Recent studies suggest that fatigue effect in dielectric optical coatings is possibly associated with the presence of strong nonlinear absorption, however, up to now there was only indirect evidence for such hypothesis. One of the reasons for that was a technical rigor to characterize nonlinear absorption losses in optical coatings and a lack of pertinent experimental data. Recent advancement of common-path interferometry and LIDT testing allows us to overcome such limitations. In this study we examine nonlinear response and fatigue effect in single- and multilayer dielectric coatings below single shot damage threshold. Although there is no quantitative model that could predict fatigue from absorptance, we found an interesting correlation between nonlinear absorption and fatigue effect under comparable experimental conditions. These results help us to understand the mechanism of fatigue in optical coatings and possibly make more durable femtosecond optics.
Standard high reflectivity coatings consist of materials with high and low refractive indexes. Typically, optical resistivity of such elements is limited by the threshold value of material with high index. Combination of two deposition methods, namely ion-beam sputtering and oblique angle deposition, was used to form high reflectivity coatings for the wavelength of 355 nm. Variation of the design of standard coating and the number of top layers, deposited at oblique angle have been investigated. Laser induced damage thresholds, optical scattering, surface roughness, spectral performance etc. were tested for the experimental samples. Analysis indicate that combination of both deposition methods allows to enhance the optical resistivity of typical high reflictivity mirrors. Introducing standard method also allows to stabilize the spectra and reduce the losses of total optical component.
Standard high reflectivity mirrors consist of layers with high and low refractive indexes. Typically, optical resistivity of such elements is limited by the threshold value of material with high index. Combination of two deposition methods, namely ion-beam sputtering and glancing angle deposition, was used to form high reflectivity mirrors for the wavelength of 355 nm. Variation of the design for standard coating and the number of top layers, deposited at oblique angle have been investigated. Laser induced damage thresholds, surface roughness, spectral performance etc. were tested for all the experimental samples. Analysis indicate that combination of both deposition methods allows to enhance the optical resistivity of typical high reflictivity mirrors. Fully sculptured thin film based mirrors also exhibit spectral instability and optical losses. Introducing standard method allows to stabilize the spectra and reduce the losses of total optical component.
Ultra-thin metal films possess big possibilities in design and manufacture of multilayer structures with sophisticated spectral performances over wide wavelength ranges. Continuous ultra-thin metal film has a smaller depolarizing effect than dielectric film, due to this reason the amount of total coating thickness and layers number in non-polarizing coatings can significant reduced. Nevertheless, depending on the substrate temperature, the deposition rate, the thickness, ultra-thin metals films can form continuous or discontinuous nano island structures. When optical components are used at oblique angles of incidence, the reflectance and transmittance of thin film exhibit strong polarization effects. However, for many applications, the polarization effects are undesirable and should be reduced. Non-polarizing coatings are widely used for a variety of photonic applications ranging from interferometry and holography. Their aim is to divide incident light into transmitted and reflected beams in a certain ratio for both s- and p-polarized light. In present work, different combinations of deposition parameters were investigated for silver thermal evaporation. Various dependencies on silver thin metal film formation are shown. The dependence on metal thickness showed different regime of film formation. Microstructure and optical properties were investigated by using of atomic force microscope, ellipsometry and spectrometer measurement A novel plate metal–dielectric three-layer, broadband nonpolarizing beam splitter (nPBS) with an ultra-thin silver film was designed.
Laser induced damage of optical coatings has been one of the most important targets during many decades of intensive research. Different techniques were used and explored with the aim to increase the resistance of multilayer systems to laser pulses. In this work, LIDT results of different “base” structures made by ion beam sputtering of Al2O3, SiO2 and their mixtures are presented, and further enhancement possibilities are discussed by applying additional layer structure using higher bandgap material – fluorides and glancing angle deposited SiO2.
Band-gap and refractive index are known as fundamental properties determining intrinsic optical resistance of multilayer dielectric coatings. By considering this fact we propose novel approach to manufacturing of interference thin films, based on artificial nano-structures of modulated porosity embedded in high band-gap matrix. Next generation all-silica mirrors were prepared by GLancing Angle Deposition (GLAD) using electron beam evaporation. High reflectivity (HR) was achieved by tailoring the porosity of highly resistant silica material during the thin film deposition process. Furthermore, the proposed approach was also demonstrated to work well in case of anti-reflection (AR) coatings. Conventional HR HfO2 and SiO2 as well as AR Al2O3 and SiO2 multilayers produced by Ion Beam Sputtering (IBS) were used as reference coatings. Damage performance of experimental coatings was also analyzed. All-silica based GLAD approach resulted in significant improvement of intrinsic laser damage resistance properties if compared to conventional coatings. Besides laser damage testing, other characteristics of experimental coatings are analyzed and discussed – reflectance, surface roughness and optical scattering. We believe that reported concept can be expanded to virtually any design of thin film coatings thus opening a new way of next generation highly resistant thin films well suited for high power and UV laser applications.
We present the investigation of a synchronously pumped optical parametric oscillator (SPOPO) based on beta barium borate (BBO) nonlinear crystal with broadband complementary chirped mirror pairs (CMPs). Three SPOPO cavity configurations with slightly different intracavity dispersion were explored. Dispersion properties of cavity mirrors were characterized using a white light interferometer and found to be the key factor determining the gap-free tuning range as well as simultaneous multiwavelength generation. The SPOPO is pumped by the second harmonic of a Yb:KGW oscillator and provides signal pulses tunable over a spectral range from 625 to 980 nm. Signal pulse duration ranges from 102 to 268 fs in various intracavity dispersion regimes. In addition, signal beam power in excess of 500 mW is demonstrated, corresponding to 27% conversion efficiency from pump to signal wave.
We present results obtained during investigation of synchronously pumped optical parametric oscillator (SPOPO) with broadband complementary chirped mirror pairs (CMP). The SPOPO based on β–BBO nonlinear crystal is pumped by second harmonic of femtosecond Yb:KGW laser and provides signal pulses tunable over spectral range from 625 to 980 nm. More than 500 mW are generated in the signal beam, giving up to 27 % pump power to signal power conversion efficiency. The plane SPOPO cavity mirror pairs were specially designed to provide 99 % reflection in broad spectral range corresponding to signal wavelength tuning (630–1030 nm) and to suppress group delay dispersion (GDD) oscillations down to ±10 fs2. Dispersion properties of designed mirrors were tested with white light interferometer (WLI) and attributed to the SPOPO tuning behaviour.
Recently, the principle of flat focusing based on one-dimensionally chirped dielectric mirrors has been proposed and experimentally demonstrated. The flat chirped mirror causes anomalous diffraction of the beam during reflection, opposite to the normal diffraction in free space propagation. The anomalous diffraction compensates the normal diffraction of the beam resulting in focusing after reflection. For a better focusing performance and for a larger near-field focal distance, a stronger anomalous diffraction is required. We show that the anomalous diffraction can be enhanced by introducing a defect layer in the chirped mirror, as the structure becomes similar to a Gires–Tournois interferometer. The focal distance can be substantially increased due to the defect layer. In our specific structure, the focal distance shows an increase from 19 to 39 μm, numerically.
In the past years the usage of mixed oxides coatings lead to an important improvement of laser damage threshold and
quality of optical elements. In this study influence of post treatment procedure - ex-situ annealing - is examined in terms
of quality, optical constants and laser induced damage threshold (LIDT) of mixed HfO2 and SiO2 coatings. Monolayer
thin films containing different fractions of HfO2 are deposited with ion beam sputtering technology (IBS.) All samples
are post annealed at different temperatures and optimal regimes are defined. Refractive index and absorption coefficient
dispersion is evaluated from transmission spectra measurements. Surface roughness of all samples is characterized
before and after deposition and annealing, using atomic force microscopy (AFM). Microstructural changes are identified
from changes in surface topography. Further, optical resistance was characterized by 5.7 ns duration pulses for 355 nm
wavelength laser radiation, performing 1-on-1 sample exposure tests with high resolution micro-focusing approach for
monolayer samples and S-on-1 tests for multilayer reflectors. Morphology of damaged sites was analyzed through
optical microscopy. Finally, conclusions about annealing effect for mixed HfO2 and SiO2 monolayer and multilayer
coatings are made.
In this work we report an experimental investigation of subsurface damage (SSD) in conventionally polished fused silica
(FS) substrates which are widely used in laser applications and directly influence performances of optical elements.
Two procedures were developed: 1 - acid etching and 2 - superpolishing. Additionally, surface roughness and total
integrated scattering (TIS) measurements were performed to find correlation between the main surface properties and
laser induced damage threshold (LIDT) as circumstantial evidence of elimination of SSD.
Different durations of acid etching have been used to study LIDT of FS substrates. These experiments revealed that the
optimal etching time is ~1 min. for a given acid concentration. Laser induced damage threshold of etched and SiO2 layer
coated FS samples increased ~3 times, while of the ones that were not coated - 4 times. It has been revealed that for nonetched
surface a single nano- to micro-scale absorbing defect ensemble most likely associated with polishing particles
within Beilby layer was dominant, while damage morphology in ~1 min etched FS sample had no point defects
observed.
More than 5 times lower roughness value (RMS) was obtained by superpolishing procedure using colloidal silica
abrasive particles. LIDT of such superpolished fussed silica substrates was also strongly increased and compared with
conventional CeO2 abrasive polishing.
We report laser-induced damage threshold (LIDT) measurements of periodically poled lithium niobate (PPLN) and
magnesium-oxide-doped PPLN (MgO:PPLN) in the femtosecond pulse duration regime at 1030 nm with 100 kHz and 75
MHz repetition rate. PPLN and MgO:PPLN crystals with broadband Nb2O5/SiO2 AR coatings for 1.4 - 1.8 um spectral
range were used. S-on-1 test for LIDT measurements were performed. S was equal to 106 and 4.56*1010 pulses for 100
kHz CPA laser system and 75 MHz oscillator, respectively. Evaluated LIDT was 20 mJ/cm2 for 290 fs pulses at 100 kHz repetition rate and 0.63 mJ/cm2 for 105 fs pulses at 76 MHz repetition rate.
We propose and demonstrate a novel functionality of chirped mirror for monochromatic light beams: a diffraction control in reflections resulting in focusing or imaging of beams. The chirped mirrors, commonly used for manipulating temporal profiles of pulses, here are applied for manipulating the spatial dispersion of a monochromatic beam. By penetrating into dielectric layers of chirped mirror, the monochromatic beam experiences the negative diffraction, therefore the beam diverge propagating in front and behind the structure in normal diffraction region can be compensated inside this structure with negative diffraction. The result is focusing or imaging of the reflected beam from a flat interface of chirped mirror without optical axis.
Mixed metal oxide coatings of ZrO2 / SiO2 , and Nb2 O5 / SiO2 as well as films of pure SiO2 , ZrO2 and Nb2 O5 have been
prepared by the Ion Beam Sputtering (IBS) technique and characterized on their physical properties. The Laser-Induced
Damage Thresholds (LIDT) of these samples have been measured at 1064 nm with the 1-on-1 mode in the nanosecond
regime. The optical resistance results obtained from laser damage probability curves indicate a decrease of the LIDT in
both sets of the mixtures when the content of the high index material is increased. By comparing the LIDT to the
bandgap values (Eg) that have been measured, a dependence of LIDT to Eg is evidenced. Following these results,
comparisons are made with the case of 500fs LIDT that have been measured on the same samples. It is found that the
same behavior is observed on both cases (ie LIDT dependence with Eg). Discussions are then conducted on the possible
physical mechanisms to explain the results and it is found that in the case of the nanosecond regime, the LIDT
dependence on Eg can be explained by a critical temperature reached during the laser damage process that exhibit a
dependence on the band-gap of the material. The influence of the film thermal conductivity on the values of the critical
temperature is studied.
Despite the growing improvement in optical polishing and deposition technologies optical resistance of the laser
components used for high-power UV applications remains insufficient in many cases. In this study influence of different
fused silica substrate preparation, post treatment processing and deposition techniques are examined in terms of surface
roughness, optical scattering and laser damage performance. The conventional techniques of polishing, etching, and
finally surface cleaning of substrates have been investigated. Further, a part of samples were also coated with SiO2
monolayer by Ion Beam Sputtering (IBS) technique. Surface quality was characterized prior to and after the treatment
and deposition processes by the means of total integrated scattering (TIS) and atomic force microscopy (AFM). The
experimental results of surface roughness measurements exhibited a good correlation between AFM and TIS methods.
Further optical resistance was characterized with 10 ns duration pulses for 355 nm wavelength laser radiation performing
1-on-1 sample exposure test with high resolution micro-focusing approach. A dominating damage precursor ensembles
produced during manufacturing processes were identified and directly compared. Finally, the conclusions about the
quality influencing factors of investigated processes were drawn.
We report on the laser damage resistance of thin films prepared by Ion Beam Sputtering. The samples are fused silica
substrates coated with single layer films of pure oxides (SiO2, Nb2O5, ZrO2, HfO2, Ta2O5, Al2O3, Sc2O3) and oxide mixtures with various ratios (Nb2O5/SiO2, ZrO2/SiO2, HfO2/SiO2, Ta2O5/SiO2, Al2O3/SiO2 and Sc2O3/SiO2). For this study the LIDT of more than 60 different samples have measured at 1030nm with pulse durations of 500fs with single pulse irradiation. The results are expressed and compared in terms of LIDT as a function of the measured band gap
energy and refractive index. For simple oxide materials a linear evolution of the LIDT with bandgap is observed, with the exception of Sc2O3 material where a very high damage threshold is observed, compared to other high index materials. In the case of mixtures, a more complex behavior is evidenced.
We report an experimental investigation in the laser-induced damage threshold (LIDT) of optical coatings materials. The
samples are single layers of Al2O3, Nb2O5, HfO2, SiO2, Ta2O5, ZrO2 deposited through different deposition techniques
(evaporation or sputtering with/without ion assistance) and mixtures of Al2O3/SiO2, Nb2O5/SiO2, HfO2/SiO2, Ta2O5/SiO2 and ZrO2/SiO2 on silica substrates. The LIDT is measured at 1030nm, 500fs in single shot mode. The results are
expressed and compared in term of LIDT as a function of bandgap and LIDT as a function of refractive index.
In this study, we report on our recent progress in research of single layer mixed zirconia-silica and niobia-silica
composite coatings prepared by Ion Beam Sputtering technique. All coatings of the same optical thickness
were characterized in terms of reflection/transmission spectrometry, X-ray diffraction, atomic force and optical
microscopy, optical back-scattering and optical resistance (laser-induced damage threshold - LIDT) in subpicosecond
mode. The optical resistance, TIS and LIDT results reveal clear dependence on high refractive index
material content in composite coating and its crystalline structure. The results are interpreted and discussed
by the means of different models available in literature.
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