Mode locking in the ring laser gyroscope: Reduced threshold for two cavity modes

This thesis was submitted for the degree of Doctor of Philosophy and awarded by Brunel University.A ring laser gyroscope is a device which employs a ring laser to measure rotation. A ring laser supports two beams propagating in opposite directions around the ring resonator. When the gyroscope is rotated, the frequencies of the two beams split by an amount proportional to the rate of rotation: the device works by measuring this frequency splitting. The main problem of ring laser gyroscope design is the frequency synchronisation - lock-in - of the two beams at low rates of rotation. Lock-in arises from weak mutual coupling caused by backscattering at the mirrors and results in a dead band around zero. One of the possible solutions to this problem is a two-mode ring laser in which two modes oscillate simultaneously and interact to reduce the dead band. The present work reviews the theory of lock-in and offers a theoretical basis for this approach, as well as providing experimental evidence to support it

Metrology of Complex Refractive Index for Solids in the Terahertz Regime Using Frequency Domain Spectroscopy

Frequency domain spectroscopy allows an experimenter to establish optical properties of solids in a wide frequency band including the technically challenging 10 THz region, and in other bands enables metrological comparison between competing techniques. We advance a method for extracting the optical properties of high-index solids using only transmission-mode frequency domain spectroscopy of plane-parallel Fabry-Perot optical flats. We show that different data processing techniques yield different kinds of systematic error, and that some commonly used techniques have inherent systematic errors which are underappreciated. We use model datasets to cross-compare algorithms in isolation from experimental errors, and propose a new algorithm which has qualitatively different systematic errors to its competitors. We show that our proposal is more robust to experimental non-idealities such as noise or apodization, and extract the complex refractive index spectrum of crystalline silicon as a practical example. Finally, we advance the idea that algorithms are complementary rather than competitive, and should be used together as part of a toolbox for better metrology.Comment: 21 pages, 11 figures, 4 appendice

Measurements of effective porosity of pharmaceutical tablets using THz TDS

The pharmaceutical industry requires a rapid nondestructive technique for monitoring porosity of tablets. Here effective porosity of compressed lactose pellets was investigated using THz time-domain spectroscopy (THz TDS)

Measuring open porosity of porous materials using THz-TDS and an index-matching medium

The porosity of porous materials is a critical quality attribute of many products ranging from catalysis and separation technologies to porous paper and pharmaceutical tablets. The open porosity in particular, which reflects the pore space accessible from the surface, is crucial for applications where a fluid needs to access the pores in order to fulfil the functionality of the product. This study presents a methodology that uses terahertz time-domain spectroscopy (THz-TDS) coupled with an index-matching medium to measure the open porosity and analyze scattering losses of powder compacts. The open porosity can be evaluated without the knowledge of the refractive index of the fully dense material. This method is demonstrated for pellets compressed of pharmaceutical-grade lactose powder. Powder was compressed at four different pressures and measured by THz-TDS before and after they were soaked in an index-matching medium, i.e., paraffin. Determining the change in refractive index of the dry and soaked samples enabled the calculation of the open porosity. The results reveal that the open porosity is consistently lower than the total porosity and it decreases with increasing compression pressure. The scattering losses reduce significantly for the soaked samples and the scattering centers (particle and/or pore sizes) are of the order of or somewhat smaller than the terahertz wavelength. This new method facilitates the development of a better understanding of the links between material properties (particles size), pellet properties (open porosity) and performance-related properties, e.g., disintegration and dissolution performance of pharmaceutical tablets

The Effect of Particle Size and Concentration on Low-Frequency Terahertz Scattering in Granular Compacts

Fundamental knowledge of scattering in granular compacts is essential to ensure accuracy of spectroscopic measurements and determine material characteristics such as size and shape of scattering objects. Terahertz time-domain spectroscopy (THz-TDS) was employed to investigate the effect of particle size and concentration on scattering in specially fabricated compacts consisting of borosilicate microspheres in a polytetrafluoroethylene (PTFE) matrix. As expected, increasing particle size leads to an increase in overall scattering contribution. At low concentrations, the scattering contribution increases linearly with concentration. Scattering increases linearly at low concentrations, saturates at higher concentrations with a maximum level depending on particle size, and that the onset of saturation is independent of particle size. The effective refractive index becomes sublinear at high particle concentrations and exceeds the linear model at maximum density, which can cause errors in calculations based on it, such as porosity. The observed phenomena are attributed to the change in the fraction of photons propagating ballistically versus being scattered. At low concentrations, photons travel predominately ballistically through the PTFE matrix. At high concentrations, the photons again propagate ballistically through adjacent glass microspheres. In the intermediate regime, photons are predominately scattered

An overview of terahertz imaging with resonant tunneling diodes

Terahertz (THz) imaging is a rapidly growing application motivated by industrial demands including harmless (non-ionizing) security imaging, multilayer paint quality control within the automotive industry, insulating foam non-invasive testing in aerospace, and biomedical diagnostics. One of the key components in the imaging system is the source and detector. This paper gives a brief overview of room temperature THz transceiver technology for imaging applications based on the emerging resonant tunneling diode (RTD) devices. The reported results demonstrate that RTD technology is a very promising candidate to realize compact, low-cost THz imaging systems

Building an end user focused THz based ultra high bandwidth wireless access network: The TERAPOD approach

The TERAPOD project aims to investigate and demonstrate the feasibility of ultra high bandwidth wireless access networks operating in the Terahertz (THz) band. The proposed TERAPOD THz communication system will be developed, driven by end user usage scenario requirements and will be demonstrated within a first adopter operational setting of a Data Centre. In this article, we define the full communications stack approach that will be taken in TERAPOD, highlighting the specific challenges and aimed innovations that are targeted

Analysis of THz scattering of compacted granular materials using THz-TDS

Scattering of terahertz radiation in compacts is of great interest due to its potential to non-destructively assess various structural elements such as particle size and defects in compacts. In this study, we isolate the scattering contributions to the loss coefficient of borosilicate glass microspheres suspended in a polytetrafluoroethylene (PTFE) compact measured by terahertz time-domain spectroscopy. The particle size and concentration of microspheres in the compacts were varied to resolve their effect on terahertz scattering

Polymer pellet fabrication for accurate THz-TDS measurements

We investigate fabrication of compacts using polytetrafluoroethylene (PTFE) and polyethylene (PE), and the effect of compaction conditions on their terahertz transmission properties. The conditions used to fabricate compressed powder samples for terahertz time-domain spectroscopy (THz-TDS) can impact the accuracy of the measurements and hence the interpretation of results. This study investigated the effect of compaction conditions on the accuracy of the THz-TDS analysis. Two polymers that are commonly used as matrix materials in terahertz spectroscopy studies were explored using a compaction simulator and a hydraulic press for sample preparation. THz-TDS was used to determine the refractive index and loss coefficient to compare the powder compacts (pellets) to the values of solid material. Sample porosity, axial relaxation and tensile strength were measured to assess the material’s suitability for terahertz spectroscopy. It was found that PTFE is the preferable material for creating THz-TDS samples due to its low porosity and high tensile strength. PE was found to show significant porosity at all compaction pressures making it an unsuitable material for the accurate determination of optical parameters from THz-TDS spectroscopy measurements. The larger particle sizes of PE resulted in compacts that exhibited significantly lower tensile strength than those made from PTFE making handling and storage difficult

Observation of spurious spectral features in mixed-powder compressed pellets measured by terahertz time-domain spectroscopy

Spurious loss features were observed in mixed-powder compressed pellets measured in transmission using terahertz time-domain spectroscopy. Loss features were identified in two types of pellets: PTFE-glass microspheres and PTFE-lactose. The features were found to be dependent on grain size and concentration. An explanation is proposed, based on varying optical thickness of the sample material