2.1 Ultrafast solid-state lasers - ETH - the Keller Group

Ref. p. 134] 2.1 Ultrafast solid-state lasers 129
[99Ude]. Knowledge of only two parameters, the comb spacing and a common offset frequency of
all modes, provides one with a set of reference frequencies, similar to the tick marks on a ruler. The
beat (difference-frequency) signal of this unknown frequency with the closest frequency in the ruler
always gives a beat signal smaller than the comb period. Thus, an optical frequency in the 100 THz
regime can be translated down to a microwave frequency in the range of 100 MHz, which can then
be measured very accurately. This can be used for an all-optical atomic clock that is expected to
outperform today’s state-of-the-art cesium clocks [02Ude]. The mode-locked laser provides a phaselocked
clockwork mediating between radio frequencies and the Terahertz frequencies of the lines in
the optical comb, effectively rendering optical frequencies countable. Details on precision frequency
measurements with mode-locked laser combs can be found in [99Rei, 01Hol, 01Cun, 02Bau].
2.1.9 Conclusion and outlook
We have shown that the technology of ultrafast lasers has become very refined and is now suitable
for application in many areas. Points of particular importance in this respect are:
– The transition from dye lasers to solid-state lasers, which can be compact, powerful, efficient
and long-lived. It has been shown that solid-state lasers can generate pulses which are even
shorter pulses than those generated in dye lasers with much more average output power.
– The development of high-power and high-brightness diode lasers for direct pumping of solidstate
lasers. This has lead not only to very efficient and compact lasers, but also to lasers with
more than 1000 W of average output power.
– The development of novel saturable absorbers such as KLM which has pushed the frontier
in ultrashort pulse duration into the two-optical-cycle regime using novel chirped mirrors for
dispersion compensation. New parameters such as the Carrier Envelope Offset (CEO) of the
electric field underneath the pulse envelope have become important for many different applications.
– The development of SEmiconductor Saturable Absorber Mirrors (SESAMs) which passively
mode-lock many different diode-pumped solid-state lasers without Q-switching instabilities and
can be optimized for operation in very different parameter regimes such as laser wavelength,
pulse duration, pulse repetition rate and power levels. SESAMs can be optimized independently
of the cavity design. This allowed for pushing the frontier in ultrafast solid-state lasers by several
orders of magnitude: femtosecond mode-locked lasers with close to 100 W of average output
power and more than 1 μJ of pulse energies and different lasers with high pulse repetition rates
up to 160 GHz have been demonstrated so far.
For the next few years we expect many new exciting developments in the field of diode-pumped
solid-state lasers:
– Very high power levels (> 100 W of average power) should become possible with passively modelocked
thin-disk lasers. Pulse durations just below 1 ps are feasible and with new materials the
regime of 200 fs or even below should become accessible with similarly high powers.
– Nonlinear frequency conversion stages (based on second-harmonic generation, sum frequency
mixing, or parametric oscillation) will be pumped with high-power mode-locked lasers to generate
short and powerful pulses at other wavelengths. The high power makes the nonlinear
conversion efficiencies very high with very simple arrangements [01Sue, 05Mar]. This will be of
interest for application in large-screen RGB display systems, for example [04Bru, 06Inn].
– Simple external pulse compression combined with novel high-average-power solid-state lasers
now allows for peak intensities as high as 12 MW with 33-fs pulses at the full pulse repetition
rate of the laser oscillator [03Sue]. This could be focused to a peak intensity of 10 14 W/cm 2 ,a
Landolt-Börnstein
New Series VIII/1B1