Improved SESAMs for femtosecond pulse generation approaching the kW average power regime

Abstract

We present semiconductor saturable absorber mirrors (SESAMs) that can potentially support femtosecond pulses from ultrafast thin disk lasers (TDLs) with high average power approaching the kW-power level and high pulse energy in the range of 100 µJ to 1 mJ at megahertz pulse repetition rates. For high-power operation, the SESAM parameters will ultimately limit the shortest pulse duration from a soliton mode-locked laser before mode locking instabilities such as multiple pulsing instabilities and continuous wave (cw) breakthrough start to occur. Currently shorter pulses are prevented due to the inverse saturable absorption that becomes stronger with shorter pulses and results in a shift of the "rollover" of the nonlinear SESAM reflectivity towards lower fluences. Here we discuss a novel SESAM design that addresses these issues and can be grown by metal-organic vapor phase epitaxy (MOVPE), an attractive epitaxial growth technology for manufacturing.

XUV Sources Based on Intra-Oscillator High Harmonic Generation With Thin-Disk Lasers: Current Status and Prospects

“…It is made of four InGaAs quantum wells (QWs), placed in two pairs in subsequent antinodes of the electric field. An embedment of the QWs in AlAsP layers ensures full strain compensation of the epitaxial structure [108]. A dielectric top-coating is added to reduce the field enhancement and prevents negative effects related to multi-photon absorption [109].…”

“…However, novel substrate-transferred directlybonded SESAMs demonstrated reduced thermal lensing under thermal load, very low imperfection and large cold surface flatness [127], which should allow for high intracavity power TDL oscillators with ultrashort pulses. Additionally, novel designs allowing the growth of strain-compensated SESAMs with multiple QWs by metal-organic vapor phase epitaxy [108] can lead in the future to industrial-grade SESAMs suitable for intraoscillator HHG. Finally, as shown in Fig.…”

Degradation mechanism of SESAMs under intense ultrashort pulses in modelocked VECSELs

“…[29][30][31][32][33][34][35][36] Among all the studies carried out to measure the damage threshold of InGaAs QW-based SESAMs, it is fairly widely accepted that the damage mechanism is dominated by two-photon absorption (TPA), especially in the case of femtosecond pulses. TPA is a non-linear process by which a carrier is excited by two photons 37 , the sum of whose energies corresponds to a very high-energy state in the conduction band of the semiconductor.…”

Peak-power scaling of femtosecond Yb:Lu_2O_3 thin-disk lasers

“…Moreover, the required large saturation fluence and modulation depth can be achieved by increasing of the number of QWs of the SESAM structure, which requires additional strain-compensating layers in order to keep the crucial high surface quality. Recently, we have successfully grown high-quality SESAMs with a large number of QWs [51] and investigated a novel direct bonding technique resulting in superior SESAM flatness and thermal properties [52]. These newly developed technologies enable SESAM structures fulfilling the requirements for high peak-power, short-pulsed modelocked operation.…”