Fiber Network : Test Equipment and Techniques

Test Equipment and Techniques

Experience tells us just what happens when you install a new communications system of any kind. Nothing. You turn it all on and wait and nothing happens! (This is actually quite a good outcome - sometimes you get smoke from one or other piece of equipment!) Then you have the task of tracking down just what is and is not happening and where the problem is. Experience also tells us that the most likely problem is that someone plugged a cable into the wrong socket! Perhaps the biggest irony of optical communications is that you can't see anything! So if we want to find out what's going on in the system in order to make it work then we appropriate test equipment on hand. In many situations a simple optical power meter is quite sufficient but in others very sophisticated equipment is essential.

Optical Power Meters (Optical Multimeter)

Optical Power Meter - Logical Structure

The simplest and most basic piece of equipment used in the field is the optical power meter. This is shown in Figure and requires very little explanation. Different models have different connector types and are specialised to either multimode or single-mode fibre. There is always a wavelength switch to adjust the power readings for the particular wavelength being received. Also there is usually a range switch which determines the range of signal power expected - although this last function can be automatically determined by the meter itself.

Optical Time-Domain Reflectometers (OTDRs)

OTDR Display – Schematic

The Optical Time-Domain Reflectometer enables us to look at a fibre link from inside the fibre. In reality it is just a radar system for looking at fibre. High intensity pulses are sent into the fibre from a specialised laser and when the pulse returns its strength is displayed on an oscilloscope screen in the form of a trace. A schematic of such a display is shown in Figure. Important considerations are as follows:

Reflections from within the Fibre

In the trace you see reflections coming from all along the fibre itself. This is the result of Rayleigh scattering. Rayleigh scattering was mentioned in the section on optical fibre as the major limiting factor in fibre attenuation. This scattering occurs backwards towards the transmitter and we can receive it and display the result.

Faults and Joints etc.

Every time there is a discontinuity or imperfection in the fibre the effect can be seen in the trace. Such events can be the presence of a connector or a splice or some more serious imperfection such as a crimp in the cable due to poor installation. In the schematic above we can see the reflections from the beginning and end of the fibre as well some imperfections in between.

Noise Floor

At the end of the fibre you see a characteristic large (4%) reflection followed by the signal dropping to the noise floor.

Measurable Parameters

From an OTDR you can quickly determine the following characteristics of the fibre link under test:

·         The length of the fibre:This is not as precise as it sounds. What you can calculate is the length of the fibre itself. Most long distance cables employ “loose tube” construction and the fibre length is between 5% and 10% longer than the cable itself.The attenuation in dB of the whole fibre link and the attenuation of separate sections of fibre (if any).

·         The attenuation characteristics of the basic fibre itself.

·         The locations of connectors, joints and faults in the cable. These locations are measured from the beginning of the fibre and can be as accurate as a few metres.

Usually you can't see the fibre close to the instrument. A “dead zone” extends from the connector at the instrument itself for about 20 metres into the fibre. So if the trace in the figure above was real we would need to have a piece of fibre about 20 metres long connecting from the OTDR to the end of the fibre under test. Often you don't have this luxury in the field. In which case many OTDRs come equipped with a red laser source which you can use to illuminate the fibre. When you look at a fibre illuminated in this way (from the side) bad joints and fibre faults will scatter the red light and you can see it. But beware YOU MUST NOT LOOK AT THE SOURCE OR AT THE END OF THE FIBRE DIRECTLY. This can damage your eyes. Before using a feature like this read the manufacturer's instructions carefully.

Advantages

The major advantage of the OTDR is that tests can be done from one end of the link and you don't need access to the other end. This means you don't need two people to do the test and you save the problem of coordinating between people. Also the testing is much quicker. So even simple tests which could be performed with a basic optical source at one end of the link and a power meter at the other are often performed with an OTDR.

Characteristics

OTDRs today are extremely sophisticated devices and come with many options. They can be large fixed laboratory instruments or small portable ones about the size of a laptop computer. Different models are available for multimode and single-mode fibres. Of course different models have different levels of sensitivity (and price). There is always a range of options for the user to control such as wavelength used for the test, timescales, pulse duration etc.

Many modern OTDRs come with additional functions such as optical power meter or laser source so that a good OTDR often has all of the function needed by a technician in the field. In addition many OTDRs offer computer output so that you can collect OTDR data in the form of digital  readings and analyse it later on a computer.

 OTDR Operational Logic

The principle of operation of a typical OTDR is shown in Figure. In the figure a circulator has been used to enable transmission and reception of the pulse from the single strand of fibre under test. Other means of signal splitting/combining are used but circulators offer the least attenuation. As might be expected the big problem with an OTDR is that the returning signal is very low level especially on long distance fibre sections. We can't use signal pulses of too high a power for many reasons and so pulses of 10-20 mW are typically sent. The problem of low return power is addressed in two ways:

1. A very sensitive APD detector is used.  Thus an APD becomes very sensitive indeed at the very low pulse rates used. The penalty for using APDs is additional noise but this is mitigated by the averaging

process.

2. A “boxcar averager” circuit is used to average many thousands of returning pulses. The averaging process removes a large amount of noise. (Most of the noise comes from the APD and its associated circuitry.) In some (very sensitive, long distance) OTDRs the averaging time can be of the order of several minutes! The averager provides logarithmic scaling of its output so that the vertical scale on the display can be displayed in dBm. The pulse rates used are quite slow! Since the optical signal propagates at approximately 5 microseconds per kilometer we have to allow 10 microseconds per kilometer of fibre length. So for 20 km of fibre we need to wait at least 200 microseconds between pulses and so a pulse rate of 5000 pulses per second would be the maximum possible.

Spectrum Analysers

Spectrum Analyser - Display Schematic

There are many occasions where we want to look at the wavelength spectrum of the signal(s) on a fibre. One such occasion would be to examine the wavelength spectrum of a WDM system to help understand system operation and to diagnose faults. A spectrum analyser scans across a range of wavelengths and provides a display showing the signal power at each wavelength.

From this display we can calculate:

1. The power levels of each channel.

2. The spectral width of each channel.

3. Any interference between channels such as crosstalk possibilities.

4. By connecting it in different places through the system we can track many potential problems such as laser drift etc.

Spectrum Analyser - Logical Structure

The logical structure of the device is shown in Figure. . Light input from the fibre is passed through a tunable Fabry-Perot filter. . The filter is scanned at quite a slow rate (perhaps 10 times per second) through the range of wavelengths that we want to examine.

·         Optical output of the FP filter is fed to an APD to convert it to electronic form.

·         The output of the APD will contain rapid variations due to modulation of the signal in each channel etc. These modulations are averaged out electronically so that the electrical signal level now represents the average power level of the optical signal (average over a few milliseconds). The electronic signal now needs to be scaled logarithmically as we need the y-axis scale to be in dBm.

·         The electronic signal is now fed to the y-axis control of an oscilloscope.

·         The x-axis is swept across in synchronism with the wavelength setting of the FP filter.

·         This results in a display similar to that in the figure.

Like OTDRs, spectrum analysers vary widely in their capabilities and prices. They range from large, very accurate and expensive laboratory instruments to small, much less expensive devices about the size of a laptop computer. You can even buy one that does not have a display and instead connects to your laptop

computer. In using one you need to be aware of the resolution (minimum width) of each wavelength measured and also of the accuracy of the instrument.

Eye Diagrams

When you think about it there is a paradox involved in much of the testing we would like to do. You have a signal that is varying extremely quickly - so quickly that we need sophisticated receiver circuitry to detect its changes of state. Yet we expect to be able to measure and display the signal very accurately - much more accurately than we could ever possibly receive it.

The secret is that we receive the signal many times (indeed millions of times) and display the aggregate. Signals when they carry information vary and therefore we can never get a good solid picture of a particular state or change of state. However we can get an excellent idea of the aggregate.

The eye-diagram has over the years become the recognised way of looking at an electronic signal and determining its “goodness” as a carrier of information. It consists of many (from hundreds to millions) of instances of the signal displayed over the top of one another. In extremely fast equipment you might get only one or two points on a trace at a single sweep. But displaying them together allows us to assess the quality of the received signal very well indeed.

Eye Diagram - Schematic

The diagram is produced by feeding the result of the analogue section of the receiver circuit to the y-axis control of an oscilloscope. The sweep is set to display one full cycle (2-bit times) and is usually triggered from the receiver's PLL circuit (the receiver's derived clock).

The following aspects of the eye are important:

1. The vertical eye opening indicates the amount of difference in signal level that is present to indicate the difference between one-bits and zero-bits. The bigger the difference the easier it is to discriminate between one and zero. Of course this is affected significantly by noise in the system.

2. The horizontal eye opening indicates the amount of jitter present in the signal. The wider the eye opening is on this axis the less problem we are likely to have with jitter.

3. The thickness of the band of signals at the zero-crossing point is also a good measure of jitter in the signal. However, you need to be careful here as the sweep is usually triggered from the receiver PLL and variations here are as much an indicator of the quality of the PLL as they are of the signal itself!

4. The best indication of signal “goodness” is just the size of the eye opening itself. The larger it is the easier it will be to detect the signal and the lower will be the error rate. When the eye is nearly closed it will be very difficult or impossible to derive meaningful data from the signal.

Real Eye Diagrams

Figure  shows two real eye diagrams of a modulated signal (actually 100 Mbps FDDI code). On the left is a very good situation with a wide open eye and very little jitter. On the right we have an example of a signal which will be relatively easy to detect but which contains a significant amount of jitter.