Characterization and alignment of microchip lasers for space applications

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2022

UNIVERSIDADE DE LISBOA FACULDADE DE CIÊNCIAS DEPARTAMENTO DE FÍSICA

Characterization and alignment of microchip lasers for space applications

Ana Luísa Antunes de Sousa

Mestrado em Engenharia Física

Versão Pública

Dissertação orientada por:

Paulo Romeu Seabra Gordo

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Acknowledgements

The work completed for this dissertation would not have been possible without my supervisor, Paulo Gordo, who accepted me to perform new laboratory work, giving me the opportunity to acquire skills and experience in the subject of microchip lasers. By inviting me to take part in this project, he provided me with the opportunity to learn with a hands-on approach what it’s like to work in this industry.

Throughout this project, he was always ready to provide helpful insight into the subject and make suggestions that propelled me into this world.

I’d like to thank all in Synopsis Planet who helped me reach the finish line of this work. Thanks to Beltran Nadal, who helped with the design and manufacturing of essential parts of this work. Thanks to Hugo Onderwater, who helped acquire necessary parts as well as help with the assembly and measurement processes from time to time. Also thanks to Prof. Rui Melicio for his advice on the dissertation.

I am grateful to my mother, who supported me during this endeavour, who was present during very difficult times and who sacrificed a lot to incentivise my academic pursuit.

Special thanks to my sister, Xana, who understood what I was going through and gave very good advice that came from someone who recently went through the same process.

I’d like to recognise my partner, Francisco, for all the emotional support given during this trying time of my life. Thank you for all the times you let me vent about my frustrations during this process, the

“career” advice you gave and the concern you showed.

I am eternally grateful for all your support.

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Abstract

The focus of this work was the implementation of an assembly process for fixed external microchip laser cavities and the characterization of optical cavities with different configurations. Two test setups were done. In the first, four different laser cavity configurations were assembled, two with CW emission and two with pulsed emission. The pumping was done in an end-pump configuration with a laser diode emitting in the 975 nm region and had QCW¹ emission to preserve the active medium. The microchip laser emitted in the 1532 nm region. Depending on the configuration, its emission was CW² or pulsed.

The active media used for this laser were two Er:Yb co-doped phosphate glasses with thicknesses of 1 mm and 1.5 mm. Both active media had HR@1550nm³ coatings as input couplers. The output coupler was externally implemented using a silica glass with a HR@1550nm coating with 98% reflectivity. The results from the CW cavities were compared. To obtain the pulsed emissions, the passive Q-switching technique was employed using a saturable absorber. This optical component was a Co-spinel glass placed inside the cavity with initial transmission of 98%. Two cavities with pulsed emission were assembled using different thicknesses for the active medium and their results were compared. The result was burst emission. It was concluded that factors such as a higher active medium thickness and the addition of the Q-switch led to a lower efficiency. In the second test setup, the previous components were glued to an aluminium substrate to produce a fixed optical cavity with pulsed emission as a commercial product. The manufacturing process proved to be very sensitive and unstable. The test setup was adapted and thrice attempted. The resulting cavity produced QCW emission.

Keywords: microchip laser; Q-switching; efficiency; erbium glass; pulsed laser emission.

1 QCW: quasi-continuous wave

2 CW: continuous wave

3 HR@1550nm: highly reflective at 1550 nm

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Resumo

O progresso científico tem-se expandido nas últimas décadas. Com a curiosidade humana veio a exploração de realidades até então distantes. A exploração espacial tem adicionado bastantes questões novas sobre o universo, mas também tem fornecido bastantes avanços tecnológicos. Cada missão espacial tem de estar equipada com os instrumentos necessários para a execução dos seus objetivos. Um instrumento muito importante é o LiDAR, utilizado para calcular distâncias entre a nave espacial e a superfície de um certo objeto. O LiDAR é um instrumento que utiliza o tempo de voo de um fotão desde que este é emitido pela fonte de luz do instrumento, ser refletido na superfície do objeto, e retornar ao instrumento, sendo detetado pelo seu detetor de fotões, para calcular esta distância. Como este instrumento utiliza luz, este tem de ter uma fonte de luz adequada. Os lasers providenciam as características de radiação requeridas para este funcionamento, visto que a sua luz é coerente.

O LiDAR é um tipo de instrumento muito usado também para aplicações na superfície da Terra. É usado para mapear terreno, podendo ser montado em aviões para varrer uma área, e também pode ser usado em carros autónomos, para que estes possam registar o que os rodeia. Por estas razões, o tipo de luz emitido pelo LiDAR tem de ser segura para a visão. Neste trabalho é contruída uma cavidade externa fixa de um microchip laser a emitir nos comprimentos de onda da região dos 1550 nm, considerados seguros para o olho.

Quatro cavidades óticas de componentes móveis de um laser microchip foram montadas usando três componentes óticos. Estes componentes foram o meio ativo, o espelho de saída e o meio saturável. O laser microchip foi ótica e longitudinalmente bombeado com um díodo laser acoplado a uma fibra ótica cujo núcleo tinha um diâmetro de 100 µm. O díodo laser emitia na região dos 975 nm, que corresponde a um pico de absorção do material utilizado como meio ativo. A emissão do bombeamento foi feita em modo QCW de forma a preservar a integridade do material do meio ativo.

Com o material do laboratório estavam disponíveis duas opções para o meio ativo do microchip laser, dependendo da configuração da cavidade desejada. O meio ativo era um vidro de fosfato co-dopado com iões de Er³⁺ na concentração de 1×10²⁰ cm^-3 e com iões de Yb³⁺ na concentração de 2×10²¹ cm⁻³, com áreas de 4×4 mm². Um dos vidros do meio ativo tinha uma espessura de 1 mm e o outro tinha uma espessura de 1.5 mm. Duas configurações do microchip foram constituídas pelo meio ativo e pelo espelho de saída. Ambas tinham emissão CW no sentido em que emitiam de forma contínua durante o pulso de bombeamento. Os seus resultados foram comparados de forma a entender o efeito de uma maior espessura no meio ativo. A cavidade cujo meio ativo tinha 1 mm de espessura tinha uma eficiência de 16.0% e a cavidade cujo meio ativo tinha 1.5 mm de espessura tinha uma eficiência de 12.5%. Concluiu- se que a espessura alargada diminuiu a eficiência do laser. Isto porque um maior volume leva a um maior número de átomos, o que leva a maiores perdas. Nas restantes características, as emissões de ambas as cavidades eram idênticas na sua forma de pulso e dependência da largura de pulso com o bombeamento.

Com a emissão da cavidade CW de meio ativo de 1.5 mm de espessura, observaram-se os modos longitudinais da radiação. Observaram-se conjuntos de picos nas regiões de 1532 nm e 1555 nm, e o número de modos de cada conjunto mostrava uma conexão com o bombeamento do laser microchip, apesar de não se ter tentado obter uma correlação. Estas cavidades emitiam em modo QCW, com a mesma frequência da emissão do bombeamento e larguras de pico na mesma ordem de grandeza. No entanto, verificou-se que a largura de pico das emissões da cavidade crescia de forma não linear com o bombeamento, não chegando a atingir o valor da largura de pico da emissão do bombeamento.

As outras duas configurações utilizaram os mesmos meios ativos e espelhos de saída, com a adição do meio saturável. As emissões resultantes foram pulsadas. As emissões foram caracterizadas usando os

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mesmos métodos para a caracterização das cavidades anteriores. Verificou-se que as eficiências das cavidades pulsadas eram menores, sendo que a cavidade com o meio ativo de 1 mm de espessura teve eficiência de 8.5% e a cavidade com meio ativo de 1.5 mm de espessura teve eficiência de 6.0%, notando-se novamente que a maior espessura do meio ativo resultou numa menor eficiência. Os modos longitudinais observados nestas cavidades encontravam-se na região dos 1532 nm, não havendo picos visíveis na zona dos 1555 nm. No sinal ótico em função do tempo observado, devido à presença do meio saturável dentro da cavidade, a sua emissão tinha uma forma de curva de sino, sendo diferente da emissão de bombeamento, que tinha uma forma quase quadrada. A largura dos pulsos, medida a meia altura destes, estava na ordem dos 10 ns. As energias e potências de pico variaram entre 9.85 µJ e 38.6 µJ e entre 966.4 W e 3185.4 W, respetivamente, dependendo da configuração da cavidade. A emissão pulsada destas cavidades era constituída por vários micro-pulsos, cujo número dependia do bombeamento.

Para a cavidade fixa foram usados o meio ativo com 1 mm de espessura, o meio saturável e o espelho de saída. Esta montagem teve somente como base a montagem das cavidades de componentes móveis realizada anteriormente. Como começo, desenhou-se a montagem em SolidWorks. Isto permitiu desenhar peças optomecânicas necessárias à montagem, caso não estivessem disponíveis, e também permitiu visualizar o aspeto do processo de montagem. Após se chegar a um desenho de montagem capaz de produzir a cavidade, fabricaram-se as peças adicionais. Estas estavam separadas em dois grupos: um grupo para segurar e aquecer o substrato, referido como sistema de aquecimento, e outro para segurar os vidros óticos, referido como pinças.

O sistema de aquecimento tinha de ser capaz de segurar o substrato a uma distância adequada da lente de focagem, cerca de 35 mm, enquanto fornecia espaço suficiente para as pinças dos vidros óticos. Este sistema também tinha de aquecer o substrato a 66 °C durante 120 minutos, visto que estas eram as especificações da cola a usar. Para isso foi usado um TEC para aquecer o substrato e um termístor para determinar a sua temperatura. O TEC foi conectado a uma fonte de alimentação e o termístor foi ligado a um controlador de TEC que reportava a temperatura do sistema no computador. Cinco peças de alumínio, a contar com o substrato, e uma peça de plástico foram desenhadas e fabricadas para o sistema de aquecimento.

As pinças para os vidros óticos tinham de ter dimensões adequadas à montagem dos vidros. A sua forma teve em conta onde estas seriam montadas de forma a dar possibilidade de ajustes de inclinação ao vidro montado. O seu volume foi minimizado de forma a evitar colisões entre peças durante a montagem e alinhamento da cavidade fixa. As pinças foram constituídas por uma peça redonda de alumínio e duas peças de plásticos que serviam de braços da pinça. Um dos braços estava fixo e o outro era móvel para segurar os vidros.

O processo de montagem da cavidade fixa provou ser bastante sensível a pequenas alterações de alinhamento e da distância entre os componentes da cavidade. Um total de três tentativas foram feitas para conseguir montar esta cavidade. A primeira tentativa iniciou-se com a colagem do meio ativo. Este procedimento envolveu a montagem do meio saturável e do espelho de saída ao mesmo tempo, após a colagem do meio ativo. Durante a colagem destes, a emissão da cavidade desapareceu e o aquecimento foi interrompido. Os dois vidros foram retirados do substrato e limpos de cola. Na segunda tentativa, o espelho de saída foi colado isoladamente com sucesso. Após a colagem do espelho de saída, tentou-se colar o meio saturável. A colagem deste teve de ser novamente interrompida devido à perda da emissão.

No processo de retirar o meio saturável da cavidade, o meio ativo descolou-se. Numa terceira tentativa, o meio ativo foi colado no substrato que já continha o espelho de saída. Após o sucesso desta colagem,

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tentou-se alinhar o meio saturável. No entanto, este alinhamento não era estável o suficiente para uma nova colagem. Deu-se por terminada a montagem da cavidade externa fixa.

O resultado desta montagem foi uma cavidade ótica externa fixa com emissão CW. A caracterização desta cavidade provou que o processo de colagem não era adequado, pois este não mantinha o alinhamento dos componentes óticos. Esta conclusão seguiu-se da observação das reflexões dos vidros óticos no alvo branco e no valor de potência ótica de saída da cavidade sem realinhamentos.

Após estas observações, houve uma tentativa de realinhar a cavidade com o sistema de bombeamento de forma a maximizar a eficiência desta. Este realinhamento permitiu obter uma maior eficiência da cavidade fixa em relação à eficiência previamente obtida antes da colagem final.

Palavras-chave: laser microchip; Q-switching; eficiência; vidro de érbio; emissão laser pulsada

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Table of Figures

Figure 1.1: General workflow followed for this work. ... 3

Figure 2.1: Representation of the stationary waves inside the optical cavity [4]. (a) The interferences of the waves traveling in opposite directions inside the cavity due to the feedback. (b) The wavelengths allowed to propagate inside the cavity according to equation (2.1). (c) The longitudinal modes of the cavity permitted to propagate, with an intensity dependant on the reflectivity of the mirrors. ... 5

Figure 2.2: Diagrams to explain the three-level system behaviour taken from [2]. ... 6

Figure 2.3: Energy diagram of Er:Yb co-doped glass [2]. ... 7

Figure 2.4: Absorption spectrum for Er:Yb phosphate glass [2] ... 7

Figure 2.5: Absorption and stimulated emission of erbium. Figure (a) represents the absorption cross section spectra of erbium for polarization σ (black) and π (red) at room temperature. Figure (b) represents the cross section for absorption (black) and stimulated emission (red) for the π polarization at room temperature. Figure (c) represents the cross section for absorption (black) and stimulated emission (red) for the σ polarization at room temperature [9]. ... 8

Figure 2.6: Example of the transmission curve of a saturable absorber [2]. ... 9

Figure 2.7: Representation of three burst pulses, each with the same pattern of micro-pulses. Taken from [10]. ... 10

Figure 2.8: Representation of a QCW signal with certain quantities associated represented in the signal [13]. In our work we used a delta of 10ms and a T of 100ms. ... 11

Figure 2.9: Representation of the peak power and the average power of a signal [14]. ... 11

Figure 2.10: Drawing of the laser cavity with pulsed emission. ... 13

Figure 3.1: Test setup for the OCMC. ... 14

Figure 3.2: Multi-mode 14-pin butterfly laser diode with electrical connections to the Laser driver and TEC driver. Between the butterfly laser and the optical table was thermal paste. The black optomechanical part acted as a clamp, keeping pressure on the butterfly laser. ... 14

Figure 3.3: Equipment used to control the pumping laser. (a) Laser diode controller model LDC4005 Thorlabs, and (b) TEC controller model TTC001 Thorlabs. ... 14

Figure 3.4: Pump laser's optical fibre end connected to the collimator held by several optomechanical parts. ... 14

Figure 3.5: Photographs of the three glasses of the optical components. (a) The active medium with 1 mm thickness. (b) The output coupler. (c) The saturable absorber. ... 14

Figure 3.6: Assembly design in SolidWorks for the OCMC. ... 14

Figure 3.7: Components of the optical table used for this work. (a) Breadboard B6090A Thorlabs where the optomechanical parts were assembled. (b) Support for the beadboard PFR6090-7 Thorlabs. (c) Fully assembled optical table. ... 14

Figure 3.8: Optomechanical components used to move in all three axis the aluminium holders of the optical components. (a) Rail XT95SP-500 Thorlabs. (b) Rail carriage XT95RC4/M Thorlabs. (c) Translation stage PT1/M Thorlabs. (d) Right-angle bracket PT102/M Thorlabs. ... 14

Figure 3.9: A set of optomechanical parts assembled to allow for translation on all three axes. The set is secure to a rail carriage and placed on the rail. ... 14

Figure 3.10: Kinematic mount used to provide tilt adjustments to the optical components. (a) Kinematic mount model KM200 Thorlabs used for the optical glasses. (b) Kinematic mount model KS1T Thorlabs used for the focusing lens. ... 14

Figure 3.11: Design of the interface between M6 taps and M4 holes (a) in SW and (b) manufactured. ... 14 Figure 3.12: Set of aluminium cookies to hold the active medium for the OCMC. (a) SW model of the boxing part. (b) SW model of the lid part. (c) Photograph of the boxing part on the left and the lid part

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on the right. (d) Photograph of the assembly of both parts that constitute the system to hold the active medium for the OCMC... 14 Figure 3.13: Assembly of the active medium in its aluminium holder in SolidWorks. The brown body is the active medium. (a) Frontal view of the assembly. (b) Cut view. ... 14 Figure 3.14: Set of aluminium cookies to hold the output coupler and the Q-switch for the OCMC. (a) SW model of the boxing part. (b) SW model of the lid part. (c) Photograph of the Viton O-ring. (d) Photograph of the boxing part. (e) Photograph of the lid part. (f) Photograph of the assembly of both parts that constitute the system to hold the output coupler and the Q-switch for the OCMC. ... 14 Figure 3.15: Assembly of the active medium in its aluminium holder in SolidWorks. The brown body is the active medium. (a) Full view of the assembly. (b) Cut view with the output coupler (green). (c) Cut view with the saturable absorber (green), the O-ring (black) and the output coupler (blue)... 14 Figure 3.16: Examples of other optomechanical parts used to secure components of the assembly of the microchip laser. Images taken from the Thorlabs website. ... 14 Figure 3.17: Photograph of the final assembly. ... 14 Figure 3.18: One of the sets of equipment used to measure optical power. (a) Photodiode power sensor model S122C Thorlabs. (b) Photodiode power sensor model PM103A Thorlabs. ... 14 Figure 3.19: Another set of equipment used to measure optical power. (a) Thermopile power sensor model 919P-003-10 Newport. (b) Optical meter 1918-R Newport. ... 14 Figure 3.20: CCD camera used to observe pumping beam. ... 14 Figure 3.21: Flowchart of assembly process of the OCMC ... 14 Figure 3.22: Examples of the coatings of the optical components at the microscope. (a) The HR@1550nm coating on the output coupler. (b) The AR@975nm and HR@1550nm coating on the active medium with 1 mm thickness. ... 14 Figure 3.23: Reflections of the pumping laser beam on the detector card due to the HR@975nm coating on the top face of the active medium glass... 14 Figure 3.24: Lack of reflection of the pumping laser beam on the detector card due to the AR@975nm coating on the top face of the active medium glass. ... 14 Figure 3.25: Alignment of the pumping laser with the assembly axis and the alignment laser. Circled from left to right: the pump laser, the white target and the alignment laser. ... 14 Figure 3.26: Overlapping of the pumping (green) and the alignment (red) beams on the detector card.

... 14 Figure 3.27: Overlapped reflections of the collimator and focusing lens on the alignment beam. ... 14 Figure 3.28: Image of the pumping beam in its focal point captured by the CCD camera. ... 14 Figure 3.29: Establishment of the focal position of the pumping system using the CCD camera and the 3D printed parts for the effect. ... 14 Figure 3.30: Placement pf the aluminium cookie containing the active medium close to the plastic 3D printed part to be close to the focal point. ... 14 Figure 3.31: Photograph of the complete assembly of the optical cavity with movable components, regardless of whether they had the Q-switch placed or not. Legend: 1 – Pumping laser; 2 – Focusing lens; 3 – Active medium in its aluminium holder; 4 – Output coupler (and Q-switch for the pulsed cavities) in its (their) aluminium holder. ... 14 Figure 3.32: Photographs of the assembly of the OCMC. Image (a) is from the perspective of the pumping laser. Image (b) is the view of the aluminium holder for the output coupler and the Q-switch.

... 14 Figure 3.33: Curve of the pumping laser's optical power vs the current applied for the pumping of the OCMCAM1CWE. ... 14 Figure 3.34: Characterization of the microchip laser emission for the OCMC with active medium 1 mm thick and CW emission... 14

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Figure 3.35: Observation of several pulses of the OCMC with active medium 1 mm thick and CW

emission. ... 14

Figure 3.36: Observation of a single pulse width of the emission of the OCMC with active medium 1 mm thick and CW emission. ... 14

Figure 3.37: Chart of the pulse FWHM of the microchip laser emission vs the pumping of the OCMC with active medium 1 mm thick and CW emission. ... 14

Figure 3.38:Characterization of the pumping laser for the OCMC with active medium 1.5 mm thick and CW emission. ... 14

Figure 3.39: Characterization of the microchip laser emission of the OCMC with active medium 1.5 mm thick and CW emission... 14

Figure 3.40: Pulse FWHM of the microchip laser emission vs pumping for the OCMC with active medium 1.5 mm thick and CW emission. ... 14

Figure 3.41: Example of the pulse shape of the OCMC with active medium 1.5 mm thick and CW emission. ... 14

Figure 3.42: Spectral emission of the OCMC with active medium 1.5 mm thick and CW emission with 306.7 mW of pumping... 14

Figure 3.43: Spectral emission of the OCMCAM1.5CWE with 341.8 mW of pumping. ... 14

Figure 3.46: Characterization of the pumping system for the OCMC with pulsed emission and active medium 1 mm thick. ... 14

Figure 3.47: Characterization of the microchip laser emission with the OCMC with CW emission and active medium 1 mm thick. ... 14

Figure 3.48: Initial peak of the emission of the OCMC with active medium 1 mm thick and pulse emission observed with the oscilloscope. ... 14

Figure 3.49: Three burst of emission of the OCMC with active medium 1 mm thick and pulsed emission. Each burst has multiple micro-pulses. The number of micro-pulses per burst increased with pumping. ... 14

Figure 3.50: One burst with several micro-pulses of the emission of the OCMC witch active medium 1 mm thick and pulsed emission. ... 14

Figure 3.51: Emission spectrum for the OCMCAM1PE with 225.6 mW of pumping. ... 14

Figure 3.52: Emission spectrum for the OCMCAM1PE with 245.0 mW of pumping. ... 14

Figure 3.53: Emission spectrum for the OCMCAM1PE with 283.7 mW of pumping ... 14

Figure 3.54: Characterization of the pumping system for the OCMCAM1.5PE. ... 14

Figure 3.55: Characterization of the emission of the microchip laser of the OCMCAM1.5PE. ... 14

Figure 3.56: Observation of three bursts of the emission of the OCMCAM1.5PE, each burst with two micro-pulses. ... 14

Figure 3.57: Isolation of one of the bursts of the emission of the OCMCAM1.5PE. ... 14

Figure 3.58: First micro-pulse of the burst of the emission of the OCMCAM1.5PE... 14

Figure 3.59: Emission spectra of the OCMCAM1.5PE with pumping 344 mW. ... 14

Figure 3.60: Emission spectra of the OCMCAM1.5PE with pumping 432 mW. ... 14

Figure 4.1: Assembly of the heating system for the substrate of the fixed optical cavity in (a) SolidWorks and (b) real assembly. ... 14

Figure 4.2: Parts of the holding and heating system of the substrate of the fixed optical cavity, their SolidWorks design and manufactured selves. ... 14

Figure 4.3: Clamp's system to hold the glasses to be assembled in (a) SolidWorks and (b) real assembly. ... 14

Figure 4.4: Parts designed and manufactured for the clamps of the glass optical components... 14

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Figure 4.5:Flowchart of the work performed for the fixed optical cavity. ... 14 Figure 4.6: Simulation in SolidWorks of the assembly of the active medium on the substrate of the fixed optical cavity. ... 14 Figure 4.7: Zoom in on the simulation on SolidWorks of the assembly of the active medium on the substrate of the fixed optical cavity. ... 14 Figure 4.8: Simulation in SolidWorks of the assembly of the output coupler and the Q-switch on the substrate of the fixed optical cavity. ... 14 Figure 4.9: Zoom in on the simulation on SolidWorks of the assembly of the output coupler and the Q-switch on the fixed optical cavity. Glass 1 is the active medium already glued. Glasses 2 and 3 are being glued. Glass 2 is the Q-switch and glass 3 is the output coupler. ... 14 Figure 4.10: Test of the fixed optical cavity as a result of the test of the full procedure. The substrate is made of plastic and the three glasses are glued on top of the substrate and parallel to each other.

Photographs (a) and (b) are of the same substrate with test glasses glued to it seen from different angles.

... 14 Figure 4.11: Alignment and gluing of the active medium on top of the substrate. Images (a) and (b) are photographs of the active medium near the focal point with glue on its edge. Images (c) and (d) are photographs of the active medium glued on the substrate... 14 Figure 4.12: Characterization of the pumping system for the fixed optical cavity. ... 14 Figure 4.13: Characterization of the fixed optical cavity before gluing the Q-switch and the output coupler to the substrate. ... 14 Figure 4.14: Three bursts of the emission of the fixed optical cavity before the gluing of the Q-switch and the output coupler. ... 14 Figure 4.15: A single burst of the emission of the fixed optical cavity before the gluing of the Q-switch and the output coupler. ... 14 Figure 4.16: A micro-pulse of a burst of the emission of the fixed optical cavity before the gluing of the Q-switch and the output coupler. ... 14 Figure 4.17: Emission spectrum of the fixed optical cavity before the final gluing of the Q-switch and the output coupler with 264.4 mW of pumping. ... 14 Figure 4.18: Emission spectrum of the fixed optical cavity before the final gluing of the Q-switch and the output coupler with 285.2 mW of pumping. ... 14 Figure 4.19: Emission spectrum of the fixed optical cavity before the final gluing of the Q-switch and the output coupler with 326.7 mW of pumping. ... 14 Figure 4.20: Gluing process of the Q-switch (the glass labelled 2) and the output coupler (the glass labelled 3) simultaneously during the first attempt at the assembly of the fixed optical cavity. The active medium has 1 mm of thickness and is labelled 1, already glued. The output coupler is the thickest glass seen to the right, secured in the L shaped arms of the clamp. The Q-switch is the thinnest glass seen in the middle, secured in the Z shaped arms of the clamp. ... 14 Figure 4.21: Gluing of the output coupler during the second attempt at the fixed optical cavity. Image (a) is the glue in place on the substrate. Image (b) is a top view of the gluing of the output coupler. .. 14 Figure 4.22: Photographs of the result of the gluing of the output coupler during the second attempt at the assembly of the fixed optical cavity. ... 14 Figure 4.23: Gluing of the Q-switch in the second attempt of the assembly of the fixed optical cavity.

... 14 Figure 4.24: Gluing of the active medium on the fixed optical cavity during the third attempt. The pumping is turned to maximum to guarantee a good alignment between the two glasses. The green glow on the active medium is due to its fluorescence. ... 14

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Figure 4.25: Fixed optical cavity still assembled on the substrate holder. Two optical components are glued: the active medium to the left, labelled 1, and the output coupler to the right, labelled 2. The third glass is the Q-switch, labelled 3, still in its clamp. ... 14 Figure 4.26: The fixed optical cavity with CW emission on the substrate holder being pumped for characterization. The green on the active medium is its fluorescence due to the pumping of the glass.

... 14 Figure 4.27: Characterization of the CW fixed optical cavity resulting from the third attempt at a fixed pulsed optical cavity. ... 14

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List of Tables

Table 2.1: Refractive indexes of the material used for the optical glass components of the laser cavity.

... 13 Table 3.1: Characteristics of the pumping laser beam for the OCMC. ... 14 Table 3.2: Pulse characteristics of the OCMC with active medium 1 mm thick and pulsed emission. 14 Table 3.3: Cavity length calculated using the free spectral range for the OCMCAM1PE. ... 14 Table 3.4: Energy and peak power results for the emission of the OCMCAM1.5PE. ... 14 Table 3.5: Cavity length calculated using the free spectral range for the OCMCAM1.5PE. ... 14 Table 3.6: Compilation of results obtained from the OCMC regarding their slope efficiency and pumping threshold. ... 14 Table 4.1: Characteristics of the pumping laser beam for the fixed optical cavity. ... 14 Table 4.2:Characetristis of the emission of the fixed optical cavity before gluing the Q-switch and the output coupler. ... 14

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List of Acronyms

Acronyms Definition

AM Active medium

CCD Charge-coupled device

CW Continuous wave

EM Electromagnetic

EMF Electromagnetic field

FWHM Full width at half measure

OC Output coupler

OCMC Optical cavity(ies) with movable component(s)

OCMCAM1.5CWE Optical cavity with movable components with active medium 1.5 mm thick and CW emission

OCMCAM1.5PE Optical cavity with movable components with active medium 1.5 mm thick and pulsed emission

OCMCAM1CWE Optical cavity with movable components with active medium 1 mm thick and CW emission

OCMCAM1PE Optical cavity with movable components with active medium 1 mm thick and pulsed emission

QCW Quasi-continuous wave

QS Q-switch

SA Saturable absorber

SW SolidWorks

TEC Thermoelectric cooler

TOF Time of flight

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1. Introduction

The sky has been an important part of everyday life ever since the beginning of humanity. The Sun provides necessary energy to sustain life and the night sky has been used as a tool to tell the passage of time, such with the seasons, as well as orientation on the Earth’s surface. Space exploration has been an ongoing quest that started in the 20^th century and will probably continue far beyond today. Such exploration has yielded great feats, such as the Moon landing and Voyage spacecrafts. The work behind the equipment required for these missions is of paramount importance in order for the mission to be safe and successful.

A great number of equipment is necessary for the success and general completion of a space mission. In order for the spacecraft to avoid collisions, it must be aware of its distance to certain objects. A way to accomplish this is to use a LiDAR, an electromagnetic radiation-based instrument that uses time of flight (TOF) of photons as they travel from the instrument’s photon emitter to the surface of an object and back to the instrument’s photon detector to determine the distance between the spacecraft and said object.

As mentioned, this instrument requires a photon emitter. For this application, lasers are used for their capability to emit coherent light. Because space is a very harsh environment, the lasers for space missions must be able to sustain certain adversities and remain functional throughout the mission’s lifetime.

Microchip lasers are often used for this purpose. These are solid-state lasers with optical cavities in the range of millimetres. They can be optically pumped using a laser diode and there are already configurations for passively Q-switched microchip lasers. These reasons make them very practical for space applications for they are compact and sturdy. Understanding how the functioning of a microchip laser changes with different optical components means the ability to choose wisely the configuration of a laser according to the specifications of the mission.

A LiDAR instrument can also be used to map surfaces on Earth, whether by airplane or by a self-driving car. For safety reasons, the wavelength of the emission of the LiDAR can not cause significant damage to the eye. Considering this, erbium ions will be used to produce the laser radiation during this work.

The erbium ions emit in the 1555 nm wavelength, which is considered “eye-safe”.

In this work, a manufacturing assembly and process will be designed and tested to produce a microchip laser cavity with pulsed emission, like the one in [1]. An erbium glass as active medium, a Co-spinel glass as a Q-switch and an external output coupler will be used for the purpose.

The materials will be assembled to make different configurations of the cavity. These will allow for the exploration of a cavity using these materials through different angles. With four different pieces of glass, four configurations of optical cavities will be assembled and their results compared to each other. This will be divided into two parts, each with its own purpose. In the first part, the study of the effects of certain aspects of the optical components will take place. In the second part, the assembly of a fixed optical cavity with the purpose of eventually becoming a part of a microchip laser will be attempted.

In the first part of this work, four optical components will be used: an erbium and ytterbium doped phosphate glass with 1 mm of thickness and an erbium and ytterbium doped glass with 1.5 mm of thickness as active media, a silica glass with a reflective coating as an output coupler and Co-spinel glass as a Q-switch. The doping concentrations and reflective coatings are the same in both glasses for the active medium. Four different cavities will be assembled and characterized.

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The first cavity with be composed by the erbium an ytterbium doped glass with 1 mm thickness as an active medium and the silica glass with a reflective coating as an output coupler. It is expected this cavity to emit CW radiation. The second cavity will use the erbium glass with 1.5 thickness as an active medium and the same output coupler. It is also expected to obtain CW emission from this configuration.

The third cavity will use the same components as the first cavity with the addition of the Co-spinel glass as a Q-switch. With this configuration, pulsed emission is expected to occur. The fourth cavity will use the same components as the second with the addition of the Q-switch. Pulsed emission is also expected.

After the assembly and alignment of each cavity during the first part of the work, characterization of their emissions will take place. Comparisons between these results will be performed.

The second part of this work will be the assembly of a fixed optical cavity by gluing the optical components to a common substrate. This will allow for an easier integration of the cavity with the other components of the microchip laser. An assembly table will be designed for this purpose and a procedure will be drawn. The resulting cavity will be characterized.

This dissertation will be divided into five chapters.

1. Introduction. In this chapter the subject of the dissertation is introduced in a concise manner.

The overall objectives are explained and a brief explanation of the methodology is given.

2. Theoretical aspects of the microchip laser. This chapter is dedicated to explaining key concepts necessary to understand how lasers work, what microchip lasers are, how to employ a passive Q-switching technique, what laser characteristics will be measured and the overall work performed for this dissertation.

3. Optical cavities with movable components. In this chapter it is detailed the material required and the procedures behind the assemblies and measurements of the optical cavities with movable components.

4. Fixed optical cavity. In this chapter it is detailed the material required and the procedures behind the assembly and measurements of the fixed optical cavity.

5. Discussion. In this chapter, the results obtained are discussed. Comparisons between all of the configurations are done and conclusions are drawn. The fixed optical cavity assembly process will be discussed as well, and improvements to it are suggested.

6. Conclusion. The final conclusions are drawn and suggestions for future work are provided.

The general workflow for this dissertation is represented next in figure 1.1.

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Figure 1.1: General workflow followed for this work.

Test setup for the OCMC

➢ Analyses of previous test setups.

➢ Design of the test setup in SW.

➢ Assembly of the OCMC.

Characterization of cavities

➢ Characterize CW cavities.

➢ Characterize pulsed emission cavities.

Rework test setup for fixed cavity

➢ Design new test setup in SW.

➢ Perform dummy test setup.

Alignment of the fixed optical cavity

➢ Align and glue AM

➢ Align QS and OC

➢ Characterize emission

➢ Glue optical components

Characterization of glued cavity

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2. Theoretical aspects of the microchip laser

2.1.

Laser principles

The word laser stands for light amplification by stimulated emission of radiation, and it functions based on three principles: stimulated emission, the optical cavity and population inversion.

2.1.1. Stimulated emission

The interactions between photons and electrons bound to atoms can be classified into three different categories: absorption, spontaneous emission and stimulated emission. To simplify the explanation of these phenomena, a two-level energy system will be considered. Each atom has multiple energy levels the electrons can occupy. Considering only two, 𝐸₁ and𝐸₂, 𝐸₂ being to highest energy level.

Absorption occurs when the photon interacts with the electron and transfers its energy to the electron, making it rise in energy levels. If an electron occupies a state in 𝐸₁ and a photon with energy 𝐸₂− 𝐸₁ interacts with the electron, the latter particle will leave the state in 𝐸₁ and climb to a state in 𝐸₂, thus becoming excited. Spontaneous emission can be seen as the reverse of absorption, when an electron emits a photon when transitioning from its original energy level,𝐸₂, to a lower energy level, 𝐸₁. The photon emitted has energy 𝐸₂− 𝐸₁.

Stimulated emission is an interaction between a photon with the right energy and an electron in an excited state. The result is a transition of the electron to a lower energy level and the emission of a photon that is a clone of the original photon. From this interaction follows the electron in a lower energy level and two photons identical to each other. In this case, the electron is in 𝐸₂ and interacts with a photon, which makes the electron transition to 𝐸₁ and emitting a second photon identical to the first photon.

2.1.2. Optical cavity

The optical cavity is the other essential element to the workings of a laser. This cavity allows for several occurrences of stimulated emission, which result in coherent light. This cavity is constituted by the optical resonator (the mirror structure) and the optical amplifier (the active medium), and it encompasses two opposite plane or curved reflectors oriented to make a right angle with the axis of the active medium [2]. This cavity can take many forms, but in this case, it is composed by two reflectors parallel to each other, and the active medium in between them. The reflectors can have the same reflectivity or not, but the light from the cavity needs to exit it. Some lasers have reflectors with different reflectivity:

one with a reflectivity very close to 100%, often called the total reflector or input coupler, and another with a lower reflectivity, often called the partial reflector or output coupler, whence the laser cavity light emerges. The optical resonator is assembled so that the radiation losses inside the cavity are compensated by the emission of the active medium [2]. The fluctuation of the losses in the cavity or fluctuations of the pump lead to fluctuations of the amplitude of the laser light [3]. It is the resonator that defines several characteristics of the laser emission, such as the spatial, directional and spectral characteristics [2]. The active medium defines other characteristics, such as wavelength and emission energy.

In the case of an optical cavity made by two plane-parallel mirrors, the radiation inside it interferes with itself as the feedback that leads to radiation gain occurs. The electromagnetic field (EMF) inside this cavity makes it so that the resulting waves can be seen as stationary, since the EMF at the mirrors is zero. Because of this, the cavity only permits the propagation of EM waves with certain wavelengths, which are determined by the cavity length. The wavelength of these waves corresponds to an integer number of half wavelengths. These are called axial or longitudinal modes [4]. The relation is as follows.

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2𝑛) = 𝐿 (2.1)

In equation (2.1), 𝐿 is the cavity length, 𝜆

⁄2 is the half wavelength, 𝑛 is the refractive index of the optical cavity and 𝑚 is an integer number.

Figure 2.1: Representation of the stationary waves inside the optical cavity [4]. (a) The interferences of the waves traveling in opposite directions inside the cavity due to the feedback. (b) The wavelengths allowed to propagate inside the cavity according to equation (2.1). (c) The longitudinal modes of the cavity permitted to propagate, with an intensity dependant on the reflectivity of the mirrors.

2.1.3. Population inversion

For the laser to work, the active medium needs to be pumped. This pump results in the electrons of the active medium to become excited (higher energy states). These states are necessary to have stimulated emission. For a simpler explanation, consider an atom model with only two energy levels, 𝐸₁ and𝐸₂. Each level has a certain number of occupied states. In a situation where no electrons ever break away from the atom, each one is either in 𝐸₁ or𝐸₂. At any given moment, there are 𝑁₁ electrons in 𝐸₁ and 𝑁₂ electrons in 𝐸₂. Systems in thermal equilibrium are found to have more occupied states in 𝐸₁ than in 𝐸₂, meaning 𝑁₁> 𝑁₂. This results in the absorption of radiation.

During this pumping, energy is provided to the electrons in 𝐸₁, making them transition to 𝐸₂. As the electrons are pumped into higher energy levels, some lose the energy they gained in the form of spontaneous emission, returning to the lower energy level. While 𝑁₂ is lower than 𝑁₁, the laser does not emit coherent light. The laser only starts to emit when population inversion occurs, which is when 𝑁₂> 𝑁₁.

While population inversion is maintained, there is gain in the active medium. When a photon with the right amount of energy encounters an electron in a higher state of energy, it can induce stimulated emission. From this follows two identical photons. Each one of these then induces another stimulated emission, from which results four identical photons. This process occurs several times until the photons reach one of the reflectors. After being reflected, they then continue to induce stimulated emissions.

Because the active medium is being constantly pumped to maintain population inversion, the photons can continue to induce stimulated emissions throughout their path within the optical cavity. This gain then reaches a point when these photons, which are identical in frequency, energy and direction, exit the cavity, by the output coupler that is partly reflective. In this way the emission of coherent light by the laser is achieved.

2.2.

Microchip Laser

The microchip laser is a plane parallel type laser diode pumped solid state laser, where the active medium is a solid, rather than a liquid or a gas. In its simplest form, this type of laser is composed of a short length active medium, ranging from a few hundreds of µm to a few mm, with the cavity reflectors

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in the form of coatings on each side of the active medium [5]. Its structure makes it a Fabry-Perot type of laser [4].

The cavity components’ geometry and the coatings on the input and output plane faces of the active medium give the microchip laser its plane parallel structure. Since the active medium is short, the reflectors are very near each other, allowing this type of laser to have very few longitudinal modes. The short cavity also excludes the possibility of an interior pumping system. For microchip lasers, the pumping is often executed in end-pumped form by a laser diode. In this end-pumped, or longitudinal pump, form, the emission of the pumping laser diode and the laser radiation obtained through the microchip laser have the same direction [2]. Due to the structure of lasers diodes, their radiation is collimated and focused onto the active medium’s input face.

2.2.1. Er:Yb co-doped phosphate glass

Microchip lasers often use Erbium glass for the advantage provided by the emission wavelength. A Er-doped phosphate glass originates an emission with wavelength around 1.54 µm, or 1550 nm, which is absorbed by water and considered “eye-safe”, making this emission appropriate for rangefinders, or LiDAR [2]. In the case of LiDAR, the wavelength brings the benefit of being incorporated in a transmission window in Earth’s and Mars’s atmospheres [6]. By making the microchip laser’s active medium of this material, the resulting emission will have 1550 nm wavelength with the added benefit of already existing tested configurations for Q-switching of lasers of this type [5, 7].

The diagrams in figure 2.2 provide assistance in an explanation of a optically pumped three-level system, such as an Er:glass. In a three-level system, most occupied states are on the ground level, level 1 in figure 2.2. During pumping of the system, electrons transition through absorption to band level 3, which, in reality, is composed of several energy levels. The multiple levels on this band permit pumping of a wider spectral range. Most of these newly occupied states in level 3 transition to level 2 by non-radiative transmissions, by emitting a phonon that transmits the energy to the lattice of the material. It is the transition from level 2 to level 1 that provides the laser radiation. If the pumping is below the threshold, the transitions from level 2 to level 1 occur by spontaneous emission. Above the pumping threshold, some of these transitions are made through stimulated emission that then turns to laser radiation [2].

Figure 2.2: Diagrams to explain the three-level system behaviour taken from [2].

The disadvantage of the three-level system is the need for more than half the electrons in level 1 to be raised to level 2 and loss of energy due to the transference of energy to the lattice in the transitions from level 3 to level 2 [2].

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For erbium glass to emit laser radiation at room temperature, it needs to be co-doped with ytterbium, due to the weak pump radiation absorption and three-level nature of erbium.

Figure 2.3: Energy diagram of Er:Yb co-doped glass [2].

Figure 2.3 represents the workings behind the Er:Yb co-doped phosphate glass as a laser active medium.

In Er:Yb co-doped glass, the photons from the pumping, with wavelength around 975 nm, are absorbed by the electrons on the level ²𝐹_7/2, making them rise to the ²𝐹_5/2 level on the Yb³⁺ ions. Due to the overlap of the Yb³⁺ ions ²𝐹_5/2 level and the Er³⁺ ions ⁴𝐼_11/2 level, the electrons from the former transition to the latter. After this transition, they decay to the Er³⁺ ions ⁴𝐼_13/2 level, where they can undergo stimulated emission and fall to the 4𝐼15/2 level, thus producing the laser radiation of wavelength around 1550 nm. This whole behaviour is possible because the transition from the Yb³⁺ ions to the Er³⁺ ions is must faster than the transition from the Yb³⁺ 2𝐹5/2 to 2𝐹7/2 levels [2].

Figure 2.4 represents the absorption spectrum of the Er:Yb where it is clear that pumping around 975 nm will lead to better absorption. Pumping for lasers using this material as active medium should be around this wavelength.

Figure 2.4: Absorption spectrum for Er:Yb phosphate glass [2]

The doping concentrations of both ions play a significant role in the workings of the laser.

Concentrations that are two low lead to a lack of energy transfer between the two ions and, consequently,

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loss of energy by spontaneous emission in the Yb³⁺ ions, as well as less absorption of the pumping due to the lack of ions. Concentrations that are too high lead to additional energy transfers and clustering, causing energy losses [8].

According to [9], the absorption and stimulated emission of the material of Er:glass is represented in figure 2.5.

Figure 2.5: Absorption and stimulated emission of erbium. Figure (a) represents the absorption cross section spectra of erbium for polarization σ (black) and π (red) at room temperature. Figure (b) represents the cross section for absorption (black) and stimulated emission (red) for the π polarization at room temperature. Figure (c) represents the cross section for absorption (black) and stimulated emission (red) for the σ polarization at room temperature [9].

From figure 2.5 it is possible to see multiple peaks of emission in both polarizations of the Er:glass. This means that the stimulated emission that occurs inside the active medium of this material does not have the same exact wavelength. Laser emission with several different wavelengths is then permitted to occur from the use of Er:glass as an active medium. The emitted wavelengths of the laser are then determined by the length characteristics of the cavity, such as cavity length of the thickness of the active medium.

2.3.

Passive Q-switching with a saturable absorber

Q-switching is a technique employed to reach emission of short high energy pulses. This is done by altering the quality factor Q of the optical cavity to store energy for later release. This factor is defined as the ratio between the energy stored and the energy lost in the optical cavity in each cycle [2].

For the deployment of this method, the quality factor of the cavity is lowered during the pumping process. Although the pumping allows for high energy storage and population inversion of levels far above threshold in the active medium, the losses are also high. This prevents the laser from emitting laser radiation. When the quality factor is reinstated to higher values, the energy stored is swiftly released in a very short time, forming a short pulse with energies far greater than if achieved without Q-switching [2].

The method of Q-switching can be employed in various ways: mechanical, electro-optical and passive Q-switches are examples of different way in which the Q-switching technique is implemented. In this work, passive Q-switching was chosen because of its advantages regarding space applications. Like the designation implies, the passive Q-switch does not required activation of some sort. These Q-switches are valued in space applications because they do not require electrical or mechanical switching devices, making the cavities that include them simpler, shorter and more robust [5]. The optical element with the Q-switch functionality is placed inside the optical cavity, between the active medium and the output coupler.

A passive Q-switch is an optical component whose transmission is dependent on the fluence of the incident radiation. An example of this behaviour is shown in figure 2.6 and it can be simply described

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as an increase of transmission with an increase of fluence. This element can be a cell filled with organic dye or a doped crystal [2].

Figure 2.6: Example of the transmission curve of a saturable absorber [2].

It is the type of transmission vs fluence curve of the material that makes it a passive Q-switch. Also, the Q-switch material must have high absorption of the cavity wavelength. At the beginning of the pumping, the fluence is low, which means the Q-switch transmission is also low. The Q-switch crystal increase the loses in the cavity [2] which makes the quality factor of the cavity low, since the feedback is low.

The low Q factor results in the accumulation of energy in the active medium (high number of excited electrons in the active medium). From this results high fluence. The consequence of high fluence has been described above. This can be referred to as the saturation or bleaching of the Q-switch, hence the name saturable absorber. Because of this, the Q-switch transmission becomes high, which raises the quality factor. The accumulated energy is released quickly, allowing the cavity to emit a short high energy pulse. After this pulse, the fluence returns to low levels, which returns the transmission of the Q-switch to low levels, thus restarting the cycle.

The characteristics of the other elements of the microchip laser, such as pumping and the reflectivity of the output coupler, have an influence over the Q-switching behaviour of the laser. Pumping of the laser can be done in a quasi-continuous wave mode, henceforth referred to as QCW, where the pumping emission is composed of short square or flat-top pulses. Short pumping pulses are preferred because their employment increases the efficiency of the resulting laser. Nevertheless, pulses that are too short do not deliver the same amount of energy, forcing the peak power of the pumping to be higher in order to compensate. The reflectivity of the output coupler needs to be taken into consideration as well. If the reflectivity of the output coupler is too high or the initial transmission of the saturable absorber is too high, the Q-switching method can lead to multiple pulses of microchip laser emission per pump pulse [2]. This multiple pulsed emission is called burst emission.

2.4.

Burst emission

Due to the characteristics of the microchip laser, its pulsed emission can, in reality, consist of multiple pulses, called a burst, per pulse of pumping. Within a burst, the pulses can be referred to as micro-pulses.

The micro-pulses are not necessarily equal between each other, each can have its own value of pulse width, peak power, energy and others. In most cases, the micro-pulses have a fixed repetition rate within the burst [10].

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Figure 2.7: Representation of three burst pulses, each with the same pattern of micro-pulses. Taken from [10].

2.5.

Alignment

The alignment of glass optical components for microchip lasers can be accomplished using a separate laser whose emission has a spectral range well within the visible spectrum. This laser, called the alignment laser, is itself aligned with the axis of the assembly of the microchip laser. Because the glass components have parallel face on each side, when they are inserted in the assembly axis, their frontal and back faces will reflect the alignment beam in a parallel form, according to the Law of Reflection.

Using a target with a hole for the alignment beam, the glass component can be tilted to make its reflections overlap the alignment beam. The overlap means the faces of the glass are perpendicular to the laser alignment beam, which means they are also perpendicular to the pumping beam.

2.6.

Laser characterization

Laser specifications can be characterized by several different methods. Different setup changes, such as the thickness of the active medium, the presence of a Q-switch and others, result in differences in the emission. The different emissions can be compared to reach conclusions regarding the elements of the cavity. Some of these include spatial and temporal coherence, monochromaticity, directionality or divergence and brightness [3]. However, these lie outside the scope of this work.

In this subsection, the measure and sequence calculation of some of laser key specifications are described.

2.6.1. Efficiency

2.6.1.1 Laser diode Efficiency

For a diode laser, the efficiency is defined as the slope of the output laser power vs electric power. Laser diode efficiency is also known as Wall-plug Efficiency [11]

2.6.1.2 Laser Slope efficiency (differential efficiency)

It is defined as the slope of the curve of the output laser power versus optical pump power. The curve starts with very low values for the output power until a certain threshold is reached. After this threshold, the curve becomes a line of proportionality between the output power and the pumping. The efficiency of the laser is the slope of this line [12].

2.6.2. Peak power

Peak power is, like the designation indicates, the power registered at the peak of the signal. In the case of lasers, it is the optical power at the peak of the laser emission. The value, at times, is not obtained through direct methods.

The signal from a QCW (quasi-continuous wave) emission with frequency 𝑓 = 1 𝑇⁄ and peak power 𝑃_{𝑝𝑒𝑎𝑘} can be represented by figure 2.8.

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Figure 2.8: Representation of a QCW signal with certain quantities associated represented in the signal [13]. In our work we used a delta of 10ms and a T of 100ms.

The definition of power is the energy rate (i.e., the energy per unit of time). In this type of signal, it is considered the power values when not at peak as negligible. This means that the energy of the pulse is concentrated in the signal at peak power, like represented in figure 2.8, with pulse width ∆𝑡. By the relation between energy and power, then

𝑃_{𝑝𝑒𝑎𝑘}= 𝐸

∆𝑡. (2.2)

Peak power is often difficult to measure directly. When using an optical power sensor, the value most frequently measured is the average power through the average of multiple samples taken in a very short time.

Figure 2.9: Representation of the peak power and the average power of a signal [14].

The average power considers the values of the energy for the ∆𝑡 interval and the values negligible for the 𝑇 − ∆𝑡 interval. Therefore, the average power can be defined as

𝑃_𝑎𝑣𝑔 =𝐸

𝑇. (2.3)

When solving equations (2.2) and (2.3) for 𝐸 and combining them, the result is

𝑃_{𝑝𝑒𝑎𝑘}∆𝑡 = 𝑃_𝑎𝑣𝑔𝑇. (2.4)

Rearranging the variables of (2.4), it becomes

∆𝑡

𝑇 = 𝑃_𝑎𝑣𝑔

𝑃_{𝑝𝑒𝑎𝑘}. (2.5)

The first fraction of equation (2.5) is known as Duty Cycle, which is the fraction of the pulse period where there is laser emission. So, to calculate the peak power using the average power measured and the duty cycle known, equation (2.6) is used [13].

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This is how to calculate the peak power using the average power and duty cycle of the QCW emission.

For pulsed emission, however, peak power must be calculated differently, since the profile of these pulses is not square or flat-top. Pulsed laser emission often has a shape akin to a bell curve, comparable to a gaussian or Sech2 functions. In these cases, the relation between the peak power and energy becomes

𝑃_{𝑝𝑒𝑎𝑘}≈ 0.94 𝐸 𝐹𝑊𝐻𝑀

(2.7)

where the FWHM replaces the pulse width in (2.2). This is an approximation to account for the different energy distribution in the emission [15, 16].

2.6.3. Pulse energy

The energy of a laser pulse is not measured directly in most cases. Instead, it is obtained through calculations using the known parameters of the emission and the optical power measured. It is an important measurement as shown in (2.7).

Power is the energy measure per unit of time. For fluctuating energy emissions, this unit is not constant.

For these cases, it is often best to measure the average power, which is the total amount of energy detected in a certain interval. Considering the interval to be the period of the pulse, the average power is simply the total energy registered during this interval over the period of the pulse, as shown in (2.3).

As such, the energy registered during the period of a pulse can be calculated with

𝐸 = 𝑃_𝑎𝑣𝑔𝑇. (2.8)

2.6.4. Cavity length

The length of the optical cavity can be determined by two methods. This length can be obtained by measuring the distance between the two mirrors of the cavity. It can also be obtained through measurement of the frequency separation of longitudinal modes, also known as free spectral range, while knowing the refractive indexes of each of the glass components.

To determine the length of the cavity using the second method, the optical path the light treks is calculated and used to the determine the physical length of the cavity.

The optical path the light treks in a certain material, 𝐿_optical, with refractive index 𝑛 and length 𝑙 is given by equation (2.9).

𝑙_{𝑜𝑝𝑡𝑖𝑐𝑎𝑙}= 𝑛𝑙 (2.9)

The microchip laser cavity can be composed by many different materials. The total optical path is simply the sum the optical paths for each material. The optical cavity of this work will be composed by the active medium, also known as AM, and the saturable absorber or Q-switch, also known as SA. The output coupler will not be taken into consideration because the light does not cross the silica glass while inside the cavity.

𝑃_{𝑝𝑒𝑎𝑘}= 𝑃_𝑎𝑣𝑔 𝐷𝑢𝑡𝑦 𝐶𝑦𝑐𝑙𝑒

(2.6)

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Figure 2.10: Drawing of the laser cavity with pulsed emission.

The laser cavity build during this work will have a configuration similar to the one shown in figure 2.10, so the optical path will be calculated using the following equation.

𝐿_{𝑜𝑝𝑡𝑖𝑐𝑎𝑙}= 𝑙₁𝑛_AM+ 𝑙₂𝑛_air+ 𝑙₃𝑛_SA+ 𝑙₄𝑛_air (2.10) The refractive indexes of the materials used in this cavity were considered to be the following.

Table 2.1: Refractive indexes of the material used for the optical glass components of the laser cavity.

Material Refractive index

Active Medium 1.52

Saturable Absorber 1.7162

Air 1

To calculate the cavity length using the free spectral range, ∆𝜆, the following approximation is used.

∆𝜆 ≈ 𝜆₀² 2𝑛𝐿

(2.11)

Reorganizing the previous equation results in the following.

𝐿 ≈ 𝜆₀² 2𝑛∆𝜆

(2.12)

𝑛_AM 𝑛_SA

𝑙2

𝑙₁ 𝑙₃

𝑙₄

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3. Optical cavities with movable components 4. Fixed optical cavity

5. Discussion

The objectives of the two parts of the work were different. However, the study of the OCMC prior to the manufacturing of the fixed optical cavity provided knowledge regarding this type of microchip laser and its alignment process. By performing this study, the laser characteristics of the fixed optical cavity were expected to be similar to the ones of the OCMC. It also gave a place to begin for the assembly procedure for the fixed optical cavity, since both test setups had many of the same parts. Even though this inspiration did not lead to a system capable of manufacturing a fixed optical cavity with pulsed emission, the result was satisfactory for a first approach, and it expected that the lessons learned lead to a successful next version of the setup.

It is important to highlight that an external fixed microchip laser cavity with QCW emission was assembled and the results of its efficiency were satisfactory when compared to the OCMC with the same configuration. The efficiency of the fixed optical cavity, with an active medium with 1 mm thickness and without Q-switch, was 16.7%, whereas the OCMCAM1CWE had a slope efficiency of 16.0%. The increase of the slope efficiency for the fixed optical cavity is likely due to the shorter cavity length. The threshold pumping of both were also similar: the OCMCAM1CWE had 29.4 mW of threshold pumping and the fixed optical cavity had 27.7 mW. The other emission characteristics were not analysed for the final fixed optical cavity due to the similarities between the fixed optical cavity and the OCMCAM1CWE and time constrains.

The attempts at the manufacturing of the fixed optical cavity provided the identification of issues with the assembly. Such brings forth the ability to resolve the issues and improve the setup.

Characterization and alignment of microchip lasers for space applications

2022