrive at much closer time intervals (時間間隔) than in step-index-fibres. This means that pulses of light can be sent at a higher frequency and thus t

36、he fibre has a higher bandwidth. It offers hundreds of times more bandwidth than step index fibre - up to about 2 GHz. 第19頁/共58頁第十九頁，編輯于星期六：二十點 五十二分。20Single-mode (單模光纖) A single-mode fibre has such a small core diameter that only one mode of light can enter the core, thus eliminating (消除) pulse bro

37、adening effects. This allows a much higher frequency of light pulses to be sent and thus allows a higher bandwidth (about 100,00 GHz). Imperfections within the material cause some pulse broadening, which increases with the spectral width of the light source. These fibres are best suited for use with

38、 a laser diode (often a single frequency fibre laser) in order to efficiently send light into the small core of the optical fibre and to enable information transmission over long distances at very bandwidths (傳輸頻帶寬,傳輸容量大).第20頁/共58頁第二十頁，編輯于星期六：二十點 五十二分。21Chromatic Dispersion We have already mentioned

39、 pulse dispersion. Another form of dispersion in optical fibres is chromatic dispersion (色散). The index of refraction of the optical fibre varies as a function of the wavelength. Therefore, according to the equation, light of different wavelengths travel at different speeds in the fibre.)(ncvmateria

40、ln()第21頁/共58頁第二十一頁，編輯于星期六：二十點 五十二分。22wwIwIIIwThe initial pulse (初始脈沖) will always contain light of different wavelengths. It is impossible to achieve a pure pulse as shown in the diagram, instead a pulse containing light of various wavelengths will be produced. 第22頁/共58頁第二十二頁，編輯于星期六：二十點 五十二分。23Chrom

41、atic Dispersion Because the initial pulse can never provide a light of only one wavelength, and because the different wavelengths of light travel at different speeds the pulse will progressively broaden (逐步擴大) as distance between the different wavelengths of light steadily increases. The further the

42、 pulse travels the more broadening will occur. Distance travelled第23頁/共58頁第二十三頁，編輯于星期六：二十點 五十二分。241 0 1 1 0 0 11 0 1 1 0 0 1?100 km100 km100 kmGetting smearedClear dataData lossUSELESSWhy is Dispersion Important?Dispersion is important because it cause data loss! 第24頁/共58頁第二十四頁，編輯于星期六：二十點 五十二分。25Chr

43、omatic Dispersion We know that dispersion will occur and we can calculate how much dispersion will occur so by design of the fibre using the right core size and right index of refraction we can compensate for (cancel out) (補償) the effects of chromatic dispersion.第25頁/共58頁第二十五頁，編輯于星期六：二十點 五十二分。Fibre

44、Optic Sensors光纖傳感器 第26頁/共58頁第二十六頁，編輯于星期六：二十點 五十二分。27Fibre Optic Sensor System A very simple fibre optic sensor system consists of a transmitter, the optical fibre, a measurement area and a receiver. Sometime the transmitter and receiver are the same device.Measurement AreaOptical FibreExternal Stimu

45、li第27頁/共58頁第二十七頁，編輯于星期六：二十點 五十二分。28Transmitted Light Signal The transmitter converts an electrical input signal into light for transmission. Depending on the nature of this signal the light may be either on or off or may linearly vary in intensity between two predetermined levels. 第28頁/共58頁第二十八頁，編輯于

46、星期六：二十點 五十二分。29Fibre Optic Sensors Fibre optic sensors are possibly the most versatile (多才多藝) sensors available, they can detect change in the following properties: Acceleration Chemicals/Gases Colour Displacement Flow Force Humidity Liquid Level Magnetic/Electric Fields Moisture Motion Proximity Ra

47、diation Sound Speed State-of-Cure Strain Surface Condition Tactile Sensing Temperature Velocity Vibration Viscosity Weight Position (linear, angular) Pressure (fluid, gas, etc.)第29頁/共58頁第二十九頁，編輯于星期六：二十點 五十二分。30Fibre Optic Sensors In a fibre optic sensor these external stimuli can cause a change in o

48、ne or more of the following properties of the light: Intensity 強度 Phase 相 Polarisation 偏振 Wavelength 波長 Spectral distribution 光譜分布 第30頁/共58頁第三十頁，編輯于星期六：二十點 五十二分。31Fibre Optic Sensors Advantages Small and Low weight Allow access into normally inaccessible areas Embedded or attached (嵌入式或附) Passive (n

49、on-electrical)High sensitivity High Bandwidth Low power Resistant to electromagnetic interference Environmental ruggedness Disadvantages New technologyExpensive Complexity ReliabilityFragilityBecause of their thinness, they do not degrade the integrity (完整性) of the composite in which they are embedd

50、ed.Optical fibres can be employed for distributed measurements (分布式測控).第31頁/共58頁第三十一頁，編輯于星期六：二十點 五十二分。32Fibre Optic Sensors: Intrinsic光纖傳感器：內(nèi)在 The fibre itself acts as the sensor. The light never leaves the fibre and is altered in some way by an external stimuli (外部刺激). Example: As the pressure incr

51、eases the fibre will become slightly deformed and experience increased microbending losses which results in a decrease in the light intensity received at the detector. A decrease in the pressure relieves stress on the fibre and hence there is an increase in transmitted light detected. 第32頁/共58頁第三十二頁

52、，編輯于星期六：二十點 五十二分。33Fibre Optic Sensor: Extrinsic光纖傳感器：外在 The fibre only acts as a light delivery and collection system (光運送和收集系統(tǒng)). The light leaves the fibre, is altered in some way, and is collected by the same (or another) fibre.第33頁/共58頁第三十三頁，編輯于星期六：二十點 五十二分。34Extrinsic: Example The sensor detect

53、s any increase or decrease in the length, l, between the two fibres. As the distance between the fibres is increased the amount of light collected by the return fibre will decrease. Conversely as the distance is decreased the light collected by the return fibre will increase. This provides a relativ

54、ely simple fibre optic sensor for determining small movements. While a sensor of this kind has problems with sensitivity to lateral movement it is a good illustration of a basic sensing technique. 第34頁/共58頁第三十四頁，編輯于星期六：二十點 五十二分。35Sensing Methods Strains on the fibre or a change in the length of sepa

55、ration between 2 fibres cause a reduction of light intensity. The polarisation (兩極分化) of the light is effected by the presence of electric and magnetic fields. Changes in temperature can cause changes in the spectral distribution (光譜分布) of the light. Strains exerted on some fibres cause changes in t

56、he light path length leading to phase (相) changes. UV light (紫外光) can be detected using a fibre which absorbs (吸收) the UV light and re-emits it as red light which is captured in the fibre. 第35頁/共58頁第三十五頁，編輯于星期六：二十點 五十二分。36Property: Intensity The simplest fibre optic sensors measure changes in light

57、intensity (光照強度). This type of sensor can be prone to errors (容易出錯) due to unexpected losses or variable losses in the fibres and due to variations in the light intensity delivered by the light source. These problems can be solved if a reference is used that can account for the loses and changes in

58、the source. 第36頁/共58頁第三十六頁，編輯于星期六：二十點 五十二分。37Property : Phase The phase or optical path length (光路長度) is used based on interferometry (干涉式). Here the light is split between 2 (or more) paths. One path is used as a reference (參考) and the other is subject to change because of the parameter being measu

59、red. Recombining the light from these 2 paths generates optical interference (光干涉). The measured property is thus related to the interferometric phase. These sensors have a limited range due to the periodic nature of the optical interference. Cant measure differences bigger than the period of the in

60、terference. Source第37頁/共58頁第三十七頁，編輯于星期六：二十點 五十二分。38Property: Wavelength & Colour Sensors based on wavelength and colour measure changes in the spectral response. Sensors based on colour tend to have a broad spectral response. They can operate at visible wavelengths. Sensors based on wavelength have a na