Dombi, Péter

Nagy, Benedek J.

Oszetzky, Dániel

Rácz, Péter

Vámos,  Lénard

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Femtosecond damage threshold at kHz and MHz pulserepetition ratesBenedek J. Nagy*, Le´na´rd Va´mos, Da´niel Oszetzky, Pe´ter Ra´cz and Pe´ter DombiMTA ”Lendu¨let” Ultrafast Nanooptics Group, Wigner Research Centre for Physics,Konkoly-Thege M. u´t 29-33, 1121 Budapest, Hungary.ABSTRACTFemtosecond laser-induced damage threshold (LIDT) measurements for different optical components are wellstudied for a set of laser pulse repetition rates spanning the range between 1 Hz and 1 kHz. Recent years sawthe advent of high-repetition-rate femtosecond systems with relatively high pulse energy. Therefore investigationof LIDT in the MHz region is essential. We performed several comparative femtosecond LIDT measurementson typically used ultrafast optical elements with different Ti:Sapphire laser systems having substantially differ-ent pulse repetition rates (a 1 kHz regenerative amplifier and a 4.3 MHz long-cavity oscillator) and found asubstantially lower MHz LIDT threshhold.Keywords: laser damage, ultrashort pulses, mirrors.1. INTRODUCTIONFemtosecond laser induced damage threshold (LIDT) has been studied in many configurations recently. As pulsedlasers with femtosecond pulse length became more available, research of the field has grown fast. There is anISO standard1 which describes a measurement procedure. Based on this or other self-consistent methods manygroups performed single-shot measurements,2, 3 multi-shot measurements,4–6 or both.2, 7–9 During multi-shotmeasurements the repetition rate of the pulses can differ significantly. Many measurements using femtosecondpulses are in the Hz-kHz region7, 10, 11 giving a huge amount of data. Without completeness a few of themperformed at 800 nm: Chen et al. measured 0.38 J/cm2 for high reflectors (HR) and 0.08-0.11 J/cm2 for chirpedmirrors.12 Starke et al. presented results in a round-robin experiment on a HR mirror between 0.2 and 0.7 J/cm2depending on number of pulses and experimental facility.13 Starke et al. also presented LIDT on differenttype HR mirrors and chirped mirrors with results of 0.2-0.5 J/cm2 and approximately 0.1 J/cm2 respectivelydepending on the sample and number of pulses.14 LIDT of TiO2/SiO2 mirror is also shown, where the resultsare 0.35-0.5 J/cm2 depending on the number of TiO2/SiO2 layer pairs and the number of pulses.Further on it is also important to investigate the LIDT with femtosecond pulses at MHz repetition rates.Jasapara et al. reported results with Ta2O5/SiO2 HR mirror between 0.16 and 0.29 J/cm2, depending on pulselength with 100 MHz repetition rate and very tightly focused (0.60 µm) beam.15 Also Angelov et al. presentedresults in the MHz repetition rate region but with picosecond pulses.16 There are high energy laser systemsoperating at this repetition rate region. E. g. passively mode-locked Yb thin disc laser17 with 65 MHz repetitionrate, 370 fs pulse width and 20.5 W average power and high power femtosecond fiber chirped-pulse amplificationsystem18 with 250 MHz repetition rate and 250 W average power.There are only very few measurements which investigated the LIDT dependence on various repetition ratesand these measurements are not matching either. Mero et al. performed measurements on a Ta2O5 single layerat 1, 10, 100 and 1000 Hz and did not observe significant changes or any trends in the LIDT.19 Hertwig etal. achieved similar results on ion-doped glass sample.20 Contrary to these results Bonse et al. demonstrateda decreasing trend in LIDT values, with measurements taken at 10, 100 and 1000 Hz on a Ta2O5/SiO2 highreflector multilayer.21 They measured 0.61, 0.45, 0.36 J/cm2 respectively. One order of magnitude increasein the repetition rate leads to approximately 25% decrease in the LIDT. Due to the sparse literature, it is animportant task to investigate the LIDT dependence on the pulse repetition rate especially for higher repetitionrates than 1 kHz.*nagy.benedek@winger.mta.huTherefore we performed several LIDT measurements of mirrors usable in ultrafast laser systems with bothkHz and MHz repetition rates at 800 nm wavelength and give a direct comparison, of the femtosecond LIDTvalues.2. METHODSWe used two solid state lasers for our measurements. One is a Coherent LEGEND regenerative amplifier andthe other is a home built long-cavity Ti:Sapphire oscillator. The parameters of the beams produced by theselasers at the measurement site are summarized in Table 1.Parameter Regenerative amplifier Ti:Sapphire oscillatorCentral wavelength (λ) 795 nm 805 nmRepetition rate 1 kHz 4.3 MHzPulse length, FWHM (τ) 91 fs (35 fs TF) 87-128 fsEnergy stability rms 0.25 % n/aPulse bandwidth, FWHM 30 nm 20 nmFocused spot size diam. 7.9 µm ± 0.2 µm 8.2 µm ± 0.2 µmTable 1: The parameters of the lasers used for damage testing (TF denotes the Fourier transform limit).In Fig. 1. a scheme is shown of the measurement setup. The beam is directed to the sample with beamsteering mirrors (M) and two pinholes are built in for proper beam alignment. The power is decreased by awheel absorption filter (WF). A shutter (SH) is built into the beamline to enable S-on-1 measurements. Thepower reaching the sample was measured with a power meter (P) between the shots. The beam is focused by anf=18 mm (L) lens on the sample which is fixed on an x-y-z translation stage (TS).Figure 1: The scheme of the setup. M: mirrors, SH: shutter, WF: wheel density filter, P: power meter, L: f=18 mmachromatic lens, TS: translation stage.There are two real-time monitoring systems shown in Fig. 1. One shown in the center is a stereo microscopebuilt into the setup. A CCD camera is attached to one of the oculars. The damage is visible with high certaintyon the CCD’s picture. The other is a thermal CCD with which the heating of the sample can also be visualized.At the kHz measurements we performed a mixture of the 1-on-1 and the S-on-1 procedures described in theISO standard.1 We were able to measure the illumination time but we were unable to stop the beam suddenlyat a damage occasion. Therefore our test was using the same number of pulses as in S-on-1 but the probabilitywas only the ratio of the damaged and all sites as in 1-on-1. The damage threshold fluence was defined as theintersection of the linear fit of the data points and the y = 0 line as shown in Fig. 2. The error is calculatedfrom the error of the linear regression. We used the shutter to control the beam and the illumination time was30 s measured by the shutter. This led to 30,000 pulses at 1 kHz.Figure 2: Damage probabilities of protected gold mirror and the determination of damage threshold value from themeasurement.At the MHz measurements we used two methods. (i) We applied approximately 35,000 pulses at 4.3 MHzwith a shutter. Further on the LIDT was calculated from five or more illuminated points. We started with anintensity below threshold and increased it at the same irradiated point until we observed the damage. So theLIDT was the mean of the last intensity without damage and the damaging intensity. Then this measurementwas repeated at different spots and the final LIDT of the sample was the mean of all former LIDT values denotingto the different points. The error of such a LIDT was given by the standard deviation of the values. (ii) Wealso performed a measurement similar to the kHz measurements formerly with 30 s irradiation time (108,000,000pulses) and measured only a little lower LIDT than in the case of method (i).We give the LIDT in the units of peak fluence that can be given in the form:F = 2Pfrepcos θw20pi, (1)where P is the average power, frep is the repetition frequency, θ is the beam angle of incidence, w0 is the beamradius (1/e2 radius, assuming a Gaussian beam).The pulse energy of the high repetition rate oscillator only allows for damaging at very tight focusing condi-tions with a 10-micron focal spot. Therefore, to decouple potential effects of the spot size on LIDT, we had toperform similar measurements with the kHz amplifier too.3. RESULTSWe tested metallic and dielectric mirrors as well as chirped mirrors, representing the most relevant cases forfemtosecond technology.. A high reflector mirror centered at 800 nm was manufactured by Optilab Ltd. We willrefer to this mirror as HR800 mirror. The high reflector mirror is coated with a dielectric layer structure. Thedielectric layer stack is a TiO2-SiO2 layer pair repeated 24 times which is a (LH)24 stack, where L denotes theSiO2 (n=1.44) low index and H the TiO2 (n=2.3) high index layer respectively. We tested protected gold andsilver mirrors as well. The protected metal mirrors all have a protecting overcoat which may be, in the mostcommon case, an approximately 100 nm thick SiO2 layer. Unfortunately the manufacturer does not provide withprecise information on the overcoat thickness. Further on we measured a chirped mirror test piece having somemanufacturing defects.The results are summarized in Table 2. In the last column the ratio of the kHz and MHz damage thresholdscan be seen. We can observe a factor of 4 to 8 for the dielectric and metallic mirrors, representing a substantialdecrease for MHz repetition rates.SampleDamage threshold (J/cm2) LIDTkHzLIDTMHz1 kHz 4.3 MHzProtected Ag mirror 1.26 ± 0.28 0.166 ± 0.005 7.6Protected Au mirror 1.20 ± 0.12 0.141 ± 0.010 8.5HR800 mirror, Optilab Ltd. 0.42 ± 0.09 0.103 ± 0.012 4.1Chirped mirror 0.16 ± 0.03 0.040 ± 0.003 4.0Table 2: LIDT peak fluence the values for the measured samples at the kHz and MHz lasers, and their ratio.4. DISCUSSIONIn all cases measurement inaccuracies of the laser power, the pulse length, the focal spot size (given by themagnification and/or the beam profiler camera evaluation software) are estimated to be less than 1%. The laserinstability, caused an error higher than the measurement inaccuracies. These errors were evaluated to accountfor a total 4-7% error in the measured damage threshold values.Since the damage fluence depends strongly on whether the beam hits a layer defect or not, the resultshave high statistical fluctuation. This can be decreased by measuring many samples with the same method orincreasing the number of measurement spots of the sample. At the kHz measurement the statistical error can becalculated from the standard error of the slope and the intersection of the linear fit. At the MHz measurementa similar type of error can be calculated as the standard deviation of the values measured at different spots.We assume that the cause of the remarkable difference between the kHz and MHz damage thresholds is thatthe damage is assisted by thermal effects. The sample has less time at the MHz measurements to relax so theLIDT decreases with increasing repetition rate. In order to illustrate the thermally assisted femtosecond damagemechanism, we show an alternative diagnostic technique. This is a thermal camera with which the thermallyassisted femtosecond damage processes can be investigated. In spite of the fact that heating effects are notexpected in the femtosecond region with kHz repetition rate, upon measuring dielectric samples the camerashows heating of the sample. The maximum temperature was nearly 70 degrees Celsius. The camera does notwork properly with metal mirrors due to the fact that metal has better heat conductivity and reflects thermalradiation as well. So the heat of the environment is seen by the camera with a very little heating point as shownin Fig. 3(b).The heat camera diagnostics will enable testing damage mechanisms and differences between kHzand MHz repetition rates in the future.(a) (b)Figure 3: Thermal camera exposure evaluation on HR800 (a) and Al mirror (b) upon illumination with 35 fs and 1 kHz.A possible reason for the smaller scaling factor of the dielectric mirrors may be the lower heat conduction.Therefore the heating effects have longer relaxation time in dielectric mirrors than the time between two pulsesreach the sample in both kHz and MHz repetition rate cases. In contrary the metallic mirrors have better heatconductivity, which leads to a shorter heating effect relaxation time less or comparable with the time betweentwo pulses at the kHz repetition rate case and much longer relaxation time as the time between two pulses inthe MHz repetition rate case.The origin of such a huge difference between kHz and MHz damage thresholds need to be investigated furtheron to identify potentially different damage mechanisms (including, for example, thermally assisted femtoseconddamage). However these measurements represent a first step to investigate an important new phenomenon. Thelow damage threshold at high repetition rate has to be taken into account in the construction of some highintensity lasers delivering femtosecond pulses at MHz repetition rates.5. CONCLUSIONSThe optical damage threshold of mirrors play an important role in laser system manufacturing and development,therefore it is essential to know the LIDT of the elements used. However, in spite of the abundance of nanosecondand picosecond data, less information is available for femtosecond damage threshold, especially for high (100kHz. . .MHz) repetition rates. For this reason we performed several femtosecond LIDT measurements on typicallyused ultrafast optical elements.The kHz results, where comparable, were well matching the earlier ones found in literature. For the first timeto our knowledge, we also compared femtosecond damage thresholds with kHz and MHz repetition rates andfound a remarkable difference with a factor of 4 to 8 lower values for MHz repetition rates, the origin of whichrequires further investigation.AcknowledgmentsPe´ter Dombi and Pe´ter Ra´cz acknowledge a ”Lendu¨let” and a postdoctoral grant of the Hungarian Academy ofSciences, respectively. 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Femtosecond damage threshold at kHz and MHz pulse repetition rates

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