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.

Typical gyro mirrors
Typical gyro mirrors
A view into our  super-clean room
A view into our super-cleanroom

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 TSBAbsorptionReflection 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