Passive mode-locking of 1.3 μm solid-state lasers is problematic for semiconductor saturable absorber mirrors
(SESAMs) not only because of difficulties in their fabrication process but also in relation to the achievable parameters
and damage resistivity. In contrast, single-walled carbon nanotube saturable absorbers (SWCNT-SAs) exhibit broadband
absorption which is controllable by varying the nanotube diameter and chirality, and require relatively simple
manufacturing processes. Here we report on steady-state mode-locked operation of a diode pumped Nd:YVO4 laser on
the 4F3/2→4I13/2 transition at 1.342 μm using a transmitting SWCNT-SA. The SWCNT-SA employed in the present work
was fabricated by SWCNTs grown by high-pressure CO conversion technique, showing broad absorption around
1.3 μm. The linear transmission at the laser wavelength was about 99%. The Nd:YVO4 laser was longitudinally pumped
by the unpolarized radiation of a 808 nm fiber-coupled laser diode. The ~1.2-m long cavity was optimized for large
fundamental mode size. Above threshold the laser operated first in the CW mode, then had a range of Q-switched modelocked
operation before reaching the regime of stable steady-state mode-locking. With an output coupler of 90%
reflectivity, the average output power in the steady-state mode-locked regime reached 0.8 W at a slope efficiency of
14.5% with respect to the incident pump power. At a repetition rate of 127 MHz this corresponds to single pulse energy
of 6.3 nJ. Such pulse energies are comparable to the best results obtained using SESAMs but the pulse duration of
16.5 ps measured in the present experiment is substantially shorter.
Passive mode-locking of Nd-lasers operating on the 4F3/2 → 4I13/2 transition is problematic for semiconductor saturable
absorber mirrors (SESAMs) not only because of difficulties in their fabrication process but also in relation to the
achievable parameters and damage resistivity. We investigate an alternative approach based on second-order nonlinearity
inside the laser cavity which utilizes negative χ(2)-lens formation in a SHG crystal assisted by nonlinear reflection of the
so-called "frequency-doubling nonlinear-mirror" (FDNLM). This approach has been previously employed only for
mode-locking of Nd-lasers emitting at 1.06 μm. Here we demonstrate passive mode-locking of a diode-pumped
Nd:YVO4 laser operating at 1342 nm based on negative χ(2)-lensing assisted by the FDNLM effect. Using a 7-mm-long
BiB3O6 (BIBO) nonlinear crystal or 10-mm-long and 1-mm-thick periodically-poled Mg-doped stoichiometric lithium
tantalate (PPMgSLT) crystal and output couplers highly-reflecting at the second-harmonic with optimized transmission
at the fundamental, we achieve average output powers in the steady-state mode-locked regime of the order of 1 W at
pulse durations in the 4-7 ps range. Such a combination of high output power and short pulse duration is superior with
respect to the results previously reported with SESAM mode-locked Nd-lasers operating on this transition. Higher
average powers have been obtained for this laser transition only by the complex additive mode-locking technique. In our
case the average power limit is set by the maximum power achievable in the fundamental transversal mode in the
continuous-wave (CW) regime. The shortest pulses (FWHM of 3.7 ps) can be very well fitted by sech2 temporal shape
assumption.
Experimental results on passive mode-locking of Nd:YVO4 laser using intracavity frequency doubling in periodically
poled KTP (PPKTP) crystal are reported. Both, negative cascaded chi-2 lensing and frequency doubling nonlinear mirror
(FDNLM) are exploited for the laser mode-locking. The FDNLM based on intensity dependent reflection in the laser
cavity ensures self-starting and self-sustaining mode-locking while the cascaded chi-2 lens process contributes to
substantial pulse shortening. This hybrid technique enables generation of stable trains of pulses at high-average output
power with several picoseconds pulse width. The pulse repetition rate of the laser is 117 MHz with average output power
ranging from 0.5 W to 3 W and pulse duration from 2.9 to 5.2 ps.
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