A laser-induced damage study is performed on blank, uncoated Rb:KTiOPO4 (RKTP) samples at 1.03 µm with two different pulse durations of ~0.3 and ~1.0 ps and a repetition rate of 100 kHz. The effect of the sample temperature is also considered.
© 2017 Optical Society of America
KTiOPO4 (KTP) is one of the most studied nonlinear optical crystals in all aspects . A major motivation for its development, and also the development of an entire family of isomorphs with mm2 point group symmetry and similar properties (RbTiOPO4 (RTP), KTiOAsO4 (KTA), RbTiOAsO4 (RTA), and CsTiOAsO4 (CTA)), has been the application in 1-µm pumped optical parametric frequency down-conversion devices, as an alternative to LiNbO3 (LN) which shows low – and often unpredictable – optical damage threshold. In particular, the photo-refractive damage is known to be less pronounced in the KTP type crystals compared to LN. The electric-field periodic poling of the ferroelectric KTP (demonstrated also for all the isomorphs except CTA ) for quasi-phase matched (QPM) frequency conversion  boosted the interest in this family of nonlinear crystals. Another significant advantage with regard to LN and the related LiTaO3 came into play: the lower coercive field. The technological development simultaneously revealed that specific material properties have to be engineered in order to obtain large aperture samples with homogeneous domain structure . Thus, the partial substitution of K by Rb was shown to overcome the major limitations of KTP with respect to QPM device fabrication while at low doping levels (< 1 mol. %) linear and nonlinear optical properties are expected to remain unchanged . Rb-doped KTP, Rb:KTP or shortly RKTP is a very promising alternative for electric-field poling because the pure Rb isomorphs are not readily available from commercial vendors with acceptable quality for high-yield periodic poling .
Extensive literature on laser induced damage threshold (LIDT) of KTP at 1.064 µm exists but it is confined to ns pulse durations and low repetition rates or single shots, e.g [6–9], i.e. it is relevant mainly to optical parametric oscillators. Bulk damage was observed in all these studies, in close relation to gray track formation which shows strong anisotropy leading to highest LIDT for E//z polarization, as well as fatigue effects (dependence on the number of pulses). However, for KTP and RKTP at 100 Hz repetition rate, the dominant effect with 8-ns long pulses was defect related surface damage .
Recent progress in high-average-power, picosecond (ps) Yb laser amplifier technology has enabled the power scaling of optical parametric amplifiers (OPAs) and in particular chirped-pulse OPAs (CPOPAs) where large aperture damage resistant crystals of the KTP family could be very useful in the spectral range from about 1.5 to beyond 4 µm, e.g . The importance of this laser pulse parameter regime  has motivated us to study LIDT for RKTP at 1.03 µm with polarization E//z (corresponding to the situation in QPM where one utilizes the maximum d33 diagonal element of the 2nd order nonlinear susceptibility tensor) and pulse durations of ~1 ps and shorter. Here we present the results of such measurements performed at 100 kHz with pulse durations of ~0.3 and ~1.0 ps, including also heating of the samples up to 90°C.
2. Experimental set-up and samples
We employed the so-called R-on-1 method to measure the LIDT . The main reason for this choice was the limited number of RKTP samples and the available surface area. Thus, the pulse energy incident on the sample was increased in a step-wise fashion until damage could be observed. The exposure time for each step amounted to 5 s.
In most experiments, a commercial ultrafast Yb-fiber laser amplifier system (Satsuma HP2, Amplitude Systemes) was used for the damage tests. The pulse duration was set to either 330 or 930 fs. In some experiments, a commercial, ~1-ps Yb:YAG laser amplifier system (Amphos 400, Amphos GmbH) was used. Both systems operated at 1.03 µm and 100 kHz.
The automatic, on-line laser damage detection setup is shown in Fig. 1. A fraction of the laser beam was split off and monitored by a reference photodiode (PD1) calibrated to the actual laser pulse energy or average power incident on the sample. A motorized half-wave plate and a polarizer were used to adjust the pulse energy incident on the sample. The pulse energy was increased in a stepwise fashion at computer controlled intervals. Monitoring the light scattered from laser-illuminated sample surfaces is commonly used in on-line damage detection, where an abrupt, drastic increase in light scattering indicates damage . To detect the occurrence of optical damage, we monitored not only the Yb-laser light scattered from the sample, but also the scatter from a visible, 633-nm HeNe laser beam propagating collinearly with the 1.03-μm pulses. The photodiode PD2 used to detect scattered light at 633 nm was located behind a short-pass filter (SPF) with a cut-off wavelength at 1 μm and a laser line interference filter. The scattered light above 1 µm was monitored by a third photodiode PD3. During ramping-up the incident 1.03-µm pulse energy, the PD2 signal stays constant while the PD3 signal increases linearly until material modifications, stimulated polariton scattering (SPS), or damage occur.
The Faraday isolator shown in Fig. 1 was used to protect the laser amplifier from backward SPS possible for ps pulse durations . The criterion for estimating the damage of the illuminated sample site was defined by a certain threshold for the scatter signal detected by PD2 (i.e. at 633 nm). The threshold level for the monitor diodes was determined empirically: it should fall into the nonlinear scatter regime, but should be at a high enough level not to confuse the catastrophic damage with the non-permanent, photorefractive modifications. Thus, the effect of scattered light in the vicinity of 1.03 μm detected by PD3 due to SPS could be eliminated. The measurements showed, however, that the drastic changes in the signals from the two photodiodes lead to very close results for the LIDT. Therefore, it can be concluded, that for the conditions of the present experiment the signal from PD3 can be equally useful for LIDT determination. Once optical damage was detected in such a way, a shutter was automatically closed to block the 1.03 µm laser beam. Figure 2 shows a representative measurement with all diode signals. After each damage test, the studied sample was translated by means of a manual 3D stage. The distance to the new test spot was at least 0.2 mm. In addition, before the next damage test, the surface of the sample was cleaned using a bulb blower air duster. To investigate the temperature dependence the sample was placed in an oven.
To ensure that the laser fluence/intensity exceeds the damage threshold the pump beam had to be focused. We used an f = 300 mm singlet lens with the sample in the focal plane. The laser beam profile was recorded by a 16-bit CCD camera located in an equivalent plane, i.e. at the same distance from the lens. To this aim, a wedge on a flip-mount was used for redirecting the beam, see Fig. 1, whereas the proper distance was adjusted by a motorized translation stage. The spot size at the front surface was ~62 µm and 83 µm (full width at half maximum, FWHM) in the case of the Satsuma and the Amphos laser, respectively, with close to Gaussian beam profiles. The measured M2 values were ≤ 1.2 for both lasers in both planes.
The pulse duration could be adjusted by the compressor of the Satsuma laser system and second-harmonic (SH) frequency resolved optical gating (FROG) was employed to characterize the pulses. In the present study we used two pulse durations: 330 and 930 fs (FWHM intensity). The pulse duration of the Amphos system was fixed at 1.3 ps. Following the technique described in , the LIDT fluence was calculated for each measurement as a peak on-axis value based on the actual laser beam profile recorded by the CCD camera. The corresponding peak on-axis intensity was obtained then by dividing by the pulse duration.
Four uncoated, blank, x × y × z = 2 × 5 × 5 mm3 RKTP samples were studied, where x corresponds to the sample length along the beam propagation direction, y is the sample width and z is the dimension along which electric field is applied for poling (in the orthorhombic KTP the correspondence between the principal optical and the crystallographic axes is xyz = abc). The samples were polished to optical finish on both x-faces using industry-standard Logitech equipment and employing SF1 Syton chemo-mechanical polishing suspension.
3. Results and discussion
Before reaching the damage threshold, an expansion of the transmitted beam profile could be seen due to self-focusing (SF). Similar self-focusing effects at such peak powers and intensities were observed and analyzed for LN in . We note that self-focusing modifies the spot size in the sample, and therefore, the fluence and intensity at the rear side. Thus, the incident fluence and intensity values cannot be used as the laser damage threshold. In this range, the scatter signal detected by PD3 still shows a linear dependence as depicted in Fig. 2 for 930 fs pulse duration (red curve). Further increase of the incident power resulted in a stronger distortion of the transmitted beam profile as detected by PD3 after 35 s, which can be attributed to the photo-refractive effect. The distortion manifested itself as characteristic rings around the main beam similar to the distortions observed in LN (cf. Figure 3 in ). The characteristic threshold in terms of average on-axis intensity amounts to ~14 kW/cm2. The changes are still visible in the transmitted HeNe beam profile after e.g. 45 s, they persist over minutes but fade away in a few 10 min, i.e. the material modification is not permanent.
Subsequent inspection by a microscope revealed that damage occurred only on the rear surface of the studied samples (cf. Figure 3). The damage morphology showed pits at random locations within the laser spot area in spite of the smooth, Gaussian laser beam profile. In Table 1, incident and internal LIDT values at the rear side are summarized for room temperature. The estimated backside values take into account Fresnel losses at the front surface and field enhancement at the backside , and were calculated as in .
The error bars in Fig. 4 represent the standard deviation of 7 measurements for a pulse duration of 330 fs and 5 measurements for a pulse duration of 930 fs. At 330 fs, before reaching the damage threshold, one could observe with the naked eye beam collapse inside the sample into a single filament due to SF with subsequent white light continuum (WLC) generation. This observation is not supported by simple calculation of the SF lengths based on the Kerr coefficient n2. For E//z polarization, cascaded second-order effects have to be taken into account in KTP . For our sample length we obtained a cascaded n2 contribution of −2.1 × 10−15 cm2/W at 1.03 µm using the Sellmeier equation from  and assuming d33 = 15 pm/V which almost cancels the intrinsic n2 value of + 2.4 × 10−15 cm2/W measured in  with a z-cut sample to avoid the cascaded effect. The observed strong SF indicates that the intrinsic Kerr effect literature data are not reliable or n2 exhibits pronounced anisotropy.
Heating up the samples to 90 °C did not affect the damage threshold. As can be seen from Fig. 5, only the photo-refractive effect was somewhat suppressed. For this measurement, the sample was placed behind the focus with a laser spot size of FWHM = 83 µm at its front surface and a wave front curvature that partially counteracted SF leading to somewhat higher LIDT values than above. This measurement was performed with the Amphos Yb:YAG laser amplifier system at a repetition rate of 100 kHz and nominal pulse duration of 1.3 ps. The incident LIDT values for the same sample at room temperature and at 90 °C, are summarized in Table 2. In both cases, the error bars represent the standard deviation of 10 measurements.
While Table 2 indicates that the LIDT is not affected by increasing the temperature, the different fluence values compared to Table 1 imply that SF cannot be ruled out for such sample lengths even for pulse durations of ~1 ps and such effects were indeed observed as explained above. Therefore, the determination of unique intrinsic LIDT values will require a more sophisticated procedure and further measurements as performed in  for LN. Thus, the main result of the present study can be considered to be the “practical” incident fluence values obtained, see Table 1, which are derived for laser and crystal parameters very common to existing high-average-power few cycle CPOPA systems [11,12].
In an earlier study we measured the LIDT of uncoated, blank LN crystals of the same length  using the same set-up and laser parameters. Table 3 shows a comparison of these measurements with the values shown before in Table 1.
For both LN and RKTP, the LIDT in terms of incident fluence values do not depend on the pulse duration within our error bars. However, this might be related to the specific repetition rate used in the present study. As shown in , thermal and cumulative effects at such high repetition rates might influence the dependence on the pulse duration by changing the conditions for SF, WLC generation and the related spectral and temporal broadening. Since n2 and the resulting SF are in general different for KTP and LN, and in all cases the results are influenced by the SF effect, it is difficult to compare the incident values measured for the different crystals in Table 3 even under similar conditions and they shall be understood more as practical limits. In general, KTP is considered to have higher damage resistivity compared to LN and this is confirmed in our study, however, the difference in this specific temporal regime is roughly only 20%. Somewhat unexpectedly, also in terms of beam distortion due to the photo-refractive effect, the two crystals showed similar behavior. However, it shall be mentioned here that the LN crystals studied in  were doped with 5% MgO.
We obtained surface damage threshold intensity and fluence values for uncoated blank RKTP in temporal regimes (pulse duration and repetition rate) where such data were previously unavailable. The shortest pulse duration for which we were able to find KTP surface LIDT estimates at 1.064 µm in the literature is 110 ps (bursts of 20 pulses with 13 ns separation, following at 3 kHz): 220 mJ/cm2 , which is very close to the present observations. It shall be emphasized, however, that comparison with literature is not justified because the present observations result from indirect damage formation. Thus, the values presented here are strictly speaking valid for a KTP length of 2 mm, and while such lengths and the used (pump) pulse durations are typical for few cycle CPOPA systems, the results shall be used with caution. Safe operation at about 50% of the given values will also help to avoid beam distortions of the amplified signal and idler caused by the additional nonlinear effects present.
From the comparison of the LIDT in RKTP and MgO-doped LN and the intermediate position in terms of effective nonlinearity of periodically poled RKTP between periodically poled LN and angle tuned KTP or LN, one can conclude that QPM in RKTP is potentially interesting for the first two stages of high-average power few cycle CPOPAs systems as the one described in . Feasibility for application in further stages with higher average powers has yet to be evaluated by damage and thermal behavior studies with larger beam sizes.
Leibniz-Gemeinschaft grant no. SAW-2012-MBI-2; European Union’s Horizon 2020 research and innovation programme under grant agreement no. 654148 Laserlab-Europe.
We thank Dr. Alexandre Mermillod-Blondin for providing the Satsuma laser.
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