Normal Single-mode Fiber Research Articles

Transmission challenges in metropolitan area optical networks

Abstract For implementing wide/metropolitan area network in optical fiber communication system the key technology that can be utilized is wavelength division multiplexing (WDM). We discuss the use of WDM in metropolitan networks along with its special feature of using optical add drop multiplexers and also design issues in implementing these systems. The issue of nonlinear crosstalk induced in metropolitan networks is considered in terms of power penalty. Also, the effect of dispersion induced in optical fibers is considered by using two types of transmission systems. One is considering the non-zero dispersion shifted fiber (NZDSF) in dispersion effected environment and second is, implementing dispersion compensation after normal single mode fiber (SMF + DCF). The effect of nonlinear cross talk which can limit transmission performance is discussed in both cases in context with different parameters like power penalty, frequency difference between signal & crosstalk and transmission distance etc.

Dispersion Turning Attenuation Microfiber for Flowrate Sensing.

We demonstrated a new optical fiber modal interferometer (MI) for airflow sensing; the novelty of the proposed structure is that an MI is fabricated based on a piece of HAF, which makes the sensitive MI itself also a hotwire. The interferometer is made by applying arc-discharge tapering and then flame tapering on a 10 mm length high attenuation fiber (HAF, 2 dB/cm) with both ends spliced to a normal single mode fiber. When the diameter of the fiber in the processing region is reduced to about 2 μm, the near-infrared dispersion turning point (DTP) can be observed in the interferometer's transmission spectrum. Due to the absorption of the HAF, the interferometer will have a large temperature increase under the action of a pump laser. At the same time, the spectrum of the interferometer with a DTP is very sensitive to the change in ambient temperature. Since airflow will significantly affect the temperature around the fiber, this thermosensitive interferometer with an integrated heat source is suitable for airflow sensing. Such an airflow sensor sample with a 31.2 mm length was made and pumped by a 980 nm laser with power up to 200 mW. In the comparative experiment with an electrical anemometer, this sensor exhibits a very high air-flow sensitivity of -2.69 nm/(m/s) at a flowrate of about 1.0 m/s. The sensitivity can be further improved by enlarging the waist length, increasing the pump power, etc. The optical anemometer with an extremely high sensitivity and a compact size has the potential to measure a low flowrate in constrained microfluidic channels.

Tapered PCF Mach–Zehnder interferometer based on surface plasmon resonance (SPR) for estimating concentration toxic metal ions (lead)

In this study, a surface plasmon resonance based on Mach–Zehnder interferometer is produced by combining two normal Single mode fibers with a Photonic Crystal Fiber SMF-PCF-SMF for measuring the refractive index (RI) for ion toxic metal lead. In the center of the sensor, a small portion (4 cm) of PCF is created, and after the process of tapering is complete, gold is deposited with a thickness of about (40 nm). A photonic crystal fiber's gold-coated core receives a deposit of toxic lead ions. The computed signal to noise ratio (SNR), sensitivity (S), resolution (R), and figures of merit (FOM): SNR is 0.18, Sensitivity is 11.11 m/RIU, Resolution is 1.7 × $${10}^{-4}$$ , and FOM is 22.87.

Spacing-Tailored Multicore Fiber Interface for Efficient FIFO Devices

Multicore fibers (MCFs) have attracted intense attention to overcome the capacity limit of single-core optical fiber communication system. However, as indispensable connection devices between the MCF and the individual single mode fiber (SMF), fan-in and fan-out (FIFO) devices are challenging to achieve overall performance in terms of coupling efficiency, crosstalk, return loss, and device compactness. Here we develop an MCF reverse-tapering technique to increase the core-to-core spacing of the MCF and construct a spacing-tailored interface between the MCF and the normal SMF bundle. The fabricated FIFO device for the small-core MCF with narrow core-to-core spacing of 35 μm features a low average coupling loss (∼0.6 dB), extraordinarily low crosstalk (<−65 dB) and a return loss (<−60 dB) without using any bridge elements, where the remarkably low crosstalk is about 20 dB lower than using special bridge fibers for the same type of MCF. The flexibility of reverse tapering of MCFs can overcome its intrinsic integration challenge and provide a powerful approach to constructing all-fiber FIFO devices with compact construction and excellent overall performance.

High sensitive Mach–Zehnder interferometer for seawater refractive index measurement

To improve the sensitivity of seawater refractive index (RI) measurement, fluorine-doped fiber (FDF)-sandwiched Mach–Zehnder interferometer (MZI) is proposed, which is fabricated by splicing a short section of FDF between two sections of single-mode fibers (SMFs) with a large lateral offset. In FDF-sandwiched MZI, the two light paths are the FDF cladding and the measured seawater. The FDF cladding has lower RI than that of normal single-mode fiber (SMF), so the FDF-sandwiched MZI has higher sensitivity than that of SMF-sandwiched MZI. The results show that the RI sensitivity of the proposed sensor can be up to −14,638 nm/RIU, which is about 10.9% higher than that of SMF-sandwiched MZI. Temperature cross-sensitivity of our sensor is only 1.7 × 10−6 RIU/°C. The proposed sensor has the potential application in real physical, chemical, and biological sensing.

Advances in Multicore Fiber Grating Sensors

In recent years, multicore fiber (MCF) has attracted increasing interest for sensing applications, due to its unique fiber structure of multiple parallel cores in a single fiber cladding, which offers a flexible configurable platform to establish diverse functional fiber devices for sensing applications. So far, a variety of discrete fiber sensors using MCF have been developed, among which one of the major categories is the MCF grating sensors. The most distinct characteristic of MCF that differs from the normal single mode fibers is that the off-center cores of a MCF are sensitive to bending, which is caused by the bending induced tangential strain in off-center waveguides through either compression or stretching. The bending sensitivity has been widely developed for bending/curvature sensing or measuring physical parameters that are associated with bending. In this paper, we review the research progress on MCF-based fiber grating sensors. MCF-based diverse fiber grating sensors will be introduced, whose working principles will be discussed, and various types of applications of the MCF grating sensors will be summarized. Finally, the challenges and prospects of MCF grating for sensing applications will be presented.

Tilted Long Period Grating Inscribed in Eccentric Dual Core Fiber for Highly Sensitive Torsion Sensing

We proposed and experimentally investigated the torsion sensing performance of tilted long period gratings (TLPG) based on eccentric dual core fiber (EDCF). The EDCF is composed of a centric core and an eccentric core with a large separation. The asymmetrical structure of the EDCF and the high refractive index modulation of TLPG can bring some directional and sensitivity advantages to structure. We tested the torsional sensing in two optical channels of EDCF with different grating parameters and found both cores of EDCF can recognize the torsion direction. When the tilt angle of the TLPG is 80°, the maximum torsional sensitivity of the centric core and eccentric core are −0.381 nm/(rad/m) and 0.812 nm/(rad/m), respectively, which are 7 times and 16 times higher than similar TLPG inscribed in normal single mode fiber. In this condition, this sensor has good structural stability and high torsion sensitivity, and has great potential in multi-channel sensing applications.

Sensing applications of fiber Bragg gratings in single mode fibers with as-drawn 25 μm diameter cladding

The sensing properties of fiber Bragg gratings (FBG) inscribed in single mode fiber with a 5 μm diameter core and 25 μm diameter cladding are studied experimentally for temperature, strain, bending, and surrounding refractive index. Compared to normal single mode fiber, the diameter of this fiber is 5 times smaller and it stretches 14.5 times more at the same applied load. Therefore, it is much more flexible and stretchable, while maintaining excellent optical quality at wavelengths near 1550 nm. In addition to a core mode back reflection resonance, strong FBGs inscribed in this fiber show a relatively small number of narrow bandwidth (0.7 nm) cladding mode resonances separated in wavelength by 2.5–6 nm. This relatively coarse spectral comb can then be used to sense many different kinds of perturbations involving core and cladding modes. In particular, unlike cladding-mode based sensors made from tilted FBGs, all resonances are of the same azimuthal order as the core mode (i.e. HE1m). This feature makes these gratings particularly sensitive to bending which causes the appearance of new resonances and reduced amplitudes of the original ones, each by up to 10 dB/mm−1 of curvature. On the other hand, the temperature sensitivities of all modes are similar to that of standard fiber (around 11 pm/oC) while strain sensitivities are somewhat higher (1.6–1.7 pm/μstrain). The surrounding refractive index sensitivity is also increased (by a factor of 3) over normal fiber, mostly due to the increased modal dispersion of the modes of the thinner cladding. Furthermore, it is possible to serially multiplex different gratings at different wavelengths by interleaving their resonance combs and preserving each grating identity in the combined spectrum.

Mode division multiplexing for multiple particles noncontact simultaneous trap.

We propose and demonstrate an optical trap on the basis of a normal single-mode fiber (SMF), which is used to trap two particles in the axial direction at the same time without contact based on mode division multiplexing technology. We design and manufacture a tapered fiber probe. The LP11 mode beam is excited by docking a normal SMF to a 980 nm SMF with a 2 µm offset. Then the beams of LP01 and LP11 are both transmitted in the fiber. To converge the LP11 mode beam, a SMF with a tapered end is used to produce a cage for trapping the first microparticle. This particle acts as a lens to converge the LP01 mode beam to trap the second microparticle. We verify the feasibility of trapping two particles simultaneously through simulation. With this function, the proposed optical trap is easier to manipulate different individual particles for comparison and testing, which can promote the development of the biological, biophysical, colloidal, and soft matter fields.

Simultaneously distributed temperature and dynamic strain sensing based on a hybrid ultra-weak fiber grating array.

A fiber-optic sensing system based on two types of ultra-weak fiber Bragg gratings (UWFBG) for simultaneous temperature and vibration sensing was proposed. Narrowband and broadband UWFBGs are alternately written into an optical fiber with equal spacing. Distributed temperature sensing is realized by demodulating the wavelength shift of the narrowband UWFBG, while distributed vibration sensing is achieved by detecting phase variation between two adjacent broadband UWFBG interference pulses. The experimental results show that the proposed hybrid UWFBG array can perform temperature and vibration sensing simultaneously. The experimentally conducted temperature measurement ranges from 20°C to 100°C, with the measurement error less than 0.1°C. Vibration signals at different temperatures can be accurately restored, and the signal-to-noise ratio (SNR) is improved by 21.1 dB compared with a normal single-mode fiber (SMF).

Chromatic dispersion measurement in optical fibers using optoelectronic oscillations

In this paper, a novel technique is introduced that enables the measurement of chromatic dispersion (CD) in optical fibers. This technique is based on a relatively low-frequency optoelectronic oscillation (OEO) to provide a fast, precise and a relatively low-cost method for CD measurement that can be implemented easily in commercial instruments. In addition, another technique is introduced to compensate for the thermal fluctuations in fiber during measurement. The proposed setup is implemented to measure CD in normal single-mode fibers with lengths of 40 km, 10 km, and 1 km. Moreover, it is implemented to measure CD in 400 m of non-zero dispersion shifted fiber to test the system ability to resolve small chromatic delays. The proposed setup can resolve delays less than 0.1 ps/nm and measure CD with precision as low as 0.005 ps/nm·km over a wavelength range from 1500 to 1630 nm in only 20 s with 5 nm steps. The precision and the speed of measurement can be improved by increasing the wavelength steps to 10 nm.

A novel polyvinyl alcohol and hypromellose gap-coated humidity sensor based on a Mach–Zehnder interferometer with off-axis spiral deformation

A novel humidity sensor based on a Mach–Zehnder interferometer (MZI) in normal single-mode fiber (SMF) was developed and investigated. The MZI is composed of a polyvinyl alcohol coated gap that serves as a sensitive beam splitter at the input end of the MZI and an off-axis spiral deformation point that acts as a beam combiner at the output end of the MZI. The coated gap is formed between the end faces of 2 SMFs, where the humidity-sensitive polyvinyl alcohol and hypromellose film is deposited on the end face of the input SMF. The off-axis spiral deformation is achieved by twisting the lead-out fiber under continuous arc discharging with the appropriate off-axis radius. When the humidity of the external environment changes, both the refractive index and the thickness of the hygroscopic film changes, so the coupling ratio of the propagation light changes accordingly, resulting in an intensity change in the output signal of the interferometer. The interferometer was applied for high-sensitive humidity sensing in the range of 20–80%, with a sensitivity of 0.21 dB/%RH and a fast rise time of 0.795 s. Thanks to its low cost, simple preparation method, and short manufacturing time, the humidity sensor proposed in this paper can be used in a wide range of applications in the fields of structural health testing, medical treatment, industrial production, and environmental monitoring.

Strain sensor based on two concatenated abrupt-tapers in twin-core fiber

In this paper, a novel and high sensitivity optical fiber sensor based on Mach-Zehnder interferometer (MZI) with two concatenated abrupt-tapers in twin-core fiber (TCF) is proposed and experimentally demonstrated. The setup includes two abrupt-tapers with different size in TCF and then spliced with two segment of normal single mode fibers (SMFs). The multiple cladding mode will be excited within the TCF, partly coupled with the core mode while in the first taper. After propagation of these coupled modes, they will be recouped in the second tapers. Through the two abrupt-tapers, we realized twice MZI. It is because of the two abrupt-tapers structure that the interference between modes is strengthened. Maximum interference resonance dips exceeds 19 dB. The interference fringe of this device would shift with the variation of strain. Therefore, we can desire the parameters through monitoring the wavelength shift of the transmission spectrum. The experimental results show that the sensitivity of the strain sensor is 6.39 pm/με when the resonance wavelength is 1466.2 nm. Especially, a higher sensitivity has achieved in this paper compared with other strain sensor, which sensitivity is up to 7.6 pm/με at 1716.5 nm with interferometer extinction ratio of 11 dB. The proposed sensor is easy-fabricated, low-cost and high sensitivity, which is very suitable for physical application of strain measurement.

Feedback Stabilized Optical Fabry–Pérot Interferometer Based on Twin-Core Fiber for Multidimension Microdisplacement Sensing

A novel optical Fabry–Perot interferometer (FPI) is proposed and experimentally demonstrated for multidimension microdisplacement sensing (MMS). The sensor structure consists of a section of twin-core fiber with a length of 2.4 cm and a section of feedback fiber (FBF) fabricated by normal single-mode fiber. The end faces of two fibers can be formed as the reflection cavity (RC) of the FPI. Mounting the sensor on translation stages, and moving the FBF in different directions to change the RC, the changing mode interference of the reflected beam can be measured. A maximum microdisplacement sensitivity of 0.954 nm/μm with a higher resolution 0.052 μm was achieved. Due to the advantages of the MMS and simple configuration, the sensor is significantly beneficial to practical application.

A chitosan-coated humidity sensor based on Mach-Zehnder interferometer with waist-enlarged fusion bitapers

Abstract A novel humidity sensor, which adopts a Mach-Zehnder interferometer (MZI) in normal single mode fiber (SMF) modified by the deposition of chitosan (a moisture-sensitive natural polymer) on the cladding, is proposed and experimentally demonstrated. It is fabricated by the fusion splicing of a segment between the two SMF with waist-enlarged fusion bitapers. This all-fiber MZI based on SMF incorporates intermodal interference between the core mode and the cladding mode. Due to the fact that it is sensitive to external refractive index and that the RI of the chitosan multi-layer film coat depends on the environmental humidity, the SMF-MZI with a chitosan coating layer of nanometer thickness is employed in humidity measuring. The sensitivity of ∼119.6 pm/RH (relative humidity unit) is achieved within the range from 10% to 90% on the experimental level. Moreover, the chitosan coat has good biocompatibility for in vivo biomedical applications like immunosensing and DNA hybridization detection in the near future.

A Hollow Core Fiber-Based Intermodal Interferometer for Measurement of Strain and Temperature

A hollow core fiber-based all-fiber intermodal interferometer is proposed for measurement of strain and temperature. The sensing structure is simply a short segment of hollow core silica fiber being sandwiched between two normal single mode fibers. The fabrication process only involves a conventional fusion splicer and a mechanical fiber cleaver. Experimental results show that sensitivities of 1.21 pm/μe and 21.30 pm/°C are achieved for strain and temperature measurements, respectively.

Micro-Fiber-Based FBG Sensor for Simultaneous Measurement of Vibration and Temperature

We propose and experimentally demonstrate an integrated in-line micro-fiber sensor based on fiber Bragg gratings to measure simultaneously the vibration and temperature information for state estimation of cable-stayed bridges. The micro vibration structure with micro beam and mass block is integrated to a normal single mode fiber by a special chemical etching method. The results reveal that both of the gratings exhibit similar temperature sensitivities (8.55 and 8.09 pm/ °C) but markedly different sensitivities for ultra-low frequency vibration. This kind of the sensing structure is a new type of sensor for simultaneous measurement of temperature and ultra-low frequency vibration particularly for monitoring the large structure health.

Simultaneous Measurement of Strain and Temperature with Hollow Core Fiber Based Intermodal Interferometer

A hollow core fiber based all-fiber intermodal interferometer is proposed for measurement of strain and temperature. The sensing structure is simply a short segment of hollow core silica fiber being spliced between two normal single mode fibers. The fabrication process only involves a conventional fusion splicer and a mechanical fiber cleaver. Experimental results show that sensitivities of-1.21 pm/με and 21.30 pm/°C are achieved for strain and temperature measurements, respectively.

Narrow linewidth Yb-doped fiber laser at 1120 nm

A 974 nm laser diode pumped 1120 nm ytterbium (Yb)-doped fiber laser with all fiber components has been experimentally demonstrated. The resonator is composed of a normal single-mode Yb-doped fiber 1 m long and a pair of fiber Bragg gratings. Up to 21.6 mW of continuous-wave output power is generated when the launched pump power is 161 mW with an optical-to-optical conversion efficiency of 13.4%. The central wavelength is 1120.89 nm with a linewidth as narrow as 0.02 nm. Additionally, a similar fiber laser with a 2 m long gain medium displays a broadened linewidth. The results of the two fiber lasers are compared and discussed in the paper.

Characteristics of bend sensor based on two-notch Mach–Zehnder fiber interferometer

A compact all-fiber modal Mach–Zehnder interferometer is presented based on two notches at different locations along normal single-mode fibers by CO2 laser irradiation. Two notches are mode splitter and mode combiner, respectively. This configuration which does not need any troublesome cleaving or aligning process is demonstrated as a bending sensor. The results suggest that the transmission loss varies and the interference fringe shifts with the bending increasing. The fluctuation of temperature dose not affects the shape of the interferometer fringe and transmission. Therefore, the proposed device offers salient advantages of simple fabrication technique and low cost. In addition, a high stability over time was expected since the irradiated notches do not degrade over time or with temperature. Due to its asymmetry, it also possesses higher sensitivity to orientation, which would be a promising unit for sensor applications.