Small-core Single-mode Fibers Research Articles

Broadband mode-selective couplers based on tapered side-polished fibers.

We propose the broadband mode-selective coupler (MSC) formed with a side-polished six mode fiber (6MF) and a tapered side-polished small core single-mode fiber (SC-SMF) or an SMF. The MSCs are designed to allow the LP01 mode in the SC-SMF and SMF to completely couple to the LP01, LP11, LP21, LP02, LP31, LP12 modes in the 6MF over a broadband wavelength range. The phase-matching conditions of the MSCs are satisfied by tapering the SC-SMF and SMF to specific diameters. The tapered fibers are side-polished to designed residual fiber thickness using the wheel polishing technique. The effective indices of the side-polished fibers are measured with the prism coupling method. The MSCs provide high coupling ratio and high mode purity. High coupling efficiencies in excess of 81% for all the higher-order modes are obtained in the wavelength range 1530-1600 nm. For the LP01, LP11, LP21, LP02, LP31, LP12 MSCs at 1550 nm, the coupling ratios are 96.2%, 99.8%, 89.5%, 85.0%, 90.9%, 96.1%, respectively, and the mode purity of the MSCs is higher than 88.0%. The loss of the MSCs is lower than 1.8 dB in the wavelength range 1530-1600 nm. This device can be applied in broadband mode-division multiplexing transmission systems.

Low short-wavelength loss fiber Bragg gratings inscribed in a small-core fiber by femtosecond laser point-by-point technology.

A femtosecond-laser-induced fiber Bragg grating (FBG) usually has a higher insertion loss at the shorter wavelength than at the reflection wavelength, i.e., so-called short-wavelength loss. High-quality FBGs are inscribed in different types of small-core single-mode fibers (SMFs) by the use of femtosecond laser point-by-point technology in order to investigate the effect of the fiber core diameter on the grating inscription efficiency and on the short-wavelength loss. A lower laser pulse energy is required to achieve the same grating reflectivity in a smaller-core fiber than in a large-core fiber. The short-wavelength loss of the small-core FBG is lower than that of the large-core FBG with the same reflectivity. Furthermore, a series of FBGs with a low short-wavelength loss are inscribed in a small-core SMF along the fiber axis to achieve so-called series-integrated FBGs (SI-FBGs). Finally, the effect of the input light direction on the reflection peak of the SI-FBGs is investigated to reduce the influence of the grating short-wavelength loss in the sensing and communication applications.

High sensitivity optical fiber sensors for simultaneous measurement of methanol and ethanol

High sensitivity volatile organic compounds (VOCs) sensors based on a tapered small core single mode fiber (TSCSMF) and a microfiber coupler (MFC) are reported. The TSCSMF had a waist diameter of ∼5.1 μm and the MFC had a waist diameter of ∼1.9 μm each and both were fabricated using a customized microheater brushing technique. Silica based materials containing immobilized Nile red prepared by sol-gel method with two different recipes (recipe I and recipe II) are investigated. Initially recipe I based coating materials were applied to the surfaces of the TSCSMF and MFC. The experimental results show that the sensor based on an MFC shows much better sensitivities of −0.130 nm/ppm and −0.036 nm/ppm to ethanol and methanol than those of the TSCSMF based sensor. The corresponding minimum detectable concentration change of the MFC based sensor are calculated to be ∼77 ppb and ∼281 ppb to ethanol and methanol respectively. Both sensors are demonstrated fast response times of less than 5 min, while the recovery times varied from 7 min to 12 min. In addition, another TSCSMF based sample (∼7.0 μm) coated with a mixed layer of sol silica and Nile red prepared by recipe II was fabricated to achieve simultaneous measurement of ethanol and methanol, employing a second-order matrix approach.

LP01 to LP11 mode convertor based on side-polished small-core single-mode fiber

An all-fiber LP01-LP11 mode convertor based on side-polished small-core single-mode fibers (SMFs) is numerically demonstrated. The linearly polarized incident beam in one arm experiences π shift through a fiber half waveplate, and the side-polished parts merge into an equivalent twin-core fiber (TCF) which spatially shapes the incident LP01 modes to the LP11 mode supported by the step-index few-mode fiber (FMF). Optimum conditions for the highest conversion efficiency are investigated using the beam propagation method (BPM) with an approximate efficiency as high as 96.7%. The proposed scheme can operate within a wide wavelength range from 1.3 μm to1.7 μm with overall conversion efficiency greater than 95%. The effective mode area and coupling loss are also characterized in detail by finite element method (FEM).

High sensitivity sol-gel silica coated optical fiber sensor for detection of ammonia in water.

A high sensitivity ammonia sensor based on a tapered small core singlemode fiber (SCSMF) structure for measurement of ammonia concentration in water is reported. Two tapered SCSMF fiber structures with different waist diameters of 23 µm and 13.5 µm are fabricated by using a customized microheater brushing technique. The silica based material prepared by the sol-gel method is used as a coating applied to the surface of the tapered fiber structures. To investigate the influence of the coating thickness on the sensitivity to ammonia in water, silica coatings with different thicknesses (2-pass and 8-pass coatings) are deposited on the surface of the fiber sensor with a waist diameter of 23 µm. Experiments demonstrate that the sensor with a thicker (8-pass) silica coating shows better sensitivity of 0.131 nm/ppm to ammonia compared to that of 0.069 nm/ppm for the thinner silica coating (2-pass). To further improve the sensor sensitivity, the taper waist diameter is reduced. For an 8-pass coating (249nm at the taper waist section) applied to a tapered SCSMF structure based fiber sensor with a reduced waist diameter of 13.5 µm. Experimental results show that the sensitivity to ammonia is significantly improved to 2.47nm/ppm. The best measurement resolution for ammonia concentration in water is estimated to be 4 ppb while the response and recovery times are less than 2 and 5 minutes respectively. The proposed sensor also offers good performance in terms of repeatability and good selectivity for sensing ammonia compared to that of other common ions and organic molecules in water.

High sensitivity refractive index sensor based on a tapered small core single-mode fiber structure.

A high sensitivity refractive index (RI) sensor based on a tapered small core single-mode fiber (SCSMF) structure sandwiched between two traditional single-mode fibers (SMF28) is reported. The microheater brushing technique was employed to fabricate the tapered fiber structures with different waist diameters of 12.5, 15.0, and 18.8 μm. Experiments demonstrate that the fiber sensor with a waist diameter of 12.5 μm offers the best sensitivity of 19212.5 nm/RIU (RI unit) in the RI range of 1.4304 to 1.4320. All sensors fabricated in this Letter show good linearity in terms of the spectral wavelength shift versus changes in RI. Furthermore, the sensor with the best sensitivity to RI was also used to measure relative humidity (RH) without any coating materials applied to the fiber surface. Experimental results show that the spectral wavelength shift changes exponentially as the RH varies from 60% to 95%. A maximum sensitivity of 18.3 nm per relative humidity unit (RHU) was achieved in the RH range of 90.4% to 94.5% RH.

Improvement of refractive index profiling of a small-core single-mode fiber under partially coherent light excitation

The refractive index profile of a non-bending small-core single-mode fiber was first reconstructed from the measured guided mode intensity profile and its spatial derivatives. By combining a rotating diffuser with a coherent laser source, the resulting partially coherent light source overcomes the speckles on the guided mode image of a straight fiber, and the corresponding refractive index profile was successfully reconstructed. Furthermore, guided mode images of a bent fiber with a fiber coil at midway were also measured under different coherency light excitations for comparison. The slight asymmetry of the reconstructed index profile of a bent fiber measured under coherent light excitation was greatly improved when measured under partially coherent light excitation. The generated additional leaky modes propagating partly in both core and cladding induced by the effects of the fundamental core mode deformation at the intermediate fiber bending structure and the mode transition at the bent-to-straight fiber section were averaged out with a low coherence transformed light source.

High-sensitivity fiber-optic refractive index sensor based on multimode interference using small-core single-mode fiber for biosensing

A fiber-optic refractive index (RI) sensor using multimode interference (MMI) composed of an unclad multimode fiber (MMF) sandwiched by small-core single-mode fibers (SMFs) has been proposed and demonstrated. The small-core SMF at the input port enhances the diffraction of the light outputted to the MMF, and that at the output port provides a steep coupling characteristic of interfered light at the end of the MMF, resulting in the generation of a sharp signal. This leads to high sensitivity to small RI changes. A very high sensitivity of less than 6 × 10−6 at an RI of more than 1.35 has been realized.

Enhanced refractive index sensor using a combination of a long period fiber grating and a small core singlemode fiber structure

An enhanced refractive index (RI) sensor based on a combination of a long period fiber grating (LPG) and a small core singlemode fiber (SCSMF) structure is proposed and developed. Since the LPG and SCSMF transmission spectra experience a blue and a red shift respectively as the surrounding RI (SRI) increases, the sensitivity is improved by measuring the separation between the resonant wavelengths of the LPG and SCSMF structures. Experimental results show that the sensor has a sensitivity of 1028 nm/SRI unit in the SRI range from 1.422 to 1.429, which is higher than individual sensitivities of either structure alone used in the experiment. Experimental results agree well with simulation results.

Enhanced Refractometer Based on Periodically Tapered Small Core Singlemode Fiber

An all-fiber refractive index (RI) sensor with a simple configuration of periodical tapers is proposed and investigated experimentally. The proposed fiber RI sensor consists of a small core fiber sandwiched between two standard singlemode fibers, with tapers periodically fabricated along the small core fiber using a focused CO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> laser beam. Such a structure can be used for RI sensing by measuring the dip wavelength shift of the multimode interference within the small core fiber cladding. An average sensitivity of 226.6 nm/RIU (RI Unit) has been experimentally achieved in the RI range from 1.33 to 1.38. The refractometer is sensitive to temperature and an experimental investigation of this sensitivity is presented. It is found that the peak shift response has a linear variation with temperature; therefore, temperature dependence can be mitigated by a suitable RI correction process. The proposed RI sensor benefits from simplicity and low cost and achieves a competitive sensitivity compared with other existing fiber-optic sensors.

A comprehensive analysis verified by experiment of a refractometer based on anSMF28–small-core singlemode fiber (SCSMF)–SMF28 fiber structure

A comprehensive theoretical model for an SMF28–small-core SMF (SCSMF)–SMF28structure based refractometer is developed based on the modal propagation analysis (MPA)method. The simulation result shows that the wavelength shift of this refractometerchanges exponentially as the surrounding refractive index (SRI) varies. The core diameterof SCSMF does not have a significant influence on the sensitivity of the refractometer butcladding diameter does have. The simulation results are verified experimentally and it isalso experimentally demonstrated that there is a maximum sensitivity of 1808 nm/RIU (refractive index unit) for an SRI range from 1.324 to 1.431 and that, as expected, thewavelength shift response is an exponential function of SRI.

Humidity sensor based on a single-mode hetero-core fiber structure

Using a small-core single-mode fiber (SCSMF), a novel relative humidity (RH) sensor based on an SMF28-SCSMF-SMF28 fiber structure was proposed in this paper. By depositing a humidity sensitive material, such as poly (ethylene oxide) (PEO) on the bare SCSMF fiber, the proposed structure can act as an RH sensor with high sensitivity. Experiments demonstrated that the proposed RH sensor with PEO coating can achieve a sensitivity of 430 nm per relative humidity unit (RHU) in the RH range from 80% to 83% RH and a sensitivity of 50 nm per RHU in the RH range from 83% to 95% RH.

Directional Bend Sensor Based on Re-Grown Tilted Fiber Bragg Grating

A novel fiber optic bend sensor is implemented by using a re-grown tilted fiber Bragg grating (TFBG) written in a small core single mode fiber with UV overexposure. The spectrum of the re-grown TFBG contrasts with that of normal TFBG by exhibiting large differences in the amplitude between neighboring symmetric (LP <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0m</sub> ) and asymmetric (LP <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1m</sub> ) cladding mode resonances, moreover each asymmetric cladding mode resonance splits into two peaks (corresponding to two orthogonal polarization states). The differential response of the three individual resonances of such group provides quantitative information about the magnitude and directions of bends in the TFBG. Numerical simulations indicate that the changes in the cladding-mode profiles in a bent fiber are responsible for this behavior through their impact on coupling coefficients. A bend sensitivity of 0.4 dB. m (for the 18th order group of cladding modes) is experimentally demonstrated within a range of 0-10.6 m <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">- 1</sup> .

High Δ, small core, single-mode fibers and their uses

High Delta, small core single-mode fibers were investigated with emphasis on clarifying both their fundamental optical characteristics and on possible uses. Preparation of such high Delta single-mode fibers through the vapor phase axial deposition process made it possible to achieve the low transmission losses of 0.82 dB/km (at 1.61 microm) for a 2.9%-Delta fiber and 0.68 dB/km (at 1.62 microm) for a 1.9%-Delta fiber. Measurements of fundamental characteristics such as transmission loss spectrum, bending loss, dispersion, stimulated Raman scattering, and frequency chirping were made for high Delta single-mode fibers. Two possible uses were investigated: a direct coupling experiment achieved 43% coupling efficiency between high Delta fiber and a LED, and an optical pulse compression experiment in the 1.5-microm wavelength region provided pedestal-free optical pulses with 1.2-ps (FWHM) width.

Optical sources for fiber transmission systems

Two types of semiconductor devices are available for use as light sources in fiber transmission systems. The simpler device, the light-emitting diode (LED), emits light in many directions and is useful with large core diameter, multimode fibers. The more complicated semiconductor laser emits more collimated light and can couple 10-100 times more power into a multimode fiber and can also be efficiently coupled to small-core single-mode fibers. LED's will be adequate for use in the more numerous data bus and low-to-medium capacity transmission systems, and will be used whenever possible because of their simpler driving circuit requirements, lower temperature sensitivity. and lower cost. Semiconductor laser with their higher power and narrower emission spectrum will be required for high-capacity and long-span transmission systems.