Abstract
High reflectivity and low loss mirrors are very much essential for state-of art sensors like Ring Laser Gyroscopes widely used for navigation of moving platforms. Surface scattering is the main source of loss for the Laser mirrors of high reflectivity. Surface scattering also lead to coupling of back reflected light leading to dead band of optical sensors at low input rotation. Super polished glass substrates have been prepared from low expansion glass ceramic material. Surface roughness is achieved about 3-5Å (RMS value) for the ceramic glass substrates prior to thin film coating. The substrates are deposited with 41 alternative layers of SiO2 and Ta2O5 of Quarter-Wave optical thickness with Electron Beam Deposition (EBD) technique. Ion beam Assisted Electron Beam Deposition (IAEBD) is used for another batch of substrates to coat identical stack of layers on glass substrates of the same surface quality. The key process parameters of both the coating processes are presented. The optical constants of the films are evaluated with spectroscopic Ellipsometer from 400nm to 1600nm. Total Integrated Scattering is estimated from the achieved surface finishing of mirrors deposited. The multi-layer coatings are characterized with UV-Vis-IR Spectrophotometer and Cavity Ring Down(CRD) loss meter. The quantitative optical loss of the mirrors is measured at 45o angle of incidence with CRD loss meter. The merits of ion assisted EB process over simple EB coating processes are evaluated against optical constants of thin films and performances of mirrors. Fitness of the processes for realizing Laser mirrors for optical sensors at 632.8nm is evaluated.
Highlights
High reflectivity thin films have several applications in optical metrology instruments and electro-optic devices like Ring Laser Gyroscopes and Gravity Sensors
The multi-layer dielectric Laser mirrors deposited with EB deposition techniques shown a degraded surface roughness compared to the uncoated substrates due to columnar and bubbles structures of the film grown
The absorption loss is evidenced with more cavity decay loss when measured with cavity ring down loss meter for mirrors realized with Electron Beam Deposition (EBD) process
Summary
High reflectivity values of the order of 99.9% or better can be achieved with dielectric multi-layer coating. Roughness values achieved in the two processes namely EBD and IAEBD technique are contemplated for the Total Integrated Scattering (TIS), which is an important parameter for the application of mirror surfaces for the application of Laser resonators[3,4]. These techniques have been found to be very useful for fabricating Laser mirrors with TiO2 and SiO2 alternatively[5]. The present work brings out the merits of coating techniques correlating, optical constants, reflectivity, total loss and TIS values
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