1. Design and Fabrication of Alumina/Silica Optical Bandpass Filter
Gaurav Dwivedia, Nancy Sharmaa, VSRS Praveen Kumara,b, P. Sunitaa,b, Mukesh Kumara,b, Neelam Kumaria,b, Vinod Karara,b, Amit L. Sharmaa,b*
aCSIR-Central Scientific Instruments Organisation, Chandigarh, India,
bAcademy of Scientific & Innovative Research (AcSIR), CSIR-CSIO, Chandigarh
* Corresponding Author: Dr. Amit L. Sharma,
ABSTRACT
Substrate
Absentee Layer
L- SiO2
H- Al2O3
An alternate stack of 26 layers using Alumina (Al2O3) and Silica (SiO2) is designed and fabricated to realize optical bandpass filter at 660 nm wavelength for 45° angle of incidence.
FilmstarTM software is used to design the filter at desired wavelength with Alumina selected as high refractive index material and Silica as low refractive index material .
Designed filter was fabricated in e-beam vacuum coating plant on BK-7 glass substrate under reactive oxygenated environment.
The fabricated optical filter is characterised with the help of UV-Vis-NIR spectrophotometer to obtain the transmittance spectrum in the visible range ( nm).
Coated substrate is subjected to Tape and Adhesion test according to MIL standard to observe hardness and durability of coated optical thin film.
Keywords: Optical Bandpass filter, Thin Film Coating, Transmission Spectra, Adhesion, Hardness Testing
Figure 3: MIL standard Cellophane tape and rubbing pad.
Figure 2: Schematic for Al2O3/SiO2 layer stack
INTRODUCTION
The designing and development of Optical bandpass filters remain focussed area for researchers and optical industries.
Its excellent environmental durability, superlative reproducibility, and low thermal stress makes it fit for applications like aviation, automobile industry, optical instrumentation and strategic etc.
In the present work, thin films are prepared on optical grade glass substrate using electron beam evaporation technique by precise monitoring and control of thickness to get the required performance.
As per designed bandpass filter alternating layers with thickness values nm for Al2O3 and nm for SiO2 were deposited and a quartz crystal thickness monitor was used to control and monitor the thicknesses of these films during deposition.
Figure 4: Schematic of e-beam vacuum coating process
Figure 5: Pictographic view of e-beam coating equipment
RESULTS
DESIGN AND METHODOLOGY
Optical filter is designed using FilmstarTM software which includes deciding number of layers, materials with their refractive index and individual layer thickness.
Vacuum grade Al2O3 and SiO2 (M/s Umicore Thin Film Products, 99.99% purity) was used for evaporation having refractive indices of 1.63 and 1.46, respectively.
BK-7 optical grade glass (SCHOTT, Malaysia) was used as the substrate with dimensions 70 x 70 mm.
Electron beam evaporation plant (PFEIFFER PLS570) was used for fabrication of filter.
Transmission spectrum of this filter was obtained using UV-Vis-NIR spectrophotometer (Perkin-Elmer, Lambda 9) in the wavelength range of nm in normal and oblique angles of incidence.
Adhesion and hardness properties of the deposited films were tested by using cellophane tape and rubbing pad respectively according to MIL standards.
Figure 4: 26 layers with quarter wave stack of alternating high (Al2O3) and low (SiO2) indices
Figure 5: Transmission spectrum of optical bandpass filter simulated in FilmstarTM software
Figure 6: Measured transmission spectrum of Alumina/Silica optical bandpass filter
Figure 7: Comparison between designed and experimental results for Alumina/Silica optical bandpass filter
Table 1: Deposition Parameters
S. No.
Deposition Parameters
Corresponding values 1
Substrate temperature
250°C 2
Initial base pressure
2.432x10-5 mbar 3
Substrate used
BK7, Glass 4
Oxygen gas pressure
2x10-4 mbar 5
Glow discharge gas
Argon (Ar)
CONCLUSIONS
Measured transmittance was obtained as 79% at 660 nm for oblique angle of incidence which is found to be consistent with designed results.
No peel off or scratch was observed on coated substrate when subjected to adhesion and hardness test according to MIL standards.
Absentee layer of high refractive index material results in creation of passband in visible region nm.
ACKNOWLEDGEMENT
Authors are thankful to The Director, CSIR-CSIO, Chandigarh for his support. Funding from CSIR through 12th FYP Supra Institutional Project (SIP) grant OMEGA/PSC0202 is gratefully acknowledged.
Figure 1: Flow chart for design and development of optical bandpass filter
DESIGN EQUATIONS
REFERENCES
Alternative stack consists of following layer order:
(HL)5 2H (LH)7
where, H represents High refractive index material (Al2O3)
L represents Low refractive index material (SiO2)
Kumar, Mukesh, et al. "Multi-layer dielectric thin-film optical filters for beam folding applications." International Conference on Optics & Photonics International Society for Optics and Photonics, 2015.
Saraf, Mohit, et al. "Alternately Stacked TiO2/Al2O3 Multilayer Based Optical Filter Fabricated by Electron Beam Evaporation Technique.“ Exploring Basic and Applied Sciences for Next Generation Frontiers (EBAS), , 2014.
Macleod, H. Angus. Thin-film optical filters. CRC Press, 2010.