3.1.1

Coding and modulation

Information is represented as a sequence of bits and in basic binary systems, there are two possible states for each bit. For instance, computers use digital 1 and 0 to represent logical TRUE and FALSE, respectively. Two things are required to communicate information. Firstly, there needs to be a system to assign meaning to bit sequences. This is known as encoding. A well-known system that many youths may have heard of is Morse code. Here, the two binary states are represented as dots • and dashes -, and each letter and numeral in the alphanumeric set (i.e. A to Z and 0 to 9) is encoded as a distinct sequence of these. Tab. 1 lists the internationally used Morse codes. As an example, the universal distress signal “SOS” is encoded as •••---•••.

Tab 1.

InternationalMorse code chart.

Secondly, a physical means of conveying the two binary states is needed. This can be conveniently actualized, in the optical context, by turning a flashlight on or off, or in the Morse code example, by turning a flashlight on briefly (e.g. 1 second) for a dot and turning it on for longer (e.g. 3 seconds) for a dash.¹² In this way, a person sending a message by means of Morse code using a flashlight would be transmitting a sequence of long and short flashes. This is known as modulation. That is, the (encoded) message modulates the communications signal (e.g. light flashes) which then travels through a channel (e.g. air) to be perceived by a receiver (e.g. another person seeing the flashes).

3.1.2

Class activity: Morse code

In this activity, students work in pairs to send Morse coded messages to one another. Each pair is provided with a flashlight, a cardboard tube, a Morse code chart such as Tab. 1, a Morse code decoding tree chart as shown in Fig. 1, two pens or pencils, and several blank sheets of paper.

Figure 1:

Morse code decoding tree.

In each pair, one student is assigned as the sender and the other, the receiver. The sender decides on a message unbeknownst to the receiver. S/he encodes the message in Morse code referring to Tab. 1 and may choose to discreetly write down the encoded sequence. The sender then transmits the encoded message by turning the flashlight on and off accordingly and shines the beam through the cardboard tube, which acts as a fiber channel, as depicted in Fig. 2a. The receiver looks at and records the sequence of flashes from the other end of the tube (i.e. the output). S/he then uses the chart in Fig. 1 to decode the message and verify it with the sender. For each encoded sequence, the receiver begins at ‘start’ and moves down the tree. Each dot or dash corresponds to a left or right traversal, respectively, to the next lower level. The node on which the traversal ends is the decoded character. For example, a ‘W, which is encoded as • - -, would have a tree traversal beginning at ‘start’ and moving left→right→right. Although it is possible to use Tab. 1 for the decoding process, the reverse lookup is unordered and therefore cumbersome. Hence, the chart in Fig. 1 is recommended as an easier alternative. After one round, the sender and receiver can swap roles if time permits.

Figure 2:

(a) Morse code class activity using a flashlight and cardboard tube. (b–c) Morse code circuit and schematic.

A number of tips can be considered. Firstly, the rule of no verbal communication should be articulated to the students. This ties the activity closer to reality, where verbal communication is not possible over long distances. Secondly, situations when the receiver decodes an incorrect message can be used as opportunities to introduce students to the existence of errors and the associated challenges of communications systems. For instance, a receiver mistaking a dot for a dash is analogous to a bit error in digital systems. Thirdly, students may discover that the manual process of sending dots and dashes is tedious and slow. This is an opportunity to highlight the need for ever faster high-speed transceiver systems, especially with exponentially increasing internet traffic.¹³ Lastly, it is helpful to show students and let them interact with a simple home-built Morse code circuit, such as in Figs. 2b–c, for them to appreciate how Morse code was telegraphically transmitted historically.

3.2.1

Reflection and refraction

Most students would already be familiar with the concept of reflection. Examples, such as bathroom mirrors, are given to reinforce the students’ realization that it is a common perceptible phenomenon. The less conspicuous concept of refraction is also introduced. An example of a real-world occurrence, such as the perceived bending of a straight straw in a glass of water as in Fig. 3a, can be given.

Figure 3:

(a) Refracted straw image. (b) Refraction of light traveling through two materials of different refractive indices n₁ < n₂.

Refraction is explained as a change in the speed of light as it travels between materials of different refractive indices. For young students, it is unnecessary (and inadvisable) to elaborate on the microscopic particle interactions that cause the change of speed. Rather, it suffices to mention that light slows down when travelling through a more optically dense material, which has a higher refractive index. The difference in refractive indices, and the resulting difference in light propagation speed, between two adjoining materials through which light travels causes it to bend at the interface, thus leading to refraction, as shown in Fig. 3b. The refractive indices n of each material are related to the trajectory angles θ of light passing through them (measured from the normal to the interface) through Snell’s law of refraction:¹²

The scenario in Fig. 3b is typical of light passing from a material of lower refractive index, e.g. air, to one of higher refractive index, e.g. water. That is, n₁ < n₂.

3.2.2

Total internal reflection

The case converse of Fig. 3b, i.e. when light travels from a higher to a lower refractive index, is also possible (see Fig. 4a). This time, the incidence angle θ₁ is smaller than refraction angle θ₂, since light travels faster after it reaches the material of lower index n₂. As the incidence angle is increased, so too will the refraction angle until is perpendicular to the normal, as in Fig. 4b. At this point, the incidence angle is known as the critical angle θ_c. If the incidence angle is increased further than the critical angle, i.e. when θ₁ > θ_c as in Fig. 4c, then no light is refracted and all light is instead internally reflected.¹⁴ The interface acts as if it were a mirror and the reflection angle θ₂ is equal to θ₁. This phenomenon is known as total internal reflection (TIR).¹⁵ It is emphasized that TIR can only occur for light traveling from within a material of higher refractive index to one of lower refractive index.

Figure 4:

Light at a refractive index interface with n₁ > n₂: (a) refraction, (b) critical refraction, and (c) total internal reflection.

Figure 5:

TIR in an optical fiber (crosssection).

3.2.3

Class activity: measurement of refractive index

The objective of this experiment is for students to measure the refractive index of gelatin. Students can work in small groups and each group is provided with a transparent square container prepared with clear gelatin, a 360° protractor,^* slitted light-emitting diode (LED) source,^† a scientific calculator, some pens or pencils, and an associated worksheet.

The experiment is set up as light passing between air and gelatin. From Eq. (1), the unknown refractive index of gelatin n_gel can be determined from the known refractive index of air (n_air ≈ 1) as well as the angles of incidence and refraction which are to be measured. This is done by placing the gelatin container on the protractor, with the edge of the container on the 0° to 180° line of the protractor. A narrow beam from the LED source is shined at an angle through the edge of the container, at the point through the center of the protractor, as depicted in Fig. 6a. The higher n_gel compared to n_air causes the light to slow down and refract when traveling through the gelatin. By measuring the incidence and refraction angles, θ_air and θ_gel, respectively, and rearranging Eq. (1), n_gel can be calculated via

Figure 6:

Refractive index measurements using (a) clear gelatin and (b) water.

This refractive index is typically about 1.34 to 1.40. Depending on whether the students are familiar with trigonometry, it may be necessary to assist them in calculating the sines of angles using the scientific calculator.

A simple extension to the activity is for the students to measure the refractive index of water. The same aforementioned procedure is used, with the container holding water instead of gelatin, as shown in Fig. 6b. The refractive indices of water (∼ 1.33) and gelatin can then be compared and a teacher/demonstrator-led discussion can follow about how gelatin, being a solid, has an index closer to that of the glass in optical fibers. Although any color of light can be used for the measurements, it is important that the same color is used for both the gelatin and water cases, since different colors (wavelengths) of light travel at different non-vacuum speeds^12,15 and will refract at slightly different angles.

3.2.4

Class demonstration: light guidance using TIR

Continuing from the refraction topic, the effect of TIR as discussed in Sec. 3.2.2 can be demonstrated. This can be achieved either by the students themselves by manipulating the incidence angle on the gelatin set-up as in Fig. 7a, or through a class demonstration of the Tyndall/Colladon experiment,⁶ where demonstrators guide laser light through a flowing water stream as in Fig. 7b. The latter requires a large transparent tank with a faucet, filled with water, a laser pointer, and a water collection bucket. A combination of both demonstrations is encouraged.

Figure 7:

TIR demonstrations using (a) a container of clear gelatin and (b) a home-built Tyndall/Colladon experiment set-up. (c) A fiber fountain used as a teaching aid for TIR.

An additional useful tool is a commercial fiber fountain, like that shown in Fig. 7c. The hairlike plastic strands provide a safe emulation of real glass fibers. Moreover, its aesthetic nature can be utilized to gain student attention.

3.3.1

Wavelength-division multiplexing

Fig. 8 portrays a simplified model of the WDM picture. Each sender, who may be communicating different types of data as illustrated by the various icons,^‡ uses a separate wavelength as indicated by the colored lines. These are all multiplexed (MUX) and transmitted simultaneously through the same fiber link. At the receiver side, the signals are demultiplexed (DEMUX) to separate each channel, and subsequently routed to each respective recipient.

Figure 8:

Multiuser system utilizing WDM on a single shared fiber link.

3.3.2

Class activity: wavelength-division demultiplexing

Demultiplexing is the process where the individual channels that were multiplexed together are separated at the receiver.¹⁴ Students learn about the separation of colors through filtering. They can be divided into small groups, with each group having a transparent container prepared with clear gelatin as per Sec. 3.2.3; three other transparent containers, each prepared with a different colored jelly;^§ and three slitted LED light sources emitting red, green, or blue.

In this activity, which was inspired by the “Laser Jello” experiment from the Exploratorium®¹⁹ and the “Transmission with Gummy Bears” activity from LASER Classroom™,²⁰ students first shine all three LED colors into the clear gelatin, noting that they are all transmitted, as shown in Fig. 9a. Thereafter, they experiment with different colored jellies to observe that a particular colored jelly transmits its correspondingly colored LED beam and not others, thus acting as a wavelength (channel) filter. Figs. 9b–d depict this effect, where switching among the colored jellies resembles the action of a tunable optical filter.¹⁸ It should be noted that because the beam colors are not exactly at the same wavelengths as the colors of the corresponding jellies, all beams are neither completely transmitted nor completely filtered out; rather it is the relative degree of transmission and filtering that matters for this pedagogy. For instance, the red jelly in Fig. 9b mostly transmits the red beam and mostly filters out the green and blue beams. In Fig. 9c, the purple colored jelly is actually closer in color to the red beam than the blue one and therefore transmits red more. Lastly, to put together the scenario of having both a multiplexed signal in a fiber and a receiver demultiplexer, the beams can be first shone through the clear gelatin (representing the fiber) and then through any of the colored jellies (representing the demultiplexer), as portrayed in Fig. 9e.^¶ This experiment can also be carried out using laser pointers instead of LED sources, in which case it is recommended to be demonstrated by experienced teachers/demonstrators, rather than by students working on their own.

Figure 9:

Multiple wavelength transmission through (a) clear gelatin and selective filtering using (b) red, (c) purple, and (d) green jellies. (e) Clear gelatin and colored jelly used to simulate a fiber and demultiplexer set-up.