Anne Hildenbrand

Accurate metrology for laser damage measurements in nonlinear crystals

Accurate laser damage measurements are more difficult to perform in nonlinear optical crystals than in glasses due to several effects proper to these materials or greatly enhanced in these materials. Before discussing these effects, we address the topic of error bar determination for probability measurements. Error bars for the measured damage probabilities are especially important when testing small and expensive samples like nonlinear crystals, where only few sites are used for each measurement. The mathematical basics for the numerical calculation of probability error bars corresponding to a chosen confidence level are presented. Effects that possibly modify the maximum light intensity obtained by focusing into a biaxial nonlinear crystal are mainly the focusing aberrations and self-focusing. Depending on focusing conditions, position of the focal point in the crystal, beam propagation direction, and polarization, strong aberrations may change the beam profile and drastically decrease the maximum intensity in the crystal. A correction factor based on former theoretical work is proposed for this effect. The characteristics of self-focusing are quickly reviewed for the sake of completeness, and a note on parasitic second harmonic generation is added at the end.

Model for nanosecond laser induced damage in potassium titanyl phosphate crystals

A model for nanosecond laser induced damage in the bulk of potassium titanyl phosphate nonlinear optical crystals is presented. In a first step, laser-induced damage precursors are produced by multiphoton absorption. In a second step, the damage precursors are activated. Damage occurs if the precursor activation rate exceeds a critical value. Basic considerations allow evaluating the parameters of the model. The validity of the model is discussed by comparing it to several experimental observations, in particular, the decrease of the laser damage threshold during second harmonic generation of 1064 nm pulses.

Laser damage of the nonlinear crystals CdSiP2 and ZnGeP2 studied with nanosecond pulses at 1064 and 2090 nm

Abstract. CdSiP2 (CSP) is a very promising nonlinear crystal for the mid-infrared spectral range with a nonlinear coefficient slightly larger than that of ZnGeP2 (ZGP). In contrast to ZGP, CSP is phase-matchable and can be employed in 1.064-μm pumped optical parametric oscillators (OPOs) without two-photon absorption. Although low damage resistivity has been reported in such initial OPO tests of CSP, no reliable data on the damage threshold of uncoated CSP exists. In this work, we compare the damage resistivity of uncoated CSP with ZGP at two wavelengths, 2.09  μm (1 kHz, 21 ns) and 1.064  μm (100 Hz, 8 ns).

Laser damage investigation in nonlinear crystals : Study of KTiOPO4 (KTP) and RbTiOPO4 (RTP) crystals

High power or miniaturized laser systems are limited by the laser damage resistance of optical components, particularly of nonlinear crystals. The laser damage of optical components depends on many factors such as wavelength, frequency, pulse duration, spot-size,... Moreover, in nonlinear crystals, the anisotropy of physical parameters may cause anisotropy of the Laser Induced Damage Threshold (LIDT). Thus, the LIDT may depend on polarization or propagation direction of the laser beam. The aim of this paper is to discuss the laser damage results of two nonlinear crystals: KTiOPO4 (KTP) and RbTiOPO4 (RTP). In general, due to its higher effective electro-optic coefficients, RTP is more used for electrooptic applications, whereas KTP is popular for second harmonic generation. Laser damage tests in KTP and RTP reveal that for both crystals the LIDT depends on the polarization. The laser damage tests were carried out at 1064nm with a nanosecond Nd:YAG laser. The tests were performed with the polarization and the propagation direction of the light along a principal crystal axis, and all configurations were tested with a parallel beam (waist diameter 75μm). As they belong to the same crystal family, RTP and KTP crystals have similar nonlinear optical properties. This work also reveals that the laser resistance of KTP and RTP is very close. Functional laser damage tests of RTP for Pockels cell applications and SHG-cut KTP were performed too. We also discuss the influence of second harmonic generation on the LIDT.

Laser damage investigation in RbTiOPO4 crystals: a study on the anisotropy of the laser induced damage threshold

Lasers for space applications require miniaturized high power components that can be operated at low voltages. RbTiOPO4 (RTP) is a highly efficient electro-optical material, which is used in particular for the realization of low voltage and high repetition rate Pockels cells. RTP can be operated in two crystal orientations (x-cut and y-cut). In both cases, the incoming linear polarization is oriented at 45o to the z-direction. In this study, laser damage is investigated in RTP crystals. More precisely, we focus on the correlation between the laser damage characteristics and the used crystal orientation. The laser damage tests were carried out at 1064 nm with a standard 6 ns Q-switched Nd:YAG laser and the polarization was oriented as for Pockels cell operation at 45o to the z-axis of the crystals. This work reveals that the Laser Induced Damage Threshold (LIDT) is two times higher for x-cut than for y-cut RTP crystals. Reflection and transmission measurements show that this LIDT anisotropy can not be explained by an evident loss mechanism like Stimulated Raman Scattering (SRS).

Laser damage investigation in KTiOPO4 (KTP) and RbTiOPO4 (RTP) crystals: threshold anisotropy and the influence of SHG

The laser damage probability of nonlinear optical crystals depends on many different factors. In addition to the fabrication process (crystal growth, cutting, polishing, coating) the damage threshold of non-linear crystals is influenced by the usage of the crystal. The anisotropy of material properties like the complex refractive index, the mechanical yield strength and the polarizability may cause anisotropy of the Laser Induced Damage Threshold (LIDT). The LIDT may depend on the propagation direction or the polarization of the light. Anisotropy in the LIDT has in fact been observed in different crystals. The dependency of the KDP-LIDT on the propagation direction and its independence on the polarization direction reported by Burnham et al. is an example that is not yet fully understood. For KTP it has been evidenced by Hu et al. that the grey-tracking threshold is polarization dependent. In this contribution we discuss the bulk laser damage resistance of two isomorphous non-linear crystals: KTiOPO4 (KTP) and RbTiOPO4 (RTP). All tests are performed using a nanosecond laser at 1064nm wavelength. For both crystals all polarization and propagation directions parallel to the principal axes have been tested. In addition we investigated two typical devices: two types of Pockels cells in RTP and a SHG-cut crystal in KTP. The results are analyzed on the basis of an anisotropic sensitivity of the crystal to the electric field and the efficiency of second harmonic generation.

Nanosecond-laser induced damage at 1064 nm, 532 nm, and 355 nm in LiB3O5

LiB3O5, short LBO, is an important nonlinear optical material for frequency conversion. As efficient frequency conversion requires high intensities nonlinear optical crystals are often subject to laser induced damage even in commercial laser systems. In this work we studied nanosecond laser induced damage in LBO at the three harmonic wavelengths of the Nd:YAG laser: 1064nm, 532nm and 355nm. Similarly to KTP and RTP a polarization dependent anisotropy of the laser induced damage threshold has been found, being strongest at 1064nm wavelength. The weakest point of this material regarding the three tested wavelengths is the bulk damage threshold at 355nm. The fatigue effect was found to be negligible at the UV wavelength and most important for the IR light. Despite the fact that the green light has been generated by external frequency doubling, it caused an intermediate fatigue effect. Our measurements also confirmed the high bulk laser damage threshold for the IR wavelength being approximately a factor 1.5 higher than the one of synthetic fused silica.

Laser damage metrology in biaxial nonlinear crystals using different test beams

Laser damage measurements in nonlinear optical crystals, in particular in biaxial crystals, may be influenced by several effects proper to these materials or greatly enhanced in these materials. Before discussion of these effects, we address the topic of error bar determination for probability measurements. Error bars for the damage probabilities are important because nonlinear crystals are often small and expensive, thus only few sites are used for a single damage probability measurement. We present the mathematical basics and a flow diagram for the numerical calculation of error bars for probability measurements that correspond to a chosen confidence level. Effects that possibly modify the maximum intensity in a biaxial nonlinear crystal are: focusing aberration, walk-off and self-focusing. Depending on focusing conditions, propagation direction, polarization of the light and the position of the focus point in the crystal, strong aberrations may change the beam profile and drastically decrease the maximum intensity in the crystal. A correction factor for this effect is proposed, but quantitative corrections are not possible without taking into account the experimental beam profile after the focusing lens. The characteristics of walk-off and self-focusing have quickly been reviewed for the sake of completeness of this article. Finally, parasitic second harmonic generation may influence the laser damage behavior of crystals. The important point for laser damage measurements is that the amount of externally observed SHG after the crystal does not correspond to the maximum amount of second harmonic light inside the crystal.

Quasi-Phase-Matched Gallium Arsenide for Mid Infrared Frequency Conversion

Progress in processing low-loss quasi-phase-matched gallium arsenide crystals allows their excellent nonlinear properties to be employed in practical mid infrared devices. This presentation will address both crystal growth aspects and the most recent devices demonstrations.

Influence of the Laser Beam Size on the Laser-Induced Damage in Thin Films and Substrates

The influence of the laser beam size on the laser-induced damage threshold (LIDT) in thin films and substrates is investigated. LIDT measurements realized with beam of different dimensions give information on laser damage precursors.