With the escalating demands for optical communication network system, the need for broadband gain medium in optical communication has increased. Among them, Cr4+:YAG crystal has shown an exceptionally successful broadband amplified spontaneous emission (ASE) light source that fully cover 1.2-1.6 £gm range (3-dB bandwidth up to 265 nm). More recently, we demonstrated the realization of a waveguiding, low-loss, and low-threshold Cr4+:YAG double-clad crystal fiber (DCF) based ultra-broadband ASE light source, optical amplifier, and laser grown by the co-drawing laser-heated pedestal growth technique. These results demonstrate the potential of the Cr4+:YAG DCF for the replacement of the erbium doped fiber in future optical communications. To further improve the efficiency of Cr4+:YAG DCF based active devices, here we show the difference in thermal expansion coefficients between a YAG core and an inner cladding creates a significant localized strain field by near-field scanning optical microscope (NSOM), which can result in optical confinement and provide the possibility to simultaneously alter the Cr3+ and Cr4+ fluorescence lifetime with varied core dimensions. The results indicate that There exists a nearly zero strain across the entire core with a diameter of ~20 £gm, which is beneficial the higher Cr4+ fluorescence lifetime (+6.43%) and emission cross section (+19.17%) as compared with 11-£gm core. In addition, we have successfully investigated the near-field modal characteristics of Cr4+:YAG DCF laser and ASE by NSOM. The results demonstrate that the Cr4+:YAG DCF laser produced nearly a single-mode (LP01) output with diffraction-limited beam quality of M2 ~1.1; for ASE, the modal weighting of LP01 decreases from 26% to 15% as the number of modes increases from 4 to 7. The results offer a guideline not only for further fabricating Cr4+:YAG DCF tunable lasers, but for efficiently coupling a broadband ASE light source into a single-mode fiber.

With the escalating demands for optical communication network system, the need for broadband gain medium in optical communication has increased. Among them, Cr4+:YAG crystal has shown an exceptionally successful broadband amplified spontaneous emission (ASE) light source that fully cover 1.2-1.6 £gm range (3-dB bandwidth up to 265 nm). More recently, we demonstrated the realization of a waveguiding, low-loss, and ultralow threshold Cr4+:YAG double-clad crystal fiber (DCF) based ultrabroadband ASE light source, optical amplifier, and laser grown by the codrawing laser-heated pedestal growth (LHPG) technique. These results demonstrate the potential of the Cr4+:YAG DCF for the replacement of the erbium doped fiber in future optical communications. In this thesis, we focus on the correlation between the nanospectroscopy and nanostructure of the Cr:YAG DCF in order to further improve its device performance. For nanospectroscopic and nanostructural characterizations, near-field scanning optical microscopy (NSOM) and high-resolution transmission electron microscopy (HRTEM) techniques have played key roles. In this thesis, we successfully prepared the HRTEM specimen of Cr:YAG DCF, which is heterostructure, ultrahard, but fragile. Here we show the first study on the nanospectroscopy and nanostructure of the nanocrystals in the inner cladding of Cr:YAG DCF by highly spatial resolved NSOM. The NSOM results were compared with those obtained by HRTEM. In addition, the difference in thermal expansion coefficients between a YAG core and an inner cladding creates a significant localized strain field beneath the core, which can result in optical confinement and provide the possibility to simultaneously control the Cr3+ and Cr4+ fluorescence with systematically varied growth parameters. This new class of strain-tunable Cr:YAG DCF opens up new opportunity to improve the performance of the Cr:YAG DCF based ultrabroadband light source, optical amplifier, and crystal fiber laser in all-optic fiber communications.

ABSTRACT For the generation of broadband tunability over 1300 nm to 1600 nm range in optical communications, Cr4+ ions have been demonstrated in a number of hosts including Cr4+:forsterite, Cr4+:Y2SiO5, and Cr4+:YAG. Since YAG is a cubic crystal with Ia-3d space group, its excellent thermal and optical properties has drawn extensive studies. In recent years, various modes of operation using Cr4+:YAG as laser gain medium have been achieved since the first tunable CW laser of Cr4+:YAG bulk gain medium was demonstrated by Shestakov, et al in 1988. Experimentally, the Cr4+:YAG crystal fiber was grown by the laser-heated pedestal growth (LHPG) method which provides outstanding crystal quality and can easily change growth conditions, such as growth speed and core diameter. A double cladding technique was developed with pure YAG, silica/YAG mixture, and pure silica as the core, inner cladding, and outer cladding, respectively. The smallest core diameter we achieved is 11 mm. In this thesis, the fabrication process of the Cr4+:YAG crystal fiber laser involving crystal fiber growth, sample preparation, and coating design will be presented in detail. In order to understand the relation between the microstructure of Cr4+:YAG crystal fibers and the growth conditions, high-resolution transmission electron microscopy (HRTEM) was employed, which reveals the nano-scale information in the core region, the inner-outer cladding interface, and the mechanism of inter-diffusion process during growth. In addition, the thesis also describes the specimen preparation procedures of crystal fibers for the HRTEM analysis. Furthermore, quantitative analysis of Cr4+:YAG crystal fiber was employed by electron-probe micro-analyzer (EPMA) and energy dispersive X-ray spectrometer (EDX), showing accurate characterization of the constitute elements and concentrations. The comparison of Cr4+ fluorescence and dopant concentration of CaO and Cr2O3 will also be presented in this thesis.

Cr4+:YAG is an attractive gain medium due to its broad 3-dB emission spectra all the way from 1253 nm to 1530 nm that just cover the low loss window of silica fiber. Such a broadband characteristic offers a potential to develop a broadband amplified spontaneous emission (ASE) light source, optical amplifier, and tunable laser. Growing the Cr4+:YAG bulk crystal into fiber form is necessary for generating larger gain by the better optical confinement of the waveguide structure. For the application of laser, it is superior to bulk crystal for reduced lasing threshold and better slope efficiency due to also the optical confinement effect and better heat dissipation. Laser heated pedestal growth (LHPG) method has been used to grow high purity crystal fibers due to its crucible free nature. A novel cladding technique, co-drawing LHPG (CDLHPG), was developed to solve core-reduction problem and obtained a double-clad fiber (DCF) structure. But the power fluctuation of heating laser caused large core variation of Cr4+:YAG DCF, and further impaired the optical performance. An innovating method for suppressing the fluctuation of heating power, sapphire tube assisted CDLHPG technique, was developed and combined with power feedback control program. By this technique, 10-£gm-core Cr4+:YAG DCFs which meet the adiabatic propagation criterion were fabricated. By comparing with ASE and optical amplifier experimental data, cross sections of pump absorption, emission, and excited-state absorptions (ESAs) of pump and signal were determined. Pump ESA loss limited the optical performance that could be solve by using cladding pump scheme. A record-low threshold Cr4+:YAG DCF laser with two slopes with respect to absorbed pump power was achieved at room temperature. The threshold pump powers were 2.5 mW and 96 mW in the low and high absorbed pump powers with the same output coupler transmittance of 3.8%, respectively. The slope efficiencies of the fiber laser were 0.4% and 6.9%, respectively. By numerical simulation, 56% slope efficiency can be achieved with a length of 7 cm and an output reflectance of 80%. Our group also firstly used the ASE as the light source of optical coherence tomography, an axial resolution of 3.5 £gm was achieved.

Pulling bulk crystal into fiber is suitable for laser, amplified spontaneous emission (ASE), and optical amplifier applications in optical communications because of its structural similarity to silica fiber. Moreover, fiber configuration can confine pump light in a small cross-sectional area with a high energy density for a long distance. Among crystal fiber growth techniques, the laser-heated pedestal growth method (LHPG) was adopted. It is crucible free and can therefore produce high-purity, low-defect-density single crystals. However, interface loss of the crystal fiber is one of the main causes of optical loss. In order to reduce the optical loss, a proper method to clad the fiber is important for high device performance. For laser application, high-efficient Nd:YAG lasers were demonstrated using gradient-index crystal fibers. We used controlled profile of the active ion resulted in index difference of 0.0284 between the center and the edge of the fiber to confine the laser beam in the center region and thus reduced the interface loss. A laser output power of 80 mW was achieved with a slope efficiency of 28.9%, which, to our knowledge, is the highest ever achieved for diode-laser-pumped Nd:YAG fiber laser. For ASE and optical amplifier applications, Cr4+:YAG crystal fiber was studied due to its fluorescent spectrum just covering the low loss window of silica optical fiber. To reduce the fiber diameter and propagation loss, a novel cladding technique, codrawing LHPG (CDLHPG), was developed. Although fused-silica-clad fiber can be made with a 29-micron-diameter core and a propagation loss of less than 0.1 dB/cm, which is a factor of 7 smaller than that of an unclad fiber, it has almost no Cr4+ fluorescence in the core area due to the entering of SiO2 in YAG. With proper controlled growth parameters of the CDLHPG method, a double-clad fiber with a core diameter of 25 micron was successfully grown. Up to 2.36 mW of ASE with a bandwidth of 265 nm was demonstrated. After splicing the double-clad fiber with conventional single mode fiber, we successfully demonstrated the first transition metal-doped fiber amplifier in the optical fiber communication band. Up to 16-dB of gross gain at 1.47 micron was achieved.

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Recently, with the escalating demands for optical communication, the need to use broadband laser light sources in optical communication network system has increased. Henceforward, the broadband characteristes of Cr4+:YAG crystal fiber possess signifies its indispensability. Furthermore, Yb3+:YAG-silica also has its advantages in high power laser domain. In this thesis, the crystal fiber grown by the laser heated pedestal growth method is used as the laser gain medium with fused silica packaging technique. Cr4+:YAG double-clad crystal fiber with a core diameter as small as 11 £gm was achieved. Moreover, a Yb3+:YAG-silica layer was formed due to the strong inter-diffusion between silica capillary and Yb3+:YAG crystal. When the silica all diffused into the Yb3+:YAG, a Yb3+:YAG-silica fiber with 125-£gm core was obtained with waveguide structure. By directly coating the optical thin films onto the end faces of the two types of fibers, the laser configuration is compact and cost effective. Besides, heat dissipation is also improved. By Cu-Al alloy packaging, a record-low Cr4+:YAG double-clad crystal fiber laser was achieved with threshold of 0.75 mW and a record-high slope efficiency of 6.9% at room temperature. And we also successfully fabricate the Yb3+:YAG-silica fiber laser with low threshold (100 mW) and high efficiency (67.2%) at room temperature. In fiber laser development, we have successfully fabricated the coating of high-reflective thin films which match the faces of fiber heterostructure (single cladding and double cladding structures). It forms a cavity with anti-reflectivity for pumping wavelength and high-reflectivity for lasing wavelength. For these reasons, low threshold, high slope efficiency, and stable laser output have been achieved. Finally, through different thin-film designs, the strain effect between thin film and heterosubstrate is significantly reduced, which facilitates the realization of high performance fiber lasers.

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.

Nous explorons quelques aspects nouveaux des lasers solides impulsionnels ou continus fonctionnant en régimes mono-fréquence ou bi-fréquence. <br /> Tout d'abord, nous avons mis au point quelques techniques pour optimiser les caractéristiques des lasers impulsionnnels. (i) Une faible modulation de la puissance de pompe a permis de rendre stable le taux de répétition avec une stabilité relative de 10-6. (ii) La durée des impulsions a été contrôlée continûment d'une dizaine de ns à quelques centaines de ns par trois méthodes différentes : l'ajustement du diamètre du faisceau laser dans l'absorbant saturable, l'ajustement du diamètre du faisceau de pompe dans le milieu actif, et, en particulier, l'utilisation des états fourchus dans un laser à deux axes. De plus, cette méthode des états fourchus a aussi permis (iii) d'augmenter l'énergie des impulsions par addition cohérente.<br /> Un laser tout compact Nd:YAG-Cr:YAG à deux fréquences dont la différence est continûment accordable jusqu'à 2,7 GHz a été construit. Les impulsions bi-fréquences sont idéales pour répondre aux besoins des applications telles que le lidar-radar Doppler. En outre, nous montrons que les impulsions bi-fréquences à 1,55 μm peuvent être obtenues en utilisant un nouvel absorbant saturable Co:ASL taillé suivant l'axe c dans un laser Er-Yb:verre. Ces impulsions sont parfaitement adaptées aux systèmes de détection en espace libre nécessitant la sécurité oculaire. Le temps de cohérence du battement dans ces lasers a aussi été étudié : il est limité à la durée de l'impulsion. Une nouvelle technique de modulation de la puissance de pompe au voisinage de la fréquence d'oscillation de relaxation du laser a été étudiée et a permis d'obtenir des battements cohérents d'impulsion à impulsion.<br /> La conversion de fréquence utilisant des effets optiques non linéaires a permis d'obtenir des sources bi-fréquences dans le visible. Des impulsions bi-fréquences vertes et rouges ont été obtenues. Une source THz dans le rouge a aussi été réalisée en utilisant un laser à deux axes Er-Yb:verre doublé dans un cristal mixte “en éventail” de PPLN. Les applications à la génération de micro-ondes et d'ondes sub-millimétriques sont discutées.