Optical fiber is an important transmission medium in optical communication system. We have designed circular stepindex fiber and circular graded-index fiber for Orbital Angular Momentum (OAM) transmission. And we calculate the existing vector modes in fiber through the software COMSOL Multiphysics. The step-index fiber has a high refractive index ring which doped with PbS quantum dots between the core and the cladding. And the refractive index difference between the ring and the cladding reaches 0.035, which weaken the degeneracy of adjacent eigenmodes. Numerical analyses show the step-index ring core fiber can support the long-distance transmission for the OAM mode of |L|=1,5,6,7 in the wavelength range of 1530nm to 1565nm (|L| refers to the mode order of orbital angular momentum, called topological charge). In order to enhance its stability, we adjust the ring to be a graded refractive index profile, which increases the effective refractive index difference between the vector modes from the same mode order. This fiber design not only increases transmission stability, but also has a better manufacturing process than the step -index fiber. From the simulation results, we can find that the graded-index fiber with a gradation rate α =8 provides a relatively large effective refractive difference. The effective refractive difference is 1.68 × 10−4 , 1.42 × 10−4 and 2.44 × 10−4 respectively corresponding to L=1, 5, and 6 at a wavelength of 1550 nm. While the effective refractive index difference is 1.65 × 10−4 , 0.9 × 10−4 and 1.52 × 10−4 for the step-index optical fiber.
High-quality lead sulfide (PbS) nanofilm was deposited on silica fiber substrate materials via atomic layer deposition (ALD) technology. The structure, morphology, and optical properties of PbS nanofilm were investigated. Scanning electron microscopy (SEM) result shows that the PbS nanomaterials had cubic phase and the size of nanoparticles were 50~100 nm. The Raman spectrum shows three peaks at 134, 425 and 966 cm-1 , which further reveal the bonding modes between PbS and silica materials. In addition, spectral characteristics of the samples show the emission peak at 379 nm, with the excitation wavelength of 250 nm at room temperature.
Optical properties of tapered optical fiber deposited with PbS are investigated, which is deposited based on atomic layer deposition technique with Pb(tmhd)2 and H2S as Pb and S precursors. After deposition, morphology of PbS thin film is characterized by a scanning electron microscope, and the composition is confirmed by X-ray photoelectron spectroscopy and energy dispersive spectrum. Raman spectrum shows a typical peak at 204 cm − 1, which is assigned to the influence of the PbS structure, and it further reveals that PbS is deposited on the surface of the tapered optical fiber successfully. With a 980-nm pump laser diode, tapered optical fiber deposited with PbS applied to fiber amplifier exhibits a wide band optical gain at 1550 nm with the largest gain of 5.6 dB.
The physical characteristics and optical properties of PbS nanoclusters are investigated by using density functional theory (DFT) of first-principles. Microstructure models of (PbS)n (n=1-9) nanoclusters and bulk materials are built on Materials Studio platform, and its energy band structures, highest occupied molecular orbital-lowest unoccupied molecular orbital gap (HOMO-LUMO gap), density of state (DOS), and optical properties are calculated, respectively. Compared to PbS bulk materials, PbS nanoclusters show a discrete energy gap as well as the DOS, because of the quantum confinement effect. It is interesting that the HOMO-LUMO gap of (PbS)n (n=1-9) shows oscillates with the increasing of the n number. However, when its size is large enough, the HOMO-LUMO gap is gradually decrease with the increasing of size (>27 atoms). And, the HOMO-LUMO gap of PbS nanoclusters of different sizes is range from 2.575 to 0.58 eV, which covers the low loss communication band of optical communication. In addition, PbS nanomaterials (NMs) with small size are synthesized by using oleylamine as ligands. Sizes of PbS NMs can be accurately controlled through control of the reaction time as well as the growth temperature. The photoluminescence (PL) spectra show strong size dependence, which is large red shift with increasing size of the NMs. This trend is basically in agreement with the theoretical calculation above. Moreover, transmission electron microscopy (TEM) further reveals the morphology of PbS NMs. PbS NMs can be used in optical fiber amplifiers and fiber lasers because of its unique optical properties in optical communication bands.
The technique of atomic layer deposition (ALD) has been introduced to fabricate PbS-doped silica fibers, whose
absorption peaks are discovered to be shifted from 1230 nm to 920 nm when the number of ALD deposition cycles
varies from 80 to 30 during optical fiber preform fabrication. This is explained by suggesting that the PbS doped in fiber
are under the 3D quantum confinement, i.e., quantum dots (QDs). An effective-mass approximat ion of the PbS QDs ’
sizes is then made to show the shift of absorption peaks can be attributed to the change of size distribution of these dots.
The structures and optical properties of (PbS)n cluster in silica optical fiber material are investigated. The microstructures models of (PbS)n (n=1-4) and PbS-(SiO2)n (n=1-6) have been built and calculated by Gaussian-03 software using density functional theory with the B3LYP level. The gap between the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) is also calculated for microstructures. Compared with the (PbS)n clusters and (SiO2)n clusters, The HOMO-LUMO gaps of (PbS)n clusters combined with (SiO2)n clusters make a big difference. The geometry structures of (PbS)n-(SiO2)4 (n=2-4) clusters are calculated by using the singles configuration interaction (CIS) method. The calculation results show that the excitation energies of (PbS)n-(SiO2)4 clusters changed as the sizes or the structures are changed. The PbS-doped silica optical fiber is fabricated, and the optical properties are measured to compare with the theoretical results.
Silver nanoparticles have been found many interesting applications, such as absorption amplifiers in dye-sensitized solar
cells. However, silver nanoparticles are easily oxidized. In order to protect silver nanoparticles, atomic layers of TiO2were deposited onto silver nanoparticles coated to a glass slide. Then the glass slide was exposed to corrosive I-/I3- solutions, and the degree of silver etching was measured via scanning electron microscopy (SEM) and ultraviolet-visible
spectroscopy (UVS). It was found that 3 nm (30 cycles) of Al2O3 with 9 nm of (90 cycles) TiO2 could completely protect
silver nanoparticles from oxidization.
A novel PbS quantum dots (QDs) fiber amplifier based on SiO2 Sol-Gel method was proposed. The QDs doped
SiO2 films was deposited onto a fused tapered fiber coupler based on standard single mode fiber (SMF). With a 980 nm
wavelength laser diode (LD) as the pump, 1550 nm signal and 980 nm pump light waves were injected into the tapered
region simultaneously, through the evanescent wave, we obtained the gain at 1576 nm wavelength as high as 5 dB. The
proposed fiber amplififier can implement the property of a small, integrated, high output, low noise, high gain, low cost,
which meet the need of the future of optical fiber communication system.
A PbSe quantum dots (QDs) fiber amplifier has been demonstrated. The PbSe QDs were synthesized via sol-gel self-assembly
method. The size of PbSe QDs was controlled to 5.5 nm through control of the reaction time as well as the
growth temperature. Transmission electron microscopy (TEM) and photoluminescence (PL) spectroscopy were used to
characterize the PbSe QDs samples. The fiber amplifier was fabricated by coating the QDs onto a tapered optical fiber
coupler. Through evanescent wave, the QDs were excited to realize optical amplification. A 1550 nm semiconductor
light emitting diode (SLED) as the signal source and a 980 nm laser diode (LD) source as the pump were injected into
the fiber coupler simultaneously.
Nano-Rare Earth Doped Fibers (NREDFs) have shown great application for optical fiber amplifiers, fiber lasers and
sensors. The rapid development of fiber communication systems has a higher requirement on the NREDFs. Atomic layer
deposition (ALD) is a chemical vapor deposition technique based on the sequential use of self-terminating gas-solid
reactions. As a film deposition technique, ALD is known for its effective material utilization at low temperatures,
accuracy thickness control, excellent step coverage, good uniformity and adhesion, good conformability. In this paper,
ALD was used to fabricate high concentration alumina-erbium co-doped amplifying fibers. Based on Modified Chemical
Vapor Deposition (MCVD) and ALD, using nanomaterials as a dopant, the alumina-erbium co-doped amplifying fibers
were fabricated. The main advantages of this novel method include good uniformity, high dispersibility, and high doping
concentration. Scanning electron microscopy (SEM), transmission electron microscopy (TEM) images and X-ray energy
dispersive spectroscopy (EDS) showed the physical and chemical features of the deposited film upon a porous silica soot
layer. Photoluminescence (PL) and absorption spectra were used to characterize the optical properties. The fibers have
high gain, low noise, high power and are independent of polarization, which make them desirable for fiber devices.
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