F. X. Kärtner

DESIGN AND FABRICATION OF DOUBLE-CHIRPED MIRRORS

We present an analytic design method for the reproducible fabrication of double-chirped mirrors to achieve simultaneously a high reflectivity and dispersion compensation over an extended bandwidth compared with those of standard quarter-wave Bragg mirrors. The mirrors are fabricated by ion beam sputtering. Use of these mirrors in a Ti:sapphire laser leads to 6.5-fs pulses directly out of the laser. The method can also be applied to the design of chirped-fiber gratings and general optical filters.

Stabilization of solitonlike pulses with a slow saturable absorber

We show that a soliton of the nonlinear Schrödinger equation perturbed by filter losses and/or the finite gain bandwidth of amplifiers can be kept stable by saturable absorbers with a relaxation time much longer than the width of the soliton. This provides for ultrashort pulse generation with a slow saturable absorber only and may have possible applications in the stabilization of soliton storage rings.

Self-starting 6.5-fs pulses from a Ti:sapphire laser

We demonstrate self-starting 6.5-fs pulses from a Kerr-lens mode-locked Ti:sapphire laser with 200-mW average output power at a pulse repetition rate of ~86 M Hz. This is to our knowledge the shortest pulse ever generated directly from a laser. For dispersion compensation we used a prism pair in combination with double-chirped mirrors, which balances the higher-order dispersion of the prism pair and therefore flattens the average total group-delay dispersion in the laser cavity. For self-starting mode locking we used a broadband semiconductor saturable-absorber mirror.

56-ps passively Q-switched diode-pumped microchip laser

We passively Q switched a diode-pumped Nd:YVO4 microchip crystal with an antiresonant Fabry-Perot saturable absorber and achieved single-frequency pulses as short as 56 ps. We can vary the pulse width from 56 ps to 30 ns and the repetition rate from 27 kHz up to 7 MHz by changing the design parameters of the saturable absorber and the pump power.

Solitary-pulse stabilization and shortening in actively mode-locked lasers

We present a theory and experiments on active mode locking in the presence of negative group-velocity dispersion (GVD) and self-phase modulation (SPM). It is shown that beyond a critical value of GVD a solitonlike pulse can be stabilized by the mode locker. The width of the soliton can be shorter than the width of the Gaussian pulse produced by the mode locker in the absence of soliton shaping. We establish analytically that the pulse shortening possible by addition of SPM and GVD is limited only by the requirement that the phase shift of the soliton per round trip be limited. Parameter ranges allowing for stable solitary-pulse formation and shortening are derived and discussed for different gain media and compared with numerical simulations and experimental results.

Passively Q-switched 180-ps Nd:LaSc 3 (BO 3 ) 4 microchip laser

We passively Q switched a Nd:LaSc(3)(BO(3))(4) microchip laser with an antiresonant Fabry-Perot saturable absorber (A-FPSA) and achieved single-frequency, 180-ps pulses with 0.1 microJ of pulse energy at a repetition rate of 110 kHz. Because of the compactness and scaling possibilities offered by the A-FPSA, the pulse width can be varied from 180 ps to 30 ns and the repetition rate from 50 kHz to 7 MHz.

Measurement of the Raman gain spectrum of optical fibers

The stimulated Raman gain spectrum of optical fibers has been measured down to 6 cm(-1) by means of short pulses. Results for parallel and perpendicular polarizations are reported. With this technique, we observe spectral oscillations arising from a Brillouin mediated coupling between cw and pulsed light.

Continuous-wave mode-locked solid-state lasers with enhanced spatial hole burning

We systematically investigate the difference between both actively and passively mode-locked lasers with Gain-at-the-End (GE) and Gain-in-the-Middle (GM) at the example of Nd:YLF lasers. The GE laser generates pulse widths approximately three times shorter than a comparable GM cavity. This is due to enhanced Spatial Hole Burning (SHB) which effectively flattens the saturated gain and allows for a larger lasing bandwidth compared to a GM cavity. We first investigate enhanced SHB by measuring the cw mode spectrum, where we have observed that the mode spacing in GE cavities depends primarily on the crystal length. This was also confirmed for a Nd:LSB crystal, where the pump absorption length was significantly shorter than the crystal length. In mode-locked operation, pulse widths of 4 ps for passive mode locking and 5 ps for active mode locking are demonstrated with GE cavities, compared to 11 ps for passive and 17 ps for active mode locking with GM cavities. Additionally, the time-bandwidth product for the GE cavity is approximately twice the ideal product for a sech2 pulse shape and cannot be improved by dispersion compensation alone, while the GM cavity has nearly ideal time-bandwidth-limited performance. The results for the GM cavity compare well to existing theories taking into account the added effect of pump-power-dependent gain bandwidth which increases the bandwidth of Nd: YLF from 360 to > 500 GHz. In a following paper [1] (called Part II) a rigorous theoretical treatment of the effects due to SHB will be presented.

Diode-pumped mode-locked Nd:glass lasers with an antiresonant Fabry–Perot saturable absorber

We demonstrate passively mode-locked diode-pumped Nd:glass lasers with different media such as silicate, phosphate, and fluorophosphate that are homogeneously or inhomogeneously broadened. An antiresonant Fabry-Perot saturable absorber starts and stabilizes the soliton mode-locked Nd:glass lasers, producing pulses as short as 130 fs at an average output power of 100 mW. With a cw Ti:sapphire pump laser we obtain pulses as short as 90 fs.

Experimental verification of soliton mode locking using only a slow saturable absorber

We demonstrate experimentally that solid-state lasers with strong solitonlike pulse shaping can be mode locked by a slow saturable absorber only, i.e., the response time is much slower than the width of the soliton. A Ti:sapphire laser mode locked by a low-temperature-grown GaAs absorber with 10-ps recovery time generates pulses as short as 300 fs without the need for Kerr-lens mode locking and critical cavity alignment. An extrapolation of this result would predict that an asymptotically equal to 100-fs recovery time of a semiconductor absorber could support pulses into the 10-fs regime.