Nanosecond Pulse Laser Set For Optical Materials And Coatings Beam Resistance Study
That is why one of the main challenges in creating nanosecond pulse laser systems is to achieve higher efficiency and reliability of their operation which directly depends on operation of individual elements.
We present results of the elements beam resistance study based on the set developed using a nanosecond pulse solid-state laser. Laser phosphate glass tests were carried out to show the possibility of using the system to monitor beam resistance of optical materials and coatings. Laser phosphate glass is used for production of large-size disk-shaped active elements as parts of high-power laser sets.
The laser set is built around a standard arrangement common for the sets generating pulses in a nanosecond duration range. The specific feature of the set is its ability to make use of a low-power oscillator and to amplify the energy of radiation (gain is not less than 10,000) and shape on the test sample an equivalent impact area featuring a uniform distribution of radiant energy density within a spot with rather large dimensions 5 Ч 5 mm.
The set design principles
The interest to the development of high-power 1 to 3 ns lasers using laser phosphate glass as the gain media remains strong since the latter part of the last century [1–4]. The advancement of the nanosecond radiation pulse laser engineering is impossible without studying the resistance of glasses to laser radiation.
The base set to determine beam resistance of any optical material includes a source of radiation (laser), a facility to measure energy (power) of the laser beam, and an equivalent impact area shaping unit. The equivalent impact area (Seq) is the laser spot area featuring uniform distribution of energy density equal to the maximum energy density in the real spot and containing the same amount of energy. As a rule, with the Gaussian distribution, Seq is measured at the level of 0.5 of the maximum radiation intensity. The equivalent impact area can be determined based on the spatial distribution of radiation energy on the test sample.
The simplest way to determine the equivalent impact area is to burn out the material, e. g. photo paper, followed by measuring the diameter of the burned surface provided that use is made of light filters to sequentially reduce radiated energy until the burn-out threshold is achieved. Plotting the relevant curve makes it possible to determine the spot diameter at the 0.5 level of the maximum intensity. A more sophisticated method lies in the film flood exposure followed by photometric measurements. Laser energy meters are based on various physical principles: calorimetric, photoelectric, etc. Up-to-date devices to measure laser power can be compatible with all types of sensors: thermocouple, pyroelectric and photodiode (LED) types.
Using the above principle, a base set has been developed (Fig.1a) to study coatings and optical media damage threshold under the impact of a nanosecond laser pulse. The set consists of the master oscillator, preamplifier, laser pulse time and spatial profile shaping arrangements, as well as the four-stage amplifier featuring a 45-mm output aperture . Master oscillator 50-ns pulse was cut using Pockels shutter to obtain a 1-ns pulse. To eliminate amplitude and phase distortions of the beam wavefront, a central 2.5–15-mm zone was isolated out from the output beam. Then, after attenuating the radiation beam it was focused on the test object using a long-focus lens to obtain a spot diameter of about 1 mm. A typical energy density distribution across the beam cross-section on the sample’s plane is shown in Fig. 1b.
Sample damage threshold Qthr is considered as the energy density equal to the average value of the maximum energy density Qmax that did not cause damage of the surface and the minimum energy density Qmin at which damages could still exist: Qthr = (Qmax – Qmin) / 2. Taking account of the fact that in this method the threshold intensity depends on the spot diameter it can be seen that shaping a small Seq is the typical disadvantage of this method. Besides, such a procedure assumed a small statistics of measurements on each sample, and this did not allow estimation of the real damage threshold, particularly with the account of random nature of distribution of admixtures and defects in the sample.
Over the years, the principle of designing the sets did not practically change: master oscillator, pulse cutting system based on the Pockels shutter, a number of amplification stages, and a spatial filter system. Although, when selecting the type of arrangement, account was taken of the dependence of the test sample damage threshold on the spot size, the sample radiation spot diameter remained small and did not exceed 1 to 2 mm. But as new results of the study become available with regard to the dependence of the beam resistance on the impact spot size , and as more effective systems to monitor radiation energy, generated pulse shape, and monitor spatial distribution of energy across the focal spot were created (use was made of more accurate models of calorimeters, photo sensors and oscilloscopes, CCD cameras were installed), the measurement systems became more sophisticated (Fig. 2) .
At the same time, measurement procedures also were upgraded. Article  presents a procedure that makes it possible to not only estimate the energy threshold density but also take account of the statistical nature of the surface damage process. Fig. 3 shows a procedure version taking account of the statistical (probabilistic) approach. Taking account of the fact that the energy distribution law ε(r) across the radiation spot and the energy density level ε(rm) are known, it is considered that the boundary along the most distant damage points outline corresponds to the energy density threshold level for the given sample. The circular outline shown in the upper part of Fig.3 stands for the damage zone determined with reference to the most distant points. Statistical approach to the measurement results treatment turns out to be quite correct.
Later, an express procedure to determine beam resistance has been elaborated. The procedure implies that the sample is exposed to not one beam but an array of beams featuring Gaussian intensity distribution . The optical diagram for shaping the array to implement this procedure is shown in Fig.4. A diffractive quality laser beam with aperture ∅24 m illuminates an array of diaphragms. The array diameter is 19 mm, diaphragms diameter is 3 mm, and the number of diaphragms is 19. The diaphragm array is located in the focal plane of lens L1 (f = 200 cm), and the array image is aligned to infinity. On the other side of the lens, in the focus, a ∅2 mm diaphragm is placed to ensure angular selection of the beam. The diaphragm is placed in a vacuum tray. Placed 1 m away from the selective diaphragm is lens L2 (f = 50 сm). Since the first lens aligns the image of the diaphragms array to infinity, lens L2 brings the image from infinity to the focal plane. In this case, the image is formed in the focus irrespective of the distance between the lenses. The beam thus formed falls onto the sample. The diameter of the array of Gaussian beams on the sample in this arrangement is ≈5 mm, and the beam effective diameter is 0.5 mm. A typical laser irradiation intensity distribution is shown in Fig. 5.
One of the latest ingenious techniques developed to determine damage thresholds for dielectric coatings applied to substrates made it possible to determine the statistics of the optical breakdown effected by a single shot laser pulse . However, the optical spot diameter in this paper was only 29 microns.
Searching for the methods to increase Seq led also to the necessity to make use of a raster-type prism-like system to focus the beam. Specifically, in paper  use was made of a raster-type system providing uniform laser pumping and shaping of a uniformly illuminated spot of the sufficient area (up to 120 mm 2). Used as an irradiation source was a Nd laser generating a 10–7 s pulse with 200 J maximum radiation energy. Apart from increasing Seq size owing to the larger duration of the pulse radiation, a higher beam resistance of the glass samples  will be also provided.
The problem of developing neodymium-doped glass high-power lasers used to study thermonuclear fusion, is still of current interest. When developing a concept for creating a high-power laser set for VNIIEF company, a module has been created in the form of a 4-channel Nd phosphate glass laser with the full output radiation power up to 12 kJ at wavelength λ = 1053 nm and pulse duration τ ≈ 3 to 5 ns . It is implied that the maximum beam load on active elements of the 4-pass power amplifier will amount to approximately 10 J/cm 2, while power margin should be 2–3-fold. Thus, active elements should withstand their own radiation density of at least 20 J/cm 2. Besides, it is implied that with 10 J/cm 2 load the power margin will provide the number of radiation pulses required for this system.
Up to now, the sets with the above characteristics and Seq meeting the research tasks were not available. Hence, the purpose of this work is to implement a test bench to perform tests for beam resistance, primarily of laser phosphate glass witness samples used in high-power laser system amplifier stages.
Carrying out tests of materials for beam resistance
The basic equipment for the designed laser set includes the following: high-power laser emitter, optical system for shaping laser beam to impact the sample, a system to align and direct the beam onto the sample, laser radiation parameters measurement system, laser radiation impact results recording equipment. The principle of operation of the laser set consists in that the pulse emission with the required time parameters and produced by a low-power oscillator passes sequentially through amplifier stages and is amplified to 10–15 J. Using an optical system, the energy is focused on the test sample to ensure energy density on the target up to 60 J/cm 2. At the same time, the system registers parameters of the operating laser radiation, and then using a microscope the availability or non-availability of micro damages is determined.
The laser set equipment (Fig. 6) is located in clean production premises (class ISO 9), in three separate zones, to ensure the required conditions for carrying out measurements and safety operation. The first zone accommodates the set’s power supply equipment (capacitive storage, charges, pump flashtubes ignition units). The second zone accommodates the set’s optical and mechanical portion including: master oscillator, amplifier quantrons, vacuum spatial filter, measurement equipment, material test samples as well as cooling system and vacuum pump. The third zone accommodates the set’s control board.
Used as a master oscillator is the diode-pump LiYF4 (yttrium-lithium tetrafluoride) oscillator generating pulses with duration of 3 to 5 ns and 10–3 energy in one pulse. The gain media in the amplifier stages is in the form of Nd-ion-doped phosphate glass featuring luminescence spectrum practically similar to the LiYF4 crystal emission spectral characteristics. Amplifier stages make use of lamp pumping.
Technical problems related to the design of a laser amplifier with high gains, namely elimination of the return flare impact on the master oscillator optics, suppression of self-oscillation in amplifiers, – these problems are solved by adding the following nonreciprocal elements to the laser set: a Faraday cell based on TGG (terbium, gallium, garnet) crystal and a passive light shutter based on YAG:Cr 4+ (yttrium aluminum garnet crystal doped with chromium). Using a λ/4 retarder in amplifier stages implements circular polarization of the beam, whereas vacuum spatial filter eliminates small-size inhomogenuities and is aligned to obtain a diverging beam. These schematic solutions altogether made it possible to raise thresholds of the beam self-focusing in the gain elements of the set’s amplification section.
Focusing a laser beam using a combination of the prism raster and a converging lens allows shaping, on the test sample, of the impact zone with the laser beam uniform distribution within the spot measuring 5 Ч 5 mm. Prism raster serves as the laser beam homogenizer and consists of 49 raster elements formed by 7 planes of each side of the K-8 glass element (Fig. 7).
Pulse parameters measuring system includes Ophir heat heads (medium power heat sensor, thermocouple sensor), and a Nova II recording meter to measure radiation pulse energy with an accuracy of ±3%. The implemented laser set design provides laser radiation stable characteristics listed in Table 1.
Safe operation of the laser set is achieved owing to the number of implemented measures. First, by placing the basic equipment in different zones: equipment power supply system and power supply control board are separated from the main room. In this case, transmission of information on the voltage across each capacitor battery to the control board is ensured using a video camera. Second, laboratory room is furnished with interlocking devices to ensure removal of the high-voltage charge from capacitors when the laboratory room entrance door is open.
As far as procedural features of measurements using this set are concerned, they are not limited to direct measurements of the spot size and power parameters. It is possible to perform measurements of the radiation density threshold values leading to the sample damage, by overlaying topographic damage pattern onto the energy density distribution across the impact spot. This task is solved using IsoMap software which allows processing of CSV file format numeric packages generated by the video camera software, and carry out visual analysis of radiation density distributions with regard to CSV files and determine the map of levels of equal energy density (Fig. 8).
Discussion of results
In the course of measurements, the following radiation parameters have been obtained:
Pulse duration 3–5 ns;
Output energy 3–15 J (it is achieved by changing the voltage across capacitive storages in the range of 0 to 5,000 V with the accuracy of up to 10 V);
Radiation energy density near the test sample, depending on the spot size, from 3 to 60 J/cm 2 (this range is obtained by attenuating the beam using a set of neutral light filters).
The measured energy density threshold (beam resistance) of witness samples in the form of disc-shaped phosphate glass gain elements meets the required specifications; the measurement error was ±8% with 8–10 irradiations of the sample. Table 2 provides experimental values of the volume beam resistance for laser phosphate glass samples of similar composition but obtained using different production methods. The data obtained made it possible to determine directions for enhancement of phosphate glass production in relation to the beam resistance.
Operational experience of the new laser set showed that it can be used to monitor beam resistance of a wide range of optical materials and coatings. Besides, the ability of obtaining a sufficiently high energy density across rather large impact spot provides a means to determine, under current conditions, the self-focusing threshold of optical materials.
Authors acknowledge the management and the staff of AO LZOS and OAO "Swabe-Research" for the joint friendly and creative work in designing the laser set. The authors also acknowledge V.A. Serebriakov – a fellow labourer of OAO VNC "GOI named after C.I Vavilov". S.Petersburg, for his contribution to this project.