VERY HIGH-PEAK-POWER LASERS
Cryogenic cooling
multiplies output of
Ti:sapphire laser

DIRK MÜLLER, STERLING BACKUS, KENDALL READ, MARGARET MURNANE

AND HENRY KAPTEYN

T he broad laser bandwidth and FIGURE 1. A cryogenically cooled Ti:sapphire laser amplifier crystal is excellent material properties of pumped by a frequency-doubled Nd:YLF laser. titanium-doped sapphire ( Ti:sapphire) have proved ideal for small- and large- 532 nm, (2.33 eV) is absorbed by the Ti:sapphire and scale ultrafast lasers and systems.¹ Incorpo- emitted as a photon at approximately 800 nm (1.55 eV).

© GARY LARSON 2004

rating cryogenic cooling into an ultrafast The rest of the energy—

Ti:sapphire laser amplifier can reduce Aggressively cooling roughly one-third of the
thermal-lens-induced aberrations by more incident power—is con-
than two orders of magnitude, resulting ultrafast lasers results in verted into heat. With
^{in additional performance improvements. a sharp reduction in thermal- a 100-W pump laser fo-} Although not common in lasers, cryogenic cused into a submilli-
cooling makes ultrafast systems more versa- _{lens-induced aberrations} meter spot, the heat load
tile and is also surprisingly cost-effective for creates very strong tem-
a variety of applications (see Fig. 1). and more-versatile systems perature gradients that
Ti:sapphire amplifier systems are typi- distort the optical prop-
cally pumped by frequency-doubled Nd: that are cost-effective for erties of the crystal, pro-
YAG or Nd:YLF lasers, and very high- ducing distortion of the
average-power lasers of this type (approxi- a variety of applications. amplified pulse. The dis-
mately 100 W) have existed for some time. tortion results from the
Previous-generation Ti:sapphire systems were incapa- changing index of refraction of the sapphire with temper-
ble of using these pump lasers fully, however, because of ature, and the physical expansion of the material, which
thermal-load limitations. In a Ti:sapphire laser amplifier, causes a bowing of the output faces.
light from the pump laser is focused on the Ti:sapphire To a first approximation, the thermal loading corre-
rod, and during amplification a photon at approximately sponds to a variable-focal-length lens in the optical sys-
tem. Proper amplifier design can compensate for this
DIRK MÜLLER is product line manager, STERLING BACKUS is CTO, lens to some extent, but large optical aberrations remain.
KENDALL READ is COO, MARGARE T MURNANE is president and _{Hence, early-generation Ti:sapphire systems were limit-}

CEO and HENRY KAPTEYN vice president at Kapteyn-Murnane

Laboratories, 1855 South 57th Court, Boulder CO 80301; e-mail: ed to about 1 W before thermal distortion became a seri-
^{dmueller@kmlabs.com; www.kmlabs.com.} ous problem. Potential applications were also limited be-