Overwrite fabrication and tuning of long period gratings

The central wavelengths of the resonance bands are critical aspect of the performance of long period gratings (LPGs) as sensors, particularly for devices designed to operate near the phase matching turning point (PMTP), where the sensitivity to measurements can vary rapidly. Generally, LPGs are characterized by their period, but the amplitude of the amplitude of the index modulation is also an important factor in determining the wavelengths of the resonance bands. Variations in fabrication between LPG sensors can increase or decrease the sensitivity of the LPG to strain, temperature or surrounding refractive index. Here, the technique of overwritten UV laser fabrication is demonstrated. It is shown that, on repeated overwriting, the resonance bands of an LPG exhibit significant wavelength shift, which can be monitored and which can be used to tune the resonance bands to the desired wavelengths. This technique is applied to periods in the range 100 to 200 µm, showing the cycle-to-cycle evolution of the resonance bands near the PMTPs of a number of cladding modes. The use of online monitoring is shown to reduce the resonance band sensor-to-sensor central wavelength variation from 10 nm to 3 nm

Design and fabrication of optical fibre long period gratings for CO₂ sensing

This thesis investigated the repeatability of the overwrite long period grating (LPG) fabrication method and highlighted the advantage it offers in its ability to tune spectral features thus allowing the manufacture of bespoke sensors. Moreover, LPGs with periods ranging from 100 - 200 μm were written and a novel technique for mapping the transmission data was presented. This method gave a unique overview into the period mediated evolution of attenuation features, which, when designing LPGs that operate at the sensitive phase matching turning point, is invaluable. Further exploration into the overwrite method revealed that the UV irradiation duty cycle used in the fabrication of LPGs was found to influence the presence of harmonics, where a duty cycle of 25% maximised coupling to 2nd order transmission features. LPGs which possessed these additional spectral features within a small wavelength range (600 - 1000 nm) were assessed for their suitability in performing multi-parameter sensing. Ionic liquids were explored as an LPG COThis thesis investigated the repeatability of the overwrite long period grating (LPG) fabrication method and highlighted the advantage it offers in its ability to tune spectral features thus allowing the manufacture of bespoke sensors. Moreover, LPGs with periods ranging from 100 - 200 μm were written and a novel technique for mapping the transmission data was presented. This method gave a unique overview into the period mediated evolution of attenuation features, which, when designing LPGs that operate at the sensitive phase matching turning point, is invaluable. Further exploration into the overwrite method revealed that the UV irradiation duty cycle used in the fabrication of LPGs was found to influence the presence of harmonics, where a duty cycle of 25% maximised coupling to 2nd order transmission features. LPGs which possessed these additional spectral features within a small wavelength range (600 - 1000 nm) were assessed for their suitability in performing multi-parameter sensing. Ionic liquids were explored as an LPG COThis thesis investigated the repeatability of the overwrite long period grating (LPG) fabrication method and highlighted the advantage it offers in its ability to tune spectral features thus allowing the manufacture of bespoke sensors. Moreover, LPGs with periods ranging from 100 - 200 μm were written and a novel technique for mapping the transmission data was presented. This method gave a unique overview into the period mediated evolution of attenuation features, which, when designing LPGs that operate at the sensitive phase matching turning point, is invaluable. Further exploration into the overwrite method revealed that the UV irradiation duty cycle used in the fabrication of LPGs was found to influence the presence of harmonics, where a duty cycle of 25% maximised coupling to 2nd order transmission features. LPGs which possessed these additional spectral features within a small wavelength range (600 - 1000 nm) were assessed for their suitability in performing multi-parameter sensing. Ionic liquids were explored as an LPG CO₂ sensitive coating. It was shown that these materials demonstrate a refractive index change upon exposure to CO₂ which was maintained following mechanical stabilisation using a gelling agent. A coating system for applying the gelled ionic liquid to the surface of an optical fibre was developed and techniques to improve the coating deposition were explored. The sensor demonstrated an 8 nm wavelength shift in response to 20% CO₂, which was reversible by reducing the partial pressure of CO₂ for 25 min.sensitive coating. It was shown that these materials demonstrate a refractive index change upon exposure to CO₂ which was maintained following mechanical stabilisation using a gelling agent. A coating system for applying the gelled ionic liquid to the surface of an optical fibre was developed and techniques to improve the coating deposition were explored. The sensor demonstrated an 8 nm wavelength shift in response to 20% CO₂, which was reversible by reducing the partial pressure of CO₂ for 25 min. sensitive coating. It was shown that these materials demonstrate a refractive index change upon exposure to CO₂ which was maintained following mechanical stabilisation using a gelling agent. A coating system for applying the gelled ionic liquid to the surface of an optical fibre was developed and techniques to improve the coating deposition were explored. The sensor demonstrated an 8 nm wavelength shift in response to 20% CO₂, which was reversible by reducing the partial pressure of CO₂ for 25 min

Fabrication, instrumentation and application for subwavelength periodic nanophotonic devices

This dissertation focuses on developing novel and efficient fabrication methodology for periodic nanostructures based nanophotonic devices, especially variable or tunable optical/photonic devices that based on photonic crystal or plasmonic crystal slabs. These nanophotonic devices are optically characterized to demonstrate the effectiveness. This dissertation starts by developing an angular-dispersion detection instrument based on a one-dimensional photonic crystal. This instrument was demonstrated to have applications in chemical sensing and imaging by monitoring the guided-mode resonance (GMR) supported in the PC sensor comprised of a one-dimensional grating structure. Exposed to solutions with different refractive indices or adsorbed with biomaterials, the PC sensor exhibited changes of the optical resonant modes. In order to fabricate tunable nanophotonic devices with continuously varying resonant wavelengths, two different approaches were explored in this dissertation. The first approach is to introducing graded geometry into the structure of the device, such as a varying period over the device surface. To accomplish this, a strain-tunable soft lithography method is developed using PDMS masters as the replicate molds. The process exploits an elastomeric mold made of PDMS to generate the designed periodic pattern in a UV curable polymer (UVCP) on glass or plastic substrates. During the imprint and curing process, the PDMS mold was mechanically deformed by a uniaxial force, which causes the periodic pattern carried on the PDMS mold to vary as designed. By control the stretching direction and magnitude of the applied force carefully, the lattice constant and arrangement can be determined. For example, by stretching the mold with a 2D array in a square lattice, rectangular and triangular lattice arrangements can be obtained. As a specific application, we have applied this programmable nanoimprint lithography method to create a linear variable photonic crystal (PC) filter with continuously tunable resonant wavelength covering a wide spectral range along its length. The other approach is incorporating materials with tunable optical properties into the constituent material of the periodic nanophotonic devices. In this dissertation, a thin layer of phase-change material, Ge2Sb2Te5ïÿý (GST), in nanometers was embedded in the waveguide layer of a photonic crystal (PC) structure. The PC structure is based on a one-dimensional grating with a zinc sulfide waveguide. The GST-incorporated PC (GST-PC) structure supports the guided-mode resonance (GMR) that selectively absorbs light at particular wavelengths. The tuning effects were experimentally demonstrated by the crystallization or re-amorphization of the GST thin film. The GST-PC device opens a new path for tuning optical resonances in the near infrared region. Potential applications include color generation, display, optical storage, optical switches, and optical filters. At last, a novel fabrication method for an ultrathin freestanding gold plasmonic membrane is proposed. The freestanding plasmonic membrane was characterized using FT-IR, and demonstrated to support extraordinary optical transmission in the mid infrared wavelength range. The effect of the thickness of gold was also investigated. This plasmonic device was utilized as a surface-based optical sensor by measuring the absorption of the stretching modes of chemical bonds in the Mid-IR

Akinetic Tuneable Optical Sources with Applications

Optical Coherence Tomography (OCT) is a modern, non-invasive imaging technique in biomedical research and medical diagnostics. It was initially developed for clinical applications in ophthalmology, providing high-resolution, cross-sectional images of the retina, retinal nerve fibre layer and the optic nerve head. Today, OCT is used for in vivo imaging of almost every type of tissue and it also branched out in fields outside medicine, like industrial or pharmaceutical applications. OCT is a continuously improving imaging technique, benefiting from the development of advanced optical components and broadband optical sources. The objective of the work presented in the thesis was the development of both short and, respectively, long cavity akinetic optical devices, employing several types of dispersive optical fibre components in the cavity, like chirped fibre Bragg gratings, single mode or dispersion compensating fibre, and actively radio-frequency tuned semiconductor optical amplifiers, used as gain media. The use of external modulators, like Fabry-Perot assemblies, rotating mirrors and other mechanical devices is therefore completely eliminated, while versatility is added in the control of the coherence length, output bandwidth, repetition rate and power. The short cavity source was developed in the 1060 nm region, the output power and bandwidth showing a slow decay with the increase of repetition rate up to 250 kHz. Without any booster, the power achieved was 2 mW at 100 kHz. A novel dual-mode-locking mechanism was developed in order to tune an akinetic swept source based on dispersive cavities at a repetition rate close to, but different from the inverse of the cavity roundtrip. Several optical source configurations emitting in the 1060 nm or 1550 nm wavelength region were developed, characterised and tested in OCT applications. For the 1550 nm swept source employing a Faraday Rotating Mirror in a dispersive cavity, sweeping rates in the range of MHz were achieved, from 782 kHz to up to 5 times this value, with proportional decrease in the tuning bandwidth. Linewidths smaller than 60 pm and output powers exceeding 10 mW were measured. OCT topographic imaging was demonstrated. The thesis ends with a proposed broadband investigation of microresonators written in silica glass employing akinetic optical sources at 1550 nm. The work presented in this thesis resulted in several peer reviewed papers, one patent application and several conference presentations, listed after the final conclusions

Microwave Photonic Signal Processing Using On-Chip Nonlinear Optics

The field of microwave photonics (MWP) emerged as a solution to the challenges faced by electronic systems when dealing with high-bandwidth RF and microwave signals. Photonic devices are capable of handling immense bandwidths thanks to the properties of light. MWP therefore employs such devices to process and distribute the information carried by RF and microwave signals, enabling significantly higher capacity compared to conventional electronics. The photonic devices traditionally used in MWP circuits have mainly comprised bulky components, such as spools of fibre and benchtop optical amplifiers. While achieving impressive performance, these systems were not capable of competing with electronics in terms of size and portability. More recently, research has focused on the application of photonic chip technology to the field of MWP in order to reap the benefits of integration, such as reductions in size, weight, cost, and power consumption. Integrated MWP however is still in its infancy, and ongoing research efforts are exploring new ways to match integrated photonic devices to the unique requirements of MWP circuits. This work investigates the application of on-chip nonlinear optical interactions to MWP. Nonlinear optics enables light-on-light interactions (not normally possible in a linear regime) which open a vast array of powerful functionalities. In particular, this thesis focuses on stimulated Brillouin scattering, resulting from the interaction of light with hypersonic sound waves, and four-wave mixing, where photons exchange energies. These two nonlinear effects are applied to implement MWP ultra-high suppression notch filters, wideband phase shifters, and ultra-fast instantaneous frequency measurement systems. Experimental demonstrations using integrated optical waveguides confirm record results

Microwave Photonic Signal Processing Using On-Chip Nonlinear Optics

The field of microwave photonics (MWP) emerged as a solution to the challenges faced by electronic systems when dealing with high-bandwidth RF and microwave signals. Photonic devices are capable of handling immense bandwidths thanks to the properties of light. MWP therefore employs such devices to process and distribute the information carried by RF and microwave signals, enabling significantly higher capacity compared to conventional electronics. The photonic devices traditionally used in MWP circuits have mainly comprised bulky components, such as spools of fibre and benchtop optical amplifiers. While achieving impressive performance, these systems were not capable of competing with electronics in terms of size and portability. More recently, research has focused on the application of photonic chip technology to the field of MWP in order to reap the benefits of integration, such as reductions in size, weight, cost, and power consumption. Integrated MWP however is still in its infancy, and ongoing research efforts are exploring new ways to match integrated photonic devices to the unique requirements of MWP circuits. This work investigates the application of on-chip nonlinear optical interactions to MWP. Nonlinear optics enables light-on-light interactions (not normally possible in a linear regime) which open a vast array of powerful functionalities. In particular, this thesis focuses on stimulated Brillouin scattering, resulting from the interaction of light with hypersonic sound waves, and four-wave mixing, where photons exchange energies. These two nonlinear effects are applied to implement MWP ultra-high suppression notch filters, wideband phase shifters, and ultra-fast instantaneous frequency measurement systems. Experimental demonstrations using integrated optical waveguides confirm record results

In-line fibre-optic laser doppler velocimeter using bragg grating interferometric filters as frequency to intensity transducers

Three dimensional complex flows particularly those of turbomachinery present challenges to current measurement technology in terms of restricted optical access, measurement accuracy for the on-axis velocity component, the need to resolve flow turbulence and measurement difficulty from close to surface or intra-channel measurements in rotating machinery. A novel non-intrusive in-line fibre-optic laser Doppler velocimeter is presented specifically for the measurement of the on-axis component of velocity. The measurement principle is based on a Doppler frequency to intensity transducer in the form of a fibre-optic Bragg grating based Fabry-Perot interferometric filter. The filters were fabricated at 514.5 nm but in principle any desired wavelength may be used thus permitting any laser wavelength source to be used. Filters with appropriate features were designed with the aid of the theoretical models based on the coupled mode theory and transfer matrix approach. The argon-ion laser emission wavelength was locked to a corresponding Doppler broadened absorption line of molecular iodine vapour while the Fabry-Perot interferometer phase was controlled in an independent feedback system using digital lock-in amplifiers. The optical frequency was stabilized to within 10 MHz for at least one hour while the phase was controlled to an equivalent of (within) ± 3 MHz in frequency. Both feedback loops utilized custom designed PID electronic circuit controllers. The bandwidth of the filter was tunable by up to 400 MHz, with a resolution of between 0.2 ms'1 and 1 ms"1, and a sensitivity range of between 0.5 [GHz]'1 and 1.7 [GHz]'1. In this technique the filter was tuned to the optical wavelength, rather than tuning the laser wavelength to match the filter. The finished instrument was applied to the measurement of the on-axis component of velocity, of a rotating disc, over an available range of up to ± 42 ms'1, limited only by the maximum velocity of the disc. The detection system was reconfigured for low velocity measurements at twice the sensitivity over a velocity range of ± 7 ms'1. This technique demonstrates a unique contribution to fluid dynamics for the measurement of the traditionally difficult in-line component of velocity.Ph

Hybrid integrated semiconductor lasers with silicon nitride feedback circuits

Hybrid integrated semiconductor laser sources offering extremely narrow spectral linewidth as well as compatibility for embedding into integrated photonic circuits are of high importance for a wide range of applications. We present an overview on our recently developed hybrid-integrated diode lasers with feedback from low-loss silicon nitride (Si3N4 in SiO2) circuits, to provide sub-100-Hz-level intrinsic linewidths, up to 120 nm spectral coverage around 1.55 um wavelength, and an output power above 100 mW. We show dual-wavelength operation, dual-gain operation, laser frequency comb generation, and present work towards realizing a visible-light hybrid integrated diode laser.Comment: 25 pages, 16 figure

Hybrid Optical Fiber Sensors for Smart Materials and Structures

There has been a rapid growth in the use of advanced composite materials in a variety of load-bearing structures, for example in aviation for structures such as rotor blades, aircraft fuselage and wing structures. Composite materials embedded with fiber-optic sensors (FOS) have been recognized as one of the prominent enabling technologies for smart materials and structures. The rapid increase in the interest in composite materials embedded with FOS has been driven by numerous applications, such as intelligent composite manufacturing/processing, and safety-related areas in aircrafts. Research has been focused recently on using several optical sensor types working together to form so called “hybrid optical fiber sensors” in order to overcome the limitations of the individual sensor technologies. The main aim of the research described in this thesis is to investigate a hybrid sensing scheme that utilizes polarimetric sensors and FBG sensors working in a complimentary fashion to measure multiple physical parameters in a composite material, with a particular focus on measuring the complex indirect parameters thermal expansion and vibration. The research described in this thesis investigates the performance of a hybrid sensing scheme based on polarimetric sensors and FBG sensors after embedding in a composite material. It is shown that the influence of thermal expansion within a composite material on embedded polarimetric sensors is the main source of errors for embedded fiber sensor strain measurements and that for practical strain sensing applications buffer coated PM-PCF are more suitable for embedding in composite. Further, using a buffer stripped PM-PCF polarimetric sensor, a measurement scheme to measure a composite material\u27s thermal elongation induced strain is proposed. A novel hybrid sensor for simultaneous measurement of strain, temperature and thermal strain is demonstrated by integrating polarimetric sensors based on acrylate coated high bi-refringent polarization maintaining photonic crystal fiber (HB-PM-PCF), and a coating stripped HB-PM-PCF sensor together with an FBG sensor. Flexible demodulation modules that can be embedded or surface attached is a challenge for composite materials containing fiber-optic sensors. In this thesis an interrogation method that allows intensity domain operation of hybrid sensor is demonstrated. Further focusing towards the miniaturization of the hybrid sensor interrogator, a miniaturized flexible interrogator for the demonstrated hybrid sensing scheme embedded in a composite material is also designed. Low frequency vibration measurements are performed for glass fibre-reinforced composite material samples with two different strain-sensitive polarimetric sensor types embedded. It is shown that the strain sensitivity of polarimetric sensors limits the vibration measurements to a certain range of vibration amplitudes. A polarimetric sensor based buffer stripped HB-PM-PCF is demonstrated for monitoring the different stages of the curing process for a Mageneto-Rheological composite material. By providing information about multiple parameters such as strain, temperature, thermal strain, vibration amplitude and vibration frequency the proposed and demonstrated hybrid sensing approach has a high potential to change the paradigm for smart material design in the future

Optical frequency comb technology for ultra-broadband radio-frequency photonics

The outstanding phase-noise performance of optical frequency combs has led to a revolution in optical synthesis and metrology, covering a myriad of applications, from molecular spectroscopy to laser ranging and optical communications. However, the ideal characteristics of an optical frequency comb are application dependent. In this review, the different techniques for the generation and processing of high-repetition-rate (>10 GHz) optical frequency combs with technologies compatible with optical communication equipment are covered. Particular emphasis is put on the benefits and prospects of this technology in the general field of radio-frequency photonics, including applications in high-performance microwave photonic filtering, ultra-broadband coherent communications, and radio-frequency arbitrary waveform generation.Comment: to appear in Laser and Photonics Review