Nageswara Lalam

Experimental analysis of EVM and BER for indoor radio-over-fibre networks using polymer optical fibre

This paper presents a theoretical investigation and experimental implementation of indoor radio-over-fibre (RoF) using polymer optical fibre. We characterise the system based on the error vector magnitude (EVM), bit error rate (BER), and the signal-to-noise ratio (SNR). We consider three modulation formats of quadrature phase shift keying (QPSK), 16-quadrature amplitude modulation (QAM) and 64-QAM. The results show the effect of modulation order on the higher acceptable EVM limit that can be linked with the BER estimation process. Furthermore, the analysis of input signal power penalty for the three modulations indicates the advantage of higher order formats. We conclude that even with linear increment of the power penalty, higher orders modulation can offer a higher bandwidth without a significant difference compared to lower orders.

Employing wavelength diversity technique to enhance the Brillouin gain response in BOTDA system

In this paper, we propose a novel wavelength diversity technique in Brillouin optical time domain analysis (BOTDA) system to improve the Brillouin gain response and increase sensing range. This technique greatly maximizes the launching pump power, without activating the nonlinear effects that limit the conventional BOTDA sensing range. The strain and temperature effects on Brillouin frequency shift have been characterized in the proposed technique with an extended sensing range up to 100 km and 1 meter spatial resolution.

High Sensitivity Refractometer Based on Reflective Smf-Small Diameter No Core Fiber Structure

A high sensitivity refractive index sensor based on a single mode-small diameter no core fiber structure is proposed. In this structure, a small diameter no core fiber (SDNCF) used as a sensor probe, was fusion spliced to the end face of a traditional single mode fiber (SMF) and the end face of the SDNCF was coated with a thin film of gold to provide reflective light. The influence of SDNCF diameter and length on the refractive index sensitivity of the sensor has been investigated by both simulations and experiments, where results show that the diameter of SDNCF has significant influence. However, SDNCF length has limited influence on the sensitivity. Experimental results show that a sensitivity of 327 nm/RIU (refractive index unit) has been achieved for refractive indices ranging from 1.33 to 1.38, which agrees well with the simulated results with a sensitivity of 349.5 nm/RIU at refractive indices ranging from 1.33 to 1.38.

Performance improvement of BOTDR system using wavelength diversity technique

In this paper, a novel technique was proposed to improve the sensing performance by employing wavelength diversity in Brillouin optical time domain reflectometry (BOTDR). This technique enables to maximize the launch pump power to achieve a higher measurement accuracy, without activating the nonlinear effects, which limit the conventional BOTDR performance. Experimentally, we have demonstrated the proposed technique, that provides measurement accuracy improvement of 3.6 times at far end of the sensing fibre compared to the conventional BOTDR system.

A simple all-fiber comb filter based on the combined effect of multimode interference and Mach-Zehnder interferometer

A polarization-dependent all-fiber comb filter based on a combination effect of multimode interference and Mach-Zehnder interferometer was proposed and demonstrated. The comb filter was composed with a short section of multimode fiber (MMF) fusion spliced with a conventional single mode fiber on the one side and a short section of a different type of optical fiber on the other side. The second type of optical fiber is spliced to the MMF with a properly designed misalignment. Different types and lengths of fibers were used to investigate the influence of fiber types and lengths on the performance of the comb filter. Experimentally, several comb filters with free spectral range (FSR) values ranging from 0.236 to 1.524 nm were achieved. The extinction ratio of the comb filter can be adjusted from 6 to 11.1 dB by varying polarization states of the input light, while maintaining the FSR unchanged. The proposed comb filter has the potential to be used in optical dense wavelength division multiplexing communication systems.

Analysis of brillouin frequency shift in distributed optical fiber sensor system for strain and temperature monitoring

In this paper, we have analyzed Brillouin frequency shift (BFS) in single mode silica optical fiber. The BFS is analyzed in conventional Brillouin optical time domain analysis (BOTDA) at operating wavelength of 1550 nm by a pump-probe technique. The effects of strain and temperature on BFS are fully characterized. We found that, the BFS change of 0.06 ΜΗz/μ-strain and 1.26 MHz/oC, respectively. The BFS changes in Brillouin gain and Brillouin loss mechanism have been analyzed. In addition, we also presented Brillouin linewidth and peak gain variations of Brillouin gain spectrum with various temperature and strain values. The results demonstrate, the BFS have a strong linear relationship with strain and temperature along the sensing fiber.

Future of distributed fiber sensors (invited paper)

Distributed fiber sensors offers an innovative technology for a spatially distributed measurement based on Raleigh, Brillouin and Raman scattering. By analyzing backscattered signal frequency, intensity and phase, one can realize a distributed measurement of strain, temperature and vibration over a tens of kilometers. This paper reviewed the history and current technology development of distributed time-domain fiber sensors in terms of spatial resolution and sensing range while present the standard operating principles. The future prospective view that can be extend further research development has also been discussed.

Perfluorinated polymer optical fiber for precision strain sensing based on novel SMS fiber structure

This paper presents an experimental investigation of a novel strain sensor based on a single-mode-multimode-single-mode (SMS) fiber structure using perfluorinated graded index polymer optical fiber as the multimode fiber section. The experimental results showed that the strain sensitivity is as high as 101 pm/με, which is 16 times higher than those obtained in silica multimode fiber based on SMS fiber structure.

Characterization of Brillouin frequency shift in Brillouin Optical Time Domain Analysis (BOTDA)

In this paper, we have characterised Brillouin frequency shift (BFS) in single mode silica fibre. In particular, BFS was characterised in conventional Brillouin Optical Time Domain Analysis (BOTDA) at operating wavelength of 1550 nm by a pump-probe technique. In addition, we also presented Brillouin gain and Brillouin loss mechanism in BOTDA sensing system. The simulation results shows, BFS have strong linear relationship with acoustic velocity and applied strain along the sensing fibre. A frequency shift of 60 MHz was obtained for each 0.1% strain (1000 μ-strain) on fibre. From the measurement results, it was evident that the each μ-strain on the fibre the frequency shift was found as 0.06 MHz. Therefore, BOTDA sensing system based on BFS is a promoting technique for structural health monitoring.