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QSFP+ AOC Cable – A Favored Solution for 40G Network

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As the need for increased network bandwidth to meet the global IP traffic demand is growing rapidly, more and more data centers are deploying 40G Ethernet links. And there are various technologies and solutions to be selected for 40G transmission. 40G QSFP+ transceivers, 40G direct attach cables (DACs) and active optical cables (40GbE QSFP+ AOC Cable) are three kinds of fiber optics used to achieve 40G interconnections in data center. Among them, 40G QSFP+ AOCs are very popular and have a broad prospect. In fact, 40G AOCs are widely used in many fields and play an important role in promoting the traditional data center to step into optical interconnection.
What’s AOC?
AOC cable is a kind of optical transceiver assemblies that is terminated with transceiver-style plugs to be used in the same ports where optical transceivers are used. And 40G AOC is a 4-channel parallel active optical cable, each channel is capable of transmitting data at dates of about 10Gb/s per direction, providing an aggregated rate of about 40Gb/s over multi-mode fiber (MMF) ribbon cables. Unlike DAC cable, AOC cables are active devices, which incorporate active electrical and optical components to boost/receive signal via optical fiber. And they are embodied the latest technology capable of providing the highest data rates.
Why QSFP+ AOC Is Favored in 40G Network?
As an alternative to optical transceivers, 40G AOCs use electrical-to-optical conversion on the cable ends to improve speed and distance performance of the cable without sacrificing compatibility with standard electrical interfaces. Nowadays, active optical cabling is one of the fastest growing technologies in the data center space. QSFP+ AOC cables are popular in 40G network for the following reasons.
Less Expensive
Compared with 40G QSFP+ transceivers, 40G AOCs are much more cost-effective solutions for the data center. Firstly, the QSFP+ transceivers terminated to the cable are cheaper than the 40G QSFP+ transceivers for the reason that there are no lasers in the terminated transceivers (the lasers in the transceivers are very expensive). In addition, the Active Optical Cable eliminates the separable interface between transceiver module and fiber cable. That’s to say, the transceivers are permanently attached to the fiber cables and no patch cables are needed. While the 40G QSFP+ transceivers need to be used together with fiber patch cords which may be expensive (as the following figure shows). Furthermore, since there are no air holes between the transceivers and cables, AOC cables can provide protection from environmental pollutants and other user trouble during installation.
Lighter and With Higher Performance
AOC cables are originally invented to replace copper technology and to facilitate high-speed data connectivity for storage, networking, and high-performance computing (HPC) applications. It is known to us that DACs are heavy, bulky and require significantly higher power, making it difficult to physically manage the data center. And the nature of electrical signals, electromagnetic interference (EMI) also limits DACs’ performance and reliability. AOC cables, however, provide lighter weight, a smaller size, EMI immunity, a lower interconnection loss, and reduced power requirements.
More Customer-Friendly
Consisting of a complete fiber-optic data link (transceivers plus cable) that can be plugged into existing ports, AOCs enable a very rapid introduction of optical connections. AOCs provide customers with access to all the great advantages of fiber (for example, high bandwidth, relatively thin, lightweight cable and so on) in a plug-in format. Furthermore, used in a largely electrical data communications infrastructure, AOCs enable the end users to literally plug into the power and security of a fiber-optics link without any special knowledge of fiber optics.
Conclusion
As people expect more information to be available at their fingertips, our communications systems will need to be quicker—and active optical cable is one of the best solutions to this challenge. Fiberstore provides a wide range of solutions for 40G network, including the above mentioned 40G QSFP+ transceivers, 40G QSFP+ DAC cable and 40G QSFP+ AOCs. We highly recommend 40G QSFP+ AOCs to be used in data center interconnections. All of our optical assemblies are tested in original-brand switch which ensures the 100% compatibility to your device. For more information or quotation, please contact us via sales@fiber-mart.com.

How to Build a Business Fiber Optic Network

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What A Business Fiber Optic Network Contains:
The essential philosophy of contemporary LAN wiring may be the idea of structured cabling. The complete networking method is separated into chunks that allow workstation wires to become concentrated. In a typical enterprise LAN system, the fiber optic network contains Telecommunication Rooms, Backbone Wiring, Work Areas and Horizontal Wiring.
On each floor, there will be a telecommunication room located on top of the other person. These telecommunication rooms hold all network equipment including routers, servers and switches. Telecommunication rooms are linked together with fiber optic cables passing through vertical shafts which are called backbone wiring/cabling or vertical wiring/cabling.
The backbone fiber optic cables typically run at 10Gbps Ethernet speed to supply enough bandwidth for the whole enterprise.
Work areas are work stations (PCs) split up into cubicles. These work areas are connected to each floor’s telecommunication room with horizontal cabling. These horizontal copper/fiber optic cables typically run at 1Gbps Ethernet speed.
How To Pull The Fiber Optic Cable Through Vertical Shaft:
The backbone cabling was once twisted pair copper cables. The good news is it is normally multimode fibers as well as single mode fibers. There are many tools available to pull the vertical backbone fiber cables. Included in this are Gopher poles, cable caster pulling tools or fish tapes. In most cases you have to put in a pulling eye to guard the fiber cables and connectors while pulling the fiber cables.
How To Terminate A Backbone Vertical Fiber Optic Cable:
The backbone fiber optic cables can be found in without termination (connector). You always have to terminate these fibers with fiber optic connectors such as ST, SC or LC connectors. The termination steps usually are not extremely hard nevertheless it does require some extensive training before you perform a fairly good job.
Fiber optic termination tools
The equipment necessary for fiber terminations are fiber optic cable strippers, Kevlar cutters, fiber cleavers, ST, SC, LC or MTRJ fiber optic connectors, fiber connector hand polishing puck, fiber polishing films and fiber inspection microscope.
Fiber optic cable termination steps
1. Strip the fiber: Fiber cables have 3mm jacket, Kevlar strength member and 0.9mm buffer coating. To get at the 0.125mm fiber cladding, you should remove the 3mm jacket having a fiber jacket stripper, then cut the Kevlar fibers having a Kevlar cutter, finally strip the 0.9mm buffer down to 0.125mm cladding having a fiber optic stripper.
2. Cleave the fiber: After stripping the fiber as a result of 0.125mm cladding, you insert the fiber into a SC, ST or LC connector, after which inject some fiber optic epoxy in to the connector using a syringe. You will then lay the connector into a hot oven for stopping the fiber epoxy so it can take the fiber tightly. After the curing process, you cleave extra fibers in the connector tip having a fiber optic cleaver.
3. Hand polishing the fiber: Within the next step, you place the connector (already with fiber fixed inside) into a hand polishing puck, which serves as a fixture while you polish the end face with the connector to get a good quality mirror like finish. Then you definitely hold the polishing puck and polish the connector over a connector lapping film in a figure 8 shape for 10~15 times. Repeat the hand polishing steps stepping from 12um, 3um to 0.5um lapping films.
4. Fiber termination quality inspection: The last step is to inspect the caliber of work. You insert the finished connector right into a fiber optic inspection microscope which zooms to 200 to 400 time level to show you all the scratches and pits which could exist around the connector end face. If everything looks perfect, then you can connect your fiber into the network.

What You Should Think About Before Selecting Fiber Cables

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Sorting through cables and connectivity options could be a frustrating exercise. It’s hard enough working through the categories and levels of copper networking cables, where most cables end with similar connector. What happens when you start looking at fiber cables? This is where things can definitely get confusing! This article tells you how to select the right kind of fiber cables.
Let’s move on off by saying that fiber optic cables can be used in a huge variety of applications, from small office LANs, to data centers, to inter-continental communication links. The information lines that connect between North America and Europe, for example, are constructed with fiber optic cable strung underneath the ocean. Our discussion in this article will focus mainly on the kinds of cables present in those small-scale networks closer to home, and in particular to pre-terminated cables which may be designed for installation, called “patch cords”, “pre-terms”, or any other similar nicknames like fiber patch cables. Prior to you buying, you should make clear the following parameters.
Multimode and Single mode
One of the first things to determine when selecting fiber optic cables is the “mode” of fiber that you’ll require. The mode of a fiber cable describes how light beams travel within the fiber cables themselves. It’s important because the two modes aren’t compatible with each other, which means that you can’t substitute one for that other.
There’s really not much variety with single mode patch cords, but there’s for multimode. You will find varieties described as OM1, OM2, OM3 and OM4 (OM means the “optical mode”). Basically, these varieties have different capabilities around speed, bandwidth, and distance, and the right type to make use of will be based mostly upon the hardware that is being used with them, and any other fiber the patch cords will be connecting to.
Fiber Optic Cable Jackets
Pre-term fiber can be used in a variety of installation environments, and as a result, may need different jacket materials. The standard jacket type is called OFNR, which means “Optical Fiber Non-conductive Riser”. This can be a long-winded way of saying, there’s no metal in it, so it won’t conduct stray electrical current, and it can be installed in a riser application (going in one floor up to the next, for instance). Patch cords are also available with OFNP, or plenum jackets, which are ideal for use in plenum environments for example drop-ceilings or raised floors. Many data centers and server rooms have requirements for plenum-rated cables, and also the local fire codes will invariably have the final say in what jacket type is required. The ultimate choice for jacket type is LSZH, which means “Low Smoke Zero Halogen”, that is a jacket produced from special compounds that provide off very little smoke with no toxic halogenic compounds when burned. Again, seek advice from the neighborhood fire code authority to be certain of the requirements from the installation before making the jacket selection.
Simplex and Duplex
Simplex and duplex have only the difference between one fiber or two, and between one connector at each end of the cable, or two connectors each and every end. Duplex patch cords are the most common type, because the method in which most fiber electronics work is they need two fibers to speak. One is used to transmit data signals, and the other receives them. However, sometimes, just one fiber is required, so simplex patch cords may be essential for certain applications. If you aren’t sure, you can always be on the safe side by ordering duplex patch cords, and just one of these two fibers.
Fiber Optic Cable Connectors
Remember what we should said at first about copper category cables? No matter what level of twisted pair you were coping with (Cat 5, 5e, etc), you always knew you would be dealing with an 8-position modular RJ-45 plug around the end from the cable. Well, with fiber patch cords, there is a few possibilities when it comes to connectors. The common connector types are FC, LC, SC, ST and MTRJ etc..
These are the most typical selections that you will find when choosing amongst patch cords. If you’re able to determine which of these characteristics you need, it is highly likely you will make the right choice when custom fiber optic cables with suitable parameters.

How to Specify Fiber-Optic Sensors

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Fiber-optic sensors work well in tight spots and in applications with a high degree of electrical noise, but care must be taken when specifying these critical components.
Sensing part presence in machines, in fixtures, and on conveyors is an important component of industrial automation. Error-proofing assembly and controlling sequence based on presence or absence of a part is often required. In many cases, one can’t just assume the part is where it should be or the nest is empty as expected, so a presence sensor must be used for verification.
Many types of sensors are available, including inductive, magnetic, capacitive, and photoelectric. Each has its own strengths and weaknesses depending on the application. Photoelectric sensors, however, have the broadest offering of types and technologies, and the widest range of applications.
Photoelectric sensors come with a variety of light-emission types (infrared, visible red, laser Class 1 and 2), sensing technologies (diffuse, background suppression, reflective, through-beam), and housing configurations (photo eye or fiber optic). This article focuses on specifying and applying fiber-optic sensors, which offer advanced capabilities and configuration options, and are great for tight spots that are too small for a photo-eye sensor.
Fiber-Optic Technology
Fiber-optic sensors, sometimes called fiber photoelectric sensors, include two devices that are typically specified separately: the amplifier, often called the electronics or fiber photoelectric amplifier; and the fiber-optic cable, which includes the optic sensor head and the fiber cable that transmits light to and from the amplifier.
The basic theory behind all photoelectric sensors is quite simple. Every photo eye has a light emitter producing the source signal and a receiver that looks for the source signal. Many different technologies exist for sensing and measuring the light transmitted to the receiver. For example, background suppression sensors look for the angle at which the light is returned, while standard photo eyes look for the amount of light, called excess gain, returned to the sensor. Other sensors monitor the time light takes to return, thus providing distance measurement.
1. A variety of fiber optic amplifiers are available, with simple to advanced configuration options.
Photo eyes house the emitter and receiver in either one optical sensor head, such as that used in diffuse and reflective units, or two optical sensor heads like those used in through-beam units. Fiber-optic sensors put all of the electronics in a single housing, with the optical heads for the emitter and receiver separated from and connected to the electronics housing via a fiber cable. The emitted and received light travels through these fiber cables, much like high-speed data in fiber-optic networks.
One benefit to this segregation is that only the sensor head needs to be mounted on the machine. The integrated fiber-optic cable is routed and plugged into the amplifier, which can be mounted in a safe place (typically a control enclosure), protecting it from the often harsh manufacturing environment.
The variety of options available for both amplifiers and fiber-optic cables is vast. Amplifiers range from basic to advanced, and machine builders continue to demand more functions, including logic and communication capabilities.
Fiber-Optic Sensor Amps
Fiber-optic amplifiers range from those with basic electronics and plug-and-play functionality to models with fully configurable electronics (Fig. 1). Some even have electronic units that can handle up to 15 fiber inputs in a manifold-like configuration. Output indication is highly desirable on fiber-optic electronics, as it shows whether the sensor is working properly, but other basic functions (Table 1) must be specified. The output format and connection to the amplifiers are important because they define the interface to the controller, and teaching the on and off setpoints is an integral part of amplifier configuration.
normally open or normally closed—as well as switching via sinking, sourcing, or push-pull. This allows the device to either sink or source the signal automatically, depending on how the circuit is wired. Electrical connection options are generally prewired with at least a two-meter length of cable, or a quick disconnect with a standard M8 or M12 multi-pin connector. Switch settings are programmed by dialing-in a potentiometer or digitally via pushbuttons.
Beyond the basics, advanced amplifier capabilities provide significant flexibility with features such as pulse outputs, on/off delays, and the ability to eliminate intermittent signals. These advanced electronics give machine builders the ability to drill down and adjust amplifier parameters as required by the application.
On/off delays are often desired to slow the reaction of the control system to changes in sensed parameters. In the case of intermittent signals, some applications present the sensor with spurious, short-term signals that aren’t consistent with overall operating conditions. The ability to eliminate these signals at the sensor frees up the controller from this task.
Most all models will provide output-status LEDs, while some offer graduated displays to provide a coarse view of signal strength and output status. More advanced units have multiline OLED displays with customized diagnostics and programming.
Filtering is an option often needed with increased sampling rates, as it provides a more resilient measurement less susceptible to ambient conditions. This stronger signal, however, requires the unit to operate at slower switching frequencies. Pulse outputs allow for stretching of the input signal, which may help when the operating frequency is too fast for a PLC input. On/off delays give machine builders the ability to add timers when the output signal starts and stops.
Advanced units provide more programming options, such as sensitivity adjustments. Using these options, machine builders can teach the machine to sense part absence, part presence, or both—even with difficult materials such as glass. This teaching function reduces or eliminates the need for programming the controller to perform these functions. They can also program the output to switch off/on inside two switch points. By way of example: For part positioning, a switch could turn on at one position and off at another such as in a fill level signal for a pump application.
Seeing the Light with Fiber Cable
Fiber-optic cables don’t conduct electricity; instead they transmit light. They come in a variety of configurations with different material types and optic head styles (Fig. 2). Table 2 lists some of the decisions to be made when specifying fiber-optic cable.
2. Options abound for fiber-optic cables and heads; making the proper selection depends heavily on application requirements.
Diffused fiber-optic cables have two leads to insert in the amplifier for the emitter and receiver light, with the two leads joined together near the single optical head. Through-beam fiber-optic cables are two separate, identical cables that are connected to the amplifier, each with their own optical head. One cable transmits the emitting light, and the other transmits the receiving light. A common mistake is only ordering one through-beam cable, as some suppliers may provide one piece per part number, while others package the required two cables.
Fiber materials are generally either plastic or glass. Plastic units are thinner, less expensive, and provide a tighter bending radius, while glass units tend to be more rugged and can handle higher operating temperatures. Plastic fibers can be cut to length with a special one-time cutter; glass fibers aren’t able to be cut once received from the supplier. The fiber jacket material can also vary from a basic extruded plastic, on up to stainless-steel braiding to operate reliably in the toughest environments.
Optical-head selection is the most crucial part of fiber-optic sensor specification, because it greatly affects the detection of the small stationary or moving parts found in most applications. Head selection differs in how the emitter and receiver optics are oriented in angle and dispersion to the object to be detected. Heads can have rounded bundles of fiber to project a circular beam, or else spread out to form a horizontal, ribbon-like projection.
Round bundles in a diffuse head can be strictly bifurcated with all emitter fibers on one half and all receiver fibers on the other. This is common, but can provide a lag in reading a part moving perpendicular to the bifurcation line. Another option is to have the emitter and receiver fibers dispersed evenly in the head to produce a more homogenous beam. Homogenous fiber mixing gives equal exposure to sending and receiving light, and provides detection independent of part travel direction.
Sensing range for fiber optics will be impacted by the amplifier, fiber cable length, and type of optical head. Thus, it is usually difficult to determine an exact working range, but suppliers typically supply an estimate. Generally speaking, through beam has longer range than diffuse. The longer the fiber cable, the shorter the range, and advanced amplifiers usually have stronger emitting signals and longer ranges as well.
Connecting Fiber-Optic Sensors
Use of distributed I/O and distributed smart devices has been increasing throughout machine automation, and fiber-optic sensors are no exception. Connecting multiple fiber-optic sensor cables to a single manifold of electronics has its advantages.
Fiber-optic amplifiers are typically single-channel standalone units. With slim housings and common DIN rail mounting, they can easily be sandwiched and stacked in a panel. One drawback may concern the routing of electrical connections for each single amplifier.
Another option is to use a fiber-optic manifold, which groups multiple fiber channels to one central control and electrical point (Fig. 3). These fiber-optic manifolds typically utilize an OLED display with menus to allow for programming of each fiber channel. Each fiber channel can be configured separately, such as setting light-on or dark-on, and switching hysteresis. This central control also enables grouping of outputs via basic AND/OR logic, which can reduce and simplify the output signal to the PLC.
3. Fiber-optic manifolds with expansion electronics simplify and reduce the number of wires to the machine controller by converting sensor signals to digital data, and combining signals logically if desired. Pictured is AutomationDirect’s new three-channel OPT2042 fiber manifold, which is expandable to 15 channels. It accepts various plastic and glass fiber optics, and transmits and receives data via IO-Link to allow full 15-channel diagnostics on a single 4-pin connector. It can also be wired with two 8-pin M12 connectors to hardwire each channel if needed—for example, in applications where the controller doesn’t support IO-Link.
Applications and Issues
Fiber optics work well, and are commonly used, in applications where significant electrical noise is generated by sources such as automated welding, variable frequency drives, and motors. Fiber cabling is immune to electrical noise, and the electronics can be mounted away from the noise in a shielded enclosure.
Another very common application is small part assembly. These operations tend to be fully automated, and thus require multiple sensors to confirm part placement (seated) and assembly verification to confirm completion of an operation. Typically, the parts are moving in and out of a stage quickly on carriers or an indexing table. Because travel tolerance is minimal, precise measurement of position becomes essential.
A fiber-optic solution provides various options in head size, orientation, and light dispersion to allow the smallest and most accurate light focus for each application, regardless of the electrical housing size. With on-board logic, one channel of a two-channel sensor can confirm a part is in place to trigger an assembly action, while the other channel is able to confirm that assembly was completed.
A common issue in fiber-optic installations concerns excessive flexing of the fibers. Since the fiber cables are bundles of individual fibers, they typically feel quite pliable, allowing an installer to easily bend the fibers beyond their recommended maximum bend radius. This can cause irrecoverable plastic deformation of the fibers, which will reduce the light transmission or, in the worst case, sever it entirely. The maximum bend radius, listed with all fibers, varies depending on fiber material, bundle size, and fiber dispersion in the bundle—and it must be adhered to in all cases.
Regardless of the application, machine builders must select the proper sensor technology. If fiber-optic sensors are used, amplifiers and fiber-optic heads must be carefully selected for the application to provide robust sensing performance.

What Types of Fiber Optic Sensing Technologies are Available?

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There are many technologies, but commercial solutions really boil down to two main categories: point sensing for which the active portion of the fiber is <= 1cm, and distributed sensing where the entire fiber, perhaps tens of kilometers long, is the sensor.
Fiber optic distributed sensors measure temperature only (Raman Optical Time Domain Reflectometry — ROTDR) or both strain and temperature (Brillioun Optical Time Domain Reflectometry — BOTDR). Spatial resolution is typically one meter or more and strain and temperature resolution are reported at about one microstrain and one degree C respectively, with sampling rates of a few seconds per measurement. The beauty of these approaches is that standard (i.e., inexpensive) telecom fiber is the sensor. The fiber is usually packaged in a tough outer jacket for deployment. Instrumentation is often US$100,000 or more, however. But still the value is very good for long range (>2 km) applications such as pipelines, tunnels, power transmission lines.
Fiber optic point sensors are found in two basic types: fiber Bragg grating (FBG) sensors and Fabry-Perot (FP) sensors. FP sensors have found an important niche in measuring strain, temperature, and particularly pressure for medical applications. They are very small (especially the pressure sensors), but only one sensor can be used per fiber.
FBG sensors for strain and temperature are also very small – as short as 2mm in a 150 micron fiber diameter or as long as a few meters for long gage strain measurements. Other properties like pressure, acceleration, displacement, humidity, and chemical presence, are measured by using a transducer to relate strain to pressure or strain to acceleration, for example. A key advantage of FBG sensors is that dozens, or even a hundred, can be used in series on a single fiber — even if they are measuring different physical properties.
Fiber Bragg grating technology is by far the most widely used fiber optic sensor technology. The versatility of the technology and relatively low cost make it a winner for many applications. At Micron Optics, well over 90% of our sensing customers use FBG based sensors. Whether they’re examining a cancer patient, monitoring a bridge, flying an airplane or pumping oil, they need the information that Micron Optics technology can glean from fundamental measurements of FBGs.

An Introduction to Fiber-Optic Sensors

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A fiber-optic sensor system consists of a fiber-optic cable connected to a remote sensor, or amplifier.
The sensor emits, receives, and converts the light energy into an electrical signal. The cable is the mechanical component that transports the light into and out of areas that are either too space constrained or too hostile back to the sensor.
Fiber-optic cable consists of a plastic or glass core surrounded by a layer of cladding material (see Figure 2). The difference in densities between these two components enables the cables to act in accordance with the principle of total internal reflection, which will be discussed later.
Glass Fibers
Optical fiber can be made of either glass or plastic. Glass optical fibers consist of a bundle of very thin glass strands, each typically measuring 0.051 mm (0.002 in.) dia. A flexible stainless steel–armored sheath is usually added to protect the bundle of cladded fibers, but for some applications a polyvinyl-chloride jacket (PVC) is used.
Glass, by nature, is very resilient, a trait that enables it to perform reliably under extreme conditions such as high temperatures or a corrosive environment. Glass fiber bundles can withstand operating temperatures as high as 450°F as standard product. Customers whose applications have operating temperatures >450°F can special-order cables capable of surviving operating temperatures as high as 1200°F.
With reasonable radius corners, glass fibers can withstand indefinite cyclic bending. Given this premise, you would think that glass fibers can stand up to sharp bending, stretching, extreme vibration, pulling, and other harsh treatment. But they can’t. In fact, they tend to break, and while a few broken strands in a bundle are generally not noticeable, when large numbers are severed there will be a proportionate loss of signal strength.
To achieve a high degree of light-coupling efficiency, fiber manufacturers optically polish the surface of the sensing face to ensure that the end of each fiber is perfectly flat. We therefore encourage customers to special-order nonstandard cable lengths rather than trying to do their own cutting to size.
Plastic Fibers
Plastic fiber-optic cable usually consists of a single strand typically 0.254–1.52 mm dia. These fibers are flexible, and excellent for applications that require repeated flexing as well as for use in extremely tight areas. They generally are sold with a cutting device that allows customers to trim to the desired length.
In recent years, Omron and certain other manufacturers have released multi-core high-flex plastic fiber. These differ from conventional plastic fibers in having multiple independent cores, a configuration that allows a bending radius as small as 1 mm and thus a flexibility close to that of electric wire. They can be bent at 90° with no reduction of light transmission, and readily conform to machine contours without the problems associated with extreme vibrations or pulling. Various vendors also offer coiled versions of plastic fibers for applications that require articulated or reciprocating motions.
When the sensor will be exposed to harsh chemicals, solvents, or high temperatures, glass fibers are preferable. But plastic fibers can be sheathed with Teflon, nylon, or polypropylene for added immunity to hostile environments.
The degree to which light energy is attenuated as it travels through optical fiber is influenced by three factors: the fiber material, the distance traveled in the fiber, and the wavelength of the light. Glass fibers perform fairly consistently at all wavelengths. Plastic fibers, however, tend to absorb light from IR LEDs. Visible LEDs, such as red, exhibit less attenuation in plastic optical fiber and are therefore in wider use.
Principle of Total Internal Reflection
The complete transmission of light through fiber optics is based on the principle of total internal reflection, which states that all the light striking a boundary between two media will be totally reflected. That is, no light energy will ever be lost across the boundary. This principle pertains only when two conditions are met:
The critical angle is less than the angle of incidence for the particular combination of materials (see Figure 3). The materials in this case are the core and the cladding of the optical fiber.
The light is in the denser medium and approaching the less dense medium. The cladding material is less dense than the core material, and as a result has a lower index of refraction.
As long as these two conditions are satisfied, the principle of total internal reflection applies whether the fiber-optic cable is bent or straight (within a defined minimum bend radius).
Sensing Modes and Fiber-Optic Assemblies
Because fiber-optic sensor systems are a derivative of photoelectric sensing technology, photoelectric sensing modes (diffuse reflective, through-beam, retroreflective) are also available for fiber optics. The two types of fiber-optic assemblies that address these sensing modes are individual and bifurcated.
Fiber-optic through-beam mode, as shown in Figure 1, requires two cables. One is attached to the emitter of the remote sensor and is used to guide light energy to a sensing location. The other is attached to the receiver of the remote sensor and is used to guide light energy from the sensing location back to the remote sensor. As with standard through-beam photoelectric sensing, the emitter and detector cables are positioned opposite each other. Sensing is achieved when the light beam that extends from the emitter to the receiver fiber-optic cable is interrupted.
A bifurcated fiber-optic assembly is used for both diffuse reflective and retroreflective sensing. In constrast to an individual cable,a bifurcated cable combines the emitter and the receiver cable assemblies into one assembly. The emitter and receiver strands are laid side-by-side along the length of the cable (see Figure 4) and are randomly mixed at the sensing point, an ideal configuration for applications that require a compact sensing tip. When an object is in front of the sensing tip of the bifurcated cable, light from the emitter cable reflects off the object and back into the receiver of the remote sensor via the receiver cable, and detection is achieved.
Benefits of Fiber Optics
Because optical fiber is essentially a passive, mechanical component of a fiber-optic sensing system, it contains neither moving parts nor electrical circuitry and is therefore completely immune to all forms of electrical interference. This characteristic makes it an ideal way to isolate the sensing system electronics (in this case the remote sensor to which it connects) from known sources of electrical interference.
Furthermore, there is no possibility of a spark, allowing its safe use even in the most hazardous sensing environments such as oil refineries, grain bins, mining operations, pharmaceutical manufacture, and chemical processing. There is also no danger of electrical shock to personnel repairing broken fibers.
Latest Developments
As industrial automation applications grow more complex, and real estate becomes more of a concern, there is a concomitant call for more sophisticated sensing devices in smaller packages. Omron, Keyence Corp. of America (Woodcliff Lake, NJ), Banner Engineering Corp. (Minneapolis, MN), and SUNX Sensors (West Des Moines, IA), among others, have begun to respond by introducing new waves of fiber-optic sensors.
These companies now offer fiber-optic amplifiers (remote sensors) with easy-to-read digital LEDs. The numerical values and percentages that are displayed allow users to monitor and precisely set up their applications. The digital display provides real-time feedback that advises of the slightest misalignment, or of dust accumulation on the cable tip that is beginning to degrade sensor performance.
Some of these new sensors also need significantly less wiring. For instance, there are configurations in which 16 sensors are connected and share a single power line. How? A master connector (from the master sensor) distributes power to the slave sensors, thus eliminating the power lines that each slave would normally require (see Photo 1). The slave sensors need only output wiring. Some of these connector designs also feature simplified installation and maintenance. Some have unique connector designs that allow users to easily detach the sensor without disturbing the cable installation or output wiring.
New dual-output fiber-optic sensors offer the performance of two sensors in one package. Certain models offer either two independent digital outputs or a combination of analog and digital output. Other types also have a “lockout” feature that prevents unwanted adjustments of or tampering with the sensor’s settings. This feature allows customers to give their employees on the shop floor a degree of autonomy without compromising their performance goals.
Most of these sensors now incorporate either a 12-bit or 16-bit CPU as well as 12-bit A/D converter that provides both higher resolution and faster response time, in some cases as fast as 20 µs. As many as four auto-teach functions enable quick sensor setup and allow the user to select the best teach method for the application.