Search

The effectiveness of ultrashort pulse compression depends on the coatings group delay dispersion (GDD) characteristics in chirped mirror (CM)-based optical systems. Current porous-layer-based CMs with low GDD oscillations are limited to less than half optical octave spectral bandwidth and the stability of their spectral parameters is still unknown. In order to address these limitations, a broadband CM (covering 400 nm bandwidth) with a porous top layer was deposited and spectral parameters were measured at different environmental conditions. The modeled refractive index of the porous layer varied by approximately 1% due to the changes in humidity. Deposited coatings exhibited low sensitivity of GDD oscillations to deposition errors of porous layer and humidity in the surrounding environment.

Back-side coated (BASIC) chirped mirror is characterized by very smooth group delay dispersion (GDD) curve over extremely wide spectral bandwidth. However, these optical elements suffer from poor flatness since they combine as thin as possible substrate and highly stressed sputtered coating. Several stress compensating techniques may be combined to produce fully stress-compensated BASIC chirped mirror. Ion beam sputtering technology was used to deposit BASIC chirped mirror based on Nb2O5/SiO2 mixtures and post-deposition annealing was implemented to compensate coating stress and fabricate optical element with sufficient flatness. Chirped mirror for 150 nm wavelength range and −300 fs average GDD was deposited on 0.35 mm thickness substrate. Final optical element featured initial flatness of uncoated substrate and close to maximum achievable GDD, considering coating thickness and wavelength range.

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.

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.

In this paper, the relation between the laser-induced damage threshold (LIDT) and the electric field intensity (EFI) distribution inside a CM is investigated experimentally. We show that it is possible to increase the LIDT values by slightly modifying the electric field of a standing wave distribution without loss of spectral and dispersion performance. Suggested CM design improvement could increase reliability and LIDT performance of both CM elements and high-power systems they are used in.

The decrease of laser-induced damage threshold (LIDT) when exposed with high number of laser pulses is a well-known phenomenon in dielectrics. In the femtosecond regime this fatigue is usually attributed to the incubation of laser-induced lattice defects. In this work, a computational model is used to combine the data from time-resolved digital holographic microscopy measurements together with results of S-on-1 laser-induced damage threshold test in order to investigate fatigue of ZrO 2 single layer coating. Two distinct damage modes were identified and shown to follow different fatigue behaviors: formation of catastrophic damage is highly nonlinear in time, while incubation of color-change mode appears to be linear in time.

The nonlinear LBO (LiB3O5) crystal is widely used in many Nd : YAG, Yb : KGW and Nd : YLF lasers in order to generate higher optical harmonics. However, the most limiting factor in such applications is the optical resistance of their coated surfaces described by the so-called Laser Induced Damage Threshold (LIDT) parameter. In this work we investigate the “fatigue” (multi-pulse) behaviour of LIDTs in LBO crystals coated with different types of (single AR@355 nm and triple AR@355+532+1064 nm wavelength) anti-reflective coatings while optimising the refractive index designs and selecting appropriate sub-layer materials. All the coatings were produced of different oxide materials (ZrO2, Al2O3, SiO2) and ZrO2-SiO2 mixtures by using the ion beam sputtering (IBS) deposition technique. The optical damage resistance of both fixed and transient refractive index coatings is of special interest. Besides the spectral properties, the resistance to laser irradiation is characterised at the wavelength of 355 nm with laser pulses of nanosecond duration. The conclusions are drawn about the AR coated LBO with the most successful designs by the means of optical resistance.

We present the flat focusing mirror - the dielectric mirrors capable to focus and image the narrow beams in reflections. The mirrors are flat, and transversally invariant, i.e. have no optical axis. We consider two different configurations of such mirrors: 1) Bragg-like mirrors, consisting of structure of specially designed dielectric layers; 2) periodically modulated surface on a subwavelength scale. We also discuss the combined case - the longitudinally chirped photonic crystals.

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 fs 2 . 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 mu m, numerically.

The laser damage resistance of an optical element in high power laser systems depends significantly on the surface quality of the optical substrate. In this experiment, commercially polished fused silica substrates were etched in argon plasma generated by a RF source and their surface roughness, flatness and optical properties were investigated. This method can be applied in a vacuum chamber prior to deposition of the multilayer coatings without breaking the vacuum. It was shown that by etching the resistance to 355 nm, laser radiation could be improved more than 8 times. However, it strongly related with primary substrate quality. The etching depth from 100 nm suggests the optimum performance of surface quality in terms of surface low roughness, high flatness, and high laser damage threshold. These results are of significant importance for the manufacture of high quality laser optics on fused silica substrates. As an example of an application of our technology, anti-reflective and polarizing optical interference coatings were deposited on etched substrates and the increase of their resistance to laser radiation was measured.

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 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.

Y. C. Cheng,1 M. Peckus,2 S. Kicas,2 J. Trull,1 C. Cojocaru,1 R. Vilaseca,1 R. Drazdys,2 and K. Staliunas1,3 1Departament de Fisica i Enginyeria Nuclear, Universitat Politecnica de Catalunya, Colom 11, 08222 Terrassa, Barcelona, Spain 2Center for Physical Sciences and Technology, Savanoriu Ave. 231, LT-02300, Vilnius, Lithuania 3Institucio Catalana de Recerca i Estudis Avancats (ICREA), Pg. Lluis Companys, 23, 08010 Barcelona, Spain (Received 21 December 2012; published 15 April 2013)

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 (SiO 2 , Nb 2 O 5 , ZrO 2 , HfO 2 , Ta 2 O 5 , Al 2 O 3 , Sc 2 O 3 ) and oxide mixtures with various ratios (Nb 2 O 5 /SiO 2 , ZrO 2 /SiO 2 , HfO 2 /SiO 2, Ta 2 O 5 /SiO 2 , Al 2 O 3 /SiO 2 and Sc 2 O 3/ SiO 2 ). 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 Sc 2 O 3 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.

Summary form only given. We explore reflections of narrow beams from one-dimensionally modulated photonic structures, where the modulation period varies across the structure (the so called chirped dielectric mirrors). It is known, that the plane waves reflected in a normal direction from the chirped mirrors, can show modified temporal dispersion [1]. Different frequency components reflect from the different depth of the structure, as illustrated in Fig. 1a. As a consequence the pulses reflected from such mirrors can obtain a chirp, or contrarily, a pulse chirp of opposite sign can be compensated and eliminated by reflection. We argue, that the character of reflection of narrow beams can be also peculiar from such a chirped structure. The plane wave components of the beam also reflect from the different depth of the structure (see illustration in Fig. 1b), which can result in modification of the spatial dispersion. The beam, in this way could obtain important spatial transformations during reflections. In particular a focalisation of the beam could be possible under particular conditions, as illustrated in Fig. 1c.

We report on the laser damage resistance of ion beam—sputtered oxide materials (Al2O3, Nb2O5, HfO2, SiO2, Ta2O5, ZrO2) and mixtures of Al2O3-SiO2, Nb2O5-SiO2, HfO2-SiO2, Ta2O5-SiO2, and ZrO2-SiO2, irradiated by single 500 fs pulses at 1030 nm. Laser-induced damage threshold (LIDT), refractive index, and bandgaps of the single-layer coatings are measured. For pure oxide materials a linear evolution of the LIDT with bandgap is observed. The results are in accordance with our simulations based on photo-ionization and avalanche-ionization. In the case of mixtures, however, deviations from the previous behaviors are evidenced. The evolution of the LIDT as a function of the refractive index is analyzed, and an empirical description of the relation between refractive index and LIDT is proposed.

Recent advances in ion beam sputtering technology enabled efficient deposition of different oxide mixture coatings. In the present investigation, coating materials ZrO2and SiO2were utilized for the synthesis of different mixtures. The goal of the conducted investigation was to explore the potential of ZrO2/SiO2mixtures, especially for applications in the range of the UV spectral. Deposited dielectric mirrors having classical quarter-wave and “rised” design with ZrO2/SiO2mixture for 266nm and 355nm showed good resistance to laser radiation.We also investigated the “fatigue” behavior of LIDT’s in LBO crystals with single, dual and triple wavelength anti-reflective coatings (AR@355, AR@532+1064 and AR@355+532+1064) in order to optimize design and layer materials. The influence of gradient refractive index profiles on damage resistance is of a special interest. We selected a few designs of antireflective coatings which demonstrated the best resistance to laser radiation.An experimental study onrugatefilter coatings showed the need for a more accurate characterization of optical properties of metal oxides and their corresponding mixtures. Santrauka Šiame darbe tiriamas jonapluoščio dulkinimo technologija dengtų ZrO2/SiO2ir jų mišinių optinių savybių pritaikymas kintamo lūžio rodiklio (Rugate tipo) optinėms interferencinėms dangoms formuoti. Mišiniuose naudojant mažą ZrO2frakciją, buvo išplėstas šios medžiagos pritaikymas UV srityje. Remiantis gautais rezultatais, suformuotos didelio atspindžio (HR) ir skaidrinančios (AR) dangos esant λ= 355nm bangos ilgio spinduliuotei, taip pat teoriškai sumodeliuotos ir suformuotos daugiabangio skaidrinimo (AR@ 355+532+ 064) Rugate tipo dangos ant LBO (LiB3O5) netiesinių kristalų. Palygintos šių dangų optinės savybės su tomis dangomis, kuriose panaudoti gryni metalo oksidai (fiksuoto lūžio rodiklio). Atliktas Rugate tipo dangų, pasižyminčių siaura didelio (R > 99,9%) atspindžio zona ties 532nm ir pralaidumu likusioje spektro dalyje, teorinis modeliavimas ir eksperimentinis garinimas. Raktiniai žodžiai:Rugate tipo dangos, metalų oksidai ir jų mišiniai, LBO kristalai, notch filtrai, jonapluoštis dulkinimas

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 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.

ZrO2-SiO2 and Nb2O5-SiO2 mixture coatings as well as those of pure zirconia (ZrO2), niobia (Nb2O5), and silica (SiO2) deposited by ion-beam sputtering were investigated. Refractive-index dispersions, bandgaps, and volumetric fractions of materials in mixed coatings were analyzed from spectrophotometric data. Optical scattering, surface roughness, nanostructure, and optical resistance were also studied. Zirconia-silica mixtures experience the transition from crystalline to amorphous phase by increasing the content of SiO2. This also results in reduced surface roughness. All niobia and silica coatings and their mixtures were amorphous. The obtained laser-induced damage thresholds in the subpicosecond range also correlates with respect to the silica content in both zirconia- and niobia-silica mixtures.

In this study, we present our recent progress in research of mixed zirconia/silica and niobia/silica coatings prepared by Ion Beam Sputtering technique. Single-layer coatings of the same optical thickness were characterized with respect to LIDT.

A novel application of chirped dielectric mirrors for narrow beam focalization is proposed and demonstrated numerically and experimentally. Analogy to temporal dispersion compensation by chirped dielectric mirrors is discussed.

A novel use of chirped mirrors for narrow beam focalization is proposed. Focusing in reflection from flat chirped mirror has been demonstrated numerically and experimentally. Analogy to temporal dispersion mirrors is discussed.