Guide to Fiber Optic Attenuator

Many components are used to enlarge the signals in today's fiber optic transmission system, like EDFA (Erbium-Doped Fiber Amplifier). However, in some cases, the power level of an optical signal should be reduced. For example, in DWDM (dense wavelength division multiplexing) systems, multiple wavelength channels arriving at a node may pass though different paths and experience different losses, their powers need to be equalized before entering the optical amplifier to get flat gain since the gain of each channel depends on the power levels of the other channels. In this case, a point reduction in optical signal strength may be required. And a component is usually used which is known as fiber optic attenuator. This article is to give a basic introduction of fiber optic attenuator in details.

Introduction

A fiber optic attenuator, also known as an optical attenuator, is a passive component that is used to reduce the power level of an optical signal by a predetermined factor in fiber optic transmission system. The intensity of the signal is described in decibels (dB) over a specific distance the signal travels. Fiber optic attenuators are generally used in single-mode long-haul application.

Working Principles of Fiber Optic Attenuators

As technologies advanced, many principles are used in the operation of fiber optic attenuator to accomplish the desired power reduction. Several operation principles of fiber optic attenuators are being introduced here.

Gap-loss Principle: in attenuator using gap-loss principle, the reduction of the optical power level is accomplished by two fibers that are separated by air to yield the correct loss. The optical signal is attenuated when it passes a longitudinal gap between two optical fibers. This kind of attenuator is also called air gap attenuators which are susceptible to dust contamination and can be sensitive to moisture and temperature variations. In addition, this attenuator is very sensitive to modal distribution ahead of transmitter. Thus, it is recommended to be used very close to the optical transmitter. The farther the air gap attenuator is placed away from the transmitter, the less effective the attenuator is, and the desired loss will not be obtained. To attenuate a signal far down the fiber path, and optical attenuator using absorptive or reflective techniques should be used. Gap-loss principle is showed as following picture.

Absorptive Principle: as the fiber optic has the imperfection to absorb optical energy and convert it to heat. This absorptive principle is used in the design of fiber optic attenuator, using the material in optical path to absorb optical energy. This principle is very simple, however, it can be an effective way to reduce the optical signal power. The following picture shows the absorptive principle.

Reflective Principle: another imperfection of fiber optic is also being used to reduce the signal power, which is reflection. The major power loss in optical fiber is caused by the reflection or scattering. The scattered light causes interference in the fiber, thereby reducing the signal power. Using reflective principle (shown in the picture below), fiber optic attenuator could be manufactured to reflect a known quantity of the signal, thus allowing only the desired portion of the signal to be propagated.

Various principles are being applied to reduce the power single. Also various types of attenuators are being manufactured to meet different applications. The following part is about the main types of the fiber optic attenuators.

Types of Fiber Optic Attenuators

Fixed and variable attenuators are the main types that are being provided in today’s market. Their characteristics are being introduced.

Fixed Attenuator, as the name implies, has a fixed attenuation level. Fixed attenuator can theoretically be designed to provide any amount of attenuation that is desired and be set to deliver a precise power output. Fixed attenuators are typically used for single-mode applications. They mate to regular connectors of the identical type for example FC, ST, SC and LC.

Variable attenuators allow a range of adjustability, delivering a precise power output at multiple decibel loss levels. Variable attenuators can be divided into two types. One is stepwise variable attenuator which can change the attenuation of the single in known steps such as 0.1dB, 0.5dB, or 1 dB. The other one is continuously variable attenuator. This kind of fiber optic attenuator produces precise level of attenuation, with flexible adjustments. It allows the operators to adjust the attenuator to accommodate the changes required quickly and precisely without any interruption to the circuit. They are also available with various fiber optic connectors.

Fiber optic attenuator, an important device to control the power level of optical signal precisely, are being designed to different operation principles and types. Getting the basic knowledge about its working principle and types could help to select the fiber optic attenuator to the required applications.

This article is originally published in http://www.fiber-optic-tutorial.com/guide-to-fiber-optic-attenuator.html

Visual Fault Locator Overview

Whether install new fiber links or troubleshooting an existing network, the faster you can locate a problem, the faster you can fix it. To locate the faults in fiber optic cables in a short time, various fiber optic testers are being invited to locate the faults of the fiber optic cable, like OTDR (optical time-domain reflectometer). However, OTDR has dead zone during the testing. Another simple and useful tester which can work in an OTDR dead zone is usually being used to work as an accessory of OTDR. It is known as VFL (visual fault locator) which can also work alone to locate the faults in fiber optic cable in a time saving manner in some situations.

Visual fault locator is now one of the most commonly used fiber optic testing devices to trace optical fibers, check fiber continuity and find faults such as breaks, bad splices and tight, sharp bends in fiber optic cable. The most popular visual fault locators are pen shape VFL and hand-held VFL, which are showed in the following picture respectively.

How VFL Works
The light used for transmit signals over fiber optic is usually at 1300 to 1650nm wavelength which is invisible to naked eyes. Unlike OTDR which measures the time of the incidence and the amplitude of the reflected pulses sent to the fiber optic cable to locate the faults, VFL uses powerful visible light at the 360 to 670nm wavelength injecting to a fiber to visually and directly locate the faults in fiber optic cable. The visible light travels along the core until it reaches a fault, where it leaks out. Light leaking through the fault can be seen through plastic coating and jackets under suitable illumination. This is how VFL locates the faults in fiber optic cable.

Visual fault locators radiate in continuous wave (CW) or pulse modes. The glint of the light source in VFL is usually at 1 or 2 Hz, kHz range is also being provided in today’s market. The output power is generally at 1 mW or less. The working distance of a VFL is usually in the range of 2 to 5 km.

How to Use VFL
VFL is very easy to use. The steps to use a VFL are provided as following:

Step One: remove the plastic connector covers from both ends of the test fiber cable.
Step Two: connect the fiber optic visual fault locator one end of the fiber. Press the tester button and observe that light emanates from the other end of the fiber. This gives a simple indication of the continuity of the fiber link.
Step Three: repeat with several other fibers. Check for light that can be seen leaking from a faulty splice. This may illustrate an easy way of carrying out visual fault finding on bad splices or joints.
Step Four: disconnect all equipment, put the plastic covers back on the connector ends and return everything to the state it was in before you started the practical so that the next group can carry out the practical in full.

Notes during the using of a VFL:

1.Never look directly into the VFL’s output.
2.Cover the VFL’s output with the dust cap when the VFL is not in use.
3.Not recommended for use on dark colored or armored cables.

Using simple but useful technical principle, visual fault locator individually can provide an economic and time saving solution to locator faults in fiber optic cables in some cases. While working as an accessory of OTDR, VFL, together with OTDR, can provide the fiber technician the best solution to locate fiber faults.

This article is originally published in: http://www.fiber-optic-tutorial.com/visual-fault-locator-overview.html

MPO/MTP Connector – Multi-fiber Connector for High Port Density

In today's transmission networks, small and multi-fiber connectors are replacing larger, older styles connectors for space saving. For example, the SC connector is gradually being replaced by its small version LC connector which allows more fiber ports per unit of rack space. To save space, multi-fiber connector is also a good solution, like MTP/MPO connectors. MTP/MPO connector allows more fiber ports per unit of rack space and also satisfies parallel optical interconnections' needs for multi-fiber connection. This article is to introduce MPO/MTP connectors in details.

MPO Connector & MTP Connector

MPO is short for the industry acronym—"multi-fiber push on". The MPO connector is a multi-fiber connector which is most commonly defined by two documents: IEC-61754-7 (the commonly sited standard for MPO connectors internationally) and EIA/TIA-604-5 (also known as FOCIS 5, is the most common standard sited for in the US). MPO connectors are based on MT ferrule which can provide quick and reliable high performance interconnections up to 4, 12, 24 or more and are usually used with ribbon fiber cables. The following picture shows diagram of MPO connectors, 12-fold (left) and 24-fold (right). The fibers for sending and receiving are colorcoded, red and green, respectively.

MTP stands for "Multifiber Termination Push-on" connector and it is designed by USConec and built around the MT ferrule. MTP connector is a high performance MPO connector designated for better mechanical and optical performance and is in complete compliance with all MPO connector standards. Some main improvements of MTP connector are as following:

• The MTP connector housing is removable;
• The MTP connector offers ferrule float to improve mechanical performance;
• The MTP connector uses tightly held tolerance stainless steel guide pin tips with an elliptical shape;
• The MTP connector has a metal pin clamp with features for centering the push spring;
• The MTP connector spring design maximizes ribbon clearance for twelve fiber and multifiber ribbon applications to prevent fiber damage;
• The MTP connector is offered with four standard variations of strain relief boots to meet a wide array of applications.

Application of MPO/MTP Connector

As mentioned, MPO/MPT connectors are compatible ribbon fiber connectors. MPO/MTP connectors cannot be field terminated, thus MTP/MPO connector is usually assembled with fiber optic cable. MTP/MPO fiber optic cable is one of the most popular MTP/MPO fiber optic cable assemblies, which are now being widely used in data center to provide quick and reliable operation during signal transmission. MPO/MTP connectors can be found in the following ap=-[poikjhnb lications:

• Gigabit Ethernet
• CATV and Multimedia
• Active Device Interface
• Premise installations
• Optical Switch interframe connections
• Interconnection for O/E modules
• Telecommunication Networks
• Industrial & Medical, etc.

MPO/MTP Connector Selection Guide

The structure of MPO/MTP connector is a little complicated. The picture above shows the components of a MPO connector. With the drive of market requests. Various types of MPO/MTP connectors are being provided. Some basic aspects should be considered during the selection of a MPO/MTP connector are as following.

First is pin option. MPO/MTP connectors have male and female design. (as showed in the picture on the left) Male connectors have two guide pins and female connectors do not. Alignment between mating ferrules of MPO/MTP connectors is accomplished using two precision guide pins that are pre-installed into the designated male connector. Second is fiber count: MPO/MTP connector could provide 4, 6, 8, 12, 24, 36, 64 or more interconnections, among which 12 and 24 are the most popular MPO/MTP connectors. In addition, like other fiber optic connectors, the selection of a MPO/MTP connectors should also consider fiber type and simplex or duplex design.

MPO/MTP Connector is a popular multi-fiber connector for high port density. It can offer ideal solution to set up high-performance data networks with the advantages of time saving and cost saving. As an important technology during migration to 40/100 Gigabit Ethernet, MTP/MPO connector is now being adopted by more and more data centers.

Note: this article is originally published in my blog (http://www.fiber-optic-cable-sale.com/), which is updated weekly. For more pictures in details about this article and knowledge about optical communication, you can follow this blog.

How Much Do You Know About LC Connector

Fiber optic connectors are used to the mechanical and optical means for cross connecting fibers. Fiber optic connectors can also be used to join fiber cables to transmitters or receivers. There have been many types of connectors developed for fiber cable. Single mode networks have used FC or SC connectors in about the same proportion as ST and SC in multimode installations. But LC connector with smaller size and higher performance has become popular and the connector choice for optical transceivers for systems operating at gigabit speeds. The following text gives a detailed introduction of LC connector.

History of LC Connector

LC stands for Lucent Connector, as the LC connector was developed by Lucent Technologies as a response to the need by their primary customers, the telcos, for a small, low insertion loss connector. Then the LC design was standardized in EIA/TIA-604-10 and is offered by other manufacturers.

Advantages of LC Connector

There are solid reasons that the LC is the preferred connector for high-performance network. From the appearance, LC connect is like a mini size of SC connector. LC connector borrows split-sleeve construction and a cylindrical ferrule (usually ceramic) from SC connector. LC connector has a push-and-latch design providing pull-proof stability in system rack mounts. The picture below shows the appearance of SC connector and LC connector.

The ferrule size of LC connector is 1.25 mm which is half the size of SC connector ferrule—2.5 mm. LC connector is rated for 500 mating cycles and its typical insertion loss is 0.25 dB. An interesting feature of the LC is that, in some designs, the ferrule can be “tuned” or rotated with a special tool after it has been assembled. This offers a considerable performance advantage. The design and performance of LC connector address the need for high density and low insertion loss.

Application of LC Connector

LC connector can be found in many places for termination and connection, especially SFP transceivers for gigabit transmission. For example, the optic interfaces of Cisco SFP transceivers are all LC connectors. Some other applications are as following:

• Telecommunication networks
• Local area networks
• Data processing networks
• Cable television
• Fiber-to-the-home
• Premises distribution

LC Connector Selection Guide

To meet the needs of market, there are various types of LC connectors provided now. During the selection of LC connector, transmission media should be the first factor to consider. LC connector favors single mode fiber optic cable. But it can also be used with multimode fiber optic cable. Signals sometimes are transferred over simplex fiber optic cable and sometime duplex fiber optic cable. Thus, LC connector has both simplex and duplex design. The picture above shows an APC simplex LC connector on the left and a UPC duplex LC connector on the right. Some other factors like polishing style (APC or UPC), hole size and cable diameter should not be ignored. For more details about LC connectors, you can visit Fiberstore which provides various LC connectors with high performance and low price.

Note: this article is originally published in my blog (http://www.fiber-optic-cable-sale.com/), which is updated weekly. For more pictures in details about this article and knowledge about optical communication, you can follow this blog.