Fibre Optics

Key success factors of Fibre Optic Valley

2009-05-21T21:54:00.000-07:00

The five key success factors of Fibre Optic Valley can be summarised as follows:

access to real enthusiasts or driving forces;

access to sufficient financing;

capacity to formulate clear goals;

critical mass of people, partners and projects in order to get a large enough pool of spent time and knowledge;

the possibility of getting people to work together towards a common vision.

Fibre Optic Valley has found it beneficial to adopt a double-head managment team: ensuring that one person does not have all the responsibility. This makes the leader(s) less vulnerable, and less exposed. It has given the cluster organisation flexibility "when storms were blowing". The combination of two people at the top has been useful and although there might be an increased risk for potential disagreements, the project participants found that the benefits of "learning from disagreeing" outweigh the negative aspects.

Another lesson learnt is that it is vital to be aware of the role an organisation such as Fibre Optic Valley can really play in supporting innovation. It is important to identify the key factors to ensure success of the relationships within the network. There is also a need for developing strong relationships between the board, which should work politically, and the actual activities within the organisation.

It is also essential to recruit the right people: people who are communicative, salespersons, who are not overly obsessed with administration, who are open to warning signals and are capable of acting upon them. It is also important to create an organisational culture, so that the team can change, without this affecting the outcome. Indeed, as the project manager said, “this is all about people”, “ We need to work strategically and view ourselves as the lubricating agent in the ongoing process”. This may be one of the vital factors of FOV success – a management that perceives needs within the innovation field and can facilitate the interchange between knowledge and the market.

SUSTAINABILITY AND TRANSFERABILITY of FIbre Optic Valley

2009-05-21T21:51:00.000-07:00

Sustainability

The Fibre Optic Valley project was still underway at the time of the case study. The legal structure is as described in the previous section, going through a reinforcement phase, broadening the competence of the members of the board, focusing on the three fields and further developing the innovative culture. The creation of the private company linked to the existing non-profit association is a sign of the commitment to the future of the project partners.

Transferability

The transferable implementation practices and innovative results presented below are the lessons learned from this project, on a generalised level. The features that made the Fibre Optic Valley approach a success and which are useful pointers for other regions launching similar projects, include:

finding an area of competence in which you have regional know-how in combination with a capacity to further develop and become more innovative within the field;

finding partners within academia, the local authorities and trade and business representatives (triple helix), and finding common and dynamic means for driving this development;

finding support among groups outside of the inner core of the project in order to succeed with the ambition and spread information on accomplishments on a broad basis;

set up measurable goals and milestones, both quantified and ‘softer’ goals.

What makes Fibre Optic Valley innovative may not be the way the organisation was set up, or the way innovations are developed. What makes Fibre Optic Valley innovative is the way in which the municipality, the university and the regional trade and industry came together and managed to adapt to a negative trend through turning ‘discarded' resources from into levers of development for a new innovative environment.

Fibre Optic Valley

2009-05-21T21:45:00.000-07:00

Fibre Optic Valley is a cluster initiative that has the ambition to become Europe’s leading area in this field of technology, thereby creating thousands of jobs by 2015. The goal of the members of the cluster is to create Europe’s most favourable development climate for new companies in the field of Fibre Optics: through creating an arena of competence and knowledge within this field. It is expected that this will attract financiers, entrepreneurs and innovators to the region.

Activities within the framework of Fibre Optic Valley aim at supporting regional growth through developing know-how and innovation in the digital communication sector. This includes offering support to business development from an initial idea to a final product.

The cluster focuses on the following three areas:

fibre to the users (FTTX): constructing the “true broadband society”;

e-services: as fibres are installed in all homes, which (capacity demanding) services are in demand?

fibre optic industry based application: e.g. fire alarms, information systems in tunnels, transporting of welding flames, and other new techniques where interesting innovations are made.

Fibre Optic Valley works to create an innovative climate in these three technology areas by co-ordinating and mobilising resources in order to offer a test bed with contracted test pilots, qualified evaluators, research, training, business models, behavioural analysis, statistical models and an advanced fibre laboratory. The cluster is based on the philosophy that the activities and ventures supported must create an added value for the co-financiers and partners. It is the business perspective that sets objectives since it is the companies who bring innovations to customers and where the new jobs are created.

Since 2000, Fibre Optic Valley has been developed through a combination of six interlinked projects support a laboratory, research and development facilities and services, a test bed, users studies, R&D driven business development, and marketing and information support. The three focus areas are the driving force of the valley with each following specific development steps and processes. Project participants underlined that the experiences gained and actions taken within one project have been the basis for the next, so that knowledge has been built up over the entire chain of projects.

The direct beneficiaries are the members of Fibre Optic Valley, who take part in projects and benefit from the added value created through the user-driven research initiated within fibre optics. Additionally, the local higher education institutes (Mid Sweden University and the Högskolan i Gävle, Gävle University6) and local research institutes have been given a better framework in which to pursue contract research in the fibre optics fields.

Indirectly the valley creates new job opportunities for the region's inhabitants in the region and increased income and revenues for the municipalities. Other small enterprises who are not members of the cluster also benefit from being in a region at the forefront of the development of new services based on fibre optic broadband. Even Ericsson7 and Iggesund8, the two largest international companies with production in the region, benefit from the project, since the actions within Fibre Optic Valley offer a more 'open innovation' environment in the region, sustaining a broader approach to new product development.

A number of striking results of Fibre Optic Valley can be highlighted:

fibre optic broadband has been installed and is accessible to actors in the region;

a research institute, Acreo, has been attracted to the area;

the construction of an apartment building fully equipped with fibre optic systems where inhabitants’ usage can function as a test environment for e-solutions;

a complete test-bed and fibre lab for worldwide use has been created.

The 'test-bed' is an open network of 300 technology testers, all with 100 Mbps triple play (Internet, telephone, TV) access into the home. The test bed also has a wider network of 1,500 testers which evaluate e-services. Here, systems developers, network operators, service developers and researchers can test new products and services, such as IP distributed HDTV, in an environment that combines technology tests with behavioural studies. Some 15 international and national systems suppliers are present in the test bed, which is the official testing ground for three EU broadband research projects (MUSE9, NOBEL10 and MUPBED11). Acreo is present in all three projects.

According to project reports, by-mid 2007, Fibre Optic Valley had indirectly supported the development of 230 new jobs in the region, and 500 in all of Sweden. Seven new enterprises in the field of fibre optics have been created in the region. Moreover, 19 doctoral students have graduated thanks to funding or studies initiated by Fibre Optic Valley. In the longer run, the expected impact of this project is to have added about 2000 new jobs in the region by 2015. Additional growth is to be created within the three focus areas: fibre to the user; e-services; fibre optic industry applications.

The European Regional Development Fund (ERDF) has been a very important financial contributor to the pipeline of project funding in favour Fibre Optic Valley. The ERDF cofinancing,
together with the long-term regional and national financing, provided a foundation for the cluster and enabled it to build capacity to launch new activities. In particular, the possibility to work on the three focus areas in parallel, fostering important synergies and cross-cutting activities, would not have been possible without the additional financial means provided by the ERDF.

ADVANTAGES OF FIBER-OPTICS

2009-05-21T21:35:00.000-07:00

In recent years there has been a significant research effort in the area of high-speed electronics for communication. Higher speeds are required in order to take full advantage of the broadband capabilities of optical fibers. In particular integrated solutions are sought for practical systems to reduce cost and improve reliability. One of the target bit-rates for integrated fiber optic receivers is 10 Gb/s, which is consistent with the SONET hierarchical specification; practical transmission systems at these extremely high data rates will open the way to unexplored territory in networking. Each of these systems will require high-speed, low-cost interface electronics.

Currently, the bandwidth of optical fiber (1400 GHz-km for 1.3 single-mode fibers) and low losses (0.15 dB/km) can not be fully exploited. A bottleneck in system throughput exists due to speed limitations of the electronics in the receiver and transmitter. This bottleneck can be circumvented by optically multiplexing several lower data-rate channels through a single fiber. Both a 9.6 Gb/s wavelength-division multiplexing (WDM) system, and a 20 Gb/s time-division multiplexing (TDM) system, have been demonstrated in laboratory experiments. These systems are capable of handling enormous data rates, because all of the high-speed processing, including amplification, can be done optically. These systems, however, are quite expensive and complicated.

Types of fiber optic gyros

2009-05-09T00:46:00.000-07:00

Two types of fiber optic gyros are being developed.

The first type is an open-loop fiber optic gyro with a dynamic range on the order of 1000 to 5000 (dynamic range is unitless), with a scale factor accuracy of about 0.5% (this accuracy number includes nonlinearity and hysterisis effects) and sensitivities that vary from less than 0.01 Degree =hr to 100 Degree =hr and higher. These fiber gyros are generally used for low-cost applications where dynamic range and linearity are not the crucial issues.

The second type is the closed-loop fiber optic gyro that may have a dynamic range of 106 and scale factor linearity of 10 ppm or better. These types of fiber optic gyros are primarily targeted at medium- to high-accuracy navigation applications that have high turning rates and require high linearity and large dynamic ranges.

The Sagnac Interferometer - Fibre optic gyro

2009-05-09T00:43:00.000-07:00

The Sagnac interferometer has been principally used to measure rotation and is a replacement for ring laser gyros and mechanical gyros. It may also be employed to measure time-varying effects such as acoustics, vibration, and slowly varying phenomena such as strain. By using multiple interferometer configurations, it is possible to employ the Sagnac interferometer as a distributed sensor capable of measuring the amplitute and location of a disturbance.

The single most important application of fiber optic sensors in terms of commercial value is the fiber optic gyro. It was recognized very early that the fiber optic gyro offered the prospect of an all solid-state inertial sensor with no moving parts, unprecedented reliability, and a potential of very low cost.

The potential of the fiber optic gyro is being realized as several manufacturers worldwide are producing them in large quantities to support automobile navigation systems, pointing and tracking of satellite antennas, inertial measurement systems for commuter aircraft and missiles, and as the backup guidance system for the Boeing 777. They are also being baselined for such future programs as the Comanche helicopter and are being developed to support long-duration space flights.

Other applications using fiber optic gyros include mining operations, tunneling, attitude control for a radio-controlled helicopter, cleaning robots, antenna pointing and tracking, and guidance for unmanned trucks and carriers.

The fiber etalons

2009-05-09T00:28:00.000-07:00

The fiber etalons in Fig. 1 can also be used as sensors for measuring strain, as the distance between mirrors in the fiber determines their transmission characteristics. The mirrors can be fabricated directly into the fiber by cleaving the fiber, coating the end with titanium dioxide, and then resplicing. An alternative approach is to cleave the fiber ends and insert them into a capillary tube with an air gap. Both of these approaches are being investigated for applications where multiple in-line fiber sensors are required.

Fig. 1. Intrinsic fiber etalons are formed by in-line reflective mirrors that can be embedded into the optical fiber. Extrinsic fiber etalons are formed by two mirrored fiber ends in a capillary tube. A fiber etalon-based spectral filter or demodulator is formed by two reflective fiber ends that have a variable spacing.

For many applications a single point sensor is adequate. In these situations an etalon can be fabricated independently and attached to the end of the fiber. Fig 2 shows a series of etalons that have been configured to measure pressure, temperature, and refractive index, respectively.

In the case of pressure, the diaphragm has been designed to deflect. Pressure ranges of 15 to 2000 psi can be accommodated by changing the diaphragm thickness with an accuracy of about 0.1% full scale. For temperature the etalon has been formed by silicon–silicon dioxide interfaces. Temperature ranges of 70 Degree to 500 Degree Kcan be selected, and for a range of about 100 Degree K a resolution of about 0.1 Degree K is achievable. For refractive index of liquids, a hole has been formed to allow the flow of the liquid to be measured without the diaphragm deflecting. These devices have been commercialized and are sold with instrument packages.

Fig 2. Hybrid etalon-based fiber optic sensors often consist of micromachined cavities that are placed on the end of optical fibers and can be configured so that sensitivity to one environmental effect is optimized.

The transmission characteristics of a fiber etalon as a function of finesse,
which increases with mirror reflectivity.

Optical fiber, wire glass of the World Wide Web

2009-05-04T10:39:00.000-07:00

Optical fiber, a technology used since the 1970s, has greatly contributed to the explosion of telecommunications at the global level, allowing communications at long distance and at speeds previously not possible. In the field of telephony and Internet, the "spider web" world woven wire glass.

An optical fiber is a wire glass or thin plastic that allows light to drive over very large distances (several hundreds or thousands of kilometers).

This technology is used in oceanic and terrestrial transmissions of data, because the light signal transits it is capable of transmitting large amounts of information.

Fiber optics, which offer a flow of information significantly higher than that of coaxial cables and support a "broadband", have multiple applications.
They can transmit both television and telephone as video or computer data.

First optical fiber can be used in telecommunications networks were built in 1970 in the laboratories of the U.S. Company Corning Glass Works by Robert Maurer, Peter Schultz and Donald Keck.

Description

In a schematic way, an optical fiber is composed of the following:

A thread of very fine glass (a few microns in diameter) and in one piece, called "the heart"

A sheath that surrounds and traps the light in the heart, reflecting multiple times, almost without loss

A protective sheath that covers several tens to several hundreds of optical fibers

A very specific connection (which does not prevent the transmission of light).

In the field of optical telecommunications, material is generally very pure silica (which produces very low optical losses) even if they are also plastic fibers.

Principle of operation

The optical fiber is a waveguide that uses REFRACTIVE properties of light.

The electrical signal to be transmitted is first converted into a light signal through a transceiver that uses an LED (light emitting diode) or a laser to produce light.

When a light beam enters one end of the fiber according to a proper angle, it undergoes multiple reflections and spreads to the other end following a zigzag course.

The light signal is then converted into an electrical signal through a detector, usually a photodiode.

When fiber is not yet fed, it is called dark fiber.

Applications

Endoscopy, to look inside the human body, or welds in aircraft engines, was the first application of fiber optics in the early 1950s with the invention of flexible fiberscopes by van Heel and Hopkins.

The invention of the laser in 1960, then allowed to transmit a light signal with enough power over a great distance.
Early attempts telecommunications optical fiber followed by 1966.

Today, more than 80% of long distance communications in the world are transported through a network of optical fiber cables of more than 25 million kilometers.

This technology is also used in the field of sensors (temperature, pressure, etc.) and imaging.

The next application of fiber optics will be the FTTH (Fiber To The Home), a system of very high-speed communication through an optical fiber network at up to the customer.
This technology should soon compete with xDSL technologies and gradually replace ADSL in our homes.

Benefits

The advantages of data transmission by fiber optics are numerous:

Loss of signal over a large distance is much lower compared to electric transmission in a metallic conductor

This technology offers transmission speeds high

Optical fibers are not sensitive to external interference (near a high voltage cable, for example)

They do not produce heat (unlike copper).

Fiber grating sensors

2009-04-30T08:27:00.000-07:00

Fluorescent-based fiber sensors is fabricated by ‘‘writing’’ a fiber grating into the core of a germanium-doped optical fiber. This can be done in a number of ways. One method, illustrated by the fig in the previous post, uses two short-wavelength laser beams that are angled to form an interference pattern through the side of the optical fiber. The interference pattern consists of bright and dark bands that represent local changes in the index of refraction in the core region of the fiber. Exposure time for making these gratings varies from minutes to hours, depending on the dopant concentration in the fiber, the wavelengths used, the optical power level, and the imaging optics.

Other methods that have been used include the use of phase masks as well as interference patterns induced by short, high-energy laser pulses. The short duration pulses have the potential to be used to write fiber gratings into the fiber as it is being drawn. Substantial efforts are being made by laboratories around the world to improve the manufacturability of fiber gratings because they have the potential to be used to support optical communication as well as sensing technology. Once the fiber grating has been fabricated, the next major issue is how to extract information.When used as a strain sensor, the fiber grating is typically attached to, or embedded in, a structure. As the fiber grating is expanded or compressed, the grating period expands or contracts, changing the grating’s spectral response.

For a grating operating at 1300 nm, the change in wavelength is about 10^-3 nm per microstrain. This type of resolution requires the use of spectral demodulation techniques that are much better than those associated with conventional spectrometers. Several demodulation methods have been suggested using fiber gratings, etalons, and interferometers. Figure below illustrates a system that uses a reference fiber grating. The reference fiber grating acts as a modulator filter. By using similar gratings for the reference and signal gratings and adjusting the reference grating to line up with the active grating, one may implement an accurate closed-loop demodulation system. An alternative demodulation system would use fiber etalons. One fiber can be mounted on a piezoelectric and the other moved relative to a second fiber end. The spacing of the fiber ends as well as their reflectivity in turn determine the spectral filtering action of the fiber etalon.

Figure: Fabrication of a fiber grating sensor can be accomplished by imaging to short-wavelength laser beams through the side of the optical fiber to form an interference pattern. The bright and dark fringes imaged on the core of the optical fiber induce an index of refraction variation resulting in a grating along the fiber core.

Fluorescent-based fiber sensors

2009-04-30T08:19:00.000-07:00

Fluorescent-based fiber sensors are widely used for medical applications and chemical sensing and can also be used for physical parameter measurements such as temperature, viscosity, and humidity. There are a number of configurations for these sensors, Fig. 20 illustrates two of the most common. In the case of the end-tip sensor, light propagates down the fiber to a probe of fluorescent material. The resultant fluorescent signal is captured by the same fiber and directed back to an output demodulator. The light sources can be pulsed, and probes have been made that depend on the time rate of decay of the light pulse.

Figure: Fluorescent fiber optic sensor probe configurations can be used to support the measurement of physical parameters as well as the presence or absence of chemical species. These probes may be configured to be single-ended or multipoint by using side etch techniques and attaching the fluorescent material to the fiber.

In the continuous mode, parameters such as viscosity, water vapor content, and degree of cure in carbon fiber reinforced epoxy and thermoplastic composite materials can be monitored. An alternative is to use the evanescent properties of the fiber and etch regions of the cladding away and refill them with fluorescent material. By sending a light pulse down the fiber and looking at the resulting fluorescence, a series of sensing regions may be time division multiplexed. It is also possible to introduce fluorescent dopants into the optical fiber itself. This approach causes the entire optically activated fiber to fluoresce. By using time division multiplexing, various regions of the fiber can be used to make a distributed measurement along the fiber length. In many cases users of fiber sensors would like to have the fiber optic analog of conventional electronic sensors. An example is the electrical strain gauge widely used by structural engineers. Fiber grating sensors can be configured to have gauge lengths from 1 mm to approximately 1 cm, with sensitivity comparable to conventional strain gauges.

Spectrally based temperature sensor

2009-04-27T05:17:00.000-07:00

Another type of spectrally based temperature sensor, shown in Fig below, is based on absorption. In this case a gallium arsenide (GaAs) sensor probe is used in combination with a broadband light source and input/output optical fibers. The absorption profile of the probe is temperature-dependent and may be used to determine temperature.

Fiber optic sensor based on variable absorption of materials such as GaAs allow the measurement of temperature and pressure.

Spectrally based fibre optic sensor

2009-04-27T04:30:00.000-07:00

Spectrally based fiber optic sensors depend on a light beam modulated in wavelength by an environmental effect. Examples of these types of fiber sensors include those based on blackbody radiation, absorption, fluorescence, etalons, and dispersive gratings.

Blackbody fiber optic sensors allow the measurement of temperature at
a hot spot and are most effective at temperatures of higher than 300 Degree C.

One of the simplest of these sensor types is the backbody sensor. A blackbody cavity is placed at the end of an optical fiber. When the cavity rises in temperature, it starts to glow and act as a light source. Detectors in combination with narrow band filters are then used to determine the profile of the blackbody curve and, in turn, the temperature, as in Fig. 2. This type of sensor has been successfully commercialized and used to measure temperature to within a few degrees C under intenseRF fields. The performance and accuracy of this sensor are better at higher temperatures and fall off at temperatures on the order of 200 Degree C because of low signal-to-noise ratios. Care must be taken to ensure that the hottest spot is the blackbody cavity and not on the optical fiber lead itself, as this can corrupt the integrity of the signal.

Blackbody radiation curves provide unique signatures for each temperature.

A linear position sensor

2009-04-27T04:19:00.000-07:00

An example of a linear position sensor using wavelength division multiplexing is illustrated in the image below. Here a broadband light source, which might be a light-emitting diode, is used to couple light into the system. A single optical fiber is used to carry the light beam up to a wavelength division multiplexing (WDM) element that splits the light into separate fibers that are used to interrogate the encoder card and determine linear position. The boxes on the card represent highly reflective patches, while the rest of the card has low reflectance. The reflected signals are then recombined and separated by a second wavelength division multiplexing element so that each interrogating fiber signal is read out by a separate detector.

Fig 1. A linear position sensor using wavelength division multiplexing decodes position by measuring the presence or absence of a reflective patch at each fiber position as the card slides by via independent wavelength separated detectors.

Fig 2. A linear position sensor using time division multiplexing measure decodes card position via a digital stream of ons and offs dictated by the presence or absence of a reflective patch.

A second common method of interrogating a position sensor using a single optical fiber is to use time division multiplexing methods. In Fig. 2 a light source is pulsed. The light pulse then propagates down the optical fiber and is split into multiple interrogating fibers. Each of these fibers is arranged so that the fibers have delay lines that separate the return signal from the encoder plate by a time that is longer than the pulse duration. When the returned signals are recombined onto the detector, the net result is an encoded signal burst corresponding to the position of the encoded card.

These sensors have been used to support tests on military and commercial aircraft that have demonstrated performance comparable to conventional electrical position sensors used for rudder, flap, and throttle position. The principal advantages of the fiber position sensors are
immunity to electromagnetic interference and overall weight savings.

Fiber optic translation sensor

2009-04-27T03:25:00.000-07:00

With two optical fibers arranged in a line, a simple translation sensor can be configured as in the image below. The output from the two detectors can be proportioned to determine the translational position of the input fiber.

Several companies have developed rotary and linear fiber optic position sensors to support applications such as fly-by-light.

These sensors attempt

(1) to eliminate electromagnetic interference susceptibility to improve safetyand
(2) to lower shielding needs to reduce weight

Closure and vibration fiber optic sensors

2009-04-27T03:17:00.000-07:00

In some respects the simplest type of fiber optic sensor is the hybrid type that is based on intensity modulation. Figure 3 shows a simple closure or vibration sensor that consists of two optical fibers held in close proximity to each other. Light is injected into one of the optical fibers; when it exits, the light expands into a cone of light whose angle depends on the difference between the index of refraction of the core and cladding of the optical fiber. The amount of light captured by the second optical fiber depends on its acceptance angle and the distance d between the optical fibers. When the distance d is modulated, it in turn results in an intensity modulation of the light captured.

Closure and vibration fiber optic sensors based on numerical aperture can be used to support door closure indicators and measure levels of vibration in machinery.

A variation on this type of sensor is shown in the image below Here a mirror is used that is flexibly mounted to respond to an external effect such as pressure. As the mirror position shifts, the effective separation between the optical fibers shift with a resultant intensity modulation. These types of sensors are useful for such applications as door closures where a reflective strip, in combination with an optical fiber acting to input and catch the output reflected light, can be used.

Intrinsic (all-fiber) fiber optic sensors

2009-04-27T03:13:00.000-07:00

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.

In this case an optical fiber leads up to a ‘‘black box’’ that impresses information onto the light beam in response to an environmental effect. The information could be impressed in terms of intensity, phase, frequency, polarization, spectral content, or other methods. An optical fiber then carries the light with the environmentally impressed information back to an optical and=or electronic processor. In some cases the input optical fiber also acts as the output fiber. The intrinsic or all-fiber sensor shown in image above uses an optical fiber to carry the light beam, and the environmental effect impresses information onto the light beam while it is in the fiber. Each of these classes of fibers in turn has many subclasses with, in some cases, sub-subclasses that consist of large numbers of fiber sensors.

Extrinsic (hybrid) fiber optic sensors

2009-04-27T03:04:00.000-07:00

Fiber optic sensors are often loosely grouped into two basic classes referred to as extrinsic, or hybrid, fiber optic sensors and intrinsic, or all-fiber, sensors. The below image illustrates the case of an extrinsic, or hybrid, fiber optic sensor.

Extrinsic fiber optic sensors consist of optical fibers that lead up to and out of a ‘‘black box’’ that modulates the light beam passing through it in response to an environmental effect.

Fiber Optic Sensors

2009-04-27T02:56:00.000-07:00

Overview of Fiber Optic Sensors

Over the past 20 years two major product revolutions have taken place due to the growth of the optoelectronics and fiber optic communications industries. The optoelectronics industry has brought about such products as compact disc players, laser printers, bar code scanners, and laser pointers. The fiber optic communications industry has literally revolutionized the telecommunications industry by providing higher-performance, more reliable telecommunication links with ever-decreasing bandwidth cost. This revolution is bringing about the benefits of high-volume production to component users and a true information superhighway built of glass. In parallel with these developments, fiber optic sensor technology has been a major user of technology associated with the optoelectronic and fiber optic communications industry. Many of the components associated with these industries were often developed for fiber optic sensor applications.

Fiber optic sensor technology, in turn, has often been driven by the development and subsequent mass production of components to support these industries. As component prices have fallen and quality improvements have been made, the ability of fiber optic sensors to displace traditional sensors for rotation, acceleration, electric and magnetic field measurement, temperature, pressure, acoustics, vibration, linear and angular position, strain, humidity, viscosity, chemical measurements, and a host of other sensor applications has been enhanced. In the early days of fiber optic sensor technology, most commercially successful fiber optic sensors were squarely targeted at markets where existing sensor technology was marginal or in many cases nonexistent. The inherent advantages of fiber optic sensors, which include

1. their ability to be lightweight, of very small size, passive, low-power, resistant to electromagnetic interference,

2. their high sensitivity,

3. their bandwidth, and

4. their environmental ruggedness, were heavily used to offset their major disadvantages of high cost and end-user unfamiliarity.

The situation is changing. Laser diodes that cost $3000 in 1979 with lifetimes measured in hours now sell for a few dollars in small quantities, have reliability of tens of thousands of hours, and are widely used in compact disc players, laser printers, laser pointers, and bar code readers. Single-mode optical fiber that cost $20=m in 1979 now costs less than $0.10=m, with vastly improved optical and mechanical properties. Integrated optical devices that were not available in usable form at that time are now commonly used to support production models of fiber optic gyros. Also, they could drop in price dramatically in the future while offering ever more sophisticated optical circuits. As these trends continue, the opportunities for fiber optic sensor designers to product competitive products will increase and the technology can be expected to assume an ever more prominent position in the sensor marketplace. In the following sections the basic types of fiber optic sensors being developed are briefly reviewed followed by a discussion of how these sensors are and will be applied.

Design Of A Low-Cost Distributed Strain Sensor Using Optical Fibers With Radiative Taps

2009-04-11T19:49:00.000-07:00

Using an analytical model for power distribution from a series of radiative taps on multimode optical fibers we show how to design a low-cost distributed strain sensor.

It is possible to fabricate radiative taps on optical fibers by using a variety of methods such as

laser micro machining,
chemical etching,
macro bending and polishing
cutting and polishing, and
making radiative Bragg gratings, etc.

A cut or indentation is made by one of these fabrication methods on the cladding of an optical fiber and a fraction of the evanescent optical field in the cladding and the cladding modes are radiated out from this cut. Thus we can sample a small portion of the optical fields inside the fiber from such a tap. Radiative taps have several uses in making fiber optic components, sensors, tapped delay lines, signal processors, and add-drop multiplexers. A series of such taps distributed along an optical fiber can be used to build devices and subsystems such as filters, equalizers long period Bragg gratings , distributed fiber optic sensors , or spectrum analyzers.

The distributed sensor we discuss is a low-cost intensity modulated sensor consisting of a number of identical taps on a large core multimode fiber. A fraction of the light coupled to the fiber is radiated out from each tap and detected by a photodetector. The output from each photodetector are monitored by a central processor. When stress or pressure is applied a few taps in the vicinity are strained. The optical power radiated by those taps changes as a result and the central processor determines the magnitude of the strain from the change. Thus unlike 1 more sophisticated sensors [12, 13] our distributed strain sensor requires neither an optical-time-domain-reflectometry equipment nor the measurement of Brillouin scattered fields.

Cutting Corners in Fibre Optics

2009-04-01T02:03:00.000-07:00

During the last two decades of the twentieth century, single-mode optical fibres rapidly became the back-bone of the world’s vast telecommunications network both on land and under the oceans because of their ability to propagate large volumes of digitised data over vast distances with minimal light loss and signal distortion. These fibres are normally encased in optical cables to protect them from environmental hazards, but these cables, such as the one shown in the picture, are flexible enough to be bent into small radii of some tens of centimetres that are encountered when they are laid in ducting under city streets and elsewhere. The light propagating in these fibres readily follows these bends with essentially no propagation loss.

Fibre core and protective layers of a typical telecommunications optical fibre

However, as optical fibre and especially optical waveguide, technology finds its way into ever more diverse areas such as medicine, remote sensing and aerospace, small radius bends become an inevitability. These tight bends are especially desirable in integrated optics planar waveguides, because they reduce the overall device size. The problem is that such bends make it difficult to confine the the light within the waveguide. Conventional theoretical understanding of this phenomena has been unable to offer any practical solutions. However, ANU researchers have recently developed a radically new model of bend loss which more strongly relates to the actual physics of the waveguide. This new approach examines the physical evolution of the waveguide mode into, along and out of the bend, and takes into account the finite structure of the cross section of a practical single-mode waveguide. Using this model, scientists believe it may be possible to engineer waveguide refractive index profiles to greatly improve light transmission in tight bends.

Light traveling down an optical fibre is confined by total internal reflection. Each time the beam hits the wall the difference in refractive index at the boundary acts like a mirror bouncing the light back. However, because the reflectivity depends on the angle of incidence, sharp bends in the fibre tend to create poor mirrors leading to high light loss.

Comparison with electrical transmission

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The choice between optical fiber and electrical (or copper) transmission for a particular system is made based on a number of trade-offs. Optical fiber is generally chosen for systems requiring higher bandwidth or spanning longer distances than electrical cabling can accommodate. The main benefits of fiber are its exceptionally low loss, allowing long distances between amplifiers or repeaters; and its inherently high data-carrying capacity, such that thousands of electrical links would be required to replace a single high bandwidth fiber cable. Another benefit of fibers is that even when run alongside each other for long distances, fiber cables experience effectively no crosstalk, in contrast to some types of electrical transmission lines. Fiber can be installed in areas with high electromagnetic interference (EMI),(along the sides of utility lines, power-carrying lines, and railroad tracks). All-dielectric cables are also ideal for areas of high lightning-strike incidence.

For comparison, while single-line, voice-grade copper systems longer than a couple of kilometers require in-line signal repeaters for satisfactory performance; it is not unusual for optical systems to go over 100 kilometers (60 miles), with no active or passive processing. Single-mode fiber cables are commonly available in 12 km lengths, minimizing the number of splices required over a long cable run. Multi-mode fiber is available in lengths up to 4 km, although industrial standards only mandate 2 km unbroken runs.

In short distance and relatively low bandwidth applications, electrical transmission is often preferred because of its

Lower material cost, where large quantities are not required
Lower cost of transmitters and receivers

Capability to carry electrical power as well as signals (in specially-designed cables)
Ease of operating transducers in linear mode.

Optical Fibers are more difficult and expensive to splice.
At higher optical powers, Optical Fibers are susceptible to fiber fuse wherein a bit too much light meeting with an imperfection can destroy several meters per second. The installation of fiber fuse detection circuity at the transmitter can break the circuit and halt the failure to minimize damage.

Because of these benefits of electrical transmission, optical communication is not common in short box-to-box, backplane, or chip-to-chip applications; however, optical systems on those scales have been demonstrated in the laboratory.

In certain situations fiber may be used even for short distance or low bandwidth applications, due to other important features:

Immunity to electromagnetic interference, including nuclear electromagnetic pulses (although fiber can be damaged by alpha and beta radiation).
High electrical resistance, making it safe to use near high-voltage equipment or between areas with different earth potentials.
Lighter weight—important, for example, in aircraft.
No sparks—important in flammable or explosive gas environments.
Not electromagnetically radiating, and difficult to tap without disrupting the signal—important in high-security environments.
Much smaller cable size—important where pathway is limited, such as networking an existing building, where smaller channels can be drilled and space can be saved in existing cable ducts and trays.

Optical fiber cables can be installed in buildings with the same equipment that is used to install copper and coaxial cables, with some modifications due to the small size and limited pull tension and bend radius of optical cables. Optical cables can typically be installed in duct systems in spans of 6000 meters or more depending on the duct's condition, layout of the duct system, and installation technique. Longer cables can be coiled at an intermediate point and pulled farther into the duct system as necessary.

Dispersion

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For modern glass optical fiber, the maximum transmission distance is limited not by attenuation but by dispersion, or spreading of optical pulses as they travel along the fiber. Dispersion in optical fibers is caused by a variety of factors. Intermodal dispersion, caused by the different axial speeds of different transverse modes, limits the performance of multi-mode fiber. Because single-mode fiber supports only one transverse mode, intermodal dispersion is eliminated.

In single-mode fiber performance is primarily limited by chromatic dispersion (also called group velocity dispersion), which occurs because the index of the glass varies slightly depending on the wavelength of the light, and light from real optical transmitters necessarily has nonzero spectral width (due to modulation). Polarization mode dispersion, another source of limitation, occurs because although the single-mode fiber can sustain only one transverse mode, it can carry this mode with two different polarizations, and slight imperfections or distortions in a fiber can alter the propagation velocities for the two polarizations. This phenomenon is called fiber birefringence and can be counteracted by polarization-maintaining optical fiber. Dispersion limits the bandwidth of the fiber because the spreading optical pulse limits the rate that pulses can follow one another on the fiber and still be distinguishable at the receiver.

Some dispersion, notably chromatic dispersion, can be removed by a 'dispersion compensator'. This works by using a specially prepared length of fiber that has the opposite dispersion to that induced by the transmission fiber, and this sharpens the pulse so that it can be correctly decoded by the electronics.

Transmitters

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The most commonly-used optical transmitters are semiconductor devices such as light-emitting diodes (LEDs) and laser diodes. The difference between LEDs and laser diodes is that LEDs produce incoherent light, while laser diodes produce coherent light. For use in optical communications, semiconductor optical transmitters must be designed to be compact, efficient, and reliable, while operating in an optimal wavelength range, and directly modulated at high frequencies.

In its simplest form, an LED is a forward-biased p-n junction, emitting light through spontaneous emission, a phenomenon referred to as electroluminescence. The emitted light is incoherent with a relatively wide spectral width of 30-60 nm. LED light transmission is also inefficient, with only about 1 % of input power, or about 100 microwatts, eventually converted into «launched power» which has been coupled into the optical fiber. However, due to their relatively simple design, LEDs are very useful for low-cost applications.

Communications LEDs are most commonly made from gallium arsenide phosphide (GaAsP) or gallium arsenide (GaAs). Because GaAsP LEDs operate at a longer wavelength than GaAs LEDs (1.3 micrometers vs. 0.81-0.87 micrometers), their output spectrum is wider by a factor of about 1.7. The large spectrum width of LEDs causes higher fiber dispersion, considerably limiting their bit rate-distance product (a common measure of usefulness). LEDs are suitable primarily for local-area-network applications with bit rates of 10-100 Mbit/s and transmission distances of a few kilometers. LEDs have also been developed that use several quantum wells to emit light at different wavelengths over a broad spectrum, and are currently in use for local-area WDM networks.

A semiconductor laser emits light through stimulated emission rather than spontaneous emission, which results in high output power (~100 mW) as well as other benefits related to the nature of coherent light. The output of a laser is relatively directional, allowing high coupling efficiency (~50 %) into single-mode fiber. The narrow spectral width also allows for high bit rates since it reduces the effect of chromatic dispersion. Furthermore, semiconductor lasers can be modulated directly at high frequencies because of short recombination time.

Laser diodes are often directly modulated, that is the light output is controlled by a current applied directly to the device. For very high data rates or very long distance links, a laser source may be operated continuous wave, and the light modulated by an external device such as an electroabsorption modulator or Mach-Zehnder interferometer. External modulation increases the achievable link distance by eliminating laser chirp, which broadens the linewidth of directly-modulated lasers, increasing the chromatic dispersion in the fiber.

Fiber-optic communication

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Fiber-optic communication is a method of transmitting information from one place to another by sending pulses of light through an optical fiber. The light forms an electromagnetic carrier wave that is modulated to carry information. First developed in the 1970s, fiber-optic communication systems have revolutionized the telecommunications industry and have played a major role in the advent of the Information Age. Because of its advantages over electrical transmission, optical fibers have largely replaced copper wire communications in core networks in the developed world.

The process of communicating using fiber-optics involves the following basic steps: Creating the optical signal involving the use of a transmitter, relaying the signal along the fiber, ensuring that the signal does not become too distorted or weak, receiving the optical signal, and converting it into an electrical signal.