CS251633B1 - Creating method of the double antireflextion layer on the pad from optical glases - Google Patents

Abstract

The essence of the method is that on the mat optical glass is first steamed in Vacuum layer of rare earth metal oxide for example yttrium or gadolinium, or hafnium oxide or magnesium oxide, whereupon it is steamed at the same optical thickness with tolerance - 30% silica layer or magnesium fluoride. Way it is designed especially for optical areas production where reflection suppression is required of monochrome light.

Description

The invention relates to a method for forming a double antireflective layer on an optical glass substrate.

To reduce the reflectance of optical surfaces in a narrow spectral band, two methods of coating the optical substrates g with a refractive index n dosud, typically by vacuum baking or sputtering, have been used. The first method consists in applying one quarter-wavelength layer

This requirement is only compromise for most optical glasses and therefore the reflection suppression is insufficient. The second method is to apply two layers to manufacturers of traditional high and low refractive index materials with different optical thicknesses, calculated from the refractive indices of the layers and the substrate, as described, for example, by Knittl, Optics of Thin Films, Wiley 1976, p. 131.

The disadvantage of this solution is that if the optical thicknesses are not observed, not only the minimum reflectance in the spectrum is shifted, but also its value increases. This is illustrated by FIG. 1, showing a double antireflective layer of TiOg and SiOg with an indication of 20% optical thickness deviations.

This property requires that each layer of optical elements be preceded by a series of tests and measurements, in which both the optical thicknesses are changed independently until we first reach the zero minimum of the spectral reflectance curve and then place that minimum at the desired wavelength.

Each of the two requirements is dependent on the thickness of the two layers, so that the correction to ensure that one of the requirements is met will violate the other. Furthermore, the reflectance at the minimum is not a linear function of the thicknesses, or their ratio, and therefore the detection of a tendency in the first correction may not lead to a decrease in the reflectance below the tolerance level in the second correction.

Regarding the last correction, i.e. placing the minimum reflectance at the desired wavelength by varying the thicknesses of both layers in the same ratio, it cannot generally be omitted. This gives at least three corrections, but practically with respect to refractive index dispersion, inhomogeneity and other factors, usually more.

Due to systematic (small) changes in production, this multi-stage correction procedure has to be repeated over time, which results in considerable losses of time and material, and especially wear of the expensive production apparatus. Another disadvantage of the double antireflection layer used hitherto is that the optical thickness of the layer adjacent to the substrate is substantially lower than a quarter-wave.

In fact, when changing its thickness by corrective action or random variations during production, the inhomogeneity of the refractive index, i.e., the non-linear dependence of the optical and geometric thickness, is applied, which makes corrections more difficult and reduces reproducibility in production.

The existing deficiencies in the coating of optical substrates are eliminated by the method of forming a double antireflective layer on the optical glass substrate, the principle being that a layer of rare earth metal such as yttrium or gedolinium, or hafnium oxide or oxide e, it is then vaporized in the same optical thickness with a tolerance of 30% of silica or magnesium fluoride.

The main advantage of this method is the replacement of the multi-stage correction procedure of pre-production and intermediate production tests by one-stage, respectively with respect to dispersion index refraction, inhomogeneity and other possible changes of layer parameters, maximum two-stage. apparatuses.

Another advantage is the replacement of the very thin layer adjacent to the backing by a layer of quarter-wave optical thickness, in which the inhomogeneities of the refractive index are applied to a lesser degree, which results in a higher reproducibility of the desired parameters of the double antireflection layer.

The prior art and the state according to the invention are illustrated in FIGS. 1 and 2, respectively, showing the reflectance spectral curves of a double antireflection layer on an optical glass substrate with a refractive index n ^ = 1.52 to suppress reflection of perpendicular incident light at wavelength λ = 633 nm, where the wavelength λ of the light in nanometers is applied to the horizontal axis, and the reflectance R as a percentage in the vertical axis. The individual curves 1 to 4 correspond to the variations of the nominal optical thicknesses as well as _{2 of the} two partial layers as follows:

t ,, tg - nominal t, + 20%, t ₂ + 20% t, - 20%, t ₂ + 20% t, + 20%, t ₂ - 20%

The graphs in Figure 1 represent the prior art where the SiO ₂ layer-forming materials with refractive index n, = 1.47 and titanium dioxide with refractive index n ₂ = 2.27 are used, while the graphs in Figure 2 represent the state of the invention wherein yttria with a refractive index n ₂ = 1.8 is used instead of titanium dioxide.

As can be seen from FIG. 1, the variation in the optical thickness of any layer results in not only a shift in the reflectance minimum in the spectrum, but in particular an increase in its value, while the double antireflection layer according to the invention does not show the reflectance values at 20%. shown in the graphs in FIG. 2.

This is a substantial advantage over the prior art. The correction intervention, nullifying the minimum spectral reflectance curve required in the pre-production tests of the prior art layer, becomes unnecessary in the double antireflection layer of the invention. In order to achieve the desired effect, i.e. to achieve a minimum reflectance at the wavelength of interest, only a correction is made to shift the minimum of the spectral curve to the desired wavelength, i.e. to reduce the optical thickness of both layers at the same ratio.

In the case of an oblique incidence of light, it is generally possible to annul the reflectance for only one of the two polarizing components of the light, or to compromise both, by double layer, as is known. The effect according to the invention is analogous to the perpendicular impact situation for each of the components of the polarization of light, as illustrated in Fig. 2.

However, the refractive indices and the optical thicknesses of the individual layers must be adapted to the given angle of incidence, as shown in Calculation Example 2 below. For refractive indices of the layers resulting from the perpendicular impact condition, the oblique impact satisfies both components Polarization of light compromise. However, the thicknesses must again be adapted to the oblique impact.

Example 1

By suppressing reflection of laser light of wavelength λ = 633 nm from optical glass surfaces with refractive indices in the range (1.46 - 1.62) below 0.25% in perpendicular impact, good results were achieved by applying quarter-wavelength layers of yttrium oxide as first and SiO ₂ as second from the pad.

The spectral course of reflectance of this double antireflective layer is shown by curve 1 in Fig. 2. For construction, silicon dioxide was first chosen as a mechanically and chemically resistant material, since this layer is exposed to the external environment.

251633 4

Hence the choice of the second material according to the following relation)

ⁿ 2

(1)

For the most commonly used optical glass BK7 with refractive index n ^ = 1.52 when light from the air with refractive index ηθ = 1, we get the value of n _g = 1.8 l, which roughly corresponds to yttrium oxide (n = 1.8). Optical tlouělky two layers are quarter-wave, i.e. t = t _g = λ / 4 = 158 nm.

Example 2

For an angle of incidence Θ = 45 ° we want to suppress the reflectance of heavy optical glass with refractive index n ^ »1.72 for light of wavelength λ = 633 nm, falling from the air with refractive index n _Q = 1, polarized parallel to the plane of incidence. Magnesium fluoride MgF _g with refractive index η, = 1, 38 will be used for the adjacent layer. The choice of layer material adjacent to the substrate results from the following procedure: For the Inclined Impact in Calculation, we consider the so-called admittance, defined by n ”sin '--F, instead of refractive indices.

í '' ύ s (2) where +1 resp. -1 we select the components of light polarized perpendicular to the plane, or parallel to the plane of incidence, for antireflection. By inversion of this relation we get for refractive index n _g = Ϊ,

C 1 + 1- (2n _o sin0 / Y ₂ ) ^Ž l (3) where Y _g is given by condition (1), which is generalized for

Which, after substituting for admittance to the right - hand side of equation (Ie) for (2), can be adjusted to

IRL "n Costa _Q (n ² -n ² sin ² a) Ty | -n ° sin ^ 0 (4)

By substituting the values of the refractive indices and the angle of incidence from the assignment we calculate the admittance of the first layer from the substrate Y _g and by substituting it into (3) the corresponding refractive index n _g . This gives a value of n _g = 1.685, which roughly corresponds to magnesium oxide MgO (n = 1.7). For the optical thicknesses of both layers t1 and _{g of} skin = -1.2

Then we substitute the values of the reflectance value R in the minimum for substrates (MgO) n _g = 1.7 according to the relation we have t, = 174 nm, t _g = 184. Finally we verify the technological value of the refractive index of the first layer from * ⁼ Κ / ^ ⁺ <* 1

We calculate the individual admittances according to (2) J we get Y _Q = 1,414, Y ^ = 1,887,

Yg = 1.869, Yj = 1.607. By substituting these values in (5), we find a residual reflectance R = 0.005%, which is negligibly small for most applications.

In particular, the invention is directed to those areas of optical manufacturing where monochromatic light reflectivity suppression is required.

Claims (1)

SUBJECT OF THE INVENTION Method for forming a double antireflective layer on an optical glass substrate, characterized in that a layer of rare earth metal, for example yttrium or gadolinium or hafnium oxide or magnesium oxide, is first vapor deposited on the optical glass substrate and then vaporized in the same optical fiber Thickness - 30% layer of silica or magnesium fluoride.