Traditional fi ber optic textiles use commercial TIR optical fi bers developed for the telecommunication industry. Such fi bers are made of transparent materials, such as glass or polymers, and optimized for low loss information transmission.
TIR optical fi bers embedded into woven composites have been applied for in- service structural health monitoring and stress– strain monitoring of industrial textiles and composites (Uskokovic et al. , 1999; D’Amato, 2002; Kuang and Cantwell, 2003). Integration of optical fi ber- based sensor elements into wearable apparel allows real- time monitoring of bodily and environmental conditions, which is important to various hazardous civil occupations, including the security and military sectors. Examples of such sensor elements can be optical fi bers with biologically- or chemically-activated claddings for biochemical detection (El-Sherif et al. , 2000), Bragg gratings and long- period gratings (Ghosh et al. , 2005) for temperature and strain measurements, as well as microbending- based sensing elements for pressure detection (Zheng et al. , 2006). Advantages of the optical fi ber sensors over other sensor types include resistance to corrosion and fatigue, fl exible and lightweight nature, immunity to E&M interference, and ease of integration into textiles.
TIR optical fi bers modifi ed to emit light sideways (Spigulis et al. , 1997) have been used to produce emissive fashion items (Lumigram), as well as backlighting panels for medical and industrial applications (Selem et al. , 2007; Lumitex).
To implement such emissive textiles, common silica (Spigulis et al. , 1997) or plastic (Harlin et al. , 2003) optical fi bers are used, in which light extraction is achieved through corrugation of the fi ber surface or fi ber microbending. Moreover, specialty fi bers capable of transverse lasing (Balachandran et al. , 1996; Shapira et al. , 2006) have been demonstrated, with additional applications in security and target identifi cation.
3.2.1 Total internal refl ection fi bers
The key functionality of a standard optical fi ber is effi cient guiding of light from an optical source to a detector. To date, all photonic textiles are made using TIR optical fi ber which, by its nature, confi nes light very effi ciently in the fi ber core.
Due to considerations of commercial availability and cost, silica glass- based telecommunication grade fi bers are often used, however they are even less suitable for photonic textiles, as such fi bers are designed for ultra- low loss transmission with virtually undetectable side leakage. Thus the main problem for the photonic textile manufacturers becomes the extraction of light from the optical fi bers. In TIR fi bers, light is guided via consecutive refl ections of light at the fi ber/air cladding interface. Only the rays within the cone defi ned by the fi ber numerical aperture are guided along the fi ber, while the rays with steeper angles of propagation leak out within small (typically sub- cm) propagation distances from the source.
3.2.2 Extraction of light from total internal refl ection fi bers
Light extraction from the core of a TIR fi ber is typically accomplished by introducing perturbations at the fi ber core/cladding interface. The two most frequently used methods to realize such perturbations are macro- bending of optical fi bers by the threads of a supporting fabric ( Fig. 3.1(a) ), or scratching of the fi ber surface to create light scattering defects ( Fig. 3.1b ).
The principal disadvantage of the macro- bending approach is high sensitivity of scattered light intensity on the value of a bend radius. In particular, insuring that the fi ber is suffi ciently bent with constant bending radii throughout the whole textile is challenging. If uniformity of the fi ber bending radii is not assured, then only a part of a textile featuring tightly bent fi bers will be lit up. This technical problem becomes especially acute in the case of wearable photonic textiles in which the local textile structure is prone to changes due to variable force loads during wear, resulting in ‘patchy’ looking non- uniformly luminescing fabrics.
Moreover, optical and mechanical properties of the commercial silica fi bers degrade irreversibly when the fi bers are bent into tight bends (bending radii of
several mm), which are necessary for effi cient light extraction, thus resulting in somewhat fragile textiles. The main disadvantage of the scratching approach is that mechanical or chemical methods used to roughen the fi ber surface tend to introduce mechanical defects into the fi ber structure, thus resulting in weaker fi bers prone to breakage. Moreover, due to the random nature of mechanical scratching or chemical etching, such post- processing techniques tend to introduce a number of randomly located very strong optical defects, which result in almost complete leakage of light at a few singular points, making the photonic textile appearance unappealing.
3.2.3 Dynamic color manipulation using total internal refl ection fi bers
To adjust the overall color of the traditional TIR fi bers, we have to individually control the intensities of light launched into the fi ber. Usually three different light sources of red, green and blue (the so- called RGB) are used ( Fig. 3.2 ). The main disadvantage of this technique is that relative intensities of the three light sources have to be monitored and maintained constantly over time to avoid color drift. In the event of failure of one of the light sources, it has to be replaced and the fi ber color has to be recalibrated by adjusting the relative intensities of all three sources.
3.1 Light extraction from TIR optical fi bers: (a) microbending in TIR fi bers; (b) surface corrugation in TIR fi bers.