Introduction

Characterization of optical fibers is very important for a number of reasons. The users of optical fibers need the fiber characteristics to design the optical fiber communication system, whereas the manufacturers need them for optimizing their fabrication processes to obtain fibers with desired characteristics. The fiber characteristics are also necessary for the development and verification of various theoretical models for predicting various performance properties of the fiber. The two most important characteristics of an optical fiber are bandwidth (or pulse dispersion) and loss. In addition, one requires knowledge of various other parameters such as refractive index profile, core diameter, and so forth for predicting losses at joints. Table 19.1 lists the various characteristics of optical fibers along with their effect on system performance.

A large number of techniques have been developed for measuring various fiber characteristics. In this and the following chapter, we briefly discuss some of the standard techniques used in fiber characterization; for more details of the various techniques, readers may consult Pal, Thyagarajan, and Kumar (1988) and Thyagarajan, Pal, and Kumar (1988b).

In Section 19.2 we discuss some general experimental considerations relevant to fiber measurements, and in Section 19.3 we discuss various techniques for the measurement of refractive index profile, spectral attenuation and pulse dispersion, or bandwidth. In Chapter 20 we discuss measurement of characteristics specific to single-mode fibers only – namely, mode field diameter, cutoff wavelength, and birefringence.

Introduction

In the preceding chapters we discussed the characteristics of optical fibers, optical sources, and optical detectors. These form the three basic units of any optical fiber communication system. In this chapter we discuss how these basic elements can be put together to build a simple point-to-point optical fiber communication link.

Let us assume that we need to transmit information between two points (see Figure 13.1). The separation between these points could range anywhere from less than a kilometer in the case of computer data links to several thousands of kilometers, for instance, in transoceanic links. In all such links, there would be a transmitter that could be either an LED or a laser diode, the transmission path consisting of optical fibers that could be either multimode or singlemode fibers, and the optical receiver that could be a PIN or APD followed by the detection electronics. The choice of the components would depend on the distance as well as on the bit rate. When the separation between the points is greater than about 50–100 km, then because of attenuation in the link or pulse dispersion, it becomes necessary to use regenerators that consist of a receiver–transmitter combination. These regenerators detect the pulse stream before the power becomes too low or the pulses become unresolvable and retime, reshape, and regenerate (and, hence, the name 3R repeater) a new set of optical pulses to be transmitted over the next part of the link. For links that are limited by loss rather than by dispersion, one can use optical amplifiers in place of regenerators.

Introduction

Although the major application of optical fibers has been in telecommunications, there is a growing application of optical fibers in sensing applications for measurement of various physical and chemical variables, including pressure, temperature, magnetic field, current, rotation, acceleration, displacement, chemical concentration, pH, and so forth. Such fiber optic sensors are finding applications in industrial process control, the electrical power industry, automobiles, and the defense sector.

One of the main advantages of a fiber optic sensor stems from the fact that optical fibers are purely dielectric and thus can be easily used in hazardous areas where conventional electrically powered sensors would not be safe. In addition, fiber optic sensors are immune to electromagnetic interference, have greater geometric versatility (i.e., they can be configured into a variety of arrangements to suit the application), and should have a very short response time. They can be multiplexed into various configurations and the information from various sensors can be transmitted over long distances by optical fibers. They can also be configured to provide spatially distributed measurements of external parameters.

In a typical optical fiber sensor, light from a source such as a laser diode or LED is guided by an optical fiber to the sensing region. Some property of the propagating light beam gets modulated by the external measurand such as pressure, temperature, magnetic field, and so forth. The modulated light beam is then sent via another (or the same) optical fiber for detection and processing.

Introduction

As discussed earlier, optical fiber communication systems are ultimately limited by either loss or dispersion. Loss leads to power levels of received signal that cannot be detected within tolerable errors, whereas dispersion leads to overlapping of adjacent pulses of light with resultant loss in resolution and, hence, information. Figure 14.1 shows a typical wavelength dependence of loss and dispersion of a single-mode fiber for both a CSF with zero dispersion around 1300 nm and a DSF with zero dispersion around 1550 nm. As evident from the figure, the effect of loss can be minimized by operating at the minimum loss wavelength around 1550 nm, whereas dispersion can be minimized by operating at the zero dispersion wavelength. Using DSFs has advantages of both minimum loss and zero dispersion. Figure 14.2 shows the maximum permissible unrepeatered length as a function of bit rate as determined by loss or by dispersion for both a CSF and a DSF (see Chapter 13 for a detailed discussion of this figure). The pulses are assumed to be Fourier transform limited. As can be seen from the figure, even at bit rates of 2.5 Gb/s, a system operating with CSFs is limited by loss rather than by dispersion. For DSF, loss-limited operation extends to almost 10 Gb/s.

In long-haul fiber optic communication systems, the effects of loss and pulse dispersion are normally overcome by using periodically spaced electronic repeaters. In these repeaters the input optical signal is first detected and converted to electrical signals.

There has always been a demand for increased capacity of transmission of information, and scientists and engineers continuously pursue technological routes for achieving this goal. The technological advances ever since the invention of the laser in 1960 have indeed revolutionized the area of telecommunication and networking. The availability of the laser, which is a coherent source of light waves, presented communication engineers with a suitable carrier wave capable of carrying enormously large amounts of information compared with radiowaves and microwaves. Although the dream of carrying millions of telephone (audio) or video channels through a single light beam is yet to be realized, the technology is slowly edging toward making this dream a reality.

A typical lightwave communication system consists of a lightwave transmitter, which is usually a semiconductor laser diode (emitting in the invisible infrared region of the optical spectrum) with associated electronics for modulating it with the signals; a transmission channel – namely, the optical fiber to carry the modulated light beam; and finally, a receiver, which consists of an optical detector and associated electronics for retrieving the signal (see Figure 1.1). The information – that is, the signal to be transmitted – is usually coded into a digital stream of light pulses by modulating the laser diode. These optical pulses then travel through the optical fiber in the form of guided waves and are received by the optical detector from which the signal is then decoded and retrieved.