Return Loss (ORL - Optical Return Loss) - distortion of the waveform in the process of its propagation through the fiber under the influence of the reflected signal.The reflected signal, caused by Fresnel reflection, greatly affects the transmission quality of an analog TV signal (especially if it has amplitude modulation).Therefore, in most cases, the ORL indicator, along with attenuation, characterizes the quality of the optical communication line.However, if no CATV transmission is planned on the PON, then the ORL measurements can be omitted.Note: 1) ORL is defined as the logarithmic ratio of base signal power to reflected power, so the higher the ORL, the better.2) The reflected signal can be reduced by using connectors with a more modern type of polishing.Regular UPC polished connectors have ORL values ​​around 50..55dB.When using connectors with APC polishing, ORL increases to 65..70dB.Stimulated inelastic scattering.There are Raman (SRS) and Brillouin (SBS) scattering.The essence of these phenomena is that at a high power density of a single signal (~10..20dBm for SBS and ~30..40dBm for SRS), the fiber refractive index is distorted, as a result of which counter (Stokes) and co-directional (anti-Stokes) signals at adjacent wavelengths (λ±Δλ).In the PON network, the SRS effect can be ignored, and the SBS effect will manifest itself only when a cable television network is added to the PON (the average power of the CATV signal after the amplifier is ~18..20dBm).Moreover, most modern TV transmitters use special SBS-C lasers (externally modulated lasers with a "smeared" frequency of radiation), which can significantly reduce the effect of SBS.Thus, during the construction and operation of a PON network, it is necessary to monitor only two indicators: attenuation in the line and return loss (ORL).Optical power meters (Optical Power Meter - ORM).Used to measure the optical power of a signal at different wavelengths.Paired with a stabilized light source, ORM is used to measure attenuation in network nodes (cable sections, splitters, welded and mechanical joints).Main characteristics of OPM: photodiode type (Ge, GeX or InGaAs), operating wavelengths (nm), photodiode sensitivity (dBm).BecauseOPM is often used "in the field", it must have a gallium arsenide alloy (InGaAs) photodiode to work stably in all temperature conditions.To work with PON OPM must "be able" to measure a signal with a power of -30 to + 20dBm at wavelengths of 1310, 1490, 1550 and 1625nm (optional).Stabilized Light Source (SLS).They play the role of inputting a signal of a given power and at a given wavelength into the optical line.Always used in conjunction with OPM, asusing one SLS is not advisable.Main characteristics: laser power (dBm), working wavelengths (nm).Typically, SLSs generate -5..-7dBm signal power, so to measure attenuation between optical end nodes, SLS must be paired with an OPM that can measure signals down to -40dBm.All SLS "can" work at 1310 and 1550nm wavelengths, which is sufficient for a PON network.However, there are models that support the 1625nm wavelength (at this wavelength it is convenient to carry out measurements in an already operating network).Attenuation analyzers (Optical Loss Test Set - OLTS).These devices are a set consisting of a power meter (OPM) and a radiation source (SLS).However, most modern OLTS are represented by one device, which has both a meter and an emitter “on board” - this approach is extremely effective for two-way testing of a communication line (if there are a pair of such devices).Visual flaw detectors (Visual Fault Locator - VFL).They are optical emitters in the visible range of the spectrum (~ 650nm).They allow detecting cable defects in optical nodes (cross-connects, couplings, PON-boxes, etc.), where visual inspection is possible.The essence of the operation of the device is that the signal at a wavelength of ~650 nm is scattered by large inhomogeneities in the fiber, i.e.observed by the operator in the form of bright spots through the cable sheath.The main characteristic of VFL is the laser power (dBm), which determines how far the light can travel through the fiber.Modern VFLs are capable of transmitting light over a distance of about 8-10 km at a laser power of 5..10dBm.Optical Time Domain Reflectometers (OTDRs).One of the most popular devices for network diagnostics and testing.Having access to one end of the fiber, the reflectometer allows you to build a graph of the signal power distribution along the optical line - a reflectogram.Analysis of the trace allows you to determine almost all the necessary parameters of the line: per unit attenuation of the fiber, power losses between two points of the network or losses on individual passive elements, return losses.In addition, the trace allows you to quickly and accurately locate splitters, connections, fiber breaks or other defects.Nevertheless, the prices for these devices are quite high, so not everyone can afford a high-quality reflectometer.The main characteristics of OTDR are: operating wavelengths (nm), pulse width (ns), dynamic range (dB), dead zone (m), resolution (m), number of measurement points.Typically, OTDR measurements are made at two wavelengths (1310 and 1550nm).However, OTDR, which is also capable of operating at 1625nm, allows for more accurate detection of fiber defects (macrobends) and can take measurements on a live network without interrupting communications.The pulse width is an indicator on which both the dynamic range and the dead zone largely depend.Wide pulses allow you to take a reflectogram from longer lines, but worsen the detail of measurements and increase the dead zone.Short pulses, on the contrary, allow one to accurately determine the location of the inhomogeneity, but at short distances.A high-quality reflectometer is capable of generating pulses with a width of 5 to 20,000 ns.Dynamic range (the most important indicator) determines the length of the fiber that the OTDR can see through.If the installer is faced with the task of enlightening a small section of the network, then OTDRs with a dynamic range of ~ 30dB are suitable;if you need to enlighten the whole PON tree, then you should choose a reflectometer with a dynamic range of at least 35dB (38-40dB is better).ORL meters.Used to measure total return loss or return loss on individual passive elements.These devices are almost never available as a separate device - instead, ORL measurement modules are built into many OLTS and OTDRs.The ORL meter is somewhat similar in operation to a reflectometer, but it determines the level of return loss more accurately.The main characteristics of the ORL meter are the operating wavelengths (nm) and the ORL range (dB).Simple ORL meters operate at two wavelengths (1310 and 1550nm), more expensive ones also operate at 1625nm.The ORL range for conventional meters is ~50dB, which corresponds to the reflection from the UPC connector.If it is necessary to measure the weakest reflected signals (from APC connectors), then the ORL range should be at least 60dB.A more rational solution is to purchase two attenuation analyzers (OLTS), which are represented by a 3 in 1 device (emitter, power meter, ORL meter).This approach allows not only to perform a two-way attenuation analysis (without swapping devices), but also to take return loss characteristics (ORL) from the line.In extreme cases, if the installer does not have a stabilized radiation source (SLS), an OLT transmitter (SFP module with a wavelength of 1490nm) or a TV transmitter transmitter (1550nm) can be used instead.In this case, the measurements will be less accurate (±1db), becauseterminal equipment transmitters are non-constant light sources.With such a scheme (Figure 3.2), measurement flexibility is lost, sincethe radiation source is tied to one point of the network.When working with OPM, one problem may arise - measuring the signal power at a specific wavelength.Ordinary OPMs have a broadband photodetector and do not have built-in filters to select one or another wavelength, i.e.they measure the group signal.If only Internet traffic is transmitted in the PON network, then there are no problems (OPM measures the signal at 1490nm).Problems appear if the network, in addition to the Internet, has a CATV signal at a wavelength of 1550nm.In this case, OPM must be connected through a CWDM bulb (light filter), which will cut off the unwanted signal.Recently, however, a more elegant solution has appeared on the market for measuring instruments - a special PON OPM.This is a pass-through power meter that is connected to the line and measures the signal at three wavelengths (1310, 1490 and 1550nm) at once, while introducing minimal (<1.5dB) attenuation into the line.Another feature of this meter will be described below.In a working PON network, measurements at wavelengths of 1310 and 1490nm are prohibited, becausethis will lead to errors in data transmission.Measurement at 1550 wavelength is only allowed if there is no CATV signal on the network.If it is still transmitted, then the backup wavelength of 1625nm (which was mentioned so much earlier) is used for PON measurements.That is why it is desirable to purchase SLS, OPM and OTDR capable of working with this wavelength.Two CWDM bulbs (one on the transmitter and one on the receiver) are enough to inject the redundant signal from the SLS into the fiber and out of the fiber at the OPM.Similarly, you can connect a reflectometer.Unfortunately, it is impossible to connect measuring equipment to an optical node without disrupting the network (this is only possible if there are free leads on the splitter or patch panel).However, it takes less than a few seconds to install the bulb in line, so subscribers are unlikely to notice the absence of the Internet.Another problem is measuring the signal strength from the ONU when connecting a new subscriber.If you connect a power meter to the ONU, then it will not show anything, becauseThe ONU does not transmit a signal without the permission of the OLT (Figure 3.7 a).If the meter is connected to the ONU through a splitter (Figure 3.7 b), then it will show an incorrect value.This happens because a conventional meter calculates the average power value over a certain time interval, and the ONU is “silent” most of the time.