4.1 What to measure?
There are a lot of parasites in optical networks, that deteriorate signal's transmission, but only two of them has special influence to the PON network: attenuation and return losses.
Attenuation — power loss of optical signal when its expansion in optical fiber and when its passing through passive network elements: splitters, mechanical and welded connections.
Figure 23 - Attenuation influence to signal amplitude
ORL — OpticalReturnLoss) — optical signal alteration, when its expansion in optical fiber under the action of return signal, that forms on mechanical and more rear on welded connections.
Figure 24 - ORL
*ORL determines as log likehood of base signal power to return signal power, that is why as higher ORL rate, as better the quality of transmitted signal.*
It is possible to decrease reflected signal, using some connectors with APC polishing in places of mechanical connections (ORL = 65…70dB) instead of standard UPC (ORL = 50…55dB). As it was mentioned above, is it most significantly to pay attention to ORL rate in case if You want to transmit CATV through PON, because reflected signal strongly influences to analogue TV quality (especially of it has amplitude modulation).
Thus, when PON network building and using, it is necessary to attend to only two rates: attenuation in the line and return losses (ORL).
4.2 With a help of what to measure?
To measure PON the same equipment is using as for FTTX, but with more absolute characteristics. That is why we will not describe here work principles of one or another devices (we hope, the reader has minimal idea about it), and will only provide with a list of necessary measuring equipment with main technical parameters, that fit to PON network measuring.
OpticalPowerMeter — ОРМ.
Main characteristics: photodiod type (Ge, GeX or InGaAs), working waive length (nm), photodiod sensibility (dBm). When OPM choosing, it is better to attend to models with gallium arsenide photodiod (InGaAs), as these meters work stable in any temperature conditions. Fro work with PONOPM must meter a signal with power from -40dBm to +20dBm om waive length 1310nm, 1490nm (1550nm and 1625nm — optional).
StabilizedLightSource — SLS.
Main characteristics: lazer power (dBm), working waive length (nm). SLSs usually generate low signal with power -5...-7dBm, so to attenuation measurement between terminal nodes of PON network, SLS should work together with OPM, that can measure a signal to -40dBm. It is desirable for SLS to support mot only standard waive length (1310 nm and 1550 nm), but also 1625 nm (on the present waive length is is comfortable to fulfill measurement in "living" network).
*There are devices on market, that combine OPM and SLS - OpticalLossTestSet — OLTS. If there is a pare of such devices, it is possible to fulfill two-side testing of the optical line very effectively.
Optical Time Domain Reflectometer — OTDR.
Main characteristics: working waive lengths (nm), impulse widths (ns), dynamical range (dB). Usually OTDR fulfills measurements in two waive lengths (1310nm and 1550nm). Nevertheless, OTDR, that can also work om waive length 1625nm, lets to detect more clear fiber defects (macro kinks) and can fulfill measurements in the working network without connection breach.
Impulse width - is a rate, makes all the difference of dynamic range. Wide impulses let take reflectogram from more long lines, but deteriorate measurements specification. Short impulses, vise versa, let to clearly specify a discontinuity location, but on small distances. Reflectometer of good quality can generate impulses with width from 5ns to 2000ns.
Dynamical range specifies fiber length, that reflectometer can "illuminate". If installer has a task to "illuminate" that small part of PON, OTDR with dynamical range ~30dB will be suitable; but if it is necessary to "illuminate" all PON tree completely, it is better to choose reflectometer with dynamical range that is not less 35dB (better 38-40dB).
*In OTDR characteristics, dynamical range specifies together with impulse width, at which it was changed, that is why at equal dynamical ranges, it is better to choose device, that has lower impulse width.*
ORL meters.The present devices are rare produced as separate device — instead ORL modules are built in many OLTS and OTDR. ORL meter is somehow similar to reflectometer, but it meters return loss level more careful.
Main characteristics: working waive lengths (nm) and ORL range (dB). Simple ORL meters work on two waive lengths (1310nm and 1550nm), more expensive work on waive length 1625 nm as well. ORL range fro middle-class meters in near 50dB. It is quite enough for PON. If there is a task to install PONCATV, it is necessary to measure the most low reflected signals and in such case ORL range must be lower than 60dB.
Exclusive standards to measuring equipment for PON networks are results of large optical loss budget (more than 25dB), installed to the very technology. If there is no task for network installer to measure signal between terminate network points ( crudely speaking, between OLT and ONU), there is no necessity to purchase the most modern and expensive meters.
4.3 How to measure?
On all optical network building stages (PON is not exclusion), it is necessary to fulfill it's quality examination - to measure attenuation per unit length of cable span, attenuations on separate passive components (adapters, splitters) and on welded connections, complete attenuations between network termination points, as well as return losses.
In most cases, network installers are economical and practical people, thus they use for measurements of only Source Stabilized Light (SLS) and of Optical Power Measurement (OPM). Connecting them to different network points, it is possible to measure attenuations, as on separate passive elements and connections, as all optical pathway power losses (Figure 25).
Fig 25 Connection sample SLS+OPM
The most economical networkers use power meter only. They use optical modules transmitters (SFP, XFP, etc.) instead of SLS. In this case measurements will be less exact (±1dB), while transceivers transmitters are not constant light sources. In such scheme (figure 26), measurements flexibility losses, while radiation source is connected to one network point.
Figure 3.2 OPM + terminal equipment connection sample as radiation source
If you plan to transmit CATV through PON, you will fatally meet one more problem - how to measure the signal power on concrete waive length. Usual OPM have wide range photodevice and don't have built filters, i.e. they measure group signal. It is necessary to connect OPM to line through CWDM flack (light filter) in such case, that will reduce undesired signal.
Figure 27. OPM connection through CWDM flack.
But there is simpler solution - its bushing-type PON instrument, that can measure signal on three waive lengths (1310nm, 1490nm, 1550nm) at the same time, bringing minimal attenuation (<1.5dB) into the line. The other exclusive side of the present meter will be specified below.
Figure 28. Bushing-type PON instrument connection sample
As a rule, when network building ends, it is important to document all important optical line rates. As such documents, usually acting taken from different network nodes reflectograms. When breaches, presence of base reflectogams let to detect fast breach place and type due to base and "breach" reflectogram comparing.
Figure 29. Reflectometer (OTDR) connection sample
But if reflectometer using in FTTX network is usual and simple, there are a lot of questions when installer is in PON network.
After the network is built and is put into operation, new question arise: how to fulfill measurements in operating network? Measurements on waive lengths 1310nm and 1490nm are not possible for reasons you can appreciate. Only 1550nm (if Your network doesn't have CATV) and reserve 1625nm(that was mentioned above) are left. That is why it is desirable to purchase SLS, OPM and OTDR, that are able to work with such waive length. To insert reserve signal from SLS to the fiber and outcome it from the fiber to OPM, it is enough to use two CWDM flasks (one on transmitting side and one on receiving side). The same connecting has reflectometer.
Figure 30. Measuring sample without connection breach.
The one more problem is signal power from ONU measuring. If connect the meter to ONU, it will show nothing, i.e. ONU does not transmit the signal without OLT "permission" (Figure 31/a). If connect the meter to ONU through the splitter (Figure 31/б), it will show incorrect rate, i.e. usual meter calculates average power rate for specified time period, and ONU is "silent" most time.
Figure 31. ONU transmitter power measurement variants
To solve this problem lets PON-meter, that was mentioned above (Figure 31/в). On waive length 1490nm it works as usual power meter, but on waive length 1310nm, it works in impulse mode, i.e. meters the peak signal amplitude, that lets to measure ONU transmitter power correctly.
Results: to build and maintain the optical network of high quality (not PON only), it is necessary to have and to operate by measuring equipment. Measuring equipment absence may cause time and subscribers loss and bring additional charges (for example, outsourcing of specialist with measuring equipment, in any, even "small" and not important case).
Within the project UA.PON, that has started more than three years ago on the territory of Ukraine, in was made a decision to build PON based on active equipment (OLT and ONU), produced by Chinese company BDCOM. BDCOM choice is not random: It is leading in China in network equipment producing and selling as on domestic, as on international market. Moreover, equipment is of quite high quality and consumer grade. And this all together with complete support of native Research and Developing Department, that may solve any program or apparatus problem in shortest terms.
Thus within the RU.PON project, the BDCOM company equipment was expected chosen.
As GEPON technologies become more and more popular worldwide, BDCOM creates a greater variety as of head stations, as of subscribers equipment.