Fiber Characterization

Brief

Teralight Integration technicians have been working on fiber characterization since long. Teralight Integration's Fiber Characterization Services will help NLC understand issues and optimize the performance of their fiber networks by identifying the limitations of each fiber. Our service will help NLC to reduce time to market for optical-based services by identifying problems proactively and providing time to take corrective actions. Teralight Fiber Characterization Services provide an accurate report of individual fiber characteristics along with future scalability and performance.

Fiber Characterization is a comprehensive technique of measurements carried out on a fiber optic cable link to determine the key performance attributes of that link which may affect current or future applications. Fiber characterization also allows the quality of the fiber optic link to be assessed, including the identification of the type and grade of fiber installed.

As NLC wants its networks should carry more and more traffic to meet the demands of the Internet and other "Bandwidth Bandit" applications, then the data rates are getting higher and higher. 10Gb/s systems are being widely deployed and people are once again starting to talk seriously about 40Gb/s channels. The next development of Ethernet will include 100 Gb/s. At these very high data rates then many aspects of the fiber infrastructure become critical including chromatic dispersion and PMD (Polarization Mode Dispersion). Also the power budgets become much tighter so loss measurements are more critical.

In addition to high data rates per channel, there is also a proliferation of multi-channel systems using DWDM technology - and as more channels are required and the data rates increase then the total grange of wavelengths being used also increases. Therefore the fiber might need to be characterized to assess its performance at wavelengths other than those it was originally intended for and tested against. For example OTDR testing at 1625nm to verify satisfactory operation at wavelengths beyond 1550nm.

In order to meet all these challenges Teralight is offering its professional Services to NLC to cater for its entire network, rings, spurs, core and edge. Teralight will be implanting 8 teams in total to cater for NLC network which is divided into 6 rings and multiple number of spurs spread all over across Pakistan. Teralight will be finishing complete work in 2 months to perform this duty.

When is Fiber Characterization required?

Fiber characterization might be required in any of following cases.

1. Dark Fiber deals
2. Network Upgrades
3. Merger & Acquisitions
4. New Networks

Dark Fiber Deals

The demand for fiber characterization services is increasing in many territories due to the increasing complexity of the competitive telecoms markets. In the telecom world, there are many fiber networks operated by different companies. However there are very few new networks being built due to the very high cost of construction and the lack of money in the telecoms industry! This means that as new customers are signed up then often fairly complex dark fiber deals are being done sometimes with several different providers, to stitch together a new fiber route whilst installing the minimum amount of new fiber. This fiber route then needs to be characterized.

Network Upgrades

Since there are few new cables going in, then network operators need to upgrade their existing infrastructure to operate at higher data rates and/or using DWDM or CWDM systems over extended wavelength ranges. It is likely that older fibers were never tested for Chromatic dispersion or PMD. At 40 GB/s data rates it may be necessary to know the chromatic dispersion precisely so that it can be compensated for with sufficient precision. Following ITU Recommendations, an upgrade from 10 GB/s to 40 GB/s means that the CD compensation has to be 16 times more precise! PMD was unlikely to be a limiting factor on the performance of 2.5 GB/s systems, but can become critical at 10 GB/s and at 40 GB/s it is likely to be a problem on many older fibers.

Merger & Acquisitions

2005 onward saw a revival of merger and acquisition activity in the world telecoms marketplace. Major Cable TV companies merged also many GSM/CDMA operators done the merger agreements. Due diligence studies by purchasers of networks may require an independent audit of the quality and value of the assets being purchased.

New Networks

Although there are few new networks being installed in mature telecoms markets, there are many territories where new networks are still being deployed.

What test should be done in Fiber Characterization?

Fiber Characterization is defined as a series of tests taken on a fiber optic span to determine the integrity of the fiber, installation practices, and performance for a desired transmission rate (OC-48 or faster) and/or Service to be implemented (DWDM). There are 5 tests taken to qualify a fiber:

1. Optical Return Loss (ORL)
   1550nm wavelength, from both fiber ends
2. OTDR
   1550nm & 1625nm wavelengths, bi-directionally averaged
3. Power Meter and Light Source (LTS)
   1550nm & 1625nm wavelengths, bi-directionally average
4. Polarization Mode Dispersion (PMD)
   1550nm wavelength, single ended
5. Chromatic Dispersion (CD)
   1520nm to 1630nm at 10nm wavelength increments, single ended

Optical Return Loss (ORL)

• ORL measures the total light reflected back to the transmitter caused by the fiber and the components including connector pairs, mechanical splices, etc.
• ORL provides a reading of the light that does not reach the opposite end of the fiber
• ORL determines the overall fiber plant efficiency
• Reflective events include all connector pairs and mechanical splices
• ORL is measured as a + dB reading
• The higher the ORL reading the better the reflections in the fiber under test

OTDR

The OTDR sends a light pulse down the fiber and measures the return signal power and travel time in order to calculate the fiber distance as well as loss of the fiber under test. Once the loss and distance are obtained they are plotted and a fiber trace is created.

As well as plotting the optical distance of a fiber, the trace will show events such as splices and connector pairs.

To accurately measure events, OTDR traces should be taken bi-directionally and averaged. Since Fiber Characterization testing is typically done on single mode fiber, typically OTDR traces are taken at 1550nm and 1625nm wavelengths to cover the C-band and L-band transmission windows.

Power Meter and Light Source

A Power Meter and Light Source combination (Loss Test Set) is the most accurate way to provide end to end loss readings on an optical span, including the fiber attenuation and the initial and end connectors of the fiber under test. A power meter and light source is used to send a continuous wave light from the source to the power meter. The difference in power is the total span loss.

Polarization Mode Dispersion

Polarization Mode Dispersion (PMD) is the result of light traveling down a fiber along different paths. Each path will have a slightly different length which will result in different arrival times for each component of light traveling. The difference in the arrival times is PMD. This “differential delay” is PMD and is measured in picoseconds (ps) of delay.

PMD is performed at 1550nm with a broadband light source. The test is performed by taking several scans of the fiber under test. The result documented is the worst case result as the total delay and coefficient of the fiber under test. The PMD is fitted to a Gaussian curve and determined to pass or fail based on industry standards.

SONET	Bit Rate	Bit Time	Delay
OC-48	2.5Gigabit/sec	401.88ps	40ps
OC-192	10Gigabit/sec	100.47ps	10ps
OC-768	40Gigabit/sec	25.12ps	2.5ps

Chromatic Dispersion

Chromatic Dispersion (CD) is a result different wavelength traveling at different speeds of the non-zero spectral width of transmitters. Since transmitters are actually made up of several wavelengths and each wavelength travels at a different speed, the difference in arrival time of each wavelength causes pulse spreading or (chromatic) dispersion. This phenomenon is measured in ps/nm.

CD tests are typically set up to measure across a variety of fiber types. For example, a typical CD set up would measure from 1520nm to 1630nm in 10nm increments. The results are then plotted and graphed. The results include the dispersion slope, zero dispersion point and delay ps/nm.

• Chromatic Dispersion is NOT a pass/fail reading
• The results determine if dispersion compensation is required
• The results are required for network engineering for proper transmission and network element requirements and placement.

SONET	Bit Rate	Total Dispersion
OC-48	2.5Gigabit/sec	16640ps/nm
OC-192	10Gigabit/sec	1040ps/nm
OC-768	40Gigabit/sec	65ps/nm

 

Link Characterization

Link characterization measures fiber performance and the quality of its interconnections, such as splices or connectors. The suites of tests used primarily depends upon the user’s methods and procedures, which could be unidirectional or bidirectional and could comprise some or all of the measurements covering the required parameters.

Test Parameters	Measuring Tools
Connector Inspection	Video Inspection Scope
Distance Measurement (Fiber Length)	OTDR Module
Connectors/Splice Measurements	OTDR Module
Reflectance Measurement	OTDR Module
ORL Measurements	OFI Module
PMD Measurements	PMD Analyzer
CD Measurements	CD Analyzer

 

Network Characterization

Network characterization provides the baseline measurements for the network before turning up the transmission system. Measurements are performed through the optical amplifiers, dispersion compensators, and any elements in line. There is a limited suites of tests compared to the ones done for the link characterization.

Test Parameters	Measuring Tools
Connector Inspection	Video Inspection Scope
PMD Measurements	PMD Analyzer
CD Measurements	CD Analyzer

 

Conclusion

As outside plant crews are increasingly required to test existing fibers for DWDM upgrades, employing the same equipment to perform fiber characterization and dispersion tests on both medium-haul and long-haul links is not the most effective approach.

Outside plant managers need to keep in mind that even as technology advances raises the level of testing accuracy and precision required for medium-haul links, employing the methods, measurement range, and equipment used for long-haul links far exceed medium-haul requirements. Modular, field upgradeable and multifunctional test instruments enable outside plant managers to expand conventional testing capability and effectively future-proof their equipment. The benefits of this approach are many: capital equipment expenditures and staff time and costs are reduced while customer satisfaction increases by permitting outside plant managers to consistently meet turn-up dates.