Ce travail de thèse concerne la réalisation d’une source VECSEL bi-fréquence émettant à 1550 nm pour les capteurs à fibres optiques à base de l’effet Brillouin. Nous présentons la conception et la réalisation de la source VECSEL bi-fréquence. Nous comparons les spécifications de notre source réalisée avec ceux recommandés pour la source utilisée avec les capteurs de Brillouin. Dans un premier temps, nous avons testé les structures VECSELs fabriquées au laboratoire LPN-CNRS dans le but de sélectionner la structure la plus performante pour notre cavité finale. La structure sélectionnée fournit une puissance de sortie ~200 mW avec un schéma d’évacuation de chaleur à travers le miroir de Bragg. Il est nécessaire de générer un décalage fréquentiel autour de 11 GHz, pour cela, dans un second temps, nous avons calculé l’accordabilité du décalage fréquentiel par plusieurs façons (la rotation de la lame biréfringente, la variation de la longueur de cavité et la variation de la température du cristal biréfringent). Nous avons trouvé qu’une accordabilité grossière (de l’ordre du GHz) peut-être assurée avec la rotation de la lame biréfringente alors qu’une accordabilité plus fine (de l’ordre de MHz) peut-être assurée avec la variation de la température du cristal ainsi que la variation de la longueur de cavité. Dans un troisième temps, nous avons démontré la possibilité de réaliser une émission bi-fréquence avec nos structures semi conductrices, néanmoins l’émission obtenue était instable. Après l’optimisation de la table optique l’émission obtenue observée sur un analyseur de spectre optique (avec une résolution de 1 GHz) était stable. Un meilleur contrôle de la stabilité de l’émission bi-fréquence est assuré avec la focalisation du diamètre du spot de pompage. Pour obtenir de faibles diamètres du spot de pompage, nous avons utilisé une diode laser de pompage monomode qui permet de pomper le mode fondamental de cavité. Les puissances de sorties ont été examinées. Nous avons remarqué qu’une puissance de sortie de 50 mW peut être obtenue en optimisant la réflectivité du miroir de sortie ainsi que la fonction de filtre et l’ISL de l’étalon Fabry-Pérot. Dans un quatrième temps nous avons examiné la stabilité de l’émission bi-fréquence avec une résolution ~ 1 GHz pour de différentes valeurs de recouvrement spatial entre les modes de cavité. Une émission bi-fréquence stable était observée avec un coefficient de recouvrement spatial allant jusqu’à 70%. Une examination de la stabilité avec une résolution de l’ordre de kHz était réalisée. Nous avons observé une largeur de raie de 200 kHz. Pour estimer la dérive du battement fréquentiel sur des temps longs, nous avons examiné l’évolution de l’enveloppe fréquentielle sur des intervalles temporels d’une minute. Une dérive de 0.8 MHz/minute était observée / In this thesis, we focus on the development of a dual-frequency VECSEL source operating at 1550 nm for Brillouin based optical fiber sensors. We present the design and the realization of this dual-frequency source. We compare the obtained results to the recommended specifications for Brillouin fiber sensors. In a first step, we have examined the output power of VECSEL structures fabricated in the LPN-CNRS labs. The selected structure ensure an output power of ~200mW using a downward heat dissipation scheme. For our application, it is crucial to ensure a frequency difference between the two cavity modes close to 11 GHz. For this reason, in a second step, we have calculated the frequency difference tunability with several ways (rotation of the birefringent crystal, temperature variation of the birefringent crystal and cavity length variation). A large tunability (GHz order) is ensured when the birefringent crystal is rotated. A weak tunability (MHz order) is ensured by varying the cavity length and the crystal temperature. In a third step, we have demonstrated the realization of a dual-frequency VECSEL at 1550 nm; however, the obtained laser emission was unstable. By optimizing our optical table, the observed dual frequency emission was stable (using a resolution ~1 GHz). A better control of the laser emission stability is ensured by a better focalization of the pump spot diameter. For this reason we have used a single mode laser diode as a pump source. This type of source ensures the pumping of the fundamental cavity modes only. By examining the influence of intra cavity elements on output powers, we have concluded that, an output power around 50 mW can be obtained using an output coupler of 99% reflectivity and a Fabry-Perot etalon with FSR= 15 nm and filter function close to 0.56 nm at 99.5% transmission. In a fourth step, we have examined the stability of the dual-frequency emission with a resolution ~1 GHz for different values of the coupling constant coefficient. A stable dual-frequency emission is obtained with a coupling coefficient between the modes up to 70 %. By investigating the stability with a kHz resolution we have obtained a FHWM close to 200 kHz.To estimate the jitter of the beat frequency obtained we have examined the evolution of the frequency envelope over a minute. A jitter of 0.8 MHz/minute was observed

Orientador: Paulo Clóvis Dainese Júnior / Dissertação (mestrado) - Universidade Estadual de Campinas, Instituto de Física Gleb Wataghin / Made available in DSpace on 2018-08-29T15:49:49Z (GMT). No. of bitstreams: 1 FlorezPenaloza_OmarEnrique_M.pdf: 35304158 bytes, checksum: f1b7fbb7d6036f3186dd9378d06e850e (MD5) Previous issue date: 2015 / Resumo: O Espalhamento Brillouin e um processo nao linear oriundo da interacao entre ondas opticas e acusticas. Este efeito foi amplamente estudado em fibras mono-modo e mais recentemente em uma grande variedade de micro-estruturas fotonicas. A habilidade de se fabricar estruturas que podem confinar ambas as ondas opticas e acusticas em dimensoes menores que o comprimento de onda criou novas oportunidades de se estudar a interacao foton-fonon. Um aspecto em particular que se torna importante em sistemas de alto confinamento e o efeito de deslocamento de borda (Shifting-Boundary), alem do efeito Elasto-Optico, mais bem entendido. Micro-fios de Silica sao ideais para estudar estes efeitos. Primeiro, quando seu diametro e menor que o comprimento de onda, o campo eletrico na superficie do guia de onda aumenta significativamente. Em segundo lugar, a interface ar-silica apresenta um alto contraste de indice de refracao, o que aumenta o espalhamento devido ao efeito de deslocamento de borda. Finalmente, a geometria cilindrica simples permite um calculo analitico da eficiencia do espalhamento Brillouin, considerando as perturbacoes tanto Elasto-Opticas como de Deslocamento de Borda. Nesta tese, estudamos teorica e experimentalmente o espalhamento Brillouin em microfios de Silica. Amostras com diametros de 0.6 a 3 ¿Êm foram fabricadas e caracterizadas utilizando um sistema de deteccao heterodina para os espalhamentos co-propagante e contra-propagante. Para o espalhamento Brillouin co-propagante, usamos a tecnica de bombeio e prova para induzir excitacao impulsiva dos modos acusticos proximos da frequencia de corte. Espalhamento devido a ondas acusticas do tipo Rayleigh foi observado e extensivamente caracterizado. Estas ondas sao particularmente interessantes, ja que a maior parte da energia acustica e concentrada proxima da superficie do guia de onda, o que aumenta a contribuicao do efeito de Deslocamento de Borda. Desenvolvemos estudos teoricos extensivos dos modos opticos e acusticos na geometria cilindrica, e aplicamos a teoria de modos acoplados para calcular a eficiencia de espalhamento para cada modo acustico. Um estudo da eficiencia de espalhamento em funcao do diametro foi feito, ajudando a entender melhor os mecanismos que determinam a evolucao do espectro Brillouin. Finalmente, fomos capazes de identificar modos nos quais o processo de espalhamento e dominado pelo efeito de Deslocamento de Borda, e modos nos quais o efeito dominante e o Elasto-Optico. Este entendimento pode contribuir no projeto de estruturas nas quais estes efeitos sao somados ou cancelados, e pode ser usado como outro mecanismo para controlar o processo de espalhamento Brillouin / Abstract: Brillouin scattering is a nonlinear process that arises from the interaction between op- tical and acoustic waves. This effect has been widely studied in standard single-mode fibers and more recently in a variety of photonics microstructures. The ability to fab- ricate structures that can confine both optical and acoustic waves in sub-wavelength dimensions has created new opportunities to study photon-phonon interaction. One particular aspect that becomes important in high-confinement systems is the effect of shifting boundaries, in addition to the better-understood elasto-optic effect. Silica mi- crowires are ideal systems to study these effects. First, when its diameter is smaller than the wavelength, the electric field overlaps strongly with the waveguide surface. Second, the air-silica interface presents high index contrast, which enhances scattering due to shifting boundary effect. Finally, the simple cylindrical geometry allows an- alytical calculation of Brillouin scattering efficiency considering both elasto-optic and shifting-boundary perturbations. In this thesis, we studied theoretically and experimentally Brillouin scattering in silica microwires. Samples with diameter ranging from 0.6 to 3 ?m were fabricated and char- acterized using heterodyne detection for both backward and forward Brillouin scattering. For forward Brillouin scattering, we used the pump and probe technique to induce impul- sive excitation of acoustic modes near cutoff. Scattering due to Rayleigh acoustic waves was observed and extensively characterized. These waves are particularly interesting as most of the acoustic energy is concentrated close to the waveguide surface, therefore enhancing the shifting boundary contribution. Theoretically, we developed extensive studies of optical and acoustic modes in cylindrical geometry, and applied coupled-mode theory to calculate the scattering efficiency for each acoustic mode. A study of the scat- tering efficiencies as a function of diameter was performed, helping better understand the mechanisms that determined the evolution of the Brillouin spectrum. Finally, we were able to identify modes in which the scattering process is dominated by shifting- boundary effect and modes in which elasto-optic dominates. This understanding may help design structures in which these effects add or cancel each other, and can be used as another mechanism to control Brillouin scattering process / Mestrado / Física / Mestre em Física / 1142161/2012 / CAPES

Acoustic modes guided by thin-film metal superlattices have been investigated using Brillouin spectroscopy. Samples were grown on both single-crystal sapphire and fused silica substrates by alternately sputtering two different metals to yield a total thickness in the range 0.3 - 0.5 μm. Structural and chemical characterization of the polycrystalline films was performed using x-ray diffraction. Rutherford backscattering and optical interferometry. Thermally excited acoustic waves in the metal film create a surface ripple which weakly interacts with light incident from a single mode argon laser. A tandem Fabry-Perot consisting of two synchronized 3-pass cavities is used to measure the frequency shift of light which is inelastically scattered from acoustic waves. The contrast ratio of this interferometer exceeds 10¹⁰ and provides sufficient stray light rejection to detect the surface Rayleigh wave and as many as 13 higher order acoustic modes. The elastic stiffness constants of the anisotropic superlattices were estimated by fitting the measured acoustic mode velocities to a parameterized acoustic model. A comparison is made between these elastic constants and those predicted from the properties of the separate bulk constituents. The dependence of bilayer wavelength on the elastic properties of both Cu/Nb and Mo/Ta superlattices over the range of roughly 10 to 200 Å was determined. The unexpected softening of Cu/Nb superlattices within a range of bilayer wavelengths near 20 Å which was reported previously is qualitatively similar to the measurements reported here. It is shown that the elastic stiffness coefficient with the largest variation is c₄₄. The stiffness variations determined for the Mo/Ta samples are much smaller than for Cu/Nb. It is suggested that this is due to either structural differences (Cu/Nb is fcc-bcc and Mo/Ta is bcc-bcc) or the smaller interfacial lattice mismatch for Mo/Ta. Interfacial strain is found to be strongly correlated with the stiffness variations of the Mo/Ta samples. However, the underlying cause of these variations in stiffness remains anomalous. This dissertation also reports the first observations of Love waves and Stoneley waves by Brillouin scattering. The purely transverse Love waves guided by Cu/Nb films were detected by elasto-optic scattering from the evanescent acoustic strain in the sapphire substrate. The stiffness coefficient c₁₂ of the hexagonally symmetric metal film cannot be determined by the other guided acoustic waves which ripple the surface. Molybdenum in contact with fused silica is predicted to support a Stoneley wave which is guided by the interface. The lowest order Sezawa made guided by a molybdenum film was found to evolve to the Stoneley wave as the film becomes thicker. These measurements together with measurements of the surface Rayleigh wave show that the stiffness of the sputtered metal films is quite homogeneous and independent of film thickness.

基於光纖非線性效應的光學信號處理在光纖通信和傳感中起著重要作用。在各種非線性效應中，光纖中的布里淵散射不僅被廣泛應用於高速通信信號的處理，而且被用於建立光纖傳感器。本文研究基於布里淵散射的光學信號處理新技術在光通信和傳感中的應用。 / 近年來由於慢光技術在實現時間延遲和光學信號處理中的廣泛應用，它吸引了廣泛的注意力。 在各種實現慢光的技術中，基於布里淵散射的慢光技術展示了很大的潛力，因為它具有在常溫工作以及與現有光纖系統兼容的優勢。但是由於布里淵泵浦和信號之間嚴格的頻率要求，大多數的研究工作是建立於一個泵浦延遲一個信號的基礎上，所以只能獲得一個被延遲的信道。本文提出了一種在一個布里淵泵浦的慢光系統中實現同時產生多個延遲信號的技術。這種技術應用了基於四波混頻的廣播效應。輸入信號被布里淵泵浦延遲的同時，延遲通過三個四波混頻泵浦的廣播效應傳遞給其他六個新產生的信道。這種慢光廣播技術可以被應用於並行光學信號處理，比如實現多信道同步以及時分複用。 / 光纖傳感技術為結構的健康提供了一種優秀的監測方法，尤其是溫度和應力的監測。在過去的二十年間，基於布里淵散射的傳感技術吸引了大批人的興趣，因為布里淵光纖傳感器擁有高分辨率，長距離監測以及監測範圍廣的優點。本文提出了一種新的基於布里淵慢光的溫度和應力傳感技術。布里淵頻移的溫度和應力相關性使得輸入光脉衝的延遲也與溫度和應力相關，因此我們通過測量這個延遲來監測溫度和應力。我們分別實現了對100米和2米單模光纖的溫度測量。隨後我們也實現了分佈式溫度和應力監測。通過設置泵浦和探測光脉衝之間的延遲時間，我們可以監測特定位置的光纖。因此，通過控制整個延遲時間，我們實現了對整個光纖的溫度和應力分佈的監測。相比于普通的布里淵光纖傳感器，我們這種技術擁有以下優點：更加直接簡單的實現監測，快速的反應時間以及實時監測的潛力。 / 波長轉換在路由和交換中起了很重要的作用。在各種波長轉換的技術中，基於四波混頻的波長轉換非常優越因為它具有對調製格式，比特率以及通信協議透明的優點。但是，四波混頻只有在各個光波的相速度匹配的情況下才能有效的產生。這種匹配條件很難在一個很寬的頻段內保持，因此四波混頻的轉換帶寬是很有限的。本文提出了一種基於零增益受激布里淵散射的方法來動態地控制四波混頻的相位匹配。 通過布里淵泵浦和斯托克斯光引入自我補償的受激布里淵增益和損耗，四波混頻的相位匹配條件可以被受激布里淵散射激發的折射率改變來靈活的控制，並且不會影響四波混頻初始的參數。我們把這種零增益受激布里淵散射應用于增大簡並四波混頻的帶寬，增強通信信息波長轉換的效果，全光調控非簡並四波混頻的帶寬，實現偏振不敏感寬帶波長轉換以及延長基於四波混頻波長轉換和色散的延遲線的最大延遲時間。 / 低噪聲寬帶放大可以通過光學參量過程來實現。雖然光參量放大器可以提供高至70分貝的增益，但是這種參量放大器經常受限於各個光波的相位不匹配。在本文中，我們把零增益受激布里淵散射用於光參量放大器來動態的控制它的增益譜。基於這種技術，我們動態地改變了傳統的“M“型增益譜，並且由此得到了非常平滑的增益譜，增益的變換量僅僅在0.1分貝以內。 / Optical signal processing based on fiber nonlinearities plays an important role in both fiber-optic communications and sensing. Among various nonlinear effects, stimulated Brillouin scattering (SBS) in optical fibers has been widely employed not only in processing of high-speed communication signals, but also in constructing fiber-optic sensors. This thesis investigates new techniques of optical signal processing based on SBS for fiber-optic communications and sensing. / In the recent years, slow light has attracted considerable interest because of its numerous applications, in realizing variable true time delay and in optical information processing. Among various slow light mechanisms, the SBS based slow light shows great potential in all-optical signal processing due to the advantages of room-temperature operation and device compatibility with existing fiber systems. However, owing to the tight requirement of spectral alignment between the SBS pump and the signal, most of the published works are for the case where one SBS pump is used to delay a single channel. Hence, only one delayed channel is obtained. In this thesis, we demonstrate a technique to simultaneously generate multiple delayed signals through four-wave mixing (FWM) wavelength multicasting in a single-pump stimulated Brillouin scattering (SBS) based slow light system. The signal delay is achieved with a SBS pump while at the same time the delay is transferred to six other channels by three FWM pumps employed for wavelength multicasting. This slow light multicasting technique may find applications in parallel optical information processing such as simultaneous multichannel synchronization and time division multiplexing. / Fiber-optic sensor techniques provide a promising approach for structure health monitoring, especially the temperature and strain monitoring. The technique based on Brillouin scattering has attracted much interest in the past two decades because Brillouin fiber sensors offer advantages of high resolution, long distance sensing, and large sensing range. In the thesis, we propose and experimentally demonstrate a new method for temperature/strain sensing using stimulated Brillouin scattering based slow light. The approach relies on temperature/strain dependence of the Brillouin frequency shift in a fiber, hence the time delay of an input probe pulse. By measuring the delay, temperature/strain sensing can be realized. We achieve temperature measurement for both a 100 m single mode fiber (SMF) and a 2 m SMF. Distributed temperature/strain sensing has been demonstrated later. The temperature/strain of a particular fiber section can be monitored by setting an appropriate relative delay between the pump and probe pulses. By controlling the relative delay, we have achieved distributed profiling of the temperature/strain along the whole sensing fiber. Compared to conventional Brillouin fiber sensors, our scheme has the merits of more straightforward implementation, fast response and potential of real-time monitoring. / Wavelength conversion plays an important role in wavelength routing and switching. Among various schemes for wavelength conversion, the one based on FWM is superior as it offers advantages in being transparent to modulation formats, bit-rates, and communication protocols. However, significant FWM can occur only if the phase velocities of the interplaying waves are matched. The matching condition can hardly be satisfied over a wide spectral range and hence the conversion bandwidth is often limited. In this thesis, we propose and experimentally demonstrate an approach to dynamically control the FWM phase matching condition by using gain-transparent SBS. By introducing self-compensation of optical gain/loss with SBS pump and Stokes waves, the FWM phase matching condition can be flexibly controlled through SBS induced refractive index change without affecting the initial parameters of the FWM. The gain-transparent scheme is employed to enlarge the degenerate FWM conversion bandwidth, enhance the performance in wavelength conversion of communication signals, all-optically manipulate non-degenerate FWM conversion bandwidth, achieve both polarization-insensitive and wideband operation in a dual orthogonal pump wavelength converter, and extend the maximum optical delay of a delay line based on FWM wavelength conversion and dispersion. / Low noise and broadband amplification are possible by using optical parametric processes. Although fiber-optic parametric amplifier (FOPA) can provide gain as high as 70 dB, its operation is often confined by phase mismatch of the interplaying fields. In this thesis, we apply gain-transparent SBS to a FOPA and dynamically control its gain profile. The conventional “M“ shape gain profile can be dynamically changed. Flattening of the gain profile to within 0.1 dB variation has been achieved. / Detailed summary in vernacular field only. / Detailed summary in vernacular field only. / Detailed summary in vernacular field only. / Detailed summary in vernacular field only. / Detailed summary in vernacular field only. / Wang, Liang. / Thesis (Ph.D.)--Chinese University of Hong Kong, 2013. / Includes bibliographical references. / Abstract also in Chinese. / ABSTRACT --- p.i / ACKNOWLEDGEMENT --- p.vi / TABLE OF CONTENT --- p.viii / Chapter 1 --- INTRODUCTION --- p.1 / Chapter 1.1 --- Overview of Optical Signal Processing --- p.3 / Chapter 1.2 --- Outline of the Thesis --- p.6 / References --- p.10 / Chapter 2 --- STIMULATED BRILLOUIN SCATTERING IN OPTICAL FIBERS --- p.15 / Chapter 2.1 --- Physical Process of Brillouin Scattering --- p.16 / Chapter 2.2 --- Stimulated Brillouin Scattering Under Steady-State Conditions --- p.19 / Chapter 2.3 --- The Brillouin Gain --- p.22 / Chapter 2.3.1 --- Complex Brillouin Gain --- p.22 / Chapter 2.3.2 --- Brillouin Gain Spectrum --- p.24 / Chapter 2.4 --- Threshold of Brillouin Scattering --- p.30 / References --- p.32 / Chapter 3 --- SLOW LIGHT BASED ON SBS IN OPTICAL FIBERS --- p.34 / Chapter 3.1 --- Introduction to Slow Light --- p.35 / Chapter 3.2 --- Slow Light based on SBS in Optical Fibers --- p.39 / Chapter 3.2.1 --- Mathematical Description --- p.39 / Chapter 3.2.2 --- Delay of Optical Signals by SBS based Slow Light --- p.42 / Chapter 3.3 --- Generation of Multichannel Delayed Pulses by FWM Assisted SBS Slow Light System --- p.46 / Chapter 3.3.1 --- Principle and Experimental Setup --- p.47 / Chapter 3.3.2 --- Results and Discussion --- p.51 / References --- p.58 / Chapter 4 --- SBS SLOW-LIGHT-BASED FIBER-OPTIC SENSOR --- p.64 / Chapter 4.1 --- Introduction to Fiber-Optic Sensors --- p.66 / Chapter 4.2 --- Principle of Fiber-Optic Sensor based on SBS Slow Light --- p.69 / Chapter 4.3 --- Temperature Sensing by SBS Slow Light for a Whole Segment of Fiber --- p.73 / Chapter 4.3.1 --- Temperature Sensing for a 100 m Single-Mode Fiber --- p.73 / Chapter 4.3.2 --- Temperature Sensing for a 2 m Single-Mode Fiber --- p.76 / Chapter References --- p.80 / Chapter 5 --- DISTRIBUTED TEMPERATURE & STRAIN SENSING USING SBS-BASED SLOW LIGHT --- p.82 / Chapter 5.1 --- Introduction to Distributed Brillouin Fiber Sensor --- p.84 / Chapter 5.2 --- Distributed Fiber-Optic Temperature Sensor Using SBS-based Slow Light --- p.91 / Chapter 5.2.1 --- Principle and Experimental Setup --- p.92 / Chapter 5.2.2 --- Results and Discussion --- p.94 / Chapter 5.3 --- Distributed Fiber-Optic Strain Sensor Using SBS-based Slow Light --- p.101 / Chapter 5.3.1 --- Principle and Experimental Setup --- p.101 / Chapter 5.3.2 --- Results and Discussion --- p.104 / References --- p.109 / Chapter 6 --- DYNAMIC CONTROL OF PHASE MATCHING IN FWM WAVELENGTH CONVERSION BY GAIN-TRANSPARENT SBS --- p.114 / Chapter 6.1 --- Phase-matching Condition in FWM --- p.116 / Chapter 6.2 --- Conversion Bandwidth Enlargement in Degenerate FWM Using Phase-Matching Control by Gain-Transparent SBS --- p.119 / Chapter 6.2.1 --- Principle and Experimental Setup --- p.120 / Chapter 6.2.2 --- Results and discussion --- p.125 / Chapter 6.3 --- Wavelength Conversion of Communication Signals Using Degenerate FWM with Gain-Transparent SBS for Phase-Matching Control --- p.131 / Chapter 6.3.1 --- Principle --- p.131 / Chapter 6.3.2 --- Wavelength Conversion for Amplitude-Modulated Signals --- p.133 / Chapter 6.3.3 --- Wavelength Conversion for Phase-Modulated Signals --- p.139 / Chapter 6.3.4 --- Discussion --- p.145 / Chapter 6.4 --- All-Optical Manipulation of Non-Degenerate FWM Conversion Bandwidth by Gain-Transparent SBS --- p.150 / Chapter 6.4.1 --- Principle and Experiment Setup --- p.151 / Chapter 6.4.2 --- Results and Discussion --- p.153 / Chapter 6.5 --- Enhanced Performance of Polarization-insensitive Wavelength Conversion through Dynamic Control of Optical Phase --- p.157 / Chapter 6.5.1 --- Principle and Experiment Setup --- p.157 / Chapter 6.5.2 --- Results and Discussion --- p.160 / Chapter 6.6 --- Extension of the Maximum Optical Delay using Gain-Transparent-SBS-Controlled FWM Wavelength Conversion and Group Velocity Dispersion --- p.165 / Chapter 6.6.1 --- Principle and Experiment Setup --- p.166 / Chapter 6.6.2 --- Results and Discussion --- p.169 / References --- p.174 / Chapter 7 --- DYNAMIC CONTROL OF GAIN PROFILE IN FIBER OPTICAL PARAMETRIC AMPLIFIER BY GAIN-TRANSPARENT SBS --- p.182 / Chapter 7.1 --- Introduction to FOPA --- p.184 / Chapter 7.2 --- Dynamic Gain Profile in FOPA Assisted by Gain-Transparent SBS --- p.187 / Chapter 7.2.1 --- Principle and Experimental Setup --- p.187 / Chapter 7.2.2 --- Results and Discussion --- p.190 / References --- p.196 / Chapter 8 --- THESIS SUMMARY AND FUTURE WORK --- p.200 / Chapter 8.1 --- Summary --- p.201 / Chapter 8.2 --- Future Work --- p.206 / References --- p.208 / APPENDICES --- p.i / Chapter Appendix A. --- List of Publications --- p.i / Chapter Appendix B. --- List of Figures --- p.iv

Includes bibliographical references (leaves 205-227) A study of phase conjugation by stimulated Brillouin scattering is presented with emphasis on the limiting factors, such as aperture and polarization losses, spatial coherence and saturation of the incident wave on the quality of phase conjugation, as well as the application of stimulated Brillouin scattering to loop phase-cojugated mirror and intracavity-SBS-cell-phase-conjugated oscillator.

Thesis (Ph.D.)--University of Adelaide, Dept. of Physics and Mathematical Physics, 2001. / Includes bibliographical references (leaves 205-227).

Random fiber lasers, a new type of fiber laser that uses disordered medium to provide distributed feedback, have drawn considerable interest in the photonics community over the past ten years. Stimulated Brillouin scattering (SBS), with a typical narrow spectral width of ~100 MHz, provides an important gain mechanism for random fiber lasers. Brillouin random fiber laser (BRFL) has shown excellent advantages in generating highly coherent photons and in ultrasound sensing. However, the accompanied large intensity noise in BRFLs hinders its further performance improvement and practical applications. In order to design a low noise BRFL, it is important to explore the fundamental physics behind BRFL and study its output characteristics. This thesis focuses on the study of random lasing mechanism in BRFL, which lays the foundation for the demonstration of a low noise BRFL. The main research results and contributions are as follows: (1) In order to understand the dynamic noise properties of BRFLs, the properties of the acoustic wave generated by BRFL, including its intrinsic spectral width, intensity dynamics, distributed spectrum and distributed intensity statistics are characterized for the first time. The characterization method is based on the SBS enhanced polarization decoupled four wave mixing process, where the pump wave, Stokes wave, probe wave and reflected probe wave are coupled through the fiber density variation induced by the acoustic wave. It is demonstrated that the intrinsic spectral width of the acoustic wave in the Brillouin gain fiber depends on the spectral convolution of pump light and Stokes light. Stochastic behaviour is introduced to the intensity dynamics of the acoustic wave when the linewidth of the pump light (or the Stokes light) is larger than several MHz. The distributed spectra of the dynamic grating are determined by the birefringence of the Brillouin gain fiber, which have maximum change on the order of 10-7 to 10-6 when the BRFL is on operation. Different proportion of optical rogue waves are detected at high gain position and low gain position near the lasing threshold, proving the nonlinear amplification of the SBS process. (2) In order to study the mode selection mechanism of the distributed random feedback and explore new physics phenomenon in BRFLs, the conventional Rayleigh scattering fiber in BRFL is replaced by the artificially controlled random scattering medium. First, weak FBG array with random spacing offers distributed feedback with varied length, which demonstrate the longitudinal mode filter function of the distributed random feedback. Single longitudinal mode operation of BRFL is realized by using appropriate length of the FBG array. Then, scattering from random fiber grating (RFG) with varied grating period is used to provide feedback for BRFL. The enhanced backscattering strength from RFG improves the slope efficiency of BRFL to 29.3% and reduces the lasing threshold to 10.2 mW. By calculating the correlation of the intensity fluctuation spectra from trace to trace, the correlation of two traces is found to be dependent on the specific two chosen traces, demonstrating the replica symmetry breaking phenomenon in photonics. (3) RFG with relatively large refractive index modulation shows potentials in improving the performance of the BRFL. In order to investigate the working mechanism of the RFG, optical frequency domain reflectometry (OFDR) with spatial resolution of 8 μm is employed to characterize the property of RFG. The backscattering strength and spectral response of RFG is highly related to the degree of randomness of RFG. Theoretically, entropy is introduced to build a quantitative relationship between the degree of randomness and backscattering strength of the RFG based on the transfer matrix method. A linear relationship between the average reflectivity of the RFG in dB scale and sub-grating’s entropy is found. Further, based on a polarization maintaining RFG, a low noise BRFL is proposed and demonstrated. Compared to Rayleigh scattering, the polarization maintaining RFG can tolerate environmental perturbation, leading to a 20 dB intensity noise suppression of the BRFL in the low frequency domain from 10 Hz to 1 kHz. (4) The dynamic properties of the slowly varying frequency drift of a dual-wavelength BRFL in polarization maintaining fiber are characterized. Two principal lasing peaks in each polarization are enabled by the combined distributed Rayleigh scattering and the Brillouin gain provided by the polarization maintaining fiber with large birefringence. Polarization dependent and polarization independent spectral variations are studied in the dual-wavelength BRFL due to the environmental perturbation and gain competition. The probability distribution of the lasing frequency exhibits a dip near the mean frequency that is caused by the spectral hole burning. By calculating the matrix of the Pearson correlation coefficient, the internal correlations between different part of random fiber laser spectra are found, which enhances the understanding of the fundamental physics of random lasing process.

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Le cadre général dans lequel s’insère ce travail de thèse est celui de l’étude de la diffusion Brillouin dans une nouvelle génération de fibres optiques à cristaux photoniques (PCFs). Ces fibres, qui présentent un arrangement périodique de micro-canaux d’air parallèles le long de la fibre, possèdent en effet des propriétés optiques et acoustiques remarquables et inédites par rapport aux fibres conventionnelles. De façon plus précise, nous montrons dans ce travail, par le biais de simulations numériques et de données expérimentales, que les fibres à cristaux photoniques offrent la possibilité de supprimer ou, à contrario, augmenter les interactions entre les photons et les phonons. Dans une première partie, nous présentons une méthode de cartographie des fluctuations longitudinales de la microstructure des fibres PCFs à l’aide d’un capteur distribué basé sur une méthode innovante d’écho Brillouin. Cette méthode, très sensible et à haute résolution, est directement intéressante pour caractériser et améliorer l’uniformité des PCFs lors de leur fabrication et également pour la détection des différentes contraintes de température et étirement induites le long des fibres. Sur le plan fondamental, notre système de mesure distribuée à haute résolution nous a également permis d’observer, pour la première fois à notre connaissance, le temps de vie des ondes acoustiques dans les fibres à cristaux photoniques et les fibres standard. Par ailleurs, sur le plan technique, nous avons développé une architecture simplifiée de capteur distribué combinant la technique des échos Brillouin et celle de la modulation différentielle par déplacement de phase avec un seul modulateur d’intensité. Nos résultats montrent une résolution centimétrique dans la zone de soudure entre deux fibres optiques à l’aide d’une impulsion de phase de 500 ps. Nous démontrons dans une deuxième partie la suppression directe et passive de la rétrodiffusion Brillouin stimulée dans une fibre optique micro structurée en faisant varier périodiquement le diamètre de la microstructure. Une augmentation de 4 dB du seuil de puissance Brillouin a été obtenue avec une variation de seulement 7% sur une période de 30m. Ce résultat est très intéressant car la diffusion Brillouin est un facteur limitant dans les systèmes de télécommunications par fibre optique et les lasers à fibre. La troisième et dernière partie est consacrée à l’étude numérique et expérimentale de la diffusion Brillouin en avant dans les fibres à cristaux photoniques. En plus de la suppression de la plupart des modes acoustiques transverses, nous montrons que cette diffusion Brillouin est fortement augmentée pour certains modes acoustiques à haute fréquence qui sont piégés au cœur de la microstructure. Nous avons également étudié une fibre à structure multi-échelle qui révèle l’excitation sélective de plusieurs phonons acoustiques à des fréquences allant jusqu’a 2GHz. Ces mesures ont étés confirmées par des simulations numériques basées sur une méthode vectorielle aux éléments finis. L’impact des irrégularités de la microstructure a aussi été mis en évidence.Mots clés : optique non linéaire, diffusion Brillouin, fibres optiques microstructurées, seuil Brillouin, capteurs Brillouin distribués. / Brillouin scattering is a fundamental nonlinear opto-acoustic interaction present in optical fibers with important implications in fields ranging from modern telecommunication networks to smart optical fiber sensors. This thesis is aimed at providing a comprehensive theoretical and experimental investigation of both forward and backward Brillouin scattering in next generation photonic crystal fibers in view of potential applications to above mentioned fields. We show in particular that these micro-structured optical fibers have the remarkable ability to either suppress or enhance photon-phonon interactions compared to what is commonly observed in conventional fibers. Firstly, this thesis provides a complete experimental characterization of several photonic crystal fibers using a novel highly-resolved distributed sensing technique based on Brillouin echoes. We perform distributed measurements that show both short-scale and long-scale longitudinal fluctuations of the periodic wavelength-scale air-hole microstructure along the fibers. Our mapping technique is very sensitive to structural irregularities and thus interesting for fiber manufacturers to characterize and improve the fiber uniformity during the drawing process. With this technique, we also report the first experimental observationof the acoustic decay time and the Brillouin linewidth broadening in both standard and photonic crystal fibers. Furthermore, we experimentally demonstrate a simplified architecture of our Brillouin echoes-based distributed optical fiber sensor with centimeter spatial resolution. It is based on differential phase-shift keying technique using a single Mach-Zehnder modulator to generate a pump pulse and a _-phase-shifted pulse with an easy and accurate adjustment of delay. These sensing techniques are also applied to distributed strain measurement. Another aspect of this thesis is the investigation of a novel method for suppressing stimulated Brillouin scattering that is detrimental to optical fiber transmissions and fiber lasers. We experimentally study several fibers and a demonstrate 4 dB increase of the Brillouin threshold in a photonic crystal fiber by varying periodically the core diameter by only7%. The efficiency of this passive technique is verified by use of our distributed sensing technique where the oscillating Brillouin frequency shift is clearly observed.Lastly, we present experimental and numerical results demonstrating the simultaneous vi Abstract frequency-selective excitation of several guided acoustic Brillouin modes in a photonic crystal fiber with a multi-scale structure design. These guided acoustic modes are identified by using a full vector finite-element model to result from elastic radial vibrations confined by the air-silica microstructure. We further show the strong impact of structural irregularities of the fiber on the frequency and modal shape of these acoustic resonances

Le cadre général dans lequel s'insère ce travail de thèse est celui de l'étude de la diffusion Brillouin dans une nouvelle génération de fibres optiques à cristaux photoniques (PCFs). Ces fibres, qui présentent un arrangement périodique de micro-canaux d'air parallèles le long de la fibre, possèdent en effet des propriétés optiques et acoustiques remarquables et inédites par rapport aux fibres conventionnelles. De fa¸con plus précise, nous montrons dans ce travail, par le biais de simulations numériques et de données expérimentales, que les fibres à cristaux photoniques offrent la possibilité de supprimer ou, à contrario, augmenter les interactions entre les photons et les phonons. Dans une première partie, nous présentons une méthode de cartographie des fluctuations longitudinales de la microstructure des fibres PCFs 'a l'aide d'un capteur distribué basé sur une méthode innovante d'écho Brillouin. Cette méthode, très sensible et à haute résolution, est directement intéressante pour caractériser et améliorer l'uniformité des PCFs lors de leur fabrication et également pour la détection des différentes contraintes de température et étirement induites le long des fibres. Sur le plan fondamental, notre système de mesure distribuée à haute résolution nous a également permis d'observer, pour la première fois à notre connaissance, le temps de vie des ondes acoustiques dans les fibres à cristaux photoniques et les fibres standard. Par ailleurs, sur le plan technique, nous avons développé une architecture simplifiée de capteur distribué combinant la technique des échos Brillouin et celle de la modulation différentielle par déplacement de phase avec un seul modulateur d'intensité. Nos résultats montrent une résolution centimétrique dans la zone de soudure entre deux fibres optiques 'a l'aide d'une impulsion de phase de 500 ps. Nous démontrons dans une deuxième partie la suppression directe et passive de la rétro-diffusion Brillouin stimulée dans une fibre optique microstructurée en faisant varier périodiquement le diamètre de la microstructure. Une augmentation de 4 dB du seuil de puissance Brillouin a été obtenue avec une variation de seulement 7% sur une période de 30m. Ce résultat est très intéressant car la diffusion Brillouin est un facteur limitant dans les systèmes de télécommunications par fibre optique et les laser à fibre. La troisième et dernière partie est consacrée 'a l'étude numérique et expérimentale de la diffusion Brillouin en avant dans les fibres à cristaux photoniques. En plus de la suppression de la plupart des modes acoustiques transverses, nous montrons que cette diffusion Brillouin est fortement augmentée pour certains modes acoustiques à haute fréquence qui sont piégés au coeur de la microstructure. Nous avons également étudié une fibre à structure multi-échelle qui révèle l'excitation sélective de plusieurs phonons acoustiques à des fréquences allant jusqu'a 2GHz. Ces mesures ont été confirmées par des simulations numériques basées sur une méthode vectorielle aux éléments finis. L'impact des irrégularités de la microstructure a aussi été mise en évidence.