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Measurements in optical cable networks

20 February 2012

 The article considers the main types of instrumentation required for construction and operation of the optical cable networks. Their operating principles, main measurement schemes and parameters of devices are described. Helpful advice is given for entry-level meter-men. There are references to specific models of devices of various types.

 

 

It's no secret that use of optical communication links strongly entered our everyday lives. There’s hardly a provider of telecommunications services that would not be apply optical fiber as communication lines. There are exceptions to the rules, of course, but they are old prejudices and sooner or later optical fiber will be used for data transmission.
Now the market proposes a huge range of products for construction of optical communication links such as cable for different laying conditions, cross-connecting equipment, and various accessories. It seems just to buy, construct and that's it. But far from it!
The main element of optical networks is optical cable, or rather an optical fiber in it. Quality of installation during construction determines the network reliability and durability, as well as minimum cost of emergency repair work. This brings up quite a logical question, "How to control quality of optical links?" And it is here that we cannot go without a total class of equipment called measuring equipment for optical networks.
The equipment includes, first of all, optical reflectometers (OTDR), optical tester, optical power meters, laser sources, sources of visible laser light (flaw detectors), identifiers of active fibers, etc.
If you still have to work with optical fiber, it is necessary to get familiar with the main types of measuring equipment. In this article we try to clear up the operating principles of those devices in detail, present typical connection schemes and some nuances.

 

Why do you need it?

 

Many people may wonder, "Why is it necessary?" as it works itself! It’s up to a person, of course, to decide whether to purchase the measuring equipment or not. But those who happened to face the problems during construction, operation or maintenance of optical networks would no doubt answer you that you cannot go without it.
First of all, when constructing the optical links (same as in other construction cases), the construction companies shall control quality of the work done, and you definitely cannot decide just by eye whether the works are performed correctly and accurately. Measuring equipment is also required for controlling various characteristics (for example, optical signal level, fading in the baseband transmission path, loss at welded connections, etc.) at pre-commissioning (commissioning) of optical networks. Repairing in case of accidents is problematic without knowing the exact damage area.
And now let’s turn out attention to the issue essence, namely, what features of optical links one shall learn in the first place and what devices can measure them.
The first and probably most important characteristic is attenuation (measured in dB) in the optical path at the operating wavelength. This value shows how optical signal fades (attenuates) while passing through a given link. It is also called "Insertion loss" or "Insertion attenuation".
Main elements which cause attenuation in the optical path are optical fiber itself (characterized by loss per length unit, dB/km), welded joints, mechanical connectors, optical splitters.
The other equally important characteristic is back reflection («Optical Return Loss» or «Back Reflection»). This value (also expressed in dB) characterizes optical power that is reflected back to the emitting source.
The source of back reflection can be mechanical connections, cracks in the fiber, as well as the free end of optical connector.

Cleanliness is a keystone to success

 

Before proceeding to measurement in fiber optics, you should remember a very important rule - optical connectors shall be kept clean. Since the fiber core diameter is about 9 microns, one cannot see pollution with unaided eye. But the fact is that pollution is always present. And it does not matter at all where and how the connector was stored, where it is old or new, pollution will be present at the ferrule end in any case. First and foremost, it will affect the accuracy of measurements that will be discussed further on. Extent of loss which can be introduced by "dirty" connectors may vary widely, and make up to several dB. Pollution also increases the value of back reflection, which is extremely undesirable at transmission of AM signal of cable television.
Surface of optical connectors can be cleaned by various methods. The easiest and most economical one is a lint-free cloth soaked in pure alcohol. It should be noted that wiping with a damp cloth shall be followed with dry wiping to remove streaks. One of the most convenient methods is application of special lint-free cleaning tapes which make cleaning of connectors fast and convenient.

 

This device allows fast and quality cleaning of the ferrule end surface from various contaminants; it is suitable for various types of connectors: SC, FC, LC, ST, and MU.

 

The cleaning process is performed in just two steps. You must first open the protective shutter and pressing tightly the connector end face towards the cleaning tape – pass along the guides, first forward and then backward. A special microscope of 200-fold magnification power to control the surface finish.

 

 

Sources of visible laser light

 

This is perhaps the simplest device that represents the source of red light (650 nm) emitting of which is introduced into optical fiber. The main purpose of this device is local detection of various damages (cracks, bends, faulty welding, etc.). Bright glow will be observed at the damage point. Standard distance at which this device can be applied is 3-5 km.

Application in optical cross (wrong fiber laying)

 

The photograph shows a bright glow at a fiber output of the protective sleeve, indicating the exceeded minimum allowable radius of pigtail band. In this case, there is radiation yield in the shell and, consequently, additional loss, especially at a wavelength 1550 nm.

 

Defects in fiber Detection of defects in pigtail

 

The following picture shows the defects of optical fiber pigtail. They are illuminated with red light and can be easily detected, even in bright daylight. Those may be micro cracks or other local damages in the fiber due to mechanical injuries; but in any case subsequent use of such a pigtail is not recommended. It should be noted that appearance of pigtail is perfectly normal, but as we apply the source of visible light all the defects become apparent at once.
These instruments are indispensable for assembly work at cross equipment, functional testing of optical patch cords with different connectors (SC, FC, and ST), pigtails, for identification of the required fibers by "highlighting" them, etc.
Their main advantages are compact size, ease of use, versatility, and most importantly – small price.

General view of devices MULTITEST MT3105 and LEADLIGHT VF-65-BU2S

 

 

Sources of laser light

 

Here’s some information about design of the given devices. Laser light source is a device the main element of which is a semiconductor laser (laser diode), number of which may be different. The most common ones are wavelengths of 1310 nm and 1550 nm, since optical signal is mainly transferred at these wavelengths. There may be various combinations of different lasers, design of some laser light sources may be provided with the source of visible laser light, as mentioned above.
But the main purpose of these devices is generation of laser radiation at a set wavelength to measure loss in the optical lines. Standard optical power is 7dBm. Additional features of the laser light sources can be generation of not only a continuous signal, but of a modulated signal of the given frequency (e.g. 270 Hz, 1 kHz, 2 kHz) for fibers identification, automatic shutdown, battery level, etc.
Output port of the sender is usually equipped with adapter FC/UPC.
Some models of these devices can be equipped with built-in red light transmitter (separate port) for visual defectoscopy.

 

General view of the light source MULTITEST MT3109



 

Optical power meters

 

This device records the level of input optical power and displays the value on the screen. The main element of the device is a photo detector.
A broadband photo detector is usually used. It registers all the optical power coming onto it within the range 800 - 1800 nm. By measuring the wavelength (calibrated), we obtain the numerical value in dBm or W. If there is radiation at several wavelengths at a time in the path, the device displays the total power value.

Typical values of the measured (calibrated) wavelengths are still the same 1310 and 1550, but there may be others as well: 850, 980, 1300, 1490 nm, etc. Dynamic range of the meter (optical power which it can measure) depends on the used sensor; a typical value for InGaAs is approximately 60-70 dB. An optimal device is chosen depending on your specific application. Power meters with a more sensitive photo detector (+6…-70 dBm) are suitable for measurements in telecommunication networks, and measuring of rather great power (+26…-50 dBm) is important for optical cable TV networks. Same with the radiation sources, the device operates from a built-in battery, has a display lighting, automatic shutoff, function of keeping the results, etc. The input optical port is usually provided with adapter FC/UPC. One of the most important functions of this device is measuring of the loss of optical signal with respect to any initial level (see details below).

  

General view of MULTITEST MT 1103, MT1108 and MT1106

 

 

Optical tester

 

This device is a light source and optical power meter within one housing. Everyone can decide himself about its advantages and disadvantage as compared to separate devices, taking into account specific character of this device application.
• compactness;
• independent operation of the source and the meter;
• similar functionality of the source and the meter.

General view of optical tester MULTITEST MT3204С

 

 

Let’s pass over to the issues of practical application of these devices. The first and most important task is to measure signal attenuation in the optical line. To do this we need both the light source and optical power meter.

 

Loss measurement by insertion loss method

 

Since the meter determines the power level only, two measurements are required to measure loss (attenuation) in an optical line. First we determine power level at the radiation source output (reference level), and then - the signal power transmitted through the line under test. Difference between these values (in dBm) or logarithmic relationship (in W) between them makes up loss in the line.
Since the meter determines the power level only, two measurements are required to measure loss (attenuation) in an optical line. First we determine power level at the radiation source output (reference level), and then - the signal power transmitted through the line under test. Difference between these values (in dBm) or logarithmic relationship (in W) between them makes up loss in the line.
The reference level is determined by direct connection of the source and meter through a connection cable (patch cord). When measuring, we set the corresponding wavelength at the source and the meter. Once the result is received, we turn over to the mode of relative loss measuring (button dB); value 00.00 dB is displayed on the meter. It allows us not to do recalculation, and the next measurement gives us attenuation value directly on the meter display.

Definition of the reference level

 

At second measurement, we connect (after cord) the section of our concern, where we need to measure the loss; right after that, we’ll obtain the loss value (dB) at the screen.

 

Measurement of loss in the line by insertion loss

 

This measurement method is very simple, practical, not time-consuming and requires no expensive equipment. We achieve small measurement error of 0.1 dB. In the absence of a measuring radiation source for measuring the attenuation, we can use any optical transmitter with a wavelength that is available in your power meter provided with a continuous radiation mode (CW).

If you have to measure the loss when both ends of an optical line are in one place (for example, hank of cable), it will be convenient to use an optical tester. Measuring principle of such device is similar to joint operation of the source and the meter. Standard measuring scheme by means of an optical tester is presented below.

 

Measuring of the reference level by a tester and setting to nominal zero

 

 

 

 

Measuring of insertion loss by means of an optical tester

 

Insertion loss introduced by the tested fiber sample is displayed on the optical tester. With the help of an optical tester (as well as devices couple power + meter) we can measure the insertion loss of not only the linear fiber sections, but of optical dividers, mechanical connections, etc.

 

 

Power measurement in optical networks

 

In addition to loss in the line, power meter allows defining the level of optical power at different points of optical network. For example, we have to measure the optical signal level at the optical receiver input of the optical cable TV network. With this view, we connect a meter in the interesting point (with the operating network), set the wavelength at which the signal is transmitted, and measure the signal level. As a result of such measurement, we receive a value in dBm. If this value corresponds to the allowable input level of the optical receiver, and coincides with the calculated design value, loss in the optical path (optical transmitter - optical receiver) is within the acceptable range (standard input level value is -7 dBm to + 3 dBm, depending on the type of optical receiver).
Moreover, if it is possible to measure the signal level not only at the receiver input, but at the optical transmitter output as well, we can quite accurately estimate the loss in the optical path.

Measurement of optical signal level in cable television

 

Note: Optical connectors with angled finish (APC are used in cable television networks, and it shall be considered since the optical power meters usually have finish of UPC type. In this case, combined optical cords shall be applied to prevent bonding of connectors with different finishes.

 

 

PON tester

Of special note is a separate type of testing devices for completely passive optical network (PON network). Testing is performed by switching into an optical line (into break), and scanning at the same time at three wavelengths – of upstream (from subscriber to head end) at wavelength 1310 nm, and downstream (from head end to subscribers) – 1490/1550 nm. It saves time and provides the most complete measuring pattern. The main difference with the optical power meters is presence of optical filters and separate photo detectors for each measured wavelength.

Appearance of tester MULTITEST MT3212 PON

 

Measurements can be displayed in different units – dBm or W

The device can save the measurement results in the internal storage thus further data analysis on PC is possible. Another very useful feature is automatic turn-off, which greatly increases the device operation time from the battery.

PON tester can be used when a PON network is put into operation to control the optical power levels, and during repair work, as well as for the network monitoring.

Details of use of PON tester can be found in article “Measurements in passive optical networks (PON)”

 

Identifier of active fibers.

 

  Device appearance

 

The above figure represents a compact device MULTITEST MT3306A for detection of active (presence of optical radiation) optical fibers. The device provides for a fast nondestructive method for determining the presence and direction of optical signal propagation in single-mode fibers. The device does not require switching off the transceiving equipment to determine the signal presence in fibers and its direction, and to assess optical power. If source modulated radiation of 270 Hz, 1 kHz or 2 kHz is used as a signal, the identifier determines the modulation frequency as well. The operating principle is to record optical signals in macro bend point. Versatility is provided by exchangeable tips for different diameters (fiber, pigtails and patch cords).

In terms of practical application, this device is very useful when searching for "active" and "dark" fibers in optical crosses and connectors in which many fibers are used and there is a great chance of accidental disconnection.

 

Measurements with optical reflectometer

 

The above methods allow measuring the level of optical loss in the line, but they are useless for detection of a specific damage point in the event of an emergency. The only way out for such situations is optical reflectometer (OTDR).

In this article we will try to highlight the key points during measurements using OTDR, pay attention to practical issues, and will not go to the theory heart.

So, what measurements can be made with OTDR?

•    within one measurement cycle it allows simultaneous (and without preparatory works) determination of a number of basic parameters of optical fiber: its length, attenuation per kilometer, presence of discontinuity, its nature and distance to them, loss in connectors, weld points, etc;

•    plenty of measurements at one end of optical fiber, in contrast to optical testers.


Similar to ant other method of measurement, reflectometry has its problematic issues:

•    heavy demands on the radiation input into fiber under test;

•    at least 30 seconds are required to receive a reflectogram of relatively good accuracy;

•    relatively high cost of measuring equipment.

 

Operating principle of reflectometer is sending a short optical pulse into the fiber under test. Because of reflections from various irregularities, backflow (back scattering) takes place. Reflectometer measures the signal delay time and level of the reflected radiation. Based on these data, a reflectogram is constructed which is a loss in the fiber – distance graph.

Let’s not go into details of the data processing method for measurement results, but consider the finished result of measurements and show what is displayed on reflectogram.

 

Irregularities in optical fiber displayed at reflectogram

 

The above figure shows the reflectogram pattern with irregularities that may occur in the fiber.

 

What characteristics of reflectometer shall be noticed when choosing the model?

Main parameter of any OTDR is its dynamic range. This parameter characterizes the range between signal transmission level and minimum reception level (usually at a signal/noise ratio = 1). A typical average value of this parameter is 34-36 dB. Model of dynamic range 28-32 dB can be used for measurements in short lines, and of range up to 40-45 dB or more – for lengthy sections or networks with high attenuation in passive elements (PON, extensive networks of CTV).

One more characteristic of any OTDR is a dead zone that is distance at reflectogram after irregularity, at which measurements cannot be made. The very first event present at any reflectogram is reflection of input connector. Since this connector is in close proximity to photo detector, reflection from it will "blind" the photo detector. This is the OTDR area falling into the dead zone.

There is a reflection dead zone and attenuation dead zone. Typical values are as follows: for reflection 1 - 3 meters, for attenuation 7 - 10 meters. It means that to make a correct measurement, distance between irregularities should be no less than 7-10 m, and two next connectors, for example, can be seen in reflectogram over 1-3 m.

  

General view of reflectometers Yokogawa AQ7275, Radiantech OT-8810 and Radiantech UFO-320

 

Influence of dead zone on reflectometric measurements

 

If it is very important to make measurements and see the literally first meter of the studied route on reflectogram, a so called "compensating coil" or "match throttle" is applied, its name may be different, but the meaning remains the same. It represents a segment of optical fiber of a certain length, usually from 100 m to 1 km. With this device all the "dead zone" falls onto the length of this fiber, after which we can see the whole beginning of the measured route. If the latest optical connector shall be seen as well, then a so called “receiving coil” is installed at the end of the line. This is the same fiber section that compensates the dead zone at signal reflection from the far end of the fiber. When measurements are made with such additional coils, our optical link will be in the middle of reflectogram that allows testing its performance with certainty.

Reflectogram with the applied match throttle and receiving coil

 

Different models may be provided with a variety of additional features. For example, a detection function for presence of radiation in fiber (active fiber), connection of the tested connector to the input optical connector of reflectometer, overlaying of several reflectograms, double-sided analysis, various notification and warning functions.

Advantages of some models include built-in light source, visible light source, optical power meter, etc., but all that directly affects the cost (not downward, of course).

 

Helpful tips on reflectometer operation

 

Often, when using a reflectometer, operator performs switching of optical connectors with different polish (UPC-APC) that is positively inadmissible. First of all, it will damage the ferrule surface of the optical connector input of reflectometer, and secondly, the measurements will not be reliable. To prevent such situations, different combined optic cords (patch cords) with different polish types at their ends shall be used. Let us remind you once again that absolutely all optic adapters (connectors) have a finite number of connections, so connection characteristics will worsen over time. Application of a patch cord at the output of optical connector of reflectometer would greatly increase this device working hours without repair. Also do not forget about the cleanliness of optical connectors: you cannot see the dirt with a naked eye, though the dirt is always there, even if an optical patch cord pigtail is just unpacked. Insufficiently clean connector connected to OTDR can strongly distort the reflectogram image, since the device really operates with very weak echoed signals.

 

Optical link damage detector

 

One of the biggest challenges for reflectometry – determination of distance to the damage point – can be successfully met using a simpler and therefore cheaper device – optical link damage detector (Fiber Ranger). This device operates by OTDR principle: it sends outgoing pulses into the line and detects the reflected power. However, it cannot perform a serious mathematical signal processing, or construct a reflectogram; it just shows the distance to the strong reflection of optical power (up to the breakage, till the fiber end, etc.). The measurement result is displayed on the device screen in meters.

The device is very useful when an optical network is operated and it is important to define the damage point quickly. Fiber Ranger is extremely user-friendly, has good sensibility characteristics (from one to several meters) and can display distance values for up to 8 events (for example, intermediate low-quality plug-type connections at optical line, strong fiber bends in cartridges, etc.). The device is provided with a built-in laser emitter of red light (650 nm) for visual detection of damages.

Appearance of MT3304N Fiber Ranger

 

 

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Nowadays, provision of quality services is one of the main criteria in telecommunication sector. DEPS Company can always help you to choose the measuring equipment that ideal for specifics of your network, so to ensure its reliable and durable operation.

 

DEPS Department for Fiber-Optic Technology and Cable Networks

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