The optical fiber used to visualize the underwater rescue-rope is made of polymethyl methacrylate (PMMA), which has no problems related to electric insulation and corrosion in an underwater environment [7]. The light attenuation rate at the core is 0.15 dB/m [8][9].

The quantitative performance indicator of the rescue-rope is the brightness that can be recognized by divers from a certain distance; therefore, the surface luminance of the optical fiber is used as the criterion.

The specifications of the light source used in the experiment are shown in Table 1, which is a 525 nm device-on-board (DOB) type green LED package [10].

The main factors influencing the selection of the light source for the underwater rescue-rope were the total luminous flux and the surface luminance (lm/mm²) within the core area of the optical fiber.

The structure of an optical fiber for communication and side emission purposes consists of a core, clad, and jacket, as shown in Figure 2.

Figure 2: 
Structure of optical fibers: (a) Communication purpose; (b) Side emission purpose

The light passes through the core. For communication, the clad shields the light that propagates into the core, whereas for side emission, the clad is applied to radiate a certain amount of light outward, at right angles to the propagation passage of the light.

In addition, for communication, the jacket is used only to protect the clad, whereas for side emission, the jacket is made of materials with a high transparent rate so that it can also radiate the light to the clad outward.

The specifications of the optical fiber used as an underwater rescue-rope are shown in Table 2. The following experimental studies were conducted to enhance the surface luminance of the side-emitting optical fiber:

- Application of a light concentrator between the light source and the optical fiber
- Shielding and reflection control of the jacket
- Application of a reflector at the end of the optical fiber

The beam angle of the LED package is generally greater than 110° unless a specially designed lens is applied. In addition, the radiant heat from the luminous surface of a high-power LED is over 100 °C, which indicates that the optical fiber may be damaged if its core and the luminous area of the LED come into direct contact [11].

As shown in Figure 3 (a), the distance between the LED package and the optical fiber should be at least 10 mm to prevent the damage to the optical fiber by the radiant heat. In this case, 60% of the luminous flux from the LED is condensed at the core and the rest dissipates. Therefore, a light concentrator between the LED package and the optical fiber is required to utilize the luminous flux fully. In this study, a multi-layer glass lens composed of a concave lens and 3-convex lens was applied as the light concentrator. Subsequently, an optimal light concentrator that focuses the radiated flux from the LED onto the core of the optical fiber was placed [12]-[14].

Figure 3: 
Method of light concentration (a) Direct coupling; (b) Insertion of the light concentrator

The beam angle of the light concentrator was narrower than 60° and its height was 10 mm. As shown in Figure 3 (b), the light concentrator was inserted between the LED package and the optical fiber. To analyze the efficiency of the light concentrator, the ratio of the luminous area of Figure 3 (a) without the concentrator; and Figure 3 (b) with one, at a 10mm from the LED was compared. As a result, the following is obtained:

In Figure 3 (a), the radius passing through the luminous flux at a beam angle of 120° is 17.32 mm (=10×tan60°) and the cross-section area of the solid angle is approximately 940 mm² (=Π×17.32²). Only the luminous flux incident on the cross-section area of 200 mm² that corresponds to the radius of 8 mm was focused on the core.

In Figure 3 (b), the radius passing through the luminous flux at a beam angle of 60° is 5.77 mm (=10×tan30°). Therefore, the luminous flux that corresponds to a beam angle of 120° was focused on the core. Similarly, by the application of a light concentrator with a beam angle of 60°, the luminous flux focused on the core was increased by 470% (=940÷200).

Even considering an absorption and reflection loss of 30% in the light concentrator, the incident luminous flux onto the core was increased by 330%. The surface luminance measured at the end of the optical fiber was increased by 170%, as the forward current of the LED was fixed at 5 A and a light concentrator was applied at a 20-m-long optical fiber.

The light radiated from the surface of the optical fiber is diffused underwater and a rescue-rope with a higher surface luminance is better recognized by divers in turbid water.

In this study, a 100-cm-long auxiliary jacket with a reflectance of 50% and a transmissivity of 20% was covered on the optical fiber at intervals of 10 cm, as shown in Figure 4. It was used to prevent the surface luminance of the optical fiber from decreasing, as more luminous flux was radiated outward with the increase in the distance of the optical fiber from the light source.

Figure 4: 
Structure of the optical fiber with auxiliary jackets

In addition, a reflector with a reflectance of 85% was covered at the end of the optical fiber to reduce the loss of luminous flux as shown in Figure 5.

Figure 5: 
Installation of the reflector at the end of the optical fiber

Figure 6 shows the surface luminance of the optical fiber after applying all the three aforementioned techniques. The surface luminance increased considerably, as the number of jackets was increased and the reflector was installed. The surface luminance was increased by 280% with the installation of auxiliary jackets. In addition, the surface luminance was increased by a maximum of 430% after applying the reflector.

Figure 6: 
Increase in luminance with the number of auxiliary jackets