Fiber Optical Networking

WDM(Wavelength Division Multiplexing) systems are popular in fiber optic network because they allow to expand the capacity of the network without laying more fiber. Capacity of a given link can be expanded by simply upgrading the multiplexer and demultiplexer at each end. By using WDM and optical amplifiers, they can accommodate several generations of technology development in their optical infrastructure without having to overhaul the backbone network.

WDM wavelengths are positioned in a grid having exactly 100 GHz (about 0.8 nm) spacing in optical frequency, with a reference frequency fixed at 193.10 THz (1552.52 nm). The main grid is placed inside the optical fiber amplifier bandwidth, but can be extended to wider bandwidths. Today’s DWDM systems use 50 GHz or even 25 GHz channel spacing for up to 160 channel operation.

Dense WDM (DWDM) uses the same 3rd transmission window (C-band) but with denser channel spacing. A typical DWDM system would use 40 channels at 100 GHz spacing or 80 channels with 50 GHz spacing. For example, FiberStore provides 50G DWDM Multiplexer Module.

Coarse WDM (CWDM) in contrast to conventional WDM and DWDM uses increased channel spacing to allow less sophisticated and thus cheaper transceiver designs. To again provide 16 channels on a single fiber CWDM uses the entire frequency band between 2nd and 3rd transmission window including both windows but also the critical area. The channels 31, 49, 51, 53, 55, 57, 59, 61 remain and these are the most commonly used.

WDM, DWDM and CWDM are based on the same concept of using multiple wavelengths of light on a single fiber, but differ in the spacing of the wavelengths, number of channels, and the ability to amplify the multiplexed signals in the optical space. DWDM systems have to maintain more stable wavelength or frequency than those needed for CWDM because of the closer spacing of the wavelengths. In addition, since DWDM provides greater maximum capacity it tends to be used at a higher level in the communications hierarchy than CWDM. These factors of smaller volume and higher performance result in DWDM systems typically being more expensive than CWDM.

DWDM transponders

DWDM transponders served originally to translate the transmit wavelength of a client-layer signal into one of the DWDM system’s internal wavelengths in the 1550 nm band. Signal regeneration in transponders quickly evolved through 1R to 2R to 3R and into overhead-monitoring multi-bitrate 3R regeneration. One dwdm transponder, with its tunable channel feature, can spare all DWDM channel 10G transceivers. It eliminates the need to purchasing individual transceivers (XFPs/Xenpaks) for each DWDM channel and greatly reduces sparing costs.

Transceivers versus Transponders

Transceivers – Since communication over a single wavelength is one-way (simplex communication), and most practical communication systems require two-way (duplex communication) communication, two wavelengths will be required (which might or might not be on the same fiber, but typically they will be each on a separate fiber in a so-called fiber pair). As a result, at each end both a transmitter (to send a signal over a first wavelength) and a receiver (to receive a signal over a second wavelength) will be required. A combination of a transmitter and a receiver is called a transceiver; it converts an electrical signal to and from an optical signal.There is usually types transceiver based on WDM technology, for example, there is CWDM XENPAK transceiver available.

Transponder – In practice, the signal inputs and outputs will not be electrical but optical instead (typically at 1550 nm). This means that in effect we need wavelength converters instead, which is exactly what a transponder is.

Transponders that don’t use an intermediate electrical signal (all-optical transponders) are in development.

Optical fibers require end-surface treatment for proper light propagation and that includes polishing their ends. Polishing is essential for almost all glass-based fibers with cladding diameters larger than 200 microns. Furthermore, all fiber connectors require polishing. The process of fiber optic polishing can occur in the field or in a technical lab, it employs a range of tools and products used to create precision fits and finishes in the delicate glass ends.

There is typical fiber optic polishing machine for fiber optic polishing. Fiber Optic Polishing Machines are used to polish the end faces of fiber optic products (cables, connectors, adapters, etc.) in order to minimize signal losses due to scattering. Polishing machines can increase productivity by providing rapid polishing of many different connector styles.

When selecting a fiber polishing machine, there are several features to consider, including adjustable pressure, changeable holders, a timer, and the ability to request custom specifications. Most polishing machines do not offer the flexibility of speed adjustment. This is partially due to the fact that most users only need to handle one type of ferrule material such as zirconia. A slight speed variation does not have significant impact on connector polish result. However, a versatile polisher should have the capability to change speed according the ferrule and polishing film material.

The polishing job typically involves fusion splicers, among other cable crimp tool and connectors are needed. It also requires 99% isopropyl alcohol, polishing (lapping) film and pad, a polishing puck, and epoxy or adhesive. Some technicians also find needle, syringe, and piano wire useful.

Several Different Polish Options On Fiber Connectors

The different polish of the fiber optic connector ferrules result in different performance of them, mainly on the back reflection (return loss). Generally, PC type is required at least 40dB return loss or higher, UPC is 50dB or higher, APC is 60dB or higher. (As we know, the higher the return loss, the better the performance). Insertion loss of them all should be less than at least 0.3dB, the lower the insertion loss the better the performance.

Things You Need To Mind During Fiber Optic Polishing

It is important not to dwell on any polishing film longer than necessary. Too much polishing can result in undesirable ferrule length, unnecessary polish film wear, and degraded polish finish due to particle accumulation. Make proper adjustments to the recommended polishing time in each step in case they are less than ideal.

Eye protection is always necessary to protect against powerful industrial lasers used in long-distance single-mode networks. Supporting tools may include a visual fault locater to troubleshoot fiber faults and breaks. A fiber-optic inspection microscope permits precision analysis of hair-fine fibers. Additionally, technicians rely upon jacket strippers, cutters, cable slitters, and fusion splicers.

Conclusion

Fiber polishing is a science but much like an art. The science of polishing is crystallized in a well designed machine while the art of polishing reside in the procedure and the continuous effort for improvement by the individual user. The procedure and the training are just as valuable as the polishing machine.

We can use magnifier to check the fiber connectors pin end, which quickly determined that the connector insertion loss is high or low, the need for re-grinding. With this method, you only need a few seconds, you can initially conclude that the connector meets the quality requirements. Than the use of instruments that measure the specific optical connector insertion loss value, and then determine wheter the quality meets the requirements, greatly reducing the time and improve efficiency.

Testing Equipment

Using fiber magnifier to check fiber optic connector pins end, we need at least the following equipment:

1. 200 times or 400 times of fiber optic magnifier(according to the type of fiber connector to check the selection of suitable fiber adapter);

2. Pure alcohole and lens paper (hairless soft paper);

3. Light source (we used here instead of incandescent bulbs);

Testing Steps

Check the following steps:

1. Remove the dust cap at the end of the connector to check;
2. Insert the connector in the magnifying glass of the adapter;
3. If you can not see the field of vision magnifier pin end, then adjust the position of magnifier adjustment knob until the pin end graphics all entered the field of vision;
4. Adjust the focal length of the magnifying glass to the right position, making the pin end graphics to clear;
5. Check the pin end, works well for grinding connector. Its face should be round, very smooth, the end of the fiber core is flush with the pin, and showed concentric ring shape; If there is dust (or defects), use lens paper (hairless soft paper) stick of pure alcohol wipe until the surface no dust (or you can see the clear flaws);
6. The other end of the connector to remove the dust cap, and make the end of the pins on the incandescent bulbs, we just checked in the connector end can see the light, otherwise the connector where a fiber optic cable has broken;
7. Repeat the above steps, check again, you will see a very bright core pin end view may find minor flaws;
8. Exchange ends of the connector, repeat the above steps to check the other end;
9. Mark the connector end of the existing problems with the tag, using appropriate methods, or grinding or re-assembled connector, and then repeat the steps above to be checked.

Analysis of test results

The use of a magnifier fiber optic connector for the inspection, we can see that a very good grinding effect fiber connector pin end face should have graphical features, it can have a variety of different types of defects that the end face of the connector graphical features. According to what we see different kinds of graphics, combined with our analysis, we can take the appropriate measures for improvement, in order to ensure the quality of the connector.

Recommended to use at least 200 times (preferably 400 times) of the optical magnifier to be checked. In order to check the accuracy, certainly with and without the use of incandescent bulbs in both cases with a magnifier to check connector end. In both cases the control of the end face of the pattern that can better determine whether defective.

For a good grinding effect connectors, we do not need any additional processing, instrumentatioin can be used directly for subsequent testing. If the connector is more obvious defects (based on experience needed to judge), its loss is likely higher, beyond the acceptable range of technical requipments, we can directly determine the quality problems. But for smaller connectors defective, the loss may be within the required range, then we need to use instrumentation to determine the actual test.

How to determine whether the effect of the connector polishing is “Good”?

If the connector pin end and core are round, smooth, while the fiber core is flush with the pin end, concentricity good, it is “good”, and without blemish.

If one connector looks “bad”, then the center or not circular, or is not smooth, or concentricity deviation is large, or the presence of other defects. For example, if the fiber has partially broken, then its will not be a full circle core.

The most serious situation is that we are under a magnifier to see the clear outline of the core of the phenomenon we call “fragmentation”. More than a brief introduction to how to determine a connector is a “good” or “bad”.

What’s the Optical Add-drop Multiplexer?

An optical add-drop multiplexer (OADM) is a device used in wavelength-division multiplexing systems for multiplexing and routing different channels of light into or out of a single mode fiber (SMF). This is a type of optical node, which is generally used for the construction of optical telecommunications networks. An OADM may be considered to be a specific type of optical cross-connect.

A traditional OADM consists of three stages: an optical demultiplexer, and optical multiplexer, and between them a method of reconfiguring the paths between the optical demultiplexer, the optical multiplexer and a set of ports for adding and dropping signals. The optical demultiplexer separates wavelengths in an input fiber onto ports. The reconfiguration can be achieved by a fiber patch panel or by optical switches which direct the wavelengths to the optical multiplexer or to drop ports. The optical multiplexer multiplexes the wavelength channels that are to continue on from demultiplexer ports with those from the add ports, onto a single output fiber.

Principles of OADM technology

General OADM node can use four port model (Figure 1) to represent, includes three basic functions: Drop required wavelength signal, Add rumored signal to other wavelengths pass through unaffected. OADM specific network process is as follows: WDM signal coming from the line contains mangy wavelength signals into OADM’s “MainInput” side, according to business required, from many wavelength signals to selectively retrieved from the end (Drop) output desired wavelength signal, relative to the end from the Add the wavelength of the input signal to be transmitted. While the other has nothing to do with the local wavelength channels directly through the OADM, and rumored signals multiplexed together, the line output from the OADM (Main Output) Output.

Figure 1 OADM basic model

OADM node technical classification

Optical drop multiplexer network technologies can be divided into two types, fixed optical drop multiplexer (Fixed OADM, FOADM) and reconfigurable optical drop multiplexer (Reconfigurable OADM, ROADM).

Fixed Optical Drop Multiplexer (FOADM)

FOADM to filter as the main component, and its function is fixed to join or retrieve certain light wavelengths. General common FOADM can be divided into three types, namely Thin Film Filter type (TFF type), Fiber Bragg Grating (FBG type) and integrated planar Arrayed Waveguide Gratings (AWG type).

Thin Film Filter (TFF FOADM)

* TFF FOADM using thin film between the filtering effect of the different refractive index.

Fiber Bragg Grating (FBG FOADM)

* FBG FOADM use of fiber Bragg grating filtering effect, with two circulator can become FOADM.

Arrayed Waveguide Gratings (AWG FOADM)

* AWG FOADM gererally used in semiconductor fabrication processes, the integration of different refractive index material is formed on a flat substrate in a planar waveguide, when different wavelength light source is incident through the couping after the import side, due to take a different path length, while the different phase delay caused by different wavelengths and thus produce certain wavelengths in the export side to form a constructive or destructive interference, making waves in the export side, the different wavelengths will follow the design on a different channel to reach, and thus achieve FOADM function.

Reconfigurable Optical Add/Drop Multiplexer (ROADM)

ROADM can always be adjusted with the distribution network to add and drop wavelength, which reconstruct the network resource allocation, the flexibility to meet the requires of modern urban network, so a flexible ROADM features, plus optical switch substantial advantage, making the current fastest growing ROADM based optical switch based ROADM (switch based OADM). ROADM mainly be the optical switch, multiplexer and demultiplexer composed, Switch-based OADM, mainly divided into Wavelength independent switch array and wavelength selection switch.

Type 1 Wavelength independent switch array

Type 2 Wavelength selective switch

All kinds of optical drop multiplexer performance comparison

OADM network applications

WDM ROADM optical fiber suitable for different network environments

OADM in the metropolitan network development tendency

1. Arbitrary choice must be retrieved, adding wavelength, the wavelength can take advantage of the limited resources, the node can be retrieved with the need to do to join the adjustment of the signal wavelength, and has a remote control functions. This can provide dynamic reconfiguration of optical communications network capable ROADM will be connected to the backbone network critical devices. And FOADM is used for wavelength demand network access will be smaller parts to reduce costs. Furthermore, ROADM use to all kinds of Tunable Laser, unable Filter, or wavelength selective optical switches and other components.

2. Must be able to convert incompatible wavelength suitable for the backbone network will be transmitted wavelengths. Therefore, OADM be combined with wavelength conversioin Transponder or other functional components.

3. Must be able to compensate for the node to make acquisistion, adding such action energy loss. Therefore, OADM optical amplifiers must be combined with functional components.

4. Wavelength signals related specifications, such as: the signal to noise ratio (S/N), the energy balance between the signal wavelength, etc., are required to meet network requirements. Therefore must be combined OADM fiber optic variable attenuator (VOA), dispersion compensation module (DCM) and other components.

It is important to understand the different varieties of core characteristics that are available within the fiber optic cabling itself, as each of these different characteristics will have different effects on your ability to transmit information reliably. Have a look at the most common fiber optics cores used in the industry nowadays.

Simplex Fiber Optic Cables

Simplex means this cable is with only one thread of fiber optic glass inside the single core. And simplex cables are with one single outer jacket. Simplex fiber optic cable is used in applications that only require one-way data transfer. For instance, an interstate trucking scale that sends the weight of the truck to a monitoring station or an oil line monitor that sends data about oil flow to a central location. There are singlemode and simplex multimode fiber optic cable available. Single-mode simplex fiber optic cable is a great option for anyone setting up a cable network that will require data to travel in one direction over long distances. Since this type of cable only carries one ray of light at a time, it’s better for long-distance transmissions. Single-mode fiber itself has a high-carrying capacity, is very reliable, and has lower power consumption than other options.

Analog to digital data readouts, interstate highway sensor relays, and automated speed and boundary sensors (for sports applications) are all great uses of Simplex fiber optic cable. This form of fiber cable can be cheaper than Duplex cables, because less material is involved. Simplex cable is compatible with any HDMI extender.

Duplex Fiber Optic Cables

Duplex fiber cable can be regarded as two simplex cables, either single mode or multimode, having their jackets conjoined by a strip of jacket material, usually in a zipcord (side-by-side) style. Use duplex multimode or singlemode fiber optic cable for applications that require simultaneous, bi-directional data transfer(One fiber transmits data one direction; the other fiber transmits data in the opposite direction). Duplex fiber is available in singlemode and multimode.

duplex fibre optic cable and Singlemode duplex cable alike are used for two-way data transfers. Larger workstations, switches, servers, and major networking hardware tends to require duplex fiber optic cable. Duplex cables can be more expensive than Simplex cables, and are compatible with any HDMI extender.

Simplex and duplex are with various cable structure types; they are different from single mode and multi mode which are related to fiber optic glass types.

Multi Fiber Cables

Both multi fiber cables and simplex cables are with a single outer jacket, but simplex only has one thread fiber glass inside the core, while multi fiber has many threads of fiber optic glass inside the core. For example, an 8-core multi fiber cable. There are ribbon type and bundle type multi fiber cables.

Single-mode fiber cables and multi-mode fiber cables are similar in many ways, with the main difference being that the glass center of single-mode cables is significantly smaller, at about 10 microns in diameter. The smaller size is what allows these cables to transmit data up to 40 miles with a bandwidth of 1Gbs.

Only need a simplex fiber cable if data will be traveling in one direction, such as with a security camera or truck weigh station. And if your data will be traveling a long distance – for instance between buildings or from one station to another – then you’re better off with a single-mode fiber cable.