Tài liệu Connectors in FTTP Networks Which Connector Do I Use in My FTTP Network? doc

In Fiber-To-The-Premise FTTP configurations, before one can choose the proper fiber connector, it is important to understand some of the basic terminology and make-up of the connectors a

Connectors in FTTP Networks

Which Connector Do I Use in

My FTTP Network?

Which Connector

Do I Use in My FTTP Network?

In Fiber-To-The-Premise (FTTP) configurations, before one can choose the proper fiber connector, it is important to understand some of the basic terminology and make-up of the connectors as well as the type of video delivery system that is to

be utilized

The Connector

A Connector is a device used to provide a semi-permanent link between

two optical fibers Connectors must be able to maintain good optical contact between the fibers at the connector interfaces In theory, only the cross sectional face of the two fibers need to touch, hence the term Physical Contact (PC) connector is used

The basic components of a connector include the body, the ferrule, the barrel (coupling nut/insert), and the boot The body is the physical shell of the connector, which houses the mechanism used to secure the fiber to the connector The ferrule is the long cylinder-like piece, which extends from the body and provides protection and alignment of the fiber Ferrules can be made

of composite materials, stainless steel, or ceramic These material types provide for a wide range in cost and performance values The barrel is the plastic or metal screw applicator or insert, which provides the method of attaching some connector types (FC, SMA, SC, D4, Bionic) to their mating couplers And finally there is the boot, the small plastic strain relief that provides protection for the fiber entering the rear of the connector body

Key performance values in connectors include Insertion Loss, Reflectance, Core Alignment (Apex Offset), and End Face Polishing Insertion Loss is the measure

of reduction in signal caused by inserting a component, such as a connector, into the optical pathway Insertion loss is measured in decibels (dB), with a lower

dB reading having better performance standards, i.e (0.10dB) is better than (0.20dB) Reflectance is the measurement of light reflected from the cleaved or polished fiber end at the glass/air interface Reflectance is expressed in dB relative

to the incident signal This value is important because some active component systems are sensitive to light reflecting back into them, such as analog video Similar terms are back reflection and optical return loss (ORL) Again a lower reading is better, i.e (-50dB) is better than (-40dB), and you will see below, that the Angled Physical Contact (APC) connectors have the lowest reading in all the connector configurations

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Alignment of the fiber cores is extremely important when connectorizing two fibers.

Two Important Alignment Issues in Connector Performance:

• Maintain Fiber to Fiber Contact

• Minimize Chance for Optical Degradation

– Potential air gap at elevated temperatures

– Opportunity for debris migration

With single-mode core diameters approaching 8um, accurate core alignment is imperative Finally there is End Face Polishing The End Face of the connector refers to the circular cross-section of the fiber where light is emitted and

received By proper polishing of the connector, the geometrical properties of the end-face can be enhanced to provide optimum fiber coupling As connectors get worn, scarred, or contaminated, these geometric properties change resulting

in increased attenuation and loss of signal strength

To improve upon the physical performance values at the connection point, there are several options to choose from One of the best ways to provide ultimate connector performance is to terminate the fiber with factory connectorized pigtails A pigtail is nothing more than a factory terminated connector with a short piece of fiber that can be fusion

or mechanically spliced onto the fiber cable Another method is by selecting high performance polishing techniques and/or utilizing Angled Physical Contact (APC) connectors Obviously not all connectors are created equal Beside field termination polishing, which has been a tried and true method of terminating fiber, there are various grades of polished connectors available from the factory Typical field terminated Physical Contact connectors will have a reflectance of (< -5dB) while an Ultra Physical Contact (UPC) connector will have a reflectance value approaching (<-55dB) Lastly by introducing a small angle to the end face (APC connectors), the reflectance properties of the connector can be improved

to (<-65dB) which as we will see is important for an analog video overlay

The Network

To understand why APC connectors are used in a PON optical network, it is important to understand the theory and technology involved when converting an analog RF video signal to an analog optical signal First, the signal remains in an analog format and is not digitized The inherent qualities of an analog RF video signal remain when converted the optical signal, but there are some disturbing effects when a high optical power is applied to the signal The characteristics of a video overlay (analog video) applied to Passive Optical Networking (PON) technology is such that the tri-plexer within the Optical Network Termination (ONT) that receives the 1550 nm optical signal converts the optical signal directly back into

an analog RF signal

To start from the beginning, the video source is first amplified electrically, and then is converted to an optical signal This optical signal is then amplified through a cascade of erbium-doped fiber amplifiers (EDFAs); this cascade is also referred to

as a video optical line termination (V-OLT) Passive wavelength division multiplexing (WDM) is then used to combine the amplified video signal onto the PON The WDM filter is not a standard 110 nm/1550 nm filter; instead, it is designed to route 110 nm and 1490 nm wavelengths to the OLT and ONT respectfully and the 1550 nm wavelength to the ONT

Which Connector Do I Use in My FTTP Network?

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The ONT is also different; it is designed to receive and route the 1550 nm wavelength to a video optical receiver called the tri-plexer, that converts the analog optical signal directly back to the radio frequency (RF) analog signal for connection

to the coaxial home wiring The primary advantage of analog format is that it can be displayed on existing televisions without the need for a set-top box (STB) converter Digital format, on the other hand, requires an STB for each television One should not confuse the term digital TV with Video over IP (Digital Format) (They are not the same and the subject of another white paper.)

Video Performance

Video performance is the quality of each channel's picture For analog channels, three primary metrics have been

established to quantitatively assess picture quality: carrier-to-noise ratio (CNR), composite second order (CSO), and composite triple beat (CTB) CNR measures the level of the video carrier signal relative to the noise A poor CNR appears

as “snow” in the picture CSO and CTB measure the level of coherent “beats” formed by mixing the individual channels, relative to each video carrier's level Poor CSO and CTB appear as lines and patterns in the picture

Based on these metrics, the Federal Communications Commission (FCC) in North America has established minimum standards for video quality when providing analog video service The minimum standards are CNR greater than

or equal to 4 dB, CSO less than or equal to –51 dB, and CTB less than or equal to –51 dB The Society of Cable

Telecommunication Engineers (SCTE) has established a standard of not worse than –5 dB for CSO and CTB, a level that produces video imperfections that many consider to be imperceptible While the FCC and SCTE requirement for CNR is 4 dB, most CATV providers seek to deliver better video quality, with a CNR greater than or equal to 47 dB

The primary reason for amplifying the original optical signal is such that the when the ONT receives the 1550 nm

wavelength, the receiver sensitivity is typically set between –6.5 and zero dBm so a carrier-to-noise ratio of 48 dB can be realized It has been shown that the best CNR metrics are between 46 dB and 52 dB (see graph below)

Figure 2 CNR Levels based upon Optical Receiver Levels

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Which Connector Do I Use in My FTTP Network?

Path Loss

The video optical path begins at the output of the EDFA and ends at the input of the ONT It consists of a WDM Filter

in the CO, optical fiber, couplers, splices, and connectors (See Figure .) Knowing the quantity and loss characteristics

of each component, it is simple math to arrive at the total path loss A typical path loss might look something like that shown in Table 1

Quantity Unit

1550 nm Wavelength Loss/Unit Loss

Required EDFA Output Power

With the path loss calculated as in Table 1, it is now easy to determine the EDFA output power necessary to achieve the optimum receiver level Based upon table 1, and a 20 km PON, the EDFA output power must be at least 20.1 dBm (–4 dBm + 24.1 dBm)

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Connectors Used in the Outside Plant

Attaching a connector to a fiber optic cable will cause

some of the light traversing through the fiber to be

lost Regardless of whether the connector was installed

in the factory or in the field, its presence will be

responsible for some light being reflected back towards

its source, the laser Commonly known as return loss

(RL), these reflections can damage the laser and degrade

the performance of the signal The degree of signal

degradation caused by RL depends on the specifications

of the laser, with some lasers being more sensitive to RL

than others Additionally, different types of applications

tolerate different degrees of RL Experience in the cable

television industry has shown video equipment tolerates

only a minimal level of optical return loss Similarly, high

bandwidth broadcast applications (such as uncompressed

HD) and long haul links between studios and transmitter

sites require minimal RL

The amount of optical return loss generated is related

to the type of polish that is used on the connector The

“angled physical contact” (APC) connector is best for

high bandwidth applications and long haul links since it

offers the lowest return loss characteristics of connectors

currently available In an APC connector, the end-face of

a termination is polished precisely at an 8-degree angle

to the fiber cladding so that most RL is reflected into the

cladding where it cannot interfere with the transmitted

signal or damage the laser source As a result, APC

connectors offer a superior RL performance of -65 dB

For nearly every application, APC connectors offer the

optical return loss performance that broadcasters require

to maintain optimum signal integrity However, it is

extremely difficult to field terminate an angled physical

contact connector at 8 degrees with any consistent level

of success Therefore, if an APC connector is damaged in

the field it should be replaced with a factory terminated

APC connector

The “ultra physical contact” (UPC) connector, while not

offering the superior optical return loss performance

ofan APC connector, has RL characteristics that are

acceptable for intra-plant serial digital video or data

transmissions When using UPC connectors, make sure

your laser’s specifications can handle the return loss your

UPC connectors will generate Offering –57 dB RL, ultra

physical contact connectors rely on machine polishing

to deliver their low optical return loss characteristics

Ultra physical contact polishing refers to the radius of

the end-face polishing administered to the ferrule, the

precision tube used to hold a fiber in place for alignment

The rounded finish created during the polishing process

allows fibers to touch on a high point near the fiber

core where light travels Unlike APC connectors, UPC

connectors can, with the proper tools and training, be

repaired in the field

Generally UPC connectors have a flat 90-degree edge

to them With APC connectors, a slight 8-degree

angle is introduced to the end face allowing improved performance at the fiber/air interface This improve performance is due to limiting the reflectance upstream, back into the downstream optical signal When reflecting

an analog optical signal into the initial downstream signal, two things occur: 1 The reflected signal tends

to degrade or attenuate the downstream signal, and 2 There is a cross modulating effect in the analog signals where when converting back to an analog RF signal there

is a ghosting or shadowing effect in the viewed signal at the television set

However, when working with the APC type of connector, extreme care must be taken in properly aligning the fibers in any electronics and/or adapters Even though these connectors are keyed, they still may require a little 'jiggle and wiggle' before the matching connectors align with each other Many times, an installer has thought connector alignment was successful only to find the power loss too great until they were wiggled into position Another word of caution would be about the actual cut of the angle Though manufactures have now standardized on the 8-degree angle, there are still some old systems out there that use a different degree of cut

Be careful to match the angles of the connectors and equipment

Figure 4a.

UPC Polish – Light is reflected

back down

to the core Return Loss = 57 dB Adequate for most applications

Figure 4b.

8º Angle Polish – Light is

reflected into the cladding Return Loss = 60 dB (0.0001% of power reflected back)

Ideal for video applications

• Maintaining Core-to-Core Contact – APC demands process control » FTTX is pushing the APC connector performance

to new levels – Feature sets within connector important

• Performance over temperature – Connectors

– Ribbon Cable Assemblies

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Which Connector Do I Use in My FTTP Network?

Figure 5 SC UPC and SC APC Connectors

Color Code

The generally accepted color code for connector bodies and/or boots is: Beige for multimode fiber, Blue for single-mode fiber, and Green for APC (angled) connectors

Passive Optical Network (PON) infrastructures deployed in fiber-to-the-premises (FTTP) networks require numerous fiber connections to achieve the distribution of services to multiple homes Although splicing has its place in these systems, use

of reliable angle-polished connectors (APCs) provides numerous advantages in terms of overall network flexibility, testing and troubleshooting

Standards

Today, the SC connector type is standard in both the inside plant and the outside plant applications The standard that dictates the rules for outside plant is GR-26

Gr-26 Issue  – UPC SC Connector Specifications

• Max 0.2 dB Insertion Loss (IL) (0. dB change over temperature ) Tuned Connectors

• Min Intramated Return Loss (ORL) 50 dB, Typical –58 dB

• Open RL 15 dB Typical

SC APC Connectors

• Typically higher IL (0.5 dB)

• Similar Temperature Implications

• Min ORL: 65 dB (67 dB open)

Conclusion

The manufacturing techniques used today have greatly improved the performance of both the UPC and APC connectors Most advertised loss characteristics range from 0.14 to 0.18 dB for both connector types, making the APC connector the connector of choice when building an FTTP network Apart from the requirements of the RF Video overlay as stated above, there are other active component systems that require a lower reflectance value (ORL) that only the APC connector can guarantee

The design considerations for overlaying an analog video network onto a PON in regards to types of connectors that are to be used has been presented The key considerations can be summarized as:

• Defined Connector Performance Requirements

• Defined Active PON Network Requirements

• Defined Passive PON Network Requirements

• Established Video Performance Requirements

• Connectors Used in the Outside Plant

It is important to note that fiber-optic connector reflectance performance is a direct result of achieving and maintaining good physical contact between fiber core end-faces

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