Author(s): Mohammad Saeed Marouf
This paper presents the measurement of the laser Threshold damage of an optical surface at different roughness coefficients. The study procedure is achieved through the manufacturing of two types of parallel-sided borosilicate plate (Glass Bk7, and SF13).
The Laser used is ND: YAG pulsed laser and the longitudinal mode TEM00 laser pulses with pulse width τ =30ns and wave length λ=1.064μm, the laser beam is focused using F=30 mm lens. The concentrated laser beam is guided by a converging lens towards the optical surface of the parallel surface plate.
Finally, we obtained a graph of roughness coefficient is drawn in relation the optical Threshold Damage of the optical surface, Before and after processing the optical surface. We were able to raise the threshold by about 15% after conducting thermal treatment of the optical surface
Optical components are considered as a key element that may limit the potential and the efficiency of new products and applications. Consequently, their specifications and the type of treatment they obtain can effectively influence the potential and efficiency of the whole physical system in many fields of the optical technology.
There are examples in laser processing or in microlithography that show that the nature of treatment the optical components received is closely related to the optical losses and the lifetime of the beam steering or imaging systems.
It is worth noting that when a low-energy laser beams changes in terms of energy, through a transparent optical element or falls on a reflective mirror or on an absorbent medium, on may observe a little effect on the optical element or absolutely nothing.
But when the intensity of the laser beams increases, there will be an irreversible effect that are observed on an optical compound or element such as stress, thermal expansion, rise in temperature, nonlinear permeability, photovoltaic effects, second vibration generation, standard optical vibration and burning of the endothermic element. All these effects are manifested when the laser pulse is sufficiently high in term of average energy, peak energy, or power density [1,2,3].
A greater the increase in the intensity of the laser beam will lead to changes in the material or the compound being used.
The first laboratory laser systems were large in size and often not suitable for industrial and military applications, which led to effort intensification in order to reduce their size and increase their effectiveness at the same time. This generates density of energy and capability to damage components that include laser resonators (such as mirrors, optical windows, beam splitters ...). Effects that modify laser output are important. They include strain absorption and damage laser inducing.
Experiments show that most lasers suffer from at least one of these effects when they work for a long time.
The heterogeneous absorption of the laser beam by an element leads to wave-front distortion and a production of intrusive modes, strain, cracking, and laser beam concentration.
The functioning of the laser system even at low output can lead to mirror deterioration and changing of laser conditions.
The action of lasers at high output levels can lead to a direct single shock and a laser induced damage.
The prolonged operation of laser beams or when it works at highpulse repetition frequency (HPRF) can lead to a cumulative laser- induced degradation of the HPRF.
Since the laser power density is the highest within the laser response and amplifier, it is important to carefully select the elements so that we obtain a system without defects as possible.
The following paragraphs will show how the treatment of an optical element will affect its effectiveness and demonstrate desirable properties to the industry.
The destruction in a crystal network occurs when it is subjected to a laser beam defined by parameters of matter, wavelength, energy, and the shape of the laser pulse (temporal and spatial shape). The destruction mechanism of the transmitting elements determined by the peak power density, while it is defined completely by the destruction of the highly absorbent materials [1-5].
Laser Damage is Associated with the Following Factors
The highest measured values for damage thresholds are that for a highly purified volumetric materials with compact and transparent close-packed materials having low refractive index.
The reason for the threshold damage of materials by laser radiation under special conditions is one of the mechanisms that will be discussed in the following paragraph:
The highest thresholds of laser vaporization can be obtained when fine polishing of the crystal so that scratches and cracks deeper than 1% of the laser wavelength are absent.
It is made from the internal feedback mechanism that amplifies sound and electromagnetic wave at the expense of the entering laser beam.
At high power levels, damage occurs from the mechanical stresses related to the electromagnetic wave.
It is the reduction of laser beam diameter less than the expected refractive index value of the non-radioactive material. It results from any process that leads to an increase in the index of refraction along with the increased light intensity that is associated with or without a high temperature.
It can be generated by the presence of impurities, electrons of conductivity, or by the crystalline network, and can cause laser damage by heating, thereby melting part of the radiant region.
It can be in the form of overlapping surface layers.
Whether it is a polishing element, unguided crystals or impurities at the boundaries of granules and the threshold of laser damage, one of the following factors is the energy density required to generate partial stress in the host crystal, which is attributed to the expansion of the heated outer shell.
Whether due to a polishing procedure, unoriented crystals, or impurities retained at the grain boundaries. The damage threshold due to a laser beam is related to one of the following factors:
Optical components can be damaged by laser irradiation with a sufficiently high energy. At any specific laser irradiation level and operation mode of the laser source, the probability of laser damage is usually higher for the surface of a component than for its bulk. Therefore, the limiting value of an optical component is frequently given by the damage threshold of its surface. Bulk damage is observed when the electrical field strength in the bulk of the component is enhanced by self- focusing, interference, scattering or other effects.
Imperfections, such as inclusions, dislocations, color centers or heterogeneities, can reduce the power-handling capability in the bulk of an optical component. Damage by a single laser pulse is often induced by defects or a mechanical stress on the coating, a contamination of the surface, or an optical absorption, which leads to a catastrophic heating of the surface.
For multiple-pulse operation, not only reversible mechanisms induced by thermal heating and distortion but also irreversible damage mechanisms induced by ageing, micro damage, moisture damage and generation or migration of defects are both observed.
The various parts of this International Standard are concerned with the determination of irreversible damage of the optical surfaces and the bulk of an optical component under the influence of a laser beam. Depending on the environmental conditions, damage is a function of material properties and laser parameters, in particular a wavelength, a spot size and an irradiation duration.
It is known that the transmission coefficient of a parallel-sided optical glass plate of type BK7, which has 5 mm in thickness, 25mm in diameter, characterized by a good optical quality, no bubbles, no cracks in the optical structure of the plate. Finally, the parallel between the two surfaces has a value 5" arc second, the irregularity coefficient is ΔN=0.3, and the flatness is N=1.
BK7 is a high quality optical glass that is used whenever the additional benefits of fused silica are not required. Since BK7 performs well in all chemical tests, and no additional or special handling is required, costs of manufacturing are reduced. It is a relatively hard material with extremely low bubble and inclusion content, while providing excellent transmittance through-out the visible and near infrared spectra and down to 350 nm in the ultraviolet.
For all standard optical glass components, or when optical glass is generically requested for custom elements, Glass Dynamics uses normal high quality BK7 optical glass [20].
The graph line of the selected sample BK7 which has not been painted in order to prevent reflection is illustrated in figure (1).
The resulting graph is drawn from the percentage changes of the Transmission with the wavelength variations measured in nanometers [20].
Figure 1: BK7 transmission with the range wavelength 350nm -2µm changes.
The figure (1) indicates that the permeability of the glass is very good and there is no absorption on the visible and the near infrared fields.
It appears below that the optical chart, of the first parallel-sided plate made of BK7 optical glass with geometric dimensions and tolerances, along with second plate which is made of SF13 optical glass, have the same specifications and measurements in Figure (2).
Figure(2-a): present the geometric design of a two-sided plate. Figure (2-b): Optical interference fringes show the quality of the optical surface by using the Fizeau Interferometer device presented in the following figure (2-c).
Figure (2-a): Present the geometric design of a two- sided plate.
Figure (2-b): Optical interference the quality of the optical show fringessurface by using the Fizeau Interferometer device presented in the following figure (2-c).
Figure (2-c): Fizeau Interferometer device
Optical glasses BK7 & SF13 were exposed to a laser beam so that the surface of each of them is located at the focal point of the positive lenses that pass the laser beam as shown in figure (3).
Figure 3: shows the two-sided parallel plate after manufacture, and how to place it on a mechanical stand against lasers
We can take the thermal effect of lasers, using laser-sensitive burning paper, measure their dimensions' spots and then calculate the area of the laser spot, [9- 17] as shown in Figure (4).
Figure (4-a): The thermal effect of lasers on sensitive burning paper
Figure (4-b): The thermal effect of laser presented as spots on the clean glass BK7
-We can take a digital image of the laser beam at a point located at the focal level of the lens on the studied optical surface of the plate, as shown in Figures (5-a,5- b).
Figure (5-a): Digital image of laser spot on the optical surface
Figure(5-b): Digital image of laser spot on the sensitive burning paper
The intensity of power projected on the glass can be calculated as shown in mathematical equations [6-9].-video1
As shown in the attached video1, the damage of the real surface Table 2 shows the experimental work of examining the glass SF13 located at the focal level of the converging lens is illustrated.
Study the intensity of energy in relation to the roughness of the optical surface using neodymium-yag laser:
Table 1 shows the experimental work on glass BK7 type. The threshold damaged which is p th[Mw/cm ] is examined in relation to the change in roughness coefficient Rn(nm) and the number of type threshold damaged p th [Mw/cm ], with the change in the roughness coefficient Rn(nm) and the number of grinding hours, for each stage of the grinding & surface finishing (polishing).
Curved energy density with the roughness coefficient of the optical surface for glass SF13, is shown in Figure (7).
Figure 6: Curved energy density p th [Mw/cm ] in relation to the optical surface roughness coefficient Ra [nm] for BK7 glass.
- Damage Threshold of the optical surface, using neodymium-yag laser for glass SF13.
Table 2 shows the experimental work of examining the glass SF13 type threshold damaged p th [Mw/cm2], with the change in the roughness coefficient Rn (nm) and the number of grinding hours, for each stage of the glaze grinding & surface finishing (polishing).
-Curved energy density with the roughness coefficient of the optical surface for glass BK7, is shown in Figure (6).
Figure 7: Curved energy density p th [Mw/cm ] in relation to the 2 optical surface Roughness Coefficient Ra [nm] for SF13 glass
-We can also draw a logarithm, which is almost straight and very close to the theoretical curve, as shown in Figure (8).
Figure 8: the graph on the left (which presents the BK7 optical glass) is very close to the theoretical curve (vertical axis is drawn on logarithmic scale).
-From the previous graphic curves, for each of the two types of the studied glass, we conclude that the optical surface absorbs the thermal energy generated by laser, otherwise the real surface contains a greater amount of cracks resulting from the processes of optical grinding during the finishing process [12-18].
When the surface is as clean and free of optical defects, as damage threshold is high
After testing the damage threshold for the two previous glass types and marking the relationship between the thermal damage threshold and the roughness coefficient Ra, we will perform thermal treatment of the same two types of glass to improve and raise this threshold in order to obtain optical glass with better physical specifications while maintaining the properties Chemical and optical glass.
This is very useful for heat-resistant glass that can be used in devices with medium and high-capacity laser beams,Because the more laser-tolerant the glass can withstand, the better.
We can avoid the problems of a burning spot on the glass surface, for different visual elements, which will worsen whenever there are bad weather factors, such as moisture and dust, especially the outer surfaces of laser optical systems.
After cleaning the studied glass samples of bk7, SF13, placed in a heat oven that is regularly controlled and slow to a temperature of 250 degrees Celsius for five hours, waiting until the heat cools regularly.
Then we do the calculations, as in paragraph 6 above, and draw the graph curves. which shows the increase of the threshold, after heat treatment as shown by tables 3 and 4
Table 3 shows the experimental work on glass BK7 type. The roughness coefficient Rn(nm) and the number of grinding hours, for each stage of the grinding & surface finishing (polishing).
-Curved energy density with the roughness coefficient of the optical surface for glass BK7, is shown in Figure (9).
Curved energy density with the roughness coefficient of the optical surface for glass SF13, is shown in Figure (10).
Figure 9: Curved energy density p th [Mw/cm ] in relation to the optical surface roughness coefficient Ra [nm] for BK7 glass, After heat treatment of glass
- Damage Threshold of the optical surface, using neodymium-yag laser for glass SF13.
Table 4 shows the experimental work of examining the glass SF13 type threshold damaged p th [Mw/cm ], with the change in the We examine the samples, in the same way as in paragraph 5. And we repeat that after each stage of the grinding,
Curved energy density with the roughness coefficient of the optical surface for glass SF13, is shown in Figure (10).
Figure 10: Curved energy density p th [Mw/cm ] in relation to the optical surface roughness coefficient Ra [nm] for SF13 glass, After heat treatment of glass
We can also explain the effect of heat treatment between the combination of the before and after heat treatment chart as in the following two figures 11-12.
Figure 11: Curved energy density p th [Mw/cm ] in relation to the optical surface roughness coefficient Ra [nm] for BK7, Before and after the heat treatment of the glass
The lower curve, blue in figure 11, is without heat treatment for samples of BK7 glass,
The upper curve in the shape in red, is after heat treatment, and the amount of increase in the values of the upper curve, is about 10%-15% more than the first value.
Figure 12: Curved energy density p th [Mw/cm ] in relation to the optical surface roughness coefficient Ra [nm] for SF13 Before and after the heat treatment of the glass.
The lower curve, red in figure 12, is without heat treatment for samples of SF13 glass,
The upper curve in the shape in blue, is after heat treatment, and the amount of increase in the values of the upper curve, is about 10%-15% more than the first value.
As a result, we note that after heat treatment, the tolerance of the optical surface, for both types of BK7&SF13 glass, has increased by an equal percentage, by about 10%-15%. As a result, we obtained better thermally treated glass, in terms of the optical surface Damage Threshold p th [MW/cm ] of lasers, as we showed the difference in the chart above, for both types of glass.
We have used ND: YAG pulse laser with longitudinal mode TEM00 with a τ=30ns pulse width, concentrated inside BK7, SF13 glass, using a lens to increase the concentration of the focal length of lens F=30mm laser beam.
We studied the change in the R n roughness coefficient with the P the Damage Threshold and charted the graph of this change. We found that the softer the surface, the higher the Damage
Threshold of damage, the better the surface and the greater the ability to load the surface for lasers, because the roughness of the surface is the main cause of its destruction, where the surface cannot dissipate the heat easily, while the surface gets rid of the heat more, and it is Its absorption of laser is less.
We also conclude that bk7 optical glass Damage Threshold is greater than SF13 glass [3-10].
We concluded that the shattering damage Threshold of optical glass becomes better the softer the glass surface, which is illustrated by the density and smoothness curve [6-9].
-We have presented a new approach for improving the interpretation of laser- damage probability curves. Indeed, laser- induced damage in optical material is often a problem of defects, and our aim was to characterize them as well as possible. Then we have proposed to consider that in a given class of defects, each defect has its own threshold, i.e. thresholds of the defect population are distributed. In this paper, a Gaussian distribution of thresholds has been considered. Other models can be used, but they require a preliminary knowledge of these defects (especially size and absorption). To illustrate our investigation, we have shown experimental results obtained on substrates polished according to different techniques. Results achieved on a BK7 & SF13 layer optical glass also shown, which establish a good agreement between theory and experiment. This model is of interest for the study of polishing and cleaning processes since it improves the knowledge on laser-damage precursors.
Bk7 glass is very important because of its strength among the types of glass, where its hardness (HKop) =520kp/mm2, low refractive dissonance nd=1,5168 and ape factor νd =64.17and made of positive lenses and optical prisms [18-20].
SF13 glass is weaker, with hardness (HKop) =380kp/mm2, a large refractive presumption nd =1,74077 and ape factor νd =27.6 and negative lenses (telephoto)[16,20].
These two types are often used together to make objective lenses in optical devices.
-The reason for the selection of these two types of glass is because of their wide spread use as beam expander, which are used in optical systems and devices that use lenses in focusing laser beam, they are also used in doublet lenses in medical devices, Industrial, remote sensing, optical communications and imaging system.
The better the process of the surfaces finishing during manufacturing, the less optical loss, so previous applications using various optical elements such as lenses and mirrors and another component become better because they will transmit lasers and optical signals without loss.
-After heat treatment of glass samples, the tolerance of the optical surface, for both types of BK7&SF13 glass, has increased by an equal percentage, by about 10%-15%. As a result, we have a better heat-treated glass, in terms of the optical surface Damage Threshold P [MW/cm2] of lasers.
The detailed experimental data used to support the findings of this study is available from the corresponding author upon request https://orcid.org/0000-0003-2657-1605
The author would like to thank Dr. M. Sayem El-Daher of the higher Institute for laser research at Damascus University Dr. A. Obed, and En. H. Hourani. for the assistance they have provided to this work.
