Wavefront Sensors/MEMSMap 510

Family of Wavefront Sensors For Instant Phase Analysis
The CLAS-2D™ Shack-Hartmann wavefront sensor combines the functions of an interferometer, beam profiler, and beam quality meter in one instrument. The system software analyzes optical aberrations including astigmatism, coma, spherical aberration, focus error/collimation, tilt and more. In addition, the CLAS-2D™ measures M 2 beam quality, MTF, Strehl Ratio, near field and far field beam divergence, and other beam parameters.
Versions are available for both CW and pulsed beams in three wavelength ranges: visible, near IR, and long IR.
Typical wavefront sensor applications are:• Laser beam diagnostics• Optical testing• Alignment• Angular measurement• Collimation• Surface measurement
Nondestructive testing with shearography
About Nondestructive Testing (NDT) by Speckle Interferometry Speckle Interferometry is a common term used for different optical measurement techniques like TV-holography (VibroMap 1000), Shearography (SNT 4045) and other techniques. With these techniques, laser light is used to detect static and/or dynamic displacements of object surfaces with very high sensitivity and accuracy. One main application of Speckle Interferometry is Nondestructive Testing - NDT. Speckle Interferometry is especially useful to detect defects in metallic and/or polymer composite materials. When the object is excited dynamically or statically, surface or sub surface defects can be detected, as many defects give inhomogeneous surface displacements when the object is excited. Typical excitation methods are thermal loading, single frequency vibration, white noise vibration, vacuum or pressure loading, mechanical loading. Typical defects are delaminations and debonds, flaws, impact damage etc. Optonor has long time experience with the use of Speckle Techniques for NDT applications. The Trondheim Group has delivered systems world wide (both TV-holography and Electronic Shearography) for industrial NDT applications. The shearography system SNT 4045 can be customized for different industrial applications, like testing of large ship structures, aircraft structures like fan ducts etc., panels and a long range of other products. In addition, the TV-holography system VibroMap 1000 can also be used for several NDT applications. You are welcome to contact us to get an evaluation of your NDT problems and requirements.
MEMSMap 510
MEMS analysis High frequency vibration measurements Static deflection measurements 3D measurements Surface profiling The MEMSMap 510 is a microscopic TV-holography system which can be used on surfaces down to less than 0.1x0.1 mm (full field). The MEMSMap 510 / MicroMap 5010 can be used for 3 types of measurements}Vibration measurements up to 240 MHz or moreStatic deflection measurements and Surface topography measurements (profiling)The MEMSMap 510 works a similar way to the VibroMap 1000. The MEMSMap can be used on both specular (shiny) objects and rough objects. The MEMSMap 510 can also be delivered with an option for in-plane deflection measurements, but for in-plane measurements, the MEMSMap can be used on rough surfaces only. The main applications for the Optonor MEMSMap 510 system are deformation- and vibration analysis of MicroMany MEMS structures and transducers have their properties and functionality directly connected to their dynamic and semistatic deflection properties, and the MEMSMap 510 is a powerful tool in the development and testing of such structures.

Chiral photonics

What is chiral photonics ?
Chiral photonics stems from the unique way in which light interacts with chiral or helical structures such as twisted optical fibres. At Chiral Photonics we have built microforming towers that allow us to fabricate devices based on fibres that can be twisted through more than 25,000 revolutions over a 1 inch length. The density of twists per inch (or periodicity) determines the extent of filtering, polarizing or scattering of light and results in three categories of chiral fibres.
Chiral gratings with a pitch of the order of 100 µm (known as chiral long-period gratings) couple light into the cladding of the optical fibre and are the basis for our sensor technology.
Chiral gratings with a pitch of the order of 10 µm (known as chiral intermediate-period gratings) scatter a portion of the light out of the fibre and are the basis for our polarizer and isolator technology.
Chiral gratings with a pitch of the order of 1 µm (known as chiral short-period gratings) reflect light within the fibre core and are useful for filtering and lasing.
The gratings can target specific wavelengths determined by their helical pitch. The gratings also have a handedness, much like a screw. This handedness can be used to selectively determine how they interact with a specific polarization of light. Unpolarized light contains a mixture of left and right circularly polarized light. Only circularly polarized light with the same handedness as the chiral structure will "feel" the structure and be scattered backwards, into the cladding or out of the fibre.
For example, a right-handed intermediate-period grating will scatter only right-handed circularly polarized light. This will leave left-handed light to be transmitted. This polarization selectivity, and the ability to simply convert circular to linear polarizations via the same chiral structures, is the basis for non-absorbing polarizers and polarization-selective lasing.
What are the advantages of chiral photonic
structures?
Chiral structures enable in-fibre devices that can displace discrete optical elements such as lasers, filters and sensors. These devices improve system transmission efficiency and robustness and ease integration. What's more, the devices are manufactured using an automated and scalable process, which, for example, promises lasers that are a fraction of the cost and three times more efficient than today's semiconductor lasers.
What are the main applications ?
Our spot-size-converting interconnects are being used by leading telecoms and datacoms companies to directly couple sub-micron-sized waveguides to standard optical fibres. These interconnects exhibit low loss (less than 0.5 dB), preservation of polarization and channel spacing below 25 µm and obviate the need for on-chip spot-size conversion, which takes up precious space. They also eliminate the air gap required for typical lens-fibre coupling, which compromises package stability, especially for waveguides with small mode-field sizes.
The company also offers a temperature sensor that can be used at temperatures up to 1100 °C without degradation. The sensors are based on pure silica fibres and are being used for turbine and combustion engine development and monitoring; geothermal, oil and gas applications (both borehole and refining processes); materials processing, such as metal smelting and welding; and solar collection.
Our in-fibre chiral polarizers exhibit excellent broadband performance, bend-insensitive stability across a broad temperature range and an ability to address a wide range of wavelengths. These polarizers are used for sensing applications – including current, biomedical, gyroscope and pressure sensors – as well as telecoms applications.
Chiral Photonics also offers twisted capillary tubes, which can be used for the study of protein molecules. Passing the protein through a twisted channel allows it to unfold and be imaged. What's more, the rotation of the protein as it passes through the channel enables 360° imaging. In addition, the capillary tubes have other microfluidic applications, including mixing and uniform heat exchange.
What can we expect to see from Chiral Photonics in
the future
?
One of the most troublesome and expensive components needed in the construction of high-power fibre lasers is the optical isolator. The current state-of-the-art is an in-line isolator that is composed of bulk optical components. The drawback of these isolators is that free-space facets in high-power applications can be easily burned – even with the slightest misalignment. These misalignments are especially hard to avoid where high optical power is present, as even minimal absorption can result in slight thermal excursions.
Chiral Photonics is developing an in-fibre chiral isolator that has no exposed facets. Although this is work in progress, we want to involve fibre laser manufacturers at an early stage as isolator pigtails are integral to the design and to that of the fibre laser into which it will be integrated.
We are also developing our first active component, a chiral fibre laser based on chiral short-period gratings, which will exploit the very efficient chiral feedback structure to enable a low-cost, single-polarization, single-frequency laser that can be used for sensing and high power, seed-laser applications.

OPTICAL COHERENCE TOMOGRAPHIC IMAGES OF RESTORED TEETH

OPTICAL COHERENCE TOMOGRAPHIC IMAGES OF RESTORED TEETH
While intraoral radiographs are highly sensitive and specific for diagnosing primary caries, they are less reliable in the detection of recurrent caries around existing restorations. OCT offers a potentially more sensitive method for detecting recurrent caries. Moreover, images of the fit and marginal adaptation restoration margins can be made and quantified.
An OCT image of the margin of a cemented, functional porcelain-fused-to-metal crown .The marginal adaptation of the metal coping to the cavosurface margin is easily identified, and the internal contours of the restoration and various enamel layers can be seen.
The intact enamel marginal ridge and the cavosurface marginal adaptation are easily identified. The dentin-composite interface and the contour of the occlusal floor of the preparation also are seen.
CONCLUSION
We have constructed a prototype clinical dental OCT system and have demonstrated the feasibility of using it in a clinical setting. Our research to date has shown that OCT is a powerful method for generating high-resolution, cross-sectional images of oral structures. We have used OCT to take images of the teeth, locate soft- and hard-tissue boundaries of the periodontium and evaluate restoration margins. Our goals in our ongoing research are to characterize normal dental structures using OCT and verify that this new technology can be used to take images of and quantify common dental problems including caries, defective restorations and periodontal disease.
Twisted optical fibres : it's time to think chiral
Taking inspiration from the helical-shaped designs found in nature, Chiral Photonics is adding a new twist to optical technologies. Dan Neugroschl, the company's president, tells Marie Freebody about the unique ways in which light interacts with chiral structures.

OPTICAL COHERENCE TOMOGRAPHY

OPTICAL COHERENCE TOMOGRAPHY
A new imaging technique, called optical coherence tomography, or OCT, creates cross-sectional images of biological structures using differences in the reflection of light. This technique uses broad-band, near-infrared light sources with considerable penetration into tissue, yet it has no known detrimental biological effect.Microstructural tissue detail is revealed by differentiating between scattered and transmitted, or reflected, photons.OTC was first proposed for use as a biological imaging system in 1991 by Huang and colleagues.Because of their collaborative work, OCT imaging now is being used in clinical practice in ophthalmology.
DENTAL OPTICAL COHERENCE TOMOGRAPHY
We have developed and tested an OCT system to make images of dental structures. Our prototype dental OCT system consists of a computer, compact diode light source, photodetector with associated electronics and handpiece that scans a fiber-optic cable over the oral tissues .The system uses a white light fiber-optic Michelson interferometer connected to a handpiece that moves the sample arm linearly to create a tomographic scan. Light from the low-coherence diode is separated by a fiber-optic splitter into sample and reference arms of the interferometer. Reflections from the reference mirror and backscattered light from the tissue are recombined at the splitter and transmitted to the photodetector. An interference signal is detected when the pathlength of light reflected from the tissue and the reference mirror is within the coherence length of the source. Because the position of the reference mirror is known, the location within the tissue of the reflected signal can be precisely determined.
A single interferometric signal measured at a specific point on the tissue gives the reflective boundary along the axis of the beam .The locations of reflected signals correspond to their axial position, while the magnitude of the signal is determined by the unique scattering characteristics of a particular tissue. Signals, therefore, are relatively high at tissue interfaces. Signal amplitudes are assigned a gray scale, or false color, value in the computer and are displayed in a linear array. These amplitude differences create a range of contrast that is characteristic of the tissue interactions with the light photons. As the handpiece scans the light across a region of clinical interest, axial signals are serially displayed. The final OCT image is a composite of many axial signal arrays in other words, the OCT image is a two-dimensional representation of the optical reflections of tissue in cross-section.
DENTAL OPTICAL COHERENCE TOMOGRAPHIC IMAGES

In previous studies, we verified the accuracy of OCT for taking in vitro images of dental structures using an animal model. We found that these images corresponded to histologic images, and we correlated probing depths to sulcular depth measurements made in OCT images.
To test the capacity of our system to take in vivo images of dental structures, we used our prototype system to take dental OCT images of healthy adults with normal dentitions and no clinical evidence of gingivitis or periodontal disease; this test was approved by the Institutional Review Board at The University of Connecticut Health Center School of Dental Medicine. Our system uses a 140-microwatt, 1310-nanometer superluminescent diode light source and detects up to 70 femtowatts of reflected light. It has an imaging depth of approximately 3 millimeters; imaging depth is limited by the amount of light that is propagated through the tissue, as well as the image acquisition time. Image acquisition time in our current system is 45 seconds.
The images we made represent the first in vivo OCT images of human dental tissue is an OCT image of the midbuccal surface of a mandibular premolar. The OCT scans were made along the long axis of the tooth near the cervical region. The images represent a labial-lingual cross-section of the tooth at a resolution determined by the diameter of the OCT beam (20 micrometers). The axial resolution of 12 µm in periodontal tissue is given by the coherence length of the light source (16 µm) separated by the refractive index of the tissue.
Our in vivo dental OCT images clearly depict anatomical structures that are important in the diagnostic evaluation of both hard and soft oral tissue. Periodontal tissue contour, sulcular depth and connective tissue attachment are visualized at high resolution using this technology. We are evaluating its clinical usefulness for periodontal assessments in ongoing clinical studies. Because OCT reveals microstructural detail of the periodontal soft tissues, it offers the potential for identifying active periodontal disease before significant alveolar bone loss occurs.OCT images of the periodontium can be stored in the patient record, providing visual documentation of disease progression, response to therapy or both. More extensive clinical studies that will correlate OCT parameters to current diagnostic assessments such as probing depths are ongoing.

NEW IMAGING TECHNOLOGY FOR DENTISTRY

A NEW IMAGING TECHNOLOGY FOR DENTISTRY

ABSTRACT
Background. Optical coherence tomography, or OCT, is a new diagnostic imaging technique that has many potential dental applications. The authors present the first intraoral dental images made using this technology.
Methods. The authors constructed a prototype dental OCT system. This system creates cross-sectional images by quantifying the reflections of infrared light from dental structures interferometrically.
Results. We used our prototype system to make dental OTC images of healthy adults in a clinical setting. These OCT images depicted both hard and soft oral tissues at high resolution.
Conclusions. OCT images exhibit microstructural detail that cannot be obtained with current imaging modalities. Using this new technology, visual recordings of periodontal tissue contour, sucular depth and connective tissue attachment now are possible. The internal aspects and marginal adaptation of porcelain and composite restorations can be visualized.
Clinical Implications. There are several advantages of OCT compared with conventional dental imaging. This new imaging technology is safe, versatile, inexpensive and readily adapted to a clinical dental environment.
In plain film radiographic techniques, such as periapical or cephalometric radiography, the radiographic source and film are stationary. All anatomical structures interposed between the X-ray source and the film are present in the image. The disadvantage of plain film radiography is that important diagnostic information often is obscured by the superimposition of regional anatomy; for example, the morphological characteristics of the mandibular condyles are obscured in a cephalometric radiograph by the dense overlying structures of the cranial base.
The term tomography first was used to describe sectional radiographic techniques. When the radiographic tube is moved during exposure synchronous with the film plate, but in the opposite direction, the image of a selected anatomical plane remains stationary on the moving film while the shadows of all other planes are blurred or obliterated. A tomographic image, thus, represents a selected "layer" or "slice" of the structure whose images have been recorded. In tomographic images, for example, the mandibular condyles are clearly visualized without the superimposition of the dense cranial base. Panoramic radiographs are the most common form of tomographic imaging used in dentistry.
Tomography now is used as a general term to describe any imaging method that produces images of selected anatomical planes within a structure. The tomographic images created by panoramic radiography and computed tomography result from the interaction of biological tissues with X-radiation photons. Recent developments in the field of optical engineering have made it possible for researchers to consider optical techniques for biomedical imaging applications. These developments include the increased availability of compact, modular diode light sources and the development of highly sensitive detectors that make it possible to distinguish very small numbers of light photons after they interact with tissue.

How Are Optical Fibers Made

How Are Optical Fibers Made ?
Now that we know how fiber-optic systems work and why they are useful -- how do they make them? Optical fibers are made of extremely pure optical glass. We think of a glass window as transparent, but the thicker the glass gets, the less transparent it becomes due to impurities in the glass. However, the glass in an optical fiber has far fewer impurities than window-pane glass. One company's description of the quality of glass is as follows: If you were on top of an ocean that is miles of solid core optical fiber glass, you could see the bottom clearly.
Making optical fibers requires the following steps:
1.Making a preform glass cylinder
2.Drawing the fibers from the preform
3.Testing the fibers
Making the Preform Blank
The glass for the preform is made by a process called modified chemical vapor deposition (MCVD).
In MCVD, oxygen is bubbled through solutions of silicon chloride (SiCl4), germanium chloride (GeCl4) and/or other chemicals. The precise mixture governs the various physical and optical properties (index of refraction, coefficient of expansion, melting point, etc.). The gas vapors are then conducted to the inside of a synthetic silica or quartz tube (cladding) in a special lathe. As the lathe turns, a torch is moved up and down the outside of the tube. The extreme heat from the torch causes two things to happen:
· The silicon and germanium react with oxygen, forming silicon dioxide (SiO2) and germanium dioxide (GeO2).
· The silicon dioxide and germanium dioxide deposit on the inside of the tube and fuse together to form glass.
The lathe turns continuously to make an even coating and consistent blank. The purity of the glass is maintained by using corrosion-resistant plastic in the gas delivery system (valve blocks, pipes, seals) and by precisely controlling the flow and composition of the mixture. The process of making the preform blank is highly automated and takes several hours. After the preform blank cools, it is tested for quality control (index of refraction).
Drawing Fibers from the Preform BlankOnce the preform blank has been tested, it gets loaded into a fiber drawing tower.
The blank gets lowered into a graphite furnace (3,452 to 3,992 degrees Fahrenheit or 1,900 to 2,200 degrees Celsius) and the tip gets melted until a molten glob falls down by gravity. As it drops, it cools and forms a thread.
The operator threads the strand through a series of coating cups (buffer coatings) and ultraviolet light curing ovens onto a tractor-controlled spool. The tractor mechanism slowly pulls the fiber from the heated preform blank and is precisely controlled by using a laser micrometer to measure the diameter of the fiber and feed the information back to the tractor mechanism. Fibers are pulled from the blank at a rate of 33 to 66 ft/s (10 to 20 m/s) and the finished product is wound onto the spool. It is not uncommon for spools to contain more than 1.4 miles (2.2 km) of optical fiber.
Testing the Finished Optical FiberThe finished optical fiber is tested for the following:
· Tensile strength - Must withstand 100,000 lb/in2 or more
· Refractive index profile - Determine numerical aperture as well as screen for optical defects
· Fiber geometry - Core diameter, cladding dimensions and coating diameter are uniform
· Attenuation - Determine the extent that light signals of various wavelengths degrade over distance
· Information carrying capacity (bandwidth) - Number of signals that can be carried at one time (multi-mode fibers)
· Chromatic dispersion - Spread of various wavelengths of light through the core (important for bandwidth)
· Operating temperature/humidity range
· Temperature dependence of attenuation
· Ability to conduct light underwater - Important for undersea cables
­ Once t­he fibers have passed the quality control, they are sold to telephone companies, cable companies and network providers. Many companies are currently replacing their old copper-wire-based systems with new fiber-optic-based systems to improve speed, capacity and clarity.
Physics of Total Internal Reflection
When light passes from a medium with one index of refraction (m1) to another medium with a lower index of refraction (m2), it bends or refracts away from an imaginary line perpendicular to the surface (normal line). As the angle of the beam through m1 becomes greater with respect to the normal line, the refracted light through m2 bends further away from the line.
At one particular angle (critical angle), the refracted light will not go into m2, but instead will travel along the surface between the two media (sine [critical angle] = n2/n1 where n1 and n2 are the indices of refraction [n1 is greater than n2]). If the beam through m1 is greater than the critical angle, then the refracted beam will be reflected entirely back into m1 (total internal reflection), even though m2 may be transparent!
In physics, the critical angle is described with respect to the normal line. In fiber optics, the critical angle is described with respect to the parallel axis running down the middle of the fiber. Therefore, the fiber-optic critical angle = (90 degrees - physics critical angle).
In an optical fiber, the light travels through the core (m1, high index of refraction) by constantly reflecting from the cladding (m2, lower index of refraction) because the angle of the light is always greater than the critical angle. Light reflects from the cladding no matter what angle the fiber itself gets bent at, even if it's a full circle!
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 within the fiber, mostly due to impurities in the glass. The extent that the signal degrades depends upon the purity of the glass and the wavelength of the transmitted light (for example, 850 nm = 60 to 75 percent/km; 1,300 nm = 50 to 60 percent/km; 1,550 nm is greater than 50 percent/km). Some premium optical fibers show much less signal degradation -- less than 10 percent/km at 1,550 nm.

Advantages of Fiber Optics

Advantages of Fiber Optics
Why are fiber-optic systems revolutionizing telecommunications? Compared to conventional metal wire (copper wire), optical fibers are:
· Less expensive - Several miles of optical cable can be made cheaper than equivalent lengths of copper wire. This saves your provider (cable TV, Internet) and you money.
· Thinner - Optical fibers can be drawn to smaller diameters than copper wire.
· Higher carrying capacity - Because optical fibers are thinner than copper wires, more fibers can be bundled into a given-diameter cable than copper wires. This allows more phone lines to go over the same cable or more channels to come through the cable into your cable TV box.
· Less signal degradation - The loss of signal in optical fiber is less than in copper wire.
· Light signals - Unlike electrical signals in copper wires, light signals from one fiber do not interfere with those of other fibers in the same cable. This means clearer phone conversations or TV reception.
· Low power - Because signals in optical fibers degrade less, lower-power transmitters can be used instead of the high-voltage electrical transmitters needed for copper wires. Again, this saves your provider and you money.
· Digital signals - Optical fibers are ideally suited for carrying digital information, which is especially useful in computer networks.
· Non-flammable - Because no electricity is passed through optical fibers, there is no fire hazard.
· Lightweight - An optical cable weighs less than a comparable copper wire cable. Fiber-optic cables take up less space in the ground.
· Flexible - Because fiber optics are so flexible and can transmit and receive light, they are used in many flexible digital cameras for the following purposes:
· Medical imaging - in bronchoscopes, endoscopes, laparoscopes
· Mechanical imaging - inspecting mechanical welds in pipes and engines (in airplanes, rockets, space shuttles, cars)
· Plumbing - to inspect sewer lines
Because of these advantages, you see fiber optics in many industries, most notably telecommunications and computer networks. For example, if you telephone Europe from the United States (or vice versa) and the signal is bounced off a communications satellite, you often hear an echo on the line. But with transatlantic fiber-optic cables, you have a direct connection with no echoes.

A Fiber-Optic Relay System

A Fiber-Optic Relay System
To understand how optical fibers are used in communications systems, let's look at an example from a World War II movie or documentary where two naval ships in a fleet need to communicate with each other while maintaining radio silence or on stormy seas. One ship pulls up alongside the other. The captain of one ship sends a message to a sailor on deck. The sailor translates the message into Morse code (dots and dashes) and uses a signal light (floodlight with a venetian blind type shutter on it) to send the message to the other ship. A sailor on the deck of the other ship sees the Morse code message, decodes it into English and sends the message up to the captain.
Now, imagine doing this when the ships are on either side of the ocean separated by thousands of miles and you have a fiber-optic communication system in place between the two ships. Fiber-optic relay systems consist of the following:
· Transmitter - Produces and encodes the light signals
· Optical fiber - Conducts the light signals over a distance
· Optical regenerator - May be necessary to boost the light signal (for long distances)
· Optical receiver - Receives and decodes the light signals
Transmitter The transmitter is like the sailor on the deck of the sending ship. It receives and directs the optical device to turn the light "on" and "off" in the correct sequence, thereby generating a light signal.
The transmitter is physically close to the optical fiber and may even have a lens to focus the light into the fiber. Lasers have more power than LEDs, but vary more with changes in temperature and are more expensive. The most common wavelengths of light signals are 850 nm, 1,300 nm, and 1,550 nm (infrared, non-visible portions of the spectrum).
Optical Regenerator As mentioned above, some signal loss occurs when the light is transmitted through the fiber, especially over long distances (more than a half mile, or about 1 km) such as with undersea cables. Therefore, one or more optical regenerators is spliced along the cable to boost the degraded light signals.
An optical regenerator consists of optical fibers with a special coating (doping). The doped portion is "pumped" with a laser. When the degraded signal comes into the doped coating, the energy from the laser allows the doped molecules to become lasers themselves. The doped molecules then emit a new, stronger light signal with the same characteristics as the incoming weak light signal. Basically, the regenerator is a laser amplifier for the incoming signal.
Optical Receiver The optical receiver is like the sailor on the deck of the receiving ship. It takes the incoming digital light signals, decodes them and sends the electrical signal to the other user's computer, TV or telephone (receiving ship's captain). The receiver uses a photocell or photodiode to detect the light.

HOW OPTICAL FIBER WORKS?

How Fiber Optics Work
You hear about fiber-optic cables whenever people talk about the telephone system, the cable TV system or the Internet. Fiber-optic lines are strands of optically pure glass as thin as a human hair that carry digital information over long distances. They are also used in medical imaging and mechanical engineering inspection.
What are Fiber Optics ?
Fiber optics (optical fibers) are long, thin strands of very pure glass about the diameter of a human hair. They are arranged in bundles called optical cables and used to transmit light signals over long distances.
If you look closely at a single optical fiber, you will see that it has the following parts:
· Core - Thin glass center of the fiber where the light travels
· Cladding - Outer optical material surrounding the core that reflects the light back into the core
· Buffer coating - Plastic coating that protects the fiber from damage and moisture
Hundreds or thousands of these optical fibers are arranged in bundles in optical cables. The bundles are protected by the cable's outer covering, called a jacket.
Optical fibers come in two types:
· Single-mode fibers
· Multi-mode fibers
Single-mode fibers have small cores (about 3.5 x 10-4 inches or 9 microns in diameter) and transmit infrared laser light (wavelength = 1,300 to 1,550 nanometers). Multi-mode fibers have larger cores (about 2.5 x 10-3 inches or 62.5 microns in diameter) and transmit infrared light (wavelength = 850 to 1,300 nm) from light-emitting diodes (LEDs).
Some optical fibers can be made from plastic. These fibers have a large core (0.04 inches or 1 mm diameter) and transmit visible red light (wavelength = 650 nm) from LEDs.
How Does an Optical Fiber Transmit Light?
Suppose you want to shine a flashlight beam down a long, straight hallway. Just point the beam straight down the hallway -- light travels in straight lines, so it is no problem. What if the hallway has a bend in it? You could place a mirror at the bend to reflect the light beam around the corner. What if the hallway is very winding with multiple bends? You might line the walls with mirrors and angle the beam so that it bounces from side-to-side all along the hallway. This is exactly what happens in an optical fiber.
The light in a fiber-optic cable travels through the core (hallway) by constantly bouncing from the cladding (mirror-lined walls), a principle called total internal reflection. 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 within the fiber, mostly due to impurities in the glass. The extent that the signal degrades depends on the purity of the glass and the wavelength of the transmitted light (for example, 850 nm = 60 to 75 percent/km; 1,300 nm = 50 to 60 percent/km; 1,550 nm is greater than 50 percent/km). Some premium optical fibers show much less signal degradation -- less than 10 percent/km at 1,550 nm.

APPLICATION OF OPTICAL FIBERS/COMMUNICATIONS

Applications
Optical fiber communication
Optical fiber can be used as a medium for telecommunication and networking because it is flexible and can be bundled as cables. It is especially advantageous for long-distance communications, because light propagates through the fiber with little attenuation compared to electrical cables. This allows long distances to be spanned with few repeaters. Additionally, the per-channel light signals propagating in the fiber can be modulated at rates as high as 111 gigabits per second,although 10 or 40 Gb/s is typical in deployed systems.Each fiber can carry many independent channels, each using a different wavelength of light (wavelength-division multiplexing (WDM)). The net data rate (data rate without overhead bytes) per fiber is the per-channel data rate reduced by the FEC overhead, multiplied by the number of channels (usually up to eighty in commercial dense WDM systems as of 2008).
Over short distances, such as networking within a building, fiber saves space in cable ducts because a single fiber can carry much more data than a single electrical cable.[vague] Fiber is also immune to electrical interference; there is no cross-talk between signals in different cables and no pickup of environmental noise. Non-armored fiber cables do not conduct electricity, which makes fiber a good solution for protecting communications equipment located in high voltage environments such as power generation facilities, or metal communication structures prone to lightning strikes. They can also be used in environments where explosive fumes are present, without danger of ignition. Wiretapping is more difficult compared to electrical connections, and there are concentric dual core fibers that are said to be tap-proof.
Although fibers can be made out of transparent plastic, glass, or a combination of the two, the fibers used in long-distance telecommunications applications are always glass, because of the lower optical attenuation. Both multi-mode and single-mode fibers are used in communications, with multi-mode fiber used mostly for short distances, up to 550 m (600 yards), and single-mode fiber used for longer distance links. Because of the tighter tolerances required to couple light into and between single-mode fibers (core diameter about 10 micrometers), single-mode transmitters, receivers, amplifiers and other components are generally more expensive than multi-mode components.
Examples of applications are TOSLINK, Fiber distributed data interface, Synchronous optical networking
Fiber optic sensors
Fibers have many uses in remote sensing. In some applications, the sensor is itself an optical fiber. In other cases, fiber is used to connect a non-fiberoptic sensor to a measurement system. Depending on the application, fiber may be used because of its small size, or the fact that no electrical power is needed at the remote location, or because many sensors can be multiplexed along the length of a fiber by using different wavelengths of light 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 an optical time-domain reflectometer.
Optical fibers can be used as sensors to measure strain, temperature, pressure and other quantities by modifying a fiber so that the quantity to be measured modulates the intensity, phase, polarization, wavelength or 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 such fiber optic sensors is that they can, if required, provide distributed sensing over distances of up to one meter.
Extrinsic fiber optic sensors use an optical fiber cable, normally a multi-mode one, to transmit modulated light 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 inside aircraft jet engines by using a fiber to transmit radiation into a radiation pyrometer located outside the engine. Extrinsic sensors can also be used in the same way to measure the internal temperature of electrical transformers, where the extreme electromagnetic fields present make other measurement techniques impossible. Extrinsic sensors are used to measure vibration, rotation, displacement, velocity, acceleration, torque, and twisting.

Other uses of optical fibers
ST connectors on multi-mode fiber.
Optical fibers are connected to terminal equipment by optical fiber connectors. These connectors are usually of a standard type such as FC, SC, ST, LC, or MTRJ.
Optical fibers may be connected to each other by connectors or by splicing, that is, joining two fibers together to form a continuous optical waveguide. The generally accepted splicing method is arc fusion splicing, which melts the fiber ends together with an electric arc. For quicker fastening jobs, a "mechanical splice" is used.
Fusion splicing is done with a specialized instrument that typically operates as follows: The two cable ends are fastened inside a splice enclosure that will protect the splices, and the fiber ends are stripped of their protective polymer coating (as well as the more sturdy outer jacket, if present). The ends are cleaved (cut) with a precision cleaver to make them perpendicular, and are placed into special holders in the splicer. The splice is usually inspected via a magnified viewing screen to check the cleaves before and after the splice. The splicer uses small motors to align the end faces together, and emits a small spark between electrodes at the gap to burn off dust and moisture. Then the splicer generates a larger spark that raises the temperature above the melting point of the glass, fusing the ends together permanently. The location and energy of the spark is carefully controlled so that the molten core and cladding don't mix, and this minimizes optical loss. A splice loss estimate is measured by the splicer, by directing light through the cladding on one side and measuring the light leaking from the cladding on the other side. A splice loss under 0.1 dB is typical. The complexity of this process makes fiber splicing much more difficult than splicing copper wire.
Mechanical fiber splices are designed to be quicker and easier to install, but there is still the need for stripping, careful cleaning and precision cleaving. The fiber ends are aligned and held together by a precision-made sleeve, often using a clear index-matching gel that enhances the transmission of light across the joint. Such joints typically have higher optical loss and are less robust than fusion splices, especially if the gel is used. All splicing techniques involve the use of an enclosure into which the splice is placed for protection afterward.
Fibers are terminated in connectors so that the fiber end is held at the end face precisely and securely. A fiber-optic connector is basically a rigid cylindrical barrel surrounded by a sleeve that holds the barrel in its mating socket. The mating mechanism can be "push and click", "turn and latch" ("bayonet"), or screw-in (threaded). A typical connector is installed by preparing the fiber end and inserting it into the rear of the connector body. Quick-set adhesive is usually used so the fiber is held securely, and a strain relief is secured to the rear. Once the adhesive has set, the fiber's end is polished to a mirror finish. Various polish profiles are used, depending on the type of fiber and the application. For single-mode fiber, the fiber ends are typically polished with a slight curvature, such that when the connectors are mated the fibers touch only at their cores. This is known as a "physical contact" (PC) polish. The curved surface may be polished at an angle, to make an "angled physical contact" (APC) connection. Such connections have higher loss than PC connections, but greatly reduced back reflection, because light that reflects from the angled surface leaks out of the fiber core; the resulting loss in signal strength is known as gap loss. APC fiber ends have low back reflection even when disconnected.

Free-space coupling
It often becomes necessary to align an optical fiber with another optical fiber or an optical device such as a light-emitting diode, a laser diode, or an optoelectronic device such as a modulator. This can involve either carefully aligning the fiber and placing it in contact with the device to which it is to couple, or can use a lens to allow coupling over an air gap. In some cases the end of the fiber is polished into a curved form that is designed to allow it to act as a lens.
In a laboratory environment, the fiber end is usually aligned to the device or other fiber with a fiber launch system that uses a microscope objective lens to focus the light down to a fine point. A precision translation stage (micro-positioning table) is used to move the lens, fiber, or device to allow the coupling efficiency to be optimized.

Fiber fuse
At high optical intensities, above 2 megawatts per square centimeter, when a fiber is subjected to a shock or is otherwise suddenly damaged, a fiber fuse can occur. The reflection from the damage vaporizes the fiber immediately before the break, and this new defect remains reflective so that the damage propagates back toward the transmitter at 1–3 meters per second (4−11 km/h, 2–8 mph).The open fiber control system, which ensures laser eye safety in the event of a broken fiber, can also effectively halt propagation of the fiber fuse.In situations, such as undersea cables, where high power levels might be used without the need for open fiber control, a "fiber fuse" protection device at the transmitter can break the circuit to prevent any damage

HISTORY OF OPTICAL FIBER

History
Fiber optics, though used extensively in the modern world, is a fairly simple and old technology. Guiding of light by refraction, the principle that makes fiber optics possible, was first demonstrated by Daniel Colladon and Jacques Babinet in Paris in the early 1840s. John Tyndall included a demonstration of it in his public lectures in London a dozen years later.Tyndall also wrote about the property of total internal reflection in an introductory book about the nature of light in 1870: "When the light passes from air into water, the refracted ray is bent towards the perpendicular... When the ray passes from water to air it is bent from the perpendicular... If the angle which the ray in water encloses with the perpendicular to the surface be greater than 48 degrees, the ray will not quit the water at all: it will be totally reflected at the surface.... The angle which marks the limit where total reflexion begins is called the limiting angle of the medium. For water this angle is 48°27', for flint glass it is 38°41', while for diamond it is 23°42'."
Practical applications, such as close internal illumination during dentistry, appeared early in the twentieth century. Image transmission through tubes was demonstrated independently by the radio experimenter Clarence Hansell and the television pioneer John Logie Baird in the 1920s. The principle was first used for internal medical examinations by Heinrich Lamm in the following decade. In 1952, physicist Narinder Singh Kapany conducted experiments that led to the invention of optical fiber. Modern optical fibers, where the glass fiber is coated with a transparent cladding to offer a more suitable refractive index, appeared later in the decade. Development then focused on fiber bundles for image transmission. The first fiber optic semi-flexible gastroscope was patented by Basil Hirschowitz, C. Wilbur Peters, and Lawrence E. Curtiss, researchers at the University of Michigan, in 1956. In the process of developing the gastroscope, Curtiss produced the first glass-clad fibers; previous optical fibers had relied on air or impractical oils and waxes as the low-index cladding material. A variety of other image transmission applications soon followed.
Jun-ichi Nishizawa, a Japanese scientist at Tohoku University, was the first to propose the use of optical fibers for communications in 1963. Nishizawa invented other technologies that contributed to the development of optical fiber communications as well. Nishizawa invented the graded-index optical fiber in 1964 as a channel for transmitting light from semiconductor lasers over long distances with low loss.
In 1965, Charles K. Kao and George A. Hockham of the British company Standard Telephones and Cables (STC) were the first to promote the idea that the attenuation in optical fibers could be reduced below 20 decibels per kilometer (dB/km), allowing fibers to be a practical medium for communication. They proposed that the attenuation in fibers available at the time was caused by impurities, which could be removed, rather than fundamental physical effects such as scattering. The crucial attenuation level of 20 dB/km was first achieved in 1970, by researchers Robert D. Maurer, Donald Keck, Peter C. Schultz, and Frank Zimar working for American glass maker Corning Glass Works, now Corning Incorporated. They demonstrated a fiber with 17 dB/km attenuation by doping silica glass with titanium. A few years later they produced a fiber with only 4 dB/km attenuation using germanium dioxide as the core dopant. Such low attenuations ushered in optical fiber telecommunications and enabled the Internet. In 1981, General Electric produced fused quartz ingots that could be drawn into fiber optic strands 25 miles (40 km) long.
Attenuations in modern optical cables are far less than those in electrical copper cables, leading to long-haul fiber connections with repeater distances of 70–150 kilometres (43–93 mi). The erbium-doped fiber amplifier, which reduced the cost of long-distance fiber systems by reducing or even in many cases eliminating the need for optical-electrical-optical repeaters, was co-developed by teams led by David N. Payne of the University of Southampton, and Emmanuel Desurvire at Bell Laboratories in 1986. The more robust optical fiber commonly used today utilizes glass for both core and sheath and is therefore less prone to aging processes. It was invented by Gerhard Bernsee in 1973 of Schott Glass in Germany.
In 1991, the emerging field of photonic crystals led to the development of photonic-crystal fiberHYPERLINK \l "cite_note-9" which guides light by means of diffraction from a periodic structure, rather than total internal reflection. The first photonic crystal fibers became commercially available in 2000. Photonic crystal fibers can be designed to carry higher power than conventional fiber, and their wavelength dependent properties can be manipulated to improve their performance in certain applications.

OPTICAL FIBER

Optical fiber
An optical fiber (or fiber) is a glass or plastic fiber that carries light along its length. Fiber optics is the overlap of applied science and engineering concerned with the design and application of optical fibers. Optical fibers are widely used in fiber-optic communications, which permits transmission over longer distances and at higher bandwidths (data rates) than other forms of communications. Fibers are used instead of metal wires because signals travel along them with less loss, and they are also immune to electromagnetic interference. Fibers are also used for illumination, and are wrapped in bundles so they can be used to carry images, thus allowing viewing in tight spaces. Specially designed fibers are used for a variety of other applications, including sensors and fiber lasers.
Light is kept in the core of the optical fiber by total internal reflection. This causes the fiber to act as a waveguide. Fibers which support many propagation paths or transverse modes are called multi-mode fibers (MMF), while those which can only support a single mode are called single-mode fibers (SMF). Multi-mode fibers generally have a larger core diameter, and are used for short-distance communication links and for applications where high power must be transmitted. Single-mode fibers are used for most communication links longer than 550 meters (1,800 ft).
Joining lengths of optical fiber is more complex than joining electrical wire or cable. The ends of the fibers must be carefully cleaved, and then spliced together either mechanically or by fusing them together with an electric arc. Special connectors are used to make removable connections.