The completely alignment-free, environmentally-stable, ultra-compact and low cost femtosecond laser source has great potential for several applications but can also serve as an ultra-short seed source for a variety of short pulse amplifiers at the one micron wavelength region. In this study, we have developed a passively mode-locked environmentally-stable Yb-doped single-clad fiber laser. The linear cavity is constructed with polarization-maintaining single-mode single-clad fiber allowing environmentally-stable configuration. A highly ytterbium-doped (~ 300 dB/m absorption @ 976 nm) polarization maintaining (PM) fiber with a mode-field diameter of 4.8 μm and a length of 38 cm is used as gain medium. This fiber is pumped through a thin-film PM-WDM by a single mode diode providing a maximum output power of 400 mW at a wavelength of 976 nm. The passive fibers used in the setup are Panda 980 PM fibers with mode field diameter of 7 μm @ 1035 nm. All PM fibers are fusion spliced with a high polarization extinction ratio and a low loss. Self-starting passive mode-locking has been achieved through the semiconductor saturable absorber mirror. The SAM is based on a non-resonant design, using a GaAs/AlAs Bragg mirror with 27 layer pairs and 26 low temperature molecular beam epitaxy grown InGaAs quantum wells in front of the mirror. AR coated device has low-intensity absorption of 45 %, a modulation depth of ~ 30 %, and a saturation fluence of 100 μJ/cm2. The SAM shows a bi-temporal impulse response with a hort relaxation time of < 200 fs and a slower part of 500 fs. The ratio of the fast and slow parts has been determined to 3:2. It is directly glued to the fiber end-facet. A chirped fiber Bragg grating (CFBG) inscribed in a polarization maintaining fiber is employed for intra cavity dispersion compensation. It possesses a measured peak reflectivity of about 33% centered at 1035 nm with spectral Gaussian bandwidth of 16 nm. Therefore, besides the intra-cavity dispersion compensation the CFBG serves as the output coupler of the linear cavity. The dispersion of the CFBG has been measured by the spectral interferometry to be -0.19 ps2 at 1035 nm. Additionally, a fiber pig-tailed thin-film isolator is used at the output of the cavity to eliminate parasitic reflection into the cavity or sub-cavity effects.
B. Ortaç, M. Plötner, T. Schreiber, J. Limpert and A. TünnermannOptics Express, vol. 15, pp. 15595-15602 (2007)Double-Clad Fibers
The experimental setup of ultra-short, high-energy Yb-doped double-clad fiber laser is shown in Figure. A diode-pumped Yb-doped double-clad fiber was used as the amplifying medium. The pumping diode was coupled in the fiber with the V-groove technique (seen in Figure), which allows high pumping efficiency, while both fiber ends remained free. The laser diode operates at 975 nm, and its maximum output power is 3.7 W. The doped fiber’s length was 4 m, which allowed complete absorption of the pump power. The core diameter of the double-clad fiber was 7 mm with a numerical aperture of ~ 0.12, and its inner cladding was a 125 µm X 125 µm square with a numerical aperture of 0.4. In this fiber concept, we have developed a passively mode-locked Yb-doped double-clad fiber laser operating in the stretched-pulse regime. We have investigated the laser’s performance in pulse duration and energy as a function of pump power. The laser produces 4.8-ps Gaussian pulses with 85 mW of average power at an 18-MHz repetition rate. This corresponds to an energy per pulse of ~ 4.5 nJ. These pulses are extracavity compressed to near-bandwidth-limited 90-fs pulses.
B. Ortaç, A. Hideur, T. Chartier, M. Brunel, C. Özkul and F. SanchezOptics Letters, vol. 28, pp. 1305-1307 (2003) Microstructure LMA Fibers
The enlargement of the mode area, and hence the reduction of nonlinearity, has been investigated as a powerful approach for development of powerful ultrashort pulse laser sources. In recent years, several low-nonlinearity single transverse-mode fiber designs have been developed, e.g., fibers based on a photonic crystal inner cladding. In particular, large-mode-area (LMA) microstructure fibers allowed for a significant performance scaling of ultrafast fiber laser systems. In our studies, we focus on the development of newly design and original pulsed laser sources delivering unprecedented performance in term of power and energy levels by exploiting the advantages of next generation of optical fibers called LMA microstructure fibers. The wide variety of record average power, pulse energy, peak power, and shortest pulse generation from passively mode-locked Yb-doped fiber laser systems by using LMA microstructure fibers is successfully developed in our group.
B. Ortaç, J. Limpert and A. TünnermannOptics Letters, vol. 32, pp. 2149-2151 (2007)
A new LMA structure has been proposed arranging small doped filaments within the core. By surrounding the filaments with fluorine-doped silica, which has a lower refractive index value than pure silica, a further reduction of the effective core NA is obtained. Truly single-mode operation has been demonstrated based on such a multifilament-core fiber with large mode-field area. In this study, we report what we believe to be the first experimental demonstration of a passively mode-locked Er/Yb-doped multifilament-core intrinsically single-mode large-mode-area fiber laser that is operated in the purely anomalous dispersion regime. One key cavity element in this laser configuration is the erbium/ytterbium-doped multifilament-core LMA fiber. A cross section of this fiber is shown in Figure. The active core consists of a hexagonal lattice of 37 doped filaments with a diameter of 1.8 μm and filament-to-filament spacing of 5.1 μm, assembling a composed core with 28 by 31 μm in diameter. Numerical modelling based on the method of finite elements has been performed. The single-transverse-mode core has a mode-field diameter as large as 37 μm at the laser emission wavelength, which corresponds to an effective mode- field area of more than 1000 μm2, and an effective numerical aperture of 0.022. In this structure, the V parameter is less than 2.4 providing robust and effectively single-mode large core guidance. The pump core has a double-D shape with dimensions 208 × 250 μm coated with a low index silicone (NA 0.37). The pump light absorption of this structure is 0.9 dB/m at 915 nm and larger than 3.4 dB/m at 976 nm. The length of the fiber inside the cavity is about 2 m to obtain efficient pump absorption.
B. Ortaç, J. Limpert, S. Jetschke, S. Unger, V. Reichel, J. Kirchhof and A. TünnermannApplied Physics B: Lasers and Optics, vol. 98, pp. 27-31 (2010) Photonic Bandgap Fibers
An air-guiding photonic bandgap (PBG) fiber usually consists of a stack of thin-walled capillaries with an extra large hole in the center, as shown in Figure. Light is guided in a well-defined wavelength range and trapped by a 2D PBG of the cladding. The dispersion characteristic of such a bandgap fiber with a well-defined transmission range is in first order determined by the Kramers-Kronig relation and waveguide dispersion. Therefore, the dispersion is anomalous over much of the transmission band. Positively-chirped picosecond pulses are compressed to femtosecond range by using hollow-core photonic bandgap fiber (HC-PBG). An air-silica photonic bandgap fiber used (Crystal Fibre AIR-10-1060) for extra-cavity pulse compensation has a core diameter of 10.5 μm and the bandgap ranges from 980 nm to 1080 nm with an attenuation of less than 0.1 dB/m. The dispersion of this fiber is -0.0625 ps2/m at 1035 nm. An adapted length of this HC-PBG fiber has been fusion spliced to output with non-optimized losses of 2.3 dB. This setup allows for the generation of femtosecond laser pulses in an all-fiber format.
B. Ortaç, M. Plötner, T. Schreiber, J. Limpert and A. TünnermannOptics Express, vol. 15, pp. 15595-15602 (2007)