Optical fibers doped with rare-earth ions as the gain medium have made possible efficient diode-pumped fiber lasers of diffraction-limited beam quality and remarkable potential for high power. Heat is efficiently dissipated from the long and thin gain medium; however, because the fiber core carries most of the optical power, intensities can readily reach values that evoke a nonlinear optical response and even cause fiber damage. This holds especially for ultrashort pulses that show at the same time high peak power.
An elegant way out is to enlarge the area of the fundamental propagation mode by a proper fiber design so that the optical power can fill a larger cross section without exciting unwanted transverse modes. Since the late 1990s, large-mode-area (LMA) fiber can be conveniently designed taking advantage of advanced photonic-crystal technology.
Femtosecond pulses at 50-W average power
By diode-pumping an LMA fiber composed of an ytterbium (Yb)-doped core, an inner cladding created by a surrounding grid of air channels in silica, an inner cladding layer of mostly air, and an outer cladding, researchers at the Friedrich-Schiller-Universität (Jena, Germany) demonstrated in 2003 powerful continuous-wave lasing in the 100-W range with 78% slope efficiency.1 Now that group, lead by Andreas Tünnermann, has extended the use of LMA photonic-crystal fiber to the generation of 50-fs pulses at 1038 nm with 73-MHz repetition by combining LMA technology with nonlinear pulse compression, using the same kind of fiber.2 The compressed pulses of about 1-mJ energy yielded a peak power of about 10 MW, with the average power topping 50 W.
The setup consisted of a passively modelocked, diode-pumped Yb-doped potassium gadolinium tungstate oscillator laser system whose output pulses passed a grating pulse stretcher and was subsequently amplified in two diode-pumped Yb-doped LMA fibers, with fundamental mode-field diameter as large as 35 mm. The large cross section kept the optical-power density low enough to allow the system to tolerate moderate pulse stretching, resulting in a fairly compact setup. Then the pulses passed through a pulse compressor consisting of two highly efficient and damage-resistant silicon gratings, yielding 300-fs pulses with up to 65-W average power. To this point, the initial spectral bandwidth of 5 nm around a 1038-nm central wavelength remained the same.
To provide final pulse shortening, the pulses were fed into a 3.5-m-long LMA fiber for further spectral broadening by self-phase modulation (SPM). The length of the fiber was optimized so that SPM and group-velocity dispersion (GVD) interplayed to provide an almost-linear frequency chirp during pulse propagation. The pulse was finally compressed by applying anomalous GVD in a chirped-mirror pair. A full-width-at-half-maximum pulse duration of 50 fs was deduced from an autocorrelation trace of the pulse obtained by second-harmonic generation (see figure).
1. F. Röser et al., Optics Lett. 30 (20), 2754 (2005).
2. F. Röser et al., Photonics West 2006, Fiber Lasers III, Session 10, PW6102-35.