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

1 MHz Akinetic Dispersive Ring Cavity Swept Source at 850 nm

A fast dual mode-locked akinetic optical swept source at 850 nm central wavelength is presented using a dispersive cavity. We demonstrate that single mode fiber can be successfully used as dispersive element to induce mode locking. A first locking condition is imposed by driving the optical gain at a high frequency, to induce mode locking, while a second locking condition involves sweeping at a rate close to resonance value. In this regime, using the same fiber length in the loop, sweeping rates of 0.5 MHz and 1 MHz are demonstrated with proportional reduction in the tuning bandwidth. The axial range of the swept source was evaluated by scanning through the channeled spectrum of a Michelson interferometer

Real-time RF Sensor Monitoring Based on Optical Injected Semiconductor Laser and Temporal Measurement

Ultrafast and real-time RF sensor monitoring is in true demand for dynamic and transient sensing scenarios. In this paper, a high resolution and real-time RF sensor interrogation system based on an optically injected semiconductor laser and temporal measurement is proposed. Eliminating the need for RF spectrum measurement, the frequency of the RF sensor can be determined from temporal measurements featuring high speed and high resolution. In addition, no temporal reference signal is required and RF sensor interrogation is achieved by direct measurement of time interval of a pair of electrical pulses. Real-time monitoring of a RF temperature sensing is experimentally demonstrated and our results show that the proposed system offers a measurement bandwidth of 6 GHz, and an interrogation speed of 1 MHz. Only a very low temporal sampling rate of 60 MS/s is required

Ferroelectric Control of Magnetism in Ultrathin HfO2\Co\Pt Layers

The recent demonstration of ferroelectricity in ultrathin HfO2 has kickstarted a new wave of research into this material. HfO2 in the orthorhombic phase can be considered the first and only truly nanoscale ferroelectric material that is compatible with silicon-based nanoelectronics applications. In this article, we demonstrate the ferroelectric control of the magnetic properties of cobalt deposited on ultrathin aluminum-doped, atomic layer deposition-grown HfO2 (tHfO2 = 6.5 nm). The ferroelectric effect is shown to control the shape of the magnetic hysteresis, quantified here by the magnetic switching energy. Furthermore, the magnetic properties such as the remanence are modulated by up to 41%. We show that this modulation does not only correlate with the charge accumulation at the interface but also shows an additional component associated with the ferroelectric polarization switching. An in-depth analysis using first order reversal curves shows that the coercive and interaction field distributions of cobalt can be modulated up to, respectively, 5.8% and 10.5% with the ferroelectric polarization reversal.status: publishe