Extremely low loss laser optics
So called supermirrors in ring laser gyroscope assemblies or certain scientific applications require coated optics with extremely low losses (i.e. absorption and scattering). These mirrors also have a maximum reflectivity with R > 99.998% and total losses < 10 ppm.
LASEROPTIK uses a modified IBS machine that is capable to produce coatings on superpolished substrates. The cleanliness of the machine and environment is maintained in a dedicated superclean room, where also the extensive substrate pre- and after-treatment takes place.
Measurement devices such as white light profilometers and high resolution microscopes (up to x 1000) for the inspection procedures are in place. A custom built cavity ring-down setup allows to determine the reflection with a precision of up to four decimal places and to refer back to the losses.
The total back scattering (TSB, as referred to in ISO 13696) of a supermirror at 633 nm taken from LASEROPTIK’s current production has been measured at the Fraunhofer Institute for Applied Optics and Precision Engineering in Jena and achieved a value of TSB = 1.1 ppm. The typical absorption and residual transmission of together < 15 ppm is equivalent to a reflectivity of at least 99.998%. For longer wavelengths even 99.999% has been achieved which is very close to the perfect laser mirror with R = 100%.
The results have been confirmed by LASEROPTIK’s in-house cavity ring-down setup and scatter measurement. The use of super-polished substrates with a surface roughness < 1 Å rms is essential for the values mentioned above. Their quality is inspected with a white light profilometer. Below some measured values:
|Scatter TSB||Absorption||Reflection CRD|
|HR 532 nm / 0°||4.9 ppm1) (int. ARS)||10.2 ppm 2)||> 99.997% 2) (T~5 ppm)|
|HR 633 nm / 0°||1.1 ppm 1)||> 99.998% (T~5 ppm)|
|HR 1064 nm / 0°||(< 1 ppm) 3)||< 2 ppm 4)||> 99.999% (T~5 ppm)|
|HR 2940 nm / 0°||24 ±12 ppm6)||99.994% 5) (T=36 ppm)|
1) Measured at IOF Jena; 2) Measured at LZH; 3) Calculated from surface roughness; 4) Measured at ILT Aachen; 5) measured by customer; 6) 1-R-T