Florian G. Bell

Dispersion in optical fibers

This chapter discusses the types of dispersion in optical fibers. Dispersion is an important factor limiting the rate of data transmission in fibers. Intramodal (chromatic) dispersion is present in both single-mode and multimode fibers.Chromatic dispersion is the most significant dispersion mechanism in single-mode fibers. Intramodal dispersion has two contributions: material and waveguide. Material dispersion is the primary type of intramodal dispersion; waveguide dispersion is less significant than material dispersion in both single-mode and multimode fibers. Over a large portion of the optical spectrum, waveguide and material dispersion have opposite signs. By suitable doping, waveguide dispersion can be increased in magnitude. When added to material dispersion, the total dispersion becomes reduced at wavelengths greater than the zero-dispersion wavelength. Fibers employing this type of compensation are known as dispersion-shifted and dispersion-flattened fibers. Intermodal dispersion occurs only in multimode fibers and presents a significant limitation to the transmission rate of multimode fibers. In multimode fibers, where the dispersion is large, significant pulse spreading occurs over relatively short distances (about 1 km).

Fundamentals of fiber optics

Since OTDRs are used almost exclusively to test optical fibers, it will be useful to review some of the basics of fiber optics before we begin discussing the details of OTDRs. * There are many fine texts on the subject of fiber optics, but this chapter concentrates on those fundamental elements of fiber optics that are likely to be important when testing optical fibers using OTDRs. † We begin by illustrating some of the problems that must be overcome to transmit information using light. Then we introduce the concepts of internal reflection, Rayleigh scattering, absorption, numerical aperture, modal properties of fiber, dispersion, and coherence. We have attempted to summarize the material in an understandable fashion, with working equations that are adequately referenced for those readers wishing a more detailed and rigorous development of the subject. Later chapters treat some of these subjects, such as chromatic dispersion, in greater detail as they relate to specialized tests that you might perform with an OTDR.

Analyzing passive networks containing splitters and couplers

Optical couplers, or splitters, are devices used to broadcast an optical signal from one fiber to many fibers. In the most general case, splitters are configured as M by N (see figure 10.1). In such devices, there are M input ports and N output ports. Optical signals on any of the input ports are branched to all the output ports. * If the splitter has only one input port it is called a 1-by- N splitter. Although construction techniques vary, one way to build a 1-by- N splitter is to combine a series of 1-by-2 splitters as shown in figure 10.2.

Performance characteristics of OTDRs

This chapter focuses on the specific performance characteristics of optical time-domain reflectometers (OTDR). Dynamic range is the single most important specification for OTDRs. There are two types of dynamic range: reflective and scattering. When measuring reflectivity, one needs an OTDR with high reflective dynamic range. When measuring splice loss over long lengths of fiber, one needs an OTDR with high scattering dynamic range. A second important figure of merit is the ability of the OTDR to resolve closely spaced events in the fiber. Dead zone is the region after a reflective event where only limited measurements may be performed. Event dead zone (EDZ) is the minimum distance after a reflective event before the presence of another reflective event can be detected. The third parameter used to specify OTDR performance is the attenuation dead zone (ADZ), or loss-measurement dead zone (LMDZ). In some applications, OTDRs analyze optical fibers that contain discrete reflective events at various locations.

Polarization mode dispersion

When a very short pulse of light is launched into one end of a fiber, it typically emerges from the opposite end somewhat broadened. * Dispersion is the term we use for this pulse broadening; in single-mode fibers, the dominant cause is typically chromatic dispersion. However, in systems operating near the fibers zero-dispersion point and using very narrow-line-width sources, the dominant source of pulse broadening can be differential group delay (DGD) due to birefringence in the optical fiber. After chromatic dispersion, DGD is the most likely effect limiting the transmission bandwidth of single-mode fiber, and it presents an inherent potential limitation in long-distance communications systems operating in the multigigabit range. 1

Measuring nonreflective events

This chapter discusses the types of events that result in a nonreflective waveform signature in optical time-domain reflectometers (OTDR). The location of a nonreflective event is typically more difficult to measure than the location of a reflective event. Distance measurements of nonreflective events offer more complexities than many operators might expect. Distance measurement errors depend on such things as the local waveform noise, offset errors, time-base errors, errors in setting the index of refraction, and interpretive errors. Estimating distance-measurement error is probably beyond the desires and capabilities of most OTDR operators. Even if an operator wanted to calculate the measurement uncertainty, the details of event marking algorithms are proprietary and are unknown to him or her. This makes it almost impossible for the operator to calculate the expected errors in the instruments measurements. The only practical way for the operator to know the measurement uncertainty is for the OTDR to calculate it specifically for each event.

Automatic event-marking algorithms and calibration

This chapter discusses the types of automatic event-marking algorithms and their calibration. Event-marking algorithms are arguably the most complex aspect of modern optical time-domain reflectometers (OTDR). They can provide the OTDR user with a tremendous range of measured data. From link loss to reflectivity to loss-measurement uncertainty and distance-measurement uncertainty, event-marking algorithms can easily inundate the user with data. They can be broadly categorized as manual, semimanual, semiautomatic, or fully optimized. Fully automatic, fully optimized event marking is the most advanced event marking available. With fully optimized event marking, the OTDR automatically acquires waveforms using multiple pulse width, averaging, gain, and bandwidth settings. It does this to optimize the acquisition parameters at each event along the fiber. After acquiring several waveforms using multiple acquisition parameters, the instrument automatically evaluates each waveform and computes an event table. The OTDR also displays a composite waveform, made by splicing sections of the various acquired waveforms. This also allows the user to perform manual measurements if desired.

Loss-measurement error

This chapter discusses the sources of loss-measurement errors. Like distance-measurement errors, loss-measurement errors are also highly dependent on noise, measurement algorithms, calibration, and methodology. The increase in number of waveform points leads to a decrease in the loss-measurement uncertainty, and the usable range over which the OTDR can make accurate loss measurements also increases. OTDRs measure loss by comparing the strength of the Rayleigh scattering on each side of the fusion splice. OTDR loss measurements, therefore, are based on the assumption that the scattering characteristics and the capture ratios of the two fibers are identical, but there are always at least minute differences between the fibers. Because of this, each loss measurement has some degree of uncertainty arising from the fact that the splice joins different types of fibers. Splicing of two optical fibers together results in optical loss, which can be broadly defined as caused by intrinsic or extrinsic effects. Extrinsic splice loss results from external factors, such as lateral misalignment, fiber bending, and contamination. Intrinsic splice loss results when the two fibers being joined have different mode-field diameters or numerical apertures.

Complications caused by reflective events

This chapter discusses some of the measurement problems associated with reflective events. The distance, after a reflection and before an optical time-domain reflectometers (OTDR) can make measurements, depends on the type of measurement one wants, the strength of the reflection, and the OTDRs bandwidth. Event dead zone is the minimum distance after a reflection before the OTDR can “see” another reflection of the same height. This distance is limited almost exclusively by the OTDRs pulse width and bandwidth. On the other hand, the loss dead zone is the distance from the leading edge of a reflective event to the point past the event where the waveform signature falls to within 0.5 dB of the normal Rayleigh backscatter signature. If another event, such as a fusion splice, is within the OTDRs loss dead zone, the OTDR is effectively unable to measure the individual losses of the two events and must instead measure only the total or grouped loss of the events. There are a number of ways to reduce the effects of reflective events. One approach is to use low-reflectance connectors, such as PC connectors or angled connectors. Another approach is to replace your connectors with fusion splices.

Fundamentals of OTDR operation

This chapter discusses some of the fundamentals of optical time-domain reflectometers (OTDR) operation. In the standard OTDR configuration, the output varies continuously as a function of the input, and averaging serves only to lower the amount of noise that we see in the waveform. Rayleigh backscattering is fundamental to OTDR operation and is the method by which OTDRs measure the end-to-end loss of a fiber-optic line as well as the discrete losses of splices and connectors. Another most important specification for an OTDR is the dynamic range that specifies the strength of the backscatter signal to the noise level. The key point to remember when comparing the dynamic range of one OTDR with that of another is that dynamic range is a complicated quantity that depends on many different parameters. To make a useful comparison, one must compare the dynamic ranges of different OTDRs under the same conditions of pulse width and averaging.