Photosensitivity and bragg grating technology for upconversion all-fiber lasers

Fiber lasers are compact lightsources with high intrinsic beam quality and high efficiency. The lasing spectrum ranges from the blue to the near infrared and is achievable using fibers with different dopants and glass compositions. All-fiber lasers are characterized by a high attainable stability and are also available in the pulsed regime. A widespread range of applications for all-fiber lasers exists in the domain of telecommunications as well as in the medical domain. The first part of this work is dedicated to the characterization of fluoride glasses regarding photosensitive effects. Fluoride glasses allow several lasing transitions, which are not available in silicate glasses. Photosensitive effects in fluoride glasses would pave the way to fabricate fiber Bragg gratings in fluoride fibers. Fiber Bragg gratings can act as laser mirrors and are a necessary tool on the way to all-fiber lasers with an output beam inside a fiber tail. In this work, doped and undoped fluoride glasses are exposed to different ultraviolet lasers and induced refractive index changes are investigated by changes of their absorption spectra. Index changes higher than 10-4 have been obtained. The second part is dedicated to demonstrate novel upconversion laser configurations in the visible spectral range using fiber Bragg grating technology in germanosilicate fibers and upconversion in fluoride fibers. Upconversion lasing in the green, orange and red spectral range were demonstrated. Tuning over 8 nm was shown for the orange laser. To minimize intracavity losses, mode field diameter adapter in germanosilicate fibers have been realized. The incorporation of an all-fiber accousto optic phase modulator inside the cavity allows pulsed operation of the upconversion laser. Mode locking is demonstrated at the red wavelength with pulse durations below 100 ps. In conclusion, the potential to fabricate Bragg gratings in Cerium doped and undoped fluoride glasses was demonstrated. Furthermore, green, orange and red lasing of an all-fiber configuration has been shown by a hybrid approach based on fiber Bragg grating technology in germanosilicate fibers and upconversion in fluoride fibers. An all-fiber mode locked upconversion laser in the red spectral range was demonstrated. We believe that such novel upconversion light sources represent a useful tool in different biomedical applications.

Photosensitivity and luminescence of chromium and bismuth doped fibers

Christian Ban

Optical fiber materials with broad-band gain in the visible or in the telecommunication window are of great interest for optical communication or the biomedical domain in order to built integrated tunable lasers, amplifiers, ultra-short pulse lasers, or broadband light sources. Possible activator ions in compact solid-state host materials for the generation of broadband luminescence, gain, and laser emission are transition-metal (TM), bismuth and rare-earth (RE) ions. Emission lines in TM and bismuth exhibit luminescence with typically several hundred of nanometers of spectral bandwidth. This is a much broader range than can be obtained from the emission of RE ions, which is restricted to less than hundred nanometers. Chromium and bismuth-doped fibers are of great interest as they are potential candidates for broadband light sources and tunable lasers. The idea of this work was to investigate the photosensitivity and luminescence properties of Cr and Bi-doped silica optical fibers in collaboration with the Fiber Optic Research Center at the Russian Academy of Sciences (FORC-RAS) who fabricated the fibers and glass samples. The photosensitivity of Cr-and Bi-doped fibers with different dopant concentrations was investigated using an ArF excimer laser emitting at 193 nm. No refractive index change was observed in pristine Cr-doped fibers. After H2-loading, refractive index change up to 2.82×10-3 was obtained in Cr-Ga-doped fiber. The hydrogen loaded annealed fiber showed an index change of 1.4×10-3. The Bi-Al-doped fibers showed permanent photo-induced index changes up to 2.0×10-4 (highest Bi concentration). Using H2-loading index changes up to 2.2×10-3 were measured without reaching saturation. Such index changes are almost comparable to index changes in H2-loaded Ge-silicate fibers and allow fabricating very strong Bragg grating fiber laser reflectors directly in the doped fibers. Stress measurements were performed to distinguish between color center and compaction that contribute to the photo-induced refractive index change. Stress changes in SMF-28 standard telecom optical fibers irradiated by a femto-second laser emitting at 264 nm showed a contribution of compaction of 70% for non hydrogen loaded fibers whereas no stress change was observed in hydrogen loaded fiber for the same photo-induced refractive index. No stress change indicates the absence of compaction leaving color centers solely responsible for the refractive index change. Irradiated annealed H2-loaded Ga-Cr-doped fiber showed no stress change as well. Although generally assumed for H2-loaded fibers, these are the only two examples that prove a color center based photosensitivity responsible for fiber Bragg gratings reported so far. In contrary, in H2-loaded Bi-Al-doped fibers the refractive index change is related to color centers and compaction. A contribution of compaction of about 41 % and 11 %, was evaluated for the pristine and the H2-loaded Bi-Al-doped fiber. Strategies to increase the luminescence, i.e. the number of active centers include e.g. temperature annealing (Cr3+ to Cr4+), changing concentrations of dopants (Bi) and/or the co-dopants (Al, P, Ge), and fiber fabrication conditions. In this work new strategies were followed: UV irradiation (193 nm), hydrogen loading, and H2-loading followed by UV irradiation. Measurements of absorption and luminescence of Bi-doped fibers with different treatments were performed. The absorption was measured from 360 to 1700 nm. The spectrum was decomposed into 8 Bi-related and 1 OH band. The irradiation increased the absorption by ∼100 dB/m and induced two supplementary absorption bands. UV irradiation combined with hydrogen loading induced three additional bands as compared to the pristine fiber. The luminescence was investigated using visible and infra-red pump lasers. The emission takes place mainly from 600 to 900 nm (3 bands) and from 900 to 1300 nm (3 bands). The luminescence shape and intensities of the pristine and irradiated fibers were similar. H2-loading combined with irradiation increased the ratio of NIR to visible fluorescence intensity depending on the pump wavelength. A side luminescence measurement showed 18 and 13 dB luminescence increase of the 1130 nm and 1390 nm bands. This luminescence enhancement might be due to an increase of the number of active bismuth centers. Up to 1.8 dB/m on-off gain was observed in the visible band around 700 nm under 676 nm excitation in the pristine fiber. However, the absorption increases simultaneously. As a result the net-gain was always negative with a trend towards a positive value below 700 nm. High reflective fiber gratings and a narrow band transmission filter were fabricated in a single mode fiber with cut-off around 920 nm. All gratings were used to realize a Bi-all-fiber laser operating at 1179 nm with a reduced linewidth of < 100 pm. Narrow linewidth lasers have a great potential for frequency doubling to the yellow.

EPFL2009

Génération de deuxième et troisième harmonique avec un laser Nd:YAG en régime pulsé libre

Sébastian Favre

Industrial laser precision machining is predominately based on pulsed Nd:YAG lasers operated at a wavelength of 1.06 µm in a free running pulse regime. The objective of this thesis work has been to investigate second and third harmonic generation for such "long pulse" Nd:YAG lasers. Visible and UV radiation can be better focused. The absorption coefficient at these wavelengths is higher in most materials. Processing at the doubled or tripled frequency increases machining precision and allows processing of new classes of materials such as, e.g., high reflective metals or ceramics. Frequency doubling and tripling of Q-switched, modelocked, or CW operated Nd:YAG lasers has been the subject of numerous publications. However, the particular problems associated with the free running pulse regime have not yet been addressed. The generation of harmonics requires a laser source that can deliver radiation with high brightness. A system based on a slab crystal with a beam quality factor of M2 ≈ 1.6 and a power of up to 1.5 kW has been used in the experiments. An external electro-optical switch has been implemented in order to precisely control the laser pulse shape and duration. The non-linear coefficient and the angular acceptance have been the main criteria for selecting LBO (LiB3O5), KTP (KTiOPO4), and MgO:LiNbO3 for the frequency conversion experiments. The conversion efficiency has been measured for these crystals as a function of the intensity at the fundamental wavelength. In contrast to simple model predictions, the efficiency does not increase linearly with intensity under long pulse conditions. An intensity dependant phase mismatch is at the origin of the deviations. A heuristic model for the efficiency prediction has been developed. It is based on a linear increase of phase mismatch with intensity and it allows describing the experimental findings with sufficient accuracy. The conversion efficiency for frequency doubling is limited to about 20% at pulse durations of 100 µsec. It decreases with increasing pulse duration. Catastrophic thermal damage is at the origin of the limitation in this pulse regime. Our experiments indicate that the damage is initiated by the frequency doubled light. Temporary color centers are created in the presence of the second harmonic light. The dynamic of the creation and annihilation of these centers has been investigated. It could be shown that finally the absorption of the fundamental harmonic (infrared) radiation at these centers results in irreversible thermal damages. A power of 140 W in the second harmonic (178 MW/cm2 on a waist of 5 µm) and 12 W in the third harmonic (61 MW/cm2 in a waist of 2.5 µm) was obtained with pulses of 200 µs. This allowed to demonstrate the feasibility of material processing with very high reproducibility and definition, which for instance allowed the design and the manufacturing of micro-grippers and several other prototypes in the field of robotics and micro-engineering.

EPFL2001

Laser-induced stress changes in optical fibers

Florian Dürr

The precise determination of residual stress profiles in optical fibers allows drawing conclusions about radiation-induced density-changes of the fibers doped core glass. The corresponding index changes partially contribute to the phenomenon of photosensitivity, i.e. the modification of the refractive index of a glass through the irradiation with an appropriate laser source. Photosensitivity is widely exploited for the fabrication of fiber Bragg gratings which are in-fiber filter devices whose spectral response can be designed with great versatility. Fiber Bragg gratings find broad application nowadays in the field of telecommunication as well as in sensing applications. The polarimetric assessment of the integrated transverse fiber birefringence serves as a means to gather information about both stress and strain profiles of the fiber. Birefringence is introduced in the optical fiber due to the photo-elastic effect. In this work, a highly sensitive polariscope with sub-micron resolution has been developed. The captured birefringence data has been interpreted to be basically strain and not stress-induced, which allowed the proper discrimination between an inelastic, stress-free strain contribution and a strain part essentially accompanied by elastic stress. In earlier interpretations of the polarimetric data, all birefringence had been attributed to be purely stress-induced, which excluded the existence of inelastic strain and even led to contradictory results in some cases. The reinterpretation in terms of strain, however, results in a consistent picture of both stress and strain-induced birefringence. The polariscope has been used in the following to determine radiation-induced stress changes for different optical fibers and irradiation sources. Femtosecond-laser induced core stress changes in SMF-28® standard telecommunication fibers have been measured tomographically and correlated with corresponding index changes. Furthermore, stress has been correlated to the refractive index change in highly germanium-doped fibers irradiated with cw-irradiation at 244 nm. The annealing behavior of stress, strain and index in this fiber has been studied. Approximately the same linear correlation between stress and index change was found for all germanium-doped fibers under investigation. In addition, stress changes were measured for phosphorus- and nitrogen-doped fibers drawn at different tensions before and after irradiation with an ArF excimer-laser. The impact of the drawing force on fiber stress and inelastic strain anisotropy has been determined as well as radiation-induced stress modifications. The observed stress increase has been compared to those obtained for germanium-doped fibers. Atomic Force Microscopy of cleaved and subsequently etched fiber end-faces is a powerful means to gather topographic information on a nanometer scale due to differential etching of the doped core and the undoped silica cladding. Within this work, the technique has been used to acquire information about etch rate changes in fibers drawn from the same nitrogen-doped preform with different drawing conditions. Furthermore, the impact of UV-irradiation on the etch rate has been determined for phosphorus- as well as nitrogen-doped fibers. Comparison of the results, with stress measurements allowed the attribution of the etch rate changes to a modification of the color center population.

EPFL2005