Light sensitivity in glass fibers enables the manufacture of grid-based networks and devices, which are essential components in optical communication networks. The intrinsic sensitivity to light in standard single-mode telecommunications (SSM) fibers is generally low (Δn ≈ 3E−5). As a result, several methods to improve the light sensitivity of fibers have been actively studied over the past decade [1, 2]. One of the main areas of research was the development of an intrinsically light-sensitive fiber by increasing the germanium (Ge) content in the core of the fiber and also co-doping with materials such as boron, phosphorus, erbium and tin [1]. The use of these special fibers results in significant splicing losses (≥ 1 dB) with SSM fiber due to mode field diameter shifts, an increase in ultraviolet (UV) induced alterations such as absorption loss and birefringence due to mesh manufacturing, and a reduction in the thermal stability of networks due to dopant migration [1, 2]. The development of new methods to increase the intrinsic sensitivity to light in SSM fiber is still attractive because of its wide application, excellent optical properties, low polarization dispersion, low cost and minimal insertion loss. Here, I discuss the results using two methods – UV sensitization and dilute hydrogen sensitization – to improve light sensitivity in SSM optical fibers for the manufacture of fiber Bragg grating (FBG) based devices. A light-sensitive semiconductor device is a semiconductor device that converts light into electric current. Current is generated when photons are absorbed in the device. Devices may include optical filters and built-in lenses and may have large or small surfaces. These are typically used in sensors for light measurement, as photometers built into the camera, or to respond to light intensities, for example when lighting street lights after dark.
The big breakthrough came with the report on holographic writing of lattices by monophoton absorption at 244 nm by Gerry Meltz et al. [13]. They demonstrated reflective networks in the visible part of the spectrum (571-600 nm) with two interfering rays outside the fiber. The scheme provided the degree of freedom needed to move the Bragg condition to longer, more useful wavelengths, which depend primarily on the angle between the interfering rays. This principle has been extended to produce reflection networks at 1530 nm, an interesting wavelength in telecommunications, which also allows the demonstration of the first fiber laser operating from the reflection of the light-sensitive fiber network [14]. The ultraviolet (UV) induced index change in untreated optical fibers was ~10−4. Since then, several developments have taken place that have increased the index change of optical fibers a hundredfold, so that efficient reflectors with a length of only one hundred wavelengths can be produced. Lemaire and colleagues [15] showed that loading glass fibers with molecular hydrogen photosensitized even standard telecommunication fibers to such an extent that networks with very high refractive index modulation could be written. The global discrete semiconductor market is highly fragmented, with many semiconductor manufacturers supplying the product. Companies continuously invest in the product and technology to promote sustainable environmental growth and avoid environmental risks. Companies also acquire other companies that deal specifically with these products in order to increase their market share.
Asian countries such as China, India, South Korea, Taiwan and Japan have a significant presence of manufacturers of these devices and have experienced lockdowns and disrupted production schedules. Sales are down as the lockdown has led to a limitation of deliveries to essentials in most economies around the world and companies have revised their revenue targets. Amid the spread of viruses, governments around the world have ordered the suspension and shutdown of consumer electronics manufacturing processes, which will negatively impact the light-sensitive semiconductor device market. However, their use in health care devices could help offset the losses. The growth rate and maximum change in the index are attractive if strong networks are to be produced in a short period of time. This suggests that reduced Germania is better than ordinary fiber in both respects. However, the maximum variation of the index is still less than that required for a number of applications and the manufacturing time is too high. The use of hot hydrogen to reduce Germania has the additional effect of increasing the loss close to 1390 nm due to the formation of hydroxyl species [49,50].
The absorption loss at 1390 nm is estimated to be ~0.66, 0.5 and 0.25 dB/(m-mol%) at 1390.1500 and 1550 nm respectively [40]. A major advantage of reduced fibers is that they are permanently sensitive to light and require minimal processing compared to hydrogenated fibers (see sections below). The incorporation of 0.1% nitrogen into germanium-doped silica fibers by the surface plasma-assisted chemical vapor deposition (SPCVD) process [51] showed high photosensitivity [52]. The effect on absorption of 240 nm is dramatic, increasing it to 100 dB/mm/mol% GeO2, doubling compared to the equivalent of doping to germanium alone. The induced changes in the refractive index would be large (2.8 × 10−3) and much greater (0.01) with a cold soaking of 7 mol%Ge;0 mol%N fiber. The Type IIA threshold would increase by a factor of ~6 compared to that of 20% molar GE fibers without nitrogen. However, there is evidence of increased absorption loss within the 1500 nm window due to the addition of nitrogen. The next fibers most sensitive to light are germania-boron or tin-doped fibers. The photosensitive semiconductor devices market is expected to register a CAGR of over 9.2% during the forecast period (2021 – 2026).
These devices are used in image sensors, which are mainly used in a large number of imaging devices and digital cameras to improve the quality of cauterization and storage of images. These imaging applications have been widely adopted in industrial, media, medical and consumer applications. The researchers were already experimenting and studying the even more bizarre phenomenon of second harmonic generation in optical fibers from German-doped silicon dioxide, a material that has a second-order nonlinear coefficient of zero responsible for the generation of the second harmonic. The observation differed markedly from another nonlinear phenomenon of sum frequency generation previously reported by Ohmori and Sasaki [4] and Hill et al. [5], which was also strange. Ulf Osterberg and Walter Margulis [6] discovered that ML-QS infrared radiation could “condition” a German-doped silica fiber after long exposure so that the second harmonic radiation (such as the Ken Hill reflective grating) reached an efficiency of nearly 5% and was quickly identified as a network formed by a nonlinear process [7,8]. Julian Stone`s observation [9] that virtually all Germania-doped silica fibers were sensitive to argon laser radiation opened up activity in the fiber network region [10,11] and determined possible connections between the two light-sensitive effects. Parent et al. [12] had pointed out the two-photon absorption of the phenomenon from the fundamental radiation at 488 nm. Light excitation can also produce negative photoconductivity, depending on the energy of the photons. Consider a simple n-type semiconductor consisting of naturally thermally generated free electrons of concentration No.
with type I and type II centers, as shown in figure 7-58(e). Now, if we use excitatory light from photon energy (Er1 − Ev) ≤ hv < (Ec − Ev) to illuminate the semiconductor, the light can only excite electrons from Ev to Er1 and create free holes to recombine with free electrons. This reduces the concentration of the majority electrons, making the conductivity lower than the original dark conductivity. For this reason, photoconductivity under such light excitation is called negative photoconductivity. 73.99 TiO2 is a semiconductor-based material known for its high sensitivity to light. It has a band gap energy of 3.0 eV. When absorbing a photon with an energy equal to or greater than this value, TiO2 can be excited to create negative electrons in the conduction band, creating positive holes in the valence band. These charge carriers react with surrounding water or oxygen to form cytotoxic ROS with powerful oxidizing agents that can attack cell membranes and other cellular components, resulting in apoptosis of cancer cells [107]. In addition to its high photoreactivity, TiO2 has attracted great interest as a potential inorganic photosensitizer for PDT due to its non-toxicity, high stability and excellent biocompatibility [108–111]. However, TiO2-NP tends to clump together in neutral aqueous solution, hindering their use in biological applications. Sensitization includes not only the sensitization of photoconductivity to a certain photonic energy, but also the awareness of spectral sensitivity. In most wideband insulators, such as polymers, only high-energy radiation such as X-rays (or at least ultraviolet light) can be absorbed to produce photocurrent.
