=Paper=
{{Paper
|id=Vol-1900/paper1
|storemode=property
|title=Multilayer dielectric stack Notch filter for 450-700 nm wavelength spectrum
|pdfUrl=https://ceur-ws.org/Vol-1900/paper1.pdf
|volume=Vol-1900
|authors=Muhammad A. Butt,Sergey A. Fomchenkov,Svetlana N. Khonina
}}
==Multilayer dielectric stack Notch filter for 450-700 nm wavelength spectrum ==
https://ceur-ws.org/Vol-1900/paper1.pdf
Multilayer dielectric stack Notch filter for 450-700 nm wavelength
spectrum
M.A. Butt1, S.A. Fomchenkov1,2, S.N. Khonina1,2
1
Samara National Research University, 34 Moskovskoe Shosse, 443086, Samara, Russia
2
Image Processing Systems Institute β Branch of the Federal Scientific Research Centre βCrystallography and Photonicsβ of Russian Academy of Sciences, 151
Molodogvardeyskaya st., 443001, Samara, Russia
Abstract
In this work, a multilayer dielectric optical notch filters design is proposed based on TiO2 and SiO2 alternating layers. Titanium dioxide (TiO2)
is selected for its high refractive index value (2.5) and Silicon dioxide (SiO2) as a low refractive index layer (1.45). These filters are
conventionally envisioned for overpowering of powerful laser beams in research experiments, to obtain good signal-to-noise ratios in Raman
laser spectroscopy. It is precarious that light from the pump laser should be blocked. This is attained by inserting a notch filter in the detection
channel of the setup. In addition to spectroscopy, notch filters are also useful in laser-based florescence instrumentation and biomedical laser
systems. The designed filter shows a high quality with an average transmission of more than 90% in 450-535 and 587-700 nm bandwidths.
And a stop band region between 536-586 nm shows a transmission of 3% only with an optical density of greater than 3, which makes it a
promising element to be used as a notch filter.
Keywords: Notch filter; Optical density; Distributed Bragg Reflector (DBR); visible spectrum
1. Introduction
Thin film optics is well-established technology. Many devices such as band pass filters, band-stop filters, polarizers and
reflectors are realized with the help of multilayer dielectric thin films [1-4]. Thin films coatings have also been used to increase
both colour and energy efficiency of glass and as reflecting mirrors coatings. However the application of single layer thin films
has increased, there are a number of applications which require multilayer films that combine the attractive properties of
numerous materials. Some of the important applications of multilayer films are in the design of computer disks, optical
reflectors, antireflection coating, optical filters, and solar cells among others. An optical filter is an element or material which is
purposefully used to change the spectral intensity distribution or the state of polarization of the electromagnetic radiation
incident on it. The change in the spectral intensity distribution may or may not depend on the wavelength. The filter possibly will
act in transmission, in reflection, or both. Notch filters are usually known as band-stop or band-rejection filters which are
designed to transmit most of the wavelengths with the low-intensity loss while diminishing the light within a specific wavelength
range to a very low level. These filters are conventionally proposed for overpowering of powerful laser beams in research
experiments to obtain good signal-to-noise ratios in Raman laser spectroscopy. It is precarious that light from the pump laser
should be blocked. This is attained by inserting a notch filter in the detection channel of the setup. In addition to spectroscopy,
notch filters can also be used in laser-based fluorescence instrumentation and biomedical laser systems. They are also used for
eye protection and as a camera accessory. These filters contain alternating layers of high (H) and low (L) refractive index
materials with precise thicknesses with good knowledge about their refractive index and absorptions. Several multilayer coatings
are deposited onto a transparent substrate. Both the multilayer and substrate contribute to the total performance of the filter.
Layers made of oxides are, as a rule, harder than those made of fluorides, sulphides or semiconductors. Therefore, they are ideal
to be used on unprotected surfaces. Semiconductor materials should be avoided in filters which have to be used over a wide
range of temperatures because their optical constants can change considerably. Distributed Bragg Reflectors (DBRs) work on the
principle of multiple reflections between high and low index materials interface. It has a Ξ»/4 thickness of the central wavelength.
The high reflection region of a DBR is known as the DBR stopband and can be attained by the refractive index contrast between
the constituent layers. A broad stop band can be realized by using high index contrast thin films. The schematic of the DBR is
shown in figure 1.
In this work, the design of a Notch filter based on TiO2/SiO2 is proposed at a central wavelength of 561 nm with an FWHM of
50 nm. Titanium dioxide (TiO2) is selected for its high refractive index value (2.5)[5] and Silicon dioxide (SiO2) as a low
refractive index layer (1.45)[5]. TiO2 is a vital dielectric material with a wide band-gap energy and high refractive index that can
make it useful in the fabrication of multilayer thin films due to its high optical properties. For instance, its high transmittance
and high refractive index in the visible region (380-760 nm) make it valuable to be employed in the production of the optical
filter and window glazing [6, 7].
In the designing of optical filters, the behaviour of the entire multilayer system is anticipated on the basis of the properties of
the individual layers in the stack [8]. Hence to attain the optimum performance, it is important to optically characterize and
accurately determine the thickness of the individual layers. We designed this filter with a less possible number of layers with
high transmission in pass band region and high reflection is obtained in the stop band. Open-source software, Open Filters, is
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Computer Optics and Nanophotonics / M.A. Butt, S.A. Fomchenkov, S.N. Khonina
used in this work to design and optimize the required filter. Transmission and reflection properties of interference filters are
dependent on materials refractive index and layer thickness of materials. Open filter calculates optical properties of filters. It
uses transfer matrix method to calculate the transmission and reflection properties of filters based on the absorption and
materials refractive indices [9]. Optimization techniques are available in this software like needle synthesis (Adding an extra
layer to give targeted transmission).
Fig. 1. Schematic of Distributed Bragg Reflector (DBR).
2. Optical density of the notch filter
A filter plate made of an isotropic material with smooth and parallel surfaces, the transmittance depends on the thickness,
optical constants of the material, the angle of incidence and polarization state of the incident light, and the degree of coherence
between multiple reflected waves [10, 11]. Optical density (OD) is used to see the blocking specification of a filter and is
associated with the amount of energy transmitted through it. It uses a logarithmic scale to describe the transmission of light
through a highly blocked optical filter, particularly useful when the transmission is extremely small. A high optical density value
indicates very low transmission of light and low optical density indicates high transmission. For instance, OD=1 relates to a
transmittance value of 0.1, and OD =8 corresponds to a transmittance value of 10-8. It can be expressed as [12]:
ππ(π‘π‘π‘π‘π‘π‘π‘π‘π‘π‘π‘π‘π‘π‘π‘π‘π‘π‘π‘π‘π‘π‘π‘π‘) = 10βππππ π₯π₯ 100
ππ
ππππ = β log οΏ½ οΏ½ β¦ β¦ β¦ β¦ β¦ β¦ β¦ β¦ β¦ β¦ β¦ β¦ β¦ . ππππ (1)
100
For the filters having OD β₯ 3 the effects of multiple reflections are insignificant because of the low reflectance and strong
absorption of the filter.
3. Filter design and discussion
Multilayer thin films have an extensive wavelength tunability which gives an optical response that is desired for a specific
application. Distributed Bragg Reflectors (DBRs)[13,14] consisting of alternating high and low refractive index material pairs
are the most commonly used mirrors in FP filters, due to their high reflectivity. However, DBRs have high reflectivity for a
selected range of wavelengths known as the stop band of the DBR. Its reflectance usually depends on the constructive or
destructive interference of light reflected at consecutive boundaries of different layers of the stack. The performance of the
multilayer devices highly depends on the interface formed between the alternating layers. Therefore an appropriate sequencing
of the layers of suitable dielectric materials and their thicknesses is critical for achieving the desired spectral response and
application. Therefore, it is important to optimize the coating conditions in the designing process [15, 16]. In our previous work,
we proposed multilayer dielectric filter based on TiO2 and SiO2 materials because of their excellent optical properties [17].
Therefore, TiO2 and SiO2 are chosen as high and low refractive index materials, respectively. The choice of materials is made on
the basis of low absorption and high index contrast in the wavelengths of interest. The notch filter is designed for visible
spectrum ranges from 450-700 nm with FWHM of 50 nm. The optimized thickness of the layers is shown in table 1. The total
thickness of the filter is estimated to be 3627 nm with a total of 27 alternating layers of TiO2 and SiO2 deposited on a substrate.
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Computer Optics and Nanophotonics / M.A. Butt, S.A. Fomchenkov, S.N. Khonina
Table 1. Layer thickness of Notch filter based on TiO2/SiO2.
Layer no. Layer Thickness Layer no. Layer Thickness
name (nm) name (nm)
1 SiO2 548 15 SiO2 147
2 TiO2 11 16 TiO2 164
3 SiO2 28 17 SiO2 149
4 TiO2 280 18 TiO2 127
5 SiO2 153 19 SiO2 165
6 TiO2 124 20 TiO2 42
7 SiO2 151 21 SiO2 25
8 TiO2 164 22 TiO2 116
9 SiO2 148 23 SiO2 80
10 TiO2 123 24 TiO2 16
11 SiO2 153 25 SiO2 39
12 TiO2 52 26 TiO2 120
13 SiO2 153 27 SiO2 227
14 TiO2 122 - - -
The transmission spectrum of the designed notch filter shows a stop band at 536 nm to 586 nm with a central wavelength at
561 nm. The line width which is measured at half of the maximum transmission is around 50 nm. The transmission in pass band
regions 450-536nm and 586-700nm is more than 90% as shown in figure 2. The transmission of such filters can be improved by
increasing the number of the layers. Whereas this designed filter has only 27 layers which can be implemented economically.
100
90
80
Transmission (%)
70
60
50
40
30
20
10 Incidence angle 0
Incidence angle 30
0
450 500 550 600 650 700
Wavelength (nm)
Fig. 2. The transmission spectrum of a notch filter at 0o and 30o of incidence light.
The designed filter has maximum transmission of 3% in the stop band. The OD of the filter is calculated by using an eq. (1)
which provides a value greater than 3.5 (Transmission is 0.0003%). It shows a promising result for the notch filter. The optical
density of the notch filter is plotted in figure 3.
5
Optical density
4
3
2
450 500 550 600 650 700
Wavelength (nm)
Fig. 3. The optical density of the designed notch filter.
rd
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Computer Optics and Nanophotonics / M.A. Butt, S.A. Fomchenkov, S.N. Khonina
4. Effect of the angle of incidence of light on the central wavelength and FWHM
In all dielectric stack filters, the transmission depends on the angle of incidence. The central wavelength of the blocking
region shifts to shorter wavelengths and FWHM increases as the angle of incidence is increased. It can be seen from figure 2,
when the angle of incidence of light increases, a noticeable increase in the FWHM of the bandwidth of stop band is seen which
shifts towards smaller wavelength. And an increase in the OD is also noticed which is around 3.9 with a slight decrease in the
transmission of the band-pass region. Table 2 summarizes the effect of the incidence angle of light on the filters FWHM and
central wavelength.
Table 2. Central wavelength and FWHM of the notch filter at different incident angles.
Angle of Incidence Central wavelength FWHM
(Degrees) (nm) (nm)
0 561 50
30 542 53
5. Conclusion
In this work, a multilayer dielectric optical notch filter design is presented which is based on TiO2/SiO2 alternating layers.
These filters provide an average transmission of more than 90% in region 450-535nm and 587-700 nm. The transmission of the
stop band 536-586 nm is around 3%. The OD of this filter is greater than 3.5 which shows the high blocking specification of a
filter and is associated with the amount of energy transmitted through it. With an increase in the incident angle of light, the
central wavelength of the notch filter shifts toward smaller wavelength.
Acknowledgements
This work was supported by the Ministry of Education and Science of the Russian Federation and the Russian Foundation for
Basic Research (grant No. 16-29-11698-ofi_m, 16-29-11744-ofi_m).
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