Ultrashort Laser Pulse Phenomena, 2nd Edition

370 Ultrashort Sources II: Examples

when pumped with InGaAs diode lasers at 942 nm. A small quantum defect

implies that a minimum amount of energy is dissipated in the crystal in the form

of heat.

The combination of diode pumping (high wall plug efficiency), broad band-

width and small quantum defect has spurred the development of short pulse, high

average power Yb:YAG sources. The main problem to be overcome in develop-

ing high average output power sources is the removal of the heat produced by

pump intensities of the order of tens of kW/cm

2

. Two solutions have been imple-

mented, which led to pulse sources at 1.03 µm, subpicosecond pulse duration,

and several tens of watts of average power:

1. A thin disk Yb:YAG laser [114] and

2. Laser rods with undoped endcaps.

The undoped endcaps allow for symmetrical heat extraction on either side

of the beam waist. Typical average powers are between 20 and 30 W [71,115].

Quantum wells are generally used for mode-locking, with the exception of a

21 W, 124 MHz repetition laser using a variation of APML [71] (cf. Section 6.3).

In a thin-disk laser, the laser material has a thickness much smaller than the

diameter of the pump and laser mode. One end face of the disk is coated for high

reflectivity and put in direct contact with a heat sink. The resulting heat flow

is longitudinal and nearly one-dimensional. Typical disks are 100 µm thick, for

10% doping with Yb. An average power of 60 W, for 810 fs pulses at a repetition

rate of 34 MHz has been obtained [116].

6.7.6. Nd:YVO

4

and Nd:YLF

Both neodymium doped lithium yttrium fluoride (YLF) and vanadate (YVO

4

)

have gained importance as diode pumped lasers. The emission bandwidth is only

slightly larger than that of Nd:YAG, hence the shortest pulse durations that are

possible with these lasers are in the range of a few picoseconds (3 ps [118] to

5 ps [119] have been reported). The absorption bandwidth of Nd:vanadate is

roughly 18 nm, as opposed to 2.5 nm for Nd:YAG, making it a preferred crystal

for diode pumping.

Nd:YLF, like Alexandrite, is a long lifetime medium (twice as long as

Nd:YAG), hence an ideal storage medium for regenerative amplifiers. Its nat-

ural birefringence overwhelms the thermal induced birefringence, eliminating

the depolarization problems of optically isotropic hosts like YAG. For example,

a 15 W cw diode array was used to pump a Nd:YLF regenerative amplifier,

amplifying at 1 kHz 15 ps, 20 pJ pulses to 0.5 mJ [120].

The main parameters of Nd:YLF and Nd:YVO

4

are summarized in Table 6.7.