DESIGN AND CHARACTERIZATION OF PHOTONIC CRYSTAL FIBER FOR SENSING APPLICATIONS

A simple structure of Photonic Crystal Fiber (PCF) for gas sensing and chemical sensing has been proposed in this paper. Index guiding properties of proposed PCF have been numerically investigated by using finite element method (FEM). From the numerical result, it is shown that the relative sensitivity and confinement loss depend on geomatrical parameters and wavelength. The relative sensitivity is increased by a increase of the diameters of central hollow core and innermost ring holes and confinement loss is decreased with a increase of the diameters of outermost cladding holes. By optimize the parmeters, the relative sensitivity is improved to the value of 20.10%. In this case, the confinement loss of the fiber is 1.09×10-3 dB/m

High Fluence Chromium and Tungsten Bowtie Nano-antennas

Nano-antennas are replicas of antennas that operate at radio-frequencies, but with considerably smaller dimensions when compared with their radio frequency counterparts. Noble metals based nano-antennas have the ability to enhance photoinduced phenomena such as localized electric fields, therefore-they have been used in various applications ranging from optical sensing and imaging to performance improvement of solar cells. However, such nano-structures can be damaged in high power applications such as heat resisted magnetic recording, solar thermo-photovoltaics and nano-scale heat transfer systems. Having a small footprint, nano-antennas cannot handle high fluences (energy density per unit area) and are subject to being damaged at adequately high power (some antennas can handle just a few milliwatts). In addition, given that nano-antennas are passive devices driven by external light sources, the potential damage of the antennas limits their use with high power lasers: this liability can be overcome by employing materials with high melting points such as chromium (Cr) and tungsten (W). In this article, we fabricate chromium and tungsten nano-antennas and demonstrate that they can handle 110 and 300 times higher fluence than that of gold (Au) counterpart, while the electric field enhancement is not significantly reduced.Te authors gratefully acknowledge the fabrication facilities provided by Australian National Fabrication Facility (ANFF ACT node, Australia). We would acknowledge the fnancial support from UNSW Canberra, Australia. We also would like to thank Te Asian Ofce of Aerospace Research and Development (AOARD US Air Force FA2386-15-1-4084), Australian Research Council (ARC LP160100253, DP170103778 and DE190100413) to provide the funding

Fiber nonlinearity mitigation using mid-span spectral inversion in long-haul coherent optical OFDM systems

Optical fiber communication systems have become the backbone of today’s communication networks due to their enormous bandwidth, over several terahertz (THz), enabling capacities of 100 Tb/s and beyond. Almost all of world’s long-haul internet traffic is carried by these optical backbone networks. Despite the fact that the internet bubble ended in the early 2000s, its traffic has been constantly increasing at an astounding rate of 75% per year. In addition to that, new emerging video-centric applications such as IPTV will continue to increase the demand on the underlying optical backbone networks. As a result, ten to twenty years from now, optical networks will have to carry vastly increased amounts of data. However, recent research shows that fundamental limits in optical backbone networks are being approached. These limits are imposed by noise generated from inline amplifiers used to boost up the signal and the intrinsic nonlinearity of conventional standard single mode fiber (S-SMF). In order to meet long-term needs and challenges, therefore, research in wideband optical subsystems enabling high capacity long-haul transmission must be urgently pursued. One approach to break through the current capacity limit is a combination of using advanced modulation formats like coherent optical Orthogonal Frequency Division Multiplexing (CO-OFDM) and fiber nonlinearity mitigation techniques. In OFDM, the orthogonal property of the sub-carriers allows formation of an almost rectangular spectrum, which increases spectral efficiency. However, at the high powers required for higher order modulation formats, the nonlinearity in the fiber causes nonlinear mixing between the subcarriers, restricting the maximum allowable power below nonlinear threshold, and hence constraining the total capacity and distance. The PhD thesis proposed using mid-span spectral inversion (MSSI) that uses optical phase conjugation (OPC) module to mitigate the fiber nonlinearity in CO-OFDM systems. Using MSSI, the spectrum of the first-half of the link (from the Tx to the OPC module) will be inverted by Four-Wave Mixing (FWM) of the OFDM signal with a pump wave. The spectrally inverted signal is then selected to pass through the second half of the link. Because the signal is spectrally inverted, the second half of the link should undo the dispersion and nonlinearity of the first half of the link. During the project, a detailed analytical formalism to describe the performance of the OPC module has been developed. This aids the design and improvement of fundamental performance of OPC module. The first experimental demonstration of using MSSI in a coherent system has been made using dual polarization CO-OFDM systems carrying 1.21-Tb/s over 800 km. A design outline for optimum performance using MSSI has been developed. Two novel methods for improving the fundamental performance of MSSI have been proposed. The first method splits the nonlinear element into two parts, inserting a notch filter to remove the pump and then reinserting the pump into the second part of the nonlinear element. The second method uses a phase shift filter between the two nonlinear elements, to improve robustness in practical implementation. Both methods offer 1 dB of maximum signal quality improvement in a 10 × 80-km 4-QAM 224-Gb/s CO-OFDM system. In summary, this work has demonstrated by simulation and experiments that MSSI offers benefits to coherent optical systems, including OFDM systems. It has developed analytical formalisms that identify the performance-limiting mechanisms in optical phase conjugators based on third order nonlinearity, and has introduced two methods of mitigating these mechanisms. MSSI fell from favour in the mid-2000s due to its complexity. However, after this successful demonstration of fiber nonlinearity compensation using MSSI in a coherent system, MSSI has now drawn considerable attention recently. Subsequently, there have been demonstrations of Raman-enhanced MSSI and multiple phase-conjugations based coherent systems at the 2014 conference on Optical Fiber Communications (OFC)

Design and numerical analysis of microstructured-core octagonal photonic crystal fiber for sensing applications

This paper presents an octagonal photonic crystal fiber (O-PCF) for liquid sensing, in which both core and cladding are microstructured. Some propagation characteristics of proposed structure have been investigated by using the full vectorial finite element method (FEM). Confinement loss and sensitivity are examined and compared with varying number of rings, core diameter, diameter of air holes in cladding ring and pitch. It is found that sensitivity is increased for the increment pitch value, air filling ratio, core diameter, inner ring diameter as well as number of rings. At the same time confinement loss is significantly decreased. It is also found that the increment of pitch by keeping the same air filling ratio increases the sensitivity and loss. Investigating the effects of different parameters, an O-PCF structure is designed which has a significantly higher relative sensitivity and lower confinement loss. Keywords: Photonic Crystal Fiber (PCF), Octagonal photonic crystal fiber (O-PCF), Evanescent field, Chemical sensor, Sensitivity and confinement los

Fiber nonlinearity mitigation using mid-span spectral inversion in long-haul coherent optical OFDM systems

Optical fiber communication systems have become the backbone of today’s communication networks due to their enormous bandwidth, over several terahertz (THz), enabling capacities of 100 Tb/s and beyond. Almost all of world’s long-haul internet traffic is carried by these optical backbone networks. Despite the fact that the internet bubble ended in the early 2000s, its traffic has been constantly increasing at an astounding rate of 75% per year. In addition to that, new emerging video-centric applications such as IPTV will continue to increase the demand on the underlying optical backbone networks. As a result, ten to twenty years from now, optical networks will have to carry vastly increased amounts of data. However, recent research shows that fundamental limits in optical backbone networks are being approached. These limits are imposed by noise generated from inline amplifiers used to boost up the signal and the intrinsic nonlinearity of conventional standard single mode fiber (S-SMF). In order to meet long-term needs and challenges, therefore, research in wideband optical subsystems enabling high capacity long-haul transmission must be urgently pursued. One approach to break through the current capacity limit is a combination of using advanced modulation formats like coherent optical Orthogonal Frequency Division Multiplexing (CO-OFDM) and fiber nonlinearity mitigation techniques. In OFDM, the orthogonal property of the sub-carriers allows formation of an almost rectangular spectrum, which increases spectral efficiency. However, at the high powers required for higher order modulation formats, the nonlinearity in the fiber causes nonlinear mixing between the subcarriers, restricting the maximum allowable power below nonlinear threshold, and hence constraining the total capacity and distance. The PhD thesis proposed using mid-span spectral inversion (MSSI) that uses optical phase conjugation (OPC) module to mitigate the fiber nonlinearity in CO-OFDM systems. Using MSSI, the spectrum of the first-half of the link (from the Tx to the OPC module) will be inverted by Four-Wave Mixing (FWM) of the OFDM signal with a pump wave. The spectrally inverted signal is then selected to pass through the second half of the link. Because the signal is spectrally inverted, the second half of the link should undo the dispersion and nonlinearity of the first half of the link. During the project, a detailed analytical formalism to describe the performance of the OPC module has been developed. This aids the design and improvement of fundamental performance of OPC module. The first experimental demonstration of using MSSI in a coherent system has been made using dual polarization CO-OFDM systems carrying 1.21-Tb/s over 800 km. A design outline for optimum performance using MSSI has been developed. Two novel methods for improving the fundamental performance of MSSI have been proposed. The first method splits the nonlinear element into two parts, inserting a notch filter to remove the pump and then reinserting the pump into the second part of the nonlinear element. The second method uses a phase shift filter between the two nonlinear elements, to improve robustness in practical implementation. Both methods offer 1 dB of maximum signal quality improvement in a 10 × 80-km 4-QAM 224-Gb/s CO-OFDM system. In summary, this work has demonstrated by simulation and experiments that MSSI offers benefits to coherent optical systems, including OFDM systems. It has developed analytical formalisms that identify the performance-limiting mechanisms in optical phase conjugators based on third order nonlinearity, and has introduced two methods of mitigating these mechanisms. MSSI fell from favour in the mid-2000s due to its complexity. However, after this successful demonstration of fiber nonlinearity compensation using MSSI in a coherent system, MSSI has now drawn considerable attention recently. Subsequently, there have been demonstrations of Raman-enhanced MSSI and multiple phase-conjugations based coherent systems at the 2014 conference on Optical Fiber Communications (OFC)

Multi-Functionality of Plasmonic Nano-Antennas

Light-matter interaction in metals creates surface plasmon polaritons that propagate at the interface between metal and dielectric and can localize light at the sub-wavelength region. Localized surface plasmon resonances can produce high electric field enhancement factors in the gap of nano-antennas, over distance scales which are beyond the diffraction limit. This extraordinary property opens opportunities for different applications such as surface enhanced Raman spectroscopy (SERS), imaging, and fabrication of ultra-compact devices. This thesis focuses on the theoretical design and experimental studies of new nano-antennas to improve their optical properties and applications (e.g., SERS). Also, I propose a new approach to the design of high fluence nano-antennas that may be suitable for high power laser applications. More specifically, the studies include:Firstly, a multilayer bowtie nano-antenna structure is proposed where a lossless silica dielectric material is combined with lossy gold metal. Then, the geometrical parameters of this structure are numerically optimized at the desired wavelength of 1053 nm and, it is shown that an adequate choice of parameters, shape and proper combination of materials can lead to a high electric field enhancement factor. The structure can produce six times higher relative electric field enhancement than a purely gold single layer bowtie nano-antenna with the same thickness.Secondly, dual polarized star gap plasmonic nano-antennas are developed: a nano-antenna with a star gap for multi-polarization is numerically analyzed and the performance of fabricated nano-antenna arrays in a quartz substrate is performed by a surface enhanced Raman spectroscopy (SERS). I observed that a star gap with a circular grating helps to operate for both linear and circular polarizations: the maximum electric field enhancement for both linear and circular polarization is 48, with the average electric field enhancement factor of about 34.75. Raman spectroscopy experiments produce surface enhanced Raman scattering enhancement of 4.18×106 for the 880 cm−1 line of ethanol for both polarizations in the presence of the star gap nano-antenna.Thirdly, I numerically investigated broadband bowtie belt nano-antennas: a linear array of bowtie nano-antennas placed between two metallic strips for theoretical realization and observation of broadband nano-antennas. This research shows that the bandwidth can be manipulated by increasing or decreasing the number of bowtie nano-antennas in the array. As an example, I designed a nanoantenna that can work from 800 to 1420nm (600nm linewidth), with an electric field enhancement factor close to 20.All the above nano-antennas are designed based on gold metal. Although noble metals based nano-antennas have the ability to enhance photo-induced phenomena such as the local electric field, leading to their use in different applications ranging from optical sensing and imaging to boosting the performance of solar cells, noble metal antennas are not good for high power applications such as nanoscale heat transfer systems. However, refractory plasmonic materials can overcome these liabilities. Therefore, finally, a new approach is proposed for developing high fluence bowtie nano-antennas where chromium (Cr) and tungsten (W) refractory materials are used instead of gold noble metal and results show that chromium and tungsten nano-antennas can handle 114 and 300 times higher fluence, respectively than their gold (Au) counterparts without a significant reduction in their electrical field enhancement capacities

The effect of the substrate on the damage threshold of gold nano-antennas by a femtosecond laser

Gold nano-antennas with silica substrate may not be suitable for high power applications such as heat resisted magnetic recording, solar thermophotovoltaics, and nano-scale heat transfer systems. When a laser beam reaches to these nano-antennas, part of the light is absorbed by the metallic regions, leading to a temperature rise of the device. If these devices reach a temperature beyond its Tamman temperature (the temperature at which sintering of atoms or molecules start to occur), the antenna can be damaged. One strategy to allow the antenna to work at higher fluences (energy density) is to employ substrates that can quickly carry the heat away from the antennas. In this paper, we show that high thermal conductivity substrates, such as diamond, can allow the antenna to withstand 20 times higher fluence than a low thermal conductivity silica substrate

Broadband and thermally stable tungsten boride absorber

Metal-based perfect absorbers that use metals such as aluminum (Al) can have their optical properties changed with temperature, subsequently affecting the device’s performance. The changes with temperature can critically limit the applications of absorbers at high powers or high temperatures. In this paper, we show a thermally stable broadband absorber based on an ultrathin layer of refractory ceramic, tungsten boride (WB). We experimentally analyze and compare the performance of the absorber with an aluminum (Al) based absorber. The multilayer perfect absorber has absorption higher than 85% in the wavelength range between 500 and 1600 nm over a large range of incident angles (up to 60°). We show that a WB absorber has significantly better temperature stability when compared with its Al counterpart, achieving stable operation up to temperatures as high as 270°C. These absorbers may find applications in solar thermo-photovoltaic energy conversion.Australian Research Council (LP160100253); University of New SouthWales (Scientia Fellowship)

Tungsten Refractory Plasmonic Material for High Fluence Bowtie Nano-antenna

In general, noble metals based nano-antennas cannot work at high power applications such as heat resisted magnetic recording, solar thermo-photovoltaics, and nano-scale heat transfer systems. These antennas are prone to being damaged at sufficiently high energy density due to their small footprint and low Tamman temperature. This paper proposes tungsten refractory plasmonic material based nano-antennas as an alternative gold nano-antennas: we show that the antennas can handle 300 times higher fluence than gold (Au) counterpart. In addition, it can achieve 7.22 higher magnitude of electric field intensity than gold antennas