M. F. Koldunov

Efficient solid state dye lasers based on polymer-filled microporous glass

This paper reports on the laser emission properties of Pyrromethene 580, Pyrromethene 597, Pyrromethene 650 and Rhodamine 11B in the novel matrix polymer-filled microporous glass (PFMPG). This host material combines the advantages of an organic environment for the dye with the superior thermooptical and mechanical properties of an inorganic glass. Laser efficiency was measured as a function of pump flux for different dye concentrations, resonator feedback, and locations on the sample. Service life, defined as the number of pulses for the output to drop to the 70% point, was recorded at 5 Hz for the higher dye concentrations. The highest efficiencies were observed for Pyrromethene 597 (is congruent to 70%), which had a service life of is congruent to 60,000 shots at 25 MW/cm2 and is congruent to 45,000 shots at 50 MW/cm2. The longest service life was measured for Rhodamine 11B (is congruent to 110,000 pulses at 25 MW/cm2), but this dye had somewhat lower efficiency (is congruent to 50 - 55%). Thermal lensing measurements were made for dye-doped PFMPG and MPMMA, and showed that the lensing is much lower in the hybrid matrix. The agreement with the theoretical modeling is very good.

Theory of laser-induced inclusion-initiated damage in optical materials

Abstract. We present the results of theoretical studies of the laser-induced damage in transparent solids containing absorbing inclusions. The investigation is based on the inclusion-initiated thermal explosion model. Key aspects of the model are considered: thermal instability initiated by the inclusions; a mechanism of photoionization of a surrounding layer of a host material by a thermal ultraviolet-radiation of laser-heated inclusions; the thermal instability kinetics, and an associated pulse-width dependence of the laser-induced damage threshold. Also, statistical features of the damage, caused by random spatial distribution of the inclusions in the materials, and a final stage of the damage process—a mechanical stress-produced crack formation—are analyzed. A comparison of the theoretical results, related to the pulse-width dependence of the damage threshold, with experimental data for some typical optical materials in a wide pulse-width range is presented.

Theory of laser-induced damage to optical coatings: inclusion-initiated thermal explosion mechanism

The theory of laser induced damage to an optical single-layer dielectric film (coating) initiated by an absorbing inclusion is presented. The damage model based on the photoionization mechanism due to thermal radiation of the laser heated inclusion is analyzed. Dependences of the laser induced damage threshold upon coating/substrate band-gap ratio and absorption cross-section ratio, and upon coating thickness are derived from a solution of heat and electron diffusion equations. The coating thickness dependence predicted is compared with experimental data.

Multishot laser damage in transparent solids: theory of accumulation effect

A phenomenological model for accumulation of irreversible changes initiated by absorbing inclusion in transparent dielectric material under laser radiation is proposed. Kinetics of the accumulation process is investigated for both highly absorbing and low absorbing inclusions. Main regularities of the accumulation effect are derived, particularly the dependence of multishot damage threshold upon a number of pulses. Results of the theoretical analysis are compared with experimental data.

Beam-quality measurements on solid state dye lasers using nonconfocal unstable resonators

This paper reports on a systematic study ofthe beam quality ofsolid-state dye lasers for different resonator designs. The resonators investigated include: multimode stable, hard-edge unstable, and unstable with a graded-reflectivity mirror (GRM) output coupler. The purpose ofthe work was to investigate the conditions under which good beam quality can be achieved, and to examine the possible trade-off between beam quality and energy extraction efficiency. Beam quality was quantified through conventional M2 measurements. A hard-edge unstable resonator of magnification ?2.5 produced a beam ofM2 ?11 - 12 at a PRF of 2Hz and a pump flux of 5 MW/cm2. The value increased to ? 18 — 19 at a pump flux of2O MW/cm2. This can be compared with an M2 of ? 140 - 150 with the multimode stable resonator. Preliminary measurements with the GRM resulted in M2 values of ? 5 to 6 at 5 MW/cm2.

Multishot laser-induced damage in optical materials: an analysis of main regularities

In this paper we analyze features of absorbing inclusion-initiated laser-induced failure of a transparent dielectric surface in a multishop irradiation regime. Ablation and thermoelastic stress-induced damage mechanisms are investigated. The ablation is shown to be the dominant mechanism in the ultrashort pulsewidth domain, whereas the latter is realized at longer pulses. The alteration of damage mechanisms at pulsewidth variation is also characteristic for intrinsic damage mechanisms. Resent experimental results obtained for the ultrashort pulse damage are critically discussed in a context of a theory developed in this paper.

Pulse-width and pulse-shape dependencies of laser-induced damage threshold to transparent optical materials

Theory of pulsewidth dependence of laser induced damage threshold (LIDT) in transparent solids is presented. The damage is supposed to be initiated by thermal explosion of absorbing inclusions. The investigation of thermal explosion is based on an analysis of the heat transfer equation and a new approach to solving this equation is developed allowing to study kinetics of thermal explosion without any modeling presentation of an absorption mechanism. It is shown that the key parameter determining a dependence of LIDT upon a laser pulsewidth, (tau) p, is the heat transfer time, (tau) , from an inclusion to a surrounding medium. At (tau) p >> (tau) a damage threshold is characterized by a laser radiation intensity, whereas at (tau) p << (tau) --by an energy density. The pulsewidth dependence of the LIDT has been investigated for rectangular and gaussian shapes of laser pulses and it has been established that the dependencies considerably differ in these two cases in a range of (tau) p approximately (tau) . An effect of damage statistics, connected with a random spatial distribution of inclusions in a material, is also investigated. For the case of one-type inclusions (single-(tau) inclusions) it is shown: the statistics does not change a functional form of the pulsewidth dependence of the LIDT and correct only the LIDT values by a spot-size factor. Theoretical results are compared with experimental data published by different research groups for the laser damage in a nanosecond-picosecond region.

Theory of laser-induced damage to optical coatings: dependence of damage threshold on physical parameters of coating and substrate materials

A problem of a laser induced damage to dielectric coatings due to thermal explosion of the absorbing inclusions is investigated. It is shown that major parameters governing damage characteristics of the coating are band gaps of coating and substrate materials. In particular, damage threshold dependences on a coating thickness and a position of the inclusion in the coating or on the coating-substrate interface are connected with an effect of this fundamental parameter. Experimental data are in qualitative agreement with theoretical predictions.

Recent progress in theoretical studies of laser-induced damage (LID) in optical materials: fundamental properties of LID threshold in the wide-pulse-width range from microseconds to femtoseconds

We present an overview of our theoretical studies on laser- induced damage (LID) in transparent optical materials including recent results on a pulsewidth dependence of the LID threshold, thermoelastic stress-induced damage criteria, damage morphology features, peculiarities of the LID at ultrashort pulse irradiation. Main attention is focused on inclusion-initiated damage, but some results are related also to intrinsic damage mechanisms. Theoretical results are compared with available experimental data on pulsewidth dependence of the LID threshold in bulk optical materials, on damage feature in thin films and on damage morphology studies. A good agreement of theoretically predicted and observed features is demonstrated, in particular, for the pulsewidth dependence of the LID threshold in a wide pulsewidth range from nanoseconds to femtoseconds.

Influence of intensity fluctuations on laser damage in optical materials

A study is reported of the influence of temporal fluctuations of laser radiation on the development of thermal explosion of absorbing inclusions and on the statistical properties of the laser induced damage in tranparent dielectrics. A fluctuation time scale in which the fluctuations effect the thermal explosion of inclusions is established. An analysis is made of the conditions ensuring control of temporal fluctuations of laser radiation so as to eliminate their influence on the experimental statistical relationships of absorbing inclusions in the bulk and on the surface of a sample.