Abstract

Recent experiments succeeded in sub-100fs pulse generation from solid-state lasers by use of a saturable single-quantum-well absorber (SQW) yielding self-starting 34fs pulses in a ti.sapphire laser [1] as well as 90fs pulses in a diode pumped Cr:LiSAF laser [2]. These experimental findings can hardly be explained by the common theoretical understanding of passive mode locking in solid-state lasers which requires a fast saturable loss for stable pulse generation. The relaxation of the absorption bleaching in a GaAs quantum well at room temperature exhibits a bitemporal behavior where the fast component of about 200fs owing to thermal ionization of the exzitons is remarkably slower than the meanwhile achieved pulse width in the experiments. Recently we have proposed a novel mechanism for mode locking a solid state laser by use of a slow off-resonant saturable two-level absorber [3]. Here we analyze the operating conditions for this new mode locking scheme by use of a slow saturable SQW-absorber and calculate the dependence of the pulse parameters on the characteristical laser and absorber parameters. The underlying principle of mode locking of this mechanism is that in a first order expansion beyond the rate equation approximation a slow saturable absorber with an arbitrary large response time T₁produces an additional intensity dependent loss $\Delta \alpha =-\gamma \frac{d\Delta n}{d\omega }I$, where Δnis the linear refractive index change owing to the near-resonant transition, γis an absorber parameter and I is the pulse intensity. The physical origin of the intensity dependent loss Δαis that due to the finite phase relaxation time τ_athe coherent medium polarization encounters a group velocity change which leads owing to the slow saturation under off-resonance with ${\frac{d\Delta n}{d\omega }|}_{{\omega }_L}>0$ to a negative fast saturable loss term Δα< 0. To study this mechanism for a SQW absorber we consider a simple 2D free carrier band filling model [4]. Using these simplifications we derive the spectrum of the complex SQW-susceptibiliy χ(ω) in dependence on the optically induced carrier density N. Expanding the time-domain round trip operators with respect to small parameters we deduce a stationary mode locking equation which governs the steady-state pulse parameters of the mode locked laser. The calculated pulse parameters in dependence on the SQW and laser parameters shows that in a reasonably extended operating region a near-resonant slow saturable SQW-absorber always yields an additional fast saturable loss which is sufficient to provide and maintain stable passive mode locking in broad band solid state lasers. Thus, off-resonant slow passive mode locking (OPSM) [3] with a slow saturable SQW-absorber represents a reliable novel method for fs-pulse generation in solid-state lasers and can explain the main findings in recent experiments [1,2].

© 1996 IEEE