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One of the most important procedures in the maintenance of a fiber optic system is fiber optic cleaning. This is required to keep quality connections between fiber optic equipment. Keeping the fiber end face and ferrule on the fiber optic connectors absolutely clean is very essential. If any particles such as dust, lint or oil get on the end face, this will jeopardize the completeness of the optical signal that is being sent over the fiber.

These particles can also cause other problems such as scratching the glass surface, instability in the laser system, and a misalignment between the fiber cores. Misalignment between the fiber cores can significantly degrade the optical signal.

Steps to Follow Before Cleaning Fiber Optics

Before cleaning fiber optic connectors, make sure the cable is disconnected from both ends and turn off any laser sources. Always wear safety glasses and inspect the connectors before you clean them. Before a connection is made, the connectors have to be inspected and cleaned. A connector housing should be used for plugging or unplugging a fiber. A protective cap should be placed on unplugged fiber connectors. The unused caps can be kept in a sealed container.

When cleaning fiber optics, the end face of the connector should never be touched and also the clean area of a tissue or swab should not be touched or reused.

When using alcohol, the portion of a tissue or fiber optic cleaning swabs where it has been applied or the dispensing tip of an alcohol bottle should never be touched. Alcohol is flammable, so it should not be used around an open spark. Also, never leave alcohol residue on the end face because this could damage the equipment.

Cleaning Fiber Ends

The fiber end should be inspected with a fiber microscope of at least 200x magnification, and if it is contaminated, it should be cleaned with the dry cleaning method.

Cletop fiber optic cleaner is recommended for the dry cleaning method. With the Cletop fiber optic cleaner, the fiber end can be cleaned with a cloth. The used cloth is then thrown away, and with just the pressing of a thumb a clean cloth will emerge. After cleaning, the fiber end should be inspected with a fiber microscope. If the fiber end is still contaminated after using the dry cleaning method, repeat the process.

Inspect the connector again with the fiber microscope, and if it is still contaminated, use the wet cleaning method, followed by a dry cleaning method until there is no residue.

With the wet cleaning method, an optical quality cleaning cloth and fluid are used. Even though isopropyl alcohol is frequently used as a cleaning fluid, it is slow at drying and can leave residue. Opticwipes cleaning cloth and fluid are recommended.

When using this method, dampen the cleaning cloth with the fluid and wipe the end face of the connector several times. Repeat this and then with a clean, dry area of the cloth, clean the fiber end.

The fiber should not be used if the cleaning is unsuccessful because the contamination could be due to scratching, improper polishing, or some other damage.

We saw in the last part of this series, “An Optical Transmitter for Every Need,”  how the use of complex optical modulation schemes affects the transmitter architecture. It is not surprising that on the receiver side, we have to rethink the traditional concepts, too.

In on/off keying (OOK), we are able to detect the signal simply with a photodiode, which converts the optical power into an electrical current I_Photo. The photocurrent IPhoto originating in the photodiode is directly proportional to the product of the optical signal S and its complex conjugate S*. In the equation in Figure 1 you can see that the result only contains the amplitude A_S. I_Photo does not provide any information on the angular frequency ω_s and the phase θ_s. Thus, the QPSK signal in the time domain on the right side cannot be directly mapped to the IQ diagram on the left without ambiguity. You can only tell that the lower curve passing zero represents the diagonal transitions between the four constellation points and the middle curve the outer transitions. The flat signal through 1 represents the cases where the phase does not change, meaning that a symbol is followed by the same one.

Figure 1. In direct detection, the photo current IPhoto only provides information on the light amplitude.

For unambiguous identification of the symbol transitions, we have to look for more sophisticated methods that enable detection of the complete electric field including phase information.

Complicating the problem is the fact that in today’s optical communication systems we operate at wavelengths in the near infrared – for example, at 1550 nm, corresponding to a frequency close to 200 THz. So the changes of the electrical field in time and space are several orders of magnitude too fast to be processed with available electronics operating in the MHz to GHz range.

A local oscillator helps
The key to solve both problems lies in measuring not the absolute phase but the phase relative to a known reference signal. In Figure 2 you can see the basic detection setup; the ideally monochromatic laser that produces the reference signal R is often referred to as the “local oscillator.”

Figure 2. Mixing the signal S with a reference signal R allows measuring the phase difference. QPSK signal mixed with different reference signals.

The signal of interest S and the reference signal R are superimposed in an optical combiner and detected with a photodiode. I_Photo is then proportional to the product of the sum of both signals (R+S) and its complex conjugate (R+S)*. The equation in Figure 2 reveals that the result holds the phase difference Δθ = θ_S – θ_R and the frequency difference Δω = ω_S – ω_R. From Δθ, we can now deduce the evolution of θ_S over time.

The reference frequency ω_R is chosen close to ω_S so that Δω is now small enough to be electronically processable.

The phase-dependent term is called the heterodyne term or beat term because it results from mixing or “beating” the two signals.

There is also a term containing the squared amplitude that has no implications as long as only the phase is modulated and the amplitude stays constant – which is the case in QPSK modulation.

At the bottom of Figure 2 we have the case without reference signal, discussed before, with only the A_S² term.

When a reference signal is added that is large in comparison to the signal itself, we see basically the beat term shifted upwards by A_R². It would be advantageous to get only the beat term without this shift.

Suppressing phase-independent terms with a balanced receiver
As shown in Figure 3, we can suppress all other phase-independent terms with a balanced receiver. Here, the signal to be detected S and the reference signal R are summed on one branch and subtracted on the second branch of a 2×2 optical combiner (which could be a fiber optical or free-space optical coupler). Each of the resulting signals is detected by one photodiode. The difference between the two photocurrents is then used. In the equation, also given in Figure 3, we can see that all other terms have cancelled out, and only the beat term remains.

Figure 3. Using a balanced receiver, only the beat term remains with doubled intensity.

An additional advantage of balanced detection is visible as well: the net photocurrent has doubled.

Taking the concept to the IQ plane – IQ demodulator
To recover both amplitude and phase, a coherent receiver should provide the in-phase (I) component and the quadrature (Q) component as two separate output signals. For this purpose, we need a second balanced detector. A single local oscillator provides the reference signal for both of them but the phase must be shifted by π/2 to obtain the Q part. Figure 4 gives, for the case of a QPSK signal, an idea of the whole setup, which is called an “IQ demodulator.”

Figure 4. IQ demodulator providing two independent measurements that both contain information on amplitude and phase.

This setup only works for coherent signals that are not polarization-division multiplexed. In addition, the signal only mixes with the component of the local oscillator signal with the same state of polarization at the detector.

Extending the concept to dual polarization
For dual polarization, we have to further develop the demodulator concept. The basic principle stays the same: after a polarization splitter, we now have two IQ demodulators, one for the x-polarization and the other one for the y-polarization. Only one local oscillator provides the reference signals for all branches.

The block diagram is given in Figure 5. You can see that there are four output signals to resolve I- and Q-coordinates, one for each polarization direction respectively. In the equations the upper indices h and v reflect the horizontal and vertical polarization state of the signal with respect to the polarization reference frame of the receiver. This polarization diversity architecture also assures that all of the signal is mixed with the local oscillator, regardless of the input state of polarization. Therefore it is commonly used, even if the signal does not use dual polarization.

Figure 5. IQ demodulator for polarization resolved measurements.1

So far, we have investigated receivers with a local oscillator of a frequency ω_R that is different from the signal’s frequency ω_S. These are called heterodyne receivers.

In homodyne receivers, the local oscillator has the same frequency as the carrier signal itself. The advantage: the above terms are not frequency dependent anymore.

Figure 6 quantifies the required electrical bandwidth for both homodyne and heterodyne receivers. For homodyne detection, where the local oscillator has the same frequency as the signal itself, half the signal’s optical bandwidth is needed. For a heterodyne receiver the needed electrical bandwidth increases with the frequency offset between local oscillator and signal.

Figure 6. The required electrical bandwidth for coherent detection depends on the frequency offset between signal and its reference.

Use a delayed copy of the signal as a reference – delay-line interferometers
After what we have seen, it seems that a local oscillator is indispensable for recovering the phase information. How about overlaying the signal with a copy of itself? This way, we also get a reference signal where ω_R = ω_S.

One could think that this effort is not very promising because it may not be clear how to gain additional information on the phase that way. But this self-homodyne approach is useful because we are interested in detecting the phase change over time. So, if you split the signal in two and overlay the signal with the delayed copy as a reference signal you get information on the phase changes.

The advantage of this measurement method is that it is not subject to inaccuracies due to slow (in comparison to the symbol rate) frequency fluctuations of an external local oscillator and of the carrier laser itself.

This kind of receiver setup is known as a delay-line interferometer. Figure 7 shows a balanced delay-line interferometer with the signal S(t) and the signal S(t+T) delayed by T.

Figure 7. Balanced delay-line interferometer.

The equation here shows that the result is dependent on the cosine of the phase difference between the original signal and its delayed copy. Due to the periodicity of this function, only phase differences between 0 and Pi can be uniquely identified and only for delays T that are approximately an integer multiple of the carrier period 2π/ω_S. This is sufficient for BPSK but for phase recovery of QPSK and higher-order modulation schemes, we have to add another delay line interferometer phase shifted by π/2 relative to the other delay-line interferometer to cover the full phase range from 0 to 2π.

Figure 8 shows the setup with an additional delay-line interferometer for receiving the two independent I and Q components. Q₁-Q₂ is measured additionally, while I₁-I₂ stays unchanged.

Figure 8. Extended delay-line interferometer for QPSK and higher order modulation formats.

Analogous to a heterodyne receiver, the delay-line interferometer can also be expanded for polarization-sensitive measurements.

With a delay-line interferometer, we don’t need an external local oscillator and therefore don’t have to deal with oscillator-introduced phase noise; we also require less signal processing. However, this approach does have disadvantages that still might lead us to prefer a heterodyne receiver.

First, to measure phase changes over time with a delay-line interferometer without clock data recovery (CDR), the delay and the sampling period need to be considerably smaller than the symbol period. Today’s symbol rates have reached a level where this can be hard to accomplish. In addition, for low-power signals the measurement sensitivity is reduced because the reference signal is also of low power and suffers from noise accumulated on the transmission link. For implementation with a sampling technique, the measurement time increases and a trigger is required. Bottom line, a homodyne receiver is not very flexible.

Until now, we have exclusively been looking at time-domain detection techniques. Alternatively, we could also detect the frequency spectrum and deduce from it via Fourier transformation the time-domain signal.

Frequency-domain detection
To recover a complex modulated optical signal from its spectrum, we have to measure the complex spectrum, meaning with amplitude and phase information.

This can be performed with a complex spectrum analyzer that separates the different optical frequency components with a dispersive element. All the frequency bands can either be detected simultaneously with multiple detectors or sequentially with a scanning narrow-band optical filter and a single detector.

For recovering phase and amplitude, we again employ a local oscillator as a reference signal. For recovering both components, we need a source emitting at two optical frequencies.

Figure 9 shows the complete setup that is needed to measure the polarization-resolved complex spectrum.

Figure 9. Setup for polarization-resolved coherent frequency detection.

The big plus of frequency-domain detection is its virtually unlimited bandwidth, meaning also unlimited time resolution. The bandwidth depends on the sweep range of the local oscillator so that we can reach bandwidths in the THz range with today’s tunable external cavity lasers. The other big advantage is that we don’t need a high-speed receiver.

On the other hand, there are also major drawbacks.

For example, it is only applicable to periodic signals because these result in the required discrete spectral peaks. Additionally, we now need a symbol or pattern clock. The precision of the recovered time-domain signal directly depends on the spectral resolution that determines the number of sidebands that can be resolved. The spectral resolution that can be reached today limits the pattern length to a few tens of symbols.

These factors and the fact that this method does not give results in real time, make frequency-domain detection inapplicable for network receivers. In fact, we would have to deal with long measurement times and fairly complex measurement setup and signal processing.

Finally, in frequency detection, all non-periodic effects are averaged out. This is also true for polarization-mode dispersion (PMD), which therefore cannot be compensated.

Preferences?
Self-homodyne setups need little signal processing and are the least sensitive to phase noise. Still, they are not very flexible, work only close to the design symbol rate, and are less sensitive than heterodyne implementations.

The heterodyne time domain detection methods offer the highest flexibility. Unlike frequency-domain detection, they can be used for real-time detection. They therefore are applicable to live signals in data networks. Equivalent-time sampling only works for repetitive signals of a limited length, for example in test and measurement scenarios.

With real-time sampling, we can reconstruct the complete signal in all domains and without limitations regarding the modulation format. Neither do we face any limitations regarding signal length in heterodyne time-domain detection. PMD and CD can be compensated during signal processing. In this case, only the signal processing is the throughput limiting factor.

At the same time, we have to be aware that we need four-channel high-speed equipment for this approach, such as a high-performance real-time digitizer with very low jitter and noise and a high effective number of bits (ENOB) over the whole frequency range.

We’ve now covered the basic concepts of constructing a receiver. In the next part of our series, we‘ll have a look at the details of a time-domain real-time sampling setup.

References
1. Block diagram from “OIF Implementation Agreement for Integrated Dual Polarization Intradyne Coherent Receivers.”

All other figures in this article are contributed by Oliver Funke, Bernd Nebendahl, and Bogdan Szafraniec.

Stephanie Michel is technical marketing engineer in the Digital Photonic Test Division of the Electronic Measurements Group at Agilent Technologies. The Electronic Measurements Group will spin out of Agilent in November 2014 under the name Keysight Technologies.

Just a few years ago there was concern that the amount of “dark fiber” would never be fully utilized. A 40-megabyte transmission was exceptional and debate was on whether copper or wireless or fiber optics would emerge as the ultimate medium and format of communications. As we stand in the middle of 2013 looking forward, speeds and capacities are measured in gigabytes and terabytes as systems and networks are combinations of copper, wireless and fiber optics. There are consumer, business, military, DOD, trans-oceanic and myriad systems and new equipment to be deployed in the coming years.

There are various standards for virtually all facets of the industry. Considered the “mother standard,” tenets of IEC 61300-3-35 have influenced numerous standards. These include: a) the importance of cleaning the fiber optic connection, b) the concept of diameter of debris or contamination and c) the area of the end face to be cleaned. d) methods of cleaning. Certain “cross-over” techniques can be detrimental to precision cleaning a fiber optic connection; debris and contamination outside common video inspection field of view (FOV) may influence results.

As capacity and bandwidth expand, deployments updated and new technicians trained, a clear understanding of several other tenets of precision cleaning the connection may also be considered. IEC 61300-3-35 has influenced TIA 455-240, Telcordia GR-2923-CORE, and SAE AR-6031. Both the SAE and Telcordia standards suggest expanding debris and contamination beyond OEM levels. Oddly enough, an OEM standard in plant may not be relative as the equipment is installed in OSP/Field Service. Nonetheless, virtually all standards are outdated by the time they are written and implemented. This paper is a suggestion with practical recommendations to look to exceed these standards lways with deference to those who created them.

WHY IS IT IMPORTANT TO PROPERLY CLEAN THE CONNECTION?

Before that question is addressed, it is important to recall there are there are many types of debris and contamination. These exist not only on a fiber optic connection, but also, in the world that is our environment. Some of these are dry Figure-1 and others are fluidic Figure-2. Debris on an end face may also be present in “combination” as on Figure-3. The cleaning procedure should strive to be a first time event. A best practice procedure can be identified that does not require multiple techniques and numerous attempts. These tenets are regularly exhibited when we wash a car, clean a window, and sanitize our hands or other common cleaning element.

When discussing a fiber optic connection, dry, fluidic, or combinations of contaminants also have height Figure-4. (12) Commonly used cleaning techniques can create fluidic contamination outside the field of view of most video inspection and software analysis Figure-2. Another technique can create a static field that attracts additional debris Figure-1. (8) It is essential to consider all potential sources and types of soils to effectively clean these connections.

There are well established tenets, present in everyday life, that point the direction to properly cleaning the fiber optic connection. Dry wipers, use of solvents and surfactants, and identification of debris or contamination are every-day choices. These are relevant to precision clean a fiber optic connection.

Ineffective cleaning fiber optic cleaning procedures may result in debris being mis-characterized as an artifact. Currently, IEC 61300-3-35 is relative to both production line applications (OEM) and field service (OSP) applications. In this work the term OEM refers to Original Equipment Manufacturer or a production line environment. The term OSP refers to Outside Plant or the installation and maintenance environment post production line. Precision cleaning is the first step to assure accurate test, measurement, or, high speed transmission of services. One precision cleaning procedure for all connections is ideal. The process should remove the widest range of debris and contamination the first time. This article is an effort to define and update inspection and cleaning procedures to facilitate existing operations and anticipate future needs.

END FACE GEOMETRY: HOW IT INFLUENCES PRECISION CLEANING

For the purpose of this document, debris is defined as a dry contaminate and contamination is defined as fluidic debris. Current standards derived from IEC 61300-3-35, characterize debris and contamination as a two dimensional measurement of microns in relation or juxtaposition to the core.

However, dry, fluidic and debris-in-combinations also has height. Video inspection easily analyzes diameter of debris or contamination in relation to the core: Zone-1. Measurement of the height of debris and contamination requires an interferometer. The interferometer reading, Figure-4, shows debris of approximately equal diameter and height. Subsequent tests and readings verify these images. Each of these aspects enter into an understanding of properly cleaning a fiber optic connection: fortunately, the actual technique is considerably easier than the “science.” However, for many, knowing why is as important as knowing how.

Existing standards for end face inspection limit the field of view to three zones on the horizontal ferrule. Implied is the end face, like debris, is two-dimensional. Of course, the end face ferrule has three dimensions with a perspective beyond “Zone-3”, usually at ~125-150 micron radius of the core that extends to a vertical.

Certain types of debris outside the IEC-Standard field of view (Zone-3) can migrate into the core. This type of migration would most likely be fluidic contamination. Fluidic contamination can be excess cleaning solvent, an oily type contaminate, or even condensation contamination wherein a connection is moved from one ambient to another. It could also be combinations of “dry” and “fluidic” types.

There are often-used cleaning techniques that wipe, or, spray-apply excess cleaner in the initial phase of cleaning. Excessive cleaning solvent can be harbored in Zone-5 and migrate in the post cleaning, post-inspection process Figure-2.(13)

While video inspection at 100-200x creates a wider field of view enabling inspection outside Zone-3, video inspection instruments may not be capable of accurate image resolution to identify clearly debris or contamination.(13)

The area outside Zone-3 may be further characterized as Zone-4 which leads to the abutment of the horizontal ferrule to the vertical ferrule which may be characterized as Zone-5. Figure-5

Since the ferrule assembly and debris are three-dimensional, there is significance for all precision cleaning applications. A jumper-side ferrule inserted through an alignment sleeve can both transfer debris from one end face to the other. Debris on the adapter can cross contaminate connection components or create a stand-off. The five-zone cleaning standard is relative to all existing connections. These include those with adapters and alignment sleeves, those with multiple fibers from ribbon cable with inter-surfaces and alignment pins, as well as expanded beam connections (Figure 10) with ball lenses, inter-surfaces, and alignment pins. In the case of the former two types, inter-surfaces and alignment pins accurately suggest there is more to precision cleaning than an end face or ball lens surface. Contamination migration is an accepted phenomenon and enters to the discussion of a best practice technique.

As capacity and speed are doubtless to increase, implementation of effective cleaning techniques that do not cause recontamination and minimize cleaning effort are base lines to future connectivity.

IF THERE ARE DIFFERENCES BETWEEN MANUFACTURING AND FIELD OPERATIONS? DOES IT MATTER?

The surface of the end face as it passes through various production line operations, typically are cleaned prior to equipment assembly. The strict control and discipline of the production floor is not mirrored in the field. Understandings of field operations personnel range from “any way we clean is good enough” to disciplined, well-trained, production line level procedures. Some OSP operators have upgraded to “zero-debris” over the complete end face. Thus, working to a higher-standard is becoming common place to stay in line with changing technologies interconnect and other technologies including fusion splice prep.

Differences between OEM and OSP/Field Service are, by and large, a result of the environment in which the precision cleaning procedure is performed. Both have quality technicians but there is no substitute for proper training as the key to success. Existing standards tend to be based on OEM practices. OSP/Field Service Standards based on the reality of the application and wide environmental differences would serve the Industry well.

Be it Fiber-to-the-Home (FTTh) or Fiber-through-the-Airframe (FttA), Fiber to the Node (FTTn) or Fiber to the Tactical Point (FttP), the fiber optic transmission and processes are very much the same. Some transmissions use single mode and others multi mode, some connections are fundamental SC-APC and others are hardened or expanded beam connections. It is the environment and debris type as well as an applications specific connection that distinguishes and amalgamates virtually all fiber optic connections.

An actual cleaning process for an expanded beam may differ as the contaminant on a ball lens may not require as much detail as a common SC or LC. However, both the expanded beam and the “Zone-5” concept on a SC or LC have the same concern: contamination migration from outside field of view or active transmission path. While the responsibility of a commercial aviation carrier or DOD operation is immense, there is also a like sense of due diligence for an entertainment, business, 911 network, or, security/traffic control installation.

It is on the production line that the most likely contaminant may be finger oil or possibly light dust as measured in the context of Arizona Test Dust in IEC 61300-3-35. The sense that complex debris is as easy to remove as simple contamination and that one cleaning technique will work as well as another is not at all accurate. Seemingly to assure this, standards suggest multiple cleanings when a first time cleaning technique is as practical as it is desirable. The technician should not have to be a geologist to determine a contaminant type and select from a cornucopia of cleaning products. Neither should that person be a chemist to determine a contaminant. Technicians are schooled to analyze test data and devise a repair. Precision cleaning is a straight-forward science with a clear path to the a simple task of doing the job right the first time.

The cleaning process should never influence a test and measurement result. A specific ambient is far too varying and debris and contamination likewise, to even begin to assert that most types of debris are one type. Precision cleaning sciences are such that one technique can clean the widest range of debris and accomplish this the first time. (13,14)

WHAT DEBRIS IS POSSIBLE

As wide a range of contaminants as imaginable should be catalogued. One such estimation was made in 2006 with a series of contaminates from Cisco?. The “Cisco Series” ranged from Arizona Test Dust to dried water. Graphite, Simethicone and dryer lint were included.(6) In recent times, testing was suggested on dust from Afghanistan, which is remarkably finer than ATD. Beach sand is coarser than ATD: variety is relevant to an effective cleaning technique. (7)

In 2011, a second series of laboratory evaluations was conducted at the ITW Chemtronics laboratory.(11) An added set of debris and contamination was applied to the end face of a 2.5mm UPC connection. In other evaluations 99.9% Isopropyl Alcohol (IPA) was compared to a proprietary HFE-7100/IPA formulation and a second proprietary HFE-7100/non-IPA formulation against a proprietary precision hydrocarbon. (2,3)

The debris included IEC Standard Arizona Test Dust and vegetable oil and added an additional ten neat contaminants and debris. (neat defined as pure, full-strength, unadulterated). The intent of the study was to create worst case debris that would lead to best practice for OEM and OSP as well as OEM that may have field service or installation capability.(11)

In addition to the dry and fluidic types of debris and contamination, a third type is also possible. Combinations of Debris such as human body oil and Arizona Test Dust/mineral oil, or, carbon black/grease as noted from field service of coal fired power plants. Silicone oils, dust other than ATD as found in some regions, and a host of diverse types of debris should be considered when creating a sense of future proof precision cleaning of existing and new transmission technologies.

The fundamental understanding of precision cleaning the fiber optic connection tends to be overly simplified. This over-simplification leads to confusion for both field service technicians as well as OEM implementation personnel all seeking to increase output and reduce costs. OSP technicians, realizing that they may not have the most effective means of cleaning tend to downplay the task, or, work tirelessly to precision clean when their analytical skills to interpret. For example, an OTDR trace would be a more meaningful use of time.

While there is an effort to categorize debris and contamination, in reality the type of debris and contamination should be open-ended. A “best practice” cleaning process should be judged by worst case debris rather than a cleaning technique that removes a simple soil. In short, “worst-case leads to best- practice. Successful removal of test debris or contamination may be judged by a series of ten successful removals using a specific technique on the same soil. In this way an applications-specific local standard can be established and fed into a data base at SAE, or, IEEE or TIA, Telordia and ultimately into IEC.

In addition to the range of dry debris types or fluidic contamination, static field attraction also plays a role. This is often termed ESA (Electrostatic Attraction) by ESD Association Standard S-2020. ESA is likely to occur when the ambient temperature ranges in the 20-50F range or >95 with relative humidity in the 35-60% range. These are not firm limits or ranges so it is not desirable that a craftsperson be a meteorologist or a geologist to determine a type of debris! Effective precision cleaning techniques should consider all types in as close as possible (98%) to a first time removal procedure.

Static field contamination should be considered as one of many debris types. Generally speaking, there are four ways to control static field contamination: 1.) is use of an air ionizer, 2.) use of a grounding strap, 3.) use of static topical coatings, 4.) dissipation by introduction of a precision cleaning solvent.

The images at right (Figure 6/7) were captured at 72F/50 percent relative humidity. The actual static field was only 78mv and attracted the light dust to the end face and an eyelash! (8)

In the world of production line electrostatic discharge control (ESD), typically there is a 2000v/in limitation. Therefore, electrostatic attraction (ESA) of debris in an OSP working environment of 78kv is an significant low number. The images above were captured by passing the 2.5mm SC/UPC end face over a dry cleaning device in three passes. The subsequent tribocharge attracted the dust to the end face that was placed within 1″ of the debris. At 38kv there was no debris attraction.

Air ionization, as mentioned in IEC 61300-3-35, is the most effective means to control ESA. Air ionization for an OEM work bench is practical and desirable. In most instances, the devices are effective within a 12″ range of the actual work.

It is conceivable that air ionization might be practical in the OSP Central Office or Head End. Such an ionization unit would have to be moved in relation to the actual connection cleaning work area. Air ionization is likely not practical for OSP operations ranging a flight deck to a cellular antenna installation or FTTx node along the side of the road.

Wrist and heel grounding straps are effective OSP devices for electronic component replacement: there is no connection from the user to the actual cleaning surface. Use of a rounding strap is not effective to control ESA in the fiber optic end face application.

Static topical coatings might be effective to control ESA on equipment surfaces, but counter-productive for fiber optic precision surfaces. Static topical coatings or materials could become contamination in themselves.

For the end face application, static dissipation is the most effective and practical means to ESA when cleaning a fiber optic connection. This means of static dissipation requires that the end face, is cleaned with a precision solvent, and immediately connected. Use of a solvent when cleaning, in effect, is the same as increased humidity which serves to control ESA.(8)

In all, the matter of ESA or static field attraction distills to how the end face is contaminated and in what environment. In short, an end face may be contaminated by ESA attraction, gravity depositing debris on the surface, or some type of accidental contact with the end face and the host debris.

In addition to various dry, fluidic and combination types, and those caused by ESA, there is also the potential for condensation contamination. The Cisco? Series of contamination includes dried water and dried IPA. (7) Condensation contamination can be exceptionally complex to remove as this type typically deposits a mono-molecular layer of debris that surface-bonds to the end face. Breaking the surface bond and residual contamination can be difficult. (1)

Condensation contamination (Figure-8) can occur when an end face is moved from one ambient extreme to another: such as 20-F to 70-F. Condensation may contain ambient debris or form a dry residue that is more difficult to remove than the original contamintion type. (1) Dried water and dried IPA is mentioned as a debris in the Cisco?-Series. (1,6,7)

It is important to be aware of all potential types of debris and contamination in each work environment. Simplification of the cleaning process is easy when the many factors impacting the process are clearly understood. This is not always the case.

WHAT IS MEANT BY “PRECISION CLEANING”?

There are numerous tenets of cleaning present in every-day life. For example: 1.) a heavily soiled automobile would not be cleaned with a dry towel: the surface and clear coat might be damaged. 2.) clothing is cleaned with water and a surfactant, 3.) commercial dry cleaning  incorporates a solvent of some type. 4.) when a display window is sprayed with a glass cleaner: it is dried.

Precision cleaning a fiber optic connection follows well-established tenets: a.) debris and contamination are attracted to moisture, b.) the connector type is identified, c.) the type of debris is categorized, and d.) a process is selected that will remove debris or contamination and not result in residual contamination that misidentifies an artifact as debris or vice versa. Ideally, this is a first time procedure while some existing standards suggest a connection be cleaned as many as five times utilizing two or three techniques. One of the major issues with using multiple cleaning techniques is that in the instance of the “first time” which may not perform the residue can create a contaminant or debris that is more difficult to remove than the original. (1)

The science of precision cleaning a fiber optic connection follows well established tenets. It is a process that removes the widest range of debris and contamination without negatively influencing the transmission surface of the end face in any way. A proposed concept that 80% of debris is dusty debris can mean that the remaining 20% can be 80% of the problem! Pareto Logic might not be the best choice when considering the needs of existing and future networks, systems and deployments. An effective best-practice cleaning procedure will consider many types of debris and contamination, as well as catalogues them for the benefit of others.

Worst case considerations lead to best practice and the science, art and craft of cleaning the connection.

WHAT COMPONENTS OF THE CONNECTION SHOULD BE CLEANED?

The concept of considering the connection as a three dimensional structure as well as a wide range of debris and contamination as three dimensional structures is the first phase. The end face horizontal surface is only part of the overall connection. In addition, there is a horizontal ferrule as well as alignment sleeve geometry. Debris deposited in the split ring or other internal structures of the adapter can immerge if consideration is not given to all components of the connector as well as the actual cleaning technique and materials themselves.

Each dimension alone or in combination can contribute to contamination failure. Surely it is unlikely that a dry-type debris on Zone-5 will migrate to Zone-1. However, repeatable test and evaluation clearly exposes that excessive fluidic contamination from Zone-5 can migrate to Zone-1. Furthermore, drying fluidic contamination is troublesome and complex. It is here that a deficiency in contemporary video inspection is disclosed: the vast majority of video inspection only views the area 125-150 micron radius of the core.

Thus, be it fluidic-type or dry-type of combination-type or unidentifiable, the technician can be offered the means to easily clean the end face.

The expanded beam connection can be influenced by debris outside the area of the ball lens Figure-9. In this image debris is evident on the surface area that could be transferred to the transmission area. There are actually three areas of concern, 1-the lens area, 2-the alignment pins, and, 3- horizontal inter-surface, and 4-areas including gasket/O-rings, 5-adapter/sleeve. In the expanded beam at image Figure-10, debris on the surface also has entered the alignment pins and is present on the outer mating ring and on O-rings.

Alignment pin contamination can create a stand-off or other damage when the connectors are mated and these are present in both expanded beam and multiple-terminus types. Figure-9. The multiple terminus type, built from ribbon fiber, has a female side without alignment pins and a male side with them. As is the case with expanded beam, debris and contamination can enter the pin areas as well as the inter-surfaces as well as what can be considered a Zone-5 from the adapter areas. Few of these connections are deployed in pristine environments. A concern is since the fibers stand-proud of the surface that debris may be lodged within the interspacing of the individual fibers. These micro surfaces and newly devised smaller expanded beam connections can be influenced by residual contamination if not properly cleaned.

Undried fluidic contamination can migrate to a ball lens transmission surface. This contamination can be a wide range of from oily, to dusty, and, combination of types. (15)

Currently, cleaning of the expanded bean connector may be done with done with anything from fresh water, dish-washing soap-and-water, to a precision cleaner. Concern arrives when the technician is given dish-washing soap to clean something other than an expanded beam! These may seem whimsical considerations, when there is lack of definition or specificity, far worse occurs outside the disciplined regimen of a production line. It is essential to understand that not all precision cleaning techniques return the same result. Best Practice is to state clearly a procedure leading to first time precision cleaning.

In fact, precision cleaning the fiber optic end face is a relatively easy science. Once debris and contamination are accepted as wide ranging, a universal procedure can be developed. A clearly defined procedure is essential to success and can be stated for virtually all connector types. This debris and contamination is measured in three dimensions that include diameter and height. Figure-11.

One cleaning process should approach the widest range of potential debris and contamination, the various components of the connector, with a technique that does not inadvertently return a negative result such as residual contamination.

This process should be accomplished in as near to a First Time Cleaning process as possible and strive to a goal between 98-100% over the complete surface and the widest range of debris and contamination.

VARIOUS CLEANING TECHNIQUES CONDENSED INTO THE CONCEPT OF 1ST TIME CLEANING

“First Time Cleaning” means more than an economy of time. In fact, first time cleaning of a simple debris such as Arizona Test Dust is not much of a challenge and, as stated, relevant to OEM and not OSP/Field Service.

For “first time cleaning” to be relevant the process must be challenged by a wide variety of debris and contamination. Consideration of Zone 4 and 5 contamination or cross-contamination from inadequate product selection is integral to the goal. Per IEC 63100-3-35, the dry process is identified as the first-choice followed by a “wet/wet-to-dry” technique if the first does not work. IEC cleaning suggests up to five times after which field replacement or other options considered.

Dry cleaning a complex debris has several significant limitations. Among those is the possibility of ESA attraction: more significantly is the reality that drying cleaning tends to move and not remove debris and contamination. Figure 11 and other interferometer images repeatedly record the height of debris may be equal or higher than the diameter. Dry cleaning may be effective on oily contamination: this is dependent on the wiping material itself. (10)

The “dry process” should be selected only when the user has access to video inspection, can identify light fluidic contamination and is schooled in the discipline to use the video inspection each time. “Dry Cleaning” tends to move debris and not remove all types. Figure-12 Even the actual cleaning motion, a twist or a figure-8, can be observed and studied.

An oily contaminant might be removed using the “dry process” with an appropriate wiper or cleaning tool. As noted, the dry technique can generate a static field that attracts additional debris. Since debris has height as well as diameter, some types of debris (metal/Cisco deries, carbon black-coalfired plants, turbine exhaust, hard stone-mining) may damage a connection during cleaning.

A “wet-to-dry” process is an advance over the “dry” technique. Adding a precision cleaner will remove a far wider range of contamination, as well as, achieve static reduction by dissipation. Image 13 is a flooded end face which is demonstrated by the common practice of drawing the end face through an IPA pad. In this case a technician cleaning during fusion splice with IPA may “cross-over” and use the technique for end face cleaning in the hope of “drying”.

However, the “wet to dry” statement and actual process is misleading and leads the worker with a false sense of confidence that something “wet” can be “dried”. (Figure-13) This is not always the case. The simple and often used technique of wiping an end face in a wetted wiper can result in contamination of Zone-5 leading to Zone-4 and ultimately Zone-1 failure. In this video, (Figure-14) gravity also plays a role in contamination of the vertical ferrule.

Especially troublesome is contamination outside the field of view is not likely to be seen. Fluidic contamination also within the adapter can migrate into Zone-3-2-1. The vague wording of prevalent “wet-to-dry” technique leaves wide misinterpretation. The phenomenon in Figure 12 is easily repeatable: a cause for additional concern. Fluidic contamination as well as dry and fluidic debris in combination is also possible. While these theses seem “sublime”, the reality is the vast majority of OSP debris and contamination are not observed. Simply stated, OSP cleaning often is done in “blind faith” and with assumption that one cleaning technique is as good as the other.

This paper argues a “best practice” to precision cleaning the fiber optic connection future-proofs the many sciences of transmission and deployment. “Worst Case” leads to “Best Practice”. Any process should be clearly defined. Therefore “Wet-to-Dry” cleaning is better updated to a third process.

A safety-net procedure is suggested reaching a higher standard than only cleaning limited debris over a limited area of the end face. The combination cleaning or hybrid process is defined as: 1.) high-probability first time cleaning, 2.) clear definition of use of minimal solvent, 3.) selection of a precision solvent with the ability to remove the widest range of debris and contamination, 4.) an integrated drying step within the actual cleaning procedure, 5.) a technique that is functional and simple yet performance based on the myriad issues discussed in this paper.

Use of a cleaning solvent is mandatory necessary to clean properly: these tenets are universal and ubiquitous.

Also, a combination or hybrid process clearly defines the wiping material as well as the amount of cleaning solvent. The technique uses less than 1ml of cleaner. The wiping material is never 100% cellulose (paper). A simple wiping material test is done by tearing the wiper: if it shreds like a paper napkin it is never acceptable. Selection of the solvent is more complex: the end user should challenge the provider to supply tests of results over a wide range of debris and contamination. Isopropyl alcohol, the industry standard for many years, is not acceptable for end face cleaning. While there may be some matters of update for the IEC 61300-3-35 mother standard, the document clearly expresses concern about even 99.9% IPA.

Why? Even highly fortified IPA, such as the type used in de-fluxing of printed circuit boards, is only capable of removing an ionic contaminant such as a salt or human body oil. Since there are many other potential contaminate types, including mineral oil, silicone oil, lanolin, and those in combinations, a solvent that removes both ionic and non-ionic debris and contamination is “best practice”.

Even the actual cleaning technique is important. For proper cleaning the end face is placed in the solvent (most typically less than 1ml or about the size of a USA quarter-dollar coin) and then lightly drawn in a straight line action so to emulsify the contaminant and automatically dry it in one motion. Debris and contamination are moved away from the initial point of contact.

The technique is not a Figure-8 action which can retrace debris over the cleaning path. The technique does not twist or turn in the initial contact: that motion can grind debris into the surface. The process is never performed on a hard surface, which can grind a gritty debris into the end face, or over a forefinger, or, palm of the hand which may transfer body oil, hand lotion or other contaminant through the wiping material to the end face.

In short, precision cleaning the end face is a clean room activity performed in a sand storm, on top of a cellular antenna, in a storm-reclamation environment or pristine production line. All are the same and it is one for all.

A precision cleaning process should encompass all aspects of the connection. (Figure 5) These include: the horizontal ferrule, the vertical ferrule, the alignment sleeve and all aspects of the adapter. In part, this is done reducing solvent use to a minimum as well as selection of an effective cleaning technique. Looking to the future is best done by updating the knowledge base now.

Training and “untraining” can be a lengthy process that is outpaced by the technology of speed and capacity. Best practice standards should be created now in anticipation of future technical advances.

Materials selection

Proper cleaning requires attention to detail in materials selection. Since the best technique is a combination of a wiping material and a precision chemical. Both fundamental components require clear definition.

While the wiping material may not be 100% cellulose (paper), it may be a combination of cellulose and polyester that is often termed a non-woven. Research carefully as there may be as many as 4,000 non-woven types available! Here again it is important to challenge the provider and require testing, demonstrations and samples. Proper non-woven wipers come in many forms: squares, rectangles, and even some cleaning platforms use them.

Certain cleanroom-grade microfibers are an advanced wiping material. Certain of a broad list of non-woven polyester/cellulose materials provide a cost-efficient alternative to cleanroom grade microfiber. Certain foam, often maligned, is also acceptable. Clues and cues to effective fiber optic precision cleaning may be garnered from the massive clean room segment.

Precision solvent selection is often controversial. The primary consideration is worker safety in the recommended dosage per an MSDS. A parallel concern is environmental impact.

Contemporary choices range from esoteric non-chlorinated formulations solvents (HFE/HFC/AK225) to precision hydrocarbons and new aqueous formulations. Solvent selection is best considered when seeking a cleaner that works on the widest debris and contamination. The selection is vast but there are few precision solvents that meet a “first-time cleaning” criteria. User safety, performance, cost of materials are considerations. Shipping is not a performance criterion: chemicals are regulated and shippable per DOT or IATA.

Chemical selection should be based on an evaluation of all available chemical types. The reality is that there are few “universal solvents” that removes a wide range of debris and contamination. There may be as many as fifteen HFE-7100 formulas: some of which use IPA while others do not. There are tradeoffs: some of the esoteric formulations are less capable cleaners and also have EPA Global Warming values. Precision hydrocarbons are flammable and are far better cleaners but are VOC like IPA. Aqueous cleaners require an active drying phase while possibly as good as precision hydrocarbons. Esoteric formulations tend to be expensive; precision hydrocarbons are close to the flammability of IPA which has been an industry-standard for many years.

IPA itself, even in 99.9% strength, is not the best choice: 1.) it is not an effective cleaner on the widest range of debris and contamination, and, 2.) it is hygroscopic meaning it quickly attracts moisture to itself to the time it reaches a 65/35% equilibrium. Storage of IPA in pump or plastic containers exacerbates the weaknesses. (2,3,4)

The actual cleaning tool is also an essential selection. There are many devices that range from reel type cassettes, to probe-type tools to precision swab tools to cleaning platforms. The producers of cassettes and probe tools claim convenience while those that developed cleaning platforms point to the larger cleaning surface as a significant aspect of the science of cleaning. Probes and swab tools have the smallest cleaning surface and have the most difficult task of precision cleaning: small surface has difficulty removing debris. Cleaning platforms work better as there is a larger surface to carry away the debris and contamination: they are easier to moisten for a best practice technique. It is highly recommended your choice be made not on convenience, but rather performance. A small inconvenience can return benefits in time, reputation, and, customer satisfaction.

Standards themselves should be fluidic and updated as new challenges are found. A standard published may not be disseminated into the field for some years and not updated for more years after that time. The fiber optic transmission industry changes quickly and the precision cleaning process should envision those changes by working from worst case to best practice rather than convenience and utility as the main goal.

CONCLUSIONS

There are various cleaning products and techniques to clean a fiber optic connection. Current standards are intertwined for the production line (OEM) and field operations (OSP). A production line is well-disciplined and trained for multiple and repeatable operations with various levels of actual cleanliness.

The field or OSP encounters a wide range of potential challenges. The actual environment at the point of a field service/OSP operation varies widely. Often, there are signifcant differences between the two disciplines.

Awareness of contaminants as well as contamination points is important. There are myriad potential sources of contamination influencing the jumper-side end face, the alignment sleeve, the back-plane end face. Even surfaces of an expanded beam connector have contamination points other than the lens. Cleaning one should not sacrifice another. Debris or contamination in an adapter or Zone-5 can be dislodged with a cleaning process that uses excessive solvent or one that does not clean a wide range of debris and contamination. Currently, there is no practical means to measure or see Zone-5 or contamination emanating from an adapter. A process change that promotes minimal solvent use protects such fouling of the connection.

In OSP a wide variety of potential debris and contamination may be present. These myriad types are not only dry, but also fluidic, and, in combinations. These have diameter as well as height. Dry debris of all types as well as fluidic contamination and those in combinations can extend beyond the existing 3-Zone standard. A higher level Five-Zone Standard anticipates “worst case” and leads to “best practice”.

There are two commonly used cleaning techniques. The “dry” method has limited effectiveness on limited types of debris. The “wet-to-dry” technique can flood the end face as well as geometry within close proximity to the ferrule. “Wet-to-dry” cleaning is an advance over “dry cleaning” but it is limited by the factors discussed in this paper. The third technique: a.) defines an amount of precision cleaner to a minimum, and, b.) dries this cleaner as an integral aspect of the process. It is an easily trainable advance that considers environment, deployment skills, and OEM production of existing and future fiber optic connections and equipment.

Multiple cleaning of an end face is not desirable, nor practical, nor necessary. A 1st Time Cleaning Standard should be the goal of all working the Industry and championed by both OEM and OSP/Field Service interests.

Select your cleaning materials as though you were working in a Class-1 Operational Cleanroom! This does not mean they have to be expensive or tedious to use. Require your provider to train use of the product through demonstrations or video or other means. Don’t let a new product or procedure be misused. Separate end face cleaning from fusion splice prep applications.

A new IEC Field Service Standard (OSP) should be written to enhance IEC61300-3-35 for OEM: all subsequent standards updated to higher levels.

Ed Forrest is the zone manager of fiber-optic products for ITW Chemtronics.

Fiberopticcleanings.com stocks a variety of fiber optic cleaning supplies, from one click cleaner to fiber optic cleaner, from fiber optic cleaning wipes to fiber optic cleaning kit – everything you need for fiber optic cleaning jobs, BUY NOW and get FREE SHIPPING on most items!

 

In order to know how effectively your fiber optic cables are transmitting, you’ll need to test each one for Optical Loss. The term “Optical Loss” describes the difference between the amount of light sent into the transmitting end of a fiber optic cable, and the amount of light that successfully makes it to the cable’s receiving end. This tutorial covers the basic steps involved in Optical Loss Testing.

 STEP 1 
Connect the test cable to the reference cable
     
 STEP 2     
Connect the test source to the transmitting end of the test cable
     
 STEP 3 
Connect the power meter to the receiving end of the reference cable
     
 STEP 4     
Using the test source, send a light signal into the test cable
     
 STEP 5     
At the receiving end of the reference cable, take a reading of the light signal with the power meter