Providing optical feedback by a resonator enhances the efficiency of nonlinear optical effects, e.g. frequency
conversion. The bow-tie cavity is known to be a very successful scheme and it has made its way into the
commercial world of second harmonic generation and parametric oscillation. We demonstrate a continuouswave
optical parametric oscillator based on a bow-tie cavity converting monochromatic pump light at 1.03 μm
wavelength to signal light being tunable from 1.25 to 1.85 μm and to corresponding idler light from 2.3 to 5.3 μm.
We observe a signal power of up to 7 W, an idler power up to 3 W, and a mode-hop free operation over 10 h
without any active stabilization. Furthermore, we have extended the tuning range of the parametric oscillator to
the terahertz region: Our system converts near-infrared pump light to a monochromatic wave with a frequency of
1.35 THz and a power of 2 μW. Now, the straightforward next development step is to reduce the footprint of such
devices. For this purpose another type of ring cavity is very promising: the whispering gallery resonator. This
system offers unequaled opportunities because of its low loss leading to a high finesse. We discuss the challenges
for transferring the parametric oscillation scheme to whispering gallery resonators, addressing the preparation
of suitable resonators with a quality factor of 107 and a finesse of 500 and locking of the pump laser to a cavity
mode for 3 hours.
The photorefractive effect in iron-doped lithium niobate crystals is investigated, using femtosecond pulses and
cw light, both at a wavelength of 532 nm, in direct comparison. For that purpose, measurements about "light-induced"
or "holographic" scattering of a single beam as well as writing of index gratings with two interfering
beams are performed.
We find that light-induced scattering is reduced for femtosecond pulses, and even absent for a sufficiently low
Fe<sup>2+</sup> concentration, in comparison and in contrast to cw light. Additional differences include a slower buildup
time and a weaker angular selectivity for the scattering of pulses. Our observations can be attributed to the
smaller temporal coherence of the pulses.
When writing index gratings into as-grown lithium niobate crystals, the saturation value of the refractive
index unexpectedly decreases with increasing pulse energy fluence. Furthermore, in oxidized crystals, writing
with femtosecond pulses turns out to be much faster than with cw light. A model about the charge excitation,
migration and trapping is proposed that explains these differences.