KTP key to better frequency doubling

Gabriel Mennerat, laser physicist from the CEA Alisé facility, told optics.org, "With an effective conversion factor [a "nonlinear coefficient of 3.3 pm/V"] 12 times larger than KDP, KTP makes it possible to achieve high conversion efficiencies with thinner crystals and at lower intensities. Also, KTP presents angular and thermal acceptances substantially larger than those of KDP, resulting in much better tolerances to input wavefront distortions, misalignment and thermal gradients."

Alisé as 1, 2, 3...

Although the KTP frequency-doubling crystal is more expensive than its KDP counterpart, the overall cost and energy demand of the whole frequency-doubled source can be reduced because the laser's beam tolerances are greater and the associated equipment is less expensive.

"This result has been possible because we can now grow sufficiently large KTP crystals," Marc-Andre Herrmann of Cristal Laser - which produced the KTP crystals for the experiment - told optics.org, "Compared with the KDP-based laser, the KTP version achieves a much better conversion efficiency and at lower energies."

Such high-energy green lasers are required to pump large-aperture chirped-pulse amplifiers for fundamental research, where the key issue is achieving very short, high-intensity pulses at the highest possible overall efficiency and repetition rate. High-peak-power lasers ("petawatt-class") ultimately have applications ranging from assessing atomic movements to the investigation of sub-atomic particles, and in the acceleration of electrons and protons for various cancer treatments.

Mennerat explained the motivation behind this demonstration with KTP: "The two main broadband amplification techniques for ultrashort pulses developed today – either laser amplification in Ti:Sa crystals or parametric amplification in nonlinear crystals like on the Alisé facility - need homogeneous and energetic pump beams in the green. In both cases, the repetition rate of the whole system is limited by thermally-induced optical aberrations in the pump-laser amplifiers. Those aberrations have a dramatic effect on frequency doubling in KDP crystals. Switching to KTP may be the key."

KTP had not been used before in this kind of laser because of its relatively low availability. In contrast, KDP is widely available in large pieces (typical apertures can be as large as 150x150 mm²) but KTP has only been available only in pieces of up to 20x20 mm² in size.

Herrmann says that the experiments at the Alisé facility show the clear strengths of KTP for high-energy frequency-doubling: "The idea was not to compare the energy achieved at 527 nm with other lasers but to compare the performance of KTP with that of KDP. The problem was the difficulty of growing KTP over the past 20 years - it's only recently become a suitable material for lasers of this type."

He added, "Some of Cristal Laser's customers have recently switched from KDP to KTP particularly because of cost advantages. Other associated components of the laser can be significantly less expensive than those used when the laser is frequency-doubled with KDP - even though KTP crystal itself is more expensive than KDP. Overall the KTP set is cheaper and has lower beam specifications but higher overall performance."

Explainer: frequency doubling

KDP or potassium dihydrogen phosphate (KH₂PO₄) had been the only nonlinear optical crystal produced in suitable dimensions for frequency conversion at very high energy (> 500 J). Other crystals such as potassium titanyl phosphate (KTiOPO₄ alias KTP) and lithium borate (LiB₃O₅) are used in smaller sizes but at lower energies.

Thanks to recent progress in crystal growth, large KTP crystal boules are now available and KTP may be also considered for frequency conversion of multi-Joule laser beams.

French company Cristal Laser supplied the Alisé facility with a couple of antireflection-coated KTP frequency doublers with an outer diameter of 38 mm and thickness ranging from 2 to 5 mm. The crystals were cut for type II phase matching in the x-y principal plane with a normal to optical surfaces tilted with an angle f=33° with respect to the x axis.

Their performance as nanosecond frequency doublers were compared to that of a 10mm-thick type I KDP doubler, cut with an angle q=41.2° from the z axis, whose length was optimized for shorter, 3-ns pulses. This 150x150mm² crystal grown by Saint Gobain Crystals and Detectors is coated at CEA with solgel antireflection coatings that also prevent surface degradation of this notably hygroscopic material.

The energy, pulse shape and beam profile of the input and residual fundamental wave as well as those of the generated second harmonic were recorded for each shot.

Taking advantage of the versatility of the Alisé front end, which can deliver frequency modulated nanosecond pulses as well as chirped pulses in the femtosecond regime, this bench is also widely used for high energy functional assessments of nonlinear crystals and novel frequency conversion schemes. The modular construction of the laser makes it possible to modify the beam profile and pulse shape and duration.

The Alisé source, which is custom-built using generally common components, is a high power Nd:glass laser with an energy up to 200 J. The beam is typically used for studies on laser physics and for ultra-high intensity interaction with solids, gases, and plasmas both within the CEA and by the wider international scientific community.

Alisé is part of the European Union's FP6 Laser Lab Europe Integrated Infrastructures Initiative. The facility also serves to support the Laser Mégajoule project and vocational training. The Alisé facility is equipped with a large aperture, frequency conversion laser bench for laser physics experiments, for instance to study the effects of various beam smoothing techniques on high energy harmonic generation.