In this dissertation, an all-fiber-based single frequency nanosecond pulsed laser system at ~ 1918.4 nm in master-oscillator-power-amplifier (MOPA) configuration is present. The nanosecond pulse seed is achieved by directly modulating a continuous wave (CW) single frequency fiber laser using a fast electro-optical modulator (EOM) driven by an arbitrary waveform generator (AWG). One piece of single mode, large core, polarization-maintaining (PM) highly thulium-doped (Tm-doped) germanate glass fiber (LC-TGF) is used to boost the pulse power and pulse energy of these modulated pulses in the final power amplifier. This laser system can work in both high power and high energy regime: in high power regime, to the best of our knowledge, the highest average power 16 W and peak power 78.1 kW are achieved for single frequency transform-limited ~2.0 ns pulses at 500 kHz and 100 kHz repetition rate, respectively: In high energy regime, nearly 1 mJ and half mJ pulse energy is obtained for ~15 ns pulses at 1 kHz repetition rate and 5 kHz repetition rate, respectively. Theoretical modeling of the large-core highly Tm-doped germanate glass double-cladding fiber amplifier (LC-TG-DC-FA) is also present for 2&mum nanosecond pulse amplification. A good agreement between the theoretical and experimental results is achieved. The model can simulate the evolution of pump power, signal energy, pulse shape and the amplified stimulated emission (ASE) in the amplifier. It can also be utilized to investigate the dependence of the stored energy in the LC-TGF on the pump power, seed energy and repetition rate, which can be used to design and optimize the LC-TG-DC-FA to achieve higher pulse energy and average power. Two channel of high energy nanosecond pulses (at 1918.4 nm and 1938 nm) are utilized to generate THz wave in a quasi-phase-matched (QPM) gallium arsenide (GaAs) based on difference frequency generation. THz wave with ~ 5.4μW average power and ~18 mW peak power has been achieved. Besides, one model is built to simulate a singly resonated THz parametric oscillator. The threshold, the dependence of output THz energy on pump energy has been investigated through this model. One pump enhanced THz parametric oscillator has been proposed. The enhancement factor of the nanosecond pulses in a bow-tie ring cavity has been calculated for different pulse duration, cavity length and the transmission of the coupler. And the laser resonances in the ring cavity have been observed by using a piezo to periodically adjust the cavity length. We also build an all-fiber thulium-doped wavelength tunable mode-locked laser operating near 2&mum. Reliable self-starting mode locking over a large tuning range (>50 nm) using fiber taper based carbon nanotube (FTCNT) saturable absorber (SA) is observed. Spectral tuning is achieved by stretching another fiber taper. To the best of our knowledge, this is the first demonstration of an all-fiber wavelength tunable mode-locked laser near 2&mum.

Envisioning novel fully monolithic fiber-optical devices, this dissertation investigates four fiber optical devices both, active and passive, that contribute to the goal of further integrating and miniaturizing fiber optics. An all phosphate glass fiber laser was designed in an effort to reduce laser intensity noise by reducing cavity losses and low mechanical strength that arise from intra-cavity fusion splices between silica fiber Bragg gratings (FBG) and phosphate active fiber in state of the art phosphate single frequency fiber lasers. Novel phosphate glass based FBGs have been fabricated utilizing high intensity laser pulses at 193 nm and a phase-mask. Net reflectivities of up to 70 % and a bandwidth of 50 pm have been achieved in the FBGs. The laser design comprised two of the novel FBGs and a short section of Er³⁺Yb³⁺ phosphate fiber to form a distributed Bragg reflector (DBR) laser. The performance of the new laser has been compared to a conventional phosphate fiber laser. Particular focus was put on the laser intensity noise due to its dependence on intra-cavity losses. Relative intensity noise (RIN) amplitudes of -80 dB/Hz have been measured for both lasers when operating at comparable output powers. For similar levels of absorbed pump power the relaxation oscillation frequencies (ROF) were shifted towards lower frequencies in the new laser. ExcessFBG scattering losses and mode-field miss-match between the active and passive fiber limited the output power of the new laser to 16 mW compared to 140 mW in the conventional laser. A monolithic all-phosphate glass fiber laser with up to 550 mW output power that is operating at a single longitudinal mode and exhibiting narrow linewidth is presented. The laser cavity has been formed by inscribing FBGs directly into heavily Er³⁺Yb³⁺ doped phosphate glass fiber using femtosecond laser pulses and a phase mask, completely eliminating the need for intra-cavity fusion splices. A linewidth of less than 60 kHz and relaxation oscillation peak amplitudes below -100 dB/Hz without active suppression of RIN have been measured. The compact form factor and higher output power combined with the low noise and narrow linewidth characteristic make this laser an ideal candidate for ranging, interferometry and sensing applications. Strong and robust Bragg gratings in optical fiber fabricated from highly photosensitive photo-thermo-refractive (PTR) glass are demonstrated. The fibers were drawn at 900 °C from a machined PTR-glass preform. A low power two beam interference pattern from a continuous wave (cw) He-Cd laser with a wavelength of 325 nm was used to write gratings into the fibers, achieving peak grating strengths of 20 dB and a spectral width of 45 pm. The gratings showed no sign of degradation when exposed to a high temperature environment of 425 °C for several hours. This is significantly higher when compared to standard Telecom FBGs which are rated for operation temperatures below 200 °C. A detailed study of novel mode-field adapters (MFA) based on multi-mode interference in graded index multi-mode fibers (GIMF) is presented. MFAs are often used in cases when low coupling losses between single mode fibers with very different mode-field diameters are needed. Here a new type of MFAs has been fabricated and characterized from a selection of commercially available single mode and graded index fibers. Compared to existing techniques the presented MFAs can be fabricated very quickly and are not limited to certain fiber types. Insertion losses of 0:5 dB over a spectral range of several hundred nm have been obtained with an ultra compact MFA with a length of 275 μm.

Carbon fiber-reinforced plastics (CFRPs) have gained widespread popularity as a lightweight, high-strength alternative to traditional materials. The unique anisotropic properties of CFRP make processing difficult, especially using conventional methods. This study investigates laser cutting by ablation as an alternative by comparing two near-infrared laser systems to a typical mechanical machining process. This research has potential applications in the automotive and aerospace industries, where CFRPs are particularly desirable for weight savings and fuel efficiency. First, a CNC mill was used to study the effects of process parameters and tool design on machining quality. Despite high productivity and flexible tooling, mechanical drilling suffers from machining defects that could compromise structural performance of a CFRP component. Rotational feed rate was shown to be the primary factor in determining the axial thrust force, which correlated with the extent of delamination and peeling. Experimental results concluded that machining quality could be improved using a non-contact laser-based material removal mechanism. Laser machining was investigated first with a Yb:YAG fiber laser system, operated in either continuous wave or pulse-modulated mode, for both cross-ply and woven CFRP. For the first time, energy density was used as a control variable to account for changes in process parameters, predicting a logarithmic relationship with machining results attributable to plasma shielding effects. Relevant process parameters included operation mode, laser power, pulse overlap, and cross-ply surface fiber orientation, all of which showed a significant impact on single-pass machining quality. High pulse frequency was required to successfully ablate woven CFRP at the weave boundaries, possibly due to matrix absorption dynamics. Overall, the Yb:YAG fiber laser system showed improved performance over mechanical machining. However, microsecond pulses cause extensive thermal damage and low ablation rates due to long laser-material interaction time and low power intensity. Next, laser machining was investigated using a high-energy nanosecond-pulsed Nd:YAG NIR laser operating in either Q-Switch or Long Pulse mode. This research demonstrates for the first time that keyhole-mode cutting can be achieved for CFRP materials using a high-energy nanosecond laser with long-duration pulsing. It is also shown that short-duration Q-Switch mode results in an ineffective cutting performance for CFRP, likely due to laser-induced optical breakdown. At sufficiently high power intensity, it is hypothesized that the resulting plasma absorbs a significant portion of the incoming laser energy by the inverse Bremsstrahlung mechanism. In Long Pulse mode, multi-pass line and contour cutting experiments are further performed to investigate the effect of laser processing parameters on thermal damage and machined surface integrity. A logarithmic trend was observed for machining results, attributable to plasma shielding similar to microsecond fiber laser results. Cutting depth data was used to estimate the ablation threshold of Hexcel IM7 and AS4 fiber types. Drilling results show that a 2.2 mm thick cross-ply CFRP panel can be cut through using about 6 laser passes, and a high-quality machined surface can be produced with a limited heat-affected zone and little fiber pull-out using inert assist gas. In general, high-energy Long Pulse laser machining achieved superior performance due to shorter pulse duration and higher power intensity, resulting in significantly higher ablation rates. The successful outcomes from this work provide the key to enable an efficient high-quality laser machining process for CFRP materials.

The dependence of thickness and concentration product (TCP) of single-wall carbon nanotubes saturable absorber (SWCNTs SA) on stabilizing and shortening pulse width in passively mode-locked erbium-doped fiber ring laser (MLEDFL) was investigated and measured. The TCP represented the amounts of SWCNTs, which the optical beam encountered when passing through the SWCNTs SA. If the TCP was smaller than 8.25 (£gm x wt%), the spectral bandwidth was below 2 nm. The pulse shaping was dominated by its own self amplitude modulation (SAM) of SWCNTs SA. With further increasing TCP, the soliton-like ML operation was achieved and the spectral bandwidth was expanded to 6 nm. For soliton-like mode locking (ML) operation, the area theorem dominated the pulse shaping. Through area theorem analysis, the estimation of SPM increased as the TCP increased. The adequate enhanced SPM for balancing the slight negative GVD was provided to generate soliton-like ML pulses shorten the pulse width. However, as the TCP increased, the soliton pulse energy decreased. The decreasing soliton pulse energy restricted the further pulse shortening. The results showed that the dependence of the pulse energy and nonlinear self phase modulation (SPM) on TCP enabled to determine the shortest pulse width in MLEDFL based on the area theorem. At optimized TCP of 70.93 (£gm x wt%), it was found that the shortest pulse width of 418 fs. In addition, based on the estimated SPM from area theorem, the nonlinear refractive index n2 was calculated at the level of 0.4 - 1 x 10^-15 m^2/W that was close to the literature values of 10^-15 - 10^-16 m2/W. It provides another way to estimate the nonlinear refractive index except for the Z-scan measurement. We could also estimate the SPM if an active Z-scan measurement was taken to obtain the nonlinear refractive index of the sample. We realized the trend of pulse energy through few samples in MLEDFL, the behavior of pulse width could be theoretically simulated based on area theorem. Hence, with the area theorem analysis, the optimized TCP of SWCNTs SA could be simulated and estimated to generate the shortest pulse width from the trends of pulse energy and estimated SPM. The significant effect of TCP on pulse energy, SPM, pulse width, and spectral bandwidth of MLFLs suggests that the TCP represents the total amount of SWCNTs in SA, which can be used as one of key parameters for characterizing the passive MLFL pulse width. Through the study of the dependence of TCP on ML pulses in MLEDFL, it may provide a guideline to fabricate an effective SWCNTs SA to generate the shortest pulse width of the MLEDFL.

Abstract During the last decade, the fast-growing communication need has promoted the development of the wavelength of 1.3 mm~1.6 mm laser light source. The Cr4+ doped YAG solid-state laser has potential to meet this super wideband demand. In addition, solid state laser has the merits of high laser beam quality, long lifetime, compact, and simple structure. In this thesis, crystal fiber was used as the laser gain medium, and coated with optical thin film at its end facets as the laser cavity. Using this configuration, the volume and cost of the laser can be appreciatively reduced, and the heat dissipation can be improved. The laser-heated-pedestal-growth method was used to grow crystal fiber, which can obtain small diameter at very fast rate and accurate control. High quality Cr4+:YAG crystal fiber with the smallest diameter of 50 mm was grown. A glass-packaged technique clothes the crystal fiber with a core diameter as small as 11 mm. Outside the glass clad Cr4+:YAG crystal fiber, Al-Cu alloy was employed as the heat sink to improve heat dissipation. After grinding, polishing, and coating of this device, the Cr4+:YAG crystal fiber laser was fabricated. Some characteristics of Cr4+:YAG crystal fiber, such as the distribution of Cr2O3 and CaO doping concentration, fluorescence intensity, refraction index, propagation loss, and absorption coefficient were measured and analyzed. In the meanwhile, some simulations of the laser output power depending on the absorption coefficient, propagation loss, output coupling, crystal fiber diameter, and crystal boundary temperature were discussed.

Recently, with the escalating demands for optical communications, the need for bandwidth in optical communication network has increased. The technology breakthrough in dry fiber fabrication opens the possibility for fiber bandwidth from 1.3 to 1.6 £gm. Cr4+:YAG double-clad crystal fiber (DCF) grown by the co-drawing laser-heated pedestal growth method has a strong spontaneous emission spectrum from 1.3 to 1.6 £gm. Such fiber is, therefore, eminently suitable for broadband optical amplifier, amplifier spontaneous emission (ASE) light source, tunable solid-state laser, and optical coherence tomography (OCT) applications. In this thesis, multilayer dielectric thin films were directly deposited by E-gun coating onto the end faces of the heterostructure Cr4+:YAG DCF. In this way we have successfully improved the extracted ASE power by the high reflection (HR) coatings. The backward ASE in the fiber reflected and propagates with gain through the fiber in the forward direction. In dual-pump scheme, as much as 1.7 mW power (DCF length is 9.5 cm) of collimated output ASE was achieved. The dual-pump scheme and HR thin films provided 1.6 time improvements of the ASE output power. For broadband optical amplifier in dual-pump and double-pass scheme, a 3.7-dB gross gain and a 0.7-dB net loss (DCF length is 8.7 cm) at 1.4-£gm signal wavelength have been successfully developed with HR coatings onto one of the Cr4+:YAG DCF end faces. In addition, we have successfully developed the Cr4+:YAG DCF fiber laser by direct HR coatings onto fiber end faces. A record-low threshold of 96 mW (DCF length is 1.6 cm) with a slope efficiency of 6.9% was achieved at room temperature. It is more than four times lower than any previously reported Cr4+:YAG lasers.

In this dissertation I report the development of high power pulsed fiber laser systems. These systems utilize phosphate glass fiber for active elements, instead of the industry-standard silica fiber. Because the phosphate glass allows for much higher doping of rare-earth ions than silica fibers, much shorter phosphate fibers can be used to achieve the same gain as longer silica fibers.This single-frequency laser technology was used to develop an all-fiber actively Q-switched fiber lasers. A short cavity is used to create large spacing between longitudinal modes. Using this method, we demonstrated the first all-fiber Q-switched fiber laser in the 1 micron region.In addition to creating high peak powers with Q-switched lasers, created even higher powers using fiber amplifier systems. High power fiber lasers typically produce spectral broadening through the nonlinear effects of stimulated Raman scattering, stimulated Brullion scattering, and self-phase modulation. The thresholds for these nonlinearities scale inversely with intensity and length. Thus, we used a short phosphate fiber gain stage to reduce the length, and a large core fiber final stage to reduce intensity. In this way we were able to generate high peak power pulses while avoiding visible nonlinearities, and keeping a narrow bandwidth.The immediate goal of developing these high power fiber laser systems was to generate narrowband terahertz radiation. Two different wavelengths were combined into the final amplifier stage at orthogonal polarizations. These were collimated and directed into a GaSe crystal, which has a very high figure of merit for THz generation. The two wavelengths combined in the crystal through the process of nonlinear difference frequency generation. This produced a narrowband beam of THz pulses, at higher powers than previous narrowband THz pulses produced by eyesafe fiber lasers.

Ce travail est consacré à l’étude des interactions d’un grand nombre de solitons dans un laser à fibre dopée erbium/ytterbium. Les impulsions interagissent entre elles et se structurent pour former des distributions plus ou moins organisées. Deux cavités ont été réalisées, l’une basée sur la rotation non-linéaire de la polarisation (RNLP) et l’autre sur le miroir optique à boucle non-linéaire (NOLM) en configuration de laser en forme de huit. Avec la RNLP, nous nous sommes intéressés d’abord à une distribution où les impulsions sont liées (cristal de solitons). Ce cristal, stable pour des puissances moyennes, devient instable lorsque la puissance augmente : il se disloque. Les solitons se réorganisent pour former un régime de verrouillage harmonique (HLM) de 50 cristaux. Nous avons ensuite cherché s’il était possible de contrôler les distributions de solitons par l’injection d’une composante continue externe. Nous avons montré que le HLM peut être forcé par l’injection de cette composante. Nous avons développé deux approches afin de modéliser un laser verrouillé en phase et soumis à l'injection d'une composante continue, l'une est scalaire et l'autre vectorielle. Ces modèles démontrent que le signal injecté peut modifier les interactions entre les solitons. Enfin, le NOLM a permis d’observer plusieurs dynamiques : pluie de solitons, gaz de solitons, liquide de solitons, poly-cristal de solitons, multi-cristal de solitons, cristal de solitons, états liés et verrouillage harmonique d’états liés. Ce laser a permis également d’observer une émission laser continue et impulsionnelle autour de 1600 nm. / This work has been devoted to study the interaction of the large number of solitons in the erbium/ytterbium doped fiber laser. The interaction of pulses with each other causes to form more or less organized distribution. Two laser cavities have been constructed, one based on the non-linear polarization rotation (NLPR) and the other based on the nonlinear optical loop mirror (NOLM) in a figure-eight laser configuration. With the NLPR, we were interested to the distribution of coherent pulses (crystal solitons). This crystal, stable for moderate power, becomes unstable at high power, which means to get loose from the initial soliton. These solitons rearrange their relative position to form one harmonic-mode locking regime (HML) of 50 crystals. Afterward, we have investigated the possibility of controlling the solitons distribution by injecting an external continuous wave (cw). It has been observed that cw could force the laser to operate at HLM regime. Moreover, we have developed two theoretical approaches, such as scalar and vectorial, to model the passively mode-locked fiber laser submitted to the cw. Both models confirm that the injected signal could modify the interactions between the solitons. Finally, the NOLM allowed us to observe several dynamics, including rain solitons, gas solitons, liquid solutions, poly-crystal solitons, multi-crystal solitons, crystal solitons, bound states and harmonic-mode locking of bound states. Furthermore, it made possible to observe the continuous as well as pulsed laser emission around 1600 nm.

A method of holographic cross-connection is proposed for an Optical Ising machine system. The designed optical Ising machine based on multi-core fiber laser is introduced, including the theory of computation, history of optical computing, the concept of Ising model, the significance of optical Ising machine, the method to achieve Ising machine optically. The cross-connection part is based on computer-generated holograms (CGH), which is produced by Gerchburg-Saxton algorithm. The coupling coefficient between two channels as well as the phase change are controlled by CGHs. The design of holograms is discussed. The instrument used to display holograms is phase-only liquid crystal spatial light modulator (SLM) from HOLOEYE company. The optical system needed in this project, such as collimation lens and relay lens, is designed using Zemax. The system is first evaluated in Zemax simulation, and then constructed experimentally. The results show that we can control amplitude and phase of the reinjection beam at Multi-core fiber. Further experiment should be done to conclude that the control of the cross coupling between channels is achieved by displaying different holograms.

Les méthodes de combinaison cohérente sont rapidement apparues comme très prometteuses dans la course à la puissance des sources lasers. Cela s’explique par le fait que la puissance autour de l’axe de propagation évolue selon une loi quadratique avec le nombre de faisceaux combinés. Mes premiers travaux ont porté sur la montée en puissance de pompage dans les systèmes de mise en phase passive par auto-organisation. Pour la première fois, nous avons mis en évidence à la fois expérimentalement et numériquement, qu’au-delà du seuil laser, le filtrage spectral intracavité dû à la structure interférométrique du système laser, est un des principaux facteurs limitant l’obtention de qualités de phasage élevées. L’augmentation du nombre d’émetteurs accentue la dégradation de l’efficacité de combinaison avec la montée en puissance, montrant l’incapacité de ce type de méthode à combiner efficacement un grand nombre d’émetteurs lasers de forte puissance. Par la suite, mes travaux ont porté sur l’étude d’une méthode innovante de phasage actif, mise au point à XLIM. Le principe de cette méthode associe un filtrage optique de type contraste de phase, à un algorithme d’optimisation réduisant les écarts de phases entre émetteurs. Les calculs et expériences ont mis en évidence la très faible sensibilité de la méthode au nombre d’émetteurs mis en jeu. Les démonstrations de combinaison cohérente de 7 à 37 émetteurs fibrés délivrant jusqu’à 5W chacun ont été faites. Ce dernier résultat constitue aujourd’hui un record en termes de nombre d’émetteurs combinés de manière active. L’efficacité de combinaison en champ lointain a été estimée à une valeur élevée de 94%, correspondant à une erreur de phase résiduelle d’environ λ/25. Le faible nombre d’itérations d’algorithme nécessaires pour converger a permis de corriger les fluctuations de phase sur une bande d’environ 1kHz. / Coherent laser beam combining techniques rapidly appeared highly promising in the field of ultra-high power laser sources. Indeed, the combined intensity around the propagation axis follows a quadratic law with the number of combined emitters. The first part of my work has been focused on passive phasing techniques, based on self-organization properties of coupled lasers. We have shown, both numerically and experimentally, that the intracavity filtering function due to the interferometric nature of the set-up, is an intrinsic reason for combining efficiency decrease far above laser threshold. The decrease goes steeper when the number of combined laser increases, making that kind of system inappropriate for coherently combining a large number of lasers delivering high power. The second part of my work consisted in studying an innovative active phasing method that associates a phase-contrast like filter with an optimization algorithm reducing phase errors between emitters. Both simulations and experiments showed the weak sensitivity of this method to the number of combined emitters. We demonstrated the phasing of 7 to 37 fiber lasers, delivering up to 5W each. To the best of our knowledge, this last result is the highest number of fiber lasers combined with an active phasing method. The combining efficiency has been estimated around 94%, corresponding to a residual phase error of λ/25. The weak number of algorithm iterations needed to reach the in-phase regime offered a bandwidth of approximately 1kHz.