Silicon Core Optical Fibers Research Articles

Broadband, tunable wavelength conversion using tapered silicon fibers extending up to 2.4 μm

Wavelength conversion via four-wave mixing holds great promise for the construction of broadband and tunable light sources at wavelengths beyond 2 μm. In this work, we design and fabricate a tapered silicon core optical fiber with a dispersion profile that supports efficient conversion spanning the telecom band up to the edge of the mid-infrared spectral region over an extended propagation length. By pumping with a fiber laser centered around 1.99 μm, a tuning range of 690 nm has been measured, although simulations predict that a bandwidth of up to 1255 nm could be observed if a suitable seed source was available. Conversion efficiencies of ∼−30 dB have been obtained over a bandwidth of 380 nm when using an input pump power of only 6 dBm, with a maximum efficiency of −18 dB achieved when the conversion overlaps the strong Raman gain of the silicon core.

Generation of stable temporal doublet by a single-mode silicon core optical fiber

A highly nonlinear single mode anomalous dispersion silicon-core fiber is designed and optimized at the operating wavelength of 2.2 µm for the purpose of generating stable temporal pulse doublets. To designate the output pulse pair as a perfect Gaussian doublet, two new parameters, dissimilarity factor () and co-similarity index (μ cs) are introduced. Different input pulse parameters such as power, pulse width and chirp are optimized to obtain Gaussian temporal doublets at the shortest optimum length (∼few cm) which is sufficiently smaller in comparison to silica fibers reported earlier. The output pulse pairs remain as a doublet for quite a good stability length. In view of serving practical purposes, the possibilities of fluctuations of input power and pulse width are included to investigate the changes in stability length and effective repetition rate (ERR). The change in ERR along the fiber length produces a remarkable change in free carrier concentration in core, which has also been taken into account for the first time as per our knowledge to obtain the temporal pulse doublet in the so designed Si-core fiber.

Specialty optical fiber fabrication: fiber draw tower based on a CO laser furnace

An experimental, laboratory-scale optical fiber drawing tower based on CO laser heating has been developed and used to fabricate speciality optical fiber. The CO laser was utilized in a symmetric four beam heating system. The localized and responsive heating time of the laser-based furnace was beneficial for manufacturing crystalline core fibers, specifically, silicon core optical fibers. Moreover, the specific absorption properties of the CO laser radiation in silica have been evaluated with the aid of finite element modeling. In comparison to the more traditional C O 2 laser, CO lasers were found to improve temperature uniformity and heating times while minimizing surface evaporation.

All-optical high-speed modulation of THz transmission through silicon core optical fibers.

High speed optical modulation of THz radiation is of interest for information processing and communications applications. In this paper infrared femtosecond pulses are used to generate free carriers that reduce the THz transmission of silicon based waveguides over a broad spectral range. Up to 96% modulation is observed from 0.5 to 7 THz in an optical fiber with a 210 µm diameter gold-doped silicon core. The observed carrier recombination time of 2.0 ± 0.2 ns makes this material suitable for high speed all-optical signal processing. These results show both enhanced modulation depth and reduced carrier lifetime when compared to the performance of a high resistivity float zone silicon rectangular guide with comparable cross sectional area.

Designing silicon-core fiber tapers for efficient supercontinuum generation in the greenhouse gas absorption region

We propose a tapered silicon-core optical fiber design for extending the long-wavelength edge of supercontinuum generation to obtain a high spectral density source across the 3–4.5 µm regime. The taper works by generating sufficient spectral broadening of the driving laser pulse to produce a series of pumps for nondegenerate four-wave mixing, and then opening up new phase-matching conditions to transfer the power from these pumps to a target region of mid-infrared wavelengths. We show, by simulation, that this taper design works effectively when pumped with a conventional 2.1 µm femtosecond fiber laser, significantly improving the spectral coverage obtained with a fixed-diameter fiber. Thus, these tapered silicon-core fibers offer a potential platform for an efficient all-fiber spectroscopy solution to measure greenhouse gases.

Theoretical Design of Mid-Infrared Graphene Optical Gas Sensor Based on Slot Si Core Fiber

Silicon core optical fibers are promising for realizing all-in-fiber optical processing and sensing applications in the mid-infrared spectral region. However, this approach requires integration with other optoelectronic materials to realize various functions. Here, we theoretically designed a graphene optical gas sensor based on silicon fibers at 8- ${\mu }\text{m}$ wavelengths. Gas molecules adsorbed on graphene serving as the charge-carrier donors or acceptors change graphene’s optical conductivity, which leads to the change in the optoelectronic properties of graphene-integrated devices. We first optimized the Fermi level of graphene and the operating wavelength so that the gas molecules can effectively change graphene’s optical conductivity. Then, we proposed a silicon slot fiber structure to enhance the optical interaction in graphene-on-silicon fiber devices. Based on the optimized silicon slot fiber structure, we designed a fiber-integrated graphene Bragg grating structure, in which the 3-dB bandwidth of the central reflection band increases linearly with the gas concentration. Our proposed device has a detection limit below one part per million and 90-fold improvement of sensitivity compared with the designed graphene-on-silica microfiber gas sensors for gaseous nitrogen dioxide detection. This letter paves the way for the mid-infrared silicon optical fiber sensors.

Ultrasmall light-spot transmission in a silicon-core fiber with a bowtie slot structure.

In this study, we propose a novel silicon-core optical fiber with a bowtie-shaped slot core structure for ultrasmall light-spot transmission. Our simulations show that this dielectric structure creates a nanosized optical spot with the high intensity and transmits it with the low loss. As an example, we obtained an optical spot with the full width at half maximum of 5 nm × 5 nm and a peak power density 167 times higher than that of the surrounding areas. The optical loss because of the scattering in the waveguides and the material absorption was estimated to be 0.58 dB/cm, which is a thousand times less than the optical losses in typical plasmonic waveguides. We believe our proposed structure will contribute to research studies in the field of near-field sensing systems. It also has application potential in nanolithography with high-power lasers.

Wavelength Conversion and Supercontinuum Generation in Silicon Optical Fibers

This paper describes the state of the art in wavelength conversion and supercontinuum generation using glass-clad silicon core optical fibers. Such semiconductor fibers have enjoyed considerable attention due to their intrinsically high third-order nonlinearities, which are markedly higher than in conventional infrared glasses. Results to date from small core silicon fibers fabricated using both the high-pressure chemical vapor deposition technique and the molten core drawing method are presented. Also discussed are directions for continued study and development, including engineering the dispersion and nonlinear properties as well as improved interconnection.

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Crystalline Silicon Optical Fibers with Low Optical Loss

Polycrystalline silicon core optical fibers have been fabricated by modified thermal annealing of amorphous silicon chemically deposited at high pressure. The resulting fibers have small-diameter cores, a geometry advantageous for optical guidance. Moreover, the combination of chemical deposition and annealing avoids difficulties associated with undesired transfer of oxygen impurities to the silicon core from the molten cladding during the drawing process. The high aspect ratio of the amorphous silicon core and the presence of the silica cladding surrounding make the design rules for annealing to optimize their polycrystalline structure different from those of conventional amorphous silicon films. We find that optimization of the annealing allows for an increase in the polycrystalline grain size and decrease in the defects in the silicon core. A low optical loss of less than 1 dB/cm at a wavelength of 2.2 μm is thus realized, much lower than that reported for small core size (<10 μm) crystalline silicon f...

Temperature characteristics of silicon core optical fiber Fabry-Perot interferometer.

Silicon core optical fiber expanded silicon photonics to specialty fiber platform. Although great challenges still exist for the fiber fabrication, the presence of semiconductor material has already given optical fiber new features and enormous possibilities for fiber-based devices and sensors. In this Letter, an all fiber silicon cavity Fabry-Perot interferometer was made by splicing silicon core silica cladding fiber with conventional single-mode silica fiber. The cavity shows high temperature sensitivity around 82pm/°C due to the larger thermo-optical coefficient of silicon material compared with that of silica material.

Ultrafast wavelength conversion via cross-phase modulation in hydrogenated amorphous silicon optical fibers

We present a characterization of the spectral modulation and wavelength shifting induced via cross-phase modulation (XPM) in a hydrogenated amorphous silicon (a-Si:H) core optical fiber. Pump-probe experiments using picosecond and femtosecond signal pulses are shown to be in good agreement with numerical simulations of the coupled nonlinear propagation equations. The large 10nm red-shifts obtained with the femtosecond probe pulses are attributed to the high Kerr nonlinearity of the a-Si:H material. Extinction ratios as high as 12 dB are measured for the conversion process at telecommunications wavelengths, indicating the potential for high-speed nonlinear optical control in a-Si:H fibers and waveguides.

On loss in silicon core optical fibers

Glass clad semiconductor core fibers have received much attention recently for their potential utility for nonlinear optics and infrared power delivery. As these fibers have progressed, it has become evident that a greater understanding as to the dominant sources of loss is needed. This work begins that discussion by investigating intrinsic and extrinsic sources of loss in silica glass clad crystalline silicon core optical fibers. Of particular interest are, to the best of our knowledge, the first lattice-fringe images of single and poly-crystalline regions of the silicon core optical fibers as well as scattering sources. Suggested herein are methods to further reduce the presence of impurities and defects that lead to scattering and dominate optical losses.

Nonlinear pulse dynamics in multimode silicon core optical fibers

Multimode propagation in silicon core optical fibers is investigated via numerical modeling of the coupled mode equations. The simulations consider spectral evolution in two fibers with different micrometer-sized cores that have experimentally been shown to exhibit nonlinear broadening. The results indicate that most of the coupled power is propagated in the fundamental mode of each fiber, with a small contribution from the higher-order modes affecting the spectral shape but not the width of the broadening.

Cladding Glass Development for Semiconductor Core Optical Fibers

Glass‐clad optical fibers comprising a crystalline semiconductor core have garnered considerable recent attention for their potential utility as novel waveguides for applications in nonlinear optics, sensing, power delivery, and biomedicine. To date, cladding compositions have relied on commercially‐available expedients and have not been tailored for the specific semiconductor core nor the application. In this work, more‐optimum silicate and nonoxide glass compositions are developed for unary (Si, Ge), binary (InSb, GaAs), and ternary (GaAlSb) semiconductor cores based on two main design criteria: (1) matching the thermal expansion coefficient between semiconductor core and glass cladding and (2) matching the viscosity‐temperature dependences such that the cladding glass draws into fiber at a temperature slightly above the melting point of the semiconductor. While this latter requirement is critical to the molten core fabrication method, which offers a practical approach to long fiber lengths at acceptable manufacturing speeds (&gt;m/s), these compositions are more broadly applicable to other semiconductor fiber processing methods. Preliminary experimental results on silicon core optical fiber are provided and show a marked diminution in oxygen content relative to analogous fibers drawn using a pure silica cladding.

Mid-IR soliton compression in silicon optical fibers and fiber tapers

Numerical simulations are used to investigate soliton compression in silicon core optical fibers at 2.3 μm in the mid-infrared waveguide regime. Compression in both standard silicon fibers and fiber tapers is compared to establish the relative compression ratios for a range of input pulse conditions. The results show that tapered fibers can be used to obtain higher levels of compression for moderate soliton orders and thus lower input powers.

Annealing of silicon optical fibers

The recent realization of silicon core optical fibers has the potential for novel low insertion loss rack-to-rack optical interconnects and a number of other uses in sensing and biomedical applications. To the best of our knowledge, incoherent light source based rapid photothermal processing (RPP) was used for the first time to anneal glass-clad silicon core optical fibers. X-ray diffraction examination of the silicon core showed a considerable enhancement in the length and amount of single crystallinity post-annealing. Further, shifts in the Raman frequency of the silicon in the optical fiber core that were present in the as-drawn fibers were removed following the RPP treatment. Such results indicate that the RPP treatment increases the local crystallinity and therefore assists in the reduction of the local stresses in the core, leading to more homogenous fibers. The dark current-voltage characteristics of annealed silicon optical fiber diodes showed lower leakage current than the diodes based on as-drawn fibers. Photons in UV and vacuum ultraviolet (VUV) regions play a very important role in improving the bulk and carrier transport properties of RPP-treated silicon optical fibers, and the resultant annealing permits a path forward to in situ enhancement of the structure and properties of these new crystalline core optical fibers.

All-optical modulation using two-photon absorption in silicon core optical fibers

All-optical modulation based on degenerate and non-degenerate two-photon absorption (TPA) is demonstrated within a hydrogenated amorphous silicon core optical fiber. The nonlinear absorption strength is determined by comparing the results of pump-probe experiments with numerical simulations of the coupled propagation equations. Subpicosecond modulation is achieved with an extinction ratio of more than 4 dB at telecommunications wavelengths, indicating the potential for these fibers to find use in high speed signal processing applications.

Polycrystalline silicon optical fibers with atomically smooth surfaces

We investigate the surface roughness of polycrystalline silicon core optical fibers fabricated using a high-pressure chemical deposition technique. By measuring the optical transmission of two fibers with different core sizes, we will show that scattering from the core-cladding interface has a negligible effect on the losses. A Zemetrics ZeScope three-dimensional optical profiler has been used to directly measure the surface of the core material, confirming a roughness of only ~0.1 nm. The ability to fabricate low-loss polysilicon optical fibers with ultrasmooth cores scalable to submicrometer dimensions should establish their use in a range of nonlinear optical applications.

Soliton propagation in tapered silicon core fibers

Numerical simulations are used to investigate soliton-like propagation in tapered silicon core optical fibers. The simulations are based on a realistic tapered structure with nanoscale core dimensions and a decreasing anomalous dispersion profile to compensate for the effects of linear and nonlinear loss. An intensity misfit parameter is used to establish the optimum taper dimensions that preserve the pulse shape while reducing temporal broadening. Soliton formation from Gaussian input pulses is also observed--further evidence of the potential for tapered silicon fibers to find use in a range of signal processing applications.