Modeling of mode-locking in a laser with spatially separate gain media

We present a novel laser mode-locking scheme and discuss its unusual properties and feasibility using a theoretical model. A large set of single-frequency continuous-wave lasers oscillate by amplification in spatially separated gain media. They are mutually phase-locked by nonlinear feedback from a common saturable absorber. As a result, ultra short pulses are generated. The new scheme offers three significant benefits: the light that is amplified in each medium is continuous wave, thereby avoiding issues related to group velocity dispersion and nonlinear effects that can perturb the pulse shape. The set of frequencies on which the laser oscillates, and therefore the pulse repetition rate, is controlled by the geometry of resonator-internal optical elements, not by the cavity length. Finally, the bandwidth of the laser can be controlled by switching gain modules on and off. This scheme offers a route to mode-locked lasers with high average output power, repetition rates that can be scaled into the THz range, and a bandwidth that can be dynamically controlled. The approach is particularly suited for implementation using semiconductor diode laser arrays.Comment: 13 pages, 5 figures, submitted to Optics Expres

Development of an irradiation laser system for phototherapy mediated by nanoparticles

Cancer is a disease associated with high morbidity and mortality, which led to the devel-opment of clinical techniques to deal with this illness. Phototherapy is one of these methods. It is seen as a good solution to this problem due to its low invasiveness and prevention of DNA damage of healthy cells with the usage of non-ion-ising radiation. Advances in this technology have been made, and, in order to increase the safety of the procedure, it has been coupled with photosensitising agents, particles characterized by high absorbance at specific wavelengths. Since these agents possess high affinity towards tumour cells, it is possible to induce necrosis or apoptosis of unhealthy tissue with localised thermal energy. Nevertheless, increased safety and optimisation are pursued so it was considered that a beam delivering system capable of almost exclusively target tumour cells, protecting healthy ones, independently of the size of the cancer, should be the next step since few literature was found on the development of these systems. Two resolutions are proposed. A zoom lens based optical system able to dynamically vary its output beam diameter, focusing on the malignant tissue at once, and a galvanometric mirror system that allows the scanning of the area of the tumour, requiring a sweeping pattern due to the smaller output beam diameter employed. Despite both solutions producing promising results, to achieve clinical implementation is still needed the development of an apparatus that integrates the zoom lens based system and the testing in the laboratory of the galvanometric mirror system.O cancro é uma doença associada a elevada morbilidade e mortalidade, o que levou ao desenvolvimento de técnicas clínicas para lidar com esta enfermidade. A fototerapia é um desses métodos e é tomada como uma boa solução para esse problema, devido a ser pouco invasiva e prevenir danos ao ADN de células saudáveis, por usar radiação não ionizante. Para aumentar a segurança do procedimento, introduziu-se nesta terapia agentes fotos-sensibilizantes, partículas caracterizadas por alta absorbância em comprimentos de onda especí-ficos. Como estes agentes possuem alta afinidade com as células tumorais, é possível induzir necrose ou apoptose de tecido não saudável com energia térmica bem localizada. No entanto, aumentar a segurança e a otimização é necessário e, por isso, foi considerado que um sistema de entrega de feixe capaz de focar quase exclusivamente células tumorais, prote-gendo as saudáveis, independentemente do tamanho do cancro, deveria ser o próximo passo, uma vez que pouca literatura foi encontrada sobre o desenvolvimento desses sistemas. Duas resoluções são propostas. Um sistema ótico baseado em lentes de zoom capaz de variar dinamicamente o diâmetro do feixe de saída, atingindo o tecido maligno de uma só vez, e um sistema de espelhos galvanométricos que permite a cobertura da área do tumor, exigindo um padrão de varrimento, devido ao menor diâmetro do feixe de saída. Apesar de ambas as soluções produzirem resultados promissores, para alcançar uma im-plementação clínica ainda é necessário o desenvolvimento de um aparelho que integre o sistema baseado em lentes de zoom e o teste em laboratório do sistema de espelhos galvanométricos

Specialized directional beaming through a metalens and a typical application

© 2017 Yongqi Fu et al., published by De Gruyter, Berlin/Boston 2017. The anomalous beaming effect of the periodic metallic corrugations functioning as a special case of kSP=2π/Λ is discussed and verified by means of both theoretical calculation and experimental probing. A metalens is designed on the basis of the special case. Unlike the conventional beaming of convergence or divergence, the metalens can realize beam collimating, which is useful for practical applications. As a typical application example of the metalens, we integrate the metalens together with a vertical cavity surface emission laser (VCSEL) on the top surface of the aperture area. Our experimental results demonstrate that the integrated metalens is capable of suppressing the divergence angle of the VCSEL for collimation use

Proof-of-concept of a single-point Time-of-Flight LiDAR system and guidelines towards integrated high-accuracy timing, advanced polarization sensing and scanning with a MEMS micromirror

Dissertação de mestrado integrado em Engenharia Física (área de especialização em Dispositivos, Microssistemas e Nanotecnologias)The core focus of the work reported herein is the fulfillment of a functional Light Detection and Ranging (LiDAR) sensor to validate the direct Time-of-Flight (ToF) ranging concept and the acquisition of critical knowledge regarding pivotal aspects jeopardizing the sensor’s performance, for forthcoming improvements aiming a realistic sensor targeted towards automotive applications. Hereupon, the ToF LiDAR system is implemented through an architecture encompassing both optical and electronical functions and is subsequently characterized under a sequence of test procedures usually applied in benchmarking of LiDAR sensors. The design employs a hybrid edge-emitting laser diode (pulsed at 6kHz, 46ns temporal FWHM, 7ns rise-time; 919nm wavelength with 5nm FWHM), a PIN photodiode to detect the back-reflected radiation, a transamplification stage and two Time-to-Digital Converters (TDCs), with leading-edge discrimination electronics to mark the transit time between emission and detection events. Furthermore, a flexible modular design is adopted using two separate Printed Circuit Boards (PCBs), comprising the transmitter (TX) and the receiver (RX), i.e. detection and signal processing. The overall output beam divergence is 0.4º×1º and an optical peak power of 60W (87% overall throughput) is realized. The sensor is tested indoors from 0.56 to 4.42 meters, and the distance is directly estimated from the pulses transit time. The precision within these working distances ranges from 4cm to 7cm, reflected in a Signal-to-Noise Ratio (SNR) between 12dB and 18dB. The design requires a calibration procedure to correct systematic errors in the range measurements, induced by two sources: the timing offset due to architecture-inherent differences in the optoelectronic paths and a supplementary bias resulting from the design, which renders an intensity dependence and is denoted time-walk. The calibrated system achieves a mean accuracy of 1cm. Two distinct target materials are used for characterization and performance evaluation: a metallic automotive paint and a diffuse material. This selection is representative of two extremes of actual LiDAR applications. The optical and electronic characterization is thoroughly detailed, including the recognition of a good agreement between empirical observations and simulations in ZEMAX, for optical design, and in a SPICE software, for the electrical subsystem. The foremost meaningful limitation of the implemented design is identified as an outcome of the leading-edge discrimination. A proposal for a Constant Fraction Discriminator addressing sub-millimetric accuracy is provided to replace the previous signal processing element. This modification is mandatory to virtually eliminate the aforementioned systematic bias in range sensing due to the intensity dependency. A further crucial addition is a scanning mechanism to supply the required Field-of-View (FOV) for automotive usage. The opto-electromechanical guidelines to interface a MEMS micromirror scanner, achieving a 46º×17º FOV, with the LiDAR sensor are furnished. Ultimately, a proof-of-principle to the use of polarization in material classification for advanced processing is carried out, aiming to complement the ToF measurements. The original design is modified to include a variable wave retarder, allowing the simultaneous detection of orthogonal linear polarization states using a single detector. The material classification with polarization sensing is tested with the previously referred materials culminating in an 87% and 11% degree of linear polarization retention from the metallic paint and the diffuse material, respectively, computed by Stokes parameters calculus. The procedure was independently validated under the same conditions with a micro-polarizer camera (92% and 13% polarization retention).O intuito primordial do trabalho reportado no presente documento é o desenvolvimento de um sensor LiDAR funcional, que permita validar o conceito de medição direta do tempo de voo de pulsos óticos para a estimativa de distância, e a aquisição de conhecimento crítico respeitante a aspetos fundamentais que prejudicam a performance do sensor, ambicionando melhorias futuras para um sensor endereçado para aplicações automóveis. Destarte, o sistema LiDAR é implementado através de uma arquitetura que engloba tanto funções óticas como eletrónicas, sendo posteriormente caracterizado através de uma sequência de testes experimentais comumente aplicáveis em benchmarking de sensores LiDAR. O design tira partido de um díodo de laser híbrido (pulsado a 6kHz, largura temporal de 46ns; comprimento de onda de pico de 919nm e largura espetral de 5nm), um fotodíodo PIN para detetar a radiação refletida, um andar de transamplificação e dois conversores tempo-digital, com discriminação temporal com threshold constante para marcar o tempo de trânsito entre emissão e receção. Ademais, um design modular flexível é adotado através de duas PCBs independentes, compondo o transmissor e o recetor (deteção e processamento de sinal). A divergência global do feixe emitido para o ambiente circundante é 0.4º×1º, apresentando uma potência ótica de pico de 60W (eficiência de 87% na transmissão). O sensor é testado em ambiente fechado, entre 0.56 e 4.42 metros. A precisão dentro das distâncias de trabalho varia entre 4cm e 7cm, o que se reflete numa razão sinal-ruído entre 12dB e 18dB. O design requer calibração para corrigir erros sistemáticos nas distâncias adquiridas devido a duas fontes: o desvio no ToF devido a diferenças nos percursos optoeletrónicos, inerentes à arquitetura, e uma dependência adicional da intensidade do sinal refletido, induzida pela técnica de discriminação implementada e denotada time-walk. A exatidão do sistema pós-calibração perfaz um valor médio de 1cm. Dois alvos distintos são utilizados durante a fase de caraterização e avaliação performativa: uma tinta metálica aplicada em revestimentos de automóveis e um material difusor. Esta seleção é representativa de dois cenários extremos em aplicações reais do LiDAR. A caraterização dos subsistemas ótico e eletrónico é minuciosamente detalhada, incluindo a constatação de uma boa concordância entre observações empíricas e simulações óticas em ZEMAX e elétricas num software SPICE. O principal elemento limitante do design implementado é identificado como sendo a técnica de discriminação adotada. Por conseguinte, é proposta a substituição do anterior bloco por uma técnica de discriminação a uma fração constante do pulso de retorno, com exatidões da ordem sub-milimétrica. Esta modificação é imperativa para eliminar o offset sistemático nas medidas de distância, decorrente da dependência da intensidade do sinal. Uma outra inclusão de extrema relevância é um mecanismo de varrimento que assegura o cumprimento dos requisitos de campo de visão para aplicações automóveis. As diretrizes para a integração de um micro-espelho no sensor concebido são providenciadas, permitindo atingir um campo de visão de 46º×17º. Conclusivamente, é feita uma prova de princípio para a utilização da polarização como complemento das medições do tempo de voo, de modo a suportar a classificação de materiais em processamento avançado. A arquitetura original é modificada para incluir uma lâmina de atraso variável, permitindo a deteção de estados de polarização ortogonais com um único fotodetetor. A classificação de materiais através da aferição do estado de polarização da luz refletida é testada para os materiais supramencionados, culminando numa retenção de polarização de 87% (tinta metálica) e 11% (difusor), calculados através dos parâmetros de Stokes. O procedimento é independentemente validado com uma câmara polarimétrica nas mesmas condições (retenção de 92% e 13%)

A stability and spatial-resolution enhanced laser absorption spectroscopy tomographic sensor for complex combustion flame diagnosis

A novel stable laser absorption spectroscopy (LAS) tomographic sensor with enhanced stability and spatial resolution is developed and applied to complex combustion flame diagnosis. The sensor reduces the need for laser collimation and alignment even in extremely harsh environments and improves the stability of the received laser signal. Furthermore, a new miniaturized laser emission module was designed to achieve multi-degree of freedom adjustment. The full optical paths can be sampled by 8 receivers, with such arrangement, the equipment cost can be greatly reduced, at the same time, the spatial resolution is improved. In fact, 100 emitted laser paths are realized in a limited space of 200mm×200 mm with the highest spatial resolution of 1.67mm×1.67 mm. The stability and penetrating spatial resolution of the LAS tomographic sensor were validated by both simulation and field experiments on the afterburner flames. Tests under two representative experiment states, i.e., the main combustion and the afterburner operation states, were conducted. Results show that the error under the main combustion state was about 4.32% and, 5.38% at the afterburner operation state. It has been proven that this proposed sensor can provide better tomographic measurements for combustion diagnosis, as an effective tool for improving performances of afterburners

Precise determination of the electron beam energy with Compton backscattered laser photons at ANKA

Energy is one of the most fundamental parameters of the synchrotrons. An innovative energy measurement method based on Compton backscattering with a transverse configuration has been successfully developed to calibrate the entire energy range of ANKA storage ring accurately. The theoretical model, numerical studies, design and implementation of the respective sub-system, as well as the measurement procedure and results are presented in this thesis

Design and Development of Multi-Emitter High Power Laser Diode Modules

The growing demand for high-power laser diode modules for laser-based material processing machines has stimulated the development of a number of architectures that, taking advantage of multiple beam combination techniques, have allowed the realization of multi-emitter devices with unprecedented performance. However, these designs typically rely on roughly approximated relations, which have reached their limit of applications. Therefore, to further scale the output power and increase the brightness, new and more accurate models are necessary. From the market point-of-view, the deployment of high-power multi-emitter modules is limited by the cost per emitted watt, which is proportional to the number of required optical elements and package assembly time. Cost reduction, therefore, requires, again, accurate models to properly optimize the package layout, but also new assembly strategies and tools. The thesis analyzes in detail these two aspects - accurate models and assembly strategies and tools - and presents for both innovative solutions to help to push the technology beyond the current state-of-the-art. In particular, for what concerns the multi-emitter model, a new relation to predict the beam quality at the pigtail fiber input by taking into account the impact of lenses and the distance between two adjacent chips in spatial beam multiplexing has introduced. The model is based on the propagation and transformation of paraxial Gaussian beams and analyzes, not only the impact of the choices on the focal length of each collimating or focusing lens but also of the truncation caused by their finite aperture. Then, as the model requires the knowledge of the individual laser chip beam characteristics, specific benches for the measurement of the near and far field emissions have been developed. The proposed model has been validated in different working conditions and found to lead to an error lower than 6%. As for the multi-emitter assembly, an industrial grade procedure has been devised and a completely new approach based on back-propagation artificial neural network to automatically determine the optimal positioning of each optical element has been developed. The neural network is trained using ray tracing of Gaussian beams, starting from the emission characteristics of the laser chips. The new tool has been tested in practical cases with the most critical of all the components, the positioning of the fast axis collimator, obtaining a reduction of the assembly time of more than 50% with respect to current automatic assembly machines. Finally, the design model and the assembly procedure have been applied to the development of a prototype of a multi-emitter module that, by exploiting spatial, wavelength, and polarization multiplexing of a plurality of chips emitting about 10W each, delivers over 300W in a 105/0.15 fiber pigtail, figures that represent a remarkable improvement over the current state-of-the-art. This result has been very challenging because it required the combination of theoretical, experimental, and technological aspects, not limited to photonics, but including also measurement theory, precision mechanics and thermal management

Laser Wire Scanner Compton Scattering Techniques for the Measurement of the Transverse Beam Size of Particle Beams at Future Linear Colliders

This archive summarizes a working paper and conference proceedings related to laser wire scanner development for the Future Linear Collider (FLC) in the years 2001 to 2006. In particular the design, setup and data taking for the laser wire experiments at PETRA II and CT2 are described. The material is focused on the activities undertaken by Royal Holloway University of London (RHUL).Comment: 61 page

High precision angle calibration for spherical measurement systems

The European Synchrotron Radiation Facility (ESRF) located in Grenoble, France is a joint facility supported and shared by 19 European countries. It operates the most powerful synchrotron radiation source in Europe. Synchrotron radiation sources address many important questions in modern science and technology. They can be compared to “super microscopes”, revealing invaluable information in numerous fields of diverse research such as physics, medicine, biology, geophysics and archaeology. For the ESRF accelerators and beam lines to work correctly, alignment is of critical importance. Alignment tolerances are typically much less than one millimetre and often in the order of several micrometers over the 844 m ESRF storage ring circumference. To help maintain these tolerances, the ESRF has, and continues to develop calibration techniques for high precision spherical measurement system (SMS) instruments. SMSs are a family of instruments comprising automated total stations (theodolites equipped with distance meters), referred to here as robotic total stations (RTSs); and laser trackers (LTs). The ESRF has a modern distance meter calibration bench (DCB) used for the calibration of SMS electronic distance meters. At the limit of distance meter precision, the only way to improve positional uncertainty in the ESRF alignment is to improve the angle measuring capacity of these instruments. To this end, the horizontal circle comparator (HCC) and the vertical circle comparator (VCC) have been developed. Specifically, the HCC and VCC are used to calibrate the horizontal and vertical circle readings of SMS instruments under their natural working conditions. Combined with the DCB, the HCC and VCC provide a full calibration suite for SMS instruments. This thesis presents their development, functionality and in depth uncertainty evaluation. Several unique challenges are addressed in this work. The first is the development and characterization of the linked encoders configuration (LEC). This system, based on two continuously rotating angle encoders, is designed improve performance by eliminating residual encoder errors. The LEC can measure angle displacements with an estimated uncertainty of at least 0.044 arc seconds. Its uncertainty is presently limited by the instrumentation used to evaluate it. Secondly, in depth investigation has lead to the discovery of previously undocumented error-motion effects in ultra-precision angle calibration. Finally, methods for rigorous characterisation and extraction of rotary table error motions and their uncertainty evaluation using techniques not previously discussed in the literature have been developed

INTENSE TERAHERTZ GENERATION VIA TWO-COLOR LASER FILAMENTATION

The main focus of this dissertation is intense terahertz (THz) generation using two-color laser mixing in air plasma. In this scheme, the fundamental and second harmonic of an ultrashort pulsed laser are combined and focused into air to create a gaseous plasma, which produces an intense THz pulse in the far field. To understand the generation process, we have developed a two-dimensional (2-D) plasma current model. Using this model, we have simulated the conditions for optimal THz generation and verified them experimentally. A full control of THz output is demonstrated by varying the phases and polarization states of the input laser pulses. We have studied how the generated THz energy scales with various focal lengths and input laser energies up to 60 mJ. For high enough energy inputs, the resulting THz saturates. This arises from inefficient laser energy coupling into the plasma, which results from plasma-induced laser defocusing in filamentation. We have overcome the saturation effect by elongating the filaments and achieved 7 µJ of THz energy with 60 mJ laser energy. This provides a conversion efficiency of 10-4 from optical to THz energy. In addition, we have investigated high-power THz generation in two-color laser filamentation with terawatt (TW) lasers including a 0.5 TW, 1 kHz repetition (rep) rate system, as well as, 2 TW and 30 TW systems, both operating at 10 Hz rep rate. In particular, our 1 kHz rep rate THz source can provide high-energy (>1 µJ), high-average power (>1 mW), intense (>1 MV/cm), and broadband (0.01~60 THz) THz radiation via two-color filamentation in air. Based on our observed scaling law, a ~30 TW laser can produce >0.1 mJ of THz radiation with multi-gigawatt (GW) peak power in ~1.5 m long filamentation. We have also studied various THz detection methods covering a broad range of THz frequency bands. We observe our THz source produces extremely broad electromagnetic (EM) radiation ranging from radio-micro waves to infrared frequencies, confirmed by our complementary THz detection methods. This source could be a useful tool for broadband linear and nonlinear THz spectroscopy